Optical modulation circuit and optical transmission system

The present invention relates to a multicarrier modulation technique for transmitting binary data in a plurality of subcarriers.

Priority is claimed on Japanese Patent Application No. 2007-177871, filed on Jul. 6, 2007, the contents of which are incorporated herein by reference.

As a technique for providing a large capacity in optical fiber transmission, the wavelength division multiplexing (WDM) technique is used in which a symbol rate for a single carrier is increased to narrow the channel spacing for an increased number of multiplexes, to thereby achieve an enlarged transmission capacity.

However, an increase in symbol rate results in a more noticeable influence of a chromatic dispersion and a polarization mode dispersion (PMD) in a transmission fiber. Therefore, a sophisticated compensation technique is required. In addition, in order to obtain necessary receiver sensitivity, more power is required. This leads to a problem of suppressing a nonlinear optical effect produced in the fiber such as self phase modulation (SPM), four wave mixing (FWM), or cross phase modulation (XPM). In addition, with the increase in symbol rate, an occupied band for a single channel is expanded. This makes it difficult to increase the number of WDM channels.

Here, symbol rate is synonymous with baud rate, which is a bit rate to be transmitted divided by the number of bits of the modulation code. For example, if it is assumed that the bit rate is B (b/s) and that an M-ary modulation code is used, then the symbol rate is given by B/log 2 (M) (symbol/sec).

To solve this problem, what used to be transmitted in a single carrier manner is divided into subcarriers at a symbol rate that does not require a sophisticated compensation technique and then the subcarriers are transmitted. Thereby, it is possible to suppress chromatic dispersion and PMD, and an influence of a nonlinear optical effect.

If an orthogonal frequency division multiplexing (OFDM) modulation, which is often used in wireless transmission, is performed on data, and continuous light emitted from a continuous light source is modulated by an intensity modulator or a single sideband (SSB) modulator driven with an OFDM signal, then optical OFDM transmission can be implemented (for example, see Patent Document 1, or Non-Patent Document 1 or 2).

Patent Document 1: Japanese Unexamined Patent Application, First Publication No. 2005-311722 Non-Patent Document 1: Arthur James Lowery et al., “Orthogonal Frequency Division Multiplexing for Adaptive Dispersion Compensation in Long Haul WDM Systems,” Optical Fiber Communication Conference 2006, PDP 39 (2006) Non-Patent Document 2: Brendon J. C. Schmidt et al., “Experimental Demonstration of 20 Gbit/s Direct-Detection Optical OFDM and 12 Gbit/s with a colorless transmitter,” Optical Fiber Communication Conference 2007, PDP18 (2007).

Technical Problem

However, the scheme using an intensity modulator or an SSB modulator has the following problems.

To generate an OFDM signal from a data signal as described in Non-Patent Documents 1 and 2, a high-speed Fourier transform circuit, and a high-speed D/A converter is needed. In the case such as in Patent Document 1, there are required as many modulators as the optical subcarriers that are intended to be generated, and an oscillator that supplies a clock with a frequency equal to the symbol rate of the data signal to a modulator drive signal generation circuit. Therefore, it is difficult to obtain a high-speed operation and control.

Patterns of optical modulation will be described with reference to FIG. 11A to FIG. 11C . In FIG. 11A to FIG. 11C , the axis of abscissa represents optical frequency, and the axis of ordinate represents optical intensity. FIG. 11A and FIG. 11B show an optical spectrum in conventional modulation schemes. In the case where an intensity modulator is driven with an OFDM signal, the optical spectrum is as shown in FIG. 11A . The identical subcarrier groups are produced on both sides of the optical carrier frequency. This results in a problem in that the occupied band is wider than in the single carrier transmission.

Furthermore, in the case where an intensity modulator and an SSB modulator are driven with an OFDM signal, the optical spectrum is as shown respectively in FIG. 11A and FIG. 11B . Therefore, to efficiently wavelength-multiplex the optical OFDM signal, it is required to use an optical filter to filter an optical carrier with a frequency of fc emitted from the continuous light source, to thereby cut out an optical OFDM signal. Alternatively, it is required to perform an SSB modulation on the optical carrier. In addition, in the case where an OFDM signal in a single sideband is suppressed by filtering, it is required to insert a guard band. This makes requirements for the electrical/optical circuit more rigid.

