Optical measurement device

Abstract

The present invention provides a light measurement device with improved light intensity detection performance. The light measurement device includes: a light receiving element capable of converting the light intensity of the light to be measured into an electrical signal, an input terminal to which the electrical signal is input, a first amplifier and a non-linear element constituting a logarithmic amplifier, a plurality of offset resistors, a switching unit, and a control unit. The inverting input terminal of the first amplifier is electrically connected to the input terminal. The plurality of offset resistors each have a different resistance value. The switching unit can switch the offset resistor electrically connected between the voltage source and the input terminal among the plurality of offset resistors. An offset current is input to the input terminal through the offset resistor electrically connected between the voltage source and the input terminal. The control unit measures the light intensity based on the output voltage value of the first amplifier.

Description

Technical Field

[0001] The present disclosure relates to an optical measurement device. Background Art

[0002] Conventionally, a device for measuring the light intensity of light to be measured has been known (for example, Patent Document 1). The photoelectric conversion circuit described in Patent Document 1 includes an amplifier in which a logarithmic amplification negative feedback circuit using a logarithmic conversion element as a load and a linear amplification negative feedback circuit using a resistor as a load are connected in parallel.

[0003] Prior Art Documents

[0004] Patent Documents

[0005] Patent Document 1: Japanese Patent Laid-Open No. 02-090025 Summary of the Invention

[0006] Problems to be Solved by the Invention

[0007] In a device for measuring the light intensity of light to be measured, it is required to improve the performance of detecting the light intensity of the light to be measured.

[0008] The present disclosure has been completed in view of the above aspects, and an object thereof is to provide an optical measurement device that improves the performance of detecting the light intensity of light to be measured.

[0009] Means for Solving the Problems

[0010] The optical measurement device according to some embodiments includes: a light receiving element that can convert the light intensity of the light to be measured into an electrical signal; an input terminal to which the electrical signal is input; a first amplifier and a non-linear element that constitute a logarithmic amplifier, and an inverting input terminal of the first amplifier is electrically connected to the input terminal; a plurality of offset resistors each having a different resistance value; a switch unit that can switch the offset resistor that electrically connects a voltage source and the input terminal among the plurality of offset resistors; and a control unit that inputs an offset current to the input terminal through the offset resistor that electrically connects the voltage source and the input terminal, and the control unit measures the light intensity based on the output voltage value of the logarithmic amplifier. With such a configuration, the light intensity of the light to be measured can be measured at high speed without distortion. Therefore, an optical measurement device that improves the performance of measuring the light intensity is provided.

[0011] In the optical measurement device according to one embodiment, the control unit measures the current value of the offset current based on the output voltage value of the logarithmic amplifier when the light receiving element is shielded from light. With such a configuration, the optical measurement device can measure the current value of the offset current.

[0012] In the optical measurement device according to one embodiment, the control unit calculates the light intensity of the measured light by subtracting the current value of the offset current calculated based on the output voltage value of the logarithmic amplifier when the light shielding element shields the light receiving element from the current value calculated based on the output voltage value of the logarithmic amplifier when the measured light is incident on the light receiving element. With such a configuration, the light intensity of the measured light can be measured over a wide range.

[0013] The optical measurement device according to one embodiment includes: a second amplifier having a non-inverting input terminal electrically connected to the voltage source; a first switch capable of switching whether to electrically connect the light receiving element to the input terminal; a second switch capable of switching whether to electrically connect the light receiving element and the inverting input terminal of the second amplifier; a third switch capable of switching whether to electrically connect the inverting input terminal and the output terminal of the second amplifier; and a fourth switch capable of switching whether to electrically connect the non-inverting input terminal of the second amplifier to a reference potential. The plurality of offset resistors are provided between the light receiving element and the output terminal of the second amplifier. With such a configuration, the optical measurement device can use the offset resistor to adjust the current value of the offset current and also use it as a feedback resistor of the second amplifier. As a result, cost reduction and reduction of the mounting area can be achieved, and an optical measurement device capable of forming both a logarithmic amplifier and a linear amplifier can be configured.

[0014] The optical measurement device according to one embodiment has: a first mode of measuring the light intensity based on the output voltage value of the logarithmic amplifier, and a second mode of measuring the light intensity based on the output voltage value of the second amplifier. In the first mode, the light receiving element and the input terminal are electrically connected through the first switch, the light receiving element and the inverting input terminal of the second amplifier are electrically separated through the second switch, the inverting input terminal and the output terminal of the second amplifier are electrically connected through the third switch, and the non-inverting input terminal of the second amplifier is electrically separated from the reference potential through the fourth switch. In the first mode, the offset current is input to the input terminal through the offset resistor electrically connected between the output terminal and the input terminal of the second amplifier. With such a configuration, for example, the user can appropriately switch the mode of the optical measurement device to the first mode or the second mode according to the measured light.

[0015] In the optical measurement device according to one embodiment, the voltage source is a digital-to-analog conversion unit. With such a configuration, the optical measurement device can input a voltage corresponding to the first mode or the second mode to the non-inverting input terminal of the second amplifier through the digital-to-analog conversion unit.

[0016] In the light measurement device according to one embodiment, in the first mode, the control unit measures the current value of the offset current based on the output voltage value of the second amplifier when the light receiving element is shielded from light. With such a configuration, the light measurement device can measure the current value of the offset current.

[0017] In the light measurement device according to one embodiment, the control unit measures the output voltage value of the logarithmic amplifier and the output voltage value of the second amplifier when the light receiving element for each current value of the offset current is shielded from light while switching the switch unit, and generates a table by associating the measured output voltage value of the logarithmic amplifier with the current value of the offset current calculated based on the measured output voltage value of the second amplifier. Based on the output voltage value of the logarithmic amplifier when the measured light is incident on the light receiving element and the table, the light intensity of the measured light is measured. With such a configuration, in the first mode, the light intensity of the measured light is measured with high precision.

[0018] The light measurement device according to one embodiment further includes: a transistor that is an N-channel depletion-type field-effect transistor, the light receiving element is a photodiode, the anode of the photodiode is electrically connected to the first switch and the second switch, the gate of the transistor is electrically connected to the input terminal, the source of the transistor is electrically connected to the cathode of the photodiode, and a positive voltage value is input to the drain of the transistor. With such a configuration, in the first mode, the light intensity of the measured light is measured with high precision.

[0019] The light measurement device according to one embodiment further includes: a fifth switch capable of switching whether the cathode of the photodiode is electrically connected to the reference potential or to the source of the transistor. In the second mode, the cathode of the photodiode is electrically connected to the reference potential through the fifth switch. With such a configuration, in the second mode, the light intensity of the measured light is measured with high precision.

[0020] Effects of the Invention

[0021] According to the present disclosure, a light measurement device with improved performance in detecting light intensity is provided. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 is a block diagram of the light measurement device according to the first embodiment of the present disclosure.

[0023] Figure 2 is Figure 1 a block diagram of the logarithmic amplifier circuit shown.

[0024] Figure 3 is a diagram showing a setting example of measurement sensitivity.

[0025] Figure 4 It is a diagram showing the waveform of the spectrum.

[0026] Figure 5 It shows Figure 1 A flowchart showing an example of the optical measurement method of the optical measurement device shown.

[0027] Figure 6 It is an optical measurement device using a linear amplifier of the first comparative example.

[0028] Figure 7 It is a diagram showing a setting example of the measurement sensitivity of the first comparative example.

[0029] Figure 8 It is an optical measurement device using a logarithmic amplifier of the second comparative example.

[0030] Figure 9 It is a diagram showing the waveforms of the spectra of the first comparative example and the second comparative example.

[0031] Figure 10 It is a block diagram of the optical measurement device according to the second embodiment of the present disclosure.

[0032] Figure 11 It is a graph showing the relationship between the output voltage value and the current value.

[0033] Figure 12 It is an example of a table showing the current value with respect to the output voltage value.

[0034] Figure 13 It shows Figure 10 A flowchart showing an example of the optical measurement method of the optical measurement device shown (Part 1).

[0035] Figure 14 It shows Figure 10 A flowchart showing an example of the optical measurement method of the optical measurement device shown (Part 2).

[0036] Figure 15 It is a block diagram of the optical measurement device according to the third embodiment of the present disclosure.

[0037] Figure 16 It shows Figure 15 A diagram showing the waveform of the output voltage value of the amplified logarithmic circuit shown.

[0038] Reference Numeral Explanation

[0039] 1, 101, 201 Optical measurement device

[0040] 2 Arithmetic device

[0041] 10 Photodiode

[0042] 20 Logarithmic Amplifier Circuit (Logarithmic Amplifier)

[0043] 21 Amplifier (First Amplifier)

[0044] 22, 23 Amplifiers

[0045] 30, 130 Switching Section

[0046] 40 AD Conversion Section

[0047] 50 Storage Section

[0048] 51 Input Section

[0049] 52 Control Section

[0050] 60 Amplifier (Second Amplifier)

[0051] 70 DA Conversion Section

[0052] C130 Capacitor

[0053] P1 Input Terminal

[0054] P2 Input Terminal

[0055] P3 Output Terminal

[0056] R1, R2, R3, R4, R5, R6, R7, R8 Resistors

[0057] R30, R130 Offset Resistors

[0058] SW1 Switch (First Switch)

[0059] SW2 Switch (Second Switch)

[0060] SW3 Switch (Third Switch)

[0061] SW4 Switch (Fourth Switch)

[0062] SW5 Switch

[0063] SW6 Switch (Fifth Switch)

[0064] SW30, SW130 Changeover Switches

[0065] T1, T2 Transistors (Nonlinear Elements)

[0066] T3 Transistor Detailed Implementation Manner

[0067] In the present disclosure, a "linear amplifier" means an amplifier that uses a fixed resistor in the feedback section of the amplifier. The fixed resistor used in the feedback section is also referred to as a "feedback resistor". The linear amplifier can be used in a transimpedance circuit.

[0068] In the present disclosure, a "logarithmic amplifier" means an amplifier that uses a non-linear element in the feedback section of the amplifier. The non-linear element is, for example, a transistor or a diode. The non-linear element used in the logarithmic amplifier converts the current value input to the non-linear element into a voltage value proportional to the logarithm of the current value. The non-linear element is also referred to as a "logarithmic conversion element". The logarithmic amplifier can be used in a transimpedance circuit.

[0069] (First Embodiment)

[0070] The optical measurement device 1 can be applied to various uses for measuring the optical intensity of the measured light. As will be described below, the optical measurement device 1 can measure a wide range of optical intensities. The optical measurement device 1 can be applied to a spectroscopic analyzer or an optical power meter that requires measurement of a wide range of optical intensities. For example, in a spectroscopic analyzer, measurement of a wide range of optical intensities from +10 [dBm] to -90 [dBm] is required. Hereinafter, the optical measurement device 1 will be described as a device applicable to a spectroscopic analyzer.

[0071] In a spectroscopic analyzer, the measured light is dispersed by a monochromator such as a diffraction grating. As Figure 1 shown, the dispersed measured light is input to the optical measurement device 1.

[0072] As Figure 1 shown, the optical measurement device 1 includes: a photodiode 10 (light receiving element), a logarithmic amplifier circuit 20, a resistor R5, a switch section 30, offset resistors R30-1 to R30-N, an AD (Analog to Digital) conversion section 40, and an arithmetic device 2. The arithmetic device 2 includes a storage section 50, an input section 51, and a control section 52. Among them, if the optical measurement device 1 is a light receiving element that can convert the optical intensity of the measured light into an electrical signal, it may also include a light receiving element other than the photodiode 10. The logarithmic amplifier circuit 20 has an input terminal P1, an input terminal P2, and an output terminal P3.

[0073] Hereinafter, without particularly distinguishing each of the offset resistors R30-1 to R30-N, they are also collectively referred to as "offset resistor R30".

[0074] The anode of the photodiode 10 is electrically connected to the input terminal P1 of the logarithmic amplifier circuit 20. The cathode of the photodiode 10 is electrically connected to the reference potential. The light to be measured is incident on the photodiode 10. The photodiode 10 converts the light intensity of the light to be measured into a photocurrent ip through the photovoltaic effect. The photocurrent ip is input to the input terminal P1 of the logarithmic amplifier circuit 20. The current value of the photocurrent ip is also referred to as "photocurrent value Ip".

[0075] The photocurrent value Ip is converted into a voltage value through a transimpedance circuit. In the present embodiment, the transimpedance circuit is a logarithmic amplifier as described below. The optical measurement device 1 calculates the light intensity of the light to be measured based on the output voltage value of the logarithmic amplifier.

[0076] As Figure 2 shown, the logarithmic amplifier circuit 20 includes: an amplifier 21 (first amplifier), an amplifier 22, an amplifier 23, a transistor T1, a transistor T2, a resistor R1, a resistor R2, a resistor R3, and a resistor R4. The transistors T1 and T2 are, for example, bipolar transistors.

[0077] The amplifier 21 and the transistor T1 are configured as a logarithmic amplifier. For example, the emitter of the transistor T1 is electrically connected to the output terminal of the amplifier 21, and the collector of the transistor T1 is electrically connected to the inverting input terminal of the amplifier 21. In addition, the transistor T1 is configured as a grounded-base bipolar transistor. For example, the base of the transistor T1 is electrically connected to the reference potential. In addition, the amplifier 21 is configured such that the inverting input terminal and the non-inverting input terminal are in a virtual short circuit state. For example, the non-inverting input terminal of the amplifier 21 is electrically connected to the reference potential.

[0078] The inverting input terminal of the amplifier 21 is electrically connected to the input terminal P1 of the logarithmic amplifier circuit 20. The current input to the input terminal P1 is also referred to as "current i1". In addition, the current value of the current i1 is also referred to as the current value I1.

[0079] The relationship between the current value I1 and the output voltage value Va1 of the amplifier 21 is represented by Equation (1) according to the characteristics of the bipolar transistor.

[0080] Va1 = -kT / q × ln(I1 / Is) Equation (1)

[0081] In Equation (1), the constant k is the Boltzmann constant. The constant k is, for example, 1.38×10 －23 [J / K]. The temperature T is the absolute temperature of the transistor T1. The electric charge quantity q is the electric charge quantity per one electron. The electric charge quantity q is, for example, 1.602×10 －19 [C]. The current value Is is the current value of the reverse saturation current of the transistor T1.

[0082] In the present embodiment, in order to cancel the current value Is in Equation (1), as described below, an amplifier 22, an amplifier 23, a transistor T2, a voltage source Vref, and a resistor R5 are used.

[0083] The amplifier 22 is a replica circuit of the amplifier 21. The amplifier 22 has the same electrical characteristics as the amplifier 21. The transistor T2 is a replica circuit of the transistor T1. The transistor T2 has the same electrical characteristics as the transistor T1. The transistor T2 is in thermal contact with the transistor T1. The transistor T2 operates at the same temperature as the transistor T1.

[0084] The amplifier 22 and the transistor T2 are configured as a logarithmic amplifier. For example, the emitter of the transistor T2 is electrically connected to the output terminal of the amplifier 22, and the collector of the transistor T2 is electrically connected to the inverting input terminal of the amplifier 22. In addition, the transistor T2 is configured as a grounded-base bipolar transistor. For example, the base of the transistor T2 is electrically connected to the reference potential. In addition, the amplifier 22 is configured such that the inverting input terminal and the non-inverting input terminal are in a virtual short circuit state. For example, the non-inverting input terminal of the amplifier 22 is electrically connected to the reference potential.

[0085] The inverting input terminal of the amplifier 22 is electrically connected to the input terminal P2 of the logarithmic amplifier circuit 20. The voltage from the voltage source Vref is input to the input terminal P2 via the resistor R5. A current i2 is input to the input terminal P2. The current value of the current i2 is also referred to as “current value I2”.

