We describe design and performance of efficient linear-to-radial polarization and mode converter. The converter is a
spatially variable retarder (SVR), comprised of eight appropriately cut half- wave plate segments. The SVRs were
applied to perform two different tasks. First, a linearly polarized Nd:YAG TEM00 beam was converted into a radiallypolarized
LG01* beam with polarization-purity of 98% and 96% measured in the near-field and far-field respectively.
The total power-loss in the transformation was 18%. Second, a 70W Nd:YAG radially-polarized beam with beamquality
M2=2.6 was converted into a linearly polarized nearly-Gaussian beam with beam-quality M2=1.4. Taking into
account power-losses, the experimental beam-brightness was increased by a factor of 2.6. The SVR manufacture was
optimized for application to high power lasers, where minimum phase-front distortion and maximum cylindrical-
polarization purity is required. SVRs have so far been successfully tested to the kW level. The proposed converging
methods can be of high practical importance due to unique properties of radially-polarized beams.
Radially-polarized beams can be strongly amplified without significant birefringence- induced aberrations. However, further improvement of the beam quality is desirable. In effort to transform the radially-polarized beam to a nearly-Gaussian beam, we consider effect of a spiral phase element (SPE) on the Laguerre-Gaussian (LG) (0,1)* beam with radial polarization, and compare this with the case when the input beam is a LG (0,1)* beam with spiral phase and uniform or undefined polarization. The LG (0,1)* beam with radial polarization, despite its identity in intensity profile to the beam with spiral phase, has distinctly different properties when interacting with the SPE. With the SPE and spatial filter, we transformed the radially-polarized (0,1)* mode with M2=2.8 to a nearly-Gaussian beam with M2=1.7. Measured transformation efficiency was 50%, and the beam brightness P/(M2)2 was practically unchanged. The SPE affects polarization state of the radially-polarized beam, leading to appearance of spin angular momentum in the beam center at the far-field.
We developed a Nd:YAG rod-based MOPA to generate high power in a radially polarized beam. Two pump-chambers
in the amplifier section produced 2.1kW @ M2=9.5, while three pump-chambers yielded 3.1kW @ M2=14. Efficiency in
the last pump-chamber was 33%. Several techniques were utilized to enhance beam-quality: a radially polarized
oscillator, special pump-chambers, external compensation of lower-order aberrations, and high-order aberration
compensation by pairing pump-chambers.
KEYWORDS: Stars, Polarization, Oscillators, Nd:YAG lasers, Monochromatic aberrations, Optical amplifiers, Transmission electron microscopy, High power lasers, Rod lasers, Solid state lasers
We developed a Nd:YAG rod-based MOPA to generate high power in a radially-polarized beam. Two pump-chambers
in the amplifier section produced 2.1kW @ M2=9.5, while three pump-chambers yielded 3.1kW @ M2=14. Efficiency in
the last pump-chamber was 33%. Several techniques were utilized to enhance beam-quality: an azimuthally-polarized
oscillator, special pump-chambers, external compensation of lower-order aberrations, and high-order aberration
compensation by pairing pump-chambers.
Corrrection of birefringence induced effects (depolarization and bipolar focusing) was achieved in double-pass amplifiers using a Faraday rotator placed between the laser rod and the retroreflecting optic. A necessary condition was that each ray in the beam retraced its path through the amplifying medium. Retrace was limited by imperfect conjugate-beam fidelity and by nonreciprocal double-pass indices of refraction.We compare various retroreflectors: stimulated Brillouin scatter phase-conjugate-mirrors (PCMs), PCMs with relay lenses to image the rod principal plane onto the PCM entrance aperture (IPCMs), IPCMs with external, adaptively-adjusted, astigmatism-correcting cylindrical doublets, and all adaptive optics imaging variable-radius-mirrors (IVRMs). Results with flashlamp pumped, Nd:Cr:GSGG double-pass amplifiers show that average output power increased fivefold with a Faraday rotator plus complete nonlinear optics retroreflector package (IPCM+cylindrical zoom), and that this represents an 80% increase over the power achieved using just a PCM. Far better results are, however, achieved with an IVRM.
