Abstract:
An optical data transmitting apparatus and method allows data transmission over a distance that surpasses a limit, which has been imposed due to group velocity dispersion (GVD) and self phase modulation (SPM) effect within optical fiber. This has been accomplished by allocating trinal duobinary symbols to optical 1, P, −1 symbols. These optical symbols have the same intensity, +1 and −1 symbols have inverted optical phases each other, and orthogonally polarized each other between ±1 and P symbols. At the receiver, conventional polarization shift keying receiver can be used to restore original binary data stream.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to an optical data transmitting apparatus and method for an optical data transmission system using optical fiber.  
         BACKGROUND OF THE INVENTION  
         [0002]    Recently, in optical fiber data transmission, an intensity modulation direct detection system is widely used and wavelength-multiplexing technique is employed for increased data transmission capacity. In this system, a combination of group velocity dispersion (GVD) and self phase modulation (SPM) occurring in optical fiber causes waveform distortion, resulting in degraded data transmission quality.  
           [0003]    GVD is phenomena that propagation speed through optical fiber varies with wavelength. Modulated signal suffers from waveform distortion because of its wide signal spectrum. Recent technology provides a compensation element for waveform distortion due to CVD Such a compensation element, however, is expensive. Thus, there have been proposals for modulated signals, which are less affected by GVD. The narrower the signal spectrum, the less the signal is affected by GVD. Thus, improvement may be expected if signal spectrum is narrowed. Optical duobinary modulation is one approach. JP-A 8-139681, which is deemed to be equal to U.S. Pat. No. 5,543,952 (Yonenaga et al.), and K. Fukuchi et al. OFC &#39;97 Technical Digest, ThH3, 1997 teach optical data transmission utilizing such optical duobinary modulation. Duobinary signal falls in a category of partial response signals. Three-level duobinary signal is known, which has three levels of amplitude. According to the optical duobinary modulation, three levels are allocated to three states +1, 0, −1 using phase and amplitude of light. 0 indicates that intensity is zero level. +1 and −1 indicate that intensity is one level, but phases are inverted to each other. At receiver, this optical duobinary modulated signal is decoded to the original two-level digital signal by normal direct detection. During propagation through optical fiber, the signal band is left compressed and waveform distortion due to GVD is low.  
           [0004]    SPM is another reason for degradation of data transmission quality. SPM is phenomena that phase modulation is applied to signal as light intensity varies with time. This problem is unavoidable if intensity modulation is employed. The above-mentioned optical duobinary signal is the same as the ordinary intensity modulated signal in that it uses light intensity. Thus, suppression of SPM effect cannot be expected. SPM increases signal spectrum bandwidth although it does not alter the signal waveform, thereby limiting density of wavelength multiplexing. SPM cooperates with GVD to induce combined SPM+GVD effect.  
           [0005]    As one approach to reduce SPM effect, polarization modulation has been proposed. According to the polarization modulation, symbols of signal are allocated to different states of polarization of light. As intensity envelope is unaltered, SPM effect is less, thus suppressing degradation of data transmission quality. JP-A 1-208920 (Fukaya) discloses an optical fiber data transmission employing such polarization modulation.  
           [0006]    SPM+GVD effect, in question, is phenomena that, during propagation through optical fiber, SPM occurs concurrently with occurrence of waveform distortion, and thus the dispersion compensation technique cannot restore the waveform. If SPM occurs concurrently with waveform distortion, light signal is subjected to frequency chirping. The presence of GVD causes the chirping to induce further waveform distortion. This further waveform distortion induces new SPM. Thus, the waveform distortion is different from distortion in the absence of SPM and this is the reason why the dispersion compensation technique is ineffective to restore the waveform distortion under the presence of SPM. The GVD+SPM is thus considered as the major obstacle to extension of data transmission distance in optical fiber data transmission over distance exceeding 1000 kilometers.  
           [0007]    From the preceding description, it is to be understood that the conventional duobinary signal is ineffective to SPM, and polarization modulation is ineffective to GVD although the former is effective to GVD and the latter is effective to SPM.  
         SUMMARY OF THE INVENTION  
         [0008]    The present invention aims at providing a signal combining the merits of them.  
