Optical spectrum analyzer

Abstract

To provide an optical spectrum analyzer capable of improving a sweep speed without lowering measurement accuracy of a spectrum of light to be measured.SOLUTION: A control unit 12 sets a measurement interval when VBW of the lowest 1 kHz band is set to be 400 μs, and sets a measurement interval when another VBW is set to be 200 μs. During the period in which the VBW of 1 kHz band is set, based on a light power signal sampled by an A / D conversion unit 18, the control unit 12 interpolates the optical power signal in the wavelength between the wavelength sampled before and the wavelength sampled next.SELECTED DRAWING: Figure 4

Description

[Technical field]

[0001] The present invention relates to an optical spectrum analyzer. [Background technology]

[0002] Conventionally, in order to measure the spectrum of the light to be measured (the optical power for each wavelength contained in the light to be measured), an optical spectrum analyzer such as that shown in Patent Document 1 has been used. The optical spectrum analyzer in Patent Document 1 inputs the light to be measured into a diffraction grating, emits diffracted light obtained by dispersing and spectroscopically dispersing the light to be measured, and rotates the diffraction grating to cause a light receiving section to receive the diffracted light whose wavelength has been swept.

[0003] However, in conventional optical spectrum analyzers, there is a trade-off between spectrum measurement accuracy and sweep speed; increasing the measurement accuracy requires slowing down the sweep speed, whereas increasing the sweep speed results in poor measurement accuracy. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent No. 3274035 Summary of the Invention [Problem to be solved by the invention]

[0005] The present invention has been made in view of the above-mentioned circumstances, and an object of the present invention is to provide an optical spectrum analyzer that can increase the sweep speed without degrading the measurement accuracy of the spectrum of the light to be measured. [Means for solving the problem]

[0006] In order to achieve the above-mentioned object, an optical spectrum analyzer according to the present invention has the following features [1] and [2]. [1] a spectroscopic unit (4) that outputs diffracted light obtained by splitting wavelengths contained in the light to be measured into directions corresponding to the wavelengths; a light receiving unit (8) that receives the diffracted light emitted from the spectroscopic unit (4), converts it into an electrical signal, and outputs it as an optical power signal; a sweeping unit (9) for causing the light receiving unit (8) to receive the wavelength-swept diffracted light; a low-pass filter (16) for removing high-frequency noise from the optical power signal; an A / D conversion unit (18) that performs A / D conversion on the optical power signal output from the low-pass filter (16); a passband setting unit (12) that sets a passband of the low-pass filter (16) in accordance with a signal level of the optical power signal; a control unit (12) that determines optical power for each wavelength contained in the measured light based on the optical power signal sampled and converted into a digital value by the A / D conversion unit (18) at each measurement interval during wavelength sweeping, a measurement interval setting unit (12) that sets the measurement interval longer when the lowest passband is set than when other passbands are set; and an interpolation unit (12) that, based on the optical power signal sampled and converted into a digital value by the A / D conversion unit (18), interpolates the optical power signal at a wavelength between a previously sampled wavelength and a next sampled wavelength during a period in which the measurement interval is set long. An optical spectrum analyzer (1). [2] In the optical spectrum analyzer (1) according to [1], An operation unit (15) is provided for selecting whether or not to perform interpolation by the interpolation unit. An optical spectrum analyzer (1).

[0007] According to the optical spectrum analyzer having the configuration described above in [1], when the lowest passband is set, the measurement interval is lengthened and the electrical signal is interpolated, so that the sweep speed can be increased without degrading the measurement accuracy of the spectrum of the measured light. According to the optical spectrum analyzer having the configuration described above in [2], the user can select whether or not to perform interpolation. Effect of the Invention

[0008] According to the present invention, it is possible to provide an optical spectrum analyzer capable of increasing the sweep speed without degrading the measurement accuracy of the spectrum of the light under measurement.

[0009] The present invention has been briefly described above. Furthermore, the details of the present invention will be further clarified by reading the following description of the embodiment of the present invention (hereinafter, referred to as "embodiment") with reference to the accompanying drawings. [Brief description of the drawings]

[0010] [Figure 1] FIG. 1 is a configuration diagram showing an embodiment of an optical spectrum analyzer according to the present invention. [Diagram 2] FIG. 2 is a circuit diagram showing details of the measurement unit shown in FIG. [Diagram 3] FIG. 3 is an example of a graph displayed on the display unit shown in FIG. [Figure 4] FIG. 4 is a time chart for explaining the sampling of the optical power signal of the optical spectrum analyzer shown in FIG. [Diagram 5] FIG. 4 is a flowchart illustrating a processing procedure of the control unit shown in FIG. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] Specific embodiments of the present invention will be described below with reference to the accompanying drawings.

