We use a detailed numerical model of stimulated thermal Rayleigh scattering to compare mode instability thresholds in cladding pumped Tm3+-doped and Yb3+-doped fiber amplifiers. The Tm-doped fiber amplifies 2040 nm light using a 790 nm pump; the Yb-doped fiber amplifies 1060 nm light using a 976 nm pump. The predicted instability threshold of the Tm-doped fiber is found to be higher than that of the Yb-doped fiber, even though its heat load is much higher. We attribute the higher threshold in part to its longer signal wavelength, and in part to stronger gain saturation.
We show how signal bandwidth affects the gain of stimulated thermal Rayleigh scattering (STRS) which is responsible for a transverse mode instability in fiber amplifiers. The gain is reduced if the coherence time of the signal is less than the group-velocity-induced walk off between the two interacting modes, usually LP01 and LP11. We derive expressions for the bandwidth required to suppress gain for short pulses, for periodically chirped continuous waves, and for general periodically modulated cases.
We use our numerical model of mode instability to analyze the influences of spontaneous thermal Rayleigh scattering (sTRS) and laser gain saturation on instability threshold powers. sTRS is stronger than the quantum noise used as the seed power for stimulated thermal Rayleigh scattering in previous studies, so the threshold is reduced by 15-25% with sTRS seeding. Gain saturation is strong in any efficient amplifier and we show how it can be exploited to raise instability thresholds be a factor of two or more while staying below the stimulated Brillouin threshold.
We show by detailed numerical modeling that stimulated thermal Rayleigh scattering can account for the modal instability observed in high power fiber amplifiers. Our model illustrates how the instability threshold power can be maximized by eliminating amplitude and phase modulation of the signal seed light and the pump light and by careful injection of the signal seed light. We also illustrate the influence of photodarkening and mode specific loss.
We studied theoretically the laser-plasma interaction, and performed experiments to investigate the mechanisms giving rise to optical damage in Borosilicate glass using nanosecond laser pulses at wavelength 1064 nm. Our experimental result shows that the optical damage process generated by nanosecond laser pulses is the result of an optically induced plasma. The plasma is initiated when the laser irradiance frees electrons from the glass. Although it may be debated, the electrons are likely freed by multi-photon absorption and the number density grows via impact ionization. Later when the electron gas density reaches the critical density, the electron gas resonantly absorbs the laser beam through collective excitation since the laser frequency is equal to the plasma frequency. The laser energy absorbed through the collective excitation is much larger than the energy absorbed by multi-photon ionization and impact ionization. Our experimental result also shows the plasma survives until the end of the laser pulse and the optical damage occurs after the laser pulse ceases. The plasma decay releases heat to the lattice. This heat causes the glass to be molten and soft. It is only as the glass cools and solidifies that stresses induced by this process cause the glass to fracture and damage. We also show the experimental evidence of the change of the refractive index of the focusing region as the density of the electron gas changes from sub-critical to overcritical, and the reflection of the over-critical plasma. This reflection limits the electron gas density to be not much larger than the critical density.
We present a physical model that may describe the observed phenomenon of modal instability in high power fiber
amplifiers. In the power range of several hundred watts, large-mode-area, cladding-pumped, Yb3+-doped fiber
amplifiers (both step-index and photonic crystal fibers), exhibit a sudden transition in the output beam profile
from the fundamental mode to a higher order mode. We show how this behavior can be caused by a thermally
induced mode coupling that leads to exponential gain of the higher order mode, and we implement a numerical
model that quantitatively predicts the instability threshold for any large-mode-area step index fiber amplifier.
We measured the single-shot and multiple-shot optical breakdown thresholds leading
to optical damage of borosilicate glass, specifically BK7 glass, at 1.064 μm. We used 8-ns, single-longitudinal-mode, TEM00 laser pulses tightly focused inside a BK7 glass
window. The radius of the focal spot was measured using surface third harmonic
generation; it is equal to 7.5 μm. With this tight focus, the laser power at the breakdown
threshold of BK7 glass is below the SBS threshold, and the effect of self focusing is
small.
We found the single-shot and multiple-shots optical breakdown thresholds to be
deterministic. At the single-shot damage threshold, the optical breakdown in BK7 glass
occurs on the trailing edge of the laser pulse, in contrast to fused silica in which the
breakdown always occurs at the peak of the laser pulse. However, the multiple-shot
damage threshold of BK7 glass occurs at the peak of the last laser pulse.
