JP2012039290A - Coherent optical time division multiplex transmission system - Google Patents

Abstract

A high-precision optical phase synchronization technique is used to perform phase synchronization between an optical modulation pulse signal and a local light emission pulse signal, thereby realizing a long-distance optical fiber transmission of an ultrafast optical multilevel modulation pulse signal.
An optical time-division multiplex multilevel modulation pulse signal obtained by optical multi-level modulation and optical time-division multiplexing of an optical pulse signal and a reference optical signal phase-synchronized with the optical pulse signal are generated and combined by a transmission device Then, these signals are propagated in the optical fiber transmission line 4, and the phase of the optical time division multiplexing multilevel modulation pulse signal and the local light pulse signal is synchronized by the receiving device 11 using the reference optical signal, The optical time division multiplexing multilevel modulation pulse signal and the local light emission pulse signal are time-division demultiplexed and coherent homodyne detected by the coherent detection circuit 7.
[Selection] Figure 3

Description

  The present invention combines a coherent optical multilevel transmission technology that simultaneously puts information on the amplitude and phase of light and an optical time division multiplex transmission technology, and realizes a high transmission rate at a low symbol rate. It is about.

  Along with a significant increase in Internet traffic, in recent optical communication fields, it is desired to increase the transmission capacity per wavelength. Recently, multi-level coherent optical transmission that realizes high frequency utilization efficiency has attracted attention as a new transmission method that satisfies such demands. In this transmission method, since multi-bit information can be transmitted with one symbol, high-speed transmission can be realized at a low symbol rate. As a result, high resistance to chromatic dispersion and polarization mode dispersion in the optical fiber can be obtained.

  Figure 1 shows recent research trends in multilevel coherent optical transmission. There are two main trends in this research. One is coherent optical transmission using CW (Continuous-Wave) light as a transmission and a local oscillation signal. Up to now, 200 km transmission of polarization multiplexed 10 Gsymbol / s, 32-QAM (Quadrature Amplitude Modulation) signal (total transmission capacity: 100 Gbit / s) has been reported as an optical transmission using this method (non-patent literature). 1). In this transmission system, optical phase-locked loop (OPLL) between the CW-transmitted optical signal in the transmitter and the CW-local light signal in the receiver is not performed. For this reason, the optical phase between the two optical signals becomes asynchronous, and the waveform distortion caused thereby is compensated by digital signal processing. On the other hand, the present inventors have applied for a high-accuracy optical phase synchronization system (Patent Document 1) of a transmission light source and a local light source in order to greatly improve the multilevel of CW-multilevel coherent optical transmission. The present patent application relates to an optical phase synchronization system in which CW light is arranged in both the transmitter and the local oscillator. We use this multi-level coherent optical transmission system using optical phase synchronization technology to transmit 160 km of 3 channels of polarization multiplexed 1 Gsymbol / s, 128 QAM signals (total transmission capacity: 42 Gbit / s). (Non-patent document 2). In this transmission, 42 Gbit / s data signals have been successfully transmitted in the optical frequency band of 4.2 GHz. At this time, the frequency utilization efficiency has reached 10 bits / s / Hz. As described above, this experimental result shows that a multilevel modulated optical signal having a large multilevel can be transmitted by a multilevel coherent optical transmission system using optical phase synchronization.

On the other hand, as another multilevel coherent optical transmission system, a coherent optical time division multiplexing transmission system using optical pulses for transmission and local oscillation signals has been newly proposed. Figure 2 shows an overview of this transmission method. This method does not use coherent CW light, but multi-level amplitude / phase modulation of coherent light pulses. As is well known, the optical time division multiplexing method is a method of expanding transmission capacity by multiplexing ultrashort optical pulses in the time domain. For example, 16 times optical time division multiplexing is performed at a symbol rate of 10 Gsymbol / s, which is often used today, and the speed is increased to 160 Gsymbol / s, and then 64 (= 2 ⁶ ) value QAM is combined with polarization multiplexing technology. If the format is used, 6 bits of information can be transmitted in one symbol, so a transmission speed of 160 Gsymbol / s × 2 × 6 bit / Symbol = 1.92 Tbit / s can be realized. In this way, compared to the conventional ultrahigh-speed optical transmission system, it is a feature of this system that a transmission rate exceeding 1 Tbit / s can be realized at a symbol rate as low as 160 Gsymbol / s. In such a transmission system as well, similarly to CW-multilevel coherent optical transmission, the carrier frequency of the transmitted data optical pulse train (frequency of the optical electric field) and the carrier frequency of the local light pulse signal generated by the receiving unit The optical phase synchronization technology is very important.

