JPH0683142B2 - Optical communication method by frequency shift keying - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an optical communication method by frequency shift keying capable of obtaining a demodulated signal having a correct code polarity on the receiving side.

[Prior Art and its Problems] In recent years, the performance and characteristics of semiconductor lasers have been improved, and semiconductor lasers that oscillate in a single-axis mode and have high spectral purity have come to be obtained. For this reason, a coherent optical transmission system using information on the frequency and phase of light has become feasible, and a highly sensitive optical communication system has become feasible.
In particular, in the case of an optical heterodyne communication method using binary frequency modulation (FSK) using frequency information, for example, the paper “Coherent Optical Fiber Transmission Modulation and Demodulation Technology” by Saito et al. Number 12 (198
2), page 2173), the output light of the semiconductor laser can be directly frequency-modulated by slightly changing the injection current of the semiconductor laser in response to the signal, and a system with low loss can be simplified. It has the feature that it can be configured.

By the way, in the case of optical heterodyne detection and demodulation of the binary frequency-modulated signal light using the local oscillation light on the receiving side, the mark signal is shifted to the low frequency side due to the relative relationship between the frequencies of the signal light and the local oscillation light. There are two cases, that is, when the space signal is on the low frequency side. However,
Conventionally, the receiving side cannot obtain correct information about the setting position of the oscillation frequency of the local oscillation light. As a result, the sign polarity (“1”, “0”) of the output signal may be inverted and correct sign information may not be obtained. It had the problem of occurring. In addition, in the case of multi-valued frequency modulation, the probability that the oscillation frequency of the local oscillation light is set correctly becomes lower, and the above problem becomes more serious.

As another optical transmission method using frequency information,
A method of converting a frequency shift keying wave to a light intensity keying wave at the receiving side using an optical frequency discriminator has been considered (Paper by Yamamoto "Basic study of various optical digital modulation / demodulation methods",
IEICE Technical Report, CS79-144, 1979). In this case, a Fabry-Perot etalon, etc. is used as the optical frequency discriminator, but it is difficult to obtain information on whether the sign polarity of the output signal is reversed or not even with this optical frequency discriminator, and the correct sign polarity is obtained. However, there is a problem that it is difficult to demodulate a signal having a.

[Object of the Invention]

An object of the present invention is to solve the above problems, and to provide an optical communication method by frequency shift keying that allows a receiving side to always demodulate a signal having a correct code polarity.

[Structure of Invention]

The present invention relates to an optical communication method by frequency shift modulation, which emits a signal light frequency-shift-modulated by a transmission signal from a transmission unit and demodulates the signal light incident on a reception unit to obtain a demodulated signal. An identification pulse signal is inserted in the signal, the identification pulse signal is detected by the receiving section, its code polarity is identified, and the signal light is demodulated with the correct code polarity with reference to the code polarity of the identification pulse signal. The feature is that it is controlled as follows.

[Action]

In the present invention, for example, when explaining an optical heterodyne communication method, the identification pulse signal is inserted in the pulse train of the transmission signal, the demodulation signal is the same mark as the transmission signal based on the code polarity of the identification pulse signal detected by the receiving unit, the space The output wavelength of the local oscillation light source with respect to the signal light is set so as to have a correct relationship so as to have the polarity.
For example, when seven codes of 1,0,1,0,0,1,0 are sequentially inserted into the signal pulse train in this order as a discrimination pulse signal in a constant cycle, the wavelength of the local oscillation light source becomes the signal light. On the other hand, if the relationship between the mark and the space is not correct and the relationship between the mark and space is reversed, the identification pulse signal on the receiving side is 0,1,0,
Since 1,1,0,1 is read, it is possible to recognize the setting error of the wavelength of the local oscillation light. Therefore, based on this recognition result, if the wavelength of the local oscillation light source is reset to the correct relationship with respect to the wavelength of the signal light, or if 1,0 of the demodulated signal is inverted by using an electric means and used, A signal with the correct code polarity can be demodulated.

