JP5378279B2 - Optical receiver - Google Patents

Description

  The present invention relates to an optical receiver. In particular, the present invention relates to an optical receiver that receives an optical OFDM (Orthogonal Frequency Division Multiplexing) signal composed of a plurality of subcarriers that are not symbol-synchronized at the receiving end.

  As the demand for data communication increases, optical transmission networks using optical signal modulation technology and optical signal multiplexing technology that enable transmission of large-capacity traffic are becoming widespread. Realization of high-capacity traffic transmission requires realization of high frequency utilization efficiency, and examples of techniques for realizing it include multilevel modulation techniques and optical frequency orthogonal multiplexing (OFDM) techniques. In particular, optical OFDM is a scheme that suppresses optical interference from an adjacent channel that may otherwise occur by using the orthogonality of optical frequencies that are satisfied between channels, and achieves high frequency utilization efficiency. Combination with value modulation is also possible (for example, see Non-Patent Document 1).

  FIG. 1 shows an example of the configuration of an optical transmitter that generates an optical OFDM signal. The optical transmitter includes a light source, an optical modulator, a clock generator, an optical delay line, and an optical coupler. The light output from each light source is called a subcarrier and is arranged with the same frequency interval (f1-f2 = f3-f2 = f4-f3 = Δf). Each subcarrier is individually modulated at the same clock frequency by each optical modulator, and becomes a signal having a spectrum spread. Here, the modulation rate is equal to the frequency interval between subcarriers. An optical OFDM signal is generated by combining these modulated subcarriers with an optical coupler. The optical delay line is used to align the timing of signals superimposed on each subcarrier, that is, to achieve symbol synchronization.

  FIG. 2 shows an example of the configuration of an optical receiver that separates an optical OFDM signal into subcarriers. The optical receiver includes a Mach-Zehnder Interferometer (MZI), an arrayed waveguide grating (AWG), and an optical demodulator. Since MZI can extract an optical signal having a desired frequency without interference, the optical OFDM signal is separated into even and odd subcarriers by MZI with delay amount Δt set to 1 / 2Δf. Here, Δf represents a frequency interval between adjacent subcarriers. The subcarriers separated evenly and oddly are further separated into subcarriers by AWG and demodulated by an optical demodulator.

Further, there is a configuration using coherent detection and digital signal processing as an optical receiver. FIG. 3 shows another example of the configuration of the optical receiver. In this case, as shown in FIG. 3, the local light that is phase-synchronized with the signal light is combined with the signal light and received by a photo detector (PD), thereby including the phase information of the optical signal. In this state, it is converted into a baseband signal. With respect to one symbol of the baseband signal, sampling is performed so that the sampling interval Δt satisfies Δt = 1 / NΔf with respect to the symbol length T in the sampling unit, and the obtained N data strings S (kΔt) are obtained. On the other hand, the signal superimposed on each subcarrier is demodulated by applying a discrete Fourier transform represented by (Equation 1) in the digital signal processing unit. Where N denotes the number of subcarriers forming an optical OFDM signal, k = 0,1, ···, N -1, n = 0,1, ···, a N-1, _{d n} is n The signal superimposed on the first subcarrier, f, is the carrier frequency of the signal superimposed on the lowest frequency subcarrier constituting the optical OFDM signal.

H. Sanjoh, et.al., 'Optical orthogonal frequency division multiplexing using frequency / time domain filtering for high spectral efficiency up to 1 bit / s / Hz,' OFC 2002, ThD1. K. Yonenaga, et.al., 'Bit-Rate-Flexible All-Optical OFDM Transceiver Using Variable Multi-Carrier Source and DQPSK / DPSK Mixed Multiplexing,' OFC 2009, OWM1.

  In order to realize long-distance and large-capacity transmission in optical fiber communication, it is important to realize high frequency utilization efficiency. On the other hand, it is also important to have high resistance to chromatic dispersion of the fiber transmission line.