  As coherent time division multiplexing optical transmission, 1,073 km transmission of 640 Gbit / s, QPSK (Quadrature Phase Shift Keying) signal has been reported (Non-patent Document 3). In this transmission system, optical phase synchronization is not performed between the optical pulse signals of the optical modulation pulse signal in the transmission unit and the local light emission pulse signal in the reception unit. For this reason, the phase between the two optical pulse signals becomes asynchronous, and the waveform distortion caused thereby is compensated by digital signal processing. Similarly, a report example of modulation / demodulation experiment of 5.1 Tbit / s, RZ-16 QAM signal using a method that compensates for waveform distortion caused by asynchronous phase fluctuation between optical modulation pulse signal and local light pulse signal by digital signal processing Yes (Non-Patent Document 4). In this system, the output optical pulse signal from the mode-locked laser is modulated into a data optical pulse train, and a part of the optical pulse signal generated by the transmitter is used as a local light pulse signal to perform coherent detection. However, the phase synchronization between the two optical pulse signals is not performed. In addition, this report only describes the experimental results when not transmitting, and optical fiber transmission has not been realized.

  As described above, in multi-level coherent optical time division multiplexing transmission using conventional optical pulse signals, optical phase synchronization between both the optical time division multiplexed multi-level modulation signal and the local pulse signal is not performed, There are no examples of successful multi-level coherent optical time division multiplex transmission.

JP 2008-244369 A

Y. Mori, C. Zhang, M. Usui, K. Igarashi, K. Katoh, and K. Kikuchi, “200-km transmission of 100-Gbit / s 32-QAM dual-polarization signals using a digital coherent receiver,” presented at the European Conference on Optical Communication (ECOC), Vienna, Austria, Paper 8. 4. 6, (2009.9). M. Nakazawa, “Challenges for FDM-QAM coherent transmission with ultrahigh spectral efficiency,” presented at the European Conference on Optical Communication (ECOC), Brussels, Belgium, Paper Tu. 1. E. 1, (2008.9). C. Zhang, Y. Mori, M. Usui, K. Igarashi, K. Katoh, and K. Kikuchi, “Straight-line 1,073-km transmission of 640-Gbit / s dual-polarization QPSK signals on a single carrier,” presented at the European Conference on Optical Communication (ECOC), Vienna, Austria, Postdeadline Paper PD2. 8, (2009.9). C. Schmidt-Langhorst, R. Ludwig, D.-D. Grob, L. Molle, M. Seimetz, R. Freund, and C. Schubert, "Generation and coherent time-division demultiplexing of up to 5.1 Tb / s single -channel 8-PSK and 16-QAM signals, "presented at the Optical Fiber Communication Conference (OFC) 2009, San Diego, CA, Postdeadline Paper PDPC6, (2009.3).

とすると、本信号の位相項は、

と表される。ここでｉはサンプリング番号、Ｔはシンボルレート、θ_Ｓ（ｉＴ）は位相変調項、θ_ｎ（ｉＴ）は光変調パルス信号と局発光パルス信号間の位相誤差を表す項である。本搬送波位相推定法ではθ_ｎ（ｉＴ）を算出してこれを式（２）より差し引くことにより、θ_Ｓ（ｉＴ）の推定を行う。θ_ｎ（ｉＴ）の算出においてはまず式（１）をM(2のベキ乗)乗して位相変調項を除去する。M乗演算により位相角はM倍されるが、このとき２π／Ｍの整数倍である位相変調成分は除去される。すなわちＡ（ｉＴ）^Ｍ∝ｅｘｐ（ｊＭθ_ｎ（ｉＴ））である。したがって位相誤差成分θ_ｎ（ｉＴ）は、このM乗して算出した位相角をMで割ることによって求まる。位相誤差成分を式（２）から差し引くことでθ_Ｓ（ｉＴ）を推定し、データ信号の波形歪を補償する。このような平均位相誤差信号を用いた歪補償はリアルタイム制御ではなく、かつ多値度が少ないうちは復調が可能であるが、多値度が16(=2⁴)以上となる場合には復調アルゴリズムが複雑になり復調が困難となる。 In the optical transmission system described in Non-Patent Document 3, the phase error between the optical modulation pulse signal and the local light emission pulse signal in the M-phase PSK format is obtained using a carrier phase estimation method by digital signal processing, and this is canceled. Compensation calculation is performed on the data signal. The compensation calculation method will be described below. The complex amplitude of the data signal A / D converted in the demodulator

