The overall goal of this SBIR program was to develop high-energy, eyesafe optical transmitters and incorporate them into lidar systems for monitoring of atmospheric clouds and aerosols. On the Phase I project we built a Nd-laser-pumped KTP OPO transmitter and incorporated it into a laboratory-level lidar system, and obtained atmospheric aerosol lidar data out to a range of 2 km. On the Phase II project we started with the Phase I concept and built it into a complete, integrated, ruggedized, fieldable aerosol lidar system.
On the Phase II program we redesigned and repackaged the Nd pump laser (changing the laser material from Nd:YAG to Nd:YLF) for improved ruggedness, greater reliability, better shielding, and higher-average-power operation. We thoroughly investigated and documented the performance of KTP and KTA OPOs in a wide variety of configurations. We chose a single-wavelength, ring KTP OPO configuration for our 1.5-mm transmitter, and built it in a compact and rugged package. We designed and built a custom transmit telescope for the system and mounted the transmit assembly on a commercial Meade 16" LX200 astronomical telescope, which serves as the receiver telescope. We conducted hard-target lidar field tests at SEO with the integrated system, using a simple Germanium PIN photodiode and delivered the system to the University of South Florida for aerosol lidar field testing, where they have employed a variety of available detectors. We began experimenting with a new type of detector, the TE-IPD, which may be favorable for this application. In summary, we started with the basic Phase I concept and built a complete fieldable lidar system from the ground up, optimizing each component as much as possible for practical field use.
The eyesafe source developed on this program combines the favorable properties of simple, multimode, Q-switched Nd lasers and the nonlinear optical crystal KTP. KTP has been under development for a number of years. Its potential as an efficient frequency-doubling crystal for Nd lasers provided the primary motivation for its development. However, researchers have also demonstrated its utility in other nonlinear applications including electro-optic modulation and optical parametric generation. In addition, a number of KTP isomorphs are currently being studied, some of which may prove to be superior to KTP in infrared transparency, resistance to photochromic damage and optical nonlinearity. For the subject application, the isomorphs, at a minimum, offer different wavelengths of operation in the noncritically phase-matched (NCPM) orientation, ranging from 1550 to 2000 nm.
The significance of this innovation is clear when viewed in contrast to other solid state lasers that operate in this wavelength region, in particular, Co:MgF2, Cr,Ho,Tm:YAG (CTH:YAG), Cr,Tm,Ho:YSGG (CTH:YSGG), Cr,Tm:YAG (CT:YAG) and Er:glass. SEO researchers have, in fact, been active participants in the effort to optimize laser systems that are based on these materials. We, and others, have found that all of the above laser materials yield less than desirable performance for one or more of the following characteristics, which are important in high-resolution, long-range, lidar systems: 1) pulse energy, 2) pulsewidth, 3) pulse repetition frequency (PRF), 4) maintenance of reliable single-pulse operation, 5) stable operation at a single wavelength, 6) efficiency, 7) compactness, and 8) reliability, especially with respect to optical damage.
The KTP OPO source developed on this program clearly outperforms all of the previously developed eyesafe laser sources. The specific advantages of KTP for the subject application are as follows:
1) When oriented for noncritical phase-matching (NCPM), a KTP OPO pumped by a 1053-nm Nd:YLF laser produces a signal output at 1550 nm and an idler output at 3280 nm. A KTP OPO pumped by a 1064-nm Nd:YAG laser produces a signal output at 1570 nm and an idler output at 3300 nm. The 1550- and 1570-nm signal wavelengths are ideal for lidar, since they fall in a region in which atmospheric transmission is excellent and high-quality optical detectors are available.
2) In the NCPM orientation the KTP is very insensitive to angular deviations in the pump-laser beam and has no "walk-off". These two properties allow the use of long KTP crystals and multi-transverse-mode operation of the pump laser. The practical consequence of this is that efficient OPO operation can be achieved with a relatively simple, compact, and alignment-insensitive pump laser.
3) With the addition of diode-laser-pumping of the Nd pump laser, the eyesafe laser can be an all-solid-state system. With diode-laser pumping the system will be more efficient and compact, and will operate for longer periods of time without maintenance.
To summarize, the Nd laser-pumped KTP OPO can provide high peak-power infrared radiation in a wavelength region that is eyesafe, has good atmospheric transmission and for which there are high-speed, high-quality detectors. The innovation in the SBIR is the development of efficient KTP OPO sources in configurations that are potentially capable of reliable operation in harsh environments including ground, airborne, and space-based lidar platforms. By employing Nd lasers as the pump source we base the eyesafe source on a well-developed laser that is already proven in both commerical and military systems. By using KTP, we take advantage of the fortuitous nonlinear properties of a now mature optical material, in a NCPM configuration that minimizes alignment sensitivity. With continuing refinement and integration into lidar hardware, these new OPO sources could provide a new generation of eyesafe transmitters for aerosol/cloud lidars and other laser-ranging systems.
The eyesafe source for the lidar consists of a flashlamp-pumped Nd:YLF laser and a ring-cavity KTP OPO.
The pump laser is a compact, rugged, single-flashlamp, Q-switched, oscillator/amplifier system based on the Compact Laser Head developed at SEO Boston. The optical head is approximately 16" x 6" x 5" in size. It is powered by an RCS-3000 power supply (Converter Power, Beverly, MA) and an SEO-built lamp driver. The unit is cooled by a Model 190 heat exchanger (IR Sources, Brookline, NH).
The output of the pump laser is 700 mJ at 10-30 Hz, at a wavelength of 1053 nm. The pulsewidth is 12-15 ns (FWHM) at full power. The beam is approximately 5.6 mm in diameter and top-hat in profile, with an M2 of 13. Typical output-energy vs. flashlamp-energy data are shown below.
The OPO, a ring-cavity design, converts the output of the Nd:YLF laser to 1550 nm. The OPO output at 20 Hz had a beam M2 of 38 and a conversion efficiency of 36%. The input-output energy data at the signal wavelength appears in the graph below, with the maximum energy exceeding 200 mJ.
Thumbnail sketches appear below, with links (click on either the photo or the label) to the full photographs. Please use the back button on your browser to return after viewing the full-size photographs
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Transmitter with transmit telescope (190 KB GIF) |
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Power supply (204 KB GIF) |
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Receive telescope with transmitter mounted on top (195 KB GIF) |
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Side view of telescope (190 KB GIF) |
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