           [0009]    According to one aspect of the present invention, there is provided an optical data transmitting apparatus comprising:  
           [0010]    a code encoder transforming a binary input data signal to obtain a duobinary signal having a center level and two extreme levels;  
           [0011]    a carrier source of a carrier light;  
           [0012]    an optical modulator modulating phase and polarization of said carrier light in response to said duobinary signal to obtain a modulated output light,  
           [0013]    said modulated output light having a first polarization state that corresponds to said center level of said duobinary signal, and second and third polarization states that correspond to said two extreme levels, respectively,  
           [0014]    said second and third polarization states sharing a common plane of polarization that is orthogonal to a plane of polarization of said first polarization state,  
           [0015]    said second and third polarization states having phases that are inverted to each other.  
           [0016]    According to another aspect of the present invention, there is provided an optical data transmitting apparatus comprising:  
           [0017]    a code converter transforming a binary input data signal to obtain a pair of mutually complementary duobinary signals;  
           [0018]    a carrier source of a carrier light;  
           [0019]    an intensity modulator separating said carrier light into first and second carrier lights, altering phases of said first and second altered carrier lights in response to said pair of mutually complementary duobinary signals, respectively, combining said phase altered first and second carrier lights to obtain a pair of mutually complementary interference output light; and  
           [0020]    means for combining said pair of mutually complementary interference output light beams in mutually orthogonal polarized relation.  
           [0021]    According to still another aspect of the present invention, there is provided an optical data transmission apparatus comprising:  
           [0022]    means for separating a binary input data signal into a first altered data signal and a second altered data signal;  
           [0023]    a polarization modulator modulating polarization of said carrier light in response to said first altered data signal to obtain polarized carrier light;  
           [0024]    a pre-coder transforming code of said second altered data signal to obtain coded second altered data signal; and  
           [0025]    a phase modulator modulating phase of said polarized carrier light in response to said coded second altered data signal.  
           [0026]    According to further aspect of the present invention, there is provided an optical data transmitting apparatus comprising:  
           [0027]    a carrier source for carrier light;  
           [0028]    means for separating a binary input data signal into a first data signal and a second data signal;  
           [0029]    a polarization modulator modulating polarization of said carrier light in response to said first data signal to obtain polarization modulated carrier light;  
           [0030]    a pre-coder transforming code of said second data signal to obtain coded second data signal; and  
           [0031]    a phase modulator modulating phase of said polarization modulated carrier light in response to said coded second data signal.  
           [0032]    According to still further aspect of the present invention, there is provided an optical transmitting apparatus comprising:  
           [0033]    a carrier source of carrier light;  
           [0034]    means for separating said carrier light into first and second orthogonally polarized lights, phase modulating said first polarized light in response to a first driving signal to obtain phase modulated first polarized light, phase modulating said second polarized light in response to a second driving signal to obtain phase modulated second polarized light, and combining said phase modulated first and second polarized lights; and  
           [0035]    a driving signal generator inputting a binary input data signal to generate said first and second driving signals.  
           [0036]    According to further aspect of the present invention, there is provided an optical transmitting apparatus comprising:  
           [0037]    a carrier source of carrier light:  
           [0038]    a polarization modulator modulating polarization of said carrier light,  
           [0039]    said polarization modulator being provided with means for inputting duobinary signal, as modulator driving signal,  
           [0040]    said polarization modulator being optical azimuth rotator and operative to optically rotate the polarization of said carrier light in response to said modulator driving duobinary signal such that said carrier light has opposite phase in response to said modulator driving signal being at the maximum or the minimum.  
           [0041]    According to further another aspect of the present invention, there is provided an optical data transmitting method comprising the steps of:  
           [0042]    transforming a binary input data signal to obtain a duobinary signal having a center level and two extreme levels; and  
           [0043]    modulating phase and polarization of carrier light in response to said duobinary signal to obtain a modulated output light,  
           [0044]    said modulated output light having a first polarization state that corresponds to said center level of said duobinary signal, and second and third polarization states that correspond to said two extreme levels, respectively,  
           [0045]    said second and third polarization states sharing a common plane of polarization that is orthogonal to a plane of polarization of said first polarization state,  
           [0046]    said second and third polarization states having phases that are inverted to each other.  
