JP6047056B2 - Multi-carrier optical transmitter and multi-carrier optical transmission method - Google Patents

Description

  The present invention relates to a multicarrier optical transmitter and a multicarrier optical transmission method applicable to an optical communication system.

  In order to further improve the transmission capacity per channel in an optical communication system, a multicarrier transmission method using a plurality of subcarriers has been actively studied. In multicarrier transmission, it is desirable to reduce the frequency interval between subcarriers as much as possible to improve optical spectrum utilization efficiency (Spectral Efficiency: SE). Actually, as shown in Non-Patent Document 1, a study of multicarrier transmission in which the frequency interval between subcarriers is narrowed to the symbol rate using Nyquist WDM (Nyquist Wavelength Division Multiplexing) or OFDM (Orthogonal Frequency Division Multiplexing). Has been actively conducted in recent years.

  In order to realize such multi-carrier transmission with high SE, it is important to accurately control the frequency interval between subcarriers. For this reason, as a light source for multicarrier transmission, a configuration in which a single CW (Continuous Wave) light is used as a seed light source and a plurality of sub-peaks are generated using an external modulator or a nonlinear optical medium is often used.

R. Schmogrow et al., "Real-time Nyquist pulse generation beyond 100 Gbit / s and its relation to OFDM" Optics Express, 2012, Vol. 20, No.1, p. 317-337.

  However, in such a configuration, it is difficult to equalize the power level between sub-peak components, and in principle, the optical power per sub-peak component after equalization is 1 / subcarrier number of the output power of the CW light source. Therefore, there is a problem that the optical power per sub-channel is reduced, which becomes a limiting factor for the transmittable distance. In addition, a separate optical separation filter for separating the sub-peak components one by one and sending them to different modulation means is required, resulting in a problem that the entire transmitter becomes complicated.

  In order to increase the optical power per subchannel signal and eliminate the need for a separation filter, it is desirable to use a separate CW light source for each subchannel. However, it is generally difficult to accurately control the frequency interval between CW light sources that oscillate independently. For example, since the oscillation frequency accuracy of a semiconductor CW light source generally used in an optical transmission system is about ± several GHz including an aging change, the frequency interval between CW light sources that operate independently is distributed on the order of about twice that. That is, this is insufficient accuracy to be applied to multi-carrier transmission with high SE.

  The present invention has been made in view of such problems, and an object of the present invention is to provide a multi-carrier optical transmitter that uses an individual CW light source for each subchannel and can control the frequency interval between subcarriers. It is to provide a multi-carrier optical transmission method.

In order to solve the above-mentioned problem, the multicarrier optical transmitter according to claim 1 is configured such that N transmission CW light sources respectively corresponding to N (N is an integer of 2 or more) subchannels, Monitor light extraction means for extracting and outputting N monitor lights from the N output lights output from the transmission CW light source, and frequency intervals between the N output lights using the N monitor lights, respectively. Frequency interval monitoring means for detecting, generating and outputting frequency interval information from each of the detected frequency intervals, and each of the N subchannels based on a transmission data signal for data modulation input from the outside Is generated by shifting the frequency of the complex electric field signal by a difference between each frequency interval of the frequency interval information and a predetermined frequency interval, thereby generating the N subchannels. Drive signal generation means for generating and outputting N drive signals respectively corresponding to the N drive signals and the N drive signals and the N output lights to generate and output N subchannel signals. An optical vector modulation unit that modulates the N output lights based on the N drive signals and shifts the frequency of the N output lights, so that the sub-vector modulation unit The optical vector modulation means for generating the N subchannel signals so as to have the predetermined frequency interval between channel signals, and the N subchannel signals output from the optical vector modulation means are multiplexed. and a light multiplexing means for transmitting a multicarrier signal, the frequency interval monitoring means, clock the monitor light that is removed from at least one of said N transmit CW light source After modulating with a signal, the modulated monitor light and the monitor light interfere with each other to obtain a beat signal, the frequency interval information includes the beat signal, and the drive signal generating means includes the The difference between the frequency intervals and a predetermined frequency interval is calculated based on the beat frequency of the beat signal .

The multicarrier optical transmitter according to claim 2 , wherein N transmission CW light sources respectively corresponding to N (N is an integer of 2 or more) subchannels, and N output from the N transmission CW light sources Monitor light extraction means for extracting and outputting N monitor lights from the output lights, and detecting frequency intervals between the N output lights using the N monitor lights, respectively, and detecting the detected frequency intervals Frequency interval monitoring means for generating and outputting frequency interval information from each of them, generating a complex electric field signal for each of the N subchannels based on a transmission data signal for data modulation input from the outside, By shifting the frequency of the complex electric field signal by a difference between each frequency interval of the frequency interval information and a predetermined frequency interval, N pieces of N subchannels respectively corresponding to the N subchannels are obtained. Drive signal generation means for generating and outputting a drive signal; and optical vector modulation means for inputting the N drive signals and the N output lights to generate and output N subchannel signals. Then, the optical vector modulation means modulates the N output lights based on the N drive signals and shifts the frequency of the N output lights, so that the predetermined channel between the subchannel signals is obtained. The N subchannel signals are generated so as to have a frequency interval, and the optical vector modulation means and the N subchannel signals output from the optical vector modulation means are combined to transmit a multicarrier signal. Optical frequency combining means, and the frequency interval monitoring means includes a reference light source, and has means for obtaining a beat signal by causing the monitor light and output light of the reference light source to interfere with each other. The information includes the beat signal, and the drive signal generation means calculates the difference between each frequency interval and a predetermined frequency interval based on the beat frequency of the beat signal, and the output light spectrum of the reference light source is: It is characterized by having sub-peaks arranged at equal frequency intervals of N or more .

The multicarrier optical transmission method according to claim 3 , wherein N output lights are output from N transmission CW light sources respectively corresponding to N (N is an integer of 2 or more) subchannels; Extracting and outputting N monitor lights from the output lights, detecting frequency intervals between the N output lights using the N monitor lights, and detecting each of the detected frequency intervals. Generating and outputting frequency interval information; and generating a complex electric field signal for each of the N subchannels based on a transmission data signal for data modulation input from the outside, and generating a frequency of the complex electric field signal Is shifted by the difference between each frequency interval of the frequency interval information and a predetermined frequency interval, thereby generating and outputting N drive signals respectively corresponding to the N subchannels. And generating and outputting N subchannel signals from the N drive signals and the N output lights, and the N output lights based on the N drive signals Generating the N subchannel signals to have the predetermined frequency interval between the subchannel signals by shifting the frequency of the N output lights, and Combining a subchannel signal and transmitting a multicarrier signal, and generating and outputting the frequency interval information includes : extracting the frequency interval information from at least one of the N transmission CW light sources. After the monitor light is modulated with a clock signal, the modulated monitor light and the monitor light interfere with each other to obtain a beat signal, and the frequency interval information includes Wherein the beat signal, the step of generating and outputting the drive signal, and calculates the difference between the predetermined frequency spacing and each of the frequency intervals on the basis of the beat frequency of the beat signal.
The multicarrier optical transmission method according to claim 4, wherein N output lights are output from N transmission CW light sources respectively corresponding to N (N is an integer of 2 or more) subchannels; Extracting and outputting N monitor lights from the output lights, detecting frequency intervals between the N output lights using the N monitor lights, and detecting each of the detected frequency intervals. Generating and outputting frequency interval information; and generating a complex electric field signal for each of the N subchannels based on a transmission data signal for data modulation input from the outside, and generating a frequency of the complex electric field signal Is shifted by the difference between each frequency interval of the frequency interval information and a predetermined frequency interval, thereby generating and outputting N drive signals respectively corresponding to the N subchannels. And generating and outputting N subchannel signals from the N drive signals and the N output lights, and the N output lights based on the N drive signals Generating the N subchannel signals to have the predetermined frequency interval between the subchannel signals by shifting the frequency of the N output lights, and Combining a subchannel signal and transmitting a multicarrier signal, and generating and outputting the frequency interval information includes: using a reference light source, and outputting the monitor light and the output light of the reference light source. Obtaining a beat signal by interfering, wherein the frequency interval information includes the beat signal, and generating and outputting the drive signal comprises: beat frequency of the beat signal Based wherein calculating the difference between each frequency interval and a predetermined frequency interval, the output light spectrum of the reference light source is characterized by having a sub-peak arranged at equal frequency intervals or N present.

