Abstract:
The present invention suppresses to a minimum the degradation of the transmission quality caused by chromatic dispersion characteristic of an optical transmission medium, and the interplay between the chromatic dispersion and non-linear optical effects in dense WDM transport systems. A baseband input data signal is pre-coded in advance by a pre-coding unit, phase modulation is carried out using a pre-coded signal by the optical phase modulating unit, and the phase modulated optical signal is converted to an RZ intensity modulated signal by the optical filter unit that performs phase-shift-keying to amplitude-shift-keying conversion. For example, an optical phase modulating unit generates an encoded DPSK phase modulated signal using a differential phase shirt keying (DPSK) format, and a phase modulated signal is converted to an RZ intensity modulated signal by the optical filter unit disposed downstream of the optical phase modulating unit.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an optical transmitter and an optical transmission system that can minimize the deterioration in the transmission quality due to the chromatic dispersion of an optical transmission medium such as an optical fiber, or due to the interaction between the chromatic dispersion and nonlinear optical effects. 
     This application is based on patent application No. 2001-199467 filed in Japan, the contents of which are incorporated herein by reference. 
     2. Background Art 
     An RZ (return-to-zero) optical intensity modulation format used along with phase modulation has been proposed having the object of minimizing the deterioration in the transmission quality due to the chromatic dispersion of an optical transmission medium such as an optical fiber, or due to the interaction between the chromatic dispersion and nonlinear optical effects. 
     For example, a citation  1 , Y. Miyamoto et al. “Duobinary carrier-suppressed return-to-zero format and its application to 100 GHz-spaced 8×43-Gbit/sec DWDM unrepeatered transmission over 163 km”, Tech. Digest of OFC 2001, paper Tu U4, 2001, discloses a technology relating to a duobinary carrier-suppressed return-to-zero (DCS-RZ) format that modulates a dual mode beat signal with an optical duobinary code. 
       FIG. 41  is a diagram for explaining the conventional structure of an optical transmitter that uses a DCS-RZ format. 
     In  FIG. 41 , a direct current bias is applied to the first push-pull type Mach-Zehnder (MZ) optical intensity modulator  91  so as to realize transmission-null when unmodulated, and the first push-pull type Mach-Zehnder optical intensity modulator  91  is complementarily driven by an electrical sine wave signal having one-half the frequency of the line rate generated by a half frequency divider  92 . 
     The intensity and phase of the CW light output from the single longitudinal mode LD  90 , which is the light source of the carrier frequency f 0 , are simultaneously modulated by the MZ optical intensity modulator  91  using the frequency multiplier function and the phase modulation function of an MZ optical intensity modulator, and a dual mode beat signal having a repetition frequency of B is generated. Here, B is the line rate. 
     At the second MZ optical intensity modulator  93 , the dual mode beat signal is modulated with data using an optical duobinary format. The input NRZ (non-return-to-zero) signal is converted to a pre-coded NRZ code by the precoder circuit  97  that is formed by the logic inversion circuit  94 , the exclusive OR circuit  95 , and the 1 bit delay circuit  96 , and the pre-coded NRZ code is differentially output. 
     The differential pre-coded NRZ code is amplified by the baseband amplifier  98 , and then converted to a complementary ternary electrical duobinary code by the low pass filter (LPF  99 ) having 3 dB bandwidth of B/4. A direct current bias is applied to the second MZ optical modulator  93  so as to realize transmission-null when unmodulated, and the second MZ optical modulator  93  modulates with a complementary ternary electrical duobinary code to generate a DCS-RZ optically modulated code. 
       FIGS. 42A through 42F  and  FIGS. 43A and 43B  show an example of the operation of the conventional technology.  FIG. 42A  shows the binary NRZ signal input generated by the binary NRZ signal generating unit  103 .  FIG. 42B  shows the NRZ data signal output from the logic inversion circuit  94  in the case that a binary NRZ signal is input.  FIG. 42C  shows the positive-phase signal output from the pre-coding circuit  97  in the case that the output NRZ data signal is input, and the logic thereof is inverted each time a space bit is input as the input NRZ signal.  FIG. 42D  shows the waveform output from the LPF  99  in the case that the pre-coded signal is input. 
     As shown by reference numeral  100  in  FIG. 41 , the logical operation of the LPF  99  is identical to that of a circuit block comprising the 1 bit delay circuit  101  and the analog AND circuit  102 . Due to the band limiting function of the LPF  99 , the complementary ternary electrical duobinary signal shown by the bold solid line is generated. 
       FIG. 42E  shows the electrical field waveform of the dual mode beat optical signal modulated by the first MZ optical intensity modulator  91  when the CW optical signal from the LD  90 , which is the light source, is input. The electrical field waveform forms an optical pulse train where the repetition frequency is equal to the line rate, and whose optical phase is alternate π phase flip for each bit. This dual mode beat optical signal is modulated with the ternary electrical duobinary signal shown in  FIG. 42D , and thereby the DCS-RZ code shown in  FIG. 42F  is generated. The phase is inverted for each mark bit, and thus an RZ intensity modulated optical data signal is obtained. 
       FIG. 43A  shows the optical spectrum of the dual mode beat signal output from the first MZ optical intensity modulator  91 . The optical carrier signal component f 0  is suppressed, and at the optical frequency fb±(B/2) (where B is the line rate), two longitudinal modes having a frequency spacing of B are generated. The two longitudinal modes are modulated with each of the optical duobinary codes by the second MZ optical modulator  93 . 
     As a result, as shown in  FIG. 43B , the optical modulated spectrum of the generated DCS-RZ optical signal is comprised of two optical duobinary signal modulated spectrum arranged at optical frequencies f 0 ±B, the carrier component is completely suppressed, and the optical modulation band is narrowed to 2B. Thereby, the tolerance with respect to chromatic dispersion is double that of the conventional RZ. 
     The above format suppresses the impairment of the optical duobinary code due to optical nonlinear effects, and thus RZ encoding can be realized while suppressing the broadening of the optical modulation band. Thus, this is suitable as a modulation format in a dense wavelength division multiplexing transmission system. 
     When considering a wavelength division multiplexing system on a binary RZ intensity modulation code, the optical nonlinear phase shift due to the cross-phase modulation from other channels is strongly depending on the signal pattern, and the interplay between chromatic dispersion and cross-phase modulation (XPM) causes the system performance to deteriorate. In order to mitigate the XPM-induced impairment, T. Miyano et al. propose an RZ-intensity-modulated phase-encoded signal in citation 2, T. Miyano, M. Fukutoku, and K. Hattori, “Suppression of degradation induced by SPM/XPG+GVM transmission using a bit-synchronous intensity modulated DPSK signal”, Digest of OECC2000, Makuhari, paper 14D3-3, pp. 580–581, 2000. 
     As described above, in a conventional optical transmitter and optical transmission system using an RZ optical intensity modulated format used with phase modulation, generally, optical modulators are necessary for each intensity modulation, phase encoding, and pulse modulation, and these optical modulators are connected in a multi-stage cascade. Thereby, the insertion loss in the modulating unit increases, and the optical output power of the modulating unit decreases. Thus, there are the problems that the optical signal shot noise increases and the SN ratio of the output of the optical transmitting unit degrades. 
     In addition, in the case of high speed transmission, the relative phase between an electrical data signal and a clock signal for each of the modulators connected in multi-stage must be precisely controlled, and in order to compensate the drift of the phase due to temperature characteristics and the like, a stable phase control must be carried out. Thereby, the problem of the control circuits and the like becoming complicated is made tangible. 
     Furthermore, in the conventional wavelength division multiplexing system, since two or more optical modulators must be installed for each channel, the number of parts increases, in particular in the case that the number of channel increases in the WDM system. This is a drawback because the cost of the optical transmitter and the optical transmission system using them increases. 
     At the same time, in the conventional RZ optical transmitter and optical transmission system using a DCS-RZ format, the optical duobinary encoding unit, which carries out the intensity data modulation and phase modulation in data encoding process requires a baseband analog processing circuit (LPF  99  shown in  FIG. 41 ) that generates a ternary opto-electrical signal converted signal depending on the line rate. 
     As the line rate increases, however, it is difficult to realize the high-speed baseband analog processing in the unit. In order to suppress waveform distortion of the ternary electrical duobinary signal, the waveform distortion due to reflected waves in the rejection band of the LPF  99  must be suppressed. At the same time, in the high frequency band, realizing ideal electrical characteristics is difficult, and in particular, terminating the reflected wave in the rejection band of the filter is difficult. In addition, when realizing the ideal roll-off characteristics of the electrical filter, frequency dependent loss and frequency dispersion of the electrical transmission line and the filters occurs as the line rate increases, and thereby the waveform distortion occurs. Thus, there is the problem that compensation of the waveform becomes difficult. 
     In addition, the conventional PSK signal that has been modulated using an RZ format can suppress the cross-phase modulation in a wavelength division multiplexing system. However, when considering an increasingly high density of the wavelength division multiplexing system equal to or above 0.4 bits/s/Hz, the optical modulation band spreads four times the line rate, and thus the cross-talk penalty increases. In addition, when considering the high-speed transmission using the conventional technology, it is necessary to increase the operating speed of the baseband signal input into the modulator. 
     However, as the line rate increases, generally there is a tendency for the breakdown voltage of the electronic device to decrease, and thus this makes difficult to realize high output operation in a driver for driving a modulator or the like becomes difficult. Furthermore, realization of high-speed operation in the pre-coding circuit as well becomes difficult, and it is necessary to redesign and remanufacture the circuit each time the line rate is increased. 
