基于超结构光纤光栅的编/解码器的发展和应用：第一部分

戴博+汪东+张大伟

摘要： 回顾了基于超结构光纤光栅的光学编/解码器的发展，介绍了基于超结构光纤光栅的编/解码器的工作原理，以及详细讲解了近期研发的两种编/解码器。采用±π/2相移的编/解码器能够隐秘地将光学码信息隐藏于编码波形中，有效地改善编码安全性。单输入多输出的编/解码器能够实现同时产生一组光学码，并且将这些光学码分配至不同的光路。

关键词： 超结构光学光栅； 光学码； 编码/解码

中图分类号： TH 741文献标志码： Adoi： 10.3969/j.issn.10055630.2014.04.020

IntroductionOptical code en/decoders are the most crucial components in plenty of optical applications.A lot of optical components have been developed into optical en/decoders，including fiberoptic delay lines（FODL），planar lightwave circuits（PLC），spatial light modulators（SLM），arrayedwaveguide grating（AWG），holographic device，and micro electromechanical systems（MEMS）[17].Among the various en/decoders，superstructured fiber Bragg grating（SSFBG）is a good candidate because of low insertion loss，polarization insensitivity，high compactness and low cost[8].In this paper，we review our past work on the development of SSFBG en/decoders.We firstly briefly introduce the operation principle of SSFBG based en/decoders and then we demonstrate two proposed novel en/decoders for security improvement and multipleoutput en/decoding.

1Operation principle of SSFBG based en/decoderA SSFBG is defined as an FBG with a rapidly varying refractive index modulation of uniform amplitude and pitch，also with a slowly varying refractiveindex modulation profile imposed along its length[9].In addition，a full complex refractiveindex modulation profile can be realized in a SSFBG by inserting phase shifts between different segments，as shown in Fig.1.After input of a short optical pulse，the SSFBG can generate a series of coherent short optical pulses whose phases are decided by the pattern of the phase shifts in the SSFBG.If the lengths of whole segments are all the same and the refractiveindex modulation is constant along the whole grating，the light can pierce the whole grating and the respective segments of the grating contribute more or less equally to the reflected response.The SSFBG thus works as an optical transversal filter to generate a binaryphaseshiftkey（BPSK）or a quaternaryphaseshiftkey（QPSK）optical code from its impulse response.Besides，it can Fig.1Superstructured FBG with phase shifts for

optical code generation/recognitionperform correlation for optical code recognition.The SSFBG can be manufactured with a single short phase mask by continuous grating writing or holographic techniques which can provide a high elasticity in generating different ultralong optical code.High accuracy phase control can be achieved as well for BPSK，QPSK or even more multiple phase level optical code.2±π/2SSFBG based en/decoder

2.1Security improvementTypically，0/πphaseshifted SSFBG（0/πSSFBG）en/decoder，which has the structure of 0 or π phase shift between adjacent chips，is always used for the temporal binary phase coding.Nevertheless，the security vulnerability of the 0/πSSFBG encoder has been revealed.The cancellation of the adjacent chips，due to the π phase shift，resulting in a dip，implies the vulnerable regularity in the encoded waveform.In consequence of this regularity，in the singleuser system，eavesdroppers can easily extract the code sequence from the encoded waveform.To address the security problem，± π/2phaseshifted SSFBG（± π/2SSFBG）en/decoder is proposed，which has the phase shift of either + π/2 or - π/2 between adjacent chips subject to the code pattern，to provide uniform encoded waveform，which is regardless of code pattern，and significantly improve the security[1011].In the experiment，we generate the Gaussian shaped optical pulse with the pulse width of 1 ps（FWHM）and inject into the 0/πSSFBG and ± π/2SSFBG encoders（CG1 and NG1）to investigate the security of the encoded waveforms，as shown in the Fig.2.We can found dips easily in the waveform，which is encoded by the 0/πSSFBG encoder.The existence of the dips infers the π phase shifts in the code pattern，resulting in the vulnerable regularity.Then，eavesdroppers can easily extract the code sequence from the encoded waveform of the 0/πSSFBG encoder.In the Fig.2（b），the encoded waveform has same peaks and valleys，and no regularity can be found，so it is hard for eavesdroppers to figure out the code sequence used in the ± π/2SSFBG encoder，which improves encoding security.

Fig.2Encoded waveforms of 0/πSSFBG and ± π/2SSFBG encoders with different chip durations（a）CG1 and（b）NG1

Fig.3Autocorrelation of the 0/πSSFBG and

± π/2SSFBG en/decoders with 31chip，

63chip and 127chip Gold code2.2Coding performanceExcept for the security performance，the coding performance of both 0/πSSFBG and ± π/2SSFBG en/decoders were also investigated by evaluating the ratio of autocorrelation intensity peak over the maximum wing level（PW）for three different code lengths.In Fig.3，we can easily see the ± π/2SSFBG en/decoder almost has the same autocorrelation performance as the 0/πSSFBG en/decoder do.When the code length increases，the autocorrelation performances of both en/decoders improve and the code capacity expands.Compared to the 31chip and 63chip cases，we found the en/decoders using 127chip Gold code behaves much better performance.Fig.4Measured and calculated correlation of 0/πSSFBG and ± π/2SSFBG en/decoders

