Abstract
Previously in this journal we have reported on fundamental transverse mode selection (TMS#0) of broad area semiconductor lasers (BALs) with integrated twice-retracted 4f set-up and film-waveguide lens as the Fourier-transform element. Now we choose and report on a simpler approach for BAL-TMS#0, i.e., the use of a stable confocal longitudinal BAL resonator of length L with a transverse constriction. The absolute value of the radius R of curvature of both mirror-facets convex in one dimension (1D) is R = L = 2f with focal length f. The round trip length 2L = 4f again makes up for a Fourier-optical 4f set-up and the constriction resulting in a resonator-internal beam waist stands for a Fourier-optical low-pass spatial frequency filter. Good TMS#0 is achieved, as long as the constriction is tight enough, but filamentation is not completely suppressed.
Highlights
Broad area lasers (BALs) are intended to emit high optical output powers
The drawback is the distribution of the high output power over a large number of transverse modes limiting the portion of the light power in the fundamental transverse mode, which ought to be maximized for the sake of good light focusability
The uppermost plot is from the confocal Broad area (semiconductor diode) lasers (BALs) without constriction, and the other ones stem from the devices with 64, 48, and 32 μm wide constriction, respectively
Summary
Broad area (semiconductor diode) lasers (BALs) are intended to emit high optical output powers (where “high” is relative and depending on the material system). In all these instances eventually low-pass spatial frequency filtering is performed. Since feedback from an external cavity may cause selfpulsation due to destabilization of the emission process [15– 19], the transverse mode selection set-up might be integrated into the laser resonator [20, 21], a concept which we presented earlier. Approaches with tapered lasers or amplifiers or similar devices are known [22–25]. In this journal we have reported on a concept for TMS#0, which has employed a twice-retracted integrated 4f set-up with an actual length of 1f forming the laser resonator [26].
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