     It is an object of the present invention to provide an optical transmitter and an optical transmission system wherein decreasing the loss and increasing the speed of the optical modulator is facilitated by using the RZ optical intensity modulation format along with phase modulation. In addition, the invention measures the increasing speed of the analog signal processing by performing a function in the optical carrier frequency domain, which has been carried out by a conventional baseband analog processing circuit. Furthermore, the invention facilitates realization of an amplifier circuit such as a driver circuit by encoding all the electric signals with a simple binary NRZ format. 
     Furthermore, it is an object of the present invention to provide an optical transmitter and an optical transmission system that use the RZ optical intensity modulation format along with phase modulation, and that make possible simultaneous PSK-ASK conversion of the wavelength division multiplexed signals by using a periodic optical conversion filter, and make possible the elimination of the synchronization function in the active high-speed signal processing by using a passive optical filter. 
     SUMMARY OF THE INVENTION 
     This summary of the invention does not necessarily describe all necessary features so that the invention may also be a sub-combination of these described features. 
     The optical transmitter of the present invention comprises: a light source; a pre-coding device that receives an NRZ signal; an optical phase modulating device that carries out optical phase modulation and that is driven by either a pre-coded NRZ signal or a differential pre-coded NRZ signal generated by the pre-coding device; and an output terminal, wherein an RZ optical signal in which a plurality of duobinary optical signal components are included in the optical RZ signal spectrum with carrier suppression is output from the output terminal. 
     In addition, the optical transmitter of the present invention comprises: a light source which is a single mode longitudinal light source; a pre-coding device that generates a differential pre-coded NRZ signal whose logic inverts each time a mark bit is input as an NRZ signal; an optical phase modulating device that modulates the optical phase of a single longitudinal mode optical signal from the light source using the differential pre-coded NRZ signal generated by the pre-coding device; and an optical filter device that converts an optical-phase modulated signal generated by the optical phase modulating device to an RZ optical intensity modulated signal. 
     Furthermore, the optical transmitter of the present invention further comprises: a dual mode beat pulse light source that generates two longitudinal mode signals which synchronizes with a data signal and which have the mode spacing between the two longitudinal modes that is an integral multiple of the data line rate, and which is mode-locked with each other; a pre-coding device that carries out code conversion such that an intensity modulated signal output from the optical transmitter has a logic identical to the input NRZ signal; and an optical phase modulating device in which an optical pulse train generated by the dual mode beat pulse light source has undergone optical phase modulation by a pre-coded NRZ signal generated by the pre-coding device. 
     The optical transmission system of the present invention comprises: an optical transmitter that comprises: a light source; a pre-coding device that receives an NRZ signal; an optical phase modulating device that carries out optical phase modulation and that is driven by either a pre-coded NRZ signal or a differential pre-coded NRZ signal generated by the pre-coding device; and an output terminal, and that outputs an RZ optical signal having the suppressed carrier component from the output terminal as an optical transmission signal; an optical phase-modulation/amplitude modulation converting device that is provided on one of either the optical transmitter or an optical receiver; wherein the optical receiver demodulates and detects the transmitted RZ optical signal as an optical intensity modulated signal, and converts the optical intensity modulated signal to an electrical signal. 
     In addition, the optical transmission system of the present invention comprises: an optical transmitter that comprises: a pre-coding device that generates a differential pre-coded NRZ signal whose logic inverts each time a mark bit is input as the NRZ signal; an optical phase modulating device that performs optical phase modulation on a signal from either a single longitudinal mode light source or a dual mode beat pulse light source using the differential pre-coded NRZ signal generated by the pre-coding device; and an optical filter device that converts an optical phase modulated signal generated by the optical phase modulating device into an RZ optical intensity modulated signal; an optical transmission medium that transmits the RZ optical intensity modulated signal output by the optical transmitter; and an optical receiver that receives the RZ optical intensity modulated signal output by the optical transmitter via the optical transmission medium, and directly detects the RZ optical intensity modulated signal to convert the RZ optical intensity modulated signal into a baseband electrical signal. 
     The optical transmission system of the present invention comprises: an optical transmitter that is provided with: a pre-coding device that carries out code conversion of the input NRZ signal such that the optical intensity modulated signal output from the optical transmitter has a logic identical to the input NRZ signal; and an optical phase modulating device that performs optical phase modulation on a signal either from a single longitudinal-mode light source or a dual mode beat pulse light source using the differential pre-coded NRZ signal generated by the pre-coding device; an optical transmission medium that transmits a phase modulated data signal with sinusoidal RZ optical intensity modulation that has been output by the optical transmitter; and an optical receiver that receives the RZ optical modulated signal output by the optical transmitter via the optical transmission medium, and after passing through an optical filter device that converts the RZ optical modulated phase-encoded signal to an optical amplitude-encoded signal, directly detects the optical intensity modulated signal to convert to a baseband electrical signal. 
     In the structure described above, the baseband input data signal is pre-coded in advance by the pre-coding device, phase modulation is carried out by the phase modulating device using the pre-coded signal, and the resultant phase encoded optical signal is converted into an RZ intensity modulated signal that has undergone the phase modulation by the optical filter device. The phase modulating device, for example, generates an encoded DPSK phase modulation signal using differential phase shift keying (DPSK), and converts the phase modulated signal into an RZ intensity signal by the optical filter device disposed downstream of the DPSK optical modulation signal device. 
     If an optical periodic filter is used as the optical filter device described above, conversion of modulation format of the wavelength division multiplexed signals can be simultaneously conducted due to the broadband characteristic of the optical filter, and thus an optical filter device for each channel becomes unnecessary. Thereby, the number of parts can be drastically reduced in the high capacity wavelength division multiplexing system having a large number of channels, and thereby the cost of the optical transmitter can be reduced. In addition, by using a passive optical filter that does not require high-speed signals, precise control of the signal phase between modulators becomes unnecessary. 
     In particular, by using a Mach-Zehnder optical intensity modulator as the phase modulating device that carries out differential phase shift keying on the single longitudinal mode optical signal using a pre-coded NRZ signal, it is possible to use only binary NRZ signals as the electric signals. Therefore, the baseband signal processing can be easily realized, and the number of optical modulators can be reduced. In addition, the analog processing function conventionally carried out in the baseband can be realized in an optical carrier frequency band using a passive optical filter, and thereby, an ideal wideband analog processing can be realized, and improvement of the reflective characteristics in the filter processing and the broadband transmission characteristics becomes possible for generation of the ultra-high-speed signal. 
     In addition, as another embodiment of the optical transmission system of the present invention, an optical filter that carries out phase modulation/RZ intensity modulation conversion is disposed on the receiver side of the optical transmission system, phase encoded signal is used as a transmission code, and an RZ pulse whose intensity has been phase encoded is used. Thereby, transmission impairments due to nonlinear cross talk such as cross-phase modulation in wavelength division multiplexing transmission system can be suppressed, and at the same time the optical modulation band can be reduced in comparison to conventional technology, and attaining higher density wavelength division multiplexing system becomes possible. 
     As another embodiment of the optical transmission system of the present invention, the phase modulating device can be structured by n phase modulators connected in series. Thereby, by using a driver for driving a modulator and a pre-coding circuit having a baseband signal processing speed of B′, it is possible to generate a phase modulated optical data signal that has undergone the n time division multiplexed RZ intensity modulation having a line rate of B=n×B′ or an RZ intensity optical data signal that has undergone phase modulation. 
     All of these multiplexed signals are bandwidth-reduced RZ signals, and can form a high-density wavelength division multiplexing system using a simple structure in comparison to conventional technology. 
     In addition, as yet another embodiment of the transmitter and the pre-coding device in the optical transmission system according to the present invention, by using a structure comprising n pre-coding circuits having a signal processing speed of B′, a delay device that delays the n output signals of the pre-coding circuit, and an exclusive OR circuit that performs exclusive OR on the n delayed output signals, it is possible to generate n time division multiplexed pre-coded signals having a line rate of B=n×B′, and an increase in the line rate of a transmission system can be easily realized. 
     According to the present invention explained above, in order to realize a high speed optical transmitter and optical transmission system using the RZ optical intensity modulation format used with phase modulation, the optical transmitter is provided with a pre-coding device that generates a differential pre-coded NRZ signal whose logic inverts each time a mark bit is input as an NRZ signal; an optical phase modulation device that carries out optical phase modulation on a single longitudinal mode optical signal generated by a light source using the differential pre-coded NRZ signal generated by the pre-coding device; and an optical filter device that converts the phase modulated optical signal generated by the optical phase modulating device to an RZ optical intensity modulated signal. Thereby, the loss of the optical modulator is decreased and enhancing the high-speed operation of the optical modulator becomes easy. 
     In addition, by carrying out an analog processing function in the optical carrier frequency domain that was conventionally carried out in baseband frequency domain, the speed of the analog signal conversion processing can be increased, and furthermore, by making all electrical signals binary NRZ format, the amplifier circuits such as drive circuits can be easily realized. 
     At the same time, by employing a periodic optical filter, simultaneous PSK-ASK conversion of the wavelength division multiplexed signals becomes possible, and by using a passive optical filter, the synchronization between several optical modulators in the active high-speed signal processing can be omitted, and it becomes possible to provide an optical transmitter and optical transmission system that uses the RZ optical intensity modulation format along with phase modulation. 
     In addition, according to the present invention, simultaneous conversion of the phase modulated wavelength division multiplexed signals to intensity modulated WDM signals becomes possible by the parallel processing of the optical filter. Furthermore, by phase modulating a dual mode beat pulse signal, an RZ phase modulated code or an intensity modulated code having a constant duty cycle can be used as the transmission code. Therefore, narrowing of the optical modulation bandwidth becomes possible, and the efficiency for frequency utilization of the wavelength division multiplexing system can be improved. At the same time, the tolerance of transmission quality degradation due to nonlinear effects can be improved. 