Our single shot damage threshold for BK7 glass is 4125
J/cm2, and our multiple shot
damage threshold ranges from 3974 J/cm2 for 2-shot damage to 3289 J/cm2 for 31-shot
damage. We also compare damage morphologies of BK7 glass with those of fused silica.
We compared the 1064 nm surface damage thresholds of fused silica polished by
three different techniques:
1. A conventional polishing technique: that uses loose Alumina abrasives (lapping)
followed by a fine Cerium oxide polish.
2. An alumina polishing process producing surfaces very close to super polished.
3. Process 2 followed by a silica polish until the silica surfaces are super polished.
We employed the same measurement technique that proved successful for the bulk
damage threshold measurement to measure the damage thresholds of bare silica surfaces
polished by the above three polishing techniques. We used an 8-nanosecond, single
transverse and longitudinal mode pulsed laser, from a Q-switched Nd:YAG laser. We
used the surface third harmonic generation technique to precisely place the focus of the
laser beam on the surface of the fused silica window, and to measure the laser focus spot
size which was found to be 8 μm in radius.
The wavelength region between 190 and 200 nm is especially relevant to semiconductor manufacturing. In
contrast to ArF excimer lasers, frequency up-converted solid-state lasers offer tuning, coherence and beam
quality characteristics that are essential to high performance semiconductor processing. This paper reviews
various methodologies for implementing pulsed non-linear optical interactions in this wavelength region
given a wide range of laser operating formats and describes the utilization of these sources for the specific
semiconductor applications of interference lithography and photoresist materials studies.
We used 9.9-ns, single-longitudinal-mode, TEM00 pulses tightly focused to an 8-micron
radius spot to measure single-shot and multiple-shot damage thresholds of pure and Nddoped
ceramic Yttrium Aluminum Garnet (YAG), and of pure, Nd-doped, Cr-doped, and
Yb-doped crystalline YAG. By tightly focusing the laser beam, we kept the damage
threshold powers below the SBS threshold, and minimized the effect of self focusing.
The size of the focus spot was measured using surface third harmonic generation.
We found both single-shot and multiple-shot damage thresholds to be deterministic. At
the single-shot damage threshold in YAG, breakdown always occurs on the trailing edge
of the laser pulse. However, for multiple-shot damage threshold, breakdown occurs at the
peak of the nth laser pulse.
Our measured damage thresholds for doped and undoped, ceramic and crystalline
YAG range from 1.1 to 2.2 kJ/cm2. We also report some damage morphologies in
crystalline YAG.
We measured the single-shot and multiple-shot damage thresholds of pure and Nddoped ceramic Yttrium Aluminum Garnet (YAG), and of pure, Nd-doped, Cr-doped, and Yb-doped crystalline YAG. We used 9.9 ns, single-longitudinal-mode, TEM00 pulses
tightly focused inside the ceramic and crystalline YAG. The 8 microns radius of the focal
spot was measured using surface third harmonic generation. With this tight focus the
damage threshold powers for both the ceramic and crystalline YAG were below the SBS
threshold, and the effect of self focusing was small.
We found the single-shot and multiple-shots damage thresholds to be deterministic.
The single-shot damage of YAG occurs on the trailing edge of the laser pulse, in contrast
to fused silica in which damage always occurs at the peak of the laser pulse. However,
the multiple-shot damage threshold of YAG occurs at the peak of the nth laser pulse.
We find the damage thresholds of doped and undoped, ceramic and crystalline YAG
range from 1.1 to 2.2 kJ/cm2. We also report some damage morphologies in YAG.
We review what we have learned in the last few years from modeling nanosecond fiber amplifiers. We have developed a number of models that treat bent fiber mode profiles and bend loss, plus gain models and models of nonlinear processes such as self phase modulation, self focusing, SRS, SBS, and four wave mixing. The models have been validated by detailed comparisons with laboratory measurements.
We employed the same measurement techniques that have proven successful for bulk
damage thresholds measurements to measure damage thresholds of bare silica surfaces polished
using various methods and to measure damage thresholds for antireflection coated silica, again for
various surface polishes. Light in a single transverse and longitudinal mode, from a Q-switched
Nd:YAG laser is focused to an 8 µm spot on the front and rear surfaces of silica windows polished
using ceria, alumina, or alumina/silica to find the damage threshold. We repeated the exercise for the
same surfaces anti reflection coated with silica/hafnia film stacks. We used surface third harmonic
generation to precisely place the focus on the surfaces. Key findings include:
1. The surface damage threshold can be made equal to the bulk damage threshold. There is a
large difference in single-pulse damage thresholds of bare silica surfaces polished using
ceria, alumina, and alumina followed by silica. The ceria polished samples have a statistical
damage threshold ranging from 50 to 450 GW/cm2. The alumina polished surfaces damage at
200-500 GW/cm2, with half the spots damaging at the bulk threshold of 500 GW/cm2. The
windows polished by alumina followed by silica damage almost universally at the bulk
damage threshold of 500 GW/cm2.
