Single-mode intra-cavity frequency-doubled CW solid-state laser with volumetric gain-grating

#The present invention relates in general to lasers delivering ultraviolet (UV) radiation by frequency-conversion of fundamental laser radiation having a wavelength in the visible or a longer-wavelength region of the electromagnetic spectrum. The invention relates in particular to semiconductor-laser pumped solid-state lasers delivering UV radiation by frequency-doubling fundamental radiation from a solid-state-gain-medium.

#There a several laser applications that require relatively high average power, for example, greater than one-hundred milliwatts (mW) average power, of continuous wave (CW) UV laser radiation at some UV wavelength between about 200 nanometers (nm) and 400 nm. Such applications include wafer and mask inspection in the semiconductor industry.

#Commercially available lasers suitable for this purpose include solid-state and optically pumped semiconductor (OPS) lasers in which UV radiation is generated by frequency conversion of fundamental wavelengths in the near infrared spectral region to third-harmonic or fourth-harmonic wavelengths. By way of example, a 1064 nanometer (nm) fundamental wavelength of neodymium-doped yttrium vanadate (Nd:YVO4) can be converted to a third-harmonic wavelength of about 353 nm or a fourth-harmonic wavelength of about 266 nm using two stages of frequency conversion. Intra-cavity (intra-resonator) frequency conversion is usually employed for generating CW harmonic radiation. The need for two intra-cavity (IC) frequency conversion stages adds cost and complexity to a laser resonator. IC-frequency tripling and quadrupling, however, are rather complex and require complex control technology to ensure that the laser output power and beam-pointing are stable. Further, the efficiency of a two-stage IC CW frequency converted laser is typically relatively low, for example less than about 3% of pump-power.

#The accompanying drawings, which are incorporated in and constitute a part of the specification, schematically illustrate a preferred embodiment of the present invention, and together with the general description given above and the detailed description of the preferred embodiment given below, serve to explain principles of the present invention.

#FIG. 1 schematically illustrates a simplified layout of a preferred embodiment of a frequency-doubled solid-state laser in accordance with the present invention including a once-folded standing-wave laser-resonator including a thin disk solid-state gain-medium backed by a reflector providing one end-mirror of the resonator, with coherent pump light directed into the gain-medium at an angle to the resonator axis and being reflected back from the reflector such that interference between the forward and reflected pump radiation forms pump-light fringes, the angle being selected such that the peaks of the pump-light fringes are aligned with antinodes of a standing wave in the resonator having a fundamental wavelength of the gain-medium, and an optically nonlinear crystal being included in the resonator for frequency-doubling the fundamental wavelength radiation to provide second-harmonic output radiation.

#FIG. 2A is a graph schematically illustrating beam-averaged gain-contours in one example of a Pr:YLF gain-medium having a thickness of 500 micrometers and suitable as the gain-medium of the laser of FIG. 1.

#Referring now to the drawings, wherein like components are designated by like reference numerals, FIG. 1 schematically illustrates a simplified layout (for calculation purposes) of a preferred embodiment 10 of a frequency-doubled solid-state laser in accordance with the present invention. The laser has a once-folded, standing-wave laser-resonator 12 having a longitudinal optical axis (z-axis) 14 and transverse axes x and y perpendicular to each other and perpendicular to the z-axis.

#Resonator 12 includes a solid-state gain-medium 16 in the form of a thin-disk gain-medium. The thin-disk gain medium preferably has a thickness between about 200 μm and 1000 μm with a thickness of 500 μm in examples described hereinbelow. Dimensions of the gain-medium are exaggerated in FIG. 1 for convenience of description. A more accurate description of dimensions and other details of the thin-disk gain-medium is presented further hereinbelow. The gain-medium is energized by pump light (pump radiation) from a diode-laser (not shown). It is critical to the operation of the present invention that the pump-light has a coherence length sufficient that the pump-light is still coherent in the gain-medium.

#The pump light is directed into the gain-medium at an angle θe to the z-axis outside the gain-medium such that the pump-light inside the gain-medium is directed at a predetermined angle θi to the z-axis. The change in angle from θe to θi is due to refraction of light at the air-gain medium interface. Gain-medium 16 has a front surface 16A, through which the pump-light is directed, and a rear surface 16B, which is coated with a reflective coating 18. The reflective coating is highly reflective at a fundamental wavelength of the gain-medium and also highly reflective for the wavelength of the pump-light. Because the pump-light is coherent, pump-light reflected back from reflective coating 18 interferes with incident pump light to form pump-light fringes in the gain-medium with the fringing being more pronounced the closer the fringes are to the reflective coating (rear surface of the gain-medium). Angle θe is selected such that the intensity peaks of the pump-light fringes in the gain-medium are aligned with antinodes of the standing-wave of the fundamental mode in the gain-medium. A detailed description of the pump-light fringe formation is provided further hereinbelow.