CN103576222B - CS-RZ to NRZ pattern upconversion fiber grating design method and device thereof - Google Patents

Abstract

The invention discloses a CS-RZ to NRZ code pattern conversion fiber grating design method and its device. The method includes a set of pseudo-random codes, and obtains CS-RZ codes and The discretized numerical sequence of the NRZ code; fast Fourier transform is performed on two sets of discretized numerical sequences to obtain the discretized numerical sequence of the power spectrum of the two; the central wavelength of the discretized numerical sequence of the power spectrum of the CS-RZ signal is blue-shifted or Redshift the wavelength width corresponding to the bit rate of half the CS-RZ signal to obtain the CS-RZ signal spectrum; subtract the CS-RZ signal spectrum from the discretized numerical sequence of the NRZ power spectrum to obtain a new set of discretized numerical sequences; select The clipping window whose width is the wavelength width corresponding to the bit rate of the two CS-RZ signals clips the new discretized numerical sequence to obtain the complete characteristic spectrum of the fiber grating; the structure of the grating is obtained by using the reverse layer stripping method. The invention has good filtering effect, can effectively suppress code pattern effect and ripple fluctuation, and obtain higher Q value.

Description

CS-RZ to NRZ Code Conversion Fiber Bragg Grating Design Method and Device

technical field

The invention relates to the technical field of optical network communication, in particular to an all-optical code conversion device from carrier suppression return-to-zero code (CS-RZ) to non-return-to-zero code (NRZ).

Background technique

On the one hand, future high-speed and large-capacity optical communication systems will integrate Dense Wavelength Division Multiplexing (DWDM) and Optical Time Division Multiplexing (OTDM) technologies to increase transmission capacity and speed. On the other hand, in order to further increase the transmission capacity of the optical communication system and increase the spectral efficiency (SE for short), many new modulation formats with high spectral efficiency and unique transmission characteristics will be adopted. New modulation technology has become one of the key technologies in high-speed systems. With the deepening of people's research on modulation technology, in addition to the traditional modulation formats such as return-to-zero (RZ) codes and non-return-to-zero (NRZ) codes, various new modulation formats continue to emerge. Carrier suppressed return-to-zero code (CS-RZ) ) is one of them. This modulation format was first proposed by MIYAMOTO in 1999, which belongs to the pseudo-multilevel modulation format. This modulation format itself has both amplitude modulation information and phase information. It is a phase-changing OOK (alternating-phaseOOK) modulation format. From the perspectives of dispersion tolerance, SPM tolerance, and nonlinear damage tolerance , CS-RZ code is a good modulation format.

The spectrum of CS-RZ is narrow, about half of that of RZ code, so the spectral efficiency is higher than that of RZ code; in addition, its chirp mainly exists in the place where two pulses meet, so the tolerance to dispersion is relatively high. Studies have shown that CS-RZ codes are significantly better than NRZ codes in alleviating the impact of nonlinear effects. It not only allows higher fiber input power, but also has a much lower power cost than NRZ codes, and is suitable for backbone networks.

Non-return-to-zero code (NRZ) has the advantages of simple generation method, narrow frequency band, tight channel wavelength spacing, and high spectral efficiency, and is suitable for applications in Dense Wavelength Division Multiplexing (DWDM) technology that requires high spectral efficiency. In addition, the NRZ pattern is also friendly to electronic devices, has strong time jitter tolerance and dispersion tolerance, but has low nonlinear tolerance and short transmission distance, so it is generally suitable for metropolitan area networks and access network.

Different OOK patterns are suitable for different networks because of their different time domain and frequency domain characteristics. Therefore, at the interface of commercial high-speed OTDM and DWDM network systems, the code conversion between various OOK patterns Technology, especially the all-optical code conversion technology has become one of the important and indispensable network interface technologies.

