Sub-picosecond green-pumped OPG/OPA using non-critical phase matching in LBO was investigated at high average powers, utilizing a novel high-power ultrafast fiber laser providing 300W of 1ps pulses at 515nm and 1.4GHz pulse repetition rate. The 4-stage collinear OPA produced about 100W of combined Signal plus Idler output powers when tuning the Signal wavelength between 740nm and 940nm. An extended spectral tuning range between 300nm and 1800nm with average power between 10W and 70W was demonstrated by using SFG of Signal plus Pump and SHG of Signal and of Idler. The OPA output pulse durations were ~0.5ps long.
Kilowatt-level narrow-linewidth SM ytterbium fiber laser operating in high-repetition-rate QCW regime was used to obtain 700 W average power at 532 nm with single-mode beam quality and wall-plug efficiency of over 23 %. To the best of our knowledge, this is ~60 % higher power than previously reported for single-mode green lasers based on other platforms, and also is ~30 % increase comparing to the previous result obtained by our group on the base of similar fiber laser platform. We have also experimentally proved that the same type of fiber laser can be used for generating of world-record levels of power at other wavelengths of visible and UV spectral ranges by employing cascaded non-linear frequency conversion. Thus, utilizing frequency tripling in 2 LBO crystals, we achieved over 160 W average power of nearly single-mode UV light at 355 nm with THG efficiency of more than 25 %. As far as we know, this is the highest output power ever reported for UV laser with nearly diffraction limited beam quality. We also conducted some preliminary experiments to demonstrate suitability of our approach for generating longer wavelengths of the visible spectrum. By pre-shifting fundamental emission wavelength in fiber Raman converter, followed by frequency doubling in NCPM LBO, we obtained average powers of 36 W at 589 nm and 27 W at 615 nm. These proof-of-concept experiments were performed with low-power pump laser and were not fully optimized with respect to frequency conversion. Our analysis indicates that employing kW-level QCW ytterbium laser with optimized SRS and SHG converters we can achieve hundreds of Watts of average power in red and orange color with single-mode beam quality.
We report a single-mode (SM) green laser based on single-pass frequency doubling of a linearly-polarized narrowlinewidth Yb fiber laser in LBO crystal, and configured to operate in a range of regimes from continuous-wave (CW) to high-repetition-rate quasi-continuous-wave (QCW). Adjusting the duty cycle, we maintained high second harmonic generation (SHG) efficiency for various output powers. Average powers of over 550W in QCW and over 350W in CW regimes were obtained with the wall-plug efficiency up to 15%, opening the possibility to creating new class of simple, compact and efficient single-mode green lasers with output power up to 1kW and above. The same approach could also be used to create high-power lasers operating at other wavelengths in ultraviolet and visible spectral ranges.
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.
In this manuscript we present our recent achievements utilizing thulium-doped photonic crystal fiber rods (PCF-rods) for lasing at 2 μm wavelength and their potential as high peak power amplifiers. Two PCF-rods with 65 μm and 80 μm core diameter were first separately characterized in CW laser oscillators. The rods were pumped with a 793 nm laser diode and produced more than 18 W output power with near diffraction limited beam quality and a slope efficiency of up to 27.8 %. Implementing an intracavity high reflectivity grating for lasing wavelength selection enabled a tuning range of 180 nm from 1810 nm to 1990 nm. Thereafter the PCF-rod with 80 μm core was used as an amplifier and produced similar output powers when seeded with up to 4 W at 1960 nm from a master oscillator power amplifier (MOPA). The slope efficiency in this case was reduced to 20.1 % mainly due to the center wavelength of 1960nm. We are in the process of characterizing these rods in a pulsed amplification configuration to surpass MW-level peak power with multi-mJ pulse energy.
Utilizing the transparency of silicon at 2 μm, we are able to ablate the backside of 500-μm thick
silicon wafers without causing any damage to the front surface using a novel nanosecond
Tm:fiber laser system. We report on our high energy/high peak power nanosecond Tm:fiber
laser and provide an initial description of the effects of laser parameters such as pulse duration
and energy density on the ablation, and compare thresholds for front and backside machining.
The ability to selectively machine the backside of silicon wafers without disturbing the front
surface may lead to new processing techniques for advanced manufacturing in solar cell and
microelectronics industries.
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.
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 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.
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 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.
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 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.
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.
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.
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