[0086] The current value I2 is set by the voltage value VREF of the voltage source Vref and the resistor R5. The current value I2 is known. The resistor R5 is composed of a fixed resistor. The resistor R5 has two terminals. One terminal of the resistor R5 is electrically connected to the input terminal P2. The other terminal of the resistor R5 is electrically connected to the voltage source Vref.

[0087] The relationship between the current value I2 and the output voltage value Va2 of the amplifier 22 is expressed by Equation (2) in the same manner as Equation (1) above.

[0088] Va2 = -kT / q × ln(I2 / Is) Equation (2)

[0089] The output terminal of the amplifier 21 is electrically connected to the inverting input terminal of the amplifier 23 via the resistor R1. The inverting input terminal of the amplifier 23 is electrically connected to the output terminal of the amplifier 23 via the resistor R2. In addition, the output terminal of the amplifier 22 is electrically connected to the non-inverting input terminal of the amplifier 23 via the resistor R3. The non-inverting input terminal of the amplifier 23 is electrically connected to the reference potential via the resistor R4.

[0090] The resistor R1 and the resistor R3 are composed of a resistance temperature detector having a positive temperature coefficient. The resistor R2 and the resistor R4 are composed of fixed resistors.

[0091] Each of the resistors R1 and R2 has two terminals. One terminal of the resistor R1 is electrically connected to the output terminal of the amplifier 21. The other terminal of the resistor R1 is connected to one terminal of the resistor R2 and the inverting input terminal of the amplifier 23. The other terminal of the resistor R2 is connected to the output terminal of the amplifier 23. Each of the resistors R3 and R4 has two terminals. One terminal of the resistor R3 is electrically connected to the output terminal of the amplifier 22. The other terminal of the resistor R3 is electrically connected to one terminal of the resistor R4 and the non-inverting input terminal of the amplifier 23. The other terminal of the resistor R4 is connected to the reference potential.

[0092] As will be described later, the light intensity of the measured light is calculated based on the output voltage value of the logarithmic amplifier circuit 20. The output voltage value of the amplifier 23, that is, the output voltage value Vo1 of the logarithmic amplifier circuit 20 is represented by Equation (3).

[0093] Vo1 = G × (Va2 - Va1)

[0094] = GkT / q × ln(I1 / I2) Equation (3)

[0095] In Equation (3), G = r2 / r1 = r4 / r3. The resistance value r1 is the resistance value of the resistor R1. The resistance value r2 is the resistance value of the resistor R2. The resistance value r3 is the resistance value of the resistor R3. The resistance value r4 is the resistance value of the resistor R4.

[0096] It can be seen from Equation (3) that the output voltage value Vo1 has temperature dependence. For example, the output voltage value Vo1 is proportional to the temperature T. The temperature dependence of the output voltage value Vo1 is reduced by using a resistance temperature detector having a positive temperature coefficient in the resistors R1 and R3 among the resistors R1 to R4.

[0097] Here, when the photodiode 10 is shielded from light, the photocurrent value Ip becomes 0 [A] or less. If the current value I1 becomes 0 [A] or less because the photocurrent value Ip becomes 0 [A] or less, since the current flowing from the emitter to the collector of the transistor T1 is very small, the output voltage value Va1 of the amplifier 21 saturates at a positive voltage value. That is, the output voltage value Va1 of the amplifier 21 saturates at a positive voltage value, and the amplifier 21 latches up. In order to suppress the latching up of the amplifier 21, in the present embodiment, an offset current ioff is input to the input terminal P1 through the switch unit 30 and the offset resistor R30. The current value of the offset current ioff is also referred to as the "offset current value Ioff". With such a configuration, the current value I1 becomes the sum of the offset current value Ioff and the photocurrent value Ip. Since the current value I1 becomes the sum of the offset current value Ioff and the photocurrent value Ip, the case where the current value I1 becomes 0 [A] or less is suppressed, and the case where the amplifier 21 latches up is suppressed.

[0098] The offset resistor R30 has two terminals. The offset resistor R30 is composed of a fixed resistor. The resistance values of each of the offset resistors R30-1 to R30-N are different from each other. For example, the resistance value of the offset resistor R30 increases by 10 times each time from the offset resistor R30-1 to the offset resistor R30-N.

[0099] The switch unit 30 can switch the offset resistor R30 electrically connected between the voltage source Vb and the input terminal P1 among the offset resistors R30-1 to R30-N. The switch unit 30 includes changeover switches SW30-1 to SW30-N.

[0100] Hereinafter, without particularly distinguishing the changeover switches SW30-1 to SW30-N, they are also referred to as the "changeover switch SW30".

[0101] The changeover switch SW30 has two terminals. The changeover switch SW30 is composed of a mechanical relay, an optoelectronic relay, an analog switch, or the like. However, the switch unit 30 may also be configured as an analog multiplexer. In the case where the switch unit 30 is configured as an analog multiplexer, one terminal of each of the changeover switches SW30-1 to SW30-N is shared.

[0102] Hereinafter, the offset resistor R30 corresponding to the changeover switch SW30-i (i is an integer from 1 to N) is also referred to as the "offset resistor R30-i".

[0103] One terminal of the changeover switch SW30-i is electrically connected to the voltage source Vb. Another terminal of the changeover switch SW30-i is electrically connected to one terminal of the offset resistor R30-i. Another terminal of the offset resistor R30-i is electrically connected to the input terminal P1. Alternatively, one terminal of the offset resistor R30-i may be electrically connected to the voltage source Vb, and another terminal of the offset resistor R30-i may be connected to one terminal of the changeover switch SW30-i. In this case, another terminal of the changeover switch SW30-i is electrically connected to the input terminal P1.

[0104] A control signal from the control unit 52 is input to the changeover switch SW30. The changeover switch SW30 becomes an on state (conductive state) or an off state (non-conductive state) based on the control signal from the control unit 52. When the changeover switch SW30-i becomes an on state, the offset resistor R30-i is electrically connected between the voltage source Vb and the input terminal P1. Further, when the changeover switch SW30-i is in an off state, the offset resistor R30-i is electrically separated from between the voltage source Vb and the input terminal P1.

[0105] The AD conversion unit 40 is an analog-digital conversion unit. The AD conversion unit 40 is electrically connected to the output terminal of the amplifier 23. The output voltage value Vo1 of the logarithmic amplifier circuit 20 is input to the AD conversion unit 40. The AD conversion unit 40 converts the output voltage value Vo1, which is an analog signal, into a digital signal. The AD conversion unit 40 outputs the digital signal to the control unit 52.

[0106] The storage unit 50 is, for example, a semiconductor memory, a magnetic memory, an optical memory, or the like. However, the storage unit 50 is not limited thereto. The storage unit 50 may also have functions as a main storage device, an auxiliary storage device, or a cache memory, for example. The storage unit 50 stores any information used in the operation of the optical measurement device 1. For example, the storage unit 50 may store various information such as a system program and an application program.

[0107] The input unit 51 includes an input interface for receiving an input from a user. The input interface is a physical key, a capacitance key, a touch screen, a microphone that accepts voice input, or the like. However, the input interface is not limited thereto.

[0108] The control unit 52 includes at least one processor, at least one dedicated circuit, or a combination thereof. The processor is, for example, a general-purpose processor such as a CPU (Central Processing Unit) or a GPU (Graphics Processing Unit), or a dedicated processor dedicated to specific processing. The dedicated circuit is, for example, an FPGA (Field-Programmable Gate Array) or an ASIC (Application-Specific Integrated Circuit). The control unit 52 controls each part of the optical measurement device 1 while performing processing related to the operation of the optical measurement device 1.

[0109] [Setting Process of Measurement Sensitivity]

[0110] Before performing the measurement of the spectrum, for example, the control unit 52 receives the input of the measurement sensitivity described below through the input unit 51. This input is input by the user from the input unit 51.

[0111] The measurement sensitivity is an index indicating at what noise level the light intensity of the light to be measured is measured. The measurement sensitivity is specified by the sensitivity of the optical measurement device 1, for example. The higher the sensitivity of the optical measurement device 1, the smaller the noise level. For example, as Figure 3 shown, the measurement sensitivity is set to sensitivity A, sensitivity B, sensitivity C, and sensitivity D. The noise level decreases in the order of sensitivity A, sensitivity B, sensitivity C, and sensitivity D. The noise levels of the optical measurement device 1 for each of sensitivity A, sensitivity B, sensitivity C, and sensitivity D are -50 [dBm], -60 [dBm], -70 [dBm], and -80 [dBm].

[0112] The higher the feedback resistance value of the feedback part of the amplifier 21, that is, the resistance value Rt of the transistor T1, the higher the sensitivity of the current value I1 with respect to the output voltage value Vо1 of the logarithmic amplifier circuit 20. That is, the higher the resistance value Rt of the transistor T1, the higher the sensitivity of the logarithmic amplifier circuit 20, and the smaller the noise level of the optical measurement device 1. However, since the higher the resistance value Rt of the transistor T1, the lower the response speed of the amplifier 21, the measurement speed of the optical measurement device 1 decreases. That is, the higher the resistance value Rt of the transistor T1, the smaller the noise level of the optical measurement device 1, but the measurement speed of the optical measurement device 1 decreases. For example, the noise level decreases in the order of sensitivity A, sensitivity B, sensitivity C, and sensitivity D, but the measurement speed decreases by one-tenth one by one in the order of sensitivity A, sensitivity B, sensitivity C, and sensitivity D.

[0113] Here, the resistance value Rt of the transistor T1 is represented by Equation (4). Equation (4) is derived by differentiating the above Equation (1). In addition, the cut-off frequency fc of the amplifier 21 is represented by Equation (5).

[0114] Rt = kT / (q×I1) Equation (4)

[0115] fc = 1 / (2π×Rt×Cj) = (q×I1) / (2π×kT×Cj) Equation (5)

[0116] In Equation (5), the capacitance value Cj is the junction capacitance value of the transistor T1. For example, when the capacitance value Cj is 1 [pF] and the current value I1 is 100 [pA], the cut-off frequency fc becomes 600 [Hz]. In addition, when the capacitance value Cj is 1 [pF] and the current value I1 is 1 [nA], the cut-off frequency fc becomes 6 [kHz].

[0117] As can be seen from Equation (4), the smaller the current value I1, the higher the resistance value Rt. In addition, as described above, the higher the resistance value Rt of the transistor T1 becomes, the higher the measurement sensitivity of the optical measurement device 1 becomes, but the measurement speed of the optical measurement device 1 decreases. As a result, the smaller the current value I1, the higher the resistance value Rt, and the higher the measurement sensitivity of the optical measurement device 1, but the measurement speed of the optical measurement device 1 decreases. In addition, as can be seen from Equation (5), the cut-off frequency fc is proportional to the current value I1. That is, the cut-off frequency fc increases in proportion to the current value I1 up to the amplification band of about several [MHz] of the amplifier 21. Therefore, the larger the current value I1, the higher the cut-off frequency fc, and the frequency band of the amplifier 21 becomes a wide frequency band.

[0118] In the present embodiment, as described above, the current value I1 becomes the sum of the offset current value Ioff and the photocurrent value Ip. In the present embodiment, the offset current value Ioff can be adjusted by switching the offset resistor R30 electrically connected between the voltage source Vb and the input terminal P1 by the switch unit 30. That is, in the present embodiment, by adjusting the offset current value Ioff, the current value I1 can be adjusted, thereby adjusting the measurement sensitivity of the optical measurement device 1, the measurement speed of the optical measurement device 1, and the frequency band of the amplifier 21.

[0119] For example, when the light intensity of the light to be measured is low, the offset resistor R30 having a large resistance value is electrically connected between the voltage source Vb and the input terminal P1 by the switch unit 30. With such a configuration, since the offset current value Ioff becomes smaller and the current value I1 becomes smaller, the measurement sensitivity of the optical measurement device 1 is improved.

[0120] For example, when the light intensity of the light to be measured is high, the offset resistor R30 with a small resistance value is electrically connected between the voltage source Vb and the input terminal P1 through the switching unit 30. With such a structure, since the offset current value Ioff becomes larger and the current value I1 becomes larger, the measurement speed of the optical measurement device 1 becomes high speed.

[0121] As Figure 3 shown, the sensitivities A, B, C, and D are set by adjusting the offset current value Ioff to 200 [nA], 20 [nA], 2 [nA], and 200 [pA], respectively. In this case, the cut-off frequencies fc of the sensitivities A, B, C, and D become 1 [MHz], 100 [kHz], 10 [kHz], and 1 [kHz], respectively. In Figure 3 this, the resistance value Rs30 is the resistance value of the offset resistor R30 that is electrically connected between the input terminal P1 and the voltage source Vb through the switching unit 30. When the voltage value VB of the voltage source Vb is 0.2 [V], by setting the resistance value Rs30 to each of 1 [MΩ], 10 [MΩ], 100 [MΩ], and 1 [GΩ], the measurement sensitivity is set to each of the sensitivities A, B, C, and D.

[0122] Hereinafter, the resistance values of each of the offset resistors R30-1, R30-2, R30-3, and R30-4 are set to 1 [MΩ], 10 [MΩ], 100 [MΩ], and 1 [GΩ], respectively. By electrically connecting each of the offset resistors R30-1 to R30-4 between the input terminal P1 and the voltage source Vb through the switching unit 30, the measurement sensitivity of the optical measurement device 1 is set to each of the sensitivities A to D. That is, each of the sensitivities A to D is set by setting each of the changeover switches SW30-1 to SW30-4 to the on state and setting the changeover switches SW30 other than each of the changeover switches SW30-1 to SW30-4 to the off state.

[0123] In the storage unit 50, the measurement sensitivity is stored in association with the switching information of the switching unit 30 for setting the measurement sensitivity. The switching information of the switching unit 30 includes the information of the changeover switch SW30 set to the on state and the information of the changeover switch SW30 set to the off state. For example, when the measurement sensitivity is Figure 3 the sensitivity A as shown, the switching information of the switching unit is the information of the changeover switch SW30-1 set to the on state and the information of the changeover switches SW30 other than the changeover switch SW30-1 set to the off state.

[0124] [Measurement process of offset current value]

[0125] By measuring the output voltage value Vo1 of the logarithmic amplifier circuit 20 when the photodiode 10 is shielded from light by the AD conversion unit 40, the control unit 52 can measure the offset current value Ioff. That is, the control unit 52 can calculate the offset current value Ioff based on the output voltage value Vo1. As will be described later, the offset current value Ioff is used for the calculation of the light intensity of the light to be measured. Sometimes due to manufacturing errors of the optical measurement device 1, etc., the offset current value Ioff as the actual value deviates from Figure 3 the offset current value Ioff set as a fixed value as shown. By measuring the offset current value Ioff, the light intensity of the light to be measured is measured with higher accuracy.

[0126] The control unit 52 calculates the offset current value Ioff1 through the output voltage value Vo1 and Equation (6). The offset current value Ioff1 is the current value calculated as the offset current value from the output voltage value Vo1.

[0127] Ioff1 = I2 × exp(Vo1 / K) Equation (6)

[0128] In Equation (6), K = GkT / q.

[0129] It can be seen from Equation (6) that the offset current value Ioff1 calculated through the output voltage value Vo1 depends on the temperature T. That is, even if the offset resistor R30 connected between the voltage source Vb and the input terminal P1 is the same, if the temperature T of the optical measurement device 1 is different during these optical measurements, the offset current value Ioff1 is also different.