In the above method, in order to realize a discrete Fourier transform represented by equation (1), within a time to perform the N sampling, the data signal d _n superimposed on each subcarrier must be constant . This means that the timing of the symbols of each data signal must be aligned, that is, the symbols must be synchronized. This is shown in FIG. However, generally, the symbol timing of the optical OFDM signal at the time of reception is shifted due to the influence of chromatic dispersion in the transmission path. This creates a transmission penalty for the optical OFDM signal and becomes a serious factor that limits the transmission distance. FIG. 5 shows the relationship between the transmission distance and the power penalty in the optical OFDM signal obtained by experiment. Here, the polarization of each subcarrier is orthogonalized. The chromatic dispersion of the transmission line is 16.6 ps / nm / km. This optical OFDM signal is composed of two subcarriers, and each subcarrier is subjected to 20 Gbps NRZ-DQPSK modulation. If there is no symbol timing shift due to chromatic dispersion in the transmission line, that is, if symbol synchronization is achieved, the optical OFDM signal has the same dispersion tolerance as the single-carrier 20Gbps NRZ-DQPSK signal (solid line connecting x) Should have. However, since the transmission path actually has chromatic dispersion, it can be confirmed that a shift in symbol timing occurs and a transmission penalty occurs (solid line connecting ▲). On the other hand, the broken line connecting ● indicates the dispersion tolerance when the symbol timing is optimized in accordance with the dispersion of the transmission line in the optical delay line of the optical transmitter. From this, it can be confirmed that the deviation of the symbol timing due to the wavelength dispersion of the transmission path greatly contributes to the penalty. For example, paying attention to the transmission distance with a penalty of 4 dB, it can be seen that the transmission distance is extended 1.5 times from 40 km to 60 km by suppressing the influence of the symbol timing shift. The influence of chromatic dispersion on the optical OFDM signal may be not only the symbol timing shift but also the signal pulse spread, but from FIG. 5, the symbol timing shift is the dominant factor for the signal waveform degradation of the optical OFDM signal. I understand.

  Even when the transmission path does not have chromatic dispersion, the optical OFDM signals generated using different clocks are not strictly symbol-synchronized, and the deviation increases with time. In this case, a similar penalty may occur. FIG. 6 shows the relationship between the symbol timing shift and the power penalty. Here, the penalty for the optical OFDM signal composed of two 10 Gbps intensity modulation signals is calculated by numerical simulation, and the broken line indicates that the polarization between subcarriers is orthogonalized. From FIG. 6, it can be confirmed that when the symbol timing deviation is about half the symbol length, a penalty of 6 dB or more occurs even if the polarization between subcarriers is orthogonalized. Thus, it can be seen that the use of different clocks and the shift in symbol timing between subcarriers caused by chromatic dispersion in the transmission path have a great influence on signal waveform degradation in optical OFDM transmission.

  The present invention has been made in view of the above points, and an object thereof is to provide an optical receiver for demodulating a data signal even when there is a synchronization shift between subcarriers constituting an optical OFDM signal at a receiving end. And

In order to solve the above problems, the optical receiver of the present invention is:
An optical receiver for receiving an optical OFDM signal,
A sampling unit that performs a plurality of samplings per symbol at a sampling timing corresponding to a symbol synchronization shift between subcarriers constituting the optical OFDM signal;
A demodulator that demodulates a data signal superimposed on each subcarrier based on information obtained by sampling;
It is characterized by having.

  According to the embodiment of the present invention, a data signal can be demodulated even when there is a synchronization shift between subcarriers constituting an optical OFDM signal at the receiving end. Accordingly, the dispersion resistance is improved, and the transmission distance can be extended.

The figure which shows an example of a structure of the optical receiver which produces | generates an optical OFDM signal The figure which shows an example of a structure of the optical receiver which isolate | separates an optical OFDM signal into each subcarrier The figure which shows another example of a structure of the optical receiver which isolate | separates an optical OFDM signal into each subcarrier Diagram showing how symbols are synchronized The figure which shows the relationship between the transmission distance and the power penalty in the optical OFDM signal The figure which shows the relationship between the shift of the symbol timing and the power penalty in the optical OFDM signal Diagram showing a situation where symbol synchronization is not achieved Diagram showing a situation where symbol synchronization is not achieved (when N = 2) The figure which shows the structure of the optical transmitter which concerns on the 1st Embodiment of this invention. The figure which shows another structure of the optical transmitter which concerns on the 1st Embodiment of this invention. The figure which shows the structure of the optical receiver which concerns on the 1st Embodiment of this invention The figure which shows the eye pattern at the time of detecting directly the optical OFDM signal comprised from two 10Gpbs intensity modulation signals The figure which shows another structure of the optical receiver which concerns on the 1st Embodiment of this invention The figure which shows the structure of the optical transmitter which concerns on the 2nd Embodiment of this invention. The figure which shows another structure of the optical receiver which concerns on the 2nd Embodiment of this invention. The figure which shows another structure of the optical receiver which concerns on the 2nd Embodiment of this invention. The figure which shows another structure of the optical receiver which concerns on the 2nd Embodiment of this invention. The figure which shows another structure of the optical receiver which concerns on the 2nd Embodiment of this invention.

