JP2004289863A - Method and apparatus for processing optical signal - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a new method, an apparatus, and a system for processing an optical signal, independently of the bit rate of the optical signal, pulse shape or the like. <P>SOLUTION: In the apparatus provided with an optical divider to divide an input optical signal into a first optical signal and a second optical signal, a waveform forming unit into which the first optical signal is input, a timing extracting unit which generates a clock pulse based on the second optical signal, an optical AND circuit into which an optical signal output from the waveform forming unit and a clock pulse output from the timing extracting are input, the timing extracting unit includes an optical waveguide structure which provides nonlinear optical effects so that chirping occurs in the second optical signal and an optical bandpass filter, into which an output optical signal output from the optical waveguide structure is supplied for extracting components other than the small components of the chirping; and the optical bandpass filter has a pass band which includes wavelength different from the wavelength of the optical signal. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

      The present invention relates to a method, apparatus and system for processing an optical signal.

  In an optical fiber communication system that has been put into practical use in recent years, a decrease in signal power due to a transmission line loss, a branch loss, or the like is compensated for using an optical amplifier such as an erbium-doped fiber amplifier (EDFA). The optical amplifier is an analog amplifier and linearly amplifies a signal. In this type of optical amplifier, the signal-to-noise ratio (S / N ratio) is reduced due to the addition of spontaneous emission (ASE) noise generated with the amplification, so that the number of relays and the transmission distance are limited. In addition, chromatic dispersion of an optical fiber and waveform deterioration due to nonlinear optical effects in the fiber are also factors that give a transmission limit. In order to overcome these limitations, a regenerative repeater that processes signals digitally is required, and its realization is desired. In particular, an all-optical regenerative repeater that performs all processing at an optical level is important in realizing a transparent operation that does not depend on a signal bit rate, a pulse shape, or the like.

  The functions required for the all-optical regenerative repeater are amplitude reproduction or re-amplification, timing reproduction or re-timing, and waveform shaping or re-shaping. Focusing on these functions, the present invention uses all-optical regeneration in an optical communication system by using chirping due to a self-phase modulation (SPM) effect received when an optical pulse propagates in an optical waveguide structure such as an optical fiber. It is intended to provide a repeater and a signal regenerator at various node points of an optical network.

  The most common timing extraction circuit of an optical signal reproducing device is to convert an input optical signal into an electrical signal once by a photodetector such as a photodiode, electrically extract a fundamental frequency, and then generate a laser beam at this frequency. This is a type of intensity modulation (OE type). OE type timing extraction circuits are used in regenerative repeaters in conventional optical communication systems. However, since the operation speed of the OE type timing extraction circuit is limited by the electronic circuit for signal processing, there is a problem that the bit rate of the input signal is fixed at a low rate.

  As another conventional timing extraction circuit, an active MLL (mode-locked laser) in which an optical modulator such as an LN (lithium niobate) modulator and an EA (electroabsorption) modulator is inserted inside in the same manner as described above. There is also known a type in which the optical modulator is modulated with a fundamental frequency reproduced electrically to reproduce a clock pulse.

  On the other hand, as an all-optical type timing extractor that performs processing at all optical levels, a Fabry-Perot laser composed of a saturable absorber and a DFB-LD (distributed feedback laser) integrating a reflection and phase modulation function unit are used. Diode) and an active MLL that applies AM or FM modulation to a nonlinear medium with signal light.

  However, when the processing of an optical signal by a conventional timing extractor is applied to wavelength division multiplexing (WDM), there is a problem that a timing extractor is required for each wavelength channel and the device scale is increased.

  Therefore, an object of the present invention is to provide a method, apparatus and system for processing a novel optical signal that does not depend on the bit rate, pulse shape, and the like of the optical signal.

  It is another object of the present invention to provide a method, apparatus and system for processing an optical signal suitable for WDM (wavelength division multiplexing). Still other objects of the present invention will become apparent from the following description.

  According to a first aspect of the present invention, an optical splitter that splits an input optical signal into a first optical signal and a second optical signal, a waveform shaper that receives the first optical signal, A timing extractor for generating a clock pulse based on the second optical signal; and an optical AND circuit to which the optical signal output from the waveform shaper and the clock pulse output from the timing extractor are input. The timing extractor is provided with an optical waveguide structure providing a non-linear optical effect such that chirping occurs in the second optical signal, and an output optical signal output from the optical waveguide structure. An optical band-pass filter for extracting a component other than the low-chirping component, wherein the optical band-pass filter has a pass band including a wavelength different from the wavelength of the optical signal.

  According to a second aspect of the present invention, a step of generating a clock pulse having a single wavelength based on an input signal light; and supplying the clock pulse to an optical waveguide structure providing a nonlinear optical effect, thereby providing a spectrum of the clock pulse. And providing a clock pulse having the spectrum spread to an optical filter having a plurality of pass bands to generate a plurality of clock pulses having a plurality of wavelengths.

  According to the third aspect of the present invention, an optical splitter for splitting a WDM signal light obtained by wavelength division multiplexing a plurality of optical signals having different wavelengths into a first WDM signal light and a second WDM signal light. A first WDM signal light is supplied to the input port, and a plurality of optical signals having different wavelengths are output from the plurality of output ports. A demultiplexer; a multi-wavelength clock generator for receiving the second WDM signal light and generating a plurality of clock pulses having a plurality of wavelengths; and a plurality of clocks connected to a plurality of output ports of the optical demultiplexer. A plurality of waveform shapers for shaping the waveforms of the plurality of optical signals based on a pulse, wherein the multi-wavelength clock generator has a clock having a single wavelength based on the second WDM signal light. A clock extractor that generates a pulse, an optical waveguide structure that is supplied with the clock pulse and spreads the spectrum of the clock pulse by a non-linear optical effect, and has a plurality of passbands. An optical filter provided to generate a plurality of clock pulses having a plurality of wavelengths.

  ADVANTAGE OF THE INVENTION According to this invention, the effect that it becomes possible to provide the method, apparatus, and system for novel optical signal processing which does not depend on the bit rate, pulse shape, etc. of an optical signal is produced.

