The above design illustrates a 3-user spectral-amplitude-coded OCDMA with two users transmitting data and the other user offline. The FBGs in the system are working as encoders/decoders for the incoherent optical signal.
In an SAC-OCDMA system based on FBGs, each user has bits of information that are modulating the broadband incoherent optical carrier to fulfill the E/O conversion (OOK modulation format). A series of FBGs are introduced to control the amplitude spectra of the broadband incoherent optical signals.
The spectral frequency pattern, with spectral chips centered about the grating frequencies, is determined by signature code properly written in the FBGs. The receiver end applies a matched series of FBGs and a balanced detector for computing correlation difference. The user’s bit stream can now be extracted.
Results – OCDMA Design
The diagram below shows the receiver-end for one of the users, where the received signal is split into an upper branch (FBGs are the same as the encoder) and lower branch (FBGs are the complement of the encoder).
Represents the receiver structure for User 1.
The optical spectrums from the two OSAs are shown below:
Represents the optical spectrum detected at the upper branch.
Represents the optical spectrum detected at the lower branch.
After the signal is reflected by FBGs at these two branches, it is photo-detected and the complementary spectra, that consists of the original data transmitted, is obtained. The Bit-Error-Rate and Eye Diagram can now be constructed and analyzed.
Represents the Eye Diagram, BER, and Q-Factor for User 1.
The Q-Factor can now be calculated for all 3 users and used to optimize overall system performance. Multiple coding schemes can also be implemented and tested using OptiSystem’s vast optical component library.