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Lasers & Sources

Laser offers robust source for space-based lidar systems

A high-energy single-frequency pump laser, capable of operating in harsh environments, represents an enabling technology for remote sensing from space.
3 December 2006, SPIE Newsroom. DOI: 10.1117/2.1200610.0444

The use of light detection and ranging (lidar) systems in ground, airborne, and space-based missions can provide earth and planetary science measurements that were previously unavailable. A number of space-based high-resolution altimetry and aerosol monitoring lidar systems have already been fielded. These include the Lidar In-space Technology Experiment (LITE);1 Clementine, which performed lunar altimetry;2 the Mars Orbiter Laser Altimeter (MOLA2);3 the Near Earth Asteroid Rendezvous (NEAR) spacecraft, which performed asteroid altimetry;4 the Geoscience Laser Altimeter System (GLAS) carried on the Ice, Cloud, and Land Elevation Satellite (ICESAT);5 and the lidar system carried aboard the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) satellite.6

Lidar systems that use direct measurement of Doppler shifts for wind velocity, as well as those that measure molecular back-scattering in the presence of a large aerosol signal using high-spectral-resolution lidar,7 both require the use of a single-frequency laser. Ozone differential absorption lidar (DIAL) systems that require the generation of two UV wavelengths have improved nonlinear conversion efficiencies when the primary pump laser is a single-frequency laser. All high-energy laser transmitters benefit from the reduced probability of optical damage if we eliminate the temporal spikes that occur in multi-mode lasers. Our goal is to develop a space-qualifiable single-frequency pump laser that meets the needs of the next generation of space-based lidar missions.

Although injection-seeded single-frequency lasers are available commercially, their designs do not meet the requirements of a space-based mission. For high reliability, the pulsed laser must only q-switch (i.e., allow light to leave the cavity) when the seed laser is in resonance with the cavity of the pulsed laser. The typical approach minimizes the time between q-switched output pulses by varying the cavity length of the pulsed laser over a number of output pulses and, therefore, does not meet the requirements for high reliability. In addition, this approach does not guarantee fast recovery from a loss of resonance due to a thermal or mechanical perturbation. Other features that are needed for spaced-based systems—but unavailable to varying degrees in commercial lasers—are conductive cooling, a mechanical design capable of surviving a space launch, vacuum operation, and radiation-hardened electronics.

Our approach to the design of a space-qualifiable single-frequency 1μm laser is shown in Figure 1. The oscillator and all of the amplifiers use Nd:YAG (neodymium-doped yttrium aluminum garnet) zigzag slabs that are pumped by diodes and conductively cooled. Waste heat is conducted directly to the base of the laser canister where it is dissipated by either external cold plates for ground-based testing or radiators in space. The ring laser incorporates a low-thermal-expansion Zerodur bench that improves the seeding and boresight stability. We designed the pumping geometry of the amplifiers to maximize the overlap between the pumped regions of the slabs and the extracting beam for higher efficiency. We also incorporated second (532nm) and third (355nm) harmonic generation crystals.

Figure 1. Design for a space-qualifiable single-frequency laser starts with a fiber-coupled 1μm laser that seeds the ring resonator (left). The output power of the pulses is increased by three amplifiers and then two nonlinear crystals (LBO) double or triple the frequency, to obtain 532 or 355nm light, respectively.

Our design achieves stable single-frequency operation of the ring oscillator with an approach that is a hybrid of the ‘ramp and fire’8,9 and the Pound-Drever-Hall10 approaches. An intracavity electro-optic modulator periodically dithers the optical path length of the ring at 10kHz. Resonances between the seed laser and ring cavity are detected by a photodiode that monitors the seed leakage through one of the cavity-forming polarizers. These resonances are used to set up the diode-pumping and q-switching sequence that ensures single-frequency operation. Further frequency stabilization is accomplished by phase locking the drive wave of the electro-optic modulator to that of the resonance signal from the photodiode. This approach yields pulses with more than 50mJ/pulse at a repetition rate of 50Hz from the ring oscillator, with a beam-quality measurement of M2 < 1.5.

The first amplifier incorporates two near-normal-incidence slabs. Both slabs are mounted to a single heat sink and pumped on one side by a dedicated bank of diode arrays. The pump arrays are positioned at the bounce points within the slabs to maximize the overlap between the 808nm pump light and 1064nm extracting beam. This design achieves over 500mJ/pulse with an M2 ∼ 1.8. The final power amplifier is a Brewster-angle design pumped from both sides to reduce thermal distortion in the slab. The diodes are again positioned at the bounce points within the slab to maximize efficiency.

The design of the electronics is another critical aspect of any space-qualifiable laser system. Our space-qualifiable electronics package features a 28V DC primary power input; autonomous startup and operation in high-power, low-power, or diagnostic modes that are controlled through an RS-232 interface; a primary power-supply design based on high-efficiency military-specified DC/DC converters and electromagnetic interference filters that have radiation-hardened equivalents; and active components—such as field-effect transistors, field-programmable gate arrays, microprocessors—that all have radiation-hardened equivalents.

Figure 2 shows a 50Hz laser that incorporates the design elements described above. The fundamental 1064nm output is frequency tripled as shown in Figure 1 This system has achieved close to 900mJ/pulse at 1064nm and 500mJ/pulse at 355nm. The true wall-plug efficiencies, which include the power for the seed laser as well as all of the control electronics, are 6.4% for 1064nm and 3.5% for 355nm. We are in the process of characterizing the performance of a 1064nm-only version of the laser that operates at 100Hz. In the future, we plan to iterate the design for higher electrical efficiency, higher optical efficiency and beam quality, and more compact packaging.

Figure 2. A space-qualifiable laser transmitter prototype that will be used for qualification and lifetime testing at Raytheon Space and Airborne Systems. The bottom box is the laser module. The upper box is the electronics module.

Fibertek wishes to acknowledge funding and support by Raytheon Space and Airborne Systems, the NOAA BalloonWinds Program, the NASA Earth Science Technology Office, and the SBIR programs of the NASA Goddard Space Flight Center, NASA Langley Research Center, and the Air Force Research Laboratories.

Floyd Hovis
Fibertek, Inc.
Herndon, VA

Floyd Hovis is an assistant vice president at Fibertek, where his primary responsibility is the development of lasers for space-based remote sensing. His interests include the development of robust single-frequency lasers, designs that improve the efficiency and lifetime of diode-pumped lasers, and the development of long-lived UV laser transmitters.