JP4515436B2 - apparatus - Google Patents

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 transmission line loss, branch loss, and the like is compensated by 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, since the signal-to-noise ratio (S / N ratio) is reduced by the addition of spontaneous emission (ASE) noise generated along with amplification, the number of relays and thus the transmission distance is limited. Waveform degradation due to the chromatic dispersion of the optical fiber and the nonlinear optical effect in the fiber is also a factor that gives a transmission limit. In order to overcome these limitations, a regenerative repeater that digitally processes signals is required, and its realization is desired. In particular, an all-optical regenerative repeater that performs all processing at the optical level is important in realizing a transparent operation that does not depend on the bit rate or pulse shape of the signal.

  The functions required for the all-optical regenerative repeater are amplitude regeneration or re-amplification, timing regeneration or retiming, and waveform shaping or reshaping. The present invention pays attention to these functions, and 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. The present invention provides a signal regenerator at various node points of a repeater or an optical network.

  The most common timing extraction circuit for optical signal regenerators is to convert the input optical signal into an electrical signal by a photo detector such as a photodiode, extract the fundamental frequency electrically, and then apply the laser light at this frequency. Intensity modulation type (OE type). The OE type timing extraction circuit is used in a regenerative repeater in a conventional optical communication system. 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, in an active MLL (mode-locked laser) in which an optical modulator such as an LN (lithium niobate) modulator and an EA (electro-absorption) modulator is inserted, as described above. There is also known a type in which a clock pulse is regenerated by modulating the optical modulator with an electrically regenerated basic frequency.

  On the other hand, as an all-optical type timing extractor that processes all at the optical level, a Fabry-Perot laser composed of a saturable absorber, and a DFB-LD (distributed feedback laser) integrated with reflection and phase modulation function units. A self-pulsation laser using a diode), and an active MLL that applies AM or FM modulation to a nonlinear medium with signal light.

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

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

  Another object of the present invention is to provide a method, apparatus and system for processing optical signals suitable for WDM (wavelength division multiplexing). Other objects of the present invention will become clear from the following description.

According to an aspect of the present invention, a multi-wavelength clock generator that receives a WDM signal light obtained by wavelength division multiplexing a plurality of optical signals having different wavelengths and generates a plurality of optical clock pulses each having a different wavelength; A clock monitoring device that monitors an optical clock pulse of one wavelength among a plurality of optical clock pulses output from the multi-wavelength clock generator, wherein the multi-wavelength clock generator includes a plurality of optical clock pulses included in the WDM signal light. A first optical filter that passes an optical signal of any one of the wavelengths, a photodetector that passes through the first optical filter and converts the optical signal into an electrical signal, and an electrical signal output from the photodetector A band pass filter that outputs an electric signal having a single frequency corresponding to the bit rate component of the optical signal that has been input and passed through the first optical filter; A mode-locked laser that generates an optical clock pulse having a single wavelength that is modulated based on the single-frequency electric signal output from the bandpass filter, and is supplied with the optical clock pulse by a nonlinear optical effect. An optical waveguide structure for diffusing the optical spectrum of the optical clock pulse, and a plurality of passbands including the plurality of wavelengths, and the optical clock pulse having the optical spectrum diffused is supplied to have the plurality of wavelengths. An apparatus is provided that includes a second optical filter that generates a plurality of optical clock pulses.

  As described above, according to the present invention, it is possible to provide a method, apparatus, and system for processing a new optical signal that does not depend on the bit rate or pulse shape of the optical signal.

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

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 the optical pulse. When it is sufficiently long (L _D >> L _NL ), the phase shift φ _NL (z, L) due to SPM (self-phase modulation) 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 becomes larger as the power slope becomes tighter. Further, the propagation distance z increases and the nonlinear length L _NL decreases (γP ₀ increases) and increases. Thus, chirping with SPM imparts a new frequency component to the optical pulse, resulting in a broadening of 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 larger, the chirp δω _NL becomes very large, so-called supercontinuum (SC). It becomes light (SC light) having a broadband spectrum called. For example, since the response time of the third-order nonlinear effect in the 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. Accordingly, when a part of the SC light is extracted using the optical bandpass 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 within the SC light band can be generated.

