JP2006287635A - Optical transmitting method and system, optical transmitter and optical receiver - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical transmitting method and system, optical transmitter and optical receiver capable of setting more channels with less crosstalk, that is, accommodating more signal light. <P>SOLUTION: Optical intensity modulators 16a and 16b generate optical pulse signals S1 and S2 for conveying data D1 and D2, respectively. Nonlinear group delay encoders 18a and 18b give to the optical pulse signals S1 and S2 nonlinear group delays that fluctuate within a signal band and are different from each other. A multiplexer 20 multiplexes output light of the nonlinear group delay encoders 18a and 18b. A demultiplexer 42 divides light inputted from the an optical fiber transmission line 30 into two parts, supplies one part to a nonlinear group delay encoder 44a and supplies the other to a nonlinear group delay decoder 44b. The decoders 44a and 44b eliminate distortion of the optical pulse signals S1 and S2, respectively. Photodiodes 46a and 46b convert output light of the decoders 44a and 44b into an electric signal, and data demodulators 48a and 48b demodulate the data D1 and D2 from output electric signals of the photodiodes 46a and 46b. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an optical transmission method and system that can be used for optical dispersion division multiplexing, an optical transmission device, and an optical reception device.

  In an optical fiber transmission line, the pulse waveform is distorted due to the wavelength dependence of the group delay. Usually, in order to eliminate this waveform distortion, a dispersion compensator that provides chromatic dispersion that cancels the accumulated chromatic dispersion of the optical fiber transmission line is disposed in the optical receiver. In order to prevent the accumulated chromatic dispersion value from becoming excessively large on the optical fiber transmission path, the optical transmission apparatus may give in advance chromatic dispersion having a sign opposite to that of the chromatic dispersion value of the optical fiber transmission path.

  A plurality of signal lights can be multiplexed and transmitted using such a dispersion-compensated optical transmission system. That is, Patent Document 1 discloses an optical dispersion division multiplexing transmission system that multiplexes signal light by wavelength dispersion. In the multiplex transmission system, an optical transmission device multiplexes a plurality of optical signals with different chromatic dispersions between the optical signals, and outputs the multiplexed signal light to an optical fiber transmission line. The optical receiving apparatus power-divides the multiplexed signal light input from the optical fiber transmission path, and gives a wavelength dispersion value corresponding to the signal desired to be received to each split light. The chromatic dispersion value given to each split light by the optical receiver is a chromatic dispersion value that makes the cumulative chromatic dispersion value of the signal light desired to be received substantially zero, for example, the chromatic dispersion value added by the optical transmitter. This is a value obtained by adding the accumulated chromatic dispersion values of the optical fiber transmission line. As a result, no chromatic dispersion remains for the desired signal light, so that the optical pulse waveform is recovered. For signal light other than the desired signal light, chromatic dispersion remains and the maximum value of the light intensity is small, so that the pulse spreads to such an extent that it can be simply regarded as noise.

  Patent Document 1 exemplifies an optical fiber and an optical fiber grating as an apparatus for adding a desired chromatic dispersion value.

It is also possible to use wavelength division multiplexing together with multiplex transmission using such chromatic dispersion. In this case, the unit for adding chromatic dispersion in the optical transmission device, the optical reception device, and both of them may be for each signal light or for each channel composed of a plurality of signal lights having adjacent wavelengths.
JP 11-215103 A

  The optical element used in the dispersion added circuit described in Patent Document 1 is an optical fiber or an optical fiber grating, and a difference in chromatic dispersion value given to signal light is small, and crosstalk is likely to occur between optical signals.

  For example, the residual chromatic dispersion value that cannot discriminate an optical pulse is, for example, about 1000 ps / nm or more for an NRZ modulation signal of 10 Gbit / s. That is, it is necessary to make the chromatic dispersion amount different by 1000 ps / nm or more between the optical receiving terminal stations. This amount of chromatic dispersion corresponds to a length of about 60 km in a single mode optical fiber, and is not realistic. Even if another optical device having a large chromatic dispersion value is used, it is inevitable to increase the size of the receiving terminal station, and it is difficult to increase the number of multiplexed channels.

  Further, when signal light directed to different receivers is input to the receiving device, a malicious receiver can receive signals addressed to other receivers by sweeping the chromatic dispersion value to be applied. .

  Therefore, a chromatic dispersion multiplexing system that is resistant to crosstalk, difficult to decode, or can easily cope with an increase in the number of users is desired.

  An object of the present invention is to provide an optical transmission method and system, an optical transmission apparatus, and an optical reception apparatus that satisfy such a demand.

  In an example of an optical transmission method according to the present invention, a group delay of a nonlinear group delay characteristic that fluctuates within a signal band is given to signal light in an optical transmission apparatus, and the nonlinear group delay characteristic of the received signal light is given to a received signal light in an optical reception apparatus. It is characterized by providing chromatic dispersion that reduces group delay.

  In another example of the optical transmission method according to the present invention, the first signal light carrying the first data is generated. A second signal light carrying the second data is generated. The first distorted signal light is generated by giving the first signal light a group delay having a first nonlinear group delay characteristic that fluctuates within the signal band. Nonlinear group delay characteristics that fluctuate within the signal band of the second signal light, and the first nonlinear group delay characteristic is a second distortion signal obtained by giving a group delay of the second nonlinear group delay characteristic. Produce light. The first and second distortion signal lights are combined and the combined light is output to the optical transmission line. The light from the optical transmission path is divided into first and second component lights. A chromatic dispersion that reduces the group delay of the first nonlinear group delay characteristic is applied to the first component light to generate a first distortion-reduced signal light. The first distortion reduction signal light is converted into a first electric signal. The first data is demodulated from the first electric signal. A second distortion-reduced signal light is generated by giving chromatic dispersion to the second component light to reduce the group delay of the second nonlinear group delay characteristic. The second distortion reduction signal light is converted into a second electric signal. Then, the second data is demodulated from the second electric signal.

