P. Crump

Theoretical and experimental investigations of the limits to the maximum output power of laser diodes

The factors that limit both the continuous wave (CW) and the pulsed output power of broad-area laser diodes driven at very high currents are investigated theoretically and experimentally. The decrease in the gain due to self-heating under CW operation and spectral holeburning under pulsed operation, as well as heterobarrier carrier leakage and longitudinal spatial holeburning, are the dominant mechanisms limiting the maximum achievable output power.

20W continuous wave reliable operation of 980nm broad-area single emitter diode lasers with an aperture of 96μm

High power broad area diode lasers provide the optical energy for all high performance solid state and fiber laser systems. The maximum achievable power density from such systems is limited at source by the performance of the diode lasers. A crucial metric is the reliable continuous wave optical output power from a single broad area laser diode, typically for stripe widths in the 90-100 μm range, which is especially important for users relying on fibered multi-mode pumps. We present the results of a study investigating the reliable power limits of such 980nm sources. We find that 96μm stripe single emitters lasers at 20°C operate under continuous wave power of 20W per emitter for over 4000 hours (to date) without failure, with 60μm stripe devices operating reliably at 10W per stripe. Maximum power testing under 10Hz, 200μs QCW drive conditions shows that 96μm stripes reach 30W and 60μm stripes 21W per emitter, significantly above the reliable operation point. Results are also presented on step-stress-studies, where the current is step-wise increased until failure is observed, in order to clarify the remaining reliability limits. Finally, we detail the barriers to increased peak power and discuss how these can be overcome.

Experimental and theoretical analysis of the dominant lateral waveguiding mechanism in 975 nm high power broad area diode lasers

For maximum fibre-coupled power, high power broad area diode lasers must operate with small lateral far field angles at high continuous wave (CW) powers. However, these structures are laterally multi-moded, with low beam quality and wide emission angles. In order to experimentally determine the origin of the low beam quality, spectrally resolved near and far field measurements were performed for a diode laser with 50 µm stripe width. Within the range measured (CW optical output powers to 1.5 W) the laser is shown to operate in just six stable lateral modes, with spatially periodic profiles. Comparisons of the measured profiles with the results of two-dimensional modal simulation demonstrate that current-induced thermal lensing dominates the lateral waveguiding, in spite of the presence of both strong built-in index guiding and gain guiding. No evidence is seen for filamentation. Building on the diagnosis, proposals are presented for improvements to beam quality.

Combination of Low-Index Quantum Barrier and Super Large Optical Cavity Designs for Ultranarrow Vertical Far-Fields From High-Power Broad-Area Lasers

When active regions that use low refractive index quantum barriers (LIQB) are combined with super large optical cavity (SLOC) designs in GaAs-based diode lasers, high-power operation with extremely narrow vertical far-fields is observed. However, LIQB designs are found to have lower slope efficiency and increased operation voltage. Comparison of experiment and finite element device simulation shows that this is due to hole accumulation at the edge of the active region. Example devices using an 8.6-μm thick SLOC deliver 30 W at 1065 nm with vertical divergence of 15.6° (95% power).

Root-Cause Analysis of Peak Power Saturation in Pulse-Pumped 1100 nm Broad Area Single Emitter Diode Lasers

Many physical effects can potentially limit the peak achievable output power of single emitter broad area diode lasers under high current, pulse-pumped operation conditions. Although previous studies have shown reliable operation to high pump levels (240 A, 300 ns, and 1 kHz), power was found to saturate. We present here results of a systematic study to unambiguously determine the sources of this power saturation. A combination of detailed measurements and finite element device simulation were used for the diagnosis. We find that the measured power saturation is dominated by electron leakage caused by band bending at high bias due to the low mobility of the p-type waveguide. However, the power saturation is only fully reproduced when longitudinal spatial hole-burning is included. Higher powers are expected to be achieved if higher energy barriers and lower confinement factors are used to mitigate leakage and longitudinal hole-burning, respectively.

975 nm high power diode lasers with high efficiency and narrow vertical far field enabled by low index quantum barriers

For optimal coupled power into fiber, high power diode lasers should operate efficiently with smallest possible vertical far field emission angle. Although waveguide and cladding layers can be designed to achieve small angles, the refractive index profile of the active region itself restricts the minimum achievable value. We show that the use of low index quantum barrier layers leads to substantially reduced far field angles, while sustaining high power conversion efficiency. 90 μm stripe lasers that use such designs have narrow vertical far field angles of 30° (95% power content), power conversion efficiency of 58% and operate reliably at 10 W output.

Volume Bragg semiconductor lasers with near diffraction limited divergence

The problem of high-brightness, narrow line semiconductor lasers sources is important for different kinds of applications. The proposed solution of the problem is the use of an external cavity with volume Bragg grating for effective angular and spectral selection. High-efficient volume Bragg gratings provide complete selection directly in space of wave vectors and serve as a diaphragm in angular space. The condition of effective selection is the provision of a substantial difference in losses for a selected mode by matching angular selectivity of a Bragg grating with divergence of the selected mode. It was proposed off-axis construction of an external cavity with a transmitting volume Bragg grating as an angular selective element and a reflecting volume Bragg grating as a spectral selective feedback. In such external cavity broad area laser diodes have shown stable near-diffraction limited generation in the wide range of pumping current. For LD with 0.5% AR-coated mirror and 150 μm stripe it was achieved 1.7 W output power with divergence of 0.62° at current exceeding six thresholds. Total LD slope efficiency in the considered external cavity is less then slope efficiency of free running diodes by 3-5% only. Spectral width of such locked LD emission was narrowed down to 250 pm in the whole range of pumping current.

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

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 ...

85% power conversion efficiency 975-nm broad area diode lasers at − 50°C, 76 % at 10°C

Optimized single stripe 975-nm broad area devices deliver 76% power conversion efficiency at 10degC. Cooling the material leads to 85% efficiency at -50degC. External differential quantum efficiency is the dominant term.

1060-nm Ridge Waveguide Lasers Based on Extremely Wide Waveguides for 1.3-W Continuous-Wave Emission Into a Single Mode With FWHM Divergence Angle of

Extremely large epitaxial waveguides with thickness t_WG = 8.6 μm enable diode lasers with very narrow vertical divergence angle. We demonstrate that when such designs are processed in ridge waveguide laser format, there is substantial interaction between the vertical and lateral waveguiding mechanisms. For very narrow stripes with width w ≪ t_WG, such interaction leads to lasing operation in the second-order mode in the vertical direction. For the case of an optimal stripe width w~t_WG single fundamental mode operation is achieved, with peak kink-free optical output power of 1.3 W and beam divergence angles (vertical, lateral) of 9° × 6° at full-width at half-maximum and 17° × 13° with 95% power content. The maximum brightness is 90 MW × cm^-2 sr^-1 for 95% power content.