1504 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 10, NO. 10, OCTOBER 1998 Experimental Demonstration of Bipolar Optical CDMA System Using a Balanced Transmitter and Complementary Spectral Encoding Cedric F. Lam, Student Member, IEEE, Dennis T. K. Tong, Ming C. Wu, Member, IEEE, and Eli Yablonovitch, Fellow, IEEE Abstract—We demonstrate a novel balanced differential optical transmitter for spectrally encoded optical code-division multiple- access (CDMA) systems. The proposed structure is suitable for making optical signaling bipolar using complementary spectral encoding. An optical CDMA link with a pair of programmable transmitter and receiver is tested at the OC-3 transmission speed (155 Mb/s) for single-channel transmission. Unmatched code rejection is also demonstrated in this work. Index Terms—Balanced receiver, balanced transmitter, bipolar signaling, optical CDMA, spectral encoding. O PTICAL CODE-DIVISION multiple-access (CDMA) was first conceived as a multiple access protocol in a local area network (LAN) environment [1], [2]. In a LAN environment where the traffic is usually bursty, an efficient multiple-access protocol that allows users to access the network asynchronously at all times is important. By using optical processing, one can alleviate the amount of electronic multiplexing bottleneck and overhead. Besides, CDMA systems also offer a security advantage over other multiple access systems. Most of the proposed optical CDMA systems [1]–[4] em- ploy coherent or noncoherent encoding of ultrashort optical pulses. The coherent approach [3] involves complicated phase and polarization synchronization and is difficult to implement. The noncoherent approach [1], [2], [4] uses delay line en- coders and pseudoorthogonal intensity codes which can never achieve true orthogonality. To reduce crosstalk, these codes are designed with long code lengths and small code weight which reduces the spectral efficiency of the system. Spectrally encoded optical CDMA first appeared in [5]. It is known that by employing complementary spectral encoding and balanced detection [6], one can achieve complete bipolar- ity and true orthogonality. Moreover bipolar signaling has a 3-dB signal to noise advantage over on–off keying systems. In this letter, we propose and experimentally demonstrate a bipolar spectrally encoded optical CDMA system using a novel balanced transmitter, which employs an array of optical switches to manipulate the optical code signature, Manuscript received April 29, 1998; revised June 17, 1998. This work was supported by the Air Force Office of Scientific Research under Grant F49620- 95-1-0534. The authors are with the Electrical Engineering Department, University of California at Los Angeles, Los Angeles, CA 90095-1594 USA. Publisher Item Identifier S 1041-1135(98)07138-9. hence offering full system configurability and the possibility of fast code hopping and enhanced security. The added security in the physical layer can be useful for high bandwidth real time secure data transmissions such as voice and motion picture systems where encryption delay is critical. The schematic diagram of the proposed system is shown in Fig. 1(a). The double balanced transmitter comprises two serially-connected broadband optical sources such as super- luminescent light-emitting diodes (LED), or the spontaneous emission output from erbium-doped fibers pumped into super-luminescent mode. Alternatively, multiwavelength laser sources could also be used, as will be demonstrated later. In either case, an intensity encoder [Fig. 1(b)] is employed to selectively transmit or block certain spectral components of the balanced transmitter output. Two identical wavelength- division-multiplexed (WDM) demultiplexers are used to disperse the outputs from the balanced transmitter. An array of 2 2 switches control the transmission of individual wavelength components from either of the two broad-band optical sources. When a switch in the array is in the BAR (or CROSS) state, wavelength from the upper (or lower) optical source of the balanced transmitter is transmitted. The states of the switches, , thus define the encoded spectrum at the output WDM multiplexer. Data modulation is achieved by direct modulation of the balanced optical sources, generating the direct and complementary encoded output spectra defined by for 0 and 1 bit. The encoded spectra from various transmitters are broadcast to the receivers through a star coupler. Each receiver consists of a decoder and a balanced detector. The decoder [Fig. 1(c)] has a similar structure as the encoder, but provides the reciprocal optical path. The same encoding switch array is used for decoding. Two photodetectors connected in a balanced fashion are used to detect the difference signal between the complementary outputs of the two WDM multiplexers at the decoder. Spectral components corresponding to the direct (or complementary) encoded spectrum of the matched transmitter are combined at the upper (or lower) photodetector. Therefore, for a matched channel, the direct and complementary encoded spectra produce a 1 and 1, respectively, at the output of the balanced detector. However, a signal from an unmatched transmitter results in the received spectrum equally split between the two complementary spectral outputs of the decoder, generating zero output at the balanced detector. 1041–1135/98$10.00  1998 IEEE