  Embodiments of the present invention will be described below with reference to the drawings.

  An optical receiver according to an embodiment of the present invention receives an optical OFDM signal composed of a plurality of subcarriers in which polarization between adjacent subcarriers is orthogonal and symbol synchronization is not achieved at the receiving end. . An optical OFDM signal that is not symbol-synchronized at the receiving end may be generated in an optical transmitter or may be generated due to chromatic dispersion by a transmission path. The optical receiver demodulates the signal superimposed on each subcarrier from the sampling sequence that performs sampling at the sampling timing characterized by the amount of deviation in symbol synchronization of the optical OFDM signal at the time of reception. And a digital signal processing unit.

  First, the principle of the present invention will be described. In general, an optical OFDM signal S (t) composed of N subcarriers is expressed as (Equation 2), and a baseband OFDM signal s (t) after coherent detection is expressed as (Equation 3).

As described above, FIG. 4 shows a signal in which symbol synchronization is established. T is the symbol length. In order to obtain the data signal d _n (t) superimposed on each subcarrier, t ₀ , t ₀ + T / N,..., T ₀ + (N−1) T / N It is necessary to sample N times at a sampling interval Δt = T / N, and to perform the discrete Fourier transform shown in (Equation 4) on the data string s (t ₀ + kΔt) obtained from the sampling. At this time, in order to execute the discrete Fourier transform shown in (Expression 4), d _n (t) in each subcarrier must be constant during sampling, that is, the value of the data being sampled. Must be immutable. Therefore, in a normal optical OFDM signal, a shift in symbol synchronization causes a penalty.

FIG. 7 shows a state where symbol synchronization is not achieved. τ _n represents the symbol synchronization shift of each subcarrier with respect to subcarrier 0. As shown in FIG. 7, the data signal d _n (t) superimposed on each subcarrier is constant in the time of the symbol length T. In the present invention, first, in a time domain where the data signal d ₀ (t) is constant, t ₀ , t ₀ + T / N,... At a sampling interval Δt = T / N from a predetermined sampling timing t ₀ as a reference. Extract the signal at t ₀ + (N−1) T / N. Then in the time domain is a data signal _d 1 (t) is constant, at same sampling interval Δt = T / N from a predetermined sampling timing _{t 1} as a _reference, t _1, t 1 + T / N, · · ·, Extract the signal at t ₁ + (N−1) T / N. This is performed in the time domain for all data signals, and a total of N ² signals are extracted. Here, t _n = t ₀ + τ _n .

In the following, for simplicity, attention is paid to the case of N = 2. At this time, the symbol asynchronous optical OFDM signal is expressed as shown in FIG. 8, and the signal to be extracted is expressed as (Equation 5). Here, T = 1 / Δf, bit information of interest in d ₀ (t) is d ₀ , adjacent bit information is d ₀ ′, bit information of interest in d ₁ (t) is d ₁ , and adjacent The bit information to be performed is d ₁ ′. Now, the phase difference between subcarrier 0 and subcarrier 1 is indeterminate. In order to remove this uncertain factor, the polarization between adjacent subcarriers is orthogonalized, and direct detection is used to extract each signal. By performing such processing, there are four unknowns d ₀ , d ₀ ′, d ₁ , d ₁ ′, whereas four extracted signals (s (t ₀ ), s (t ₀ ) + Τ ₁ ), s (t ₀ + T / 2), and s (t ₀ + τ ₁ + T / 2)), it is possible to obtain data signals d ₀ and d ₁ by performing digital signal processing. .

Even if the symbol synchronization deviation τ _n is not known in advance by the optical receiver, the four signals (s (t ₀ ), s (t ₀ + τ ₁ ), Since s (t ₀ + T / 2) and s (t ₀ + τ ₁ + T / 2)) can be extracted, the data signals d ₀ and d ₁ can be obtained. In the above description, the principle of the present invention has been described focusing on the case where N = 2. However, a data signal can be obtained in the same procedure when N> 2.

