Raphael Lallouz

High-energy Nd:Cr:GSGG lasers based on phase and polarization conjugated multiple-pass amplifiers

Lasers based on Nd:Cr:GSGG low-energy oscillator/highenergy multiple-pass amplifiers produced 1.7 J pulses in a M2?2 divergence beam at 2.4% electrical efficiency. Thermal lensing and birefringence correction were major factors driving the amplifier design. Essential components in achieving a moderate average power output were an intra-amplifier telescope to correct for lowest order thermal focusing, a phase conjugate mirror to correct for higher order thermal lensing aberrations, and a linear optics derived polarization conjugator to correct for birefringence.

Working beyond the static limits of laser stability by use of adaptive and polarization-conjugation optics

Strong thermo-optical aberrations in flash-lamp-pumped Nd:Cr:GSGG rods were corrected to yield TEM(00) output at twice the efficiency of Nd:YAG. A hemispherical resonator operating at the limit of stability was employed. As much as 3 W of average power in a Gaussian beam (M(2) approximately 1) was generated. Unique features were zero warm-up time and the ability to vary the repetition rate without varying energy, near- and far-field profiles, or polarization purity. Thermal focusing and astigmatism were corrected with a microprocessor-controlled adaptive-optics backmirror composed of discrete elements (variable-radius mirror). A reentrant resonator coupled polarizer losses back into the laser rod and corrected depolarization.

Low-threshold phase conjugate mirrors based on position-insensitive tapered waveguides

The potential for substantial threshold reduction in stimulated Brillouin scatter phase conjugate mirrors (PCMs) was systematically studied using tapered hollow glass waveguides filled with carbon disulfide. Using a low-threshold PCM of 100 cm straight section length and 10 μm inner diameter, the threshold was reduced by a factor of 1000 compared to the bulk liquid threshold. Use of a tapered input alleviated alignment sensitivity normally encountered with small-input-diameter fibers, and allowed for the fabrication of wide-field-of-view PCMs (and Brillouin amplifiers) of a type useful in many applications. Performance of the low-threshold PCM was found to meet theoretical expectations.

Nonlinear optical isolators based on high-reflectivity Brillouin mirrors and their application to advanced lasers

A new type of optical isolator, the nonlinear isolator (NLI), was developed to replace polarization sensitive isolation devices in pulsed, solid state laser systems. The NLI was based on a high-reflectivity, stimulated Brillouin scatter (SBS) mirror configured as a Newtonian telescope. Transmission through the NLI was found to be limited to the SBS threshold power. Input powers tested were 50 to 100 times the threshold power. The NLI was operated in a 10-Hz repetition rate laser without introducing noticeable far-field distortion. Partial pulse compression could occur in the NLI. A single triple-pass phase-conjugated YAG amplifier that incorporated the NLI, plus a 2-mJ SLM oscillator, produced 0.83-J pulses of 2.5-ns duration.

Dynamic Correction of Thermal Focusing in Nd:YAG Confocal Unstable Resonators by Use of a Variable Radius Mirror.

Excellent beam quality and divergence stability over a wide pump power range was demonstrated in a Q-switched, Nd:YAG, positive branch confocal unstable resonator by using a one degree-of-freedom, adaptive optic. Unlike single-element flexible-membrane mirrors, the variable radius mirror (VRM) consisted of a lens and mirror, whose separation determined the VRMs effective radius of curvature. This simple method enabled low cost and efficient thermal focusing compensation. The VRM was demonstrated by producing a 300-mJ Q-switch or 1-J free-running at a beam quality factor M(2) that varied between 1.2 and 1.8 as the average output power varied between 0 and 33 W.

