Optically Amplified Optical Code Division Mxmtiple Access(CDMA) Experiments

Abstract

suw(ARp Optical CDMA has recognized advantages for bursty, concurrent, asynchronous, non-blocking digital data communications as might be encountered in a high performance computer environment (1,2,3,4). Also, it is believed to be protocol transparent (4). However, optical CDMA tends to require laser pulsewidths much narrower than the bit time, whether coherent (4,5) or incoherent (6,7) linear codes are used (this is the origin of the "time penalty"). In addition, the concurrent communication requires broadcasting which, together with the fiber optic implementation of the codes, gives link losses in excess of the link margin for most optical communications transceiver candidates (which generaly are designed for point-to-point applications). Since 1990 we have been concentrating on matrix CDMA codes which have significantly reduced time penalties (8,9). The link loss problem can be solved by means of fiber optic amplifiers. We have integrated a 4x4 matrix CDMA system breadboard (9) with a 1550nm communications grade laser, an Ar/Ti:Sapphire 980 nm pump, and an erbium doped fiber; see Figure 1. The fiber doped amplifier (FDA) was a linear configuration. The laser diode was gain switched with a step recovery diode (SRD) and signal generator at various frequencies between 100and 500MHz. Without optical amplification, the correlation signal was below the electronic noise of the detector. With optical amplification, it has good signal to noise (SNR) and signal to clutter characteristics as shown in Figure 2. This paper discusses the analysis and experiments of optical CDMA enabled by optical amplifiers of the FDA type.