  An optical transmission system according to the present invention includes a first signal light generator that generates first signal light that carries first data, and a second signal light that generates second signal light that carries second data. And a first nonlinear group delay encoder that generates a first distorted signal light by giving the first signal light a group delay having a first nonlinear group delay characteristic that fluctuates within the signal band. And a second non-linear group delay characteristic that fluctuates within the signal band, and a second non-linear group delay characteristic that is different from the first non-linear group delay characteristic is given to the second signal light. A second nonlinear group delay encoder that generates second distortion signal light, an optical multiplexer that combines the first and second distortion signal lights and outputs the combined light to an optical transmission line, and the light An optical demultiplexer that divides light from the transmission path into first and second component lights, and the first component light A first nonlinear group delay decoder for generating a first distortion-reduced signal light by providing chromatic dispersion for reducing the group delay of the first nonlinear group delay characteristic; and the first distortion-reduced signal light as a first distortion A first photoelectric converter for converting to an electrical signal; a first data demodulator for demodulating first data from the first electrical signal; and a second nonlinear group delay characteristic for the second component light. A second non-linear group delay decoder that generates a second distortion-reduced signal light by providing chromatic dispersion that reduces the group delay of the second, and a second non-linear group delay decoder that converts the second distortion-reduced signal light into a second electrical signal. A photoelectric converter and a second data demodulator that demodulates second data from the second electric signal are provided.

  An optical transmission apparatus according to the present invention includes a first signal light generation apparatus that generates a first signal light that carries first data, and a first signal light that varies within the signal band of the first signal light. And a first non-linear group delay encoder that generates a first distorted signal light by giving a group delay having a non-linear group delay characteristic.

  An optical receiver according to the present invention is an optical receiver in an optical transmission system that transmits a signal by wavelength dispersion division multiplexing, and is a group of nonlinear group delay characteristics that vary within a signal band due to signal light from the optical transmission path. Nonlinear group delay decoder for reducing distortion of signal light by giving delay, photoelectric converter for converting output signal light of nonlinear group delay decoder into electric signal, and data demodulator for demodulating data from the electric signal It is characterized by comprising.

  According to the present invention, since not only the magnitude of the group delay value but also a group delay with a positive inclination and a group delay with a negative inclination are given to the signal light, more channels can be set with less crosstalk. That is, more signal light can be accommodated. By making the nonlinear group delay encoder or decoder programmable, it becomes possible to flexibly cope with network changes.

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

  FIG. 1 shows a schematic block diagram of an embodiment of the present invention. The optical transmission device 10 includes a laser diode 12 that is a light source. The optical demultiplexer 14 divides the output laser light of the laser diode 12 into two, and supplies one to the light intensity modulator 16a and the other to the light intensity modulator 16b. The light intensity modulator 16a modulates the intensity of the laser light from the optical demultiplexer 14 according to the data D1, and outputs an optical pulse signal S1 carrying the data D1 to the nonlinear group delay encoder 18a. Similarly, the optical intensity modulator 16b modulates the intensity of the laser light from the optical demultiplexer 14 according to the data D2, and outputs an optical pulse signal S2 carrying the data D2 to the nonlinear group delay encoder 18a.

  The nonlinear group delay encoders 18a and 18b are optical elements that give different nonlinear group delays to the optical pulse signals S1 and S2. In the conventional optical dispersion division multiplexing transmission system, a constant group delay is given to the optical pulse signal within the band. However, the nonlinear group delay encoders 18a and 18b of the present embodiment are within the band of the optical pulse signals S1 and S2. Is given to the optical pulse signals S1 and S2. In other words, the nonlinear group delay encoders 18a and 18b generate distorted signal light in which the pulse waveforms of the optical pulse signals S1 and S2 are deteriorated. The structure of the nonlinear group delay encoders 18a and 18b and the necessary group delay characteristics will be described later.

  The optical multiplexer 20 combines the output lights of the nonlinear group delay encoders 18a and 18b. The combined light from the optical multiplexer 20 is output light from the optical transmitter 10, is applied to the optical fiber transmission path 30, propagates through the optical fiber transmission path 30, and enters the optical receiver 40.

  In the optical receiver 40, the optical demultiplexer 42 divides the light input from the optical fiber transmission line 30 into two, and supplies one to the nonlinear group delay decoder 44a and the other to the nonlinear group delay decoder 44b. The nonlinear group delay decoder 44a has a nonlinear group delay characteristic that cancels the nonlinear group delay given to the optical pulse signal S1 by the nonlinear group delay encoder 18a. The nonlinear group delay decoder 44b has a nonlinear group delay characteristic that cancels the nonlinear group delay given to the optical pulse signal S2 by the nonlinear group delay encoder 18b. Accordingly, when the waveform distortion in the optical fiber transmission line 30 is ignored, the nonlinear group delay decoder 44a restores the optical pulse signal light S1, and the nonlinear group delay decoder 44b restores the optical pulse signal light S2.

  The photodiode 46a converts the output light of the nonlinear group delay decoder 44a into an electric signal, and the data demodulator 48a demodulates the data D1 from the output electric signal of the photodiode 46a. Similarly, the photodiode 46b converts the output light of the nonlinear group delay decoder 44b into an electrical signal, and the data demodulator 48b demodulates the data D2 from the output electrical signal of the photodiode 46b.