  As an example, an mth-order Super-Gaussian type optical pulse

Is input to the optical fiber, the equation (2)

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

It is. The state of the expressions (3)-(5) is shown in FIGS. In FIGS. 1A and 1B, the solid line indicates the case of a Gaussian pulse (m = 1), and the broken line indicates the case of a super Gaussian pulse when m = 3. Chirp is generated along the slope of the pulse, and δω <0 at the head and δω> 0 at the tail (up-chirp). Further, 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 applying chirp by SPM to the optical pulse. It is particularly important that the vicinity of the center of the large chirp slope can be distinguished from the vicinity of the peak and the bottom of the chirp. By using this, for example, minute power fluctuations and accumulated noise near the peak and skirt can be removed using an optical filter.

  That is, first, an optical pulse is propagated in an optical fiber to forcibly generate SPM and frequency-separated into a large part and a small part of a chirp, and then a spectral component in the vicinity of a small chirp (δω to 0) is emitted as light. A band stop filter (BSP) is used to remove intensively.

  A method for performing all-optical 2R (Rian Prefication and Reshaping) reproduction in this way has already been described in Japanese Patent Application No. 2000-34454 (filed on Feb. 14, 2000). If the signal pulse is a short pulse with sufficiently large peak power, SC light is generated by chirping by SPM. In such SC light as well, relatively small chirp noise components are concentrated near the center wavelength of the spectrum (the wavelength of the input signal light). Therefore, if the optical bandpass filter extracts the spectral component outside the band including much noise as described above, the clock pulse from which the noise is removed can be reproduced.

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

It is represented by Here, ω represents the optical angular frequency, c represents the speed of light in vacuum, and n ₂ and A _eff represent the nonlinear refractive index and effective core area of the fiber, respectively. Since the nonlinear coefficient of a conventional DSF (dispersion shifted fiber) is as small as about γ = 2.6 W ⁻¹ km ^−1, a length of several km to 10 km or more is necessary to obtain a sufficient chirp. In order to generate a sufficiently large chirp with a shorter length, it is effective to increase the light intensity by increasing n ₂ or decreasing the mode field diameter (MFD), and hence A _eff in 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. A large n ₂ value of 5 × 10 ⁻²⁰ m ² / W or more was obtained when the GeO ₂ addition concentration was 25 to 30 mol% (in the case of a normal silica fiber, n _{2 to} 3.2 × 10 ⁻²⁰ m). 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 core shape. When the relative refractive index difference Δ is about 2.5 to 3% in the GeO ₂ -doped fiber, a fiber having an MFD of about 4 μm is obtained. As a combined effect of these effects, a fiber with a large γ value of 15 W ⁻¹ km ⁻¹ or more has been obtained.

  Further, in order to make the dispersion length sufficiently longer than the nonlinear length or to perform chirp compensation, it is desirable that the GVD of such a fiber can be arbitrarily adjusted. This point can also be achieved by setting the above parameters as follows. First, in normal DCF, generally, when Δ is increased under the condition that MFD is constant, the dispersion value increases in the normal dispersion region. On the other hand, the dispersion decreases when the core diameter is increased, and conversely, the dispersion increases when the core diameter is decreased. Accordingly, when the core diameter is increased while the MFD is set to a certain value in a given wavelength band, the dispersion can be made zero. Conversely, a desired normal dispersion fiber can be obtained.

By such a method, a highly nonlinear dispersion shifted fiber (HNL-DSF) or DCF (dispersion compensating fiber) with 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 compared with a normal DSF. As described above, a normal DSF requires a length of about 10 km, but such a fiber can achieve the same effect with a length of about 1 to 2 km. Actually, since the loss is reduced by the shortening, a shorter length is sufficient.