[0130] Therefore, the control unit 52 measures the offset current value Ioff1, for example, before performing the light intensity measurement process or periodically. The control unit 52 can also measure the offset current value Ioff1 for each offset resistor R30, that is, for each measurement sensitivity. In this case, the control unit 52 measures the output voltage value Vo1 of the logarithmic amplifier circuit 20 when the photodiode 10 is shielded from light while switching the offset resistor R30 electrically connected between the voltage source Vb and the input terminal P1 through the switch unit 30. The control unit 52 calculates the offset current value Ioff1 for each offset resistor R30 through the output voltage value Vo1 and Equation (6). The control unit 52 stores the offset current value Ioff1 associated with the offset resistor R30, that is, the measurement sensitivity, in the storage unit 50.

[0131] [Measurement process of light intensity]

[0132] The control unit 52 measures the output voltage value Vo1 of the logarithmic amplifier circuit 20 when the light to be measured is incident on the photodiode 10 through the AD conversion unit 40. The control unit 52 calculates the current value I1 based on the measured output voltage value Vo1 and Equation (7). Additionally, Equation (7) is derived from the above Equation (3). Also, the current value I2 in Equation (7) is known as described above. The control unit 52 obtains the offset current value Ioff1 associated with the measurement sensitivity from the storage unit 50. The control unit 52 calculates the photocurrent value Ip by subtracting the obtained offset current value Ioff1 from the current value I1 according to Equation (8). The control unit 52 calculates the light intensity Pin based on the photocurrent value Ip and Equation (9).

[0133] I1 = I2 × exp(Vo1 / K) Equation (7)

[0134] Ip = I1 - Ioff1 Equation (8)

[0135] Pin = Ip / S Equation (9)

[0136] In Equation (7), K = GkT / q. Also, in Equation (9), the light reception sensitivity S is the light reception sensitivity of the photodiode 10.

[0137] Figure 4 is a graph showing the waveform of the spectrum. In Figure 4 the horizontal axis represents the optical wavelength [nm]. Also, the vertical axis represents the light intensity [dBm]. In a spectrum analyzer, the spectrum is measured by continuously measuring the light intensity with respect to the optical wavelength. That is, when the optical measurement device 1 is applied to a spectrum analyzer, in the optical measurement device 1, the light intensity is measured while scanning the optical wavelength.

[0138] The waveform W1 represents the light intensity calculated by setting Ip = I1 without subtracting the offset current value Ioff1 from the current value I1 according to Equation (8) and calculating from Equation (9). The waveform W2 is the result calculated by subtracting the offset current value Ioff1 from the current value I1 according to Equation (8) to calculate the photocurrent value Ip and then calculating based on the calculated photocurrent value Ip and Equation (9).

[0139] In this way, in the optical measurement device 1, the control unit 52 subtracts the offset current value Ioff1 from the current value I1 according to Equation (8) to calculate the photocurrent value Ip. With such a structure, the light intensity of the light to be measured can be measured over a wide range.

[0140] [Operation of the Optical Measurement Device]

[0141] Figure 5 is a representation of Figure 1The light measurement method can also be implemented as a light measurement program that is executed by a processor such as the control unit 52. The light measurement program can also be stored in a non-transitory computer-readable medium. When the control unit 52 detects an input for measuring sensitivity through the input unit 51, it starts Figure 5 The process of step S10 is shown.

[0142] In the process of step S10, the control unit 52 receives an input of the measurement sensitivity through the input unit 51. The user inputs the measurement sensitivity from the input unit 51 according to the light intensity of the light to be measured. Figure 3 For example, when the light intensity of the light to be measured is low, the user inputs sensitivity D from the input unit 51 . Also, when the light intensity of the light to be measured is high and the user wishes to increase the measurement speed, the user inputs sensitivity A from the input unit 51 .

[0143] Before executing the process of step S11, the photodiode 10 is set to a light-shielded state. In the process of step S11, the control unit 52 switches the offset resistor R30 electrically connected between the voltage source Vb and the input terminal P1 through the switch unit 30, and measures the output voltage value V01 of the logarithmic amplifier circuit 20 when the photodiode 10 is light-shielded.

[0144] In the process of step S12 , the control unit 52 calculates the offset current value Ioff1 for each offset resistor R30 using the output voltage value V01 and the above equation (6). The control unit 52 stores the offset current value Ioff1 associated with the offset resistor R30 , ie, the measurement sensitivity, in the storage unit 50 .

[0145] In the process of step S13, the control unit 52 obtains the switching information of the switch unit 30 for setting the measurement sensitivity received in the process of step S10 from the storage unit 50. The control unit 52 controls the switch unit 30 according to the obtained switching information of the switch unit 30. For example, the measurement sensitivity received in the process of step S10 is Figure 3 In the case of the sensitivity A shown in FIG. 1 , the control unit 52 turns the switch SW30 - 1 on and turns the switches SW30 other than the switch SW30 - 1 off.

[0146] Before executing the process of step S14, the photodiode 10 is set to a state where the light to be measured can pass through the photodiode 10. In the process of step S14, the light to be measured is input to the photodiode 10. In the process of step S14, the control unit 52 measures the output voltage value V01 of the logarithmic amplifier circuit 20 when the light to be measured is incident on the photodiode 10 through the AD converter 40.

[0147] In the process of step S15, the control unit 52 acquires the offset current value Ioff1 associated with the measurement sensitivity received in the process of step S10 from the storage unit 50. In the process of step S15, the control unit 52 calculates the light intensity of the measured light by using the output voltage value Vo1 measured in the process of step S14, the offset current value Ioff1 acquired from the storage unit 50, and the above formulas (7) to (9).

[0148] In addition, if the control unit 52 executes the processes of steps S11 and S12 in advance, it may not execute the processes of steps S11 and S12.

[0149] In addition, the control unit 52 may execute the processes of steps S11 and S12 regularly at an arbitrary timing. In this case, when executing the process of step S11, the user may manually shield the photodiode 10. Alternatively, the optical measurement device 1 may be configured to automatically shield the photodiode 10 when executing the process of step S11.

[0150] The effects of the optical measurement device 1 of the present embodiment will be described while comparing the optical measurement device 301 of the first comparative example and the optical measurement device 401 of the second comparative example.

[0151] <First Comparative Example>

[0152] Figure 6 This is the optical measurement device 301 that uses the linear amplifier of the first comparative example. The optical measurement device 301 includes a photodiode 10, an AD conversion unit 40, an amplifier 302, feedback units 303-1 to 303-N, a resistor R304, a resistor R305, and a DA (Digital to Analog) conversion unit 306. In addition, the resistor R304 and the resistor R305 are used in the same way as the resistor R6 and the resistor R7 described later. Figure 10 In addition, the DA conversion unit 306 is used in the same way as the DA conversion unit 70 described later. Figure 10 The anode of the photodiode 10 is electrically connected to the inverting input terminal of the amplifier 302. When the measured light is incident on the photodiode 10, the photocurrent ip flows from the photodiode 10 toward the inverting input terminal of the amplifier 302.

[0153] The amplifier 302 and the feedback units 303-1 to 303-N constitute a linear amplifier. For example, the feedback units 303-1 to 303-N are electrically connected between the output terminal of the amplifier 302 and the inverting input terminal of the amplifier 302.

[0154]

[0155] ​The feedback units 303-1 to 303-N are electrically connected in parallel. The feedback units 303-1 to 303-N each include capacitors C303-1 to C303-N, resistors R303-1 to R303-N, and switches SW303-1 to SW303-N.

[0156] Hereinafter, without particularly distinguishing each of the feedback units 303-1 to 303-N, they are also collectively referred to as "feedback unit 303". In addition, without particularly distinguishing each of the capacitors C303-1 to C303-N, they are also collectively referred to as "capacitor C303". In addition, without particularly distinguishing each of the resistors R303-1 to R303-N, they are also collectively referred to as: resistor R303:. In addition, without particularly distinguishing each of the switches SW303-1 to SW303-N, they are also collectively referred to as "switch SW303".

[0157] The capacitor C303 has two terminals. The resistor R303 has two terminals. The switch SW303 has two terminals.

[0158] Hereinafter, each of the capacitor C303, resistor R303, and switch SW303 included in the feedback unit 303-i (i is an integer from 1 to N) is also referred to as "capacitor C303-i", "resistor R303-i", and "switch SW303-i".

[0159] One terminal of the capacitor C303-i is electrically connected to one terminal of the resistor R303-i. The other terminal of the capacitor C303-i is electrically connected to the other terminal of the resistor R303-i. One terminal of the resistor R303-i is electrically connected to the inverting input terminal of the amplifier 302. The other terminal of the resistor R303-i is electrically connected to one terminal of the switch SW303-i. The other terminal of the switch SW303-i is electrically connected to the output of the amplifier 302.

[0160] In the first comparative example, the AD conversion unit 40 is electrically connected to the output terminal of the amplifier 302. The output voltage value Vo302 of the amplifier 302 is input to the AD conversion unit 40. In the first comparative example, the light intensity of the measured light is measured based on the output voltage value Vo302 of the amplifier 302.

[0161] In the optical measurement device 301 of the first comparative example, the measurement sensitivity is set by the resistance value of the resistor R303, that is, the feedback resistor, connected between the output terminal and the inverting input terminal of the amplifier 302. If the resistance value of the feedback resistor becomes higher, the sensitivity of the optical measurement device 301 becomes higher, and the noise level of the optical measurement device 301 becomes smaller. For example, as Figure 7As shown, the measurement sensitivity of the optical measurement device 301 is set to high sensitivity, medium sensitivity, and low sensitivity. In the order of low sensitivity, medium sensitivity, and high sensitivity, the resistance value of the feedback resistor becomes higher, and the noise level of the optical measurement device 301 becomes smaller. However, if the resistance value of the feedback resistor becomes higher, the response speed of the amplifier 302 decreases, so the measurement speed of the optical measurement device 301 decreases. That is, if the resistance value of the feedback resistor becomes higher, the noise level of the optical measurement device 301 becomes smaller, but the measurement speed of the optical measurement device 301 becomes low speed. For example, as Figure 7 shown, in the order of low sensitivity, medium sensitivity, and high sensitivity, the measurement speed of the optical measurement device 301 becomes high speed, medium speed, and low speed and decreases.

[0162] In the first comparative example, the cut-off frequency fc of the amplifier 302 is represented by Equation (10).

[0163] fc = 1 / (2π×Rf303×Cf303) Equation (10)

[0164] In Equation (10), the feedback capacitance value Cf303 is the capacitance value of the capacitor C303, that is, the feedback capacitor, which is electrically connected between the output terminal and the inverting input terminal of the amplifier 302. In addition, the feedback resistance value Rf303 is the resistance value of the resistor R303, that is, the feedback resistor, which is electrically connected between the output terminal and the inverting input terminal of the amplifier 302. For example, when the feedback capacitance value Cf303 is 1 [pF] and the feedback resistance value Rf303 is 100 [MΩ], the cut-off frequency becomes 1.6 [kHz].

[0165] It can be seen from Equation (10) that if the feedback resistance value Rf303 becomes smaller, the cut-off frequency fc becomes higher, and the frequency band of the amplifier 302 becomes a wide frequency band. In addition, as described above, if the feedback resistance value Rf303 becomes smaller, the measurement sensitivity of the optical measurement device 301 becomes lower. That is, if the feedback resistance value Rf303 becomes smaller, the measurement sensitivity of the optical measurement device 301 becomes lower, but the frequency band of the amplifier 302 becomes a wide frequency band. For example, as Figure 7 shown, in the order of high sensitivity, medium sensitivity, and low sensitivity, the cut-off frequency becomes higher in the order of low frequency band, medium frequency band, and high frequency band, and the frequency band becomes a wide frequency band.

[0166] In addition, it can be seen from Equation (10) that if the feedback capacitance value Cf303 becomes smaller, the cut-off frequency fc becomes higher, and the frequency band of the amplifier 302 becomes a wide frequency band. However, if the feedback capacitance value Cf303 becomes smaller, the high-frequency noise of the amplifier 302 becomes larger. As a result, the feedback capacitance value Cf303 is limited to a certain value, and the cut-off frequency fc, etc. are adjusted by the feedback resistance value Rf303.

[0167] Here, in the optical measurement device 301 of the first comparative example, the light intensity of the measurable light to be measured is limited by the power supply voltage of the linear amplifier composed of the amplifier 302 and the feedback unit 303. In the first comparative example, since the light intensity of the measurable light to be measured is limited by the power supply voltage of the linear amplifier, when the light intensity of the light to be measured is large, it is required to reduce the feedback resistance value Rf303. In addition, in the first comparative example, when the light intensity of the light to be measured is small, in order to improve the measurement sensitivity of the optical measurement device 301, it is required to increase the feedback resistance value Rf303. That is, when the optical measurement device 301 is applied to a spectral analyzer, in the optical measurement device 301, when the light intensity of the light to be measured is large, it is required to reduce the feedback resistance value Rf303, and when the light intensity of the light to be measured is small, it is required to increase the feedback resistance value Rf303. With such a structure, in the first comparative example, it is required to switch the switch SW303 according to the light intensity of the light to be measured, so as to switch the resistor R303 connected between the output terminal and the inverting input terminal of the amplifier 302. Therefore, in the first comparative example, when scanning the optical wavelength during the measurement of the spectrum, it is required to switch the operation of the switch SW303 according to the light intensity of the light to be measured. In the first comparative example, by requiring the operation of the switch SW303 to be switched, the measurement time of the spectrum becomes longer. Furthermore, in the optical measurement device 301, it is required to stop the measurement of the spectrum before the output voltage value Vо302 of the amplifier 302 converges after the switch SW303 is switched. In the optical measurement device 301, by stopping the measurement of the spectrum before the output voltage value Vо302 of the amplifier 302 converges, the measurement time of the spectrum becomes longer.

[0168] Compared with such a first comparative example, in the optical measurement device 1 of the present embodiment, as can be seen from the above formula (7) and the above formula (8), the output voltage value Vо1 becomes the logarithm of the photocurrent value Ip. Since the output voltage value Vо1 becomes the logarithm of the photocurrent value Ip, in the optical measurement device 1, the light intensity of the measurable light to be measured is not limited by the power supply voltage of the logarithmic amplifier circuit 20. With such a structure, in the optical measurement device 1, it is not necessary to switch the operation of the switch SW303 during the measurement of the spectrum as in the first comparative example. In addition, in the optical measurement device 1, it is not required to stop the measurement of the spectrum before the output voltage value Vо302 of the amplifier 302 converges as in the first comparative example. Therefore, in the optical measurement device 1 of the present embodiment, compared with the optical measurement device 301 of the first comparative example, the measurement time of the spectrum is shortened.

[0169] <Second Comparative Example>

[0170] Figure 8This is the optical measurement device 401 that uses the logarithmic amplifier of the second comparative example. The optical measurement device 401 includes a photodiode 10, a logarithmic amplifier circuit 20, and a resistor R5. The optical measurement device 401 has a resistor R403 to replace the switch unit 30 and the offset resistor R30 of the present embodiment.

[0171] In the second comparative example, in order to suppress the situation where the amplifier 21 locks when the photodiode 10 is shielded from light, a bias current ib is used. The current value of the bias current ib is also referred to as the "bias current value Ib". The bias current ib, like the Figure 1 shown offset current ioff, is input to the input terminal P1. The bias current value Ib is set by the voltage of the voltage source V402 and the resistance value of the resistor R403. The resistor R403 is composed of a fixed resistor. The resistor R403 has two terminals. One terminal of the resistor R403 is electrically connected to the input terminal P1. The other terminal of the resistor R403 is electrically connected to the voltage source 402.

