JP5406348B2 - Optical orthogonal frequency division multiplexing transmission circuit - Google Patents

Description

  The present invention relates to a transmission / reception circuit for optical orthogonal frequency division multiplexing (OFDM) transmission capable of realizing high band utilization efficiency in the field of optical communication.

  Research and development of a wavelength division multiplexing (WDM) transmission system has been actively conducted as means for increasing the bandwidth utilization efficiency of an optical communication system. When the channel interval is Δf [Hz] and the bit rate per channel is BR [bit / s], the band use efficiency E of the WDM transmission system is expressed as E = BR / Δf.

  FIG. 1 shows a configuration example of a WDM system (Non-Patent Document 1). Data signals from a plurality of optical transmitters 1-1 to J (equal frequency interval Δf) are multiplexed by using a wavelength multiplexer 2 such as an arrayed-waveguide grating (AWG). The multiplexed signal is transmitted through the optical fiber 3, then demultiplexed for each frequency by a wavelength demultiplexer 4 such as AWG, and received by a plurality of optical receivers 5-1 to 5 -J.

A.R.Chraplyvy and R.W.Tkach, “Terabit / second Transmission Experiments,” IEEE J. Quantum Electron., Vol. 34, no. 11, pp.2103-2107, 1998.

  In the case of normal non-multilevel IM-DD (Intensity Modulation-Direct Detection) binary WDM transmission (WDM transmission using only strength 0, 1 as a data signal), in order to avoid inter-channel crosstalk in FIG. , Δf> BR is indispensable. For this reason, there is a problem that the bandwidth utilization efficiency deteriorates.

  Conventionally, signals for optical OFDM transmission are generated and demodulated mainly by electrical signal processing. However, optical OFDM transmission mainly using electric signal processing has a problem that it is limited by the speed of electric signal processing. Actually, in the case of electrical signal processing, the bit rate of each channel remains on the order of Mb / s.

  The present invention has been made in view of such a problem. By making the channels orthogonal to each other, it is possible to achieve high band utilization efficiency even in binary transmission, and to set the bit rate of each channel to Gb / s. It is an object of the present invention to provide a receiver configuration for optical OFDM transmission using optical signal processing that can be increased in order.

  In order to achieve the above object, the present invention is an optical receiver for optical orthogonal frequency division multiplexing transmission, which splits the optical orthogonal frequency division multiplexed light into N branches. A light intensity splitter, N optical delay lines for delaying N lights from the light intensity splitter, and N inputs and N outputs for performing a fast Fourier transform on the N lights from the optical delay lines. An optical fast Fourier transform circuit; and N time gate elements that gate the N lights from the optical fast Fourier transform circuit and output a demodulated channel signal for each channel. To do.

  According to a second aspect of the present invention, there is provided an optical receiver for optical orthogonal frequency division multiplexing transmission, wherein the optical orthogonal frequency division multiplexed light is phase-modulated and the frequency is within the time slot length T of the channel signal. A first optical phase modulator that shifts as a function of time, an optical frequency demultiplexer that demultiplexes the light from the optical phase modulator into N optical pulses for each predetermined frequency, and the optical frequency For the N optical pulses from the demultiplexer, N second optical phase modulators each performing phase modulation whose sign is opposite to that of the first optical phase modulator, and the second optical phase N optical delay lines that respectively delay N optical pulses from the modulator and N optical pulses from the optical delay line are subjected to fast Fourier transform processing, and a demodulated channel signal is output for each channel. N-input N-output optical fast Fourier transform circuit And wherein the door.

  According to a third aspect of the present invention, there is provided an optical receiver for optical orthogonal frequency division multiplexing transmission, wherein the optical orthogonal frequency division multiplexed light is divided into N paths within the time slot length T of the channel signal. An optical switch that performs switching, N optical delay lines that delay N optical pulses from the optical switch, and N optical pulses from the optical delay line are subjected to fast Fourier transform processing for each channel. And an N-input N-output optical fast Fourier transform circuit for outputting the demodulated channel signal.

The invention according to claim 4 is the optical receiver according to any one of claims 1 to 3, wherein the number of the optical fast Fourier transform circuits is N = 2 ^L (L is an integer of 1 or more). When L is 1, it is composed of one 2-input 2-output directional coupler. When L is 2 or more, two N / 2-input N / 2-output lights A fast Fourier transform circuit and N / 2 two-input two-output directional couplers, and each of the M optical fast Fourier transform circuits (M = 1, 2,...). .., N / 2) are connected to the inputs of the Mth directional coupler among the N / 2 directional couplers, respectively. To do.

The invention according to claim 5 is the optical receiver according to any one of claims 1 to 3, wherein the number of the optical fast Fourier transform circuits is N = 2 ^L (L is an integer equal to or greater than 1). When L is 1, it is composed of one 2-input 2-output symmetric Mach-Zehnder interferometer. When L is 2 or more, two N / 2-input N / 2-output lights A fast Fourier transform circuit and N / 2 two-input two-output symmetric Mach-Zehnder interferometers, and each of the two optical fast Fourier transform circuits (M = 1, 2,...). .., N / 2) are connected to the inputs of the Mth symmetric Mach-Zehnder interferometer among the N / 2 symmetric Mach-Zehnder interferometers, respectively.

The invention according to claim 6 is the optical receiver according to any one of claims 1 to 3, wherein the number of the optical fast Fourier transform circuits is N = 2 ^L (L is an integer equal to or greater than 1). When L is 1, it is composed of one 2-input 2-output directional coupler or a symmetric Mach-Zehnder interferometer. When L is 2 or more, two N / 2 inputs N / 2 output optical fast Fourier transform circuit and a combination of N / 2 2-input 2-output directional coupler and symmetric Mach-Zehnder interferometer, each of the two optical fast Fourier transform circuits. The Mth output (M = 1, 2,..., N / 2) is output from the combination of the N / 2 directional couplers and the symmetric Mach-Zehnder interferometer, or the Mth directional coupler. Connect to the input of a symmetric Mach-Zehnder interferometer, respectively Characterized in that it is configured.

