The invention presents a method and apparatus for utilizing a single frequency, self seeding laser pulse signal transmitted to create a target beam to be projected toward an atmospheric target whereby a portion will be reflected back upon itself, and a local oscillator beam constructed to a shape approximating a low level elongated square wave pulse to be heterodyned with the returning target beam. A radio frequency voltage drives an acousto-optic modulator and amplitude modulates the local oscillator beam to form an optical output that is a flat top pulse resembling a square wave function. The local oscillator beam and returning target beam are polarization adjusted to match and are then combined and focused onto detector. The system is effective for use as an aircraft velocimeter.

Solid state pulsed coherent laser radar for short range velocimetry applications

We present the generation of 12.1 ns frequency chirped pulses from a distributed feedback laser diode via injection current modulation and demonstrate its application to absorption-based methane sensing at 1650.9 nm. The chirp spectral distribution is found to be significantly impacted by the transient dynamics of these short pulses. A simplified, two-part model is used to evaluate the chirp property, suggesting spectral mixing and oscillation. The chirp enables intra-pulse self-calibrated gas sensing by taking the amplitude ratio between the absorption peak and a reference, which is demonstrated for methane concentrations from 0 to 22.6% with a detection limit of 25 ppm. It also provides resistance to power fluctuation from various sources. These short pulses offer high spatial and temporal resolution and can potentially enable reflection-based, highly multiplexed gas sensing systems.

12-ns Frequency Chirped Pulse for Self-Calibrated Gas Sensing

As part of a laser radar program, a single-mode ruby laser transmitter has been developed having the following properties: Master Oscillator: power output ≈ 20 kW; pulse width =1μs; repetition rate = 1 pps. Power Amplifier: (measured gain 24 dB). This paper describes results obtained with this laser transmitter. The techniques to determine the key properties of this laser transmitter are described along with the actual results that have been obtained. The particular characteristics of interest are 1) coherence of the laser, 2) frequency shift of "chirp" during the pulse, and 3) method of achieving the single mode.

I. Goldstein

Papers

7B3 - Characteristics of a traveling-wave ruby single-mode laser as a laser radar transmitter