Experimental investigation of factors limiting slow axis beam quality in 9xx nm high power broad area diode lasers

Abstract

GaAs-based broad-area diode lasers are needed with improved lateral beam parameter product (BPPlat) at high power. An experimental study of the factors limiting BPPlat is therefore presented, using extreme double-asymmetric (EDAS) vertical structures emitting at 910 nm. Continuous wave, pulsed and polarization-resolved measurements are presented and compared to thermal simulation. The importance of thermal and packaging-induced effects is determined by comparing junction -up and -down devices. Process factors are clarified by comparing diodes with and without index-guiding trenches. We show that in all cases studied, BPPlat is limited by a non-thermal BPP ground-level and a thermal BPP, which depends linearly on self-heating. Measurements as a function of pulse width confirm that self-heating rather than bias-level dominates. Diodes without trenches show low BPP ground-level, and a thermal BPP which depends strongly on mounting, due to changes in the temperature profile. The additional lateral guiding in diodes with trenches strongly increases the BPP ground-level, but optically isolates the stripe from the device edges, suppressing the influence of the thermal profile, leading to a BPP-slope that is low and independent of mounting. Trenches are also shown to initiate strain fields that cause parasitic TM-polarized emission with large BPPlat, whose influence on total BPPlat remains small, provided the overall polarization purity is >95%.

High Beam Quality in Broad Area Lasers via Suppression of Lateral Carrier Accumulation

“…For this investigation, broad area devices were fabricated that make use of extreme-double-asymmetric vertical designs with extremely thin p-side, analogous in construction to [1] but with compositions modified for emission at λ = 969 nm. The devices contain no lateral index guiding, with the electrical stripe being defined purely using implantation of the electrical contact layer.…”

“…The intention is that any carriers in the vicinity of the active region that diffuse into the device edges (for example electron-hole pairs within the quantum well) will rapidly re-combine, preventing LCA from occurring. The influence of the modified carrier profile on completed devices was assessed using continuous wave (CW) measurements at heat sink temperature of 25°C, with beam quality determined by imaging near and far field profiles onto a moving slit, as described in detail in [1].…”

“…Although improved lateral (in-plane) beam quality is in strong demand to enable improved performance in laser material processing systems, the limiting effects are not fully understood [1]. Specifically, the relative importance of carrier profile, temperature, and filamentation effects remains unclear, even when detailed experimental information is available on the beam profiles of individual guided modes.…”

“…Following the approach presented in [1], rather than presenting a detailed study of a single device as in [2], we instead use measurements of a series of diagnostic lasers with varied construction to help identify the importance of carrier accumulation at the device edges. Specifically, tailored implantation profiles are used to manipulate the lateral carrier distribution, and the resulting changes in measured lateral beam parameter product BPP lat are analyzed (where BPP lat = w 95% θ 95% /4, with w 95% and θ 95% denoting the lateral near field width and far field angle containing 95% power, respectively.…”

A modified structure with asymmetric and doping barrier interlayers of GaAs-based laser diodes with both small vertical divergence angle and low threshold current

“…Small beam divergence always means reducing optical confinement factor to enlarge effective spot size, which leads to high threshold current. A great number of proposals can be found in literature on how to improve beam quality and reduce beam divergence angle, for example, by increasing waveguide layer thickness and inserting undoped and symmetric low index layers between waveguide layers and cladding layers [4][5][6][7][8]. By using these methods, although vertical far-field (VFF) angle can be depressed to a large extent, threshold current is often increased greatly.…”