We report on the design, fabrication and testing of a 1645 nm injection-seeded and locked Er:YAG laser resonator with single-frequency output operating at a methane line with > 500 μJ/pulse at 4-7 kHz pulse repetition frequencies with a pulse width < 1 μsec. The state-of-the-art technology for lidar methane sensing uses Optical Parametric Oscillator (OPO)/Optical Parametric Amplifier (OPA)–based systems. A key innovation of our system is the use of resonantly 1532 nm pumped Er:YAG gain crystals, which results in improved efficiency and a reduced footprint compared with the current OPO systems. Another feature adapted in our system is the high bandwidth injection locking technique which includes fast piezoelectric mirror and in-house developed FPGA locking algorithm, capable of active locking and wavelength control for each pulse as pulse repetition frequencies up to 10 kHz. The single frequency laser output follows the seed diode wavelength and which scans across the targeted methane absorption line.
We report on the cause and corrective actions of three amplifier crystal fractures in the space-qualified laser systems used in NASA Goddard Space Flight Center’s (GSFC) Ice, Cloud, and Land Elevation Satellite-2 (ICESat-2). The ICESat-2 lasers each contain three end-pumped Nd:YVOO4 amplifier stages. The crystals are clamped between two gold plated copper heat spreaders with an indium foil thermal interface material, and the crystal fractures occurred after multiple years of storage and over a year of operational run-time. The primary contributors are high compressive loading of the NdYVO<sub>4</sub> crystals at the beginning of life, a time dependent crystal stress caused by an intermetallic reaction of the gold plating and indium, and slow crack growth resulting in a reduction in crystal strength over time. An updated crystal mounting scheme was designed, analyzed, fabricated and tested. Thee fracture slab failure analysis, finite-element modeling and corrective actions are presented.
Fibertek has developed an injection locked, resonantly pumped Er:YAG solid-state laser operating at 1.6 μm capable of pulse repetition rates of 1 kHz to 10 kHz for airborne methane and water differential absorption lidars. The laser is resonantly pumped with a fiber-coupled 1532 nm diode laser minimizing the quantum defect and thermal loading generating tunable single-frequency output of 1645-1646 nm with a linewidth of < 100 MHz. The frequency-doubled 1.6 μm Er:YAG laser emits wavelengths in the 822-823 nm spectrum, coincident with water vapor lines. Various cavity designs were studied and optimized for compactness and performance, with the optimal design being an injection seeded and locked five-mirror ring cavity. The laser generated 4 W of average power at pulse repetition frequencies (PRFs) of 1 kHz and 10 kHz, corresponding to 4 mJ and 400 μJ pulse energies, respectively. The 1645 nm was subsequently frequency doubled to 822.5 nm with a 600 pm tuning range covering multiple water absorption lines, with a pulse energy of 1 mJ and a pulse repetition frequency of 1 kHz. The resonator cavity was locked to the seed wavelength via a Pound Drever Hall (PDH) technique and an analog Proportional Integral Derivative (PID) Controller driving a high-bandwidth piezoelectric (PZT)-mounted cavity mirror. Two seed sources lasing on and off the methane absorption line were optically switched to tune the resonator wavelength on and off the methane absorption line between each sequential output pulse. The cavity locking servo maintained the cavity resonance for each pulse.
We report on the completion of the space qualification testing program for NASA Goddard Space Flight Center’s
(GSFC) Ice, Cloud, and Land Elevation Satellite 2 (ICESat-2) program. This paper describes the final performance
results of the fully integrated (laser and electronics) flight laser system with an emphasis on the system design evolution
from a breadboard demonstration to a fully space-qualified laser system. The 532 nm ICESat-2 laser transmitter
generates diffraction limited pulse energies of 1 mJ, pulsewidths of < 1.5 ns, and 10 kHz pulse repetition frequency and
has minimum lifetime of 1 trillion pulses on-orbit. A combination of engineering design units and correlated structural
thermal optical analysis was used to systematically improve reliability and performance over the operating environment.
