Offset zeroing of optical power meters

Abstract

Provided are an optical power measurement method, an offset calibration method, and an optical power meter suitable for applying the offset calibration method. The optical power measurement method, the offset calibration method, and the optical power meter are characterized by two temperature sensors for more accurately predicting optical power offset. The first temperature sensor is positioned to read the temperature of a photodiode, and the second temperature sensor is positioned to read the temperature of a PCB ground plane.

Description

Technical Field

[0001] This description relates generally to optical power meters and, more particularly, to offset nulling. Background Art

[0002] The optical power of light is typically measured using a photodiode, which converts the optical power received at the surface of the photodiode into a photocurrent.

[0003] Even when no light is incident on the photodetector, photodiodes typically generate electrical noise, known as "dark current." Therefore, the total current flowing out of the photodiode is the sum of the photocurrent and the dark current. If not accounted for, the dark current can introduce an offset into the optical power measurement, affecting the linearity or implicit uncertainty of the measurement. Dark current is known to vary significantly with the temperature of the photodiode.

[0004] When using a photodiode to measure optical power, it is known in the art to perform a prior step of offset nulling to eliminate optical power offsets caused by dark current and other circuit components. Due to temperature variations in the offset, this offset nulling is only valid for the moment the offset nulling is performed, and it is generally recommended to repeat the offset nulling step each time the optical power meter is used. This offset nulling is also sensitive to optical power meter warm-up, and care should be taken to perform the offset nulling step after the recommended warm-up time. Offset nulling can be performed automatically by blocking the input light or switching the input electronic circuit. Some disadvantages of this offset nulling (manual or automatic) are that performing this operation requires interrupting the measurement process, which must be performed very carefully and requires some additional time and additional hardware.

[0005] An alternative solution is to perform a factory offset null calibration at a given room temperature (23°C ± 1°C) after a given warm-up time. This factory offset null calibration allows avoiding the need to repeat the offset null step in the field for each new optical power measurement, as long as the optical power meter is used close to this given room temperature. Any measurement performed outside of this narrow temperature range will be subject to offset null errors.

[0006] However, handheld optical power meters are designed for outdoor use in high humidity and temperature ranges. When sealed, they may require several hours of warm-up time. Without this warm-up time, the factory offset zero calibration cannot be reliable, and such warm-up time is unacceptable in the industry.

[0007] Therefore, there remains a need for an offset nulling calibration method that allows optical power measurements to be made under varying ambient temperatures without requiring long warm-up times. Summary of the Invention

[0008] Provided are an optical power measurement method, an offset calibration method, and an optical power meter suitable for applying the offset calibration method. The optical power measurement method, the offset calibration method, and the optical power meter are characterized by two temperature sensors for more accurately predicting optical power offset. The first temperature sensor is positioned to read the temperature of a photodiode, and the second temperature sensor is positioned to read the temperature of a PCB ground plane.

[0009] From the optical power offset values ​​obtained for two different temperature points, a numerical model can be derived that takes into account the thermal laws of the photodiode, the transimpedance amplifier, and the entire electronic amplification circuit. This derived numerical model can then be later applied to predict the optical power offset of subsequent optical power measurements obtained with different photodetector and ground plane temperature values.

[0010] Using the proposed numerical model, an optical power meter can be calibrated by measuring the optical power offset at only two different temperature points. In one embodiment, these measurements can be read during factory calibration or field self-calibration during a warm-up procedure. Advantageously, the offset calibration step does not require any specific or stable temperature set point.

[0011] Advantageously, once the offset calibration process is complete, the optical power meter can be used with near-zero warm-up time compared to prior art methods. In some implementations, the proposed calibration method may not require any additional steps beyond those already required, for example, to perform wavelength calibration, thereby reducing calibration time.

[0012] According to one aspect, there is provided a method for measuring optical power, comprising:

[0013] At the temperature measurement point,

[0014] Reading the raw optical power value from an optical power meter including a photodetector and an amplifying circuit;

[0015] Read the measured photodetector temperature value (T pd );as well as

[0016] Read the measured ground plane temperature value (T gnd );

[0017] According to the predetermined parameters associated with the photodetector and the amplifying circuit of the optical power meter, the measured photodetector temperature value (T pd ) and the measured ground plane temperature value (T gnd ), determine the measured optical power offset value;

[0018] An optical power measurement value is derived from the raw optical power value and the determined measured optical power offset value.

[0019] The predetermined parameters may be obtained from the preceding steps of:

[0020] At the first temperature point:

[0021] Read the first photodetector temperature value (T pd0 );as well as

[0022] Read a first ground plane temperature value (T gnd0 );

[0023] For the first amplification gain setting and for the second amplification gain setting:

[0024] Read the optical power offset value (A0, B0);

[0025] At a second temperature point different from the first temperature point:

[0026] Read the second photodetector temperature value (T pd1 );as well as

[0027] Read a second ground plane temperature value (T gnd1 );

[0028] For the first amplification gain setting and for the second amplification gain setting:

[0029] Read the optical power offset value (A1, B1);

[0030] From the optical power offset value (A0, B0; A1, B1), the photodetector temperature value (T pd0 ,T pd1 ) and the ground plane temperature (T gnd0 ,T gnd1 ) derive the predetermined parameters associated with the photodetector and the amplification circuit of the optical power meter.

