Pulsed 2-Micron Laser Transmitter
sensors
Pulsed 2-Micron Laser Transmitter (LAR-TOPS-334)
For Coherent 3-D Doppler Wind Lidar Systems
Overview
Innovators at the NASA Langley Research Center (LaRC) have designed a Pulsed 2-Micron Laser Transmitter for Coherent 3-D Doppler Wind Lidar Systems. The design produces a compact, efficient, long-lifetime laser transmitter as needed for use in space, while also having potential applications as an airborne or ground-based wind measurement tool. The new laser transmitter leverages two-decades of research and development expertise at NASA LaRC in the field of 2-micron lasers, particularly those for coherent Doppler wind lidar remote sensing. The invention is a conductively cooled, highly efficient, pulsed 2-Micron Tm(Thulium):Fiber pumped Ho(Holmium):YAG laser transmitter that provides optimal atmospheric transmission for wind based measurements. This innovation provides a transformational improvement in weather forecasting by minimizing severe weather losses and offering better understanding of atmospheric and atmosphere-ocean processes, helping many investigations including climate change.
The Technology
The new NASA LaRC Pulsed 2-Micron Laser Transmitter for Coherent 3-D Doppler Wind Lidar Systems is an innovative concept and architecture based on a Tm:Fiber laser end-pumped Ho:YAG laser transmitter. This transmitter meets the requirements for space-based coherent Doppler wind lidar while reducing the mission failure risks. A key advantage of this YAG based transmitter technology includes the fact that the design is based on mature and low-risk space-qualified YAG host crystal. The transmitter operates at a 2096 nm wavelength using Ho:YAG, resulting in high atmospheric transmission (>99%), versus a transmitter operating at 2053 nm using co- doped Tm:Ho:LuLiF, which suffers limited transmission (90%) due to water vapor interference. In-band pumping through Tm:Fiber pump Ho:YAG architecture offers lower quantum defect from 1908 to 2096 nm (9.1%) compared to traditionally used co-doped Tm:Ho:LuLiF of 792 to 2051 nm (61%). The transmitter has an efficient pump compared to LuLF, since YAG has 27% higher pump absorption and 52% lower reabsorption of the emitted 2-micron, resulting in higher efficiency and lower heat load. Being isotropic, YAG is amenable for spatial-hole burning mitigation which supports linear cavity architecture without compromising injection seeding quality. This attribute is important in designing a compact, stable, high seeding efficiency laser. A folded linear cavity for long pulse (>200 ns), transform limited line-width (2.2 MHz) and high beam quality (M2 = 1.04) - the most critical parameters for coherent detection - are easier to achieve using YAG compared to LuLF. Lower heat load results in high repetition rate (>300 Hz) operation, which allows higher probability of wind measurements through broken clouds, off clouds, and below clouds, thus reducing errors and increasing science data product quantity and quality.
Benefits
- Enables wind speed measurements through cloud layer: this transmitter has the ability to measure wind speed under the cloud layer from space, an important missing data element in current weather models (about 60% of the Earth's surface is covered by clouds at any instant)
- Improved signal-to-noise ratio: the system achieves 200 pulses-per-second (pps), while current methods can only emit signals at 10 pps. Faster pulsing increases the likelihood that pulses can span clouds to gauge wind speed
- Compact and highly efficient: the transmitter uses a pumping frequency 10X closer to the output frequency than current methods, so much less input energy is wasted as heat. Current systems pass only 15% of the input power to output signals, while this new system passes over 95%, decreasing power and weight
Applications
- Meteorology: airborne or ground-based or space-based wind measurement for weather forecasting
- Remote sensing: coherent 3-D Doppler wind lidar from satellites