[0012] As shown in FIG. 1, an optical spectrum analyzer 1 is a device that measures and displays the optical power for each wavelength contained in pulsed light as light to be measured.

[0013] The optical spectrum analyzer 1 includes an incident section 2, a collimator 3, a diffraction grating 4 (=spectroscope section), a condenser 5, a slit 6, a condenser 7, a light receiving section 8, and a motor 9 (=sweep section). The incident section 2 is composed of, for example, an optical fiber, and emits pulsed light. The pulsed light is incident on the collimator 3. The collimator 3 converts the pulsed light from the incident section 2 into parallel light. The parallel light emitted from the collimator 3 is incident on the diffraction grating 4.

[0014] The diffraction grating 4 disperses and spectroscopically separates the parallel light emitted from the collimator 3. In this embodiment, the diffraction grating 4 is provided so as to be rotatable around a rotation axis 10 perpendicular to the paper surface by a motor 9. A plurality of fine grooves (not shown) are formed on the surface of the diffraction grating 4 along the direction of the rotation axis, which are aligned in a direction perpendicular to the rotation axis 10. When the parallel light from the collimator 3 is incident on the diffraction grating 4, the diffraction grating 4 outputs diffracted light that has been dispersed and spectroscopically separated. The diffracted light is light that has been obtained by dispersing the wavelengths contained in the pulsed light by the diffraction phenomenon into directions according to the wavelengths. The diffracted light is incident on the condenser 5. The motor 9 rotates the diffraction grating 4, and causes the light receiving unit 8 to receive the diffracted light that has been wavelength-swept. That is, the diffracted light whose wavelength changes continuously as the diffraction grating 4 rotates is received by the light receiving unit 8.

[0015] The condenser 5 condenses the diffracted light at a position where a slit 6, which will be described later, is disposed. The diffracted light that has passed through the slit 6 is incident on the condenser 7. The condenser 7 condenses the incident diffracted light at a position where a light receiving unit 8 is disposed. The light receiving unit 8 is composed of a photodiode, and a current corresponding to the optical power of the incident diffracted light flows through it.

[0016] The optical spectrum analyzer 1 further includes a measurement unit 11, a control unit 12, a memory 13, a display unit 14, and an operation unit 15. As shown in Fig. 2, the measurement unit 11 includes a current-voltage conversion unit 16, an amplifier unit 17, and an A / D conversion unit 18. The current-voltage conversion unit 16 converts the current flowing through the light receiving unit 8 into a voltage and outputs it as an optical power signal (=electrical signal). The current-voltage conversion unit 16 also functions as a low-pass filter that removes high-frequency noise from the optical power signal. The current-voltage conversion unit 16 is provided with a changeable pass bandwidth (VBW: Video Band Width) and gain of the low-pass filter, and the VBW and gain are controlled by the control unit 12 described later.

[0017] Next, a detailed configuration of the current-voltage conversion unit 16 will be described. The current-voltage conversion unit 16 has an amplifier Amp1, a plurality of elements 161-16n (n is an integer) connected between the input and output of the amplifier Amp1, and switches SW1-SWn for turning on and off the connections between the plurality of elements 161-16n and the amplifier Amp1. The elements 161-16n are composed of a capacitor and a resistor. The VBW and gain of the current-voltage conversion unit 16 can be changed by controlling the on / off of the switches SW1-SWn to switch the elements 161-16n connected to the amplifier Amp1.

[0018] The amplifier 17 amplifies the optical power signal output from the current-voltage converter 16. In this embodiment, the amplifier 17 has a plurality of amplifiers Amp21 to Amp2m (m is an integer) each having a different gain. The output of each of the plurality of amplifiers Amp21 to Amp2m is connected to the A / D converter 18. The analog optical power signal from which high frequency noise has been removed by the current-voltage converter 16 and which has been amplified by the amplifier 17 is input to the A / D converter 18.

[0019] The A / D conversion unit 18 includes a plurality of A / D converters, each having an input to the corresponding A / D converter. The outputs of the plurality of amplifiers Amp21 to Amp2m are connected to the inputs of the plurality of A / D converters. The A / D conversion unit 18 samples and A / D converts the analog optical power signals input to the inputs of the plurality of A / D converters, and outputs digital values ​​of the signal levels of the optical power signals to the control unit 12.

