JPH03110438A - Frequency characteristic measuring instrument for photodetecting circuit - Google Patents

Abstract

PURPOSE:To speedily measure the frequency characteristics of the photodetecting circuit by accurately control and sweep the frequency of a beat signal obtained by making reference light and laser light to be controlled incident on the photodetecting element and synchronizing the sweeping for the frequency response of the photodetecting circuit by a spectrum analyzer at the same time. CONSTITUTION:The beat signal corresponding to the frequency difference between the reference light 108 and the output light of a laser diode 101 to be controlled is obtained from circuit 81 to be measured. The beat signal and the output of a 1st local oscillator 111 are mixed by a mixer 110 to generate an intermediate frequency signal. The frequency of the laser diode 101 is controlled by using a PLL to hold the frequency of the intermediate frequency signal constant. In this state, the oscillation frequency of the local oscillator 111 is swept to sweep the oscillation frequency of the diode 101. Consequently, the frequency of the beat signal is tracked by a heterodyne type PLL at all times, so the beat frequency can accurately be swept in a short time and the level of the detected beat signal is recorded. The sweep needs to be performed only once when the spectrum analyzer and the sweeping speed of the beat frequency are made coincident, and started and measured at the same time.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a device for measuring the frequency characteristics of a high-speed/wideband photodetector circuit, and in particular, the present invention relates to a device for measuring the frequency characteristics of a high-speed/wideband photodetector circuit, and in particular, to measure the frequency characteristics in a short time and with a good S/N ratio. This invention relates to a frequency characteristic measuring device for a light receiving circuit that can measure the frequency characteristics of a light receiving circuit.

[Conventional technology]

In recent years, signal transmission speeds in optical fiber communications have continued to increase, and modulation and speeds exceeding 10 Gb/s have already been achieved.
Demodulation technology is becoming a reality. In order to perform such high-speed, broadband optical fiber transmission, it is necessary to correctly evaluate the modulation characteristics of the semiconductor laser diode on the transmitter side as well as the frequency characteristics (also called frequency response) of the light receiving circuit on the receiver side. It is essential.

Conventionally, methods for evaluating the frequency characteristics of high-speed, wide-band photodetector circuits (including those configured integrally as photodetectors) ranging from several GHz to several tens of GHz have been based on methods in which the oscillation optical frequencies are extremely close and the polarization is The light from two semiconductor laser diodes in the same state is combined, enters the photodetector circuit under test, and detects the beat signal, while changing the temperature or injection current of one semiconductor laser diode to adjust its oscillation frequency. A method is used to record the intensity of the beat signal by sweeping it (for example, Kawanishi, Saruwatari, ``
``Evaluation of high frequency characteristics by heterodyne detection of long wavelength photodetectors'', 1985 National Conference of Semiconductor and Materials Division, Institute of Electrical Communication, 376, or Yonishi et al., ``Method for measuring frequency characteristics of photodetector circuits'', Patent Application No. 1988-274412. ).

FIG. 2 is a diagram showing the configuration of a conventional frequency characteristic measuring device based on such a method. In this figure, the first DFB (distrib
output light from the laser diode 21 and the second D, which is also oscillating in a single longitudinal mode.
The output light from the I"B laser diode 22 is transmitted through isolators 23 and 24 to prevent reflection from affecting the oscillation frequency, and then combined by an optical coupler 25 so that the polarization states of both output lights match. and enters the light receiving circuit 26 to be measured.

Here, the first and second DFB laser diodes 21
．． In order to prevent the oscillation light frequency from changing due to temperature fluctuations, temperature control devices 27.2 and 22 are respectively installed.
8 is added. Further, the electrical output of the light receiving circuit to be measured 26 is input to a spectrum analyzer 29 for measuring and recording its frequency components.

Next, the principle of measuring the frequency characteristics of the light receiving circuit under test using the conventional apparatus shown in FIG. 2 will be explained. Now, if the oscillation optical frequency of the first DFB laser diode 21 is ν and the oscillation optical frequency of the second DFB laser diode 22 is ν, then each optical electric field is E l= A ICOS (2E ν+t+φ2)
...=(1) Et=AtCos (2yr v
It can be written as tt+φt) −・・・−・(2). however,
A + , A t are electric field amplitudes, φ1. φ is the initial phase. The electrical output obtained by the light-receiving circuit 26 under test generates an electrical signal proportional to the incident light power in principle, so the output is V = RI E + + E
t l ...... is given by (3). However, R is a coefficient determined by the light receiving circuit. It can be seen that by substituting equation (+) and equation (2) into equation (3) above, a beat signal having a frequency rb given by the following equation, in addition to the DC component, can be obtained.

rb=lν1-shear1...
(4) At this time, if the frequency component of the beat signal is measured by temporally sweeping the measurement frequency band with the spectrum analyzer 29, a result as shown in FIG. 3(A) can be obtained.