High-repetition-rate oscillators based on athermal glass rods and on birefringence correction techniques

A high-repetition-rate oscillator, based on a flashlamp-pumped Q-100-clad athermal phosphate glass rod in a stable cavity, was developed to act as the driver for a high-repetition-rate Nd:glass laser system. Although the athermal properties compensated to a large extent for thermal lensing, birefringence losses in a conventional linear cavity containing a polarizer, as required for Q-switched operation, reduced pulse energy by up to a factor of 3 when the repetition rate was increased from single shot to 50% of fracture-limit pump conditions. Several birefringence compensation techniques were evaluated with the best results obtained in a reentrant (Y-cavity) design, with a Faraday rotator to partially compensate for birefringence and a mirror to reinsert polarizer-rejected light, so that the light quadruple passed the laser head per cavity round-trip and egressed through the output mirror (the only oscillator exit port). Birefringence losses were then reduced to 20%. High-repetition-rate TEM 00 operation was obtained by using a long (223-cm) cavity, and by placing the Faraday rotator/laser head together with a large mode selection aperture next to the curved back mirror.

Comparison between Nd:YAG and Nd:YLF laser oscillators, end pumped by high-brightness diode laser arrays

With the goal of developing high efficiency diffraction limited, high repetition rate lasers, a 2 watt quasi-cw diode laser (SDL 2272-P1) was used to end pump Nd:YAG and Nd:YLF slabs. The 1 X 200 micrometers diode stripe was coupled via two lenses and an anamorphic prism pair to the TEM00 mode of the cavity with an efficiency of 83%. A total light to light conversion efficiency of 44% was achieved in the free-running Nd:YAG laser with a slope efficiency of 48% when pumped at 808 nm. The measured efficiency in the free-running Nd:YLF when pumped in the (sigma) polarization at 809 nm was 32% with a slope efficiency of 34%. When pumped in the (pi) polarization, the corresponding Nd:YLF values were 25% and 28%, respectively. In both Nd:YAG and Nd:YLF, the output laser beam was diffraction limited at repetition rates of up to 1 kHz.

Brillouin scatter and Faraday effect isolators/nonreciprocal rotators for high-fluence multiple-pass amplifiers

A high-energy phase-conjugate mirror (PCM)/Nonlinear Isolator (NLI) based on pressurized methane was tested to the 5 J level, using 200 ns, 300 MHz bandwidth pulses from a phase conjugated multiple Nd:glass amplifier. Performance was good when a 50 cm focal length lens was employed. With a 25 cm lens, PCM performance was good except when gas breakdown occurred. In the presence of gas breakdown, reflectivity and return beam quality dropped. NLI isolation was aided by breakdown. A model was developed that shows that if breakdown occurs for a short focal length lens, then increasing the focal length raises the breakdown threshold but does not eliminate the problem at all input energies. This is a consequence of the fact that as input energy increases, the amount of energy leaking into the focal region increases faster than the breakdown threshold increase due to temporal broadening of the leaked pulse. High reflectivity (> 80%) single-cell tight-focus PCMs are feasible for use at the 10 J level, but as an NLI appears limited compared to glass based Faraday rotators.

Faraday rotators for use in multijoule Nd:Cr:GSGG and Nd:glass, phase and polarization conjugated multiple-pass amplifiers

High fluence solid-state lasers were built using Nd:Cr:GSGG or Nd:glass in oscillator/multiple-pass phase and polarization conjugated amplifier configurations. Beam path control, thermal stress induced birefringence correction, and isolation were achieved using Faraday rotators. Damage threshold on material and on pulse duration were investigated. Terbium glass was found to have a damage threshold five times greater than that of TGG. The damage threshold of both Terbium doped materials was virtually independent of pulse duration.

Comparison between rod and slab architectures for high-power-diode side-pumped Q-switched Nd:YAG lasers

A comparison between two side-pumping configurations of Nd:YAG Q-switched lasers is presented, based on experimental results and theoretical models developed by us. The first architecture, experimentally and theoretically tested, used a close-coupled pumping design where bare diode lasers were placed next to a Brewster cut rod. This laser produced 10 W average power at 10 kHz repetition rate with 30 ns pulse width and good beam quality, 12 percent light to light conversion efficiency was achieved with the added advantage of simplicity. The second arrangement, theoretically tested, was a zig-zag slab layout where the diode light was collimated with micro-lenses prior to being absorbed in a strip like are inside the slab. This design eliminated the problem of thermal induced birefringence, through the Cartesian symmetry of the thermal gradients developed inside the slab, while averaging out pump nonuniformities through the zig-zag path, thus yielding high beam fill factor within the pump volume.