JPS63200631A - Optical communication method using optical heterodyne detection - Google Patents

Abstract

PURPOSE:To realize a high optical reception sensitivity with low noise at the reception side by using an image rejection mixer. CONSTITUTION:In the process of obtaining a demodulation signal from a received signal light, the polarized state of the signal light and the local oscillation light is adjusted and they are made coincident after the adjustment, the synthesized light is separated into two orthogonal linearly polarized light components, which are subject to optical heterodyne detection respectively, two intermediate frequency signals having a phase difference of 90 deg. obtained by the light heterodyne detection are coupled and the combined intermediate frequency signal is divided into two signals based on the frequency arrangement of the signal light and the local oscillation light. As a means to realize the method, an image rejection mixer 19 is used. Thus, a semiconductor laser is used for the light source at the sender side in the frequency shift keying (FSK) optical heterodyne detection optical communication system and even when the frequency shift is selected large at FSK modulation, the band required for the reception side is suppressed lower.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an optical communication method, and particularly to an optical heterodyne detection optical communication method for extracting a demodulated signal by optically heterodyne detecting received signal light.

[Conventional technology]

Conventionally, optical communication methods that use optical heterodyne detection on the receiving side of optical signals have lower performance compared to optical communication methods that use optical direct detection.
Because it has the great advantage of increasing optical reception sensitivity by 10 to 100 times or more, it is expected to be an effective communication method for long-distance optical communication trunk systems.

In this optical communication method using optical heterodyne detection,
For example, frequency shift keying (FSX) using frequency information,
In other words, the information signal on the optical transmitter side is marked and spaced.
In the case of the FSX optical heterodyne detection optical communication method, in which the signal light is frequency-shift modulated so that the mark becomes a mark signal and the space becomes a space signal, the receiving side performs optical heterodyne detection to extract the demodulated signal. Direct frequency modulation becomes possible if a semiconductor laser is used as the transmission light source. In this case, since there is no need to use an external modulator on the transmitting side, insertion loss can be avoided, and a long-distance transmission system can be constructed.

[Problem that the invention seeks to solve]

In the above-mentioned conventional optical heterodyne detection optical communication method, when a semiconductor laser is used as a transmission light source on the transmitting side, there is a problem that code amount interference occurs between mark and space signals due to spectrum expansion, resulting in deterioration of transmission characteristics. There is. Normally, in order to avoid the influence of this spectrum broadening, the transmitting side increases the frequency shift during FSK modulation. For example, the document by Emura et al. from the 2612 National Conference of the Institute of Electronics and Communication Engineers in 1988.
In the case of the method described in "Characteristics of FSK optical heterodyne single filter detection method using DFB-LD", the high frequency component of the intermediate frequency signal on the receiving side has a considerably high frequency.Usually, the optical receiving circuit The noise characteristics deteriorate as the frequency increases. Therefore, if a large amount of frequency deviation is taken on the transmitting side, the optical receiving sensitivity will deteriorate on the receiving side due to the noise in the high frequency range of the optical receiving circuit. , there is a problem with speeding up.

An object of the present invention is to eliminate such drawbacks, and in the FSX optical heterodyne detection optical communication system, even when a semiconductor laser is used as the transmission light source on the transmitting side and a large frequency shift is taken during FSX modulation, the reception can be improved. An object of the present invention is to provide an optical heterodyne detection optical communication method that allows signals to be received using a low intermediate frequency on the side, and that can easily achieve high optical reception sensitivity and high speed.

[Means for solving problems]

The present invention provides an optical heterodyne detection optical communication method in which an optical transmission side transmits signal light frequency shift modulated by an information signal, and an optical reception side obtains a demodulated signal from the received signal light. is set within the frequency band of the signal light, the polarization states of the signal light and the local oscillation light are adjusted, and the signal light with the adjusted polarization state and the local oscillation light are combined, and a result of this combination is obtained. The combined light is divided into two orthogonal linearly polarized components, and each of these two linearly polarized components is optically heterodyne-detected to form two intermediate frequency signals having a phase difference of 90° from each other. the combined intermediate frequency signal is divided into two signals based on the frequency arrangement of the signal light and the local oscillation light, each of these two signals is demodulated, and the two demodulated signals are combined. The feature is that a demodulated signal is obtained by doing this.

[Effect]

The optical heterodyne detection method according to the present invention adjusts the polarization state of the signal light and the local oscillation light in the process of obtaining a demodulated signal from the received signal light, and then combines the signal light and the local oscillation light with the adjusted polarization states. The combined light is divided into two orthogonal linearly polarized components, and each of these two linearly polarized components is optically heterodyne-detected. The combined intermediate frequency signal is divided into two signals based on the frequency arrangement of the signal light and the local oscillation light.As a means to realize these methods, For example, an image rejection mixer is used.

