JP2012039662A - Optical transmitter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To realize small-sized and highly stable cross-phase modulation (XPM) suppression means with increased number of channels.SOLUTION: An optical transmitter on a transmission side comprises: wavelength alternate division means for dividing a wavelength multiplex optical signal into two series of wavelength multiplex optical signals; delay time application means for applying delay time different between the two separated series of wavelength multiplex optical signals; an optical switch for switching an output port for each optical signal applied the delay time; wavelength alternate multiplexing means for alternately arranging channels of the two series of wavelength multiplex optical signals and multiplexing the arranged signals on a wavelength multiplex optical signal with a channel interval f; and wavelength multiplexing means for wavelength-multiplexing the optical signal switched to each output port. Letting ΔT as a basic delay time difference, the delay time application means sets ΔT for the delay time difference between wavelength multiplex optical signals to which the divided optical signals are adjacent.

Description

  The present invention relates to an optical transmission method and apparatus for transmitting wavelength division multiplexed optical signals.

  A configuration example of an optical transmission system that linearly relays wavelength-multiplexed signal light (WDM (Wavelength Division Multiplexing) signal light) using an optical amplifier will be described with reference to FIG. FIG. 12 is a diagram illustrating a configuration example of a conventional optical transmission system. In FIG. 12, reference numeral 1 is a transmission system, reference numeral 2 is a reception system, reference numeral 3 is a linear relay system, reference numeral 4 is an optical fiber, reference numeral 5 is an optical amplifier, and reference numeral 6 is XPM (Cross Phase Modulation) suppression means. .

  In a WDM optical transmission system, one of the factors that deteriorate transmission characteristics is cross-phase modulation (XPM) in an optical fiber that is a transmission path. This XPM is a phenomenon in which the phase of a certain channel is modulated depending on the intensity of another optical signal due to the third-order nonlinear optical effect of the optical fiber, and this phase modulation has a waveform via the chromatic dispersion of the optical fiber. Transmission characteristics are degraded due to distortion.

  Since the power of the optical signal is the highest when entering the transmission line such as the optical fiber 4 from the linear relay system 3, the transmission characteristic degradation due to XPM is greatest near the entrance end of the transmission line of each span. When chromatic dispersion compensation is performed in each span, the timing and intensity of other channels with respect to each channel are always the same at the transmission line entrance end, so the influence of XPM in which the phase modulation effect is accumulated is the worst condition.

  On the other hand, if the timing between the channels at the transmission line entrance end is different for each span and the strength of other channels with respect to each channel changes, the effect of XPM is suppressed because the phase modulation effect is averaged.

  Until now, as a method of suppressing this XPM, a method of setting a delay time difference between channels has been proposed. In FIG. 12, each linear relay system 3 is provided with XPM suppression means 6.

  FIG. 13 is a diagram illustrating a configuration of a conventional XPM suppression unit. Here, reference numeral 12 is fiber Bragg grating (FBG), and reference numeral 13 is an optical circulator. The FBG has a configuration in which the reflected wavelength differs depending on the position. By passing the WDM optical signal through the optical isolator and the FBG, a different delay time can be given for each channel.

  By providing this XPM suppression means 6 in each linear relay system, the timing between channels shifts at the transmission path entrance end of each span. As a result, the intensity of other channels with respect to each channel changes and the phase modulation is averaged, so that the influence of XPM is suppressed.

In the prior art, as shown in FIG. 14, the delay time difference ΔT between the first channel and the nth channel (n is an integer of 1 to N) is increased by one symbol time slot T ₀ ( T ₀ , 2T ₀ ,..., MT ₀ : m is an integer).

  If the time slot of one symbol is 100 ps (corresponding to a bit rate of 10 Gbit / s) and about 10 channels, the maximum delay time is 100 ps × 10 = 1 ns. When the FBG is used as the delay time providing means as in the reference, the length of the FBG is about 10 cm, which can be realized.

G. Bellotti et al. , “10 × 10 Gb / s cross-phase modulation suppressor for multispan transmissions using WDM narrow-band fiber Bragg grating”, IEEE Photonics Technol. 12, no. 10, pp. 1403-1405, 2000

  In the conventional technique described above, when the number of channels increases to 100 or more, the FBG length becomes 1 m or more, which is difficult to manufacture. Therefore, there is a problem that the number of wavelengths that can be transmitted by the optical transmission apparatus is limited to about 10.