[0172] In the second comparative example, the output voltage value Vo1 of the logarithmic amplifier circuit 20 is also measured by the AD conversion unit 40. Among them, in the second comparative example, different from the present embodiment, the offset current value Ioff1 is not measured based on the output voltage value Vo1. In the second comparative example, the photocurrent value Ip calculated by substituting the current value I1 obtained by the output voltage value Vo1, the current value I2, and the above formula (7) into the above formula (9) is used to measure the light intensity Pin. That is, in the second comparative example, instead of subtracting the offset current value Ioff1 from the current value I1 according to the above formula (8), the light intensity Pin is measured by setting I1 = Ip according to the above formula (9).

[0173] In the second comparative example, the current value I1 becomes the sum of the photocurrent value Ip and the bias current value Ib. In the second comparative example, also referring to the above formula (4), as described above, the smaller the current value I1, the higher the resistance value Rt, and the higher the measurement sensitivity of the optical measurement device 401. In addition, referring to the above formula (5), as described above, the larger the current value I1, the higher the cut-off frequency fc, and the frequency band of the amplifier 21 becomes a wide frequency band. With such a structure, in the second comparative example, by adjusting the bias current value Ib, the current value I1 is adjusted to adjust the measurement sensitivity of the optical measurement device 401, the measurement speed of the optical measurement device 401, and the frequency band of the amplifier 21.

[0174] In the second comparative example, in order to adjust the bias current value Ib, for example, it is considered to adopt a digital-to-analog conversion section in the voltage source V402 to make the voltage value of the voltage source V402 variable. Here, the voltage range of the voltage source V402 is limited by the power supply voltage of the optical measurement device 401. As a result, in order to set the bias current value Ib in a wide range, it is required to set the resistance value of the resistor R403 low. If the resistance value of the resistor R403 is set low, a larger bias current value Ib can be set.

[0175] However, if the resistance value of the resistor R403 is set low, the gain, i.e., the noise gain, observed from the non-inverting input terminal side of the amplifier 21 increases. Therefore, even if the voltage value of the voltage source V402 is reduced to improve the measurement sensitivity, the noise level cannot be reduced. In the second comparative example, in order to suppress the increase in the noise gain of the amplifier 21, it is required to increase the resistance value of the resistor R403 to some extent. For example, the resistance value of the resistor R403 is set to several [GΩ].

[0176] Thus, in the second comparative example, in order to suppress the increase in the noise gain of the amplifier 21, it is required to increase the resistance value of the resistor R403 to some extent. As a result, in the second comparative example, the bias current value Ib cannot be set large. In the second comparative example, since the bias current value Ib cannot be set large, the frequency band of the amplifier 21 cannot be set to a wide frequency band. If the frequency band of the amplifier 21 cannot be set to a wide frequency band, in the optical measurement device 401, as described with reference to Figure 9 As will be described below, in the optical wavelength where the light intensity of the light to be measured becomes low, distortion occurs in the waveform.

[0177] Figure 9 Waveforms showing the spectra of the first comparative example and the second comparative example. In Figure 9 the horizontal axis is the optical wavelength [nm]. In addition, the vertical axis is the light intensity [dBm]. The waveform W3 is the waveform of the spectrum measured by the optical measurement device 301 of the first comparative example. The waveform W4 is the waveform of the spectrum measured by the optical measurement device 401 of the second comparative example. In the waveform W4, if compared with the waveform W3, distortion occurs in the range of the optical wavelength where the light intensity becomes low.

[0178] Relative to such a second comparative example, in the optical measurement device 1 of the present embodiment, as Figure 1As shown, various offset current values Ioff can be set by the offset resistor R30 with different resistance values. In the optical measurement device 1, it is possible to suppress the increase in the noise gain of the amplifier 21 due to electrically connecting the offset resistor R30 with a high resistance value between the voltage source Vb and the input terminal P1, and to set a lower offset current value Ioff without reducing the voltage value VB of the voltage source Vb. In addition, in the optical measurement device 1, by electrically connecting the offset resistor R30 with a lower resistance value between the voltage source Vb and the input terminal P1, a larger offset current value Ioff can be set. As a result, it is possible to suppress the increase in the noise gain of the amplifier 21 and set a larger offset current value Ioff while setting the frequency band of the amplifier 21 to a wide band. With such a configuration, in the optical measurement device 1, as Figure 9 shown, distortion generation in the waveform is suppressed in the range of optical wavelengths where the light intensity is low.

[0179] In addition, in the second comparative example, as described above, the offset current value Ioff1 is not measured based on the output voltage value Vo1. That is, in the second comparative example, different from the present embodiment, instead of subtracting the offset current value Ioff1 from the current value I1 according to the above formula (8), the light intensity Pin is measured by setting I1 = Ip according to the above formula (9). As a result, in the second comparative example, it is impossible to measure a photocurrent value Ip smaller than the bias current value Ib.

[0180] Relative to such a second comparative example, in the optical measurement device 1 of the first embodiment, the control unit 52 calculates the photocurrent value Ip by subtracting the offset current value Ioff1 from the current value I1 according to the above formula (8). Further, the control unit 52 calculates the light intensity Pin by using the calculated photocurrent value Ip and the above formula (9). In this way, for calculating the photocurrent value Ip by subtracting the offset current value Ioff1 from the current value I1 according to the above formula (8), the offset current value Ioff1 can be made larger than the bias current value Ib. By increasing the offset current value Ioff1, the measurement speed of the optical measurement device 1 can be set to high speed.

[0181] As described above, in the optical measurement device 1 of the first embodiment, the performance of detecting the light intensity of the light to be measured is improved.

[0182] (Second Embodiment)

[0183] As Figure 10As shown, the optical measurement device 101 includes a photodiode 10, a logarithmic amplifier circuit 20, a resistor R5, an AD conversion unit 40, and an arithmetic device 2. The optical measurement device 101 includes an amplifier 60 (second amplifier), a resistor R6, a resistor R7, a DA conversion unit 70, a switch SW1 (first switch), a switch SW2 (second switch), a switch SW3 (third switch), a switch SW4 (fourth switch), and a switch SW5. The optical measurement device 101 includes a switch unit 130, offset resistors R130-1 to R130-N, and capacitors C130-1 to C130-N.

[0184] Hereinafter, without particularly distinguishing each of the offset resistors R130-1 to R130-N, they are also collectively referred to as "offset resistor R130". Further, without particularly distinguishing each of the capacitors C130-1 to C130-N, they are also collectively referred to as "capacitor C130".

[0185] Each of the switches SW1 to SW5 includes a mechanical relay, an optoelectronic relay, or an analog switch, etc. The switches SW1 to SW5 may also be configured as an analog multiplexer. Further, in Figure 10 the switches SW1 and SW2 are shown as independent switches, but the switches SW1 and SW2 may also be configured as one switch. In this case, the switches SW1 and 2 are switches that switch the connection destination of the anode of the photodiode 10. For example, it may have the same structure as the switch SW5. Further, in Figure 10 the switches SW2 and SW3 are shown as independent switches, but the switches SW2 and SW3 may also be configured as one switch. In this case, the switches SW2 and 3 are switches that switch the connection destination of the inverting input terminal of the amplifier 60 and may have the same structure as the switch SW5, for example.

[0186] The switch SW1 can switch whether to electrically connect the input terminal P1 and the anode of the photodiode 10. For example, the switch SW1 has two terminals. One terminal of the switch SW1 is electrically connected to the input terminal P1. The other terminal of the switch SW1 is electrically connected to the anode of the photodiode 10. The switch SW1 becomes an on state or an off state based on a control signal from the control unit 52. If the switch SW1 becomes an on state, the input terminal P1 and the photodiode 10 are electrically connected. Further, if the switch SW1 becomes an off state, the input terminal P1 and the photodiode 10 are electrically separated.

[0187] The switch SW2 can switch whether to electrically connect the anode of the photodiode 10 and the inverting input terminal of the amplifier 60. For example, the switch SW2 has two terminals. One terminal of the switch SW2 is electrically connected to the anode of the photodiode 10. The other terminal of the switch SW2 is electrically connected to the inverting input terminal of the amplifier 60. The switch SW2 becomes an on state or an off state based on a control signal from the control unit 52. If the switch SW2 becomes an on state, the anode of the photodiode 10 and the inverting input terminal of the amplifier 60 are electrically connected. If the switch SW2 becomes an off state, the anode of the photodiode 10 and the inverting input terminal of the amplifier 60 are electrically separated.

[0188] The switch SW3 can switch whether to electrically connect the inverting input terminal of the amplifier 60 and the output terminal of the amplifier 60. For example, the switch SW3 has two terminals. One terminal of the switch SW3 is electrically connected to the inverting input terminal of the amplifier 60. The other terminal of the switch SW3 is electrically connected to the output terminal of the amplifier 60. The switch SW3 becomes an on state or an off state based on a control signal from the control unit 52. If the switch SW3 becomes an on state, the inverting input terminal of the amplifier 60 and the output terminal of the amplifier 60 are electrically connected. If the switch SW3 becomes an off state, the inverting input terminal of the amplifier 60 and the output terminal of the amplifier 60 are electrically separated.

[0189] The switch SW4 can switch whether to electrically connect the non-inverting input terminal of the amplifier 60 to the reference potential. For example, the switch SW4 has two terminals. One terminal of the switch SW4 is electrically connected to the non-inverting input terminal of the amplifier 60 via the resistor R6. The other terminal of the switch SW4 is electrically connected to the reference potential. The switch SW4 becomes an on state or an off state based on a control signal from the control unit 52. If the switch SW4 becomes an on state, the non-inverting input terminal of the amplifier 60 is electrically connected to the reference potential. If the switch SW4 becomes an off state, the non-inverting input terminal of the amplifier 60 is electrically separated from the reference potential.

[0190] The switch SW5 can switch to electrically connect the output terminal of the amplifier 60 to the AD conversion unit 40 or to electrically connect the output terminal P3 of the logarithmic amplifier circuit 20 to the AD conversion unit 40. For example, the switch SW5 has three terminals. One terminal of the switch SW5 is electrically connected to the output terminal of the amplifier 60. One terminal of the switch SW5 is electrically connected to the output terminal P3 of the logarithmic amplifier circuit 20. One terminal of the switch SW5 is electrically connected to the AD conversion unit 40. The switch SW5 switches to electrically connect the output terminal of the amplifier 60 to the AD conversion unit 40 or to electrically connect the output terminal P3 of the logarithmic amplifier circuit 20 to the AD conversion unit 40 based on a control signal from the control unit 52.

[0191] The output terminal of the amplifier 60 is electrically connected to the switch SW5. The inverting input terminal of the amplifier 60 is electrically connected to the switch SW2 and the switch SW3. The non-inverting input terminal of the amplifier 60 is electrically connected to the resistor R6 and the resistor R7.

[0192] Each of the resistors R6 and R7 has two terminals. Each of the resistors R6 and R7 is composed of a fixed resistor. One terminal of the resistor R6 is electrically connected to the non-inverting input terminal of the amplifier 60. The other terminal of the resistor R6 is electrically connected to the switch SW4. One terminal of the resistor R7 is electrically connected to the non-inverting input terminal of the amplifier 60. The other terminal of the resistor R7 is electrically connected to the DA conversion unit 70.

[0193] The DA conversion unit 70 is a digital-to-analog conversion unit. The DA conversion unit 70 is electrically connected to the non-inverting input terminal of the amplifier 60 via the resistor R7. The control signal from the control unit 52 is input to the DA conversion unit 70 as a digital signal. The DA conversion unit 70 converts the input digital signal into an analog signal. The DA conversion unit 70 outputs the analog signal to the non-inverting input terminal of the amplifier 60 via the resistor R7. Through such a DA conversion unit 70, a voltage corresponding to the first mode or the second mode described later is input to the non-inverting input terminal of the amplifier 60.

[0194] The offset resistor R130 has two terminals. The offset resistor R130 is composed of a fixed resistor. The resistance values of each of the offset resistors R130-1 to R130-N are different. For example, the resistance value of the offset resistor R130 increases by 10 times successively from the offset resistor R130-1 toward the offset resistor R130-N.

[0195] The capacitor C130 has two terminals. The capacitor C130 is electrically connected in parallel with the offset resistor R130. For example, one terminal of the capacitor C130 is electrically connected to one terminal of the offset resistor R130. The other terminal of the capacitor C130 is electrically connected to the other terminal of the offset resistor R130.

[0196] In the second mode described later, the capacitor C130 functions as the feedback capacitor of the amplifier 60. The capacitance value of the capacitor C130 can be appropriately set according to the response characteristics of the amplifier 60 in the second mode described later.

[0197] The switch unit 130 can switch the offset resistor R130 electrically connected between the output terminal of the amplifier 60 and the photodiode 10 among the offset resistors R130-1 to R130-N. Further, the switch unit 130 can switch the capacitor C130 electrically connected between the output terminal of the amplifier 60 and the photodiode 10 among the capacitors C130-1 to C130-N. The switch unit 130 includes switching switches SW130-1 to S130-N.

[0198] Hereinafter, without distinguishing the switching switches SW130-1 to SW130-N, they are also collectively referred to as "switching switch SW130".

[0199] The switching switch SW130 has two terminals. The switching switch SW130 includes a mechanical relay, an optoelectronic relay, an analog switch, or the like. However, the switch unit 130 may also be configured as an analog multiplexer. In the case where the switch unit 130 is configured as an analog multiplexer, one terminal of each of the switching switches SW130 to S130-N is shared.

[0200] Hereinafter, each of the offset resistor R130 and the capacitor C130 corresponding to the switching switch SW130-i (i is an integer from 1 to N) is also referred to as "offset resistor R130-i" and "capacitor C130-i".

[0201] One terminal of the switching switch SW130-i is electrically connected to the output terminal of the amplifier 60. The other terminal of the switching switch SW130-i is electrically connected to one terminal of the offset resistor R130-i and one terminal of the capacitor C130-i. The other terminal of the offset resistor R130-i and the other terminal of the capacitor C130-i are electrically connected to the anode of the photodiode 10. However, one terminal of the switching switch SW130-i may also be electrically connected to the anode of the photodiode 10. In this case, the other terminal of the switching switch SW130-i is electrically connected to one terminal of the offset resistor R130-i and one terminal of the capacitor C130-i. Further, the other terminal of the offset resistor R130-i and the other terminal of the capacitor C130-i are electrically connected to the output terminal of the amplifier 60.

[0202] The control signal from the control unit 52 is output to the switching switch SW130. The switching switch SW130 becomes on or off based on the control signal from the control unit 52. If the switching switch SW130-i becomes on, the offset resistor R130-i and the capacitor C130-i are connected between the output terminal of the amplifier 60 and the anode of the photodiode 10. Additionally, if the switching switch SW130-i becomes off, the offset resistor R130-i and the capacitor C130-i are electrically separated from between the output terminal of the amplifier 60 and the anode of the photodiode 10.

[0203] The optical measurement device 101 has a first mode and a second mode. The first mode, similar to the first embodiment, is a mode in which the light intensity of the measured light is measured based on the output voltage value Vo1 of the logarithmic amplifier circuit 20 as shown. Figure 2 The second mode is a mode in which the light intensity of the measured light is measured based on the output voltage value Vo2 of the amplifier 60 as described below.

[0204] [First Mode]

[0205] The control unit 52 receives an input for switching the mode of the optical measurement device 101 through the input unit 51. This input is input by the user from the input unit 51. If the control unit 52 receives this input through the input unit 51, it switches the mode of the optical measurement device 101 to the first mode.

[0206] The switches SW1 to SW4 of the control unit 52 appropriately output control signals to switch the connection state of the optical measurement device 101 to the connection state corresponding to the first mode. In the first mode, the control unit 52 sets the switch SW1 to on, the switch SW2 to off, the switch SW3 to on, and the fourth switch to off.

