Jordan P. Leidner

Beam Quality Improvement in Broad-Area Semiconductor Lasers via Evanescent Spatial Filtering

In semiconductor lasers, the unpumped cladding region outside the optical waveguide is nominally lossy. Since the evanescent tail of higher-order waveguide modes extends further into the waveguide cladding, the modal loss increases for high spatial frequency perturbations, creating a weak spatial filter. Increasing the effective laser cavity length in a broad-area laser enhances the effect of this spatial filter, resulting in improved beam quality. Simulations predict that by optimizing the lateral index step and output coupler reflectivity, a factor of two increases the beam brightness with only a 10% penalty in electrical-to-optical efficiency.

Filamentation-Free Broad-Area Laser Design for Efficient Coupling to Single-Mode Fiber

Although the quest for high-power, diffraction-limited, semiconductor lasers has arguably met with limited success, the drive for higher brightness diode lasers continues, driven by fiber laser systems. Using tailored refractive-index and injection-current profiles, broad-area laser beam propagation method simulations predict simultaneous suppression of free-carrier-induced self-focusing and far-field mode compression at high powers. Asymmetric gain from tailored current profiles generate unconventional, null-less modes and lead to distributed gain- and loss-filtering mechanisms for increased beam quality output. A 5.2-7.7 W of filament-free power enables high-efficiency (70%) coupling into SMF-28, resulting in 3.1-5.5 W of fiber-coupled power for erbium-doped fiber amplifiers pumping with 0.72-W/A slope efficiency. The concept is shown to be robust against the shape of the profiles, inhomogeneities in both injection current and refractive index, and thermal lensing with proper submount design.

Three fiber designs for mitigating thermal mode Instability in high power fiber amplifiers (Conference Presentation)

Large-mode-area Yb-doped fiber amplifiers operating at high powers are of interest in many areas of manufacturing, processing, and defense as they bypass the catastrophic power density limits of single-mode fiber devices. As power is increased, thermal mode instability limits the beam quality of these devices due to thermally induced, long-period gratings that couple light out of the fundamental mode. In this work, three fiber designs for delaying the onset threshold of thermal mode instability are investigated using a spatio-temporally resolved model based on that developed by Shadi Naderi, modified for additional physics of temperature dependent modes and our design criteria. The model includes amplifier gain, temperature dependent mode shapes and effective indexes, cross- and self-phase modulation terms, and spatially-dependent coupling between fiber modes based on thermals determined by deposition and diffusion. The three designs include a trefoil multi-core design, a clad linear index-graded design, and a confined gain design. The three independent designs result in over 27%, 50%, and 100% improvement in threshold, respectively. All designs were approached based on the objective to reduce the overlaps between the fundamental mode and the higher-order modes, as well as reduce the higher-order mode overlaps with the gain dopant and thus the area of greatest thermal deposition. All three fibers are designed to have a fundamental mode of similar spatial extent to that of the fundamental mode a of a 50 micron core optical fiber with 0.06NA.

Spectral Narrowing and Stabilization for Broad-Area Lasers via Modified Delivery Fiber

A proof-of-concept experiment for fiber-based broad-area laser stabilization is experimentally demonstrated using coupling optics and a 1-nm wide, 98% reflectivity fiber Bragg grating. The output spectrum of a 3-W, 100-μm wide, 2-mm long commercial broad-area laser was narrowed by more than 10× to 0.26-nm full-width half-max. In addition, the system showed 7%-13% enhanced output power. The system also shows potential for slow-axis beam quality improvement in the desired operating regime. A pump delivery fiber scheme is proposed for integration of the diode feedback into a pump system package for enhanced-brightness fiber pumping.

Tapered Multi-Mode Interference Waveguide for High-Power Self-Organizing Single-Mode Semiconductor Laser Arrays

A 1 × N, tapered, multi-mode interference (MMI) coupler is numerically studied for use in a self-organizing semiconductor laser-array system. The MMI structure is linearly tapered in a way such that the single-port side is wider than the N-port side. The single-port waveguide is N times wider than the other ports to maintain intensity similar to that in each of the N-port waveguides. Using such a device, the N-ports of a semiconductor laser array can be coherently combined in a self-organizing architecture. This modified MMI design increases the output power threshold for catastrophic optical damage while maintaining only single-mode excitation of a wide single port output. The device is nonadiabatic with a single-pass power transmission of 98%. A power imbalance up to 40% in a single port reduces the device efficiency by less than 1%, while a waveguide roughness as large as 70 nm rms detracts only 3% from the device efficiency. When designed to work at 1 μm wavelength, the structure has a full-width half-maximum pass-band of 10 nm, which can provide wavelength selectability when used in a self-organized semiconductor laser-array system.

Spectral narrowing and stabilization of 10-W broad-area laser with fiber pump delivery system (Conference Presentation)

The emission spectrum of a 10-W commercial broad-area laser was narrowed and stabilized by coupling a portion of its far-field emission to a single-mode fiber Bragg grating. The feedback produced in this external cavity configuration stabilized the peak wavelength of the laser with respect to temperature and produced a narrow spectrum compatible with pumping ytterbium-doped fiber lasers. The spectral narrowing was achieved at multiple far field locations and stabilized over a wide temperature band, indicating a robust and manufacturable design. This proof-of-concept experiment can be translated into a pump delivery fiber configuration that integrates the diode feedback into a pump system package for spectrally bright pumping.

Tailored Transverse Cavity Designs for >7W Filamentation-Free Semiconductor Lasers

Using tailored refractive-index and injection-current profiles, broad-area laser simulations demonstrate suppression of free-carrier-induced self-focusing. 7.7W of spatially stable power enables high-efficiency (>70%) coupling into SMF-28, resulting in 5.5W of fiber-coupled power for EDFA pumping.