Optical transmitter and optical transmitter unit

The optical transmitter includes a light source; a signal processor; an optical modulator that modulates output light from the light source in accordance with a transmitting signal subjected to digital signal processing in the signal processor and outputs the modulated light as a light signal to a transmission path; and a control circuit that controls a carrier-wave frequency of the light signal output from the optical modulator, the signal processor including a mapping circuit that maps the transmitting signal to electric-field information according to a modulating scheme, and a phase rotating circuit that provides a phase rotation having a constant cycle to an electric-field phase of the electric-field information to which the mapping circuit maps the transmitting signal, the control circuit controlling the cycle of the phase rotation that the phase rotating circuit provides and thereby controlling the carrier-wave frequency of the light signal output from the optical modulator.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2010-269020, filed on Dec. 2, 2010, the entire contents of which are incorporated herein by reference.

FIELD

The embodiments discussed herein are an optical transmitter, and an optical transmitter unit that includes a number of optical transmitters and carries out wavelength multiplexing.

BACKGROUND

In accordance with increase in data traffic, a high-capacity trunk light communication network has been demanded, and high-speed communication of 40 Gbps (Giga bit per second) or 100 Gbps are putting into practice. For high-speed communication in an optical communication network, one of the techniques recently focused is optical transmission and optical reception using digital signal processing.[Non-Patent Literature 1] D.-S. Ly-Gagnon, et al. “Coherent Detection of Optical Quadrature Phase-Shift Keying Signals With Carrier Phase Estimation”, IEEE JLT, vol. 24, no. 1, pp. 12-21, January 2006[Non-Patent Literature 2] D. McGhan, “Electronic Dispersion Compensation”, OFC2006, OWK1

In general, a wavelength locker having, for example, an etalon filter is used to stably output light having a constant oscillation frequency (wavelength) from a transmitting light source used in a wavelength multiplexed optical transmission system. Considering aged deterioration, the accuracy of control by such a wavelength locker is several GHz. Fluctuation of this several GHz causes lowering the transmission capability in an optical transmission system and hinders high-density intervals of wavelengths to be multiplexed in a single optical fiber. In other words, frequency stability of a transmission light source is factors limiting the transmitting distance in an optical transmission system and a transmission capacity of a single optical fiber.

An oscillation frequency (wavelength) of a transmission light source is generally controlled by temperature, which takes time to make it difficult to flexibly change the wavelength arrangement particularly during the operation of the optical transmission system. More specifically, such control may take several minutes and the oscillation frequency to be controlled may have poor stability. In addition, it is difficult to accomplish complex and precise control while the system is working.

SUMMARY

As one aspect of the embodiments, an optical transmitter including a light source; a signal processor that carries out digital signal processing on a transmitting signal to be transmitted; an optical modulator that modulates output light from the light source in accordance with the transmitting signal subjected to the digital signal processing in the signal processor and outputs the modulated light as a light signal to a transmission path; and a carrier-wave frequency control circuit that controls a carrier-wave frequency of the light signal output from the optical modulator, the signal processor including a mapping circuit that maps the transmitting signal to electric-field information according to a modulating scheme, and a phase rotating circuit that provides a phase rotation having a constant cycle to an electric-field phase of the electric-field information to which the mapping circuit maps the transmitting signal, the carrier-wave frequency control circuit controlling the cycle of the phase rotation that the phase rotating circuit provides and thereby controlling the carrier-wave frequency of the light signal output from the optical modulator.

As another aspect, an optical transmitter unit including a plurality of the above optical transmitters; and an optical coupler that carries out wavelength multiplexing by combining a plurality of the light signals one output from each of the plurality of optical transmitters to the transmission path, wherein a plurality of the light sources, provided one for each of the plurality of optical transmitters, oscillate output light having a same frequency, a plurality of the carrier-wave frequency control circuits, provided one for each of the plurality of optical transmitters, fine-adjust the carrier-wave frequencies of the light signals output from the optical modulators by controlling cycles of the phase rotation that the corresponding phase rotating circuits provide.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, various embodiments will now be described with reference to the drawings.

(1) Basic Configuration of an Optical Transmitter:

FIG. 1is a block diagram illustrating the basic configuration of an optical transmitter. The optical transmitter1having the basic configuration ofFIG. 1includes a laser light source11, a signal processing circuit12, a DAC13, a driver14, an IQ modulator15, and a carrier-wave frequency control circuit16.

The laser light source (light source)11oscillates output light having a predetermined frequency fC.

The signal processing circuit (signal processor)12carries out digital signal processing on a transmitting signal which is input from an external device and which is binary data, and is exemplified by a DSP (Digital Signal Processor). The signal processing circuit12has functions of a modulating scheme mapping circuit121and a phase rotating circuit122.

The modulating scheme mapping circuit121constellation-maps a transmitting signal which is input from an external device and which is binary data to electric-field information in accordance with a modulating scheme such as QPSK (Quadrature Phase Shift Keying), QAM (Quadrature Amplitude Modulation), and OFDM (Orthogonal Frequency Division Multiplexing). If a modulation scheme such as RZ (Return to Zero) or NRZ (Non Return to Zero) is adopted, it is sufficient that the transmitting data in the form of binary data is simply converted into the electric-field information.

The phase rotating circuit122provides a phase rotation of a predetermined cycle to the electric-field phase of the electric-field information, to which the modulating scheme mapping circuit121maps the transmitting signal. Specifically, the phase rotating circuit122provides, upon receipt of an amount Δfaof frequency control from the carrier-wave frequency control circuit16, a phase rotation θ=2πΔfat to the electric-field phase.

The DAC (digital/analog converting circuit)13converts a digital signal from the signal processing circuit12into an analog signal.

The driver (modulator driving circuit)14amplifies the signal from the DAC13, and drives the IQ modulator15using the amplified signal.

The optical modulator (optical modulating section)15modulates the output light from the laser light source11in accordance with the transmitting signal after being subjected to the digital processing by the signal processing circuit12and the processing by the DAC13and the driver14, and then outputs the modulated light, being regarded as a light signal, to the transmission path3.

The carrier-wave frequency control circuit16controls the carrier-wave frequency of the light signal output from the optical modulator15. Specifically, for the control of the carrier-wave frequency of the light signal output from the optical modulator15, the carrier-wave frequency control circuit16outputs an amount Δfaof frequency control to the phase rotating circuit122to control the cycle of phase rotation θ=2πΔfat that the phase rotating circuit122applies. The carrier-wave frequency control circuit16fine-adjusts the carrier-wave frequency of a light signal output from the optical modulator15in the electric band of the optical transmitter1of the first embodiment through the above frequency control. The electric band of the optical transmitter1depends on the band properties of the DAC13, the driver14, and the optical modulator15.

In the optical transmitter1, which has the above basic configuration, the carrier-wave frequency control circuit16outputs an amount Δfaof frequency control to the phase rotating circuit122, so that the cycle of the phase rotation θ=2πΔfat that the phase rotating circuit122provides is controlled. This fine-adjusts the carrier-wave frequency of the light signal output from the optical modulator15to a value of the sum of the frequency fCof the output light from the laser light source11and an amount Δfaof frequency control within the electric band of the optical transmitter1.

Accordingly, use of the amount Δfaof frequency control makes the optical transmitter1possible to control the carrier-wave frequency of the light signal output from the optical modulator15more precisely and faster than the conventional method that directly controls the laser light source11. This ensures the stability of the oscillation frequency of the laser light source11, so that the transmission performance of an optical communication system using the optical transmitter1is improved. In addition, wavelength multiplexing intervals come to be dense to improve the rate of using the band of the transmission path (optical fiber)3and high-capacity transmission is realized.