2. There are strong conditioning effects for these surfaces. The ceria polished surfaces have
reduced thresholds for multiple pulses. The alumina polished surfaces attain the bulk damage
threshold at most locations using multiple pulse annealing.
3. The underlying polishes strongly affect the damage thresholds for the AR coatings. The
alumina plus silica polished samples have the highest thresholds, with statistical variations
from 150-380 GW/cm2. The alumina polished samples damage at only 50 GW/cm2, but with
annealing the threshold rises to 200 GW/cm2, while the ceria polished samples damage at 50-200 GW/cm2 with no strong multiple shot effect.
4. We found there was no beam size variation of the damage threshold irradiance for the bare
alumina/silica polished samples.
5. We showed that air breakdown does not limit the surface irradiance, silica breakdown does.
6. We recorded damage morphologies for the different surfaces.
We are interested in maximizing the performance of fiber lasers and amplifiers,
particularly for amplification of ps-ns pulses. The observed pulse energies from large
mode area fiber amplifiers routinely exceed the reported bulk damage threshold of silica.
We have undertaken a program to establish the intrinsic damage thresholds of silica that
are relevant for fiber applications. We use a single transverse / single longitudinal mode
Q-switched Nd:YAG laser focused to an 8-µm spot several Rayleigh ranges deep in silica
windows for the nanosecond measurement, and a Q-switched, mode locked Nd:YAG
laser for the picoseconds measurements. Our key findings include:
1. The damage threshold is deterministic rather than statistical for both ns and ps
pulses. The threshold varies less than 1% from location to location.
2. The intrinsic damage threshold of silica is 475±25 GW/cm2 (fluence = 3850
J/cm2) for 8 ns pulses and approximately 3 times higher for 14 ps pulses.
3. There is no difference in damage thresholds among Corning's A0, B1, C1, D1,
D2, and D5 grades of silica.
4. A tight focus is required to avoid large self focusing corrections and to avoid SBS
for the 8-ns pulses.
5. Damage morphologies are reproducible from pulse to pulse but change with focal
spot size and pulse duration. In all cases, damage appears to begin exactly at the
focus and then move upstream approximately one Rayleigh range.
6. The dependence of the damage threshold fluence on pulse duration is nearly linear
for pulse durations longer than 50 ps. The square root of duration dependence
reported by several investigators for the 50 ps to 10 ns range is refuted.
7. The variation of damage fluence with pulse duration from 20 fs to 20 ns and
beyond is well described by a single electron avalanche rate equation with three
fixed rates for the avalanche, multiphoton ionization, and electron recombination
terms.
8. Our damage threshold is consistent with the most reliable DC field breakdown
threshold.
9. We verified in detail the self focusing corrections and the SBS thresholds for our
measurement conditions.
10. The damage threshold is affected little by mechanical strain at levels similar to
those in polarization-preserving fiber.
Sandia National Laboratories' program in high-power fiber lasers has emphasized development of enabling technologies
for power scaling and gaining a quantitative understanding of fundamental limits, particularly for high-peak-power,
pulsed fiber sources. This paper provides an overview of the program, which includes: (1) power scaling of diffraction-limited
fiber amplifiers by bend-loss-induced mode filtering to produce >1 MW peak power and >1 mJ pulse energy
with a practical system architecture; (2) demonstration of a widely tunable repetition rate (7.1-27 kHz) while
maintaining constant pulse duration and pulse energy, linear output polarization, diffraction-limited beam quality, and
<1% pulse-energy fluctuations; (3) development of microlaser seed sources optimized for efficient energy extraction; (4)
high-fidelity, three-dimensional, time-dependent modeling of fiber amplifiers, including nonlinear processes; (5)
quantitative assessment of the limiting effects of four-wave mixing and self-focusing on fiber-amplifier performance; (6)
nonlinear frequency conversion to efficiently generate mid-infrared through deep-ultraviolet radiation; (7) direct diode-bar
pumping of a fiber laser using embedded-mirror side pumping, which provides 2.0x higher efficiency and much
more compact packaging than traditional approaches employing formatted, fiber-coupled diode bars; and (8)
fundamental studies of materials properties, including optical damage, photodarkening, and gamma-radiation-induced
darkening.
