1. Introduction

Diode-pumped vertical external cavity surface emitting lasers (VECSELs) have emerged recently as an important category of power-scalable semiconductor laser [1]. They are capable of multi-Watt output in a circularly symmetric, TEM00 beam [2], and have the potential to replace a number of mainframe lasers in many research and industrial applications through offering an efficient and compact technology of great wavelength flexibility. VECSELs have so far been very successful in the near-infrared, not only with impressive power scaling [3], but also with high average power sub-picosecond modelocking [4], single frequency operation [5], and frequency doubling to selected wavelengths in the violet to orange [6–9]. Blue devices in particular are now available commercially [7].

Many applications in areas including biophotonics and atom optics would benefit from similar, compact, high-performance sources in the red region of the visible spectrum. The technique of photodynamic therapy (PDT), for example, requires a monochromatic source for selective and efficient excitation of specific photosensitizers, and the efficiency and ease with which a high quality laser beam can be coupled into fibres is ideal for flexible endoscopes [10]. The majority of PDT sensitizers are activated at red wavelengths and some diode lasers are therefore suitable. However, for research purposes, high power and wavelength tuning is essential and so far the only lasers widely available for this function have been dye lasers [11]. The ideal solution is a compact, tuneable solid-state source with high beam quality at red wavelengths, performance which can be provided by VECSEL technology. Although 1.3µm VECSELs, such as our own GaInNAs devices [12] could in principle reach the red region by frequency doubling, achieving direct red operation offers advantages including higher output power in the highest quality beam and more straightforward continuous wavelength tuning. As this report will show, the GaInP/AlGaInP/GaAs material system is capable of high performance at red wavelengths.

The first demonstration of a red VECSEL was reported by Müller et al., where an GaInP/AlGaInP/GaAs structure, pumped with an argon laser at 514nm, achieved pulsed operation at low temperatures with a peak output power of 200mW [13]. Later, low power (55mW) continuous wave (CW) operation was achieved at -35°C when pumped by a 630nm dye laser [14]. Here we report high power, continuous-wave operation of a red VECSEL around room temperature. To our knowledge, this is the first VECSEL where high-power output has been achieved directly at visible wavelengths. The laser is based on a GaInP/AlGaInP structure capillary-bonded to an intracavity diamond heatspreader, and is optically pumped with a commercial, frequency-doubled, vertically polarized Nd:YVO₄ laser at 532nm. This all-solid-state red laser may be made more compact in the near future with the advent of commercial multi-Watt GaN blue/violet diodes [15]; as planar absorbers, VECSELs are ideal for diode pumping.

2. VECSEL structure

The AlGaInP alloy system offers lattice-matching to GaAs, and therefore, as with previously reported infrared VECSELs, it is convenient to use an AlGaAs distributed Bragg reflector (DBR) for high reflectivity at the signal wavelength, on top of which is the monolithically-grown AlGaInP gain region. At shorter wavelengths however, compromises must be made in the composition of the mirror, as we can no longer take advantage of the superior refractive-index-contrast of lattice-matched GaAs and AlAs since the former will absorb at the signal wavelength. A compromise is reached by introducing aluminium to the high refractive index layer and compensating for the loss in reflectivity due the reduced refractive-index-contrast by increasing the number of layer pairs.

The VECSEL structure was grown on a 2” diameter GaAs substrate, orientation [100], by molecular beam epitaxy (MBE). The DBR has 40 pairs of Al_0.45Ga_0.55As/AlAs quarter wavelength layers and the gain region has 20 Ga_0.46In_0.54P compressively-strained quantum wells, grouped in pairs and separated by (Al_0.6Ga_0.4)_0.51In_0.49P barriers so that they coincide with the antinodes of the electric field standing wave for resonant periodic gain (RPG). There is no strain compensation in the structure. The RPG wavelength, which determines the output wavelength of the VECSEL, was set at 670nm. The quantum wells were 6nm thick to achieve a room temperature peak emission wavelength at 660nm. The off-set between the quantum well and RPG wavelengths, which shift at different rates with temperature, is set to optimise performance with increasing temperature induced in the gain region by optical pumping.