  The group delay characteristic that periodically varies within the band of the optical pulse signal can be realized by a ring resonator. That is, by adopting a single ring resonator or a plurality of serially connected ring resonators as the nonlinear group delay encoders 18a and 18b and the nonlinear group delay decoders 44a and 44b, it is necessary for this embodiment. Nonlinear group delay characteristics can be obtained.

  A nonlinear group delay optical element that can be used as the nonlinear group delay encoders 18a and 18b or the nonlinear group delay decoders 44a and 44b will be described. FIG. 2 shows a plan view of an example of a nonlinear group delay optical element from a single ring resonator, and FIG. 3 shows a sectional view taken along line AA of FIG.

  A Mach-Zehnder interferometer 52 is disposed on the substrate 50. The Mach-Zehnder interferometer 52 connects two input / output two optical multiplexer / demultiplexers 54 and 56, two output ports of the optical multiplexer / demultiplexer 54, and two input ports of the optical multiplexer / demultiplexer 56, respectively. Two arms 58 and 60 are provided. An input waveguide 62 is connected to one input port of the optical multiplexer / demultiplexer 54, and an output waveguide 64 is connected to one output port of the optical multiplexer / demultiplexer 56. The other output port of the optical multiplexer / demultiplexer 56 is connected to the other input port of the optical multiplexer / demultiplexer 54 via the waveguide 66.

  As a result, the ring resonator shown in the figure is arranged such that the ring waveguide composed of the waveguide 66 and the arm 58 is disposed close to the main waveguide composed of the input waveguide 62, the arm 60, and the output waveguide 64. It is configured to optically couple to the main waveguide at two locations of the optical multiplexer / demultiplexers 54 and 56.

  A thin film heater 68 as a phase shifter is disposed on the arm 60, and a voltage controller 70 applies a voltage to the thin film heater 68. The optical path length of the arm 60 is adjusted by the thin film heater 68, and as a result, the optical coupling rate between the waveguides 62 and 64 and the waveguide 66 can be adjusted. A thin film heater 72 is disposed on the waveguide 66, and the voltage controller 74 applies a voltage to the thin film heater 72. The optical path length of the arm 66, that is, the optical path length of the ring waveguide can be adjusted by the thin film heater 72. By adjusting the optical path length and coupling efficiency by the thin film heaters 68 and 72, the resonance frequency, period and group delay amount of the periodic group delay characteristic can be adjusted.

The group delay characteristics of the ring resonator as shown in FIGS. 2 and 3 are expressed by the following equation τ (ω) = T (1−K ² ) / {1 + K ² −2K cos (ωT−φ)}.
However,
K = (1-κ) ^1/2
κ = 0.5 + 0.5cosθ
Given in. ω is the optical frequency, T is the time required for the signal light to make one round of the ring waveguide, and φ is the phase shift amount (relative value) by the thin film heater 72. The free spectral region FSR (Free Spectral Region) is represented by 1 / T. θ is a phase shift amount (relative value) by the thin film heater 68. κ represents the coupling rate of the Mach-Zehnder interferometer 52.

  More complex group delay characteristics can be obtained by serially connecting a plurality of ring resonators configured as shown in FIG. FIG. 4 shows a configuration example of a nonlinear group delay optical element including three serially connected ring resonators. Three ring resonators 82, 84, and 86 are arranged along the linear input / output waveguide 80. The ring waveguides 82b, 84b, 86b of the ring resonators 82, 84, 86 are connected to the input / output waveguide 80 via Mach-Zehnder interferometer type optical couplers 82a, 84a, 86a having the same configuration as the Mach-Zehnder interferometer 52. Photocouple.

  Optical phase shifters 82c, 84c, 86c are arranged on the ring waveguides 82b, 84b, 86b. By setting the optical phase shift amounts φ1, φ2, and φ3 of the optical phase shifters 82c, 84c, and 86c to different values, the resonance frequencies of the ring resonators 82, 84, and 86 can be shifted. By adjusting the coupling coefficient of the optical couplers 82a, 84a, 86a, the group delay amount in each of the ring resonators 82, 84, 86 can be adjusted.

  The signal light input side of the input / output waveguide 80 becomes the input waveguide 80a, and the signal light output side becomes the output waveguide 80b.

  The total group delay characteristic of the nonlinear group delay optical element shown in FIG. 4 is equal to the sum of the group delay characteristics of the individual ring resonators. By shifting the resonance frequency of each ring resonator, a high-order group delay characteristic can be obtained.

  FIG. 5 shows the power spectrum of the pulse signal light output from the light intensity modulators 16a and 16b and the group delay characteristics of the nonlinear group delay encoders 18a and 18b configured as shown in FIG. The horizontal axis indicates the wavelength (nm), and the vertical axis indicates the light intensity of the pulse signal light output from the light intensity modulators 16a and 16b and the relative group delay of the nonlinear group delay encoders 18a and 18b. The upper part shows the total group delay characteristics of the nonlinear group delay encoders 18a, 18b, and the middle part shows the group delay characteristics of the individual ring resonances 82, 84, 86 constituting the nonlinear group delay encoders 18a, 18b. The power spectrum of the signal light is shown in the lower part.

