JPH0775337B2 - Coherent optical transmission method and device - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a coherent optical transmission method and apparatus for performing long-distance and large-capacity transmission of various signals using light.

2. Description of the Related Art As a signal transmission method using an optical fiber, a brightness modulation method that uses light as power is the mainstream. However, in recent years
The so-called coherent transmission method, which performs optical transmission using the amplitude, frequency, and phase of light, has been receiving attention because of its high reception impression and large transmission capacity.

There are various methods of performing coherent transmission, but a method of using a semiconductor laser as a light source and changing the current of the semiconductor laser to perform frequency modulation of light (optical FM method) is often used.

The optical FM method is often used because the frequency of the light is modulated by directly changing the current of the semiconductor laser with a signal (optical F
This is because M) can be done easily.

In addition, a method of performing optical frequency shift keying (optical FSK method) in a similar method is also used for digital signal transmission.

Regarding the light source, distributed feedback semiconductor laser (DFB-LD)
By using, a light source with a narrow spectral line width can be easily obtained.

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention As described in the conventional example, in the coherent transmission method, the optical FM transmission method in which the current of the semiconductor laser is directly modulated with a signal is mainly used. However, a drawback of this method is that the amount of oscillation frequency change due to current modulation of the semiconductor laser depends on the frequency of the modulation current (frequency dependence of FM modulation sensitivity of the semiconductor laser). The FM modulation sensitivity of this semiconductor laser is large in the frequency range of 100 kHz to several tens of MHz, changes by about two digits, and has complicated frequency dependence.

Therefore, when the current of the semiconductor laser is driven by the signal including this frequency region to perform the optical FM, the signal obtained by FM demodulating the received light has the frequency characteristic corresponding to the above frequency dependency. As mentioned above, this frequency dependence is 2
It is difficult to electrically compensate because there is a change in the order of magnitude and the change is complicated.

Similar problems also exist in the method of performing frequency shift keying of light with a digital signal (optical FSK). Digital signals occupy a wide band from a relatively low frequency to a transmission clock frequency. As a result, the part of the FM modulation sensitivity that has a large frequency dependency is included in the band, and the demodulated signal waveform becomes distorted. In order to discriminate such a waveform and reproduce the original digital signal without error, it is necessary to use a larger value than in the case without distortion.
Requires S / N ratio. Further, when a semiconductor laser having a large frequency dependency is used, the identification may not be possible.

In view of such conventional problems, the present invention can easily and accurately compensate for the frequency dependence of the FM modulation sensitivity of a semiconductor laser, and even if the coherent transmission characteristics are poor, the characteristics of the transmission signal itself can be improved. It is an object of the present invention to provide a coherent optical transmission method and device capable of maintaining the same.

Means for Solving the Problems The coherent optical transmission method of the present invention modulates a signal to be transmitted in advance, and modulates the power source for driving a semiconductor laser with the modulation signal in such a small amount that a luminance modulation component of output light can be ignored. Then, the frequency of light is modulated and transmitted.

In the coherent optical transmission device of the present invention, a modulator for modulating a signal to be transmitted, and a luminance modulation component of output light are minutely current-modulated by the output of the modulator to a negligible amount to perform frequency modulation of oscillation light. An optical transmitter having a semiconductor laser, and a semiconductor laser for local oscillation that oscillates at a frequency equal to or close to the oscillation frequency of the semiconductor laser,
Light having a photoelectric conversion element that mixes the received light from the semiconductor laser and the semiconductor laser light for local oscillation to perform photoelectric conversion, an FM demodulator that FM demodulates the output, and a demodulator that demodulates the output And a receiver.

Further, the coherent optical transmission method of the present invention electrically FSKs a digital signal once, and directly modulates the current of the semiconductor laser with the signal by such a small amount that the luminance modulation component of the output light can be ignored, and changes the frequency of the light. Modulate and transmit.

Further, in the coherent optical transmission device of the present invention, a modulator for FSK-modulating a digital signal, and a luminance modulation component of output light at the modulator output are minutely current-modulated to such an extent that they can be ignored, and frequency modulation of oscillated light is performed. An optical transmitter having a semiconductor laser, a local oscillation semiconductor laser that oscillates at a frequency equal to or close to the oscillation frequency of the semiconductor laser, and a received light from the semiconductor laser and the local oscillation semiconductor laser light are mixed. Photoelectric conversion element for performing photoelectric conversion, an FM demodulator for FM modulating its output, and FSK for its output
And an optical receiver having an FSK demodulator for demodulation.

The present invention limits the band required for transmission to the high frequency region by premodulating the signal to be transmitted. By modulating the driving current of the semiconductor laser with this signal by a small amount,
This is to perform coherent transmission by performing optical FM without using the region where the frequency dependence of the FM modulation sensitivity of the semiconductor laser changes intricately.