It is a figure which shows the structural example of the conventional wavelength division multiplex transmission system. It is a figure which shows the structural example of an optical orthogonal frequency division multiplexing transmission system. It is a figure which shows the spectrum of an optical orthogonal frequency division multiplexing transmission signal. It is a figure which shows the structural example of the transmitter for optical orthogonal frequency division multiplexing transmission systems which concerns on a 1st reference form. It is a figure which shows the structural example of the transmitter for optical orthogonal frequency division multiplexing transmission systems which concerns on a 2nd reference form. It is a figure which shows the structural example of the transmitter for optical orthogonal frequency division multiplexing transmission systems which concerns on a 3rd reference form. It is a figure which shows the structural example of the transmitter for optical orthogonal frequency division multiplexing transmission systems which concerns on a 4th reference form. It is a figure which shows the structural example of the receiver for optical orthogonal frequency division multiplexing transmission systems which concerns on the 1st Embodiment of this invention. It is a figure which shows the structural example of the optical circuit for implement | achieving an optical discrete Fourier transform. It is a figure which shows the structural example of the optical circuit for implement | achieving optical fast Fourier transform. It is a figure which shows the structural example of the optical circuit for implement | achieving optical fast Fourier transform. It is a figure which shows the structural example of the optical circuit for implement | achieving optical fast Fourier transform. It is a figure which shows the mode of the optical pulse in the receiver for orthogonal frequency division multiplexing transmission systems of FIG. It is a figure which shows the structural example of the receiver for optical orthogonal frequency division multiplexing transmission systems which concerns on the 2nd Embodiment of this invention. It is a figure which shows the structural example of the receiver for optical orthogonal frequency division multiplexing transmission systems which concerns on the 3rd Embodiment of this invention. It is a figure which shows the structural example of the receiver for optical orthogonal frequency division multiplexing transmission systems which concerns on 5th reference form. It is a figure which shows the structural example of the receiver for optical orthogonal frequency division multiplexing transmission systems which concerns on 6th reference form. It is a figure which shows the structural example of the receiver for optical orthogonal frequency division multiplexing transmission systems which concerns on 7th reference form. It is a figure which shows the structural example of the receiver for optical orthogonal frequency division multiplexing transmission systems which concerns on an 8th reference form. FIGS. 8A and 8B are diagrams illustrating signal spectra of respective parts when N = 4 and the optical Fourier transform circuit unit is configured in FIG. 11, in which FIG. 8A is a signal spectrum at the optical input unit 502, and FIG. The signal spectrum in the waveguide 44-3 is shown. FIG. 13 is a diagram illustrating a result of measuring error rate characteristics of signals in the optical output units 512-1 to 512 when N = 4 in FIG. 8 and the optical Fourier transform circuit unit is configured in FIG. FIG. ⁹ is a diagram illustrating an eye pattern at an error rate of 10 ⁻⁹ of the signal d ₁ (t) when N = 4 in FIG. 8 and the optical Fourier transform circuit unit is configured in FIG.

  Hereinafter, the present invention will be described in detail with reference to embodiments shown in the drawings.

First, an outline of optical orthogonal frequency division multiplexing transmission will be described. FIG. 2 shows a configuration example of an optical orthogonal frequency division multiplexing transmission system. Here, the reference carrier optical frequency is f ₀ , the optical frequency interval between the nth channel and the (n + 1) th channel is Δf _n , the time slot time length of each channel is T, and the synchronized channel signal is d _n (t) To do. N is an integer of 2 or more, and n = 0, 1,..., N−1, t represents time. _{Further, Δf n = k n / T} : is the _(k n 1 or more integer).

In the optical transmitters 11-1 to 11 -N, the optical frequency in which the phase between the oscillation optical frequencies is maintained in the optical frequency region is modulated using the respective channel signals d _n (t). While maintaining the phase between the oscillation optical frequencies and the synchronization between the channel signals d _n (t), the modulated signal lights from the optical transmitters 11-1 to 11 -N are combined using the optical intensity combiner 12. The combined signal S (t) is expressed by the following equation.

The modulated waveband signal S _n (t) for each subcarrier (carrier wave) is expressed by the following equation.

The cross-correlation function CC [S _n (t) S _{n + 1} (t)] between the modulation waveband signals S _n (t) and S _{n + 1} (t) for adjacent subcarriers is adjacent as shown in the following equation. The subcarriers are orthogonal to each other.

Thus, when k _n = 1 regardless of n, the minimum frequency interval in which adjacent channels are orthogonal is obtained, and the band use efficiency E = 1 is realized. The spectral shape of each subcarrier at this time is shown in FIG.

When attention is paid to one subcarrier, the frequency component of this subcarrier becomes 0 at the position of the center frequency of the other subcarrier, and the other subcarriers are not affected. Hereinafter, in all the embodiments, the discussion proceeds for the case of k _n = 1 for simplicity. In this case, S (t) can be transformed as follows.

That is, the signal light S, the inverse discrete Fourier transform of the channel signal _{d n:} the (IDFT Inverse Discrete Fourier Transform). Thereafter the signal light S that has passed through the optical fiber 13, through the optical discrete Fourier transform element 14, is demodulated into original d _n, is incident to the optical receiver 15-1～N. The optical discrete Fourier transform element 14 performs processing represented by the following equation.

Equations 4 and 5 are the same as those in the case of k _n = 1 regardless of n, even if one of _n or k _n ≧ 2 (E <1). Discuss.

  Thus, by processing in the optical domain for both the transmitter and the receiver, there is no speed limitation due to electrical signal processing, and the bit rate of each channel can be increased to the Gb / s order. Thereby, further increase in capacity of optical orthogonal frequency division multiplexing transmission can be realized.

  In addition, by using optical orthogonal frequency division multiplexing transmission, time demultiplexing processing using ultrashort optical pulsing in a time division multiplexing (TDM) system (high-speed time demultiplexing for each time slot T / N). It is not necessary to use (processing), and only low-speed processing for each time slot T is performed.

(First reference form)
FIG. 4 shows a configuration example of a transmitter according to the present embodiment. The transmitter 100 includes a light source 102 in which phases between a plurality of oscillation light frequencies are maintained in an optical frequency region, an optical frequency demultiplexer 104 that demultiplexes light from the light source for each optical frequency, and demultiplexed light. Are modulated by channel signals, optical modulators 106-1 to 106-N, optical frequency combiners (or optical intensity combiners) 108 that combine the modulated signal light, and optical outputs that output the combined signal light. Part 110. The transmitter 100 can be configured on a single substrate using, for example, waveguide technology.