(First embodiment)
The operation of the optical transceiver and the state of signal light in the first embodiment of the present invention will be described with reference to FIGS. FIG. 9 shows the configuration of the optical transmitter 10. The optical transmitter 10 includes a light source 101, an intensity modulator 103, a clock generator 105, and a polarization beam combiner (PBC) 107. Each light source 101 generates continuous light having different carrier frequencies, and the frequency interval Δf satisfies the relationship of Δf = 1 / T with respect to the symbol length T of the signal. In each modulator 103 operated by different clock generators 105, data signals d ₀ (t) and d ₁ (t) are given to the respective continuous lights by intensity modulation. Note that there may be a difference in the clock frequency of different clock generators 105. Further, the same clock generator may be used for each modulator 103. Here, the carrier frequency of each subcarrier is f, f + Δf. The optical signal to which the data has been added is multiplexed by the PBC 107 so that the polarizations of the subcarriers are orthogonal. In this manner, a symbol asynchronous optical OFDM signal is generated.

  In the first embodiment, the optical OFDM signal does not necessarily need to be symbol asynchronous. Therefore, as shown in FIG. 10 or Non-Patent Document 2, continuous light transmitted from a single light source 111 is converted into a plurality of subcarriers by an intensity modulator 112, and each modulation operated by a clock generator 115 is performed. An optical OFDM signal generated by giving independent data signals to each other by the device 113, that is, an optical OFDM signal in which symbol synchronization is established may be used. In this case as well, the same clock generator may be used for each modulator 113.

FIG. 11 shows the configuration of the optical receiver 20. The optical receiver 20 includes an MZI 203, a PD 205, a sampling unit 207, and a digital signal processing unit 209. The optical OFDM signal transmitted from the optical transmitter is in a state where a symbol synchronization shift τ has occurred in the optical receiver 20 because symbol synchronization is not achieved or due to wavelength dispersion in the transmission path. The symbol synchronization shift here means a time difference between data signal sequences superimposed on each subcarrier. Such an optical OFDM signal is demultiplexed by the MZI 203 having a free spectral range (FSR) of 2Δf, and each optical signal is directly detected by the PD 205. The sampling unit 207 performs sampling twice with a time difference τ per symbol for the electrical signal that has been directly detected and becomes only the light intensity information, and sends the information obtained therefrom to the digital signal processing unit 209. Data signals d ₀ (t), d ₁ superimposed on each subcarrier are obtained by performing digital computation in the digital signal processing unit 209 based on a total of four data, two from each sampling unit 207. (T) is obtained.

  Below, the state of the signal in each function part is demonstrated. A baseband signal s (t) in an optical OFDM signal composed of two subcarriers is generally expressed as (Equation 6).

When this signal is demultiplexed by the MZI 203 whose FSR is 2Δf, a signal transmitted from each of the constructive port and the destructive port of the MZI 203 is expressed by (Expression 7).

FIG. 8 shows a baseband signal superimposed on each subcarrier, and it is assumed that the signal in the optical receiver 20 has a symbol synchronization shift of τ. Here, bit information of interest in d ₀ (t) is d ₀ , adjacent bit information is d ₀ ′, bit information of interest in d ₁ (t) is d ₁ , and adjacent bit information is d ₁ ′. . In such a situation, the sampling unit 207 performs sampling at t = t ₀ , t ₀ + τ with respect to the electrical signal transmitted from the MZI 203 represented by (Equation 7) and having only the light intensity information by the PD 205. Do. The data signals A, B, C, and D obtained at this time are expressed by (Equation 8), (Equation 9), (Equation 10), and (Equation 11), respectively.

Here, A and B represent data information sampled at the eye opening timing t = t ₀ of the subcarrier of interest with respect to the optical signals output from the constructive port and the destructive port of the MZI 203, respectively. Incidentally, t ₀ is the eye opening timing as a reference for the sampling, it may be at a timing when the eye opening is improved. C and D represent data information sampled at timing t = t ₀ + τ with respect to the optical signals output from the constructive port and the destructive port of the MZI 203, respectively.

