JPH0327859B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method for accurately measuring the frequency characteristics of a light receiving circuit used on the receiving side in a high-speed optical transmission system.

[Conventional technology]

Conventionally, in order to evaluate the frequency characteristics of a light receiving circuit (including one that is integrally configured as a light receiving element), an IM (intensity modulation) signal whose frequency is swept or a short pulsed optical signal is used as an optical signal. A method has been used to observe the frequency characteristics or pulse waveform of an electrical signal that is incident and photoelectrically converted by the light receiving circuit.

[Problem that the invention seeks to solve]

However, in the former case, it is essential to generate the optical IM signal over a high frequency and to calibrate the level of the generated modulation signal. In this case, there is a limit to the bandwidth of the semiconductor laser (LD) that generates and modulates the optical signal and the external modulator, or there is a need for calibration using a wideband optical receiving system, resulting in wideband optical reception. It was basically difficult to evaluate the circuit. In the latter case, it is necessary to generate a sufficiently small optical pulse compared to the measured band and evaluate its pulse width, and in addition, it is necessary to Fourier transform the obtained electrical signal waveform. It was impossible.

As described above, the conventional method requires a technology for increasing the speed of the optical transmission system that performs intensity modulation of the incident light itself in accordance with the measurement band, and also requires a technology for calibrating the optical signal waveform. Therefore, not only did the configuration of the band measurement system become complicated, but there were also problems with the measurable band and its accuracy.

[Means for solving problems]

The frequency characteristic measuring method of the present invention has been developed in view of the above-mentioned problems, and the method combines the output lights of two semiconductor lasers and inputs them into the light receiving circuit that is the object to be measured. The oscillation frequency difference between the two semiconductor lasers is swept by applying a physical change such as a DC bias current change or a temperature change to at least one of the two semiconductor lasers, and the electrical output of the light receiving circuit is measured using a spectrum analyzer.

[Effect]

Let the time amplitudes of two semiconductor lasers oscillating at a single frequency be v ₁ (t), v ₂ (t), and v ₁ (t), v ₂ (t)
is expressed as below.

v ₁ (t) acosω ₁ t ...(1) v ₂ (t) bcosω ₂ t ...(2) After combining these two lights and entering the light receiving circuit that is the object to be measured, this light receiving circuit The heterodyne detection output v ₃ (t) obtained as the electrical output is: v ₃ (t) = (acosω ₁ t + bcosω ₂ t) ² = a ² cos ² ω ₁ t + b ² cos ² ω ₂ t + 2abcosω ₁ tcosω ₂ t = a ² cos ² ω ₁ t+b ² cos ² ω ₂ t +ab{cos (ω ₁ −ω ₂ )+cos (ω ₁ +ω ₂ )} (3), and a difference frequency component is generated in the third term. Two semiconductor lasers are CW (Continuous Wave)
Since it is in an oscillation state, a and b are constant. Furthermore, ω ₁ and ω ₂ can be varied by changing the temperature of the semiconductor laser or the DC bias current. For example, by changing the temperature of the semiconductor laser by about 1°C, it is possible to perform a frequency sweep of 10 GHz or more. . At this time, variations in the values of a and b are usually almost negligible. Therefore, the beat intensity (∞ab) when the output lights of two semiconductor lasers are combined
is constant within the range of frequency difference required to evaluate the receiving system (approximately 50 GHz), and the frequency-detection output characteristic when this beat light is input to the light receiving circuit is:
Accurately represents the frequency characteristics of the light receiving circuit.

〔Example〕

Hereinafter, the present invention will be explained in detail together with examples.

FIG. 1 is a block diagram showing one embodiment of the present invention, and FIG. 2 is a perspective view showing the structure of a semiconductor laser used in this embodiment. Distributed feedback semiconductor lasers (DFB-LD) 101 and 102 oscillate independently, and are integrally configured as a DFB-LD array 100. When integrated into an array in this way, the semiconductor laser 10
It is easy to make the difference in the oscillation wavelengths of 1 and 102 within about 1 Å and to have a single wavelength, which facilitates the operation control described later. Semiconductor laser 101,1
02 are provided with upper electrodes 103, 104 and a common lower electrode 105, respectively. Lenses 106 and 117 for narrowing down the output light are arranged at the light output sections at both ends of the semiconductor laser 101, and lenses 107 and 118 for narrowing the output light are arranged at the light output sections at both ends of the semiconductor laser 102. ing. In front of the lenses 106, 107, 117, 118, isolators 108, 109, 11
9 and 120 are arranged to prevent the output light and oscillation spectrum of the semiconductor lasers 101 and 102 from becoming unstable due to reflection. Two isolators 10 of these four isolators
In front of the filters 8 and 109, elements whose transmitted light intensity changes depending on the frequency of incident light, such as optical filters 110 and 111 such as Fabry-Perot etalons, are arranged. As shown in FIG. 3, the peaks of the frequency-transmitted light intensity of the optical filters 110 and 111 are shifted from each other. Optical filter 110,1
Further ahead of 11 is a photodiode 112,
113 is arranged, and the photodiode 11
The output terminals of 2 and 113 are each connected to a differential amplifier 11.
It is connected to the non-inverting input terminal and the inverting input terminal of No. 4. The output terminal of differential amplifier 114 is connected to one input terminal of each of electric mixers 115 and 126. The other input terminal of the electric mixer 115 is connected to a power source for a DC bias current applied to the semiconductor laser 101, and the output terminal of the electric mixer 115 is connected to the upper electrode 103 of the semiconductor laser 101. electric mixer 126
The other input terminal of the electric mixer 127 is connected to the output terminal of the electric mixer 127 , and the output terminal of the electric mixer 126 is connected to the upper electrode 104 of the semiconductor laser 102 .