  According to the present invention, it is possible to provide a multicarrier optical transmitter that uses an individual CW light source for each subchannel and can control the frequency interval between subcarriers. In particular, by using the optical vector modulation means as a frequency shifter and data modulator to control the frequency interval between subcarriers, it is not necessary to control the frequency of the CW light source for transmission itself, and multicarrier optical transmission can be performed with a simple configuration. Can be realized.

It is a block diagram which shows the structure of the multicarrier optical transmitter which concerns on the 1st Embodiment of this invention. It is a block diagram which shows the structure of the frequency interval monitoring means which concerns on the 1st Embodiment of this invention. It is a block diagram which shows the structure of the drive signal generation means which concerns on the 1st Embodiment of this invention. It is a figure which illustrates typically the relationship of each spectrum of the output CW light of the transmission CW light source in the 1st Embodiment of this invention, and the output optical signal of an optical vector modulation means. It is a block diagram which shows the structure of the frequency interval monitoring means which concerns on the 2nd Embodiment of this invention. It is a block diagram which shows the structure of the frequency interval monitoring means which concerns on the 3rd Embodiment of this invention. It is a block diagram which shows the structure of the drive signal generation means which concerns on the 3rd Embodiment of this invention. FIG. 6 schematically shows the relationship between the output CW light of the transmission CW light source, the output spectrum of the optical modulator, and the output optical signal of the optical vector modulation means in the multicarrier optical transmitter according to the third embodiment of the present invention. It is a figure explaining. It is a block diagram which shows the structure of the frequency interval monitoring means which concerns on the 4th Embodiment of this invention. It is a block diagram which shows the structure of the drive signal production | generation means which concerns on the 4th Embodiment of this invention. In the multicarrier optical transmitter according to the fourth embodiment of the present invention, the relationship between the output CW light of the transmission CW light source, the output spectrum of the reference light source, and the spectrum of the output optical signal of the optical vector modulation means is schematically described. It is a figure to do. It is a block diagram which shows the structure of the frequency interval monitoring means which concerns on the 5th Embodiment of this invention. It is a block diagram which shows the structure of the frequency interval monitoring means which concerns on the 6th Embodiment of this invention. It is a block diagram which shows the structure of the drive signal generation means which concerns on the 6th Embodiment of this invention. In the multicarrier optical transmitter which concerns on the 6th Embodiment of this invention, the relationship of each spectrum of the output CW light of a transmission CW light source, the output light spectrum of a reference light source, and the output optical signal of an optical vector modulation means is typically shown. It is a figure explaining. It is a figure which illustrates a polarization multiplexing IQ modulator.

  In this specification, the “frequency of the transmission CW light source” refers to the frequency of the CW light output from the transmission CW light source, and the “subcarrier frequency” refers to the effective signal light of each subchannel after data modulation. Refers to the correct carrier frequency. Usually, “subcarrier frequency” = “center frequency of optical signal spectrum of each subchannel after data modulation” (however, when using some special modulation methods such as single sideband modulation, etc. The formula does not hold). In a conventional individual CW light source type multicarrier transmitter, “frequency of transmission CW light source” = “subcarrier frequency”. However, the basic technical idea of the present invention is to control the “subcarrier frequency” without controlling the “frequency of the transmission CW light source”. Specifically, as described in detail in the following description of the embodiment, the above-described control is made possible by using an optical vector modulation means as a frequency shifter and data modulator. Therefore, in the embodiment described below, the “frequency of the transmission CW light source” and the “subcarrier frequency” basically do not match. In all the drawings in this specification, the arrows simply indicate the flow of signals, and the number of arrows does not necessarily match the number of actual physical wirings.

(First embodiment)
FIG. 1 is a block diagram showing a configuration of a multicarrier optical transmitter 100 according to the first embodiment of the present invention. In addition, although all the embodiments in this specification show an example of a 4-carrier transmitter, the same configuration can be applied to any number of carriers of 2 or more. In FIG. 1, a multicarrier optical transmitter 100 includes transmission CW light sources 101 to 104, monitor light extraction means 111 to 114, optical vector modulation means 121 to 124, optical multiplexing means 130, output port 140, frequency Interval monitoring means 150 and drive signal generation means 160 are provided.

The transmission CW light sources 101 to 104 output CW lights having optical frequencies f _{1 to} f ₄ corresponding to the subchannels, respectively. In this example, f ₁ <f ₂ <f ₃ <f ₄ . The monitor light extraction means 111 to 114 tap a part (for example, 5%) of the output light of the transmission CW light sources 101 to 104 and send it to the frequency interval monitoring means 150 as monitor light. The frequency interval monitor unit 150 detects the frequency interval between the output lights of the transmission CW light sources 101 to 104 from the monitor light obtained by the monitor light extraction units 111 to 114, and the frequency interval including the beat signal from the detected frequency interval. Information is generated and sent to the drive signal generating means 160. The drive signal generation unit 160 generates drive signals for the optical vector modulation units 121 to 124 based on the frequency interval information obtained from the frequency interval monitor unit 150 and a transmission data signal input from the outside. Based on the input drive signal and the output light of the transmission CW light source, the optical vector modulation units 121 to 124 generate subchannel signals so that the subchannel signals have a predetermined frequency interval. The subchannel signals output from the optical vector modulation units 121 to 124 are combined by the optical combining unit 130 and output from the output port as a multicarrier (4-carrier) optical signal.

As the transmission CW light sources 101 to 104, for example, a distributed feedback laser diode or a distributed Bragg reflection laser diode can be used. As the optical vector modulation means 121 to 124, for example, a polarization multiplexing IQ modulator using a Mach-Zehnder Modulator (MZM) as shown in FIG. 16 can be used. FIG. 16 shows an MZM _XI corresponding to the X polarization In-phase (I) component, an MZM _XQ corresponding to the X polarization Quadrature (Q) component, an MZM _YI corresponding to the Y polarization I component, and Y A polarization multiplexed IQ modulator 1600 with MZM _YQ corresponding to the polarization Q component is shown. As shown in FIG. 16, the polarization multiplexing IQ modulator 1600 includes four parallel MZMs respectively corresponding to the I component and Q component of two orthogonally polarized waves (X polarized wave and Y polarized wave). Can arbitrarily modulate the intensity and phase of each polarization component of the output optical electric field.

As a material for the modulator, a material using lithium niobate (LiNbO ₃ : LN) is most widely used. However, a material using a semiconductor material such as indium phosphorus (InP) or a polymer material is used. In recent years, however, development has progressed. Since the effect of the present invention does not depend on the selection of the modulator material, the selection of the modulator material is arbitrary.

  As the optical multiplexing means 130, for example, a wavelength-independent optical multiplexer such as a 4 × 1 combiner or a multimode interferometer-type coupler may be used. In order to avoid optical loss, wavelength multiplexing / demultiplexing such as an arrayed waveguide grating may be used. A waver may be used.

  In addition, although the arrow showing the drive signal of the optical vector modulation means 121-124 was each represented by one each, as above-mentioned, the arrow shows the flow of a signal and does not represent the number of physical wiring. When the polarization multiplexing IQ modulator 1600 shown in FIG. 16 is used as the optical vector modulation means 121 to 124, the number of physical wirings of the drive signal output from the drive signal generation means 160 is four per optical vector modulation means (respectively X polarization I component, X polarization Q component, Y polarization I component, and Y polarization Q component) are each 16 in total.

  Further, in order to minimize the difference between the processing time of the frequency interval monitoring means 150 and the drive signal generation means 160 and the arrival time of the light wave from the monitor light extraction means 111 to 114 to the optical vector modulation means 121 to 124, the monitor light extraction is performed. An optical delay line may be provided between the means 111 to 114 and the optical vector modulation means 121 to 124 as necessary.

  FIG. 2 is a block diagram showing the configuration of the frequency interval monitoring means according to the first embodiment of the present invention. As shown in FIG. 2, the frequency interval monitoring means 200 (corresponding to the frequency interval monitoring means 150 shown in FIG. 1) includes optical branching devices 212 and 213, optical couplers 221 to 223, and balanced light receiving devices 231 to 233. With. The optical couplers 221 to 223 are general 2 × 2 3 dB couplers, and the balanced light receivers 231 to 233 include a balanced PD and a transimpedance amplifier. In addition, in all the embodiments described in this specification including this embodiment, a balanced photoreceiver is used to obtain high sensitivity. However, if sufficient sensitivity is obtained, it is replaced with a single-ended photoreceiver. Also good.

