Optical Spectrum Analyzer

Abstract

To provide an optical spectrum analyzer that can stably measure spectrum of pulsed light while suppressing a decrease in sweep rate.SOLUTION: An A / D conversion unit 17 includes a plurality of inputs to which outputs of a plurality of amplifiers Amp21-Amp2m are connected. A control unit 12 selects one of the amplifiers Amp21-Amp2m whose output is not saturated based on an optical power signal amplified by the plurality of amplifiers Amp21-Amp2m. The control unit 12 takes an average value of the optical power signal sampled by the A / D conversion unit 17 and amplified by the selected amplifier Amp21-Amp2m while being swept from previous wavelength to next wavelength as the optical power signal for the next wavelength.SELECTED DRAWING: Figure 2

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] In recent years, there has been a demand for measurement using pulsed light as the light to be measured in order to suppress heat generation from the light source. Therefore, an optical spectrum analyzer has been proposed that performs A / D conversion of the optical spectrum signal (electrical signal) converted by the light receiving unit at the timing when the pulsed light is emitted (Patent Document 2).

[0004] However, with conventional optical spectrum analyzers, it was necessary to synchronize the period of the pulsed light with the sampling in the A / D conversion section, which made the measurement system complicated. In addition, the sweep speed was limited by the frequency of the pulsed light, which slowed down the sweep itself. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent No. 3274035 [Patent Document 2] JP 2022-152858 A Summary of the Invention [Problem to be solved by the invention]

[0006] The present invention has been made in consideration of the above-mentioned circumstances, and an object of the present invention is to provide an optical spectrum analyzer that can stably measure the spectrum of pulsed light while suppressing a decrease in sweep speed. [Means for solving the problem]

[0007] In order to achieve the above object, the optical spectrum analyzer according to the present invention comprises the following components: ] It is characterized by: [1] a spectroscopic unit (4) that outputs diffracted light obtained by dispersing wavelengths contained in the pulsed light to be measured in directions corresponding to the wavelengths; a light receiving unit (8) that receives the diffracted light emitted from the spectroscopic unit (4) and converts it into an electrical signal; a sweeping unit (9) for causing the light receiving unit (8) to receive the wavelength-swept diffracted light; an amplifier (16) that amplifies the electrical signal converted by the light receiving unit (8) and outputs it as an optical power signal; an A / D conversion unit (17) that performs A / D conversion on the optical power signal output from the amplifier unit (16); a control unit (12) that determines an optical power for each wavelength included in the pulsed light based on the optical power signal sampled and converted into a digital value by the A / D conversion unit (17) during wavelength sweeping, The amplifying section (16) has a plurality of amplifiers (Amp21 to Amp2m) having mutually different gains, The A / D conversion unit (17) has a plurality of A / D converters to which the outputs of the plurality of amplifiers (Amp21 to Amp2m) are respectively connected, The control unit (12) an amplifier selection unit that selects one of the amplifiers (Amp21 to Amp2m) whose output is not saturated, based on the optical power signal that is amplified by the plurality of amplifiers (Amp21 to Amp2m) and converted into a digital value by the plurality of A / D converters, The average value or peak value of the digital values ​​of the optical power signals sampled by the A / D conversion unit (17) and output by the amplifiers (Amp21 to Amp2m) selected by the amplifier selection unit while the wavelength is being swept from the previous wavelength to the next wavelength is calculated as the optical power of the previous wavelength or the optical power of the next wavelength. year, the A / D conversion unit (17) samples the optical power signal at a sampling time shorter than a period of the pulsed light. An optical spectrum analyzer (1).