Then, the phase term of this signal is

It is expressed. Here, i is a sampling number, T is a symbol rate, θ _S (iT) is a phase modulation term, and θ _n (iT) is a term representing a phase error between the optical modulation pulse signal and the local light emission pulse signal. In this carrier phase estimation method, θ _n (iT) is calculated and subtracted from the equation (2) to estimate θ _S (iT). In calculating θ _n (iT), first, the expression (1) is raised to the power of M (2 to the power of 2) to remove the phase modulation term. Although the phase angle is multiplied by M by the M-th power calculation, the phase modulation component that is an integer multiple of 2π / M is removed at this time. That is, A (iT) ^M ∝exp (jMθ _n (iT)). Therefore, the phase error component θ _n (iT) is obtained by dividing the phase angle calculated by raising the power to M to M. By subtracting the phase error component from the equation (2), θ _S (iT) is estimated, and the waveform distortion of the data signal is compensated. Distortion compensation using such average phase error signal is not real-time control and can be demodulated while the multilevel is small, but if the multilevel is 16 (= 2 ⁴ ) or higher, it is demodulated. The algorithm becomes complicated and demodulation becomes difficult.

  Similarly, in Non-Patent Document 4, since the compensation of the waveform distortion of the data signal caused by the phase error between the optical modulation pulse signal and the local light emission pulse signal on the digital signal processing device in the demodulation unit is performed on the software, The compensation accuracy is not sufficient. As a result, it has succeeded in generating an optical time division multiplexed pulse signal that has been optically multi-level modulated in the 16 QAM format, but has not succeeded in optical fiber transmission.

  Therefore, the present invention has been newly devised so that the high-precision optical phase synchronization technology disclosed in Patent Document 1 can be applied to a coherent optical time division multiplex pulse transmission system, and using this, an optical modulation pulse signal and local light emission are used. The purpose is to realize phase-synchronization between pulse signals and realize long-distance optical fiber transmission of ultra-high speed and high multilevel modulation coherent optical pulse signals.

  According to the solution of the present invention, the transmission unit includes a port for optical multilevel modulation and optical time division multiplexing of the optical pulse signal, and a port for generating a reference optical signal phase-synchronized with the optical pulse signal. After combining the optical time division multiplexed multilevel modulation pulse signal output from the reference optical signal and the reference optical signal, these signals are simultaneously propagated through the optical fiber transmission line, and the optical time transmission is performed using the reference optical signal in the receiving unit. The phases of the division multiplexed multilevel modulation pulse signal and the local light pulse signal are synchronized, and the optical time division multiplexed multilevel modulation pulse signal and the local light pulse signal are time-division multiplexed and coherent homodyne detection (signals) by a coherent detection circuit. There is provided a coherent optical time division multiplex transmission system characterized in that the carrier frequency of light and the frequency of local light are the same).

  By using the present invention, high-accuracy phase synchronization between the optical multilevel modulation pulse signal and the local light emission pulse signal is realized, and the transmission rate per channel is increased by improving the multilevel in coherent optical time division multiplexing transmission. , And a long distance can be realized. Further, in the present invention, since an ultra-high speed optical transmission system can be constructed at a low symbol rate, an inexpensive optical device that operates at low speed can be used. Therefore, the cost of the entire optical fiber communication system can be reduced.