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

Now, consider a case where an optical pulse U (z, T) having a width T ₀ and a peak power P ₀ propagates through an optical fiber. Here, T is the time in the coordinate system that moves with the light pulse. The chromatic dispersion β ₂ of this optical fiber is not so large, and the dispersion length L _D = T ₀ ² / | β ₂ | is smaller than the nonlinear length L _NL = 1 / γP ₀ (γ is a third-order nonlinear constant) for an optical pulse. If sufficiently long (L _D »L _NL) is, SPM phase shift by (self phase modulation) φ _NL (z, L) can be expressed as follows.

Here, z _eff = [1-exp (−αz)] / α is an effective (non-linear) interaction length.

At this time, the chirp δω _NL is given by:

Since | U (0, T) | ² corresponds to the peak power, according to the equation (2), the chirping in each part of the optical pulse increases as the power gradient becomes steeper. Further, the propagation distance z becomes longer and the nonlinear length L _NL becomes shorter (γP ₀ becomes larger) and becomes larger. Thus, chirping by SPM imparts new frequency components to the light pulse and consequently broadens the spectrum.

In particular, when the pulse to be used is a pulse of several ps or shorter, and the peak power is several watts or more, the chirp δω _NL becomes very large, which is a so-called supercontinuum (SC). Light (SC light) having a so-called broadband spectrum. For example, since the response time of the third-order nonlinear effect in an optical fiber is on the order of femtoseconds, each spectral component of the SC light is almost completely synchronized with the original input signal pulse. Therefore, when a part of the SC light is extracted using the optical band-pass filter, one or a plurality of pulses synchronized with the input signal pulse can be extracted. This indicates that a pulse synchronized with an input signal pulse of an arbitrary wavelength in the band of the SC light can be generated.

  As an example, a super-Gaussian light pulse of the m-th order is used.

When inputting into the optical fiber, from the equation (2),

It is. In particular, for a normal Gaussian pulse (m = 1),

It is. (A) and (B) in FIG. 1 show the states of the equations (3) to (5). In FIGS. 1A and 1B, a solid line shows a case of a Gaussian pulse (m = 1), and a broken line shows a case of a super Gaussian pulse when m = 3. Chirp occurs along the slope of the pulse, and δω <0 at the beginning and δω> 0 at the tail (up chirp). The Gaussian pulse has a substantially linear chirp near the peak of the pulse.

  FIGS. 1A and 1B show that the time component of the pulse can be resolved on the spectrum by giving the chirp of the light pulse by SPM. What is particularly important is that it is possible to distinguish between the vicinity of the large slope of the chirp and the vicinity of the small peak and the bottom of the chirp. By using this, for example, minute power fluctuations near the peak and the tail and accumulated noise can be removed using an optical filter.

  That is, first, an optical pulse is propagated through an optical fiber to forcibly generate an SPM, and frequency-separated into a large portion and a small portion of the chirp. The band-stop filter (BSP) is used to remove intensively.

  The method of performing all-optical 2R (lean pre-figuration and reshaping) reproduction in this manner has already been described in Japanese Patent Application No. 2000-34454 (filed on Feb. 14, 2000). If the signal pulse is a short pulse having sufficiently large peak power, SC light is generated by chirping by SPM. Even in such SC light, relatively small chirp noise components are concentrated near the center wavelength of the spectrum (wavelength of the input signal light) as described above. Therefore, if the optical band-pass filter is used to extract from the spectral components outside the band including much noise, the clock pulse from which the noise has been removed can be reproduced.

  In order to effectively generate chirp due to SPM, it is effective to use a dispersion flat fiber (DFF) that gives a small normal dispersion independent of wavelength and to increase the γ value of the optical fiber. The DFF can be obtained, for example, by appropriately controlling the core diameter and the relative refractive index difference. On the other hand, γ of the optical fiber is

Is represented by Here, ω represents the optical angular frequency, c represents the speed of light in a vacuum, and n ₂ and A _eff represent the nonlinear refractive index and effective core area of the fiber, respectively. The nonlinear coefficient of a conventional DSF (dispersion-shifted fiber) is as small as about γ = 2.6 W ⁻¹ km ⁻¹ , so that a length of several km to 10 km or more is required to obtain a sufficient chirp. In order to generate a short and sufficiently large chirp, it is effective to increase the light intensity by increasing n ₂ or decreasing the mode field diameter (MFD), that is, A _eff in the equation (6). As a means for increasing n ₂ , there is a method of adding fluorine or the like to the clad and adding GeO ₂ or the like to the core at a considerably high concentration. When the additive concentration of GeO ₂ is 25 to 30 mol%, a large n ₂ value of 5 × 10 ⁻²⁰ m ² / W or more is obtained (n _{2 to} 3.2 × 10 ⁻²⁰ m in a normal silica fiber). ² / W). On the other hand, it is possible to reduce the MFD by designing the relative refractive index difference between the core and the clad and the design of the core shape. When the relative refractive index difference Δ in the GeO ₂ -doped fiber is about 2.5 to 3%, a fiber having an MFD of about 4 μm is obtained. As a total effect of these effects, a fiber having a large γ value of 15 W ^-1 km ^-1 or more has been obtained.

  In addition, in order to make the dispersion length sufficiently longer than the nonlinear length or to perform chirp compensation, it is desired that the GVD of such a fiber be arbitrarily adjustable. This point can also be set by setting the above parameters as follows. First, in a normal DCF, when Δ is generally increased under the condition that the MFD is fixed, the variance value increases in the normal variance region. On the other hand, as the core diameter increases, the dispersion decreases, and conversely, as the core diameter decreases, the dispersion increases. Therefore, when the MFD is set to a certain value in a given wavelength band, the dispersion can be made zero by increasing the core diameter. Conversely, it is also possible to obtain a desired normal dispersion fiber.