[0008] According to the optical spectrum analyzer having the configuration described in [1] above, the optical power signal sampled while the wavelength is being swept from the previous wavelength to the next wavelength is highly likely to include an optical power signal sampled at the peak timing. Moreover, since the period during which the amplifier is switched is eliminated, the period during which sampling is possible while the wavelength is being swept from the previous wavelength to the next wavelength can be extended, and the possibility of sampling at the peak timing of the optical power signal can be further increased. This makes it possible to stably measure the spectrum of the pulsed light while suppressing a decrease in the sweep speed. According to the optical spectrum analyzer having the configuration described above in [2], it is possible to further increase the possibility of sampling at the timing of the peak of the optical power signal. Effect of the Invention

[0009] According to the present invention, it is possible to provide an optical spectrum analyzer that can stably measure the spectrum of pulsed light while suppressing a decrease in sweep speed.

[0010] 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]

[0011] [Figure 1]FIG. 1 is a diagram showing the configuration of 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 flowchart illustrating a processing procedure of the control unit shown in FIG. [Diagram 5] FIG. 5 is a time chart of the optical power signal output from the current-voltage converter of FIG. [Figure 6] FIG. 6 is a circuit diagram showing an example of a measurement unit of an optical spectrum analyzer in a comparative example. [Figure 7] FIG. 7 is a time chart showing periods during which continuous sampling is possible and periods during which continuous sampling is not possible for the optical spectrum analyzer of the present embodiment shown in FIG. 1 and the spectrum analyzer of the comparative example shown in FIG. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] Specific embodiments of the present invention will be described below with reference to the accompanying drawings.

[0013] As shown in FIG. 1, an optical spectrum analyzer 1 is a device that measures and displays the optical power of each wavelength contained in pulsed light.

[0014] 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 to be measured. 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.

[0015] The diffraction grating 4 disperses and splits 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 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 split. The diffracted light is light that has wavelengths contained in pulsed light split into directions according to the wavelengths by a diffraction phenomenon. The diffracted light is incident on the condenser 5. The motor 9 rotates the diffraction grating 4, and the wavelength-swept diffracted light is received by the light receiving unit 8, which will be described later. That is, the diffracted light whose wavelength changes continuously as the diffraction grating 4 rotates is received by the light receiving unit 8.

[0016] 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.

[0017] The optical spectrum analyzer 1 further includes a measurement unit 11, a control unit 12, a memory 13, and a display unit 14. As shown in Fig. 2, the measurement unit 11 includes a current-voltage conversion unit 15, an amplifier unit 16, and an A / D conversion unit 17. The current-voltage conversion unit 15 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 15 also functions as a low-pass filter that removes high-frequency noise from the optical power signal. The current-voltage conversion unit 15 is provided such that the pass bandwidth (VBW: Video Band Width) and gain of the low-pass filter are changeable, and the VBW and gain are controlled by the control unit 12 described later.

[0018] Next, a detailed configuration of the current-voltage conversion unit 15 will be described. The current-voltage conversion unit 15 has an amplifier Amp1, a plurality of elements 151-15n (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 151-15n and the amplifier Amp1. The elements 151-15n are composed of capacitors or resistors. The VBW and gain of the current-voltage conversion unit 15 can be changed by controlling the on and off of the switches SW1-SWn to switch the elements 151-15n connected to the amplifier Amp1.

[0019] The amplifier 16 amplifies the optical power signal output from the current-voltage converter 15. In this embodiment, the amplifier 16 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 17. The analog optical power signal from which high frequency noise has been removed by the current-voltage converter 15 and which has been amplified by the amplifier 16 is input to the A / D converter 17.

[0020] The A / D conversion unit 17 includes a plurality of A / D converters, each having an input to the 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 17 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.

[0021] 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.

[0022] Next, the operation of the optical spectrum analyzer 1 described above will be described below with reference to the flowchart in Fig. 4. The control unit 12 starts a sweep in response to an operation of an operation unit (not shown). 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).

[0023] Next, the control unit 12 determines whether or not the diffraction grating 4 has reached the measurement end point based on 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 15 (S3).

