Optically tunable external cavity ring laser

A detailed investigation of the design, fabrication, and characterization of a novel optically tunable external cavity ring laser is described here. The external cavity laser (ECL) utilizes a semiconductor optical amplifier as the gain medium in a ring configuration, and a surface photonic crystal incorporating a superstrate layer of an azobenzene solution as the wavelength-selective element. Wavelength tuning is realized through optical illumination of a thin layer of azobenzene, which causes a refractive index change of the azobenzene molecules produced by photon-induced, cis???trans isomerization, thus leading to controlled tuning of the external cavity laser emission. The ECL system is stable, compact, and exhibits single-mode emission at the Bragg wavelength of the surface photonic crystal. The lasing threshold of the ECL with respectto current is ~186 mA, and the emission spectrum has a line-width as narrow as 0.05 nm, which is limited by the resolution of the spectrometer. Under a driving current of 200 mA, an output power of 500 mW is obtained, with a side-mode-suppression ratio (SMSR) of 21.8 dB. While exposed to optical illumination from a continuous Nd:YVO4 laser, the ECL demonstrates single-mode emission over a tuning wavelength range of 842 to 848 nm. The emission peak shift occurs on a timescale of minutes, and is completely reversible upon termination of the pump Nd:YVO4 laser. This optically tunable external cavity ring laser represents a departure from conventional tunable ECLs insofar as the elimination of mechanically movable parts is concerned. The result is an easy and accurate approach to remotely wavelength tuning the laser. The wavelength-selective element of the surface photonic crystal is fabricated on a plastic substrate using a replica-molding technique, which allows for inexpensive submicron structure production over large surface areas. Due to the control it provides over laser emission wavelength, as well as its reduced weight and complexity, compactness, and attractiveness for mass production, this optically tunable external cavity ring laser is expected to find applications in optically integrated systems and future photonic circuits