General approach to high power, coherent visible and ultraviolet light sources

Publication: ResearchPh.D. thesis – Annual report year: 2009

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The main goal of this project is to develop a generic approach to synthesise any wavelength in the visible and UV spectral region based on sum frequency generation. The approach is based on a hybrid system combining solid state and semiconductor technology. The generation of light in the UV spectral region require nonlinear materials with a transparency range extending into the ultraviolet, the ability to sustain high photon energies and with the ability to obtain phasematching for the desired nonlinear conversion process. In this project experiments are conducted using three differently co-doped GdCOB crystals. The crystals are optimized for noncritical phasematching in the blue-UV spectral region through co-doping with Lu and Sc, a nonlinear coefficient for these crystals of 0.78, 0.81 and 0.89 pm/V are measured, which is comparable to LBO. The ability to adjust the noncritical phasematching by co-doping of these crystals makes them promising candidates for generation of light in the blue-UV region. A novel method for cavity dumping based on nonlinear frequency conversion is investigated. A high finesse laser is constructed with an intracavity nonlinear material inserted in a beam waist. The nonlinear material is phasematched to support sum frequency generation between the 1342nm circulating field in the cavity and a single pass passively Q-Switched 1064nm laser, effectively converting the circulating power whenever a single pass pulse is present. Furthermore the Q-Switched laser can easily be frequency doubled in a single pass configuration, therefore the nonlinear cavity dumping approach is suggested for the generation of 340nm UV light, using 532nm pulses to cavity dump a 946nm Nd:YAG laser. Furthermore experiments are conducted tripling a Q-switched 1064nm laser to 355nm by cascaded second harmonic and sum frequency generation using periodically poled KTP and BBO for the SHG and SFG process, respectively. The 355nm light is used to promote different photo induced reactions. The main limitation of reaching any desired wavelength in the visible spectrum using sum frequency generation is the limited laser lines available from efficient solid state lasers. One fundamental way to overcome this limitation is to use semiconductor lasers to provide one of the fundamental fields. The problem of using semiconductor lasers for nonlinear frequency conversion has previously been the lag of coherence of these devices. This problem can, however, to a large extent be solved using external cavity tapered diode lasers, which allows for the generation of coherent radiation at the watt power level. Using differently doped semiconductor materials these devices can potentially cover the wavelength range from the red and into the infrared spectral range. These devices are very efficient, however, the available devices in the visible region are still very inefficient, therefore a generic approach using high finesse solid state lasers with intracavity nonlinear materials and single pass tapered diode was sought to cover the shorter wavelength range. In this project more then 300mW of 488nm power is generated by direct sum frequency mixing of a solid state laser and a single pass external cavity tapered diode laser. The performance of the device is compared to systems where the output of the tapered diode laser is spatially filtered and to an all solid state laser system based on mixing with a single frequency Ti:Sapphire laser. Finally experiments with a semiconductor disk laser used as the high finesse cavity laser and sum frequency mixing with a single pass solid state laser is coniv ducted. These experiments show that it is possible to design systems exploiting the benefits of semiconductor based lasers and nonlinear sum frequency generation to cover large parts of the optical spectrum, which has previously been difficult to access due to the lag of efficient, coherent light sources
Original languageEnglish
Publication dateSep 2009
Place of publicationKgs. Lyngby, Denmark
PublisherTechnical University of Denmark (DTU)
Number of pages110
ISBN (print)978-87-993734-0-6
StatePublished
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