[0031] According to another aspect, there is provided an offset calibration method comprising:

[0032] At the first temperature point:

[0033] Read the first photodetector temperature value (T pd0 );as well as

[0034] Read a first ground plane temperature value (T gnd0 );

[0035] For the first amplification gain setting and for the second amplification gain setting:

[0036] Read the optical power offset value (A0, B0);

[0037] At a second temperature point different from the first temperature point:

[0038] Read the second photodetector temperature value (T pd1 );as well as

[0039] Read a second ground plane temperature value (T gnd1 );

[0040] For the first amplification gain setting and for the second amplification gain setting:

[0041] Read the optical power offset value (A1, B1);

[0042] The photodetector temperature value (T pd0 ,T pd1 ), ground plane temperature (T gnd0 ,T gnd1 ) and the optical power offset value (A0, B0; A1, B1) are used to derive parameters associated with the photodetector and the amplification circuit of the optical power meter.

[0043] According to another aspect, there is provided an optical power meter comprising:

[0044] Photodetector, amplifier circuit and analog-to-digital converter for reading raw optical power value;

[0045] Used to measure the temperature value of the photodetector (T pd ) a first temperature sensor associated with the photodetector;

[0046] Used to measure the ground plane temperature (T gnd ) is associated with a ground plane of the amplifying circuit; and

[0047] A processing unit configured to:

[0048] According to the predetermined parameters associated with the photodetector and the amplifying circuit, the temperature value of the photodetector (T pd ) and the ground plane temperature value (T gnd), determine the measured optical power offset value; and

[0049] An optical power measurement value is derived from the raw optical power value and the determined measured optical power offset value.

[0050] It should be noted that the measured temperature point may be different from both the first temperature point and the second temperature point.

[0051] Furthermore, the raw optical power value may be read using a first amplification gain setting, a second amplification gain setting, or a third amplification gain setting that is different from both the first temperature point and the second temperature point.

[0052] In this specification, unless otherwise indicated, modifiers such as "substantially" and "approximately" that modify a value, condition, relationship, or characteristic of one or more features of an embodiment should be understood to mean that the value, condition, relationship, or characteristic is defined to be within an acceptable tolerance for proper operation of the embodiment in the context of its intended application.

[0053] In this description, the terms "light" and "optical" are used to refer to radiation in any appropriate region of the electromagnetic spectrum. More specifically, the terms "light" and "optical" are not limited to visible light, but may include, for example, the infrared wavelength range. For example, in some embodiments, the wavelength of the optical signal measured by the optical power meter may be in the range of approximately 800 nm to approximately 1650 nm.

[0054] Additional features and advantages of the present invention will become apparent to those of ordinary skill in the art from a reading of the following description taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0055] FIG. 1 (Prior Art) is a schematic diagram illustrating a circuit of an optical power meter according to a prior art embodiment.

[0056] Figure 2 is a schematic diagram illustrating an optical power meter according to one embodiment.

[0057] Figure 3 is a flow chart illustrating a calibration method for characterizing an optical power offset of an optical power meter according to one embodiment;

[0058] Figure 4 is a flow chart illustrating an optical power measurement method according to one embodiment;

[0059] Figure 5 is a schematic diagram showing a photodiode equivalent circuit model known in the prior art;

[0060] Figure 6 is with Figure 5 The equation corresponding to the photodiode equivalent circuit.

[0061] Figure 7 is a graph showing a graphical representation of the thermal law governing the reverse saturation current of a photodiode.

[0062] Figure 8 is a graph showing the thermal law governing the shunt resistance.

[0063] Figure 9 is a graph showing the variation of the dark current of the photodetector with the reverse voltage.

[0064] It will be noted that similar features are identified by similar reference numerals throughout the drawings. In order not to unduly obscure the drawings, certain elements may not be indicated in some drawings if they have been identified in previous drawings. It should be understood herein that the elements of the drawings are not necessarily drawn to scale. Some mechanical or other physical components may also be omitted in order not to obscure the drawings.

[0065] The following description is provided to provide a comprehensive understanding of the methods, devices, and / or systems described herein. Various changes, modifications, and equivalents of the methods, devices, and / or systems described herein will suggest themselves to those of ordinary skill in the art. Descriptions of well-known functions and structures may also be omitted to enhance clarity and brevity.

[0066] Although some features may be described with respect to various exemplary embodiments, the aspects are not necessarily limited thereto, such that features from one or more exemplary embodiments may be combined with other features from one or more exemplary embodiments. DETAILED DESCRIPTION

[0067] FIG1 shows a conventional circuit for an optical power meter 10. The optical power meter 10 includes a photodetector 12 implemented as a photodiode (such as a PIN photodiode or any other type of pn junction photodiode, without limitation to the semiconductor material), an amplifier circuit 14, and an analog-to-digital (A / D) converter 16. Ideally, the digital count at the output of the analog-to-digital converter 16 is linearly related to the optical power incident on the photodetector 12. The amplifier circuit includes a transimpedance amplifier TZ amplifier having a plurality of selectable linear amplification gain settings G_low and G_high, the selectable linear amplification gain settings also referred to herein as scale number i, implemented by amplification gain setting resistors, which may be software or firmware selectable via switches Sw_low and Sw_high. Of course, the number of amplification gain settings may vary.

[0068] When measuring optical power using the optical power meter 10, it is known in the art to perform a prior step of offset nulling to eliminate optical power offsets caused by dark current and other circuit components. This is performed by reading the output of the A / D converter 16 while placing a nulling cap on the photodetector (so as to block any incident light). This process can be repeated for each scale number i. The value obtained by this process can be referred to as the offset value offset i .

[0069] Then, the offset value is subtracted from the raw optical power value raw read by the optical power meter 10. i To obtain optical power measurement net i :

[0070] net i =raw-offset i (1)

[0071] Where i is the scale number used for measurement, net i is the optical power measurement obtained by scaling i, raw is the actual value read on the A / D converter 16 for a given optical power and offset i is the offset value for scale i.