FIG. 2 is a block diagram showing a first embodiment of the apparatus 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 amplified to a power sufficient to generate a required chirping by the optical amplifier 6 and then input to the optical fiber 2. 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 with the chirping passes through an optical bandpass filter (BPF) 8 whose passband center wavelength is λ _S ′, and extracts components other than the components with small chirping. Thus, the reproduction pulse 10 having the center wavelength λ _{S ′} is output.

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

It is desirable that the center wavelength λ _S ′ of the pass band of the BPF 8 is sufficiently set 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 passband of the BPF 8 be set appropriately according to the required pulse width and shape. Basically, it is set substantially equal to the spectral 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 the input is about P ₀ = 1 W, since L _D = 25 km and L _NL = 0.05 km, L _D >> L _NL holds. Therefore, the influence of dispersion on chirp is ignored here.

At this time, the chirp δω is 1.68 THz (13 nm) when L = 0.5 km. If P ₀ = 2W under the same conditions, 3.33 THz (26 nm) is obtained. For short pulses of 5 to 10 ps, for example, even a signal of about 40 Gb / s, a peak power of about 1 W can be realized relatively easily, 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 embodiment of the apparatus 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''. Thus, by using a _{multi-modal BPF} , 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 wave guide), an interleaver filter, a fiber grating connected in tandem, 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 intensity fluctuations in a signal pulse before processing according to the present invention, and the horizontal axis represents time (T). Considering the chirping of each part of this pulse by SPM, the parts a and a ′ are first derived from spontaneous emission (ASE) noise of an optical amplifier or waveform distortion due to fiber transmission. The parts indicated by a and a ′ should be essentially at the zero level, and thus the strength is low. Therefore, the chirp received in the optical fiber is small in the portions indicated by a and a ′.

  Next, as shown by b and b ', fluctuations in the vicinity of the peak of the pulse and on the slope are mainly due to beat noise between the signal pulse and the ASE of the optical amplifier, waveform distortion due to optical fiber transmission, etc. Therefore, the slope is equal to or slightly tighter than the slope of the pulse itself. In this case, since the slope is zero in the peak and valley portions of the intensity fluctuation, the chirp in the vicinity thereof is small.

  However, there will also be extremely peaky fluctuation components as indicated by c and c ′. At such singularities, the chirp is larger than that in the slope of the pulse itself, and the spectrum is located outside that of the main slope.

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

FIG. 5 shows the entire transmission band (pass band) extracted by the present invention. Here, two transmission bands of interest with respect to the wavelength λ _S are given to the signal spectrum of the center wavelength λ _S. The stopband between the two transmission bands is given by, for example, an optical bandstop filter, and the stopband outside the two transmission bands is given by, for example, an optical bandpass filter. In this case, the stop band by the optical band stop filter is narrower than the pass band of the optical band pass filter. A noise band indicated by a, a ′, b and b ′ in FIG. 4 can be removed by a stop band between the two transmission bands, and 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 or a high-order filter such as a double cavity multilayer filter can be used.

By the way, in FIG. 5, the spectrum is displayed with the highest intensity near the center. However, if the dispersion and the power of the pulse are set appropriately, the shape of the spectrum can be made flat. This is the case when normal dispersion (β ₂ > 0) is set, etc. (in this case, the pulse is chirped 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 and the like is large). By applying the present invention to such a flat chirp spectrum, a constant output independent of the input peak power can be obtained, so that fluctuations near the peak can be effectively suppressed.

