Heterostructure Waveguide design for high-power narrow far-field laser emission

Abstract

The heterostructure waveguiding and the resulting emitted vertical beam profile (VBP) is one of current issues in semiconductor lasers design and technology. The motivation is the need for improved coupling efficiency with external receivers and fibers. Generally it is accepted that the vertical beam divergence reduction can be achieved by enlarging the guided mode size [or the effective heterostructure waveguide thickness d <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">eff</inf> = d/Γ, where d is quantum well (QW) thickness and Γ is QW confinement factor], which simultaneously increases the catastrophic optical damage (COD) threshold. This relation is ambiguous, however, and depends on the heterostructure waveguide design details. For the waveguides of similar values of d <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">eff</inf> the decisive for VBP is the optical field distribution: wide evanescent tails are crucial for reduction of emitted beam divergence rather than the mode full width at half maximum (FWHM). The example is shown in Fig.1, where the refractive index profiles and calculated optical field distributions of TM <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</inf> mode guided by the Double-Barrier Separate-Confinement-Heterostructure (DBSCH) [1] of d <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">eff</inf> = 0.68 µm are compared with those for the Step-Index Broadened Waveguide Large-Optical-Cavity (STBW-LOC) heterostructure of d <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">eff</inf> = 0.72 µm (based on the published data [2]). In STBW-LOC and other LOC and SuperLOC waveguides the field distribution is accordingly wide but rather tightly confined within the waveguide layers [2–5]. In DBSCH, insertion of thin, wide-gap (low index) barrier layers at the interfaces between waveguide and cladding layers (of conventional separate confinement heterostructure (SCH)) causes a local guiding / antiguiding competition allowing for the optical confinement control. In the case of designed local antiguiding dominance a weakening of the optical confinement leads to formation of evanescent tails of the field distribution deeply penetrating cladding layers. This is like widened, ‘diffused’ edges of an emitting aperture of laser diode (LD) and leads to the vertical beam divergence reduction and COD level increase. Corresponding calculated VBPs are shown in Fig.2 for LDs based on both STBW-LOC and DBSCH structures. They are in high degree Gaussian-like. The calculated beam divergence of DBSCH LDs (Θ <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">⊥</inf> = 14.4° FWHM) is definitely lower than that of STBW-LOC LDs (Θ <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">⊥</inf> = 27.6° FWHM), despite similar d <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">eff</inf> values. Calculated Θ <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">⊥</inf> of 27.6° for STBW-LOC structure is in good agreement with the experimental 27° [2], which is a proof of credibility of this modeling. Further possible Θ <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">⊥</inf> decrease by LOC enlargement is limited by optical and recombination losses caused by carrier accumulation in the waveguide layers [3–5].