If the signal to be transmitted is a digital signal,
FM modulation of the semiconductor laser light is performed without using the region in the relatively low frequency range where the frequency dependence greatly changes by modulating the driving current of the semiconductor laser by a small amount with an electrically FSK-modulated signal. Coherent transmission is performed.

Embodiment An embodiment of a coherent transmission device according to the present invention is shown in FIG.

The optical transmitter 100 will be described first. The signal 1 to be transmitted is modulated by the modulator 4 using the output 3 of the carrier oscillator 2. The correction circuit 5 corrects the frequency characteristic of the modulator output as described later, and the current of the semiconductor laser 6 is modulated. The current of the semiconductor laser 6 is minutely changed within a range in which the brightness modulation component of the output light can be ignored, and the laser oscillation output light 7 undergoes frequency modulation corresponding to the current. The amount of change in current is set to such an extent that the frequency modulation component is not affected by the brightness modulation component when the frequency modulation light is demodulated. By doing so, the amount of FM deviation of the laser becomes proportional to the amount of current. As the semiconductor laser 6, a wavelength-stabilized single mode LD (for example, DFB-LD) is used.

Here, how the laser light is modulated will be described with reference to FIG. In FIG. 2, the vertical axis represents the FM modulation sensitivity of the semiconductor laser, and the horizontal axis represents the current modulation frequency. A shows the frequency dependence of the FM modulation sensitivity of a typical semiconductor laser. In the low frequency region, the change in frequency due to heat is main, and in the high frequency region, the change in carrier density is main. Middle area (number 10)
MHz), the two are balanced and have flat characteristics. B indicates the frequency band of the signal obtained by modulating the transmission signal, and a region not affected by heat is used as the FM modulation sensitivity.

The light thus frequency-modulated is guided to the transmission medium 8. An optical fiber is often used as the transmission medium 8, but any medium that transmits light may be used.

The light transmitted by the medium 8 is guided to the optical receiver 101. The received light 9 is mixed by the mixer 12 with the output light 11 from the local oscillation semiconductor laser 10 that oscillates at a frequency equal to or close to the oscillation frequency of the semiconductor laser 6 in the optical transmitter 100, and is mixed in the photoelectric conversion element 13. Be guided. In this mixing or photoelectric conversion part, an automatic polarization plane controller or a polarization diversity method is used to avoid fluctuations in the polarization plane due to fiber transmission, but the present invention is essentially Any method may be used as it is not relevant. Also,
When the local oscillation frequency is equal to the oscillation frequency of the semiconductor laser in the transmitter, homodyne detection is performed, and when the frequencies are close but not equal, heterodyne detection is performed. The photoelectric conversion element 13 extracts the beat components of the two mixed laser lights and outputs them as an electric signal.
By demodulating the output by the FM demodulator 14, a signal modulated by the modulator in the optical transmitter 100 can be obtained.
The original signal is reproduced by demodulating this signal by the demodulator 15.

Here, the FM modulation sensitivity of the semiconductor laser will be described again. As described with reference to FIG. 2, in the present invention, FM
The part where the frequency dependence of the modulation sensitivity changes intricately is not used, but the part where the frequency dependence is used is used. Therefore, it is necessary to correct the influence of this frequency dependence.

This correction is (1) so that the frequency characteristic of the modulator output 5 in the optical transmitter has a frequency characteristic opposite to the frequency dependency of the laser, and (2) the same after FM demodulation by the optical receiver. Make a correction, which is possible.

The correction method of (1) provides better transmission characteristics. This is because the frequency shift amount of the optical FM becomes constant regardless of the modulation signal frequency, and thus it is possible to compensate for the S / N of the signal in a good state. The correction circuit 5 in FIG. 1 is a circuit for performing the correction by the first method described here. When the correction is performed by the second method, the correction circuit is the FM demodulation circuit 14 and the demodulator.
Will be placed between 15.

For the modulation method of the modulator in the optical transmitter, AM,
FM, PM, etc. are conceivable, but in coherent transmission, the C / N and waveform distortion characteristics are not so good at present, so it is optimal to use FM modulation. At this time, in the light condition, FM−
It becomes FM modulation. Needless to say, the transmission band is wider than that of AM-FM modulation, but considering the S / N of the demodulated signal and the effect of distortion, FM-FM is suitable.

Next, FIG. 3 shows an embodiment in which the transmission signal is a digital signal.

The optical transmitter 102 will be described first. The digital signal 20 to be transmitted is FSK modulated by the FSK modulator 21. The frequency characteristic of the modulator output is corrected by the correction circuit 22, and the current of the semiconductor laser 23 is modulated. Due to the current modulation of the semiconductor laser 23, the laser oscillation output light 24 undergoes frequency modulation corresponding to the current.