  Here, the signal light is RZ modulated light having a center wavelength of 1550 nm, a bit rate of 10 Gbit / s, and a pulse width of 30 ps. The phase shift amounts of the three ring resonators 82, 84, 86 constituting the nonlinear group delay encoders 18a, 18b are 0, π / 2 (rad) and π (rad), respectively, and the optical couplers 82a, 84a, The optical coupling ratio of 86a is 0.85. The ring resonators 82, 84 and 86 have the same FSR, but their resonant frequencies are slightly shifted.

  In the calculation example shown in FIG. 5, the FSR of the nonlinear group delay encoders 18a and 18b is 8 GHz, which is 4/5 times the signal light bit rate of 10 Gbit / s. The resulting group delay is about 250 ps peak-to-peak. If the band of the optical signal of A (Gbit / s) is defined as 4 × A (GHz), the band of the signal light of 10 Gbit / s is 40 GHz. Therefore, the signal band includes a positive slope portion and a negative slope portion of the group delay characteristics of the nonlinear group delay encoders 18a and 18b, more specifically, a plurality of periods.

  In order to reduce the crosstalk between the data D1 and D2, it is preferable that the group delay characteristics given to the signal lights S1 and S2 by the encoders 16a and 16b have a more complicated shape.

  In this embodiment, it is possible to provide a nonlinear group delay having a complicated shape that varies within the signal band. On the receiving side, it is impossible or difficult to receive a signal unless a nonlinear group delay that flattens the nonlinear group delay applied on the transmitting side is given.

  Actually, the decoders 44a and 44b need only reduce the fluctuation range of the nonlinear group delay provided by the encoders 16a and 16b. For example, when the encoders 16a and 16b are composed of three ring resonators 82, 84 and 86 and exhibit nonlinear group delay characteristics as shown in the upper part of FIG. 5, the corresponding decoders 44a and 44b are arranged in the upper part of FIG. What is shown is a non-linear group delay characteristic having the same period that is a valley at the peak of the non-linear group delay characteristic shown, and a peak at the valley, for example, a single ring resonator. In this combination, the group delay characteristic after the decoder approaches flat. Since the degradation of the optical signal waveform depends on the magnitude of the wavelength differential of the group delay, the degradation of the optical signal is less if the group delay characteristic after the decoder is made flat.

  FIG. 6 shows a case where the encoder has the nonlinear group delay characteristic shown in the upper part of FIG. 5 and the decoder is composed of a single ring resonator, and the valley of the group delay characteristic of the encoder and the peak of the group delay characteristic of the decoder. Shows the group delay characteristics of the encoder and decoder and the group delay characteristics of their combination. At the same time, the power spectrum of the signal light is shown at the bottom. The horizontal axis represents wavelength, and the vertical axis represents relative group delay and light intensity. The top stage shows the group delay characteristic of the combination of the encoder and decoder, the second stage shows the group delay characteristic of the encoder, and the third stage shows the group delay characteristic of the decoder. When the fluctuation range of the combined group delay characteristics of the encoder and decoder becomes small as shown in FIG. 6, the receiving apparatus can receive the signal.

  FIG. 7 shows a case where the encoder has the nonlinear group delay characteristic shown in the upper part of FIG. 5 and the decoder is composed of a single ring resonator, and the peak of the group delay characteristic of the encoder and the peak of the group delay characteristic of the decoder. The group delay characteristics of the encoder and the decoder and the group delay characteristics of their combination are shown when. The horizontal axis indicates the wavelength, and the vertical axis indicates the relative group delay. The top stage shows the group delay characteristic of the combination of the encoder and decoder, the second stage shows the group delay characteristic of the encoder, and the third stage shows the group delay characteristic of the decoder. The fluctuation range of the combined group delay characteristic of the encoder and decoder is larger than that in the case of FIG. 6 and also larger than the group delay characteristic of the encoder. In this case, even after decoding, the waveform is greatly degraded and signals cannot be received.

  In the present embodiment, since the resonance frequency and period (or FSR) are added as parameters in addition to the group delay amount, it is difficult to decode compared to the conventional example in which only the group delay amount is different, and crosstalk can be reduced. It becomes easy to increase the number of users.

  In the nonlinear group delay optical element shown in FIGS. 2 and 4, the resonance frequency can be adjusted by adjusting the phase shifters 72, 82c, 84c, 86c on the ring waveguide. That is, it is possible to tune the non-linear group delay characteristic imparted to the signal light before transmission. By using the function that can easily change the nonlinear group delay characteristic, the transmission device can easily change the destination of the signal light, and the reception device can easily change the received signal light. That is, it becomes possible to flexibly cope with changes in the network configuration.

  The number of ring resonators constituting the nonlinear group delay encoder and the number of ring resonators constituting the nonlinear group delay decoder may be the same or different. The number of ring resonators may be four or more. Further, the coupling rate of the variable optical coupler of the ring resonator may be any coupling efficiency that can provide a group delay gradient enough to consider an optical signal of an unintended channel as noise. The value of can be taken. In addition, in a configuration in which a plurality of ring resonators are serially connected, ring resonators having different FSRs may be used, and even when the same FSR is provided, it is not necessary to arrange resonance frequencies at equal intervals.

  This embodiment is applicable to both RZ optical transmission and NRZ optical transmission. FIG. 8 shows a characteristic diagram in the case of NRZ optical transmission. The optical signal is NRZ modulated signal light having a center wavelength of 1549.841 nm and a bit rate of 10 Gbit / s. The nonlinear group delay encoder was a three-ring resonator, the nonlinear group delay decoder was a one-ring resonator, and the FSR of the ring resonator was 25 GHz. The peak-to-peak value of the group delay of the encoder and decoder was about 80 ps. The combined group delay characteristics of the encoder and decoder are shown at the top, the group delay characteristics of the encoder and decoder are shown at the middle, and the optical power spectrum of the signal light is shown at the bottom. The horizontal axis indicates the wavelength, and the vertical axis indicates the relative group delay of the encoder and decoder and the light intensity of the optical spectrum. In this example, the signal band is 10 (GHz) × 4 = 40 GHz. The FSR of the encoder and decoder is smaller than the signal band, and the group delay of the encoder varies within the signal band.