In front of the isolator 120 is a light intensity modulator 1.
22 are provided. Optical intensity modulator 122 modulates the light output from isolator 120 using an electrical signal from amplifier 123. Optical mixer 121
isolator 119 and optical intensity modulator 122
The output light is input to the light receiving circuit 124 which is the object to be measured, and the output signal of the light receiving circuit 124 is sent to the spectrum analyzer 1.
25. Note that the signal source 116 generates a minute current waveform, and this signal is superimposed on the DC bias current in the electric mixer 127 and applied to the upper electrode 104 of the semiconductor laser 102.
is applied to amplifier 123.

Next, the operation of this embodiment will be explained. First, when the same DC bias current is applied to the upper electrodes 103 and 104, the semiconductor laser 10
Laser light is output from 1 and 102. Out of the output light of the semiconductor laser 101, the light focused by the lens 106 is passed through the isolator 108 and the optical filter 110.
The photodiode 112 converts the signal into an electrical signal. Furthermore, out of the output light of the semiconductor laser 102, the light focused by the lens 107 passes through an isolator 109 and an optical filter 111, and is converted into an electrical signal by a photodiode 113. At this time,
The oscillation frequency-transmitted light intensity characteristics of the optical filters 110 and 111 are shifted from each other as shown in FIG. 3, and the transmitted light intensities match at the frequency fo. Therefore, the output of the differential amplifier 114 becomes as shown in FIG. The output of this differential amplifier 114 is passed through an electric mixer 115 to the semiconductor laser 101.
It is also fed back to the injection current of the semiconductor laser 102 through the electric mixer 126. Due to this feedback, in the steady state, the oscillation frequencies of the semiconductor lasers 101 and 102 are fixed at the point fo where the output of the differential amplifier 114 is zero.

In such a steady state, a sawtooth wave (for example, several tens of Hz) is generated from the signal source 116 as shown in FIG.
~ several KHz) is superimposed on the DC bias current in the electric mixer 127 and applied to the upper electrode 104 of the semiconductor laser 102 . Note that this current waveform may be any waveform other than a sawtooth wave, for example, a sine wave, a triangular wave, or any other current waveform that is not very steep. This superimposed minute current causes the semiconductor laser 10 to
The oscillation frequency of 2 changes, and the oscillation frequency of the two semiconductor lasers 1 changes.
A change is added to the oscillation frequency difference between 01 and 102. Here, the feedback system comprised of numerals 106 to 115 and 126 is designed to have a slow time constant that does not respond to this superimposed current. Therefore, it is possible to instantaneously sweep the frequency difference using the superimposed current while suppressing slow fluctuations (approximately 0.1 Hz or less) in the oscillation frequency difference.

By the way, when the current injected into the semiconductor laser is changed in this way, not only the oscillation frequency but also the output light intensity changes. If this fluctuation in the output light intensity cannot be ignored, it is necessary to calibrate the optical output using the optical intensity modulator 122 and the amplifier 123. FIG. 6 is a graph showing the operation of the light intensity modulator 122, in which the horizontal axis represents the voltage applied to the light intensity modulator 122, and the vertical axis represents the transmitted light intensity. In the optical intensity modulator 122, the optical output which is fluctuating as shown in the waveform 601 due to the superimposed current from the signal source 116 is transmitted to the amplifier 12 using the current from the signal source 116.
The applied voltage signal 602 is modulated by the applied voltage signal 602, which is appropriately amplified by 3. That is, by adjusting the modulation intensity in the optical intensity modulator 122 using the amplifier 123 and selecting the operating point, the output optical intensity can be made constant as shown by the straight line 603. This light is sent to the semiconductor laser 1 in the optical mixer 121.
It is combined with the output light of a constant intensity outputted by 01 and inputted to the light receiving circuit 124. At this time, the light receiving circuit 12
4 output electrical signal to spectrum analyzer 125
The frequency-output amplitude characteristic of the light receiving circuit 124 can be measured by inputting the signal to the signal.

Note that since the variation in the current injected by the signal source 116 is minute, the variation in the output light intensity of the semiconductor laser 102 due to the injection current is usually negligible, and in such a case, the light intensity modulator 12
2 and amplifier 123 are unnecessary.

