Application of a Doppler optical coherence technique to boundary layer sounding

Abstract

We have applied the principle of optical coherence tomography (OCT) to obtain range resolution with a continuous-wave Doppler Lidar. The system we have realized is built around a DFB laser diode operating at 1.55 μm and can thus employ standard communications technology in its construction, with an erbium-doped fibre amplifier being used to amplify the output power to 1 W. In essence, the system amounts to a Michelson-Morley interferometer, with one arm being the measurement path to atmosphere and the other arm being an optical fibre delay loop within the instrument. When the two arms are the same length to within the coherence length of the source, then a narrow-band beats spectrum is obtained. The OCT arrangement allows range resolution to be independent of range but at the cost (relative to a focused system) of a greatly reduced signal strength. Nevertheless, usable signals can be obtained from clear air in conditions with visibility as great as 30-40 km. This paper will describe the system as constructed and the numerical techniques necessary to extract a range-resolved signal. A problem with the use of commercial single mode fibre is the control of polarization to enable heterodyning of the received signal. The paper will describe a simple technique where the slow drift in the frequency of the master oscillator can be translated into a drift in polarization of this received signal: over a period of 20-30 s, a good wind measurement can thus be obtained. Finally the paper will suggest how, with increased processing power, the system could be rearranged to extract radial wind speeds at many ranges simultaneously.