  As the light source 102, a semiconductor or optical fiber mode-locked laser, a semiconductor gain switch light source, or a direct current light source and a light intensity modulator (an interferometer type modulator using an electro-optic effect of a ferroelectric material or a semiconductor material, or a semiconductor EA). (Electro-Absorption) modulator). Alternatively, an optical frequency comb generator that combines a direct current light source and an optical phase modulator (ferroelectric material) can be used. As the optical frequency demultiplexer 104 and the optical frequency multiplexer 108, an arrayed waveguide grating, an asymmetric Mach-Zehnder interferometer, or a multistage cascade connection thereof can be used. As the optical modulators 106-1 to 106-N, those made of a ferroelectric material or a semiconductor material can be used. Further, as the light intensity combiner 108, a multi-mode interference (MMI) coupler, a star coupler, a directional coupler is connected in multiple stages, or a symmetrical Mach-Zehnder interferometer is connected in multiple stages. Can be used.

4, the optical frequency demultiplexer 104, the optical frequency multiplexer (or optical intensity combiner) 108 and the optical output unit 110, and the connections of the respective components are stable and have low loss (˜ 0.01 dB / cm) and can be fabricated using quartz-based glass waveguide technology on silicon or glass substrates, with length and phase adjustable precisely. As the phase adjustment unit, a thin film heater of chromium or tantalum nitride (thermo-optic phase shifter, capable of precise phase adjustment of 0.01π or less) deposited on quartz glass can be used. In addition, a polymer waveguide, a ferroelectric waveguide such as LiNbO ₃ (LN), KTa _1-x Nb _x O ₃ (KTN), a semiconductor waveguide, a silicon waveguide, a waveguide alone such as an optical fiber, or the like It is also possible to use a hybrid configuration using a plurality.

For example, an optical frequency demultiplexer 104, an optical frequency combiner (or optical intensity combiner) 108, an optical output unit 110, and a silica-based glass waveguide are used for the connection of each component, and other components are used. Can be configured to hybridly integrate devices of other materials into a silica-based glass waveguide. Since the phase between a plurality of frequencies is fixed in the mode-locked laser or the optical frequency comb generator of the light source 102, it is branched by the optical frequency demultiplexer 104, and each frequency component in the optical modulators 106-1 to 106-N. Is modulated (photoelectric conversion) with a period T using each channel signal d _n (t). Each modulated signal light is combined using the optical frequency combiner 108 or combined using the optical intensity combiner 108, whereby the signal light S (t) represented by Formula 1 is output at the optical output unit 110. Can be obtained.

  Here, when the optical frequency multiplexer 108 is used, low-loss multiplexing is possible, but since it is a frequency-dependent element, there is a disadvantage that the modulation signal light band is narrowed. Further, when the light intensity combiner 108 is used, the loss increases due to the merge loss, but since it is not a frequency dependent element, there is no reduction degradation of the modulated signal light band. Since optical orthogonal frequency division multiplexing is not a signal modulation but a multiplexing method, the modulation in the optical modulators 106-1 to 106-N is not limited to binary intensity modulation, and may be binary or multilevel. Regardless of the value, any of intensity modulation, phase modulation, frequency modulation, polarization modulation, and the like may be used.

(Second reference form)
FIG. 5 shows a configuration example of a transmitter according to the present embodiment. The optical transmitter according to the present embodiment is for simply simulating transmission characteristics using only one modulator in an optical orthogonal frequency division multiplexing transmission system. The optical transmitter 200 includes a light source 202 in which a phase between a plurality of oscillation optical frequencies is held in an optical frequency region, an optical modulator 204 that modulates light from the light source with a specific channel signal, and an optical signal obtained by modulating the modulated signal light. An optical frequency demultiplexer 206 that demultiplexes each frequency, optical delay lines 208-1 to N that delay the demultiplexed signal light, and an optical frequency multiplexer that multiplexes the modulated signal light (or optical intensity). And a light output unit 212 that outputs the combined signal light. The optical transmitter 200 can be configured on one substrate using, for example, a waveguide technique.

The light modulator 204 modulates light from the light source 202. In this case, the signal for driving the optical modulator 204 may be only one of the channel signals d _n (t). Next, the signal light modulated using the optical frequency demultiplexer 206 is demultiplexed for each wavelength. Since the data signals of the respective wavelengths are modulated using the same modulator, it is necessary to separate them in time and reduce the data correlation between the respective wavelengths. For this reason, the lengths of the optical delay lines 208-1 to 208 -N are set to different values, and the difference in length based on the minimum length is set to be pTc ₀ / n _eff . Here, p is 0 or an even number of 2 or more, c ₀ is the speed of light in vacuum, n _eff is N pieces of N that connect the output of the optical frequency demultiplexer and the input of the optical frequency multiplexer (or optical intensity combiner). This is the equivalent refractive index of the part. The reason why p is not an odd number equal to or greater than 1 is to avoid the same data from continuing between wavelengths. Finally, signals of respective wavelengths are combined (or combined) using an optical frequency combiner (or optical intensity combiner) 210, and a simulation signal of the signal light S (t) is obtained at the optical output unit 212. . In addition to the delay lines, the lengths of the waveguides corresponding to the respective wavelengths are set to be equal.

(Third reference form)
FIG. 6 shows a configuration example of a transmitter according to the present embodiment. This optical transmitter 300 includes a light source 302 having a specific oscillation light frequency, a light intensity splitter 304 that branches N light from the light source, and optical phase modulators 306-1 to 306-1 that phase-modulate the branched light. An optical modulator 308-1 to N that modulates phase-modulated light whose frequency is shifted with a channel signal, an optical frequency combiner (or optical intensity combiner) 310 that combines the modulated signal light, A light output unit 312 for outputting the waved signal light. The optical transmitter 302 can be configured on a single substrate using, for example, waveguide technology.

The light having a specific oscillation light frequency f ₀ from the light source 302 is N-branched using the light intensity splitter 304 and is incident on the optical phase modulators 306-1 to 306-1. The light intensity splitter 304 can be configured by reversing the input and output of the light intensity combiner. Optical phase modulators 306-1 to 306-1 perform the following phase modulation on the phase φ _n (t) of the light of each channel.

The frequency f _n (t) of the light given the phase modulation of Equation 6 is shifted from the original frequency f ₀ and is given by the following equation.

Then, the optical modulators 308-1 to 30-N modulate the frequency components f _n (t) of the respective channels using the respective channel signals d _n (t), and the optical frequency multiplexer (or the optical intensity combiner). 310 center frequency

By combining (or combining) each time, the light output unit 312 can obtain the signal light S (t) represented by Expression 1.

  The optical frequency multiplexer 310 has a lower element loss than the optical intensity combiner 310. Center frequency at optical frequency multiplexer 310

Each combination may result in non-uniform frequency spacing, but an arrayed waveguide grating, an asymmetric Mach-Zehnder interferometer, or an optical frequency multiplexer that cascades them is not suitable for non-uniform frequency spacing. It can also handle waves.