The light thus frequency-modulated is guided to the transmission medium 25. An optical fiber is often used as the transmission medium 25, but any medium that transmits light may be used.

The light transmitted by the medium 25 is guided to the optical receiver 103. The received light 26 is mixed with the output light 28 from the local oscillation semiconductor laser 27 that oscillates at a frequency equal to or close to the oscillation frequency of the semiconductor laser 23 in the optical transmitter 102 by the mixer 29, and then the photoelectric conversion element 30. Be guided. The photoelectric conversion element 30 detects the beat component of the two mixed laser beams and outputs 31 as an electric signal. By demodulating the output 31 by the FM demodulator 32, an FSK modulated signal can be obtained. By demodulating this signal by the FSK demodulator 33, the original digital signal is reproduced.

The power spectrum of a digital signal is usually spread over a wide band from a considerably low frequency part to a transmission clock. Therefore, as in the present invention, once the digital signal is FSK-modulated to eliminate the low frequency component, the frequency characteristic can be easily compensated as in the case of the analog described above.

It is assumed that the signal to be transmitted is once PCM encoded as a digital signal. In this case, the signal to be transmitted is
After being PCM-encoded once, it is FSK-modulated, and then optical FM is performed for transmission. Therefore, even if the characteristics of the optical transmission system (that is, the coherent transmission characteristics) are slightly poor, the signal to be transmitted is transmitted with good characteristics.

EFFECTS OF THE INVENTION When the present invention is used, the frequency dependence of the FM modulation sensitivity of the semiconductor laser can be easily and accurately compensated, so that any frequency dependence of the semiconductor laser can be used. Further, even if the coherent transmission characteristic is poor, the characteristic of the transmission signal itself can be kept good.

[Brief description of drawings]

FIG. 1 is a block diagram of an embodiment of the present invention, FIG. 2 is a frequency dependence characteristic diagram of FM modulation sensitivity of a semiconductor laser, and FIG. 3 is a block diagram of another embodiment of the present invention. 100 ... Optical transmitter, 2 ... Carrier oscillator, 4 ... Modulator, 6 ... Semiconductor laser, 8 ... Transmission medium, 101 ...
Optical receiver, 10 ... local oscillation semiconductor laser, 12 ... mixer, 13 ... photoelectric conversion element, 14 ... FM demodulator, 15 ... demodulator, 102 ... optical transmitter, 20 ... digital Signal, 21 …… F
SK modulator, 23 ... Semiconductor laser, 25 ... Transmission medium, 10
3 …… Optical receiver, 26 …… Received light, 27 …… Local oscillation semiconductor laser, 29 …… Mixer, 30 …… Photoelectric conversion element, 32 ……
FM demodulator, 33 ... FSK demodulator.

Claims (5)

[Claims] 1. A signal to be transmitted is pre-modulated, a driving current of a semiconductor laser is modulated by the modulation signal by such a small amount that a luminance modulation component of output light can be ignored, and the frequency of light is modulated and transmitted. A coherent optical transmission method characterized by the above. 2. A modulator for modulating a signal to be transmitted, and a semiconductor laser for performing frequency modulation of oscillated light by current-modulating the output light of the modulator so that a luminance modulation component of the output light is negligible. An optical transmitter having the same, a local oscillation semiconductor laser that oscillates at a frequency equal to or close to the oscillation frequency of the semiconductor laser, and received light from the semiconductor laser and the semiconductor laser light for local oscillation are mixed to perform photoelectric conversion. A coherent optical transmission device comprising: a photoelectric conversion element, an FM demodulator for performing FM demodulation of its output, and an optical geophone having a demodulator for demodulating its output. 3. The coherent optical transmission device according to claim 2, wherein an FM modulator is used as the modulator. 4. A digital signal is temporarily FSKed electrically, and the current of the semiconductor laser is directly modulated by the signal by such a small amount that the brightness modulation component of the output light can be ignored, and the frequency of the light is modulated for transmission. A coherent optical transmission method characterized by the above. 5. A modulator for FSK modulating a digital signal,
An optical transmitter having a semiconductor laser in which the intensity modulation component of the output light is negligibly current-modulated by the modulator output to frequency-modulate the oscillation light, and the oscillation frequency of the semiconductor laser is equal to or A semiconductor laser for local oscillation that oscillates at a close frequency, a photoelectric conversion element that performs photoelectric conversion by mixing received light from the semiconductor laser and the semiconductor laser light for local oscillation, and an FM demodulator that FM demodulates its output. And an optical receiver having an FSK demodulator for FSK demodulating the output thereof, and a coherent optical transmission device.