By such a method, a highly nonlinear dispersion-shifted fiber (HNL-DSF) or DCF (dispersion compensating fiber) having γ = 15 W ⁻¹ km ⁻¹ or more is realized. For example, a fiber with γ = 15 W ⁻¹ km ⁻¹ can achieve the same efficiency with a length of about 2.6 / 15 to ¹ / 5.7 as compared with a normal DSF. As described above, a normal DSF requires a length of about 10 km, but with such a fiber, the same effect can be achieved with a length of about 1 to 2 km. In practice, the shorter the length, the shorter the loss.

FIG. 2 is a block diagram showing a first preferred embodiment of the device according to the present invention. An optical fiber 2 is used as an optical waveguide structure that provides a nonlinear optical effect. GVD of the optical fiber 2 is beta _2, the optical fiber 2 gives normal dispersion and third-order nonlinear optical effects, for example, the supplied optical signal.

The signal pulse 4 as an optical signal having the center wavelength λ _S is input to the optical fiber 2 after being amplified by the optical amplifier 6 to a power sufficient to generate required chirping. In the optical fiber 2, chirping occurs due to SPM, and the spectrum is expanded (spread). The output optical signal output from the optical fiber 2 given the chirping passes through an optical band-pass filter (BPF) 8 having a passband center wavelength of λ _S ′, and components other than the component having a small chirp are extracted. Then, the reproduction pulse 10 having the center wavelength λ _{S ′} is output.

  The small chirping component mainly includes a variation (for example, waveform deterioration due to GVD) from the zero point of the off-power (0 sign) component and a small slope component near the peak of the pulse. Since these components determine the optical signal-to-noise ratio (OSNR), by removing these portions by the BPF, the OSNR reduction due to power fluctuation, extinction ratio degradation, noise accumulation, etc. of these components is improved. can do. Therefore, the present invention enables wavelength conversion and clock pulse extraction with an improvement in OSNR.

The center wavelength λ _S ′ of the pass band of the BPF 8 is desirably set sufficiently from the center wavelength λ _S of the signal pulse so that a component having a small chirp including noise is not included in the output signal pulse 10. Further, it is desirable that the width and shape of the pass band of the BPF 8 are appropriately set according to the required pulse width and shape. Basically, it is set substantially equal to the spectrum shape of the input signal pulse 4.

Here, in order to evaluate the feasibility of the present invention, the degree of chirp is estimated. For example, consider a case where a pulse of T ₀ = 5 ps propagates through a normal dispersion fiber of β ₂ = 1 ps ² / km and γ = 20 W ⁻¹ km ⁻¹ . If input is made at about P ₀ = 1 W, L _D = 25 km and L _NL = 0.05 km, so that L _D ≫L _NL holds. Therefore, the effect of dispersion on chirp is ignored here.

At this time, the chirp δω is 1.68 THz (13 nm) when L = 0.5 km. Assuming that P ₀ = 2 W under the same conditions, it is 3.33 THz (26 nm). For a short pulse of 5 to 10 ps, a peak power of about 1 W can be relatively easily realized even with a signal of about 40 Gb / s, for example, and the present invention can be implemented even in consideration of the above estimation. It is.

FIG. 3 is a block diagram showing a second preferred embodiment of the device according to the present invention. In this embodiment, an optical bandpass filter 8 having a plurality of (two in the figure) passbands is used. The center wavelength of the passband is lambda _S'and λ _S''. By using a _multimodal BPF in this manner, an output WDM signal pulse including two optical signals having center wavelengths λ _{S ′} and λ _{S ″} can be obtained. As the multimodal BPF, an AWG (arrayed waveguide), an interleaver filter, a tandem-connected fiber grating, or the like can be used.

  Next, the principle of noise removal according to the present invention will be described with reference to FIG.

  FIG. 4 shows the intensity fluctuation in the signal pulse before processing according to the present invention, and the horizontal axis is time (T). Considering the chirping of each part of this pulse by SPM, first, the parts a and a 'are derived from spontaneous emission (ASE) noise of the optical amplifier or waveform distortion due to fiber transmission. The parts indicated by a and a 'should originally be at zero level and therefore have low intensity. Therefore, the portions indicated by a and a 'receive less chirp in the optical fiber.

  Next, as shown by b and b ', fluctuations near the pulse peak and on the slope are mainly caused by beat noise between the signal pulse and the ASE of the optical amplifier, waveform distortion due to optical fiber transmission, and the like. Since the band has a band equal to or slightly wider than the band, the slope is equal to or slightly greater than the slope of the pulse itself. In the peak and valley portions of the intensity fluctuation in this case, the inclination becomes zero, and the chirp in the vicinity is small.

  However, as shown by c and c ′, there will be extremely peaky fluctuation components. At such a singular point, the chirp is larger than on the slope of the pulse itself, and the spectrum is located outside that of the main slope.

  Accordingly, the light pulse extracted by the BPF from the spectral component excluding the chirp component having a relatively small chirp and the chirp component having an extremely large chirp generated from the singular point portion has the intensity fluctuation due to noise suppressed. .

FIG. 5 shows the entire transmission band (pass band) extracted according to the present invention. Here, two transmission bands of interest for the wavelength λ _S are given to the signal spectrum of the center wavelength λ _S. The stop band between the two transmission bands is provided, for example, by an optical band stop filter, and the stop band outside the two transmission bands is provided, for example, by an optical band pass filter. In this case, the stop band of the optical band stop filter is narrower than the pass band of the optical band pass filter. A stop band between the two transmission bands can remove noise components indicated by a, a ', b, and b' in FIG. 4, and can be indicated by c and c 'by a stop band outside the two transmission bands. Noise components can be removed.

  As the optical band stop filter, for example, a narrow band fiber grating can be used. As the optical bandpass filter, reflection by a fiber grating can be used, or a higher-order filter such as a double-cavity type multilayer filter can be used.