〔Example〕

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

FIG. 1 shows an apparatus configuration according to the first embodiment of the present invention, and FIG. 2 shows a relative relationship in wavelength between a local oscillation light and a signal light and a demodulated identification pulse signal. In FIG. 1, 1 is a transmitter, 2 is a receiver, and the transmitter 1 and the receiver 2 are connected by an optical fiber transmission line 3.

The transmission unit 1 is configured by connecting the multiplex conversion circuit 4, the drive circuit 5, and the light source unit 6 in cascade. The multiple conversion circuit 4 includes, for example, 100 Mb / s signals 7, 8, 9, 1 of the first to fourth series.
0 is input, and the transmission signal of 400 Mb / s is time-division multiplexed here.
It will be 11. At this time, the identification pulse signal 12, which also serves as a frame synchronization signal, is inserted into the transmission signal 11 at regular intervals. Identification pulse signal 12 is a pulse train P ₁ as example shown in Figure 2
It is a signal of repetition of P ₇ to P ₇ and is inserted every 125 μs in the pulse train of the transmission signal 11, that is, one every 50,000 pulses. The transmission signal 11 thus obtained is amplified to a required level by the drive circuit 5 and then sent to the light source unit 6. The light source unit 6 is configured by using a semiconductor laser, and directly outputs the signal light 13 which is binary frequency shift keyed by an applied current having strength corresponding to the transmission signal 11. The signal light 13 propagates through the optical fiber transmission line 3 and then enters the receiver 2.

In the receiver 2, the incident signal light 13 is combined with the local oscillation light 16 provided from the local oscillation light source 15 in the multiplexing circuit 14, and thereafter, optical heterodyne detection is performed in the photodetector 17 and converted into the intermediate frequency signal 18. To be done. The intermediate frequency signal 18 is amplified to a required level by an amplifier 19 and then given to a demodulation circuit 20 and a frequency discriminator 21. Demodulation circuit 20 is an intermediate frequency signal
18 is converted into a demodulation output 22 in the base band, and the demodulation output 22 is output from the receiving section 2 and the code identification circuit 23. The code identifying circuit 23 detects the identification pulse signal 12.

Reference numeral 24 is a wavelength control processor, and the wavelength control processor 24 is a frequency error signal 25 from the frequency discriminator 21 and a code identification circuit 23.
The input signal is output to output a command signal to the temperature control circuit 26 and the applied current control circuit 27 that control the operation of the local oscillation light source 15. The wavelength control processor 24 is a well-known program control device using a microprocessor. Further, the local oscillation light source 15 is composed of the same semiconductor laser as the light source unit 6, and the oscillation wavelength is controlled by the temperature control circuit 26 and the applied current control circuit 27. Therefore, in the steady operation of the local oscillation light source 15, the wavelength control processor 24 activates the temperature control circuit 26 and the applied current control circuit 27 based on the frequency error signal 25 from the frequency discriminator 21 to change the wavelength of the local oscillation light 16 to the signal light. It is carried out by stabilizing at 13 wavelengths.

Next, in the above configuration, the identification pulse signal 12 in the demodulation output 22
The control operation based on the state will be described with reference to FIG. 2A shows the case where the relative relationship between the local oscillation light 16 and the signal light 13 is correct, and FIG. 2B shows the case where the relative relationship is not correct. As shown in FIG. 2 (A), when the relationship between the wavelength λ _L of the local oscillation light 16 and the wavelength λ s of the signal light 13 is correct, the identification pulse signal included in the demodulation output 22 obtained from the intermediate frequency 18 12 is a pulse with the correct polarity sign
It is composed of P _{1 to} P ₇ . On the other hand, as shown in FIG. 2B, the wavelength λ _L of the local oscillation light 16 and the wavelength λ s of the signal light 13
When the relationship is not correct, the signal included in the demodulation output 22 is the inverted identification pulse signal with the sign polarity reversed.
Since it is 28, the code identifying circuit 23 has a feature that it is not detected as the identification pulse signal 12.

