JPS62290205A - Light receiving circuit - Google Patents

Abstract

PURPOSE:To realize a high sensitivity circuit for detection by providing a photodetector, a transmission line connected to the photodetector in parallel and an amplifier element combined to the photodetector, and permitting the transmission line to resonate in parallel with an electrostatic capacity components that the photodetector and the amplifier element have and with plural frequencies. CONSTITUTION:At frequencies (f1, f2 and f3) in which the absolute value of an inductive reactance component in the transmission line 52 and that of a capacitive reactance component become equal, a parallel resonance occurs between the transmission line 52 and the electrostatic capacity C1. At these plural resonance frequencies, an impedance Zin which is viewed from the input level stage (terminals 53-53') of an amplifier A in an equiavalent circuit becomes a resistance equivalent to the negative resistance RL of the photodetector, and the impedance Zin at the time of resonance becomes the maximum value RL, compared to the impedance Zin at other frequencies. Consequently, at the resonance frequencies (f1, f2 and f3) a current arising in the photodetector is effectively converted into a voltage only by the negative resistance RL, therefore, noise in the photodetector can be reduced.

Description

Detailed Description of the Invention 3. Detailed Description of the Invention [Field of Industrial Application] The present invention relates to an optical receiving circuit used in an optical communication system, particularly an optical heterodyne detection type optical communication system.

[Conventional technology]

In recent years, the characteristics of semiconductor lasers have improved, and it has become possible to obtain semiconductor lasers that oscillate in a single-axis mode and have high spectral purity. For this reason, it has become possible to realize an optical heterodyne detection method that uses information on the frequency and phase of light, and highly sensitive systems have come to be realized.

In the optical heterodyne detection method, the transmitter transmits information by modulating the amplitude, frequency, or phase of three signal lights corresponding to the information signal, and the receiver uses a local oscillation light source to transmit information. The signal is demodulated by heterodyne detection of the light.

In order to increase the sensitivity of such an optical heterodyne detection method, it is important to reduce noise in the optical receiving circuit.

Normally, in optical heterogain detection, the frequency difference between the signal light and the local oscillation light, that is, the intermediate frequency, is
It is desirable to set the value to at least double.

However, the noise in the optical receiving circuit increases as the frequency becomes higher due to the influence of the capacitance of the photodetector and the amplifying element.

Therefore, even in optical heterodyne detection, there is a drawback that sensitivity degradation is likely to occur due to the influence of noise in the optical receiving circuit.

Therefore, as a method to reduce the noise of the optical receiving circuit near the intermediate frequency, for example, the paper r400M by Harushita et al.
b/s Optical FSK Long Distance Transmission Experiment” (Proceedings of the 1985 IEICE Information and Systems Division National Conference 280
), there is a tunable optical receiver circuit in which an inductance is connected in parallel to the photodetector of the optical receiver circuit to cancel the influence of capacitance. This tunable optical receiver circuit is
The inductance connected in parallel to the photodetector resonates in parallel with the parasitic capacitance of the photodetector and amplification element near the intermediate frequency.The capacitance component is removed near the intermediate frequency, and the light The noise has been sufficiently reduced as an optical receiver circuit for heterodyne detection. As a result, wavelength 1.
It uses an optical frequency shift keying (optical FSX) method with a transmission rate of 400 Mb/s in the 5 μm band, achieving high optical reception sensitivity of -49 dBm.

[Problem that the invention seeks to solve]

Now, in the optical heterodyne detection communication system, since information such as the frequency of the optical signal light can be used, the optical transmitter transmits a large number of optical signals by high-density frequency multiplexing, and the optical receiver transmits one optical signal. Optical heterodyne detection is performed in the receiving circuit,
Another feature is that optical frequency division multiplex communication in which signal components for each intermediate frequency are extracted and demodulated for each signal can be easily performed.

On the other hand, in the above-mentioned tunable optical receiver circuit using an inductance, parallel resonance between the parasitic capacitance of the photodetector and the inductance occurs only at one frequency.

Therefore, in this tunable optical receiving circuit, the noise reduction of the optical receiving circuit is achieved only in a certain intermediate frequency band.
In other frequency bands, there is no effect of low noise.

- Therefore, when this tunable optical receiver circuit is used in an optical receiver circuit for optical heterodyne communication that performs optical frequency multiplex communication,
Although it is possible to increase the sensitivity to a certain specific signal light,
High sensitivity cannot be expected for other signal lights, and the performance of the communication system is determined by the characteristics of the signal light with the lowest sensitivity, so there is no advantage of using a tunable optical receiver circuit. There was a drawback that it could not be done.

