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Overview

OCDMA with Super-Structured Fiber Bragg Grating (SSFBG)

Description

One important application of FBGs is the generation of codes for Optical Code Division Multiple Access (OCDMA). OCDMA technology based on super-structured FBGs gratings (SSFBGs) for code generation and recognition offers great promise for a variety of network applications, particularly in the metro and access sectors.
This application example illustrates design and characterization of SSFBG-based OCDMA encoders and decoders on the example of simple unipolar and bipolar 7-chip SSFBGs designed to encode 2.5 ps sechant-shaped pulses [1]. For this purpose, a fiber grating module is used that allows simulating the index gratings written into the core of single-mode optical fibers. The coupled-mode theory is used to describe the coupling of the co- or counter-propagating fundamental and cladding modes.

Typical Results

Figure 1 shows the spectra at the reflection port for three encoders: a pure amplitude-modulated (unipolar coded) grating M7U-1 containing seven discrete sections of grating (spatial chips), which exhibit either full or zero refractive index modulation, and two different phase-modulated (bipolar coded) gratings M7B-1 and M7B-2 with a uniform amplitude refractive index level along the whole length but with discrete pi-jumps in phase at the boundaries of adjacent spatial chips. Figure 2 represents the M7B-1 coded signal.
In the setup represented in Figure 3, pulses with 2.5 ps width are first encoded by a bipolar code encoder and then decoded by two different bipolar code decoders. The first decoder is the correct one, with the grating profile being spatially reversed with respect to the encoding grating. The second decoder is a wrong one - its grating profile does not match the profile of the encoding grating (Figure 4).
It can be observed that with a correct grating decoder, the threshold power can be safely used to detect the arrived bits with a good BER level. However, with a wrong grating decoder, the detection of the arrived bits becomes a problem due to many extra pulses. Thus, the customer receives only the bits that have been addressed specifically to him.

Keywords

Optical Code Division Multiple Access (OCDMA), Fiber Bragg Grating, super-structured FBG (SSFBG)