Here, the temperature of the first DFB laser diode 21 is
By changing the temperature using the temperature control device 27, the frequency ν1 can be changed, and as a result, the frequency "b" of the beat signal can be changed.
When measured, the results are shown in FIGS. 3(B) and 3(C). On the other hand, the electrical signal strength measured by the spectrum analyzer corresponds to the frequency response of the light receiving circuit under test 25 at each frequency "b. Therefore,
While sweeping the temperature of the first DPB laser diode 21 and slowly sweeping the frequency ν, repeat the sweep measurement of the frequency component of the beat signal by the spectrum analyzer 29, and record the maximum value of each measurement value. , the frequency characteristics of the light receiving circuit 25 to be measured can be measured. This state is shown in FIG. 3(D), and the broken line shown in the figure corresponds to the frequency characteristic of the light receiving circuit to be measured.

[Problem to be solved by the invention]

By the way, in the above-mentioned conventional frequency characteristic measuring device, per one sweep of the spectrum analyzer 29,
Since the frequency response at only one frequency value can be measured, there is a problem in that it takes a considerable amount of time to measure the frequency characteristics. This is not a problem that can be solved simply by increasing the sweep speed. That is, the optical frequency ν
When the sweep speed of , is increased, spectrum analyzer 2
3 (E) between the maximum value recorded by one measurement of 9 and the maximum value recorded by the next one measurement.
As a result, the signal-to-noise ratio (S/N) of the measured frequency characteristics deteriorates.

For example, suppose the line width of the beat signal is 3 MIIz or less and the resolution of the spectrum analyzer 29 is 3 MHz.

If one sweep time [is 5 (lns), the sweep speed V of optical frequency ν1 can be simply calculated as follows: V =!9=(3Mllz)/(50ras)=6
0 MHz/s - From (5), it must be 60 MHz/s or less. That is, it takes 84 seconds or more to measure the frequency characteristics in the 5 GHz band.

Furthermore, the S/N of the frequency characteristic measured by the conventional frequency characteristic measuring device described above is generally poor even when the sweep speed V of the optical frequency ν1 is sufficiently slow.

This is because the oscillation light frequency of the semiconductor laser diode 21 still has frequency fluctuations of several Mllz even in a state where the temperature has been stabilized to about I/100 degrees. That is, since the beat frequency itself fluctuates by about Mtlz, unevenness as shown in FIG. 3(E) still occurs between multiple consecutive sweep measurements by the spectrum analyzer 29.

This invention was made in view of the above-mentioned circumstances, and its purpose is to overcome the drawbacks of the prior art as described above, and to improve the frequency characteristics of a high-speed, wide-band photodetector circuit in a short time and with good quality. It is an object of the present invention to provide an excellent frequency characteristic measuring device for a light receiving circuit that can measure with a high S/N ratio.

[Means to solve the problem]

In order to solve the above-mentioned problems, this invention generates a beat signal obtained by inputting a reference light and an output light of a controlled semiconductor laser diode into a light receiving circuit to be measured or a reference light receiving circuit.
An intermediate frequency signal is generated by mixing the output of the first local oscillator for frequency sweeping with a mixer, and controlled using a PLL (phase synchronized loop) so that the intermediate frequency signal always has a constant frequency. The optical frequency is swept by sweeping the oscillation frequency of the first local oscillator while controlling the oscillation frequency of the semiconductor laser. It is characterized by sweeping measurements using a spectrum analyzer synchronized with the sweep of an oscillator, or a recording device that combines a detector and a storage scope.

