Guy N. Pearson

High-energy in-fiber pulse amplification for coherent lidar applications

An Er:Yb codoped fiber amplifier chain for the generation of pulses for coherent lidar applications at a wavelength near 1.5 microm is reported. The final 1.8-m-long power amplification stage had a 50-microm core diameter and yielded a 23-dB energy gain, resulting in 0.29-mJ, 100-ns pulses at a repetition rate of 4 kHz with no Brillouin scattering and an M2 of 2.1.

Analysis of the performance of a coherent pulsed fiber lidar for aerosol backscatter applications

The antenna and the Doppler estimation characteristics of a coherent pulsed lidar intended for short-range aerosol backscatter applications have been analyzed. The system used fiber-optic interconnects and operated at a wavelength of 1.548 microm. The range dependence of the signal for various bistatic and monostatic antenna configurations has been determined. The system operated in a low-pulse-energy, high-pulse-repetition-rate mode, and the Doppler estimates from the return signal were achieved with a multipulse accumulation procedure. The expected performance of the accumulation in this low-photocount regime was compared with the data obtained from the system, and a reasonable level of agreement was demonstrated.

All-fiber 1.15-mJ pulsed eye-safe optical source

We report a MOPA (master oscillator - power amplifier) pulsed optical fiber source emitting high-brightness radiation (M2 = 1.65) in the “eye-safe” 1.55 μm region. A high pulse energy of 1.15 mJ was reached at low repetition rates while the maximum average output power was 2.2 W at a wavelength of 1562 nm.

Chirp-pulse-compression three-dimensional lidar imager with fiber optics

A coherent three-dimensional (angle-angle-range) lidar imager using a master-oscillator-power-amplifier concept and operating at a wavelength of 1.5 microm with chirp-pulse compression is described. A fiber-optic delay line in the local oscillator path enables a single continuous-wave semiconductor laser source with a modulated drive waveform to generate both the constant-frequency local oscillator and the frequency chirp. A portion of this chirp is gated out and amplified by a two-stage fiber amplifier. The digitized return signal was compressed by cross correlating it with a sample of the outgoing pulse. In this way a 350-ns, 10-microJ pulse with a 250-MHz frequency sweep is compressed to a width of approximately 8 ns. With a 25-mm output aperture, the lidar has been used to produce three-dimensional images of hard targets out to a range of approximately 2 km with near-diffraction-limited angular resolution and submeter range resolution.

Evaluating the precision of a transverse excitation atmospheric based CO2 Doppler lidar system with in situ sensors

A ground based, mobile, scanning Doppler lidar system used for atmospheric boundary layer research is described. The system is designed to operate safely in urban areas and has an eye safe wavelength of 10.6 µm, a range capability of up to 9 km (dependent on atmospheric conditions) and a range resolution of 112 m. Results are presented from an intercomparison campaign that was undertaken in September 2002 to compare several UFAM (UK Universities Facilities for Atmospheric Measurements) instruments with traditional in situ balloon borne instrumentation. A comparison between lidar and balloon derived horizontal wind profiles is given along with an estimation of the magnitude of the random error in the lidar Doppler velocity estimates. Measurements were made over heterogeneous terrain up to a height of 1 km, with a maximum velocity difference of 0.53 m s-1 between the derived wind profiles over a height range of 400 m. The magnitude of the random error in the velocity estimates was found to be 0.41 m s-1 for a signal to noise greater than -6 dB. Keywords: Doppler lidar, atmospheric boundary layer

All-fiber 0.4-mJ high-coherence eye-safe optical source

We report a narrow-linewidth (Δλ = 0.4 nm) optical pulsed MOPA (master oscillator - power amplifier) source emitting 0.41 mJ pulses (1.5 kW peak power) in the eye-safe range (λ = 1548 nm). Pulse duration and repetition rate were 90 ns and 5 kHz respectively.

Long range 3D active imagery with a scanned single element 1.5 μm coherent lidar system

Recent progress on the development of a long-range, high-resolution 3D active imaging sensor is described. Diffraction limited angular resolution of 20μrad and sub-metre down range resolution are demonstrated at stand-off ranges of 8km. A scanned single pixel arrangement was employed using an all-fiber coherent lidar operating in a chirp-pulse-compression mode. The monostatic antenna had an aperture of 150mm and the image was built up using a piezoelectric tip/tilt stage positioned prior to the final expansion of the beam. Transmit/receive multiplexing was achieved with a fiber optic circulator. Examples of recently acquired images consisting of 150x150 pixels with 1000, 30cm range cells per pixel at a stand-off range 8km are presented.

ATLAS : an airborne active linescan system for high resolution topographic mapping

High resolution ground mapping is of interest for survey and management of long linear features such as roads, railways and pipelines, and for georeferencing of areas such as flood plains for hydrological purposes. ATLAS (Airborne Topographic Laser System) is an active linescan system operating at the eyesafe wavelength of 1.5μm. Built for airborne survey, it is currently certified for use on a Twin Squirrel helicopter for operation from low levels to heights above 500 feet allowing commercial survey in built up areas. The system operates at a pulse repetition frequency of 56kHz with a line completed in 15ms, giving 36 points/m2 at the surface at the design flight speed. At each point the range to the ground is measured together with the scan angle of the system. This data is combined with a system attitude measurement from an integrated inertial navigation system and with system position derived from differential GPS data aboard the platform. A recording system captures the data with a synchronised time-stamp to enable post-processed reconstruction of a cloud of data points that will give a three-dimensional representation of the terrain, allowing the points to be located with respect to absolute Earth referenced coordinates to a precision of 5cm in three axes. This paper summarises the design, harmonisation, evaluation and performance of the system, and shows examples of survey data.

Pulsed Doppler lidar at QinetiQ

Recent developments in pulsed Doppler lidar technology for range-resolved aerosol and hard-target imaging applications are presented. Systems based upon CO2 and fiber-optic technologies at wavelengths of 10.6 μm and 1.5 μm respectively are described. Data are presented showing aspects of system and component development as well as recent field deployments.