Our objective is to understand the mechanism that generates catastrophic optical damage in pulsed fiber amplifiers. We measured optical damage thresholds of bulk fused silica at 1064 nm for 8 ns and 14 ps pulses. The 8 ns pulse is single longitudinal mode from a Q-switched laser, and the 14 ps pulse is from a Q-switched mode-lock laser. The beams in both cases are TEM00 mode, and they are focused to a 7.5 μm spot inside a fused silica window. The pulse-to-pulse energy variations are 1% for 8 ns pulses and 5% for 14 ps pulses. Under these conditions optical damage is always accompanied by plasma formation at the focal spot; we found the damage threshold fluences are 3854 ± 85 J/cm2 for the 8 ns pulses and 25.4 ± 1.0 J/cm2 for the 14 ps pulses. These fluences are corrected for self focusing. Both damage thresholds are deterministic, in contrast to the claim often made in the literature that optical damage is statistical in the nanosecond range. The measured damage threshold fluences for 8 ns and 14 ps pulses do not fit a square root of pulse duration scaling rule. We interpret the damage in terms of plasma formation initiated by multiphoton ionization and amplified by an electron avalanche. The damage threshold irradiance can be matched with a simple rate equation model that includes multiphoton ionization, electron avalanche, and electron-hole recombination. The damage morphologies are dramatically different in the nanosecond and picosecond cases because of the large difference in deposited energy. However, both morphologies are reproducible from pulse to pulse. We also measured surface damage thresholds for silica windows polished by different methods. We find that cerium oxide polished surfaces damage at approximately 40% of the bulk threshold, with a large statistical spread. Surfaces prepared using an Al2O3 polish damaged between 50% and 100% of the bulk damage limit, with a substantial fraction at 100%. Surfaces polished using first the Al2O3 polish and then an SiO2 polish exhibit surface damage values equal to the bulk damage value at nearly every point. We also measured damage thresholds for different sized focal spots. Some earlier reports have claimed that damage thresholds depend strongly on the size of the focal spot, but we find the surface threshold is independent of the spot size.
We report a ZGP OPO/OPA system capable of producing >30 mJ at a signal wavelength of 3.4 μm. Our pump source
for the OPO/OPA is a 2.05-μm, diffraction-limited Ho:YLF master-oscillator power-amplifier (MOPA) producing ~150
mJ energy per pulse at repetition rate from single shot to ~500 Hz (>100 mJ at 1 kHz). The Ho-MOPA output was split
~2:1 into two beams: one (~50 mJ per pulse) pumping an image-rotating RISTRA ZGP OPO to generate a 3.4-μm signal
wave with >10 mJ of pulse energy and near diffraction-limited beam quality, and the other to pump a ZGP OPA stage to
increase the signal wave to ~30 mJ per pulse.
The objective of this work is to understand catastrophic optical damage in nanosecond
pulsed fiber amplifiers. We used a pulsed, single longitudinal mode, TEM00 laser at 1.064
µm, with 7.5-nsec pulse duration, focused to a 7.45-&mgr;m-radius spot in bulk fused silica.
Our bulk damage threshold irradiance is corrected to account for self focusing. The pulse
to pulse variation in the damage irradiance in pure silica is less than 1%. Damage is
nearly instantaneous, with an induction time much less than 1 ns. These observations are
consistent with an electron avalanche rate equation model, using reasonable rate
coefficients. The bulk optical breakdown threshold irradiance of pure fused silica is
5.0x1011 ±7% Watts/cm2. We also measured the surface damage threshold irradiance of
1% Yb3+ doped fused silica preform of Liekki Yb1200 fiber, and found it is equal to that
of pure silica within 2%.
The optical damage morphology is reproducible from pulse to pulse. To facilitate the
morphology study we developed a technique for locating the laser focus based on the
third harmonic signal generated at the air-fused silica interface. This gives a very small
uncertainty in focal position (~ 10 &mgr;m) which is important in interpreting the damage
structure. The surface third harmonic method was also used to determine the laser focus
spot size and verify beam quality.
Earlier reports have claimed that the damage irradiance depends strongly on the size
of the focal spot. We varied the focal volume to look for evidence of this effect, but
found none.
The design and optimization of high-power fiber amplifiers requires a simulation tool capable of including a wide range of effects simultaneously, including mode distortion
and loss due to bending, spatially-dependent saturable gain, guiding from arbitrary index of refraction profiles and self-focusing. In addition, the nonlinear effects are power
dependent and thus will distort the pulse shape. We have constructed a numerical model to address these issues and serve as a platform for data analysis and system optimization.