(2) The Optical Transmitter of the First Embodiment:

FIG. 2is a block diagram schematically illustrating the configuration of an optical transmitter1A of the first embodiment. The optical transmitter1A includes a laser light source11, a signal processing circuit12, DACs13I and13Q, drivers14I and14Q, an IQ optical modulator15A, and the carrier-wave frequency control circuit16.

The laser light source11oscillates output light having a predetermined frequency fC.

The signal processing circuit (signal processor)12carries out digital signal processing on a transmitting signal which is input from an external device and which is binary data. The signal processing circuit12has functions of a modulating scheme mapping circuit121, a phase rotating circuit122, an I-channel predistortion compensating circuit123I, and Q-channel predistortion compensating circuit123Q.

Likewise the optical transmitter1ofFIG. 1, the modulating scheme mapping circuit121constellation-maps a transmitting signal which is input from an external device and which is binary data to electric-field information in accordance with a modulating scheme such as QPSK, QAM, and OFDM. The electric-field information E1, to which the modulating scheme mapping circuit121maps the transmitting signal, includes an I (In-phase) component and a Q (Quadrature-phase) component, and is expressed by Formula E1=I+j·Q=A(t)·exp(jθ(t)) where the term “j” represents the imaginary unit; the term “A(t)” represents an electric-field intensity (amplitude); the term “θ(t)” represents an electric-field phase; and the term “t” represents time.

The phase rotating circuit122provides a phase rotation of a predetermined cycle to the electric-field phase of the electric-field information E1, to which the modulating scheme mapping circuit121maps the transmitting signal. Specifically, the phase rotating circuit122applies, upon receipt of an amount Δfaof frequency control from the carrier-wave frequency control circuit16, a phase rotation θ=2πΔfat to the electric-field phase of the electric-field information E1similarly to the optical transmitter1ofFIG. 1.

As illustrated inFIG. 4, which is a block diagram illustrating the configuration of the phase rotating circuit122, the phase rotating circuit122includes an integrator circuit122aand a multiplexer122b. The integrator circuit122aintegrates, upon receipt of the amount Δfaof frequency control from the carrier-wave frequency control circuit16, an amount Δω(=2πΔfa) of phase rotation per discrete time unit T and thereby calculates a complex electric field information exp(−j(ΔωnT)) that rotates the phase by ΔωnT during a discrete time nT (here, n is an integer). The multiplexer122bmultiplies the complex electric field information exp(−j(ΔωnT)), which the integrator circuit122acalculates, by the electric-field information E(t) (=E1) from the modulating scheme mapping circuit121, and thereby obtains and outputs electric-field information E′(t)=E(t)*exp(jΔωnT), to which the electric-field phase of the electric-field information E(t) is rotated by ΔωnT(=2πΔfat).

The I-channel predistortion compensating circuit123I compensates the I component of the electric-field information, to which the phase rotating circuit122provides the phase rotation, for prospective deterioration in signal quality of the I component of the electric-field information, which deterioration is caused by incompletion of a transmitting system corresponding to the I component. Similarly, the Q-channel predistortion compensating circuit123Q compensates the Q component of the electric-field information, to which the phase rotating circuit122provides the phase rotation, for prospective deterioration in signal quality of the Q component of the electric-field information, which deterioration is caused by incompletion of a transmitting system corresponding to the Q component. Here, an example of the transmitting system corresponding to the I component includes DAC13I, the driver14I, and a phase modulator151I that are to be detailed below; and an example of the transmitting system corresponding to the Q component includes the DAC13Q, the driver14Q, and a phase modulator151Q that are to be detailed below. The I-channel predistortion compensating circuit123I and the Q-channel predistortion compensating circuit123Q compensate for, for example, a loss variation between I and Q signals, skew, a band variation, linearity. As detailed below, the compensation is accomplished by multiplying (convolving) the electric-field information from the phase rotating circuit122by the reversed function of a waveform distortion of the optical transmitter1A (the transmitting system).

The DACs13I and13Q convert digital signals (I component and Q component) from the I-channel predistortion compensating circuit123I and the Q-channel predistortion compensating circuit123Q, respectively into analog signals.

The drivers14I and14Q amplify signals from the DACs13I and13Q, and drive the respective corresponding phase modulator151I and151Q (seeFIG. 5) in the IQ modulator15A using the amplified signals.

The IQ modulator (optical modulator)15A modulates output signal from the laser light source11using the transmitting signals (the I component and the Q component) processed by the DACs13I and13Q and the drivers14I and14Q after being subjected to the digital processing by the signal processing circuit12, and outputs the modulated light, being regarded as a light signal, to the transmission path3. As illustrated inFIG. 5, the IQ modulator15A includes the phase modulator151I for an I component, the phase modulator151Q for a Q component, and a phase shifter152.FIG. 5is a block diagram schematically illustrates the configuration of the IQ modulator15A. The phase shifter152provides a predetermined phase difference (π/2) between a pair of light signals propagating through the phase modulator151I and the phase modulator151Q, and is disposed on the side of the phase modulator151Q. The phase modulator151I and the phase modulator151Q phase-modulate the pair of light signals that have the predetermined phase difference provided on the basis of the transmitting signals (the I component and the Q component) from the drivers14I and14Q, and output the modulated signals to the transmission path3.

Likewise the optical transmitter1ofFIG. 1, the carrier-wave frequency control circuit16controls the carrier-wave frequency of the light signal output from the IQ modulator15A. Specifically, for the control of the carrier-wave frequency of the light signal output from the optical modulator15, the carrier-wave frequency control circuit16outputs an amount Δfaof frequency control to the phase rotating circuit122to control the cycle of phase rotation θ=2πΔfat that the phase rotating circuit122applies. Through the above frequency control, the carrier-wave frequency control circuit16fine-adjusts the carrier-wave frequency of the light signal output from the IQ modulator15A within the electric band of the optical transmitter1A of the first embodiment as illustrated in, for example,FIG. 3. The electric band of the optical transmitter1A depends on the band properties of the DACs13I and13Q, the drivers14I and14Q, and the IQ modulator15A.FIG. 3illustrates a transmitting spectrum of the optical transmitter1A of the first embodiment.

In accordance with an error signal from an optical receiver2A or2B (seeFIGS. 9-11) in communication with the optical transmitter1A, the carrier-wave frequency control circuit16fine-adjusts the carrier-wave frequency within the electric band of the optical transmitter1A by adjusting the rotating frequency that the phase rotating circuit122applies so that the error at the receiver end is resolved. Here, an error signal is related to the result of quality of a received signal received by the optical receiver2A or2B, and represents occurrence of deterioration in quality of received signal in the receiver end, as to be detailed below with reference toFIGS. 10 and 11. Such an error signal may be transmitted from the optical receiver2A or2B to the optical transmitter1A through a reverse-direction channel; through a control channel of the optical communication system; or through frequency modulating (to be detailed below), that is a novel function of the optical transmitter1A. Alternatively, the carrier-wave frequency may be fine-adjusted on the basis of a light spectrum or a light frequency measured by means of a transmitter output, a relay, or a receiver.

Further alternatively, the carrier-wave frequency control circuit16may superimpose a pilot signal onto a frequency adjusting value Δfa. Specifically, the carrier-wave frequency control circuit16may have a function of superimposing a pilot signal onto a carrier-wave frequency of a light signal that the IQ modulator15A outputs through frequency modulation by controlling the cycle of the phase rotation θ=2πΔfat that the phase rotating circuit122applies to thereby control the carrier-wave frequency of the light signal output from the IQ modulator15A, and transmits the superimposed signal to the receiver end through the transmission path3. Besides, the carrier-wave frequency control circuit16may superimpose a result of detecting a quality of a received signal of the reverse-direction channel, specifically being the above error signal and serving as a pilot signal, onto the carrier-wave frequency through frequency modulation. If such frequency modulation is carried out, the light receiver2B should have a function of demodulating a pilot signal subjected to the frequency modulation from the carrier wave as to be detailed below with reference toFIG. 11.