At present, WandiLi, JianWang, etc. have reported two technical solutions for the code conversion from NRZ to CS-RZ:

1. Active format conversion technology using semiconductor optical amplifiers to form fiber loop mirrors (W.D.Li, M.H.Chen, Y.Dong, and S.Z.Xie, "All-optical format conversion from NRZtoCSRZ and between RZ and CSRZ using SOA-based fiber loop mirror," IEEE Photon.Technol.Lett.16, 203-205 (2004).);

2. Using the non-transparent properties of periodic polarization inversion lithium niobate (PPLN) optical waveguide to realize the active pattern conversion technology of NRZ-to-CSRZ (JianWang, JunqiangSun, XinliangZhang, and DexiuHuang, /ProposalforPPLN-Basedall-opticalNRZ-to- CSRZ, RZ-to-CSRZ, RZ-DPSK-to-CSRZ-DPSK, and RZ-DPSK-to-CSRZ-DPSK format conversions, 0 IEEE Photonics Technol. Lett. 20, 1039-1041 (2008); For optical code conversion, only YuYu et al. have reported a passive code conversion technology scheme using Mach-Zehnder interference (fiber delay interferometer, DI) cascaded narrowband filters (YuYu, Xinliang Zhang, and DexiuHuang, 'All-OpticalFormatConversion from Cs-RztoNrzat40gbit/S', Opticsexpress, 15(2007), 5693-98, YuYu, XinliangZhang, and DexiuHuang, 'SimultaneousAll-OpticalMulti-ChannelRzandCsrztoNrzFormatConversion', OpticsCommunications, 35) (2091) Working in a passive way, although compared with active code conversion devices, it has the advantages of simple structure, high cost performance, and stable performance. Its disadvantages are: 1. Due to the use of interferometer structures such as DI, the adjustment operation is more difficult, And the cost is relatively high; 2. The scheme of using interferometer cascaded filters actually uses two filters to perform secondary clipping on the spectrum to realize the code conversion function, and does not provide a direct clipping with only one filter Formed technology leads to relatively complex existing technology and relatively high cost, which is not conducive to batch use in commercial systems; 3. The filtering effect is not good, and the pattern effect is obvious. The symmetrical spectral response obtained by combining the two filters is not a complete CS - The best spectral response for conversion from RZ to NRZ, the best spectral response should be specifically designed according to the spectrum difference between CS-RZ and NRZ.

Contents of the invention

The purpose of the present invention is to overcome the deficiencies of the prior art, to provide a passive CS-RZ to NRZ all-optical code conversion device and its design method based on the knife-type spectral response fiber grating, which is different from the existing The device for performing secondary spectrum clipping based on interferometer cascaded narrow-band filters only needs the fiber grating filter designed by the design method of the present invention without adding any other active or passive devices.

In order to achieve the above purpose, the following technical solutions are adopted:

A kind of CS-RZ to NRZ pattern conversion fiber grating design method, comprises the following steps:

Given a set of pseudo-random codes with a code stream length of 2 ⁿ -1, the discretized numerical sequences of CS-RZ codes and NRZ codes are given when the duty cycle is a certain value and the signal rate is a constant value;

Fast Fourier transform is performed on two sets of discretized numerical sequences, and the discretized numerical sequences of the power spectra of the two are obtained in decibels;

Blue-shift or red-shift the central wavelength of the discretized numerical sequence of the power spectrum of the CS-RZ signal by half the wavelength width corresponding to the bit rate of the CS-RZ signal, and obtain the CS-RZ signal spectrum after the central wavelength is blue-shifted or red-shifted ;

Subtract the blue-shifted or red-shifted CS-RZ signal spectrum from the discretized numerical sequence of the NRZ power spectrum to obtain a new set of discretized numerical sequences;

Selecting a clipping window whose clipping width is the wavelength width corresponding to the bit rate of the two CS-RZ signals clips the new discretized numerical sequence to obtain the complete characteristic spectrum of the fiber Bragg grating;

According to the characteristic spectrum of the fiber grating, the structure of the grating is obtained by using the method including discrete reverse layer stripping.

The CS-RZ to NRZ code conversion fiber grating device designed according to the above design method is used to convert the CS-RZ signal to be converted at the set signal rate into an NRZ signal at the same signal rate, including a knife-shaped spectral response The fiber grating is composed of a fiber grating main body and two ports, the first port of the grating and the second port of the grating.

Further, a circulator is also included, the circulator is connected with the fiber grating, the CS-RZ signal to be converted enters from the circulator, enters the fiber grating, returns to the circulator, and is output from the circulator.

Further, the signal rate is greater than or equal to 10Gbit/s and less than or equal to 200Gbit/s.

Compared with prior art, the beneficial effect of the present invention is:

1. The conversion device designed and obtained by the design method of the present invention is simple in structure and easy to use and operate: the whole conversion device is simplified as a fiber grating with knife-shaped spectral response, which is the most simplified CS-RZ to NRZ all-optical code type conversion device.