Enhanced birefringence compensation together with dynamic thermal focus correction was demonstrated in a high-power, high-pulse-energy oscillator based on two flashlamp-pumped, 1x10cm Nd:YAG rods. Birefringence compensation was achieved by 90¡ polarization rotation with either a quartz rotator between the rods or with a Faraday Rotator between forward and backward resonator passes. An Imaging Variable Radius Mirror ÑIVRM composed of discrete optics was used as the resonator rear mirror. Imaging was required to re-image the rod principal plane onto itself so as to minimize the birefringence compensation's bifocal tensing sensitivity. Dynamic control of the IVRM radius of curvature was required to compensate thermal focusing variations. Integration into a Reentrant configuration, enabled average output power of 114W @ 20Hz with 3% overall efficiency in a polarized beam. Pulse energy dropped by only 3% compared to "cold" operation.
Flashlamp pumped oscillators utilizing Nd:Cr:GSGG or Nd:YAG rods were stabilized against varying levels of thermal focusing by use of a Variable Radius Mirror (VRM). In its simplest form, the VRM consisted of a lens followed by a concave mirror. Separation of the two elements controlled the radius of curvature of the reflected phase front. Addition of a concave-convex variable-separation cylindrical lens pair, allowed astigmatism to be corrected. These distributed optical elements together with a computer controlled servo system formed an adaptive optic capable of correcting the varying thermal focusing and astigmatism encountered in a Nd:YAG confocal unstable resonator (0 - 30 W) and in Nd:Cr:GSGG stable (hemispherical or concave- convex) resonators so that high beam quality could be maintained over the entire operating range. By utilizing resonators designed to eliminate birefringence losses, high efficiency could also be maintained. The ability to eliminate thermally induced losses in GSGG allows operating power to be increased into the range where thermal fracture is a factor. We present some results on the effect of surface finish (fine grind, grooves, chemical etch strengthening) on fracture limit and high gain operation.
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.
We report the development of a high-power Er:strengthened- glass laser emitting at the eye-safe 1.535 μm wavelength. The flashlamp pumped Cr:Yb:Er:glass produced 330 mJ output @ 0.45% slope efficiency. Thermo-optical measurements indicated strong thermal lensing, of 16 diopter/kW and mild birefringence induced depolarization of 5% at 200 W. In terms of radial and birefringence elastooptical coefficients these data determine the values of 0.075 ± 0.002 and 0.0094, respectively. For a hemispherical resonator configuration a TEM00 beam was achieved.
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.
In this work, five fundamental concepts were combined to a low development of high efficiency, low divergence, narrow bandwidth, flashlamp pumped oscillators capable of operation over a broad operating range. These concepts were: flashlamp pumped Nd:Cr:GSGG to achieve high efficiency, a 'Reentrant Cavity' to eliminate birefringence losses, a variable radius back mirror in a hemispherical cavity to achieve maximum Gaussian beam fill factor, a very high damage threshold, spectrum narrowing output coupler fabricated using a stack of uncoated etalons to form a resonant reflector, a cylindrical zoom lens to completely eliminate astigmatism. The results were successful, and yielded an oscillator that produced 10 mJ, TEM00 300 MHz bandwidth, 75 ns pulses, over a repetition rate of 1-20 Hz, and at a slope efficiency of 2 percent. These techniques were also successfully applied to a YLF oscillator. They may, in part, be adapted for use to unstable resonators.
Laser based on Nd:Cr:GSGG low-energy oscillator/multiple-pass amplifiers produced 1.7 J pulses in a M2 approximately equals 2 divergence beam at 2.4% electrical efficiency. Thermal lensing and birefringence correction were major factors.
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