           [0047]    According to further aspect of the present invention, there is provided an optical data transmitting method comprising the steps of:  
           [0048]    transforming a binary data signal to obtain a pair of mutually complementary duobinary signals;  
           [0049]    separating carrier light into first carrier light and second carrier light;  
           [0050]    modulating phases of said first and second carrier lights in response to said pair of mutually complementary duobinary signals, respectively, to obtain phase modulated first and second carrier lights;  
           [0051]    combining said phase modulated first and second carrier lights to interfere with each other to obtain a pair of mutually complementary interference output lights; and  
           [0052]    combining said pair of mutually complementary interference output lights in mutually orthogonal polarized relation.  
           [0053]    According to further aspect of the present invention, there is provided an optical data transmitting method comprising the steps of:  
           [0054]    separating a binary input data signal into a first data signal and a second data signal;  
           [0055]    inputting said first data signal to obtain a pair of mutually complementary interference output lights:  
           [0056]    transforming code of said second data signal to obtain coded second data signal;  
           [0057]    inputting one of said pair of mutually complementary interference output lights and said coded second data signal to modulate phase of said one interference output light in response to said coded second data signal to obtain phase modulated interference output light; and  
           [0058]    combining said phase modulated interference output light and the other of said pair of mutually complementary interference output light in mutually orthogonal polarized relation.  
           [0059]    According to further aspect of the present invention, there is provided an optical data transmitting method comprising the steps of:  
           [0060]    separating a binary input data signal into first data signal and second data signal;  
           [0061]    modulating polarization of carrier light in response to said first data signal to obtain polarization modulated carrier light;  
           [0062]    transforming code of said second data signal to obtain code transformed second data signal; and  
           [0063]    modulating phase of polarization modulated carrier light in response to said code transformed second signal.  
           [0064]    According to further aspect of the present invention, there is provided an optical data transmitting method comprising the steps of:  
           [0065]    separating carrier light into first and second orthogonally polarized lights;  
           [0066]    phase modulating said first polarized component in response to a first driving signal to obtain phase modulated first polarized component;  
           [0067]    phase modulating said second polarized light in response to a second driving signal to obtain phase modulated second polarized light;  
           [0068]    combining said phase modulated first and second polarized lights; and  
           [0069]    inputting a binary input data signal to generate said first and second driving signals.  
           [0070]    According to further aspect of the present invention, there is provided an optical data transmitting method comprising the steps of:  
           [0071]    transforming binary input data signal to obtain duobinary signal as a modulator driving signal; and  
           [0072]    rotating polarization of carrier light, using optical azimuth rotator, in response to said modulator driving signal such that said carrier light has opposite phase in response to said modulator driving signal being at the maximum or the minimum. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0073]    [0073]FIG. 1 is a schematic view of a first embodiment of an optical data transmitting apparatus according to the present invention.  
         [0074]    [0074]FIG. 2 is a schematic view of a second embodiment according to the present invention.  
         [0075]    [0075]FIG. 3 illustrates states of two interference output lights in the first embodiment.  
         [0076]    [0076]FIG. 4 is a circuit diagram illustrating one example of pre-coder.  
         [0077]    [0077]FIG. 5 is a schematic view a third embodiment of the present invention.  
         [0078]    [0078]FIG. 6 illustrates waveforms.  
         [0079]    [0079]FIG. 7 is a schematic view of a fourth embodiment of the present invention.  
         [0080]    [0080]FIG. 8 illustrates polarization modulation in the fourth embodiment.  
         [0081]    [0081]FIG. 9 is a circuit diagram illustrating one example of a driving signal generator.  
         [0082]    [0082]FIG. 10 is a circuit diagram illustrating another example of a driving signal generator.  
         [0083]    [0083]FIG. 11 illustrates two reference levels.  
         [0084]    [0084]FIG. 12 is a schematic view of a first modification of the fourth embodiment.  
         [0085]    [0085]FIG. 13 is a schematic view of a second modification of the fourth embodiment.  
         [0086]    [0086]FIG. 14 is a schematic view of a fifth embodiment of the present invention.  
         [0087]    Table I illustrates code conversion rule employed by pre-coder of the first embodiment.  
         [0088]    Table II illustrates code conversion rule employed by driving signal generator of the fourth embodiment.  