  In addition, since the loss differs between the channel reflected by the grating near the incident end of the FBG and the channel reflected by the grating far from the incident end, the wavelength dependency of the loss increases as the number of channels increases. It was.

  Conventionally, since the XPM suppression means 6 has a large apparatus scale as described above, only a configuration in which the XPM suppression means 6 is arranged separately from other means (for example, the optical amplifiers 5 of each linear relay system in FIG. 12) has been proposed. There is a problem that it cannot be incorporated into other means.

  The present invention has been made to solve such a problem, and can dramatically increase the number of channels from about 10 to more than 100, and can be integrated with an optical integrated circuit such as a PLC (Planer Light Wave Circuit). Since it is possible to realize a compact and highly stable XPM suppression means, and because the circuit scale is reduced, it can be installed inside other means (wavelength selective switch, wavelength cross connect, etc.). In addition, an object of the present invention is to provide an optical transmission method and apparatus in which other means can be reduced in size and cost.

The present invention has one input port and a plurality of output ports, inputs a wavelength multiplexed optical signal obtained by multiplexing optical signals of different N wavelengths, where N is an integer of 2 or more, and outputs an output port for each wavelength. In the optical transmission apparatus that selects and outputs the wavelength division means for separating the wavelength-multiplexed optical signal and the channel of the wavelength-division multiplexed optical signal with the channel spacing f ₀ (Hz), the channel spacing 2f _{0 is} doubled. (Hz) Wavelength alternating separation means for separating into two series of wavelength multiplexed optical signals, delay time giving means for giving different delay times between the two series of wavelength multiplexed optical signals separated, and each light given delay time An optical switch that switches the signal output port and two wavelength-multiplexed optical signal channels with a channel interval of 2f ₀ (Hz) are alternately arranged to multiplex the wavelength-multiplexed optical signal with a channel interval of f ₀ (Hz). Multiple hands And wavelength multiplexing means for wavelength multiplexing the optical signal switched to each output port, and the delay time providing means is configured such that the basic delay time difference is ΔT, and the adjacent wavelength multiplexed optical signals of the separated optical signals are adjacent to each other. The delay time difference is set to ΔT.

  When the optical signal is RZ modulated, it is desirable to set the basic delay time difference ΔT so that the intensity peaks of the optical signals between adjacent channels do not overlap.

When the optical signal is NRZ modulated, it is desirable that m is a natural number, T ₀ is a time slot per symbol, and the basic delay time difference ΔT is mT ₀ .

  According to the present invention, the number of channels, which has been a problem in the prior art, can be dramatically increased from about 10 to 100 or more.

  Further, since optical integration technology such as PLC (Planer Light wave Circuit) can be applied, a small and highly stable XPM suppression means can be realized.

  Furthermore, since the circuit scale is reduced, it can be incorporated in other means (wavelength selective switch, wavelength cross connect, etc.). At that time, the other means can be reduced in size and cost.

It is a figure which shows the delay time in 1st embodiment. It is a figure which shows the relationship between a channel period and Q value improvement amount. It is a figure which shows the structural example of the XPM suppression means of 1st embodiment. k = is a diagram showing the relationship between the delay time mT ₀ and Q value improvement amount in the case of RZ format 2 when. It is a figure which shows the structural example of the XPM suppression means of 2nd embodiment. It is a figure which shows the conventional wavelength selective switch. It is a figure which shows the wavelength selective switch of 3rd embodiment. It is a figure which shows the conventional wavelength selective switch. It is a figure which shows the wavelength selective switch of 4th embodiment. It is a figure which shows the conventional optical cross-connect apparatus. It is a figure which shows the optical cross-connect apparatus of 5th embodiment. It is a figure which shows the structural example of the conventional optical transmission system. It is a figure which shows the structure of the conventional XPM suppression means. It is a figure which shows the conventional delay time.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
(First embodiment)
The delay time in the first embodiment of the present invention is shown in FIG. In FIG. 1, the horizontal axis represents the channel (wavelength), and the vertical axis represents the delay time. In the first embodiment, assuming that the basic delay time difference is ΔT (sec), the first optical channel adjacent to the separated optical signal and the nth channel (n is an integer of 1 to N) are adjacent to each other. Delay time increased by ΔT, 2ΔT, 3ΔT, ..., kΔT, 0, ΔT, 2ΔT, 3ΔT, ..., kΔT, 0, ΔT, ... (k is an integer greater than or equal to 1 and less than or equal to N / 2) Set to repeat. FIG. 1 shows a case where k = 3.