[0072] Due to the temperature dependence of optical power offset, this offset nulling is only valid for the moment of offset nulling, and it is generally recommended to repeat the offset nulling step each time the optical power meter is used. This offset nulling is also sensitive to optical power meter warm-up, and care should be taken to perform the offset nulling step after the recommended warm-up time.

[0073] Of course, it is used to indicate the optical power offset value. i The units of the raw optical power value raw and the optical power measurement value net may vary. For example, these values ​​may be expressed in units of measurement representing physical quantities, such as watts (including milliwatts, microwatts, etc.) or decibel-milliwatts (dBm), or may be expressed in counts of readings read directly at the output of the A / D converter (which will then be converted into measurement units).

[0074] Now refer to Figure 2 and Figure 3 , an optical power meter and calibration method are provided herein that allow prediction of optical power offsets that will be read by the optical power meter at temperature points other than those used for calibration.

[0075] Figure 2An optical power meter 11 is shown that includes a photodetector 12 implemented as a photodiode (such as a PIN photodiode or any other type of pn junction photodiode (including but not limited to planar diffused, low capacitance planar diffused, Schottky and avalanche photodiode), without limitation to semiconductor materials (including but not limited to Si, Ge, InGaAs, etc.), an amplifier circuit 14, an A / D converter 16, a processor 18 and a data storage area 20. The optical power meter 11 includes features similar to those of the optical power meter 10, and similar features will not be repeatedly described. In addition to selectable linear amplification gain settings G_low and G_high - also referred to herein as scale number i, the amplifier circuit can optionally include a bias setting Vbias, which is also controllable in software or firmware via an actuator Sw_bias to apply a reverse bias voltage Vbias to the photodetector 12. As is known in the art, a reverse bias voltage can be used to improve high signal linearity.

[0076] In optical power meter 11, first temperature sensor 22 and second temperature sensor 24 are positioned to measure the actual temperature Tpd of the photodetector and the actual temperature Tgnd of the PCB ground plane, respectively, to more accurately predict optical power excursion. Note that the PCB ground plane temperature Tgnd represents the actual temperature of the amplifier circuit, including the transimpedance amplifier and the amplifier gain setting resistor.

[0077] A calibration procedure is applied to the optical power meter 11 in order to derive a numerical representation of the optical power offset based on the actual photodetector temperature Tpd and the ground plane temperature Tgnd read from the temperature sensors 22, 24. Advantageously, the calibration procedure can be performed once at the factory, and its results can then be used to predict the optical power offset based on the temperature values ​​(Tpd, Tgnd) read at the temperature sensors. Of course, it can also be repeated at any other time during the service life of the optical power meter 11. It can also be repeated periodically at predetermined time intervals, or repeated at requested recalibration to account for aging.

[0078] The calibration procedure does not exclude the use of prior art offset nulling and its legacy applications (certificates of conformity, etc.), which may replace the calibration method presented here at any time.

[0079] However, it will be appreciated that the thermal variation of the optical power offset is specific to each photodetector, and therefore to each individual optical power meter 11 , which is therefore individually calibrated.

[0080] Using the proposed numerical model, the proposed calibration procedure measures the actual optical power offset values ​​at two different temperature points. For example, these measurements can be taken during a warm-up procedure, where the temperature of the optical power meter 11 is typically varied from room temperature to a higher steady-state operating temperature. Advantageously, such a procedure does not require any additional or controlled temperature set points. Of course, additional and / or stable temperature points can be used instead, as appropriate for the actual implementation.

[0081] Then, a calibration procedure is used to derive parameters associated with the photodetector and amplification circuitry of the optical power meter, including thermal laws thereof, based on the numerical representations of the parameters associated with the photodetector and amplification circuitry of the optical power meter.

[0082] In one embodiment, the optical power offset i The numerical value of is expressed as follows:

[0083] net i =raw-Kel i *(Ib(Tgnd)+Id(Tpd))-Vo(Tgnd) (2) where i is the scale number used for measurement, net i is the optical power measurement obtained by scaling i, raw is the actual value read on the A / D converter 16 for a given optical power, Tgnd is the actual temperature of the PCB ground plane that can be read on the temperature sensor 24, Tpd is the actual temperature of the photodetector that can be read on the temperature sensor 22, Kel i is the amplifier gain for scale i (expressed in counts / A), Ib is the transimpedance amplifier input bias as a function of temperature Tgnd, Id is the photodetector dark current as a function of temperature Tpd, and Vo is the amplifier circuit offset as a function of temperature Tgnd.

[0084] The photodetector dark current can be expressed as:

[0085] Id(Tpd)=Id0*10 m*Tpd (3a)

[0086] where Id0 is the photodetector dark current at Tpd = 0°C, and m is the photodetector dark current exponential slope and thermal law.

[0087] Or equivalently, using the exponential e basis function:

[0088]

[0089] Or more generally:

[0090] Id(Tpd)=Id0*10 m*(Tpd-T0) (3c)

[0091] Where T0 is the reference temperature corresponding to Id0.

[0092] As will be described below, the general behavior of optical power offset expressed in numerical terms can be based on photodetector and transimpedance amplifier data sheet information, while specific parameters of each optical power meter (such as Ib, Id0, and Vo) can be derived from calibration procedures.

[0093] For each possible scale i, the amplification gain Kel i These values ​​can be obtained, for example, from design parameters of the amplifier circuit or by a previous electrical calibration of the amplifier circuit.