It is a figure which shows the research trend of the multi-value coherent optical transmission in recent years. It is a block diagram which shows the structure of the conventional coherent light time division multiplex transmission system. It is a block diagram which shows the whole structure of the coherent optical time division multiplexing transmission system of this invention. FIG. 2 is a block diagram showing a detailed configuration of a phase synchronization and local pulse signal generator used in the first embodiment of the present invention. FIG. 5 is a block diagram showing a detailed configuration of a phase synchronization and local pulse signal generator used in a second embodiment of the present invention. (a) The optical comb signal generated in the transmitter and (b) the frequency relationship of the local light comb signal generated in the phase synchronization and local light pulse signal generator and (9) the heterodyne beat spectrum of both. It is. (a) Optical spectrums of a local light comb signal and a reference optical signal in an optical phase synchronization experiment performed in the second embodiment. (b) It is the figure which expanded and showed the optical spectrum of (a) partially. 6 is a heterodyne beat signal spectrum of a reference optical signal and a sixth harmonic of an optical comb signal in an optical phase synchronization experiment performed in the second embodiment. 9 is an SSB phase noise spectrum of the beat signal spectrum of FIG. This is a constellation map of 10 Gsymbol / s 32 QAM data signal. 10 Gsymbol / s 32 Bit error rate characteristics with respect to the reception power of QAM data signal.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. FIG. 3 is a block diagram showing the configuration of the coherent optical time division multiplexing transmission system according to the first embodiment of the present invention. The configuration of the transmission system of this embodiment includes a transmission device 10, an optical fiber transmission line 4, and a reception device 11.

The transmitter 10 includes an optical pulse signal and reference optical signal generator, an optical IQ modulator 2, an encoder 6, an optical time division multiplexing circuit 3, a polarization modulator composed of a CW light source 12 a, an optical comb modulator 13 a, and optical filters 14 a and 14 b. A wave multiplexing circuit 15, an optical coupler 16a, and a clock signal source 5 are provided.
The optical comb modulator 13a is driven by the signal R [Hz] supplied from the clock signal source 5, modulates the CW optical signal output from the CW light source 12a, and generates an optical comb signal composed of a plurality of sidebands. . The optical comb signal is pulse-curved by an optical filter 14a to generate a Fourier-limited optical pulse having a repetition frequency R [Hz], where the Fourier-limited pulse is called a Transform Limited (TL) pulse, For example, in the case of a Gaussian pulse, the product of the time pulse width Δτ and the spectrum width Δν satisfies ΔτΔν≈0.44.

  The optical filter 14b extracts one sideband of K-order (K is a positive integer) harmonic from the optical comb signal, and this signal is used as a reference optical signal. However, the sideband to be extracted needs to be a signal having a frequency that does not overlap with the spectrum of the TL optical pulse signal.

The TL optical pulse signal is input to the optical IQ modulator 2, where it is subjected to data modulation by a 2 ^M (M is a positive integer) multilevel modulation signal generated by an encoder 6 phase-synchronized with a clock signal source 5, and MR [bit / s] optical multi-level modulation pulse signal train is output. For example, since 16 QAM signal is 16 = 2 ⁴ , M = 4. The optical time division multiplexing circuit 3 time-division-multiplexes the optical multi-level modulated pulse signal sequence with N-value multiplicity and multiplies the transmission speed by N times, and optical time-division multiplexing with a transmission speed of NMR [bit / s] A multi-level modulation pulse signal train is output. The polarization multiplexing circuit 15 multiplexes the data optical pulse signal sequence on two orthogonal polarization axes, and outputs a 2 NMR [bit / s] polarization multiplexed light time division multiplexed multilevel modulation pulse signal sequence.

  The optical coupler 16a combines the 2 NMR [bit / s] polarization multiplexed optical time division multiplexed multilevel modulation pulse signal sequence generated in the above process with the reference optical signal, and these optical signals are transmitted to the optical fiber transmission line 4. To join.

In the transmitter 10, a narrow line width laser is suitable as the CW light source 12a, and for example, a fiber laser, a narrow line width semiconductor laser, or a solid state laser may be used. As the optical comb modulator 13a, for example, an optical modulator using an electro-optic effect such as LiNbO ₃ or an EA (Electro-Absorption) modulator is used. The encoder 6 is composed of, for example, a field programmable gate array (FPGA) circuit and a D / A converter. Further, an arbitrary waveform generation device that generates a fixed pattern may be used as the encoder 6.

  The optical fiber transmission line 4 is a transmission line composed of various optical fibers having arbitrary dispersion and polarization mode dispersion.