The second term on the right side in (Equation 8) and the first term on the right side in (Equation 11) are crosstalk from adjacent subcarriers. From this data signal, in order to obtain the intensity information d _{0 ^2,} d _{1 2} of the superimposed data signal to each subcarrier, it performs digital signal processing as described below.

  FIG. 12 shows an eye pattern when an optical OFDM signal composed of two 10 Gbps intensity modulation signals is directly detected by a PD. Here, the polarization between subcarriers is in an orthogonal state. The two figures show eye patterns when τ = 0 ps and τ = 50 ps, respectively. From FIG. 12, it can be confirmed that crosstalk does not occur at τ = 0 ps, but crosstalk from adjacent subcarriers occurs at τ = 50 ps. Further, it can be confirmed that the crosstalk power is 0 at the minimum and about 1/4 of the signal power at the maximum.

In intensity modulation, each signal can be normalized as 0 ≦ d ₀ (t) ² ≦ 1, 0 ≦ d ₁ (t) ² ≦ 1, so d ₀ ² = 0 or 1, d ₁ ² = 0 or 1 and 0 ≦ d ₀ ′ ² ≦ 1, 0 ≦ d ₁ ′ ² ≦ 1. From this, it can be seen that the power of the crosstalk is 0 ≦ (d ₀ ′ −d ₀ ) ² ≦ 1, 0 ≦ (d ₁ −d ₁ ′) ² ≦ 1. Further, from (Equation 9), if B <1/2 (pattern 1), d ₁ ² = 0, and if B ≧ 1/2 (pattern 2), d ₁ ² = 1. Therefore, in pattern 1, 2B = In d ₁ ′ ² and pattern 2, 2B = (d ₁ ′ ± 1) ² . Power crosstalk superimposed on subcarriers 0 Therefore, when the pattern ^{_{1 (d 1 '-d 1)}} 2/2 = B, when the pattern _{2, (d 1' -d 1} ) 2/2 = [ ^{2-√ (2B)] 2} /2 , and the it is possible to calculate the crosstalk power from B. Similarly, the crosstalk power superimposed on the subcarrier 1 can be calculated from C. In summary, the relationship between the crosstalk power and B and C can be expressed as (Equation 12).

As described above, the data B and C obtained by sampling are digitally calculated as shown in (Equation 12), whereby the crosstalk power superimposed on each subcarrier can be calculated. Further, the data signals d ₀ and d ₁ can be obtained by subtracting the crosstalk power obtained in (Equation 12) from the data A and D similarly obtained by sampling. In summary, the data signals d ₀ and d ₁ superimposed on the optical OFDM signal having the symbol synchronization deviation τ are digitally expressed by (Expression 13) with respect to the data A, B, C, and D obtained by sampling. It is obtained by performing an operation.

In the optical receiver 20 of FIG. 11, it has been considered that the sampling unit 207 performs sampling twice per symbol, but the optical receiver 21 may be configured as shown in FIG. The optical receiver 21 includes an MZI 213, an optical coupler 214, a PD 215, a sampling unit 217, and a digital signal processing unit 219. The optical coupler 214 splits the optical signal output from each of the constructive port and the destructive port of the MZI 213 into two branches. The branched optical signal is directly detected by the PD 215. At this time, the sampling unit 1 (217 ₁ ) acquires the data A and performs clock extraction for the signal of the subcarrier 0. By performing clock synchronization with the sampling unit 3 (217 ₃ ) based on the clock, the data B can be acquired by the sampling unit 3 (217 ₃ ). Similarly, the sampling unit 4 (217 ₄ ) acquires the data D and performs clock extraction for the signal of the subcarrier 1. By performing clock synchronization with respect to the sampling unit 2 (217 ₂ ) based on the clock, the data C can be acquired by the sampling unit 2 (217 ₂ ). As described above, the data A is acquired at the timing t ₀ in the sampling unit 1 (217 ₁ ), the data C is acquired at the timing t ₀ + τ in the sampling unit 2 (217 ₂ ), and the timing is acquired in the sampling unit 3 (217 ₃ ). t ₀ data B is obtained, the data D is acquired at timing _{t 0} + tau in the sampling unit 4 (217 _4). With such a configuration, even when the symbol synchronization shift τ changes with time or is uncertain, the data signals d ₀ and d ₁ can be demodulated as long as the clock can be extracted from the signal. become. Further, since sampling is performed at the eye opening timing in each sampling unit 217, it is not necessary to perform high-speed sampling even when the symbol synchronization shift τ is very small.