[0207] In the first mode, the anode of the photodiode 10 and the input terminal P1 are electrically connected through the switch SW1, and the anode of the photodiode 10 and the inverting input terminal of the amplifier 60 are electrically separated through the switch SW2. Additionally, in the first mode, the inverting input terminal and the output terminal of the amplifier 60 are electrically connected through the switch SW3, and the non-inverting input terminal of the amplifier 60 is electrically separated from the reference potential through the switch SW4.

[0208] When the control unit 52 measures the output voltage value Vo1 of the logarithmic amplifier circuit 20 in the first mode, it outputs a control signal to the switch SW5. The control unit 52 outputs a control signal to the switch SW5, electrically separates the output terminal of the amplifier 60 from the AD conversion unit 40 through the switch SW5, and electrically connects the output terminal P3 of the logarithmic amplifier circuit 20 to the AD conversion unit 40. With such a structure, the control unit 52 can measure the output voltage value Vo1 through the AD conversion unit 40.

[0209] When the control unit 52 measures the output voltage value Vo2 of the amplifier 60 in the first mode, it outputs a control signal to the switch SW5. The control unit 52 outputs a control signal to the switch SW5, electrically connects the output terminal of the amplifier 60 to the AD conversion unit 40 through the switch SW5, and electrically separates the output terminal P3 of the logarithmic amplifier circuit 20 from the AD conversion unit 40. With such a structure, the control unit 52 can measure the output voltage value Vo2 of the amplifier 60 through the AD conversion unit 40.

[0210] In the first mode, the inverting input terminal of the amplifier 60 and the output terminal of the amplifier 60 are electrically connected through the switch SW3, so that the amplifier 60 has the function of a voltage follower. Since the amplifier 60 has the function of a voltage follower, the output voltage value Vo2 of the amplifier 60 becomes equal to the voltage of the non-inverting input terminal of the amplifier 60. The voltage input to the non-inverting input terminal of the amplifier 60 is set by the DA conversion unit 70, the resistor R6, and the resistor R7. In the first mode, the DA conversion unit 70 has the function of Figure 1 the voltage source Vb as shown. The output voltage value Vo2 of the amplifier 60, since the amplifier 60 has the function of a voltage follower, corresponds to Figure 1 the voltage value VB of the voltage source Vb as shown.

[0211] In the first mode, the switch unit 130 can switch the offset resistor R130 electrically connected between the output terminal of the amplifier 60 and the input terminal P1 among the offset resistors R130-1 to R130-N. In the first mode, through the offset resistor R130 electrically connected between the output terminal of the amplifier 60 and the input terminal P1 by the switch unit 130, the offset current ioff is input to the input terminal P1 through the switch SW1.

[0212] [First Mode: Measurement Sensitivity Setting Process]

[0213] The control unit 52 receives the input of the measurement sensitivity in the first mode through the input unit 51, for example, before the spectral measurement process. The measurement sensitivity in the first mode is the measurement sensitivity of the optical measurement device 101 set in the first mode. The measurement sensitivity in the first mode can also be the sensitivity as described above with reference to Figure 3 and so on.

[0214] In the second embodiment, in the storage unit 50, the measurement sensitivity of the first mode, the switching information of the switching unit 130 for setting the measurement sensitivity to the first mode, and the information of the digital signal input to the DA conversion unit 70 for setting the measurement sensitivity to the first mode are stored in association with each other. The switching information of the switching unit 130 includes the information of the switching switch SW130 set to the on state and the information of the switching switch SW130 set to the off state.

[0215] If the control unit 52 receives the measurement sensitivity of the first mode through the input unit 51, the control unit 52 acquires the switching information of the switching unit 130 associated with the measurement sensitivity of the first mode and the information of the digital signal input to the DA conversion unit 70 from the storage unit 50. Based on the acquired information of the digital signal, the control unit 52 outputs the digital signal to the DA conversion unit 70, thereby setting the output voltage value of the DA conversion unit 70. In addition, based on the acquired switching information of the switching unit 130, the control unit 52 controls the switching unit 130 in the same manner as the switching unit 30 in the first embodiment.

[0216] [First mode: Table creation process]

[0217] However, sometimes the actual relationship between the output voltage value Vо1 and the current value I1 deviates from the relationship represented by the above formula (7). In Figure 11 is a graph showing the relationship between the output voltage value Vо1 and the current value I1. In Figure 11 the horizontal axis is the output voltage value Vо1 [V]. In addition, the vertical axis is the current value I1 [A]. In Figure 11 the dashed line represents the relationship between the output voltage value Vо1 and the current value I1 calculated by the above formula (7). The solid line represents the actual relationship between the output voltage value Vо1 and the current value I1. As Figure 7 shown, a deviation occurs between the solid line and the dashed line. This deviation amount varies due to temperature.

[0218] Here, in the state where the photodiode 10 is shielded from light, the photocurrent value Ip becomes 0 [A] (Ip = 0), and the current value I1 is equal to the offset current value Iоff (I1 = Iоff). Furthermore, in the first mode, as described above, the output voltage value Vо2 of the amplifier 60 corresponds to Figure 1 the voltage value VB of the voltage source Vb shown. Therefore, in the second embodiment, the control unit 52 can measure the current value I1 by measuring the output voltage value Vо2 of the amplifier 60 when the photodiode 10 is shielded from light by the AD conversion unit 40.

[0219] The control unit 52 changes the output voltage value of the DA conversion unit 70 and switches the switch unit 130, thereby setting the offset current value Ioff, that is, the current value I1. For each set offset current value Ioff, the control unit 52 measures the output voltage values Vo1 and Vo2 when the photodiode 10 is shaded through the AD conversion unit 40 by appropriately switching the switch SW5. For example, for each set offset current value Ioff, the control unit 52 outputs a control signal to the switch SW5. Through the switch SW5, the output terminal of the amplifier 60 is electrically connected to the AD conversion unit 40, and the output terminal P3 of the logarithmic amplifier circuit 20 is electrically separated from the AD conversion unit 40. For each set offset current value Ioff, the control unit 52 measures the output voltage value Vo2 of the amplifier 60 through the AD conversion unit 40. In addition, for each set offset current value Ioff, the control unit 52 outputs a control signal to the switch SW5, electrically separates the output terminal of the amplifier 60 from the AD conversion unit 40 through the switch SW5, and electrically connects the output terminal P3 of the logarithmic amplifier circuit 20 and the AD conversion unit 40. For each set offset current value Ioff, the control unit 52 measures the output voltage value Vo1 of the logarithmic amplifier circuit 20 through the AD conversion unit 40.

[0220] For each set offset current value Ioff, the control unit 52 generates by associating the measured output voltage value Vo1 with the current value I1 calculated based on the measured output voltage value Vo2 Figure 12 the table shown. The control unit 52 calculates the offset current value Ioff, that is, the current value I1, through the measured output voltage value Vo2 and Equation (11).

[0221] Ioff = Vo2 / Rs130 Equation (11)

[0222] In Equation (11), the resistance value Rs130 is the resistance value of the offset resistor R130 electrically connected between the output terminal and the input terminal P1 of the amplifier 60 through the switch unit 130.

[0223] In Figure 12 the table shown, for the output voltage value Vo1, the output voltage value Vo1 measured as digital data through the AD conversion unit 40 is used. In Figure 12 , each of AAA, BBB, and CCC of the output voltage value Vo1 as digital data is associated with each of aaa, bbb, and ccc of the current value I1. The control unit 52 stores the generated table in the storage unit 50.

[0224] Here, the range of the offset current value Ioff that can be set by the change in the output voltage value of the DA conversion unit 70 and the switching of the switch unit 130 is the same as the range of the photocurrent value Ip corresponding to a wide range of optical intensities from +10 [dBm] to -90 [dBm]. The range of optical intensities from +10 [dBm] to -90 [dBm] is the same as the measurement range of the second mode described later. The control unit 52 can set multiple offset current values Ioff in a wide range and generate a table from the output voltage value Vo1 and the output voltage value Vo2 measured for each set offset current value Ioff.

[0225] Instead of the above formula (7), the control unit 52 can use the generated table to calculate the optical intensity of the measured light. The current value I1 calculated by the above formula (7) deviates from the actual current value I1 as described above. By using the table, the optical intensity of the measured light is calculated using the actual relationship between the output voltage value Vo1 and the current value I1. With such a configuration, the optical intensity of the measured light is measured with high precision.

[0226] Here, when all of the switching switches SW130-1 to SW130-N are in the off state, the offset current value Ioff becomes 0 [A] (Ioff = 0), and the current value I1 becomes equal to the photocurrent value Ip (I1 = Ip). It is also possible to create a table by inputting the measured light with a known optical intensity into the photodiode 10 and measuring the output voltage value Vo1 of the logarithmic amplifier circuit 20 corresponding to the photocurrent value Ip calculated from the optical intensity of the known measured light while changing the optical intensity of the measured light. However, it takes effort to input the measured light with a known optical intensity into the photodiode 10. In addition, without preparing the measured light separately, it is not possible to correspond to the change in temperature of the deviation amount generated between the current value I1 calculated according to the above formula (7) and the actual current value I1. For such a configuration, in the present embodiment, it is possible to set Ioff in a wide range by changing the output voltage value of the DA conversion unit 70 and switching the switch unit 130 in a state where the photodiode 10 is shielded from light, without inputting the measured light with a known optical intensity into the photodiode 10. Furthermore, in the present embodiment, a table can be created by measuring the output voltage value Vo1 and the output voltage value Vo2 while switching the switch SW5. Therefore, in the present embodiment, a table can be easily created. Thus, in the present embodiment, for example, by creating a table before performing the optical intensity measurement process or periodically, it is also possible to correspond to the change in temperature of the deviation amount generated between the current value I1 calculated according to the above formula (7) and the actual current value I1.

[0227] In addition, to create a table, multiple offset current values Ioff are set within a wide range. For each set offset current value Ioff, the output voltage value Vo1 of the logarithmic amplifier circuit 20 and the output voltage value Vo2 of the amplifier 60 are measured, and sometimes time is spent on this. Here, with respect to the change in temperature of the deviation amount generated between the current value I1 calculated according to the above formula (7) and the actual current value I1, the offset of the output voltage value Vo1 is dominant. Therefore, for the setting of one offset current value Ioff, the control unit 52 can also measure the output voltage value Vo1 of the logarithmic amplifier circuit 20 and the output voltage value Vo2 of the amplifier 60. Further, the control unit 52 calculates the output voltage value Vo1 corresponding to the measured output voltage value Vo2 and the offset current value Ioff calculated by the above formula (11) from the table. The control unit 52 calculates the difference between the calculated output voltage value Vo1 and the measured output voltage value Vo1, and adds this difference to each of the output voltage values Vo1 in the table. By adding this difference to each of the output voltage values Vo1 in the table, the change in the deviation amount caused by the above offset can be reflected in the table. With such a configuration, the time for creating the table can be shortened. Here, the output voltage value of the DA conversion unit 70 and the offset current value Ioff set by the switch unit 130 may also be the same value as the current value I2.

[0228] [First Mode: Measurement Process of Offset Current Value]

[0229] Similar to the first embodiment, the control unit 52 measures the offset current value Ioff1, for example, before performing the light intensity measurement process or periodically. In the second embodiment, the control unit 52 outputs the output voltage value corresponding to each measurement sensitivity to the DA conversion unit 70, and while setting the offset current value Ioff corresponding to each measurement sensitivity through the switch unit 130, measures the output voltage value Vo1 of the logarithmic amplifier circuit 20 when the photodiode 10 is shielded from light through the AD conversion unit 40. The control unit 52 sets the offset current value Ioff corresponding to each measurement sensitivity by switching the offset resistor R130 electrically connected between the output terminal and the input terminal P1 of the amplifier 60. The control unit 52 calculates the offset current value Ioff1 for each measurement sensitivity based on the output voltage value Vo1 and the above table. The control unit 52 stores the offset current value Ioff1 associated with the measurement sensitivity in the storage unit 50. However, similar to the first embodiment, the control unit 52 calculates the offset current value Ioff1 based on the output voltage value Vo1 and the above formula (6).

[0230] [First Mode: Measurement Process of Light Intensity]

[0231] Before performing the measurement process of the light intensity, the control unit 52 outputs a control signal to the switch SW5. The control unit 52 outputs a control signal to the switch SW5, electrically separates the output terminal of the amplifier 60 from the AD conversion unit 40 through the switch SW5, and electrically connects the output terminal P3 of the logarithmic amplifier circuit 20 and the AD conversion unit 40.

[0232] The control unit 52 measures the output voltage value Vо1 of the logarithmic amplifier circuit 20 when the measured light is incident on the photodiode 10 through the AD conversion unit 40. The control unit 52 calculates the light intensity of the measured light based on the measured output voltage value Vо1, the table stored in the storage unit 50 Figure 12 shown, the offset current value Iоff1 associated with the measurement sensitivity, the above formula (8), and the above formula (9). That is, the control unit 52 uses Figure 12 the shown table to calculate the light intensity of the measured light instead of the above formula (7). With such a structure, as described above, the light intensity of the measured light is measured with high precision. However, similar to the first embodiment, the control unit 52 can also calculate the light intensity of the measured light based on the output voltage value Vо1 and the above formulas (7) to (9).

[0233] [Second Mode]

[0234] The control unit 52 receives an input for switching the mode of the light measuring device 101 to the second mode through the input unit 51. This input is input by the user from the input unit 51. If the control unit 52 receives this input through the input unit 51, it switches the mode of the light measuring device 101 to the second mode.

[0235] The control unit 52 appropriately outputs control signals to the switches SW1 to SW4 to switch the connection state of the light measuring device 101 to the connection state corresponding to the second mode. In the second mode, the control unit 52 sets the switch SW1 to the off state, the switch SW2 to the on state, the switch SW3 to the off state, and the fourth switch to the on state.

[0236] In the second mode, through the switch SW1, the anode of the photodiode 10 and the input terminal P1 are electrically separated, and through the switch SW2, the anode of the photodiode 10 and the inverting input terminal of the amplifier 60 are electrically connected. Additionally, in the second mode, through the switch SW3, the inverting input terminal and the output terminal of the amplifier 60 are electrically separated, and through the switch SW4, the non-inverting input terminal of the amplifier 60 is electrically connected to the reference potential.

[0237] When the control unit 52 measures the output voltage value Vo2 of the amplifier 60 in the second mode, it outputs a control signal to the switch SW5. The control unit 52 outputs a control signal to the switch SW5, electrically connects the output terminal of the amplifier 60 to the AD conversion unit 40 through the switch SW5, and electrically disconnects the output terminal P3 of the logarithmic amplifier circuit 20 from the AD conversion unit 40. With such a configuration, the control unit 52 can measure the output voltage value Vo2 of the amplifier 60 through the AD conversion unit 40.

[0238] In the second mode, the switching unit 130 can switch the offset resistor R130 that electrically connects between the output terminal of the amplifier 60 and the inverting input terminal of the amplifier 60 among the offset resistors R130-1 to R130-N. The offset resistor R130 electrically connected between the output terminal and the inverting input terminal of the amplifier 60 and the amplifier 60 constitute a linear amplifier. That is, the offset resistor R130 electrically connected between the output terminal and the inverting input terminal of the amplifier 60 functions as a feedback resistor of the amplifier 60. Hereinafter, the resistance value of the offset resistor R130 electrically connected between the output terminal and the inverting input terminal of the amplifier 60 in the second mode is also referred to as "feedback resistance value Rf130".