FIG. 6 is a block diagram showing a third embodiment of the apparatus according to the present invention. The input signal light 12 having the center wavelength λ _S is supplied to the timing extraction optical circuit 14 and the clock pulse 16 having the center wavelength λ _{S ′} is extracted. The clock pulse 16 is amplified to a power sufficient to generate the required chirping by the optical amplifier 6 and then input to the optical fiber 2 as an optical waveguide structure that provides 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 with chirping is passed through the optical bandpass filter 8 whose passband center wavelength is λ _{S ″, and} components other than the component with small chirping are extracted. The output clock pulse 18 having the center wavelength λ _{S ″} is output.

According to this embodiment, even when there are various noises and waveform distortions in the clock pulse 16 generated by the timing extraction optical circuit, they can be suppressed and a higher quality clock pulse can be obtained. Further, 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 passband, a clock pulse having the same wavelength as the original signal wavelength can be generated. Furthermore, a clock pulse having an arbitrary wavelength can be generated by appropriately setting the center wavelength of the pass band of the optical band pass filter 8.

  FIG. 7 is a block diagram showing an embodiment of the system according to the present invention. This system includes a first optical fiber transmission line 20 that transmits an optical signal, an optical repeater 22 that receives an optical signal transmitted through the first optical fiber transmission line, 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 light supplied from the optical repeater 22 to the second optical fiber transmission line 24. The signal is output signal light 28 having a wavelength λ _{S ′} .

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

The input signal light 26 is amplified by the optical amplifier 30 and then branched into first and second optical signals by the optical branching device 32. The first optical signal is input to the 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 a 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 apparatus according to the present invention is applied to the timing extractor 34, it is possible to obtain a high-quality clock pulse in which noise and waveform distortion are suppressed, 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 contained in the signal light is converted into a clock pulse. It corresponds to the wavelength of the pulse 36. Further, by using such an optical gate, a waveform shaping effect can be obtained together with an optical AND operation.

  In the embodiment shown in FIG. 7, the apparatus according to the present invention is applied to the timing extractor 34, but the apparatus according to the present invention can also 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 shifted fiber (DSF). . Each of the optical fiber transmission lines 20 and 24 may be an optical amplification repeater transmission line including at least one optical amplifier. In this case, the attenuation of the optical signal can be compensated by the optical amplifier, so that long distance transmission is possible.

  In carrying out the present invention, it is desirable to compensate in advance for waveform distortion due to GVD and nonlinear effects in the transmission line in order to effectively generate chirp. Accordingly, each of the optical fiber transmission lines 20 and 24 may include an appropriate dispersion compensator or an optical phase commoner, 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 improvement of the S / N ratio in the middle of the transmission path is applied, and the present invention is also applied to the receiver. In this case, the reception sensitivity can be improved.

  By the way, conventionally, in order to generate an optical clock, the input signal light is once converted into an electric signal by an optical / electric converter, and then the clock is electrically extracted and converted into an optical signal again by a laser. The method that occurs is the most common. However, in this generation method, first, the transmission speed of the input signal light is limited by the electric band limitation. In addition, in order to generate an optical clock having a plurality of wavelengths, a number of lasers for generating a clock are required. Therefore, the configuration scale of the wavelength division multiplexing optical clock generator increases 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 at the optical stage, there is no electrical band limitation. Referring to FIG. 8, an example of the configuration of the timing adjuster according to the all-optical clock generation method is shown. WDM signal light (multi-wavelength input signal light) obtained by wavelength division multiplexing optical signals of different wavelengths λ ₁ ,..., Λ _n is divided into optical signals of each wavelength by the optical demultiplexer 42. Each optical signal is supplied to a timing adjuster 44. The timing adjuster 44 includes a clock extraction circuit 48 that extracts a clock based on an 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 is adjusted by the optical AND circuit 46 is wavelength-division multiplexed again by the optical multiplexer 50, and WDM signal light (multi-wavelength output signal light) is output from the optical multiplexer 50.

  According to this prior art, since the all-optical clock generation method is applied, there is no limitation on the electrical band, but an optical clock is still required for the number of wavelengths, so that the apparatus is roughly proportional to the number of wavelengths of WDM signal light. Scale increases.