  FIG. 2 shows the relationship between the channel period k and the XPM improvement effect. In FIG. 2, the horizontal axis represents the channel period k, and the vertical axis represents the Q value improvement amount. Here, a WDM signal obtained by multiplexing an optical signal of a modulation format RZ-DQPSK with a bit rate of 40 Gbit / s with a channel spacing of 50 GHz and transmitted at 80 GHz with a power of −2 dBm and a transmission path of 80 km × 6 spans with a wavelength dispersion of 1 ps / nm / km is transmitted. Is the calculation result of If the channel period k is 2 or more, the Q value can be improved by suppressing the XPM.

  FIG. 3 is a configuration example of the XPM suppression unit of the first embodiment. Reference numeral 7 denotes wavelength separation means, reference numeral 8 denotes delay time giving means, and reference numeral 9 denotes wavelength multiplexing means. Different delays between the first channel and the nth channel (n is an integer from 1 to N) of the optical signal separated by the delay time providing unit 8 after the wavelength division unit 7 separates the wavelength multiplexed optical signal. The optical signal given the delay time is wavelength-multiplexed by the wavelength multiplexing means 9 given time.

  At this time, in the delay time providing means 8, the delay time difference between the first channel and the nth channel (n is an integer not less than 1 and not more than N) of the separated optical signal is expressed as ΔT, 2ΔT, 3ΔT,. As shown in FIG. 1, kΔT, 0, ΔT, 2ΔT, 3ΔT,..., kΔT, 0, ΔT,... (k is an integer not less than 2 and not more than N / 2) are set to repeat increasing for each k channel. Delay time can be realized. Although FIG. 3 shows the case of k = 3, other periods may be set.

  An arrayed waveguide grating can be used as the wavelength separation means 7 and the wavelength multiplexing means 9. By integrating the waveguides of the wavelength demultiplexing means 7, the wavelength multiplexing means 9, and the delay time providing means 8 on a planar lightwave circuit (PLC), a small XPM suppression means can be realized.

FIG. 4 shows the relationship between the delay time ΔT and the Q value in the RZ (Return to Zero) format when k = 2. In FIG. 4, the horizontal axis represents the delay time, and the vertical axis represents the Q value improvement amount. When the intensity peaks of optical signals between adjacent channels overlap (ΔT = mT ₀ ), the Q value improvement amount is relatively small. Therefore, in the RZ format, it is effective to set the delay time ΔT so that the intensity peaks of the optical signals between adjacent channels do not overlap. On the other hand, in the case of NRZ (Non Return to Zero) format, if mT ₀ is used, the XPM suppression effect is large.

(Second embodiment)
A second embodiment of the present invention will be described with reference to FIG. FIG. 5 is a diagram illustrating a configuration example of the XPM suppression unit according to the second embodiment. Reference numeral 10 denotes a wavelength alternating separation means, and reference numeral 11 denotes a wavelength alternating multiplexing means.

The wavelength alternate separation means 10 divides the channels of the wavelength multiplexed optical signal alternately and separates them into two series (A, B) wavelength multiplexed optical signals having double channel spacing (for example, f ₀ (Hz) → 2f ₀ ( Hz)). The wavelength alternately separating means arranged to form M-stage tree, the wavelength-multiplexed optical signal of the 2 ^M-sequence channel spacing of 2 ^M times the wavelength-multiplexed optical signal 2 ^M f ₀ (Hz) of the channel spacing f ₀ (Hz) To separate.