[0024] An example of VBW setting executed by the control unit 12 in S3 will be described. 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 band of the VBW, 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 15. 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 15 to reach a high signal level, and as a result, the sweep time 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.

[0025] 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.

[0026] Next, the control unit 12 starts an A / D read process in which the digital values ​​of the optical power signal sampled by the A / D conversion unit 17 at a sampling period shorter than the period of the pulsed light are sequentially read out and stored in the memory 13 (S4). In S4, the control unit 12 reads out the digital values ​​of the optical power signal amplified by all of the amplifiers Amp21 to Amp2m and stores them in the memory 13. Thereafter, the control unit 12 performs an amplifier selection process to select one of the amplifiers Amp21 to Amp2m whose output is not saturated based on the digital values ​​of the optical power signals that have been read out (S5).

[0027] Details of this amplifier selection process will be described with reference to Fig. 5. Fig. 5 shows the optical power signal output from the current-voltage conversion unit 15. Saturation levels Lo (o is any integer from 1 to m) to Lo+2 in the figure indicate the input signal levels at which the output of each of the amplifiers Amp2o to Amp2o+2 is saturated. Since the optical power signal does not exceed the saturation level Lo of the amplifier Amp2o, the digital value of the optical power signal amplified by the amplifier Amp2o does not include a maximum value (=saturation value), and the peak of the optical power signal can be captured.

[0028] Because the optical power signal exceeds the saturation levels Lo+1 and Lo+2 of the amplifiers Amp2o+1 and Amp2o+2, the digital values ​​of the optical power signals amplified by the amplifiers Amp2o+1 and Amp2o+2 contain a maximum value, and the peak of the optical power signal cannot be captured. In the amplifier selection process, the control unit 12 selects one of the amplifiers Amp21 to Amp2m that does not contain a maximum value in the digital value.

[0029] If the outputs of all the amplifiers Amp21 to Amp2m are saturated and the amplifiers Amp21 to Amp2m cannot be selected (N in S6), the control unit 12 stops the A / D readout process, returns to S3, and changes the gain by changing the VBW of the current-voltage conversion unit 15. If the amplifiers Amp21 to Amp2m can be selected (Y in S6), the control unit 12 determines whether the measurement interval per point according to the minimum value of the VBW has elapsed and the next wavelength has been reached (S7). If the next wavelength has been reached (Y in S7), the control unit 12 stops the A / D readout process and sets the wavelength according to the output of the encoder at this time as the next wavelength reached in S7 (S8). Next, the control unit 12 performs a filter process (S9).

[0030] The period from when the A / D readout process is started in S4 to when the A / D readout process is stopped in S8 is included in the period during which the wavelength is swept from the previous wavelength to the next wavelength. In the filtering process, the control unit 12 calculates the average value of the digital values ​​of the optical power signals amplified by the amplifiers Amp21 to Amp2m selected in the amplifier selection process during this period (S9). The control unit 12 stores the average value calculated by the filtering process in the memory 13 as the optical power signal of the next wavelength set in S8 (S10), and then returns to S2.

[0031] Returning to S2, when the diffraction grating 4 reaches the measurement end point (Y in S2), the digital values ​​of the optical power signals for each discrete wavelength between the measurement start point and the measurement end point have been stored in the memory 13. The control unit 12 connects together these optical power signals for each discrete wavelength to generate a display waveform showing the optical power for each continuous wavelength as shown in Fig. 3, displays it on the display unit 14 (S11, S12), and ends the process.

[0032] Next, the effects of the optical spectrum analyzer 1 of the above-mentioned embodiment will be described. Conventional optical spectrum analyzers sample the optical power signal and read the digital value every time the next wavelength is reached while sweeping from the measurement start point to the measurement end point. For this reason, sampling of the optical power signal may be performed at the timing when the pulsed light is off, making it impossible to stably measure the spectrum of the pulsed light.