(4th reference form)
FIG. 7 shows a configuration example of a transmitter according to the present embodiment. The optical transmitter according to the present embodiment is for simply simulating transmission characteristics using only one modulator in an optical orthogonal frequency division multiplexing transmission system. The optical transmitter 400 includes a light source 402 having a specific oscillation optical frequency, an optical modulator 404 that modulates light from the light source with a specific channel signal, an optical intensity splitter 406 that splits the modulated signal light into N branches, Optical phase modulators 408-1 to 408 -N for phase-modulating the branched signal light, optical delay lines 410-1 to 410 -N for delaying the phase-modulated signal light respectively, and optical frequency multiplexing for multiplexing the delayed signal light (Or a light intensity combiner) 412 and a light output unit 414 for outputting the combined signal light. The optical transmitter 400 can be configured on one substrate using, for example, a waveguide technique.

Light having a specific oscillation light frequency f ₀ from the light source 402 is modulated using the light modulator 404. Only one of the channel signals d _n (t) is required to drive the optical modulator 404. Next, the modulated signal light is branched into N using the light intensity branching device 406 and is incident on the optical phase modulators 408-1 to 408 -N. In the optical phase modulators 408-1 to 408 -N, the phase φ _n (t) and the frequency f _n (t) of the light of each channel are expressed by Expression 6 and Expression 7, respectively. Since the data signals of the respective branches are modulated using the same modulator, it is necessary to separate them in time and reduce the data correlation between the wavelengths. Therefore, the lengths of the optical delay lines 410-1 to 410 -N are set to different values, and the difference in length based on the minimum length is set to be pTc ₀ / n _eff . Here, p is 0 and an even number greater than or equal to 2, c ₀ is the speed of light in vacuum, n _eff is the N parts that connect the output of the optical intensity splitter and the input of the optical frequency multiplexer (or optical intensity combiner) Is the equivalent refractive index. Finally, the modulated signal light of each channel is combined (or combined) using the optical frequency combiner (or optical intensity combiner) 412, and the simulation signal of the signal light S (t) is output from the optical output unit 414. can get.

(First embodiment)
FIG. 8 shows a configuration example of a receiver according to the first embodiment of the present invention. The optical receiver 500 includes an optical input unit 502 to which optical orthogonal frequency division multiplexed signal light is input, an optical intensity splitter 504 that splits the input signal light into N, and a delayed signal light. Optical delay lines 506-1 to N, optical fast Fourier transform circuit 508 that performs Fourier transform processing on the delayed signal light, time gate elements 510-1 to N that perform gate processing on the signal light subjected to Fourier transform processing, and demodulation. And optical output units 512-1 to 512-N for outputting channel signals. The optical receiver 500 can be configured on one substrate using, for example, a waveguide technique.

On the receiving side, when the channel signals d _n (t) are demodulated collectively from the optical orthogonal frequency division multiplexed signal light S (t), the optical discrete Fourier transform shown in Expression 5 is required. An optical fast Fourier transform that can reduce the amount of calculation (multiplication amount) of the optical discrete Fourier transform and can simplify the configuration also in terms of hardware will be described. For simplicity, the case where N = 4 will be described. In this case, the optical discrete Fourier transform represented by Expression 5 is expressed by the following expression.

Here, the channel signal d _n (t) can be modified as follows.

Therefore, Expression 8 can be transformed as the following expression.

  In Equation 10, the number of multiplications is reduced from 16 to 8 as compared with Equation 8. This is the optical fast Fourier transform. Since the multiplication is halved, it is possible to reduce the amount of calculation, that is, to simplify the optical circuit configuration for realizing the calculation.

  FIG. 9 shows a configuration example of an optical circuit for realizing the calculation of Expression 8, and FIGS. 10 and 11 show configuration examples of the optical circuit for realizing Expression 10.

FIG. 9 shows an optical circuit for realizing the optical discrete Fourier transform. This optical circuit 20 includes four input waveguides 21-1 to 21-4, four light intensity splitters 22-1 to 2-4, 16 phase adjusting units 23-1 to 16-16, and 16 waveguides. It comprises waveguides 24-1 to 16, four light intensity combiners 25-1 to 25-4, and four output waveguides 26-1 to 26-4. The lengths of the input waveguides 21-1 to 21-4, the lengths of the waveguides 24-1 to 16, and the lengths of the output waveguides 26-1 to 26-4 are set to be equal. This is to maintain the phase relationship in Equation 8. By adjusting the phases of a total of 16 (generally N ² ) phase adjusting units 23-1 to 23-16, the relationship of Expression 8 is satisfied. Since the relative phases of the other 15 phase adjustment units need only match one of the phase adjustment units 23-1 to 23-16, the required minimum number of phase adjustment units is 15 (generally N a ² -1). In this way, the optical discrete Fourier transform can be realized using an actual optical circuit. In addition, this optical circuit has a fault which has many cross | intersection parts and a waveguide loss becomes high.

FIG. 10 shows an optical circuit for realizing the optical fast Fourier transform. The optical circuit 30 includes four input waveguides 31-1 to 31-4, seven phase adjusting units 32-1 to 7, four directional couplers 33-1 to 3-4, and four outputs. It consists of waveguides 34-1 to 34-4. The lengths of the input waveguides 31-1 to 31-4, the lengths of the output waveguides 34-1 to 34, and the lengths of the waveguides connecting the directional couplers 33-1 to 3-4 are set to be equal. To do. This is to maintain the phase relationship in Equation 10. The transfer matrix f _cpl of the directional couplers 33-1 to 3-4 is expressed by the following equation when the strength coupling factor κ is 0.5 (3 dB coupler).

  Therefore, when the phase of the phase adjusters 32-1 to 32-7 is set to −π / 2 (transfer expression exp (jπ / 2)), the calculation of Expression 10 (optical fast Fourier transform) is performed using the configuration of FIG. realizable. Details are as follows. (F0, F1) and (F2, F3) in FIG. 10 are expressed by the following equations.

Therefore, (d ₀ (t), d ₂ (t)), (d ₁ (t), d ₃ (t)) are expressed by the following equations.

The above two expressions agree with Expression 10 except for a constant multiple factor.

  The constant multiplication factor in the directional coupler transfer matrix and the vector representation showing the output of FIG. 10 does not affect the demodulation of the signal. The configuration example of FIG. 10 is an exemplification, and the installation location and the set amount of the phase adjustment units 32-1 to 32-1 may be changed. Compared to FIG. 9, in the configuration of FIG. 10, the number of waveguides is reduced from 24 to 4, and the number of phase adjusting units is reduced from 15 to 7. This reduction effect becomes more prominent as the number of N, which is normally set to a multiple of 2, increases. As the directional couplers 33-1 to 33-4, a multi-mode interference (MMI) coupler, a star coupler, or the like can be used.

