GaAs based high power broad area lasers are the most efficient source of optical energy and are used in many industrial applications. Despite considerable improvement in power and efficiency in recent years, further improvement is needed due to the high demand from industry. We review here progress in vertical epitaxial layer design, showing how higher performance is enabled by migrating from asymmetric large optical cavity (ASLOC) designs to the newly developed extreme-triple-asymmetric (ETAS) vertical structure. Building on earlier studies at 940 nm, we focus on gain-guided lasers that have operating wavelength 970 nm, have 90 μm stripe width and 4 mm resonator length. We can emphasize the positive impact of epitaxial layer design, without need for advanced lateral structures. We show how design improvement increases conversion efficiency ηΕ at 12 W output power from 56% to 66%, whilst peak (saturation) power increases from Popt = 14 to 19 W in continuous wave (CW) mode for p-down single emitters on CuW carriers (thermal resistance 3 K/W). Progress in epitaxial design also leads to smaller lateral beam parameter product (BP Plat) at higher bias, leading to lateral brightness Popt/BPPlat < 3 W/mm × mrad. Specifically, in these most recent ETAS structures, by design BPPlat increases more slowly with self-heating, and this leads directly to lower BPPlat at high bias. We will also review options for further increased performance, include efforts to understand and improve BPPlat, which is also limited by a non-thermal ground level BPP0 (here ∼ 1 mm × mrad).
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 1 kW-emitting diode-laser bars optimized for higher conversion efficiency and smaller far-field angle Θ95% power content), as needed, e.g., for solid-state laser pumping (wavelength λ= 940 nm). First, we review the latest high-efficiency designs, targeting reduced series resistance Rs and less power saturation and then discuss developments for high brightness via tailored chip-internal heat distribution. Recent results include conversion efficiency η of 66% and far-field width Θ 95%= 8.8° at 1 kW (thermal resistance Rth ~ 0.02 K/W), as well as 64% efficiency and 10.8° divergence at Rth ~ 0.05 K/W, equivalent to CW operation with advanced packaging.
Over the last decades considerable efforts have been undertaken to increase output power, conversion efficiency and beam quality of GaAs based broad-area diode lasers by optimizing the epitaxial layer design as well as the lateral device structure. In this respect the reduction of current spreading is essential to meet future requirements for high power diode lasers. Lateral current spreading enhances the accumulation of carriers at the edges of the active region defined by the contact stripes which results in additional leakage current and lasing of higher-order lateral modes, reducing efficiency and beam quality. We address this issue by implementing a tailored deep implantation scheme as a current block, implanting O and Si, using two-step epitaxy. This work elucidates the effects of buried current apertures, fabricated by Si and O doping at different doses on the optoelectronic properties of broad area lasers. It will be shown how deep O- and Si-implantation significantly suppresses current spreading, leading to lower threshold currents and higher efficiency.
Widely-tunable lasers without moving parts are attractive light sources for sensors in industry and biomedicine. In contrast to InP based sampled grating (SG) distributed Bragg reflector (DBR) diode lasers which are commercially available, shorter wavelength GaAs SG-DBR lasers are still under development. One reason is the difficulty to integrate gratings with coupling coefficients that are high enough for functional grating bursts with lengths below 10 μm. Recently we have demonstrated > 20 nm wide quasi-continuous tuning with a GaAs based SG-DBR laser emitting around 975 nm. Wavelength selective reflectors are realized with SGs having different burst periods for the front and back mirrors. Thermal tuning elements (resistors) which are placed on top of the SG allow the control of the spectral positions of the SG reflector combs and hence to adjust the Vernier mode. In this work we characterize subsections of the developed SG-DBR laser to further improve its performance. We study the impact of two different vertical structures (with vertical far field FWHMs of 41° and 24°) and two grating orders on the coupling coefficient. Gratings with coupling coefficients above 350 cm-1 have been integrated into SG-DBR lasers. We also examine electronic tuning elements (a technique which is typically applied in InP based SG-DBR lasers and allows tuning within nanoseconds) and discuss the limitations in the GaAs material system
In this work, a widely tunable hybrid master oscillator power amplifier (MOPA) diode laser with 6.2 W of output power at 971.8 nm will be presented. The MO is a DBR laser, with a micro heater embedded on top of the DBR grating for wavelength tunability. The emitted light of the MO is collimated and coupled into a tapered amplifier using micro cylindrical lenses, all constructed on a compact 25 mm ⨯ 25 mm conduction cooled laser package. The MOPA system emits light with a measured spectral width smaller than 17 pm, limited by the spectrometer, and with a beam propagation factor of M21/e2= 1.3 in the slow axis. The emission is thus nearly diffraction limited with 79% of the total power within the central lobe (4.9 W diffraction limited). The electrically controlled micro-heater provides up to 5.5 nm of wavelength tunability, up to a wavelength of 977.3 nm, while maintaining an output power variation of only ± 0.16 % for the entire tuning range.
Er3+/Yb3+-codoped 92SiO2-8TiO2 planar waveguides, with 1.2 mol% Er and molar ratio Er/Yb of 2, were fabricated by rf-sputtering technique. The active films were deposited on silica-on-silicon and v-SiO2 substrates. The parameters of preparation were chosen in order to optimize the waveguides for operation in the NIR region with particular attention to the minimization of the losses. The thickness of the waveguides and the refractive index at 632.8 and 543.5 nm were measured by an m-line apparatus. The losses, for the TE0 mode, were evaluated at 632.8 and 1300 nm. The structural properties were investigated with several techniques such as Secondary Ion Mass Spectrometry, Energy Dispersive Spectroscopy and Raman Spectroscopy. All waveguides were single-mode at 1550 nm. An attenuation coefficient of 0.5 dB/cm at 632.8 nm and 0.1 dB/cm at 1300 nm were measured. The emission of 4I13/2 → 4I15/2 of Er3+ ion transition with a 40 nm bandwidth was observed upon excitation at 981 and 514.5 nm in the TE0 mode. Back energy transfer from Er3+ to Yb3+ was demonstrated. Photoluminescence excitation spectroscopy was used to obtain information about the effective excitation efficiency of Er3+ ions by co-doping with Yb3+ ions.