  In the combination of the encoder and decoder of the group delay characteristic shown in FIG. 8, the peak of the group delay characteristic of the decoder matches the valley of the group delay characteristic of the encoder, and the peak-to-peak value of the total group delay characteristic is sufficient. The wavelength differentiation is also small. Degradation of the signal light after decoding was within an allowable value, and the signal could be received.

  FIG. 9 is a characteristic diagram in the case where the peak of the group delay characteristic of the decoder is located near the peak of the group delay characteristic of the encoder while using the encoder and the decoder having the same signal light and the same group delay characteristic as in FIG. . The combined group delay characteristics of the encoder and decoder are shown at the top, the group delay characteristics of the encoder and decoder are shown at the middle, and the optical power spectrum of the signal light is shown at the bottom. The horizontal axis indicates the wavelength, and the vertical axis indicates the relative group delay of the encoder and decoder and the light intensity of the optical spectrum.

  In the combination of the encoder and decoder of the group delay characteristic shown in FIG. 9, since the peak of the group delay characteristic of the decoder substantially matches the peak of the group delay characteristic of the encoder, the fluctuation of the total group delay characteristic becomes large. The two peak value is about 100 ps. Degradation of the signal light after decoding exceeds the allowable range, and the signal cannot be received.

  For example, with respect to the nonlinear group delay encoder 18a, the group delay characteristic of the nonlinear group delay decoder 44a is changed to the characteristic shown in FIG. 8, and the group delay characteristic of the nonlinear group delay decoder 44b is changed to the characteristic shown in FIG. The device 48a can demodulate the data D1, and the data 48b cannot demodulate the data D1. For the data demodulator 48b, the component of the signal light S1 output from the decoder 44b becomes noise light.

  FIG. 10 shows the relationship between the received light intensity and the bit error rate (BER) in the case of FIGS. The horizontal axis represents the received light intensity, for example, the logarithmic value of BER. The characteristic before encoding indicates the BER value of the output signal light of the light intensity modulator 16a. The characteristic after encoding indicates the BER value of the output signal light of the encoder 18a. The corresponding characteristic after decoding indicates the BER value of the signal light after decoding for the combination of the encoder and decoder shown in FIG. The characteristic after decoding that does not correspond indicates the BER value of the signal light after decoding for the combination of the encoder and the decoder shown in FIG. From FIG. 10, it can be seen that the signal light transmitted by wavelength dispersion multiplexing transmission can be separated by appropriately combining the periodic group delay characteristics of the encoder and the decoder.

  The signal light before encoding exhibits an “error-free operation” in which the BER linearly decreases in the drawing with respect to the increase in received light intensity, whereas the signal light after encoding has a marked BER at the same received light intensity. It is getting worse, and the improvement of BER is slow as the received light intensity increases. When the decoder corresponding to the encoder is used, although the power penalty is about 3 dB, the BER shows an error-free operation, so that it can be seen that the waveform is recovered. It can be seen that when a decoder that does not correspond to an encoder with a large group delay wavelength differential value is used, the signal is greatly degraded even after decoding. As a result, data can be transmitted to an optical receiver having a desired decoder.

  FIG. 10 shows an example of the operation, and it is possible to return the reception penalty to zero (the same quality as the original transmission signal) by optimizing the elements and the operating conditions.

  FIG. 11 shows a schematic block diagram of the second embodiment of the present invention. In this embodiment, different signals can be transmitted from one optical transmitter to a plurality of optical receivers using wavelength division multiplexing.

  The optical transmission device 110 includes a laser diode 112 that is a light source. The optical demultiplexer 114 divides the output laser light of the laser diode 112 into four parts, and supplies the divided lights to the light intensity modulators 116a to 116d. Each of the light intensity modulators 116a to 116d modulates the intensity of the laser beam from the optical demultiplexer 114 according to the data D1 to D4, and the optical pulse signals S1 to S4 carrying the data D1 to D4 are nonlinear group delay encoders 118a to 118a. To 118d.

  The non-linear group delay encoders 118a to 118d are composed of non-linear group delay optical elements similar to the non-linear group delay encoders 18a and 18b. The non-linear group delay encoders 118a to 118d perform different non-linear group delays periodically varying within the band of the optical pulse signals S1 to S4. Give to S1-S4.

  The optical multiplexer 120 multiplexes the output lights of the nonlinear group delay encoders 118a to 118d. The multiplexed light by the optical multiplexer 120 is output light from the optical transmission device 110 and is applied to the optical fiber transmission line 130.

  The optical demultiplexer 140 divides the signal light propagated through the optical fiber transmission line 30 into four parts and supplies the divided lights to the optical receivers 142a to 142d.

  Each of the optical receivers 142a to 142d includes a non-linear group delay decoder 144a to 144b, a photodiode 146a to 146d, and a data demodulator 148a to 148d. The nonlinear group delay decoders 144a to 144b have nonlinear group delay characteristics that cancel or reduce the group delay applied to the signal light by the nonlinear group delay encoders 118a to 118d, respectively. For example, the nonlinear group delay decoder 144a has a nonlinear group delay characteristic that cancels the nonlinear group delay applied to the optical pulse signal S1 by the nonlinear group delay encoder 118a, and the nonlinear group delay decoder 144b includes the nonlinear group delay encoder 118b. It has a nonlinear group delay characteristic that cancels the nonlinear group delay applied to the optical pulse signal S2.