We have numerically investigated the behavior of the LP01 fundamental mode of a step-index, multimode
(MM) fiber as the optical power approaches the self-focusing limit (Pcrit). The analysis includes the effects
of optical gain and fiber bending and are thus applicable to coiled fiber amplifiers. We find that at powers
below Pcrit, there exist stably propagating power-dependent modifications of the LP01 mode, in contrast to
some previous solutions that exhibited large-amplitude oscillations in beam waist along the fiber. For the
first time, to our knowledge, we show that in a MM fiber amplifier seeded with the low-power LP01
eigenmode, the transverse spatial profile will adiabatically evolve through power-dependent stationary
solutions as the beam is amplified toward Pcrit. In addition, for a given value of the nonlinear index, Pcrit is
found to be nearly the same in the bulk material and in a step-index fiber. These conclusions hold for both
straight and bent fibers, although the quantitative details are somewhat different.
Recently the use of "vortex" beams of high azimuthal mode number has been proposed as a way of increasing the maximum peak power through-put of optical fibers beyond the few MW allowed for Gaussian beams by self-focusing. We report a numerical investigation of these and other schemes using a beam propagation approach that includes
a Kerr-type nonlinearity.
The objective of this work is to understand catastrophic optical damage in nanosecond pulsed fiber
amplifiers. We used a pulsed, single longitudinal mode, TEM00 laser at 1.064 &mgr;m, with 7.5-nsec pulse
duration, focused to a 7.45-&mgr;m-radius spot inside a fused silica window, to measure the single shot optical
breakdown threshold irradiances of 4.7E11 and 6.4E11 W/cm2 respectively for pure fused silica, and for a
1% Yb3+ doped fused silica preform of Liekki's Yb1200 fiber. These irradiances have been corrected for
self focusing which reduced the area of the focal spot by 10% relative to its low field value. Pulse to pulse
variations in the damage irradiance in pure silica was >2%. The damage induction time appears to be much
less than 1 ns.
We found the damage morphology was reproducible from pulse to pulse. To facilitate our morphology
study we developed a technique for locating the position of the focal waist based on the third harmonic
signal generated at the air-fused silica interface. This gives a precise location of the focal position (± 10
&mgr;m) which is important in interpreting the damage structure. The surface third harmonic method was also
used to determine the diameter of the focal waist.
Earlier reports have claimed the damage irradiance depends strongly on the size of the focal waist. We
varied the waist size to look for evidence of this effect, but to date we have found none. We have also
studied the temporal structure of the broadband light emitted upon optical breakdown. We find it consists
of two pulses, a short one of 16 ns duration, and a long one of several hundred ns. The brightness, spectra,
and time profiles of the white light provide clues to the nature of the material modification.
We report experimental realization of a new type of optical parametric oscillator, in which oscillation is achieved by
polarization rotation in a linear retarder, followed by nonlinear mixing of the two polarized beams. The mixing is
performed by a type II degenerate parametric down conversion in a PPKTP crystal, pumped at 1064nm by a pulsed
Nd:YAG laser. A single, linearly polarized beam is generated at the degenerate wavelength. The output spectrum has a
narrow linewidth (below the instrumentation bandwidth of 1nm), and is highly stable at degeneracy with respect to
variations in the crystal temperature. The frequency locking is explained in terms of balanced roundtrip phase-matching
condition.
The design and optimization of high-power fiber lasers and amplifiers requires a detailed understanding of several important physical processes, both linear and nonlinear. The influence of bending on the overlap of the propagating mode as well as its resistance to deleterious nonlinear effects such as self-focusing must be accurately predicted. To this end we have developed a number of models, both analytic and numerical, that allow us to treat these effects in detail.
We have numerically modeled an efficient method of doubling the 1064 nm wavelength of a Q-switched Nd:YAG laser using a lambda-doubling nanosecond optical parametric oscillator (LDOPO). The LDOPO cavity is based on the four-mirror nonplanar RISTRA geometry, denoting rotated-image singly-resonant twisted rectangle, and contains a single type-II KTP crystal. By using the polarization-rotating properties of this cavity, and modifying its geometry to incorporate polarization-selective mirrors with angles of incidence near Brewster's angle, this
design obtains stable, singly-resonant oscillation at degeneracy. If the pump laser is injection-seeded, and the LDOPO contains an intra-cavity etalon for single-longitudinal-mode oscillation, the phase of the wavelength-doubled 2128 nm light remains locked to the phase of the pump, independent of cavity length, so active frequency stabilization is not required. Numerical analysis indicates that a pulse-injection-seeded LDOPO can obtain 1064 nm to 2128 nm conversion efficiency exceeding 61%. However, analysis of a complete system incorporating a primary low-energy LDOPO that pulse-injection-seeds a secondary higher-energy LDOPO indicates total 1064 nm to 2128 nm efficiency of approximately 57%. A 2128 nm lambda-doubling system having conversion efficiency > 50% may offer a cost-effective alternative to conventional two micron laser sources such as Tm:Ho:YAG.