         [0089]    Table III illustrates code conversion rule employed by driving signal generator of the fourth embodiment.  
         [0090]    Table IV illustrates code conversion rule employed by driving signal generator of the fourth embodiment.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0091]    Referring to the accompanying drawings, the present invention is described along with embodiments.  
         [0092]    (First Embodiment)  
         [0093]    [0093]FIG. 1 illustrates a first preferred implementation of the present invention, which employs an optical duobinary modulation technique using a push-pull type Mach-Zehnder optical intensity modulator, which has been mentioned in U.S. Pat. No. 5,543,952. The Mach-Zehnder (NZ) intensity modulator is one kind of interferometers, thus it has two output ports, which are complementary to each other. According to conventional system, one output of the interferometer is allowed to utilize, while the other output is allowed to diverge. According to this first embodiment, the polarization of the other half output is rotated  90  degrees and superimposed on the one half, thereby providing modulated light having a complemented intensity.  
         [0094]    A device in the form of a semiconductor laser  1  is used as a carrier source of carrier light The semiconductor laser  1  produces radiation having a 1.55 μm (micrometers) wavelength band. The output of the laser  1  propagates through a polarization maintaining (PM) optical fiber to a LiNbO3 optical intensity modulator  30  of the push-pull drive MZ interferometer type. The PM optical fiber can transmit the light with its linear propagation unaltered. The modulator  30  separates or divides the incident light at a Y-branch  40  so that half the light travels through each optical path to a 3 dB coupler  43 . The optical paths include phase modulating sections  41  and  42 , respectively, each of which performs phase modulation in response to the corresponding one of duobinary signals  25  and  26 . Superimposition at the 3 dB coupler  43  outputs interference lights  50  and  51 . All of the above-mentioned components of the modulator  30  are integrated on a single substrate and PM fiber pig-tails attached to the input and output.  
         [0095]    Binary (two-level) input data signal  22  is impressed on an input of a first logical inverter (NOT circuit)  11   a.  A pre-coder or elementary encoder  7  codes the output of the inverter  11   a.  The precoded signal is divided into one half fed to a first low pass filter (LPF)  12  to produce a first duobinary signal  25 . The other half the precoded signal is impressed on an input of a second logical inverter  11   b.  The output of the inverter  11   b  is fed to a second LPF  13  to produce a second duobinary signal  26 .  
         [0096]    [0096]FIG. 4 illustrates a circuit diagram of the pre-coder  7 . TABLE I illustrates truth value table showing code conversion rule between a logical input a i , a logical output b i , and a feedback b i−1 . The LPFs  12  and  13 , each take the form of a fifth order Bessel-Thompson low-pass filter that has a cutoff frequency roughly as high as one fourth (¼) a bit rate. The pre-coder  7  and LPF  12  that are used herein are well known as a pre-coder for duobinary process and a (1, 1) converter, respectively. They perform the conversion as follows:  
         [0097]    Input of pre-coder  7 : a i   
         [0098]    Output of pre-coder  7 : b i =a i (Ex-OR) b i−1  Ex-OR: Exclusive OR  
         [0099]    Output of (1, 1) converter: c i =b i +b i+1 .  
         [0100]    Various forms of implementing this conversion are known although specific description of them is hereby omitted.  
         [0101]    The logical inverter  11   b  and LPF  13  path creates the second duobinary signal  26 . This duobinary signal  26  is the same as the duobinary signal  25 , but has the inverted polarity to the latter. This process of producing the second duobinary signal  26  is appropriate and superior to process of producing the signal  26  by inverting the polarity of the first duobinary signal  25 . This is because inverting the signal  25  that is analog might cause deterioration of waveform. If desired, the inverter may be provided for the final output of a receiver side.  