For image rejection mixer, 1986
Electronics Letters (Electroni)
cs Letters), Volume 22, No. 15, 825-82
On page 6, see the article “Optical Heterodyne Image Rejection Mixer (OPT)” by Darcy (T, E, DARCIF) and Glance (B, GLANCE).
ICAL HETERODYNE IM Hachie E-REJ
ECTION MIXER)”.

FIG. 2 is a diagram showing the configuration of the image rejection mixer. The image rejection mixer first mixes the signal light 31 and the local oscillation light 6 into the first . The polarization state is adjusted by second polarization controllers 4 and 7. The light whose polarization state has been adjusted is multiplexed by an optical multiplexer 8. This combined light is divided by a polarization beam splitter 9 into two orthogonal linearly polarized components.

At this time, the light intensities of both polarized light components are adjusted to be equal. The two orthogonal components obtained here are respectively the first and second orthogonal components. When the second optical receiver 10.11 performs optical heterodyne detection, the obtained 1° second intermediate frequency signal 2
L22 will have a phase difference of 90° from each other.

At this time, which intermediate frequency signal advances by 90° depends on the frequency arrangement of the signal light 3 and the local oscillation light 6.

For example, if the oscillation frequency of the signal light 3 is higher than the oscillation frequency of the local oscillation light 6 and the first intermediate frequency signal 21 leads the second intermediate frequency signal 22 by 90 degrees, the signal light 3 When the second intermediate frequency signal 22 is brought to a lower frequency side than the local oscillation light 6, the second intermediate frequency signal 22 leads by 90° compared to the first intermediate frequency signal 21.

Now, if two intermediate frequency signals of equal magnitude and with a phase difference of 90 degrees are input to the 3 dB coupler 12 which has a 90 degree hybrid characteristic, there are two output signals output from the 3 dB coupler 12. Output port 2
3.24 is output from only one of them. At this time 2
Which terminal of the two output ports 23 and 24 a signal is output from depends on the phase relationship between the two input intermediate frequency signals.

As described above, in the image rejection mixer, the frequency arrangement of the signal light 3 and the local oscillation light 6 allows the demodulation system to
It can be seen that the output terminal of the B coupler 12 from which the signal is output changes.

〔Example〕

Next, embodiments of the present invention will be described with reference to the drawings.

FIG. 1 is a block diagram showing an optical communication device for implementing an embodiment of the present invention. The image rejection mixer used in this embodiment is the one shown in FIG. In Figure 1, on the transmitting side, a signal light source 1 that oscillates in a single axis mode
The output of is binary frequency modulated with the signal from the signal source 20. After the frequency-modulated signal light 3 propagates through the optical fiber 2, it is applied to the image rejection mixer 19 on the receiving side.

On the other hand, as shown in FIG. 3, the oscillation frequency of the local oscillation light source 5 on the receiving side is within the frequency band of the signal light and is set between the oscillation frequency of the mark signal and the oscillation frequency of the space signal. has been done. Therefore, the mark signal and the space signal are arranged symmetrically with respect to the local oscillation light. Also, the frequency deviation amount of the mark signal and space signal is IG
Hz.

In the image rejection mixer 19, the polarization state of the signal light 3 is controlled by the first polarization controller 4, and the polarization state of the local oscillation light 6, which is the output from the local oscillation light source 5, is controlled by the second polarization controller 7. The polarization state is adjusted. That is, the first . The polarization state is adjusted by a second polarization controller 4.7.

The light whose polarization state has been adjusted is multiplexed by an optical multiplexer 8. This combined light is split by a polarization beam splitter 9 into two orthogonal linearly polarized components.

At this time, the light intensities of both polarized light components are adjusted to be equal.

As mentioned above, the first. The polarization state was adjusted by the second polarization controllers 4 and 7, so that the combination of the optical multiplexer 8 and the polarization beam splinter 9 operated as an optical 90° hybrid.

The two orthogonal linearly polarized components are the first . They are received separately by a second optical receiver 10.11. The two orthogonal components are respectively the first . When optical heterodyne detection is performed by the second optical receiver 10.11, the first degree and second intermediate frequency signals 21.22 obtained there will have a phase difference of 90 degrees from each other.

Now, two intermediate frequency signals 21 and 22, which are equal in magnitude and whose phases are different from each other by 90°, are input to the 3 dB coupler 12 having a 90° hybrid characteristic.

