JPH03134536A - Measuring apparatus of frequency characteristic - Google Patents

Abstract

PURPOSE:To detect the frequency characteristic of a photodetector in a wide frequency band by making it unnecessary to measure a beat frequency electrically with using a frequency stabilizing laser of a known frequency and a variable frequency laser. CONSTITUTION:Lights output from a frequency stabilizing laser 11 and a variable frequency laser source 12 are, through an optical fiber, combined by a coupler 3. The combined light is incident upon a photodetector 4 to be measured through an optical fiber. The photodetector 4 detects the beat signal and the electric power of the beat signal is measured by a power meter 6. Then, the frequency characteristic of the light detected by the photodetector 4 is displayed by a frequency characteristic display 7, for example, with an output of the power meter 6 being shown on an axis of ordinate and a sweep output of a signal generator 8 being indicated on an axis of abscissa. The frequency of the beat signal output from the photodetector 4 is corresponding to the output of the signal generator 8. Therefore, if the power of the output from the photodetector is measured by the power meter 6, the frequency characteristic of the photodetector 4 can be detected in a wide band.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to improving the characteristics of a device for measuring optical frequency response characteristics such as a photodetector.

<Prior Art> FIG. 8 is a configuration block diagram showing a conventional frequency characteristic measuring device. Two lights having very similar wavelengths emitted from laser light sources 1 and 2 are combined by a coupler 3, and the beat signal thereof is detected by a photodetector to be measured 4. Laser light source 1.2
By controlling the current and temperature of the oscillation frequency and changing its oscillation frequency, it is possible to sweep the beat frequency over a wide band. By measuring the output of the photodetector 4 with the spectrum analyzer 5, the frequency characteristics of various light receiving elements and light receiving circuits used as the photodetector 4 can be measured over a relatively wide band.

<Problems to be Solved by the Invention> However, in the frequency characteristic measuring device configured as described above, the beat frequency and level of two laser beams must be measured. When measuring frequency with a spectrum analyzer, a beat frequency oscillator is usually required, and when measuring the level, detection is performed with a diode, so there is a problem that the measurable frequency range is limited to, for example, 100 GHz.

The present invention has been made to solve such problems, and an object of the present invention is to realize a frequency characteristic measuring device that can detect the frequency characteristics of a photodetector over a wide band.

Means for Solving the Problems> The frequency characteristic measuring device according to the present invention includes a stabilized laser light source that stabilizes the frequency by controlling the frequency of a semiconductor laser according to the absorption spectrum line of the standard material, and a variable frequency laser light source that can control the frequency of a semiconductor laser and then change the frequency; a signal generator that sweeps the frequency of the variable frequency laser light source;
A combining means for combining the output light of the stabilized laser light source and the output light of the variable frequency laser light source, and a power of a beat signal output from a photodetector to be measured into which the output light of the combining means is incident. The photodetector is characterized in that it includes a power meter for measurement, and a display that inputs the output of the power meter and the output of the signal generator to display the frequency characteristics of the photodetector.

Effect> The frequency of the beat signal output from the photodetector corresponds to the output of the signal generator, so if you measure the power of the photodetector output with a power meter, you can easily display the frequency characteristics on the display. Can be done.

<Example> The present invention will be explained in detail below using the drawings.

FIG. 1 is a configuration block diagram showing an example of an optical frequency characteristic device according to the present invention. The same parts as in FIG. 8 are given the same symbols. 11 is a frequency-stabilized laser light source that stabilizes the frequency of a semiconductor laser by controlling the frequency of the semiconductor laser according to the absorption spectrum line of the standard substance; 12 is a frequency-stabilized laser light source that controls the frequency of the semiconductor laser according to the absorption spectrum line of the standard substance and then changes the frequency; 8 is a signal generator that sweeps the frequency of the variable frequency laser light source 12 with a sawtooth wave or the like; 3 is a signal generator that combines the output light of the stabilized laser light source 11 and the output light of the variable frequency laser light source 12; A polarization-maintaining fiber coupler constitutes a wave combining means and combines the output lights of both light sources 11 and 12 by aligning their leakage planes, and 4 is a light detection device to be measured such as a photodiode into which the output light of the coupler 3 is input. 6 is an AC power meter that measures the electrical signal power output from the photodetector 4; 7 is an AC power meter that inputs the output of the power meter 6 and the output of the signal generator 8 to display the frequency characteristics of the photodetector 4; This is an indicator that displays

FIG. 2 is a structural block diagram showing the stabilized laser light source 11 of FIG. 1. LDI is a semiconductor laser, 'I' S1
is a temperature sensor that measures the temperature of this semiconductor laser LDI, PEI is a Bertier element that cools or heats this semiconductor laser LDI, and CTI is a temperature sensor that drives the Bertier element PE based on the output of the temperature sensor TSI to measure the temperature of the semiconductor laser LDI. TBI is a constant temperature bath that stores these and reduces temperature fluctuations.