Therefore, if the identification pulse signal 12 is not detected by the code identification circuit 23 within a predetermined time (for example, about 3 seconds) after the intermediate frequency signal 18 of an appropriate level is input to the demodulation circuit 20, the wavelength control processor 24 Determines that the sign polarity is inverted, and operates the temperature control circuit 26 to change the wavelength of the local oscillation light 16 emitted from the local oscillation light source 15 toward the other tuning wavelength range. As a result, the local oscillation light
When the wavelength relationship between 16 and the signal light 13 is correct as shown in FIG. 2 (A) and the identification pulse signal 12 is detected by the code identification circuit 23, the wavelength control processor 24 determines that the intermediate frequency
Only the wavelength control of the locally oscillated light 16 is performed so that the center frequency f _{I of} 18 is kept at a predetermined value (for example, 800 MHz), and a steady operation state is entered.

As described above, by including the identification pulse signal 12 in the transmission signal 11, the identification pulse signal 12 in the code identification circuit 23
It is possible to judge whether or not the sign polarity is reversed in the detection state of. Therefore, if the wavelength of the locally oscillated light 16 is controlled based on such a judgment, the demodulated signal having the correct code polarity can always be obtained in the receiver 2.

The operation principle of the code identification circuit 23 is the same as that of a frame synchronization signal detection circuit used in a normal pulse transmission device, and the pulse output is extracted every 50,000 pulses in the pulse train of the demodulation output 22 and is determined in advance. The operation of determining whether or not a signal that matches the identification pulse signal 12 is obtained. This technique is well known.

FIG. 3 shows only the receiving section of the device configuration according to the second embodiment of the present invention. The transmitter is the same as that in the first embodiment, so the description thereof is omitted. In this embodiment, a code inversion circuit 29 is provided in the next stage of the demodulation circuit 20, the demodulation output 22 of the demodulation circuit 20 is input to the code identification circuit 23 and the code inversion circuit 29, and the output signal of the code identification circuit 23 is It is configured to be applied to the sign inversion circuit 29. The other circuit configuration and the operation of each component are the same as those in the first embodiment.

In the above configuration, when the code identification circuit 23 detects the inverted identification pulse signal 28 shown in FIG. 2 (B) in the demodulation output 22 from the demodulation circuit 20, the code inversion circuit 29 outputs the demodulation output 22. The code polarity can be inverted, and the demodulation output 30 having the correct code polarity can be output from the receiving unit 2. When the code identification circuit 23 does not detect the inverted identification pulse signal 28, the code inversion circuit 29 does not operate, and the demodulation output 22 is output as it is from the reception section 2 as the demodulation output 30.

According to the second embodiment, when the sign polarity of the identification pulse signal 12 is inverted, the entire "1", "0" of the demodulation output 22 is directly
Can be inverted to obtain a demodulated output with the correct code polarity. In this embodiment, the local oscillation light 16 and the signal light 13 are
This has the advantage that there is no need to re-set the positional relationship of the wavelengths of.

FIG. 4 shows only the receiving section of the device configuration according to the third embodiment of the present invention, and FIG. 5 shows the characteristics of the optical frequency discriminator and the demodulated identification pulse signal. The transmitter is the same as that in the first embodiment, so the description thereof is omitted. In this embodiment, the receiving unit 2
The signal light 13 incident on is passed through the optical frequency discriminator 31 and given to the photodetector 17, converted into a demodulation output 22 in the baseband by the photodetector 17, and then electrically amplified to a required level by the amplifier 19. Is amplified to. The optical frequency discriminator 31 is
A Fabry-Perot etalon is used, and it has a light transmission characteristic 32 as shown in FIG. 5 (A), and can convert the frequency shift modulation wave 33 into a light intensity modulation wave 34. In this case signal light
It is necessary to match the wavelength λs of 13 and the central wavelength λc of the light transmission characteristic 32 as shown in FIG. When the wavelength λs and the central wavelength λc are combined as shown in FIG. 5 (B), the obtained light intensity modulated wave 35 is inverted as compared with the case of FIG. 5 (A). The inverted discrimination pulse signal 28 will be included in the demodulation output 22. Reference numeral 23 is the code identifying circuit, and 29 is the code inverting circuit, which are configured similarly to the case of the second embodiment.