  The photodiodes 146a to 146d convert the output light of the nonlinear group delay decoders 144a to 144d into electric signals, and the data demodulators 148a to 148d demodulate the data D1 to D4 from the output electric signals of the photodiodes 146a to 146d.

  In this manner, in the embodiment shown in FIG. 11, the optical transmitter 110 can transmit the data D1 to D4 to the optical receivers 142a to 142d located at different locations, respectively. Since data can be received only when the group delay characteristics of the nonlinear group delay encoder and the nonlinear group delay decoder correspond to each other, the group delay characteristics of the nonlinear group delay encoders 118a to 118d and the nonlinear group delay decoders 144a to 144d are appropriately selected. Thus, it is possible to set a state in which the optical receiving devices 142a to 142d can selectively receive only the data D1 to D4, respectively.

  FIG. 12 shows a schematic block diagram of the third embodiment of the present invention. In this embodiment, a plurality of channels can be set between a plurality of optical transmitters and a plurality of optical receivers using wavelength division multiplexing.

  Each of the optical transmitters 210a to 210d includes laser diodes 212a to 212d, optical intensity modulators 216a to 216d, and nonlinear group delay encoders 218a to 218d. The light intensity modulators 216a to 216d respectively modulate the intensity of the laser light from the laser diodes 212a to 212d according to the data D1 to D4, and the optical pulse signals S1 to S4 carrying the data D1 to D4 are nonlinear group delay encoders 218a to 218a. Output to 218d.

  Similarly to the nonlinear group delay encoders 118a to 118d, the nonlinear group delay encoders 218a to 218d give the optical pulse signals S1 to S4 different nonlinear group delays that periodically vary within the band of the optical pulse signals S1 to S4. The output lights of the nonlinear group delay encoders 218 a to 218 d become the output signal lights of the optical transmitters 210 a to 210 d and are supplied to the optical multiplexer 220.

  The optical multiplexer 220 multiplexes the output signal lights of the optical transmission devices 210a to 210d. The multiplexed light from the optical multiplexer 220 propagates through the optical fiber transmission line 230 and is divided into four by the optical demultiplexer 240. Each split light by the optical demultiplexer 240 enters the optical receivers 242a to 242d.

  The configurations and functions of the optical receivers 242a to 242d are the same as the configurations and functions of the optical receivers 142a to 142d of the embodiment shown in FIG. Each of the optical receivers 242a to 242d includes a non-linear group delay decoder 244a to 244b, a photodiode 246a to 246d, and a data demodulator 248a to 248d. The nonlinear group delay decoders 244a to 244b have nonlinear group delay characteristics that cancel or reduce the group delay given to the signal light by the nonlinear group delay encoders 218a to 218d, respectively.

  The photodiodes 246a to 246d convert the output light of the nonlinear group delay decoders 244a to 244d into electric signals, and the data demodulators 248a to 248d demodulate the data D1 to D4 from the output electric signals of the photodiodes 246a to 246d.

  As the nonlinear group delay encoders 218a to 218d, by adopting optical elements that can easily change their nonlinear group delay characteristics, for example, optical elements composed of one or more ring resonators as described above, the optical transmission apparatuses 210a to 210d The destination of the optical signal can be easily changed. Conversely, as the non-linear group delay decoders 244a to 244d, optical elements that can easily change their non-linear group delay characteristics, for example, optical elements composed of one or more ring resonators as described above can be used to receive optical signals. The devices 242a to 242d can easily change the received optical signal. In this embodiment, such a feature can flexibly cope with a change in the network configuration.

The coupling rate of the variable optical coupler of the ring resonator used as the nonlinear group delay optical element may be any coupling efficiency that can provide an inclination of the group delay so that an optical signal of an unintended channel can be regarded as noise. Any value in the range of less than 1 can be taken. In addition, in a configuration in which a plurality of ring resonators are serially connected, ring resonators having different FSRs may be used, and even when the same FSR is provided, it is not necessary to arrange resonance frequencies at equal intervals.
(Other nonlinear group delay optical devices)

  Instead of a ring resonator, a Fabry-Perot optical resonator may be used. FIG. 13 is a schematic configuration diagram of a Fabry-Perot optical resonator. An x% reflector 312 is fixed on the fixed plate 310, and a 100% reflector 314 parallel to the reflector 312 is placed on the fixed plate 310 via a fine movement table 316. x is greater than 0 and less than 100. The distance L between the reflectors 312 and 314 is adjusted by the fine movement table 316. This configuration is a Fabry-Perot resonator having a resonator length of 2L. The optical path length of the ring waveguide of the ring resonator corresponds to 2L, the phase shifter of the ring resonator corresponds to the fine movement table 316 that adjusts the distance between the reflecting mirrors 312 and 314, and the coupling rate of the variable optical coupler is This corresponds to the transmittance (100−x) (%) of the x% reflecting mirror 312. The input / output characteristics of the Fabry-Perot resonator shown in FIG. 13 are equivalent to those of the ring resonator shown in FIG. Therefore, the Fabry-Perot optical resonator having the configuration shown in FIG. 13 can also realize nonlinear group delay characteristics that vary within the band.