         [0102]    In the modulator  30  of the push-pull type, halves of the light travelling along one and the other optical paths of MZ interferometer are phase adjusted in the opposite directions. Referring to FIG. 3, the operation is further described. FIG. 3 illustrates two interference output lights  50  and  51 , each having three different states that correspond to three amplitude levels, namely, +1, 0, and −1, of duobinary code of an input electric signal, respectively. This input electric signal does not contain any DC component. Adjustment to align the amplitudes and timings of duobinary signals  25  and  26  and appropriate setting of a DC bias adjuster  27  result in the phase modulation as illustrated in FIG. 3. In this Figure, a less bold vector represents light out of each optical path of the MZ interferometer, while a bold vector, which results from two less bold vectors, represents interference output light. It is now understood that the interference output lights  50  and  51  are complementary to each other such that the absolute value of the vector sum is always 1 (one). For simplicity of illustration, relative phase difference between the outputs  51  and  52  has been neglected.  
         [0103]    Twisting one of PM fiber pig-tails for the light outputs  50  and  51  through 90 degrees prior to superimposition at a polarization beam splitter (PBS)  10  results in production of a modulated signal beam  55  (output light) that has a complemented constant intensity. This signal beam  55  is not completely duobinary because it includes an intensity-modulated optical component as well as a duobinary optical component. This does not pose any problem since, upon receiving the transmitted signal, the intensity-modulated component is not used and only the duobinary optical component is used.  
         [0104]    Experiment has been conducted with an optical transmission system incorporating the first embodiment. The experimental conditions are as follows:  
         [0105]    Bit rate is 5 Gb/s. Optical path includes optical fiber with GVD of −1.5 ps/nm/km, optical amplifiers, and dispersion compensating optical fibers (DCF), each having about 20 km in length and GVD of +17 ps/nm/km. The optical amplifiers are disposed at a regular distance of 40 km, and DCF are disposed at a regular distance of about 240 km. In this experiment, the optical fiber having GVD as low as −1.5 ps/nm/km is used. This attributes to the fact that, if an optical fiber with completely zero GVD were used, signal and noise that are added at each optical amplifier would cause creation of light resulting from four-wave mixing, thereby increasing possibility of restricting data transmission performance.  
         [0106]    The result of this experiment shows that, according to the first embodiment, data transmission distance has extended as far as about 6,000 km. This embodiment is advantageous over the conventional intensity modulation system, optical duobinary modulation system, and conventional polarization modulation system. This is because the conventional intensity modulation provided data transmission as far as about 3,000 km, and the optical duobinary modulation system and conventional polarization modulation system each provided data transmission as far as about 5,000 km. The transmission allowable distance was restrained owing mainly to SPM+GVD effect.  
         [0107]    (Second Embodiment)  
         [0108]    According to the first embodiment, the intensity-modulated light  51  is not optically duobinary coded, and thus exhibits less resistivity to dispersion. If this signal is also modulated like optical duobinary, so that its change in waveform becomes gradual against chromatic dispersion, the transmission capability of the system against SPM+GVD effect is increased. Thus, according to the second embodiment, for increased resistivity against chromatic dispersion, this signal is subjected to phase modulation, which will be described below.  
         [0109]    [0109]FIG. 2 illustrates the second embodiment. This embodiment is substantially the same as the first embodiment of FIG. 1 except the provision of a second pre-coder  7   b,  a phase shifter  14 , and a phase modulator  15 .  
         [0110]    Input digital data signal  22  is divided into first data signal and second data signal.  
         [0111]    The first data signal is fed to an optical transmitter for producing output lights  50  and  51 . The optical transmitter used in this embodiment is substantially the same as its counterpart in the first embodiment. The output light  51  passes through the phase modulator  15  and it is superimposed with the other output light  50  at a PBS  10 . The setting of a DC bias adjuster  27  is such that square-law detection of the light  50  will provide a signal train having the same sign as that of the input data signal  22 . In FIG. 2, the reference numeral  7   a  denotes a first pre-coder that corresponds to the pre-coder  7  of the first embodiment.  
         [0112]    The second data signal is fed to the phase modulator  15  via the second pre-coder  7   b  and phase shifter  14 . The second pre-coder  7   b  encodes the second data signal to feed the result to the phase shifter  14 . The phase modulator  15  is driven by signal from the phase shifter  14 . The phase shifter  14  adjusts the driving signal such that a delay of 0.5 bit is provided to the timing with respect to timing of intensity modulation of the light  51 , and thus the phase is inverted at the center of a bit slot where optical intensity is zero.  
