JP4130342B2 - Optical signal processing device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to interference between channels in wavelength division multiplexing (WDM) transmission suppression possible optical signal processing equipment.
[0002]
[Prior art]
In a wavelength division multiplexing optical transmission system, it is necessary to multiplex wavelengths with high density in order to effectively use a band. However, if the frequency interval between channels is narrowed, interference between channels increases and signal quality deteriorates.
[0003]
As a means for avoiding such interference, a configuration using a polarization multiplexing system in which the polarizations of adjacent channels are orthogonal to each other (for example, T. Ito et al., “6.4 Tb / s (160 × 40 Gb / s) WDM) transmission transmission with 0.8 bit / s / Hz spectral efficiency ”, 26th European Conferencing on Optical Communication (ECOC 2000), narrower band input to each receiver (1.1), and postdeadline paper 1.1 For example, S. Bigo et al., “5.12 Tbit / s (128 × 40 Gbit / s WDM) transition over 3 × 100 km of Teralign ht ™ fiber ”, 26th European Conference Optical Communication (ECOC 2000), postdeadline paper 1.2). In the latter paper, in order to suppress interference between adjacent channels, it is described that the wavelength arrangement is unequal.
[0004]
The method of narrowing the band of the optical transmission signal is also known as single sideband (SSB) transmission or residual sideband (VSB) transmission, as disclosed in US Pat. No. 6,088,147 and US Pat. No. -213830), U.S. Pat. No. 6,262,834, and Japanese Patent Laid-Open No. 2001-264710 (or U.S. Publication No. 2002075546).
[0005]
[Problems to be solved by the invention]
In the polarization multiplexing method, a circuit for tracking the polarization is required on the receiving side, and the configuration of the receiving apparatus becomes complicated. Further, when the transmission distance is increased, the orthogonal state of polarization between adjacent channels is not maintained. That is, the polarization multiplexing system is difficult to apply to long-distance transmission.
[0006]
In the method of limiting the band of the optical transmission signal, if the band is limited too much, the signal quality deteriorates, and eventually the transmission characteristics deteriorate. That is, there is a limit to the increase in density due to the band limitation.
[0007]
The present invention, even without the orthogonal polarization between adjacent channels, and an object thereof is to provide an optical signal processing equipment that interference can be effectively suppressed between channels.
[0008]
[Means for Solving the Problems]
An optical signal processing device according to the present invention includes a wavelength separation filter that extracts an optical signal having a predetermined wavelength from wavelength division multiplexed signal light including optical signals having a plurality of wavelengths, and light having a predetermined wavelength extracted by the wavelength separation filter. A chromatic dispersion medium that extends the pulse waveform of the signal on the time axis according to its wavelength, a clock extraction device that extracts a clock from the input light or output light of the chromatic dispersion medium, and chromatic dispersion according to the clock extracted by the clock extraction device And an optical gate device for extracting a time slot at the center of the pulse from the output light of the medium.
[0009]
In such a configuration, the wavelength dispersion medium moves the light components of other wavelengths mixed in the optical signal of the predetermined wavelength separated by the separation device forward and backward on the time axis. The optical gate device extracts a predetermined time slot portion, typically the central portion, from the optical signal output from the chromatic dispersion medium. As a result, coherent components due to crosstalk from other wavelengths can be suppressed, and reception characteristics of signals carried by optical signals of a predetermined wavelength can be improved and stabilized. Even when the wavelength interval is narrowed so that the spectrum overlaps between wavelengths in WDM transmission, each signal can be received satisfactorily. That is, the wavelength interval can be narrowed.
[0019]
【Example】
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
[0020]
FIG. 1 shows a schematic block diagram of an embodiment of the present invention applied to a 40 Gb / s, n-wavelength WDM transmission system.
[0021]
The transmitting terminal station 10 outputs to the optical fiber transmission line 12 wavelength division multiplexed signal light (WDM signal light) composed of signal light of n wavelengths (λ1 to λn) each having 40 Gb / s. FIG. 2 shows an example of the optical spectrum of the WDM signal light. The horizontal axis indicates the wavelength, and the vertical axis indicates the light intensity. The WDM signal light propagates through the optical fiber transmission line 12 and enters the receiving terminal station 14.
[0022]
In the receiving terminal station 14, the WDM signal light is processed as follows. That is, the optical WDM separation device 20 composed of a wavelength separation element such as an arrayed waveguide grating (AWG) separates the signal light of each wavelength λ1 to λn. FIG. 3 shows the relationship between the filter characteristic of wavelength λ2 and the optical spectrum of transmission signals of wavelengths λ1, λ2, and λ3 in the optical WDM demultiplexer 20. In FIG. 3, the optical filter characteristic for separating the wavelength λ2 is indicated by a wavy line. Usually, the optical filter characteristic is set so as to extract the optical spectrum component of the target signal light. In the illustrated example, since both sidebands are transmitted, the optical filter characteristics are set so as to extract a range including the optical carrier frequency, the upper sideband, and the lower sideband. The filter characteristics of other wavelengths are the same as those shown in FIG. 3 except for the difference in the center wavelength.