[0239] In the second mode, the switching unit 130 can switch the capacitor C130 that electrically connects between the output terminal of the amplifier 60 and the inverting input terminal of the amplifier 60 among the capacitors C130-1 to C130-N. The capacitor C130 electrically connected between the output terminal and the inverting input terminal of the amplifier 60 functions as a feedback capacitor of the amplifier 60. Hereinafter, the capacitance value of the capacitor C130 electrically connected between the output terminal and the inverting input terminal of the amplifier 60 in the second mode is also referred to as "feedback capacitance value Cf130".

[0240] [Second Mode: Measurement Sensitivity Setting Process]

[0241] For example, before measuring these spectra, the control unit 52 receives the input of the measurement sensitivity in the second mode through the input unit 51. The measurement sensitivity in the second mode is the measurement sensitivity of the optical measurement device 101 set in the second mode.

[0242] In the second mode, due to the resistance value of the offset resistor R130, i.e., the feedback resistance value Rf130, electrically connected between the output terminal and the inverting input terminal of the amplifier 60, the measurement sensitivity of the optical measurement device 101 changes. The higher the feedback resistance value Rf130 becomes, the higher the measurement sensitivity of the optical measurement device 101 becomes, and the smaller the noise level of the optical measurement device 101 becomes. However, the higher the feedback resistance value Rf130 becomes, the lower the response speed of the amplifier 60, so the measurement speed of the optical measurement device 101 becomes lower. In addition, the cut-off frequency fc of the amplifier 60 is expressed by Equation (12).

[0243] fc = 1 / (2π × Rf130 × Cf130) Equation (12)

[0244] In Equation (12), as described above, the feedback capacitance value Cf130 is the capacitance value of the capacitor C130, i.e., the feedback capacitor, electrically connected between the output terminal and the inverting input terminal of the amplifier 60.

[0245] It can be seen from Equation (12) that the lower the feedback resistance value Rf130 becomes, the higher the cut-off frequency fc becomes, and the frequency band of the amplifier 60 becomes a wide frequency band. In addition, the smaller the feedback capacitance value Cf130 becomes, the higher the cut-off frequency fc becomes, and the frequency band of the amplifier 60 becomes a wide frequency band. However, if the feedback capacitance value Cf130 becomes smaller, the high-frequency noise of the amplifier 60 becomes larger. Therefore, in the present embodiment, the feedback capacitance value Cf130 is limited to a certain value, and the cut-off frequency fc is adjusted by the feedback resistance value Rf130.

[0246] In short, in the second mode, the higher the feedback resistance value Rf130 becomes, the higher the measurement sensitivity of the second mode becomes. The higher the measurement sensitivity of the second mode becomes, the narrower the frequency band of the amplifier 60 becomes, and the lower the measurement speed of the optical measurement device 101 becomes. In addition, in the second mode, the lower the feedback resistance value Rf becomes, the lower the measurement sensitivity of the second mode becomes. The lower the measurement sensitivity of the second mode becomes, the wider the frequency band of the amplifier 60 becomes, and the higher the measurement speed of the optical measurement device 101 becomes.

[0247] Therefore, in the second mode, for each measurement sensitivity of the second mode received by the input unit 51, the maximum feedback resistance value Rf130 is set. The feedback resistance value Rf130 is adjusted based on the output voltage value Vo2 of the amplifier 60 so as not to exceed the maximum feedback resistance value Rf130. In addition, the data of the maximum feedback resistance value Rf130 may also be stored in the storage unit 50 in association with the measurement sensitivity of the second mode.

[0248] Here, in the second mode, in the light intensity measurement process, as described later, the light intensity of the measured light is calculated based on the output voltage value Vo2 of the amplifier 60. The greater the light intensity of the measured light, the greater the output voltage value Vo2. If the output voltage value Vo2 increases to a certain extent, since the power supply voltage of the linear amplifier is limited, it may not be possible to accurately calculate the light intensity of the measured light based on this output voltage value Vo2. That is, in the second mode, when the light intensity of the measured light increases to a certain extent, it is desirable to decrease the feedback resistance value Rf130 and decrease the output voltage value Vo2.

[0249] As an example, in the light intensity measurement process described later, the control unit 52 measures the output voltage value Vo2 of the amplifier 60 through the AD conversion unit 40. At this time, the control unit 52 determines whether the absolute value of the output voltage value Vo2 is within a specified range. The upper limit value of the specified range can be set based on the maximum rated voltage value of the output voltage value Vo2 of the amplifier 60, or it can also be set based on the maximum value of the absolute value of the output voltage value Vo2 that ensures the measurement accuracy of the light intensity. The lower limit value of the specified range can be set according to the feedback resistance value Rf130 that is one level higher than the current feedback resistance value Rf130 and the upper limit value of the specified range, or it can also be set based on the minimum value of the absolute value of the output voltage value Vo2 that ensures the measurement accuracy of the light intensity.

[0250] When the control unit 52 determines that the absolute value of the output voltage value Vo2 is within the specified range, in the light intensity measurement process, as described later, the light intensity of the measured light is measured based on the output voltage value Vo2.

[0251] On the other hand, when the control unit 52 determines that the absolute value of the output voltage value Vo2 exceeds the specified range, the control unit 52 controls the switch unit 130 to switch the current feedback resistance value Rf130 to a feedback resistance value Rf130 that is one level lower. When the absolute value of the output voltage value Vo2 exceeds the specified range, since the power supply voltage of the limiting linear amplifier is limited, there is a high possibility that the light intensity of the measured light cannot be accurately calculated based on this output voltage value Vo2. When the absolute value of the output voltage value Vo2 exceeds the specified range, by switching the current feedback resistance value Rf130 to a feedback resistance value Rf130 that is one level lower, the output voltage value Vo2 can be made smaller than the current voltage value. With such a structure, the light intensity of the measured light is accurately calculated based on the output voltage value Vo2 after the feedback resistance value Rf130 is switched.

[0252] In addition, when the control unit 52 determines that the absolute value of the output voltage value Vo2 is lower than the specified range, it controls the switch unit 130 to switch the current feedback resistance value Rf130 to the next higher-level feedback resistance value Rf130. When the absolute value of the output voltage value Vo2 is lower than the specified range, due to the influence of noise or the like, it is highly likely that the light intensity of the measured light cannot be accurately calculated based on this output voltage value Vo2. By switching the current feedback resistance value Rf130 to the next higher-level feedback resistance value Rf130 when the absolute value of the output voltage value Vo2 is lower than the specified range, the measurement sensitivity of the light measurement device 101 becomes higher than the current measurement sensitivity. With such a structure, based on the output voltage value Vo2 after switching the feedback resistance value Rf130, the light intensity of the measured light is accurately calculated.

[0253] [Second Mode: Measurement Processing of Light Intensity]

[0254] Before performing the measurement processing of the light intensity, the control unit 52 outputs a control signal to the switch SW5. The control unit 52 outputs a control signal to the switch SW5, and electrically connects the output terminal of the amplifier 60 to the AD conversion unit 40 through the switch SW5, and electrically disconnects the output terminal P3 of the logarithmic amplifier circuit 20 from the AD conversion unit 40.

[0255] The control unit 52 measures the output voltage value Vo2 of the amplifier 60 when the measured light is incident on the photodiode 10 through the AD conversion unit 40. The relationship between the output voltage value Vo2 and the photocurrent value Ip is expressed by Equation (13).

[0256] Vo2 = -Rf130 × Ip + Voff Equation (13)

[0257] In Equation (13), as described above, the feedback resistance value Rf130 is the resistance value of the offset resistor R130 electrically connected between the output terminal of the amplifier 60 and the inverting input terminal of the amplifier 60. The voltage value Voff is the output voltage value Vo2 of the amplifier 60 when the potential difference between the inverting input terminal and the non-inverting input terminal of the amplifier 60 is 0 [V], that is, the output offset voltage value of the amplifier 60.

[0258] To reduce the voltage value Voff in Equation (13), in the Figure 10 shown structure, at the non-inverting input terminal of the amplifier 60, a voltage obtained by dividing the voltage output from the DA conversion unit 70 by the resistor R6 and the resistor R7 is input. The voltage value Voff is about several [mV]. The resistance value of the resistor R6 is set to about several [Ω], for example, and the resistance value of the resistor R7 is set to be sufficiently larger than the resistance value of the resistor R6. In addition, as will be described later, the voltage value Voff is stored in the storage unit 50 in association with the feedback resistance value Rf130.

[0259] The control unit 52 calculates the photocurrent value Ip based on the voltage value Voff obtained from the storage unit 50, the measured output voltage value Vo2, and Equation (13). The control unit 52 calculates the light intensity Pin of the measured light based on the calculated photocurrent value Ip and the above Equation (9).

[0260] Here, when the user wishes to measure the light intensity of pulsed light, the user can switch the mode of the light measurement device 101 to the second mode by inputting, from the input unit 51, an input for switching the mode of the light measurement device 101 to the second mode. For example, compared with a logarithmic amplifier, in the linear amplifier in the second mode, it is possible to easily measure the time average value of the light intensity of pulsed light. For example, in a linear amplifier, by increasing the feedback capacitance value Cf130 of the linear amplifier, it is possible to measure the time average value of the light intensity of pulsed light. In this case, in the second mode, the control unit 52 can also, via the switch unit 130, electrically connect a capacitor C130 having a feedback capacitance value Cf130 corresponding to the period of the pulsed light between the output terminal of the amplifier 60 and the inverting input terminal of the amplifier 60. Additionally, a low-pass filter can be provided between the output terminal of the amplifier 60 and the AD conversion unit 40. By measuring the output voltage value of the amplifier 60 via the low-pass filter by the AD conversion unit 40, it is possible to measure the time average value of the light intensity of pulsed light.

[0261] In addition, the 1 / f noise of the logarithmic amplifier in the first mode is sometimes greater than the 1 / f noise of the linear amplifier in the second mode. As a result, sometimes in the first mode, the measurement sensitivity of the light measurement device 101 cannot be set to as high a sensitivity as in the second mode. In this case, when the user wishes to set the measurement sensitivity of the light measurement device 101 to a higher sensitivity because the light intensity of the measured light is small, the mode of the light measurement device 101 can be switched to the second mode.

[0262] [Second Mode: Adjustment Process of Voltage Value Voe and Measurement Process of Voltage Value Voff]

[0263] When the photodiode 10 is in a light-shielded state, Equation (13) is expressed as Equation (14).

[0264] Vo2 = Voff Equation (14)

[0265] From Equation (14), it can be seen that by measuring the output voltage value Vo2 of the amplifier 60 when the photodiode 10 is in a light-shielded state, the voltage value Voff can be measured.

[0266] Here, the voltage value Voff is expressed by Equation (15).

[0267] Voff = (1 + Rf130 / Rpd) × (Voe + r6 / (r6 + r7) × Vo3) Equation (15)

[0268] In Equation (15), the voltage value Voe is the input offset voltage value of the amplifier 60. The resistance value Rpd is the parallel resistance value of the photodiode 10. The resistance value r6 is the resistance value of the resistor R6. The resistance value r7 is the resistance value of the resistor R7. The output voltage value Vo3 is the output voltage value output by the DA conversion unit 70.

[0269] The control unit 52 can also adjust the output voltage value of the DA conversion unit 70 to cancel the voltage value Voe. For example, when the feedback resistance value Rf130 is at its minimum value, due to the reduced influence of the noise gain of the linear amplifier, the correspondence between the voltage value Voff and the voltage value Voe is represented by Equation (16).

[0270] Voff ≒ Voe + r6 / (r6 + r7) × Vo3 Equation (16)

[0271] From Equation (16), it can be seen that when the feedback resistance value Rf130 is at its minimum value, by measuring the voltage value Voff, the output voltage value Vo3 used to cancel the voltage value Voe can be obtained.

[0272] The control unit 52 controls the switch unit 130 to set the feedback resistance value Rf130 to its minimum value, and measures the output voltage value Vo2 of the amplifier 60, that is, the voltage value Voff, when the photodiode 10 is in the light-shielded state through the AD conversion unit 40. The control unit 52 calculates the output voltage value Vo3 of the DA conversion unit 70 for setting the voltage value Voff to 0 [V] from Equation (16). The control unit 52 determines the digital signal input to the DA conversion unit 70 in order to output the calculated output voltage value Vo3 to the DA conversion unit 70. The control unit 52 cancels the voltage value Voe by inputting this digital signal to the DA conversion unit 70. The control unit 52 stores the information of this digital signal in the storage unit 50.

[0273] With such a structure, when the feedback resistance value Rf130 is at its minimum value, the error of the voltage value Voff from 0 [V] becomes smaller. However, when the feedback resistance value Rf130 is other than its minimum value, the voltage value Voff is amplified due to the noise gain error, so the error from 0 [V] becomes larger.

[0274] Therefore, while switching the switch unit 130, the control unit 52 measures, for each feedback resistance value Rf130, the output voltage value Vo2 of the amplifier 60 when the photodiode 10 is in the light-shielded state, i.e., the voltage value Voff, through the AD conversion unit 40. The control unit 52 stores the measured voltage value Voff and the feedback resistance value Rf130 at the time when the voltage value Voff is measured, in association with each other, in the storage unit 50. The control unit 52 stores the measured voltage value Voff and the switching information of the switch unit 130 used to set the feedback resistance value Rf130 at the time when the voltage value Voff is measured, in association with each other, in the storage unit 50.

[0275] Alternatively, in order to cancel the voltage value Voe, the control unit 52 may not adjust the output voltage value Vo3 of the DA conversion unit 70. In this case, as described above, the control unit 52 measures the voltage value Voff for each feedback resistance value Rf130, and stores the measured voltage value Voff and the feedback resistance value Rf130 at the time when the voltage value Voff is measured, in association with each other, in the storage unit 50.

[0276] The control unit 52 may also periodically adjust the output voltage value Vo3 of the DA conversion unit 70 to cancel the voltage value Voe. Alternatively, the control unit 52 may also periodically measure the voltage value Voff for each feedback resistance value Rf130. The voltage value Voe varies according to the temperature of the optical measurement device 101. In order to cancel the voltage value Voe, the output voltage value Vo3 of the DA conversion unit 70 is periodically adjusted, so as to accurately measure the light intensity of the light to be measured in the second mode. In addition, by periodically measuring the voltage value Voff for each feedback resistance value Rf130, the light intensity of the light to be measured in the second mode is accurately measured.

[0277] [Operation of Optical Measurement Device]

[0278] Figure 13 and Figure 14 represents Figure 10 a flowchart showing an example of the optical measurement method of the optical measurement device 101 shown. The optical measurement method may also be implemented as an optical measurement program for causing a processor such as the control unit 52 to execute. The optical measurement program may also be stored in a non-temporary computer-readable medium. When the control unit 52 detects, through the input unit 51, an input for switching the mode of the optical measurement device 101, the control unit 52 starts Figure 13 the process of step S20 shown.

[0279] In the process of step 20, the control unit 52 receives an input for switching the mode of the optical measurement device 101 through the input unit 51. The control unit 52 determines whether this input is an input for switching to the first mode (step S21). When the control unit 52 determines that this input is an input for switching to the first mode (step S21: "Yes"), it proceeds to the process of step S22. On the other hand, when the control unit 52 determines that this input is an input for switching to the second mode (step S21: "No"), it proceeds to the process of step S32.

[0280] In the process of step S22, the control unit 52 appropriately outputs control signals to switches SW1 to SW4 to switch the connection state of the optical measurement device 101 to the connection state corresponding to the first mode.

[0281] In the process of step S23, the control unit 52 receives an input of the measurement sensitivity of the first mode through the input unit 51.

