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 OP-SDLs. 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 high-quality 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 its constituent layers, so the GaInNAs/GaAs gain regions and GaAs/AlAs DBRs together act as a gain mirror.
GaInNAs/GaAs quantum-wells (QWs) have traditionally been used as gain regions in edge-emitting semiconductor lasers operating at around 1.3μm. As a downside for the use of dilute nitride materials, it should be noted that incorporation of nitrogen (N) within the GaInAs lattice leads to the creation of non-radiative recombination centers that have a detrimental effect on the laser efficiency, ultimately limiting the maximum power. The power scaling depends largely on the efficiency of heat removal from the gain region. This is commonly achieved by using a transparent high thermal conductance material, such as diamond, silicon carbide (SiC), or sapphire as a heat spreader, which is placed between the semiconductor sample and a metallic heat-sink. Our research was aimed primarily at developing high quality GaInNAs/GaAs gain structures incorporating a large number of QWs that would allow high power operation. The first trial resulted in the demonstration of 1230nm OP-SDL emitting ∼1.2W of optical power at room temperature4 and >300mW of red radiation by frequency doubling.5
After further optimization, the laser efficiency and high power operation have been improved significantly.6 The gain mirrors were grown on an n-type GaAs (100) substrate by solid source molecular beam epitaxy (MBE) equipped with a radio frequency plasma source for incorporating nitrogen into the crystal lattice. The optimized structure consisted of a 30-pair GaAs/AlAs DBR and a gain region made of ten GaInNAs quantum wells with relatively low (∼0.6–0.7%) nitrogen content. Compared to the first trial structure, the number of QWs and the N content of the QWs had been decreased in order to reduce the amount of non-radiative recombination centers caused by nitrogen incorporation. Another optimization concerned the plasma source that has been operated at lower powers (∼200W).
Gain chips measuring 2.5×2.5mm2 have been cut from the as-grown SDL wafer and capillary-bonded with water to a ∼3×3×0.3mm3 type IIa natural diamond heat spreader. The bonded chips were fixed between two copper plates with indium foil in between to ensure good thermal and mechanical contact. The mounted samples were attached to a water-cooled copper heat-sink and implemented in linear (see Figure 1) or V-shaped SDL cavities. As Figure 2 shows, by using a 2.5% output coupler we have obtained a record high output power of 3.5W of 1220nm emission with the threshold and the slope efficiency of ∼2.7W and ∼20%, respectively. The temperature of the mount in these measurements was 15°C and the pump beam diameter on the gain element was ∼180μm.
The gain chip also has been used in a Z-shaped cavity to demonstrate efficient intracavity frequency doubling.7 The total output power emitted at 612nm reached a value of ∼2.7W, yielding a conversion efficiency of 7.4% from pump to red radiation. We have also demonstrated an 8nm tuning range for the output spectrum.
ROSSANA HERNANDEZ
ESTADO SOLIDO
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