We review the status of commercially available USP (ultra short pulse) laser amplifiers and their key components, analyzing how technology innovation and industrial/scientific applications are pushing boundaries in performance and reliability. These improvements will be instrumental in facilitating adoption in challenging environments like defense applications. We provide examples of performance requirements for various applications and describe the challenges to overcome to increase performance and reliability of fielded USP lasers and amplifiers.
The performance of advanced laser systems for defense and aerospace applications rely heavily on the capabilities of the system building blocks. Due to the sensitive nature of the end applications, such components also often require domestic US sources to ensure supply chain security and facilitate engagement in the product development cycle.
Coherent maintains a full range of domestic critical component manufacturing capabilities to support the defense and aerospace laser industry, including optical fiber, semiconductor diode lasers, crystals, optical isolators, coatings and freeform optics, all from US-based manufacturing locations.
Coherent has also expanded the internal manufacturing capabilities, enabling the manufacture of complete laser component assemblies and subsystems, allowing contract partners to leverage our internal laser manufacturing expertise. We will review our latest component capabilities and discuss how these components map to critical defense applications.
Coherent delivers superior component products ranging from laser diodes to custom optics to specialty optical fibers and advanced optical fiber assemblies that meet your application requirements. In this video, Timothy McComb, Business Development Manager Components at Coherent Inc. presents our new products in the areas of multi kW and ultrafast fibers as well as fiber-coupled single-emitter diode laser pump modules.
We demonstrate the use of kW class, high energy pulsed nanosecond lasers for surface cleaning applications. A hybrid ablation-detachment process allows removal rates of semi-transparent paint from primer at 40,000 mm3 /kW.min. The process shows a threshold at which a nonlinear increase in removal rate is observed for energies above 100mJ for most paints. The impact of pulsed laser de-painting on substrates is examined.
We present a 400 W Q-switched 532 nm laser using several techniques to control the pulse duration and shape. The laser consists of two separate intracavity frequency-doubled cavities, incoherently combined to give 50 mJ total pulse energy. By extending the cavity length the pulse duration can be increased from 66 ns to 140 ns at 8 kHz with no significant power reduction. Varying the repetition rate and the triggering delay for each cavity’s Q-switch allows further adjustment of the pulse duration. The pulse shape can be controlled by adjusting this delay and attenuating the individual cavities.
We report detailed characterization results of Yb-doped Chirally-Coupled-Core (3C) fibers fabricated with Direct Nanoparticle Deposition (DND) technique. Two types of 3C fibers with core/clad geometries of 34/250μm and 55/400μm and another 25/250μm conventional large-mode-area (LMA) fiber are measured and the results are compared in terms of modal content, transmission spectrum, etc. A picosecond fiber amplifier is built based on 55/400μm 3C fiber, showing robust single-mode operation with peak power >1MW with no sign of stimulated Raman scattering (SRS).
We demonstrate flexible performance in a fiber MOPA system based on nLIGHT’s PFL seed laser platform and chirally coupled core (3C®) fiber. The 33μm core, 27μm MFD 3C fiber used in these demonstrations is fabricated in volume at nLIGHT’s Finland facility. A variety of pulse formats are amplified to nonlinearity-limited peak power <300kW, including single pulses in the 50ps to 1ns regime at a variety of repetition rates from 10’s of kHz to MHz. Beam quality in these 3C based MOPAs is exceptional with M2<1.15 and circularity <95% at all power levels. Beam pointing often evident in other LMA fiber technologies due to higher order mode content is minimal in these fiber MOPAs. Burst mode operation of the seed laser system using flexible burst packet repetition rates (10’s of kHz to several MHz) and adjustable pulse-to-pulse spacing within bursts (<10ns to 100ns) is demonstrated and amplified in the same 3C fibers. Bursts of up to ten 50ps pulses amplified to total energies exceeding 160μJ are demonstrated at 200kHz burst repetition rate and 32W average power at high efficiency (74% slope). Bursts of up to five 500ps pulses are also amplified to up to 360μJ total energy. In both cases, the varying degree of pulse saturation win a burst and mitigation paths are reviewed.