2. The filtering effect is good, which can effectively suppress the pattern effect and ripple fluctuation, and obtain a higher Q value at the same time.

3. Transparent to WDM signals. The CS-RZ signal of WDM, the NRZ signal obtained after the code type conversion directly through the present invention is still a WDM signal, there is no additional demultiplexing process, and it can be directly multiplexed without adding a wavelength division multiplexer such as AWG. Transmission in WDM system.

4. Simultaneous multi-channel conversion function: Constructing a multi-channel fiber grating can realize multi-channel CS-RZ to NRZ code conversion at the same time, for example, 4 channels are used in the simulation experiment.

5. Full grating passive working mode, energy saving and environmental protection, stable performance, no additional noise, highly compatible with existing optical fiber communication systems.

Description of drawings

Fig. 1 is a basic principle diagram of the present invention;

Fig. 2 is the power spectrum of CS-RZ signal;

Fig. 3 is the graph that the power spectrum of CS-RZ signal is clipped by fiber grating spectrum;

Figure 4 is the power spectrum of NRZ;

Fig. 5 is a comparison chart of CS-RZ and NRZ pattern power spectrum;

Fig. 6 is a schematic diagram of fiber grating characteristic spectrum clipping and a filtering order optimization diagram.

Fig. 7 is the variation curve of the converted NRZ pattern Q value with the filtering order;

Fig. 8 is the variation curve of the standard deviation of converted gained NRZ pattern 1 code along with the filtering order;

Fig. 9 shows the refractive index modulation function and local chirp of the fiber grating;

Fig. 10 is a comparative figure of target transmission spectrum and simulated transmission spectrum;

Figure 11 is a waveform diagram of a 20Gbit/s CS-RZ signal;

Fig. 12 is the transmission spectrum of the fiber grating input by CS-RZ signal of 20Gbit/s;

Figure 13 is a waveform diagram of the output NRZ after the 20Gbit/s CS-RZ signal is input;

Figure 14 is the eye diagram of converted NRZ after 20Gbit/s CS-RZ signal input;

15 is a schematic diagram of a 40Gbit/s all-optical CS-RZ to NRZ code conversion device in Embodiment 2;

Figure 16 is the eye diagram of the NRZ pattern signal after 40Gbit/s all-optical CS-RZ conversion;

Fig. 17 is the reflection spectrum of 4-channel fiber grating;

Figure 18 shows the refractive index modulation function and local chirp of 4-channel 40Gbit/s all-optical CS-RZ to NRZ code conversion fiber grating.

detailed description

The present invention will be described in detail below in conjunction with the accompanying drawings and specific implementation methods. The schematic implementation and description of the present invention are used to explain the present invention, but are not intended to limit the present invention.

As shown in Figure 1, the passive CS-RZ to NRZ all-optical pattern conversion device based on the knife-type spectral response fiber grating of the present invention is used to convert the CS-RZ signal to be converted into the same signal rate at the set signal rate The NRZ signal includes a fiber grating with a knife-type spectral response, which is composed of a fiber grating main body 2 and two ports, the first port 1 of the grating and the second port 3 of the grating. The spectral response of the main body of the fiber grating, that is, the transmission spectrum or the reflection spectrum is based on the spectral structure of the CS-RZ signal to be converted at a specific rate and the spectral structure characteristics of the NRZ signal at the corresponding bit rate, and the two sidebands of the CS-RZ spectrum are selected. One is the center wavelength, that is, the frequency corresponding to the carrier frequency redshift or blueshift half the bit rate, and calculate the difference between the NRZ power spectrum and the CS-RZ power spectrum, based on the asymmetric line of the power spectrum difference with respect to the center wavelength , which are obtained by designing different processing methods on the left and right sides of the central wavelength. The specific rate here refers to the CS-RZ signal whose rate is greater than or equal to 10Gbit/s and less than or equal to 200Gbit/s. The refractive index modulation distribution and chirp distribution of the main body of the fiber grating are obtained by inversion of the spectral response according to the causal relationship, and the fiber grating can be written and fabricated by public methods.