In the case of wavelength-multiplexing an optical OFDM signal that is produced by modifying continuous light emitted from a continuous light source by means of an intensity modulator or an SSB modulator driven with an OFDM signal, the aforementioned problems arise. Therefore, it is required to provide a guard band. This prevents effective utilization of frequency bands of light.

The present invention has been achieved against such a backdrop, and has an object to provide an optical modulation circuit and an optical transmission system capable of reducing the number of oscillators and the drive frequency that are required in generation of optical subcarriers to half or less those of the conventional one, capable of narrowing an occupied band (for example, see FIG. 11C ), and capable of suppressing chromatic dispersion and PMD, and an influence of a nonlinear optical effect, in optical transmission.

Technical Solution

An optical modulation circuit of the present invention comprises: a continuous light source which generates continuous light with a constant power; a modulator drive signal generation unit which modulates data to be transmitted into modulator drive signals such as to generate optical subcarriers which have m (m≧1) and l (l≧1) (in total, l+m) different frequencies respectively before and after a frequency of the continuous light and on which different data is superimposed; and an optical modulation unit which modulates the continuous light from the continuous light source into the optical subcarriers using the modulator drive signals.

In the optical modulation circuit of the present invention, the modulator drive signal generation unit may generate the modulator drive signal which superimposes phase-shift keying, intensity modulation, and quadrature amplitude modulation for every frequency of the optical subcarriers.

The optical modulation circuit of the present invention may comprise: an optical multicarrier generation unit which generates optical multicarriers with co-phased n (n≧2) different frequencies based on the continuous light from the continuous light source; an optical dividing unit which divides the optical multicarrier for each of the n different frequencies so as to output n optical carriers; and an optical combining unit connected to the optical modulation unit, wherein the modulator drive signal generation unit modulates data to be transmitted into n of the modulator drive signals such as to generate optical subcarriers which further have the m (m≧1) and l (l≧1) (in total, l+m) different frequencies respectively before and after each of the frequencies of the n optical carriers and on which different data is superimposed, the modulation unit uses the n modulator drive signals to further modulate the n optical carriers into l+m of the optical subcarriers, and the optical combining unit combines a total of n×(l+m) of the optical subcarriers.

In this manner, the optical multicarrier generation portion is used to generate optical multicarriers. Thereby, it is possible to decrease the bit rate and the band of the signal generated by every optical modulator compared with the case where the same number of optical subcarriers at the same transmission speed is generated without using the optical multicarrier generation portion. Therefore, it is possible to mitigate requirements such as an electric circuit speed of the modulator drive signal generation portion and an operating band of the optical modulators.

The optical modulation circuit of the present invention may make the symbol rate of the optical subcarriers equal to the distance between the optical subcarriers, to thereby put the optical subcarriers and the distance between the optical subcarriers in an orthogonal state. Consequently, it is possible to detect the optical subcarriers without receiving linear interference and also to narrow the necessary band of light compared with that in a single carrier transmission.

In the optical modulation circuit of the present invention, the multicarrier generation unit may impart a double-sideband with carrier suppression modulation by a sinusoidal wave to the continuous light source.

In the optical modulation circuit of the present invention, the optical modulation unit may comprise n optical IQ-modulators.

In the optical modulation circuit of the present invention, the modulator drive signal generation unit may convert transmission data into two pieces of parallel data: A(t) and B(t), may generate an I-phase signal resulting from A(t)+B(t) modified with a clock signal with an angular frequency of w and a Q-phase signal resulting from A(t)−B(t) modified with a signal that is out of phase by n/2 from the clock signal, and may apply the I-phase signal and the Q-phase signal on corresponding electrodes of each of the optical IQ-modulators.