By the way, in FIG. 5, the spectrum is displayed in a form in which the intensity near the center is the largest, but it is possible to flatten the shape of the spectrum by appropriately setting the dispersion and the power of the pulse. This is the case when the normal dispersion (β ₂ > 0) is set (in this case, the pulse chirps while spreading in a rectangular shape), and in the extreme case, it is called a supercontinuum (Supercontinuum). (In this case, the contribution of not only SPM but also four-wave mixing is large). If the present invention is applied to such a flat chirp spectrum, a constant output independent of the input peak power can be obtained, so that the fluctuation near the peak can be effectively suppressed.

FIG. 6 is a block diagram showing a third preferred embodiment of the device according to the present invention. The input signal light 12 having the center wavelength λ _S is supplied to the timing extracting optical circuit 14, and the clock pulse 16 having the center wavelength λ _{S ′} is extracted. The clock pulse 16 is amplified by the optical amplifier 6 to a power sufficient to generate the required chirping, and then input to the optical fiber 2 as an optical waveguide structure for providing a nonlinear optical effect. In the optical fiber 2, chirping occurs due to SPM, and the spectrum is expanded. The output optical signal output from the optical fiber 2 given the chirping passes through the optical band-pass filter 8 having the center wavelength of the pass band λ _{S ″, and the} components other than the component having a small chirping are extracted. , An output clock pulse 18 having a center wavelength λ _{S ″} is output.

According to this embodiment, even when the clock pulse 16 generated by the timing extraction optical circuit has various noises and waveform distortions, they can be suppressed and a higher quality clock pulse can be obtained. Also, even when the wavelength of the clock pulse generated by the timing extraction optical circuit is different from the original signal wavelength (λ _S ≠ λ _{S ′} ), the optical bandpass filter 8 is set so that λ _{S ″} = λ _S. By setting the center wavelength of the pass band, a clock pulse having the same wavelength as the original signal wavelength can be generated. Further, by appropriately setting the center wavelength of the pass band of the optical band-pass filter 8, a clock pulse having an arbitrary wavelength can be generated.

  FIG. 7 is a block diagram showing an embodiment of the system according to the present invention. In this system, a first optical fiber transmission line 20 for transmitting an optical signal, an optical repeater 22 to which an optical signal transmitted by the first optical fiber transmission line is input, and an output from the optical repeater 22 And a second optical fiber transmission line 24 for transmitting an optical signal.

The optical signal supplied from the first optical fiber transmission line 20 to the optical repeater 22 is the input signal light 26 having the wavelength λ _S , and the optical signal supplied from the optical repeater 22 to the second optical fiber transmission line 24. The signal is an output signal light 28 having a wavelength λ _{S ′} .

  The optical repeater 22 can include the configuration of the device according to the present invention. More specifically, it is as follows.

After being amplified by the optical amplifier 30, the input signal light 26 is split by the optical splitter 32 into first and second optical signals. The first optical signal is input to a waveform shaper 38, and the timing extractor 34 according to the present invention generates a clock pulse 36 based on the second optical signal. The wavelength of the clock pulse 36 is λ _{S ′} . The optical signal output from the waveform shaper 38 and the clock pulse 36 output from the timing extractor 34 are input to the optical AND circuit 40. As a result, the output signal light 28 is output from the optical AND circuit 40. Timing extractor 34 may be provided, for example, by the embodiment shown in FIG.

  According to this embodiment, since the device according to the present invention is applied to the timing extractor 34, a high-quality clock pulse in which noise and waveform distortion are suppressed can be obtained, and effective 3R reproduction can be realized. is there. When an optical gate such as a NOLM (nonlinear optical loop mirror) or a Mach-Zehnder interferometer is used as the optical AND circuit 40, the information of the signal light is converted into a clock pulse. It matches the wavelength of the pulse 36. Also, by using such an optical gate, a waveform shaping effect can be obtained together with the optical AND operation.

  In the embodiment shown in FIG. 7, the device according to the present invention is applied to the timing extractor 34, but the device according to the present invention may be applied to the waveform shaper 38.

  As each of the optical fiber transmission lines 20 and 24, a single mode silica fiber (SMF) can be used, and examples thereof include a 1.3 μm zero dispersion fiber and a 1.55 μm dispersion shift fiber (DSF). . Each of the optical fiber transmission lines 20 and 24 may be an optical amplification relay transmission line including at least one optical amplifier. In this case, since the attenuation of the optical signal can be compensated for by the optical amplifier, long-distance transmission becomes possible.

  In practicing the present invention, in order to generate chirp effectively, it is desirable to compensate in advance for waveform distortion due to GVD or non-linear effects in the transmission path. Therefore, each of the optical fiber transmission lines 20 and 24 may include an appropriate dispersion compensator or an optical phase concentrator, or may perform optical soliton transmission.

  According to the present invention, high-quality timing reproduction is possible. Therefore, when the present invention is applied to a repeater, the S / N ratio can be improved in the middle of a transmission path, and the present invention can be applied to a receiver. In such a case, the reception sensitivity can be improved.

  By the way, conventionally, in order to generate an optical clock, an input signal light is once converted into an electric signal by an optical / electrical converter, then the clock is electrically extracted, and the clock is converted again into an optical signal by a laser. The method that occurs is most common. However, in this generation method, first, the transmission speed of the input signal light is limited by the electrical band limitation. In addition, in order to generate an optical clock having a plurality of wavelengths, lasers for clock generation are required for the number of wavelengths. Therefore, the configuration scale of the optical clock generator for wavelength division multiplexing becomes larger in proportion to the number of wavelengths. Further, since the electric circuit depends on the format of the signal light, there is almost no flexibility with respect to the waveform of the input signal light.

On the other hand, according to the all-optical clock generation method in which all processing is performed in the optical stage, there is no electrical band limitation. FIG. 8 shows an example of the configuration of a timing adjuster according to the all-optical clock generation method. The WDM signal light (multi-wavelength input signal light) obtained by wavelength division multiplexing the optical signals of different wavelengths λ ₁ ,..., Λ _n is divided by the optical demultiplexer 42 into optical signals of each wavelength. Each optical signal is supplied to the timing adjuster 44. The timing adjuster 44 includes a clock extraction circuit 48 for extracting a clock based on the input optical signal, and an optical AND circuit 46 to which the extracted clock and the input optical signal are supplied. The optical signal whose timing has been adjusted by the optical AND circuit 46 is again subjected to wavelength division multiplexing by the optical multiplexer 50, and the optical multiplexer 50 outputs a WDM signal light (multi-wavelength output signal light).