[0282] Before performing the process of step S24, the photodiode 10 is set to a light-shielded state. In the process of step S24, the control unit 52 changes the output voltage value of the DA conversion unit 70 and switches the switch unit 130, thereby setting the offset current value Ioff, that is, the current value I1. For each set offset current value Ioff, the control unit 52 appropriately switches the switch SW5 and measures the output voltage value Vo1 and the output voltage value Vo2 when the photodiode 10 is light-shielded through the AD conversion unit 40.

[0283] In the process of step S25, the control unit 52 associates the output voltage value Vo1 measured in the process of step S24 with the current value I1 calculated based on the output voltage value Vo2 measured in the process of step S24 for each set offset current value Ioff, and generates Figure 12 the table shown. The control unit 52 calculates the offset current value Ioff, that is, the current value I1, through the measured output voltage value Vo2 and the above formula (11).

[0284] In the process of step S26, the control unit 52 outputs a control signal to the switch SW5, electrically disconnects the output terminal of the amplifier 60 from the AD conversion unit 40 through the switch SW5, and electrically connects the output terminal P3 of the logarithmic amplifier circuit 20 to the AD conversion unit 40.

[0285] Before the process of step S27 is performed, the photodiode 10 is set to a light-shielded state. In the process of step S27, the control unit 52 acquires from the storage unit 50 the switching information of the switch unit 130 for setting each measurement sensitivity, and the information of the digital signal input to the DA conversion unit 70 for setting each measurement sensitivity. Based on the acquired information, the control unit 52 outputs an output voltage value corresponding to each measurement sensitivity to the DA conversion unit 70, and while setting the offset current value Ioff corresponding to each measurement sensitivity through the switch unit 130, measures the output voltage value Vo1 of the logarithmic amplifier circuit 20 through the AD conversion unit 40.

[0286] In the process of step S28, the control unit 52 calculates the offset current value Ioff1 through the output voltage value Vo1 measured in the process of step S27 and the table generated in the process of step S25. The control unit 52 stores the offset current value Ioff1 associated with each measurement sensitivity of the first mode in the storage unit 50.

[0287] In the process of step S29, the control unit 52 acquires from the storage unit 50 the switching information of the switch unit 130 for setting the measurement sensitivity of the first mode received in the process of step S23, and the information of the digital signal input to the DA conversion unit 70 for setting the measurement sensitivity of the first mode. Based on the acquired information, the control unit 52 controls the DA conversion unit 70 and the switch unit 130.

[0288] Before the process of step S30 is performed, the photodiode 10 is set to a state where the light to be measured can pass through the photodiode 10. In the process of step S30, while scanning the light wavelength, the light to be measured is input to the photodiode 10. In the process of step S30, the control unit 52 measures the output voltage value Vo1 of the logarithmic amplifier circuit 20 when the light to be measured is incident on the photodiode 10 through the AD conversion unit 40.

[0289] In the process of step S31, the control unit 52 calculates the light intensity of the light to be measured through the output voltage value Vo1 measured in the process of step S30, the table generated in the process of step S25, the above formula (8), and the above formula (9).

[0290] In the process of step S32, the control unit 52 appropriately outputs control signals to the switches SW1 to SW4 to switch the connection state of the optical measurement device 101 to the connection state corresponding to the second mode.

[0291] In the process of step S33, the control unit 52 receives the input of the measurement sensitivity of the second mode through the input unit 51.

[0292] In the process of step S34, the control unit 52 outputs a control signal to the switch SW5, electrically connects the output terminal of the amplifier 60 to the AD conversion unit 40 through the switch SW5, and electrically disconnects the output terminal P3 of the logarithmic amplifier circuit 20 from the AD conversion unit 40.

[0293] Before performing the process of step S35, the photodiode 10 is set to a light-shielded state. In the process of step S35, the control unit 52 controls the switch unit 130 to set the feedback resistance value Rf130 to the minimum value, and measures the output voltage value Vo2 of the amplifier 60 when the photodiode 10 is in the light-shielded state, that is, the voltage value Voff, through the AD conversion unit 40. In the process of step S35, in order to set the voltage value Voff to 0 [V], the control unit 52 determines the digital signal input to the DA conversion unit 70. The control unit 52 inputs the determined digital signal into the DA conversion unit 70 to cancel the voltage value Voe.

[0294] Before performing the process of step S36, the photodiode 10 is set to a light-shielded state. In the process of step S36, while switching the switch unit 130, for each feedback resistance value Rf130, the control unit 52 measures the output voltage value Vo2 of the amplifier 60 when the photodiode 10 is in the light-shielded state, that is, the voltage value Voff, through the AD conversion unit 40.

[0295] In the process of step S37, the control unit 52 obtains from the storage unit 50 the data of the maximum feedback resistance value Rf130 associated with the measurement sensitivity of the second mode received in the process of step S33. The control unit 52 controls the switch unit 130 to set the feedback resistance value Rf130 to the intermediate value of the minimum feedback resistance value Rf130 and the obtained maximum feedback resistance value Rf130. The minimum feedback resistance value Rf130 is the minimum feedback resistance value Rf130 that can be set through the offset resistor R130.

[0296] Before performing the process of step S38, the photodiode 10 is set to a state where the measured light can pass through the photodiode 10. In addition, before performing the process of step S38, the control unit 52 outputs the digital signal determined in the process of step S35 to the DA conversion unit 70. In the process of step S38, the measured light is input to the photodiode 10. In the process of step S38, the control unit 52 measures the output voltage value Vo2 of the amplifier 60 when the measured light is incident on the photodiode 10 through the AD conversion unit 40.

[0297] After performing the process of step S38, the control unit 52 proceeds to Figure 14 the process of step S39 shown.

[0298] In the process of step S39, the control unit 52 determines whether the absolute value of the output voltage value Vо2 measured in the process of step S38 is within a specified range. When the control unit 52 determines that the absolute value of the output voltage value Vо2 is within the specified range (step S39: "Yes"), it proceeds to the process of step S40. On the other hand, when the control unit 39 determines that the absolute value of the output voltage value Vо2 is outside the specified range (step S39: "No"), it proceeds to the process of step S41.

[0299] In the process of step S40, the control unit 52 calculates the photocurrent value Ip by using the voltage value Vоff measured in the process of step S36, the measured output voltage value Vо2, and the above formula (13). In the process of step S40, the control unit 52 calculates the optical intensity Pin of the measured light by using the photocurrent value Ip and the above formula (9).

[0300] In the process of step S41, the control unit 52 determines whether the absolute value of the output voltage value Vо2 measured in the process of step S38 exceeds the specified range. When the control unit 52 determines that the absolute value of the output voltage value Vо2 exceeds the specified range (step S41: "Yes"), it proceeds to the process of step S42. On the other hand, when the control unit 52 does not determine that the absolute value of the output voltage value Vо2 exceeds the specified range (step S41: "No"), that is, when the absolute value of the output voltage value Vо2 is lower than the specified range, it proceeds to the process of step S44.

[0301] In the process of step S42, the control unit 52 determines whether the current feedback resistance value Rf130 is the minimum feedback resistance value Rf130. When the control unit 52 determines that the current feedback resistance value Rf130 is the minimum feedback resistance value Rf130 (step S42: "Yes"), it proceeds to the process of step S40. On the other hand, when the control unit 52 determines that the current feedback resistance value Rf130 is not the minimum feedback resistance value Rf130 (step S42: "No"), it proceeds to the process of step S43.

[0302] In the process of step S43, the control unit 52 controls the switch unit 130 to switch the current feedback resistance value Rf130 to the feedback resistance value Rf130 at the next lower level. After executing the process of step S43, the control unit 52 returns to the process of step S38.

[0303] In the process of step S44, the control unit 52 determines whether the current feedback resistance value Rf130 is the maximum feedback resistance value Rf130 obtained in the process of step S37. When the control unit 52 determines that the current feedback resistance value Rf130 is the maximum feedback resistance value Rf130 (step S44: "Yes"), it proceeds to the process of step S40. On the other hand, when the control unit 52 determines that the current feedback resistance value Rf130 is not the maximum feedback resistance value Rf130 (step S44: "No"), it proceeds to the process of step S45.

[0304] In the process of step S45, the control unit 52 controls the switch unit 130 to switch the current feedback resistance value Rf130 to the feedback resistance value Rf130 of the next higher level. After executing the process of step S45, the control unit 52 returns to the process of step S38.

[0305] In addition, if the control unit 52 executes each of the processes of, for example, steps S24 to S25, steps S26 to S28, and steps S35 and S36 in advance, it may not execute each of the processes of steps S24 to S25, steps S26 to S28, and steps S35 and S36.

[0306] In addition, the control unit 52 may execute the processes of steps S24 to S25, steps S26 to S28, and steps S35 and S36 regularly at an arbitrary timing. When executing steps S24 to S25, S26 to S28, and S35 and S36, as described above in the first embodiment, the user can manually shield the photodiode 10, or the optical measurement device 101 can be configured to automatically shield the photodiode 10.

[0307] As described above, in the second embodiment, the offset resistor R130 can be used for adjusting the offset current value Ioff and used as the feedback resistor of the amplifier 60. With such a structure, an optical measurement device 101 that can achieve cost reduction and reduction of the installation area and constitutes both a logarithmic amplifier and a linear amplifier can be provided. In addition, since the optical measurement device 101 has the first mode and the second mode, for example, the user can appropriately switch the mode of the optical measurement device 101 to the first mode or the second mode according to the light to be measured.

[0308] Other structures and effects of the optical measurement device 101 of the second embodiment are the same as those of the optical measurement device 1 of the first embodiment.

[0309] (Third Embodiment)

[0310] As Figure 15As shown, the optical measurement device 201 includes a photodiode 10, a logarithmic amplifier circuit 20, a resistor R5, a resistor R6, a resistor R7, an AD conversion unit 40, an arithmetic device 2, an amplifier 60, a DA conversion unit 70, a switch SW1, a switch SW2, a switch SW3, a switch SW4, a switch SW5, a switch unit 130, offset resistors R130-1 to R130-N, and capacitors C130-1 to C130-N. The optical measurement device 201 includes a transistor T3, a resistor R8, and a switch SW6 (fifth switch).

[0311] The transistor T3 is a depletion-type N-channel field effect transistor (FET: Field Effect Transistor). The transistor T3 is, for example, a depletion-type N-channel junction field effect transistor (JFET: Junction Field Effect Transistor). However, the transistor T3 may also be a depletion-type N-channel MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor).

[0312] The gate of the transistor T3 is electrically connected to the input terminal P1 of the logarithmic amplifier circuit 20. The drain of the transistor T3 is electrically connected to a voltage source Vcc. The voltage source Vcc provides a voltage having a voltage value VCC. The voltage value VCC is a positive voltage value. The voltage having the voltage value VCC is input to the drain of the transistor T3. The source of the transistor T3 is electrically connected to the voltage source Vee via the resistor R8. The voltage source Vee provides a voltage having a voltage value VEE. The voltage value VE is a negative voltage value. The voltage having the voltage value VEE is input to the source of the transistor T3 via the resistor R8. In addition, in the transistor T3, the drain and the source cannot be functionally distinguished. Therefore, in the description of the present embodiment, even if the drain and the source of the transistor T3 are replaced, the transistor T3 has the same function as before the replacement.

[0313] The resistor R8 has two terminals. The resistor R8 is composed of a fixed resistor. One terminal of the resistor 8 is electrically connected to the source of the transistor T3. The other terminal of the resistor R8 is electrically connected to the voltage source Vee. The resistance value of the resistor R8 is appropriately set based on the desired source voltage value of the transistor T3.

[0314] The switch SW6 is composed of a mechanical relay, an optical relay, an analog switch, or the like. The switch SW6 may also be configured as an analog multiplexer.

[0315] The switch SW6 can be switched to electrically connect the cathode of the photodiode 10 to the reference potential or to the source of the transistor T3. For example, the switch SW6 has three terminals. One terminal of the switch SW6 is electrically connected to the cathode of the photodiode 10. One terminal of the switch SW6 is electrically connected to the source of the transistor T3. One terminal of the switch SW6 is electrically connected to the reference potential. The switch SW6 switches, based on the control signal from the control unit 52, whether to electrically connect the cathode of the photodiode 10 to the reference potential or to the source of the transistor T3.

[0316] Similar to the second embodiment, the optical measurement device 201 has a first mode and a second mode.

[0317] [First Mode]

[0318] Similar to the second embodiment, the control unit 52 receives, through the input unit 51, an input for switching the mode of the optical measurement device 201 to the first mode. When the control unit 52 receives this input, it appropriately outputs control signals to the switches SW1 to SW4 and, similar to the second embodiment, switches the connection state of the optical measurement device 201 to the connection state corresponding to the first mode.

[0319] Similar to the second embodiment, when the control unit 52 measures the output voltage value Vо1 of the logarithmic amplifier circuit 20 in the first mode, it outputs a control signal to the switch SW5. The control unit 52 outputs a control signal to the switch SW5, electrically disconnects the output terminal of the amplifier 60 from the AD conversion unit 40 through the switch SW5, and electrically connects the output terminal P3 of the logarithmic amplifier circuit 20 to the AD conversion unit 40.

[0320] Similar to the second embodiment, when the control unit 52 measures the output voltage value Vо2 of the amplifier 60 in the first mode, it outputs a control signal to the switch SW5. The control unit 52 outputs a control signal to the switch SW5, electrically connects the output terminal of the amplifier 60 to the AD conversion unit 40 through the switch SW5, and electrically disconnects the output terminal P3 of the logarithmic amplifier circuit 20 from the AD conversion unit 40.

[0321] [First Mode: Measurement Process of Light Intensity]

[0322] Similar to the second embodiment, before performing the measurement process of light intensity, the control unit 52 outputs a control signal to the switch SW5. The control unit 52 outputs a control signal to the switch SW5, electrically disconnects the output terminal of the amplifier 60 from the AD conversion unit 40 through the switch SW5, and electrically connects the output terminal P3 of the logarithmic amplifier circuit 20 to the AD conversion unit 40.

[0323] In the third embodiment, before executing the light intensity measurement process, the control unit 52 outputs a control signal to the switch SW6. The control unit 52 outputs a control signal to the switch SW6, electrically separates the cathode of the photodiode 10 from the reference potential through the switch SW6, and electrically connects the cathode of the photodiode 10 to the source of the transistor T3.

[0324] The gate of the transistor T3 is electrically connected to the anode of the photodiode 10 via the switch SW1. The source of the transistor T3 is electrically connected to the cathode of the photodiode 10 via the switch SW6. The voltage value of the anode with respect to the cathode of the photodiode 10 becomes the same as the voltage value of the gate with respect to the source of the transistor T3. Here, since the transistor T3 is a depletion-type transistor, it can be turned on even when the voltage value of the gate with respect to the source of the transistor T3 is a negative voltage value. With such a structure, a reverse bias can be applied to the photodiode 10. In addition, the reverse bias applied to the photodiode 10 is maintained at the voltage value of the gate with respect to the source of the transistor T3. That is, the transistor T3 has the function of a bootstrap circuit.

[0325] In this way, since the reverse bias applied to the photodiode 10 is maintained at the voltage value of the gate with respect to the source of the transistor T3, the capacitance value of the photodiode 10 observed from the input terminal P1 of the logarithmic amplifier 20 is equivalently reduced. Since the capacitance value of the photodiode 10 is equivalently reduced, as described with reference to Figure 16 the recovery time as described below is reduced.

[0326] In Figure 16 it shows Figure 15 the waveform of the output voltage value Vо1 of the logarithmic amplifier 20 shown. In Figure 16 the horizontal axis is time [msec]. In addition, the vertical axis is the output voltage value Vо1 [V]. At time 0 [msec], the current value I1 rapidly decreases from 10 [mA] to 10 [nA].