[0094] Please note that Figure 2 The optical power meter 11 has two gain settings, both of which are used in the calibration method. Of course, the optical power meter can use more than two gain settings. In this case, any two of the gain settings can be used in the calibration method, but it has been found that using the highest gain and the lowest gain can provide more reliable results.

[0095] The processor 18 may implement processing steps of the optical power measurement method, such as determining an optical power offset value based on predetermined parameters stored in the data storage area 20 and deriving an optical power measurement value from the raw optical power value read by the A / D converter 16 .

[0096] Furthermore, in some embodiments, the calibration method may be implemented by software embedded in the optical power meter 11 and executed by the processor 18. In this case, the processor may further implement the processing steps of the calibration method. In other embodiments that may be suitable for, for example, factory calibration, the calibration method may be implemented by an external computing device (such as a personal computer, laptop computer, tablet computer, smartphone, etc.) that is temporarily connected to the optical power meter 11 during the calibration process. In any case, the parameters derived from the calibration method are stored in the data storage area 20 for later use in the optical power measurement method.

[0097] The data storage area 20 may further store computer instructions, for example in the form of software or firmware, for execution by the processor 18 to perform the processing steps of the optical power measurement method and optionally the processing steps of the calibration method. As such, the data storage area 20 may include, for example, EPROM, EEPROM, flash memory, or any other technology of non-volatile memory, whether read-write or read-only.

[0098] Calibration method:

[0099] Figure 3A calibration method for characterizing the optical power offset of an optical power meter according to an embodiment is shown. Figure 2 11 describes the calibration method of the optical power meter.

[0100] The offset measurement is performed at a first temperature point through steps 102 , 103 , 104 and 105 , and is repeated at a second temperature point through steps 106 , 107 , 108 and 109 .

[0101] When no light is incident on the photodetector, the optical power offset value is read by recording the actual value raw.Light may be blocked from reaching the photodetector, for example, by placing a zeroing cap (as known in the art) on the photodetector receptacle.

[0102] At the first temperature point, a first photodetector temperature value Tpd0 is read using the temperature sensor 22 associated with the photodetector 12 (step 102), and a first ground plane temperature value Tgnd0 is read using the temperature sensor 24 associated with the ground plane of the amplifier circuit, i.e., the ground plane of the amplifier circuit 14 (step 103). Still at the first temperature point, an optical power offset value A0 is read for a first amplifier gain setting (e.g., G_high) (step 104), and an optical power offset value B0 is read for a second amplifier gain setting (e.g., G_low) (step 105).

[0103] Then, at the second temperature point, the second photodetector temperature value Tpd1 is read using the temperature sensor 22 associated with the photodetector 12 (step 106), and the second ground plane temperature value Tgnd1 is read using the temperature sensor 24 associated with the ground plane of the amplifier circuit, i.e., the ground plane of the amplifier circuit 14 (step 107). Still at the second temperature point, the optical power offset value A1 is read for the first amplifier gain setting (e.g., G_high) (step 108), and the optical power offset value B1 is read for the second amplifier gain setting (e.g., G_low) (step 109).

[0104] It is noted that the order in which the measurements are read is irrelevant as long as the measurements of steps 102, 103, 104, 105 are read at one and the same temperature point and the measurements of steps 106, 107, 108, 109 are read at another and the same temperature point.

[0105] In one embodiment, the first temperature point and the second temperature point can be obtained during a warm-up procedure, wherein the temperature of the optical power meter 11 is typically changed from room temperature to a higher steady-state operating temperature. For example, the first temperature point can be obtained when the optical power meter 11 is turned on (when the internal temperature of the optical power meter is at ambient temperature) or after a certain small period of time has passed, and the second temperature point can be obtained after a certain longer period of time has passed (and the internal temperature has reached a different level). For example, the second temperature point can be obtained after a given warm-up time, such as 15 minutes, 30 minutes, or even several hours after the unit is turned on.

[0106] This procedure is not very sensitive to ambient temperature or ambient temperature stability, but may still be more accurate if the ambient temperature is about 23°C or higher and the variation is less than ±1°C. For better results, a minimum temperature difference (such as greater than 2°C) may be set between the first temperature point and the second temperature point (e.g., |Tpd1–Tpd0|>2°C or |Tgnd1–Tgnd0|>2°C).

[0107] Then, in step 110, parameters are derived from the optical power offset values, photodetector temperature values, and ground plane temperature values ​​read in steps 102, 103, 104, 105, 106, 107, 108, and 109, and these parameters are associated with the photodetector 12 and amplification circuit 14 of the optical power meter 11 for later use in the numerical representation of the optical power offset as a function of the photodetector temperature Tpd and the ground plane temperature Tgnd.

[0108] In one embodiment, the derived parameters include the amplifier input bias current Ib, the photodetector dark current Id0 at a reference temperature, and the amplifier circuit offset Vo(Tgnd) as a function of temperature. Implementation details for deriving numerically represented parameters are described below.

[0109] In step 112 , the parameters thus derived may then be saved for later use, for example in the data storage area 20 .

[0110] The calculation method can be implemented in optical power meter software or firmware executed by the processor 18 embedded in the optical power meter 11 or in software executed by an external computer (such as a personal computer or laptop). In addition to deriving parameters, the software or firmware can also control the execution of steps 102 to 109 by triggering the reading of optical power offset values ​​and temperature values.

[0111] Optical power measurement method:

[0112] Figure 4An optical power measurement method according to one embodiment is shown, and an optical power measurement value can be derived from the optical power measurement method while taking into account the optical power offset of the optical power meter using the numerical value derived from the calibration method described above. Figure 2 The optical power meter 11 describes the optical power measurement method.