[0023]
In FIG. 3, the shaded portion, specifically, the optical spectrum portion of the optical signal having the wavelengths λ1 and λ3 that has entered the optical filter characteristics for extracting the wavelength λ2, is crosstalk with respect to the wavelength λ2, that is, optical Thus, it becomes a coherent part and becomes noise light with respect to the signal light of wavelength λ2. Since it is coherent, the level of this noise light varies greatly depending on the transmission state. This makes it difficult to receive signal light of wavelength λ2. Conventionally, as shown in FIGS. 2 and 3, the wavelength interval is set so that the optical spectra do not overlap between adjacent wavelengths. In the present embodiment, since this overlapping portion can be effectively suppressed as will be described later, there is no problem even if there is an overlap between adjacent wavelengths as shown in FIGS.
[0024]
The signal light having the wavelength λ 1 separated by the optical WDM separator 20 is input to the optical TDM separator 22. The optical TDM demultiplexer 22 demultiplexes the 40 Gb / s signal light having the wavelength λ1 from the optical WDM demultiplexer 20 into four 10 G / s signal lights, and applies them to the interference suppression devices 24 a, 24 b, 24 c, and 24 d. To do. Since the interference suppression devices 24a, 24b, 24c, and 24d have the same configuration, only the internal configuration of the interference suppression device 24a is illustrated.
[0025]
In the interference suppression device 24 a, 10 Gb / s signal light from the optical TDM separation device 22 is input to the wavelength dispersion medium 30. The wavelength dispersion medium 30 acts to expand the pulse waveform of incident light on the time axis according to the wavelength. The dispersion amount of the chromatic dispersion medium 30 only needs to be large enough not to overlap with the light pulses in the adjacent time slots. As in this embodiment, when 40 G / s is down to 10 Gb / s, the dispersion amount of the chromatic dispersion medium 30 may be about 100 ps / nm. If there is no problem with clock extraction by the clock extraction device 34 to be described later, the dispersion amount of the chromatic dispersion medium 30 may be so large that it overlaps with the optical pulse of the adjacent time slot.
[0026]
FIG. 4 shows an example of chromatic dispersion characteristics of the chromatic dispersion medium 30. The chromatic dispersion medium 30 is made of, for example, an optical fiber, and the chromatic dispersion characteristic of the optical fiber generally varies in a parabolic manner with respect to the wavelength (frequency). The horizontal axis represents frequency and the vertical axis represents group delay. The greater the group delay, the slower the light, and the smaller the group delay, the faster the light propagates. In the present embodiment, a location where a change in dispersion with respect to wavelength (frequency) is large is used. For example, as shown in FIG. 5, suppose that f0 is the optical carrier frequency of wavelength λ1, f2 <f0 <f1, and as shown in FIG. 4, the wavelength that increases the group delay in the order of f2, f0, and f1. It is assumed that a dispersion medium is used as the wavelength dispersion medium 30.
[0027]
FIG. 6 is a schematic waveform diagram of the input light pulse and the output light pulse of the wavelength dispersion medium 30 under the conditions as shown in FIG. Since the accumulated chromatic dispersion of the optical fiber transmission line 12 is adjusted to zero or almost zero, the components of the frequencies f0, f1, and f2 are evenly distributed in the optical pulse in the input pulse of the chromatic dispersion medium 30. When this optical pulse is input to the wavelength dispersion medium 30, the component of the frequency f2 propagates first with respect to the component of the frequency f0, and the component of the frequency f1 is delayed. Therefore, as shown in FIG. 6 as an output pulse, the waveform of the optical pulse on the time axis is flattened as a whole, but there is a component of frequency f0 at the center of the optical pulse on the time axis, and the preceding part. The components of the frequency f2 are gathered in the first and the components of the frequency f1 are gathered in the subsequent portion. This means that parts that may interfere with crosstalk from adjacent wavelengths are collected before and after the light pulse. That is, the optical power of the leading part and the trailing part of the output pulse shown in FIG. 6 is likely to fluctuate due to the occurrence of interference, but the optical power of the central part is stabilized in a state according to the signal value to be carried.
[0028]
The demultiplexer 32 divides the output light of the wavelength dispersion medium 30 into two, and applies one to the clock extraction device 34 and the other to the optical gate device 36. The clock extraction device 34 extracts a clock from the optical pulse signal from the duplexer 32 and controls the gate operation of the optical gate device 36 with the extracted clock. The optical gate device 36 transmits the central portion of the optical pulse signal from the demultiplexer 32, that is, about 20 ps of the portion of the pulse waveform frequency f0 shown as the output pulse in FIG. 6 according to the clock from the clock extracting device 34. To do. As a result, a portion having a high SNR (signal-to-noise ratio) that stably conveys the signal can be extracted. The output light of the optical gate device 36 is applied to the optical receiving device 38a as the output light of the interference suppression device 24a.
[0029]
The optical receiving device 38a regenerates a signal from the output light of the interference suppressing device 24a by a known method. Since the unstable portion due to interference with other wavelengths is suppressed by the interference suppression device 24a, signal discrimination processing is facilitated.
