2392 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 16, NO. 10, OCTOBER 2004 Experimental Demonstration of VCSEL-Based Chaotic Optical Communications Min Won Lee, Yanhua Hong, and K. Alan Shore Abstract—Message encoding and decoding using chaotic vertical-cavity surface-emitting lasers (VCSELs) has been experi- mentally demonstrated. A 200-MHz message has been successfully encoded in an external-cavity VCSEL transmitter and decoded in a stand-alone VCSEL receiver. Message recovery has been achieved with 9-dB signal-to-noise ratio. Index Terms—Chaos, cryptography, encoding, semiconductor lasers, synchronization. V ERTICAL-CAVITY surface-emitting lasers (VCSELs) subject to optical feedback exhibit some nonlinear behavior, such as transverse mode dynamics and polariza- tion mode hopping dynamics [1]–[3], which do not arise in edge-emitting laser diodes. On the other hand, VCSELs subject to delayed optical feedback show similar sensitivity to optical injections and similar chaotic optical intensity fluctuations as found in edge-emitters [4], [5]. Recently, edge-emitting lasers diodes have been used in a number of demonstrations of optical communications based on chaotic data encryption [6]–[10]. The occurrence of chaotic dynamics in VCSELs offers potential for their use in such an application. Theoretical studies of chaos synchronization and chaos data encryption in VCSELs have been forthcoming [11]–[13], but little experimental work has been reported. A fundamental requirement for such an applica- tion is the capability to effect chaos synchronization between the transmitter and receiver of such a system. A specific feature of VCSELs which militates against an experimental demon- stration of chaos synchronization is the polarization switching which may occur when VCSELs are subject to optical feedback or optical injection [14]. Recently, an experimental demonstra- tion of the chaos synchronization of mutually coupled VCSELs was reported [15], but such a configuration does not provide the capability for chaotic optical communications. However, chaos synchronization has now been achieved experimentally using unidirectionally coupled VCSELs [16] in a configuration which allows chaotic optical data encryption to be performed. In this letter, we describe an experimental demonstration of message encoding and decoding using unidirectionally coupled chaotic VCSELs. The chaotic transmitter utilizes the polarization dy- namics of a VCSEL subject to optical feedback. The receiver is a stand-alone VCSEL. A 200-MHz message is decoded when the receiver is synchronized to the chaotic transmitter. Manuscript received February 27, 2004; revised May 27, 2004. This work was supported by the European Union OCCULT (Optical Chaos Communication Using Laser Transmitters) Project (IST-2000-29 683) and by the UK EPSRC under Grant GR/S22936/01. The authors are with the School of Informatics, University of Wales, Bangor LL57 1UT, Wales, U.K. (e-mail: alan@informatics.bangor.ac.uk). Digital Object Identifier 10.1109/LPT.2004.834446 Fig. 1. Schematic diagram of the experimental setup. BS: Beam splitter. M: Mirror. OI: Optical isolator. CA: Coupling attenuator. NDF: Neutral density filter. PD: Photodetector. The experimental setup shown in Fig. 1 uses VCSELs emit- ting at 850 nm as the transmitter laser (TL) and the receiver laser (RL). The TL and RL are pumped by low-noise current sources and have their temperatures stabilized at 23.1 C and 26.2 C, respectively, using temperature controllers with 0.01 C ac- curacy. Because of the low power characteristics of VCSELs, the TL and RL are pumped well above threshold with drive cur- rents of 6.73 mA ( , where is the threshold current of the free-running laser) and 7.57 mA , respectively. An aspheric lens is used to collimate the VCSEL emission. The col- limated beam of the TL is coupled to the receiver through beam splitters BS1 and BS3. The TL is subject to optical feedback by a mirror which forms an external cavity of length 60 cm corre- sponding to 4 ns roundtrip time. Using the neutral density filter, the optical feedback intensity is adjusted to around 2% of the TL output power which allows the TL to be rendered chaotic. At the bias current indicated above, the free-running TL lases in one polarization. In the chaotic regime generated by optical feedback, the TL exhibits dynamics in two orthogonal linearly polarized states, that is transverse electric (TE) and transverse magnetic (TM) dynamics. The TE emission is selected using the half-wave plate (HWP1) and the polarization beam-splitter (PBS1). The selected TE dynamics is coupled to the receiver through the isolator OI1 ( 40-dB isolation), HWP2, and PBS2 which ensure unidirectional coupling. A 200-MHz sine-wave- form signal which directly modulates the TL through a bias-tee is used as the message to be transmitted. The receiver consists of a stand-alone VCSEL, thus forming an open-loop configura- tion. A part of the transmitted signal is coupled to the RL and 1041-1135/04$20.00 © 2004 IEEE