[Effect]

According to the above configuration, a beat signal having a frequency component corresponding to the difference in optical frequency between the reference light and the output light of the controlled semiconductor laser diode is detected from the light receiving circuit to be measured or the reference light receiving circuit, and The frequency is used to sweep the optical frequency of the controlled semiconductor laser diode. Here, in order to control the frequency of the beat signal over a wide frequency band by the oscillation frequency of the first local oscillator, which is a variable frequency oscillator in the microwave region, the PL
However, in order to use a single semiconductor laser diode with a wide linewidth ranging from several MHz to several tens of MHz as an optical frequency variable oscillator, special measures are required. That is, the beat signal and the output of the first local oscillator are first mixed in a mixer to generate an intermediate frequency signal having the difference or sum of their respective frequencies, and the frequency of this intermediate frequency signal is always a constant value. In this way, the optical frequency of a semiconductor laser diode is controlled using a PLL. By sweeping the oscillation frequency of the first local oscillator while such a PLL is operating, the oscillation optical frequency of the semiconductor laser diode is swept, and as a result, the frequency of the beat signal changes to the so-called hetero gain type. PLL (
Since the beat frequency is constantly tracked by frequency offset locking (also called frequency offset locking), it is possible to sweep the beat frequency accurately in a short time. Further, the beat signal detected by the light-receiving circuit under test is recorded by temporally sweeping the signal strength thereof by a recording device such as a spectrum analyzer. Therefore, for example, if you match the sweep speed of this spectrum analyzer with the sweep speed of the beat frequency and start both sweeps at the same time and perform measurements, you can record the desired frequency characteristics with just one sweep of the spectrum analyzer. You can.

〔Example〕

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a diagram showing the configuration of a frequency characteristic measuring device for a light receiving circuit according to a first embodiment of the present invention.

In this figure, the controlled semiconductor laser diode +01
For example, a 1.5 μm band DBRLD (laser diode) having an active layer region and a distributed reflection layer region is used. A current of 8011 IA is injected into the active layer region by the current injection device 102, and a control current of about 30 A+A is supplied to the distributed reflection layer region by the control current injection device +03. In this case, the semiconductor laser diode 101 oscillates in a single longitudinal mode at approximately 193.5 TIIz (near 1,535 μm), and the spectral linewidth is 16M.
Hz. Furthermore, by varying the control m current value to the distributed reflective layer region by means of the controlled current injection device 103, the control m current value of 1,9 GHz/
The oscillation frequency of the semiconductor laser diode 101 can be changed at a rate of mA. Note that in this embodiment, the semiconductor laser diode 101 has l/100
The temperature is stabilized at around °C.

The output light of the controlled semiconductor laser diode 101 is transmitted through the isolator 104 in order to prevent the oscillation frequency from fluctuating due to reflected light. Then, this output light is input to an optical fiber, mixed with the reference light obtained from the reference light source 108 at the optical coupler 106 in such a state that the polarization states of both lights match, and then mixed with the reference light obtained from the reference light source 108 at the optical coupler 106.
1. Here, 15CIl is used as the light receiving circuit.
A photodiode (Lasert) with a band above z
ron QDEUC+1-035-001) 10
Amplifier 109 having a band of 16 G fiz or more in 7
I'm using something that connects. In this way, the beat signal obtained from the light receiving circuit under test 81 has an oscillation light frequency of the controlled semiconductor laser diode ν8 and an optical frequency of the reference light ν8.
, then the frequency rb given by equation (4) mentioned above is
have. The reference light source 108 is a narrow spectrum LD light source (N5L- manufactured by Santic Co., Ltd., which is a semiconductor laser diode with an external resonator to narrow the spectral linewidth to several + kllz and improve the stability of the optical frequency. 155), but a single semiconductor laser diode may also be used. Furthermore, in this embodiment, a normal single-mode optical fiber coupler is used as the optical coupler 106, and a normal single-mode optical fiber is used for the optical wiring, so that the polarization state of both lights mixed by the optical coupler 106 is After initial adjustment so that they match, a stable beat signal output can be obtained without particularly controlling the polarization states of both lights. If optical wiring is performed using fibers, it is possible to minimize the mismatch between the polarization states of both lights and obtain an extremely stable beat signal.

The beat signal output from the light-receiving circuit under test 81 is branched by a signal branching circuit 82, and a portion thereof is guided to a spectrum analyzer 83 (+11'71000 series manufactured by rlEWLETT PACKARD), and its frequency components are recorded. On the other hand, the other beat signal branched by the signal branching circuit 82 is a hetero gain type P L
L is used to control and sweep the oscillation optical frequency of the controlled semiconductor laser diode lO1.

Next, the operation of this heterodyne type PLL will be explained.