[0113] The optical power measurement is now performed at a measurement temperature point which may be different from both the first temperature point and the second temperature point. Figure 3 The calibration method derives the numerical representation and its parameters to retrieve the optical power offset read when there is no incident light on the photodetector. i , without actually performing an offset measurement at this temperature.

[0114] Therefore, at this measurement temperature point, the photodetector temperature value Tpd is read using the temperature sensor 22 associated with the photodetector 12 (step 202), and the ground plane temperature value Tgnd is read using the temperature sensor 24 associated with the ground plane of the amplification circuit 14 (step 204). Still at this measurement temperature point, the raw optical power value raw is read for a given amplification gain setting (e.g., G_high, G_low, or any other) (step 206).

[0115] Then, the optical power offset value offset is determined based on the predetermined parameters (Ib, Id0, Vo(Tgnd)) (208), the read photodetector temperature value Tpd, and the read ground plane temperature value Tgnd using, for example, Equation 2 in combination with Equations 3a, 3b, or 3c, or any equivalent thereof. i (Step 210), and from the original optical power value raw and the determined optical power offset value offset i Export optical power measurement net i :

[0116] net i =raw-Kel i *(Ib(Tgnd)+Id0*10 m*Tpd )-Vo(Tgnd) (2a)

[0117] Numerical representation:

[0118] As can be noted from Equation 2, the optical power offset i The proposed numerical representation of takes into account two different offset sources: the first term takes into account the sum of the input currents, as amplified by the chosen scaling gain Kel; and the second term takes into account other contributions to the offset (Vo) from components of the amplification circuit.

[0119] The amplifier circuit offset, Vo, accounts for circuit offsets, including amplifier output offset, resistor divider offset, reference offset, and A / D converter offset. The thermal law of the amplifier circuit offset, Vo, can be assumed to vary linearly with temperature. The amplifier circuit offset, Vo, is independent of scale. The value of Vo and its thermal law can be determined through a calibration procedure.

[0120] Kel i is a constant representing the amplification gain (expressed in counts / A) for scale i, which represents the transfer function of the amplifier circuit.

[0121] Ib is the input bias current of the transimpedance amplifier. The thermal law of the input bias current, Ib, can also be assumed to vary linearly with temperature. The value of Ib and its thermal law can be determined through a calibration procedure.

[0122] Id is the photodetector dark current which can be expressed as an exponential function of temperature and can be determined through a calibration procedure.

[0123] The current I, which is amplified by the transimpedance amplifier and causes an optical power shift in the absence of incident light, is defined as:

[0124]

[0125] where Vio is the input offset voltage of the transimpedance amplifier, and Rsh is the photodiode shunt resistance.

[0126] According to photodiode manufacturers and the optics industry:

[0127]

[0128] For precision amplifiers, Vio is very small (less than μV) and very stable with temperature (nV / °C), and this second term of Equation 4 becomes proportional to Id.

[0129] Figure 5 The photodiode equivalent circuit model known in the art is shown in FIG. 1 (see Hamamatsu datasheet). Based on this model and the basic Shockley diode equation (Equation 6), Figure 6 The equation to find the output current I of the photodetector is O , and the diode current I D is represented as follows:

[0130]

[0131] in:

[0132] Id is the diode current,

[0133] Is is the photodiode reverse bias saturation current (or scale current),

[0134] Vd is the voltage across the diode,

[0135] e is the electron charge

[0136] k is the Boltzmann constant

[0137] T is the absolute temperature of the photodiode

[0138] η is the ideality factor, also called the quality factor, or sometimes the emissivity.

[0139] It is important to note that the Shockley equation is applicable to any type of pn junction photodiode. It will be appreciated that if the methods described herein are applied to other types of photodetectors, including PIN photodiodes, the example numerical models described herein to represent the behavior of photodiodes may be modified and / or adapted to account for the differences in the behavior of such other types of photodetectors.

[0140] It is important to note that the saturation current Is is not constant but varies with temperature. This variation is dominated by the temperature coefficient of the photodiode.

[0141] The thermal law of Is is revealed from Experiment 15 conducted at the California Institute of Technology - Solid State Physics Laboratory: Temperature dependence of the saturation current of a junction diode:

[0142]

[0143]

[0144] in:

[0145] Idiff is the diffusion current,

[0146] Igen is the generated current,

[0147] n i is the intrinsic carrier concentration in the semiconductor material,

[0148] Eg is the gap energy between the valence band and the conduction band,

[0149] k is the Boltzmann constant,

[0150] T is the absolute temperature of the diode junction.

[0151] Equivalently:

[0152]

[0153] If the diffusion current dominates the saturation current, then x = 1. If the generation current dominates, then x = 2. Again equivalently, by applying the logarithm:

[0154]

[0155] Figure 7 A graphical representation of Equation 10 is shown.

[0156] Based on the results of this experiment at Caltech, it can be shown that the thermal law governing Id is as follows:

[0157] Id=exp mT+常数 =Id0*exp m*Tpd (11)

[0158] in:

[0159] Tpd is the temperature of the photodetector;

[0160] Id0 is the vertical intercept of the graph (which represents the dark current at 0°C); and

[0161] m represents the slope of the logarithmic current.

[0162] This exponential expression is convenient because photodiode manufacturers often provide datasheet graphs of Rsh in base-10 logarithmic format, as shown, for example, in Figure 8 middle.