FIG. 7 is a block diagram showing another embodiment of the present invention, and FIG. 8 is a perspective view showing the structure of a semiconductor laser used in this embodiment. In FIGS. 7 and 8, the same or corresponding parts as in FIGS. 1 and 2 are given the same reference numerals, and detailed explanation thereof will be omitted. In this embodiment, the upper electrode 103,
Heating element layers 509 and 510 are formed on the heating element layers 509 and 510 through an insulating layer 513, respectively, and each heating element layer 509 and 510 is provided with a terminal 50 for conducting electricity.
5,506,507,508 are connected.
Signal sources 531 and 532 are connected to terminals 505, 506 and terminals 507, 508, respectively. Note that 511, 512 and 514 are the upper electrodes 103, 104 and the lower electrode 1, respectively.
This is the terminal for the lead wire connected to 05.

Next, the operation of this embodiment will be explained. code 10
The operation in which the oscillation frequencies of the two semiconductor lasers 101 and 102 are stabilized by the feedback circuits 6 to 115 and 126 is exactly the same as in the first embodiment. Next, from this stable state, an oscillation frequency difference is given between the semiconductor lasers 101 and 102, but the method of giving this oscillation frequency difference is different from the first embodiment. In this embodiment, a current is passed through the heating element layer 509 or 510 to generate heat, and this heat generation causes the semiconductor laser 101,
The oscillation frequency is changed by the temperature change of the semiconductor laser 102, and the difference in the oscillation frequency between the two semiconductor lasers 101 and 102 is changed. In this case as well, the code 1
The feedback circuits shown at 06-115 and 126 are configured to have slow time constants that do not respond to current waveforms such as sawtooth waves applied to signal sources 531 or 532.

In this embodiment, the temperature dependence of the oscillation frequencies of the semiconductor lasers 101 and 102 is used to sweep the oscillation frequency difference. The temperature dependence of the oscillation frequency of a semiconductor laser is 10 GHz/deg or more, and therefore, the temperature change required to generate the frequency difference (about several + GHz) necessary for measuring the characteristics of the light receiving circuit 124 is several degrees Celsius. . In this temperature range, the outputs of the semiconductor lasers 101 and 102 are approximately constant, and the intensity of light incident on the light receiving circuit 124 is kept constant without any particular calibration. Therefore, the output of the light receiving circuit 124 is transmitted to the spectrum analyzer 125.
, the display of the spectrum analyzer 125 will represent the frequency characteristics of the light receiving circuit 124.

Note that in the above two embodiments, the isolator 1
08, 109 or photodiode 112,
113 etc., the oscillation frequencies of the two semiconductor lasers 101 and 102 in a steady state may not match, and the feedback system may become stable at a position where there is some oscillation frequency difference. Even in such a case, measurements similar to those described above can be performed by adjusting the magnitude of the signal waveform to be applied, the DC bias current of the semiconductor lasers 101 and 102, etc.

Furthermore, although distributed feedback type semiconductor lasers are used as the semiconductor lasers 101 and 102, they are not limited to this.

〔Effect of the invention〕

As explained above, according to the frequency characteristic measurement method of the present invention, the output light of two semiconductor lasers is combined and input to the light receiving circuit that is the object to be measured, while at least one of the two semiconductor lasers is combined. The oscillation frequency difference between the two semiconductor lasers is swept by applying physical changes such as DC bias current changes and temperature changes, and the electrical output of the photodetector circuit is measured using a spectrum analyzer. The frequency characteristics of a light receiving circuit can be measured with high precision without using special techniques such as optical pulse generation technology. That is, it is superior to conventional measurement methods in terms of both simplicity and accuracy.

[Brief explanation of drawings]

FIG. 1 is a configuration diagram showing one embodiment of the present invention, and FIG.
The figure is a perspective view showing the structure of the semiconductor laser used in this example, FIG. 3 is a graph showing the oscillation frequency vs. transmitted light intensity characteristics of optical filters 110 and 111, and FIG. 4 is the output of the differential amplifier 114. Graph showing the characteristics, FIG. 5 is a waveform diagram of the superimposed current, FIG. 6 is a graph showing the operation of the optical intensity modulator 122, FIG. 7 is a configuration diagram showing an embodiment of the present invention, and FIG. 1 is a perspective view showing the structure of a semiconductor laser used in an example. 101,102...Semiconductor laser, 116,5
31,532...signal source, 121...optical mixer,
124... Light receiving circuit, 125... Spectrum analyzer, 509, 510... Heat generating layer.

Claims (1)

[Claims] 1. The output light of the two semiconductor lasers is combined and inputted to the light receiving circuit to be measured, while physically changing at least one of the two semiconductor lasers. 1. A method for measuring frequency characteristics of a light receiving circuit, comprising sweeping the oscillation frequency difference of a semiconductor laser and measuring the electrical output of the light receiving circuit with a spectrum analyzer. 2. The method for measuring frequency characteristics of a light receiving circuit according to claim 1, wherein the physical change is a DC bias current change. 3. The method for measuring frequency characteristics of a light receiving circuit according to claim 1, wherein the physical change is a temperature change.