  According to this conventional technique, although the electrical band limitation is eliminated because the all-optical clock generation method is applied, since the optical clock is still required for the number of wavelengths, the device is almost in proportion to the number of wavelengths of the WDM signal light. The scale increases.

  As described above, as the number of wavelengths in the WDM increases, the number of optical clocks becomes necessary as the number of wavelengths increases in the current state of the optical clock generator, and a method and apparatus capable of collectively generating a multi-wavelength clock are provided. Expected.

  Therefore, an object of the following embodiments of the present invention is to provide a method of collectively generating multi-wavelength optical clocks based on single-wavelength signal light.

  To this end, according to the present invention, there is provided a method for obtaining a plurality of clock pulses. In this method, first, a clock pulse having a single wavelength is generated based on an input signal light. Then, a clock pulse is supplied to the optical waveguide structure that provides the nonlinear optical effect, and the spectrum of the clock pulse is spread. Then, a clock pulse having a spread spectrum is supplied to an optical filter having a plurality of pass bands, and a plurality of clock pulses having a plurality of wavelengths are generated.

  Hereinafter, the principle of this method will be described in detail focusing on spectrum spreading.

The relationship between the nonlinear effect γ (Kerr effect) that causes a phase change proportional to the intensity of signal light and the electric field of an optical pulse propagating in a centrally symmetric nonlinear optical waveguide having chromatic dispersion β ₂ is expressed by the following nonlinear Schrodinger equation: Can be described by

Here, L _d and L _n1 respectively represent a dispersion length and a nonlinear length defined by the following equation. Further, the light U represents an optical electric field standardized by the peak power of the optical pulse, ξ represents a nonlinear optical waveguide length standardized by the dispersion length, and τ represents a time standardized by the incident pulse width T ₀ .

Since the solution of this equation is significantly different from chromatic dispersion, it will be described in the following three ways. (A) chromatic dispersion is positive (normal dispersion region: β ₂ > 0), (b) chromatic dispersion is zero (zero dispersion region: β ₂ = 0), and (c) chromatic dispersion is negative (abnormal dispersion region: β ₂ <0).

(A) Wavelength dispersion is positive (normal dispersion region: β ₂ > 0)
In the normal dispersion region, when the nonlinear effect is large (L _d >> L _n1 ), the Kerr effect of the nonlinear effect becomes dominant near the incident end of the nonlinear optical waveguide, and a nonlinear frequency chirp occurs. This frequency chirp is eventually linearized and accumulated by the group velocity dispersion. This accumulated chirp causes the spectrum to spread flat. L _d / L _n1 defines the spectral spread rate, and the larger this value is, the higher the spectral spread rate can be achieved.

  Details of this principle can be referred to in the literature “OSA, J. Opt. Soc. Am. B (vol. 1, no. 2, pp. 139-149, 1984)” and the like.

(B) Zero chromatic dispersion (zero dispersion wavelength: β ₂ = 0)
When there is no chromatic dispersion, the signal light pulse propagates in the nonlinear optical waveguide without changing the pulse waveform at all, and the nonlinear frequency chirp caused by the nonlinear Kerr effect accumulates. When the incident pulse is of a Gaussian type, this accumulated amount is obtained analytically and is given by the following equation.

  Here, z indicates the effective length of the nonlinear optical waveguide. The smaller the nonlinear length (the larger γP), the greater the spread of the spectrum since a larger chirp occurs. Similarly, it can be seen that the spectrum becomes more diffuse as the effective length becomes larger.

  For details, reference can be made to the literature “Phys. Rev. A, (vol. 17, no. 4, pp. 1448-1453, 1978)” and the like.

(C) Negative wavelength dispersion (abnormal dispersion region: β ₂ <0)
In the anomalous dispersion region, an analytical solution exists. When the light pulse incident on the nonlinear optical waveguide satisfies L _d / L _n1 = 1, there is only one eigenvalue, and the eigen solution is as follows.

This is called a fundamental optical soliton, and its waveform and spectrum do not change in the nonlinear optical waveguide. When L _d / L _n1 > 1, there are a plurality of eigenvalues, and thus a superposition of a plurality of eigensolutions is obtained as an analytical solution. These are called higher order optical solitons. In the anomalous dispersion region, the spectrum can be spread in the following two ways.

(I) Soliton adiabatic compression The basic optical soliton is pulse-compressed without chirp during propagation in a nonlinear optical waveguide in which the magnitude of chromatic dispersion in the longitudinal direction is small. In this process, the spectrum is spread. The spread spectrum efficiency is obtained by the ratio of the dispersion value at the input end to the dispersion value at the output end of the nonlinear optical waveguide.

  Details of this principle can be referred to, for example, in the document “OSA, J. Opt. Soc. Am. B (vol. 5, no. 3, pp. 709-713, 1988)”.

(Ii) High-order soliton compression During propagation in a nonlinear optical waveguide, high-order optical solitons periodically change with a period called a soliton length as one cycle. Near the entrance end of the nonlinear optical waveguide, pulse compression always occurs due to the nonlinear effect, so that the spectrum is spread. If the length of the nonlinear optical waveguide is adjusted to the length at which the spectrum spreads the most, the spectrum can be spread.

  This principle can be referred to in the document “OSA, Opt. Lett. (Vol. 8, no. 5, pp. 289-291, 1983)”.

FIG. 9 is a block diagram showing a fourth preferred embodiment of the device according to the present invention. This device includes a clock extraction unit 52, an optical amplification unit 54, a spread spectrum unit 56, and a multi-wavelength optical clock generation unit 58. The clock extracting unit 52 generates a clock pulse having a single wavelength (λ _S ) based on the input signal light. The optical amplifier 54 includes an optical amplifier 60 that amplifies the clock pulse output from the clock extractor 52, and an optical bandpass filter (BPF) 62 through which the clock pulse amplified by the optical amplifier 60 passes. The optical bandpass filter 62 is provided for removing noise added by the optical amplifier 60, and the center wavelength of the passband is set to λ _S.

