Multiwatt VECSEL Output Is Wavelength-Tunable
Optically pumped, vertical-external-cavity surface-emitting lasers (VECSELs) produce multiwatt outputs with good beam quality, but until now their spectral characteristics have been less than optimal. Their linewidths are too wide for many applications, and the central wavelength tends to drift with thermal changes of the laser.
Recently, researchers at the University of Arizona and Areté Associates, both in Tucson, and at Philipps Universität Marburg in Germany inserted a birefringent filter into the resonator of an optically pumped VECSEL and obtained a spectrally narrow, multiwatt, fundamental-mode, polarized output that they were able to tune across 20 nm.
Birefringent filters were intensely studied several decades ago as a means of controlling the wavelength of dye lasers. In its simplest form, a birefringent filter is a wave plate that is inserted into the resonator at Brewster’s angle. Only a narrow range of wavelengths, determined by the orientation of the wave plate’s axes, can pass through both Brewster surfaces with zero loss. Because other wavelengths experience loss, they cannot reach threshold, and the laser lases only at the selected wavelengths.
Figure 1. Two external mirrors, one ~5 cm and the other ~17.5 cm from the semiconductor chip, formed the laser resonator. Images ©2006, American Institute of Physics.
The researchers configured their resonator in a V-shape with the gain medium — nine InGaAs double quantum wells — and the underlying distributed Bragg reflector (DBR) at the apex of the V (Figure 1). Each double well was positioned at a peak of the intracavity standing wave to enhance coupling the population inversion to the circulating power. The reflectivity of the DBR was >0.999, while the reflectivities of the two external mirrors were 0.92 and >0.999 for the output coupler and maximum reflector, respectively.
A fiber-coupled diode laser provided the 808-nm pump power, which was focused to a 500-μm-diameter spot on the VECSEL chip, corresponding to the ~430-µm-diameter TEM00 mode diameter at the chip. The scientists avoided deleterious intracavity etalon effects by applying an antireflection coating to the top surface of the chip.
Figure 2. The output power was reduced only slightly by the insertion of the birefringent filter (a), but the spectral quality was improved significantly (b). The traces in (b) show several orientations of the birefringent filter; they are not simultaneous. By rotating the filter around a normal to its surface, the researchers continuously tuned the laser across its ~20-nm gain bandwidth (c). In (c), the calculated quantum-well gain spectrum is shown as a solid line. The stability of the wavelength tuning is shown in (d), where all traces were taken at 24 W of pump power and a heat sink temperature of 10 °C.
They observed a minor reduction in output power when the birefringent filter was inserted and a slight decrease in slope efficiency (Figure 2a). But the filter narrowed the spectrum to ~1 nm and reduced its noise significantly (Figure 2b). By adjusting the filter’s orientation, the scientists tuned the laser across its entire ~20-nm gain bandwidth (Figure 2c). They observed a stable, narrow spectral output across the tuning range (Figure 2d).
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