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En/Decoder for Spectral Phase-Coded OCDMA System Based on Amplitude Sampled FBG Meng Yan, Minyu Yao, Hongming Zhang, Li Xia, and Ye Zhang
Abstract—A novel en/decoder for spectral phase-coded optical code-division multiple-access system based on amplitude sampled fiber Bragg grating (FBG) is proposed. Both equivalent chirp and equivalent phase shift are achieved by amplitude sampling. Compared with previous en/decoder based on step chirped FBG, it is easier to fabricate. The performance of the proposed en/decoder is verified by both numerical simulation and experimental demonstration. Index Terms—Equivalent chirp, equivalent phase shift, fiber Bragg grating (FBG), optical code-division multiple-access (OCDMA), spectral phase coding (SPC). Fig. 1. Conventional SCFBG en/decoder for SPC-OCDMA system.
I. INTRODUCTION PTICAL code-division multiple-access (OCDMA) technology has recently received much interest due to its capability of enabling multiple users to share the bandwidth and communicate simultaneously. In an OCDMA system, each user is assigned an address code. At the transmitter, data bits are encoded into pseudorandom signals by an encoder. At the receiver, a decoder is used to recover the intended user’s data and reject the interference from other users. Various schemes have been proposed to implement an OCDMA system [1]–[6]. Among them, spectral phase coding (SPC) accomplishes encoding/decoding in frequency domain by introducing different phase shifts to different wavelength components of a coherent light source, such as ultrashort optical pulses from a modelocked laser [4]–[7]. Spatial light modulator, integrated ring resonator, and fiber Bragg grating (FBG) can all serve as SPCOCDMA en/decoders, while FBG has advantages in its compactness, low cost, and all-fiber structure. En/decoder based on step-chirp FBG (SCFBG) was experimentally demonstrated in [7], and a theoretical analysis was given in [8]. Such SCFBG consists of a series of uniform subgratings with different central wavelengths, and contains phase shifts between certain adjacent subgratings. In order to realize accurate phase shift in the FBG’s rapidly varying refractive index profile, nanometer precision of the FBG fabricating device is needed. In this letter, we propose a novel en/decoder based on amplitude sampled FBG (ASFBG). Both equivalent chirp and equivalent phase shift are achieved by amplitude sampling. Because no real phase shift exists in the FBG’s refractive index modulation, demanding highprecision fabricating device is not required. The proposed en/decoder differs from that in [9] in two aspects: it is designed for
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Manuscript received September 30, 2007; revised January 31, 2008. This work was supported by the National Natural Science Foundation of China (60477021). The authors are with the Electronic Engineering Department, Tsinghua University, Beijing 100084, China (e-mail:
[email protected]). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LPT.2008.921113
Fig. 2. Structure of proposed en/decoder based on ASFBG.
the SPC-OCDMA system and it combines equivalent phase shift and equivalent chirp together. II. SPECTRAL PHASE ENCODER/DECODER STRUCTURE The conventional SCFBG en/decoder for SPC-OCDMA system is illustrated in Fig. 1. Two SCFBGs are used in both subgratings encoder and decoder. Each SCFBG consists of ). G1 and G2 with different central wavelengths (here, have opposite chirp on the central wavelengths of the subgratings ( and denote the sign of chirp). G2 contains phase shifts between certain adjacent subgratings, while G1 does not. When a coherent ultrashort optical pulse is reflected by (G1, G2), the overall effect is a bipolar SPC. If the decoder matches the encoder, the phase shifts placed on different wavelength components by the encoder are removed, and the original ultrashort optical pulse is reconstructed [correctly decoded signal (CDS)]. Otherwise, the phase shifts are merely rearranged, and the decoded signal remains a low-intensity noise-like burst [incorrectly decoded signal (IDS)]. Realizing accurate phase shifts in the FBG’s rapidly varying refractive index profile requires nanometer precision of the FBG fabricating device. To eliminate this stringent requirement, we propose a novel en/decoder based on ASFBG (sketched in Fig. 2). The dark area is where the refractive index modulation exists, i.e., where grating exists. The ASFBG consists of a series of sampled subgratings (separated by vertical dashed lines in ). Fig. 2), each with its own sampling period (
1041-1135/$25.00 © 2008 IEEE
YAN et al.: EN/DECODER FOR SPECTRAL PHASE-CODED OCDMA SYSTEM BASED ON ASFBG
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Fig. 3. Fabricating ASFBG with an amplitude mask and a phase mask.
Fig. 5. Calculated (dotted) and measured (solid) reflection spectrum of ASFBG.
Fig. 4. Sampling profile of ASFBG.