[0033] In contrast, the optical spectrum analyzer 1 of this embodiment uses the average value of the optical power signal sampled multiple times while moving from the previous wavelength to the next wavelength as the optical power signal of the next wavelength. The optical power signal sampled multiple times is likely to include an optical power signal sampled at the timing of the optical power signal peak. Therefore, even without a configuration in which the period of the pulsed light is synchronized with the sampling period of the A / D conversion unit, the peak of the optical power signal can be sampled, the spectrum of the pulsed light can be stably measured, and a decrease in the sweep speed can be suppressed.

[0034] Moreover, in the optical spectrum analyzer 1 of this embodiment, the A / D conversion unit 17 has multiple inputs, and the outputs of multiple amplifiers Amp21 to Amp2m can be input. The effects of this configuration will be described with reference to Figures 6 and 7. Figure 6 shows an example of a measurement unit 100 of an optical spectrum analyzer as a comparative example. In this figure, parts equivalent to those in Figure 2 are given the same reference numerals and detailed descriptions thereof will be omitted.

[0035] In the optical spectrum analyzer of the comparative example, the A / D conversion unit 101 has only one input, and only one output selected by the switches SW21-SW2m from the amplifiers Amp21-Amp2m can be A / D converted. Therefore, in the optical spectrum analyzer of the comparative example, as shown in Fig. 7, in the measurement interval T1 from the previous wavelength to the next wavelength, the switches SW1-SW1n and SW21-SW2n are first switched to set the VBW and select the amplifiers Amp21-Amp2m. During the period in which the switches SW1-SW1n and SW21-SW2n are switched, a continuous sampling disabled period T2 occurs in which continuous sampling cannot be performed.

[0036] After the continuous sampling disabled period T2 has elapsed, the A / D conversion unit 101 starts sampling. If the sampled digital value subsequently contains a maximum value (saturation value), it is necessary to change the selected amplifiers Amp21-Amp2m by switching the switches SW21-SW2n again. During the period in which the switches SW21-SW2n are being switched, a continuous sampling disabled period T3 occurs during which continuous sampling cannot be performed. For this reason, the amplifiers Amp21-Amp2m are selected in sequence, and the continuous sampling disabled period T3 occurs intermittently until an amplifier Amp21-Amp2m whose output is not saturated is selected.

[0037] In the optical spectrum analyzer of the comparative example, as shown in FIG. 5, there is a risk that the timing at which the optical power signal reaches its peak will overlap with the continuous sampling disabled period T3, making it impossible to capture the peak of the optical power signal and therefore impossible to stably measure the spectrum of the pulsed light.

[0038] In contrast, the optical spectrum analyzer 1 of this embodiment selects the amplifiers Amp21 to Amp2m using software. Therefore, as shown in Fig. 7, the optical spectrum analyzer 1 of this embodiment requires a continuous sampling disabled period T2 for setting the VBW, but can eliminate the continuous sampling disabled period T3 for switching the amplifiers Amp21 to Amp2m, and can provide a longer continuous sampling enabled period than the comparative example. Therefore, the optical spectrum analyzer 1 of this embodiment can sample the peak of the optical power signal and can stably measure the spectrum of the pulsed light.

[0039] Furthermore, the optical spectrum analyzer 1 of this embodiment samples the optical power signal at a sampling period shorter than the period of the pulsed light, which further increases the possibility of sampling at the peak timing of the optical power signal.

[0040] 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.

[0041] In the above embodiment, the control unit 12 performs filtering during wavelength sweeping, but this is not limited to the above. Filtering may be performed after wavelength sweeping is completed.

[0042] In the above-described embodiment, the control unit 12 sets the average value of the optical power signals successively sampled during the sweep from the previous wavelength to the next wavelength as the optical power signal of the next wavelength, but this is not limited to this. Instead of the average value, a peak value may be set as the optical power signal of the next wavelength. Also, during the sweep from the previous wavelength to the next wavelength, the average value of the optical power signals successively sampled may be set as the optical power signal of the previous wavelength.