  As described above, the number of optical clocks is increased as the number of wavelengths in WDM increases, and the present situation in optical clock generators is to provide a method and apparatus capable of generating multi-wavelength clocks collectively. Expected.

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

  To achieve this object, according to the present invention, a method for obtaining a plurality of clock pulses is provided. In this method, first, a clock pulse having a single wavelength is generated based on input signal light. A clock pulse is then applied to the optical waveguide structure that provides the nonlinear optical effect, and the spectrum of the clock pulse is diffused. Then, a clock pulse whose spectrum is spread 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 diffusion.

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

Here, L _d and L _n1 represent the dispersion length and nonlinear length defined by the following equations, respectively. In addition, the light U represents the optical electric field normalized by the peak power of the optical pulse, ξ represents the nonlinear optical waveguide length normalized by the dispersion length, and τ represents the time normalized by the incident pulse width T ₀ .

Since the solution of this equation is greatly different from chromatic dispersion, it will be explained 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), (c) chromatic dispersion is negative (abnormal dispersion region: β _2). <0).

(A) Positive chromatic dispersion (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 nonlinear frequency chirp occurs. This frequency chirp is eventually linearized and accumulated by group velocity dispersion. The spectrum spreads flatly due to the accumulated chirp. A spread spectrum ratio is defined by L _d / L _n1 , and a larger spread spectrum ratio can be achieved as this value increases.

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

(B) Zero chromatic dispersion (zero dispersion wavelength: β ₂ = 0)
When there is no chromatic dispersion, the signal light pulse propagates through the nonlinear optical waveguide without changing the pulse waveform at all, and the nonlinear frequency chirp generated by the nonlinear Kerr effect is accumulated. This accumulation amount is obtained analytically when the incident pulse is Gaussian and is given by the following equation.

  Here, z indicates the effective length of the nonlinear optical waveguide. The smaller the non-linear length (the larger γP), the larger the chirp is generated and the more the spectrum is diffused. Similarly, it can be seen that the spectrum spreads as the effective length increases.

  Details can be referred to in the literature “Phys. Rev. A, (vol. 17, no. 4, pp. 1448-1453, 1978)”.

(C) Chromatic dispersion is negative (abnormal dispersion region: β ₂ <0)
There is an analytical solution in the anomalous dispersion region. When an optical pulse incident on the nonlinear optical waveguide satisfies L _d / L _n1 = 1, there is only one eigenvalue, and its eigensolution is as follows.

This is called a basic optical soliton, and its waveform and spectrum do not change in the nonlinear optical waveguide. Further, the L _d / L _n1> 1, since a plurality of eigenvalues exists superposition of a plurality of specific solutions, obtained as analytical solution. These are called higher order optical solitons. In the anomalous dispersion region, the spectrum can be diffused by the following two methods.

(I) Soliton adiabatic compression Basic optical solitons are pulse-compressed without being chirped during propagation in a nonlinear optical waveguide in which the size of the chromatic dispersion value in the longitudinal direction is small. In this process, the spectrum spreads. 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 through a nonlinear optical waveguide, the high-order optical soliton changes periodically with a length called a soliton length as one period. In the vicinity of the incident end of the nonlinear optical waveguide, pulse compression always occurs due to the nonlinear effect, so that the spectrum is diffused. Spectral diffusion is possible by adjusting the length of the nonlinear optical waveguide to a length at which the spectrum is most diffused.

  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 embodiment of the apparatus according to the present invention. This apparatus includes a clock extraction unit 52, an optical amplification unit 54, a spread spectrum unit 56, and a multiwavelength optical clock generation unit 58. The clock extraction unit 52 generates a clock pulse having a single wavelength (λ _S ) based on the input signal light. The optical amplification unit 54 includes an optical amplifier 60 that amplifies the clock pulse output from the clock extraction unit 52, and an optical bandpass filter (BPF) 62 through which the clock pulse amplified by the optical amplifier 60 passes. The optical band pass filter 62 is provided to remove noise added by the optical amplifier 60, and the center wavelength of the pass band is set to λ _S.

