1.  Field of the Invention
The present invention generally relates to a direct modulation phase shift keying (DM-PSK) transmission system for coherent optical fiber transmission in which an injection current supplied to a semiconductor laser is directly modulated and, more particularly, to a DM-PSK transmission system presenting useful features in automatic frequency control, demodulation, and phase noise suppression. 
An intensity modulation/direct detection (IM/DD) system in which intensity-modulated light is directly detected by a photodiode to be converted into an electrical signal is generally practiced today as one of the optical fiber transmission systems.  On the other hand, to meet requirements for increased transmission capacity, increased transmission distance, etc.,  coherent optical fiber transmission systems are being earnestly studied.  According to this system, coherent light from a semiconductor laser is used as the carrier, the carrier is modulated with its frequency, phase, etc.  modified, and the light received by the receiver is mixed with local light so that heterodyne detection or homodyne detection is performed.  Accordingly, a great enhancement in the reception sensitivity over that obtained in the IM/DD system can be achieved. 
Further, since precise frequency selection can be achieved relatively easily after the optical detection has been performed, i.e., after the optical signal has been converted into an electrical signal, frequency-division multiplexing at a higher density becomes possible so that the transmission capacity of a single optical transmission line can be largely increased. 
2.  Description of the Related Art
A differential phase shift keying (DPSK) transmission system and a continuous phase frequency shift keying (CPFSK) transmission system are known as systems for having information to be transmitted carried by wave parameters of the light emitted from a semiconductor laser and suitable for high speed transmission. 
A block diagram of the DPSK transmission system is shown in FIG.  26.  On the transmitter side, reference numeral 201 denotes a transmission light source formed of a semiconductor laser oscillating at fixed amplitude and frequency and 202 denotes a phase modulator for modulating the phase of light from the transmission light source 201.  In order that the demodulation by means of a one-bit delayed signal is performed on the receiver side, input data is previously modified on the transmitter side into a differential code by a differential coder 203 and the code is supplied to the phase modulator 202 through an amplifier 204.  The light transmitted to the receiver side through an optical fiber 205 is combined in an optical coupler 206 with local light emitted from a local light source 207 formed of a semiconductor laser oscillating at fixed amplitude and frequency and the combined light is input to an optical detecting circuit 208 formed of a photodiode.  When the combined light of the received signal light and the local light is input to the optical detecting circuit 208, an IF signal (intermediate-frequency signal) which carries the transmitted information in the form of phase shift, for example, is produced by virtue of the square law detection characteristic of the photodiode, and this IF signal is input to a demodulator 209.  In the demodulator 209, the input IF signal is split into two portions, and one portion is delayed by a time corresponding to one bit (one time slot, i.e., the reciprocal of the bit rate) in a delay circuit 210 and mixed with the other intact portion of the IF signal in a mixer 211, and thereby, the transmitted information is regenerated.  Since, demodulation is achieved in the DPSK transmission system through comparison of one signal with the other signal preceding it by one bit in the demodulator 209, the transmitter side when performing the phase modulation is required to apply differential coding to the input signal. 
A block diagram of a CPFSK transmission system is shown in FIG.  27.  On the transmitter side, reference numeral 221 denotes a transmission light source of which oscillation frequency is variable and 222 denotes a modulation circuit for modulating the oscillation frequency of the transmission light source 221.  The modulation circuit 222 controls the shift amount of the oscillation frequency in accordance with the input signal such that the phase shift amount between different signs becomes over .pi.. The light transmitted to the receiver side through an optical fiber 223 is combined with local light from a local light source 225 in an optical coupler 224 and then subjected to optical-electrical conversion in an optical detecting circuit 226.  The IF signal produced through the conversion is input to a demodulator 227, wherein it is demodulated by being mixed with a signal a predetermined time .tau.  delayed by a delay circuit 228.  The delay time .tau.  provided in the delay circuit 228 is dependent on the modulation index m. These parameters are in the following relationships.  EQU .tau.=T/2m, m=.DELTA.F/B,
where T represents the time of one time slot, .DELTA.  F represents the frequency shift amount, and B represents the bit rate.  Thus, in the CPFSK transmission system as described above, the transmitter side performs direct modulation of the frequency, not using an external modulator, and the receiver side detects the phase shift to thereby regenerate the transmitted information. 