An object of the present invention is to reduce the noise of an optical receiving circuit by eliminating the influence of capacitance of a photodetector and amplifying element in each intermediate frequency band even when frequency multiplexing transmission of light is performed. The object of the present invention is to realize a high-bandwidth optical receiver circuit for optical heterodyne detection suitable for optical frequency multiplexing transmission, which can obtain high optical reception sensitivity for signal light.

[Means for solving problems]

In the present invention, the transmitter transmits the emotion α by modulating the amplitude 9 frequencies or the phase of the transmit signal light corresponding to the information signal, and the receiver transmits the transmit signal using a local oscillation light source. An optical receiving circuit for optical heterodyne detection communication that performs heterodyne detection of a signal light and demodulates the signal, comprising: a photodetector, a transmission line connected in parallel to the photodetector, and an amplification element coupled to the photodetector. The transmission line is characterized in that it resonates in parallel with capacitance components of the photodetector and amplification element at a plurality of frequencies.

[Effect]

The effects of the present invention will be described below.

In order to reduce the noise of the pre-reception A3 circuit, it is sufficient to cancel the capacitive reactance caused by the parasitic capacitance of the photodetector, the amplifier element, etc. with the inductive reactance.

Therefore, in the optical receiving circuit of the present invention, a transmission line is used as an inductive reactance that cancels the capacitive reactance and resonates in parallel at a plurality of frequency points.

FIG. 5 shows an equivalent circuit of a photodetector section of an optical receiving circuit for explaining the present invention in detail.

In the equivalent circuit of Fig. 5, current source (■) indicates the current generated when light enters the photodetector Q, and capacitor C5 indicates the capacitance component parasitic to the photodetector, amplifier element, etc.
2 is a bypass capacitor with a large capacitance value, resistor RL is a load resistance of the photodetector, and amplifier A is an ideal amplifier with input 1 impedance infinite and gain G. Also, in Figure 5, 52
The line shown by is a distributed constant transmission line, and if the characteristic impedance of this transmission line 52 is 20, the phase constant is β, and the line length is l, the input impedance Z seen from the terminals 51-51' is ZR= J Zo tan β! becomes.

FIG. 6 also shows the transmission line 52, capacitor C1, capacitor C! The frequency characteristics of the Nyorori actance component X are shown. Here, since the phase constant β of the transmission line 52 has a roughly proportional relationship to the frequency f, the reactance component of the transmission line 52 has a roughly tangent function relationship with respect to a change in frequency.

Now, in FIG. 6, the frequencies (f+, f2+
r3), parallel resonance occurs between the transmission line 52 and the capacitance C1. At such multiple resonant frequencies, the input stage (terminal 53-5) of amplifier A in the equivalent circuit of FIG.
The impedance Z in seen from 3') is only the resistance of the load resistance RL of the photodetector, and the impedance Z at resonance is the maximum value RL compared to the impedance Z in at other frequencies. becomes.

Therefore, at this resonant frequency (f+, fz, f*), the current generated in the photodetector is effectively converted into voltage only by the load resistor RL, so the noise in the optical receiving circuit is improved. .

Therefore, in order to use this optical receiving circuit as an optical heterodyne optical receiving circuit for optical frequency multiplexing, each intermediate frequency obtained by optical heterodyne detection of each signal light and local oscillation light is
f, , f, , f in FIG.

If the resonant frequency is set to a value such as , high optical reception sensitivity can be obtained for each signal light.

〔Example〕

Next, the present invention will be explained using examples.

FIG. 1 is a circuit diagram of an optical receiving circuit which is an embodiment of the present invention.

In FIG. 1, 1 is a photodetector, 2 is an amplification element coupled to the photodetector 1, and 3 is connected in parallel to the photodetector l, and the electrostatic capacitance parasitic to the photodetector 1 and the amplification element 2 is Transmission lines for generating parallel resonance, C3, C4, Cs, Cb
are bypass capacitors for DC cut, RL, RD
is the load resistance.

In this optical receiving circuit, the photodetector 1 has InGaAS-PI.
N photodiode and GaAs-FET as amplifier element 2
was used. The load resistance R of the photodetector 1 is set to 10Ω,
Bypass capacitors C's, Ca, CS, and C had a capacitance of 0.33 μF. In addition, the parasitic capacitance of the light output device 1 and the amplification element 2 is about 29F, and the transmission line 3 has this capacitance component and 300MHz and 1
A microstrip line with a characteristic impedance of 100Ω and a vA path length of 1001 mm was used on an alumina substrate so that parallel resonance occurred at about GHz.

Figure 2 shows the noise characteristics of this optical receiver circuit. Figure 2 shows
The horizontal axis represents frequency, and the vertical axis represents input equivalent noise current density, which is an index of noise characteristics.

