Low-loss smile-insensitive external frequency-stabilization of high power diode lasers enabled by vertical designs with extremely low divergence angle and high efficiency

Abstract

Broad area lasers with narrow spectra are required for many pumping applications and for wavelength beam combination. Although monolithically stabilized lasers show high performance, some applications can only be addressed with external frequency stabilization, for example when very narrow spectra are required. When conventional diode lasers with vertical far field angle, Θ V 95% ~ 45° (95% power) are stabilized using volume holographic gratings (VHGs), optical losses are introduced, limiting both efficiency and reliable output power, with the presence of any bar smile compounding the challenge. Diode lasers with designs optimized for extremely low vertical divergence (ELOD lasers) directly address these challenges. The vertical far field angle in conventional laser designs is limited by the waveguiding of the active region itself. In ELOD designs, quantum barriers are used that have low refractive index, enabling the influence of the active region to be suppressed, leading to narrow far field operation from thin vertical structures, for minimal electrical resistance and maximum power conversion efficiency. We review the design process, and show that 975 nm diode lasers with 90 µm stripes that use ELOD designs operate with Θ V 95% = 26° and reach 58% power conversion efficiency at a CW output power of 10 W. We demonstrate directly that VHG stabilized ELOD lasers have significantly lower loss and larger operation windows than conventional lasers in the collimated feedback regimes, even in the presence of significant (≥ 1 µm) bar smile. We also discuss the potential influence of ELOD designs on reliable output power and options for further performance improvement.

Development of high-power diode lasers with beam parameter product below 2 mm×mrad within the BRIDLE project

“…In these designs, low divergence is achieved by combining a 4.8 µm thick SLOC with a custom triple-quantum-well active region, which uses low-index quantum barriers (LIQB) to suppress waveguiding due to the active region. These ELoD2 designs are reviewed in detail in [17] and were shown in [18] to lead to high lateral beam quality in free-standing semiconductor tapered amplifiers, attributed to the suppression of filamentation effects due to the "SLOC+QLIB" approach. The ELoD2 design also makes use of low aluminum content in its waveguide layer, which helps minimize electrical resistance.…”

High differential efficiency tilted wave laser

“…To enable these opportunities, laser diodes with thick or ultrathick vertical waveguides have a significant advantage as a broad output aperture on a chip facet increases acceptance of light reflected from an external mirror allowing robust external resonators at low mirror reflectivity suitable for high efficiency operation. Further advantages of lasers with a thick vertical waveguide include suppressing catastrophic optical mirror damage [3], beam filamentation, thermal "smile" of a laser bar, what is of high importance for both high brightness and wavelength stabilization of the bars by external diffraction gratings [4]. However, until recently in the most of the cases lasers with moderate waveguide thickness were applied with the vertical beam divergence of ~10-25 o full width at half maximum (FWHM).…”

“…It should be noted that, in order to apply the advanced concept of low beam divergence lasers, multiple QWs (MQWs) must be used to counteract the reduced optical confinement factor [3,4,6,11,14,16]. To keep the gain region compact allowing the vertical mode selection [3,6,11], on the one hand, and avoiding defect formation in stacked InGaAs QWs [17], on the other hand, strain compensation by GaAsP barriers is usually applied.…”

Manufacturing‐tolerant compact red‐emitting laser diode designs for next generation applications

“…Quantum well laser diodes with low far-field divergence remain a topic of concentrated research and development effort with applications such as optical interconnects and data networks, pump sources [1] and next generation holographic red-green-blue displays requiring compact, high power, visible light sources with high spatial and spectral coherence [2]. Many types of designs for lasers with narrow vertical far-fields have been investigated, including large and super large optical cavity ([S]LOC) [3], photonic bandgap crystal (PBC) structures [4] and designs with extra layers in the optical waveguide [5][6][7][8].…”