In the coherent optical fiber transmission systems of heterodyne detection type shown in FIG.  26 and FIG.  27, if the center frequency of the IF signal deviates from the established frequency, the demodulated waveform is distorted and the reception sensitivity is deteriorated.  Therefore, it is preferred that automatic frequency control (AFC) is provided for the center frequency of the IF signal. 
Automatic frequency control means of an asynchronous type applicable to the DPSK transmission system and CPFSK transmission system will be described with reference to FIG.  28.  In this example, it is adapted such that the IF signal from the optical detecting circuit 208 (226) is split into two portions and one of which is converted into a voltage signal in a frequency-to-voltage conversion circuit 231, the voltage signal is converted into a current signal in a voltage-to-current conversion circuit 232, and this current signal is negatively fed back to the local oscillation light source 207 (225).  Reference numeral 233 denotes a bandpass filter for cutting off undesired signals other than the IF signal produced by optical detection. 
The asynchronous AFC is simple in the circuit configuration but has a demerit that its frequency controlling accuracy is low. 
As for demodulators, there are those of synchronous type and asynchronous type.  In case of polyphase PSK modulation and demodulation, for example, the synchronous type provides better reception sensitivity than the asynchronous type, and therefore, realization of synchronous demodulators are strongly desired, An example of a synchronous demodulator so far proposed is shown in FIG.  29.  A detected signal from an optical detecting circuit 208 is passed through the bandpass filter 233, which has a passband ranging from the frequency f.sub. IF +B to the frequency f.sub. IF -B, and then split into two paths, of which one is input to a mixer 264 and the other is input to a frequency doubler 261.  The signal with the frequency 2f.sub. IF passed through the frequency doubler 261 and a bandpass filter 262 is treated by a frequency halver 263 to have its frequency halved.  When a phase modulated signal is treated so as to have its frequency doubled and then halved, the modulation component is removed therefrom and, hence, the carrier is regenerated.  Then, by mixing this carrier and the IF signal having the modulation component intact in the mixer 264, synchronous demodulation is achieved.  However, since it is difficult to realize a frequency halver 263 capable of securing a definite phase state, the synchronous demodulation is not easy to attain. 
In coherent optical transmission systems, a semiconductor laser is usually used for the transmission light source or the local oscillation light source.  Therefore, it is effective for improvement of the reception sensitivity to cope with difficulty of the phase noise produced therein.  An example of phase noise suppression means so far proposed will be described below with reference to FIG.  30.  The circuit shown is that disclosed in OFC '89 TU17.  Light from a transmission light source 271 is split by a beam splitting circuit 272 and one of the split beams is subjected to phase modulation by a phase modulator 273.  The other of the beams split by the beam splitting circuit 272 has its frequency shifted by .DELTA.  f in a frequency shifter 274.  The beams from the phase modulator 273 and the frequency shifter 274 are combined in an optical coupling circuit 275 to be transmitted to the receiver side through an optical fiber 276.  The light transmitted to the receiver side is combined with local light from a local oscillation light source 277 in an optical coupling circuit 278 and the combined light is input to an optical detecting circuit 279 to be converted into an electric signal.  This signal is amplified by an amplifier 280 and then split into two paths, one of which is input to a bandpass filter 281 allowing the IF signal to pass therethrough and the other of which is input to a bandpass filter 282 allowing a signal with a frequency of f.sub.2 (=f.sub.1 +.DELTA.f) therethrough.  Here, f.sub.1 represents the center frequency of the IF signal.  The signals from the bandpass filters 281 and 282 are mixed in a mixer 283 and the transmitted information is regenerated through a demodulation circuit 284. 
For reference, the spectrum of the signal input to the bandpass filter 281, 282 is shown in FIG.  31.  Thus, by transmitting the carrier with its frequency shifted together with the signal light and, on the receiver side, having these signals mixed, the phase noise in the transmission light source can be suppressed.