  The receiving device 11 includes optical couplers 16b and 16c, an optical filter 14c, a polarization demultiplexing circuit 17, a clock signal regeneration circuit 18, a phase synchronization and local light pulse signal generator 19, a coherent detection circuit 7, and a decoder 8.

  The optical filter 14c extracts only the reference optical signal from the transmitted polarization multiplexed light time division multiplexed multilevel modulation pulse signal sequence and the reference optical signal. The polarization multiplexing / demultiplexing circuit 17 generates an optical time division multiplexed multilevel modulation pulse signal sequence of one polarization axis from a polarization multiplexed optical time division multiplexed multilevel modulation pulse signal sequence multiplexed on two orthogonal polarization axes. Is demultiplexed. The polarization demultiplexing circuit 17 uses, for example, a polarization controller and a polarizer. The clock signal regeneration circuit 18 regenerates and outputs a clock signal (sine wave) of R [Hz] included in the transmitted optical time division multiplexed multilevel modulation pulse signal sequence. The phase synchronization and local light pulse signal generator 19 outputs a local light pulse signal having a repetition frequency R [Hz] that is phase-locked to the reference optical signal extracted by the optical filter 14c. The optical time division multiplexed multilevel modulation pulse signal sequence after polarization demultiplexing is incident on the coherent detection circuit 7 together with a local light pulse signal having a repetition frequency R [Hz] phase-synchronized with the optical pulse signal sequence, and is time-divisionally divided. Demultiplexing and coherent homodyne detection are performed simultaneously. The coherent detection circuit 7 includes, for example, a 90 ° optical hybrid circuit and an optical differential detector. The optical multilevel modulation pulse signal sequence output from the coherent detection circuit 7 is then demodulated by the decoder 8. The decoder 8 is composed of, for example, an A / D converter and an FPGA circuit. Further, as the decoder 8, an off-line demodulating circuit combining a digital oscilloscope (A / D converter) and digital signal processing by software may be used. In the figure, a solid line and a broken line represent optical and electrical signal paths, respectively.

  FIG. 4 shows a detailed configuration of the phase synchronization and local pulse signal generator 19. The phase synchronization and local pulse signal generator 19 includes a CW light source 12b, an optical comb modulator 13b, optical filters 14d and 14e, optical couplers 16d and 16e, a photodetector 20, a phase comparator 21, and a negative feedback circuit 22.

The CW light source 12b is preferably a frequency variable laser having a narrow line width and a high-speed frequency response characteristic. For example, a fiber laser having a LiNbO ₃ optical phase modulator as a frequency adjustment mechanism is used. This laser is disclosed by the inventors (Patent Document 1).

The output signal from the CW light source is optically comb modulated using the optical comb modulator 13b, and a plurality of sidebands are generated. The optical comb modulator 13b is driven at the frequency R [Hz] of the clock signal extracted by the clock signal regeneration circuit 18 in FIG. Here, as the optical comb modulator 13b, for example, an optical modulator using an electro-optic effect such as LiNbO ₃ or an EA modulator is used. The optical comb signal is pulse carved by the optical filter 14d to generate a TL local pulse having a repetition frequency R [Hz].

The optical filter 14e extracts one sideband of the K + 1 order harmonic from the optical comb signal. The sideband signal and the reference optical signal extracted by the optical filter 14c in FIG. 3 are combined by the optical coupler 16e, and the combined light is incident on the photodetector 20. The beat signal of both lasers is output from the photodetector 20. The phase comparator 21 outputs, as an error signal, the phase difference between the phase of the beat signal and the frequency R [Hz] clock signal extracted by the clock signal recovery circuit 18 in FIG. For example, a double balanced mixer (DBM) is used as the phase comparator 21. This error voltage signal is negatively fed back to the frequency adjusting mechanism of the CW light source 12b. With the above operation, the phase of the beat signal output from the photodetector 20 is synchronized with the frequency R [Hz] clock signal. As a result, the carrier frequency f _L of the CW light source 12b in the phase synchronization and local light pulse signal generator 19 is completely phase-synchronized with the carrier frequency f _T of the CW light source 12a in the transmitter 10 and output from the optical filter 14d. The optical pulse signal is a signal that is phase-synchronized with the optical time-division multiplexed multilevel modulation pulse signal sequence transmitted through the optical fiber transmission line 4. In the figure, a solid line and a broken line represent optical and electrical signal paths, respectively.

