a thermospheric lidar for he 1083 nm, density and doppler measurements chad g. carlson, gary...
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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 EngineeringUniversity 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 conceptResonance 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
Resonance Fluorescence Lidar
Number of photoelectrons
at altitude z
Number of transmitted
photons
Probability of receiving
(z = 300-800 km)
Systemefficiency
Probability of scattering
•Power-aperture product: Seek to maximize system SNR by scaling power and aperture•SNR can also be increased by increasingthe integration time, τ, or range bin size Δz
Noise
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
Assuming 50 W, 0.5 m2 aperture, 10 min of integration time, and a range bin of 100 km
Measuring temperature
< 0.02 nm FWHM~ 4 GHz
•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
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
1083 nm master oscillator
• 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
2.8 mW
Two stage preamplifier
• Two stages needed to generate sufficient gain given available seed power and co-propagating configuration
2.8 mW 130 mW
• Single-clad Yb198 fiber from INO
• Gain = 17.4 dB• Efficiency = 30%
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
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
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