Furthermore, the carrier-wave frequency control circuit16may superimpose dither for detecting quality of a received signal onto the carrier-wave frequency of a light signal that the IQ modulator15A outputs by controlling the cycle of the phase rotation θ=2πΔfat that the phase rotating circuit122applies and transmit the superimposed dither to the receiver end (the optical receiver2A or2B) through the transmission path3.

When the carrier-wave frequency is to be adjusted beyond the electric band of the transmitter, the carrier-wave frequency control circuit16may carry out control using the phase rotating circuit122and control of the oscillation frequency of the laser light source11in combination with each other. Specifically, the carrier-wave frequency control circuit16may use both the fine adjustment that controls the carrier-wave frequency of a light signal output from the IQ modulator15A through controlling the cycle of the phase rotation that the phase rotating circuit122applies and rough adjustment that directly controls the frequency of the output light from the laser light source11. The oscillation frequency of the laser light source11generally used in a wavelength multiplexed optical system is controllable by means of oscillation frequency adjustment based on changing an oscillation frequency grid and controlling the temperature. The carrier-wave frequency control circuit16accomplishes the above rough adjustment using oscillation frequency adjustment based on changing an oscillation frequency grid and controlling the temperature.

Likewise the optical transmitter1ofFIG. 1, in the above optical transmitter1A of the first embodiment, the carrier-wave frequency control circuit16outputs an amount Δfaof frequency control to the phase rotating circuit122, so that the cycle of the phase rotation θ=2πΔfat that the phase rotating circuit122provides is controlled. This fine-adjusts the carrier-wave frequency of the light signal output from the IQ modulator15A to a value of the sum of the frequency fCof the output light from the laser light source11and an amount Δfaof frequency control within the electric band of the optical transmitter1A.

Accordingly, use of the amount Δfaof frequency control makes the optical transmitter1A possible to control the carrier-wave frequency of the light signal output from the IQ modulator15A more precisely and faster than the conventional method which directly controls the laser light source11. This ensures the stability of the oscillation frequency of the laser light source11, so that the transmission performance of an optical communication system using the optical transmitter1A is improved. In addition, wavelength multiplexing intervals come to be dense to improve the rate of using the band of the transmission path (optical fiber)3and high-capacity transmission is realized.

In addition, differently from a conventional method that directly controls the laser light source11, the optical transmitter1A can carry out complex frequency control such as superimposing a pilot signal onto a carrier-wave frequency by means of frequency control, and superimposing dither for detecting quality of a received signal onto the carrier-wave frequency. This makes it possible to transmit a pilot signal (e.g., an error signal) without using a control channel, and to ensure various advantages, such as improvement in sensitivity of error detection at a receiver end with the aid of dither as detailed below with reference toFIGS. 10 and 11.

In addition, since the optical transmitter1A of the first embodiment has functions of the I-channel predistortion compensating circuit123I and the Q-channel predistortion compensating circuit123Q, deterioration of signal quality caused by incompletion of the DACs13I and13Q, the drivers14I and14Q, and the phase modulators151I and151Q is compensated beforehand. This enables the optical transmitter1A to accomplish high-quality light transmission.

Furthermore, since the carrier-wave frequency control circuit16concurrently uses the fine adjustment that controls the carrier-wave frequency by the phase rotating circuit122and rough adjustment to directly control the frequency of the output light from the laser light source11, the optical transmitter1A can precisely adjusts the carrier-wave frequency even beyond the electric band of the transmitter.

(3) Optical Transmitter of the Second Embodiment:

FIG. 6is a block diagram schematically illustrating an optical transmitter1B according to a second embodiment. The optical transmitter1B ofFIG. 6is similar in configuration with the optical transmitter1A except for the signal processing circuit12having a function of a transmission-path predistortion compensating circuit124.FIG. 6omits the laser light source11, the drivers14I and14Q, and the IQ modulator15A. InFIG. 6, parts and elements having the same reference numbers as the foregoing description represent the same or similar parts and elements, so repetitious description is omitted here.

The transmission-path predistortion compensating circuit124is arranged between the121and the phase rotating circuit122, and compensates the electric-field information E1, to which the modulating scheme mapping circuit121maps the transmission signal, for prospective deterioration in signal quality due to transmission through the transmission path (optical fiber)3. As detailed below, the compensation is accomplished by multiplying (convolving) the electric-field information E1from the modulating scheme mapping circuit121by the reversed function of a waveform distortion due to the optical transmission through the transmission path3.

Accordingly, the optical transmitter1B of the second embodiment ensures the same effects as those of the optical transmitter1A of the first embodiment. In addition, since the signal processing circuit12has a function of the transmission-path predistortion compensating circuit124, the waveform distortion caused by optical transmission through the transmission path3can be compensated beforehand, so that the optical transmitter1B of the second embodiment can attain higher-quality optical transmission.

Hereinafter, description will now be made in relation to the principle of the signal processing in the optical transmitter1B ofFIG. 6, that is, controlling the carrier-wave frequency by the phase rotating circuit122with reference to following Formulae (1) through (5).

As described above, the electric-field information E1after the mapping of the transmitting signal by the modulating scheme mapping circuit121is represented by Formula (1).
E1=A(t)·exp(jθ(t))  (1)

where, the term “j” represents the imaginary unit; the term “A(t)” represents an electric-field intensity (amplitude); the term “θ(t)” represents an electric-field phase; and the term “t” represents time.

The electric-field information E2after the transmission-path predistortion compensating circuit124compensates for distortion of the electric-field information E1is expressed by following Formula (2).
E2=h1(t)*E1(2)

where, the symbol “*” represents calculation of convolving; the term “h1(t)” represents the reversed function of waveform distortion of the transmission path3.

Electric-field information E3obtained through the phase rotation based on the amount Δfaof frequency control on the electric-field information E2by the phase rotating circuit122is expressed following Formula (3)
E3=exp(j2πΔfat)·E2(3)

Electric-field information E4obtained through compensation on the electric-field information E3by the I-channel predistortion compensating circuit123I and the Q-channel predistortion compensating circuit123Q is expressed following Formula (4).
E4=h2(t)*E3(4)

where, the term “h2(t)” represents a reversed function of a waveform distortion of the optical transmitter1B (transmitting system) of the second embodiment.

The intensity Psigof a signal obtained by processing on the electric-field information E4by the DACs13I and13Q, the drivers14I and14Q, and the phase modulators151I and151Q is expressed by following Formula (5)

where, the term “fc” represents the frequency of output light that the laser light source11oscillates; the term “P” represents a photoelectric-field intensity; and the term “h2(t)′” represents a function of the waveform distortion of the optical transmitter1B (transmitting system) and establishes the relationship h2(t)′*h2(t)=1.

(4) Optical Transmitter According to a Third Embodiment

FIG. 7is a block diagram schematically illustrating the configuration of the optical transmitter1C according to the third embodiment. The optical transmitter1C adopts a polarization multiplexing scheme, and includes the laser light source11, the signal processing circuit12, the DACs13XI,13XQ,13YI, and13YQ, the drivers14XI,14XQ,14YI, and14YQ, a polarization multiplexed IQ modulator15C, and the carrier-wave frequency control circuit16.

The laser light source11oscillates output light having a predetermined frequency fC.