The optical splitters 212 and 213 split the two channels (monitor lights 202 and 203) excluding both ends of the input monitor lights 201 to 204 (optical frequencies f _{1 to} f ₄ ), respectively. The optical couplers 221 to 223 cause the monitor lights of adjacent channels to interfere with each other, and the balance light receivers 231 to 233 output beat signals 241 to 243 due to interference in order to detect the frequency interval of the monitor light of the adjacent channels. Thereby, the frequency interval monitoring means 200 outputs frequency interval information including the beat signals 241 to 243. Frequency _{f Beat_21} of the beat signal _{241~243, f} beat_32, for _{f Beat_43} the following formulas (1) to (3) holds.
f _beat — ₂₁ = f ₂ −f ₁ (Formula 1)
f _beat — ₃₂ = f ₃ −f ₂ (Formula 2)
f _beat — ₄₃ = f ₄ −f ₃ (Formula 3)

  FIG. 3 is a block diagram showing the configuration of the drive signal generation means according to the first embodiment of the present invention. As shown in FIG. 3, the drive signal generation means 300 (corresponding to the drive signal generation means 160 shown in FIG. 1) includes a complex electric field signal generation circuit 310, beat frequency detection circuits 321 to 323, and a frequency control circuit 330. And multiplication circuits 341 to 344 and output interface circuits 351 to 354.

The complex electric field signal generation circuit 310 is a digital circuit, and maps a transmission data signal for externally input data modulation to electric field information in accordance with a modulation method such as QPSK or quadrature amplitude modulation (QAM). Then, after generating a complex electric field symbol sequence for each subchannel, processing such as pulse shaping and pre-equalization is performed to generate complex electric field signals E _{1 to} E ₄ corresponding to each subchannel. When the polarization multiplexing configuration is used, each of the complex electric field signals E _{1 to} E ₄ is a complex vector composed of two components corresponding to the X polarization and the Y polarization.

Beat frequency detection circuits 321 to 323 are analog input / digital output circuits, and beat frequencies f beat — ₂₁ , f of beat signals 301 to 303 (corresponding to beat signals 241 to 243 shown in FIG. 2) sent from the frequency interval monitoring means. _{beat_32} and _{fbeat_43} are output as digital values. Specifically, as the beat frequency detection circuits 321 to 323, a general frequency counter circuit using a frequency dividing circuit or a counting circuit can be used. As another method, after the time waveform of the beat signal is digitized using an analog-to-digital converter (ADC), a spectrum is obtained using a Fourier transform circuit, and the peak position is obtained. A circuit for obtaining the value of the beat frequency is also conceivable.

Frequency control circuit 330 is a digital circuit, the beat frequency _{f beat_21, f beat_32, f beat_43} the desired frequency interval _Δf ideal_21, Δf ideal_32, compared to Delta] f _{Ideal_43,} calculates a compensation frequency δf ₁ ~δf ₄ based on the difference The compensation signal exp (2πδf _n t) (n = 1, 2, 3, 4) is output as a digital signal. Specifically, the compensation frequency is determined so that the following relationships (Equation 4) to (Equation 6) are satisfied.
δf ₂ −δf ₁ = Δf _ideal — ₂₁ −f _beat — ₂₁ (Formula 4)
δf ₃ −δf ₂ = Δf _ideal — ₃₂ −f _beat — ₃₂ (Formula 5)
δf ₄ −δf ₃ = Δf _ideal — ₄₃ −f _beat — ₄₃ (Formula 6)

In (Equation 1) to (Equation 3), δf _{1 to} δf ₄ are not uniquely determined, and the degree of freedom remains. However, for example, δf ₁ = 0 is simply set, and (Equation 4) to (Equation 4) to (Equation 4) to (Equation 4) equation 6) than δf _2, δf _3, may be obtained a delta] f _4. Further, the maximum value of | δf _n | may be determined as small as possible as follows.
1. With δf ₁ = 0, the values of δf ₂ , δf ₃ , and δf ₄ are obtained from (Expression 4) to (Expression 6).
2. A maximum value δf _MAX and a minimum value δf _min of δf _{1 to} δf ₄ are obtained.
3. n = 1, 2, 3, 4, δf _n ′ = δf _n − (δf _MAX + δf _min ) / 2 is obtained, and δf _n ′ is used as a new δf _n .

The multiplication circuits 341 to 344 are digital circuits, and the complex electric field signal E _n (n = 1, 2, 3, 4) output from the complex electric field signal generation circuit 310 and the compensation signal exp ( j2πδf _n t) is multiplied and output. The complex multiplication operation is equivalent to rotating at an angular velocity 2Paiderutaf _n in the complex plane, on the spectrum corresponds to the operation for delta] f _n by the frequency shift spectra of the complex electric field signal E _n. The time t is actually discretized.

The output interface circuits 351 to 354 are digital input / analog output circuits, a digital-to-analog converter (DAC) for converting a complex electric field signal subjected to phase rotation into an analog signal, and an analog signal to It includes a wideband amplifier circuit (driver amplifier) that amplifies, and outputs drive signals 361 to 364 of the optical vector modulation means. As described above, the number of physical wirings that actually output the drive signals 361 to 364 is 16 per arrow, ie, 16 in total. Specifically, for example, the X polarization component of _{E 1,} the Y polarization component, respectively _{E 1X,} when expressed as _{E 1Y,} the one arrow for outputting a drive signal 361 in practice Real _{[E 1X} · exp (j2πδf ₁ t)], Imag [E _1X · exp (j2πδf ₁ t)], Real [E _1Y · exp (j2πδf ₁ t)], Imag [E _1Y · exp (j2πδf ₁ t)] are output. It corresponds to a total of 4 physical wirings.

FIG. 4 is a diagram schematically illustrating the relationship between the spectra of the output CW light of the transmission CW light source and the output optical signal of the optical vector modulation unit in the first embodiment. The dotted line in FIG. 4 represents the frequency of the transmission CW light source, and the alternate long and short dash line represents the subcarrier frequency. When the optical vector modulation means is driven by a complex electric field signal subjected to phase rotation, that is, a drive signal E _n · exp (j2πδf _n t), with respect to the input CW light having the frequency f _n, the subchannel signal output from the optical vector modulation means Becomes E _n · exp {2π (f _n + δf _n ) t}. This is equivalent to the signal obtained by modulating a complex electric field signal E _n after shifting the CW light frequency by delta] f _n for the frequency f _n of the input CW light. That is, the optical vector modulation means has both a frequency shifter function and a data modulation function (complex electric field modulation function based on transmission data). Thus the frequency spacing between the sub-channel signals outputted from the optical vector modulator means is a _Δf ideal_12, Δf ideal_23, Δf ideal_34 is more, the desired value (Equation 1) to (6).

  The desired value of the frequency interval is not necessarily equal, but in the case of general Nyquist WDM or OFDM, it is equal. For example, in the case of Nyquist WDM having a modulation symbol rate of 32 Gbaud for each subchannel (the Nyquist pulse shaping is performed in the complex electric field signal generation circuit 310 shown in FIG. 3), the optical signal spectrum of each subchannel is a rectangle having a total width of about 32 GHz. A close spectrum is obtained, and a frequency interval of about 35 GHz (guard band of about 3 GHz) is used. In the case of OFDM, frequency interval = modulation symbol rate.

  As described above, in this embodiment, the frequency interval between the subcarriers is detected by detecting the frequency interval of the output light between the transmission CW light sources and driving the optical vector modulation signal with the drive signal to which the phase rotation based on this is added. It is possible to feed-forward compensate for the deviation from the desired value. There is no need to perform special frequency control for each transmission CW light source itself, and the optical vector modulation means serves both as a frequency shifter function and a data modulation function. A multicarrier optical transmitter capable of controlling the frequency interval between carriers can be provided.

If the range of the value of the compensation frequency δf _n (n = 1, 2, 3, 4) in this embodiment, that is, the compensable range is ± δf _range (δf _range > 0), δf _range is shown in FIG. The sampling rate f _s of the DAC included in the output interface circuits 351 to 354 is limited. From the sampling theorem, the signal spectrum of the driving signal _{E n · exp (j2πδf n t} ) ( corresponding to 361 to 364 shown in FIG. 3) _is required to fall within the scope of _{-f s / 2~ + f s /} 2 is there. That is, _{assuming that} the signal spectrum full width of the complex electric field signal En before the phase rotation is B, δf _range ≈ f _s / 2−B / 2. For example, in the case of Nyquist WDM with a symbol rate of 32 Gbaud, the sampling rate is generally f _s = 64 GHz so that the sampling rate is twice the symbol rate, and the signal spectrum full width is about B to 32 GHz because of the symbol rate, so δf _range ≈ It is about 16 GHz. Since the frequency fluctuation of the laser diode for high-density WDM (Dense WDM: DWDM) is about ± 2.5 GHz or less, δf _range ≈16 GHz can be said to be a sufficient compensation range even when a margin is expected. However, in general, the response characteristics of the driver amplifier and the optical modulator deteriorate as the frequency (absolute value) increases. Therefore, it is desirable that | δf _n | is as small as possible.