Different delay times are given between the 2 ^M- sequence wavelength-multiplexed optical signals separated by the delay time assigning means 8. The wavelength alternating multiplexing means 11 alternately multiplexes the channels of two series of wavelength multiplexed optical signals having the same channel spacing and multiplexes them into a wavelength multiplexed optical signal having a half channel spacing (for example, 2f ₀ (Hz) → f ₀ ( Hz)). The wavelength alternating multiplexing means 11 is arranged in M stages to multiplex a 2 ^M- series wavelength multiplexed optical signal with a channel interval of 2 ^M (Hz) into a wavelength multiplexed optical signal with a channel interval of f ₀ (Hz).

In the delay time providing means 8, the delay time difference between the first wavelength multiplexed optical signal of the separated optical signal and the nth (n is an integer of 1 to N) wavelength delay optical signals is expressed as ΔT, 2ΔT, ^{3ΔT, ..., (2 M -1} ) by setting the [Delta] T, [Delta] T the delay time difference for each channel period M, 2ΔT, 3ΔT, ..., (2 M -1) it is possible to [Delta] T, the first As in the embodiment, XPM can be suppressed. Although FIG. 5 shows the case of M = 2, other stages may be used.

  As the wavelength alternate separation means 10 and the wavelength alternate multiplexing means 11, an arrayed waveguide grating can be used. By integrating the wavelength alternate separation means 10, the wavelength alternate multiplexing means 11 and the delay time providing means 8 on the planar lightwave circuit (PLC), a small XPM suppression means can be realized.

The basic delay time ΔT is set so that the intensity peaks of the optical signals between adjacent channels do not overlap in the case of the RZ format and mT _{0 in} the case of the NRZ format. Also good.

(Third embodiment: wavelength selective switch having XPM suppression means)
A third embodiment of the present invention will be described with reference to FIGS. 6 and 7 show a conventional wavelength selective switch and the wavelength selective switch of the third embodiment, respectively. Here, reference numeral 14 denotes a 1 × N optical switch.

  The wavelength selective switch according to the third embodiment includes wavelength separation means 7, 1 × N optical switch 14, and wavelength multiplexing means 9-1 to 9-4, and includes one input port and a plurality of output ports. Have. After the wavelength multiplexed optical signal is separated by the wavelength separation means 7, the output port of each optical signal can be switched by the 1 × N optical switch 14. The switched optical signal is output from each output port wavelength-multiplexed by the wavelength multiplexing means 9-1 to 9-4.

  In the conventional wavelength selective switch, the settings regarding the optical path length of each optical signal are set to be almost the same, so there is almost no delay time difference between the optical signals, and the influence of XPM cannot be reduced.

  On the other hand, the wavelength selective switch of the third embodiment is provided with a delay time giving means 8 for giving different delay times between the channels of the separated optical signals. If the basic delay time difference is ΔT (sec) in the delay time giving means 8, the delay time difference between the first channel and the nth channel (n is an integer from 1 to N) of the separated optical signal is ΔT. 2ΔT, 3ΔT,..., KΔT, 0, ΔT, 2ΔT, 3ΔT,..., KΔT, 0, ΔT,... (K is an integer of 1 or more and N / 2 or less) By doing so, the delay time as shown in FIG. 1 can be realized as in the first and second embodiments. Therefore, the influence of XPM can be suppressed by using the wavelength selective switch of the third embodiment. In FIG. 7, k = 3, but other periods may be set.

  By integrating the wavelength separation means 7, the 1 × N optical switch 14, the wavelength multiplexing means 9-1 to 9-4 and the delay time giving means 8 on the planar lightwave circuit (PLC), a compact size can be obtained. A wavelength selective switch having an XPM suppression effect can be realized.

(Fourth embodiment: wavelength selective switch including XPM suppression means)
A fourth embodiment of the present invention will be described with reference to FIGS. 8 and 9 show a conventional wavelength selective switch and a wavelength selective switch according to the fourth embodiment, respectively. Here, reference numeral 20 denotes a diffraction grating, reference numerals 21, 21-1 and 21-2 denote mirrors, and reference numeral 22 denotes a spatial wavelength alternating separation means.