This optical receiving circuit has a frequency of 300 MHz, 1.1
Parallel resonance occurs at GHz, and the input-referred noise current density at these frequencies is approximately 2 pA/pA at a frequency of 300 MHz and approximately 2.5 at a frequency of 1.1 GHz.
It is smaller than pA/mouth.

These values are 10Ω for the load resistance RL of photodetector 1.
This value is almost close to the thermal noise value determined by 1.3 pA/J], and it was possible to eliminate the influence of capacitance caused by the photodetector 1 and the like at the parallel resonance points of 300 MHz and 1.1 GHz.

By the way, the noise in a conventional optical receiving circuit that does not use a resonator is about 1 opAi7Tli at 300 MHz and about 20 pA/unit at IGHz. , a significant reduction in noise was achieved.

, Now, using this optical receiver circuit, the wavelength is 1.55 μm.
.

Optical frequency multiplexed optical heterodyne detection was performed using two waves of signal light using a binary phase shift keying (PSK) method with a transmission rate of 140 Mb/s.

FIG. 3 shows a simple block diagram of the optical heterodyne detection system. In the block diagram of FIG. 3, the first light source 4 and the second light source 5 are Fupley-Perot type semiconductor laser diodes with a wavelength of 1.55 μm that undergo optical feedback to achieve single-axis mode oscillation and narrow spectrum width. The oscillation frequency difference between the two light sources 4 and 5 was set to 800 MHz by controlling the temperature of the semiconductor laser and controlling the injection current for each light source 4.5. The light emitted from these light sources is subjected to binary phase shift keying (PSK) modulation by first and second optical modulators 6.
After multiplexing, the signals were input into a single mode fiber 10.

On the other hand, the local oscillation light source has the first oscillation light source. Using the third light source 11 having the same configuration as the second light source, the second optical multiplexer 12 multiplexes the local oscillation light and the two signal lights, and then the optical receiver circuit 13 shown in FIG. , optical heterodyne detection was performed.

Here, by adjusting the oscillation frequency of the third light source 11 used as a local oscillation light source, the peak values of the two intermediate frequency outputs 14 generated in the optical receiving circuit 13 are each 300 MHz.
, 1.1 GHz.

After the outputs 14 of these two intermediate frequencies are sufficiently amplified by the amplifier 15, they are split, and one is output at the center frequency 300.
MHz bandpass filter 16, the other center JID
is guided to a band pass filter 17 of 1.1 GHz, and the intermediate frequencies are 300 MHz and 1.1 GHz, respectively.
The intermediate frequency components were extracted.

Thereafter, each intermediate frequency component was demodulated by first and second differential PSK demodulators 18 and 19.

Now, as a result of measuring the optical reception sensitivity (bit error rate 10-') at a transmission rate of 140 Mb/S with the above configuration, it was found that the first light source 4 with the intermediate frequency set to 300 MHz
The optical receiving sensitivity for the signal light from the second light source 5 with the intermediate frequency set to 1.1 C;Hz is -59.0 dBm, and the optical receiving sensitivity for the signal light from the second light source 5 whose intermediate frequency is set to 1.1 C;Hz is -59.0 dBm. Using an optical receiver circuit with low noise near IGHz, we were able to achieve equally high sensitivity for each optical signal.

FIG. 4 shows a block diagram of an optical heterodyne detection system in which the optical receiving circuit shown in FIG. 1 is applied to another optical heterodyne detection system using a different modulation method.

The optical receiver circuit 13 used here is similar to the one shown in FIG. 1, but in this application example, a dual frequency shift keying (F S Performing filter demodulation.

In the block diagram of FIG. 4, the light source 20 for transmission and the light source 21 for local oscillation oscillate in a single axis mode at a wavelength of 1.55 μm, and the oscillation frequency is stabilized by adjusting the injection current and temperature according to 1111. A distributed feedback semiconductor laser diode (DFB-LD) was used.

Furthermore, in the transmission light 'R20, the bias current was slightly changed by the modulation signal 28 to perform binary FSX modulation. During this modulation, the binary signal mark “1” and the space “
The frequency shift corresponding to 0'' was set to 800 MH2.

After this FSX signal light passes through a single mode fiber 10, it is combined with local oscillation light by an optical multiplexer 12, and optically heterodyne detected by an optical receiver circuit 13.

Here, the peak value of the intermediate frequency of the space "0" in the intermediate frequency output 14 of the optical receiving circuit 13 is 300 MH
z, the peak value of the intermediate frequency of mark “1” is 1.1 GH
The oscillation frequency of the light source 21 for local oscillation was adjusted so that z.