The laser system qualification and acceptance test programs included electromagnetic interference (EMI), vibration, and
thermal vacuum (TVAC) testing. This paper presents key laser performance results and lessons learned on the multi-year
laser development to facilitate future space-qualified laser developments, improve reliability, and increase performance.
Fibertek is under contract from NASA Goddard to build four space qualified laser transmitters for the ICESat-2 (Ice, Cloud, and Land Elevation Satellite) program, a second generation orbiting laser altimeter. Pertinent laser parameters driving the design included laser wall plug efficiency, laser reliability, a relatively narrow linewidth with wavelength tunability, high beam quality (M<sup>2</sup><1.6), short pulsewidths (<1.5ns), and energy scalable from 250 μJ to 900μJ in predefined steps. The laser design employs fiber coupled 880nm diodes and end-pump Nd:YVO4 slabs as the gain medium in a master oscillator/power amplifier (MOPA) architecture with an LBO second harmonic generator (SHG). Following the SHG is a telescope that sets the final beam size and divergence requirements. The first laser built will be the Integration and Test Laser (ITL) used for qualification of the design. The ITL will set the baseline parameters for the flight laser builds. The ITL will also validate the design for the telescope and will be subjected to the full environmental testing required for a space hardened flight laser. Environmental testing includes vibration, thermal vacuum conditions, and electromagnetic interference (EMI). Our presentation will address the measured laser parameters from ITL as compared to the as designed laser.
A number of ICESat-2 system requirements drove the technology evolution and the system architecture for the laser transmitter Fibertek has developed for the mission.. These requirements include the laser wall plug efficiency, laser reliability, high PRF (10kHz), short-pulse (<1.5ns), relatively narrow spectral line-width, and wave length tunability. In response to these requirements Fibertek developed a frequency-doubled, master oscillator/power amplifier (MOPA) laser that incorporates direct pumped diode pumped Nd:YVO<sub>4</sub> as the gain media, Another guiding force in the system design has been extensive hardware life testing that Fibertek has completed. This ongoing hardware testing and development evolved the system from the original baseline brass board design to the more robust flight laser system. The final design meets or exceeds all NASA requirements and is scalable to support future mission requirements.
Fibertek has designed and is building two space-based lasers for NASA’s CATS-ISS mission. This space-based lidar system requires lasers capable of provide 4-5 kHz output at 1064 nm, 532 nm and 355 nm with each wavelength having ~2-2.5 mJ pulse energy. The lasers will be based on the ISS for a mission lasting up to 3 years.
The increasing use of lidar remote sensing systems in the limited power environments of unmanned aerial vehicles and
satellites is motivating laser engineers and designers to put a high premium on the overall efficiency of the laser
transmitters needed for these systems. Two particular examples upon which we have been focused are the lasers for the
ICESat-2 mission and for the Laser Vegetation Imaging Sensor-Global Hawk (LVIS-GH) system. We have recently
developed an environmentally hardened engineering unit for the ICESat-2 laser that has achieved over 9 W of 532 nm
output at 10 kHz with a wall plug efficiency to 532 nm of over 5%. The laser has a pulse width of <1.5 ns and an M<sup>2</sup> of
<1.5. For the LVIS-GH lidar, we recently delivered a 4.2 W, 2.5 kHz, 1064 nm laser transmitter that achieved a wall
plug efficiency of 8.4%. The laser has a pulse width of 5 ns and an M<sup>2</sup> of 1.1 We provide an overview of the design and
environmental testing of these laser transmitters.
Electra is an electron beam pumped laser being developed at the Naval Research Laboratory as an inertial
confinement fusion (ICF) driver. Two opposing 500 kV, 100 kA electron beams pump the main amplifier, which
achieves energies of 730 J over a 100 ns pulse at 248 nm when run in an oscillator configuration. KrF lasers have been
shown to have intrinsic efficiencies of greater than 12% and, based on that, wall plug efficiencies of >7% are projected
for an IFE system based on our established improvements in laser physics and pulsed power technologies. The Electra
main amp has run at rep-rates of 1 Hz, 2.5 Hz, and 5 Hz in runs exceeding 10,000 shots.
