High-power, efficient semiconductor laser bars are demanded in many applications including pumping solid-state lasers and fibers. A narrow beam divergence is essential for increasing coupling efficiency and realizing an overall simple, cost-effective system. In kilowatt-class laser bars with 4 mm resonator length containing multiple broad-area emitters (with stripe width varying within 90–1200 μm) that are fabricated using conventional processing techniques, a strong thermal lens is generated within the individual emitters during laser operation. The lensing effect becomes stronger with increasing operating power. This allows a large number of lateral modes to be guided within the resonator and contribute to the laser emission, consequently deteriorating the beam quality (i.e. leading to larger lateral beam divergence angle). An approach to reduce the lateral divergence of the bar by modifying the in-plane structure of the emitters is presented. Based on simulation results, multiple lateral emitter structures have been developed and measured in quasi-continuous wave mode at low and high heat conditions with thermal resistance of 0.02 K/W and 0.05 K/W, respectively, comparable to continuous-wave testing with advanced coolers. Experimental results show that the improved lateral structures lead to enhanced power-current performance and improved beam divergence. A reduction of around 20% (~2°) in the bar lateral beam divergence angle at 95% power content has been achieved in testing at 800 W, with a simultaneous 5%-points gain in conversion efficiency with the highest performance lateral emitter structure.
KEYWORDS: Semiconductor lasers, High power lasers, Temperature metrology, Continuous wave operation, Near field, Optical testing, Near field optics, Thermography
Results of an extended series of experimental studies into the beam parameter product (BPP) of high-power diode lasers are summarized, covering efforts to clearly diagnose the limiting factors and develop novel device technology to address these limits. We review diagnostic studies, separating BPP empirically into bias-dependent (thermal) and bias-independent (non-thermal) terms for convenience of analysis. First, we use monolithically grating-stabilized lasers to confirm the presence of a well-defined series of guided modes, rather than filaments. Second, we present results from a series of custom devices and tests (guided by targeted simulations). Third, we show that effects driven by thermal lensing and current spreading dominate the variation in BPP with bias. The residual bias-independent BPP background remains around 30- 50% of the total, and is most likely partly limited by gain-guiding effects. Fourth, the presence of longitudinal temperature variation due to non-uniform optical intensity along the resonator further degrades the bias-independent background level. Lateral current blocking technology is shown to reduce current spreading, and improve the bias-dependent BPP. Thermal engineering also improves bias-dependent BPP, achieved by varying epitaxial layer structure and by targeted changes in bar layout, clarified using measurements in thermography cross-referenced to simulation. In summary, we contend that experimental studies have allowed the effects that limit lateral BPP to be largely clarified, so that research efforts can now focus on developing device technology suitable for reducing BPP without other penalties. The background level to BPP remains an open topic, and further study is needed to better understand and address this.
We present 940nm GaAs-based high-power broad-area diode lasers that use an enhanced self-aligned lateral structure "eSAS", implemented within an extreme-triple-asymmetric vertical structure with a thin p-side. In this structure, two-step epitaxial growth with intermediate selective etching is used to introduce current-blocking structures consisting of n-doped GaAs and InGaP layers outside the laser stripe, whose location, thicknesses and doping concentrations are precisely defined. These blocking structures confine current to the device center, thus reducing carrier losses in the edges and limiting the detrimental effects of lateral current spreading and carrier accumulation on beam quality, without compromising conversion efficiency, output power or polarization purity. We present results of eSAS single-emitters as well as bars with multiple emitters, in comparison to gain-guided reference devices. In addition, we demonstrate optimized blocking structures with improved current blocking, which are crucial for the realization of the eSAS structure.
We present progress in high power GaAs-based single-pass semiconductor tapered optical amplifiers and modules tailored for coherent beam combining (CBC) in master-oscillator power-amplifier configuration. Amplifier design is first studied, by varying device geometry and epitaxial structure in 976nm devices. Epitaxial structures with large vertical near field and low wave-guiding from the active region enable higher CBC efficiency. However, changes to in-plane geometry did not improve performance. Overall, CBC of tapered amplifiers is stable, reproducible and robust, motivating next the development of 1064nm CBC-ready stand-alone sub-modules. Design, construction and test results from the pilot-series fabrication of these amplifier modules are presented.
We have devised an oxide- and regrowth-free approach for current confinement in vertical-cavity surface-emitting lasers (VCSELs) which uses the photons in the cavity to optically define a current path through the device. For this purpose, an optical switch, implemented as a phototransistor (PT), is epitaxially integrated into the cavity. The PT layers become locally conductive where the highest photon density is reached in the resonator and establish the current aperture. By introducing additional photons with an external laser beam into the resonator, we show here that this current aperture can be manipulated. We demonstrate the possibility of redefining the location of the current aperture and discuss the consequences on the light-current characteristics of the devices.
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