Cherry May N. Mateo

Semiconductor membrane external-cavity surface-emitting laser (MECSEL)

Optically pumped semiconductor disk lasers are an important class of solid state lasers. Despite all their advantages, however, they suffer from heat incorporation into the active region caused by the excess energy of the pump photons. To overcome the limits of common methods in thermal management, we realized a semiconductor membrane external-cavity surface-emitting laser (MECSEL) consisting of a diamond heat spreader sandwiched active region design without a monolithically integrated distributed Bragg reflector (DBR). This diamond-sandwich approach improves the heat dissipation out of the active region and makes generally low-heat conductive DBRs obsolete. In an AlGaInP-based system, we demonstrate 595 mW output power at a wavelength of 657 nm and heatsink temperature of 10°C. The MECSEL enables a variety of new material combinations for new laser wavelengths and further potential for power scaling.

Enhanced efficiency of AlGaInP disk laser by in-well pumping

The performance of a 665-nm GaInP disk laser operated continuous-wave at 15°C both in-well-pumped at 640 nm and barrier pumped at 532 nm is reported. The efficiency with respect to the absorbed power was enhanced by 3.5 times when using a 640-nm pump instead of a 532-nm pump. In-well pumping which is based on the absorption of the pump photons within the quantum-well heterostructures of the gain region instead of short-wavelength absorption in the barrier and spacer regions reduces the quantum defect between pump and laser photon and hence the heat generation. A slope efficiency of 60% with respect to the absorbed pump power was obtained by in-well pumping at 15°C. Continuous-wave laser operation was further demonstrated at heat sink temperatures of up to 55°C. Both the measurement of photoluminescence and COMSOL simulation show that the overall heat load in the in-well pumped laser is smaller than in the barrier-pumped laser. These results demonstrate the potential of optical in-well pumping for the operation of red AlGaInP disk lasers if combined with means for efficient pump-light absorption.

2.5 W continuous wave output at 665 nm from a multipass and quantum-well-pumped AlGaInP vertical-external-cavity surface-emitting laser

An output power of 2.5 W at a wavelength of 665 nm was obtained from a quantum-well (QW) and multipass-pumped AlGaInP-based vertical-external-cavity surface-emitting laser operated at a heat sink temperature of 10°C. Intracavity frequency doubling resulted in an output power of 820 mW at a wavelength of 333 nm. To the best of our knowledge, these are the highest continuous wave output powers from this type of laser both at the fundamental wavelength and in frequency-doubled operation. In fundamental wavelength operation, further power scaling by increasing the pump-spot size increased the output power to 3.3 W. However, at this power level, the laser was highly unstable. When the laser was operated at 50% pump duty cycle, a reproducible and stable peak output power of 3.6 W was obtained. These results demonstrate the potential of optical QW pumping combined with multipass pumping for the operation of AlGaInP-based semiconductor disk lasers.

Gain chip design, power scaling and intra-cavity frequency doubling with LBO of optically pumped red-emitting AlGaInP-VECSELs

The wide range of applications in biophotonics, television or projectors, spectroscopy and lithography made the optically-pumped semiconductor (OPS) vertical external cavity surface-emitting lasers (VECSELs) an important category of power scalable lasers. The possibility of bandgap engineering, inserting frequency selective and converting elements into the open laser cavity and laser emission in the fundamental Gaussian mode leads to ongoing growth of the area of applications for tuneable laser sources. We present an AlGaInP-VECSEL system with a multi quantum well structure consisting of compressively strained GaInP quantum wells in an AlxGa1-xInP separate confinement heterostructure with an emission wavelength around 665 nm. The VECSEL chip with its n-λ cavity is pumped by a 532nm Nd:YAG laser under an angle to the normal incidence of 50°. In comparison, a gain chip design for high absorption values at pump wavelengths around 640nm with the use of quantum dot layers as active material is also presented. Frequency doubling is now realized with an antireflection coated lithium borate crystal, while a birefringent filter, placed inside the laser cavity under Brewsters angle, is used for frequency tuning. Further, power-scaling methods like in-well pumping as well as embedding the active region of a VECSEL between two transparent ic heaspreaders are under investigation.

Efficiency and power scaling of in-well and multi-pass pumped AlGaInP VECSELs

We report a continuous wave operation of a quantum-well and multi-pass-pumped AlGaInP based red vertical-external cavity surface-emitting laser emitting at 660 nm. The laser output power was 1.5 W with a slope efficiency of 35 %. The critical role of optimizing the sample design both for the pump and laser wavelengths, pump spot size, and the number of pump light passes were experimentally investigated.