         [0113]    The second embodiment may be evaluated such that the intensity-modulated light  51  is encoded to optical duobinary coding employing an encoding technique using serially connected intensity modulator and phase modulator, which is described in the before mentioned K. Fukuchi et al., OPC &#39;97 Technical Digest, ThH3, 1997.  
         [0114]    (Third Embodiment)  
         [0115]    [0115]FIG. 5 illustrates a portion of the third embodiment. The before-mentioned publication K. Fukuchi et al., OFC &#39;97 Technical Digest, ThH3, 1997 discloses an optical duobinary modulation system including a phase modulator and an intensity modulator which are connected in series. The third embodiment employs such an optical duobinary modulation system. The third embodiment results from replacing the intensity modulator with a polarization modulator. This system provides a modulated signal beam with an unaltered intensity. This configuration allows processing of a two-level digital signal instead of a three-level duobinary electric signal.  
         [0116]    A semiconductor laser  1  produces radiation having 1.55 μm wavelength band. With its linear polarized state maintained, the laser beam propagates through a PM optical fiber to a LiNbO3 polarization modulator  2 . The output of the modulator  2  is transmitted to a LiNbO3 phase modulator  3 . The polarization modulator  2  outputs a mutually orthogonalpolarized optical wave in response to two levels [1, 0], which a digital electric signal input  22  assumes. The phase modulator  3  modulates optical phase to [π, 0] (relative values) in response to values [1, 0] of the input signal  22 . The polarization modulator  2  launches the linear polarized optical wave into the phase modulator  3  at an incident angle of 45 degrees.  
         [0117]    Two-level data signal  22  is split into two, one being fed to the polarization modulator  2 , the other being logically inverted at an inverter  11   a  and then encoded by a pre-coder  7 . The encoded two-level data signal is delayed by 0.5 bit at a delay circuit  9  and then fed to the phase modulator  3 . Considering the signal propagation delay time, this delay circuit  9  provides the delay of 0.5 bit to the timing of the phase modulation with respect to the timing of polarization modulation so that the phase is inverted at the center of a bit slot of the polarization modulation. The configuration provides a modulated signal (output light)  56  having an unaltered intensity. As described above, according to this third embodiment, the light issued by a source of light is subjected to polarization modulation and then to phase modulation in this order. With regard to the order of processing, the light may be subjected first to phase modulation and then to polarization modulation. If the timing of driving signal is appropriately adjusted, this modified configuration provides substantially the same result. With regard to the position of delay circuit  9 , its position relative to the inverter  11   a  and pre-coder  7  is not limited to the illustrated example.  
         [0118]    As suggested in the above-mentioned K. Fukuchi et al., this configuration assures improved performance if polarization modulation is appropriately tuned. According to this embodiment, DC bias voltage applied to polarization modulator  2  is tuned to shift the operating point of the modulator in such a direction as to increase the optical intensity at cross points of eye pattern, which is provided after separation at PBS. The shift of bias ranging from 0.1 Vπ to 0.2 Vπ has proved to be effective.  
         [0119]    Using this configuration of this third embodiment, the optical fiber transmission experiment was conducted under the same conditions as the experiment conducted with the configuration of the first embodiment. The result of the experiment shows that the data transmission distance has extended as far as about 7,000 km.  
         [0120]    According to the third embodiment, the bias point of the polarization modulator  2  is shifted to modulate wave shape upon polarization modulation. Substantially the same result may be obtained by modulating wave shape of driving signal applied to the modulator as shown in FIG. 6. Operating an amplifier under gain saturated condition or utilizing non-linear transmission characteristic of a diode, for example, are other effective measures to provide substantially the same result. If the diode characteristic is used, the cross points of eye pattern shift downward (toward low intensity side). In this case, an inverter amplifier may be used to invert the polarity.  
         [0121]    (Fourth Embodiment)  
         [0122]    Referring to FIG. 7, the fourth embodiment is described. A semiconductor laser  1  produces radiation having 1.55 μm wavelength band. With its linear polarized state maintained, the laser beam propagates through a PM optical fiber to a first phase modulator  4 . The light output of the modulator  4  is fed to a second phase modulator  5 . Each of the modulators  4  and  5  allows incident linear polarized wave light to fall thereon obliquely at 45 degrees with respect to an optical axis thereof, resulting in performing polarization modulation. The term “optical axis” is herein used to mean the direction of polarization plane that is subjected to phase modulation within electric field. The optical axes of the first and second phase modulators  4  and  5  cross each other at right angles.  