In the optical modulation circuit of the present invention, the modulator drive signal generation unit may convert transmission data into 2n pieces of parallel data of A 1 ( t ), A 2 ( t ), . . . , An(t) and B 1 ( t ), B 2 ( t ), . . . , Bn(t), may generate an I-phase signal resulting from adding the 2n pieces of parallel data for k=1 to n, which have been modified respectively with n clock signals with angular frequencies of ω 1 , ω 2 , . . . , ωn and a Q-phase signal resulting from adding Ak(t)−Bk(t) for k=1, 2, . . . , n, each of which has been modified respectively with a signal that is out of phase by π/2 from a clock signal with an angular frequency of ωk for k=1 to n, and may apply the I-phase signal and the Q-phase signal on corresponding electrodes of each of the IQ-modulators.

In the optical modulation circuit of the present invention, the modulator drive signal generation unit may convert transmission data into four pieces of parallel data: I 1 ( t ), I 2 ( t ), Q 1 ( t ), and Q 2 ( t ), may generate: an I-phase signal in which a signal resulting from Q 2 ( t )−Q 1 ( t ) modified with a clock signal with an angular frequency of ω is added to a signal resulting from I 1 ( t )−I 2 ( t ) modified with a signal that is out of phase by π/2 from the clock signal; and a Q-phase signal in which a signal resulting from I 1 ( t )+I 2 ( t ) modified with a clock signal with an angular frequency of ω is added to a signal resulting from Q 2 ( t )+Q 1 ( t ) modified with a signal that is out of phase by π/2 from the clock signal, and may apply the I-phase signal and the Q-phase signal on corresponding electrodes of each of the IQ-modulators.

Furthermore, another aspect of the present invention can be an optical transmission system comprising: an optical transmitter comprising an optical modulation circuit of the present invention; an optical transmission path which transmits an optical subcarrier emitted from the optical transmitter; and an optical receiver which converts the optical subcarrier having been transmitted over the optical transmission path into an electrical signal.

At this time, the optical receiver may comprise Mach-Zehnder interferometer filters connected in multiple stages, optical couplers, and photoelectric converters. Alternatively, the optical receiver may comprise the Mach-Zehnder interferometer (MZI) filters connected in multiple stages, the optical couplers, optical gate circuits, and the photoelectric converters.

Furthermore, seeing the present invention from an aspect of an optical modulation method, the present invention is an optical modulation method in which an optical modulation circuit: generates continuous light with a constant power from a continuous light source; modulates data to be transmitted into modulator drive signals such as to generate optical subcarriers which have m (m≧1) and l (l≧1) (in total, l+m) different frequencies respectively before and after a frequency of the continuous light and on which different data is superimposed, by a modulator drive signal generation unit; and modulates the continuous light from the continuous light source into the optical subcarriers using the modulator drive signals, by an optical modulation unit.

According to the present invention, it is possible to implement multicarrier transmission capable of reducing the number of oscillators and the drive frequency that are required in generation of optical subcarriers to half or less those of the conventional one, capable of narrowing an occupied band, and capable of suppressing a chromatic dispersion and a PMD, and an influence of a nonlinear optical effect.

FIG. 1 is a configuration diagram showing an optical modulation circuit of a first embodiment.

FIG. 2 is a configuration diagram showing an optical multicarrier generation portion of the first embodiment.

FIG. 3 is a configuration diagram showing a modulator drive signal generation circuit of the first embodiment.

FIG. 4A is a diagram showing an output spectrum of the optical modulation circuit of the first embodiment.

FIG. 4B is a diagram showing an output spectrum of the optical modulation circuit of the first embodiment.

FIG. 5 is a configuration diagram showing an optical transmission system of a second embodiment.

FIG. 6 is a configuration diagram showing an optical receiver of the second embodiment.

FIG. 7A is a diagram showing a spectrum of an optical signal after passing through an MZI of the second embodiment.

FIG. 7B is a diagram showing a spectrum of an optical signal after passing through the MZI of the second embodiment.

FIG. 8 is a configuration diagram showing an optical receiver applied to a QPSK of the second embodiment.

FIG. 9 is a configuration diagram showing an optical modulation circuit of a third embodiment.

FIG. 10 is a configuration diagram showing an optical modulation circuit drive signal generation portion of the third embodiment.

