Binh T. Do

Bulk and surface laser damage of silica by picosecond and nanosecond pulses at 1064 nm

We measured bulk and surface dielectric breakdown thresholds of pure silica for 14 ps and 8 ns pulses of 1064 nm light. The thresholds are sharp and reproducible. For the 8 ns pulses the bulk threshold irradiance is 4.75 +/- 0.25 kW/microm2. The threshold is approximately three times higher for 14 ps pulses. For 8 ns pulses the input surface damage threshold can be made equal to the bulk threshold by applying an alumina or silica surface polish.

Bulk optical damage thresholds for doped and undoped, crystalline and ceramic yttrium aluminum garnet

We measured the bulk optical damage thresholds of pure and Nd-doped ceramic yttrium aluminum garnet (YAG), and of pure, Nd-doped, Cr-doped, and Yb-doped crystalline YAG. We used 9.9 ns, 1064 nm, single-longitudinal mode, TEM00 pulses, to determine that the breakdown thresholds are deterministic, with multiple-pulse thresholds ranging from 1.1 to 2.2 kJ/cm2.

Deterministic nanosecond laser-induced breakdown thresholds in pure and Yb3+ doped fused silica

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.

Rate equation model of bulk optical damage of silica, and the influence of polishing on surface optical damage of silica

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.

Nanosecond, 1064 nm damage thresholds for bare and anti reflection coated silica surfaces

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.

Nanosecond laser-induced breakdown in pure and Yb3+-doped fused silica

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 Liekkis 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.

Properties of optical breakdown in BK7 glass induced by an extended-cavity femtosecond laser oscillator

Using an extended-cavity femtosecond oscillator, we investigated optical breakdown in BK7 glass caused by the accumulated action of many laser pulses. By using a pump-probe experiment and collecting the transmitted pump along with the reflected pump and the broadband light generated by the optical breakdown, we measured the build-up time to optical breakdown as a function of the pulse energy, and we also observed the instability of the plasma due to the effect of defocusing and shielding created by the electron gas. The spectrum of the broadband light emitted by the optical breakdown and the origin of the material modification in BK7 glass was studied. We developed a simple model of electromagnetic wave propagation in plasma that is consistent with the observed behavior of the reflection, absorption, and transmission of the laser light.

Power scaling of fiber-based amplifiers seeded with microchip lasers

We summarize the performance of mode-filtered, Yb-doped fiber amplifiers seeded by microchip lasers with nanosecond-duration pulses. These systems offer the advantages of compactness, efficiency, high peak power, diffraction-limited beam quality, and widely variable pulse energy and repetition rate. We review the fundamental limits on pulsed fiber amplifiers imposed by nonlinear processes, with a focus on the specific regime of nanosecond pulses. Different design options for the fiber and the seed laser are discussed, including the effects of pulse duration, wavelength, and linewidth. We show an example of a microchip-seeded, single-stage, single-pass fiber amplifier that produced pulses with 1.1 MW peak power, 0.76 mJ pulse energy, smooth temporal and spectral profiles, diffractionlimited beam quality, and linear polarization.

Design and Performance of a High-Repetition-Rate Single-Frequency Yb:YAG Microlaser

We describe the design and performance of a high-repetition-rate single-frequency passively Q-switched Yb:YAG microlaser operating near 1030 nm. By using short cavity length, an intracavity Brewster polarizer, and an etalon output coupler, we are able to produce ~1-ns-long single-frequency pulses at repetition rates up to 19 kHz without shot-to-shot mode hopping. The lasers output spatial mode is TEM00 and its pulse energy varies between 31 μJ and 47 μJ depending on repetition rate. Its peak optical-to-optical efficiency is 22%.

Picosecond-nanosecond bulk damage of fused silica at 1064nm

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 Cornings 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.