We report on the single-frequency operation of an optically pumped external cavity semiconductor laser. An output power of up to 400 mW is obtained in a single spatial and longitudinal mode and with a tuning range exceeding 10 nm. The laser has been stabilized electronically to a reference cavity with a relative linewidth of less than 5 kHz.
© 2004 Optical Society of America
The optically pumped vertical external cavity surface-emitting laser (VECSEL) combines the techniques developed for optically pumped solid state lasers with the advantages of quantum well engineered semiconductor lasers . Optical pumping provides a well defined gain volume enabling good control of the spatial mode. In the cases where this is possible by direct pumping with a high-power laser diode it is both an efficient and economical technique, which has enabled the development of compact high-power solid-state laser systems. These advantages are exemplified in the commercially available diode pumped Nd:YAG and Nd:YVO4 systems. One of the drawbacks of this general class of lasers, however, is the limited range of wavelengths available. A notable exception is the widely tunable Ti:Sapphire laser, which has gained great popularity despite not being pumped directly by a laser diode. On the other hand, semiconductor lasers provide – at least in principle – a wide wavelength coverage in the near-ir range. However, the mode quality is generally poor and the power in a single spatial mode is rather limited.
In the VECSELs used in the present work the gain medium is an optically pumped semiconductor with a quantum well engineered structure similar to that of the low-power and electrically driven VCSEL lasers. However, rather than forming the laser in a monolithic structure as in the VCSEL, the semiconductor constitutes one mirror and the gain medium in a laser resonator formed by an additional one or more mirrors. This external cavity geometry affords a number of additional advantages: the laser mode is determined by the external cavity, the beam size on the gain medium can be made large allowing high-power operation , and the open cavity geometry allows the insertion of intra-cavity elements for wavelength selection .
The gain structure used in the present work, which is shown in Fig. 1(a), is designed for 808 nm pumping and emission around 970 nm. The structure is based on a GaAs wafer onto which a Bragg stack is first grown. This consists of typically 30 quarter-wave pairs of AlAs and GaAs layers and will eventually form one of the end mirrors of the laser cavity. Grown on top of the Bragg mirror are typically 12 In0.16GaAs quantum wells. For the structure used in the present work they are approximately 8 nm thick and located at the anti-nodes of the standing wave field set up by the Bragg mirror. On either side of the quantum wells is an appropriate GaAsP strain compensating layer. On top of the gain region is then an Al0.3GaAs layer and finally an In0.48GaP cap layer to prevent oxidation of the material.
The laser wavelength is determined by the width and composition of the quantum wells in the gain medium. The barrier regions between the wells is chosen to have a bandgap, such that an appropriate laser pump source can be absorbed efficiently. Hence, electron-hole pairs are created in the barrier regions and diffuse into the quantum wells where the laser action takes place between the lowest bound states of the conduction and valence band wells.
The small overall thickness of the gain region affords an additional operational advantage. There is no need to match the pump spot size to that of the laser cavity throughout an extended gain medium. There is therefore no penalty from excessive divergence associated with the use of high-NA optics to image the pump light from the output of a large multi-mode fiber to the spot size of the TEM00 mode of the laser cavity. This in effect turns the VECSEL into an efficient mode converter.
The basic external cavity configuration is shown in Fig. 1(b). This set-up provides full control of the cavity mode size, is easy to align and provides ample space for intra-cavity elements for single-frequency selection. It is worth noting that the linear cavity geometry does not introduce the well-known problems of spatial hole burning usually associated with linear lasers. This is due to the resonant periodic gain, i.e. the fact that the quantum wells are positioned at the anti-nodes of the laser field, and the proximity to the Bragg mirror at the end.