(Second Embodiment)
The operation of the optical transceiver and the state of signal light in the second embodiment of the present invention will be described with reference to FIGS. FIG. 14 shows the configuration of the optical transmitter 12. The optical transmitter 12 includes a light source 121, a phase modulator 123, a clock generator 125, and a PBC 127. Each light source 121 generates continuous light having different carrier frequencies, and the frequency interval Δf satisfies the relationship of Δf = 1 / T with respect to the symbol length T of the signal. In each modulator 123 operated by different clock generators 125, data signals d ₀ (t) and d ₁ (t) are given to the respective continuous lights by differential phase shift keying (DPSK). . There may be a difference in the clock frequency of the different clock generators 125. The same clock generator may be used for each modulator 123. Here, the carrier frequency of each subcarrier is f, f + Δf. The optical signal to which data is added is multiplexed by the PBC 127 so that the polarizations of the subcarriers are orthogonal to each other. In this manner, a symbol asynchronous optical OFDM signal is generated.

  In the second embodiment, the optical OFDM signal does not necessarily need to be symbol asynchronous. Accordingly, as in the first embodiment, continuous light transmitted from a single light source is converted into a plurality of subcarriers by an intensity modulator, and independent data signals are given by a clock generator. A generated optical OFDM signal, that is, an optical OFDM signal with symbol synchronization may be used.

FIG. 15 shows the configuration of the optical receiver 22. The optical receiver 22 includes an MZI 221, an MZI 223, a PD 225, a sampling unit 227, and a digital signal processing unit 229. The optical OFDM signal transmitted from the optical transmitter is in a state in which the symbol synchronization shift τ has occurred in the optical receiver 22 because symbol synchronization is not achieved or due to chromatic dispersion in the transmission path. By passing such an optical OFDM signal through the MZI 221 having an FSR of Δf, the DPSK signal superimposed on each subcarrier is converted into an intensity modulation signal, and the intensity modulation signal is further converted into an MZI 223 having an FSR of 2Δf. The signal demultiplexed by is directly detected by PD225. Since the signal detected by the PD 225 is an intensity modulation signal similar to that of the first embodiment, the processing of the sampling unit 227 and the digital signal processing unit 229 after detection is the same as that of the first embodiment. By performing the operation, data signals d ₀ and d ₁ superimposed on each subcarrier are obtained. Note that d ₀ ² and d ₁ ² = 1 indicate that the phase difference between adjacent bits is 0, and d ₀ ² and d ₁ ² = 1 indicate that the phase difference between adjacent bits is π.

As shown in FIG. 16, the optical receiver 23 may have a configuration in which an MZI 233 having an FSR of 2Δf is arranged in the previous stage and an MZI 231 having an Δf is arranged in the subsequent stage. Also in this configuration, regarding the processing of the sampling unit 237 and the digital signal processing unit 239 after detection by the PD 235, the same operation as in the case of FIG. 15 is performed, so that the data signals d ₀ and d ₁ superimposed on the subcarriers. It is possible to obtain

Also in the second embodiment, as in the first embodiment, instead of performing sampling twice per symbol by the optical receiver 22 in FIG. 15, an optical receiver 24 is configured as shown in FIG. Also good. By configuring the optical receiver 24 as shown in FIG. 17, data can be extracted as long as the clock can be extracted from the signal even when the symbol synchronization shift τ changes over time or is uncertain. Demodulation of the signals d ₀ and d ₁ is possible. In addition, since sampling is performed at the eye opening timing in each sampling unit 247, it is not necessary to perform high-speed sampling even when the symbol synchronization deviation τ is very small.

Further, instead of performing sampling twice per symbol by the optical receiver 23 of FIG. 16, the optical receiver 25 may be configured as shown in FIG. By configuring the optical receiver 25 as shown in FIG. 18, data can be extracted as long as the clock can be extracted from the signal even when the symbol synchronization shift τ changes with time or is uncertain. Demodulation of the signals d ₀ and d ₁ is possible. Further, since sampling is performed at the eye opening timing in each sampling unit 257, it is not necessary to perform high-speed sampling even when the symbol synchronization deviation τ is very small.