The signal processing circuit12carries out digital signal processing on a transmitting signal which is input from an external device and which is binary data, and has functions of a 1:2 DMUX120, modulating scheme mapping circuits121X and121Y, the phase rotating circuits122X and122Y, the I-channel predistortion compensating circuits123XI and123YI, and the Q-channel predistortion compensating circuits123XQ and123YQ.

The 1:2 DMUX120serves as a divider that separates the transmitting signal in the form of binary data input from an external device into two branch signals corresponding one to each of polarization components X and Y orthogonal to each other.

The modulating scheme mapping circuits121X and121Y correspond to the polarization components X and Y separated by the 1:2 DMUX120, respectively.

The modulating scheme mapping circuit121X constellation-maps a signal of the polarization component X to electric-field information according to a modulation scheme such as QPSK, QAM, and OFDM. The electric-field information Ex1, to which the modulating scheme mapping circuit121X maps the transmitting signal, includes an I component and a Q component, and is expressed by Ex1=xI+j·xQ. Similarly, the modulating scheme mapping circuit121Y constellation-maps a signal of the polarization component Y to electric-field information according to a modulation scheme such as QPSK, QAM, and OFDM. The electric-field information Ey1, to which the modulating scheme mapping circuit121Y maps the transmitting signal, includes an I component and a Q component, and is expressed by Ey1=yI+j·yQ.

The phase rotating circuits122X and122Y are arranged downstream modulating scheme mapping circuits121X and121Y, respectively, and correspond to the polarization components X and Y, respectively.

The phase rotating circuit122X applies phase rotation having a predetermined cycle to an electric-field phase of the electric-field information Ex1, to which the modulating scheme mapping circuit121X maps the transmitting signal. Specifically, likewise the optical transmitters1and1A ofFIGS. 1 and 2, the phase rotating circuit122X applies, upon receipt of an amount Δfaof frequency control from the carrier-wave frequency control circuit16, phase rotation θ=2πΔfat to the electric-field phase of the electric-field information Ex1. Similarly, the phase rotating circuit122Y applies phase rotation having a predetermined cycle to an electric-field phase of the electric-field information Ey1, to which the modulating scheme mapping circuit121Y maps the transmitting signal. Specifically, the phase rotating circuit122Y applies, upon receipt of an amount Δfaof frequency control from the carrier-wave frequency control circuit16, phase rotation θ=2πΔfat to the electric-field phase of the electric-field information Ey1. The phase rotating circuits122X and122Y each have the same configuration as the phase rotating circuit122ofFIG. 4.

The I-channel predistortion compensating circuit123XI and the Q-channel predistortion compensating circuit123XQ are arranged the downstream of the phase rotating circuit122X and correspond to the polarization component X.

The I-channel predistortion compensating circuit123XI compensates the I component of the electric-field information, to which the phase rotating circuit122X provides the phase rotation, for prospective deterioration in signal quality of the I component of the polarization component X, which deterioration is caused by incompletion of a transmitting system corresponding to the I component of the polarization component X. Similarly, the Q-channel predistortion compensating circuit123XQ compensates the Q component of the electric-field information, to which the phase rotating circuit122X provides the phase rotation, for prospective deterioration in signal quality of the Q component of the polarization component X, which deterioration is caused by incompletion of a transmitting system corresponding to the Q component of the polarization component X. Here, an example of the transmitting system corresponding to the I component of the polarization component X includes the DAC13XI, the driver14XI, and a phase modulator151XI that are to be detailed below; and an example of the transmitting system corresponding to the Q component of the polarization component X includes the DAC13XQ, the driver14XQ, and a phase modulator151XQ that are to be detailed below. As detailed below, the compensation is accomplished by multiplying (convolving) the electric-field information from the phase rotating circuit122X by the reversed function of a waveform distortion of the optical transmitter1C (the transmitting system).

The I-channel predistortion compensating circuit123YI and the Q-channel predistortion compensating circuit123YQ are arranged the downstream of the phase rotating circuit122Y and correspond to the polarization component Y.

The I-channel predistortion compensating circuit123YI compensates the I component of the electric-field information, to which the phase rotating circuit122Y provides the phase rotation, for prospective deterioration in signal quality of the I component the polarization component Y, which deterioration is caused by incompletion of a transmitting system corresponding to the I component of the polarization component Y. Similarly, the Q-channel predistortion compensating circuit123YQ compensates the Q component of the electric-field information, to which the phase rotating circuit122Y provides the phase rotation, for prospective deterioration in signal quality of the Q component of the polarization component Y, which deterioration is caused by incompletion of a transmitting system corresponding to the Q component of the polarization component Y. Here, an example of the transmitting system corresponding to the I component of the polarization component Y includes the DAC13YI, the driver14YI, and a phase modulator151YI that are to be detailed below; and an example of the transmitting system corresponding to the Q component of the polarization component Y includes the DAC13YQ, the driver14YQ, and a phase modulator151YQ that are to be detailed below. As detailed below, the compensation is accomplished by multiplying (convolving) the electric-field information from the phase rotating circuit122Y by the reversed function of a waveform distortion of the optical transmitter1C (the transmitting system).

The DACs13XI and13XQ correspond to the polarization component X, and convert digital signals (I component and Q component) from the I-channel predistortion compensating circuit123XI and the Q-channel predistortion compensating circuit123XQ, respectively, into analog signals. Similarly, the DACs13YI and13YQ correspond to the polarization component Y, and convert digital signals (I component and Q component) from the I-channel predistortion compensating circuit123YI and the Q-channel predistortion compensating circuit123YQ, respectively, into analog signals.

The divers14XI and14XQ correspond to the polarization component X, and amplify signals from the DACs13XI and13XQ and drive the respective corresponding phase modulator151XI and151XQ (seeFIG. 8) in the polarization multiplexed IQ modulator15C using the respective amplified signals. Similarly, the divers14YI and14YQ correspond to the polarization component Y, and amplify signals from the DACs13YI and13YQ and drive the respective corresponding phase modulator151YI and151YQ (seeFIG. 8) in the polarization multiplexed IQ modulator15C using the respective amplified signals.

The polarization multiplexed IQ modulator (optical modulator)15C is a polarization multiplexed modulator that modulates the polarization component X and Y, which are orthogonal each other, independently of each other. The polarization multiplexed IQ modulator15C modulates the output light from the laser light source11in accordance with the I component and Q component of two branch signals (polarization components X and Y) subjected to digital processing in the signal processing circuit12, and outputs the modulated light, regarded as a light signal, to the transmission path3.

As illustrated inFIG. 8that illustrates the configuration of the polarization multiplexed IQ modulator15C, the polarization multiplexed IQ modulator15C includes the phase modulator151XI for the I component, the phase modulator151XQ for the Q component, and a phase shifter152X that deal with the polarization component X; and the phase modulator151YI for the I component, the phase modulator151YQ for the Q component, and a phase shifter152Y that deal with the polarization component Y; and a polarization coupler153. It is to be noted thatFIG. 8is a block diagram schematically illustrating the configuration of the polarization multiplexed IQ modulator15C.

The phase shifter152X provides a predetermined phase difference (e.g., π/2) between a pair of light signals propagating through the phase modulator151XI and the phase modulator151XQ, and is disposed on the side of the phase modulator151XQ. The phase modulator151XI and the phase modulator151XQ perform phase modulation based on the transmitting signals (I component and Q component) from the drivers14XI and14XQ on the pair of light signals to which the predetermined phase difference is provided.