The ability of the optically pumped VECSEL to efficiently produce up to several Watts of output power in a circular near-TEM00 mode is well documented [3,4]. Combined with established techniques for frequency doubling this has already led to fixed wavelength commercial products in the blue part of the spectrum with an output power of up to 200 mW . Separate from this quest for high-power operation the flexibility offered by the external cavity has enabled high repetition rate passive mode-locking of a VECSEL [6,7]. In this paper we present results for high-power single frequency and tunable operation of an optically pumped VECSEL around 970 nm. We report single spatial and longitudinal mode operation with a power of up to 400 mW and a tuning range in excess of 10 nm. Electronic stabilization has been implemented and has enabled us to demonstrate a linewidth of 5 kHz relative to a reference cavity. These results demonstrate the potential for this new type of laser to provide key operating parameters such as power, linewidth and mode quality comparable to those of the Ti:Sapphire laser, but with the benefit of high efficiency, compactness and reduced operating cost offered by the direct diode pumping. Although the tuning range of the individual VECSEL is less than that of the Ti:Sapphire laser, wavelength coverage in the 750–1100 nm range is supported by the AlGaAs and InGaAs material systems. Optical pumping in this wavelength range is readily achieved with commercially available high-power diode lasers in the 670–810 nm region.
Although most demonstrations of VECSELs to date have been in the 850–1000 nm range this technology has the potential for providing a wide wavelength coverage throughout the visible and near-ir. AlGaInP/GaAs and AlGaInN/GaN based systems can potentially wide coverage in the visible region as high-power pump sources become available at shorter wavelengths . A GaInNAs based laser pumped with a standard 810 nm source has already been demonstrated around 1320 nm .
2. Thermal management
The main challenge in high power operation of an optically pumped VECSEL is the removal of heat from the gain volume [4,10]. Excessive heating of the semiconductor structure leads to a reduction in the gain as well as a differential shift of the wavelengths of the gain and the Bragg mirror. The thermal conductivity of the GaAs substrate is relatively poor so if a gain medium of the structure shown in Fig. 1a is only cooled from the back it would typically have a temperature differential of several tens of degrees across it for 10 W pumping. One of the solutions described in the literature is to grow the structure upside down, i.e. to start with the cap layer nearest the substrate, then the quantum well gain region and then finish off with the Bragg mirror . Finally the substrate is removed and the heat can be removed directly through the Bragg mirror. In the present work we have adopted an alternative approach by removing the heat through the top of the structure. This is achieved by bonding the gain medium to a high thermal conductivity material of good optical quality [4,8].
The choice of material for this optical heat spreader is unfortunately rather limited. Sapphire would have been ideal from the material quality point of view. It is also easily accessible, capillary bonds  readily to the semiconductor gain medium, and relatively inexpensive. However, its thermal conductivity is comparable to that of GaAs so the additional cooling provided is insufficient.
We have reported satisfactory power scaling results with SiC heat spreaders . SiC offers an order of magnitude higher thermal conductivity than GaAs but it can be difficult to source material of high optical quality making it harder to capillary bond SiC to the gain medium. Furthermore, SiC is birefringent. This is not a significant limitation as far as high-power broad-band operation is concerned as the laser basically picks a polarization to suit the orientation of the SiC crystal. However, we force the laser into single frequency operation by inserting further optical elements, such as a birefringent filter, into the cavity and it has proved difficult to achieve smooth tuning with a birefringent heat spreader.
We have solved all the problems with the optical heat spreader by having it manufactured from single crystal diamond. It has an exceptionally high thermal conductivity, very good optical quality and is not birefringent. We have used 350–700 µm thick diamond heat spreaders both with parallel surfaces and with a small wedge. The surface away from the gain medium is AR coated with a reflectivity of <10-4 at the lasing wavelength to reduce any etalon effect from the heat spreader. The diamond is mounted in a water cooled and temperature stabilized copper heat sink.
A VECSEL system is set up in the basic cavity configuration shown in Fig. 1(b) using the 970 nm gain medium shown schematically in Fig. 1(a). The laser is pumped with a fiber coupled 808 nm laser diode delivering up to 25 W out of a 200 µm diameter 0.22 NA fiber. The pump light is focused with a high-NA lens system to an approximately 100 µm diameter spot impinging on the gain medium at an angle of around 45°. The opening angle of the cavity is kept as small as possible to minimize astigmatism. The folding mirror has a reflectivity of >99.8% in the range 950–1050 nm and is mounted on a piezo-electric transducer to enable rapid electronic control of the cavity length. The output coupler is flat and can be chosen with a reflectivity in the range 95–99%. The overall cavity length is approximately 180 mm.