In order to make maximum use of the above compensation frequency range, the response band of the wideband amplifier circuit included in the output interface circuits 351 to 354 shown in FIG. 3 and the optical vector modulation means 121 to 124 shown in FIG. , Both need to be ˜f _s / 2 or more. Further, with respect to the operation band of the balance light receiver 231 shown in FIG. 2 and the beat frequency detection circuits 321 to 323 shown in FIG. 3, it is necessary to cover at least a _range of ± δf _range centered on the desired value Δf _ideal of the frequency interval. There is.

In addition, when the complex electric field signal generation unit 310 performs pre-equalization to preliminarily compensate for signal distortion due to band limitation of the output interface circuits 351 to 354 and the optical vector modulation units 121 to 124, the optimal pre-equalization filter coefficient is compensated. It depends on the frequency δf _n . For this reason, a path for outputting information on the compensation frequency δf _n from the frequency control circuit 330 to the complex electric field signal generation circuit 310 may be provided as indicated by a broken line arrow in FIG.

  In the overall configuration shown in FIG. 1, when the frequency interval of the transmission CW light sources 101 to 104 varies, the frequency interval shift is performed in the optical vector modulation units 121 to 124 through the frequency interval monitoring unit 150 and the drive signal generation unit 160. It takes a certain amount of time to compensate. Therefore, an optical delay line such as a fiber coil may be provided between the monitor light extraction units 111 to 114 and the optical vector modulation units 121 to 124.

(Second Embodiment)
FIG. 5 is a block diagram showing the configuration of frequency interval monitoring means according to the second embodiment of the present invention. The overall configuration of the multicarrier optical transmitter of the second embodiment is the same as that of the first embodiment shown in FIG. 1 except that the frequency interval monitoring unit 150 in the first embodiment is replaced with a frequency interval monitoring unit 500. This is the same as the configuration of the multicarrier optical transmitter according to FIG. As shown in FIG. 5, the frequency interval monitoring unit 500 includes optical branching units 512 and 513, optical couplers 521 to 523, balance light receivers 531 to 533, a clock source 550, analog multipliers 561 to 563, and And low-pass filters 571 to 573.

The optical splitters 512 and 513 split the two channels (monitor lights 502 and 503) of the input monitor lights 501 to 504 (optical frequencies f _{1 to} f ₄ ), excluding both ends, respectively. The optical couplers 521 to 523 cause the monitor lights of adjacent channels to interfere with each other, and the balance light receivers 531 to 533 output beat signals due to the interference. The beat signals output from the balance light receivers 531 to 533 are mixed (multiplied) with the clock signal (sin wave) of the frequency f _ref output from the clock source 550 in the analog multipliers 561 to 563, respectively, and the low-pass filter 571. After the high frequency component is cut through ˜573, it is output as beat signals 541 to 544. Frequency _{f Beat_21} of the beat signal _{541~543, f} beat_32, for _{f Beat_43,} it holds the following equation (7) to (9).
f _beat — ₂₁ = f ₂ −f ₁ −f _ref (Expression 7)
f _beat — ₃₂ = f ₃ −f ₂ −f _ref (Equation 8)
f _beat — ₄₃ = f ₄ −f ₃ −f _ref (Equation 9)

That is, in the present embodiment, the beat frequency is lowered compared to the first embodiment by using the superheterodyne method. Therefore, a slower circuit can be used as the beat frequency detection circuits 321 to 323 in the drive signal generation means shown in FIG. However, in this embodiment, since there is no means for determining whether the beat frequency f _beat is positive or negative, it is necessary to always satisfy f _beat > 0. For example, when the output frequency interval (f _{n + 1} −f _n ) between CW light sources is expected to be 30 GHz at the minimum, it is necessary to satisfy f _ref <30 GHz.

In the present embodiment, in determining the compensation frequencies δf _{1 to} δf ₄ , it is necessary to determine so as to satisfy the following (Expression 10) to (Expression 12) instead of the above (Expression 4) to (Expression 6). There is.
δf ₂ −δf ₁ = Δf _ideal — ₂₁ −f _beat — ₂₁ −f _ref (Equation 10)
δf ₃ −δf ₂ = Δf _ideal — ₃₂ −f _beat — ₃₂ −f _ref (Equation 11)
δf ₄ −δf ₃ = Δf _ideal — ₄₃ −f _beat — ₄₃ −f _ref (Equation 12)

(Third embodiment)
FIG. 6 is a block diagram showing a configuration of frequency interval monitoring means according to the third embodiment of the present invention. The overall configuration of the multicarrier optical transmitter of the third embodiment is that the frequency interval monitoring unit 150 in the first embodiment is replaced with the frequency interval monitoring unit 600, and the drive signal generation unit 160 in the first embodiment. 1 is the same as the configuration of the multicarrier optical transmitter according to the first embodiment shown in FIG. As shown in FIG. 6, the frequency interval monitoring means 600 includes an optical splitter 612, optical 90-degree hybrid circuits 621 to 623, balance light receivers 631 to 636, a clock source 650, an optical modulator 662, Low pass filters 671-676.

In the present embodiment, the desired subcarrier spacing is a regular intervals, i.e., a _{Δf ideal_21 = Δf ideal_32 = Δf ideal_43} = Δf ideal. Further, the frequency f _ref of the clock source 650 is adjusted to a desired subcarrier interval, that is, f _ref = Δf _ideal .

As the optical modulator 662, for example, a linear LN phase modulator can be used. The optical modulator 662 is driven by a clock signal (sin wave) output from the clock source 650, and clock-modulates the monitor light 602 (optical frequency f ₂ ). The clock signal is amplified by an amplifier (driver amplifier) as necessary, and then input to the optical modulator 662. The output signal spectrum of the optical modulator 662, sub-peak occurs at _{f ref} intervals on either side of frequency _{f 2.} The output light of the optical modulator 662 is 3 branched by the optical branching device 612, monitor light 601, 603, and 604 in the optical 90-degree hybrid circuit 621, 622, 623, respectively (optical frequency, respectively _{f 1, f 3, f 4} ) Interfere with. The balance light receivers 631 to 636 receive the I component or Q component outputs of the optical 90-degree hybrid circuits 621 to 623, respectively, and output them as analog electric signals. The outputs of the balance light receivers 631 to 636 are output as beat signals 641 to 646 after high frequency components are cut by the low pass filters 671 to 676. The beat frequency (low frequency component) of each beat signal 641 to 646 is expressed by the following (Expression 13) to (Expression 15).
f _beat — ₁ = f ₁ − (f ₂ −f _ref ) = f ₁ − (f ₂ −Δf _ideal )
(Formula 13)
f _beat — ₃ = f ₃ − (f ₂ + f _ref ) = f ₃ − (f ₂ + Δf _ideal )
(Formula 14)
f _beat — ₄ = f ₄ − (f ₂ + 2f _ref ) = f ₄ − (f ₂ + 2Δf _ideal )
(Formula 15)

That is, the primary and secondary side peaks of the clock-modulated monitor light 602 (the output light of the optical modulator 662) serve as reference light, and beat signals between the reference light and the monitor lights 601, 603, and 604 are obtained. . The time waveforms of the beat signals 641 to 646 to be output are cos (2πf _{beat — 1} t), sin (2πf _{beat — 1} t), cos (2πf _{beat — 3} t), sin (2πf _{beat — 3} t), cos (2πf _{beat — 4} t) and sin, respectively. (2πf _{beat — 4} t).

In the present embodiment, the beat frequency is distributed on both the positive and negative sides centering on zero. This is because, in the present embodiment, the beat frequencies are all zero in an ideal state where the frequencies of the transmission CW light sources are originally arranged at intervals of Δf _ideal (= f _ref ). The range in which the beat frequency is distributed coincides with the frequency accuracy (including the change over time) of the transmission CW light source. For this reason, the balance light receivers 621 to 626 used in the third embodiment have a narrow band (low speed) compared to the balance light receivers used in the first and second embodiments described so far. Things can be used. However, in the third embodiment, since it is necessary to determine whether the beat frequency is positive or negative (frequency shift direction), an optical 90-degree hybrid circuit is used instead of an optical coupler, and an I component and a Q component (cos component) of the beat signal are used. And sin component) can be monitored simultaneously. Further, if a balanced optical receiver 631 to 636 having a sufficiently narrow operating band is used, the low-pass filters 671 to 676 are unnecessary.

