Optically pumped semiconductor disk lasers for high-power wide wavelength tuning

Doktorsavhandling, 2012

In this work I present and verify a strategy that makes it possible to change the wavelength, or color, of an external cavity semiconductor laser over a very large range. This is achieved by a careful design and thorough optimization of the active component — the gain element of the laser. This thesis concerns the so-called optically pumped semiconductor disk laser (OP-SDL), which represents a relatively new class of lasers showing great promise for future applications. The advantages include the wavelength versatility that is common for most semiconductor lasers, but also adds the ability to deliver multi-Watt output powers into a nearly diffraction-limited beam, and a free-space external cavity for the easy insertion of various optical elements. These properties have generated great interest in the OP-SDL for use in life science, metrology, entertainment applications, forensics, and many other fields. Recently, efforts have also been made to extend the tuning range for use in spectroscopic applications such as intra-cavity laser absorption spectroscopy. This thesis focuses on the design of the gain element of an OP-SDL, and how to obtain a wide tuning range while keeping the output power at a high level. The design strategy has been to balance the effects of the spectral dependencies of material gain, subcavity resonance, and spatial overlap of quantum wells with the optical field. Experimental evaluations show that the strategy has been successful and a tuning range of 43 nm, with a maximum output power of 2.6 W was obtained. With improved thermal management the peak output power was increased to more than 7.5 W with a tuning range of 32 nm. Furthermore, two new measurement techniques were developed. One technique very accurately measures the active mirror reflectance, or the spectral reflectance of an optically pumped gain element, and is particularly useful for evaluating a fabricated gain element. The other technique is for the full characterization of a laser beam, yielding the amplitude and phase distributions, and is well suited for the high-intensity beam from an OP-SDL.