Femtosecond pulses were generated and amplified via chirped pulse amplification in Tm:fiber. The mode-locked oscillator centered at 1975 nm produced 800 fs transform limited pulses with 40 pJ energy at 60 MHz repetition rate. Subsequently, a soliton self-frequency shift in a thulium-doped fiber pumped with a 793 nm diode was used to amplify pulses to 3 nJ, shift the center wavelength, and reduce the pulse duration to 150 fs. This pulse was tuned to 2020 nm to match the center wavelength of a chirped Bragg grating. The pulses were stretched to >160 ps pulses, amplified to 85 nJ in single-mode Tm:fiber and recompressed to 400 fs.
Optical fibers that support single mode operation while achieving large mode areas are key elements for scaling up
optical powers and pulse energies of fiber lasers. Here we report on a study of the modal properties of a new-generation
of polarization maintaining large-mode-area photonic crystal fibers based on the spectrally and spatially resolved (S2)
imaging technique. A fiber designed for Tm fiber laser system single mode operation in the 2μm spectral range is
demonstrated for coiling diameters smaller than 40cm. At shorter wavelengths in the 1.3μm range, efficient higher order
mode suppression requires tide coiling to about 20cm diameters.
We demonstrate single-mode, highly polarized output from a thulium-doped photonic crystal
fiber (PCF) with 50 μm core diameter and an ultra-large mode area >1000 μm2. To our
knowledge, this is the largest mode area of any flexible PCF and is capable of enabling the
generation of high peak powers. In a Q-switched oscillator configuration, this fiber produces
peak powers as high as 8.9 kW with 435 μJ, 49 ns pulses, >15 dB polarization extinction ratio
and quasi diffraction-limited beam quality. The pulse energy was scaled to >1 mJ in amplifier
configuration.
We have developed an integrated Tm:fiber master oscillator power amplifier (MOPA) system
producing 100 W output power, with sub-nm spectral linewidth at -10 dB level, >10 dB
polarization extinction ratio, and diffraction-limited beam quality. This system consists of
polarization maintaining fiber, spliced together with fiberized pump combiners, isolators and
mode field adaptors. Recent advances in PM fibers and components in the 2 μm wavelength
regime have enabled the performance of this integrated high power system; however further
development is still required to provide polarized output approaching kilowatt average power.
We report on a Tm:fiber master oscillator power amplifier system producing 100 W output power, with
>10 dB polarization extinction ratio and diffraction-limited beam quality. To our knowledge, this is the
highest polarized output power from an integrated Tm:fiber laser. The oscillator uses polarization
maintaining (PM) single mode fiber with 10/130 μm core/cladding diameters, and the amplifier uses large
mode area PM fiber with 25/400 μm core/cladding diameters. The oscillator and amplifier are pumped
using 793 nm diodes spliced with pump combiners, and the oscillator is spliced to the amplifier via a
mode field adaptor.
Q-switched operation in a thulium doped photonic crystal fiber (PCF) is shown for the first time. The slope
efficiencies of 25.9, 31.9 and 33 % were observed for 10, 20 and 50 kHz repetition rates. The high peak powers of
>5 kW in the PCF oscillator was used to introduce modulation instability initiated >350 nm broadening in ~50 m
SMF-28 fiber.
We describe lasing in polarizing thulium doped PCF fiber for the first time. The ~4 m long fiber had 50/250 μm
core/cladding diameters and hole-diameter to pitch ratio of 0.18. CW lasing was achieved by end pumping with a
793 nm diode in an oscillator configuration. Slope efficiencies of ~35 % have been obtained with single mode beam
quality (M2 <1.15). The polarization extinction ratio (PER) was >13 dB without any intra-cavity polarizing
elements. This fiber, with MFD of ~36 μm and ~5.8 dB/m cladding pump absorption, is an attractive option for
high energy pulsed amplifiers in the 2 μm wavelength regime.
KEYWORDS: Silica, High power lasers, Chalcogenide glass, Laser systems engineering, Selenium, Fiber lasers, Semiconductor lasers, Near infrared, Thermal optics, Absorption
Chalcogenide samples with varying selenium concentrations, As10Se90, As30Se70, As40Se60, and As50Se50, were
characterized for high power transmission using a Tm:fiber laser system. The Tm:fiber laser oscillator consists of a
LMA fiber with 25/400 μm core/cladding diameters pumped by 793 nm diode. The output beam was collimated to
a 3 mm beam diameter, and transmitted through the chalcogenide samples at CW powers up to 23 W. We measure
the transmission as a function of incident power, as well as some initial characterization of surface damage from
nanosecond pulses at 2 μm. Furthermore, we utilize a wavefront sensor to characterized the thermal lens induced by
the Tm:fiber laser.
We describe the generation and amplification of femtosecond pulses at 2-μm wavelength in thulium doped fiber. The
mode-locked oscillator is a ring cavity based on single-mode Tm:fiber producing stable soliton pulses at 70 MHz
repetition rate with 40 pJ pulse energy, centered at ~1.97 μm wavelength with ~8 nm (FWHM) spectral width. These
pulses seed a Tm:fiber based Raman amplifier, which increases the energy up to 9 nJ. The spectrum is broadened up to
40 nm (FWHM) and the center wavelength can be shifted from
~1.97- 2.15 μm. The Raman solitons are inherently
time-bandwidth limited with pulse durations <150 fs.