3. Experimental set-up and results

A 4×4mm² sample was cleaved from the VECSEL wafer and bonded to a 4×4mm², 250µm-thick single-crystal diamond platelet, using the method of liquid capillary bonding reported in our previous work [16–18]. This diamond platelet behaves as an intracavity heatspreader for surface cooling of the VECSEL wafer. The thermal conductivity of single crystal diamond is ~30W.cm-1.K-1 [19].

The sample and heatspreader were clamped in a brass, water-cooled mount and optically pumped with up to 3.5W of power at 532nm in a 75µm diameter focus spot. The VECSEL cavity, shown in Fig. 1., consisted of the VECSEL gain structure, a high reflector folding mirror with radius of curvature 100mm, and a plane output coupler with 2% transmission at the signal wavelength. The arms of the cavity were 55mm and 250mm long, respectively, and the TEM00 mode diameter at the VECSEL wafer was calculated to be ~72µm.

With the VECSEL sample mount cooled to 0°C, 390mW output power was achieved with a slope efficiency of 17%, as shown in Fig. 2. The beam divergence is 10^-3 rad with a beam propagation ratio (M²) measured to be less than 1.05, indicating close-to-ideal fundamental mode operation.

 

Fig. 1. Experimental set-up of the red VECSEL. A birefringent filter was inserted in the cavity in order to tune the laser output spectrum. f: focal length; HR: high reflector; ROC: radius of curvature; OC: output coupler.

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The output beam from the VECSEL was passed through a broadband polarizing beamsplitter cube and the transmitted power was measured with the rotation of the cube in order to determine the polarization. The VECSEL output was found to be linearly polarized with a measured extinction ratio of ~300:1, limited by the maximum extinction ratio of the cube. It was established that the polarization was always orientated parallel to either the major (0,-1,-1) or minor (0,-1,1) flat axes in the plane of the crystal. The detailed origin of this behaviour is the subject of ongoing work.

 

Fig. 2. Power transfer of the laser with the brass mount cooling water at 0°C. Inset: horizontal profile of the laser beam measured after the output coupler.

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For tuning purposes, the VECSEL sample was rotated in its mount until the polarization was conveniently horizontal, and then a 2mm-thick, single-plate quartz birefringent filter was inserted in the long arm of the cavity at Brewster’s angle, as shown in Fig. 1. With a 2% output coupler, the output wavelength of the laser was tuned, by rotation of the filter, over a range ~5nm around a centre wavelength ~674nm, whilst maintaining an output power >100mW with a peak output power of 160mW at 674nm (Fig. 3). Replacing the output coupler with a high reflector allowed a greater tuning range of almost 10nm, although in this case the pump power was reduced and therefore the temperature of the gain region was lower. A typical free-running laser spectrum (taken with a spectrometer of resolution ~0.3nm) at the peak of the tuning curve is shown in Fig. 3. No effort was made to control the linewidth in these early measurements; with only one plate, the birefringent filter had little effect in this respect. Single-frequency operation should however be readily achievable [5].

 

Fig. 3. Tuning curves of the laser at two output couplings, with a birefringent filter inserted in the cavity. The laser output spectrum, measured at high power, is also shown.

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5. Conclusions

For the first time to our knowledge, high power CW laser operation has been achieved from a VECSEL emitting directly at visible wavelengths. Almost 400mW of output power was obtained in a TEM00 beam with a measured M2 of 1.05. The output power was limited by thermal rollover beyond a pump power of ~3.3W, however a larger pump focus would allow greater power scaling, albeit with a higher threshold [20]. The output beam was found to be linearly polarized parallel to either the major (0,-1,-1) or minor (0,-1,1) flat axes in the plane of the crystal. Using a single-plate birefringent filter it was possible to tune the output wavelength by up to ~10nm around 674nm. We are aware that the wavelength of the 2s-2p transition of the neutral lithium atom, 670.8nm, lies within this tuning range, with important implications for atom optics. This technology is now ripe for further wavelength extension in the 635–670nm range and (by frequency doubling) into the ultraviolet. In addition, further power-scaling to output of >1W is anticipated in the near future [20].