Single-Mode Diode-Pumped Micro-Crystal Laser Continuously Tuneable Over 200 GHz in the Visible Spectral Region

Abstract

Tuneable single-frequency lasers are decisive components of absolute distance interferometers for high-precision distance measuring without any ambiguity. Due to the noise they create, tuneable VIS diode lasers with an external resonator are no longer applied for measuring distances up to 100 m with acceptable measuring intervals of a few seconds. Therefore a tuneable single frequency VIS microcrystal laser, continuously tuneable over 200 GHz, was developed and tested as part of an interferometer. Nd-doped laser crystals were used for the laser and, by doubling the intracavity frequency with a KTP, a wavelength of 532 nm was reached The semi-monolithical arrangement was pumped longitudinally with a laser diode. In order to achieve a stable Single Longitudinal Mode Operation with Nd:YVO4- and Nd:LSB (NdxLa1-xSc3(BO3)4) crystals in a compact design, an Intracavity Solid State Etalon was implemented for the frequency selection This was achieved with an etalon with a Free Spectral Range of 1000 GHz and a reflectivity of 30 % for an Nd:YVO4 crystal with a thickness of 0,2 mm, that is, for an Nd:LSB crystal with a thickness of 0,3 mm at a reflectivity of 40 %. The selectivity was chosen in such a way that, due to the inertia of the homogeneously expanded medium, tuneable frequency regions of 60 GHz were shown with a stationary etalon only by changing the resonator length (see fig. 1). Scan rates of > 100 Hz were achieved in the interferometer set up by means of a sinus-formed external drive. The instantaneous frequency of the interference signal, identical with the instantaneous change of the laser frequency, follows the drive signal without delay (see fig. 2). This is of great importance for the driver electronics of the measuring system mentioned above. In order to achieve a tuneable frequency region of > 200 GHz, it was necessary to vary the etalon as well. It was the first time that a continuous variation over 240 GHz, i. e. covering half of the linewidth of the Nd:YVO4 crystal, were proved for a diode-pumped solid state laser. The tuning range was only limited by the mechanical resonator driver In principle it should be possible to vary over an even larger region using an Nd:LSB crystal with a greater linewidth that is with frequency doubling nearly 2000 GHz.