We have demonstrated an all-fiber thulium laser system that, without any intracavity polarizing elements or freespace
components, yielded a stable polarization extinction ratio (PER) of ~18 dB. The system is based on singlemode
polarization-maintaining silica fiber and its cavity is formed from each a high and low reflectivity
femtosecond laser written fiber Bragg grating resonant at 2054 nm. The output of the fiber is not only highly
polarized, but maintains a narrow linewidth of 78 pm at its maximum output power of 5.24 W. The high PER
without any polarizing elements in the cavity is of great interest and makes the systems useful for spectral beam
combining and other applications which require polarization dependent optical elements.
We present a Tm:fiber based broadband ASE source which was used for atmospheric CO2 detection. The average
spectral power of this source was limited to ~6.1 μW/nm which was the main limitation in detection of trace
concentrations of gases. This shortcoming was overcome by using an ultrashort pulsed Raman amplifier system with
maximum of ~127 μW/nm of spectral power density which was able to provide sensitivity better than 300 ppm for CO2.
In addition, improving the average power of the ASE provided an essential tool in lab to characterize optical elements
with sharp spectral features around 2 μm.
Amplified ultrashort pulses at 2 μm are of great interest for atmospheric sensing, medical, and materials processing
applications. We describe the generation and amplification of femtosecond 2 μm pulses in thulium doped silica fiber.
Mode-locked eye-safe laser pulses at ~2 μm were generated in a Tm:fiber oscillator using a single-walled carbon
nanotube saturable absorber. Stable mode-locking was achieved at a repetition rate of 70 MHz with soliton pulses
reaching energies of ~40 pJ with a spectral bandwidth of ~8 nm. Autocorrelation measurements indicated bandwidth
limited pulses of ~500 fs duration. This oscillator was used to seed a Tm:fiber amplifier in both free space and fiber
coupled configurations. Effects of dispersion compensation and pulse amplification are reported.
A polarization-maintaining (PM), narrow-linewidth, continuous wave, thulium fiber laser is demonstrated. The laser
cavity is formed from two femtosecond-laser-written fiber Bragg gratings (FBGs) and operates at 2054 nm. The
laser output possesses both narrow spectral width (78 pm) and a high polarization extinction ratio of ~18 dB at 5.24
W of output power. This laser is a unique demonstration of a PM thulium fiber system based on a two FBG cavity
that produces high PER without any free-space elements. Such a narrow linewidth source will be useful for
applications such as spectral beam combining which often employ polarization dependent combining elements.
We report on a thulium doped silica fiber ASE source for absorption spectroscopy of CO2. The average spectral power
of this source was 2.3-6.1 μW/nm. This low spectral power of this source posed limitation in the sensitivity of the
system which was overcome by using an ultrashort pulsed Raman amplifier system with 50-125 μW/nm average spectral
power. This system produced CO2 sensitivity better than 300 ppm making measurement of CO2 possible at standard
atmospheric concentrations.
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.
Cobalt doped ZnSe and ZnS crystals have been studied to determine their effectiveness for passive Q-switching for 700-800nm spectral range (Alexandrite laser). Samples were prepared using Bridgeman technique for single-step growth of Co doped crystals as well as after growth thermal diffusion of Co in undoped crystals. ZnS:Co:Cr crystals, which have been produced using the Bridgeman technique, show maximum initial absorption coefficients of 17 cm-1 at 725nm. Experimental results are reported on effective thermal diffusion of Co2+ in ZnSe and ZnS polycrystals and thermal diffusion constants of cobalt ions in ZnSe and ZnS are estimated. The nonlinear saturation properties of cobalt doped ZnSe and ZnS crystals have been investigated experimentally. The induced transparency measurements were performed using electro-optically Q-switched, alexandrite laser radiation at 731, 741, and 778 nm with a pulse duration of about 70 ns. The induced transmission measurements were analyzed using a four-level absorber model and the absorption cross sections have been estimated at both 731nm and 741nm to be 9.5 × 10-18 cm2 and 8.2 × 10-18 cm2, respectively. Absorption cross sections calculated from saturation measurements at 4A2→4T1(4P) transition are in agreement with results earlier reported for mid-infrared spectral region 4A2→4T2 of Co2+ ions. The described Co-doped crystals are very
promising as passive Q-switches for alexandrite laser resonators. Co2+ centers feature high cross section of saturation and their absorption bands are nicely matched to the spectral emission of the tunable alexandrite laser. An efficient ZnS:Co:Cr passive Q-switching of the alexandrite laser cavity was realized with output energy of 15 mJ and 50 ns pulse duration.
An optical fiber with a section of its cladless core immersed in a fluid uses a refractive index (RI), n, matching method to measure n and (delta) n/(delta) (lambda) (dispersion) of the fluid. Current methods of measuring n and (delta) n/(delta) (lambda) often require light transmission through the fluid; thus turbidity, which diffuses and attenuates the light, restricts RI analysis. This device, which senses the minimum light remaining within the fiber core, is immune to such limitations. Temperature, a factor in any precise measurement of n, is the only accurate measurement needed; no critical system calibration is required. Tests were performed on clear and turbid edible oil samples, and the method matched the +/- .0001 RI unit accuracy of the Abbe refractometer. A tunable laser was used to measure the oil RI within the wavelength range of 543 to 633 nm, and the data fitted to Cauchy's dispersion equation. Calculations, using (lambda) equals 589.3 nm, agree with Abbe measurements of the oil samples.
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