     In addition, by narrowing the optical modulation bandwidth on the receiver side, the chromatic dispersion tolerance can be improved. Furthermore, an increase in the system line rate can be realized without enhancing the operation speed of the pre-coding circuit, the modulator, and the driver for the modulator. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram showing an embodiment of the optical transmitter according to the present invention. 
         FIG. 2  is a diagram showing the internal structure of each block of the optical transmitter shown in  FIG. 1 . 
         FIG. 3  is a diagram for explaining the operation of the MZ optical modulator shown in  FIG. 2 . 
         FIGS. 4A through 4F  are diagrams for explaining in detail the operation according to the present embodiment shown in  FIG. 1  and  FIG. 2 . 
         FIGS. 5A through 5C  are diagrams for explaining the optically modulated spectrum shown in  FIG. 2  (τ=1/B). 
         FIGS. 6A through 6C  are diagrams for explaining the optically modulated spectrum shown in  FIG. 2  (τ=/(2B)). 
         FIG. 7  is a diagram for explaining the operation in the case that the optical phase modulating unit shown in  FIG. 2  is realized by another structure. 
         FIG. 8  is a diagram showing another embodiment of the optical filter unit shown in  FIG. 2 . 
         FIG. 9  is a diagram showing an embodiment of the optical transmission system according to the present invention. 
         FIG. 10  is a diagram for explaining the experimental system for the optical transmission system according to the present invention. 
         FIGS. 11A through 11D  are diagrams showing the operation of the experimental system shown in  FIG. 10 . 
         FIG. 12  is a diagram for explaining another embodiment of the optical transmission system according to the present invention. 
         FIG. 13  is a diagram for explaining a further embodiment of the optical transmitter according to the present invention. 
         FIG. 14  is a diagram showing one embodiment of the optical transmission system using the optical transmitter shown in  FIG. 13 . 
         FIGS. 15A through 15F  are diagrams for explaining the operation of the optical transmitter and the optical transmission system shown in  FIGS. 13 and 14  using the waveform of each part. 
         FIGS. 16A through 16E  are diagrams for explaining the operation of the optical transmitter and the optical transmission system shown in  FIGS. 13 and 14  using the optical spectrum of each part. 
         FIG. 17  is a diagram showing a second embodiment of the optical transmission system using the optical transmitter shown in  FIG. 13 . 
         FIGS. 18A through 18G  are diagrams for explaining the operation of the optical transmitter and the optical transmission system shown in  FIG. 17  using the waveform of each part. 
         FIGS. 19A through 19E  are diagrams for explaining the operation of the optical transmitter and the optical transmission system shown in  FIG. 17  using the optical spectrum of each part. 
         FIG. 20  is a block diagram showing another embodiment of the optical transmitter according to the present invention. 
         FIG. 21  is a block diagram showing an embodiment of the optical transmission system using the optical transmitter shown in  FIG. 20 . 
         FIGS. 22A through 22C  are diagrams for explaining an example of the structure of the optical receiver according to the optical transmission system in  FIG. 21 . 
         FIGS. 23A ,  23 I, and  23 J are diagrams for explaining the enlargement of the chromatic dispersion tolerance of the optical receiver used in the optical transmission system in  FIG. 20  based on the optical signal spectrum. 
         FIGS. 24D ,  24 G, and  24 H are diagrams for explaining the enlargement of the chromatic dispersion tolerance of the optical receiver used in the optical transmission system in  FIG. 20  based on the optical signal spectrum. 
         FIGS. 25A and 25B  are diagrams showing examples of the structure of the optical receiver shown in  FIG. 23A ,  23 I,  23 J,  24 D,  24 G, and  24 H. 
         FIGS. 26A through 26C  are diagrams for explaining the operation of the optical receiver shown in  FIG. 25B . 
         FIG. 27  is a diagram showing an example of the structure for realizing an optical filter having a polarization independent transfer function used in the present invention. 
         FIGS. 28A and 28B  are diagrams for explaining the experimental results using the structure shown in  FIG. 27 . 
         FIG. 29  is a diagram for explaining the internal structure of the pre-coding unit and the phase modulating unit used in the present invention. 
         FIGS. 30A through 30J  are diagrams for explaining the operation of the pre-coding unit and the phase modulating unit shown in  FIG. 29 . 
         FIG. 31  is a diagram for explaining another structure of the pre-coding unit used in the present invention. 
         FIGS. 32A and 32B  are diagrams for explaining the effect of the optical transmission system according to the present invention. 
         FIGS. 33A and 33B  are diagrams showing the calculation result for explaining the effect of the optical transmission system according to the present invention. 
         FIG. 34  is a diagram for explaining another embodiment of the optical transmission system according to the present invention. 
         FIG. 35  is a diagram for explaining the internal structure of the optical transmitter shown in  FIG. 34 . 
         FIGS. 36A to 36D  are diagrams for explaining the operation of the embodiment according to the present invention shown in  FIG. 34  and  FIG. 35 . 
         FIG. 37  is a diagram for explaining another embodiment of the optical transmission system according to the present invention. 
         FIGS. 38A through 38E  are diagrams for explaining the operation of the optical transmission system shown in  FIG. 37  using the optical signal spectra. 
         FIG. 39  is a diagram for explaining another embodiment of the optical receiver used in the optical transmission system shown in  FIG. 37 . 
         FIGS. 40A through 40C  are diagrams for explaining the operation of the optical transmission system shown in  FIG. 37  using the optical signal spectra. 
         FIG. 41  is a diagram for explaining the structure of a conventional optical transmitter. 
         FIGS. 42A through 42F  are diagrams for explaining the operation of the conventional optical transmitter shown in  FIG. 41 . 
         FIGS. 43A and 43B  are diagrams for explaining optically modulated spectrum of the conventional optical transmitter shown in  FIG. 41   
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following embodiments do not restrict the interpretation of the claims relating to the present invention, and the combination of all the features explained in the embodiments is not always indispensable means of solving the problem. 
       FIG. 1  is a block diagram showing an embodiment of the optical transmitter according to the present invention. In  FIG. 1 , the binary NRZ electrical signal output from the NRZ signal generating unit  1  is input into the pre-coding unit  2 . The pre-coding unit  2  carries out signal processing so that the optical signal output from the optical transmitter matches the input NRZ signal. The differential pre-coded NRZ signal generated by the pre-coding unit  2  is amplified as necessary, and input into the optical phase modulating unit  3 . 
     The single longitudinal mode CW optical signal emitted by the light source  4  (LD: a laser diode) is appropriately phase modulated by the optical phase modulating unit  3 , and subsequently input into the optical filter unit  5  that converts the optical phase modulated signal into an RZ intensity modulated signal. The output of the optical filter unit  5  is optically amplified as necessary, and is output as the output signal of the optical transmitter according to the present invention at a predetermined optical power. 
       FIG. 2  shows the detailed structure of each of the blocks of the optical transmitter shown in  FIG. 1 . In  FIG. 2 , blocks having the same reference numerals as the blocks shown in  FIG. 1  are the same as those in  FIG. 1 . 
     The NRZ electrical signal output from the NRZ signal generating unit  1  is input into the pre-coding unit  2 . The pre-coding unit  2  is formed by an exclusive OR circuit (EXOR  21 ), a 1-bit delay circuit  22 , and a differential output circuit  23 . The pre-coding unit  2  differs from the conventional pre-coding unit  97  ( FIG. 41 ) in that there is not a logic inversion circuit  94  for inverting the logic of the input data signal. 
     The pre-coding unit  2  shown in  FIG. 2  carries out differential encoding, thereby a differentially pre-coded encoded NRZ signal whose logic is inverted each time a mark bit is input as the input NRZ signal is differentially output. The differentially encoded NRZ signal is input into the optical phase modulating unit  3 . The optical phase modulating unit  3  amplifies the signal as necessary by using the baseband amplifier  31 , and supplies it to the MZ optical intensity modulator  32 . 
       FIG. 3  is a diagram for explaining the operation of the MZ optical intensity modulator  32  shown in  FIG. 2 . As shown in  FIG. 3 , the differentially encoded NRZ signal generated by the pre-coding unit  2  is amplified to equal amplitudes, preferably amplified up to half-wave voltage of the MZ optical intensity modulator  32 . Here, the direct current bias is applied to the MZ optical intensity modulator  32  so as to realize transmission-null when unmodulated. 
     Returning to  FIG. 2 , following the operating conditions described above, the MZ optical intensity modulator  32  generates the DPSK optical signal that is encoded using a DPSK (differential phase shift keying) format. The DPSK optical signal is input into the Mach-Zehnder interferometer (MZI) type optical filter that forms the optical filter unit  5  and whose rejection band center frequency matches the optical carrier frequency of the DPSK optical signal. The MZI optical filter is formed by  3  dB directional couplers ( 52  and  53 ) disposed at the input port  51  and the cross output port  56 , and an optical delay element  54  having a delay of τ(s). In the case that an optical signal input into the through port  5  is Ein (i.e., input optical signal electric field), the output optical electric field of the MZI optical filter is given by the following equation:
 
 E ( t )=( E in/2)· exp{−j (ωτ+φ)/2}·sin{(ωτ+φ)/2}  (Equation 1)
 
where ω is the optical frequency, τ is the amount of delay of the optical delay element  54 , and φ is the relative phase between the two optical signals propagated through the waveguide in the Mach-Zehnder optical filter.
 
       FIGS. 4A through 4F  are diagrams for explaining in detail the operation of the embodiment of the present invention. Here, the case is shown in which the delay amount τ of the delay element is equal to T 0 /2 (T 0 =1/B, where B is the line rate). 
