Achieving high brightness (where brightness is defined as optical power per unit area per unit solid angle) in semiconductor lasers is important for various applications, including direct-laser processing and light detection and ranging for next-generation smart production and mobility. Although the brightness of semiconductor lasers has been increased by the use of edge-emitting-type resonators, their brightness is still one order of magnitude smaller than that of gas and solid-state/fibre lasers, and they often suffer from large beam divergence with strong asymmetry and astigmatism. Here, we develop a so-called ‘double-lattice photonic crystal’, where we superimpose two photonic lattice groups separated by one-quarter wavelength in the x and y directions. Using this resonator, an output power of 10 W with a very narrow-divergence-angle (<0.3°) symmetric surface-emitted beam is achieved from a circular emission area of 500 μm diameter under pulsed conditions, which corresponds to a brightness of over 300 MW cm−2 sr−1. In addition, an output power up to ~7 W is obtained under continuous-wave conditions. Detailed analyses on the double-lattice structure indicate that the resonators have the potential to realize a brightness of up to 10 GW cm−2 sr−1, suggesting that compact, affordable semiconductor lasers will be able to rival existing gas and fibre/disk lasers. An optimized design for a broad-area surface-emitting photonic-crystal laser leads to high brightness of over 300 MW cm–2 sr–1 and an output power of 10 W under pulsed excitation.

Double-lattice photonic-crystal resonators enabling high-brightness semiconductor lasers with symmetric narrow-divergence beams.

The application of wireless backhaul communication and power transfer to outdoor small cells (SCs) could significantly decrease their installation cost. In this paper, the concept of indoor optical wireless power transfer to SCs is investigated in the absence of ambient light, i.e., during darkness hours. An experimental study is conducted by the use of up to four red laser diodes (LDs), a crystalline silicon solar panel and cell placed at $5.2$  m. A value of $69$ % is measured for the fill factor of the solar panel. Also, a total power efficiency of $3.2$ % is measured for an optical wireless (OW) link with an average efficiency of two LDs of $26.8$ %, a solar cell efficiency of $13.3$ % and only $10.6$ % of geometrical losses. A comparison of this link with a state-of-the-art inductive power transfer system shows an improvement of the total power efficiency by $2.7$ times. Another OW link is implemented with a divergence of full width at $36.8$ % of the peak intensity of $3$ and $5.75$  mrad along the small and large axes of the beam, respectively. The experimental levels of harvested power are in the order of mW, whereas approximately $1$  W is required for the operation of a SC. Therefore, a $42$ laser-based transmitter is designed both analytically and by the use of the simulation tool Zemax. The respective results show the feasibility of delivering $7.2$  W of optical power to a solar cell of up to $30$  m distance with geometrical losses of only $2$ %, but a beam enclosure is also required due to eye safety restrictions.

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Indoor Optical Wireless Power Transfer to Small Cells at Nighttime

Design and technology developments targeted at increasing both power conversion efficiency and optical output power of GaAs-based diode lasers are under intense study worldwide, driven by the demands of commercial laser systems The conversion efficiency at the operation point is known to be limited by electrical and optical losses in the p-side waveguide In this paper an ‘extreme, double asymmetric’ design to mitigate the impact of the p-side waveguide is studied and compared with a more conventional design An increase of the efficiency at the highest power is demonstrated, but it is less than expected from simulations

https://iopscience.iop.org/article/10.1088/0268-1242/29/4/045010/pdf

Comparative theoretical and experimental studies of two designs of high-power diode lasers

A simplest saturable absorber, in the form of an unpumped section, is introduced into a Fabry-Perot semiconductor laser with a strongly asymmetric broadened waveguide structure incorporating a relatively thick (80 nm) active layer. This allows for suppression of trailing oscillations and a decrease in the optical pulse width compared to the uniformly biased structure. Single optical pulses of ~80 ps full width at half maximum (FWHM) and ~35 W peak power (~3 nJ pulse energy, Eopt), practically without trailing edge oscillations, were experimentally achieved under room temperature conditions by absorber-assisted gain-switching, using pumping current pulses of ~1.3 ns FWHM and ~17 A amplitude. The laser emission has a narrow (13 degrees FWHM in the transverse direction) far field.

Performance improvement by a saturable absorber in gain-switched asymmetric-waveguide laser diodes

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 ...

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

High-power broad-area diode lasers are the most efficient light sources, with 90-μm stripe GaAs-based 940-980 nm single emitters delivering > 10 W optical output at a power conversion efficiency ηE(10 W) > 65%. A review of efforts to increase ηE is presented here and we show that for well-optimized structures, the residual losses are dominated by the p -side waveguide and nonideal internal quantum efficiency ηi . The challenge in measuring efficiency to sufficient precision is also discussed. We show that ηE can most directly be improved using low heat sink temperature THS with ηE(10 W) reaching > 70% at THS = -50 °C. In contrast, increases in ηE at THS = 25 °C require improvements in both material quality and design, with growth studies targeting increased ηi and reduced threshold current and design studies seeking to mitigate the impact of the p-side waveguide. “Extreme, double asymmetric” (EDAS) designs are shown to substantially reduce p-side losses, at the penalty of increased threshold current. The benefit of EDAS designs is shown here using diode lasers with 30-μm stripes, (in development as high beam quality sources for material processing). Efficiency increases of ~ 10% relative to conventional designs are demonstrated at high powers.

Efficient High-Power Laser Diodes

We report on 100-mum-wide 975-nm distributed-feedback lasers with a cavity length of 2 mm, which deliver 2.4 W within a 14deg vertical far-field angle (full-width at half-maximum) and 35% power conversion efficiency in combination with a spectral linewidth of 0.27 nm (95% power content). The epitaxial design is a compromise between a narrow vertical far-field and a good power conversion efficiency, as required for application as a pump source. Narrow linewidth operation is sustained to 100degC, enabled by use of a grating with a high coupling coefficient.

Narrow Vertical Far-Field 975-nm Broad-Area DFB Lasers for Wide Temperature Range Operation

Efficient DFB broad-area lasers enable spectrally-resolved measurements of lateral field profiles to high powers, indicating brightness is limited by both increasing numbers of guided modes and degradation of beam quality of low order modes.

http://ieeexplore.ieee.org/iel7/6985793/6987411/06987431.pdf

C. M. Schultz

Papers

Efficient High-Power Laser Diodes

Narrow Vertical Far-Field 975-nm Broad-Area DFB Lasers for Wide Temperature Range Operation

Studies of limitations to lateral brightness in high power diode lasers using spectrally-resolved mode profiles