The branched beat signal of frequency rb is sent to mixer 110 (
Watkins Johnson M87C: RFi range approx. 1
8GIlz)'s RF terminal. On the other hand, mixer 11
The output (frequency r) of the first local oscillator III is input to the LO terminal of 0, and as a result, the IF terminal of the mixer 110 outputs an intermediate frequency signal having a frequency rIF given by the following equation.

rlF= l fb-f l −
--- (5) The first local oscillator III is a synthesized sweeper (IIEWL) that can change and sweep the frequency over a wide frequency band.
ETTPA (11P8340A manufactured by JARD Corporation: variable frequency range 10M11z -26.5GHz) was used.

The intermediate frequency signal thus obtained is amplified by an amplifier +12 having a band of approximately 1G11z, and then inputted to a phase comparator 3113. On the other hand, as a signal whose phase should be compared with this intermediate frequency signal, the output of the second local oscillator 114 oscillating at a certain reference frequency rr is used. Here, as the phase comparator 113, the frequency division ratio M=
A digital phase comparator with a phase comparison range of ±2π×2560 was used, which was constructed by combining 20 prescalers, an 8-bit synchronous counter, a full adder, etc.

Further, in this embodiment, the reference frequency rr is set to 260
MIIz was set, and the phase comparison was designed to be performed at a crawling period of 4.3 MIIz. Note that by synchronizing the reference clocks of the second local oscillator 114 and the first local oscillator Ill, it is also possible to control the beat frequency with higher precision.

The error signal output from the phase comparator 113 is processed through a loop consisting of a low-pass filter for reducing ripples due to clock cycle components of phase comparison and a second-order lag lead filter for expanding the lock range of the PLL. It is fed back to the control current injection device +03 via the filter 115, thereby changing the control current to the distributed reflection layer region of the controlled semiconductor laser diode 101. In this way, the oscillation frequency of the controlled semiconductor laser diode 101 is controlled so that the frequency rIF of the intermediate frequency signal becomes equal to the reference frequency [r, and frequency offset locking is realized.

Next, it will be explained that the beat frequency "b" is swept by sweeping the oscillation frequency of the first local oscillator 111 in a state where such frequency offset locking is realized.

Locking state, that is, intermediate frequency ftF
is equal to the reference frequency "r" generated by the second local oscillator 114.
0! The oscillation frequency ν1 is 4 due to the magnitude relationship with the optical frequency ν of the reference light and the magnitude relationship between the beat frequency rb defined by the above-mentioned equation (4) and the oscillation frequency f of the first local oscillator 111. A possible case is a street. FIG. 4 illustrates these relationships. The phase comparator determines whether it will be in the state of ■ or ■, or whether it will be in the state of ■ or ■! Controlled semiconductor laser diode for the positive and negative phase difference detected in 13 (see Figure 1)! Depending on whether the oscillation frequency of 01 is increased or decreased, -
decided at will. For example, in this embodiment, if the phase of the intermediate frequency signal leads the phase of the output signal of the second local oscillator 114, then the controlled semiconductor laser diode! Since the configuration is such that feedback is applied in the direction of decreasing the oscillation frequency ν1 of O1, ■ or ■
Locked in either relationship. By comparing the beat frequency rb in the rocking state with the oscillation frequency f of the first local oscillator Ill, it is possible to determine whether the relationship is (1) or (2). In other words, it is uniquely determined which of the relationships 1 to 2 in FIG. 4 is locked.

Here, among the frequency values shown in FIG. 4, ν and fr are fixed values serving as a reference. Therefore, by sweeping the oscillation frequency r of the first local oscillator 111, the oscillation optical frequency ν1 of the controlled semiconductor laser diode 101 can be swept, and as a result, the beat frequency rb can be swept. For example, if the oscillation frequency of the first local oscillator 111 is continuously swept from "," to "," and the beat frequency is locked according to the frequency relationship shown in "■" in FIG.
It will be continuously swept up to r+'t.

In the above manner, the controlled semiconductor laser diode lo
The oscillation optical frequency of t is controlled. At this time, the sweep frequency range of the spectrum analyzer 83 is set from rr+r+ to rr+rt, the sweep time is made to match that of the synthesized sweeper used as the first local oscillator 21, and the trigger signal supply circuit 84 causes the spectrum analyzer 83 If the frequency sweep of the synthesized sweeper and the frequency sweep of the synthesized sweeper (both are single sweeps) are synchronized and started at the same time, one sweep of the spectrum analyzer 83 can be performed by the light receiving circuit 8! It is possible to record the frequency characteristics of For convenience, this will be referred to as synchronous measurement.