Sandia National Laboratories has developed high-energy all-solid-state UV sources for use in laboratory tests of the feasibility of satellite-based ozone DIAL. These sources generate 320 nm light by sum-frequency mixing the 532 nm second harmonic of an Nd:YAG laser with the 803 nm signal light derived from a self-injection-seeded image-rotating optical parametric oscillator (OPO). The OPO cavity utilizes the RISTRA geometry, denoting rotated-image singly-resonant twisted rectangle. Two configurations were developed, one using extra-cavity sum-frequency mixing, where the sum-frequency-generation (SFG) crystal is outside the OPO cavity, and the other intra-cavity
mixing, where the SFG crystal is placed inside the OPO cavity. Our
goal was to obtain 200 mJ, 10 ns duration, 320 nm pulses at 10 Hz
with near-IR to UV (1064 nm to 320 nm) optical conversion efficiency of 25%. To date we've obtained 190 mJ at 320 nm using extra-cavity SFG with 21% efficiency, and >140 mJ by intra-cavity SFG with efficiency approaching 24%. While these results are encouraging, we've determined our conversion efficiency can be enhanced by replacing self-seeding at the signal wavelength of 803 nm with pulsed idler seeding at 1576 nm. By switching to idler seeding and increasing the OPO cavity dimensions to accommodate flat-top beams with diameters up to 10 mm, we expect to generate UV energies approaching 300 mJ with optical conversion efficiency approaching 25%. While our technology was originally designed to obtain high pulse
energies, it can also be used to generate low-energy UV pulses with high efficiency. Numerical simulations using an idler-seeded intra-cavity SFG RISTRA OPO scaled to half its nominal dimensions yielded 560 μJ of 320 nm light from 2 mJ of 532 nm pump using an idler-seed energy of 100 μJ.
We have built and tested a highly efficient source of pulsed 320 nm light based on intra-cavity sum-frequency-generation in a self-injection-seeded image-rotating nanosecond optical parametric oscillator. The four-mirror nonplanar ring optical cavity uses the
RISTRA geometry, denoting rotated-image singly-resonant twisted rectangle. The cavity contains a type-II xz-cut KTP crystal pumped by the 532 nm second harmonic of Nd:YAG to generate an
803~nm signal and 1576 nm idler, and a type-II BBO crystal to sum-frequency mix the 532 nm pump and cavity-resonant 803 nm signal to generate 320 nm light. The cavity is configured so pump light passes first through the BBO crystal and then through the KTP crystal with the 320 nm light exiting through the output coupler following the BBO sum-frequency crystal. The cavity output coupler is designed to be a high reflector at 532 nm, have high transmission at 320 nm, and reflect approximately 85% at 803 nm. With this configuration we've obtained 1064 nm to 320 nm optical-to-optical conversion efficiency of 24% and generated single-frequency λ = 320 nm pulses with energies up to 140 mJ.
During the past several years Sandia National Laboratories has
carried out proof-of-concept experiments to demonstrate tunable,
efficient, high-energy ultraviolet nanosecond light sources for
satellite-based ozone DIAL. We designed our UV sources to generate
pulse energies > 200 mJ at 10 Hz in the range of 308-320 nm with optical-to-optical efficiency approaching 25%. We use sum-frequency generation to mix the 532 nm second harmonic of Nd:YAG with near-IR light derived from a self-injection-seeded image-rotating nonplanar-ring optical parametric oscillator. Laboratory configurations using extra- and intra-cavity sum-frequency generation were designed and tested, yielding 1064 nm to 320 nm conversion efficiencies of 21% and 23% respectively, with pulse energies of 190 mJ and 70 mJ. These
energies and efficiencies require pump depletion in the parametric
oscillator of at least 80% and SFG efficiency approaching 60%. While the results reported here fall slightly short of our original goals, we believe UV pulse energies exceeding 250mJ are possible with additional refinements to our technology. Although the sources tested to date are laboratory prototypes with extensive diagnostics, the core components are compact and mechanically robust and can easily be packaged for satellite deployment.