As shown in Figure 1, the CS-RZ signal to be converted enters the first port 1 of the grating and passes through the main body 2 of the fiber grating. From the perspective of the frequency domain, the main body 2 of the fiber grating is equivalent to a filter, and its knife-shaped spectral response is The transmission spectrum or reflection spectrum will clip the spectrum of the CS-RZ signal to be converted at a specific rate. The clipping center is a certain sideband of the selected CS-RZ spectrum, and different clipping amounts are obtained on the left and right sides. Clipping The magnitude of the amount is determined by the knife-shaped spectral response. Spectrum conversion is completed by one-time cutting of the knife-shaped spectrum response, the pulse shape adjustment in the time domain is realized, and the code type conversion is completed to obtain the NRZ signal. If the knife-type spectral response is the transmission spectrum of the fiber grating, the transmitted light contains the NRZ signal obtained by code conversion, and the NRZ signal is output from the second port 3 of the grating. Conversely, if the knife-shaped spectrum should be the reflection spectrum of the fiber grating, the reflected light contains the NRZ signal obtained by code conversion, and the NRZ signal is output from the first port 1 of the grating. As shown in FIG. 2 , it is the power spectrum of the CS-RZ signal input from the first port 1 of the grating. As shown in Figure 3, when the CS-RZ signal passes through the FBG main body 2, the FBG spectrum is clipped, and the black dotted line represents the characteristic spectrum of the FBG. As shown in FIG. 4 , it is the power spectrum of NRZ output from the second port 3 of the fiber grating.

The knife-shaped transmission or reflection spectrum of the fiber grating is designed according to the spectral structure of the CS-RZ signal to be converted at a specific rate, the red shift or blue shift of the central wavelength, and the spectral structure of the NRZ signal at the corresponding bit rate. The spectrum of the input CS-RZ signal to be converted has no obvious line spectrum at the carrier because the carrier is suppressed, but there is a strong line spectrum on the left and right sides of the carrier, which is called the CS-RZ code. Sidebands, the frequency difference between the two sidebands is equal to the bit rate. The spectrum of an NRZ code contains a continuum part and a strong discrete line spectrum at the carrier. At frequencies that are integer multiples of the bit rate, the residual line spectrum is much weaker than the line spectrum at the carrier. There are two biggest differences between the spectral structure of CS-RZ and NRZ signals: 1. The spectral carrier of CS-RZ is suppressed, and there are two strong sidebands, that is, two strong discrete line spectra, while the NRZ spectrum There is a strong discrete line spectrum only at the carrier, and the first-order sidebands are so weak that they can be ignored. 2. After the central wavelength is red-shifted or blue-shifted, the spectrum of CS-RZ loses its left-right symmetry, while the spectrum of NRZ is left-right symmetric.

As a band-pass filter, the optical fiber grating of the present invention is designed for the two major differences in the spectra of the two, and retains one sideband in the CS-RZ spectrum and filters out the other sideband. Taking the selected reserved sideband as the center, compare the CS-RZ spectrum after the center wavelength is red-shifted or blue-shifted with the NRZ spectrum centered on the carrier wavelength, and obtain different filtering curves on the left and right sides of the center wavelength, thus obtaining The knife-shaped spectral response curve of a fiber grating. The optimized design shows that the conversion effect of the first-order filter is the best.

The structural parameters of the fiber grating of the present invention, that is, the refractive index modulation distribution and the chirp distribution are obtained through spectral response inversion according to the causal relationship between the grating structure and the spectral response. Spectral response is the specific performance of FBG filtering characteristics, and its essence is uniquely determined by the structure of FBG, and the two satisfy the causal relationship. Therefore, after the required spectral response is determined, all structural parameters of the grating can be calculated according to the spectral response according to known algorithms. According to the refractive index modulation distribution and chirp distribution, a public method, such as point-by-point writing method, can be used to write and fabricate, so as to obtain the fiber grating of the present invention.

The design process of the fiber grating spectral response is given below:

Embodiment one:

The following takes a single-channel 20Gbit/s all-optical CS-RZ to NRZ code conversion design method as an example.

Step 1: Given a set of pseudo-random codes with a stream length of 2 ⁿ -1, according to the public method, a 67% duty cycle and a discrete CS-RZ code and NRZ code are given when the signal rate is 20Gbit/s Digital description, that is, discretized numerical sequences S _CS-RZ (t) and S _NRZ (t).