FIG. 11A is a diagram showing an optical spectrum in a conventional modulation scheme.

FIG. 11B is a diagram showing an optical spectrum in a conventional modulation scheme

FIG. 11C is a diagram showing an optical spectrum in an optical modulation circuit of the present invention.

1 continuous light source

2 optical multicarrier generation portion

3 optical dividing portion

4 , 4 - 1 to 4 - n optical IQ-modulator

5 optical combining portion

6 , 6 - 1 to 6 - n modulator drive signal generation circuit

10 serial/parallel converter

11 , 11 - 1 to 11 - n arithmetic circuit

12 , 12 - 1 to 12 - n oscillator

20 optical transmitter

30 optical transmission path

40 optical receiver

41 - 1 to 41 - 11 Mach-Zehnder interferometer filter

42 - 1 to 42 - 4 optical coupler

43 - 1 to 43 - 8 balanced O/E converter

44 - 1 , 44 - 2 parallel/serial converter

45 - 1 to 45 - 4 optical gate circuit

46 - 1 to 46 - 8 optical coupler

47 - 1 to 47 - 4 local oscillator light source

48 - 1 to 48 - 4 90° phase shifter

First Embodiment

An optical modulation circuit using a multi-wavelength light source as a first embodiment of the present invention will be described with reference to FIG. 1 to FIG. 4B .

In the present embodiment, the multi-wavelength light source is made of a continuous light source 1 and an optical multicarrier generation portion 2 .

FIG. 1 is a diagram showing a configuration of the optical modulation circuit of the present embodiment. As shown in FIG. 1 , the optical modulation circuit of the present embodiment includes: a continuous light source 1 for outputting continuous light with an angular frequency of ωc with a constant power; an optical multicarrier generation portion 2 for generating n (n≧2) optical multicarriers from an optical carrier emitted from the continuous light source 1 ; an optical dividing portion 3 for dividing the optical multicarriers according to their wavelength; optical IQ-modulators 4 - 1 to 4 - n for generating optical subcarriers in which phase-shift keying is superimposed on the respective divisions of the optical multicarrier; an optical combining portion 5 for combining the optical subcarriers that have been divided and modulated; and modulator drive signal generation circuits 6 - 1 to 6 - n for generating a modulator drive signal from data.

Here, the present embodiment is characterized in that a multi-wavelength light source is used to generate an optical multicarrier. Thereby, compared with the case of generating the same number of optical subcarriers with the same bit-rate without using a multi-wavelength light source, a bit rate and a band of a signal generated by a single optical IQ-modulator is made small. Consequently, it is possible to relax requirements such as an electric circuit speed of the modulator drive signal generation circuits 6 - 1 to 6 - n and an operating band of the optical IQ-modulators 4 - 1 to 4 - n.

Next is a description of an operation of the first embodiment. For convenience' sake, the description is for the case where the number of optical multicarriers emitted from the multi-wavelength light source is n=2 and the number of optical subcarriers generated by the optical IQ-modulator is l+m=2, that is, for the case where the total number of optical subcarriers generated is n×(1+m)=4, by way of example.

It is assumed that each bit rate of data # 1 and data # 2 to be transmitted is B (b/s). A variable ωc denotes an angular frequency of the optical carrier emitted from the continuous light source 1 . A variable Δωs denotes a difference in angular frequency between the optical carrier and the optical multicarrier generated by a Mach-Zehnder optical modulator as shown in FIG. 2 . The following description is for the case where the frequency spacing between the optical subcarriers is equal to the symbol rate of the optical subcarriers. At this time, the relation Δωs=2Δω is satisfied.

The Mach-Zehnder optical modulator is used to push-pull drive and modulate the light emitted from the continuous light source 1 at a minimum bias point by means of a clock signal which is a sinusoidal wave with an angular frequency of Δωs. Thereby, the optical carriers with an angular frequency of ωc is suppressed. As a result, two optical multicarrier to which a double-sideband with carrier suppression modulation is imparted are generated as shown in Equation 1. This optical signal is put as s(t).

