A Thermospheric Lidar for He 1083 nm, Density and Doppler Measurements Chad G. Carlson, Gary Swenson, Lara Waldrop, Peter D. Dragic Department of Electrical and Computer Engineering University of Illinois at Urbana-Champaign

Introduction Metastable He atoms, pumped by photo electrons have a large resonant cross section Gerrard et al. [1997] modulated and simulated the concept Resonance between 250 and 700 km UofI has developed a key enabling element, a 1083 laser, 50 W, CW, solid state (MOPA), narrow band (< 1 MHz) for Doppler sampling Simulations for Arecibo, PR and Jicamarca, Peru Transmitter and Receiver (bistatic) Summary

Metastable He(23S)

Resonance Fluorescence Lidar Number of transmitted photons Probability of scattering Probability of receiving (z = 300-800 km) Number of photoelectrons at altitude z System efficiency Noise Power-aperture product: Seek to maximize system SNR by scaling power and aperture SNR can also be increased by increasing the integration time, τ, or range bin size Δz

Signal to noise simulations Bistatic imaging receiver with 1% QE SNR scales as the square root of power- aperture product, i.e. 3% QE, 100 W of power and Starfire Optical Range telescope (10 m2 aperture) → >10x increase in SNR Given a CW transmitter in a bistatic configuration Assuming 50 W, 0.5 m2 aperture, 10 min of integration time, and a range bin of 100 km

Measuring temperature Doppler lidar requires narrow linewidth operation, i.e. delta function sampling of the lineshape function 3 frequency technique can measure winds and temperatures with good SNR and small error -- Requires a tunable transmitter < 0.02 nm FWHM ~ 4 GHz Metastable 1083 nm transition consists of three lines that are Doppler broadened by temperature

A 1083 nm lidar transmitter Master oscillator - power amplifier configuration Overall gain = 36 dB 10 W CW single mode output Narrow linewidth (~ 150 kHz) and tunable

A 1083 nm lidar transmitter

1083 nm transmitter

Ref: Price, Personal Communication, 2006 1083 nm master oscillator 2.8 mW Seed laser is a distributed Bragg reflector (DBR) laser diode provided by J.J. Coleman’s group at U of I Single frequency and tunable over several nanometers with temperature, gain current, and phase sections Free-space coupling efficiency of 25% into Hi1060 Ref: Price, Personal Communication, 2006

Acousto Optic Modulator

Two stage preamplifier 2.8 mW 130 mW Single-clad Yb198 fiber from INO Gain = 17.4 dB Efficiency = 30% Two stages needed to generate sufficient gain given available seed power and co-propagating configuration

Power amplifier 7 m LMA-YDF-10/130 fiber (0.44 NA) provided by Nufern is single mode at 1083 nm Counter propagating configuration with coupling efficiency of 87% Beam divergence with 10x telescope is less than 250 μrad Gain = 18.9 dB Efficiency = 68% 130 mW 10 W Talk about etandue matching Low beam divergence and beam stability -- Important for minimizing pixel spread that lowers SNR

1083 nm imaging receiver With CW beam, an imaging receiver provides range information A CCD or InGaAs array is placed at the focal plane of a large telescope Resolution depends on baseline between receiver and transmitter Point out green beam in image

Image of Bistatic Lidar Sim

Recent progress and future work Pulsed operation at low PRF Self-phase modulation effects due to pulsed operation SBS suppressing fiber for higher power with narrow linewidth Operational lifetime considerations due to photodarkening

Low PRF operation Briefly explain pulse pumping experiment Maintained 170 uJ of output power for measurement

Conclusion Thanks to enabling fiber technology, a thermospheric Doppler lidar to measure temperatures and winds from 300-800 km has been developed Work is ongoing to make the first detailed measurements of the 1083 emission in the thermosphere and improve the current system Thank you!