  FIG. 7 is a block diagram showing the configuration of the drive signal generating means according to the third embodiment of the present invention. In the configuration of the drive signal generation means 700 shown in FIG. 7, all parts except the beat frequency detection circuits 721 to 726 and the frequency control circuit 730 are the same as the drive signal generation means shown in FIG. Will be omitted.

ADC is used as the beat frequency detection circuits 721 to 726, and the time waveform of the beat signals 701 to 706 (corresponding to the beat signals 641 to 646 shown in FIG. 6) simply sent from the frequency interval monitoring means 600 is converted into a digital signal. Output. As described above, since the beat frequency is reduced to near zero by using modulation in the optical region, the ADC may be low speed. Frequency control circuit 730 is a digital circuit, and generates a compensation signal from the time waveform of the input beat signal _{exp (j2πδf n t) (n} = 1,2,3,4) is output. The compensation signal may be generated by the calculation of (Equation 16) below for i = 1, 3, and 4.
exp (j2πδf _n t) = exp (−j2πf _{beat_nt} )
= Cos (2πf _{beat_nt} ) −j * sin (2πf _{beat_nt} )
(Formula 16)

Note that δf ₂ is always zero. Therefore, the multiplication circuit 742 may be omitted. As another method, a method may be considered in which each input beat signal is subjected to Fourier transform to obtain a peak frequency, and δf _n is determined after appropriate operation.

FIG. 8 shows the spectrum of each of the output CW light of the transmission CW light source, the output spectrum of the optical modulator, and the output optical signal of the optical vector modulation means in the multicarrier optical transmitter according to the third embodiment of the present invention. It is a figure which illustrates a relationship typically. The dotted line in FIG. 8 represents the frequency of the transmission CW light source, and the alternate long and short dash line represents the subcarrier frequency. In the present embodiment, the sub-peaks appearing at intervals of f _ref (= Δf _ideal ) in the output spectrum of the optical modulator are used as a reference, and the deviation from there is detected as the beat frequency. As in the first embodiment, this frequency shift is compensated by phase rotation in the digital domain.

  The multi-carrier optical transmitter according to the present embodiment uses a clock modulation in the optical domain, which is lower than the multi-carrier optical transmitter according to the first and second embodiments described so far. ) Receiver and ADC can be used, and the technical difficulty can be reduced. Specifically, when the frequency accuracy (including the change over time) of the transmission CW light source is about ± 2.5 GHz with respect to a desired value, the operating band of the light receiver and the ADC is about ± 2.5 GHz (± 5 GHz with a margin in mind) Degree).

  In the present embodiment, the monitor light 602 is used as a reference, but the selection of the monitor light used as the reference is arbitrary. However, since a large modulation voltage amplitude is required to obtain sufficient intensity at a high-order sub-peak in clock modulation, it is desirable to select a channel near the center as a reference.

Further, in the present embodiment, f _ref = Δf _ideal is set, but both may be shifted by a known amount to f _ref = Δf _ideal + f _gap and the frequency control circuit 730 may appropriately perform correction. In this case, f _{beat — 1} , f _{beat — 3,} and f _{beat — 4} are _expressed by the following (Expression 17) to (Expression 19).
f _beat — ₁ = f ₁ − (f ₂ −f _ref ) = f ₁ − {f ₂ − (Δf _ideal + f _gap )} (Expression 17)
f _beat — ₃ = f ₃ − (f ₂ + f _ref ) = f ₃ − {f ₂ + (Δf _ideal + f _gap )} (Formula 18)
f _beat — ₄ = f ₄ − (f ₂ + 2f _ref ) = f ₄ − {f ₂ +2 (Δf _ideal + f _gap )}
(Formula 19)

Therefore, in order to compensate for f _gap , the operation when generating the compensation signal in the frequency control circuit 730 is as follows (Equation 20), and for n = 1, 3 and 4, m = + 1, −1, -2.
exp (j2πδf _n t)
= Exp (-j2 [ _pi ] f _{beat_nt} ) * exp (j2 [ _pi ] mf _begin t)
= {Cos (2πf _{beat —nt} ) −j * sin (2πf _{beat —it} )} * exp (j2πmf _gap t) (Formula 20)

As an example of setting f _ref ≠ Δf _ideal , for example, in the case of Nyquist WDM of 32 Gbaud and Δf _ideal = 35 GHz, the clock source 650 is shared with the symbol rate clock source used in the drive signal generation unit 700, and f _ref = 32 GHz ( It is conceivable that f _gap = 3 GHz).

In this embodiment, monitor light 601, 603, 604 is output to the signal light side input port of the two input ports of the optical 90-degree hybrid circuit, and output light of the optical modulator 662 is input to the local oscillation side input port. It is assumed that each input. When this is reversed, the output light of the optical modulator 662 is input to the signal light side input port, and the monitor lights 601, 603, and 604 are input to the local oscillation side input port, respectively, (Equation 13) to (Equation 15) The sign on the right side of) is reversed. In this case, the same effect can be obtained if the calculation when the compensation signal is generated in the frequency control circuit 730 is expressed by the following (Equation 21).
exp (2πδf _n t)
= Exp (2πf _{beat_nt} )
= Cos (2πf _{beat_nt} ) + j * sin ( _{2πfbeat_nt} ) (Formula 21)

(Fourth embodiment)
FIG. 9 is a block diagram showing the configuration of the frequency interval monitoring means according to the fourth embodiment of the present invention. The overall configuration of the multicarrier optical transmitter according to the fourth embodiment is that the frequency interval monitoring unit 150 in the first embodiment is replaced with the frequency interval monitoring unit 1000, and the drive signal generating unit in the first embodiment. The configuration is the same as that of the multicarrier optical transmitter according to the first embodiment shown in FIG. In FIG. 9, the frequency interval monitoring unit 900 includes an optical branching unit 910, optical 90-degree hybrid circuits 921 to 924, balance light receivers 931 to 938, low-pass filters 971 to 978, and a reference light source 980.

In the present embodiment, the desired subcarrier spacing is a regular intervals, i.e., a _{Δf ideal_21 = Δf ideal_32 = Δf ideal_43} = Δf ideal. The reference light source 980 is a light source that generates reference light having sub-peaks arranged at a frequency interval f _ref . However, one frequency f _{ref — 0} of the sub- _peaks is adjusted in advance so as to stand in the vicinity of f ₁ . As a configuration of the reference light source 980, for example, as schematically illustrated in FIG. 9, a configuration in which CW light is clock-modulated with a frequency f _ref and a sub-peak is output may be used, or a non-linear medium may be used. You may use the structure which raises a subpeak. Since the reference light source itself is not used for data transmission, the optical power per sub-peak component may be small as long as the frequency interval is locked to f _ref , and even if there is a variation in intensity for each sub-peak component. Good. In the present embodiment, f _ref = Δf _ideal .

The output light of the reference light source 980 is branched into four by the optical branching unit 910 and is made to interfere with the monitor lights 901 to 904 in the optical 90-degree hybrid circuits 921 to 924, respectively. The balance light receivers 931 to 938 receive the I component or Q component outputs from the optical 90-degree hybrid circuits 921 to 924, respectively, and output them as analog electric signals. The outputs of the balance light receivers 931 to 938 are output as beat signals 941 to 948 after high frequency components are cut by the low pass filters 971 to 978. The beat frequencies (low frequency components) of the beat signals 941 to 948 are expressed by the following (Expression 22) to (Expression 25).
f _beat — ₁ = f ₁ −f _ref — ₀ (Equation 22)
f _beat — ₂ = f ₂ − (f _ref — ₀ + f _ref ) = f ₂ − (f _ref — ₀ + Δf _ideal ) (Expression 23)
f _beat — ₃ = f ₃ − (f _ref — ₀ + 2f _ref ) = f ₃ − (f _ref — ₀ + 2Δf _ideal ) (Equation 24)
f _beat — ₄ = f ₄ − (f _ref — ₀ + 3f _ref ) = f ₄ − (f _ref — ₀ + 3Δf _ideal ) (Equation 25)

The time waveforms of the output beat signals 941 to 948 are cos (2πf _{beat — 1t} ), sin (2πf _{beat — 1t} ), cos (2πf _{beat —} 2t), sin (2πf _{beat —} 2t), cos (2πf _{beat —} 3t) and sin, respectively. (2πf _{beat —} 3t), cos (2πf _{beat — 4t} ), sin (2πf _{beat — 4t} ).