The phase shifter152Y provides a predetermined phase difference (e.g., π/2) between a pair of light signals propagating through the phase modulator151YI and the phase modulator151YQ, and is disposed on the side of the phase modulator151YQ. The phase modulator151YI and the phase modulator151YQ perform phase modulation based on the transmitting signals (I component and Q component) from the drivers14YI and14YQ on the pair of light signals to which the predetermined phase difference is provided.

The Polarization Beam Combiner (PCB)153combines the modulated signal of the polarization component X from the phase modulators151XI and151XQ and the modulated signal of the polarization component Y from the phase modulators151YI and151YQ, and outputs the combined signal to the transmission path3.

Similarly to the optical transmitters1,1A, and1B, the carrier-wave frequency control circuit16controls the carrier-wave frequency of the light signal (X polarization and Y polarization) output from the polarization multiplexed IQ modulator15C. The carrier-wave frequency control circuit16outputs an mount Δfaof frequency control to the phase rotating circuits122X and122Y to control the cycles of the phase rotation θ=2πΔfat that the phase rotating circuits122X and122Y apply, so that the carrier-wave frequency of the light signal (X polarization and Y polarization) output from the polarization multiplexed IQ modulator15C can be controlled. As a result of the above frequency control, the carrier-wave frequency control circuit16fine-adjusts the carrier-wave frequency of the light signal (X polarization and Y polarization) output from the polarization multiplexed IQ modulator15C within the electric band of the optical transmitter1C. The electric band of the optical transmitter1C depends on the band properties of the DACs13XI,13XQ,13YI, and13YQ, the drivers14XI,14XQ,14YI, and14YQ, and the polarization multiplexed IQ modulator15C.

Similarly to the first embodiment, the carrier-wave frequency control circuit16fine-adjusts the frequency of the carrier wave within the electric band of the optical transmitter1C by adjusting the rotating frequencies that the phase rotating circuits122X and122Y apply in accordance with an error signal from an optical receiver2A or2B (seeFIGS. 9-11) in communication with the optical transmitter1C, so that the error at the receiver end is resolved.

Alternatively, the carrier-wave frequency control circuit16may superimpose a result (i.e., the above error signal) of detecting a quality of a pilot signal exemplified by a received signal onto the carrier-wave frequency of the light signal (X polarization and Y polarization) output from the polarization multiplexed IQ modulator15C through frequency modulation similarly to the first embodiment. In the optical transmitter1C ofFIG. 7, the carrier-wave frequency control circuit16outputs the same amount Δfaand Δfbof frequency control to the phase rotating circuits122X and122Y. Alternatively, different amounts Δfaof frequency control may be provided to the phase rotating circuits122X and122Y.

Similarly to the first embodiment, the carrier-wave frequency control circuit16may superimpose dither for detecting quality of a received signal onto the carrier-wave frequency of a light signal (X polarization and Y polarization) that the polarization multiplexed IQ modulator15C outputs by controlling the cycles of the phase rotations θ=2πΔfat that the phase rotating circuits122X and122Y apply to control the carrier-wave frequency of a light signal (X polarization and Y polarization) that the polarization multiplexed IQ modulator15C outputs, and transmit the superimposed dither to the receiver end (the optical receiver2A or2B) through the transmission path3.

Similarly to the first embodiment, when the carrier-wave frequency is to be adjusted beyond the electric band of the transmitter, the carrier-wave frequency control circuit16may carry out control using the phase rotating circuits122X and122Y and control of the oscillation frequency of the laser light source11in combination with each other. Specifically, the carrier-wave frequency control circuit16uses both the fine adjustment that controls the carrier-wave frequency of a light signal (X polarization and Y polarization) output from the polarization multiplexed IQ modulator15C through controlling the cycles of the phase rotations that the phase rotating circuits122X and122Y apply, and rough adjustment that directly controls the frequency of the output light from the laser light source11, so that the carrier-wave frequency of a light signal output from the polarization multiplexed IQ modulator15C can be controlled.

Likewise the optical transmitter1ofFIG. 1, in the above optical transmitter1C of the third embodiment, the carrier-wave frequency control circuit16outputs an amount Δfaof frequency control to the phase rotating circuits122X and122Y, so that the cycle of the phase rotation θ=2πΔfat that the phase rotating circuits122X and122Y apply is controlled. This fine-adjusts the carrier-wave frequency of the light signal (X polarization and Y polarization) output from the polarization multiplexed IQ modulator15C to a value of the sum of the frequency fCof the output light from the laser light source11and an amount Δfaof frequency control in the electric band of the optical transmitter1C.

Accordingly, use of the amount Δfaof frequency control makes the optical transmitter1C possible to control the carrier-wave frequency of the light signal output from the polarization multiplexed IQ modulator15C more precisely and faster than the conventional method which directly controls the laser light source11, similarly to the optical transmitter1A of the first embodiment. This ensures the stability of the oscillation frequency of the laser light source11, so that the transmission performance of an optical communication system using the optical transmitter1C is improved. In addition, wavelength multiplexing intervals come to be dense to improve the rate of using the band of the transmission path (optical fiber)3, and high-capacity transmission is realized. In particular, the optical transmitter1C, which adopts the polarization multiplexing scheme, can double the bit-rate per baud rate in the optical communication system.

In addition, differently from a conventional method that directly controls the laser light source11, also the optical transmitter1C can carry out complex frequency control such as superimposing a pilot signal onto a carrier-wave frequency by means of frequency control, superimposing dither for detecting quality of a received signal onto the carrier-wave frequency. This makes it possible to transmit a pilot signal (e.g., an error signal) without using a control channel, and to ensure various advantages, such as improvement in sensitivity of error detection at the receiver end with the aid of dither as detailed below with reference toFIGS. 10 and 11.

Besides, the optical transmitter1C of the third embodiment, the carrier-wave frequency control circuit16may output different amounts Δfaand Δfbof frequency control to the phase rotating circuits122X and122Y, as described above. Thereby, different pilot signals can be superimposed onto the polarization components X and Y through frequency modulation, so that an amount information to be transmitted by the pilot signals can be doubled.

Also in the optical transmitter1C, since the signal processing circuit12has functions of the I-channel predistortion compensating circuits123XI and123YI, and the Q-channel predistortion compensating circuit123XQ and123YQ, it is possible to compensate for distortion of signal quality due to incompletion of the DACs13XI,13XQ,13YI, and13YQ; the drivers14XI,14XQ,14YI, and14YQ, and the phase modulators151XI,151XQ,151YI, and151YQ beforehand. This enables the optical transmitter1C to accomplish high-quality light transmission.

Furthermore, since the carrier-wave frequency control circuit16uses both the fine adjustment that controls the carrier-wave frequency by the phase rotating circuits122X and122Y and rough adjustment that directly controls the oscillation frequency of the output light from the laser light source11, the optical transmitter1C can precisely adjusts the carrier-wave frequency even beyond the electric band of the transmitter.

Also in the optical transmitter1C, transmission-path predistortion compensating circuits being the same as the transmission-path predistortion compensating circuit124of the optical transmitter1B illustrated inFIG. 6may be disposed between the modulating scheme mapping circuit121X and the phase rotating circuit122X and between the modulating scheme mapping circuit121Y and the phase rotating circuit122Y. The transmission-path predistortion compensating circuits compensate the electric-field information, to which the modulating scheme mapping circuit121X or121Y maps the transmitting signal, for deterioration in signal quality, which deterioration is caused when transmission through the transmission path3. This configuration compensates for distortion in waveform caused by optical transmission through the transmission path3beforehand, and higher-quality light transmission can be realized.