3.1 Power scaling
With no additional intra-cavity elements inserted the laser operates with a bandwidth of approximately 2 nm around 970 nm. Fig. 2 shows the output power as a function of pump power for a laser operating with a 2% output coupler. With an initial slope efficiency of 33% thermal roll-over is reached at an output power in excess of 2W and stable operation is achieved up to about 3W when the heat sink for the gain medium is cooled below 4° C. For the heat sink operating at 10° C the thermal roll-over starts at a slightly lower pump power and the maximum output power is correspondingly lower.
3.2 Spatial mode
The spatial mode of the VECSEL is determined by the overlap of the pump spot and the cavity mode structure on the gain medium. Unfortunately the cavity optimization procedure for optimum output power carried out for the data in Fig. 2 does not correspond to a pure TEM00 output. In order to eliminate the higher order mode content the pump spot is reduced slightly, effectively introducing an aperture in gain medium. This optimization, which is associated with a drop of up to approximately 25% in output power, is monitored by observing and ultimately eliminating a beat note around 250 MHz between transverse cavity modes. The resulting output beam profile is shown in Fig. 3.
3.3 Narrow-band operation
The laser bandwidth is narrowed to ~0.1 nm by the insertion of a single-plate birefringent filter. This enables smooth tuning of the laser wavelength over the gain bandwidth. Fig. 4 shows data for the laser operating with a 2% output coupler, a pump power of 5.5 W. The tuning curve is smooth over more than 10 nm with no evidence of a modulation corresponding to the free spectral range of the diamond heat spreader (~0.3 nm). This is due to a combination of the wedge and the AR coating on the diamond.
3.4 Single-frequency operation
A single longitudinal laser mode is selected using a solid etalon with a free spectral range of ~200 GHz and matched surface reflectivities of 25%. The additional loss introduced by the etalon results in a further reduction of the output power by about 25%. A typical tuning curve for this configuration is also shown in Fig. 4.
The etalon is mounted on the axis of a galvanometer in order to enable electronic control of the tilt angle and thereby the selected cavity mode. In order to long-term stabilize the etalon to the selected cavity mode we have implemented a new technique described elsewhere  employing a birefringent etalon. Appropriate polarization analysis of the light reflected from an etalon designed as a high-order quarter-wave plate yields an electronic signal appropriate for stabilization through a feedback to the galvanometer .
3.5 Frequency stabilization
The optically pumped VECSEL is inherently a low-noise laser source due to the stable operation of the pump diodes. However, an certain amount of acoustic noise is inevitably coupled into the cavity structure and will tend to broaden the laser linewidth. We have performed initial experiments locking the laser frequency to the side of the transmission of a stable external cavity by electronic feedback to the piezo-electric transducer onto which the laser cavity fold mirror is mounted. By measuring the slope of the cavity transmission at the lock point we can relate residual intensity fluctuations of the transmission to the noise on the optical frequency. The data shown in Fig. 5 demonstrate an rms linewidth of less than 5 kHz relative to the cavity with a finesse of ~80. The spectral analysis of this residual error reveals that the noise is almost exclusively in the band from dc to 3.5 kHz.
We have demonstrated single frequency high power operation of an optically pumped VECSEL in the 970 nm range. An output power of a few hundred mW and a single-frequency tuning range in excess of 10 nm combined with a pure TEM00 mode and a narrow linewidth makes this type of laser an ideal source for high-resolution laser spectroscopy applications. With the steady progress in the development of VECSEL gain media in the visible and near-ir region this technology offers the enticing prospects of obtaining the performance otherwise only available from Ti:Sapphire or dye lasers from this compact semiconductor system.
This work was supported by the UK Engineering and Physical Sciences Research Council (EPSRC), by Scottish Enterprise under the Proof of Concept scheme and by the Royal Society of Edinburgh.
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