  Also in the multicarrier optical transmitter according to the present embodiment, the beat frequency is distributed on both the positive and negative sides centering on zero, similarly to the multicarrier optical transmitter according to the third embodiment described above. As 931 to 938, a narrow band (low speed) can be used as compared with the balance light receiver used in the first and second embodiments described above. In addition, since it is necessary to determine whether the beat frequency is positive or negative (frequency shift direction), the 90-degree optical hybrid circuit is used instead of the optical coupler, and the I and Q components (cos component and sin component) of the beat signal are monitored simultaneously. The configuration is such that it can be done. Further, if a balanced optical receiver 931 to 938 having a sufficiently narrow operating band is used, the low-pass filters 971 to 978 are unnecessary.

  FIG. 10 is a block diagram showing the configuration of the drive signal generating means according to the fourth embodiment of the present invention. The drive signal generation unit 1000 shown in FIG. 10 has the drive signal generation unit 700 according to the third embodiment shown in FIG. 7 except that the beat frequency detection circuits 1021 to 1028 are increased by one set (two). It is equivalent to the configuration of

ADC is used as the beat frequency detection circuits 1021 to 1028, and the time waveform of the beat signals 1001 to 1008 (corresponding to the beat signals 941 to 948 in FIG. 9) simply sent from the frequency interval monitoring means 900 is converted into a digital signal and output. . As described above, since the beat frequency is reduced to near zero by using modulation in the optical region, the ADC may be low speed. Frequency control circuit 1030 is a digital circuit, the compensation signal exp (j2πδf _n t) from the time waveform of the input beat signal (n = 1, 2, 3, 4) generates and outputs. What is necessary is just to produce | generate a compensation signal by the calculation shown by the following (Formula 26).
exp (j2πδf _n t)
= Exp (-j2πf _{beat_nt} )
= Cos (2πf _{beat_nt} ) −j * sin (2πf _{beat_nt} ) (Equation 26)

As another method, a method may be considered in which each input beat signal is subjected to Fourier transform to obtain a peak frequency, and δf _n is determined after appropriate operation. However, when Fourier transform is used, there is a drawback that the amount of calculation increases.

FIG. 11 is a graph showing the spectrums of the output CW light of the transmission CW light source, the output spectrum of the reference light source 980, and the output optical signal of the optical vector modulation means in the multicarrier optical transmitter according to the fourth embodiment of the present invention. It is a figure which illustrates a relationship typically. The dotted line in FIG. 11 represents the frequency of the transmission CW light source, and the alternate long and short dash line represents the subcarrier frequency. In the present embodiment, the sub-peaks appearing at intervals of f _ref (= Δf _ideal ) in the output light spectrum of the reference light source 980 are used as a reference, and the deviation from there is detected as the beat frequency. Similar to the second embodiment, this frequency shift is compensated by phase rotation in the digital domain.

  The multicarrier optical transmitter according to the present embodiment uses a low-speed (narrowband) light receiver and ADC by using clock modulation in the optical domain, similarly to the multicarrier optical transmitter according to the third embodiment described above. Can be used, and the technical difficulty can be reduced. Specifically, when the frequency accuracy (including the change over time) of the transmission CW light source is about ± 2.5 GHz with respect to a desired value, the operating band of the light receiver and the ADC is about ± 2.5 GHz (± 5 GHz with a margin in mind) Degree).

Note that, similarly to the multicarrier optical transmitter according to the third embodiment described above, in the multicarrier optical transmitter according to the present embodiment, f _ref = Δf _ideal + f _gap is obtained by shifting f _ref and Δf _ideal by a known amount. The frequency control circuit 1030 may be configured to appropriately correct. The multi-carrier optical transmitter according to the third embodiment is also handled when the signal light side input port and the local oscillation side input port of the optical 90-degree hybrid circuit are reversed and the sign of f _beat is reversed. It is the same as the case of.

(Fifth embodiment)
FIG. 12 is a block diagram showing a configuration of frequency interval monitoring means according to the fifth embodiment of the present invention. The overall configuration of the multicarrier optical transmitter of the fifth embodiment is that the frequency interval monitoring means 150 in the first embodiment is replaced with a frequency interval monitoring means 1200, and the drive signal generating means 160 in the first embodiment. The configuration is the same as that of the multicarrier optical transmitter according to the first embodiment shown in FIG. 1 except that is replaced with the drive signal generating means according to the fifth embodiment. The configuration of the drive signal generation means according to the fifth embodiment is the same as the configuration of the drive signal generation means according to the third embodiment shown in FIG. The content of the calculation is different.

  As shown in FIG. 12, the frequency interval monitoring unit 1200 includes optical branching units 1212 and 1213, optical 90-degree hybrid circuits 1221 to 1223, balance light receivers 1231 to 1236, a clock source 1250, and an optical modulator 1262. 1264 and low-pass filters 1271-1276.

In the present embodiment, the desired subcarrier spacing is a regular intervals, i.e., a _{Δf ideal_21 = Δf ideal_32 = Δf ideal_43} = Δf ideal. Further, the frequency f _ref of the clock source 1250 is set to a desired subcarrier interval, that is, f _ref = Δf _ideal .

As the optical modulators 1262 and 1264, for example, linear LN phase modulators can be used. The optical modulators 1262 and 1264 are driven by a clock signal (sin wave) output from the clock source 1250, and clock-modulate the monitor light 1202 (optical frequency f ₂ ) and 1204 (optical frequency f ₄ ), respectively. That is, only the monitor light of the even subchannel is clock-modulated. The clock signal is amplified by an amplifier (driver amplifier) as necessary, and then input to the optical modulators 1262 and 1264. In the output signal spectrum of the optical modulators 1262 and 1264, sub-peaks occur at intervals of f _ref on both sides of the frequencies f ₂ and f ₄ .

The output light from the optical modulator 1262 and the monitor light 1203 are branched into two by optical splitters 1212 and 1213, respectively. In the optical 90-degree hybrid circuits 1221, 1222, and 1223, the monitor light 1201 and the output light of the optical modulator 1262, the output light of the optical modulator 1262 and the monitor light 1203, and the monitor light 1203 and the output light of the optical modulator 1264, respectively. Interfered. The balance light receivers 1231 to 1236 receive the I component or Q component outputs of the optical 90-degree hybrid circuits 1221 to 1223, respectively, and output them as analog electric signals. The outputs of the balance light receivers 1231 to 1236 are output as beat signals 1241 to 1246 after high frequency components are cut by the low pass filters 1271 to 1276. The beat frequency (low frequency component) of each of the beat signals 1241 to 1246 is expressed by the following (Expression 27) to (Expression 29).
f _beat — ₁₂ = f ₁ − (f ₂ −f _ref ) = f ₁ − (f ₂ −Δf _ideal )
(Formula 27)
f _beat — ₂₃ = (f ₂ + f _ref ) −f ₃ = (f ₂ + Δf _ideal ) −f ₃
(Formula 28)
f _beat — ₃₄ = f ₃ − (f ₄ −f _ref ) = f ₃ − (f ₄ −Δf _ideal )
(Formula 29)

That is, in all adjacent subchannel pairs, beat signals are obtained by interference between the odd-numbered subchannel monitor light and the primary subpeak of the clock-modulated even-numbered subchannel monitor light. The time waveforms of the output beat signals 1241 to 1246 are cos (2πf _{beat — 12} t), sin (2πf _{beat — 12} t), cos (2πf _{beat — 23} t), sin (2πf _{beat — 23} t), cos (2πf _{beat — 34} t), sin, respectively. (2πf _{beat — 34} t).

  Also in the present embodiment, since the beat frequency is distributed on both the positive and negative sides centering on zero as in the third embodiment, the balance light receivers 1231 to 1236 are the first and second embodiments described above. A narrow band (low speed) can be used as compared with the balance light receiver used in the embodiment. In addition, since it is necessary to determine whether the beat frequency is positive or negative (frequency shift direction), the 90-degree optical hybrid circuit is used instead of the optical coupler, and the I and Q components (cos component and sin component) of the beat signal are monitored simultaneously. The configuration is such that it can be done. Further, if a balanced optical receiver 1231 to 1236 having a sufficiently narrow operating band is used, the low-pass filters 1271 to 1276 are unnecessary.

