Proposal Title: Development of a Hybrid RF / Laser Radar
The objective of this program is to develop a new class of laser radar, or lidar, that uses commercial, off-the-shelf advanced fiber-optic components and that enhances performance by applying modern radar concepts. Anticipated advantages include size reduction, less power consumption, and sensor flexibility. Overall system cost reduction will also be a goal.
The explosive growth in the fiber-optic telecommunications field has produced numerous advances in fiber-optic technology in the 1550 nm band. Examples of these advances include single-mode laser diode modules, multi-GHz electro-optic modulators and detectors, and optical fiber amplifiers. Many of these advances represent enabling technologies for the next generation of laser radars. With the proliferation of this technology, new applications have emerged that demonstrate principles with ready application to laser radar. Examples here include subcarrier modulation and microwave and millimeter wave transport over fiber. As background, the 1550 nm operating band is special as it corresponds to the location of minimum attenuation in silica optical fibers and is the operational band of erbium-doped fiber amplifiers (EDFAs).
We propose to explore laser radar designs that exploit these technologies and to build and field test a prototype laser radar altimeter. The anticipated properties of this prototype laser radar include an operating wavelength around 1550 nm, with a modest peak transmitter power, and a range accuracy of less than 10 cm (two feet).
Since the coherence length for most lasers is still too short for long-range remote sensing applications, the approach we are proposing is optically incoherent, relying on coherence at the RF level.
Range accuracy, characterized by the one-sigma RMS range error, depends on both signal bandwidth and signal-to-noise ratio (SNR) for SNRs greater than ten. To improve the range accuracy, we can trade bandwidth for transmit power; i.e., to achieve a range accuracy of 10 cm rather than producing a high-power, short (4 ns) pulse, we can modulate a moderate power optical carrier with an RF signal to obtain a linear-FM chirp for pulse compression. The system we envision would use conventional microwave radar signals to modulate the optical carrier prior to amplification and transmission. Upon reception, a preamplifier followed by a photodetector recovers the microwave signals for subsequent radar signal processing.
The principal advantage of this approach over conventional laser radar systems is an increased PRF resulting in a reduced sample spacing. This is accomplished by independently varying the transmitted pulse width (to achieve the minimum SNR) and the pulse bandwidth (to achieve the desired range accuracy).
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