         [0123]    Referring to FIG. 8, operation of each of the phase modulators  4  and  5  is explained. Suppose that the optical axis of the first phase modulator  4  is p, and the optical axis of the second phase modulator  5  is s. When driving signals  23  and  24  to these modulators  4  and  5  are zero, i.e., phase modulation is not requested, a vector P 0  represents the polarized state of the output beam. Let us consider the case when the driving signals  23  and  24  drive the phase modulators  4  and  5 . The vector P 0  may be resolved into a p-axis vector component P 0 p and a s-axis vector component P 0 s. These vector components are subjected to phase modulations by the phase modulators  4  and  5 , respectively. In other words, they are subjected to a phase difference π. The phase modulated vector components are composed into a resultant vector. Suppose the first phase modulator  4  only is switched on (Op=1). The direction of the vector component P 0 p is inverted into an inverted vector component P 0 p over-bar. The vector components P 0 p over-bar and P 0 s make a resultant vector P 1 . The polarized wave represented by this resultant vector P 1  is orthogonal to the polarized wave represented by the vector P 0 . Suppose now that the second phase modulator  5  only is switched on (O s =1), the direction of the vector component P 0 s is inverted into an inverted vector component P 0 s over-bar. The vector components P 0 s over-bar and P 0 p make a resultant vector P −1.  The polarized wave represented by this resultant vector P −1  is orthogonal to the polarized wave represented by the vector P 0 . The vectors P 1  and P −1  represent the same polarized wave but inverted phases.  
         [0124]    Similarly to the processing to obtain a two-level encoded signal in the first embodiment, digital data signal  22  is impressed on an input of a logical inverter (NOT circuit)  11   a . A pre-coder  7  codes the output of the inverter  11   a  to provide a two-level digital signal. This two-level signal is fed to a driving signal generator  8 .  
         [0125]    The driving signal generator  8  converts a current signal (b i ) of the two-level signal and the one bit preceding signal (b i−1 ) thereof by a code conversion rule represented by a truth value table under the heading TABLE II into the driving signals O p  (23) and O s  (24).  
         [0126]    If zero (0) level continues over two bits, the first phase modulator  4  is switched on (O p =1). If one (1) level continues over two bits, the second phase modulator  5  is switched on (O s =1). If the same level fails to continue over two bits, both the first and second phase modulators  4  and  5  are switched off (O p =O s =0). FIGS. 9 and 10 show circuits implementing this Boolean operation. In the circuit arrangement of FIG. 10, a three-level signal obtained after initial stage (1, 1) conversion is fed, as input, to first and second discriminators (D-FF). The first discriminator (D-FF) produces a high level of a two-level output O p  if and only if the input signal exceeds a first discrimination level V ref1  (see FIG. 11). The second discriminator (D-FF) produces a high level of a two-level output O s  if and only if the input signal exceeds a second discrimination level V ref2  (see FIG. 11) that is lower than the first discrimination level V ref1.    
         [0127]    Referring to the truth value table 2, if b i  has the mark ratio of  {fraction (1/2,)} O   s  and O p  have the dropped mark ratio of  {fraction (1/4,)} respectively. This is disadvantageous in that the drivers are required to posses superior low frequency cut-off characteristic. To cope with this, if b   i−1 =0 and b i =0 or if b i−1 =1 and b i =0, both O s  and O p  are allowed to take the level of 1. Thus, both O s  and O p  have the mark ratio of  {fraction (1/2,)} providing good DC balance, allowing the use of ordinary drivers. In this case, truth-value tables under the headings TABLE III and TABLE IV are used. In phase modulation with phase difference of π, inverting the polarity of driving signals provide the same result, thus allowing modulation with the relation that O   p =b i and O s =b i−1  or the relation that O s =b i  and O p =b i−1  maintained. Very simple circuits may implement this modulation. If O s =O p = 1 , the output polarized wave becomes the phase inverted version of P 0  and thus may be represented by P 0  over-bar. At the receiving end, P 0  and P 0  over-bar are recognized as the same. Thus, the original codes are restored without any problem. With the preceding configuration according to the fourth embodiment, the modulated signal  57  with unaltered intensity can be obtained at all times.  