As described above, the configuration of the drive signal generation unit according to the present embodiment is the same as the configuration of the drive signal generation unit according to the third embodiment shown in FIG. The drive signal generating means will be described. Also in the drive signal generation means according to the present embodiment, ADC is used as the beat frequency detection circuits 721 to 726 similarly to the drive signal generation means 700 according to the third embodiment, and beats sent simply from the frequency interval monitoring means are used. The time waveforms of the signals 701 to 706 (corresponding to the beat signals 1241 to 1246 in FIG. 12) are converted into digital signals and output. As described above, since the beat frequency is reduced to near zero by using modulation in the optical region, the ADC may be low speed. Frequency control circuit 1230 is a digital circuit, and generates a compensation signal from the time waveform of the input beat signal _{exp (j2πδf n t) (n} = 1,2,3,4) is output. The compensation signal is obtained by the calculation represented by the following (Equation 26).
exp (j2πδf ₁ t) = 1 (Equation 30)
exp (j2πδf ₂ t)
= Exp (j2πf beat — ₁₂ t) * exp (j2πδf ₁ t)
= {Cos (2πf _{beat — 12t} ) + j * sin (2πf _{beat — 12t} )} * exp (j2πδf ₁ t) (Expression 31)
exp (j2πδf ₃ t)
= Exp (j2πf beat — ₂₃ t) * exp (j2πδf ₂ t)
= {Cos (2πf _{beat — 23} t) + j * sin (2πf _{beat — 23} t)} * exp (j2πδf ₂ t) (Expression 32)
exp (j2πδf ₄ t)
= Exp (j2πf beat — ₃₄ t) * exp (j2πδf ₃ t)
= {Cos (2πf _{beat —34} t) + j * sin (2πf _{beat —34} t)} * exp (j2πδf ₃ t) (Expression 33)

From (Expression 27) to (Expression 33), it is understood that the following (Expression 34) to (Expression 37) are obtained.
δf ₁ = 0 (Formula 34)
δf ₂ = f ₁ + Δf _ideal −f ₂ (Formula 35)
δf ₃ = f ₁ + 2Δf _ideal −f ₃ (Formula 36)
δf ₄ = f ₁ + 3Δf _ideal −f ₄ (Expression 37)

That is, the compensation signal _{exp (j2πδf n t) (n} = 1,2,3,4), can be compensated so as to align with Delta] f _ideal intervals subcarrier frequency relative to the _{f 1.} Since δf ₁ is always zero, the multiplication circuit 741 may be omitted. In addition to the above, the frequency control means 730 may calculate the peak frequency by Fourier transforming each of the input beat signals, and determine δf _n after appropriate operation. There is a drawback that the amount increases.

  The multi-carrier optical transmitter according to the present embodiment, like the multi-carrier optical transmitters according to the third and fourth embodiments described above, uses low-speed (narrow band) light reception by using clock modulation in the optical domain. The use of a vacuum vessel and an ADC, reducing the technical difficulty. Specifically, when the frequency accuracy (including the change over time) of the transmission CW light source is about ± 2.5 GHz with respect to a desired value, the operating band of the light receiver and the ADC is about ± 2.5 GHz (± 5 GHz with a margin in mind) Degree).

Note that, similarly to the multicarrier optical transmitter according to the third embodiment described above, in the multicarrier optical transmitter according to the present embodiment, f _ref = Δf _ideal + f _gap is obtained by shifting f _ref and Δf _ideal by a known amount. The frequency control circuit 730 may be configured to appropriately correct. In the multicarrier optical transmitter according to this embodiment, the handling when the signal light side input port and the local oscillation side input port of the optical 90-degree hybrid circuit are reversed and the sign of f _beat is reversed is also possible. This is the same as the case of the multicarrier optical transmitter according to the third embodiment described above.

  In the multicarrier optical transmitter according to the third embodiment described above, sub-peaks up to the second order of the output of the optical modulator 662 are used. However, in the multicarrier optical transmitter according to the present embodiment, only the first-order subpeak is used. Since it is not used, the voltage amplitude of the clock signal for raising the sub-peak can be reduced. Even when the number of subcarriers is further increased, beat detection using only the primary subpeak can be performed by applying clock modulation to the monitor light of all even-numbered subchannels.

(Sixth embodiment)
FIG. 13 is a block diagram showing the configuration of the frequency interval monitoring means according to the sixth embodiment of the present invention. The overall configuration of the multicarrier optical transmitter according to the sixth embodiment is that frequency interval monitoring means 150 in the first embodiment is replaced with frequency interval monitoring means 1300, and drive signal generating means in the first embodiment. Except for the point that 160 is replaced with drive signal generation means 1400, the configuration is the same as that of the multicarrier optical transmitter according to the first embodiment shown in FIG. As shown in FIG. 13, the frequency interval monitoring unit 1300 includes an optical splitter 1310, optical couplers 1321 to 1324, balance light receivers 1331 to 1334, low-pass filters 1371 to 1374, and a reference light source 1380.

Desired subcarrier spacing in the present embodiment has a regular intervals, i.e., a _{Δf ideal_21 = Δf ideal_32 = Δf ideal_43} = Δf ideal. The reference light source 1380 is a light source that generates reference light having sub-peaks arranged at a frequency interval f _ref . However, when a single frequency of sub-peaks was _{f ref_0, f ref_0} is _{f 1} near and to always _{f n} is the sub-peak closest to _f n (n = 1, 2, 3, 4) Adjust in advance so that it is on the negative side on the frequency axis. That is, adjustment is made in advance so that the following (formula 38) to (formula 41) always hold.
f _{ref — 0} <f ₁ (Formula 38)
(F ₁ + f ₂ ) / 2 <f _{ref — 0} + f _ref <f ₂ (Formula 39)
(F ₂ + f ₃ ) / 2 <f _{ref — 0} + 2f _ref <f ₃ (Equation 40)
(F ₃ + f ₄ ) / 2 <f _{ref — 0} + 3f _ref <f ₄ (Formula 41)

In adjusting f _{ref — 0,} a margin is provided as appropriate so that the above equation can be satisfied even when the fluctuation range due to the change in f _n with time is taken into consideration.

As a configuration of the reference light source 1380, for example, as schematically illustrated in FIG. 13, a configuration in which CW light is clock-modulated with a frequency f _ref and a sub-peak is output may be used, or a non-linear medium may be used. You may use the structure which raises a subpeak. Since the reference light source itself is not used for data transmission, the optical power per sub-peak component may be small as long as the frequency interval is locked to f _ref , and even if there is a variation in intensity for each sub-peak component. Good. In the present embodiment, f _ref = Δf _ideal .

The output light of the reference light source 1380 is branched into four by an optical branching device 1310 and is made to interfere with monitor lights 1301 to 1304 by optical couplers 1321 to 1324, respectively. The optical couplers 1321 to 1324 are general 2 × 2 3 dB couplers, and their outputs are output as beat signals 1341 to 1344 via balanced light receivers 1331 to 1334 and low-pass filters 1371 to 1374. From (Equation 38) to (Equation 41), the following (Equation 42) to (Equation 45) hold for the frequency difference f _beat between the monitor light and the reference light.
f _beat — ₁ = f ₁ −f _ref — 0> 0 (Formula 42)
_{f beat_2 = f 2 - (f} ref_0 + f ref)> 0 ( Equation 43)
_{f beat_3 = f 3 - (f} ref_0 + 2f ref)> 0 ( Equation 44)
_{f beat_4 = f 4 - (f} ref_0 + 3f ref)> 0 ( Equation 45)

That is, since f _beat is always positive and it is not necessary to discriminate between positive and negative, simple optical couplers 1321 to 1324 can be used instead of the optical 90-degree hybrid circuit. The frequency of the beat signal 1341-1344 is the _{f beat_1 ~f beat_4.}

  FIG. 14 is a block diagram showing the configuration of the drive signal generation means according to the sixth embodiment of the present invention. The configuration of the drive signal generation unit 1400 shown in FIG. 14 is the same as the configuration of the drive signal generation unit according to the first embodiment shown in FIG. 3 except that there is one beat frequency detection circuit 1421-1424. It is equivalent.

The beat frequency detection circuits 1421 to ₁₄₂₄ use frequency counters and output f _{beat — 1} to f _{beat — 4} as digital values, respectively. Frequency control circuit 1430 is a digital circuit, which generates a compensation signal from the value of the beat frequency _{f beat_1 ~f beat_4 exp (j2πδf n} t) (n = 1,2,3,4) output. Specifically, a compensation signal is generated by the calculation shown in the following (Equation 46).

exp (j2πδf _n t) = exp {j2π (f _ofst −f _beat — _n ) t} (Formula 46)
Here, the value of the offset frequency f _ofst is arbitrary, but from the viewpoint of making the maximum value of | δf _n | as small as possible, f _ofst is an intermediate value of the distribution of f _beat — _n (from the above (Expression 42) to (Expression 45), It is desirable that the value be close to a positive value.

