Advances in OptoElectronics

Volume 2019, Article ID 2719808, 7 pages

https://doi.org/10.1155/2019/2719808

## 1D Confocal Broad Area Semiconductor Lasers (Confocal BALs) for Fundamental Transverse Mode Selection (TMS#0)

Correspondence should be addressed to Henning Fouckhardt; ed.lk-inu.kisyhp@rahkcuof

Received 11 March 2019; Accepted 16 April 2019; Published 17 June 2019

Academic Editor: Vasily Spirin

Copyright © 2019 Henning Fouckhardt et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### 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 4*f* 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 *= 2*f *with focal length* f*. The round trip length 2*L *= 4*f* again makes up for a Fourier-optical 4*f* 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.

#### 1. Introduction

Broad area (semiconductor diode) lasers (BALs) are intended to emit high optical output powers (where “high” is relative and depending on the material system). As compared to conventional narrow stripe lasers, the higher power is distributed over a larger transverse cross-section, thus avoiding catastrophic optical mirror damage (COMD). Typical BALs have emitter widths of around 100 *μ*m.

The drawback is the distribution of the high output power over a large number of transverse modes (in cases without countermeasures) limiting the portion of the light power in the fundamental transverse mode (mode #0), which ought to be maximized for the sake of good light focusability.

Thus techniques have to be used to support, prefer, or select the fundamental transverse mode (transverse mode selection TMS#0) by suppression of higher order modes already upon build-up of the laser oscillation.

In many cases reported in the literature, either a BAL facet, the transverse effective refractive index distribution, or the pump current distribution is modified [1–8]. Or an external cavity is employed [7–14]. In all these instances eventually low-pass spatial frequency filtering is performed. Since feedback from an external cavity may also cause self-pulsation due to destabilization of the emission process [15–19], the transverse mode selection set-up might also be* integrated* into the laser resonator [20, 21], a concept which we presented earlier. Moreover, approaches with tapered lasers or amplifiers or similar devices are known [22–25].

*Previously* in this journal we have also reported on a concept for TMS#0, which has employed a twice-retracted integrated 4*f* set-up with an actual length of 1*f* forming the laser resonator [26]. One facet has incorporated the spatial frequency filter, while the other one has housed a film-waveguide lens as the 1D Fourier-transform element. Experimental results have shown good TMS#0. The best one-dimensional beam quality parameter measured has been = 1.47.

A technological disadvantage of the latter approach has been the sophisticated preparation of the film-waveguide lens with a necessary dry-etch depth precision better than (i.e., below) 20 nm. Here we propose a simpler resonator design.

#### 2. Concept and Laser Design

*In this contribution,* we propose and report on the realization of a confocal BAL resonator with (in top-view) a bow-tie-shaped beam constriction of minimal width* a* defining the smallest transverse beam width half-way between the cylindrical facets with Fresnel reflection. These mirror-facets are both convex in 1D (viewed from outside the resonator), giving a stable resonator.

Typically confocal resonators are not employed for semiconductor lasers. An early contribution with a so-called confocal resonator is given in [27]. But one of the mirror-facets had been convex, while the other one had been concave or plane, yielding an unstable resonator. In our case, only mirror-facets, which are convex in 1D (see above) and of equal absolute value for the radius of curvature, are employed.

A confocal resonator is defined by the following equation:where* R* is the absolute value of the radius of curvature of both facets,* L* the resonator length, 2*L* the round trip length, and* f* the common (absolute value of the) focal length of the curved mirror-facets.

Both 1D curved mirror-facets perform a 1D spatial Fourier-transform, each from their respective front to their back focal plane. Since the resonator length is 2*f*, these focal planes coincide with the plane in the longitudinal middle of the resonator, called the “middle plane” from now on.

Rays with low propagation angles with respect to the optical axis account for low spatial frequencies and thus for the fundamental transverse mode (#0). They are Fresnel-reflected back into the resonator at the cylindrical mirror-facets with a reflectivity of about 31%. Rays with larger propagation angles, which correlate with larger spatial frequencies and higher transverse modes, are blocked by the transverse constriction, the latter thus acting as a 1D low-pass spatial frequency filter, intended to support the fundamental transverse mode.

Figure 1 contains a light microscope image of one of our confocal BAL resonators in top-view with a bow-tie-shaped, dry-etched laser ridge and a constriction with a width of (in this case) 32 *μ*m in the middle plane [28]. The absolute value of the radius of curvature is* R* = 1 mm for both convex facets, identical to the resonator length* L*. To the best of our knowledge, the spatial resolution of the etch process does not affect the symmetry of the bow-tie shape.