A Thermospheric Lidar for He 1083 nm, Density and Doppler Measurements Chad G. Carlson, Gary Swenson, Lara Waldrop, Peter D. Dragic Department of Electrical.

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

Metastable He(23S)

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

A 1083 nm lidar transmitter

1083 nm transmitter

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

Acousto Optic Modulator

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

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

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!