The optical signal s(t) is divided into

(Equation 2)

s 1( t )=cos(ω c+Δωs ) t   (2)

and

(Equation 3)

s 2( t )=cos(ω c−Δωs ) t   (3)

by the optical dividing portion 3 . Here, s 1 is input to the optical IQ-modulator 4 - 1 , and s 2 is input to the optical IQ-modulator 4 - 2 .

Next is a description of an operation of the optical IQ-modulator 4 - 1 . The modulator drive signal generation circuit 6 - 1 has a configuration as shown in FIG. 3 . Allowing the data to be input as D 1 ( t ), D 1 ( t ) is divided into four pieces of parallel data: I 1 u ( t ), I 1 d ( t ), Q 1 u ( t ), and Q 1 d ( t ) by a serial/parallel converter 10 .

The divided signals are transformed into four outputs: I 1 u ( t )+I 1 d ( t ), I 1 u ( t )−I 1 d ( t ), Q 1 u ( t )+Q 1 d ( t ), and, Q 1 d ( t )−Q 1 u ( t ) by an arithmetic circuit 11 . They are modulated pairwise by a sinusoidal wave with an angular frequency of Δω that has been emitted from an oscillator 12 and by a sinusoidal wave with a π/2 phase delay, respectively. The modulated outputs are added and output as ID 1 ( t ) and QD 1 ( t ), which are respectively expressed as Equation 4 and Equation 5.

When the optical IQ-modulator 4 - 1 is driven with these signals, an optical signal that is output from the optical IQ-modulator 4 - 1 is generated as an output signal S 1 ( t ). The output signal S 1 ( t ) is an optical subcarrier in which phase-shift keying is superimposed on angular frequencies of ±Δω from an angular frequency ωc+Δωs of the optical carrier of s 1 ( t ), which is expressed as Equation 6.

Similarly, an output signal S 2 ( t ) of the optical IQ-modulator 4 - 2 is expressed as Equation 7.

S 1 and S 2 that have been output respectively from the optical IQ-modulators 4 - 1 and 4 - 2 are combined by the optical combining portion 5 . The resultant signal is then output from the optical modulation circuit, with a spectrum as shown in FIG. 4A . As shown in FIG. 4A , according to the present invention, the signal band is narrowed more than in single carrier transmission, and also the symbol rate for a single subcarrier is lowered. Therefore, chromatic dispersion and PMD tolerances improve. Furthermore, precoding the input date makes it possible to superimpose a differential phase-shift keying code.

For convenience' sake, the description above has been for the case where the number of optical multicarriers emitted from the multi-wavelength light source is two, by way of example. However, in the more general case where the number of optical multicarrier is n, the output signal is expressed as Equation 8.

where ωk denotes an angular frequency of the optical multicarrier that has been output from the multi-wavelength light source. An ωk satisfies Equation 9, where l, m (l, m≦n) are variant integers.

(Equation 9)

|ω l−ωm|= 4 |l−m|Δω   (9)

In this case, S 1 to Sn that have been output respectively from the optical IQ-modulators 4 - 1 to 4 - n are combined by the optical combining portion 5 . The resultant signal is then output from the optical modulation circuit, with a spectrum as shown in FIG. 4B .

Furthermore, if the optical multicarrier generation portion has a configuration in which, instead of a Mach-Zehnder modulator, a phase-shift modulator is driven with a sinusoidal wave with an angular frequency of Δωs, it is possible to generate three optical multicarriers: ωc, ωc+Δωs, and ωc−Δωs. In addition, if the phase-shift modulator and the intensity modulator are serially connected and driven with synchronized clock signals, it is possible to generate n optical multicarriers.

Second Embodiment

A second embodiment of the present invention will be described with reference to FIG. 5 to FIG. 8 . In the present embodiment, the description is for the case where the distance between the optical subcarriers is equal to the symbol rate of the optical subcarriers.

