Demonstration of a Simple Cdma Transmitter and Receiver Using Sampled Fibre Gratings

A short pulse reflected off a single sampled fibre grating is used as a transmitter, and a second sampled fibre grating with a photodiode is used as a receiver. This provides a simple encoding/decoding solution, applicable to optical CDMA networks. Introduction Optical code division multiplexing (CDM) has been proposed as an alternative to time-division multiplexing (TDM) and wavelength-division multiplexing (WDM) for over a decade. To date, optical encoding/decoding has only been demonstrated with fibre delay lines or bulk-optic approaches /1,2/. Fibre Bragg gratings may be more convenient to generate the codes necessary for CDMA. Cascaded fibre gratings provide time-resolved changes in the frequency domain and have been proposed for frequency-hopping CDMA /3,4/. We propose grating-based direct-sequence (DS) code generation, using a single binary-sampled fibre Bragg grating for encoding and a second sampled fibre grating for decoding. To our knowledge, this is the first demonstration of encoding and decoding with fibre gratings for CDMA. Experimental Demonstration The principle of encoding and decoding is shown in Fig 1. An erbium fibre ring laser (EFRL) provides short pulses, which are reflected off grating 1 to provide the transmitted code. After transmission, grating 2 operates as a matched filter to despread the code sequence. The two gratings have to be carefully wavelength matched, and their output is detected over a bandwidth of 7 GHz and displayed on an oscilloscope. Figure 1: Experimental Setup The EFRL is actively harmonically modelocked and wavelength tunable /5/. A pulse generator drives the EFRL with 400 ps pulses at a 125.1 MHz repetition rate, resulting in 40 ps optical pulses. Grating Design In DS-CDMA, each single data-bit is transmitted as a code sequence consisting of N binary chips. Thus for a constant chip rate C, the data bit-rate becomes B = C/N. Such a code sequence can readily be generated from a sampled grating, ie a grating whose refractive index modulation changes with a slowly varying envelope along its length L /6/. A grating with low overall reflectivity has an impulse response corresponding to the spatial structure, eg the impulse response of a uniform grating of length L is a square pulse of duration 2nL/c. To show the feasibility of this direct-sequence approach, we generated a 7-chip pseudo random binary m-sequence (1110010, see eg /7/) of chip duration 260 ps. Each of the ‘1’ chips is realised using a uniform refractive index modulation of length 26.9 mm, resulting in an overall grating length of 188.9 mm (upper trace in Fig 2). Figure 2: Designed transmitter grating (grating 1) and its measured pulse response