  The clock pulse output from the optical amplification unit 54 is supplied to the spectrum spreading unit 56. The spectrum spread unit 56 includes a nonlinear optical waveguide 64 that diffuses the spectrum of the clock pulse by a 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 either normal dispersion, anomalous dispersion, or zero dispersion. In any 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 a plurality of pass bands lambda _1, ..., a lambda _n. A clock pulse having a spread spectrum passes through the multimodal optical wavelength filter 66, thereby generating a plurality of clock pulses having a plurality of wavelengths.

  Thus, according to the present embodiment, a plurality of clock pulses having a plurality of wavelengths can be obtained in a lump, so that the scale of an apparatus 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 each timing adjuster 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.

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 that has passed through the filter 74 is supplied to the multi-wavelength clock extractor 70.

The multiwavelength clock extractor 70 ₁ a plurality of wavelengths according to the present invention lambda, ..., for generating a plurality of clock pulses having a lambda _n. Each of these clock pulses is supplied to a corresponding timing adjuster 68. 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, thereby adjusting the timing of each optical signal. The optical signal whose timing has been adjusted is wavelength division multiplexed by the optical multiplexer 50, and the resulting WDM signal light 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 WDM wavelengths is increased, the clock pulse can be supplied by one multi-wavelength clock extractor 70, so that the apparatus configuration can be simplified.

  FIG. 11 is a block diagram showing a sixth preferred embodiment of the device according to the present invention. This device relates to a method for monitoring optical clock pulses. In the conventional method, since an 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, the waveform, spectrum shape, and quality of all the remaining clock pulses are monitored only by monitoring a clock pulse of at least one wavelength from among a plurality of generated clock pulses. 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 pulses and monitors them. The reason why this monitoring method is possible is based on the fact that in the present invention, the spectrum of a single-wavelength clock pulse extracted from the clock is spread, so that the replica of the clock pulse occurs at each wavelength.

  FIG. 12 is a block diagram showing a seventh embodiment of the apparatus 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-prefixation 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 an optical signal from the optical amplifier 80 and a 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 therethrough. The filter 84 has a pass band whose center wavelength is the wavelength of the corresponding optical signal. In the NOLM 82, the waveform of the optical signal is shaped by the non-linearity, 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 apparatus 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 an optical branching unit composed of an optical coupler or the like. 72 divides the light into first and second WDM signal lights. The first WDM signal light is amplified by the optical amplifier 86 and then supplied to the WDM-TDM converter 88. 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 the TDM-WDM converter 90. The converter 90 converts the TDM signal into WDM signal light based on the supplied 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 that is supplied with the light from the light source 92 and the WDM signal light amplified by the optical amplifier 86. Including. By supplying the WDM signal light and the CW light to the NOLM 94, the WDM signal light is converted into a time division multiplexed signal having an intermediate wavelength λ _i . At the time of this conversion, waveform shaping of the signal is performed by the action of NOLM94. The obtained TDM signal is output through the optical bandpass filter 96 having the intermediate wavelength λ _i as the center wavelength of the passband.

  The TDM-WDM converter 90 includes a NOLM 98 to which the TDM signal from the 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 operates so that a plurality of clock pulses are synchronized with the TDM signal by shifting the timing for each wavelength.

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

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

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

The WDM signal light supplied to this apparatus 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.

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

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

FIG. 15 is a block diagram showing a specific configuration example of the apparatus shown in FIG. More specifically, the internal configuration of the clock extraction unit 52 shown in FIG. 9 is shown, and an optical fiber 116 is used as the nonlinear optical waveguide 64 for spectrum spreading. The clock extraction unit 52 includes a photodetector (PD) 118, an electrical bandpass 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 it into an electrical signal. By passing this electrical signal through the band-pass filter 120, the bit rate component of the optical signal is extracted. In this way, an electric signal having a single frequency corresponding to the bit rate can be obtained. The mode-locked laser 122 is modulated by the obtained electrical signal, and an optical clock pulse having a single wavelength λ _i is obtained.