BSl is a beam splitter that separates the output light of the semiconductor laser into two directions, and UMI is this beam splitter BS.
The acousto-optic modulator CLI receives the output light from one side of the acousto-optic modulator UMI and constitutes a modulation means. The enclosed absorption cell, PDI, is a photodetector that receives the transmitted light of this absorption cell CLI, A1 is an amplifier that inputs the output electrical signal of this photodetector PCI, and LAI is a lock-in that inputs the electrical output of this amplifier A1. An amplifier, Cr2, is a PID controller that inputs the output of this lock-in amplifier LAI and constitutes a control means for controlling the current of the semiconductor laser LDI, SWI is a switch whose one end is connected to the acousto-optic modulator UMI, and SGI is a switch thereof. a signal generator S, one of whose outputs serves as a reference signal for a lock-in amplifier LAI, and whose other output turns on and off the switch SWI at a frequency f (for example, 2 kHz);
G2 is the frequency fo connected to the other end of the switch SWI.
(e.g. 80 MHz).

FIG. 5 is a configuration block diagram without showing the variable frequency laser light source 12 of FIG. 1. To explain only the parts that are different from FIG. 2, SW2 is a mode changeover switch inserted between the output of the PID controller CT2 and the input of the laser LDI.
W3 is a mode changeover switch whose one end is connected to the output of the signal generator 8, and S81 is a mode changeover switch that inputs the output from the other end of this changeover switch SW3 and the output of the temperature sensor TSI, and connects the difference to the input of the temperature control means CTI. This is a subtraction circuit.

FIG. 6 is a block diagram showing the electrical power meter 6 of FIG. 1. As shown in FIG. C is a capacitor that cuts the DC component of the photodetector 4 output, R is a resistor that converts the output current of capacitor C into heat, and 61.62 is a thermocouple and its converter that measure the amount of heat generated by resistor R. .

The operation of the frequency characteristic measuring device configured as described above will be explained in detail below.

In FIG. 1, output lights from a frequency stabilized laser 11 and a variable frequency laser light source 12 are combined by a coupler 3 via an optical fiber, and the combined light enters a photodetector under test 4 via an optical fiber. .

Here, if the frequency stabilized laser 11 is oscillating at a frequency f1, the output optical electric field amplitude at this time is expressed by the following formula (%) (1) Also, if the variable frequency laser light source 12 is oscillating at a frequency f2, The output optical electric field amplitude is expressed by the following equation.

E = e sin (2πf2 + θ2) 2 (2) However, θ1. θ2 is phase noise. (1).

When the two lights expressed by equation (2) are combined by the coupler 3, the electric field amplitude of the combined light is expressed by the following equation.

E 1E = e sin (2gf1t+θ1)+1
2 1 e 5in (2πft+θ) ・ (3)
2 2 2 The photodetector 4 into which the combined light is incident generates an output proportional to the square of the electric field, so its output I can be calculated using the following formula % )) ) ) ] (4) In equation (4), [] The last term in the equation indicates that the component of the difference f - f2 between the oscillation frequencies of the two lights is output at a level proportional to the product of the respective electric field amplitudes. Photodetector 4
detects this beat signal and measures its electric power with the power meter 6.0For example, when the oscillation frequency of the variable frequency laser light source 12 is swept from fl and the output of the electric power meter 6 drops by 3 dB at f1+Δf, 3dB of this photodetector 4: IP! This becomes the 4-attenuation frequency.

Next, the operation of the frequency stabilized laser shown in FIG. 2 will be explained. The semiconductor laser LDI is controlled to a constant temperature via the Vertier element PEI by a control means C'T'l which inputs a temperature detection signal in the constant temperature bath TBI. semiconductor laser LDI
The output light is separated into two directions by the beam splitter BSI, the reflected light becomes output light to the outside, and the transmitted light enters the acousto-optic modulator UMI. When the switch SWI is on, the acousto-optic modulator UMI is driven by the output of the signal generator SG2 at the frequency f, so the frequency ν. Most of the incident light is diffracted and undergoes a frequency (Doppler) shift, and the first-order diffracted light has a frequency ν. -f, light enters the absorption cell CL1. When the switch SWI is off, all incident light has a frequency of 0th order diffracted light and enters the absorption cell CLI at ◇. Since the switch SWI is driven by the single frequency tarok of the signal generator SGI, the light incident on the absorption cell CLI is subjected to frequency modulation of the modulation frequency f 11 +modulation source fo. Absorption cell CLI with one acousto-optic transformer UM
When light modulated by 1 is incident, the amount of transmitted light is modulated only at the absorption signal location, and a signal appears at the output, as shown in the operation explanatory diagram of FIG. This signal is converted into an electrical signal by the photodetector PDI and then sent to the lock-in amplifier L via the amplifier A1.
Synchronize with frequency fl on A1! If one flow is applied, a first-order differential waveform as shown in the frequency characteristic curve diagram of FIG. 4 is obtained. Semiconductor laser LDI by PID controller CT2
If the output of the lock-in amplifier LAI is controlled to the center of the first-order differential waveform by controlling the current of the semiconductor laser LDI
The output light has a stable frequency locked to νsfo/2.

