One important application of FBGs is the generation of codes for Optical
Code Division Multiple Access (OCDMA). The OCDMA technology based on superstructured 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 the 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 . For this, 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.
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.
represents the M7B-1 coded signal.
In the setup represented in
pulses with a 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, as its grating profile
does not match the profile of the encoding grating
We can observe 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, superstructured FBG (SSFBG)
Similar demonstrations are available in VPItransmissionMaker Optical Systems and on the VPIphotonics Forum.
 P.C. Teh, P. Petropoulos, M. Ibsen, and D.J. Richardson, "A Comparative Study of the Performance of Seven and 63-Chip Optical Code-Division Multiple-Access Encoders and Decoders Based on Superstructured Fiber Bragg Gratings", J. Lightwave Technol. 19 (9), 1352-1365 (2001).