  The clock pulse output from the optical amplifier 54 is supplied to the spread spectrum unit 56. The spectrum spreading section 56 includes a nonlinear optical waveguide 64 that spreads the spectrum of the clock pulse by the nonlinear optical effect. As the nonlinear optical waveguide 64, an optical fiber, a semiconductor optical amplifier, or the like can be used. When an optical fiber is used as the nonlinear optical waveguide 64, the optical fiber provides one of normal dispersion, anomalous dispersion, and zero dispersion. In either case, the spectrum of the clock pulse is spread according to the principle described above.

The clock pulse having the spread spectrum is supplied to the multi-wavelength optical clock generator 58. The clock generator 58 includes a multimodal optical wavelength filter 66 having a plurality of pass bands. The center wavelengths of the plurality of pass bands are λ ₁ ,..., Λ _n . When the clock pulse having the spread spectrum passes through the multi-peak optical wavelength filter 66, a plurality of clock pulses having a plurality of wavelengths are generated.

  As described above, according to the present embodiment, a plurality of clock pulses having a plurality of wavelengths can be collectively obtained, so that the scale of a device to which WDM is applied can be reduced.

  FIG. 10 is a block diagram showing a fifth preferred embodiment of the device according to the present invention. In this apparatus, a timing adjuster 68 from which the clock extractor 48 is removed is used in place of the timing adjusters 44 in the prior art shown in FIG. Instead, in this embodiment, a multi-wavelength clock extractor 70 to which the present invention is applied is used. The multi-wavelength clock extractor 70 is provided by, for example, the apparatus shown in FIG.

A part of the WDM signal light supplied to the optical demultiplexer 42 is branched by the optical coupler 72 and supplied to the optical bandpass filter 74. Optical band-pass filter 74, the wavelength lambda _1, ..., passing optical signals of any wavelength lambda _i is selected from lambda _n. The optical signal passing through the filter 74 is supplied to the multi-wavelength clock extractor 70.

The multi-wavelength clock extractor 70 generates a plurality of clock pulses having a plurality of wavelengths λ ₁ ,..., Λ _n according to the present invention. These clock pulses are supplied to the corresponding timing adjusters 68, respectively. In each timing adjuster 68, the optical signal and the clock pulse supplied from the optical demultiplexer 42 are supplied to the optical AND circuit 46, whereby the timing of each optical signal is adjusted. The optical signal subjected to the timing adjustment is wavelength division multiplexed by the optical multiplexer 50, and the WDM signal light obtained as a result is output from the optical multiplexer 50.

  As each of the optical demultiplexer 42 and the optical multiplexer 50, for example, an arrayed waveguide grating (AWG) can be used. As the optical AND circuit 46, a non-linear loop mirror or an optical switch having a Mach-Zehnder configuration can be used.

  According to this embodiment, even if the number of wavelengths of WDM is increased, the supply of clock pulses can be completed by one multi-wavelength clock extractor 70, so that the device configuration can be simplified.

  FIG. 11 is a block diagram showing a sixth preferred embodiment of the device according to the present invention. The device relates to a method for monitoring an optical clock pulse. According to the conventional method, since the optical clock is generated for each wavelength, each optical clock pulse must be monitored.

In the multi-wavelength clock extractor 70 to which the present invention is applied, only the clock pulse of at least one wavelength is monitored from the generated clock pulses, and the waveform, spectrum shape and quality of all the remaining clock pulses are monitored. I understand. Therefore, in this embodiment, the multiwavelength clock extractor 70 and the clock monitoring device 76 is connected, a clock monitoring device 76, a plurality of wavelengths lambda _1, ..., any monitoring wavelength selected from lambda _n lambda _It receives _mon clock pulse and monitors the clock pulse. The reason that this monitoring method becomes possible is that the present invention spreads the spectrum of a single-wavelength clock pulse extracted by a clock, so that the clock pulse is duplicated at each wavelength.

  FIG. 12 is a block diagram showing a seventh preferred embodiment of the device according to the present invention. This embodiment corresponds to a specific example of the embodiment shown in FIG. 10, and a 3R regenerator 78 is used instead of each timing adjuster 68 shown in FIG. The 3R regenerator 78 includes an optical amplifier 80 for re-amplification and a waveform shaper 81 for reshaping and retiming.

  The optical amplifier 80 amplifies the optical signal of each wavelength output from the optical demultiplexer 42. The amplified optical signal is supplied to the waveform shaper 81. The waveform shaper 81 includes a nonlinear optical loop mirror (NOLM) 82 to which the optical signal from the optical amplifier 80 and the clock pulse from the multi-wavelength clock extractor 70 according to the present invention are supplied, and an optical signal output from the NOLM 82 And an optical bandpass filter 84 that passes. The filter 84 has a pass band centered on the wavelength of the corresponding optical signal. In the NOLM 82, the waveform of the optical signal is shaped by the nonlinearity, and the phase noise of the optical signal is removed by synchronization with the clock pulse.

  The optical signal output from the 3R regenerator 78 is wavelength division multiplexed by the optical multiplexer 50, and the resulting WDM signal light is output from the optical multiplexer 50.

FIG. 13 is a block diagram showing an eighth embodiment of the device according to the present invention. When WDM signal light obtained by wavelength division multiplexing a plurality of optical signals having different wavelengths λ ₁ ,..., Λ _n is supplied to this apparatus, first, the WDM signal light is converted into an optical splitter comprising an optical coupler or the like. The light is divided into first and second WDM signal lights by 72. The first WDM signal light is supplied to a WDM-TDM converter 88 after being amplified by an optical amplifier 86. The converter 88 converts the supplied WDM signal light into a time division multiplexed signal (TDM signal).