3C fiber technology advances the performance frontier of practical, high-pulse-energy fiber lasers by providing very large core fibers with the handling and packaging benefits associated with single mode fibers. First-generation fibers demonstrate scaling to > 240 W average power coincident with 100-kW peak power in 1-mJ, 10-ns pulses while maintaining single-mode beam quality, polarized output, and efficiencies > 70%. Peak powers over 0.5 MW with negligible spectral distortion can be achieved with sub ns, near-transform-limited pulses. In-development second-generation 3C Yb-fiber based on core sizes around 55 μm1 have produced >8 mJ, 13 ns pulses with peak powers exceeding 600 kW.
We demonstrate a robust, compact, low-cost, pulsed, linearly polarized, 1064 nm, Yb:fiber laser system capable of generating ~100 kW peak power pulses and >17 W average power at repetition rates of 80 – 285 kHz. The system employs a configurable microchip seed laser that provides nanosecond (~1.0 – 1.5 ns) pulse durations. The seed pulses are amplified in an all-fiber, polarization maintaining, large mode area (LMA) fiber amplifier optimized for high peak power operation. The LMA Yb:fiber amplifier enables near diffraction limited beam quality at 100 kW peak power. The seed laser, fiber amplifier, and beam delivery optics are packaged into an air-cooled laser head of 152×330×87 mm3 with pump power provided from a separate air-cooled laser controller. Due to the high peak power, high beam quality, spectral purity, and linearly polarized nature of the output beam, the laser is readily frequency doubled to 532 nm. Average 532 nm powers up to 7 W and peak powers exceeding 40 kW have been demonstrated. Potential for scaling to higher peak and average powers in both the green and infrared (IR) will be discussed. This laser system has been field tested and demonstrated in numerous materials processing applications in both the IR and green, including scribing and marking. We discuss recent results that demonstrate success in processing a diverse array of representative industrial samples.
We have demonstrated a pulsed 1064 nm PM Yb:fiber laser system incorporating a seed source with a tunable pulse repetition rate and pulse duration and a multistage fiber amplifier, ending in a large core (>650 μm2 mode field area), tapered fiber amplifier. The amplifier chain is all-fiber, with the exception of the final amplifier’s pump combiner, allowing robust, compact packaging. The air-cooled laser system is rated for >60 W of average power and beam quality of M2 < 1.3 at repetition rates below 100 kHz to 10’s of MHz, with pulses discretely tunable over a range spanning 50 ps to greater than 1.5 ns. Maximum pulse energies, limited by the onset of self phase modulation and stimulated Raman scattering, are greater than 12.5 μJ at 50 ps and 375 μJ at 1.5 ns , corresponding to >250 kW peak power across the pulse tuning range. We present frequency conversion to 532 nm with efficiency greater than 70% and conversion to UV via frequency tripling, with initial feasibility experiments showing >30% UV conversion efficiency. Application results of the laser in scribing, thin film removal and micro-machining will be discussed.
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.
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.
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.
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.
790nm-pumped Tm-doped fibre lasers provide a number of distinct benefits for integration into next generation DIRCM
systems. Incorporation of Tm-doped fibre technology into mid-IR laser systems has been demonstrated in two main
architectures to date; in early works the fibre laser was used as a low quantum defect pump source for Q-switched solidstate
holmium laser which was subsequently shifted to the mid-IR using a ZnGeP2 OPO [1] and more recently, a pulsed
fibre laser systems was used for directly pumping the OPO [2]. He we present two fibre laser systems for integration into
DIRCM systems. Firstly we present a 70W MOPA system (pump power limited) operating at 1908nm with 53% slope
efficiency from the amplifier stage for pumping Ho:YAG. Secondly we present a pulsed fibre laser system producing
over 4kW peak power at 1910nm using all single-mode fibres.
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