Here, when the transmission signal is a mark signal, the output signal output from the 3 dB coupler 12 is transmitted to the first output port 2.
3 and is amplified by the first amplifier circuit 13 and then demodulated by the first detection circuit 15. On the other hand, when the transmission signal is a space signal, the output signal output from the 3 dB coupler 12 is output from the second output port 24, amplified by the second amplifier circuit 14, and then sent to the second detection circuit. It is demodulated by circuit 16.

1st. The second demodulated signals 25 and 26 are differentially combined by the differential amplifier 17 to obtain the final demodulated signal 1B.

In the optical communication device used in this embodiment, the signal light source 1
As the local oscillation light source 5, a distributed feedback semiconductor laser with a wavelength of 1.55 μm was used. The bit rate of the transmission signal was 100 Mb/s, and the amount of frequency shift during binary frequency modulation was IGH2. A 2×2 optical fiber coupler was used for the optical multiplexer 8.

On the receiving side, the first. The band of the second amplifier circuits 13 and 14 was 300 MH2 to 700 MH2, and the oscillation frequency of the local oscillation light source 5 was controlled so that both the mark and space intermediate frequency signals were centered in their respective bands. Also the first
．． An envelope detection circuit was used for the second detection circuits 15 and 16.

In this embodiment, the combined spectrum spread of both the signal light source 1 and the local oscillation light source 5 is approximately 50 MH2 at half width.
However, since a large frequency shift of I GHz was adopted on the transmitting side, there was no effect of spectrum broadening at all. In addition, despite the large frequency deviation compared to IGH2, the use of an image rejection mixer on the receiving side allows the receiving band to be set to a low frequency, resulting in high optical reception of -56 dBm with an error rate of 10-q. Sensitivity was obtained. This reception sensitivity is more than 1 dB higher than when receiving using a high frequency region with a normal heterodyne detection system, and is an improvement of more than 10 dB compared to a direct detection system.

Next, other embodiments of the present invention will be described with reference to the drawings.

FIG. 4 is a block diagram of an optical communication device used in this embodiment. The image rejection mixer 19 used in this embodiment is the one shown in FIG. In Figure 4,
The signal from the signal source 20 on the transmitting side has four signal levels, and the signal light source 1 is four-level frequency modulated with information of 100 Mb/s. After the frequency-modulated signal light 3 propagates through the optical fiber 2, it is applied to the image rejection mixer 19 on the receiving side.

On the other hand, as shown in FIG.
．． 4, it is placed between signal level 2 and signal level 3. Signal level 1 and signal level 2 are placed on the low frequency side with respect to locally oscillated light, and signal level 3 and signal level 4 are placed on the high frequency side with respect to locally oscillated light.

The frequency deviation between signal level 1 and signal level 2 is 500M
H2) The amount of frequency deviation between signal level 2 and signal level 3 is I
GH2) The amount of frequency deviation between signal level 3 and signal level 4 is 500 MH. And these signal levels 1 to 4
are arranged symmetrically with respect to the local oscillation light.

In the image rejection mixer 19, the polarization state of the signal light 3 is controlled by the first polarization controller 4, and then the signal light 3 is input to the optical multiplexer 8. This optical multiplexer 8 is connected to a second polarization controller 7 for transmitting local oscillation light 6 which is an output from the local oscillation light source 5.
The light whose polarization state has been adjusted is also incident, and the signal light 3 and the local oscillation light 6 are combined. This combined light is divided into two orthogonal linearly polarized components by a polarization beam splitter 9, which are separately received by first and second optical receivers 10 and 11 and subjected to heterodyne detection. Here, as mentioned above, the first and second
The polarization state is adjusted by the polarization controllers 4 and 7, so that the combination of the optical multiplexer 8 and the polarization beam splitter 9 operates as an optical 90° hybrid. 1st. The output of the second optical receiver 10.11 is input to a 3 dB coupler 12.

The output signal output from the 3dB coupler 12 has the local oscillation light 6 arranged as shown in FIG. 23, whose intermediate frequency center frequency is ICl3 for signal level 1, 500MH2 for signal level 2, and signal level 3 and signal level 4 are output from the second output port 24 of 3dB coupler 12, Regarding the intermediate frequency center frequency, signal level 3 is 500 MH2) signal level 4 is ICl3. These signal levels 1 to 4 are amplified by the first and second amplification circuits 13 and 14, and then the first and second amplification circuits 13 and 14 correspond to the respective bands. Second. Third. Fourth demodulation system 31, 32°3
3.34 and individually demodulated. These demodulation systems 31, 3
2゜33, 34 are composed of a bandpass filter, an amplifier circuit, and a detection circuit, and the respective demodulation systems 3], 32
．． The output from 33.degree. 34 is input to a decoder 35 to obtain the final demodulated signal 18.