[0163] According to Equation 5:

[0164]

[0165] Where Rsh0 is the shunt resistance at the reference temperature of 0°C ( Figure 8 (vertical intercept plot in ):

[0166] Similarly, the slope m can be calculated using the graph values:

[0167]

[0168] The photodiode dark current is then found to be:

[0169] Id=Id0*10 m*Tpd (14)

[0170] And this thermal law is used in equations 3a, 3b and 2a.

[0171] For a given photodetector family, the slope m has been shown to represent a reliable value. The photodetector family is defined by the type of semiconductor, its technology, its package and its diameter or surface area (see, for example, Figure 8, which shows an example of shunt resistance as a function of ambient temperature (taken from a Hamamatsu datasheet).

[0172] The Id0 specific to an individual photodetector can be determined using a calibration procedure.

[0173] Calibration method:

[0174] The following describes the specific embodiments Figure 2 The following detailed calculation method is provided as an example only, and it should be understood that those skilled in the art can easily design other calculation details that will equivalently derive all necessary parameters associated with the photodetector and amplification circuit of the optical power meter.

[0175] Calibration method example 1:

[0176] In this example, the optical power offset values ​​A0, A1, B0, B1, C0, C1, and A0 are read for two or more amplification gain settings, including low scale offset, low scale, high scale, and high scale offset. 偏置 .

[0177] Table 1 shows the values ​​that will be read according to this example:

[0178]

[0179] Table 1

[0180] In Table 1, reading A0 偏置 , A0, B0 and C0 represent the raw optical power values ​​read at the first temperature point T0; readings A1, B1 and C1 represent the raw optical power values ​​read at the second temperature point T1; Tpd0 and Tgnd0 respectively represent the temperature values ​​read on the temperature sensor 22 and the temperature sensor 24 at the first temperature point T0; and Tpd1 and Tgnd1 respectively represent the temperature values ​​read on the temperature sensor 22 and the temperature sensor 24 at the second temperature point T1.

[0181] Processor 18 or another external processor then derives numerical model parameters associated with photodetector 12 and amplification circuit 14 of optical power meter 11 to obtain a numerical representation of the optical power offset. This can be performed by solving equations 2 or 3 using the values ​​read at the two temperature points.

[0182] For example, by applying Equation 2 to A0, B0, A1, and B1, we obtain:

[0183]

[0184]

[0185]

[0186]

[0187] By subtracting Equations 15 and 16, we obtain:

[0188]

[0189] Likewise, from equations 17 and 18 we have:

[0190]

[0191] In this embodiment, it is assumed that the transimpedance amplifier input bias Ib is a constant value or a linear function of temperature (temperature drift is expressed in A / °C). Of course, other temperature-dependent behaviors can be considered if necessary (which can be verified in the transimpedance amplifier data sheet). In the case where the temperature difference between T0 and T1 is small (for example, in practice less than about 2°C), the transimpedance amplifier input bias Ib can be assumed to be constant:

[0192] Ib(Tgnd0)=Ib(Tgnd1) (21)

[0193] Then, by subtracting equations 19 and 20, Id0 is obtained, which can be saved as one of the numerical representation parameters:

[0194]

[0195] Id0 is substituted in Equation 19 to obtain the value of Ib:

[0196]

[0197] From this value, the value of Ib as a function of temperature Tgnd can be retrieved, for example, from the thermal law specification provided in the transimpedance amplifier data sheet. This value of Ib(Tgnd0) can be saved as one of the numerically represented parameters.

[0198] Assuming Vo's linear heat law:

[0199] Vo(Tgnd)=Vo(Tgnd0)+δV*(Tgnd-Tgnd0) (24)

[0200] The amplifier circuit offset Vo can be derived as:

[0201] Vo(Tgnd0)=A0-Kel 高 *(Ib(Tgnd0)+Id(Tpd0)) (25a)

[0202] or

[0203] Vo(Tgnd0)=B0-Kel 低 *(Ib(Tgnd0)+Id(Tpd0)) (25b)

[0204] and

[0205]

[0206] or

[0207]

[0208] These values ​​of Vo(Tgnd0) and δVo may be saved as numerically expressed parameters.

[0209] verify:

[0210] To improve reliability, the derived value of Ib may also be verified against typical values ​​and / or minimum and maximum expected values ​​(e.g., as found in a transimpedance amplifier datasheet).

[0211] Likewise, the derived value of Id can be verified from the photodetector datasheet:

[0212]

[0213] Furthermore, according to definitions from photodiode manufacturers and the optics industry:

[0214]

[0215] The shunt resistor value Rsh (which can be derived from the value of Id) should also meet the minimum shunt resistor value as obtained from the photodetector datasheet value (see Figure 8 ).

[0216]

[0217] In one embodiment, this can be achieved by applying Vbias on a high gain scale (reading A0 偏置 The photodetector saturation current Is is derived from the value of

[0218]

[0219] where τ is the division between Is provided by the photodetector manufacturer and Ibias corresponding to the Vbias value (see e.g. Figure 9 , which shows an example of dark current varying with reverse voltage (taken from a Hamamatsu datasheet).