Semiconductor membrane laser concept (MECSEL) applicable to various materials towards new emission wavelengths

Since the first realization [1] of a semiconductor disk laser, also called vertical external-cavity surface-emitting laser (VECSEL), the performance of these compact laser systems has been substantially improved. Advantageous properties including high output power, wavelength flexibility due to bandgap engineering, near-diffraction limited beam quality and the possibility to insert intra-cavity elements make them interesting for many applications. The interplay of gain and cavity resonance, the limited charge carrier confinement and the low thermal conductivity of the thick AlAs/AlGaAs distributed Bragg reflector (DBR) lead to a strongly temperature-dependent performance. Strategies such as substrate removal, intra-cavity heat spreaders, flip-chip processes or compound mirrors have improved the thermal management. Abandoning all semiconductor parts of the VECSEL except the active region itself, the heat flow can be further optimized by sandwiching the active region in-between two heat spreaders. Theoretical investigations have shown a great potential of this novel approach [2, 3].

Novel semiconductor membrane external-cavity surface-emitting laser

Optically pumped semiconductor vertical external-cavity surfaceemitting lasers (VECSELs) exhibit many desirable properties1, 2 and have therefore become an important stand-alone class of solid-state lasers over the last 20 years. For example, VECSELs can be used nowadays to reach 100W-level continuous wave output.3 However, a large quantum defect (resulting from the energy difference between pump and laser photons) means that heat is incorporated into the active region of VECSELs. This gives rise to a strongly temperature-dependent performance4 caused by the interplay of gain and cavity resonance and the limited charge-carrier confinement. The limited charge-carrier confinement is a particular challenge in the aluminum gallium indium phosphide (AlGaInP) material system, i.e., in which the thermal conductivity5, 6 is low and the laser structure is based on a thick distributed Bragg reflector (DBR). Indeed, the thermal conductivity of this type of DBR is an order of magnitude lower than well-conducting metals (i.e., which are often used as backside heatsinks) and two orders of magnitude worse than diamond (commonly used for the backside or as an intracavity heat spreader).7 In addition, the semiconductor structure itself—with a thickness of several micrometers (for the active region and the DBR)—and the substrate (with a typical thickness of 350 m) impede the heat flow out of the active region. To overcome the heat flow problems and to improve the performance of VECSELs, numerous thermal management strategies have been previously proposed. Such approaches include changes to the heat spreader arrangement,8 removing the substrate,1 flip-chip processes,9 or the insertion of compound mirrors.10 According to the natural progression of these Figure 1. Picture of the semiconductor membrane external-cavity surface-emitting laser (MECSEL) in operation. From left to right, the out-coupling/resonator mirror, diamond-sandwiched semiconductor gain membrane (integrated into a brass mount), birefringent filter, and pump optics with a 532nm pump laser beam (behind the birefringent filter), and a highly reflective resonator can be seen (as illustrated schematically in Figure 2).

The optically pumped semiconductor membrane external-cavity surface-emitting laser (MECSEL): a concept based on a diamond-sandwiched active region

Semiconductor disk lasers with all their advantages1 became an important stand-alone class of solid-state lasers during the last years. However, these systems suffer from heat incorporation into the active region caused by the excess energy of the pump photons. To overcome this limitation we realized the semiconductor membrane external-cavity surface-emitting laser as a diamond heat spreader sandwiched active region design. A detailed process description towards the MECSEL2 approach is given as well as fundamental performance values. Furthermore, parasitic lateral lasing effects are discovered and investigated. Nevertheless, the MECSEL approach indicates enormous potential to revolutionize the semiconductor based disk lasers regarding available output powers at room temperature and material combinations.

Schemes for efficient QW pumping of AlGaInP disk lasers

Keys to high-power operation of disk lasers are a thin active layer, a small Stokes shift and an efficient cooling, best realized with a limited number of QWs which are pumped close to the laser wavelength and which are in close contact with one or two diamond heat sinks. To get sufficient pump absorption many passes of the pump radiation are needed. This can be realized either by taking advantage of intrinsic resonances (designed for the pump radiation) or by an external multi-pass optics (known from Yb disk lasers) or a combination of both. The various options will be discussed and some results for AlGaInP disk lasers will be presented.

Improved gain chip holder design for high efficient, high power AlGaInP-VECSEL

In AlGaInP based VECSELs, a low thermal conductivity of the substrate with included distributed Bragg reflector leads to a strong temperature-dependent performance due to the limited charge-carrier confinement. For efficient heat removal, a good bonding between VECSEL-chip and intra-cavity heat spreader is indispensable. Here, a new designed sample holding device which allows improved bonding is presented. With this device, the laser performance of a barrier-pumped AlGaInP VECSEL emitting at 665 nm could be improved tremendously which resulted in an output power of more than 1W at a heatsink temperature of 10°C. We present a full characterization of the laser system including a comparison between standard and the new device.