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.
NPO-20466
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Heterodyne Doppler Lidar Using Pseudonoise Code
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Overview
The document discusses the development of a compact, low-power Doppler lidar system utilizing semiconductor diode and fiber laser technology, specifically designed for measuring boundary layer wind and dust opacity profiles on Mars. The project is spearheaded by a team from the Jet Propulsion Laboratory (JPL) at the California Institute of Technology, with the aim of enhancing meteorological understanding of the Martian lower atmosphere.
The lidar operates at a wavelength of 1.5 micrometers, which is considered eye-safe, making it suitable for deployment in various environments. The technology employs coherent detection and introduces a novel approach by integrating PN-coding with a receiver/frequency processor. This combination allows the lidar to function effectively in turbulent atmospheres, where aerosol correlation times are short, thus providing accurate measurements of wind and opacity.
The motivation behind this development stems from the need for a lightweight and energy-efficient instrument that can be deployed on the Martian surface. Traditional Doppler lidars typically rely on gas or solid-state lasers, which are less electrically efficient and not ideal for the compact design required for Mars missions. The use of semiconductor diode lasers addresses these limitations, offering high electrical efficiency and the ability to obtain range-gated profiles.
The document emphasizes the novelty of this lidar system, highlighting that it is the first of its kind to utilize PN-coding for atmospheric wind profiling. This advancement could significantly improve the quality of data collected on Mars, contributing to a better understanding of its atmospheric dynamics.
Additionally, the document notes that the technology has not yet been commercialized or used for its intended purpose, and there are plans for future public disclosures and potential publication in conference proceedings. The work is conducted under the auspices of NASA, and the findings are expected to be of interest to the broader scientific community, particularly those involved in atmospheric research and planetary exploration.
In summary, this document outlines a pioneering effort to create a miniature Doppler lidar for Mars, which could revolutionize the way atmospheric data is collected and analyzed, both on Mars and potentially on Earth.