  Next, a second embodiment of coherent optical time division multiplexing transmission will be described in detail. The configuration of the coherent optical time division multiplexing transmission system in the second embodiment is the same as the configuration in the first embodiment (FIG. 3) except for the phase synchronization and local pulse signal generator 19. Here, the configuration and operation of the phase synchronization and local pulse signal generator 19 will be described in detail.

  In the optical phase synchronization operation in the first embodiment, one harmonic of the transmitted reference optical signal and the local light comb signal generated in the phase synchronization and local pulse signal generator 19 is converted into an optical filter 14c. And 14e need to be extracted with high accuracy. This is because the beat signal generated by the remaining adjacent harmonic components that cannot be removed by the optical filter is mixed as noise in the optical phase synchronization control band, and the phase of the desired beat signal (IF signal) is set as the reference signal. This is because it is difficult to synchronize the phase. However, by using the second embodiment, a highly accurate optical phase synchronization operation can be performed even when the extraction of the optical signal as described above is insufficient.

FIG. 5 shows a detailed configuration of the phase synchronization and local pulse signal generator 19 used in the second embodiment. The phase synchronization and local pulse signal generator 19 includes a CW light source 12b, an optical phase modulator 23, an optical comb modulator 13b, optical filters 14d and 14f, optical couplers 16f and 16g, a photodetector 20, a phase comparator 21, and a negative. A feedback circuit 22 and microwave signal generators 24a and 24b are provided. In the present embodiment, the optical phase modulator 23 is operated by a signal having an arbitrary frequency S [Hz] output from the microwave signal generator 24a instead of the clock signal extracted from the transmission signal, and an arbitrary repetition frequency is set. A local light comb signal is generated. Here, the CW light source 12b is preferably a frequency variable laser having a narrow line width and a high frequency response characteristic. For example, a fiber laser including a LiNbO ₃ optical phase modulator as a frequency adjustment mechanism is used. This laser is disclosed by the inventors (Patent Document 1). As the optical phase modulator 23, for example, an optical modulator using an electro-optic effect such as LiNbO ₃ is used.

6A and 6B show the relationship between the frequency of the optical comb signal output from the optical comb modulator 13a in FIG. 3 and the frequency of the local light comb signal output from the optical phase modulator 23 in FIG. Show. In addition, the broken line and the solid line in the figure indicate the optical comb signals before and after transmission through the optical filter. Here, since the bandwidth of the optical filter is wide, three reference optical signals ((1), (2), (3) in FIG. 6 (a)) of K order and (K ± 1) order are extracted. Furthermore, the case where three harmonic signals (4), (5), (6) in FIG. 6 (b) are extracted from the local light comb signal. Think. The three harmonic signals of the local light comb signal extracted by the optical filter 14f and the three reference optical signals extracted by the optical filters 14b and 14c in FIG. 3 are combined by the optical coupler 16g. The combined optical signal is incident on the photodetector 20. The photodetector 20 outputs a plurality of beat signals between the two signals. FIG. 6C shows a beat signal output from the photodetector 20. However, in this figure, the desired beat signal used for optical phase synchronization ((1) in FIG. 6 (c)) and two residual beat signals adjacent to it ((2), (3 in FIG. 6 (c)) )) Only. Here, in the present embodiment, the desired beat signal used for optical phase synchronization is only the signal (1) of the frequency f _T -f _L + KR-JS [Hz], and the other remaining beat signals are It becomes a noise signal. However, by appropriately setting the frequency S [Hz] for driving the optical phase modulator 23, the residual component can be placed at a frequency far enough from the signal (1). In particular, it is not possible to extract only one harmonic of the reference optical signal and the local light comb signal by setting S [Hz] so that the residual component is placed at a position sufficiently away from the control band for optical phase synchronization. Even in this case, optical phase synchronization can be realized. The phase comparator 21 performs phase comparison between the beat signal output from the photodetector 20 and the reference signal output from the microwave signal generator 24b, and outputs the phase difference as an error voltage signal. The error signal is negatively fed back to the frequency adjustment mechanism of the CW light source 12b, so that the beat signal output from the photodetector 20 is phase-synchronized with the reference microwave signal. Here, the beat signal of (1) is phase-synchronized with the reference signal by setting the frequency U of the reference signal output from the microwave signal generator 24b to U = | KR-JS | [Hz]. As a result, the carrier frequency f _L of the CW light source 12b in the phase synchronization and local light pulse signal generator 19 is completely phase-synchronized with the carrier frequency f _T of the CW light source 12a in the transmitter 10 and output from the optical filter 14d. The optical pulse signal is a signal that is phase-synchronized with the optical time-division multiplexed multilevel modulation pulse signal sequence transmitted through the optical fiber transmission line 4.