Furthermore，we analyze the hybrid use of both en/decoders，i.e.0/πSSFBG encoder to ±π/2SSFBG decoder and ± π/2SSFBG encoder to 0/πSSFBG decoder.In the experiment，a 1 ps Gaussian shaped optical pulse is injected into the encoder and the decoder followed the encoder directly.Both measured and calculated decoded waveforms are shown in Fig.4.The decoded waveforms of 0/πSSFBG and ± π/2SSFBG en/decoders are in the topleft and bottomright box and the waveforms of the hybrid use is in the bottomleft and topright box.If the decoder matches the encoder，then generates an autocorrelation high peak.Otherwise，a crosscorrelation low power signal is produced.The hybrid use of the 0/πSSFBG and ± π/2SSFBG en/decoders also results in the low power level crosscorrelation.The P/W is larger than 7.The good correlation guarantees the coding performance of both en/decoders and promises the hybrid use of the 0/πSSFBG and ± π/2SSFBG en/decoders.The code recognition in SSFBG is aperiodic correlation，but the code sets used in the OCDMA system are periodic correlation properties.The number of codes with good aperiodic correlation is limited.The hybrid use of both kinds of en/decoders is capable of reusing the same codes.According to the experimental measurement，even though 0/πSSFBG and ± π/2SSFBG en/decoders use the same code，they still perform good crosscorrelation.It allows the same code being used twice in the same system.Therefore，the hybrid use makes it possible to expand the number of the available codes.3Singleinput multipleoutput SSFBG encoder/decoderIn this section，we review a SSFBG en/decoder，which has a single input and multiple outputs.The en/decoder is able to generate a group of independent optical codes and distribute the encoded and decoded signal into different optical paths.

3.1StructureFig.5 shows the block diagrams and schematic diagrams of the singleinput multipleoutput（SIMO）en/decoder[12].It consists of an optical circulator array，a set of optical tunable delay lines（OTDL），a group of SSFBG en/decoders and variable optical attenuators（VOA）.All the circulators have three ports and they are placed orderly.Port 1 of the first circulator is used as the input port.All Port 3s of the circulators are connected to the OTDLs and VOAs for the outputs.The SSFBG en/decoders and the circulators are connected serially.Port 2 of the circulator is connected to one side of the SSFBG and Port 1 of the nextstage circulator is connected to the other side.Since the SSFBGs are designed with relatively low reflectivity，avoiding multireflection of the light inside the gratings，the transmission of the SSFBGs en/decoders has very low loss and slight distortion.

Fig.5（a）Block diagrams of the SIMO en/decoder.（b）Schematic diagrams of the SIMO en/decoder.E and D stands for

encoder and decoder.The first inferior，1，2…M，is marshalling sequence.The second inferior，C1，C2…CN，is code number

During the encoding，an optical pulse passes through circulators and SSFBGs orderly.The encoded signals are reflected from the SSFBGs into Port 2′s of the circulators and output from Port 3′s of the circulators.The TODLs and VOAs are designed to adjust the temporal delay and balance the power.Consequently，the encoded signals could simultaneously be generated from the output ports of the SIMO encoder.In the decoding，an encoded signal is input into the SIMO decoder then passes through all the circulators and SSFBGs，which is used in the SIMO decoder have spatially reversed structures of those used in the SIMO encoder to form pairs of matched en/decoders，which can be easily realized by spatially reversely writing the gratings using the same mask.Only when the encoded signal passes through a matched decoder，a needleshape optical pulse is generated.Otherwise，only very low noiselike signals could be generated.The decoded signals are output simultaneously from the SIMO decoder and the needleshape decoded signal can be acquired from the corresponding output port.

3.2Coding performanceThe mathematical investigation of the coding performance about the SIMO en/decoder has been done.In the calculation，we design 16 pairs of SSFBGs with 31chip Gold codes and 16 pairs of SSFBGs with 63chip Gold codes.We calculate autocorrelation and crosscorrelation functions of any two pairs of SSFBGs when they are placed in the different stages of the SIMO en/decoder.Fig.6 shows the normalized peak power of the decoded signals and PC，which is an very important parameter which could evaluate the coding performance.When the SSFBG is moved from the first stage（conventional backtoback en/decoding）to the sixteenth stage in the SIMO decoder，the coding performance（PC）slightly degrades.The average degradation of 8 stages is only 5.3% for 31chip SSFBG and 7.1% for 63chip SSFBG，while the average degradation of 16 stages is 17.6% and 19.4% for 31chip and 63chip SSFBGs respectively.Although the coding performance degrades，it is not difficult to distinguish the autocorrelation signal from the crosscorrelation noises.

Fig.6Normalized power of the decoded signals and the ratio of the autocorrelation and crosscorrelation functions with the

change of the positions of the SSFBG in the SIMO decoder.SSFBGs with（a）31chip Gold codes and（b）63chip Gold codes

Besides，We keep the four pairs of SSFBGs in the first four stages of the SIMO en/decoder and detect encoded and decoded waveforms.A sequence of 10 GHz optical pulses with pulse width of 2 ps（FWHM）is produced by a modelocked laser diode（MLLD），launched into the SIMO encoder.From the first four output ports of the SIMO encoder，four encoded signals can be obtained simultaneously，as shown in Fig.7（a）.The encoded waveforms are noiselike signals.Then the encoded signal is input into the SIMO decoder respectively.Then，the decoded signals are output from the first four output ports of the SIMO decoder.The sixteen measured and calculated decoded signals are illustrated in Fig.7（b）.Optical pulses with highintensity peaks are obtained only from the output ports when SSFBGs match.However，from other output ports，we can only obtain very lowintensity noiselike signals.The perfect coding performance indicates that the SIMO en/decoder is viable to generate and recognize a set of independent optical codes simultaneously.

Fig.7（a）Encoded and（b）decoded waveforms

4ConclusionsIn this paper，the development of the SSFBG based en/decoders is reviewed.The security of encoding can be significantly improved by using ±π/2 phase shift instead of 0/π phase shift.The SIMO en/decoder is able to generate and recognize a group of optical codes and distribute the encoded and decoded signals into different optical paths.The technology of SSFBG based en/decoders develops rapidly and it makes SSFBG based en/decoders a promising device in the optical code based applications.References：

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