FIG. 5(a) is a diagram showing the frequency characteristics of the light receiving circuit synchronously measured according to the first embodiment of the present invention. In this case, fr, r+ and ft are each 260MHz
, 13, 74 GHz and 18, 74 GHz, and the sweep time was 5 seconds.

On the other hand, FIG. 5(b) shows that in the first embodiment shown in FIG. 1, the oscillation frequency of the first local oscillator 111 is changed from rl to r without using the trigger signal supply circuit 84.

Spectrum analyzer (repeated sweep with a sweep time of 50 m5ec) 83
The results of measuring the frequency components of the output are recorded using the maximum value trace function. The characteristics measured by this method are based on the conventional equipment shown in FIG.
603A),
That is, it can be considered in the same way as that measured as shown in FIG. 3(D). For convenience, such measurements are
We will call this asynchronous measurement.

As is clear from comparing Figures 5 (a) and (b), synchronous measurement makes it possible to measure frequency characteristics with good S/N in a short measurement time, which was previously impossible. . Even though the measurement time is 1/40th, the measurement is performed with a good S/N ratio, and thus the above-described first embodiment provides a remarkable effect.

In the first embodiment described above, only the sweep start time of the spectrum analyzer 83 is controlled by the trigger signal, but a part of the output of the first local oscillator I11 is monitored to control the sweep speed of the spectrum analyzer 83. Alternatively, by using a sweep synchronization circuit that operates to constantly control the measurement frequency band, it is also possible to perform synchronized measurements by operating like a so-called scalar network analyzer. In addition, in the same embodiment, when the beat signal output intensity of the measurement light receiving circuit 81 becomes extremely small, the control of the PLL is lost and measurement becomes difficult. That is, there is a limit to the dynamic range of measurement, for example, 23 dB in this example. However, by using an AGC amplifier that operates at a higher speed than the sweep speed of the first local oscillator I11 for the amplifier 112, it is possible to further expand the dynamic range during characteristic measurement.

Next, FIG. 6 is a diagram showing the configuration of a second embodiment of the present invention. This second embodiment differs from the first embodiment described above in that the unused output of the two outputs of the optical coupler 106 is transferred to a reference light receiving circuit other than the light receiving circuit under test 81. The oscillation light frequency of the controlled semiconductor laser diode 101 is controlled using the beat signal obtained from the laser diode 92 .

Next, the operation of this second embodiment will be explained only with respect to the parts that are different from the above-described first embodiment.

One output of the optical coupler 106 is input to the light receiving circuit under test 91 to generate a second beat signal, which is recorded by the spectrum analyzer 83.

On the other hand, the other output of the optical coupler 106 is connected to the reference light receiving circuit 92.
and supplies a first beat signal used for optical frequency control of the semiconductor laser diode IO1. Here, the reference light receiving circuit 92 performs the following in the frequency range to be measured:
It is assumed that the frequency characteristic is such that a beat signal output sufficient for frequency offset locking operation can be generated.

The operation of sweeping the optical frequency using the first beat signal thus obtained is as already explained in the first embodiment.

According to the second embodiment described above, when measuring in a band including a frequency region in which the light-to-electrical conversion efficiency of the photodetector circuit 91 to be measured is extremely low (that is, the frequency characteristics are poor),
Without losing optical frequency offset locking,
A stable sweep operation can be achieved. That is,
In addition to all the advantages described in the first embodiment, frequency characteristics can be measured over a wide dynamic range.

Regarding the frequency characteristics of optical input/output devices that modulate the intensity or frequency of light depending on the intensity of light,
By providing a circuit that converts the electrical intensity change signal by some means whose frequency response is known at the subsequent stage of the optical device, and treating the entire optical device as a light-receiving circuit to be measured, the above-mentioned first or second method can be achieved. The fact that it can be evaluated based on the examples can be easily inferred by analogy. In addition, the first
Alternatively, in the second embodiment, the mixer 110 was described as being used for frequency down-conversion, but it is also possible to use this for up-conversion to configure a frequency characteristic measuring device for a light receiving circuit with a similar configuration. . In addition, the controlled semiconductor laser diode +01
Although the method of controlling the oscillation optical frequency by controlling current to the distributed reflection layer region using a DBRLD has been described as an example, the semiconductor LD used in the present invention is not limited to this in any way. . Furthermore, in measurements that do not require very high sensitivity, instead of using the spectrum analyzer 83, a detector that detects the beat signal and outputs its intensity, and a storage scope or plotter that sweeps the signal intensity changes over time can be used. It is also possible to record by a recording device that is a combination of recording devices.