[0327] The waveform W5 is the waveform of the output voltage value Vо1 in the optical measurement device without the transistor T3. The waveform W6 is the waveform of the output voltage value Vо1 in the optical measurement device 201 with the transistor T3.

[0328] In the case where the current value I1 rapidly decreases, as Figure 2 shown, the amplifier 21 cannot respond to the rapid change of the current value I1, and sometimes from Figure 2The current continues to flow from the collector to the emitter of the transistor T1. If the current continues to flow from the collector to the emitter of the transistor T1, the voltage value input to the input terminal P1 becomes negative. If the voltage value input to the input terminal P1 becomes negative, since the current flowing from the emitter to the collector of the transistor T1 is very small, the output voltage value Va1 of the amplifier 21 saturates at a positive voltage value. If the output voltage value Va1 of the amplifier 21 saturates at a positive voltage value, the output voltage value Vо1 of the logarithmic amplifier circuit 20 saturates at a negative voltage value.

[0329] If the output voltage value Vо1 saturates at a negative voltage value, in the optical measurement device without the transistor T3, as in the waveform W5, the output voltage value Vо1 requires a recovery time before returning to the voltage value corresponding to the current value I1. The recovery time of the waveform W5 is about 0.2 [msec].

[0330] In contrast, in the present embodiment, since the electrostatic capacitance value of the photodiode 10 can be equivalently reduced by the transistor T3, as in the waveform W6, the recovery time is reduced.

[0331] Here, by increasing the offset current value Iоff, the recovery time can be reduced. However, if the offset current value Iоff is increased, the noise of the logarithmic amplifier circuit 20 sometimes becomes large. In the present embodiment, the offset current value Iоff can be not increased, and the recovery time can be reduced by adding the transistor T3. With such a structure, the increase in the noise of the logarithmic amplifier circuit 20 can be suppressed, and the recovery time can be reduced.

[0332] In addition, by reducing the light-receiving area of the photodiode 10 to reduce the electrostatic capacitance value of the photodiode 10, the recovery time can be reduced. However, if the light-receiving area of the photodiode 10 is reduced, the coupling efficiency of the light to be measured to the photodiode 1 is reduced. In the present embodiment, the light-receiving area of the photodiode 10 can be not reduced, and the recovery time can be reduced by adding the transistor T3. With such a structure, the reduction in the coupling efficiency of the light to be measured to the photodiode 10 can be printed, and the recovery time can be reduced.

[0333] [First mode: Setting process of measurement sensitivity]

[0334] The source voltage value of the transistor T3 is determined by the current characteristics of the transistor T3 and the resistance value of the resistor R8. For example, the source voltage value of the transistor T3 is about 1 [V]. Since the source voltage value of the transistor T3 becomes 0 [V] or more, if the source of the transistor T3 is electrically connected to the cathode of the photodiode 10, a dark current with a current value of several [nA] to several tens [pA] sometimes occurs.

[0335] Therefore, in the third embodiment, the control unit 52 adjusts the output voltage value of the DA conversion unit 70 as described later. In order to output the adjusted output voltage value to the DA conversion unit 70, for each measurement sensitivity, the information of the digital signal input to the DA conversion unit 70 is stored in the storage unit 50.

[0336] Similar to the second embodiment, before the spectral measurement process, for example, the control unit 52 receives the input of the measurement sensitivity in the first mode through the input unit 51. If the control unit 52 receives this input, it acquires the information of the digital signal for adjusting the output voltage value of the DA conversion unit 70 associated with the measurement sensitivity in the first mode. Based on the acquired information, the control unit 52 outputs the digital signal to the DA conversion unit 70. In addition, similar to the second embodiment, the control unit 52 acquires the switching information of the switch unit 130 associated with the measurement sensitivity in the first mode from the storage unit 50. Similar to the second embodiment, the control unit 52 controls the switch unit 130 according to the acquired switching information of the switch unit 130.

[0337] [First mode: Table creation process]

[0338] When the control unit 52 creates the table as Figure 12 shown, it outputs a control signal to the switch SW6. The control unit 52 outputs a control signal to the switch SW6, electrically connects the cathode of the photodiode 10 to the reference potential through the switch SW6, and electrically separates the cathode of the photodiode 10 from the source of the transistor T3. With such a structure, the table as Figure 12 shown can be created without being affected by the dark current.

[0339] [First mode: Offset current value measurement process]

[0340] Similar to the second embodiment, the control unit 52 measures the offset current value Ioff1, for example, before performing the light intensity measurement process or periodically.

[0341] In the third embodiment, before performing the offset current value measurement process, the control unit 52 outputs a control signal to the switch SW6. The control unit 52 electrically separates the cathode of the photodiode 10 from the reference potential through the switch SW6, and electrically connects the cathode of the photodiode 10 to the source of the transistor T3.

[0342] As in the second embodiment, the control unit 52 outputs the output voltage value corresponding to each measurement sensitivity to the DA converter 70, and sets the offset current value Ioff corresponding to each measurement sensitivity through the switch unit 130, while measuring the output voltage value Vo1 of the logarithmic amplifier circuit 20 when the photodiode 10 is shielded from light through the AD converter 40. Furthermore, the control unit 52 calculates the offset current value Ioff1 for each measurement sensitivity using the output voltage value Vo1 and the above table.

[0343] The control unit 52 stores the offset current value Ioff1 associated with the measurement sensitivity in the storage unit 50. However, the control unit 52 may calculate the offset current value Ioff1 from the output voltage value V01 and the above-mentioned equation (6) as in the first embodiment.

[0344] Here, in the third embodiment, the offset current value Ioff1 may be different from the offset current value Ioff1 due to the influence of the dark current. Figure 3 The offset current value Ioff1 for each sensitivity setting shown in FIG. 1 is greatly different. The offset current value Ioff1 may vary due to the output voltage value of the DA conversion unit 70. Therefore, the control unit 52 changes the offset current value Ioff1 to the same value as the output voltage value of the DA conversion unit 70. Figure 3 The digital signal input to the DA converter 70 is determined when the offset current value shown is the same as the value shown. For example, the measurement sensitivity in the first mode is Figure 3 In the case of the sensitivity A shown in FIG. 1 , the resistance value of the offset resistor R130 electrically connected between the output terminal of the amplifier 60 and the input terminal P1 is set to be equal to Figure 3 The resistance value Rs30 shown is the same as 1 [MΩ]. When the measurement sensitivity of the first mode is sensitivity A, the control unit 52 determines the digital signal input to the DA converter 70 when the offset current value I0ff1 becomes 200 [nA]. The control unit 52 stores the information of the determined digital signal in the storage unit 50 in association with the measurement sensitivity of the first mode.

[0345] The other processes in the first mode are the same as those in the second embodiment.

[0346] [2nd Mode]

[0347] As in the second embodiment, the control unit 52 receives an input for switching the mode of the optical measurement device 201 to the second mode through the input unit 51. Upon receiving the input, the control unit 52 appropriately outputs a control signal to the switches SW1 to SW4 to switch the connection state of the optical measurement device 201 to the connection state corresponding to the second mode, as in the second embodiment.

[0348] The control unit 52, similar to the second embodiment, outputs a control signal to the switch SW5 when measuring the output voltage value Vo2 of the amplifier 60 in the second mode. The control unit 52 outputs a control signal to the switch SW5, electrically connects the output terminal of the amplifier 60 to the AD conversion unit 40 through the switch SW5, and electrically disconnects the output terminal P3 of the logarithmic amplifier circuit 20 from the AD conversion unit 40.

[0349] [Second Mode: Light Intensity Measurement Processing]

[0350] The control unit 52, similar to the second embodiment, outputs a control signal to the switch SW5 before performing the light intensity measurement processing. The control unit 52 outputs a control signal to the switch SW5, electrically connects the output terminal of the amplifier 60 to the AD conversion unit 40 through the switch SW5, and electrically disconnects the output terminal P3 of the logarithmic amplifier circuit 20 from the AD conversion unit 40.

[0351] Here, as described above, sometimes since the source voltage value of the transistor T3 becomes 0 [V] or more, a dark current with a current value of several [nA] to several tens [pA] is generated.

[0352] Therefore, the control unit 52 outputs a control signal to the output switch SW6 before performing the light intensity measurement processing. The control unit 52 outputs a control signal to the switch SW6, electrically connects the cathode of the photodiode 10 to the reference potential through the switch SW6, and electrically disconnects the cathode of the photodiode 10 from the source of the transistor T3. With such a structure, in the optical measurement device 201, the generation of the above-mentioned dark current is suppressed, and the noise of the optical measurement device 201 is suppressed from becoming large.

[0353] Other processes in the second mode are the same as those in the second embodiment.

[0354] [Operation of Optical Measurement Device]

[0355] The optical measurement method of the optical measurement device 201 in the third embodiment can be referred to Figure 13 and Figure 14 and is executed as described above.

[0356] Among them, in the third embodiment, in the process of step S24, before measuring the output voltage value Vo1 and the output voltage value Vo2, the control unit 52 outputs a control signal to the switch SW6. The control unit 52 outputs a control signal to the switch SW6, electrically connects the cathode of the photodiode 10 to the reference potential through the switch SW6, and electrically disconnects the cathode of the photodiode 10 from the source of the transistor T3.

[0357] In addition, in the third embodiment, in the process of step S26, the control unit 52 controls not only the switch SW5 but also the switch SW6. In the process of step S26, the control unit 52 outputs a control signal to the switch SW6. By outputting a control signal to the switch SW6, the control unit 52 electrically separates the cathode of the photodiode 10 from the reference potential and electrically connects the cathode of the photodiode 10 to the source of the transistor T3.

[0358] In addition, in the third embodiment, in the process of step S34, the control unit 52 controls not only the switch SW5 but also the switch SW6. In the process of step S34, the control unit 52 outputs a control signal to the switch SW6. By the switch SW6, the cathode of the photodiode 10 is electrically connected to the reference potential, and the cathode of the photodiode 10 is electrically separated from the source of the transistor T3.

[0359] The other structures and effects of the optical measurement device 201 of the third embodiment are the same as those of the optical measurement device 1 of the first embodiment or the optical measurement device 101 of the second embodiment.

[0360] The embodiments of the present disclosure have been described with reference to the respective drawings and examples. However, note that those skilled in the art can easily make various deformations or changes based on the present disclosure. Therefore, it should be noted that these deformations or changes are also included in the scope of the present disclosure. For example, the functions included in each structural part or each step can be reconfigured in a logically consistent manner, multiple structural parts or steps can be combined into one, or divided.

[0361] For example, in the above embodiment, the case where the transistor T1 as a non-linear element is electrically connected between the output terminal and the non-inverting input terminal of the amplifier 21 has been described. However, a non-linear element other than the transistor T1 may also be electrically connected between the output terminal and the non-inverting input terminal of the amplifier 21.

[0362] For example, Figure 1 an analog-to-digital conversion unit may be adopted in the voltage source Vb shown. By adopting an analog-to-digital conversion unit in the voltage source Vb, the voltage value VB can be made variable.

[0363] For example, Figure 1 the optical measurement device 1 shown may also have Figure 15 the transistor T3, the resistor R8, and the switch SW6 shown.

[0364] For example, Figure 1 the optical measurement device 1 shown may also generate Figure 12The shown table is used to measure the light intensity of the light to be measured instead of the above formula (7). When the table is generated in the optical measurement device 1, the light to be measured with a known light intensity is input to the photodiode 10 in a state where the offset current value Ioff is set to 0 [A]. That is, the current value I1 becomes equal to the known photocurrent value Ip. While changing the known photocurrent value Ip, that is, the current value I1, the control unit 52 measures the output voltage value Vo1 of the logarithmic amplifier circuit 20 through the AD conversion unit 40. The control unit 52 generates a table by associating the photocurrent value Ip and the output voltage value Vo1.

Claims

1. A light measuring device, comprising: a light receiving element capable of converting the light intensity of the light to be measured into an electrical signal; an input terminal to which the electrical signal is input; a first amplifier and a non-linear element constituting a logarithmic amplifier, the inverting input terminal of the first amplifier being electrically connected to the input terminal; a plurality of offset resistors each having a different resistance value; a switch unit capable of switching the offset resistor that electrically connects the voltage source and the input terminal among the plurality of offset resistors; and a control unit, an offset current is input to the input terminal through the offset resistor that electrically connects the voltage source and the input terminal, the control unit measures the light intensity based on the output voltage value of the logarithmic amplifier, the control unit adjusts the offset current by switching the offset resistor that electrically connects the voltage source and the input terminal, so that the measurement sensitivity of the light measuring device can be switched step by step.

2. The light measuring device according to claim 1, wherein the control unit measures the current value of the offset current based on the output voltage value of the logarithmic amplifier when the light receiving element is shielded from light.

3. The light measuring device according to claim 1 or 2, wherein the control unit subtracts the current value of the offset current calculated based on the output voltage value of the logarithmic amplifier when the light receiving element is shielded from light from the current value calculated based on the output voltage value of the logarithmic amplifier when the light to be measured is incident on the light receiving element, so as to calculate the light intensity of the light to be measured.

4. The light measuring device according to claim 1, wherein the light measuring device comprises: a second amplifier having a non-inverting input terminal electrically connected to the voltage source; a first switch capable of switching whether to electrically connect the light receiving element and the input terminal; a second switch capable of switching whether to electrically connect the light receiving element and the inverting input terminal of the second amplifier; a third switch capable of switching whether to electrically connect the inverting input terminal and the output terminal of the second amplifier; and a fourth switch capable of switching whether to electrically connect the non-inverting input terminal of the second amplifier to a reference potential, the plurality of offset resistors are provided between the light receiving element and the output terminal of the second amplifier.

5. The light measuring device according to claim 4, wherein the light measuring device has: a first mode of measuring the light intensity based on the output voltage value of the logarithmic amplifier, and a second mode of measuring the light intensity based on the output voltage value of the second amplifier, in the first mode, through the first switch, the light receiving element and the input terminal are electrically connected, through the second switch, the light receiving element and the inverting input terminal of the second amplifier are electrically separated, through the third switch, the inverting input terminal and the output terminal of the second amplifier are electrically connected, and through the fourth switch, the non-inverting input terminal of the second amplifier is electrically separated from the reference potential. In the first mode, the offset current is input to the input terminal through the offset resistor electrically connected between the output terminal and the input terminal of the second amplifier.

6. The optical measurement device according to claim 5, wherein the voltage source is a digital-to-analog conversion unit.

7. The optical measurement device according to claim 5, wherein in the first mode, the control unit measures the current value of the offset current based on the output voltage value of the second amplifier when the light receiving element is shielded from light.

8. The optical measurement device according to claim 7, wherein while switching the switch unit, the control unit measures the output voltage value of the logarithmic amplifier and the output voltage value of the second amplifier when the light receiving element is shielded from light for each current value of the offset current, and generates a table associating the measured output voltage value of the logarithmic amplifier with the current value of the offset current calculated based on the measured output voltage value of the second amplifier. The control unit measures the light intensity of the measured light based on the output voltage value of the logarithmic amplifier when the measured light is incident on the light receiving element and the table.

9. The optical measurement device according to claim 7 or 8, wherein the optical measurement device further includes a transistor which is a depletion-type N-channel field effect transistor. the light receiving element is a photodiode. the anode of the photodiode is electrically connected to the first switch and the second switch. the gate of the transistor is electrically connected to the input terminal, the source of the transistor is electrically connected to the cathode of the photodiode, and a positive voltage value is input to the drain of the transistor.

10. The optical measurement device according to claim 9, wherein the optical measurement device further includes a fifth switch capable of switching whether to electrically connect the cathode of the photodiode to the reference potential or to the source of the transistor. In the second mode, the cathode of the photodiode is electrically connected to the reference potential through the fifth switch.

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