The signal processing in the optical transmitter1C, which adopts a polarization multiplexing scheme, is carried out along Formulae (1) through (5) described above for the signal processing in the second embodiment. It should be noted that, when a polarization multiplexing scheme is adopted, the electric-field information E in Formula (1) through (5) uses Exand Eyserving as information of X polarization and Y polarization as taught in following Formula (6), and the function and the reversed function h(=h1(t), h2(t) or h2(t)′) related to distortion in waveform is replaced with the following function considering polarization as Formula (7) below.

(5) Optical Communication System and Optical Receiver:

Next, referring toFIG. 9, description will now be made in relation to an optical communication system adopting the above optical transmitters1, and1A through1C, and referring toFIGS. 10 and 11, description will now be made in relation to the configuration of the optical receivers2A and2B that the optical communication system adopts.FIG. 9is a block diagram schematically illustrating the configuration of the optical communication system of this embodiment;FIG. 10is a block diagram schematically illustrating the configuration of the optical receiver2A; andFIG. 11is a block diagram schematically illustrating the configuration of the optical receiver2B.

As illustrated inFIG. 9, the optical communication system adopting the optical transmitters1, and1A through1C includes an optical transmitter Tx, an optical receiver Rx, the transmission path3, and a number (four inFIG. 9) of repeaters4. Here, the optical transmitter Tx is one of the above optical transmitters1, and1A through1C, and the optical receiver Rx is the optical receiver2A or2B that are to be detailed below with reference toFIGS. 10 and 11. The transmission path3(optical fiber) connects the optical transmitter Tx and the optical receiver Rx, which are interposed by two or more repeaters4.

As illustrated inFIG. 10, the optical receiver (an example of the optical receiver)2A includes an OE21and a decision/error-correcting circuit22.

The OE (optical/electric converting circuit)21receives a light signal that the optical transmitter Tx outputs through the transmission path3, and converts the light signal into an electric signal.

The decision/error-correcting circuit22recognizes the electric signal from the OE21, and monitors the number of error corrections of the electric signal.

The number of error corrections monitored by the decision/error-correcting circuit22is regarded as an error signal described above, which is transmitted from the optical receiver Rx (2A) to the optical transmitter through a reverse-direction channel, through the control channel of the optical communication system, regarding the error signal as a pilot signal, or through the use of the above frequency modulation. In the optical transmitter Tx, which receives the number of error corrections from the optical receiver Rx (2A) in the above manner, the carrier-wave frequency control circuit16adjusts the rotation frequencies of the phase rotating circuits122,122X, and122Y such that the number of error corrections is minimized for fine-adjustment of the carrier-wave frequency within the electric band of the optical receiver Tx.

As illustrated inFIG. 11, the optical receiver (another example of the optical receiver, a digital coherent receiver)2B includes a local oscillator light source23, a 90-degree hybrid circuit24, an OE25, an ADC26, and a signal processing circuit27.

The local oscillator light source23oscillates local oscillation light, and outputs the oscillated light.

The 90-degree hybrid circuit24combines the local oscillated light from the local oscillator light source23and the light signal that the optical transmitter Tx transmit through the transmission path3, and outputs two pairs of light signals having light phases having a phase shift of 90 degrees.

The OE (optical/electric converting circuit)25converts the two pairs of light signals from the 90-degree hybrid circuit24into electric signals.

The ADC (analog/digital converting circuit)26converts the electric signals from the OE25into digital signals.

The signal processing circuit27carries out the digital signal processing on the two pairs of digital signals from the ADC26, and has functions of a waveform distortion compensating circuit271, a carrier-wave phase synchronizing circuit272, a decision circuit273, and the signal quality monitor274.

The waveform distortion compensating circuit271compensates for distortion in waveform of the digital signal from the ADC26.

The carrier-wave phase synchronizing circuit272has a function of compensating for a frequency difference (offset) and phase difference between the received optical signal and the local oscillator light-source (see, J. C. Rasmussen et al., “Digital Coherent Receive Technology for 100-Gps Optical Transport System”, FUJITSU, vol. 60, no. 5, p. 476-483, September, 2009); and demodulating a frequency.

The function of compensating for a frequency difference (offset) and phase difference between the received optical signal and the local oscillator light-source carries out compensation for the frequency difference (offset) and phase difference on the digital signal whose distortion in waveform is compensated by the waveform distortion compensating circuit271. There is a possibility of occurring a frequency offset in the range of a wavelength accuracy between the laser light source11in the optical transmitter Tx and the local oscillator light source23of the light receiver2B. An ordinary light source for wavelength multiplexing may have a frequency offset as large as several GHz at maximum. A large frequency offset makes it difficult to demodulate a received signal. Therefore, the function of compensating for frequency offset obtains an estimated value of a frequency offset, feeds back the estimated value based on which the oscillation frequency of the local oscillator light source23is fine-adjusted or compensated for through digital processing, so that the frequency offset is compensated for.

A phase difference between the laser light source11of the optical transmitter Tx and the local oscillator light source23of the optical receiver2B is left after the compensation for the frequency offset between the laser light source11and the local oscillator light source23. The function of estimating the phase of a carrier estimates the phase difference of the carrier wave and compensates for the phase difference in order to prepare for data decision.

The function of demodulating a frequency demodulates, when the pilot signal is superimposed through the above frequency modulation in the optical transmitter Tx, the pilot signal from the carrier wave. Consequently, the carrier-wave phase synchronizing circuit272can obtain an estimated value of the frequency offset through the function of compensating for a frequency offset, so that the offset value of the light-source frequency can demodulate a pilot signal superimposed through the frequency modulation by the optical transmitter Tx.

The decision circuit273carries out data decision on a received signal on the basis of the signal obtained by the carrier-wave phase synchronizing circuit272, and outputs the result of the decision as a data signal.

The signal quality monitor274monitors the quality of a signal obtained by the carrier-wave phase synchronizing circuit272.

The signal quality monitored by the signal quality monitor274is regarded as the above error signal, which is transmitted from the optical transmitter Rx (2B) to the optical transmitter Tx through a reverse-direction channel, through the control channel of the optical communication system, regarding the error signal as a pilot signal, or through the use of the frequency modulation. In the optical transmitter Tx, which receives the signal quality from the optical receiver Rx (2B) adjusts the rotation frequency of the phase rotating circuits122,122X, and122Y such that the signal quality comes to the best and fine-adjusts the carrier-wave frequency within the electric band of the optical transmitter Tx.

In order to easily find the optimum carrier-wave frequency, it is effective to transmit a signal obtained by superimposing a dither onto a carrier-wave frequency by the carrier-wave frequency control circuit16. In this case, the dither superimposed to the carrier-wave frequency is monitored on the basis of the number of error corrections obtained by the decision/error-correcting circuit22or of the quality information obtained by the signal quality monitor274. Then, the carrier-wave frequency control circuit16adjusts the rotation frequency of the phase rotating circuits122,122X, and122Y such that the amplitude of the dither monitored comes to be the minimum, and fine-adjusts the carrier-wave frequency within the electric band of the optical transmitter Tx. Concurrently, the amplitude of the monitored dither is regarded as the error signal, which is transmitted from the optical receiver Rx to the optical transmitter Tx.

Using of the optical transmitters1,1A through1C in combination with the optical receivers2A and2B, the carrier wave frequency can be precisely controlled at a high-speed such that the number of error corrections of a received signal is minimized, that the quality of a received signal is best, or that the dither amplitude is minimized.

(6) First Example of an Optical Transmitter Unit:

Hereinafter, a description will now be made in relation to examples (first through fifth example) of an optical communication unit that accomplishes high-density wavelength multiplexing using the optical transmitters1,1A through1C having the above configuration.