FIG. 15 shows respective spectra of the output CW light of the transmission CW light source, the output light spectrum of the reference light source 1380, and the output optical signal of the optical vector modulation means in the multicarrier optical transmitter according to the sixth embodiment of the present invention. It is a figure explaining typically the relationship. The dotted line in FIG. 15 represents the frequency of the transmission CW light source, and the alternate long and short dash line represents the subcarrier frequency. In the present embodiment, the beat signal is detected with reference to the subpeaks appearing at intervals of f _ref (= Δf _ideal ) in the output light spectrum of the reference light source 1380, but the subcarrier frequency is finally added to the subpeak frequency itself of the reference light source. Instead of adjusting the frequency, the frequency compensation is performed so that the subcarrier frequency is adjusted to the subpeak frequency + the offset frequency f _ofst .

As in the third embodiment described above, in this embodiment, f _ref and Δf _ideal may be shifted by a known amount to obtain f _ref = Δf _ideal + f _gap , and the frequency control circuit 1430 may appropriately perform correction. .

Multi-carrier optical transmitter 100
Transmission CW light source 101-104
Monitor light extraction means 111-114
Optical vector modulation means 121-124
Optical multiplexing means 130
Output port 140
Frequency interval monitoring means 150, 200, 300, 500, 600, 900, 1200, 1300
Drive signal generating means 160, 700, 1000, 1400
Optical splitters 212, 213, 512, 513, 612, 910, 1212, 1213, 1310
Optical couplers 221-223, 521-523, 1321-1324
Balance light receivers 231 to 233, 531 to 533, 631 to 636, 931 to 938, 1231 to 1236, 1331 to 1334
Complex electric field signal generation circuit 310, 710, 1010
Beat frequency detection circuits 321-323, 721-726, 1021-1028, 1421-1024
Frequency control circuit 330, 730, 1030, 1430
Multiplier circuits 341 to 344, 741 to 744, 1041 to 1044, 1441 to 1444
Output interface circuits 351 to 354, 751 to 754, 1051 to 1054, 1451 to 1454
Clock source 550, 650, 1250
Analog multipliers 561 to 563
Low-pass filters 571-573, 671-676, 971-978, 1271-1276, 1371-1374
Optical 90 degree hybrid circuit 621-623, 921-924, 1221-1223
Optical modulator 662, 1262, 1264
Reference light source 980, 1380
Polarization multiplexing IQ modulator 1600

Claims (4)

N transmission CW light sources respectively corresponding to N (N is an integer of 2 or more) subchannels;
Monitor light extraction means for extracting and outputting N monitor lights from the N output lights output from the N transmission CW light sources;
A frequency interval monitoring means for detecting frequency intervals between the N output lights using the N monitor lights, and generating and outputting frequency interval information from each of the detected frequency intervals;
A complex electric field signal for each of the N subchannels is generated based on a transmission data signal for data modulation input from the outside, and the frequency of the complex electric field signal is set to a predetermined frequency interval of the frequency interval information. Drive signal generating means for generating and outputting N drive signals respectively corresponding to the N subchannels by shifting by a difference from the frequency interval;
Optical vector modulation means for inputting the N drive signals and the N output lights to generate and output N subchannel signals, wherein the optical vector modulation means includes the N drive signals. The N sub-channel signals are modulated so as to have the predetermined frequency interval between the sub-channel signals by modulating the N output lights based on a signal and shifting the frequency of the N output lights. Generating the light vector modulating means;
Optical multiplexing means for multiplexing the N subchannel signals output from the optical vector modulation means and transmitting a multicarrier signal, and the frequency interval monitoring means includes the N transmission CW light sources. Means for modulating the monitor light extracted from at least one of the signals with a clock signal and then obtaining a beat signal by causing the modulated monitor light and the monitor light to interfere with each other, and the frequency interval information includes A multi-carrier optical transmitter comprising a beat signal, wherein the drive signal generation means calculates the difference between each frequency interval and a predetermined frequency interval based on a beat frequency of the beat signal . N transmission CW light sources respectively corresponding to N (N is an integer of 2 or more) subchannels;
Monitor light extraction means for extracting and outputting N monitor lights from the N output lights output from the N transmission CW light sources;
A frequency interval monitoring means for detecting frequency intervals between the N output lights using the N monitor lights, and generating and outputting frequency interval information from each of the detected frequency intervals;
A complex electric field signal for each of the N subchannels is generated based on a transmission data signal for data modulation input from the outside, and the frequency of the complex electric field signal is set to a predetermined frequency interval of the frequency interval information. Drive signal generating means for generating and outputting N drive signals respectively corresponding to the N subchannels by shifting by a difference from the frequency interval;
Optical vector modulation means for inputting the N drive signals and the N output lights to generate and output N subchannel signals, wherein the optical vector modulation means includes the N drive signals. The N sub-channel signals are modulated so as to have the predetermined frequency interval between the sub-channel signals by modulating the N output lights based on a signal and shifting the frequency of the N output lights. Generating the light vector modulating means;
Optical multiplexing means for multiplexing the N subchannel signals output from the optical vector modulation means and transmitting a multicarrier signal;
The frequency interval monitoring means includes a reference light source, and has means for obtaining a beat signal by causing the monitor light and output light of the reference light source to interfere with each other, and the frequency interval information includes the beat signal, The drive signal generation means calculates the difference between each frequency interval and a predetermined frequency interval based on the beat frequency of the beat signal, and the output light spectrum of the reference light source has N or more equal frequency intervals. A multi-carrier optical transmitter characterized by having sub-peaks arranged side by side . Outputting N output lights from N transmission CW light sources respectively corresponding to N (N is an integer of 2 or more) subchannels;
Extracting and outputting N monitor lights from the N output lights; and
Detecting each frequency interval between the N output lights using the N monitor lights, and generating and outputting frequency interval information from each of the detected frequency intervals;
A complex electric field signal for each of the N subchannels is generated based on a transmission data signal for data modulation input from the outside, and the frequency of the complex electric field signal is set to a predetermined frequency interval of the frequency interval information. Generating and outputting N drive signals respectively corresponding to the N subchannels by shifting by a difference from a frequency interval;
Generating and outputting N subchannel signals from the N drive signals and the N output lights, and modulating the N output lights based on the N drive signals, Generating the N subchannel signals to have the predetermined frequency interval between the subchannel signals by shifting the frequency of the N output lights; and
Combining the N subchannel signals and transmitting a multicarrier signal, and generating and outputting the frequency interval information from at least one of the N transmission CW light sources. After the extracted monitor light is modulated with a clock signal, the modulated monitor light and the monitor light interfere with each other to obtain a beat signal, and the frequency interval information includes the beat signal, The step of generating and outputting the drive signal calculates the difference between each frequency interval and a predetermined frequency interval based on the beat frequency of the beat signal . Outputting N output lights from N transmission CW light sources respectively corresponding to N (N is an integer of 2 or more) subchannels;
Extracting and outputting N monitor lights from the N output lights; and
Detecting each frequency interval between the N output lights using the N monitor lights, and generating and outputting frequency interval information from each of the detected frequency intervals;
A complex electric field signal for each of the N subchannels is generated based on a transmission data signal for data modulation input from the outside, and the frequency of the complex electric field signal is set to a predetermined frequency interval of the frequency interval information. Generating and outputting N drive signals respectively corresponding to the N subchannels by shifting by a difference from a frequency interval;
Generating and outputting N subchannel signals from the N drive signals and the N output lights, and modulating the N output lights based on the N drive signals, Generating the N subchannel signals to have the predetermined frequency interval between the subchannel signals by shifting the frequency of the N output lights; and
Combining the N subchannel signals and transmitting a multicarrier signal;
Comprising a step of generating and outputting the frequency interval information, by using a reference light source, comprising the step of obtaining a beat signal by interfering the output light of the reference light source and the monitor light, the frequency interval information Includes the beat signal, and the step of generating and outputting the drive signal calculates the difference between each frequency interval and a predetermined frequency interval based on the beat frequency of the beat signal, and outputs the reference light source The multicarrier optical transmission method, wherein the optical spectrum has sub-peaks arranged at equal frequency intervals of N or more .

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