Fibertek reports on the design and performance of two high power ruggedized Thulium fiber laser systems designed for spaceborne and airborne applications. The spaceflight system is a 100 W average power, linearly polarized, 1940 nm thulium doped fiber laser (TDFL) packaged in a hermetic 11-liter module. The airborne system is an all-fiber, narrowlinewidth, master oscillator power amplifier architecture with staged thulium fiber amplifiers with 80 W peak power and 20 W average power in a quasi-continuous temporal waveform. The airborne laser includes programmable digital phase codes with bit rates up to several GHz. We will discuss the overall system performance and environmental qualification testing of both systems.
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.
This paper describes progress toward a space -based 51 W average power amplifier for deep space PPM and Earth GEO links. We demonstrated a broadband WDM amplification at 50W with flat gain across a 25 nm bandwidth. Similarly, for 5 W amplifier we demonstrated a flat gain across a 32 nm bandwidth. These amplifiers demonstrate the feasibility for multi-channel space optical communications links. To increase the bandwidth GEO links to multi-Tbps and deep space links to > Gbps. The laser supports kW/channel SBS limited peak power for PPM and achieves an optical-to-optical efficiency of > 40%. In a separate but related effort for a deep space uplink beacon, we achieved 500 W average power, 2.6 kW peak power PPM (2,2) for a 1 μm uplink transmitter. Reliable SBS free operation is achieved with phase modulation resulting in 26 GHz transmitter linewidth. Uplink transmitter is optimized for 65 usec (pulsewidth) slot size—achieving fastest possible rise/fall times (<10 usec) and pulse uniformity.
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 NdYVO4 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 present the results of a three-year operational-aging test of a specially designed prototype flight laser operating at 1064 nm, 10 kHz, 1ns, 15W average power and externally frequency-doubled. Fibertek designed and built the q-switched, 1064nm laser and this laser was in a sealed container of dry air pressurized to 1.3 atm. The external frequency doubler was in a clean room at a normal air pressure. The goal of the experiment was to measure degradation modes at 1064 and 532 nm separately. The external frequency doubler consisted of a Lithium triborate, LiB3O5, non-critically phase-matched crystal. After some 1064 nm light was diverted for diagnostics, 13.7W of fundamental power was available to pump the doubling crystal. Between 8.5W and 10W of 532nm power was generated, depending on the level of stress and degradation. The test consisted of two stages, the first at 0.3 J/cm2 for almost 1 year, corresponding to expected operational conditions, and the second at 0.93 J/cm2 for the remainder of the experiment, corresponding to accelerated optical stress testing. We observed no degradation at the first stress-level and linear degradation at the second stress-level. The linear degradation was linked to doubler crystal output surface changes from laser-assisted contamination. We estimate the expected lifetime for the flight laser at 532 nm using fluence as the stress parameter. This work was done for NASA’s Ice, Cloud, and land Elevation Satellite-2 (ICESat-2) LIDAR at Goddard Space Flight Center in Greenbelt, MD with the goal of 1 trillion shots lifetime.
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 (M2<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.
The design of space-flight hardware is typically required to be at a Technology Readiness Level (TRL) of 6 before the build of the actual flight hardware can begin. At the early design phase the "relevant environment" for TRL-6 is frequently not well defined. For the ICESat-2 laser relevant environment was defined as the qualification levels in GEVS (General Environmental Verification Standard, GSFC-STD-7000). Our approach to dealing with the high-frequency content of the 14.1 grms random vibration levels in GEVS was a flexure mounted canister design that filtered the highfrequency content. In our talk we will discuss the program and system level implications of this design approach.
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:YVO4 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.
A laser diode module (LDM) space certification and qualification program was developed for NASA’s Ice, Cloud and Land Elevation Satellite-2, ICESat-2 mission. The ICESat-2 laser transmitter is a high performance diode-pumped solid state laser that requires high reliability, high efficiency and high brightness fiber coupled LDMs, capable of supporting a 27,000 hour mission life. The test centric LDM space certification and qualification programs consisted of several key phases including a technology plausibility study, laser diode and LDM pedigree reviews, environmental acceptance and qualification testing, and extensive life testing. The intent of the plausibility study was to analytically and experimentally demonstrate that a commercial-off-the-shelf (COTS) LDM design was capable of being space-certified. A pedigree review of the laser diode population was conducted to reject out-of-family laser diodes from the population. The laser diode pedigree review was a statistical analysis of several laser diode performance metrics (efficiency, operating current, etc.) with outliers being rejected. All LDMs underwent environmental acceptance testing including vibration, thermal cycling and an extended burn-in. The primary purpose of the acceptance testing was to highlight internal workmanship issues. The pedigree review of the acceptance tested LDMs was conducted to reject out-of-family LDMs in statistical analysis of several performance metrics (operating current, coupling efficiency, etc.). A sub-set of the flight-certified LDMs will be exposed to environmental qualification testing and will subsequently be tested to failure to determine the LDM capability. Multiple LDMs are being life tested under flight-like conditions and show no signs of degradation with run times of 22,000 hours, which is over 80% of the mission life. Details of the LDMs space certification and qualification programs are presented.
Fibertek has developed an environmentally hardened Technology Readiness Level-6 laser transmitter system for the NASA Ice, Cloud and land Elevation Satellite-2 (ICESat-2). The laser transmitter generates over 9 W of 532 nm output with a pulse repetition rate of 10kHz and a FWHM pulse width of < 1.5 ns with an expected lifetime of > 1 trillion shots. This paper presents the results of the Structural, Thermal and Optical analysis, details on the NASA General Environmental Verification Specification testing requirements, and the success of the laser transmitter performance through vibration and thermal vacuum testing.
We report on the successful delivery of a 30 W solid-state sodium Guide Star Laser System (GLS) to the W. M. Keck
Observatory in 2009, and the demonstration of a 55 W GLS delivered to the Gemini South Observatory in 2010. This
paper describes the GLS performance results of both the Keck I and Gemini South GLSs with an emphasis on the system
design and delivered performance. The 589 nm output was generated via Sum Frequency Mixing (SFM) of 1064 nm
and 1319 nm Nd:YAG lasers in a LBO (Lithium Triborate) nonlinear crystal. The Keck GLS underwent extensive
testing and has demonstrated consistent performance with a CW mode-locked output of > 30 W and measured beam
quality of M2 < 1.2 while locked to the sodium D2a transition. The Keck GLS was installed on the telescope in late 2009
and first light on the sky was achieved in early 2010. Factory testing of the Gemini South GLS shows a CW modelocked
output of > 55 W and measured M2 ~1.2 while locked to the sodium D2a line center. The Gemini South GLS has
produced a maximum power of 76 W at 589 nm with 85 W of 1319 nm and 110 W of 1064 nm as inputs to the SFM,
representing a single-pass conversion efficiency of 39%.