The second step: perform fast Fourier transform on the two sets of discretized numerical sequences of S _CS-RZ (t) and S _NRZ (t), and use decibels as a unit to obtain the discretized numerical sequence SPCT _{org_CS-RZ} ( Δλ) and SPCT _NRZ (Δλ). As shown in Figure 5, the solid line represents SPCT _NRZ (Δλ), and the dashed line represents SPCT _{org_CS-RZ} (Δλ)), namely:

S_FFT _CS-RZ = FFT(S _CS-RZ (t))

S_FFT _NRZ = FFT(S _NRZ (t))

SPCT _{org_CS-RZ} (Δλ)＝10*log(S_FFT _CS-RZ ×S_FFT _CS-RZ ^* /T)

SPCT _NRZ (Δλ)＝10*log(S_FFT _NRZ ×S_FFT _NRZ ^* /T)

Among them, S_FFT _CS-RZ ^* is the complex conjugate of S_FFT _CS-RZ , S_FFT _NRZ ^* is the complex conjugate of S_FFT _NRZ , T is the truncation time determined by the length of the code stream, Δλ=λ-λ _c , λ _c is the center wavelength That is, the carrier wavelength.

Step 3: Blueshift or redshift the center wavelength of SPCT _{org_CS-RZ} (Δλ) of the CS-RZ signal Obtain the CS-RZ signal spectrum SPCT _CS-RZ (Δλ) after the center wavelength is blue-shifted or red-shifted. Where B _1rd is the corresponding wavelength width when the signal bit rate is converted into nm, that is, the bandwidth between two sidebands of the CS-RZ spectrum. When the signal bit rate is 20Gbit/s, B _1rd =0.16nm. That is to say:

${\mathrm{<span\; class="notranslate">\; SPCT</span>}}_{\mathrm{<span\; class="notranslate">\; C</span>}\mathrm{<span\; class="notranslate">\; S</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; R</span>}\mathrm{<span\; class="notranslate">\; Z</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; \&Delta;</span>}\mathrm{<span\; class="notranslate">\; \&lambda;</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>{\mathrm{<span\; class="notranslate">\; SPCT</span>}}_{\mathrm{<span\; class="notranslate">\; o</span>}\mathrm{<span\; class="notranslate">\; r</span>}\mathrm{<span\; class="notranslate">\; g</span>}<span\; class="notranslate">\; \_</span>\mathrm{<span\; class="notranslate">\; C</span>}\mathrm{<span\; class="notranslate">\; S</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; R</span>}\mathrm{<span\; class="notranslate">\; Z</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; \&Delta;</span>}\mathrm{<span\; class="notranslate">\; \&lambda;</span>}<span\; class="notranslate">\; \&PlusMinus;</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; B</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}\mathrm{<span\; class="notranslate">\; r</span>}\mathrm{<span\; class="notranslate">\; d</span>}}<span\; class="notranslate">\; )</span>$

Among them, the positive and negative signs correspond to the blue shift and red shift, respectively. The solid line in Figure 6 shows the blue shift of the central wavelength. The blue shift is taken as an example for design below. For the red shift, it can be processed by referring to the blue shift process, and will not be described here.

Step 4: Subtract the CS-RZ blue shift from the discretized numerical sequence SPCT _NRZ (Δλ) of the NRZ power spectrum The discretized numerical sequence SPCT _CS-RZ (Δλ) of the power spectrum obtains a new set of discretized numerical sequence SPCT _NRZ-CS-RZ (Δλ), as shown by the dotted line in FIG. 6 . The left and right sides of the central wavelength in Figure 6 are asymmetrical, and the SPCT _grt and SPCT _grt are designed according to the left and right sides of the asymmetric SPCT _NRZ-CS-RZ , forming a knife-shaped spectral response as a whole.

which is:

SPCT _NRZ-CS-RZ (Δλ)＝SPCT _NRZ (Δλ)-SPCT _CS-RZ (Δλ)

Step 5: According to the value of the power spectrum difference SPCT _NRZ-CS-RZ (Δλ) in the range of ±B _1rd (±0.16nm) around the center wavelength, divide it into left and right parts for processing. The left half of SPCT _NRZ-CS-RZ is greater than zero. As a filter, because it does not have the function of power amplification, its transmittance or reflectivity cannot exceed 0dB, so the left half of the fiber grating characteristic spectrum is designed to be close to 0dB A value of ; the right half of SPCT _NRZ-CS-RZ is less than zero and can be directly used as a part of the characteristic spectrum of the fiber Bragg grating. In this way, the clipping window with a clipping width of ±B _1rd is selected to clip the SPCT _NRZ-CS-RZ to obtain the complete characteristic spectrum SPCT _grt of the fiber grating. For the 20Gbit/sCS-RZ code, B _1rd =0.16nm to obtain the characteristic spectrum SPCT _grt As shown by the solid line in Figure 6:

$\left\{\begin{array}{cc}{\mathrm{<span\; class="notranslate">\; SPCT</span>}}_{\mathrm{<span\; class="notranslate">\; g</span>}\mathrm{<span\; class="notranslate">\; r</span>}\mathrm{<span\; class="notranslate">\; t</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; \&Delta;</span>}\mathrm{<span\; class="notranslate">\; \&lambda;</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>{\mathrm{<span\; class="notranslate">\; SPCT</span>}}_{\mathrm{<span\; class="notranslate">\; N</span>}\mathrm{<span\; class="notranslate">\; R</span>}\mathrm{<span\; class="notranslate">\; Z</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; C</span>}\mathrm{<span\; class="notranslate">\; S</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; R</span>}\mathrm{<span\; class="notranslate">\; Z</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; \&Delta;</span>}\mathrm{<span\; class="notranslate">\; \&lambda;</span>}<span\; class="notranslate">\; )</span>& \mathrm{<span\; class="notranslate">\; 0</span>}<span\; class="notranslate">\; \&le;</span>\mathrm{<span\; class="notranslate">\; \&Delta;</span>}\mathrm{<span\; class="notranslate">\; \&lambda;</span>}<span\; class="notranslate">\; \&le;</span>{\mathrm{<span\; class="notranslate">\; B</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}\mathrm{<span\; class="notranslate">\; r</span>}\mathrm{<span\; class="notranslate">\; d</span>}}\\ {\mathrm{<span\; class="notranslate">\; SPCT</span>}}_{\mathrm{<span\; class="notranslate">\; g</span>}\mathrm{<span\; class="notranslate">\; r</span>}\mathrm{<span\; class="notranslate">\; t</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; \&Delta;</span>}\mathrm{<span\; class="notranslate">\; \&lambda;</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span><span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 0.01</span>}\mathrm{<span\; class="notranslate">\; d</span>}\mathrm{<span\; class="notranslate">\; B</span>}& <span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; B</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}\mathrm{<span\; class="notranslate">\; r</span>}\mathrm{<span\; class="notranslate">\; d</span>}}<span\; class="notranslate">\; \&le;</span>\mathrm{<span\; class="notranslate">\; \&Delta;</span>}\mathrm{<span\; class="notranslate">\; \&lambda;</span>}<span\; class="notranslate">\; <</span>\mathrm{<span\; class="notranslate">\; 0</span>}\\ {\mathrm{<span\; class="notranslate">\; SPCT</span>}}_{\mathrm{<span\; class="notranslate">\; g</span>}\mathrm{<span\; class="notranslate">\; r</span>}\mathrm{<span\; class="notranslate">\; t</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; \&Delta;</span>}\mathrm{<span\; class="notranslate">\; \&lambda;</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span><span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 40</span>}\mathrm{<span\; class="notranslate">\; d</span>}\mathrm{<span\; class="notranslate">\; B</span>}& \mathrm{<span\; class="notranslate">\; \&Delta;</span>}\mathrm{<span\; class="notranslate">\; \&lambda;</span>}<span\; class="notranslate">\; \&le;</span><span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; B</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}\mathrm{<span\; class="notranslate">\; r</span>}\mathrm{<span\; class="notranslate">\; d</span>}}\\ {\mathrm{<span\; class="notranslate">\; SPCT</span>}}_{\mathrm{<span\; class="notranslate">\; g</span>}\mathrm{<span\; class="notranslate">\; r</span>}\mathrm{<span\; class="notranslate">\; t</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; \&Delta;</span>}\mathrm{<span\; class="notranslate">\; \&lambda;</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span><span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 40</span>}\mathrm{<span\; class="notranslate">\; d</span>}\mathrm{<span\; class="notranslate">\; B</span>}& \mathrm{<span\; class="notranslate">\; \&Delta;</span>}\mathrm{<span\; class="notranslate">\; \&lambda;</span>}<span\; class="notranslate">\; \&Greater\; Equal;</span>{\mathrm{<span\; class="notranslate">\; B</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}\mathrm{<span\; class="notranslate">\; r</span>}\mathrm{<span\; class="notranslate">\; d</span>}}\end{array}\right.$