[0220] The voltage drop Vd across the photodiode can be calculated using Equation 6:

[0221]

[0222] from Figure 2 The circuit derives the input voltage at the transimpedance amplifier:

[0223] Vio=Vd+ΔVs=Vd+Vs2-Vs1

[0224] =Vd+S*(Tgnd1-Tpd1-Tgnd0+Tpd0)(31)

[0225] Where Vs2 and Vs1 are the Seebeck induced thermovoltages in response to the temperature difference across the photodetector terminal material, and S is the Seebeck absolute coefficient (tables usually provide the material coefficient relative to platinum, from which the Seebeck absolute coefficient can be derived). The Seebeck voltage is a result of the photodetector assembly procedure (soldering, connectors, mounting, etc.). For an ideal assembly, the temperature drop across the photodetector terminals is minimal and equal for both terminals allowing the theoretical assumption ΔVs=0. Equation 31 represents the worst case, where Vs1=0 and Vs2 is the maximum. For gold, silver, and copper, the Seebeck coefficient is 6.5 μV / K relative to platinum. The S of platinum itself is approximately -5 μV / K at room temperature. At room temperature we can apply S=1.5 μV / K for calculations (after a symbolic convention to indicate that the higher temperature end has a lower voltage).

[0226] The last term in Equation 31 can provide the temperature calibration between the two temperature sensors being compared.

[0227] For verification purposes, Vio should be within the valid value range for the given temperature, which can be derived from the transimpedance amplifier datasheet. If Vio is found to be inconsistent with the transimpedance amplifier datasheet (not within the valid value range), the following alternative method can be used.

[0228] Calibration method example 2:

[0229] In actual implementations, either of the methods in Examples 1 and 2 can be used to derive numerically represented parameters. In some embodiments, they can also be combined, such that under certain conditions, the calculation in Example 1 is performed, while under certain other conditions, the calculation in Example 2 is performed. For example, if the derived absolute value of the photodetector dark current Id is greater than the photodetector reverse bias saturation current Is (|Id|>Is), the calculation in Example 2 can be performed. Otherwise, the calculation in Example 1 is performed.

[0230] In the following calculations, typical values ​​are used for Vio and ΔVs, as can be derived from the transimpedance amplifier datasheet and the photodetector datasheet, respectively.

[0231] The voltage drop Vd across the photodiode can then be derived as:

[0232] Vd=max|Vio|+ΔVs (32)

[0233] The photodiode dark current Id can be derived from Equation 6:

[0234]

[0235] and

[0236]

[0237] This value of Id0 may be saved as one of the numerical representation parameters.

[0238] The values ​​of Vo(Tgnd0) and δVo can be derived from equations 25a, 25b, 26a and 26b above and stored as numerically represented parameters.

[0239] The value of Ib(Tgnd0) can be derived from Equation 23 above and saved as a numerical representation parameter.

[0240] Alternatively, a linear heat law can be used for Ib:

[0241] Ib(Tgnd)=Ib(Tgnd0)+δIb*(Tgnd-Tgnd0) (35)

[0242] and

[0243]

[0244] If used, the value of δIb can also be saved as a numerical representation parameter.

[0245] In some embodiments, a reverse bias voltage, Vbias, can be optionally applied to the photodetector at the lowest gain setting (low bias gain setting). In this case, the photodetector dark current, Id0bias, will be different from the dark current without the reverse bias voltage (as calculated above for low and high scales). However, the slope, m, can be assumed to be the same.

[0246] In an embodiment employing a reverse bias voltage, the value of Id0bias can be derived as follows.

[0247] By applying Equation 2 to C0 and C1, we obtain:

[0248]

[0249]

[0250] By subtracting equations 37 and 38 and assuming Ib(Tgnd0)=Ib(Tgnd1):

[0251]

[0252] When calculating the optical power offset for low bias gain settings i and optical power value net i This value can be used in place of Id0 in Equation 2 or 2a when . The other numerical values ​​indicate that the parameters (i.e., Is, Vo, and δVo) apply to all amplifier gain settings, including the offset setting.

[0253] Likewise, if the photodetector reverse bias saturation current Is is being derived at the bias voltage, this value for the photodetector dark current Id0bias can be used:

[0254]

[0255] Technology Advantages: Improved linearity specifications, improved no-zero temperature range, reduced warm-up time, improved thermal stability, possible automatic offset nulling and / or factory zeroing.

[0256] Manufacturing Advantages: It can be used to perform complete characterization of each photodetector, transimpedance amplifier, and assembled optical power meter; a statistical database can be built from it and part quality information can be updated promptly; reduced warm-up time and / or automatic offset zeroing reduce manufacturing time, thereby reducing manufacturing costs.

[0257] User benefits: Shorter measurement times and improved measurement uncertainty over the wide temperature range in which the optical power meter can operate.

[0258] Science (Lab): Improving measurement uncertainty.

[0259] In some embodiments, the proposed calibration procedure can be implemented in post-acquisition processing so that it does not interfere with the continuous acquisition sampling, which makes it suitable for high-speed or protocol detection power meters.

[0260] The above-described embodiments are intended to be exemplary only. Accordingly, the scope of the present invention is intended to be limited only by the following claims.

Claims

1. A method for measuring optical power, comprising: At the temperature measurement point, reading a raw optical power value from an optical power meter (11) including a photodiode as a photodetector (12) and an amplifying circuit (14) having a transimpedance amplifier (TZ); Using a first temperature sensor (22), a measured photodetector temperature value (T pd );as well as Using a second temperature sensor (24), a measured ground plane temperature value (T gnd ); The measured optical power offset value is determined by applying a numerical model that takes into account the thermal laws of the photodiode, the transimpedance amplifier (TZ) and the entire amplification circuit (14), wherein by applying the numerical model, the measured photodetector temperature value (T pd ) and the measured ground plane temperature value (T gnd ), determining the measured optical power offset value; An optical power measurement value is derived from the raw optical power value and the determined measured optical power offset value.