FIG. 11 shows an optical circuit having another configuration for realizing the optical fast Fourier transform. The optical circuit 40 includes four input waveguides 41-1 to 41-4, four symmetric Mach-Zehnder interferometers 42-1 to 4-4, five phase adjusting units 43-1 to 4-5, and four The output waveguides 44-1 to 44-4 are configured. The lengths of the input waveguides 41-1 to 41-4, the lengths of the output waveguides 44-1 to 4-4, and the lengths of the waveguides connecting the symmetric Mach-Zehnder interferometers 42-1 to 4-4 are equal. Set. This is to maintain the phase relationship in Equation 10. The transfer matrix f _smzi of the symmetric Mach-Zehnder interferometers _{42-1 to 42-4} has an intensity coupling ratio κ of the directional coupler constituting the symmetric Mach-Zehnder interferometer of 0.5, and the phase setting of the phase adjusting units 43-1 to 4-4. When the quantity φ is π / 2, it is expressed by the following formula.

  Therefore, when the phase of the phase adjustment unit 43-5 is set to π / 2 (transfer expression exp (−jπ / 2)), the calculation of formula 10 (optical fast Fourier transform) can be realized using the configuration of FIG. . Details are as follows. (F0, F1) and (F2, F3) in FIG. 11 are expressed by the following equations.

Therefore, (d ₀ (t), d ₂ (t)), (d ₁ (t), d ₃ (t)) are expressed by the following equations.

The above two expressions agree with Expression 10 except for a constant multiple factor.

  The configuration example in FIG. 11 is an exemplification, and the installation location and the set amount of the phase adjustment unit 43-5 can be changed. Compared to FIG. 10, it can be seen that the number of phase adjustment units is further reduced from 7 to 5 in the configuration of FIG. 11. This reduction effect becomes more prominent as the number of N increases.

  10 and 11, in order to realize the optical fast Fourier transform, any number of directional couplers and symmetric Mach-Zehnder interferometers may be used as long as the total number is N. In general, an optical fast Fourier transform circuit with 2N inputs and 2N outputs can be configured as shown in FIG. 12 by combining two optical fast Fourier transform circuits with N inputs and N outputs.

  The optical circuit 50 includes 2N input waveguides 51-1 to 2N, two N input N output optical fast Fourier modulation circuits 52-1 and 52-2, 2N optical fast Fourier modulation circuits, Waveguides 53-1 to 2N for connecting the directional couplers, N directional couplers (or symmetric Mach-Zehnder interferometers) 54-1 to N, 2N output waveguides 55-1 to 2N, It is composed of In this figure, the phase adjustment unit is omitted. The lengths of the input waveguides 51-1 to 2N, the lengths of the waveguides 53-1 to 2N, and the lengths of the output waveguides 55-1 to 2N are set to be equal. This is to maintain the phase relationship in Equation 10. As other optical circuit configurations for realizing the optical fast Fourier transform, an N input N output MMI coupler, a star coupler, and the like are also conceivable.

FIG. 13 shows the state of an optical pulse for one time slot in each part of FIG. Specifically, FIG. 13A shows a state of an optical pulse in the input waveguide portion of FIG. 8, and FIG. 13B shows a state of an optical pulse at the output of the delay line in FIG. FIG. 13C shows the state of the optical pulse at the output of the optical fast Fourier transform circuit of FIG. 8, and FIG. 13D shows the state of the optical pulse in the output waveguide portion of FIG. Yes. Also in this figure, for simplicity, a case where N = 4 will be described. The input optical pulse shown in FIG. 13A is N-branched by the optical intensity splitter 504, and different delays are given by Tc ₀ / (Nn _eff ) in the optical delay lines 506-1 to 506-N (FIG. 13B). ). In the optical fast Fourier transform circuit 508, a phase corresponding to the optical fast Fourier transform is given to each optical pulse (FIG. 13C). When only the part where each pulse overlaps is taken out in time by gating by the time gate elements 510-1 to 510- _N, d _n (t) corresponding to Equation 10 is obtained from the optical orthogonal frequency division multiplexed signal S (t). Can be demodulated.

  Note that as the time gate elements 510-1 to 510-N, an interferometer type modulator using an electro-optic effect of a ferroelectric material or a semiconductor material, or a gating using a semiconductor optical amplifier (SOA: Semlconductor Optical Amplifier), Although gating in the electrical region after photoelectric conversion of signal light can be used, a semiconductor EA modulator that can set a short gate time window is considered to be optimal at present.

In the case where N = 4 in FIG. 8 and the optical Fourier transform circuit unit is configured in FIG. 11, an experiment was performed to verify the operation of FIG. A 10 Gbit / s × 4 channel optical OFDM signal was incident on the optical input unit 502 of FIG. Each channel is modulated with a separate data signal, and the frequency difference between each channel is 10 GHz. FIGS. 20A and 20B show signal spectra in the optical input unit 502 and the output waveguide 44-3, respectively. FIG. 20B shows that the spectrum of d ₁ (t) is extracted clearly. Further, FIG. 21 shows error rate characteristics of signals (corresponding to d ₀ (t), d ₂ (t), d ₁ (t), and d ₃ (t), respectively) at the light output units 512-1 to 512-4. The result of having measured is shown. ● indicates a reference error rate characteristic when only d ₁ (t) is incident on the optical input unit 502. Further, the eye pattern shown in FIG. 22 shows an eye pattern having an error rate of d ₁ (t) of 10 ⁻⁹ . From the result of FIG. 21, using the configuration of FIG. 8, an error rate of 10 ⁻⁹ is achieved for any of d ₀ (t), d ₂ (t), d ₁ (t), and d ₃ (t). It can be seen that the optical OFDM signal is accurately separated.

(Second Embodiment)
FIG. 14 shows a configuration example of a receiver according to the second embodiment of the present invention. This optical receiver 600 includes an optical input unit 602 that receives optical orthogonal frequency division multiplexed signal light, an optical phase modulator 604 that performs phase modulation on the input signal light, and a phase modulation that causes a frequency shift. An optical frequency demultiplexer 606 that demultiplexes the signal light, an optical phase modulator 608-1 to N that cancels chirping of the demultiplexed signal light, and an optical delay line 610 that delays the demultiplexed signal light. -1 to N, an optical fast Fourier transform circuit 612 that performs Fourier transform processing on the delayed signal light, and light output units 614-1 to 614 -N that output demodulated channel signals. The receiver 600 can be configured on a single substrate using, for example, waveguide technology.