       FIG. 4A  shows the input NRZ baseband electrical signal (binary), and  FIG. 4B  shows the input pre-coded NRZ baseband electrical signal output from the pre-coding unit  2  in the case that the signal shown in  FIG. 4A  is input thereto.  FIG. 4C  shows the generated DPSK optical modulated signal that has been modulated by the MZ optical intensity modulator  32  with the pre-coded NRZ signal.  FIG. 4D  shows the DPSK optical signal at point C ( FIG. 2 ) in the MZI optical filter that has been delayed by τ by the optical delay element  54 .  FIG. 4E  shows the DPSK optical signal (the output signal from the Mach-Zehnder interferometer type optical filter) output from the MZI optical filter. The dashed line is the logical electrical field envelope, and the solid line is the DPSK optical signal for the case that optical phase modulation has been carried out with a bandwidth limit applied to the pre-coded NRZ electrical signal. Finally,  FIG. 4F  shows the directly detected waveform. 
     Moreover,  FIG. 4E  shows the RZ optical signal output from the through port  56  of the 3 dB directional coupler  53  of the MZI optical filter  51  according to the above Equation 1. The dashed line is the logical electrical field envelope, and the solid line is the electrical field envelope in the case that optical phase modulation has been carried out with a bandwidth limit applied to the pre-coded NRZ electrical signal. 
     As can be understood from  FIGS. 4E and 4F , the MZI optical filter outputs a signal in accordance with a phase change rule that is identical to that of the DCS-RZ signal, whose phase is inverted with each mark bit. In the optical filter unit  5  shown in  FIG. 2 , the optically modulated signal of the rejection band is separated by the 3 dB directional coupler  53  and output from the cross port  56 . Thereby, by terminating the cross port  56  by means of angled polishing or the like, the reflected wave in the optical filter rejection band does not return to either the through port  55  and the input port  51 , and thus the reflection can be sufficiently reduced. 
       FIGS. 5A through 5C  and  FIGS. 6A through 6C  show the optically modulated spectra at the input port and output port of the MZI optical filter in  FIG. 2 .  FIGS. 5A through 5C  show the case where the delay amount τ=T 0 , and  FIGS. 6A through 6C  show the case where τ=T 0 /2. 
       FIGS. 5A and 6A  show the DPSK optically modulated spectra at the point A shown in  FIG. 2 . The ordinate is the scale for the spectral intensity, and the abscissa is the scale for the optical frequency.  FIGS. 5B and 6B  show the frequency response characteristics of the MZI optical filter  51  shown in  FIG. 2 . The ordinate is the scale for the transmittance (dB), and the abscissa is the scale for the optical frequency. 
       FIGS. 5A through 5C  show a sine wave frequency response of period B (where B is the line rate).  FIGS. 6A through 6C  show a sine wave frequency response of period  2 B.  FIGS. 5C and 6C  show the converted RZ signal optical modulated spectra. The ordinate is the scale for the spectral intensity and the abscissa is the scale for the optical frequency. Here, the carrier frequency component is entirely suppressed, and the modulation spectrum has the modulation bandwidth of  2 B. 
     As explained above, the RZ optical signal output from the optical filter unit  5  has the same coding rule as that used in a DCS-RZ format, as can be understood from the optical phase change rule of the waveform response shown in  FIGS. 4A through 4F  and the optically modulated spectra shown in  FIGS. 5A through 5C  and  FIGS. 6A through 6C . 
       FIG. 7  is a diagram for explaining the operation in the case that the optical phase modulating unit  3  shown in  FIG. 2  is realized by another structure. The difference between the example shown in  FIG. 7  and the embodiment shown in  FIG. 2  is that the optical pulse is chirped so that the optical phase thereof changes over time and that optical intensity thereof does not change. 
     In  FIG. 7 , the optical phase responses for each of the points (a, b, c, d) is output corresponding to each of the points (a, b, c, d) in the baseband pre-coded NRZ electrical signal input. That is,  FIG. 7  shows response characteristics in which the optical phase changes linearly. These types of characteristics can be easily realized by an optical phase modulator which modulates the refractive index of the straight optical waveguide formed, for example, on LiNbO 3  by means of an electro-optical effect. 
       FIG. 8  is a diagram for showing another embodiment of the optical filter unit  5  shown in  FIG. 2 . The embodiment shown in  FIG. 8  differs from the embodiment shown in  FIG. 2  in that instead of the  3  dB directional couplers  52  and  53 , the Y-branching waveguides  58  and  59  are used in the optical branching unit of the Mach-Zehnder interferometer type filter that forms the optical filter unit  5 . 
     In  FIG. 8 , the DPSK optical signal input from the optical input port  57  is branched into two paths by the Y-branching waveguide  58  such that the power is 50% in each. One optical signal passes through the optical delay element  54  having a delay amount T, is multiplexed with another optical signal by the Y-branching waveguide  59 , and the multiplexed optical signal is output from the output port  60 . 
     Below, the optical transmission system using the optical transmitter described above will be explained.  FIG. 9  is a diagram showing an embodiment of the optical transmission system according to the present invention. Because the optical transmitter  61  used is identical to the embodiment described above, the explanation will be omitted in order to avoid repetition. 
     In  FIG. 9 , the binary NRZ input electrical signal is converted to an RZ signal by the optical transmitter  61 , is optically amplified by the optical amplifier  62  as necessary to a predetermined signal power, and is subsequently supplied to the optical transmission medium  69 . The optical transmission medium  69  can be formed from only an optical fiber transmission line  63 . Alternatively, the optical transmission medium  69  can be formed by optical transmission lines  63  and optical amplifier repeater devices  64  that directly amplify and repeat the optical signals from the optical transmission lines  63 . 
     The output signal of the optical transmission medium  69  is input into the optical receiver  65 . In the optical receiver  65 , the input optical signal is pre-amplified by the optical amplifier  62 , and then as necessary, input into the dispersion compensating circuit  66  that compensates the chromatic dispersion and the polarization mode dispersion. Thereby, waveform distortion due to the dispersion of the optical transmission medium  69  (chromatic dispersion or polarization mode dispersion) is compensated. The output of the dispersion compensating circuit  66  is directly detected by the optical signal direct detecting element  67 , and converted to a baseband electrical signal. Depending on necessity, the baseband electrical signal is equalized and amplified, and then timing extraction, and identification are carried out by the clock/data recovery (CDR) circuit  68 , thereby the transmitted data is regenerated. 
       FIG. 10  is a diagram for explaining the experimental system for the optical transmission system according to the present invention shown in  FIG. 9 . 
     In  FIG. 10 , a single mode longitudinal optical signal generated by a light source (DFB-LD)  70  in the 1.55 μm band was input into a push-pull MZ LiNbO 3  modulator  32 . The experimental signal used in the experimental system shown here was a 42.7 Gbit/s M sequence pseudorandom (PN) signal (with the pattern length of 2 7 −1). Here, the pre-coding unit  2  converted the PN signal to an identical PN signal, and thus the pre-coding unit  2  is not shown in  FIG. 10 . 
     In  FIG. 10 , the 4-channel M sequence PN 2 7 −1 stage NRZ optical signals modulated at 10.66 Gbit/s (below, written as “10.7 Gbit/s”), which had been generated by an NRZ pulse pattern generator  71 , had the same pattern with appropriate relative phase relationships. It was input into a 4 bit interleaving multiplexing circuit (4:1 MUX)  72 , multiplexed so as to generate an M sequence PN 2 7 −1 stage signal at 42.64 Gbit/s (below, written as “42.7 Gbit/s”), and this signal was differentially output. 
     The differentially output signal was amplified up to an amplitude equal to or less than the half-wave voltage in the first MZ optical intensity modulator  32  by the amplifier  31 , and then input into the first MZ optical intensity modulator  32 . Moreover, the traveling wave type MZ LiNbO 3  optical intensity modulator disclosed in citation 3, K. Noguchi, H, Miyazawa, and O. Mitomi, “LiNbO 3  high-speed modulators”, Tech. Dig. of CLEO Pacific Rim &#39;99, paper FS2, pp. 1267–1268, 1999, was used as the first MZ optical intensity modulator  32 . 
     The bias circuit  73  applied a direct current bias to the MZ optical intensity modulator  32  so as to realize transmission-null when unmodulated, and the MZ optical intensity modulator  32  output a 42.7 Gbit/s PN 2 7 −1 stage DPSK optical modulated code. 
     The 42.7 Gbit/s DPSK optical modulated code was input into the Mach-Zehnder interferometer (MZI) type optical filter  74  formed on a silica waveguide. As an MZI filter  74 , a periodic optical filter having a frequency spacing of 100 GHz and a delay time of 10 ps was used. The temperature of the MZI filter  74  was controlled, and thereby the rejection frequency of the MZI filter  74  was adjusted to match the carrier frequency of the DFB-LD. By operating the MZI optical filter  74  with this type of operating conditions, the MZI filter  74  output a DCS-RZ code. In addition, the output DCS-RZ code was amplified by an EDFA optical amplification post-amplifier (an erbium doped optical fiber amplifier)  75 , transmitted through a 1.55 μm zero dispersion optical fiber transmission line  76 , and then input into an optical receiver. 
     At the optical receiver, the signal was amplified by the EDFA optical amplification pre-amplifier  77 . Subsequently, the signal was directly detected by the optical signal direct detecting element  67 , and converted to a binary NRZ baseband electrical signal. The converted signal was supplied to the CDR circuit  68 , and the 42.7 Gbit/s NRZ data signal was regenerated by the CDR circuit  68  was further demultiplexed into four 10.7 Gbit/s data signals by the 4 bit interleaving demultiplexing circuit  78 , and each of the error rates thereof was measured by 10.7 Gbit/s error rate measuring device  79 . 
       FIGS. 11A through 11D  are diagrams showing the operation of the experimental system shown in  FIG. 10 . 
