Jason Farmer

Volume Bragg semiconductor lasers with near diffraction limited divergence

The problem of high-brightness, narrow line semiconductor lasers sources is important for different kinds of applications. The proposed solution of the problem is the use of an external cavity with volume Bragg grating for effective angular and spectral selection. High-efficient volume Bragg gratings provide complete selection directly in space of wave vectors and serve as a diaphragm in angular space. The condition of effective selection is the provision of a substantial difference in losses for a selected mode by matching angular selectivity of a Bragg grating with divergence of the selected mode. It was proposed off-axis construction of an external cavity with a transmitting volume Bragg grating as an angular selective element and a reflecting volume Bragg grating as a spectral selective feedback. In such external cavity broad area laser diodes have shown stable near-diffraction limited generation in the wide range of pumping current. For LD with 0.5% AR-coated mirror and 150 μm stripe it was achieved 1.7 W output power with divergence of 0.62° at current exceeding six thresholds. Total LD slope efficiency in the considered external cavity is less then slope efficiency of free running diodes by 3-5% only. Spectral width of such locked LD emission was narrowed down to 250 pm in the whole range of pumping current.

85% power conversion efficiency 975-nm broad area diode lasers at − 50°C, 76 % at 10°C

Optimized single stripe 975-nm broad area devices deliver 76% power conversion efficiency at 10degC. Cooling the material leads to 85% efficiency at -50degC. External differential quantum efficiency is the dominant term.

Control of optical mode distribution through etched microstructures for improved broad area laser performance

Etching microstructures into broad area diode lasers is found to lead to more uniform near field and increased power conversion efficiency, arising from increased slope. Self-consistent device simulation indicates that this improvement is due to an increase in the effective internal injection efficiency above threshold—the nonuniform near field leads to regions of inefficient clamping of the carrier density in the laser stripe. Measurements of spontaneous emission through the substrate confirm the predicted carrier profile. Both experiment and theory show that improved overlap between carrier and power distributions correlates with improved slope.

Diode Laser Efficiency Increases Enable > 400-W Peak Power From 1-cm Bars and Show Clear Path to Peak Powers in Excess of 1-kW

Peak optical power from single 1-cm diode laser bars is advancing rapidly across all commercial wavelengths. Progress in material performance is reviewed and we show that current trends imply there is no fundamental barrier to achieving peak powers of 1-kW per 1-cm diode laser bar. For bars with such high peak powers, commercially available reliable devices would be expected to deliver ~ 300-W per bar. Progress to date has allowed us to demonstrate > 400-W peak output from single 1-cm diode laser bars at emission wavelengths from 800-nm to 980-nm. The available range of emission wavelengths has also been increased, with 90-W bars shown at 660-nm and 24W at 1900-nm, complementing the 100-W bar previously demonstrated at 1470-nm. Peak power is seen to correlate closely peak efficiency. Further advances in diode laser efficiency and low thermal resistance packaging technology continue to drive these powers higher. The most critical improvements have been the reduction in the diode laser operating voltage through optimization of hetero-barriers (leading to 73% efficient 100-W bars on copper micro-channel) and a reduction in packaging thermal resistance by optimizing micro-channel performance (leading to < 0.2-oC/W thermal resistance).

Extending the wavelength range of single-emitter diode lasers for medical and sensing applications: 12xx-nm quantum dots, 2000-nm wells, > 5000-nm cascade lasers

Diode lasers supply high power densities at wavelengths from 635-nm to 2000-nm, with different applications enabled by providing this power at different wavelengths. As the range of available wavelengths broadens, many novel medical and atmospheric applications are enabled. Traditional quantum well lasers provide high performance in the range 635- nm to 1100-nm range for GaAs-based devices and 1280-nm to 2000-nm for InP, leaving a notable gap in the 1100 to 1280-nm range. There are many important medical and sensing applications in this range and quantum dots produced using Stranski-Krastanow self-organized MBE growth on GaAs substrates provide an alternative high performance solution. We present results confirming broad area quantum dot lasers can deliver high optical powers of 16-W per emitter and high power conversion efficiency of 35% in this wavelength range. In addition, there are growing applications for high power sources in wavelengths > 1500-nm. We present a brief review of our current performance status in this wavelength range, both with conventional quantum wells in the 1500-nm to 2500-nm range and MOCVD grown quantum cascade lasers for wavelengths > 4000-nm. At each wavelength, we review the designs that deliver this performance, prospects for increased performance and the potential for further broadening the availability of novel wavelengths for high power applications.

Diode laser bars deliver > 400-W peak CW power from 800-nm to 980-nm enabling wide range of applications

Peak optical power from single 1-cm diode laser bars is advancing rapidly across all commercial wavelengths. Progress to date has allowed us to demonstrate > 400-W peak output from single 1-cm diode laser bars at emission wavelengths from 800-nm to 980-nm. The available range of emission wavelengths has also been increased, with 90-W bars shown at 660-nm, 37W at 1910-nm and 25W at 2070-nm, complementing the 100-W bar previously demonstrated at 1470-nm. Peak power is seen to correlate closely peak power conversion efficiency. Further advances in diode laser efficiency and low thermal resistance packaging technology continue to drive these powers higher. The most critical improvements have been the reduction in the diode laser operating voltage through optimization of hetero-barriers (leading to 74% efficient 100-W bars on micro-channel at 975-nm) and a reduction in packaging thermal resistance by optimizing microchannel performance (leading to < 0.2-°C/W thermal resistance). We have also recently extended our high efficiency designs to shorter wavelengths, now delivering over 70% efficiency at 790-nm. Ever-increasing power levels (projected to eventually exceed 1-kW per bar) reduce the cost in Euro per W of diode laser systems, enabling broader application in military, industrial and medical markets. In addition, increasing availability of high powers at new wavelengths is enabling many new applications.

High-efficiency high-power 808-nm laser array and stacked arrays optimized for elevated temperature operation

Operation of 808-nm laser diode pumps at elevated temperature is crucial to many applications. Reliable operation at high power is limited by high thermal load and low catastrophic optical mirror damage (COMD) threshold at elevated temperature range. We demonstrate high efficiency and high power operation at elevated temperatures with high COMD power. These results were achieved through device design optimization such as growth conditions, doping profile, and materials composition of the quantum-well and other layers. Electrical-to-optical efficiency as high as 62 percent was obtained through lowered threshold current and lowered series resistance and increased slope efficiency. The performance of single broad-area laser diodes scales to that of high power single bars on water-cooled copper micro-channel heatsinks or conductively-cooled CS heatsinks. No reduction in bar performance or significant spectral broadening is seen when these micro-channel coolers are assembled into 6-bar and 18-bar cw stacks for the highest power levels.

Stabilization of Lateral Mode Transients in High-Power Broad Area Semiconductor Lasers

In this work, the temporal fluctuations of lateral modes in high-power broad area semiconductor lasers are investigated in the time domain. Index guiding (in the form of etched holes) is introduced as a method of stabilizing and controlling the lateral modes. Spatial control of the lateral modes and subsequent reduction of filamentation / smoothing of the near-field profile has already been predicted and experimentally shown to improve the efficiency of broad area laser diodes. Here, the method is shown to also dampen temporal instabilities (transients) of the lateral modes.

Mode Control for High Performance Laser Diode Sources

We report on recent progress in the control of optical modes toward the improvement of commercial high-performance diode laser modules. Control of the transverse mode has allowed scaling of the optical mode volume, increasing the peak output power of diode laser emitters by a factor of two. Commercially-available single emitter diodes operating at 885 nm now exhibit >25 W peak (12 W rated) at >60% conversion efficiency. In microchannel-cooled bar format, these lasers operate >120 W at 62% conversion efficiency. Designs of similar performance operating at 976 nm have shown >37,000 equivalent device hours with no failures. Advances in the control of lateral modes have enabled unprecedented brightness scaling in a fiber-coupled package format. Leveraging scalable arrays of single emitters, the conductively-cooled nLIGHT PearlTM package now delivers >80 W peak (50 W rated) at >53% conversion efficiency measured from a 200-μm core fiber output and >45 W peak (35 W rated) at >52% conversion efficiency measured from a 100-μm fiber output. nLIGHT has also expanded its product portfolio to include wavelength locking by means of external volume Bragg gratings. By controlling the longitudinal modes of the laser, this technique is demonstrated to produce a narrow, temperature-stabilized spectrum, with minimal performance degradation relative to similar free-running lasers.

Etched Micro-structures for Control of Optical Mode Distribution for Improved Broad Area Laser Performance

Etching micro-structures into broad area diode lasers leads allows for independent control of the optical modes. Appropriately designed microstructures are found to lead to more uniform near field and increased power conversion efficiency.