  The optical phase modulator 604 performs the following phase modulation on the phase θ (t) of the signal light that is optical orthogonal frequency division multiplexed from the optical input unit 602, with a being a constant.

Frequency f of the light given a phase modulation of the formula 13 (t) is shifted from the original frequency f _0, it is given by the following equation.

  The optical frequency demultiplexer 606 demultiplexes the signal light f (t) whose frequency is shifted by phase modulation for each optical center frequency 2aT / N. One time slot length of each demultiplexed signal light is T / N. Thereafter, the N optical phase modulators 608-1 to 608 -N are synchronized with each other, and the following phase modulation is performed within a time slot having the same time length T / N in the period T.

Since the signs of Expressions 13 and 15 are opposite, the chirping of the signal light is canceled in the optical phase modulators 608-1 to 608 -N. Next, the optical delay lines 610-1 to 610 -N are given different delays by Tc ₀ / (Nn _eff ), and then the optical fast Fourier transform circuit 612 having N inputs and N outputs has a phase corresponding to the optical fast Fourier transform. Given to each light pulse. As the output of the optical output units 614-1 to 614 -N, the channel signal d _n (t) corresponding to Equation 10 can be demodulated from the optical orthogonal frequency division multiplexed signal S (t). Note that the temporal extraction of only the portion where the pulses overlap is completed by filtering in the optical frequency demultiplexer 606. Therefore, the receiver of FIG. 14 has an advantage that a time gate element is unnecessary compared with the configuration of FIG.

(Third embodiment)
FIG. 15 illustrates a configuration example of a receiver according to the third embodiment of the present invention. The optical receiver 700 has an optical input unit 702 to which optical orthogonal frequency division multiplexed signal light is input, an optical switch 704 that switches the input signal light to each path, and delays the switched signal light. Optical delay lines 706-1 to 706 -N, an optical fast Fourier transform circuit 708 that performs Fourier transform processing on the delayed signal light, and optical output units 710-1 to 710 -N that output demodulated channel signals are provided. The receiver 700 can be configured on a single substrate using, for example, waveguide technology.

In the optical switch 704, a time slot having the same time length T of the optical orthogonal frequency division multiplexed signal S (t) is spatially transmitted in a time period T to N different output paths for each portion of the time length T / N. Switching is performed. The optical pulses extracted by switching are given different delays by Tc ₀ / (Nn _eff ) by the optical delay lines 706-1 to 706 -N, and then the phase corresponding to the optical fast Fourier transform in the optical fast Fourier transform circuit 708. Is given to each light pulse. As an output of the optical output units 710-1 to 710 -N, the channel signal d _n (t) corresponding to Equation 10 can be demodulated from the optical orthogonal frequency division multiplexed signal S (t). Note that, by switching in the optical switch 704, temporal extraction of only a portion where each pulse overlaps is completed. Therefore, the receiver of FIG. 15 has an advantage that a time gate element is unnecessary compared with the configuration of FIG. Since the optical switch 704 is required to have high speed, an optical switch made of a ferroelectric material or a semiconductor material is usually used.

(5th reference form)
FIG. 16 illustrates a configuration example of a receiver according to the present embodiment. The optical receiver 800 includes an optical input unit 802 to which optical orthogonal frequency division multiplexed signal light is input, an optical intensity splitter 804 that splits the input signal light into N branches, and phase-modulates the branched signal light. Optical phase modulators 806-1 to 806 -N, optical intensity splitters 808-1 to 808 -N for further branching the phase-modulated signal light by N, optical delay lines 810-1 to 810 -N for respectively delaying the branched signal light, Optical intensity combiners 812-1 to 81-N that combine the delayed signal light, time gate elements 814-1 to N that perform gate processing on the combined signal light, and an optical output unit 816-1 that outputs a demodulated channel signal -N. The receiver 800 can be constructed on a single substrate using, for example, waveguide technology.

In the light intensity splitter 804, the input signal light is N-branched with equal intensity. Each phase modulator 806 performs the following phase modulation on the phase φ _n (t) in a time slot having the same time length T in the period T.

Next, the light intensity splitter 808 further splits the light from the phase modulator 806 into N branches with equal intensity. The branched light is given different delays by Tc ₀ / (Nn _eff ) by the optical delay line 810, and then combined by the light intensity combiner 812, and the optical orthogonal frequency division multiplexed signal by the time gate element 814. By performing time gating that periodically extracts only the portions of the time length T / N that overlap in time from the time slots of the same time length T of S (t) every time T, Equations 5 and 8 The channel discrete signal d _n (t) can be demodulated by directly executing the optical discrete Fourier transform operation represented by:

(Sixth reference form)
FIG. 17 shows a configuration example of a receiver according to the present embodiment. This optical receiver 900 further branches an optical input unit 902 to which optical orthogonal frequency division multiplexed signal light is input, an optical intensity splitter 904 that splits the input signal light into N, and a branched signal light. Optical intensity splitters 906-1 to 906-1, optical phase shifters 908-1 to N for adjusting the phase of the branched signal light, optical delay lines 910-1 to N for delaying the phase adjusted signal light, and Light intensity combiners 912-1 to 91-N that combine the delayed signal light, time gate elements 914-1 to N that perform gate processing on the combined signal light, and an optical output unit 916-1 that outputs a demodulated channel signal -N. The receiver 900 can be constructed on a single substrate using, for example, waveguide technology.

In the light intensity splitter 904, the input signal light is N-branched with equal intensity. The N-branched signal lights are further N-branched at equal intensity in the light intensity splitters 906-1 to 906-1. The light intensity branching unit 904 and 906-1~N may be combined into one input ^{N 2} 1 one light intensity branching unit outputs. The signal lights from the N branch outputs of the N light intensity splitters 906-1 to 906-1 to N are input to the N phase shifters 908-1 to N, respectively. Each phase shifter 908 performs the following phase shift.

Then, the phase-shifted light is given a delay whose length difference is different by Tc ₀ / (Nn _eff ) by the optical delay line 910, and then combined by the light intensity combiner 912, and by the time gate element 914, Time gating is performed in which only a portion of the time length T / N that overlaps in time from a time slot of the same time length T of the optical orthogonal frequency division multiplexed signal S (t) is periodically extracted every time T. Thereby, the optical discrete Fourier transform operation represented by the equations 5 and 8 can be directly executed, and the channel signal d _n (t) can be demodulated. The order of arrangement of the phase shifters 908-1 to N and the optical delay lines 910-1 to N may be reversed from that in FIG.