<Effect of Embodiment of the Present Invention>
According to the embodiment of the present invention, a data signal can be demodulated even when there is a synchronization shift between subcarriers constituting an optical OFDM signal at the receiving end. Accordingly, the dispersion resistance is improved, and the transmission distance can be extended. Further, the data signal can be demodulated even when there is a difference in the clock frequency between the subcarriers.

  Although the embodiments of the present invention have been described above, the present invention is not limited to the above-described embodiments, and various modifications and applications can be made within the scope of the claims.

10, 11, 12 Optical transmitter 20, 21, 22, 23, 24, 25 Optical receiver

Claims (7)

An optical receiver for receiving an optical OFDM signal,
A sampling unit that performs a plurality of samplings per symbol at a sampling timing corresponding to a symbol synchronization shift between subcarriers constituting the optical OFDM signal;
A demodulator that demodulates a data signal superimposed on each subcarrier based on information obtained by sampling;
Have
When the optical OFDM signal is composed of two subcarriers whose optical polarization is modulated and whose polarizations are orthogonal to each other,
The sampling unit
Using a Mach-Zehnder interferometer with a delay amount set to 1 / ( 2Δf ) (Δf represents a frequency interval between the two subcarriers), self-delay interference is performed on the optical OFDM signal,
Predetermined sampling timings t ₀ and t ₀ + having a timing difference equal to the symbol synchronization shift τ between the subcarriers with respect to the optical signals output from the output ports of the constructive port and the deconstructive port of the Mach-Zehnder interferometer conducted a Oite sampling to τ,
The demodulator is therefore the following formulas, the optical receiver you demodulated data signal superimposed on each subcarrier.
However,
d ₀ ² and d ₁ ² are the intensity information of the data signal superimposed on each subcarrier,
A and B are information sampled at timing t _{0 with} respect to the optical signals output from the constructive port and the deconstructive port of the Mach-Zehnder interferometer, respectively .
C and D are information sampled at timing t ₀ + τ with respect to the optical signals output from the constructive port and the deconstructive port of the Mach-Zehnder interferometer, respectively . The sampling unit
The optical signal output from the constructive port of the Mach-Zehnder interferometer is bifurcated , and sampling is performed by performing clock extraction at the eye opening timing on the first optical signal that is one of the optical signals after the branching. Sampling is performed at timing t ₀ ,
Before SL Mach an optical signal output from the deconstructive port Zehnder interferometer bifurcated, with respect to the second optical signal which is one of the optical signal after branching, by performing clock extraction in eye opening timing, Sampling is performed at the sampling timing t ₀ + τ,
Sampling is performed by matching the timing of the second optical signal with the timing of the third optical signal, which is the other optical signal after branching of the optical signal output from the constructive port of the Mach-Zehnder interferometer. Sampling is performed at timing t ₀ + τ,
For the fourth optical signal which is the other optical signal after branching of the optical signal output from the deconstructive port of the Mach-Zehnder interferometer, the Rukoto combined sampling timing of the first optical signal, implementing sampled at sampling timing t _0, the optical receiver of claim 1. An optical receiver for receiving an optical OFDM signal,
A sampling unit that performs a plurality of samplings per symbol at a sampling timing corresponding to a symbol synchronization shift between subcarriers constituting the optical OFDM signal;
A demodulator that demodulates a data signal superimposed on each subcarrier based on information obtained by sampling;
Have
When the optical OFDM signal is composed of two subcarriers that are optically differential phase modulated and whose polarizations are orthogonal to each other,
The sampling unit
Using the first Mach-Zehnder interferometer with a delay amount set to 1 / Δf (Δf represents the frequency interval between the two subcarriers), self-delay interference is performed on the optical OFDM signal Then, the optical OFDM signal is converted into an intensity modulation signal,
Using a second Mach-Zehnder interferometer with a delay amount set to 1 / ( 2Δf ) , self-delay interference is performed on the intensity modulated signal,
A predetermined sampling timing t ₀ having a timing difference equal to a symbol synchronization shift τ between the subcarriers with respect to the optical signals output from the output ports of the constructive port and the deconstructive port of the second Mach-Zehnder interferometer; the Oite sampling conducted to t ₀ + τ,
The demodulator is therefore the following formulas, the optical receiver you demodulated data signal superimposed on each subcarrier.