[0020] The control unit 12 is composed of, for example, an FPGA (Field Programmable Gate Array) or a DSP (Digital Signal Processor), and controls the entire optical spectrum analyzer 1. The control unit 12 rotates the motor 9 at a uniform speed, and causes the light receiving unit 8 to receive the wavelength-swept diffracted light. The control unit 12 acquires an optical power signal for each discrete wavelength. Then, the control unit 12 connects together the optical power signals acquired for each discrete wavelength, and causes the display unit 14 to display the signal level (dB) of the optical power signal for each continuous wavelength as shown in FIG. 3. The memory 13 stores digital values ​​of the optical power signal, etc.

[0021] The control unit 12 is composed of, for example, an FPGA (Field Programmable Gate Array), and controls the entire optical spectrum analyzer 1. The control unit 12 rotates the motor 9 at a constant speed, and causes the wavelength-swept diffracted light to be received by the light receiving unit 8. The control unit 12 acquires an optical power signal for each discrete wavelength. Then, the control unit 12 connects together the optical power signals acquired for each discrete wavelength, and causes the display unit 14 to display the optical power (dB) for each continuous wavelength as shown in FIG. 3. The memory 13 stores digital values ​​of the optical power signals, etc.

[0022] The operation unit 15 allows the user to input a minimum value of the VBW, which will be described later.

[0023] Next, an example of VBW setting will be described with reference to FIG. 3. Before that, the relationship between the signal level of the optical power signal and the VBW will be described. The lower the signal level of the optical power signal, the smaller the signal-to-noise ratio (SN ratio) becomes, and the more susceptible it becomes to noise. On the other hand, the lower (narrower) the VBW band, the wider the band in which high-frequency noise can be cut, and therefore the effect of high-frequency noise can be suppressed. However, when the optical power signal is a pulse signal, the lower the band of the VBW, the longer the rise time and fall time of the pulse output from the current-voltage conversion unit 16. For this reason, if the VBW is lowered when the signal level of the optical power signal is high, it takes time for the output of the current-voltage conversion unit 16 to reach a high signal level, and as a result, the sweep time until the peak of the signal is captured becomes longer. Conversely, if the VBW is increased when the signal level of the optical power signal is low, the sweep time becomes shorter, but the effect of high-frequency noise cannot be suppressed.

[0024] Therefore, the control unit 12 sets a low VBW according to the low signal level of the optical power signal. This makes it possible to suppress the influence of high frequency noise while preventing the sweep time from becoming long. When the VBW is set, the gain is also set. In this embodiment, the user can set the minimum value of the VBW by operating the operation unit 15. When the signal level of the optical power signal is in the lowest range, the control unit 12 sets the VBW to the minimum value of the VBW set by the user.

[0025] In the example shown in FIG. 3, the minimum value of the VBW is set to a 1 kHz band (the passband is 1 kHz or less). The minimum 1 kHz band is set when the signal level of the optical power signal is in the lowest range of -70 dB to -50 dB. The 2 kHz band is set when the signal level of the optical power signal is -50 dB to -35 dB. The 3 kHz band is set when the signal level of the optical power signal is -35 dB to -20 dB. The 4 kHz band is set when the signal level of the optical power signal is -20 dB to 0 dB.

[0026] Next, the measurement interval of the optical spectrum analyzer 1 will be described. The control unit 12 controls the A / D conversion unit 18 to sample the optical power signal at each measurement interval during wavelength sweeping. As described above, the lower the VBW, the longer the rise time and fall time of the optical power signal. Therefore, in order to measure the spectrum with high accuracy, the lower the VBW, the longer the measurement interval needs to be. For example, when the VBW is in the 1 kHz band, the measurement interval needs to be 400 μs or more, and when the VBW is in the 2 kHz band, the measurement interval needs to be 200 μs or more.

[0027] In conventional optical spectrum analyzers, the measurement interval is set to 400 μs to match the 1 kHz band, which is the minimum VBW value. Therefore, if it is necessary to sample optical power signals at M wavelengths (M is an integer) between the measurement start point and the measurement end point, the sweep time from the measurement start point to the measurement end point is 400 μs × M.

[0028] In contrast, the optical spectrum analyzer 1 of this embodiment is set to a measurement interval of 200 μs in accordance with the VBW=2 kHz band. Therefore, the sweep time from the measurement start point to the measurement end point can be 200 μs×M, half the time of the conventional spectrum analyzer. However, if the measurement interval is set to 200 μs during the period when the VBW is set to 1 kHz, it is not possible to measure the spectrum with high accuracy.