(Seventh reference form)
FIG. 18 illustrates a configuration example of a receiver according to the present embodiment. The optical receiver 1000 includes an optical input unit 1002 to which optical orthogonal frequency division multiplexed signal light is input, an optical intensity splitter 1004 that branches the input signal light into N, and phase-modulates the branched signal light. Optical phase modulators 1006-1 to 1006-1 to N, optical frequency demultiplexers 1008 to 1 to N that demultiplex signal light that has been phase-modulated and frequency shifted, and light that cancels chirping of the demultiplexed signal light Phase modulators 1010-1 to 1010 -N, optical delay lines 1012-1 to 1012 -N for delaying the demultiplexed signal light, optical intensity combiners 1014-1 to 1014 -N for combining the delayed signal lights, and demodulated And optical output units 1016-1 to 1016 -N for outputting channel signals. The receiver 1000 can be constructed on a single substrate using waveguide technology.

The optical phase modulators 1006-1 to 1006-1 to 1006-1 to perform the following phase modulation with respect to the phase θ _n (t) of each light from the optical intensity splitter 1004, with b _n as a constant.

The frequency f _n (t) of light given the phase of Equation 18 is shifted from the original frequency f ₀ and is given by the following equation.

In the optical frequency demultiplexers 1008-1 to 1008 -N, demultiplexing is performed for each center frequency 2b _n T / N. One time slot length of each demultiplexed signal is T / N. Thereafter, the N optical phase modulators 1010 are synchronized with each other, and the following phase modulation is performed within a time slot having the same time length T / N in period T.

Since the first terms of Equation 18 and Equation 20 are opposite in sign, signal chirping is canceled in the optical phase modulator 1010. In the second term of Equation 20, a phase corresponding to the optical discrete Fourier transform is given to each optical pulse. The optical pulses are given different delays by Tc ₀ / (Nn _eff ) by the optical delay line 1012, and then combined by the optical intensity combiner 1014, and the optical orthogonal frequency division multiplexed signal S is output as the output of the optical output unit 1016. From (t), the channel signal d _n (t) corresponding to Equations 5 and 8 can be demodulated. Note that temporal extraction of only the portion where each pulse overlaps has been completed by filtering in the optical frequency demultiplexers 1008-1 to 1008 -N. Therefore, the receiver of FIG. 18 has an advantage that a time gate element is unnecessary compared with the configurations of FIGS.

(Eighth reference form)
FIG. 19 shows a configuration example of a receiver according to the present embodiment. This optical receiver 1100 includes an optical input unit 1102 to which optical orthogonal frequency division multiplexed signal light is input, an optical switch 1104 that switches the input signal light to each output path, and N-branch of the switched signal light Optical intensity splitters 1106-1 to 1106-1 to N, optical phase shifters 1108-1 to N for adjusting the phase of the branched signal light, and optical delay lines 1110-1 to N for delaying the phase adjusted signal light, respectively. , Optical intensity combiners 1112-1 to 1112-1 to combine delayed signal lights and optical output units 1114-1 to N to output demodulated channel signals. The receiver 1100 can be constructed on a single substrate using waveguide technology.

  In the optical switch 1104, the time slot having the same time length T of the optical orthogonal frequency division multiplexing signal S (t) is spatially transmitted in a time period T to N different output paths for each portion of the time length T / N. Switching is performed. The N paths are N-branched with equal intensity in the light intensity dividers 1106-1 to 1106-1. The signal lights from the N branch outputs of the N light intensity splitters are input to the N phase shifters 1108-1 to 1108-1, respectively. Each phase shifter 1108 performs the following phase shift.

Then, the phase-shifted light is given delays that differ in length by Tc ₀ / (Nn _eff ) by the optical delay line 1110, and then combined by the light intensity combiner 1112. As a result, the channel signal d _n (t) corresponding to Equations 5 and 8 is demodulated by the optical discrete Fourier transform from the optical orthogonal frequency division multiplexed signal S (t) as the output of the optical output units 1114-1 to 1114 -N. Can do. Note that, by switching in the optical switch 1104, temporal extraction of only a portion where each pulse overlaps is completed.

Each synchronization channel signal d _n (t) on the transmission side may be not only a parallel signal but also a signal obtained by time-separating every N bits from a signal having a bit rate N times that of d _n (t).

Each channel signal d _n (t) demodulated on the receiving side may be processed as a parallel signal as it is, but is processed after being time-multiplexed with a signal having a bit rate N times that of d _n (t). Of course, it doesn't matter.

  While the present invention has been described with respect to several specific embodiments, the embodiments described herein are merely illustrative in view of the many possible embodiments to which the principles of the present invention can be applied. It is not intended to limit the scope of the invention. The configuration and details of the embodiment exemplified here can be changed without departing from the spirit of the present invention. Further, the illustrative components and procedures may be changed, supplemented, or changed in order without departing from the spirit of the invention.

3 and 13 Optical fiber 20 Optical discrete Fourier transform circuit 21-1 to 4 Input waveguide 23-1 to 16 Phase adjustment unit 24-1 to 16 Waveguide 26-1 to 4 Output waveguide 30 Optical fast Fourier transform circuit 31- 1-4 Input Waveguide 32-1-7 Phase Adjuster 33-1-4 Directional Coupler 34-1-4 Output Waveguide 40 Optical Fast Fourier Transform Circuit 41-1-4 Input Waveguide 42-1-4 Symmetric Mach-Zehnder interferometers 43-1 to 5-5 phase adjustment unit 44-1 to 4 output waveguide 50 optical fast Fourier transform circuit 51-1 to 2N input waveguide 53-1 to 2N waveguide 55-1 to 2N output waveguide DESCRIPTION OF SYMBOLS 100 Optical OFDM transmitter 106-1-N Optical modulator 110 Optical output part 200 Optical OFDM transmitter 208-1-N Optical delay line 212 Optical output part 300 Optical OFDM transmitter 306-1-N Optical phase Modulators 308-1 to N Optical modulator 312 Optical output unit 400 Optical OFDM transmitter 408-1 to N Optical phase modulator 410-1 to N Optical delay line 414 Optical output unit 500 Optical OFDM receiver 506-1 to N Optical delay line 510-1 to N Time gate element 512-1 to N Optical output unit 600 Optical OFDM receiver 602 Optical input unit 604 Optical phase modulator 608-1 to N Optical phase modulator 610-1 to N Optical delay line 614-1 to N optical output unit 700 optical OFDM receiver 702 optical input unit 706-1 to N optical delay line 710-1 to N optical output unit 800 optical OFDM receiver 802 optical input unit 810-1 to N optical delay line 816-1 to N optical output unit 900 optical OFDM receiver 902 optical input unit 908-1 to N optical phase shifter 910-1 to N optical delay line 916-1 to N optical output unit 1000 optical OFD Receiver 1002 Optical input unit 1010-1 to N Optical phase modulator 1012-1 to N Optical delay line 1016-1 to N Optical output unit 1100 Optical OFDM receiver 1102 Optical input unit 1108-1 to N Optical phase shifter 1110- 1 to N optical delay line 1114-1 to N optical output unit