That is: in the clipping window, in the region of _{0≤Δλ≤B 1rd} , let SPCT _grt (Δλ)=SPCT _NRZ-CS-RZ (Δλ), in the region of -B _{1rd ≤Δλ} <0, the left half of the characteristic spectrum Partially designed to be a value close to 0dB, set SPCT _grt (Δλ)=-0.01dB; outside the clipping window, set SPCT _grt (Δλ) to a tiny value (-40dB) close to zero, but it cannot be taken as zero, so that the next step Grating design and physical implementation. In this way, a knife-shaped spectral response is obtained in the clipping window, and the optimization calculation shows that the best filtering effect can be obtained by using the fiber grating with this knife-shaped spectral response for first-order filtering, which is the completion of CS-RZ to NRZ Optimal spectral response for pattern conversion.

Figure 7 and Figure 8 are respectively the curves of the converted NRZ pattern Q value and the standard deviation of 1 code with the filtering order. The curves in the figure respectively illustrate that the first-order filtering can obtain the largest Q value and the smallest standard deviation of 1 code. That is to say, the first-order filtering is adopted, which can suppress the pattern effect well while obtaining the maximum Q value.

As we all know, if the converted NRZ signal needs to be output from the first port of the grating, the reflection spectrum r ² (Δλ) of the fiber Bragg grating is equal to the characteristic spectrum SPCT _grt (Δλ). output from the second port of the grating, the transmission spectrum t ² (Δλ) of the fiber grating is equal to the characteristic spectrum SPCT _grt (Δλ), that is:

10*log(r ² (Δλ))=SPCT _grt (Δλ), then the first port of the grating is the output port

Or 10*log(t ² (Δλ))=SPCT _grt (Δλ), then the second port of the grating is the output port

where t ² (Δλ)+r ² (Δλ)=1.

Step 6: According to the characteristic spectrum SPCT _grt (Δλ) of the fiber grating, the structure of the grating is designed by using, for example, a discrete reverse layer stripping method. The refractive index modulation distribution and chirp distribution shown in Fig. 9 complete the grating design process. It can be seen from Figure 10 that,

The CS-RZ signal to be converted of the 20Gbit/s code conversion device of the present invention enters the first port 1 of the grating, passes through the main body 2 of the fiber grating, and the spectrum of the CS-RZ signal to be converted is cut by the transmission spectrum of the fiber grating. Complete spectrum conversion, realize pulse stretching, complete code conversion, obtain 20Gbit/s NRZ signal and output it from the second port 3 of the grating, and the center wavelength of NRZ signal is blue-shifted relative to CS-RZ signal Where B _1rd is the corresponding wavelength width when the signal bit rate is converted into nm, that is, the bandwidth between two sidebands of the CS-RZ spectrum. When the signal bit rate is 20Gbit/s, B _1rd =0.16nm. Figure 11 is the waveform of the input CS-RZ code signal, Figure 12 is the transmission spectrum of the fiber grating, Figure 13 is the waveform of the output NRZ code signal, and Figure 14 is the eye diagram of the NRZ code signal. The transmission spectrum of the fiber grating is designed according to the difference between the 20Gbit/sCS-RZ signal spectrum and the 20Gbit/sNRZ signal spectrum. According to the requirement of eliminating the sideband at +0.16nm from the carrier wave of the 20Gbit/sCS-RZ signal spectrum, its The bandwidth is designed to be 0.32nm, and the filtering effect greater than 40dB is provided at ±0.16nm to achieve the purpose of code conversion. The eye diagram of the converted NRZ pattern signal is shown in Figure 14. The eye diagram is wide open and the Q value is 64.39.