We've generated high-quality flat-top spatial profiles from a
modified Continuum Powerlite 9010 Nd:YAG laser using the
Gaussian-to-flat-top refractive beam shaper available from Newport
Corporation. The Powerlite is a flashlamp-pumped, Q-switched,
injection-seeded Nd:YAG laser manufactured in 1993 that delivers
1.6 J at 10 Hz using an oscillator and two 9 mm diameter
amplifier rods. While its pulse energy is impressive, its beam
quality is typically poor, an all too common characteristic of
research-grade Nd:YAG lasers manufactured in the late 1980's and
early 1990's. Structure in its near-field spatial fluence profile
is reminiscent of round-aperture diffraction that is superposed
with additional hot spots. These characteristics are largely
due to poor beam quality from the oscillator coupled with
over-filled amplifier rods, and reflect a design philosophy from
the era of organic dye lasers. When these older laser systems are
used for tasks like pumping optical parametric oscillators
(OPO's), or for other applications demanding good beam quality,
their designs are simply inadequate.
To improve the 9010's beam quality we spatially filter the
oscillator beam and remove the resulting Airy rings with an iris,
then collimate and magnify the remaining central disk so its
diameter is appropriate for input to the refractive shaper. The
output of the beam shaper is then double-pass amplified through
two amplifier rods with thermally induced focusing compensated by
a negative lens before the first pass and by a convex mirror
before the second pass. Using this approach we've obtained
single-pass energy exceeding 250 mJ with little degradation of the
flat-top profile and 950 mJ after double pass amplification. After double-passing the two amplifier rods the beam suffers some degradation in symmetry and uniformity, but is still much improved compared to the beam obtained using the 9010's original factory configuration. We find the modified 9010's flat-top profile improves conversion efficiency when used for our applications in crystal nonlinear optics.
We have developed public domain numerical models of nonlinear three-wave mixing in birefringent crystals that include diffraction and dispersion. They are suitable for detailed and realistic modeling of mixing for both a single crystal pass and for multiple passes appropriate for a crystal in a resonant cavity. We routinely compare our models with laboratory devices, usually achieving excellent agreement.
We’ve demonstrated a nanosecond KTA OPO utilizing a high Fresnel-number quasi-monolithic image-rotating
nonplanar-ring optical cavity to efficiently generate 1550 nm light with beam quality M2~4. The OPO was
pumped at 1064 nm and injection-seeded at 1550 nm and was tested using either one or two 10×10×17 mm3 KTA
crystals. Total measured conversion efficiencies were as high as 45% and 55% respectively, with corresponding
1550 nm energies of approximately 135 mJ and 170 mJ. While energy and efficiency were high, agreement with
numerical models that included walkoff, diffraction, and geometry of the nonplanar-ring, was poor near the
oscillation threshold. Single-crystal oscillation revealed different thresholds for each KTA crystal. When tested
by observing unphasematched 2ω generation, each crystal appears to contain a single ferroelectric domain,
suggesting that refractive index inhomogeneity, or some other type of defect, prevents perfect phasematching.
A satellite-based UV-DIAL measurement system would allow continuous global monitoring of ozone concentration in the upper atmosphere. However such systems remain difficult to implement because aerosol-scattering return signals for satellite-based lidars are very weak. A suitable system must produce high-energy UV pulses at multiple
wavelengths with very high efficiency. For example, a nanosecond system operating at 10 Hz must generate approximately 1 J per pulse at 308-320 nm. An efficient space-qualified wavelength-agile system based on a single UV source that can meet this requirement is probably not available using current laser technology. As an alternative, we're pursuing a multi-source approach employing all-solid-state modules that individually generate 300-320 nm light with pulse energies in the range of 50-200 mJ, with transform-limited bandwidths and good beam quality. Pulses from the individual sources can be incoherently summed to obtain the required single-pulse energy. These sources use sum-frequency mixing of the 532 nm second harmonic of an Nd:YAG pump laser with 731-803 nm light derived from a recently-developed, state-of-the-art, nanosecond optical parametric oscillator. Two source configurations are under development, one using extra-cavity sum-frequency mixing, and the other intra-cavity sum-frequency mixing. In either configuration, we hope to obtain sum-frequency mixing efficiency approaching 60% by carefully matching the spatial and temporal properties of the laser and OPO pulses. This ideal balance of green and near-IR photons requires an injection-seeded Nd:YAG pump-laser with very high beam quality, and an OPO exhibiting unusually high conversion efficiency and exceptional signal beam quality. The OPO employs a singly-resonant high-Fresnel-number image-rotating self-injection-seeded nonplanar-ring cavity that achieves pump depletion > 65% and produces signal beams with M2 ≈ 3 at pulse energies exceeding 50 mJ. Pump beam requirements can be met in the laboratory using a commercial Nd:YAG laser system, but only after extensive modifications.