〔Effect of the invention〕

As described in detail above, according to the present invention, the frequency of the beat signal obtained by inputting the reference light and the output light of the controlled semiconductor laser diode into the light receiving circuit under test or the reference light receiving circuit is always accurately determined. The frequency characteristics of the light receiving circuit can be measured in a short time and with a good S/N ratio by sweeping while controlling and at the same time synchronizing the sweep for measuring the frequency response of the light receiving circuit using a spectrum analyzer, etc. This has the effect of being able to.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing the configuration of a first embodiment of the present invention, FIG. 2 is a block diagram showing the configuration of a conventional frequency characteristic measuring device for a light receiving circuit, and FIG. 3 is an example of measuring frequency characteristics according to the conventional example. FIG. 4 is a diagram for explaining the beat frequency control operation according to the present invention. FIG. 5 is a diagram showing the frequency characteristics of the light receiving circuit measured according to the first embodiment of the present invention. , FIG. 6 is a block diagram showing the configuration of a second embodiment of the present invention. 81.91... Light receiving circuit under test, 82...
・Signal branch circuit, 83...Spectrum analyzer (recording device)
, 84...Trigger signal supply circuit (sweep synchronization circuit), 92...Reference light receiving circuit, 01...Controlled semiconductor laser diode, 03
...Control current injection device, 06...Optical coupler, +08...Reference light source, 0...Mixer, l...First local oscillator, 2...Amplifier, 113...Phase comparator, 4...Second local oscillator, 5...Loop filter.

Claims (2)

[Claims] (1) Frequency characteristics of a light receiving circuit that uses a semiconductor laser diode that oscillates in a single longitudinal mode as an optical frequency variable oscillator and utilizes the frequency difference between the optical frequency of the output light of the semiconductor laser diode and the optical frequency of the reference light source. The measurement device includes: an optical coupler that combines the output light of the semiconductor laser diode and the reference light; a signal branching circuit that branches a beat signal obtained as a result of inputting the output light of the optical coupler into the light receiving circuit under test; a first local oscillator whose oscillation frequency can be swept; a mixer that generates an intermediate frequency signal by mixing a portion of the beat signal extracted by the signal branch circuit with the output of the first local oscillator; and a reference. a second local oscillator that oscillates at a frequency; a phase comparator that detects a phase difference between the output of the second local oscillator and the intermediate frequency signal and outputs an error signal according to the phase difference; oscillation frequency control means for controlling the oscillation frequency of the semiconductor laser diode so that the frequency of the intermediate frequency signal and the reference frequency are always equal to each other according to a signal; A frequency characteristic of a light receiving circuit, comprising: a recording device that performs sweep measurement and records; and sweep synchronization means that synchronizes the sweep measurement of the recording device and the sweep of the oscillation frequency of the first local oscillator. measuring device. (2) Frequency characteristics of a light receiving circuit that uses a semiconductor laser diode that oscillates in a single longitudinal mode as an optical frequency variable oscillator and utilizes the frequency difference between the optical frequency of the output light of the semiconductor laser diode and the optical frequency of the reference light source. The measuring device includes: an optical coupler that combines the output light of the semiconductor laser diode and the reference light; a reference light receiving circuit that receives the output light of one of the optical couplers and generates a first beat signal; and an oscillation frequency. a first local oscillator capable of sweeping;
a mixer that mixes the beat signal of the first local oscillator and the output of the first local oscillator to generate an intermediate frequency signal; a second local oscillator that oscillates at a reference frequency; a phase comparator that detects a phase difference with a signal and outputs an error signal according to the phase difference; and a semiconductor laser diode configured to always make the frequency of the intermediate frequency signal equal to the reference frequency by the error signal. oscillation frequency control means for controlling the oscillation frequency of the optical coupler; and a recording device for temporally sweeping measuring and recording the intensity of a second beat signal obtained as a result of inputting the other output light of the optical coupler into the light receiving circuit to be measured. and a sweep synchronization means for synchronizing the sweep measurement of the recording device and the sweep of the oscillation frequency of the first local oscillator.