A low-power, miniature Doppler lidar instrument is being developed for use in measuring opacity (from dust) and wind profiles in the Martian atmosphere. The instrument could also be used on Earth to measure turbulence in the atmospheric boundary layer, for assessments of urban and regional air quality, and perhaps for studying aircraft wing-tip vortices. The instrument is being designed to measure wind velocity component along its line of sight with a precision of 1 m/s and to perform ranging at distances from 3 km to a maximum of 10 km (or less, depending on the concentration of airborne dust).

There are other Doppler lidar systems that produce range-gated measurements of opacity and wind velocities, but those systems are unacceptably large and power-hungry for the intended application. The other systems contain, variously, gas or solid-state lasers operating in pulse mode. The heart of the transmitter in the present developmental instrument is a diode laser, which is chosen for compactness and because the electrical efficiencies of diode lasers are generally greater than those of gas and solid-state lasers. On the other hand, diode lasers are not suitable for pulsed operation at the peak power levels and pulse-repetition frequencies needed for range gating in the intended application; therefore, in the present instrument, range gating is achieved by use of a pseudonoise code.

The instrument will be highly electrically efficient. The diode laser in the transmitter will operate with a conversion efficiency approaching 40 percent. The design of the instrument will incorporate recent developments in high-speed, low-power receiver electronics.

The transmitting diode laser will be modulated with the pseudonoise code - a prescribed pseudorandom sequence of "on" and "off" states - with each "on" or "off" state lasting 1 to 2 µs. The receiver will perform heterodyne detection; it will include a beam splitter, which will enable the use of a local-oscillator diode laser modulated and frequency-shifted (to measure the Doppler effect) separately from the transmitting diode laser. The entire transmitter/local-oscillator package will be compatible with fiber optics.

For the original Mars-wind-profiling application, the desired transmitter power is about 200 mW. The spectral width of the transmitted light must be no more than about 1 MHz. At present, these requirements can be satisfied by use of a diode laser master oscillator and a fiber laser amplifier, and compact diode-laser devices that satisfy these requirements are expected to be developed during the next few years.

The receiver will include a unique, high-speed, low-noise photomixer with a matched amplifier and a high-speed, low-power, 12-bit analog-to-digital converter followed by circuitry that computes fast Fourier transforms and circuitry that processes power spectra.

This work was done by Robert Menzies, Greg Cardell, and Hamid Hemmati of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com under the Electronic Components and Systems category.


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Heterodyne Doppler Lidar Using Pseudonoise Code

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This article first appeared in the August, 2000 issue of NASA Tech Briefs Magazine.

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