2. The optical power measurement method according to claim 1, further comprising the following steps: At the first temperature point: Read a first photodetector temperature value (T pd0 );as well as Read a first ground plane temperature value (T gnd0 ); For the first amplification gain setting and for the second amplification gain setting: Read the optical power offset value (A0, B0); At a second temperature point different from the first temperature point: Read a second photodetector temperature value (T pd1 );as well as Read a second ground plane temperature value (T gnd1 ); For the first amplification gain setting and for the second amplification gain setting: Read the optical power offset value (A1, B1); From the optical power offset value (A0, B0; A1, B1), the photodetector temperature value (T pd0 , T pd1 ) and the ground plane temperature (T gnd0 , T gnd1 ) derive the numerical model.

3. The optical power measurement method according to claim 1 or 2, wherein the numerical model comprises: The input bias current of the transimpedance amplifier (TZ), the dark current of the photodetector at a reference temperature, and the amplifier circuit offset as a function of the ground plane temperature. The optical power measurement method according to claim 2 , wherein the measurement temperature point is different from both the first temperature point and the second temperature point. 5 . The optical power measurement method according to claim 2 , wherein the raw optical power value is read using a measurement amplification gain setting that is different from both the first amplification gain setting and the second amplification gain setting.

6. A method for characterizing an optical power offset of an optical power meter (11), the method comprising: At the first temperature point: Using a first temperature sensor (22), a first photodetector temperature value (T pd0 ); as well as Using a second temperature sensor (24), a first ground plane temperature value (T gnd0 ); For the first amplification gain setting and for the second amplification gain setting: Read the optical power offset value (A0, B0); At a second temperature point different from the first temperature point: Using the first temperature sensor (22), a second photodetector temperature value (T pd1 );as well as Using the second temperature sensor (24), a second ground plane temperature value (T gnd1 ); For the first amplification gain setting and for the second amplification gain setting: Read the optical power offset value (A1, B1); The photodetector temperature value (T pd0 , T pd1 ), ground plane temperature (T gnd0 , T gnd1 ) and the optical power offset values ​​(A0, B0; A1, B1) to derive a numerical model of the optical power meter (11), the numerical model taking into account the thermal laws of the photodiode, the transimpedance amplifier (TZ) and the entire amplification circuit (14); At the temperature measurement point, Read the measured photodetector temperature value (T pd );as well as Read the measured ground plane temperature value (T gnd ); According to the derived numerical model, the measured photodetector temperature value (T pd ) and the measured ground plane temperature value (T gnd ), determine the measured optical power offset value.

7. The method according to claim 6, further comprising: At the measuring temperature point, Read the original optical power value; An optical power measurement value is derived from the raw optical power value and the determined measured optical power offset value.

8. The method according to claim 6 or 7, wherein the derived numerical model comprises: The input bias current of the transimpedance amplifier (TZ), the dark current of the photodetector at a reference temperature, and the amplifier circuit offset as a function of the ground plane temperature. 9 . The method of claim 7 , wherein the measured temperature point is different from both the first temperature point and the second temperature point.

10. The method of claim 7, wherein the raw optical power value is read using a measured amplification gain setting that is different from both the first amplification gain setting and the second amplification gain setting.

11. An optical power meter (11), comprising: a photodiode as a photodetector (12), an amplification circuit (14) having a transimpedance amplifier (TZ), and an analog-to-digital converter (16) for reading the raw optical power value; Used to measure the temperature value of the photodetector (T pd ) a first temperature sensor (22) associated with the photodetector (12); Used to measure the ground plane temperature (T gnd ) a second temperature sensor (24) associated with a ground plane of the amplifying circuit (14); and A processing unit (18), the processing unit being configured to: The measured optical power offset value is determined by applying a numerical model that takes into account the thermal laws of the photodiode, the transimpedance amplifier (TZ) and the entire amplification circuit (14), wherein by applying the numerical model, the photodetector temperature value (T pd ) and the ground plane temperature value (T gnd ), determining the measured optical power offset value; as well as An optical power measurement value is derived from the raw optical power value and the determined measured optical power offset value.

12. The optical power meter according to claim 11, wherein the processing unit is further configured to: take over: A first photodetector temperature value (T pd0 ); a first ground plane temperature value (T ) associated with the ground plane of the amplifying circuit of the optical power meter and read at the first temperature point; gnd0 ); optical power offset values ​​(A0, B0) read for the first amplification gain setting and for the second amplification gain setting at the first temperature point; a second photodetector temperature value (T ) associated with the photodetector of the optical power meter and read at a second temperature point; pd1 ); a second ground plane temperature value (T ) associated with the ground plane of the amplifying circuit of the optical power meter and read at the second temperature point; gnd1 ); as well as optical power offset values ​​(A1, B1) read for the first amplification gain setting and for the second amplification gain setting at the second temperature point; and From the photodetector temperature value (T pd0 , T pd1 ), ground plane temperature (T gnd0 , T gnd1 ) and optical power offset values ​​(A0, B0; A1, B1) to derive the numerical model.

13. The optical power meter according to claim 11, wherein the numerical model comprises: The input bias current of the transimpedance amplifier (TZ), the dark current of the photodetector at a reference temperature, and the amplifier circuit offset as a function of the ground plane temperature.

14. The optical power meter according to claim 12, wherein the numerical model comprises: The input bias current of the transimpedance amplifier (TZ), the dark current of the photodetector at a reference temperature, and the amplifier circuit offset as a function of the ground plane temperature. 15 . The optical power meter according to claim 14 , wherein a measurement temperature point at which the measured optical power offset value is determined is different from both the first temperature point and the second temperature point. 16 . The optical power meter of claim 12 , wherein the raw optical power value is read using a measurement amplification gain setting that is different from both the first amplification gain setting and the second amplification gain setting.

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