In this example, four-level frequency modulation is performed, so the band occupied by the signal is 2 GHz as shown in Figure 5.
However, by using an image rejection mixer, the bandwidth required on the receiving side can be reduced to 1.
．． It can be kept down to 2GH'Z. When transmitting a four-value signal, the signal light source 1 may be modulated at 50 Mb/s in order to transmit information at 100 Mb/s. Therefore, the reception band can be narrowed and reception sensitivity can be improved. Therefore, it was possible to demodulate information at 100 Mb/s using extremely weak light of -58 dBm.

Since FSK modulation with a constant transmission level is used here, it is possible to allocate this improved reception sensitivity to the transmission path, and long-distance transmission can be realized.

In addition to the above-described embodiments, various modifications of the present invention can be considered. For example, as shown in FIG. 6, it is also possible to set the frequencies of the two lights so that the signal light is arranged asymmetrically with respect to the local oscillation light. In this case, since the frequency range of the signal output from the first output port of the 3dB coupler and the frequency range of the signal output from the second output port are different, it is possible to suppress the influence of leakage in the image band. can.

In the case of multilevel FSK modulation, the first . It is also possible to use a frequency-discriminative detector to demodulate the output from the second output port.

〔Effect of the invention〕

As explained above, according to the present invention, in the FSK optical heterodyne detection optical communication system, a semiconductor laser is used as a light source on the transmitting side, and even when a large amount of frequency shift is taken during FSK modulation, the signal is not required on the receiving side. It is possible to keep the bandwidth narrow. As a result, it is possible to achieve high optical reception sensitivity with low noise on the receiving side, and there is also an effect that speeding up is facilitated.

[Brief explanation of the drawing]

Fig. 1 is a block diagram showing an optical communication device for realizing an embodiment of the present invention, Fig. 2 is a block diagram of an image rejection mixer, and Fig. 3 shows signal light and local oscillation light. FIG. 4 is a block diagram showing an optical communication device for realizing another embodiment of the present invention. FIGS. 5 and 6 are frequency allocations of signal light and local oscillation light. FIG. l...Signal light source 2...Optical fiber 3...Signal light 4...First polarization controller 5...Local oscillation light source 6... - Local oscillation light 7... Second polarization controller 8... Optical multiplexer 9... Polarization beam splitter 10... First optical receiver 11... ... Second optical receiver 12 ... 3 dB amplifier 13 ... First amplifier circuit 14 ... Second amplifier circuit 15 ... First detection circuit 16 ... Second detection circuit 17 ... Differential amplifier 18 ... Demodulated signal 19 ... Image rejection mixer 20 ...
...Signal source 23...First output port 24...Second output port 25...First demodulated signal 26...Second demodulated signal 31.32, 33.34 ... Demodulation system 35 ...
decoder

Claims (2)

[Claims] (1) In the optical heterodyne detection optical communication method, in which the optical transmission side transmits a signal light frequency shift modulated by an information signal and the optical reception side obtains a demodulated signal from the received signal light, the oscillation frequency of the local oscillation light is determined. The signal light is set within the frequency band of the signal light, the polarization states of the signal light and the local oscillation light are adjusted, and the signal light with the adjusted polarization state and the local oscillation light are combined, and the signal light is obtained by this combination. The combined light is divided into two orthogonal linearly polarized components, each of these two linearly polarized components is optically heterodyne-detected to form two intermediate frequency signals with a phase difference of 90° from each other, and these two intermediate frequency signals are The combined intermediate frequency signal is divided into two signals based on the frequency arrangement of the signal light and the local oscillation light, each of these two signals is demodulated, and the two demodulated signals are combined. An optical heterodyne detection optical communication method characterized by obtaining a demodulated signal by. (2) In the optical heterodyne detection optical communication method according to claim 1, an image rejection mixer is used to adjust the polarization states of the signal light and the local oscillation light, and these polarization states are adjusted. The signal light and the local oscillation light are combined, the combined light obtained by this combination is divided into two orthogonal linearly polarized components, and each of these two linearly polarized components is optically heterodyne detected to detect a phase difference of 90° from each other. forming two intermediate frequency signals, combining these two intermediate frequency signals, and dividing the combined intermediate frequency signal into two signals based on the frequency arrangement of the signal light and the local oscillation light. Characteristic optical heterodyne detection optical communication method.