This paper will present an overview of the Electra accomplishments and highlight recent research, including
integrating Electra's amplifiers into a durable full laser system, interferometric measurements of the near field spatial
distortions in the amplifiers and their effect on the far field profile, and spatially and temporally resolved temperature
measurements of the electron beam transmission foil.
The first results are reported from a repetitively pulsed, electron-beam-pumped angularly multiplexed krypton fluoride (KrF) laser system. This laser system, called Electra, was constructed at the U.S. Naval Research Laboratory. The technologies developed on Electra are scalable to a full-size fusion power plant beam line and should meet the inertial fusion energy (IFE) requirements for durability, efficiency, and cost. As in a full-size fusion power plant beam line, Electra is a multistage laser system that consists of a commercial discharge laser, a 175-keV electron-beam-pumped (40-ns flat-top) preamplifier, and a 500-keV (100-ns flat-top) main amplifier. Angular multiplexing is used in the optical layout to provide pulse length control and to maximize laser extraction from the amplifiers. The laser system initially demonstrated 452 J in a single shot and 1.585 kJ total energy in a one-second, 5-Hz burst. The preamplifier alone produces a 25-J KrF output with two angularly multiplexed beams. Extraction volumes were calculated for both a single-pass and a double-pass angularly multiplexed amplifier. A standard ray trace must be used to calculate the extraction volumes for the double-pass amplifier with focusing elements.
Electra is a repetitively pulsed, electron beam pumped Krypton Fluoride (KrF) laser at the Naval Research Laboratory that is developing the technologies that can meet the Inertial Fusion Energy (IFE) requirements for durability, efficiency, and cost. The technologies developed on Electra should be directly scalable to a full size fusion power plant beam line. As in a full size fusion power plant beam line, Electra is a multistage laser amplifier system which, consists of a commercial discharge laser (LPX 305i, Lambda Physik), 175 keV electron beam pumped (40 ns flat-top) preamplifier,
and 530 keV (100 ns flat-top) main amplifier. Angular multiplexing is used in the optical layout to provide pulse length control and to maximize laser extraction from the amplifiers. Single shot yield of 452 J has been extracted from the initial shots of the Electra laser system using a relatively low energy preamplifier laser beam. In rep-rate burst of 5 Hz for durations of one second a total energy of 1.585 kJ (average 317 J/pulse) has been attained. Total energy of 2.5 kJ has been attained over a two second period. For comparison, the main amplifier of Electra in oscillator mode has demonstrated at 2.5 Hz rep-rate average laser yield of 270 J over a 2 hour period.
Electra is a repetitively pulsed, electron beam pumped Krypton Fluoride (KrF) laser at the Naval Research Laboratory
that is developing the technologies that can meet the Inertial Fusion Energy (IFE) requirements for durability, efficiency,
and cost. Electra in oscillator mode has demonstrated single shot and rep-rate laser energies exceeding 700 J with 100 ns
pulsewidth at 248 nm. Continuous operation of the KrF laser has lasted for more than 2.5 hours without failure at 1 Hz
and 2.5 Hz. The measured intensity and energy per shot is reproducible in rep-rate runs of 1 Hz, 2.5 Hz and 5 Hz for
greater than thousand shot durations. The KrF intrinsic efficiency is predicted to be 12% with measurements and
modeling (Orestes Code). In addition we have compared Orestes with initial results of 23 J for the Electra Pre-Amplifier.
The positive agreement between Orestes and our results lead allow us to predict that large KrF laser systems will meet
the efficiency requirements for inertial fusion energy driver. The focal profile measurements show for single shot
conditions recovery in less than 200 ms, the time needed for 5 Hz operation. Rep-rate focal profile measurements at 1 Hz
show reproducibility in spatial extent and energy.