In a frequency characteristic measuring device having such a configuration, the oscillation frequency of the laser is not modulated, so the output is very stable even momentarily.

Next, the operation of the variable frequency laser light source shown in FIG. 5 will be explained. In control mode, selector switch SW2 is on, SW
3 is off, the operation is the same as in Figure 2 above,
The output frequency of the semiconductor laser LDI is locked to the absorption frequency of the standard material.

In the sweep mode, changeover switch SW2 is turned off to cut off the frequency control loop, and after holding the laser injection current at this time, SW3 is turned on and the output of signal generator 8 is used to sweep the temperature of the laser LDI and its oscillation frequency. do. In this case, when the temperature changes by 1° C., the oscillation frequency of the semiconductor laser LDI changes by about 1 OG HZ.

Next, the operation of the power meter 6 shown in FIG. 6 will be explained.

The DC component of the photodetector 4 output is connected to capacitor C!・The optical output current I containing only the beat component is converted into heat W by the resistor R. Here, eat W=I R (5) This calorific value W, which is eat , is measured using the thermocouple 61 and the converter 62.

The frequency characteristic display 7 displays the optical frequency characteristic of the photodetector 4 on the screen by using, for example, the output of the power meter 6 as a vertical axis input and the sweep output of the signal generator 8 as a horizontal axis input.

According to the frequency characteristic measuring device with such a configuration, since it uses a frequency stabilized laser whose frequency is known and a variable frequency laser light source, there is no need to electrically measure the frequency from Analyzer is not required.

In addition, the power of the beat signal is detected by heat, and the frequency characteristics of the photodetector 4 can be measured up to a frequency that can be converted into heat by the resistance R of the power meter, so it can be detected electrically like a spectrum analyzer. Measurements can be made over a much wider band than in the conventional case. For example, if the temperature of the laser is changed by 60° C., it is possible to measure up to around 600 GHz.

Furthermore, since a spectrum analyzer is not required, the frequency characteristics of the photodetector can be measured with a simple configuration.

In the above embodiment, as a variable frequency laser light source, the transmission wavelength of the Fabry-Bérot etalon is set to the second wavelength.
Control the output light wavelength of the stabilized laser light source as shown in the figure,
By controlling the output wavelength of a semiconductor laser to one of the transmission wavelengths of the Fabry-Bello etalon, and then changing the output wavelength stepwise by changing the current or temperature of the semiconductor laser, it is possible to obtain a frequency-stabilized laser light source. Since the frequency difference can be accurately determined, measurement becomes easy.

Furthermore, by changing the standard material of the absorption cell CLI, it is possible to measure the frequency characteristics of the photodetector in various wavelength bands as shown in the example below.

Furthermore, in the above embodiment, the measurement band is several tens of GHz.
In cases where the damage is relatively low, a diode detector may be used as the power meter 6 shown in FIG.

Moreover, it is also possible to measure the frequency characteristics of not only the photodetector under test but also a photodetection circuit.

Furthermore, if it is desired to make measurements while keeping the optical power incident on the photodetector 4 constant, a variable attenuator 9 may be inserted between the coupler 3 and the photodetector 4 as shown in FIG.

Here, the second combined output of the coupler 3 is transmitted to the low-speed photodetector 1.
0, and the controller 11 controls the attenuation amount of the variable attenuator 9 in accordance with the output.

<Effects of the Invention> As described above, according to the present invention, a frequency characteristic measuring device capable of detecting the frequency characteristics of a photodetector over a wide band can be realized with a simple configuration.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing an embodiment of the optical frequency characteristic device according to the present invention, FIG. 2 is a partial block diagram showing the frequency-stabilized laser light source 11 of FIG. 1, and FIG. An operation explanatory diagram for explaining the operation of the device, Fig. 4 is the second
Figure 5 is a partial block diagram showing the variable frequency laser light source 12 in Figure 1; Figure 6 is a partial block diagram showing the power meter 6 in Figure 1. , FIG. 7 is a block diagram showing a modification of the optical frequency characteristic device according to the present invention, and FIG. 8 is a block diagram showing a conventional frequency characteristic measuring device. 3... Multiplexing means, 4... Photodetector to be measured, 6...
Power meter, 7...Display device, 8...Signal generator,
11... Stabilized laser light source, 12... Variable frequency laser light source, LDI... Semiconductor laser. Figure 2 (Figure 2 Round 7 number 1+, 1, live display handheld, 7.-.

Claims (1)

[Claims] A stabilized laser light source that stabilizes the frequency by controlling the frequency of a semiconductor laser according to the absorption spectrum line of the standard material, and a variable source that can change the frequency after controlling the frequency of the semiconductor laser according to the absorption spectrum line of the standard material. a frequency laser light source, a signal generator for sweeping the frequency of the variable frequency laser light source, a combining means for combining the output light of the stabilized laser light source and the output light of the variable frequency laser light source, and the combining means a power meter for measuring the power of a beat signal output from a photodetector to be measured that receives output light from the photodetector; A frequency characteristic measuring device characterized by comprising a display device for displaying information.