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High Peak Power 1.9 um Thulium-Doped Solid-State Lasers for Next-Generation Compact and Robust High Peak-Power Blue Lasers


A need exists for high pulse energy high repetition-rate lasers for LIDAR transmitters. LIDAR systems have been shown to be a powerful tool to remotely probe various oceanographic and atmospheric processes. Each system generally requires specialized transmitters at often hard to achieve wavelengths. Often the lasers available to hit these wavelengths are not suited for high peak-power operation. A Thulium and Holmium 2 um system, together, is ideally suited to cover this requirement. It has a large tuning range, and in combination with one or two frequency conversion steps, it can cover a large part of the electro-magnetic spectrum. Furthermore, the long-lived upper states required for energy storage can facilitate high pulse energy operation. There is even more compelling motivation for the Navy that is driving the development of this high peak-power 1.9 -um solid state laser. There has been a requirement for a high peak-power blue laser system solution to be operated in pulsed mode with high repetition rate for standoff oceanographic sensing applications. A combination of the intrinsic wide tuning range of Thulium and a two stage second-harmonic generation (SHG) can be utilized to generate light optimized for different ocean scenarios. The current state-of-the-art blue laser solutions with high peak-power capability usually incorporate complex, inefficient, and relatively bulky and heavy multiple-stage non-linear processes that require second or third harmonic generations and/or optical parametric oscillation approaches. These approaches often do not meet the stringent space, weight, power (SWaP) and performance and reliability requirements set forth by the Navy. Therefore, it is anticipated that frequency quadrupling of a compact high peak-power 1.9 um laser would potentially result in a blue laser with high peak-power with 50% reduction in size and weight, thereby circumventing many of the drawbacks of the more conventional approaches. NASA Langley Research Center has been actively pursuing coherent Doppler wind LIDAR development for 3-D winds measurement for the last fifteen years. The intensive research efforts have led to significant advancement of a 2 -um laser transmitter, and the NASA team has recently demonstrated a Holmium- (Ho) and Thulium- (Tm) doped solid state laser and amplifier at ~2um emission wavelength with 250 millijoules (mJ) per pulse at a pulse repetition rate of 10 Hz [1] and a Ho:YLF slab amplifier with 125 mJ at 350 Hz [2]. These are Holmium-based systems at wavelengths slightly longer than 2 um. For some Navy applications such as the aforementioned application in the blue spectral range, it is desirable to use wavelengths shorter than 1.9 um. To cover this desired wavelength range, further development of Tm-based solid-state lasers with high peak-power and repetition rate is needed. It is therefore the goal of this program to seek the development of a compact, ruggedized, high energy, high repetition rate, 1.9 um, Thulium laser that will meet the size, weight, power performance and reliability requirements stated in the following. The performance objectives of the laser solution are: 1. High repetition rate -- Threshold: 200 Hz; Objective: 250 Hz. 2. High peak-power -- Threshold: 80 mJ per pulse with pulse width no more than 100 ns. 3. Wavelength -- Threshold: between 1.850 um and 1.960 um; Objective: 1.890 um. 4. Line width of less than or equal to 2 Angstroms. 5. Polarization -- single polarization >100:1. 6. Wall plug efficiency -- Threshold 10%; Objective 20%. 7. Laser beam quality M-squared less than 3. 8. Light weight. (Total weight including the laser head, cooling system, power supply, and control system) -- Threshold: less than 50 lbs; Objective: less than 35 lbs. 9. Small volume. (Total volume for the cooling system, power supply, control system and laser head) -- Threshold: less than 2 cubic feet; Objective: less than 1 cubic foot. 10. Electrical power requirement -- Threshold: less than 1.5 kW; Objective: less than 1 kW. 11. Ability to be ruggedized and packaged to withstand the shock, vibration, pressure, temperature, humidity, electrical power conditions, etc. encountered in a system built for airborne use. 12. Reliability: Mean time between equipment failure -- 300 operating hours. 13. No cryogenic cooling allowed. PHASE I: Design and determine the feasibility of a viable robust 1.9 um solid-state laser system which meets or exceeds the requirements specified in the Description section. Identify technological and reliability challenges of the design approach, and propose viable risk mitigation strategies. PHASE II: Design, fabricate, and deliver a laser system prototype based on the design from Phase I. Test and fully characterize the system prototype. PHASE III: Finalize the design and fabricate a ruggedized laser system solution and assist to obtain certification for flight on a NAVAIR R&D aircraft. Identify transition partners and create a business plan that will provide a robust, compact and flexible LIDAR transmitter to the oceanographic and atmospheric sciences community.
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