By the second WDM signal light from the optical splitter 72 is passed through the optical bandpass filter 74, a plurality of wavelengths lambda _1, ..., the optical signals of any wavelength lambda _i is selected from lambda _n are obtained, The optical signal is supplied to a multi-wavelength clock extractor 70 to which the present invention is applied. The multi-wavelength clock extractor 70 generates a plurality of clock pulses having a plurality of wavelengths λ ₁ ,..., Λ _n based on the supplied optical signal.

  The TDM signal from the WDM-TDM converter 88 and the plurality of clock pulses from the multi-wavelength clock extractor 70 are supplied to a TDM-WDM converter 90. The converter 90 converts the TDM signal into a WDM signal light based on the supplied plurality of clock pulses and outputs the WDM signal light.

The WDM-TDM converter 88 includes a light source 92 that outputs CW (continuous wave) light having an intermediate wavelength λ _i , and a NOLM 94 to which the light from the light source 92 and the WDM signal light amplified by the optical amplifier 86 are supplied. Including. By WDM signal light and the CW light is supplied to the NOLM94, WDM signal light is converted into time-division multiplexed signal having an intermediate wavelength lambda _i. At the time of this conversion, the waveform of the signal is shaped by the operation of the NOLM 94. TDM signal obtained is output through the optical bandpass filter 96 centered wavelength of the passband of the intermediate wavelength lambda _i.

  TDM-WDM converter 90 includes a NOLM 98 to which the TDM signal from WDM-TDM converter 88 is supplied. A plurality of clock pulses from the multi-wavelength clock extractor 70 are supplied to the NOLM 98 via the delay circuit 100. The delay circuit 100 acts so that the plurality of clock pulses are shifted in timing for each wavelength and are synchronized with the TDM signal.

  By supplying the TDM signal and the plurality of clock pulses to the NOLM 98, the TDM signal is converted into the WDM signal light and the phase noise of the signal is removed. The WDM signal light thus obtained is output from this device.

  According to this embodiment, basically, only a multi-wavelength clock extractor and two NOLMs according to the present invention can collectively reproduce multi-wavelength signals without passing through electric signals. As a result, it is possible to simplify the device configuration and flexibly cope with an increase in the number of wavelengths.

  FIG. 14 is a block diagram showing a ninth embodiment of the device according to the present invention. Here, the device 110 according to the invention is used as a multi-wavelength light source in an optical add / drop device. Specifically, it is as follows.

The WDM signal light supplied to this device is divided into optical signals of wavelengths λ ₁ ,..., Λ _n by an optical demultiplexer 102, and each optical signal is supplied to an optical switch 104 for optical add / drop.

  An insertion signal processing device 106 is provided to supply an optical signal to be inserted into the optical switch 104. The optical modulator 108 operates according to the signal output from the insertion signal processing device 106. The light modulators 108 are provided for the number of wavelengths, and are supplied with clock pulses from the device 110 according to the present invention. The optical signal obtained by operating the optical modulator 108 by the insertion signal is inserted into the transmission line by the optical switch 104.

  On the other hand, a branch signal processing device 112 is provided to process the optical signal branched by the optical switch 104. The optical signal output from the optical switch 104 is wavelength division multiplexed by the optical multiplexer 114, and the resulting WDM signal light is output from this device.

  In a conventional optical add / drop device, a plurality of light sources are required to obtain a clock pulse or CW light to be supplied to a plurality of optical modulators. Therefore, it has been difficult to reduce the size of the device. On the other hand, according to the embodiment shown in FIG. 14, since only one device 110 according to the present invention is required to obtain a plurality of clock pulses, the device can be reduced in size and cost, and Reliability can be improved.

FIG. 15 is a block diagram showing a specific configuration example of the device shown in FIG. More specifically, FIG. 9 shows the internal configuration of the clock extracting unit 52 shown in FIG. 9, in which an optical fiber 116 is used as the nonlinear optical waveguide 64 for spread spectrum. The clock extracting unit 52 includes a photo detector (PD) 118, an electric band-pass filter 120, and a mode-locked laser (MLL) 122. The photodetector 118 receives an optical signal having a single arbitrary wavelength as input signal light, and converts this into an electric signal. When the electric signal passes through the band-pass filter 120, the bit rate component of the optical signal is extracted. In this manner, a single-frequency electric signal corresponding to the bit rate can be obtained. Mode-locked laser 122 by the resulting electrical signal is modulated, the optical clock pulses having a single wavelength lambda _i is obtained.

  As described above, in the present embodiment, since the optical clock pulse is reproduced by the mode-locked laser 122 based on the bit rate component extracted electrically, a plurality of clearer clock pulses can be generated.

  The present invention includes the following supplementary notes.

(Appendix 1)
An optical splitter for splitting a WDM signal light obtained by wavelength division multiplexing a plurality of optical signals having different wavelengths into a first WDM signal light and a second WDM signal light;
An optical demultiplexer having an input port and a plurality of output ports, the first WDM signal light being supplied to the input port, and a plurality of optical signals having different wavelengths being output from the plurality of output ports; ,
A multi-wavelength clock generator that receives the second WDM signal light and generates a plurality of clock pulses having a plurality of wavelengths;
A plurality of waveform shapers that are connected to the plurality of output ports of the optical demultiplexer and perform waveform shaping of the plurality of optical signals based on the plurality of clock pulses;
The multi-wavelength clock generator,
A clock extractor for generating a clock pulse having a single wavelength based on the second WDM signal light;
An optical waveguide structure that is supplied with the clock pulse and spreads the spectrum of the clock pulse by a nonlinear optical effect;
An apparatus comprising: an optical filter having a plurality of pass bands and supplied with a clock pulse in which the spectrum is spread to generate a plurality of clock pulses having a plurality of wavelengths.

(Appendix 2)
The apparatus according to claim 1, wherein
An apparatus further comprising an optical multiplexer that performs wavelength division multiplexing on the optical signal whose waveform has been shaped.

(Appendix 3)
The apparatus according to claim 1, wherein
An apparatus wherein each of said plurality of waveform shapers includes a non-linear loop mirror.