Embodiment two

The following takes a multi-channel 40Gbit/s all-optical CS-RZ to NRZ code conversion design method as an example. The device is shown in FIG. 15 , the circulator 4 includes three ports A, B, and C, and the fiber grating includes two ports, the first port 1 of the grating and the second port 2 of the grating. The WDM CS-RZ signal to be converted is input from the A port of the circulator 4, output from the C port of the circulator, enters the first port 1 of the grating, is reflected by the main body of the fiber grating, and is output from the first port 1 of the grating, and enters from the C port of the circulator Circulator, and output from the B port of the circulator to get 40Gbit/sWDMNRZ signal. Figure 16 is the eye diagram of the converted NRZ pattern signal, and Figure 17 is the reflection spectrum of the 4-channel fiber grating. Because the code-converted signal is output from the first port 1 of the grating, the characteristic spectrum here is the reflection spectrum of the fiber grating. The characteristic spectrum (reflection spectrum) of the fiber grating is designed according to the left and right parts of the spectrum obtained by blue-shifting the center wavelength of the 40Gbit/sCS-RZ signal by 0.16nm and the difference between the left and right parts of the spectrum of the 40Gbit/sNRZ signal. The signal spectrum is separated from the carrier +0.32nm sideband requirements, the bandwidth of each channel is designed to be 0.60nm, and the filter effect is greater than 40dB at ±0.32nm, and the channel spacing is set to 1.6nm (corresponding to 200GHz) to eliminate crosstalk between channels. To achieve the purpose of code conversion. The eye diagram of the converted NRZ code signal is shown in Figure 16. The eye diagram has a large opening, and the calculated Q value is 53.68.

Wherein the design of 4-channel fiber grating only needs the number of channels to be 4, the bandwidth of each channel is 0.60nm, the duty cycle is 50%, and the channel spacing is 1.6nm (corresponding to 200GHz), according to the fiber grating provided in the above-mentioned embodiment 1 The design steps can be carried out by referring to the public multi-channel fiber grating design method, and will not be repeated here. Figure 18 shows the grating structure of a 40Gbit/s all-optical CS-RZ to NRZ code conversion fiber grating with 4 channels and a channel spacing of 200GHz, which includes the refractive index modulation function and local chirp.

The technical solutions provided by the embodiments of the present invention have been introduced in detail above, and the principles and implementation modes of the embodiments of the present invention have been explained by using specific examples in this paper. The descriptions of the above embodiments are only applicable to help understand the embodiments of the present invention At the same time, for those of ordinary skill in the art, according to the embodiment of the present invention, there will be changes in the specific implementation and application scope. In summary, the content of this specification should not be construed as limiting the present invention.

Claims (4)

1. a kind of CS-RZ to NRZ pattern conversion fiber grating design method, it is characterized in that, may further comprise the steps: Given a set of pseudo-random codes with a code stream length of 2 ⁿ -1, the discretized numerical sequences of CS-RZ codes and NRZ codes are given when the duty cycle is a certain value and the signal rate is a constant value; Fast Fourier transform is performed on two sets of discretized numerical sequences, and the discretized numerical sequences of the power spectra of the two are obtained in decibels; Blue-shift or red-shift the central wavelength of the discretized numerical sequence of the power spectrum of the CS-RZ signal by half the wavelength width corresponding to the bit rate of the CS-RZ signal, and obtain the CS-RZ signal spectrum after the central wavelength is blue-shifted or red-shifted ; Subtract the blue-shifted or red-shifted CS-RZ signal spectrum from the discretized numerical sequence of the NRZ power spectrum to obtain a new set of discretized numerical sequences; Selecting a clipping window whose clipping width is the wavelength width corresponding to the bit rate of the two CS-RZ signals clips the new discretized numerical sequence to obtain the complete characteristic spectrum of the fiber Bragg grating; According to the characteristic spectrum of the fiber grating, the structure of the grating is obtained by using the method including discrete reverse layer stripping. 2. according to the CS-RZ that design method design described in claim 1 obtains to NRZ pattern conversion fiber grating device, for the NRZ signal that the CS-RZ signal to be converted of setting signal rate is converted into the same signal rate, its It is characterized in that it includes a fiber grating with knife-type spectral response, which is composed of a fiber grating main body and two ports, a first port of the grating and a second port of the grating. 3. The CS-RZ to NRZ code conversion fiber grating device according to claim 2, characterized in that: it also includes a circulator, the circulator is connected with the fiber grating, and the CS-RZ signal to be converted enters from the circulator , and then enter the fiber grating, return to the circulator, and output from the circulator. 4. The fiber grating device for converting CS-RZ to NRZ code according to claim 2, wherein the signal rate is greater than or equal to 10Gbit/s and less than or equal to 200Gbit/s.