A part of the output signal from the CW light source 12b subjected to the optical phase synchronization operation as described above is optically comb-modulated using the optical comb modulator 13b to generate a plurality of sidebands. The optical comb modulator 13b is driven at the frequency R [Hz] of the clock signal extracted by the clock signal regeneration circuit 18 in FIG. Here, as the optical comb modulator 13b, for example, an optical modulator using an electro-optic effect such as LiNbO ₃ or an EA modulator is used. The optical comb signal is pulse carved by an optical filter 14d to generate a TL local pulse having a repetition frequency R [Hz] synchronized with the received time division multiplex modulation pulse signal sequence. In the figure, the solid line and the broken line represent the paths of the optical signal and the electric signal, respectively.

  Next, an example of an optical phase synchronization and coherent time division multiplexing transmission experiment performed in the second embodiment will be described. First, the results of the optical phase synchronization experiment will be described. In this experiment, the transmitter 10 performs optical comb modulation using a clock signal of R = 9.95328 [GHz], and uses K = 13th harmonic (detuning from the carrier frequency: 13R [GHz]) as an optical reference signal. It extracted with the optical filter 14b. The phase synchronization and local light pulse signal generator 19 outputs a signal of frequency S = 2R-0.00167 [GHz] from the microwave signal generator 24a, and operates the optical phase modulator 23 according to this signal to generate an optical comb signal. I let you. Here, the detuning frequency is 1.67 MHz, but this value may be any value as long as it is 1 [MHz] or more, which is the control band of the optical phase synchronization circuit. FIG. 7A shows the optical spectrum of the optical reference signal extracted by the optical filter 14c and the optical comb signal extracted by the optical filter 14f. In FIG. 7A, an optical comb signal before passing through the optical filter 14f is also shown. In this experiment, a band-pass optical filter having a full width at half maximum of 1 nm is used as the optical filter 14f. 0.01002 [GHz]) is extracted. However, since the band of the present optical filter is wide, it is understood that only one harmonic cannot be extracted, and a plurality of harmonic components remain. FIG. 7 (b) is an enlarged view of a part of FIG. 7 (a). From this figure, the desired beat signal stands at | KR-JS | = 13R-6S = 9.9633 [GHz], while the beat signal between the 7th harmonic of the remaining local light comb and the reference optical signal is 9.94159. You can see how you stand at [GHz]. This extra beat signal is not mixed into the control band (1 [MHz]) of optical phase synchronization because it is 22 [MHz] away. Therefore, the residual beat signal does not affect the optical phase synchronization operation at all.

  The phase of the beat signal between the sixth harmonic of the optical comb signal and the optical reference signal is compared with the reference signal output from the microwave generator 24b, and the difference is detected by the DBM as an error signal. Is done. The error signal is negatively fed back to the CW light source 12b through the negative feedback circuit 22. In this experiment, by setting the frequency of the reference signal output from the microwave generator 24b as U = 13R-6S = R + 0.01002 = 9.9633 [GHz], the CW in the phase synchronization and local light pulse signal generator 19 is set. The carrier frequency of the light source 12b and the carrier frequency of the CW light source 12a in the transmitter 10 have been completely phase-synchronized. As a result, the local light pulse signal output from the optical filter 14d is a signal that is phase-synchronized with the optical time-division multiplexed multilevel modulation pulse signal sequence transmitted through the optical fiber transmission line 4. FIG. 8 is a heterodyne beat signal spectrum of the reference optical signal and the sixth harmonic of the optical comb signal in an optical phase synchronized state. The line width of the obtained beat signal spectrum is 10 Hz or less and its purity is very high. FIG. 9 is an SSB (Single Sideband) phase noise spectrum of the spectrum of FIG. 8 measured by an electric spectrum analyzer. From this, the effective value of the phase noise is evaluated to be 1.7 degrees, and the accuracy of the optical phase synchronization that can be realized by the phase synchronization and local pulse signal generator 19 is very high. For example, in the case of a 64-value QAM signal, the allowable phase noise obtained as the phase difference between adjacent symbols is 4.7 degrees, so the optical phase synchronization circuit realized in this experiment is sufficient to demodulate the 64 QAM signal. Has performance.