SNLO is free, public domain software developed at Sandia National Laboratories. It is intended to assist you in selecting the best nonlinear crystal for your particular application and to predict its performance. This paper briefly describes its functions and how to use them.
We have generated the second, third, fourth, and fifth harmonics of the output of a Yb-doped fiber amplifier seeded by a passively Q-switched Nd:YAG microchip laser. The fiber amplifier employed multimode fiber (25 μm core diameter, V ~ 7.4) to provide high-peak-power pulses, but diffraction-limited beam quality was obtained by use of bend-loss-induced mode filtering. The amplifier output had a pulse duration of 0.97 ns and smooth, transform-limited temporal and spectral profiles (~500 MHz linewidth). We obtained high nonlinear conversion efficiencies using a simple optical arrangement and critically phase-matched crystals. Starting with 320 mW of average power at 1064 nm (86 µJ per pulse at a 3.7 kHz repetition rate), we generated 160 mW at 532 nm, 38 mW at 355 nm, 69 mW at 266 nm, and 18 mW at 213 nm. The experimental results are in excellent agreement with calculations. Significantly higher visible and UV powers will be possible by operating the fiber amplifier at higher repetition rates and pulse energies and by further optimizing the nonlinear conversion scheme.
Design and laboratory characterization of a highly efficient all
solid state 200 mJ UV light source for ozone dial measurements
We are carrying out initial laboratory tests of an all solid state
UV source designed to generate 200 mJ pulses of 320 nm light based
on sum-frequency mixing of the 532 nm second harmonic of an Nd:YAG
pump laser with 803 nm light derived from a nanosecond optical
parametric oscillator. The sum-frequency stage was designed to
obtain mixing efficiency approaching 60% by carefully matching
the spatial and temporal properties of the 532 nm and 803 nm
pulses. The nearly equal balance of 532 nm to 803 nm photons
required an injection-seeded Nd:YAG pump laser and an OPO
exhibiting exceptional signal beam quality and unusually high
conversion efficiency. The OPO was designed to meet these
requirements by employing a high Fresnel-number image-rotating
nonplanar ring cavity producing signal beams with M-squared
of approximately 3
that was pulsed self-injection seeded with the goal of obtaining
greater than 75% conversion of pump energy into the signal and
idler. This design can easily be modified to produce other UV
wavelengths in the range of 300 nm to 320 nm.
We show by computer simulation that high beam quality can be achieved in high energy, nanosecond optical parametric oscillators by using image-rotating resonators. Lateral walk off between the signal and idler beams in the nonlinear crystal creates correlations across the beams in the walk off direction, or equivalently, a restricted acceptance angle. This tends to improve beam quality in the walk off plane. We show that image rotation or reflection can be used to improve beam quality in both planes. The lateral walk off can be due to birefringent walk off in type II mixing or due to noncolinear mixing in type I or type II mixing.
SNLO is public domain software developed at Sandia National Labs. It is intended to assist in the selection of the best nonlinear crystal for a particular application, and in predicting its performance. This paper briefly describes its functions and how to use them.
The effect of the pump, signal, and idler wave phases on three-wave nonlinear parametric mixing is investigated in a series of single-pass- gain experiments. Measurements are made with two angle-tuned KTP crystals in a 532 nm pumped, walkoff-compensated, optical parametric amplifier that is seeded by an 800 nm cw diode laser. In one of the measurements the second crystal is orientated to have its effective nonlinearity deff. of opposite sign to that of the first crystal, so that all mixing that occurred in the first crystal is canceled by the second when the phase mismatch (Delta) kcrystal 1 equals (Delta) kcrystal 2 equals 0. Efficient two-crystal amplification is subsequently restored by selecting the correct phase relationship for the three waves entering the crystal by inserting a dispersive plate between the crystals. The experimental results are explained in a straightforward manner with diagrams involving the three input wave polarizations. These results demonstrate that walkoff-compensated geometries require phase correction to achieve efficient mixing in the second crystal whenever the nonlinear interaction involves two extraordinary waves (e-waves). One practical application of this work may be lower oscillation thresholds and enhanced performance in walkoff- compensated optical parametric oscillators which use two e-waves.
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