     In the MZ optical intensity modulator  32 , the DPSK optical signal is generated by being push-pull driven by the 42.7 Gbit/s NRZ signal.  FIG. 11A  shows the modulated directly detected waveform of the DPSK optical signal and  FIG. 11B  shows the optical modulation spectrum of the DPSK optical signal. In addition,  FIG. 11C  shows the directly detected waveform of the output of the MZI filter  74 , and  FIG. 11D  shows the optical modulated spectrum of the optical signal output from the MZI filter  74 . 
     As can be understood from  FIG. 11C , at 42.7 Gbit/s, the optical phase modulated signal is converted to an RZ intensity modulated signal. In addition, it can be understood from the optically modulated spectrum of  FIG. 11D  that the modulated spectrum of a DCS-RZ code was obtained in which the carrier frequency f 0  (193.307 THz) is suppressed. 
     As a result of evaluating the error rate characteristics when using a PN 2 7 −1 stage NRZ signal, it was confirmed that there were no errors at 42.7 Gbit/s, and a receiver sensitivity of −27 dBm was obtained at the bit error rate of 10 −9 . From the above, it can be confirmed that the present format is a DCS-RZ format following a DCS-RZ optical encoding rule. 
     In addition, the optical loss in the pass band of the MZI filter  74  is approximately 2 dB, the amount of reflective attenuation is equal to or less than −40 dB, and compared to the structure using an MZ optical modulator, it was possible to form an extremely low loss and wide band modulation system. 
       FIG. 12  is a diagram for explaining another embodiment of the optical transmission system according to the present invention. 
     In the embodiment shown in  FIG. 12 , the optical transmission medium  69  and the optical receiver  65  are identical to those of the embodiments shown in  FIG. 9 . The only difference is that the optical transmitter  61  uses a wavelength division multiplexing transmission scheme. 
     Specifically, the optical transmitter  61  has the number of channels (CH#1 to CH#n) of the wavelength division multiplexing system. In the case of using the optical transmitter that outputs the DCS-RZ signal shown in  FIG. 2 , the optical carrier frequencies (f 01  to f 0n ) of respective channels are located so as to match the central optical frequency of the rejection band of the optical filter unit  5 . In each of the optical transmitters  61 , the different carrier signals of respective channels are modulated using a DCS-RZ modulation format in each of the optical transmitters. An MZI type optical filter can be used as the optical filter units  5  that are disposed in the optical transmitters  61  of the respective channels. The RZ modulated signal generated by the optical transmitters  61  for each of the channels is optically amplified as necessary by an optical amplifier  62 , and then input into the wavelength-division-multiplexing-light multiplexing filter  80 , and wavelength division multiplexed. 
     The DCS-RZ optical signals simultaneously wavelength division multiplexed by the wavelength-division-multiplexing-light multiplexing filter  80  are amplified by the EDFA optical amplification post-amplifier  62  depending on necessity, and the amplified DCS-RZ optical modulated code is supplied at a predetermined transmission channel power for transmission to the optical transmission medium  69 . The optical transmission medium  69  can be a linear repeatered transmission line that is formed by optical fibers and optical amplifier repeater devices for performing direct optical amplification and transmission. 
     The output of the optical transmission medium  69  is optically amplified by the EDFA optical amplification post-amplifier  62 , subsequently input into the wavelength-division-multiplexing-light demultiplexing optical filter  81 . The wavelength division multiplexed DCS-RZ signal is demultiplexed by wavelength into each of the channels (f 01 to f   0n ), and input into the optical receivers  65 . The operation in each optical receiver  65  is identical to that of the embodiment shown in  FIG. 9 , and thus the explanation thereof is omitted. 
     Moreover, in the transmitter  61  using the wavelength division multiplexing transmission scheme, only an example is shown in which a plurality of channels are multiplexed simultaneously by a wavelength-division-multiplexing-light multiplexing filter  80  and converted to RZ intensity modulated signals, but the same effect can be attained by making the polarization of the adjacent wavelength channels orthogonal. 
       FIG. 13  is a block diagram showing a further embodiment of the optical transmitter according to the present invention. Here, instead of the LD that generates a single longitudinal mode signal for the optical transmitter shown in  FIG. 1 , a dual mode beat pulse generating unit  4 ′ is used. Thereby, the duty cycle of the RZ pulse signal can be uniform between pulses, and thus the transmission quality can be improved. 
     In the figure, the blocks having reference numerals identical to those of the blocks shown in  FIG. 1  are identical to those of  FIG. 1 . A dual mode beat pulse signal whose repetition frequency is equal to the line rate is generated by the dual mode beat pulse generating unit  4 ′. The details of this unit will be explained below because a structure identical to the two beat generating pulse unit  91  shown in  FIG. 41  can be used. 
     Moreover, the mode-locked semiconductor laser that has a two mode oscillation disclosed in citation K. Sato, A Hirano, N. Shimizu, T Ohno, and H Ishii, “Dual mode operation of semiconductor mode-locked lasers for anti-phase pulse generation”, OFC&#39;2000, 320/ThW3-1, 2000, can be used. 
     The dual mode beat pulse is PSK modulated by the phase modulating unit  3 . The structure of this unit can be selected from the structures shown in either  FIG. 3  or  FIG. 7 . The pre-coding unit  2  can be structured identically to the pre-coding unit  2  shown in FIG.  35 . The connections between the phase modulating unit  3  and the pre-coding unit  2  are identical to those in  FIG. 35  when using the phase modulating unit  3  shown in  FIG. 2 . 
     The advantage of using the dual mode beat pulse generating unit  4 ′ is that the loss in the phase modulation/amplitude modulation converting optical filter used as an optical filter unit  5  is largely decreased in comparison to the case of using a CW light source such as those in  FIGS. 1 and 2 . In addition, the duty cycle of the generated pulse train can be determined by a dual mode beat pulse, and thus fluctuations of the duty cycle due to the input data pattern can be suppressed. Thereby, in particular the tolerance with respect to optical nonlinear effects in the optical fiber can be improved in comparison to using the optical transmitter shown in  FIGS. 1 and 2 . In addition, there are the advantages that the symmetry of the two generated duobinary optically modulated spectra can be improved, and thus the chromatic dispersion tolerance characteristics can be improved. 
       FIG. 14  is a diagram showing another embodiment of the optical transmission system according to the present invention. In addition,  FIGS. 15 ,  16 , and  20  are diagrams for explaining the operation thereof. 
     In  FIG. 14 , the output of the optical transmitter shown in  FIG. 13  is input into the optical transmission medium  69 . A pre-coding circuit whose delay time is 1 bit (n=1) is used as a pre-coding unit  2 . In addition, a phase modulation/amplitude modulation converting Mach-Zehnder interferometer type optical filter whose delay time is 1 bit is used as an optical filter unit  5 . A single mode fiber can be used as an example of the optical transmission medium  69 . In addition, a direct detection receiver can be used as an optical receiver  66 . 
       FIGS. 15A through 15F  show the waveforms for each of the parts in  FIG. 14 . In  FIG. 15A , a binary NRZ data electrical signal (line rate B) is input into a pre-coding unit  2 . In the dual mode beat pulse signal generating unit  4 ′ in  FIG. 14 , in the case, for example, of using a mode-locked semiconductor laser, a sine wave synchronized with the data signal and having a repetition frequency of B equal to the line rate B is input. The dual mode beat pulse signal generating unit  4 ′ generates the dual mode beat pulse having the modulated spectrum shown in  FIG. 16A , and inverts the phase thereof for each bit as shown in  FIG. 15B . 
     When this dual mode beat pulse is modulated at the timing such as that in  FIG. 15C  by the phase modulating unit  3  in  FIG. 13 , as shown in  FIG. 16B , an optical signal spectrum having an optical modulated band of 3B in which the carrier component is suppressed is output, and thereby the phase modulated RZ signal as shown in  FIG. 15D  is generated. In the case that this phase modulated RZ signal is used as a transmission code, the optically modulation band can be reduced in comparison to the technology disclosed in the citation (Miyano et al.). This signal is input into the phase modulation/amplitude modulation converting optical filter used as the optical filter unit  5 . 
     A Mach-Zehnder interferometer type optical filter whose FSR (free spectral range) is equal to the line rate B can be considered as the phase modulation/amplitude modulation converting optical filter, and the optical signal output from the port (shown by the solid line) having transmittance characteristics in which, as shown in  FIG. 16C , the center frequencies of the rejection bands coincide with the carrier frequency f 0  can be considered. At this time, as shown in  FIG. 16D , as an optical signal spectrum, the DCS-RZ signals in which two optical duobinary signal spectra are arranged at a frequency difference of B can be obtained, and the intensity modulated waveform shown in  FIG. 16E  can be obtained. The phase of this output optical signal is inverted at each mark bit. 
     In contrast, as shown in  FIG. 16C , the optical signal output from a port (shown by the dashed line) having transmittance characteristics such that the center frequencies of the pass bands coincided with the carrier frequency f 0  can be considered. Here, as an optical signal spectrum, a duobinary RZ signals in which three optical duobinary signal spectra are arranged at the frequency difference B as shown in  FIG. 16E  are obtained, and an intensity modulated waveform such as that shown in  FIG. 15F  is obtained. This output optical signal has phase modulating rules identical to a duobinary signal. 
     Either of the bandwidths of the RZ signals described above is narrow, respectively equal to or less than 2B or 3B. Therefore, it is understood that the bandwidth-reduced RZ intensity modulated signal in comparison with the conventional code can be generated by using the NRZ code as a baseband signal. 
       FIG. 17  shows an embodiment of the optical transmission system using the optical transmitter shown in  FIG. 13 . This differs from the embodiment shown in  FIG. 14  in that the amount of delay of the pre-coding unit  2  and the amount of delay of the MZI optical filter used as the optical filter unit  5  are selected so that each takes 2 time slots. 
