Tsuei Lian Wang

High-Power Optically Pumped Semiconductor Laser at 1040 nm

We demonstrate near-diffraction limited (M 2 ¿ 1.5) output up to 23.8 W with optical-to-optical efficiency 27% and slope efficiency 32.4% and 40.7 W of multimode output from an optically pumped semiconductor laser at 1040 nm. Temperature-dependent photoluminescence measurements confirm accurate epitaxial growth according to the design thereby enhancing the effective gain.

Doppler-free spectroscopy of mercury at 253.7 nm using a high-power, frequency-quadrupled, optically pumped external-cavity semiconductor laser

We have developed a stable, high-power, single-frequency optically pumped external-cavity semiconductor laser system and generate up to 125 mW of power at 253.7 nm using successive frequency doubling stages. We demonstrate precision scanning and control of the laser frequency in the UV to be used for cooling and trapping of mercury atoms. With active frequency stabilization, a linewidth of <60 kHz is measured in the IR. Doppler-free spectroscopy and stabilization to the 6(1)S(0)-6(3)P(1) mercury transition at 253.7 nm is demonstrated. To our knowledge, this is the first demonstration of Doppler-free spectroscopy in the deep UV based on a frequency-quadrupled, high-power (>1 W) optically pumped semiconductor laser system. The results demonstrate the utility of these devices for precision spectroscopy at deep-UV wavelengths.

Heat Management in High-Power Vertical-External-Cavity Surface-Emitting Lasers

The thermal properties of a high-power vertical-external-cavity surface-emitting laser (VECSEL) are studied experimentally, focusing on the generation, distribution, and removal of excess heat under extreme pumping conditions. Different heat-spreading and heat-transfer approaches are analyzed. The performance of the device is optimized yielding a maximum emitted power beyond 70 W from a single spot. Finally, the potential for power-scaling in VECSELs and its restrictions are examined.

On the measurement of the thermal impedance in vertical-external-cavity surface-emitting lasers

A detailed and systematic analysis of the loss mechanisms in vertical-external-cavity surface-emitting lasers is presented with the goal to correctly determine the amount of pump power that is converted to heat. With this input, the accuracy of a recently proposed method for measuring the thermal impedance based on roll-over characteristics is shown to be very high for devices with and without dielectric coating. Potential errors arising from non-heating losses can be determined by performing experiments with different out-coupling mirrors.

Influence of the spatial pump distribution on the performance of high power vertical-external-cavity surface-emitting lasers

The performance of a 1040 nm vertical-external-cavity surface-emitting laser is studied as function of the size and shape of the pumped area. The input-output characteristics of the device are monitored while simultaneously tracking the temperature in the active region. It is shown that the pump spot shape plays a crucial role in optimizing the laser output. Improvements up to a factor of 5 are found for a super-Gaussian in comparison to the standard Gaussian shape. For the large pump-spot sizes needed for high output powers, it turns out that the power-scalability breaks down due to the suppressed lateral heat flow.

Predictive Microscopic Modeling of VECSELs

Based on fully microscopically computed material gain and carrier recombination rates, we compute the output characteristics of optically pumped Vertical External Cavity Surface Emitting Lasers. Very good agreement with experimental results is obtained where, besides broadenings introduced to model sample imperfections, the only experimental fit parameter is a correction to the pump spot diameter to account for the nonideal profile of the used pump.

Strategies for power scaling VECSELs

Strategies for power scaling VECSELs, including improving thermal management, increasing the quantum well gain/micro-cavity detuning that increases the threshold but increases roll-over temperature, and double-passing the excess pump via reflection from a metalized reflector at the back of a transparent distributed Bragg reflector (DBR) were studied. The influence of the heat spreader thickness and the pump profile on the temperature rise inside the active region was investigated using commercial finite element analysis software. Improvement was observed in optical efficiency of the VECSEL devices with a transparent DBR by double passing the pump light. Higher dissipated power at maximum output power was found in devices with larger spectral detuning between the quantum well gain and the micro-cavity detuning.

Influence of non-radiative carrier losses on pulsed and continuous VECSEL performance

We investigate experimentally and theoretically the influence of non-radiative carrier losses on the performance of VECSELs under pulsed and CW pumping conditions. These losses are detrimental to the VECSEL performance not only because they reduce the pump-power to output-power conversion efficiency and lead to increased thresholds, but also because they are strong sources of heat. This heating reduces the achievable output power and eventually leads to shut-off due to thermal roll-over. We investigate the two main sources of non-radiative losses, defect recombination and Auger losses in InGaAs-based VECSELs for the 1010nm-1040nm range as well as for InGaSb-based devices for operation around 2μm. While defect related losses are found to be rather insignificant in InGaAs-based devices, they can be severe enough to prevent CW operation for the InGaSb-based structures. Auger losses are shown to be very significant for both wavelengths regimes and it is discussed how structural modifications can suppress them. For pulsed operation record output powers are demonstrated and the influence of the pulse duration and shape is studied.

High Peak Power Operation of a 1- μ m GaAs-Based Optically Pumped Semiconductor Laser

We report room-temperature high-peak-power operation of an optically pumped semiconductor laser based on the InGaAs/GaAs material system. We present the design of the semiconductor structure and optimization strategies to extract the maximum pulsed peak power. The gain structure was pumped by a 775-nm Alexandrite laser with a pulsewidth adjustable from 400 ns to 1 μs and a repetition rate of 3 Hz. A new record peak power of 400 W at a wavelength of 1020 nm was obtained with a Gaussian-shaped submicrosecond pulse. An optical-to-optical efficiency of 28% is demonstrated at maximum power. The key parameters limiting the output power are discussed.

Power scaling of cw and pulsed IR and mid-IR OPSLs

We present an overview of the quantum design, growth and lasing operation of both IR and mid-IR OPSL structures aimed at extracting multi-Watt powers CW and multi-kW peak power pulsed. Issues related to power scaling are identified and discussed. The IR OPSLs based on InGaAs QW bottom emitters targeted at wavelengths between 1015nm and 1040nm are operated in CW mode (yielding a maximum power of 64W) and pulsed (peak power of 245W). The mid-IR top emitter OPSLs designed to lase at 2μm are based on a novel lattice mismatched growth using InGaSb QWs and yield a maximum peak power of 350W pulsed.