  The conventional wavelength selective switch shown in FIG. 8 serves as the diffraction grating 20 and the 1 × N optical switch 14 that serve as the wavelength separation unit 7 and the wavelength multiplexing units 9-1 to 9-4 in the third embodiment. The mirror 21 is composed of one input port and a plurality of output ports. After the wavelength multiplexed optical signal is separated by the diffraction grating 20, the output port of each optical signal is switched by the mirror 21. The switched optical signal is output again from each output port wavelength-multiplexed by the diffraction grating 20.

  In the conventional wavelength selective switch shown in FIG. 8, since the settings for the optical path length of each optical signal are set to be almost the same, there is almost no delay time difference between the optical signals, and the influence of XPM is reduced. I can't.

On the other hand, the wavelength selective switch of the fourth embodiment shown in FIG. 9 has the diffraction grating 20 that functions as the wavelength separation means 7 and mirrors 21-1 and 21-2 that function as the 1 × N optical switch 14. The wavelength-multiplexed optical signals with channel spacing f ₀ (Hz) are alternately divided into two series of wavelength-multiplexed optical signals (even channels and odd channels) with double channel spacing 2f ₀ (Hz). Spatial wavelength alternating separation means 22, an even channel mirror 21-2 and an odd channel mirror 21-1 are added. Further, a difference is provided in the distance between the spatial wavelength alternate separation means 22 and the even channel mirror 21-2 and the distance between the spatial wavelength alternate separation means 22 and the odd channel mirror 21-1. Therefore, different delay times are given between the even channel and the odd channel.

That is, the delay time generated between the odd-numbered channel mirror 21-2 and the spatial wavelength alternating separation means 22 is t 1, and occurs between the even-numbered channel mirror 21-1 and the spatial wavelength alternating separation means 22. When the delay time is t2, the delay time difference ΔT is
| T1-t2 | = ΔT
It becomes.

The output signals of the even-numbered channel and odd-numbered channel optical signals given the delay times are switched by the mirrors 21-1 and 21-2, respectively, and again input to the spatial wavelength alternate separation means 22 to alternately arrange the channels. It is multiplexed onto a wavelength-multiplexed optical signal having a channel interval of f ₀ (Hz). Thereafter, the optical signal is again wavelength-multiplexed by the diffraction grating 20 and emitted from each output port.

  Thereby, the delay time difference ΔT (sec) between the even channel and the odd channel can be set. Therefore, the influence of XPM can be suppressed by using the wavelength selective switch of the fourth embodiment.

  As the 1 × N optical switch, a liquid crystal type mirror or a MEMS type mirror can be used.

  Also in the fourth embodiment, since the delay time difference can be set only by adjusting the distance between the spatial wavelength alternate separation means 22 and the mirrors 21-1 and 21-2, a small wavelength selective switch having an XPM suppression effect is provided. Can be realized.

(Fifth embodiment: optical cross-connect device provided with XPM suppression means)
A fifth embodiment of the present invention will be described with reference to FIGS. 10 and 11. 10 and 11 show a conventional optical cross-connect device and an optical cross-connect device according to the fifth embodiment, respectively. Here, reference numerals 15-1 to 15-6, 15-11 to 15-16, and 15-21 to 15-26 denote wavelength selective switches, reference numeral 17 denotes XPM suppression means, and reference numerals 18, 18-1, and 18-2 denote light. It is a cross-connect device. In FIG. 11, the wavelength selective switches 15-22 to 15-24 are hidden behind the optical cross-connect device 18-1 and cannot be seen.

  The optical cross-connect devices 18, 18-1, and 18-2 include a plurality of wavelength selective switches (WSS) 15-1 to 15-6, 15-11 to 15-16, and 15-21 to 15-. 26, and has a plurality of input ports and a plurality of output ports. The wavelength selection switches 15-1 to 15-3, 15-11 to 15-13, and 15-21 to 15-23 in the previous stage are used to switch the output port of the optical signal for each wavelength, and the wavelength selection switches 15-4 to 15 in the subsequent stage are switched. The optical signals switched by 15-6, 15-14 to 15-16, and 15-24 to 15-26 are wavelength-multiplexed and output from each output port.

  In the conventional optical cross-connect device 18 shown in FIG. 10, since the settings relating to the optical path length of each optical signal are set to be almost the same, there is almost no delay time difference between the optical signals and the influence of XPM is reduced. I can't do it.