     In  FIG. 17 , the output of the optical transmitter shown in  FIG. 13  is input into the optical transmission medium  69 . A two bit pre-coding circuit having a delay time n=2 is used as a pre-coding unit  2 , and in addition, a phase modulation/amplitude modulation converting Mach-Zehnder interferometer type optical filter circuit having a delay time of 2 bits is used as the optical filter unit  5 . As one example of the optical transmission medium  69 , a single mode fiber is used. In addition, a direct detection receiver is used as the optical receiving unit  66 . 
       FIGS. 18A through 18G  show the waveforms for each of the blocks shown in  FIG. 17 . In  FIG. 18A , a binary NRZ data electrical signal (line rate B) is input into the pre-coding unit  2 . In the dual mode beat pulse generating unit  4 ′ shown in  FIG. 17 , in the case, for example, of using a mode-locked semiconductor laser, a sine wave having a repetition frequency of B that is equal to the line rate B synchronized with the data signal is generated. The dual mode beat pulse generating unit  4 ′ generates a dual mode beat pulse having the modulated spectrum as shown in  FIG. 19A , and inverts the phase thereof for each bit, as shown in  FIG. 18C . 
     When the phase modulating unit  3  shown in  FIG. 17  performs phase modulation on this dual mode beat pulse using a pre-coded signal at the timing such as that in  FIG. 18B , as shown in  FIG. 19B , an optical signal spectrum having the optical modulation band of 3B in which the carrier component is suppressed is output, and a phase modulated RZ signal as shown in  FIG. 18D  is generated. In the case that this phase modulated RZ signal is used as a transmission code, the optical modulation band can be reduced in comparison to the technology of the above citation (Miyano et al.). This signal is input into the phase modulation/amplitude modulation converting optical filter that forms the optical filter unit  5 . Here, a Mach-Zehnder interferometer type optical filter whose FSR is equal to half the line rate B can be considered as the phase modulation/amplitude modulation converting optical filter. An optical signal output from a port (shown by the solid line) having transmittance characteristics in which the rejection band center frequencies thereof are arranged equally around the carrier frequency f 0  as shown in  FIG. 19C  can be considered. At this time, the optical signal spectrum of this port has a signal spectrum such as that shown in  FIG. 19E , and an RZ intensity modulated optical signal such as that shown in  FIG. 18G  can be obtained. 
     In contrast, an optical signal output from a port (shown by the dashed line) having transmittance characteristics such that the pass band center frequencies are arranged equally at the carrier frequency f 0  as shown in  FIG. 19C . At this time, the optical signal spectrum shown in  FIG. 19D  is obtained, and the intensity modulated frequency as shown in  FIG. 18F  is obtained. 
     Either of the bandwidths of the RZ signals described above is narrow, respectively equal to or less than 2B or 3B, and it is understood that use the NRZ code can be used as a baseband signal, and the bandwidth-reduced RZ intensity modulated signal in comparison with the conventional code can be generated. 
       FIG. 20  is a block diagram showing another embodiment of the optical transmitter according to the present invention. The embodiment shown in  FIG. 20  differs from the embodiment shown in  FIG. 13  in that the optical filter unit  5  for converting phase modulated signals to intensity modulated RZ signals is not present. Therefore, the difference is only that the signal transmitted through the optical transmission medium line (not illustrated) is an RZ intensity modulated phase-encoded signal. 
       FIG. 21  is a block diagram showing another embodiment of the optical transmission system according to the present invention using the optical transmitter shown in  FIG. 20 . This differs from the optical transmission system shown in  FIG. 15  in that the phase modulation/amplitude modulation converting optical filter  60  is formed inside the receiver disposed at the output of the optical transmission medium  69 . A 1-bit delay Mach-Zehnder interferometer type optical filter is used as the phase modulation/amplitude modulation converting optical filter  60 , and in the case of considering the frequency setting identical to that of  FIG. 16C , the optical output of the two arms of the Mach-Zehnder interferometer type optical filter becomes a complementary optical intensity modulated output signal as shown in  FIGS. 15E and 15F . 
     Specifically, in the case that a direct detection receiver carries out regeneration by using this type of delay detection, it can be understood that the alternating phase modulation by the dual mode beat pulse on the transmitting side does not influence the result of the demodulation of the data. In the optical receiving unit  66  in  FIG. 21 , as shown in  FIG. 22A , the received optical phase modulated RZ signal can be converted to the intensity modulated signal shown in  FIG. 15E  using the MZI optical filter, and can be received at the normal direct detection receiver. In addition, as shown in  FIG. 22B , the received optical phase modulated RZ signal is converted to the intensity modulated signal shown in  FIG. 15F  using the MZI optical filter, directly detected and then regenerated, and can be demodulated after inverting the logic thereof. This inversion operation can be carried out at the input of the pre-coding unit  2  of the optical transmitter. 
     In addition, as shown in  FIG. 22C , differential reception of the two outputs shown in  FIGS. 16D and 16E  can be carried out using two photo detectors. The receiver sensitivity when carrying out differential reception can be improved by a 3 dB in comparison to the case where differential reception is not carried out. 
       FIG. 23A  through  FIG. 26C  are diagrams for explaining another embodiment of the present invention. As can be understood from  FIGS. 16D and 16E , the optical duobinary component is included in the demodulated intensity modulated signal components. Therefore, as shown in  FIGS. 23A ,  23 I,  23 J,  24 D,  24 G, and  24 H, a band pass filter such as the ones in  FIGS. 23I and 24G  is provided in the receiver, and by direct detection, as shown in  FIGS. 23J and 24H , an arbitrary optical duobinary signal component included in the demodulated signal can be obtained, and the chromatic dispersion tolerance can be increased. 
     In this manner, the optical duobinary signal spectrum can be extracted from the signal spectrum of the received optical phase modulated RZ signal by filtering, and thereby, the chromatic dispersion tolerance can be almost doubled in comparison to the case of using the receiver shown in  FIG. 22A . 
     Moreover, in  FIGS. 24D ,  24 G, and  24 H, the optical duobinary component of the upper side band is extracted, but the optical duobinary component of the lower side band can also be extracted. In addition, in  FIGS. 23A ,  23 I, and  23 J, the optical duobinary component that includes the carrier frequency is extracted, but either one of the upper or lower optical duobinary components can be extracted. Specifically, by using a phase modulated code that has been RZ intensity modulated as the transmission code, the tolerance with respect to the optical nonlinear effects on the optical transmission medium can be improved, and at the receiving side, by carrying out phase modulation/amplitude modulation conversion and then limiting the signal band in the optical carrier frequency domain, the chromatic dispersion tolerance of the transmission line ban be improved. 
       FIGS. 25A and 25B  are examples of a concrete structure that adds a band limiting function to the optical filter unit and thereby improves the chromatic dispersion tolerance of the demodulated signal. 
     In  FIG. 25A , the RZ phase modulated signal that is transmitted over the optical transmission medium is amplified by the optical pre-amplifier  251 , and then is converted to an intensity modulated signal by the MZI optical filter  252 . Here, by disposing the optical band pass filter  253  having a transfer function that is nearly square, as shown in  FIGS. 23I and 24G , between the MZI optical filter  252  and the direct detection receiver  254 , the optical duobinary component can be extracted. 
     In contrast,  FIG. 25B  is an example in which the equivalent functions of the MZI filter  252  in  FIG. 25A  and the band limiting optical band pass filter  253  are realized by one Gaussian filter  255 , which is easily realized. Specifically, like  FIG. 25A , the RZ phase modulated signal that is transmitted over the optical transmission medium is amplified by the optical pre-amplifier  251 , and then the center frequency of the Gaussian filter  255  is matched with the center frequency of the desired optical duobinary component, and the phase modulation/amplitude modulation conversion and the band limiting function can be realized by a single device. 
       FIGS. 26A through 26C  are diagrams showing a graphical representation of the results of numerical calculation of a concrete experimental example of  FIG. 25B . 
       FIG. 26A  is an example of the calculation in the case of the frequency arrangement shown in  FIGS. 23A ,  23 I, and  23 J, and shows with the dashed line the modulated spectrum of an RZ phase modulated signal modulated with a M sequence pseudo-random signal having a line rate of 43 Gbit/s. The solid line shows the optical duobinary component extracted by one Gaussian filter whose full width at half maximum is 24 GHz. The waveform of the extracted signal that has been directly detected is shown in  FIGS. 26B and 26C . According to  FIGS. 26B and 26C , it can be confirmed that the original PN 2 7 −1 stage signal has been demodulated as the demodulated waveform with good eye opening has been obtained, and that the demodulation of an optical duobinary signal having a low inter-symbol interference is possible. 
       FIG. 27  is a diagram showing an example of the structure of a phase modulation/amplitude modulation converting optical filter used in the transmitter or the receiver according to the present invention. 
     In the case of using a Mach-Zehnder interferometer type optical filter, there is the problem that the transmittance characteristics change due to the input polarization state. In particular, in the case of using this type of optical filter on the receiver side, for example, in the case of using an optical fiber as an optical transmission medium, there is the problem that the receiver characteristics fluctuate due to changes in the polarization state after transmission. The structure of a phase modulation/amplitude modulation converting filter that solves this problem is shown in  FIG. 27 . 
     In  FIG. 27 , the optical phase modulated signal is input into the port  1  of the circulator  271 , and output from the port  2  of the circulator  271 . The optical phase modulated signal from the port  2  is input into the polarization beam coupler/splitter  272 , and separated into two linearly polarized wave components which are orthogonal to each other. The polarization axis of one of the optical phase modulated signal components of the split polarization components is rotated by 90°. 