However,
d ₀ ² and d ₁ ² are the intensity information of the data signal superimposed on each subcarrier,
A and B are information sampled at timing t _{0 with} respect to the optical signals output from the constructive port and the deconstructive port of the second Mach-Zehnder interferometer, respectively .
C and D are information sampled at timing t ₀ + τ with respect to the optical signals output from the constructive port and the deconstructive port of the second Mach-Zehnder interferometer, respectively . The sampling unit
To the second Mach-optical signal output from the constructive port of Zehnder interferometer bifurcated, the first optical signal which is one of the optical signal after branching, by performing clock extraction in eye opening timing Accordingly, to implement sampled at sampling timing _{t 0,}
Before SL and bifurcated output optical signal from the deconstructive port of the second Mach-Zehnder interferometer, the second optical signal which is one of the optical signal after branching, perform clock extraction with eye opening timing Thus , sampling is performed at the sampling timing t ₀ + τ,
The sampling of the second optical signal is synchronized with the timing of the third optical signal which is the other optical signal after the branching of the optical signal output from the constructive port of the second Mach-Zehnder interferometer. To perform sampling at the sampling timing t ₀ + τ,
For the fourth optical signal which is the other optical signal after branching of the optical signal output from the deconstructive port of said second Mach-Zehnder interferometer, Ru combined sampling timing of said first optical signal The optical receiver according to claim 3 , wherein sampling is performed at sampling timing t ₀ . An optical receiver for receiving an optical OFDM signal,
A sampling unit that performs a plurality of samplings per symbol at a sampling timing corresponding to a symbol synchronization shift between subcarriers constituting the optical OFDM signal;
A demodulator that demodulates a data signal superimposed on each subcarrier based on information obtained by sampling;
Have
When the optical OFDM signal is composed of two subcarriers that are optically differential phase modulated and whose polarizations are orthogonal to each other,
The sampling unit
Using the first Mach-Zehnder interferometer with a delay amount set to 1 / ( 2Δf ) (Δf represents the frequency interval between the two subcarriers), self-delayed interference is performed on the optical OFDM signal And
Using the second and third Mach-Zehnder interferometers with a delay amount set to 1 / Δf, optical signals output from the output ports of the constructive port and the deconstructive port of the first Mach-Zehnder interferometer, respectively. by carrying out the self-delay interference against converts each intensity-modulated signal,
Wherein for each intensity-modulated signal, and performs a predetermined Oite sampling the sampling timing t ₀ and t ₀ + tau with timing difference equal to the symbol synchronization error tau between the sub-carrier,
The demodulator is therefore the following formulas, the optical receiver you demodulated data signal superimposed on each subcarrier.
However,
d ₀ ² and d ₁ ² are the intensity information of the data signal superimposed on each subcarrier,
A and B are information sampled at timing t _{0 with} respect to the intensity modulation signals output from the second and third Mach-Zehnder interferometers, respectively .
C and D are information sampled at timing t ₀ + τ with respect to the intensity modulation signals output from the second and third Mach-Zehnder interferometers, respectively . The sampling unit
And bifurcated output intensity modulated signal from the second Mach-Zehnder interferometer, the first intensity modulated signal which is one of the intensity-modulated signal after branching, by performing clock extraction in eye opening timing , Sampling at sampling timing t ₀ ,
By bifurcating the intensity modulation signal output from the third Mach-Zehnder interferometer and performing clock extraction at the eye opening timing on the second intensity modulation signal which is one of the intensity modulation signals after the branching Sampling is performed at sampling timing t ₀ + τ,
Synchronizing the sampling of the second intensity modulation signal with the timing of the third intensity modulation signal which is the other intensity modulation signal after branching of the intensity modulation signal output from the second Mach-Zehnder interferometer. To perform sampling at the sampling timing t ₀ + τ,
To the third fourth intensity modulated signal which is the other of the intensity modulation signal after branching the output intensity modulated signal from the Mach-Zehnder interferometer, Ru combined sampling timing of the first intensity-modulated signal The optical receiver according to claim 5 , wherein sampling is performed at sampling timing t ₀ . Prior Symbol demodulator, by discrete Fourier transform the information obtained by sampling at predetermined sampling timing, the demodulated data signal, the optical receiver according to any one of claims 1 to 6 vessel.