This work compares the effects of ablation on GaAs, Al, and Ti samples exposed to two different regimes: a focused 800nm, 14.13mJ pulse of 55fs duration from a Ti: Sapphire laser with an intensity of 2×1016 W cm-2 and a focused 1064nm, 14.61mJ pulse of 10ns duration from a Nd: YAG laser with an intensity of 2×1010 W cm-2. The craters are examined using optical microscopy, white light interferometry, and scanning electron microscopy. Among the effects examined in this paper are the conduction of energy throughout the material, formation of nanodroplets outside of the crater, nanopits in the center of the crater, and the effects of phase explosion inside the crater.
A tunable master oscillator power amplifier (MOPA) fiber laser system based on thulium doped silica fiber designed for
investigation of multi-kilometer propagation through atmospheric transmission windows existing from ~2030 nm to
~2050 nm and from ~2080 nm to beyond 2100 nm is demonstrated. The system includes a master oscillator tunable over
>200 nm of bandwidth from 1902 nm to beyond 2106 nm producing up to 10 W of linearly polarized, stable, narrow
linewidth output power with near diffraction limited beam quality. Output from the seed laser is amplified in a power
amplifier stage designed for operation at up to 200 W CW over a tuning range from 1927 - 2097 nm. Initial field tests of
this system at the Innovative Science & Technology Experimental Facility (ISTEF) laser range on Cape Canaveral Air
Force Station, Florida will be discussed. Results presented will include investigation of transmission versus wavelength
both in and out of atmospheric windows, at a variety of distances. Investigations of beam quality degradation at ranges
up to 1 km at a variety of wavelengths both in and out of atmospheric transmission windows will be also presented.
Available theoretical models of atmospheric transmission are compared to the experimental results.
Beams from three frequency stabilized master oscillator power amplifier (MOPA) thulium fiber laser systems were
spectrally beam combined using a metal diffraction grating. Two of the laser oscillators were stabilized with guided
mode resonances filters while the third was stabilized using a gold-coated diffraction grating. Each system was
capable of producing a minimum of 40 W output powers with slope efficiencies between 50-60 %. The three lasers
undergoing combination were operating at wavelengths of 1984.3, 2002.1, and 2011.9 nm with spectral linewidths
between 250-400 pm. Beam combining was accomplished by spatially overlapping the spectrally separated beams
on a water-cooled gold-coated diffraction grating with 600 lines/mm. Beam quality measurements were completed
using M2 measurements at multiple power levels of the combined beam. Power levels of 49 W were achieved before
thermal heating of the metal diffraction grating cause degradation in beam quality. The combining grating was
~66% efficient for the unpolarized light corresponding to a total optical-to-optical efficiency of 33% with respect to
launched pump power.
A volume Bragg grating is used in two different configurations to control the output spectrum of a thulium doped silica
fiber laser. When used in a direct feedback configuration on the end of a bidirectionally pumped resonator, a power of up
to 159 W with 54% slope efficiency is produced with a narrow output spectrum centered at 2052.5 nm with <1.5 nm
full-width at 10 dB down from spectral peak. Maximum laser linewidth is limited by the VBG reflectivity width. The
VBG based laser is compared to a laser resonator based on a standard HR mirror and is able to maintain stable spectrally
narrow operation while the HR mirror laser has a wide and varied spectral output over 20-30 nm. Both lasers have
similar slope efficiency, threshold and power performance with any difference attributed to lack of AR coatings on the
VBG. In a second cavity, the VBG is used in a tunable configuration by rotating the VBG away from normal incidence.
Tuning range was found to be >100 nm from 1947 nm to 2052.5 nm with output powers as high as 48 W and up to 52%
slope efficiency. Tuning range is determined by VBG center wavelength on the long wavelength end and by the VBG
aperture size on the short end. In both system configurations, M2 is maintained at less than 1.2 at all power levels and
long term operating stability at full power is demonstrated.
Guided mode resonance filters (GMRF) were used to spectrally-stabilize and line-narrow the output
spectrum from Tm fiber lasers operating in the 2 μm wavelength regime. The GMRFs were placed in the output path
of an amplified spontaneous emission (ASE) light source and the transmitted light was measured as a notch in the
spectrum on resonance. The GMRFs were characterized to determine their peak reflectivity, resonance wavelength,
and spectral linewidth of each element. These measurements showed various resonance wavelengths and linewidths
varying from 0.50-1.5 nm depending on the individual GMRF parameters. Using GMRFs as feedback elements in
Tm fiber laser oscillators resulted in output powers up to 10 W and slope efficiencies of 30-45% with respect to
launched 790 nm pump power. In order to scale to higher powers and maintain narrow linewidths, a master
oscillator power amplifier (MOPA) setup was employed with a GMRF stabilized master oscillator. In addition to the
laser and amplifier characteristics, thermal and damage testing of the GMRFs is reported.