According to the above configuration, the demodulation output 22 output from the amplifier 19
When the code identification circuit 23 detects the inverted identification pulse signal 28 shown in FIG. 5 (B), the code inversion circuit 23 is activated to invert "1", "0" of the demodulation output 22 to obtain the code polarity. You get the correct demodulation output of 30. When the code identification circuit 23 does not detect the inverted identification pulse signal 28, the demodulation output 22 is directly output from the receiving section 2 as the demodulation output 30.

According to the third embodiment, the optical frequency discriminator 31 is used to convert the signal light 13 which is a frequency shift modulated wave into a light intensity modulated wave for demodulation. In this case, the center wavelength λc of the light transmission characteristic of the optical frequency discriminator 31 can be arbitrarily changed by adjusting the interval between the first and second etalon plates 36, 37 of the Febri-Perot etalon. In this embodiment,
Information related to the demodulation output 22 is fed back to the optical frequency discriminator 31 via the pulse peak value detection circuit 38 and the drive circuit 39,
In the demodulation output 22, the pulse crest value detection circuit 38, the drive circuit 39, and the etalon plate interval control electrostrictive element (not shown) are operated so that the mark value becomes almost maximum under the minimum space value. .

Although the embodiments of the present invention have been described above, the present invention is not limited to these embodiments, and it goes without saying that various modifications and changes can be made within the scope of the present invention.

For example, in the above embodiment, the identification pulse signal 12 is common to the frame synchronization signal, but it may be provided separately, and the code length and code configuration can be arbitrarily selected as long as they are appropriate. Further, in the embodiment, an example of binary frequency shift keying is shown, but the present invention can be similarly applied to the case of multivalues of three values or more.

In the first embodiment, the wavelength control processor 24 is activated when the identification pulse signal 12 is not detected by the code identification circuit 23 within a predetermined period after the intermediate frequency signal 18 is generated. It is also possible to activate the wavelength control processor 24 upon detection. This also applies to the second and third embodiments, and the wavelength relationship between the signal light 13 and the local oscillation light 16 is determined by using a code identification circuit that can detect either the identification pulse signal 12 or the inverted identification pulse signal 28. May be confirmed, and then the sign inversion circuit 29 may be operated.

〔The invention's effect〕

As described above, according to the present invention, in the optical communication method by frequency shift keying, the identification pulse signal is included in the transmission signal on the transmission side, and the identification pulse signal is extracted on the reception side to invert the code polarity. Since it is possible to determine whether or not it is possible to demodulate a signal having a correct code polarity on the receiving side, it is possible to easily realize an optical communication device having high reliability with respect to the code polarity. Demonstrate.

[Brief description of drawings]

FIG. 1 is a block diagram showing a first embodiment of the present invention, and FIG. 2 is an explanatory diagram showing a relative relationship in wavelength between local oscillation light and signal light, an intermediate frequency signal, and a demodulated identification pulse signal. FIG. 3 is a block diagram of essential parts showing a second embodiment of the present invention, FIG. 4 is a block diagram of essential parts showing a third embodiment of the present invention, and FIG. 5 is an operating characteristic of the optical frequency discriminator and a discrimination pulse signal. FIG. 1 …… Sending unit 2 …… Reception unit 3 …… Optical fiber transmission line 6 …… Light source unit 11 …… Transmission signal 12 …… Identification pulse signal 13 …… Signal light 14 …… Combining circuit 15 …… Local oscillation light source 16 …… Local oscillation light 18 …… Intermediate frequency signal 20 …… Demodulation circuit 22 …… Demodulation output 23 …… Sign identification circuit 24 …… Wavelength control processor 29 …… Sign inversion circuit 31 …… Optical frequency discriminator

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Claims (1)

[Claims] 1. An optical communication method using frequency shift modulation, wherein a signal light, which is frequency-shift-modulated by a transmission signal, is emitted from a transmitter, and the signal light incident on a receiver is demodulated to obtain a demodulated signal. An identification pulse signal is inserted in the transmission signal, the identification pulse signal is detected by the receiving section, its code polarity is identified, and the signal light is demodulated with the correct code polarity with reference to the code polarity of the identification pulse signal. An optical communication method using frequency shift keying, which is characterized in that it is controlled.