To begin with, an optical transmitter unit according to the first example will now be described with reference toFIGS. 12 and 13.FIG. 12is a block diagram illustrating the configuration of an optical transmitter unit of the first example; andFIG. 13is a diagram illustrating a transmission spectrum of the optical transmitter unit ofFIG. 12.

As illustrated inFIG. 12, the optical transmitter unit100A of the first example includes a number (n inFIG. 12, where n is a natural number) of optical transmitters Tx1through Txnand an optical coupler101. The optical transmitters Tx1through Txnare the same one of the optical transmitters1,1A through1C illustrated inFIGS. 1-8. The optical transmitters Tx1through TxninFIG. 12is the optical transmitter1illustrated inFIG. 1, andFIG. 12omits the DAC13and the carrier-wave frequency control circuit16.

The optical coupler101combines light signals from optical transmitters Tx1through Txnfor wavelength multiplexing and outputs the wavelength multiplexed signal to the transmission path3.

The respective laser light sources11of the optical transmitters Tx1through Txnoscillate output light having the same frequency fc.

The respective carrier-wave frequency control circuits16of the optical transmitters Tx1through Txnfine-adjust the carrier-wave frequency of light signals output from the optical modulators15,15A, or15C by controlling the cycles of the phase rotations that the phase rotating circuits122,122X, or122Y apply.

The respective carrier-wave frequency control circuits16of the optical transmitters Tx1through Txnoutput different amounts Δfa1through Δfanof frequency control to the phase rotating circuits122,122X, or122Y. As illustrated inFIG. 13, the center frequencies of the carrier waves output from the optical modulators15,15A, or15C of the optical transmitters Tx1through Txnare adjusted to fC+Δfa1, fC+Δfa2, . . . , fC+Δfanin the electric bandwidth.

According to the optical transmitter unit100A of the first example, use of the functions of the respective carrier-wave frequency control circuits16of the optical transmitters Tx1through Txncan precisely adjust the carrier-wave frequencies of the light signals output from the Tx1through Txnwithin the electric bandwidth at a high speed. Thereby, the intervals of wavelength multiplexing come to be highly dense to improve the using rate of the band of the transmission path (optical fiber)3, so that high-capacity transmission can be realized.

A single laser light source11may be shared by the optical transmitters Tx1through Txn, which makes it possible to further simplify the configuration of the optical transmitter unit100A.

(7) Second Example of the Optical Transmitter Unit:

The optical transmitter unit according to a second example will now be described with reference toFIGS. 14 through 16B.FIG. 14is a block diagram illustrating the configuration of an optical transmitter unit of the second example;FIG. 15is a diagram illustrating a transmission spectrum of the optical transmitter unit ofFIG. 14; andFIGS. 16A and 16Bare diagrams explaining wavelength arrangement change (defragmentation) of the optical transmitter unit ofFIG. 14.

As illustrated inFIG. 14, the optical transmitter unit100B of the second example includes a number (n inFIG. 14) of optical transmitters Tx1through Txn, and a variable-band optical coupler102.

The optical transmitters Tx1through Txnare the same one of the optical transmitters1,1A through1C illustrated inFIGS. 1-8.

The variable-band optical coupler102combines light signals from the optical transmitters Tx1through Txnfor wavelength multiplexing and outputs the wavelength multiplexed signal to the transmission path3, and can vary the band.

In the optical transmitter unit100B of the second example, the respective carrier-wave frequency control circuits16of optical transmitters Tx1through Txncontrol the cycles of the phase rotations that phase rotating circuit122applies, so that wavelength arrangement can be adjusted in accordance with the bit rate of the transmitting signal (binary data).

Accordingly, when signals having different bit rates are to be transmitted through a single optical fiber, the functions of the carrier-wave frequency control circuits16of the optical transmitters Tx1through Txnprecisely fine-adjust the arrangement of wavelengths corresponding to the bit rates of optical path P1through P6(i.e., the transmitting signal from the optical transmitters Tx1through Txn) at high speed as illustrated inFIG. 15. Thereby, the intervals of wavelength multiplexing come to be highly dense to improve the using rate of the band of the transmission path (optical fiber)3, so that high-capacity transmission can be realized.

At that time, the carrier-wave frequency control circuits16of the optical transmitters Tx1through Txnmay adjust the wavelength arrangement by using both the above fine adjustment function and a rough adjustment function that directly controls the frequency of the output light from the laser light source11. As detailed above, the rough adjustment is accomplished by adjusting the oscillation frequency by means of changing the oscillation frequency grid or controlling the temperature. Such a combination of the fine adjustment and rough adjustment can precisely adjust the carrier-wave frequency within a large width exceeding the electric band of the optical transmitters.

Here, after an optical network having high-dense wavelength arrangement as illustrated inFIGS. 15 and 16Aare practically used for years, demands and requests of the respective optical paths may be changed. In this case, correction and change of the wavelength arrangement of the optical network can improve the using efficiency of wavelength resources.

In the optical transmitter unit100B of the second example can correct and change the wavelength arrangement of the optical network using the above fine and rough adjustment functions. If an optical path is moved and changed within at least the electric band, the use of the fine adjustment of the carrier-wave frequency control circuits16can precisely change the wavelength arrangement hitlessly (that is, without disconnection of the optical path) at a high speed.

For example, in an optical network being in a state of wavelength arrangement of optical paths P1through P6as illustrated inFIG. 16A, the optical paths P2and P3come to be not required any longer while the optical path P4comes to require a broader band. In this case, as illustrated inFIG. 16B, for example, after the bit rate of the optical path P4is changed, the optical paths P4through P6are moved by the above fine and rough adjustment functions, so that the optical paths P1, P4through P6are high-densely arranged. In addition, in the example ofFIG. 16B, new optical paths P7and P8are added in a frequency band subsequent to the optical path P6and high-densely arranged by the above fine and rough adjustment functions.

According to the optical transmitter unit100B of the second example, the fine and rough adjustment functions of the respective carrier-wave frequency control circuits16of the optical transmitters Tx1through Txnprecisely change the wavelength arrangement at a high speed. Even if the demand and the requirement of each optical path changes after a long-term operation of the optical network, the wavelength arrangement of the optical network is corrected and changed, so that the using rate of wavelength resource can be improved.

(8) Third Example of an Optical Transmitter Unit:

A description will now be made in relation to an optical transmitter unit according to a third example with reference toFIGS. 17 and 18.FIG. 17is a block diagram schematically illustrating an optical transmitter unit of the third example; andFIG. 18is a diagram illustrating a transmitting spectrum of the optical transmitter unit ofFIG. 17.

As illustrated inFIG. 17, an optical transmitter unit100C of the third example includes a number (n inFIG. 17) of optical transmitters Tx1through Txnand an optical coupler101, similarly to the optical transmitter unit100A of the first example illustrated inFIG. 12.

The optical transmitters Tx1through Txnare the same one of the optical transmitters1,1A through1C illustrated inFIGS. 1-8. The optical transmitters Tx1through TxninFIG. 17is the optical transmitter1illustrated inFIG. 1likewiseFIG. 12, andFIG. 17omits the DAC13and the carrier-wave frequency control circuit16.

The optical coupler101combines light signals from optical transmitters Tx1through Txnfor wavelength multiplexing and outputs the wavelength multiplexed signal to the transmission path3.

The respective laser light sources11of the optical transmitters Tx1through Txnare different light source out of frequency synchronization with one another for each sub-channel or each sub-channel group.

The respective carrier-wave frequency control circuits16of the optical transmitters Tx1through Txnfine-adjust the carrier-wave frequency of light signals output from the optical modulators15,15A, or15C by controlling the cycles of the phase rotations that the phase rotating circuits122,122X, or122Y apply.