     Two polarization separated signals are coupled to either a TE propagatiori mode or a TM propagation mode of the phase modulation/amplitude modulation converting optical filter. The optical signals are propagated together in the opposite directions, and have undergone phase modulation/amplitude modulation conversion with the same transmission mode. The converted intensity modulation signal is again input into the polarization beam coupler/splitter  272 , is polarization-combined, and then output from the port  1  of the polarization beam coupler/splitter  272 , and input into the port  2  of the circulator  271 . The demodulated intensity modulation signal that has been polarization-combined can be extracted from the port  3  of the circulator. 
     Due to having this type of structure, no matter the polarization state of the input optical phase modulated signal, it is always only sensitive to filter characteristics in one direction of polarization. Thereby, phase modulation/amplitude modulation conversion that is not influenced by the polarization sensitivity of the phase modulation/amplitude modulation converting filter can be realized. 
       FIGS. 28A and 28B  present as a graph an experimental example of the phase modulation/amplitude modulation converting optical filter in the 1.58 μm band. Here, an MZI interference filter having FSR of 50 GHz formed on a silica waveguide is used as an optical filter. In the case that the structure shown in  FIG. 27  is not used, when the input polarization changes, a maximum shift in the transfer function of approximately 9 GHz can be observed. 
     In contrast, by using the structure shown in  FIG. 27 , as shown in  FIG. 28B , a transfer function of the optical filter that is almost completely insensitive to the input polarization can be realized, and it is confirmed that a stable phase modulation/amplitude modulation signal conversion can be realized. 
       FIG. 29  is a block diagram showing yet another embodiment of the phase modulation device and the pre-coding device used in the optical transmitter according to the present invention, and here a multi-stage structure where n=2 is shown. 
     The difference between this embodiment and the optical phase modulation unit of the embodiment shown in  FIG. 2  is that the optical phase modulating unit  3  comprises two DPSK modulating units  301  and  302  connected in series and the two input data signals can be time division multiplexed in the processing block of the optical carrier frequency band, and this is desirable for the realization of the high speed operation. 
       FIGS. 30A to 30J  are diagrams for explaining the operation of the transmitter shown in  FIG. 29 .  FIGS. 30A and 30B  are two independent multiplexed digital electrical signals D 1  and D 2  that have the same bit rate, and are binary NRZ signals. In addition, D 1  and D 2  are time division multiplexed to generate the binary NRZ multiplexed signal shown in  FIG. 30C .  FIG. 30D  shows the data generated by pre-coding this multiplexed signal. The signal generated by the DPSK modulation of D 1  and D 2  using time division multiplexing is phase modulated in conformity to  FIG. 30D . The optical transmitter shown in  FIG. 29  generates the multiplexed phase modulated signal by carrying out the processing using the processing block of the optical carrier frequency band instead of carrying out multiplexing in the baseband. 
     Moreover, in  FIGS. 30A to 30J ,  FIGS. 30E and 30F  are pre-coded NRZ data P 1  and P 2  that have been obtained by converting D 1  and D 2  using the pre-coding unit  2  shown in  FIG. 35 , and the code is inverted each time a mark bit is input. The pre-coded NRZ data P 2  is delayed by T/2 (where T is one time slot of the input data signals D 1  and D 2 ) as a relative delay difference in comparison to P 1 . The signal INPUT is phase modulated, as shown in  FIG. 30E , by the DPSK modulator  301  based on P 1 , and input into the DPSK modulator  302 . The DPSK modulators  301  and  302  carry out phase modulation using P 2  in accordance with a DPSK format in the manner shown in  FIG. 30G  at a timing delayed by T/2 in comparison to the phase modulation timing of D 1  shown in  FIG. 30F . As a result, the DPSK modulators  301  and  302  output a signal shown in  FIG. 30H . It can be understood that a phase modulated signal, which is the same as the phase modulated signal obtained from one of the DPSK modulator  301  and  302 , is generated using the multiplexed data shown in  FIG. 30D  that is obtained by the time division multiplexing of D 1  and D 2  and the pre-coding of the multiplexed signal. The signal shown in  FIG. 30H  and the 1 bit delayed signal shown in  FIG. 301  are combined using a Mach-Zehnder interferometer type modulator, and when phase modulation/amplitude modulation conversion is carried out, as shown in  FIG. 30J , a signal has been demodulated that is identical to the original multiplexed signal shown in  FIG. 30C . 
       FIG. 31  is a block diagram showing yet another embodiment of the pre-coding device (in the case of a 1 bit delay) used in the optical transmitter according to the present invention. The embodiment shown is basically the same as the embodiment shown in  FIG. 29 , but instead of carrying out exclusive OR of a plurality of delayed pre-coded multiplexed signals in the optical phase modulator using the carrier frequency, this is carried out using the baseband. 
     Here, the two synchronized independent NRZ signal  1  and NRZ signal  2  having identical line rates of B are each pre-coded by the pre-coding unit  2  shown in  FIG. 35 . At this time, the delay time of the delay element is set equal to the time slot T 0  of NRZ signal  1  among the multiplexed NRZ  1  and  2  signals. One of the pre-coded NRZ signal is delayed by the delay element, the relative delay difference of each of the phase modulation timings is set equal to T 0 /2, and the exclusive OR is processed by the exclusive OR circuit  204 . The output of this exclusive OR circuit  204  is equal to the pre-coded signal after the NRZ  1  input signal and the NRZ  2  input signal are multiplexed by bit interleaving 
     As can be understood from the above explanation, a pre-coding unit  2 , which is difficult to operate at high speed, can be realized by performing parallel processing using a pre-coding circuit that operates as a low speed. As a result, increasing the speed of the pre-coding unit  2  can be easily realized. 
     Here, the optical carrier frequencies (f 01  to f 0n ) of the transmitter are set in agreement with each of the channel grids of the wavelength division multiplexing system shown in  FIG. 34 .  FIG. 34  is a diagram showing an embodiment in which the channel spacing is equal. In each optical transmitter, the optical carrier signal from the light source  4  is modulated using a DPSK optical modulation format by the MZ optical intensity modulator  32 . Baseband signal processing by the DPSK optical modulation format is identical to that in  FIG. 2 , and thus the explanation thereof is omitted. 
     The DPSK optical modulated signals modulated by the optical transmitter for each channel are wavelength division multiplexed by the wavelength-division-multiplexing-light multiplexing filter  80 . After the wavelength division multiplexed DPSK optical modulation code is optically amplified according to necessity, it is input into the optical filter unit  83 . The rejection band optical center frequency of the optical filter unit  83  is set the optical carrier frequency of each channel, and in addition, the period thereof is set to the grid period of the wavelength division multiplexed channels. 
     By setting the operating conditions of the optical filter unit  83  in the above manner, the periodic optical filter unit  83  simultaneously converts the wavelength division multiplexing DPSK optical modulated signals to a wavelength division multiplexing DCS-RZ optical modulated signals. As an example of the periodic optical filter  83 , an MZI type optical filter can be used as an optical filter unit  83 . 
     Subsequently, the simultaneously converted DCS-RZ optical modulation code is optically amplified by the optical amplifier  62  according to necessity, and input into the optical transmission medium  69  at a predetermined transmission channel power. The optical transmission medium  69  can be, for example, a linear repeating transmission line having an optical fiber that is optically directly amplified and repeated by an optical amplifier repeater. The output of the optical transmission medium  69  is optically amplified, and then input into the wavelength demultiplexing filter  81 , the DSC-RZ code is wavelength demultiplexed by each channel, and input into the optical receiver  65  after wavelength demultiplexing. The operation in the optical receiver  65  is identical to that of the embodiment shown in  FIG. 12 , and thus the explanation thereof is omitted. 
     Moreover, here, in the transmitter  85  that uses the wavelength division multiplexing transmission scheme, only an example was given in which a plurality of channels are simultaneously converted to RZ intensity modulated signals by the wavelength-division-multiplexing-light multiplexing filter  80 , but the same effect can be attained by making the polarization of the adjacent wavelength channels orthogonal. 
       FIGS. 36A through 36D  are diagrams for explaining the operation of the embodiments shown in  FIG. 34  and  FIG. 35 . 
     The carrier signals ( FIG. 36A ) disposed at equal spacing in the optical frequency domain are each modulated using the DPSK format, and the output of the wavelength-division-multiplexing-light multiplexing filter  80  becomes a wavelength division multiplexed DPSK optical signal spectrum such as that in  FIG. 36B . The optical filter unit  5 , which is one essential component of the present invention, can be realized by using a periodic optical filter  83  such as that in  FIG. 36C .  FIG. 36D  shows the optically modulated spectrum of the converted wavelength division multiplexing DCS-RZ code. 
     As explained above, by using the periodicity of one periodic optical filter  83 , the wavelength division multiplexing DPSK optical signals can be converted simultaneously to wavelength division multiplexing DCS-RZ optical modulated signals. 
     Moreover, according to the embodiments of the present invention, the wavelength division multiplexing having equal spacing was assumed in the explanation, but unequal spacing can be used, and in this case, optical filters having equal transfer functions in the optical signal band of the respective channels are used. 
       FIG. 37  is a diagram for explaining yet another embodiment of the optical transmission system according to the present invention. The differences between this embodiment and the embodiment shown in  FIG. 34  are that the transmission format is a phase encoded RZ format using a dual mode beat pulse and that a phase modulation/amplitude modulation converting periodic optical filter  70  is disposed at the receiving end. The optical transmission medium can be a linear repeating transmission line having an optical fiber that is optically amplified and repeated by the optical amplifier repeater  62 . The structures shown in  FIGS. 22A through 22C  and  FIGS. 25A and 25B  are used as an optical receiver. 