A similar non-linear group delay characteristic can also be realized by using a Mach-Zehnder interferometer (MZI), which is a method of branching light waves into two and causing interference again. FIG. 14 shows a schematic block diagram of a nonlinear group delay optical element using two asymmetric MZIs. For example, an asymmetric optical delay 414 having two optical paths 414a and 414b having different optical path lengths between an asymmetric MZI 410 having an optical path length difference ΔL between arms and an asymmetric MZI 412 having an optical path length difference −ΔL between arms. Deploy. The phase difference Δφ caused by the optical path length difference ΔL of the asymmetric MZI 410 is
Δφ = 2πnΔL / λ
It is. Where n is the effective refractive index and λ is the wavelength of the light wave. As a result, there are alternately wavelengths λ at which Δφ is 0 or π. Therefore, the ratio of the light wave propagating through the optical paths 414a and 414b of the asymmetric optical delay device 414 varies depending on the wavelength. Since the asymmetric MZI 412 has a structure opposite to that of the asymmetric MZI 410, the lights from the optical paths 414a and 414b are combined and output from the output port. Thereby, it is possible to obtain a group delay characteristic in which the group delay increases or decreases periodically with respect to the wavelength.

  The nonlinear group delay characteristic in which the group delay is maximized (convex upward) at the resonance frequency has been described. However, it is also possible to use a nonlinear group delay characteristic in which the group delay is minimal (convex downward) at the resonance frequency. Therefore, an upward convex group delay characteristic may be used as an encoder, and a downward convex group delay characteristic may be used as a decoder, or vice versa.

  Although the invention has been described with reference to specific illustrative embodiments, various modifications and alterations may be made to the above-described embodiments without departing from the scope of the invention as defined in the claims. This is obvious to an engineer in the field to which the present invention belongs, and such changes and modifications are also included in the technical scope of the present invention.

1 shows a schematic block diagram of an embodiment of the present invention. The top view of an example of a nonlinear group delay optical element is shown. Sectional drawing seen from the AA line of FIG. 2 is shown. Shows a configuration example of a nonlinear group delay optical element composed of three serially connected ring resonators. Indicates the power spectrum of the pulse signal light output from the light intensity modulators 16a and 16b and the group delay characteristics of the nonlinear group delay encoders 18a and 18b. The group delay characteristics of the encoder and the decoder when the peak of the group delay characteristic of the encoder matches the valley of the group delay characteristic of the decoder, and the group delay characteristics of their combination are shown. The group delay characteristics of the encoder and the decoder when the peak of the group delay characteristic of the encoder and the peak of the group delay characteristic of the decoder are slightly shifted, and the group delay characteristics of their synthesis are shown. The characteristic figure in the case of NRZ optical transmission is shown. The characteristic diagram in the case where the peak of the group delay characteristic of the decoder is located near the peak of the group delay characteristic of the encoder is shown. The relationship between the received light intensity and the bit error rate (BER) in the case of FIGS. The schematic block diagram of 2nd Example of this invention is shown. The schematic block diagram of 3rd Example of this invention is shown. 1 is a schematic configuration diagram of a nonlinear group delay optical element including a Fabry-Perot optical resonator. FIG. 2 shows a schematic block diagram of a nonlinear group delay optical element using two asymmetric MZIs.

Explanation of symbols

10: Optical transmitter 12: Laser diode 14: Optical demultiplexers 16a, 16b: Optical intensity modulators 18a, 18b: Non-linear group delay encoder 20: Optical multiplexer 30: Optical fiber transmission line 40: Optical receiver 42: Light Demultiplexers 44a and 44b: Non-linear group delay decoders 46a and 46b: Photodiodes 48a and 48b: Data demodulator 50: Substrate 52: Mach-Zehnder interferometer 54, 56: Optical multiplexer / demultiplexer 58, 60: Arm 62: Input waveguide 64: output waveguide 66: waveguide 68: thin film heater 70: voltage controller 72: thin film heater 74: voltage controller 80: input / output waveguides 82, 84, 86: ring resonators 82a, 84a, 86a: Mach-Zehnder interference Meter type optical couplers 82b, 84b, 86b: Ring waveguides 82c, 84c, 86c: Phase shifter 110: Optical transmitter 112: Ray Diode 114: Optical demultiplexers 116a to 116d: Optical intensity modulators 118a to 118d: Nonlinear group delay encoder 120: Optical multiplexer 130: Optical fiber transmission line 140: Optical demultiplexers 142a to 142d: Optical receivers 144a to 144d : Nonlinear group delay decoders 146a to 146b: photodiodes 148a to 148d: data demodulators 210a to 210d: optical transmitters 212a to 212d: laser diodes 216a to 216d: optical intensity modulators 218a to 218d: nonlinear group delay encoder 220: optical coupling Demultiplexer 230: Optical fiber transmission line 240: Demultiplexers 242a to 242d: Optical receivers 244a to 244d: Non-linear group delay decoders 246a to 246b: Photodiodes 248a to 248d: Data demodulator 310: Surface plate 312: x% Reflector 314 100% reflector 316: fine movement table 410, 412: asymmetric MZI
414: Asymmetric optical delay devices 414a, 414b: optical paths having different optical path lengths

Claims (19)