FIG. 5 is a diagram showing a configuration of an optical transmission system of the present embodiment. The optical IQ-modulators 4 - 1 and 4 - 2 , the optical multicarrier generation portion 2 , and the modulator drive signal generation circuit 6 - 1 and 6 - 2 in the optical transmitter 20 of the present embodiment are configured similarly to those in the first embodiment, which are shown in FIG. 1 and FIG. 3 . In this configuration, it is possible to superimpose phase-shift keying on each optical subcarrier. In the present embodiment, quaternary differential phase-shift keying (DQPSK) is employed.

It is assumed that the bit rate of the data # 1 and the data # 2 that are respectively input to the modulator drive signal generation circuits 6 - 1 and 6 - 2 is B (b/s). Therefore, data with 2B (b/s) in total is transmitted. The optical subcarrier signal that has been output from the optical transmitter 20 propagates through an optical transmission path 30 and is input to an optical receiver 40 .

Similarly to the first embodiment, signals parallelized from the data # 1 by the serial/parallel converter 10 are denoted as I 1 u ( t ), I 1 d ( t ), Q 1 u ( t ), and Q 1 d ( t ). Signals parallelized from the data # 2 by the serial/parallel converter 10 are denoted as I 2 u ( t ), I 2 d ( t ), Q 2 u ( t ), and Q 2 d ( t ). In the present embodiment, Δωs=B/4 (Hz).

As shown in FIG. 6 , the optical receiver 40 is made of: Mach-Zehnder interferometer filters (MZIs) 41 - 1 to 41 - 11 with a free spectral range (FSR) of one of B/2 (Hz), B (Hz), and B/4 (Hz); optical branching portions 42 - 1 to 42 - 4 ; and balanced O/E converters 43 - 1 to 43 - 8 . An optical signal that has been input to the optical receiver 40 is first divided into two pairs of subcarriers by the MZI 41 - 1 with an FSR of B/2, as shown in FIG. 7A and FIG. 7B . FIG. 7A shows a spectrum at point A in FIG. 6 . FIG. 7B shows a spectrum at point B in FIG. 6 .

Next, the optical signal is input to the MZIs 41 - 2 and 41 - 3 with an FSR of B (Hz), where the adjacent optical subcarriers are separated. Furthermore, each optical subcarrier is divided into two optical signals with a half power respectively by the optical branching portions 42 - 1 to 42 - 4 . Each of the divided optical subcarriers is then input respectively to the MZIs 41 - 4 to 41 - 11 with an FSR of B/4 (Hz). Thereby, a 1-bit delay is imparted to each optical subcarrier. The optical subcarriers are then converted into pieces of binary data I 1 u ( t ), I 1 d ( t ), Q 1 u ( t ), Q 1 d ( t ), I 2 u ( t ), I 2 d ( t ), Q 2 u ( t ), and Q 2 d ( t ) respectively by the balanced O/E converters 43 - 1 to 43 - 8 .

The pieces of binary data are demodulated into the original data # 1 and the original data # 2 by the parallel/serial converters 44 - 1 and 44 - 2 .

Here, the present embodiment is characterized in that, because the distance between the optical subcarriers and the symbol rate of the optical subcarriers take the same orthogonal state, it is possible to separate and detect optical subcarriers without receiving linear interference and that the operating band of the necessary electrical/optical circuit are relaxed compared with those of single-carrier transmission.

Furthermore, if optical gate circuits 45 - 1 to 45 - 4 , which have been driven with a sinusoidal wave with the same frequency as the symbol rate for a single optical subcarrier, are inserted directly before the optical couplers 42 - 1 to 42 - 4 respectively, it is possible to suppress interference, thus improving transmission quality.

In the case where QPSK is superimposed on each optical subcarrier instead of DQPSK, it is not possible to perform a differential detection. Therefore, instead of performing a differential detection by use of the MZIs 41 - 4 to 41 - 11 with a 1-bit delay, the optical subcarriers are received after they are mixed with local oscillator light (local light) with a frequency substantially equal to that of the optical subcarriers. Thereby, it is possible to demodulate the optical subcarriers.