[0029] Therefore, as shown in Fig. 4, during the period when the VBW is set to 1 kHz, the optical power signal sampling is skipped and the measurement interval is set to 400 μs. However, this does not provide enough wavelengths to sample. Therefore, based on the optical power signal sampled by the A / D conversion unit 18, during the period when the measurement interval is set to 400 μs, the electrical power signal at the wavelength between the previously sampled wavelength and the next sampled wavelength is interpolated. For the interpolation, a well-known interpolation method such as the least squares method can be used.

[0030] As a result, even if the VBW is set to the 1 kHz band and the measurement interval is set to 400 μs, it is possible to obtain optical power signals with the same number of samples as when the measurement interval is set to 200 μs, enabling accurate spectrum measurement.

[0031] Next, the operation of the optical spectrum analyzer 1 described above will be described below with reference to the flowchart in FIG. 5. Here, for simplicity, a case will be described where the minimum value of the VBW is set to the 1 kHz band. The control unit 12 starts sweeping in response to the operation of the operation unit 15. At this time, it is assumed that the diffraction grating 4 is at the measurement start point. The control unit 12 controls the motor 9 to rotate the diffraction grating 4 at a constant speed toward the measurement end point (S1). The rotation speed of the motor 9 is set so that it takes a sweep time of 200 μs×M to reach the measurement end point from the measurement start point.

[0032] Next, the control unit 12 judges whether the diffraction grating 4 has reached the measurement end point from the output of an encoder (not shown) attached to the motor 9 (S2). If the diffraction grating 4 has not reached the measurement end point (N in S2), the control unit 12 controls the on / off of the switches SW1 to SWn to set the VBW of the current-voltage conversion unit 16 according to the signal level of the optical power signal (S3). The signal level of the optical power signal is judged from the optical power signal A / D converted by the A / D conversion unit 18.

[0033] Next, if the VBW set in S3 is in the 1 kHz band (Y in S4), the control unit 12 sets the measurement interval to 400 μs (S5). On the other hand, if the VBW set in S3 is not in the 1 kHz band (N in S4), the control unit 12 sets the measurement interval to 200 μs (S6).

[0034] Next, the control unit 12 reads out the digital values ​​obtained by A / D converting the optical power signals amplified by all of the amplifiers Amp21 to Amp2m, and performs an amplifier selection process to select one of the amplifiers Amp21 to Amp2m whose output is not saturated based on the read out digital values ​​(S7). Next, the control unit 12 determines whether the measurement interval set in S5 or S6 has elapsed and the next wavelength has been reached (S8). If the next wavelength has been reached (Y in S8), the control unit 12 performs an A / D read process to have the A / D conversion unit 18 sample the optical power signals, read out the digital values ​​of the A / D converted optical power signals, and store them in the memory 13 (S9).

[0035] Next, the control unit 12 sets the wavelength corresponding to the output of the encoder as the next wavelength reached in S8, and stores the digital value of the optical power signal read out in S9 in the memory 13 as the optical power signal of the next wavelength (S10), before returning to S2.

[0036] Returning to S2, when the diffraction grating 4 reaches the measurement end point (Y at S2), the optical power signals for each discrete wavelength between the measurement start point and the measurement end point are stored in the memory 13. That is, the optical power signals sampled at the times indicated by the black circles in FIG. 4 are stored in the memory 13. Next, the control unit 12 interpolates the electrical power signals at the wavelengths between the previously sampled wavelength and the next sampled wavelength during the period in which the measurement interval is set to 400 μs, based on the sampled optical power signals (S11). That is, the optical power signals indicated by the white circles in FIG. 4 are interpolated and stored in the memory 13.

[0037] Next, the control unit 12 connects together the optical power signals sampled or interpolated for each discrete wavelength to generate a display waveform showing the optical power for each continuous wavelength as shown in FIG. 3, and displays it on the display unit 14 (S12, S13), thereby completing the process.

[0038] According to the above-described embodiment, when the lowest VBW is set, the measurement interval is lengthened and the electrical signal is interpolated, so that the sweep speed can be increased without degrading the measurement accuracy of the spectrum of the light under measurement.