In contrast, a fifth embodiment of an optical cross connect apparatus 18-1 and 18-2 shown in FIG. 11, first, the channel of the wavelength division multiplexed optical signal channel spacing f ₀ (Hz) by the wavelength alternately separating means 10 Are alternately divided into two series of wavelength multiplexed optical signals (even channel and odd channel) with a channel spacing of 2f ₀ (Hz) twice, and between the two wavelength multiplexed optical signals separated by the delay time providing means 8 Give different delay times.

Next, the even-numbered channel optical cross-connect device 18-2 and the odd-numbered channel optical cross-connect device 18-1 select and output an output port for each wavelength. After that, the even-numbered channels and the odd-numbered channels are alternately arranged by the wavelength alternate multiplexing means 11 and multiplexed on the wavelength-multiplexed optical signal having the channel interval f ₀ (Hz).

  In this way, the delay time providing means 8 can set the delay time difference ΔT (sec) between the even channel and the odd channel. Therefore, the influence of XPM can be suppressed by using the optical cross-connect devices 18-1 and 18-2 of the fifth embodiment.

  In the fifth embodiment, the wavelength selective switches 15-11 to 15-16 and 15-21 to 15-26 are used as the optical switches in the optical cross-connect devices 18-1 and 18-2. A switch or an optical cross-connect device having a configuration other than that shown in FIG. 11 can also be applied.

In the fifth embodiment, two optical switches are required, but an optical cross-connect device for 2f ₀ (Hz) can be used instead of an optical cross-connect device for channel spacing f ₀ (Hz). Therefore, it is possible to realize an optical cross-connect device that has excellent filter transmission characteristics, is easy to manufacture, and has an XPM suppression effect.

  According to the present invention, the present invention can be used to realize a small and highly stable XPM suppressing means. In addition, an apparatus incorporating the XPM suppression means of the present invention can be provided in a small size and at a low cost.

DESCRIPTION OF SYMBOLS 1 Transmission system 2 Reception system 3 Linear relay system 4 Optical fiber 5 Optical amplifier 6, 17 XPM suppression means 7 Wavelength separation means 8 Delay time provision means 9, 9-1 to 9-4 Wavelength multiplexing means 10 Wavelength alternating separation means 11 Wavelength Alternate multiplex means 12 Fiber Bragg grating 13 Optical circulator 14 1 × N optical switches 15-1 to 15-6, 15-11 to 15-16, 15-21 to 15-26 Wavelength selective switches 18, 18-1, 18 -2 Optical cross-connect device 20 Diffraction gratings 21, 21-1, 21-2 Mirror 22 Spatial wavelength alternating separation means

Claims (3)

It has one input port and a plurality of output ports, N is an integer of 2 or more, a wavelength multiplexed optical signal in which optical signals of different N wavelengths are multiplexed, and an output port is selected for each wavelength. In the optical transmission device that outputs,
Wavelength separation means for separating the wavelength multiplexed optical signal;
Wavelength alternating separation means for alternately dividing the channels of the wavelength multiplexed optical signal with channel spacing f ₀ (Hz) and separating it into two series of wavelength multiplexed optical signals with double channel spacing 2f ₀ (Hz);
Delay time giving means for giving different delay times between two separated wavelength-multiplexed optical signals;
An optical switch that switches the output port of each optical signal given a delay time;
Wavelength alternately multiplexing means for multiplexing the wavelength-multiplexed optical signal of the channel spacing 2f ₀ (Hz) of the two series of wavelength division multiplexed optical signal channels by placing the channel on alternating spaces f ₀ (Hz),
Wavelength multiplexing means for wavelength-multiplexing the optical signal switched to each output port, and
The delay time giving means is
If the basic delay time difference is ΔT,
A delay time difference between adjacent wavelength-division multiplexed optical signals of the separated optical signal is set as ΔT. When the optical signal is RZ modulation,
The optical transmission apparatus according to claim 1, wherein the basic delay time difference ΔT is set so that intensity peaks of optical signals between adjacent channels do not overlap. When the optical signal is NRZ modulated,
The optical transmission apparatus according to claim 1, wherein m is a natural number, T ₀ is a time slot per symbol, and the basic delay time difference ΔT is mT ₀ .

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