A fluid-cooled laser amplifier module (100) is disclosed which comprises: a casing; a plurality of slabs (110) of optical gain medium oriented in parallel in the casing for cooling by a fluid stream (154, 156); a polarisation rotator (120) disposed between a first group of one or more slabs (111) of the optical gain medium and a second group of one or more slabs (112) of the optical gain medium; optical windows (150, 152) for receiving an input beam or pulse (130) for amplifying by the slabs and for outputting the amplified beam or pulse (140); and fluid stream ports (155, 157) for receiving and discharging the fluid stream for cooling the slabs.

A single longitudinal mode ring Raman laser including: a pump source outputting a pump light power, resonantly coupled to a first ring resonator; a optical measurement and piezo-actuator for stabilising the resonant coupling of the pump light power to a first ring resonator; a first ring resonator including a Raman gain medium, wherein the Raman gain medium receives the pump light power and undergoes Raman lasing generating resonated Stokes power at the corresponding Stokes output wavelength; the first ring resonator acting as a feedback loop for the pump light power and the resonated Stokes power and outputting a portion of the Stokes power as the laser output.

A multipass laser amplifier includes a mirror, a mirror device, a gain crystal, and refractive or diffractive beam-steering element. The gain crystal is positioned on a longitudinal axis of the multipass laser amplifier between the mirror and the mirror device. The beam-steering element is positioned on the longitudinal axis between the gain crystal and the mirror device. The beam-steering element has no optical power and deflects a laser beam, by refraction or diffraction, for each of multiple passes of the laser beam between the first mirror and the mirror device, such that each pass goes through the gain crystal for amplification of the laser beam and goes through a different respective off-axis portion of the beam-steering element. The no optical power of the beam-steering element enables maintaining a large beam size in the gain crystal, thereby facilitating amplification to high average power.

A high average and peak power single transverse mode laser system is operative to output ultrashort single mode (SM) pulses in femtosecond-, picosecond- or nanosecond-pulse duration range at a kW to MW peak power level. The disclosed system deploys master oscillator power amplifier configuration (MOPA) including a SM fiber seed, outputting a pulsed signal beam at or near 1030 nm wavelength, and a Yb crystal booster. The booster is end-pumped by a pump beam output from a SM or low-mode CW fiber laser at a pump wavelength in a 1000-1020 nm wavelength range so that the signal and pump wavelengths are selected to have an ultra-low-quantum defect of less than 3%.

A laser system and method generate milliwatt-power pump light by a fiber-coupled laser diode with a single-mode integrated fiber housed in a pump enclosure. The milliwatt-power pump light is conveyed from the single-mode integrated fiber out of the first enclosure into one end of a single-mode fiber cable that is external to the pump enclosure. The milliwatt-power pump light is conveyed from an opposite end of the external single-mode fiber cable into one end of a single-mode resident fiber disposed internally within a laser-head enclosure. A crystal housed in the laser-head enclosure is pumped with the milliwatt-power pump light that exits into free space from an opposite end of the single-mode resident fiber onto a face of the crystal, to produce stable milliwatt-power single-mode laser light having a frequency stability of less than 3 MHz per minute. The stable milliwatt-power single-mode laser light is emitted from the laser-head enclosure.

A CO.sub.2 laser that generates laser-radiation in just one emission band of a CO.sub.2 gas-mixture has resonator mirrors that form an unstable resonator and at least one spectrally-selective element located on the optical axis of the resonator. The spectrally-selective element may be in the form of one or more protruding or recessed surfaces. Spectral-selectivity is enhanced by forming a stable resonator along the optical axis that includes the spectrally-selective element. The CO.sub.2 laser is tunable between emission bands by translating the spectrally-selective element along the optical axis.

An optical amplifier comprises a gain medium having an input surface and an output surface wherein the output surface is larger than the input surface. The gain medium may be frustum shaped. The optical amplifier includes a negative diverging lens to receive an extraction laser beam and to cause the laser beam to expand as the beam passes through the gain medium. The amplifier further comprises a positive collimating lens configured to receive the expanding amplified beam and reduce the divergence. The gain medium can be pumped by counter-propagating radiation. The fluence of the laser beam within the gain medium is configured to be near constant along the length of the gain medium and may be within 1.5-2.0 F.sub.SAT. The gain medium may be doped with dopant to provide gain, with larger concentration of dopants proximal the input surface and smaller concentration proximal the output surface.

A method for intracavity frequency conversion includes end-pumping a solid-state gain medium in a laser resonator with a pump laser beam to generate an intracavity laser beam circulating in the laser resonator, and frequency-converting a portion of the intracavity laser beam in a nonlinear crystal, located in the laser resonator, to generate a frequency-converted laser beam. The method controls the output power and at least one output beam parameter of the frequency-converted laser beam by adjusting (a) the pump power and (b) a resonator loss imposed on the intracavity laser beam. Taking advantage of both the pump laser beam and the intracavity laser beam contributing to thermal lensing in the gain medium, this control scheme is capable of controlling the output power and the output beam parameter(s) independently of each other.

An extreme ultraviolet light generation apparatus includes a first light source outputting first excitation light, a laser oscillator including an active medium and performing laser oscillation by irradiating the active medium with the first excitation light to output the laser light, a measurement instrument measuring a pulse energy and a pulse time width of the laser light, a temperature regulator that adjusts a temperature of a cooling medium that cools the first light source, and a processor. The processor controls the temperature regulator to adjust the temperature of the cooling medium so that the pulse energy measured by the measurement instrument falls within a target range of the pulse energy, and adjust a current value of a current supplied to the first light source so that the pulse time width measured by the measurement instrument falls within a target range of the pulse time width.

A device and a method for measuring thermal load caused by excited state absorption in laser gain crystal are disclosed. Thermal focal lengths on the tangential and sagittal planes of the laser gain crystal are obtained by obtaining the threshold when the pump power is decreased, the optimal operating point, and cavity parameters of the single-frequency laser. Individual ABCD matrices of the laser gain crystal on the tangential plane and the sagittal plane are obtained based on thermal focal length. The thermal load corresponding to the threshold when the pump power is decreased, the ESA thermal load corresponding to the threshold when the pump power is decreased, and the ESA thermal load at the optimal operating point are obtained