In an optical transmission device, a group delay of a nonlinear group delay characteristic that fluctuates within a signal band is given to signal light,
An optical transmission method characterized in that in an optical receiver, chromatic dispersion is applied to received signal light to reduce group delay of the nonlinear group delay characteristic.   2. The optical transmission method according to claim 1, wherein the fluctuation frequency of the nonlinear group delay characteristic is smaller than the bandwidth of the signal band. Generating a first signal light carrying the first data;
Generating second signal light carrying second data;
A first distorted signal light is generated by giving the first signal light a group delay of a first nonlinear group delay characteristic that fluctuates within the signal band;
Nonlinear group delay characteristics that fluctuate within the signal band of the second signal light, and the first nonlinear group delay characteristic is a second distortion signal obtained by giving a group delay of the second nonlinear group delay characteristic. Produce light,
Combining the first and second distortion signal lights and outputting the combined light to an optical transmission line;
Splitting light from the optical transmission path into first and second component lights;
Providing the first component light with chromatic dispersion that reduces the group delay of the first nonlinear group delay characteristic to generate a first distortion-reduced signal light;
Converting the first distortion-reducing signal light into a first electric signal;
Demodulating the first data from the first electrical signal;
Providing the second component light with chromatic dispersion that reduces the group delay of the second nonlinear group delay characteristic to generate a second distortion-reduced signal light;
Converting the second distortion-reducing signal light into a second electrical signal;
2. An optical transmission method comprising demodulating second data from the second electric signal.   4. The optical transmission method according to claim 3, wherein the fluctuation frequency of the first and second nonlinear group delay characteristics is smaller than the bandwidth of the signal band. A first signal light generator for generating a first signal light carrying first data;
A second signal light generator for generating a second signal light for carrying second data;
A first nonlinear group delay encoder that generates a first distorted signal light by giving a group delay of a first nonlinear group delay characteristic that fluctuates within the signal band to the first signal light;
A second non-linear group delay characteristic that fluctuates within the signal band of the second signal light, which is different from the first non-linear group delay characteristic, is given a group delay of a second non-linear group delay characteristic. A second nonlinear group delay encoder that generates two distortion signal lights;
An optical multiplexer that combines the first and second distortion signal lights and outputs the combined light to an optical transmission line;
An optical demultiplexer that divides light from the optical transmission path into first and second component lights;
A first nonlinear group delay decoder that generates a first distortion-reduced signal light by giving chromatic dispersion to the first component light to reduce group delay of the first nonlinear group delay characteristic;
A first photoelectric converter that converts the first distortion-reducing signal light into a first electrical signal;
A first data demodulator that demodulates first data from the first electrical signal;
A second nonlinear group delay decoder that generates a second distortion-reduced signal light by applying chromatic dispersion that reduces the group delay of the second nonlinear group delay characteristic to the second component light;
A second photoelectric converter that converts the second distortion reduction signal light into a second electrical signal;
An optical transmission system comprising: a second data demodulator that demodulates second data from the second electrical signal.   6. The optical transmission system according to claim 5, wherein a fluctuation frequency of the first and second nonlinear group delay characteristics is smaller than a bandwidth of the signal band.   6. The optical transmission system according to claim 5, wherein the first nonlinear group delay encoder includes a nonlinear group delay optical element capable of changing a nonlinear group delay characteristic.   8. The optical transmission system according to claim 7, wherein the first nonlinear group delay decoder comprises a nonlinear group delay optical element capable of changing a nonlinear group delay characteristic.   6. The optical transmission system according to claim 5, wherein each of the first and second nonlinear group delay encoders comprises a nonlinear group delay optical element capable of changing a nonlinear group delay characteristic.   10. The optical transmission system according to claim 9, wherein each of the first and second nonlinear group delay decoders includes a nonlinear group delay optical element capable of changing a nonlinear group delay characteristic. A first signal light generator for generating a first signal light carrying first data;
And a first nonlinear group delay encoder that generates a first distorted signal light by giving the first signal light a group delay having a first nonlinear group delay characteristic that fluctuates within the signal band. An optical transmitter.   The optical transmission apparatus according to claim 11, wherein a fluctuation frequency of the first nonlinear group delay characteristic is smaller than a bandwidth of the signal band.   13. The optical transmission device according to claim 11, wherein the first nonlinear group delay encoder includes a nonlinear group delay optical element capable of changing a nonlinear group delay characteristic. Furthermore,
A second signal light generator for generating a second signal light for carrying second data;
A second nonlinear group delay characteristic that fluctuates within the signal band of the second signal light, and the first nonlinear group delay characteristic is a second nonlinear group delay characteristic that gives a group delay of the second nonlinear group delay characteristic. A second non-linear group delay encoder that generates a distorted signal light of
The optical transmission device according to claim 11, further comprising: an optical multiplexer that combines the first and second distortion signal lights.   15. The optical transmission device according to claim 14, wherein a fluctuation frequency of the first and second nonlinear group delay characteristics is smaller than a bandwidth of the signal band.   15. The optical transmission apparatus according to claim 14, wherein each of the first and second nonlinear group delay encoders includes a nonlinear group delay optical element capable of changing a nonlinear group delay characteristic. An optical receiver in an optical transmission system for transmitting a signal by wavelength dispersion division multiplexing,
A nonlinear group delay decoder that reduces the distortion of the signal light by giving the signal light from the optical transmission line a group delay of a nonlinear group delay characteristic that varies within the signal band;
A photoelectric converter that converts the output signal light of the nonlinear group delay decoder into an electrical signal;
An optical receiver comprising a data demodulator that demodulates data from the electrical signal.   The optical receiver according to claim 17, wherein the fluctuation frequency of the nonlinear group delay characteristic is smaller than the bandwidth of the signal band.   18. The optical receiver according to claim 17, wherein the nonlinear group delay decoder comprises a nonlinear group delay optical element capable of changing a nonlinear group delay characteristic.

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