After the intermediate frequency output 14 having these two peaks is sufficiently amplified by the amplifier 15, it is branched, and one is sent to a band-pass filter 16 with a center frequency of 300 MHz, and the other is sent to a band-pass filter 17 with a center frequency of 1.1 GHz. space “0” with a center frequency of 300 MHz, respectively.
” signal component, center frequency is 1.1 GHz mark “l
” signal component was extracted.

After that, these signal components are demodulated by the first and second envelope detectors 22.23, and then the first and second outputs 25° 26 from the first and second envelope detectors 22.23 are A differential amplifier 24 was used to convert the binary FSK signal light.

Now, with the above configuration, the optical reception sensitivity (bit error rate 10-') for binary FSK signal light at a transmission rate of 140 Mb/s was measured. As a result, the wave number for medium opening is 300.
The optical receiving sensitivity using only the output 25, which is the signal component of the MHz space “0”, is -53 dBm1 with a center frequency of 1.1.
The optical reception sensitivity using only the output 26, which is the signal component of the GHz mark a1'', was -52.5 dBm, and there was almost no difference between the two optical reception sensitivities.

In addition, when using the output 27 obtained by combining these two outputs 25.26 with the differential amplifier 24 and performing binary demodulation, the optical reception sensitivity is -56 dBm, which is excellent as an FSX optical heterodyne detection method. We were able to obtain optical reception sensitivity.

The present invention can have various other embodiments in addition to the above-described embodiments.

For example, as the photodetector 1 of the optical receiving circuit, InGa
In addition to As-PIN photodiodes, avalanche photodiodes (APDs), photoconductive detectors, photomultiplier tubes, and the like can be used.

In addition to GaAs-FET, as the amplification element 2,
A bipolar transistor or the like may also be used.

Furthermore, as a transmission line that removes parasitic capacitance components in the photodetector 1 and the like and resonates in parallel, in addition to the microstrip line, a Levher line, a coaxial line, etc. may be used.

Further, in order to adjust the parallel resonance frequency, a capacitor with a small capacitance value may be connected in parallel to the photodetector 1.

In addition, the optical frequency multiplexing optical heterodyne detection method shown in the application example is a case of two-wave multiplexing using two signal lights, but since there are theoretically an infinite number of parallel resonant frequencies of the optical receiving circuit, Optical frequency multiplexing is also possible.

C Effects of the Invention] As described above, in the optical receiving circuit according to the present invention, the parasitic capacitance component in the photodetector and the amplification element is removed, and there are a plurality of frequencies that resonate in parallel. Therefore, the noise of the optical receiving circuit can be reduced at each resonant frequency, and when optical frequency multiplexing transmission is performed using the optical heterodyne detection method, high optical receiving sensitivity can be obtained for each signal light. .

[Brief explanation of drawings]

Fig. 1 is a circuit diagram of an optical receiver circuit that is an embodiment of the present invention, Fig. 2 is a diagram showing the noise characteristics of the optical receiver circuit of Fig. 1, and Fig. 3 is a diagram showing the noise characteristics of the optical receiver circuit of Fig. 1. Figure 4 is a block diagram of the FSX optical heterodyne detection system using the optical receiver circuit of Figure 1, and Figure 5 is an equivalent circuit of the optical receiver circuit of the present invention. FIG. 6 is a diagram for explaining the principle of the optical receiving circuit of the present invention. 1... Photodetector 2... Amplifying element 3... Transmission line, i, 5.11, 20.21... Light source. 6.7... Optical modulator 8.12... Optical multiplexer 10... Single mode fiber 13... Optical receiving circuit 14... Output 15... Amplifier 16.17... Band Pass filters 18, 19... Differential PSK demodulator 22, 23... Envelope detector 24... Differential amplifier 25, 26, 27... Output 28... Modulation signals RL, Ro...・Load resistance C+, Cz...Capacitor C3, C-, Cs, Cb...Bypass capacitor A...Amplifier agent Yoshiyuki Iwasa 1--...
Photodetection n 2 ------- 1 Amplification element 3 ------- A Transmission line 3 each PL, RD --- 1 Load resistor C3, C4, C5, C6 ------- )' Ino Fig. 1 52---Item transmission 4i path I ---Electric/historical source RL--it load low resistance +, Cz----Capacitor A--1- -・Amplifier Figure 5 Beni S + - Behe Hachibi

Claims (1)

[Claims] (1) The transmitter transmits information by modulating the amplitude, frequency, or phase of the transmit signal light in accordance with the information signal, and the receiver performs heterodyne detection of the transmit signal light using a local oscillation light source. An optical receiving circuit for optical heterodyne detection communication that demodulates a signal by demodulating a signal, comprising: a photodetector; a transmission line connected in parallel to the photodetector; An optical receiving circuit characterized in that the line resonates in parallel with the capacitance components of the photodetector and the amplifying element at a plurality of frequencies.