ring resonator and fiber Bragg grating (FBG) can all serve as .... 2005, pp. 253-255. [4] Z.Jiang, D.S.Seo, S.-D.Yang et
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REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) < CDS and IDS is about 10, 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 as that shown in Fig.1. The incident 3ps ultrashort optical pulse is from a mode-locked fiber laser. Fig.7 shows the reflection 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.
3
As a proof-of-principle, we realize 15-chip en/decoder in this letter. However, en/decoder with longer address code can be realized through same approach. For a given light source with specific spectrum width, increasing the code length (N) requires us to reduce the channel spacing, namely the channel bandwidth. According to (1), we should make each subgrating longer. For instance, if N is doubled, the 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 -5dB (subgrating reflectivity ≈ 30%) 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. Acknowledgement: The work is supported by National Natural Science Foundation of China, 60477021.
Fig.7: reflection spectrum of ASFBG en/decoder (x: 0.4nm/div, y: 5dB/div).
REFERENCES [1] [2] [3] Fig.8: (a) correctly decode signal; (b) incorrectly decoded signal (x: 100ps/div, y: 20mV/div).
IV. CONCLUSION AND DISCUSSION In this letter, we propose a novel ASFBG-based en/decoder for 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 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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[6]
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