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

  The present invention includes the following supplementary notes.

(Appendix 1)
An optical branching device for branching 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 that are connected to a plurality of output ports of the optical demultiplexer and that perform waveform shaping of the plurality of optical signals based on the plurality of clock pulses;
The multi-wavelength clock generator is
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 diffuses the spectrum of the clock pulse by a nonlinear optical effect;
An optical filter having a plurality of passbands and supplied with clock pulses in which the spectrum is spread to generate a plurality of clock pulses having a plurality of wavelengths.

(Appendix 2)
The apparatus according to appendix 1, wherein
An apparatus further comprising an optical multiplexer for wavelength-division multiplexing the waveform-shaped optical signal.

(Appendix 3)
The apparatus according to appendix 1, wherein
Each of the plurality of waveform shapers includes a non-linear loop mirror.

(Appendix 4)
An optical branching device for branching 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 WDM signal light based on the plurality of clock pulses,
The multi-wavelength clock generator is
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 diffuses the spectrum of the clock pulse by a nonlinear optical effect;
An optical filter having a plurality of passbands and supplied with clock pulses in which the spectrum is spread to generate a plurality of clock pulses having a plurality of wavelengths.

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

FIG. 1A and FIG. 1B are diagrams for explaining chirping when a Gaussian pulse and a super Gaussian pulse propagate through an optical fiber. FIG. 2 is a block diagram showing a first embodiment of the apparatus according to the present invention. FIG. 3 is a block diagram showing a second embodiment of the apparatus according to the present invention. FIG. 4 is a diagram for explaining the intensity fluctuation in the pulse removed according to the present invention. FIG. 5 is a diagram showing how noise is removed by the optical filter in the embodiment of the present invention. FIG. 6 is a block diagram showing a third embodiment of the apparatus 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 embodiment of the apparatus 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 embodiment of the apparatus according to the present invention. FIG. 13 is a block diagram showing an eighth embodiment of the apparatus according to the present invention. FIG. 14 is a block diagram showing a ninth embodiment of the apparatus according to the present invention. FIG. 15 is a block diagram showing a specific configuration of the apparatus shown in FIG.

Explanation of symbols

8 Optical bandpass filter 20 Optical fiber 32 Optical splitter 38 Timing extractor 40 Optical AND circuit

Claims (1)

A multi-wavelength clock generator for receiving a WDM signal light obtained by wavelength division multiplexing a plurality of optical signals having different wavelengths and generating a plurality of optical clock pulses each having a different wavelength;
A clock monitoring device that monitors an optical clock pulse of one wavelength among a plurality of optical clock pulses output from the multi-wavelength clock generator ;
The multi-wavelength clock generator is
A first optical filter that passes an optical signal of any one wavelength among a plurality of wavelengths included in the WDM signal light;
A photodetector that passes through the first optical filter and converts an optical signal into an electrical signal;
A band-pass filter that inputs an electric signal output from the photodetector and outputs an electric signal having a single frequency corresponding to a bit rate component of the optical signal that has passed through the first optical filter;
A mode-locked laser that is modulated based on the single-frequency electrical signal output from the band-pass filter and generates an optical clock pulse having a single wavelength;
An optical waveguide structure that is supplied with the optical clock pulse and diffuses an optical spectrum of the optical clock pulse by a nonlinear optical effect;
A second optical filter that has a plurality of passbands including the plurality of wavelengths, is supplied with the optical clock pulse in which the optical spectrum is spread, and generates a plurality of optical clock pulses having the plurality of wavelengths;
A device comprising: