Intracavity Heat Spreader Research Articles

High-power quasi-CW diode-pumped 750-nm AlGaAs VECSEL emitting a peak power of 29.6 W and an average power of 8.5 W.

A peak output power of 29.6 W and an average output power of 8.5 W at a wavelength of 750 nm were demonstrated in quasi-CW multi-mode operation using an AlGaAs-based vertical external-cavity surface-emitting laser (VECSEL) diode-pumped at a wavelength of 675 nm. The comparatively low bandgap of the barrier material that was tuned to the pump-photon energy allowed a good compromise between low heat generation due to the quantum defect and strong absorptance of the pump radiation. The limitations for the average output power came mainly from insufficient heat flow from the intra-cavity heat spreader to the heat sink. These results show the potential for power scaling of diode-pumped VECSELs and the importance of effective heat removal.

High average output power from a backside-cooled 2-µm InGaSb VECSEL with full gain characterization.

We compare the gain and continuous wave lasing properties of two InGaSb-based vertical external cavity surface emitting lasers (InGaSb VECSEL) with different heat management approaches operating at a center wavelength of around 2μ m. To date, intracavity heatspreaders have been required for good average output power, which have many trade-offs, especially for passive modelocking. Here we demonstrate a record high average output power of 810 mW without an intracavity heatspreader using a backside-cooled non-resonant VECSEL chip optimized for modelocking. In addition, we introduce and demonstrate an optical characterization for a wavelength range of 1.9 to 3μ m to precisely measure wavelength-dependent gain saturation and spectral gain. Gain characteristics are measured as a function of wavelength, fluence, pump power and temperature. Small signal gain of more than 5%, small saturation fluences and broad gain bandwidths of more than 90 nm are demonstrated. In comparison to a commercial VECSEL chip with an intracavity heatspreader, we have obtained similar average output power even though our VECSEL chip is designed for antiresonance.

Ti:sapphire thin-disk laser symmetrically cooled by curved single crystal diamond heat spreaders

We report on the demonstration of a symmetrically cooled thin-disk laser concept using Ti:sapphire for the gain medium and plano-convex shaped single crystal diamond as intracavity heat spreaders. This allowed us to increase the absorbed pump intensity by at least a factor of five compared to a previously reported Ti:sapphire thin-disk oscillator.

DBR‐free semiconductor disc laser on SiC heatspreader emitting 10.1 W at 1007 nm

We report a distributed Bragg reflector-free semiconductor disc laser which emits 10 W continuous wave output power at a wavelength of 1007 nm when pumped with 40 W at 808 nm, focused into a 230 μm diameter spot on the gain chip. By introducing a birefringent filter plate in the laser cavity the wavelength could be tuned from 995 to 1020 nm. The laser consisted of a gain chip located at the beam waist of a linear concentric resonator with an output coupling of 2.15%. The gain chip consists of a 1.574-μm-thick resonant periodic gain structure, with ten In 0.13 Ga 0.87 As quantum wells embedded in strain-compensating GaAs 0.94 P 0.06 barrier layers, van der Waals bonded to a silicon carbide intra-cavity heat spreader.

Compact Split Disk Laser With SiC Wafer and <formula formulatype="inline"><tex Notation="TeX">${\rm Nd}{:}{\rm YVO}_{4}$</tex></formula> Bonding via Liquid Capillarity

The design of split disk laser is proposed and demonstrated with SiC wafer and uncoated Nd:YVO4 disk bonding via liquid capillarity. Using the SiC wafer as an intracavity heat spreader, the thermal effects and thermal induced distortions are greatly relieved. Depending on the excellent thermal and optical properties of SiC, a high slope efficiency of 43.3% and 5.3 W CW output power is demonstrated at room temperature without active cooling. No delamination of split disks occurred under lasing conditions. The measured temperature difference agreed with the numerical simulations.

Flip Chip Quantum-Dot Semiconductor Disk Laser at 1200 nm

In this letter, we present the first 2-W flip chip quantum-dot (QD) semiconductor disk laser operating at 1200 nm. Compared to other techniques used for thermal management, e.g., intracavity heat spreader approach, the flip chip design preserves undisturbed optical spectrum and exhibits low intracavity losses. The latter is particularly essential for power scaling of lasers with QD gain media.

Thermal effects in 2.x μm vertical-external-cavity-surface-emitting lasers

The thermal behavior of vertical-external-cavity-surface-emitting lasers (VECSELs) is investigated. The temperature distribution in operating VECSELs has been experimentally determined for various operating conditions and different cooling schemes. The implementation of the thermoreflectance technique for the thermal analysis of VECSELs is demonstrated. This technique allows for high resolution mapping of a temperature increase resulting from the optical pumping of the VECSEL. The influence of a heatspreader on the VECSEL temperature is investigated. It is demonstrated that the use of an intracavity heatspreader bonded to the VECSEL chip causes a pronounced decrease of the temperature of the device. From the heat balance in the device, the lowering of the temperature of the VECSEL during operation is predicted. This is experimentally confirmed.

Power scaling of Nd:YVO_4 and Nd:GdVO_4 disk lasers using synthetic diamond as a heat spreader

A newly developed low-birefringence synthetic diamond is shown to be an effective intracavity heat spreader in Nd:YVO4 and Nd:GdVO4 disk lasers. A cw output power of 25.7 W from only one double pass of the pump is reported. The diamond heat spreader is shown to increase the pump power density at which fracture occurs.

Control of surface and bulk crystalline quality in single crystal diamond grown by chemical vapour deposition

In order to improve the performance of existing technologies based on single crystal diamond grown by chemical vapour deposition (CVD), and to open up new technologies in fields such as quantum computing or solid state and semiconductor disc lasers, control over surface and bulk crystalline quality is of great importance. Inductively coupled plasma (ICP) etching using an Ar/Cl gas mixture is demonstrated to remove sub-surface damage of mechanically processed surfaces, whilst maintaining macroscopic planarity and low roughness on a microscopic scale. Dislocations in high quality single crystal CVD diamond are shown to be reduced by using substrates with a combination of low surface damage and low densities of extended defects. Substrates engineered such that only a minority of defects intersect the epitaxial surface are also shown to lead to a reduction in dislocation density. Anisotropy in the birefringence of single crystal CVD diamond due to the preferential direction of dislocation propagation is reported. Ultra low birefringence plates (< 10 − 5 ) are now available for intra-cavity heat spreaders in solid state disc lasers, and the application is no longer limited by depolarisation losses. Birefringence of less than 5 × 10 − 7 along a direction perpendicular to the CVD growth direction has been demonstrated in exceptionally high quality samples.

Investigation of thermal management in optically pumped, antimonide VECSELs

In this work, an optical modulation spectroscopy technique: thermoreflectance has been employed to study temperature distributions on the surface of optically pumped vertical external cavity surface emitting lasers (VECSELs). The thermoreflectance is based on the relative change of reflectivity of the sample induced by the change of its temperature. In this case the temperature change of the sample is induced by operating the pump in pulse mode. We have investigated samples with and without intracavity heatspreader. The resulting temperatures and temperature distributions on the sample surface have been measured and compared with theoretical calculations. The results confirm the effectiveness of silicon carbide (SiC) heatspreader approach. This is also a first report on the direct temperature mapping on VECSELs, with micron spatial resolution.

Thermal Management in 2.3-$\mu{\hbox {m}}$ Semiconductor Disk Lasers: A Finite Element Analysis

Finite element analysis is used to study heat flow in a 2.3-mum semiconductor disk laser (or vertical-external-cavity surface-emitting laser) based on GalnAsSb-AlGaAsSb. An intra-cavity diamond heatspreader is shown to significantly improve thermal management-and hence power scalability-in this laser compared to the substrate thinning approach typically used in semiconductor disk lasers operating around 1 mum. The parameters affecting the performance of an intracavity heat-spreader are studied in the context of a 2.3-mum semiconductor disk laser: the thermal impedance at the interface between the semiconductor gain material and the heatspreader is found to be much more important than the mounting arrangements for the gain-heatspreader composite; power scaling with pump spot radius-increasing the pump power at constant pump intensity-is found to be intrinsically limited; and the pump wavelength is predicted to have less affect on thermal management than might be expected. Direct pumping of the quantum wells is found to significantly reduce the temperature rise per unit pump power.

Dilute nitrides tailor the wavelength of semiconductor disk lasers

Interest is growing rapidly in efficient and compact optically pumped semiconductor disk lasers (OP-SDLs) since they provide a practical solution for generating high-power radiation at visible wavelengths. In particular, red-green-blue SDLs are thought to satisfy the demanding requirements of laser displays and projectors. Other applications, such as spectroscopy and biomedicine, are also expected to benefit greatly from the wavelength versatility and excellent beam quality offered by OPSDLs. Also known as vertical external cavity surface-emitting lasers,1, 2 OP-SDLs combine many of the advantages of traditional solid-state lasers with the versatility offered by semiconductor gain materials. Such laser sources can deliver Watt-level diffraction-limited output beams in a broad spectral range determined by the gain material. The key element of an OP-SDL is the semiconductor gain mirror, which is placed in an external laser cavity configuration, as shown in Figure 1. This arrangement allows a nonlinear crystal to be placed into the cavity for frequency conversion. The most notable results demonstrated with visible SDLs have been obtained in the blue-green region by frequency doubling 940nm and 1060nm laser radiation.3 However, the development of frequency-doubled OP-SDLs at red-orange wavelengths has been hindered by the lack of suitable semiconductor materials for fabricating high-quality gain mirrors operating at around 1200–1250nm. To tackle this problem, we have developed gallium indium nitride arsenide / gallium arsenide (GaInNAs/GaAs) gain regions that can be integrated with highquality gallium arsenide / aluminum arsenide (GaAs/AlAs) distributed Bragg reflectors (DBRs). DBRs act as mirrors for a specific wavelength range related to the optical thickness of Figure 1. Schematic of an OP-SDL with an intra-cavity heat-spreader.

Effect of the Cavity Resonance-Gain Offset on the Output Power Characteristics of GaSb-Based VECSELs

In spite of elaborate heat-sinking employing intracavity heat-spreaders, the maximum output power of current (AlGaIn)(AsSb)-based vertical-external-cavity surface-emitting lasers (VECSELs) is still limited by thermal rollover (e.g., 0.8 W at 20degC at a peak emission wavelength of 2.35 mum). To identify the mechanisms underlying this thermal limitation, VECSEL structures with different cavity resonance-gain offsets have been fabricated and characterized. The resulting data have been analyzed using a thermooptical model. The temperature-dependent spectral alignment of quantum-well gain and microcavity resonance (so-called ldquogain offsetrdquo) is found to be one of the key parameters in optimizing high-power performance. A 45% increase in maximum room-temperature output power is obtained if the gain offset is chosen such that the gain spectrum matches the cavity resonance at operating pump power, i.e., at elevated active region temperatures.

Effect of Heat Spreader Location on Lasing Properties of End-Pumped Vertical-External-Cavity Surface-Emitting Lasers

We characterize the lasing properties of two configurations of end-pumped vertical-external-cavity surface-emitting lasers: one with an intracavity diamond heat spreader, the other with an extracavity heat spreader. A pump-power-limited CW output power of 9.2 W at 1060 nm and a slope efficiency of ∼50.3% are achieved with the extracavity heat spreader. With the intracavity heat spreader, the output power and slope efficiency are 7.6 W and 42.2%, respectively. The difference in the slope efficiency can be explained by the difference in absorption efficiency, caused by different spatial distributions of the absorbed pump light.

Effect of the properties of an intracavity heat spreader on second harmonic generation in vertical-external-cavity surface-emitting laser

We investigate the effect of the properties of an intracavity diamond heat spreader on the lasing performance of vertical-external-cavity surface-emitting lasers. We compared two intracavity diamonds with different average birefringences. When a polarization selective element is inserted into the cavity, the infrared output powers are reduced by 13% and 7%, respectively, for the samples with higher and lower average birefringences. The pump-power limited maximum green output powers were changed from 3to5.2W by rotating the samples with lower average birefringence. In order to explain the variation in green output power, the additional round-trip loss caused by diamond birefringence was analyzed theoretically and experimentally. We found that the loss can be reduced when axes of the birefringence are aligned to the axes of the polarization selective element.

Optically pumped GaSb-based VECSEL emitting 0.6 W at 2.3 μm

We report on the operation and beam profile analysis of an optically pumped GaSb-based vertical-external-cavity surface-emitting laser at 2.33 /spl mu/m. To compensate for the low thermal conductivity of the laser chip, an intracavity heat spreader made from polycrystalline chemical vapor deposition diamond was bonded to the top surface of the chip. In this configuration, at -18/spl deg/C, a maximum continuous-wave output power of 0.6 W in a multitransverse mode beam (M/sup 2//spl ap/2.7) was achieved, limited by the available pump power. Optimizing the resonator for TEM/sub 00/ laser emission (M/sup 2//spl ap/1.1), an output power exceeding 0.4 W was observed at the same temperature.