As for a specific configuration therefor, optical couplers 46 - 1 to 46 - 8 are provided instead of the MZIs 41 - 4 to 41 - 11 with a 1-bit delay, as shown in FIG. 8 . By use of local oscillator light sources 47 - 1 to 47 - 4 and 90° phase shifters 48 - 1 to 48 - 4 , two beams of local oscillator light whose phases are 90° different from each other are mixed with the two branched optical subcarriers, respectively. Thereby, it is possible to perform a heterodyne or homodyne detection on each channel by use of local oscillator light.

Third Embodiment

An optical modulation circuit of a third embodiment of the present invention will be described with reference to FIG. 9 and FIG. 10 . In the second embodiment, the description has been for the case where a plurality of optical subcarriers on which differential phase-shift keying (DQPSK) is employed are generated. However, in the third embodiment, the case where phase-shift keying is superimposed on a plurality of optical subcarriers will be described.

The optical modulation circuit of the third embodiment has a configuration that includes: a continuous light source 1 for outputting continuous light with a constant power; an optical IQ-modulator 4 for generating subcarriers on which phase-shift keying is employed; and a modulator drive signal generation circuit 6 for generating a modulator drive signal from data, as shown in FIG. 9 . A configuration of the modulator drive signal generation circuit 6 is shown in FIG. 10 .

An optical carrier with an angular frequency of ωc emitted from the continuous light source 1 is incident on the optical IQ-modulator 4 , and is divided into two paths # 1 and # 2 . A signal in the path # 1 is modulated with a drive signal I(t) expressed by Equation 10. A signal in the path # 2 is modulated with a drive signal Q(t) expressed by Equation 11. The drive signals I(t) and Q(t) are generated by the modulator drive signal generation circuit 6 .

Here, the data superimposed on an upper sideband of the optical carrier is denoted by an adscript u. The data superimposed on a lower sideband of the optical carrier is denoted by an adscript d. For example, I 2 u ( t ) denotes data of an in-phase component superimposed on the subcarrier with ω+2Δω.

Furthermore, to the signal in the path # 2 is imparted a π/2 phase delay with respect to the optical carrier. Therefore, the combination of the optical signals in the two paths yields an output signal Sout(t) expressed by Equation 12.

As described above, 2m optical subcarriers on which phase-shift keying is superimposed are generated with the angular frequency ωc of the optical carrier emitted from the continuous light source 1 at their center. According to the present invention, the signal band is more narrowed than in single carrier transmission. In addition, the symbol rate for a single subcarrier is lowered. Therefore, chromatic dispersion and PMD tolerances improve. Furthermore, in the present embodiment, Iku and Qku may be in a signal constellation of quadrature amplitude modulation (QAM).

Fourth Embodiment

In the same configuration as that of the optical modulation circuit, as shown in FIG. 1 , described in the first embodiment, the circuit configurations corresponding to the modulator drive signal generation circuits 6 - 1 and 6 - 2 are each replaced with the circuit configuration for generating a plurality of optical subcarriers, as shown in FIG. 10 , described in the third embodiment. With the adjustment of the number of carriers generated by the optical multicarrier generation portion 2 and the optical IQ-modulators 4 - 1 and 4 - 2 , it is possible to generate an optional number of optical subcarriers with the frequency of the optical carrier of the continuous light source 1 at their center.

Fifth Embodiment

With the optical modulation circuit described in the fourth embodiment, an optical receiver as shown in FIG. 6 made of Mach-Zehnder interferometer filters connected in multiple stages, optical couplers, and photoelectric converters is used. Thereby, it is possible to constitute an optical transmission system in which intensity modulation or phase-shift keying is superimposed on an optional number of optical subcarriers.

Furthermore, an optical gate circuit that has been driven with a sinusoidal wave with the same frequency as the symbol rate for a single optical subcarrier may be inserted directly before each optical branching portion.

According to the present invention, it is possible to implement multicarrier transmission capable reducing the number of oscillators and the operating frequency of the electrical/optical circuit that are required in generation of optical subcarriers to half or less those of the conventional one, capable of narrowing an occupied band, and capable of suppressing a chromatic dispersion and a PMD, and an influence of a nonlinear optical effect. Therefore, it is possible to reduce the amount of hardware when an optical transmission system is constructed, and also to improve communication quality.