We report the performance of an actively Q-switched Tm fiber laser system. The laser was stabilized to sub-nanometer
spectral width using each of two feedback elements: a blazed reflection grating and a volume Bragg grating. Maximum
pulse energy using the reflection grating was 325 μJ pulses at 1992 nm (< 200 pm width) with a 125 ns duration at a 20
kHz repetition rate. Maximum pulse energy using the volume Bragg grating was 225 μJ pulses at 2052 nm (<200 pm
width) with a 200 ns duration also at 20 kHz. We also report the laser's performance as an ablation source for LIBS
experiments on copper.
We have designed and developed a grating based thulium (Tm) doped fiber laser with ~150 nm tuning range which is
used as the master oscillator in a master oscillator power amplifier (MOPA) thulium fiber laser system. Due to thermal
instability in the grating used for tuning, the MO could produce a power up to 4.5 W, beyond which the oscillator
became unstable. Injecting the seed laser into a bidirectionally pumped large mode area (LMA) Tm fiber amplifier, a
stable, tunable, narrow linewidth high beam quality amplified signal of >100 W was achieved. In the absence of stable
and sufficiently high power from the seed laser, the amplifier could not be tested to its full potential. The amplifier was
also, converted into an oscillator to investigate its power handling capability. An excellent beam quality and ~200 W of
power were achieved by running the power amplifier as an oscillator. Operation stability of the oscillator was measured
to be more than one hour with a minimum power fluctuation of 0.5%. Currently efforts are underway to increase the seed
laser power to ~10 W, large enough to reduce ASE and mitigate feedback to the master oscillator to demonstrate a 200
W, tunable (150 nm) and narrow linewidth (0.15 nm) MOPA system.
The MOPA system will be one of a number of new state-of-the-art high power lasers to be located at the
Innovative Science & Technology Experimentation Facility, creating a unique laser range facility for next generation
studies and tests across a broad range of sciences and technologies.
Eye-safe, high power tunable narrow linewidth lasers are important for various applications such as atmospheric
propagation measurements. We have investigated two techniques of generating narrow linewidth thulium 2-μm fiber
lasers, utilizing a reflective volume Bragg grating (VBG), and a guided mode resonance filter (GMRF) as a cavity end
mirror. A stable narrow linewidth (50 pm), tunable (from 2004 nm to 2054 nm) thulium doped fiber laser using a
reflective VBGg was demonstrated. A CW power of 17 W was achieved. Using a GMRF as an end mirror we showed a
narrow linewidth (~30 pm) laser with an output power of 5.8W, and at a slope efficiency of 44%.
Eye-safe, high power, tunable, narrow linewidth lasers are key technologies for a number of applications, including
atmospheric propagation measurements. Since the atmosphere has narrow line transmission windows it is important to
have a tunable linewidth source which can be matched to the transmission window. We have developed a stable narrow-linewidth
(0.3 nm), tunable (from 1947 nm to 2108 nm) large mode area thulium doped fiber laser. Using this as a seed
source, a master oscillator power amplifier with ~100 W output power will be presented.
2-micron solid-state lasers operating at moderate to high pulse energies require high power quasi-CW laser diode arrays (LDAs) operating at a nominal wavelength of 792 nm with pulse durations of at least one millisecond. This long pulse duration is one of the main causes of limited lifetimes for these arrays. Such relatively long pulse durations cause the laser diode active region to experience high peak temperatures and drastic thermal cycling. This extreme localized heating and thermal cycling of the active regions are considered the primary contributing factors for both gradual and catastrophic degradation of LDAs. This paper describes the thermal characteristics of various LDA packages, providing valuable insight for improving their heat dissipation and increasing their lifetime. The experiment includes both direct measurement of thermal radiation of the LDA facet using a high resolution IR camera and indirect measurement of LDA active region temperature by monitoring the wavelength shift of the near-IR light. The result of thermal measurements on different quasi-CW LDA packages and architectures is reported.
Several Tm,Ho-doped LuLF and YLF crystals were grown using the Czochralski method. Crystals were pulled in a furnace with atmospheric control system. Pulling and rotation rates were 1 mm/hr and 15 rpm, respectively. High purity (>= 99.99%) fluorides were used as starting materials. High vacuum (approximately equals 10-5 torr) prior to the growth and CF4 gas during the growth were applied. Single crystals of up to 20 mm in diameter and 80 mm in length were successfully grown. Various laser rods of sizes 4 X 3 X 2.7 mm3 were prepared from the grown crystal boules. The crystal ends were Brewster- cut to minimize the reflection losses inside the cavity. Two quasi-CW LD-arrays of 6 X 60 W each side-pump the crystals. The pump beam is focused using two lens ducts of length 64 mm to a waist of 2.5 X 2.5 mm2. The nearly hemispherical laser cavity is formed by a flat high reflector and a 300 mm radius of curvature 5% transmission output coupler. At room temperature, up to 13.5 mJ (9.9 mJ) with a slope efficiency of 10.5% (7.5%) has been demonstrated at 1 Hz for 5% Tm, 0.5% Ho:LuLF (5% Tm, 0.5% Ho:YLF).