     Here, the optical carrier frequency (f 01  to f 0n ) of the transmitter are set to match each of the channel grids of the wavelength division multiplexing system shown in  FIG. 37 .  FIGS. 38A through 38E  show examples in which the wavelengths are equally spaced (3B, which is three times the line rate). Each of the optical transmitters can have the structure shown in  FIG. 20 . Here, the phase modulating unit  3  can be any of the structures shown in  FIGS. 2 ,  7 , and  29 . The baseband processing using the DPSK optical modulation format is identical to that in  FIGS. 2 and 29 , and thus the explanation thereof is omitted. 
     The DPSK-CS-RZ signals ( FIG. 38 ) modulated by the optical transmitters of respective channels is wavelength division multiplexed by the wavelength-division-multiplexing-light multiplexing filter  81 . The wavelength division multiplexed DPSKCS-RZ optical transmission code is optically amplified according to necessity, and then input into the optical transmission medium  69 . The output of the optical transmission medium  69  is input into the optical phase modulation/amplitude modulation converting periodic optical filter  70 . The central frequency of the rejection band of the optical phase modulation/amplitude modulation converting periodic optical filter  70  is set to the optical carrier frequency of each of the channels, and in addition, the period thereof is set to the grid period of the wavelength division multiplexed channels. In  FIGS. 38A through 38E , the FSR is set equal to the line rate B.  FIG. 38A  shows the spectrum of the dual mode beat pulse signals generated by each of the optical transmitters in  FIG. 37 .  FIG. 38B  shows the spectrum of a signal that is wavelength division multiplexed by the wavelength-division-multiplexing-light multiplexing filter  81  after phase modulating the dual mode beat pulse signal in each of the optical transmitters shown in  FIG. 37 . 
       FIG. 38C  is a diagram showing the relationship between the optical carrier frequency f 0i  (i=1 to n) of each of the optical transmitters and the phase modulation/amplitude modulation converting periodic filter  70 . 
     In  FIGS. 38A to 38E , a 1-bit delay Mach-Zehnder optical filter is used as the optical periodic filter  70 . If the pass band arrangement for the optical periodic filter is selected as shown by the solid line in  FIG. 38C , then as shown in  FIG. 38D , differential output RZ optical intensity modulated signals are generated by simultaneous conversion, and it is output from the optical periodic filter  70 . In addition, if the pass band arrangement is selected as shown by the dashed line in  FIG. 38C , then as shown in  FIG. 38E , sum output RZ optical intensity modulated signals are generated by simultaneous conversion, and it is output from the optical periodic filter  70 . One of the above RZ optical intensity modulated signals generated by simultaneous conversion is demultiplexed by wavelength by a wavelength-division-multiplexing-light multiplexing/demultiplexing filter  82 , and demodulated at each of the optical receivers. 
     The FSR should be set equal to or greater than the line rate B. For example, considering WDM having a line rate of 43 Gbit/s and a 100 GHz spacing, demodulation is possible even if the FSR of the optical phase modulation/amplitude modulation converting periodic optical filter  70  (MZI optical filter) is 50 GHz. In addition, in the case that an MZI filter can be used as the optical phase modulation/amplitude modulation converting periodic optical filter  70 , polarization insensitive optical phase modulation/amplitude modulation converting filter shown in  FIG. 27  is preferably used. 
       FIG. 39  is a diagram showing an embodiment in the case that the function of the phase modulation/amplitude modulation converting periodic optical filter  70  and the function of the wavelength-division-multiplexing-light multiplexing/demultiplexing filter  82  are realized by one optical filter. In addition,  FIGS. 40A through 40C  are diagrams for explaining the conversion operation of the filter shown in  FIG. 39 . 
     In  FIG. 39 , like the receiving unit in  FIG. 37 , the wavelength division multiplexing RZ phase encoded signal is input into the receiver  66  shown in  FIG. 39  after being transmitted over an optical transmission medium such as an optical filter. In the optical receiver  66 , the wavelength division multiplexed signals are amplified together by the optical amplifying pre-amplifiers  62 , and input into the wavelength demultiplexer  180 . The center frequency of the pass band in each of the channels of the wavelength demultiplexer  180  is set to the carrier frequency of each of the channels. Here, the wavelength division multiplexing having equal spacing was assumed in the explanation, but unequal spacing can be used. 
     The pass band shape of the wavelength demultiplexer is a Gaussian, and its FWHM is set from 0.5 to 0.6 times the channel line rate. In the example shown in  FIGS. 26A through 26C , the FWHM is set to 0.56 times the line rate. By setting in this manner, the wavelength division multiplexing RZ phase modulated signals are simultaneously converted into wavelength division multiplexed duobinary signals. 
     The advantage of this scheme is that realization of optical phase modulation/amplitude modulation converting filter is easy and the loss in the pass band is low. In addition, a Gaussian filter whose pass band is insensitive to the input polarization can be used. For example, when the array waveguide grating filter disclosed in the citation, H. Takahashi, K. Oda, H. Toba, and Y. Inoue, “Transmission characteristics of arrayed waveguide N×N wavelength multiplexer”, IEEE J. Lightwave Technol, 13, No. 3, pp. 447–455, 1995, is used, a wavelength division multiplexing demultiplexer using a Gaussian filter having uneven spaces or even spaces can be realized. 
     When the optical phase modulation/amplitude modulation converting filter has the above operating conditions, the periodic filter simultaneously converts the wavelength division multiplexing DPSK optical modulated signals to wavelength division multiplexing RZ signals. 
     Moreover,  FIGS. 32A ,  32 B,  33 A and  33 B are diagrams for explaining the effect of the optical transmission system according to the present invention using the phase modulation format as the transmission format. 
       FIGS. 32A and 32B  show the models of two computer simulations. The computing conditions are as follows: line rate, 43 Gbit/s; number of channels, 4; wavelength division multiplexing channel spacing, 100 GHz; and signal wavelength, C band. The fiber transmission line was a 200 km optical amplifier and repeater transmission line, and each span comprised of a 100 km dispersion shifted fiber (loss, 0.21 dB; dispersion value, +2 ps/nm/km; dispersion slope, 0.07 ps/km/nm). The dispersion of the first span is compensated by the optical amplifier repeater such that the center wavelengths of channel 2 and channel 3 have zero dispersion, and the output of the first span is input into the second span. 
     A calculation model ( FIG. 32A ) in which an RZ format that carries out phase modulation/amplitude modulation conversion in the transmitter as shown in  FIG. 14  is used, is compared with a calculation model ( FIG. 32B ) in which an RZ format that carries out phase modulation/amplitude modulation conversion in the receiver as shown in  FIG. 21  is used. 
     Here, a Mach-Zehnder (MZI) type optical filter whose FSR is equal to the line rate B (=43 GHz) was used as the phase modulation/amplitude modulation converting filter  60 . Each of the optical carrier signals is modulated by the optical transmitter  61  shown in  FIG. 13  or  FIG. 20 , then wavelength division multiplexed by a wavelength division multiplexing filter at a 100 GHz spacing, and input into the fiber transmission line  63 . The output of the fiber transmission line  63  is optically amplified, demultiplexed by wavelength, and then the chromatic dispersion of the transmission line is compensated by the chromatic dispersion compensating device D ( 64 ). In  FIG. 32A , the signal received by the direct detection receiver  66  is regenerated. In  FIG. 32B , the dispersion compensated optical signal is input into the MZI filter, which serves as the phase modulation/amplitude modulation converting optical filter  60 , and after being converted to an intensity modulated signal, the signal received by the direct detection receiver  66  is regenerated. 
       FIG. 33A  shows for each of the channels the tolerance range of the eye opening penalty within 1 dB in the case that the channel power and the total dispersion (i.e., the total of the dispersion of the fiber transmission line and the dispersion of the dispersion compensating device D) change for the structure shown in  FIG. 32A . 
     In the case of linear transmission where the channel power is equal to or less than 2 dBm, the dispersion tolerance of each of the channels becomes about 80 ps/nm, which is almost twice that of the dispersion tolerance in comparison with normal RZ format can be realized. As can be understood from these results, the RZ format transmission scheme described above has a wide dispersion tolerance. The allowable channel power for which the transmission characteristics for all channels remain within an eye opening penalty of 1 dB is determined by channels 2 and 3, and is approximately +5 dBm. 
       FIG. 33B  shows for each of the channels the tolerance range of the eye opening penalty within 1 dB in the case that the channel power and the total dispersion (i.e., the total of the dispersion of the fiber transmission line and the dispersion of the dispersion compensating device D) changes for the structure shown in  FIG. 32B . 
     In the case of linear transmission where the channel power is equal to or less than 2 dBm, the dispersion tolerance of each of the channels becomes about 80 ps/nm, which is almost twice that of the normal RZ format, and a dispersion tolerance equivalent to the case shown in  FIG. 32A  can be realized. As can be understood from these results, the RZ format transmission scheme described above has a wide dispersion tolerance in comparison to conventional technology. In addition, allowable channel power for which the transmission characteristics for all channels remain within an eye opening penalty of 1 dB is determined by channels 2 and 3, and is approximately +8 dBm. 
     As can be understood from these results, the allowable channel power can be improved by approximately 3 dB over that of the scheme shown in  FIG. 32A , and the tolerance with respect to degradation due to non-linear effects can be improved. Furthermore, because the optimal dispersion value does not depend on the channel power, an optical amplifier repeater system whose channel power has a wide dynamic range can be realized using a wavelength division multiplexing technology. 
     When compared to the  FIGS. 33A and 33B , in the case that the optical transmission line is linear, identical characteristics are obtained that do not depend on the position of the phase modulation/amplitude modulation converting optical filter  60 , but the channel power is increased, and in the case that the optical transmission line is non-linear, it can be understood that in comparison to  FIG. 33C , in  FIG. 32B  a robust transmission characteristics can be realized due to the non-linear effects, and it can be understood that a new effect is obtained thereby.