  Next, the results of a 150 km transmission experiment of a polarization multiplexed, 10 Gymbol / s, 4-light time division multiplexed 32 RZ / QAM (400 Gbit / s) signal using the above-described phase locked loop are shown. FIGS. 10 (a) and 10 (b) are constellation maps of 10 Gsymbol / s (one polarization and 1-time division multiplexed signal) and 32 QAM data signals before and after transmission. Since the S / N of the optical signal deteriorates after 150 km transmission, it can be seen that each symbol point of the constellation is spread, but each is clearly separated. FIG. 11 shows the bit error rate characteristics with respect to the reception power of the 10 Gsymbol / s data signal. After 150 km transmission, the error is error-free when the received power is -27 dBm or higher.

  As described above, coherent optical time division multiplex transmission using a modulation scheme with a high multilevel can be realized by using a highly accurate phase synchronization technique between the optical multilevel modulation pulse signal and the local light emission pulse signal.

  By using the present invention, the technology that was previously phase-locked between signal light and local light using continuous light is applied to highly accurate phase-locking technology between multilevel modulation pulse signals and local light pulse signals. In coherent light time division multiplex transmission, the transmission rate per channel can be increased by improving the multilevel. Also, the transmission distance can be extended by combining compensation techniques for pulse waveform distortion in conventional ultrahigh-speed optical transmission. Furthermore, since an inexpensive optical device that operates at a low speed is used in the present invention, an inexpensive and high-speed optical transmission system can be constructed as a whole.

DESCRIPTION OF SYMBOLS 1 Optical pulse light source 2 Optical IQ modulator 3 Optical time division multiplexing circuit 4 Optical fiber transmission line 5 Clock signal source 6 Encoder 7 Coherent detection circuit 8 Decoder 9 Local pulse light source 10 Transmitter 11 Receiver 12a, b CW light source 13a, b Optical comb modulators 14a, b, c, d, e, and f Optical filter 15 Polarization multiplexing and differentiation circuit 16a, b, c, d, e, f, and g Optical coupler 17 Polarization demultiplexing circuit 18 Clock signal recovery circuit DESCRIPTION OF SYMBOLS 19 Phase synchronization and local light-emission pulse signal generator 20 Photodetector 21 Phase comparator 22 Negative feedback circuit 23 Optical phase modulator 24a, b Microwave signal generator

Claims (3)

  The transmission unit has a port 1 for optical multilevel modulation and optical time division multiplexing of an optical pulse signal, and a port 2 for generating a reference optical signal phase-synchronized with the optical pulse signal. After the division multiplexed multi-level modulation pulse signal and the reference optical signal are combined, the combined wave is propagated through an optical fiber transmission line, and the optical time division multiplexed multi-level modulation pulse is received using the reference optical signal at the receiving unit. Port 3 of the optical PLL that synchronizes the phase of the signal and the local light pulse signal, and a port for time-division demultiplexing and coherent homodyne detection of the optical time division multiplexed multilevel modulation pulse signal and the local light pulse signal by a coherent detection circuit A coherent optical time division multiplexing transmission apparatus and a coherent optical time division multiplexing transmission system.   2. The coherent optical time division multiplexing according to claim 1, further comprising a circuit for polarization multiplexing the optical time division multiplexed multilevel modulation pulse signal in the transmission unit, and a circuit for polarization demultiplexing in the reception unit. Transmission device and coherent optical time division multiplexing transmission system.   3. The coherent optical time division multiplex transmission apparatus according to claim 1, wherein an optical comb generator is used as means for generating an optical pulse signal and a reference optical signal in the transmitter and a local light pulse signal in the receiver. And coherent optical time division multiplexing transmission system.

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