26 June 1992 Transverse-mode filtering of wide-stripe semiconductor lasers using an external cavity
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Semiconducft lasers which can produce high output jxwer in a single fundamental lateral mode have in recent years become a topic of signifxant research pj ss frr maximum energy transf& when coupling the laser output to other optical components h as waveguide, optical fibers, or beam forming optics Many applications, such as free-spe commuthcation, highSpCCd optical printing, and biomedical laser technology, could greatly benefit from advances in high power, single mo& laser thos Attempts at producing fundamental mode operation from varus types of laser arrays have met with only limited success. Wide stripe miconductcr lasers can produce high 1x,w& (-8 W), and require only the most basic fabricatiou steps. However, stripe widths of more than 1-2 μn produce multiple transverse m spectra and filamentation due to carrierinduced index variations. Filamentaüon can degrade the farfie1d psofile and cause angular beam steering at high ouqit power. Using an anti-reflection coated wide stripe laser within a simple external cavity, we have produced both high output power and fundamental moik operation in the lateral dimension by filtering out higher order modes in pulsed operation. Mode discrimination is realized using an unstable cavity in the lateral dimension. The unstable cavity was fonned using a cylindrical lens within the external cavity which causes the higher order mxies to be more kssy than the fundamental. A simple Gaussian beam wopagaon model was used to xedict the effts of cylindrical lens pIzement within the external cavity. With this inokl we were able to calculate an overlap integral, or modal coupling term for each transverse mode. In our model the coupling term was considered to be spatially independent and was used to estimate the effective reflectivity of the external cavity system. Ung the cakulated effective reflectivity the threshold gath requirement for ch of the higher order modes was calculated , in order to predict the optimum placement of the cylindrical lens. Threshold gain was plotted as a fimction of the effective reflectivity (mxJal overlap) in order to predict the optimum placement of the cylindrical lens. In experiment, a 100 μm wide stripe laser was coupled to the anamorphic external cavity. WIth this cocfiguration, we achieved a nearly diffraction-limited beam at greater than 250 mW; this reprents a ftcr of ten improvement over the external cavity without the cylindrical lens. Beam quality was maintained to 3 times the original (uncoated) threshold injection current with negligible steering effects.
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Morris Burt Snipes, Morris Burt Snipes, John Gerard McInerney, John Gerard McInerney, "Transverse-mode filtering of wide-stripe semiconductor lasers using an external cavity", Proc. SPIE 1634, Laser Diode Technology and Applications IV, (26 June 1992); doi: 10.1117/12.59157; https://doi.org/10.1117/12.59157

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