         [0128]    Using this configuration of this fourth embodiment, the optical fiber transmission experiment was conducted under the same conditions as the experiment conducted with the configuration of the first embodiment. The result of the experiment shows that the data transmission distance has extended as far as about 8,000 km.  
         [0129]    Referring to FIGS. 12 and 13, modifications of the fourth embodiment are explained.  
         [0130]    In FIG. 12, light from the source of light is divided by a wave separating polarization beam splitter (PBS) into two polarized halves, each of which is subjected to phase modulation, and superimposed to each other at a wave combining polarization beam splitter (PBS). In the third embodiment, the orthogonal polarized waves propagate through the same space. In this configuration, they propagate through different spaces, respectively. This configuration requests maintenance of an optical path length between the two PBSs of the wavelength order, Thus, integration cannot be avoided for implementation of this configuration.  
         [0131]    In FIG. 13, an optical modulator utilizing the Pockels effect is used. This modulator include a crystal that exhibits the Pockels effect, which has electrodes to allow orthogonal application of two independent electric fields used to modulate laser beam. According to this configuration, the Pockels modulator plays the role of the modulators in the third embodiment. In this case, modification of the phase modulator to the travelling wave type has proved to be difficult. Thus, it has to be of the lumped-constant type, and satisfactory operation speed cannot be expected. The same manner of converting data input to driving signals as used in the third embodiment may be used in each of these modifications.  
         [0132]    With regard to a driving signal generator for driving two phase modulators associated with first and second polarized waves of carrier, the signal processing time reduces, the manufacturing cost decreases, and the reliability increases by taking the following measures. Firstly, an input data signal is divided into a first data signal and a second data signal. Secondly, the second data signal is delayed by one bit. Thirdly, the first polarized wave of carrier is phase shifted by the angle of π in response to the first data signal, while the second polarized wave of carrier is phase shifted by the angle of π in response to the delayed second data signal.  
         [0133]    (Fifth Embodiment)  
         [0134]    [0134]FIG. 14 illustrates the fifth embodiment. This embodiment employs an optical rotator, which induces optical rotation, as a modulator for modulating polarized wave. Faraday effect is used in this embodiment to modulate angle of optical rotation in response to input electric signal. The term “Faraday element” is herein defined to mean a modulation element utilizing the Faraday effect.  
         [0135]    Output optical beam of a laser light source  1  propagates through a PM optical fiber to a Faraday element  6 . With the Faraday element  6 , three levels of duobinary signal are allocated to three values δ=0, δ=π/2, and δ=π of the optical rotation angle δ, respectively. It must be noted that δ=0, and δ=π define the same polarization state of carrier with inverted phases relative to the output beam of the laser light source  1 , and δ=0 and δ=π/2 define orthogonal polarized waves.  
         [0136]    With this configuration, a modulated output light  58  with an unaltered or constant intensity has been obtained, which exhibits signal spectrum similar to that of optical duobinary signal.  
         [0137]    Using this configuration of this fifth embodiment, the optical fiber transmission experiment was conducted under the same conditions as the experiment conducted with the configuration of the first embodiment. The result of the experiment shows that the data transmission distance has extended as far as about 7,000 km.  
         [0138]    In the embodiments thus far described, LiNbO03 modulator or Faraday element has been used as phase modulator, and polarization modulator. Any devices of the electrical type or mechanical type or optical type may be used instead as long as they can modulate phase or polarization at rate around signal speed. With regard to the material, such devices may be made of semiconductor or organic material or non-organic material or optical fiber. As the phase modulator, a push-pull drive MZ type light intensity modulator may be used.  
         [0139]    In the preceding description, various circuit examples of pre-coder have been illustrated. The structure of the pre-coder is not limited to them. Appropriate combinations of logic circuits or analog circuits may be employed to perform the expected function.  
         [0140]    In the preceding embodiments, in translating three levels of duobinary signal to +1, P, −1 of carrier light, phase of polarization that is orthogonal to P is inverted immediately before or after appearance of P. If P continues, carrier light with the orthogonal polarization is not used as an output. Thus, in this case, the phase inversion is not needed.