[0039] The present invention is not limited to the above-described embodiment, and can be appropriately modified, improved, etc. In addition, the material, shape, size, number, arrangement location, etc. of each component in the above-described embodiment are arbitrary as long as the present invention can be achieved, and are not limited.

[0040] In the above embodiment, the control unit 12 performs the interpolation process after the sweep is completed, but this is not limited to the above, and the interpolation process may be performed during the sweep.

[0041] In the embodiment described above, the control unit 12 always performs the interpolation process, but this is not limited to this. The user may be able to select whether or not to perform the interpolation process by operating the operation unit 15.

[0042] In the above-described embodiment, the light to be measured is pulsed light, but the present invention is not limited to this. The light to be measured may be continuous light that is continuously turned on.

[0043] In the above-described embodiment, the A / D conversion unit 18 has a plurality of inputs to which the outputs of the plurality of amplifiers Amp21 to Amp2m are connected, but this is not limited thereto. For example, the input of the A / D conversion unit 18 may be unified, and the amplifiers Amp21 to Amp2m connected to the input of the A / D conversion unit 18 may be switched by a switch. Also, the amplifier may be unified, and the gain of the single amplifier may be switched by switching a resistor connected to the amplifier.

[0044] Here, the features of the above-described embodiments of the optical spectrum analyzer according to the present invention will be briefly summarized and listed in the following [1] and [2]. [1] a spectroscopic unit (4) that outputs diffracted light obtained by splitting wavelengths contained in the light to be measured into directions corresponding to the wavelengths; a light receiving unit (8) that receives the diffracted light emitted from the spectroscopic unit (4), converts it into an electrical signal, and outputs it as an optical power signal; a sweeping unit (9) for causing the light receiving unit (8) to receive the wavelength-swept diffracted light; a low-pass filter (16) for removing high-frequency noise from the optical power signal; an A / D conversion unit (18) that performs A / D conversion on the optical power signal output from the low-pass filter (16); a passband setting unit (12) that sets a passband of the low-pass filter (16) in accordance with a signal level of the optical power signal; a control unit (12) that determines optical power for each wavelength contained in the measured light based on the optical power signal sampled and converted into a digital value by the A / D conversion unit (18) at each measurement interval during wavelength sweeping, a measurement interval setting unit (12) that sets the measurement interval longer when the lowest passband is set than when other passbands are set; and an interpolation unit (12) that, based on the optical power signal sampled and converted into a digital value by the A / D conversion unit (18), interpolates the optical power signal at a wavelength between a previously sampled wavelength and a next sampled wavelength during a period in which the measurement interval is set long. Optical spectrum analyzer (1). [2] In the optical spectrum analyzer (1) according to [1], An operation unit (15) is provided for selecting whether or not to perform interpolation by the interpolation unit. Optical spectrum analyzer (1). [Explanation of symbols]

[0045] 1 Optical spectrum analyzer 4 Diffraction grating (spectral section) 8 Light receiving section 9 Motor 12 Control section (passband setting section, measurement interval setting section, interpolation section) 15 Control section 16 Current-to-voltage conversion section (low-pass filter) 18 A / D conversion section

Claims

1. A spectroscopic unit (4) that outputs diffracted light obtained by splitting wavelengths contained in the light to be measured into directions corresponding to the wavelengths; a light receiving unit (8) that receives the diffracted light emitted from the spectroscopic unit (4), converts it into an electrical signal, and outputs it as an optical power signal; a sweeping unit (9) for causing the light receiving unit (8) to receive the wavelength-swept diffracted light; a low-pass filter (16) for removing high-frequency noise in the optical power signal; an A / D converter (18) that performs A / D conversion on the optical power signal output from the low-pass filter (16); a passband setting unit (12) that sets a passband of the low-pass filter (16) in accordance with a signal level of the optical power signal; a control unit (12) that determines optical power for each wavelength contained in the measured light based on the optical power signal sampled and converted into a digital value by the A / D conversion unit (18) at each measurement interval during wavelength sweeping, a measurement interval setting unit (12) that sets the measurement interval longer when the lowest passband is set than when other passbands are set; and an interpolation unit (12) that, based on the optical power signal sampled and converted into a digital value by the A / D conversion unit (18), interpolates the optical power signal at a wavelength between a previously sampled wavelength and a next sampled wavelength during a period in which the measurement interval is set long. Optical Spectrum Analyzer (1).

2. An optical spectrum analyzer (1) according to claim 1, An operation unit (15) is provided for selecting whether or not to perform the interpolation by the interpolation unit. Optical Spectrum Analyzer (1).

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