Optical and Quantum Electronics 28 (1996) 1239-1257 INVITED PAPER A novel tunable dual-wavelength external- cavity laser diode array and its applications CI-LING PAN, CHI-LUEN WANG* Institute of Electro-Optical Engineering, National Chiao Tung University, Hsinchu, 30010 Taiwan, ROC Received 27 December 1995; accepted 25 March 1996 We review the principles and characteristics of a novel tunable external-cavity laser diode source. It can operate as either a continuous wave (CW) single-wavelength, a CW dual-wavelength, a single-wavelength mode-locked, or a tunable dual-wavelength mode-locked laser. The gain medium is a commercial laser diode array. The telescopic external cavity consists of a grazing-incidence grating, a lens and a stripe-mirror. The external cavity provides functions of wavelength selection, dispersion compensation and spatial mode control. The characteristics of the laser under CW or mode-locked operation are described. The dual-wavelength laser output exhibits beat note beyond 7THz. The tunable multi-terahertz beat frequency is characterized by a non-collinear intensity autocorrelation technique. A dual-wavelength interferometer that uses a tunable dual-wavelength laser as the light source is demonstrated. Finally, theoretical analysis is presented, which demonstrates that the dual-wavelength output of the laser is stable if the modes at the two wavelengths correspond to different array lateral modes. 1. Introduction Owing to their applications in coherent communication, spectroscopy and optical metrology, tunable external cavity semiconductor lasers [1] have been extensively studied in the past two decades. A tunable multi-wavelength laser is also desirable. It is useful as the light source in wavelength division multiplexing (WDM) systems, dual-wavelength interferometers, differ- ential light detection and ranging (lidar) systems and two-colour optical data storage systems. Furthermore, synchronized two-colour pulses from a single mode-locked laser are of interest for applications such as dual-wavelength pump-probe experiments and difference frequency generation. Generally, stable multi-wavelength generation from a single laser with a homo- geneously broadened media is likely only if the lasing modes utilize independent gain regions; i.e. the spatial hole-burning mechanism is operative. For example, these modes could occupy different stripes in a non-phase-locked index-guided laser array. A multi-wavelength selection mechanism would also be needed for tuning the desirable output wavelength as well as the spectral separation of the wavelengths. In this paper, we review advances in a tunable dual-wavelength *Present address: Precision Instrument Development Centre, 20 R&D Road VI, Hsinchu Science-Based Industrial Park, Hsinchu 300, Taiwan. 0306-8919 ~ 1996 Chapman & Hall 12:39