Claims (7)

An optical receiver for optical orthogonal frequency division multiplexing transmission,
An optical intensity splitter for N-branching the optical orthogonal frequency division multiplexed light;
N optical delay lines for respectively delaying the N lights from the light intensity splitter;
An N-input N-output optical fast Fourier transform circuit for performing fast Fourier transform processing on N light beams from the optical delay line;
And gated N pieces of light from the light fast Fourier transform circuit, an optical receiver, characterized in that a N time gate element for outputting a channel signal demodulated for each channel ,
The optical fast Fourier transform circuit has N = 2 ^L (L: integer greater than or equal to 1) inputs and outputs,
When L is 1, it is composed of one 2-input 2-output symmetric Mach-Zehnder interferometer,
When L is 2 or more, it is composed of two N / 2 input N / 2 output optical fast Fourier transform circuits and N / 2 two input two output symmetric Mach-Zehnder interferometers. The M-th (M = 1, 2,..., N / 2) output of the fast Fourier transform circuit is used as the M-th symmetric Mach-Zehnder interferometer among the N / 2 symmetric Mach-Zehnder interferometers. An optical receiver configured to be connected to each of the inputs . An optical receiver for optical orthogonal frequency division multiplexing transmission,
A first optical phase modulator that phase-modulates the optical orthogonal frequency division multiplexed light and shifts the frequency as a function of time within the time slot length T of the channel signal;
An optical frequency demultiplexer for demultiplexing the light from the optical phase modulator into N optical pulses for each predetermined frequency;
N optical phase modulators for N optical pulses from the optical frequency demultiplexer, each of which performs phase modulation with a sign opposite to that of the first optical phase modulator;
N optical delay lines for respectively delaying the N optical pulses from the second optical phase modulator;
And an N-input N-output optical fast Fourier transform circuit that performs fast Fourier transform processing on N optical pulses from the optical delay line and outputs a demodulated channel signal for each channel. Receiver. An optical receiver for optical orthogonal frequency division multiplexing transmission,
An optical switch that switches the optical orthogonal frequency division multiplexed light to N paths within the time slot length T of the channel signal;
N optical delay lines that respectively delay N optical pulses from the optical switch;
And an N-input N-output optical fast Fourier transform circuit that performs fast Fourier transform processing on N optical pulses from the optical delay line and outputs a demodulated channel signal for each channel. Receiver. The optical receiver according to claim 2 , wherein
The optical fast Fourier transform circuit has N = 2 ^L (L: integer greater than or equal to 1) inputs and outputs,
When L is 1, it is composed of one 2-input 2-output directional coupler,
When L is 2 or more, it is composed of two N / 2 input N / 2 output optical fast Fourier transform circuits and N / 2 two input two output directional couplers. The M-th (M = 1, 2,..., N / 2) output of the fast Fourier transform circuit is used as the M-th directional coupler among the N / 2 directional couplers. An optical receiver characterized in that the optical receiver is configured to be connected to each of the inputs. The optical receiver according to claim 2 , wherein
The optical fast Fourier transform circuit has N = 2 ^L (L: integer greater than or equal to 1) inputs and outputs,
When L is 1, it is composed of one 2-input 2-output symmetric Mach-Zehnder interferometer,
When L is 2 or more, it is composed of two N / 2 input N / 2 output optical fast Fourier transform circuits and N / 2 two input two output symmetric Mach-Zehnder interferometers. The M-th (M = 1, 2,..., N / 2) output of the fast Fourier transform circuit is used as the M-th symmetric Mach-Zehnder interferometer among the N / 2 symmetric Mach-Zehnder interferometers. An optical receiver configured to be connected to each of the inputs. The optical receiver according to claim 2 , wherein
The optical fast Fourier transform circuit has N = 2 ^L (L: integer greater than or equal to 1) inputs and outputs,
When L is 1, it is composed of one 2-input 2-output directional coupler or symmetric Mach-Zehnder interferometer,
When L is 2 or more, it is composed of a combination of two N / 2 input N / 2 output optical fast Fourier transform circuits, N / 2 two input two output directional couplers, and a symmetric Mach-Zehnder interferometer. The M-th (M = 1, 2,..., N / 2) outputs of the two optical fast Fourier transform circuits are connected to the N / 2 directional couplers and a symmetric Mach-Zehnder. An optical receiver configured to be connected to an input of an Mth directional coupler or a symmetric Mach-Zehnder interferometer, respectively, in the combination of interferometers. An optical receiver for optical orthogonal frequency division multiplexing transmission,
An optical intensity splitter for N-branching the optical orthogonal frequency division multiplexed light;
N optical delay lines for respectively delaying the N lights from the light intensity splitter;
An N-input N-output optical fast Fourier transform circuit for performing fast Fourier transform processing on N light beams from the optical delay line;
N time gate elements that gate N light from the optical fast Fourier transform circuit and output a demodulated channel signal for each channel;
An optical receiver comprising:
The optical fast Fourier transform circuit has N = 2. ^L (L: integer greater than or equal to 1) inputs and outputs,
When L is 1, it is composed of one symmetric Mach-Zehnder interferometer,
When L is 2 or more, it is composed of a combination of two N / 2 input N / 2 output optical fast Fourier transform circuits, N / 2 two input two output directional couplers, and a symmetric Mach-Zehnder interferometer. The M-th (M = 1, 2,..., N / 2) outputs of the two optical fast Fourier transform circuits are connected to the N / 2 directional couplers and a symmetric Mach-Zehnder. Each of the interferometer combinations is configured to connect to an input of an Mth directional coupler or a symmetric Mach-Zehnder interferometer,
An optical receiver characterized in that the N / 2 input N / 2 output optical fast Fourier transform circuit is a symmetric Mach-Zehnder interferometer.

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