[0043] 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 dispersing wavelengths contained in the pulsed light to be measured in directions corresponding to the wavelengths; a light receiving unit (8) that receives the diffracted light emitted from the spectroscopic unit (4) and converts it into an electrical signal; a sweeping unit (9) for causing the light receiving unit (8) to receive the wavelength-swept diffracted light; an amplifier (16) that amplifies the electrical signal converted by the light receiving unit (8) and outputs it as an optical power signal; an A / D conversion unit (17) that performs A / D conversion on the optical power signal output from the amplifier unit (16); a control unit (12) that determines an optical power for each wavelength included in the pulsed light based on the optical power signal sampled and converted into a digital value by the A / D conversion unit (17) during wavelength sweeping, The amplifying section (16) has a plurality of amplifiers (Amp21 to Amp2m) having mutually different gains, The A / D conversion unit (17) has a plurality of A / D converters to which the outputs of the plurality of amplifiers (Amp21 to Amp2m) are respectively connected, The control unit (12) an amplifier selection unit that selects one of the amplifiers (Amp21 to Amp2m) whose output is not saturated, based on the optical power signal that is amplified by the plurality of amplifiers (Amp21 to Amp2m) and converted into a digital value by the plurality of A / D converters, While the wavelength is swept from the previous wavelength to the next wavelength, an average value or a peak value of the digital values ​​of the optical power signals sampled by the A / D conversion unit (17) and output by the amplifiers (Amp21 to Amp2m) selected by the amplifier selection unit is set as the optical power of the previous wavelength or the optical power of the next wavelength. Optical spectrum analyzer(1). [2] In the optical spectrum analyzer (1) according to [1], the A / D conversion unit (17) samples the optical power signal at a sampling time shorter than a period of the pulsed light. Optical spectrum analyzer(1). [Explanation of symbols]

[0044] 1 Optical spectrum analyzer 4 Diffraction grating (spectral section) 8 Light receiving section 9 Motor (Sweeping section) 12 Control section (amplifier selection section) 16 Amplification section 17 A / D conversion section Amp21~Amp2m Amplifiers

Claims

[Claim 1] A spectroscopic unit (4) that outputs diffracted light obtained by dispersing wavelengths contained in the pulsed light to be measured in directions corresponding to the wavelengths; a light receiving section (8) that receives the diffracted light emitted from the spectroscopic section (4) and converts it into an electrical signal; a sweeping unit (9) for causing the light receiving unit (8) to receive the wavelength-swept diffracted light; an amplifier (16) that amplifies the electrical signal converted by the light receiving unit (8) and outputs it as an optical power signal; an A / D converter (17) that performs A / D conversion on the optical power signal output from the amplifier (16); a control unit (12) that determines an optical power for each wavelength included in the pulsed light based on the optical power signal sampled and converted into a digital value by the A / D conversion unit (17) during wavelength sweeping, The amplifier (16) has a plurality of amplifiers (Amp21 to Amp2m) having mutually different gains, The A / D conversion unit (17) has a plurality of A / D converters to which the outputs of the plurality of amplifiers (Amp21 to Amp2m) are respectively connected, The control unit (12) an amplifier selection unit that selects one of the amplifiers (Amp21 to Amp2m) whose output is not saturated, based on the optical power signal that is amplified by the plurality of amplifiers (Amp21 to Amp2m) and converted into a digital value by the plurality of A / D converters, While the wavelength is swept from the previous wavelength to the next wavelength, an average value or a peak value of the digital value of the optical power signal sampled by the A / D conversion unit (17) and output by the amplifier (Amp21 to Amp2m) selected by the amplifier selection unit is set as the optical power of the previous wavelength or the optical power of the next wavelength; The A / D converter (17) samples the optical power signal at a sampling time shorter than a period of the pulsed light. Optical Spectrum Analyzer (1).

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