The respective carrier-wave frequency control circuits16of the optical transmitters Tx1through Txnoutput different amounts Δfa1through Δfanof frequency control to the phase rotating circuits122,122X, or122Y. As illustrated inFIG. 18, the intervals of the center frequencies of the light signals output from the optical modulators15,15A, or15C are adjusted to a value of constant times the symbol rate (N×symbol rate, where N is a natural number).

Accordingly, even if the oscillation frequency of the laser light sources11are out of synchronization in the optical transmitter unit100C, the use of the functions of the respective carrier-wave frequency control circuits16can adjust the intervals of the center frequencies of the light signals output from the optical transmitters Tx1through Txnto a value of constant times the symbol rate. Thereby, the frequencies of light signals output from the optical transmitters Tx1through Txnare synchronized, so that OFDM (Orthogonal Frequency Division Multiplexing) can be realized.

(9) Fourth Example of an Optical Transmitter Unit:

A description will now be made in relation to the optical transmitter unit according to a fourth example with reference toFIGS. 19 and 20.FIG. 19is a block diagram schematically illustrating the configuration of the optical transmitter unit of the fourth example; andFIG. 20is a diagram illustrating a transmitting spectrum of the optical transmitter unit ofFIG. 20.

As illustrated inFIG. 19, the optical transmitter unit100D of the fourth example includes a number (n+1 inFIG. 19) of Tx1, Tx2, . . . , Txm, Txm′, . . . , and Txn, an AWG103, and the coupler104. Here, the number “m” represents a natural number of n or less.

The AWG (Arrayed Waveguide Gratings)103is an optical coupler having fixed filtering bands that includes n ports1through n and that combines light signals input through the ports1through n from the optical transmitters Tx2through Txnfor wavelength multiplexing and outputs the wavelength multiplexed signal to the transmission path3. As illustrated inFIG. 20, filtering bands are fixedly allocated one to each of the ports1through n.

Here, the optical transmitters Txmand Txm′ transmit signals having bit rates lower than those of the remaining optical transmitters, and are connected to a port m of the AWG103via a coupler104that combines signals from the optical transmitters Txmand Txm′.

The optical transmitters Tx1through Txm−1, and Txm+1through Txnare connected to ports1through m−1, abd m+1 through n of the AWG103, respectively.

The carrier-wave frequency control circuits16of the optical transmitters Tx1through Txm−1, and Txm+1through Txncontrols the cycles of the phase rotations that the phase rotating circuits122,122X, or122Y apply, so that one of the filtering bands of the AWG103is shared by two or more of the optical transmitters. Specifically, in the optical transmitter unit100D ofFIG. 19, the functions of the carrier-wave frequency control circuits16precisely adjust the carrier-wave frequencies of two optical transmitters Txmand Txm′ on a particular path input into the port m at a high speed. Thereby, the filtering band corresponding to the port m of the AWG103is shared by two optical transmitters Txmand Txm′, as illustrated inFIG. 20.

Accordingly, in the optical transmitter unit100D of the fourth example, the use of the AWG103makes it possible to combine two or more optical transmitters Txmand Txm′ having low bit rates by the coupler104, and the functions of the respective carrier-wave frequency control circuits16control the carrier-wave frequencies of the optical transmitters Tx1through Txn. Thereby, one of filtering bands of the AWG103is shared by such two or more optical transmitters Txmand Txm′ having low bit rates, so that the respective filtering band of the AWG103can be efficiently used.

The example ofFIGS. 19 and 20assume that one of the filtering bands of the AWG103is shared by two optical transmitters Txmand Txm′. However, the number of optical transmitters should by no means be limited to two. Alternatively, the carrier-wave frequencies of the optical transmitters Tx1through Txnmay be controlled by the functions of the carrier-wave frequency control circuits16such that one of the filtering bands is shared by three or more optical transmitters.

(10) Fifth Example of an Optical Transmitter Unit:

Description will now be made in relation to an optical transmitter unit according to a fifth example, and an optical receiver unit corresponding to the optical transmitter unit with reference toFIGS. 21 and 22.FIG. 21is a block diagram illustrating the configurations of the optical transmitter unit of the fifth example and the corresponding optical receiver unit, andFIG. 22is a diagram illustrating the transmission spectrum of the optical transmitter unit ofFIG. 21.

As illustrated inFIG. 21, an optical transmitter unit100E of the fifth example includes a number (n inFIG. 21) of optical transmitters Tx1through Txn, an AWG103, and couplers105and106.

The optical receiver unit200corresponding to the optical transmitter unit100E is connected to the optical transmitter unit100E via the transmission path (optical fiber)3, and includes a number (n inFIG. 21) optical receivers Rx1through Rn, an optical branching filter201, and couplers202and203.

The optical transmitters Tx1through Txnare the same one of the optical transmitters1,1A through1C ofFIGS. 1 through 8.

The AWG103is an optical coupler having fixed filtering bands that includes n ports1through n and that combines light signals input via the couplers105and106from the optical transmitters Tx1through Txnfor wavelength multiplexing and outputs the wavelength multiplexed signal to the transmission path3. As illustrated inFIG. 22, filtering bands are fixedly allocated one to each of the ports1through n.

The modulation scheme of the modulating scheme mapping circuits121of the optical transmitters Tx1through Txnin the optical transmitter unit100E adopt an Orthogonal Frequency Division Multiplexing (OFDM) scheme (seeFIG. 22).

If a light signal from one of the optical transmitters Tx1through Txn(in the example ofFIG. 22, the optical transmitters Tx2and Tx5) extends two filtering band of the AWG103, the carrier-wave frequency control circuits16of the optical transmitters Tx2and Tx5conduct the following control.

Specifically, the carrier-wave frequency control circuits16control the cycles of the phase rotations that the phase rotating circuits122,122X, or122Y provide, and thereby control the carrier-wave frequencies of the light signals output from the optical modulators15,15A or15C of the optical transmitters Tx2and Tx5such that the particular sub-carrier of the light signals from the optical transmitters Tx2and Tx5position at the guard band between the two filtering bands as illustrated inFIG. 22. At that time, the optical transmitters Tx2and Tx5prohibit use of the sub-carrier disposed at the guard bands.

The optical receivers Rx1through Rxnin the optical receiver unit200is, for example, the same one of the optical receivers2A and2B ofFIGS. 10 and 11.

The optical branching filter201receives a wavelength multiplexed light signal from the optical transmitter unit100E through the transmission path3, separates the received wavelength multiplexed light signal into n signals, and outputs the n signals one from each of the ports1through n to the optical receivers Rx1through Rxnvia the couplers202and203.

The coupler202corresponds to the coupler106of the transmitter end, and divides a signal from the port1of the optical branching filter201into two signals, one of which is output to the optical receiver Rx1. The coupler203corresponds to the coupler105of the transmitter end, combines the other signal from the coupler202, the signal from the port2of the branching filter201, and outputs the combined signal to the optical receiver Rx2.

According to the optical transmitter unit100E of the fifth example, the optical transmitters Tx1through Txneach carry out high-dense multiplexing through an OFDM scheme regardless of the filtering bandwidth of the AWG103, and precisely control the carrier-wave frequencies of the respective channels (optical transmitters Tx1through Txn) at a high speed such that only particular sub-carrier of each particular channel is disposed at the position of the guard band. Then, use of the particular sub-band positioning at the guard band is prohibited. Thereby, it is possible to surely avoid vain transmission processing through the sub-carrier of the guard band, so that transmission efficiency by OFDM in the optical network can be further enhanced.

The preferable embodiments of the present invention are described as above. The present invention should by no means be limited to the foregoing embodiments, and various changes and modifications can be suggested without departing from the gist of the present invention.