[0030]
The interference suppression devices 24b to 24d operate in the same manner as the interference suppression device 24a. The operations of the optical receiving devices 38b to 38d are the same as the operation of the optical resin device 38a.
[0031]
In this way, in this embodiment, even if the optical spectrum overlaps between adjacent wavelengths and there is a possibility of partial interference, the filter characteristics of wavelength separation and the optical spectrum components of the adjacent wavelengths are further extracted. Even if it is as wide as possible, the coherent portion can be suppressed by the interference suppression devices 24a to 24d, so that the reception performance of the transmission signal can be improved. In other words, the wavelength interval can be narrowed.
[0032]
Although omitted in FIG. 1, the other wavelengths λ2 to λn are also processed by the processing system similar to the processing system of wavelength λ1.
[0033]
For example, signal light obtained by limiting a carrier suppression RZ signal (CS-RZ signal) of 42.7 Gb / s to a bandwidth of 35 GHz is multiplexed at an interval of 40 GHz and is incident on the optical fiber transmission line 12. In this case, good reception performance is achieved by setting the half-width of the separation optical filter in the optical WDM separation apparatus 20 to 45 GHz, the chromatic dispersion of the chromatic dispersion medium 30 to 100 ps / nm, and the gate width of the optical gate apparatus 36 to 20 ps. did it.
[0034]
In the embodiment shown in FIG. 1, the clock extraction device 34 extracts the clock from the output light pulse of the wavelength dispersion medium 30, but extracts the clock from the output light of the optical TDM separation device 22 to control the optical gate device. You may do it. In this case, the clock extracting device must supply the clock to the optical gate device 36 after being delayed by a time considering the time delay in the wavelength dispersion medium 30. Although clock extraction becomes easy, it is necessary to adjust the delay time in the clock extraction device for each wavelength.
[0035]
Usually, an optical fiber is used as the wavelength dispersion medium 30. In that case, the amount of chromatic dispersion and the average delay time are likely to vary depending on the ambient temperature, optical fiber length, and the like. In general, the amount of chromatic dispersion of the chromatic dispersion medium is not so large as to overlap adjacent time slots. In such a situation, the configuration in which the clock is extracted from the output light of the wavelength dispersion medium 30 as in the clock extraction device 34 of the illustrated embodiment is more configured than the clock extraction from the input light of the wavelength dispersion medium 30. And adjustment becomes easy.
[0036]
There is a chromatic dispersion medium that has little expansion and contraction, such as a chirped fiber grating. When this is used as the chromatic dispersion medium 30, there is little problem even if the clock is extracted from the input light of the chromatic dispersion medium 30. .
[0037]
When handling an optical pulse signal with a small duty ratio, the TDM separator 22 may be omitted.
[0038]
Although the embodiment of RZ transmission has been described, the TDM separator 22 essentially has a function of converting an NRZ signal into an RZ signal. Therefore, the present invention is also applicable to NRZ transmission.
[0039]
【The invention's effect】
As can be easily understood from the above description, according to the present invention, since the coherent crosstalk from the adjacent channel can be effectively removed, the wavelength interval of the WDM transmission can be shortened and / or the transmission distance can be reduced. Can be long.
[Brief description of the drawings]
FIG. 1 is a schematic block diagram of an embodiment of the present invention.
FIG. 2 is an example of an optical spectrum of a WDM signal.
FIG. 3 is a schematic diagram of optical filter characteristics for separating signal light having a wavelength of λ2.
FIG. 4 is a diagram illustrating group delay characteristics of the wavelength dispersion medium 30;
FIG. 5 is a schematic diagram of an optical spectrum of signal light having a wavelength of λ1.
6 is a schematic diagram of an input pulse waveform and an output pulse waveform of the wavelength dispersion medium 30. FIG.
[Explanation of symbols]
10: transmitting terminal station 12: optical fiber transmission line 14: receiving terminal station 20: optical WDM demultiplexing device 22: optical TDM demultiplexing devices 24a, 24b, 24c, 24d: interference suppression device 30: chromatic dispersion medium 32: demultiplexer 34 : Clock extractor 36: Optical gate devices 38a, 38b, 38c, 38d: Optical receiver

Claims (2)

A wavelength separation filter (20) for extracting an optical signal of a predetermined wavelength from wavelength division multiplexed signal light composed of input optical signals of a plurality of wavelengths;
A chromatic dispersion medium (30) for extending the pulse waveform of the optical signal of a predetermined wavelength extracted by the wavelength separation filter (20) on the time axis according to the wavelength;
A clock extraction device (34) for extracting a clock from input light or output light of the chromatic dispersion medium (30);
An optical signal processing device comprising: an optical gate device (36) for extracting a time slot at a central portion of a pulse from the output light of the chromatic dispersion medium (30) according to the clock extracted by the clock extraction device (34) . The down-converter (22) for reducing an optical signal having a predetermined wavelength extracted by the wavelength separation filter (20) to a low rate and supplying the optical signal to the chromatic dispersion medium (30). optical signal processing apparatus.

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