(Appendix 4)
An optical splitter for splitting a WDM signal light obtained by wavelength division multiplexing a plurality of optical signals having different wavelengths into a first WDM signal light and a second WDM signal light;
A first converter for converting the first WDM signal light into a time division multiplexed signal,
A multi-wavelength clock generator that receives the second WDM signal light and generates a plurality of clock pulses having a plurality of wavelengths;
A second converter for converting the time-division multiplexed signal into a WDM signal light based on the plurality of clock pulses,
The multi-wavelength clock generator,
A clock extractor for generating a clock pulse having a single wavelength based on the second WDM signal light;
An optical waveguide structure that is supplied with the clock pulse and spreads the spectrum of the clock pulse by a nonlinear optical effect;
An apparatus comprising: an optical filter having a plurality of pass bands and supplied with a clock pulse in which the spectrum is spread to generate a plurality of clock pulses having a plurality of wavelengths.

(Appendix 5)
The apparatus according to claim 4, wherein
The apparatus wherein each of the first and second transducers includes a non-linear loop mirror.

FIGS. 1A and 1B are diagrams for explaining chirping when a Gaussian pulse and a super Gaussian pulse propagate in an optical fiber. FIG. 2 is a block diagram showing a first preferred embodiment of the device according to the present invention. FIG. 3 is a block diagram showing a second preferred embodiment of the device according to the present invention. FIG. 4 is a diagram for explaining intensity fluctuation in a pulse removed according to the present invention. FIG. 5 is a diagram showing how noise is removed by the optical filter according to the embodiment of the present invention. FIG. 6 is a block diagram showing a third preferred embodiment of the device according to the present invention. FIG. 7 is a block diagram showing an embodiment of the system according to the present invention. FIG. 8 is a block diagram showing a configuration example of a conventional timing adjuster using an all-optical clock generator. FIG. 9 is a block diagram showing a fourth preferred embodiment of the device according to the present invention. FIG. 10 is a block diagram showing a fifth preferred embodiment of the device according to the present invention. FIG. 11 is a block diagram showing a sixth preferred embodiment of the device according to the present invention. FIG. 12 is a block diagram showing a seventh preferred embodiment of the device according to the present invention. FIG. 13 is a block diagram showing an eighth embodiment of the device according to the present invention. FIG. 14 is a block diagram showing a ninth embodiment of the device according to the present invention. FIG. 15 is a block diagram showing a specific configuration of the device shown in FIG.

Explanation of reference numerals

Reference Signs List 8 optical band-pass filter 20 optical fiber 32 optical splitter 38 timing extractor 40 optical AND circuit

Claims (11)

An optical splitter for splitting an input optical signal into a first optical signal and a second optical signal;
A waveform shaper to which the first optical signal is input;
A timing extractor for generating a clock pulse based on the second optical signal;
An optical AND circuit to which an optical signal output from the waveform shaper and a clock pulse output from the timing extractor are input,
The timing extractor is
An optical waveguide structure that provides a nonlinear optical effect so that chirping occurs in the second optical signal;
An output optical signal output from the optical waveguide structure is supplied, and an optical band-pass filter that extracts components other than the small component of the chirping,
The apparatus wherein the optical bandpass filter has a passband including a wavelength different from the wavelength of the optical signal. The apparatus according to claim 1, wherein
The waveform shaper is
An optical waveguide structure for providing a nonlinear optical effect such that chirping occurs in the first optical signal;
An output optical signal output from the optical waveguide structure is supplied, and an optical band-pass filter that extracts components other than the small component of the chirping,
The apparatus wherein the optical bandpass filter has a passband including a wavelength different from the wavelength of the optical signal. Generating a clock pulse having a single wavelength based on the input signal light;
Supplying the clock pulse to an optical waveguide structure providing a nonlinear optical effect to spread the spectrum of the clock pulse;
Supplying the spectrally spread clock pulses to an optical filter having a plurality of passbands to generate a plurality of clock pulses having a plurality of wavelengths. 4. The method according to claim 3, wherein
The method further comprising amplifying a clock pulse provided to the optical waveguide structure. 4. The method according to claim 3, wherein
The method wherein the optical waveguide structure is an optical fiber. The method of claim 5, wherein
A method wherein the optical fiber provides one of normal dispersion, anomalous dispersion and zero dispersion. A clock extractor that generates a clock pulse having a single wavelength based on the input signal light;
An optical waveguide structure that is supplied with the clock pulse and spreads the spectrum of the clock pulse by a nonlinear optical effect;
An optical filter that has a plurality of pass bands and is supplied with a clock pulse in which the spectrum is spread to generate a plurality of clock pulses having a plurality of wavelengths. The apparatus according to claim 7, wherein:
An apparatus further comprising an optical amplifier for amplifying a clock pulse supplied to the optical waveguide structure. The apparatus according to claim 7, wherein:
The device wherein the optical waveguide structure is an optical fiber. The device according to claim 9,
An apparatus in which the optical fiber provides one of normal dispersion, anomalous dispersion, and zero dispersion. An optical splitter for splitting a WDM signal light obtained by wavelength division multiplexing a plurality of optical signals having different wavelengths into a first WDM signal light and a second WDM signal light;
An optical demultiplexer having an input port and a plurality of output ports, wherein the first WDM signal light is supplied to the input port, and a plurality of optical signals having different wavelengths are output from the plurality of output ports; ,
A multi-wavelength clock generator that receives the second WDM signal light and generates a plurality of clock pulses having a plurality of wavelengths;
A plurality of waveform shapers connected to a plurality of output ports of the optical demultiplexer and performing waveform shaping of the plurality of optical signals based on the plurality of clock pulses;
The multi-wavelength clock generator,
A clock extractor for generating a clock pulse having a single wavelength based on the second WDM signal light;
An optical waveguide structure that is supplied with the clock pulse and spreads the spectrum of the clock pulse by a nonlinear optical effect;
An apparatus comprising: an optical filter having a plurality of pass bands and being supplied with a clock pulse in which the spectrum is spread to generate a plurality of clock pulses having a plurality of wavelengths.

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