For each subgrating, the amplitude sampling will generate first-order reflection channel that centers at , according to
And the channel bandwidth is inversely proportional to the subgrating’s length (1) By cascading these subgratings, equivalent chirp is obtained in their first-order reflection channels. At the same time, an can be introduced to each subequivalent phase shift , grating independently by shifting its sampling profile by according to
Particularly, an equivalent [9].
phase shift is achieved when
III. SIMULATION AND EXPERIMENTAL RESULTS as a 15-chip m-sequence [101011001000111], Denoting each user in the SPC-OCDMA system can take a different as its own address code. Setting cyclic-shifted version of both channel bandwidth and channel spacing 0.22 nm, we design the ASFBG en/decoder. The 54-mm-long ASFBG consists of 15 3.6-mm-long subgratings. The amplitude of refractive and the effective mode index index modulation . The FBG’s central wavelength nm. In order to prevent overlapping between first- and second-order reflection channels of different subgratings, the sampling duty cycle is set to 0.5, which actually eliminates all even-order reflection channels. Because the ASFBG has a complex sampling profile, an amplitude mask is used in the fabricating process. As illustrated in Fig. 3, the amplitude mask is inserted between the ultraviolet (UV) source and the phase mask to realize selective
Fig. 6. CDS and IDS (simulated).
UV exposure along the fiber, which consequently results in amplitude sampling. The interleaved opaque and transparent pattern of the amplitude mask is obtained by lifting off thin metal film from the silica glass substrate. Fig. 4 plots the sampling profile of the ASFBG. Basically, the sampling period decreases from 0.40 to 0.16 mm, so the central wavelengths of the first-order reflection channels range from 1551.4 to 1554.6 nm. The singular points in Fig. 4 are where we change phase the sampling period by half to produce equivalent shifts. The reflection spectrum of ASFBG is shown in Fig. 5. We can see that the measured spectrum matches well with the calculated one, which validates our design. The simulated decoded signal is plotted in Fig. 6. In simulation, the incident optical pulse is a transform-limited Gaussianshape pulse with 2-ps full-width at half-maximum (FWHM), and its central wavelength is 1553 nm. CDS is obtained when address code is used in both encoder and decoder, while IDS in the encoder and in the decoder. is obtained when using is the two-chip rightward cyclic-shifted version of . Although the en/decoder’s reflection spectrum has some ripples (see Fig. 5), we can still obtain pulse-like CDS and noise-like IDS after decoding. The contrast ratio between CDS and IDS is about ten, indicating good en/decoding performance of the ASFBG en/decoder. We also investigate the en/decoding capability of proposed ASFBG by experiment. The experimental setup is similar to that shown in Fig. 1. The incident 3-ps ultrashort optical pulse is from a mode-locked fiber laser. Fig. 7 shows the reflection
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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 20, NO. 10, MAY 15, 2008
Fig. 7. Reflection spectrum of ASFBG en/decoder (x: 0.4 nm/div; y : 5 dB/div).
As a proof-of-principle, we realize a 15-chip en/decoder in this letter. However, an en/decoder with a longer address code can be realized through the same approach. For a given light source with specific spectrum width, increasing the code length ( ) requires us to reduce the channel spacing, namely the channel bandwidth. According to (1), we should make is doubled, the each subgrating longer. For instance, if total grating length will be quadrupled. So, the scalability of the proposed device is mainly limited by the maximal grating length achievable. It is worth noting that our analysis is based on weak grating approximation, so it is valid only when the grating is weak. As a result, the ASFBG we fabricated exhibits low reflectivity and high power loss. Because the reflection channels of different subgratings distribute side-by-side in spectrum domain, the loss of the en/decoder is determined by the reflectivity of the subgratings. Making the subgratings stronger will reduce the loss. But as the reflectivity grows, the contrast ratio between CDS and IDS decreases and system performance deteriorates [8]. So, there is a trade-off between performance and power loss. Further numerical simulation results indicate that the loss of the en/decoder can be improved to 5 dB (subgrating reflectivity ) without obvious degradation on the CDS-to-IDS contrast ratio and system performance. Finally, the proposed en/decoder shows irregular group delay ripple (GDR). The GDR can adversely affect the system performance, especially when the decoder and encoder do not match well, or there are multiple simultaneous users in the system. REFERENCES
Fig. 8. (a) Correctly decoded signal; (b) IDS (x: 100 ps/div; y : 20 mV/div).
spectrums of two ASFBGs we use. They correspond to different address codes. Fig. 8 shows the CDS and the IDS observed with oscilloscope at the receiver. The experiment results prove that the ASFBG can achieve expected en/decoding performance. IV. CONCLUSION AND DISCUSSION In this letter, we propose a novel ASFBG-based en/decoder for a spectral phase-coded OCDMA system. Both equivalent chirp and equivalent phase shift are achieved by amplitude sampling. The ASFBG is fabricated with an amplitude mask and a uniform phase mask. Because no real phase shift exists, only micrometer precision of the amplitude mask is required, which can be conveniently achieved by lithography. Each address code requires a dedicate amplitude mask. The en/decoding performance of the ASFBG is verified by both numerical simulation and experimental investigation. Compared with a conventional SCFBG en/decoder, the ASFBG en/decoder can deliver identical en/decoding performance, is easier to fabricate, and has great potential of mass production.
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