Simon C. Woods

Phase-diversity wave-front sensing with a distorted diffraction grating

We describe a novel wave-front sensor comprising a distorted diffraction grating, simple optics, and a single camera. A noniterative phase-diversity algorithm is used for wave-front reconstruction. The sensor concept and practical implementation are described in detail, and performance is validated against different Zernike modes and a representative atmospheric phase map.

Optical parametric amplification of mid-infrared radiation using multi-layer glass-bonded QPM GaAs crystals

Non-linear optical wavelength conversion of near-infrared lasers within optical parametric oscillators (OPOs) offers a route to powerful tunable sources in the mid-infrared (mid-IR). Engineered quasi-phasematched (QPM) non-linear optical materials based on gallium arsenide (GaAs) offer an alternative to conventional birefringently phasematched single-crystal materials such as ZnGeP2, which are currently used in mid-IR OPOs. QPM GaAs crystals have been assembled from commercially available, high-optical quality 100-micron thickness gallium arsenide (GaAs) wafers using a novel glass-bonding (GB) process. This uses thin layers of an infrared transmitting glass (refractive index matched to GaAs) deposited onto each GaAs wafer, which, when heated under pressure, fuse the wafers together to form a monolithic structure. By varying the thickness of the deposited glass layers, the dispersion in the glass can be used to compensate for variations in GaAs wafer thickness and to fine tune the phasematching wavelengths of the QPM crystal. GBGaAs crystals with up to 100 layers have been designed and built for wavelength conversion from 2 &mgr;m into the mid-IR. We report the performance of these crystals used as optical parametric amplifiers (OPAs) in the mid-IR, when pumped by a 2.094 &mgr;m source, and compare these results to measurements for a ZGP OPA. In addition, the dependence of conversion within GBGaAs crystals on the polarisation state of the amplifier seed beam has been investigated along with the temperature dependence of the optimum operating wavelength. Good agreement between experimental results and performance predictions obtained from a numerical model is observed.

Improvement and commissioning of a novel technology for the measurement of laser-beam profiles

Measurement of the laser beam propagation factor M2 is essential in many laser applications including materials processing, laser therapy, and lithography. In this paper we describe the characterisation of a prototype device using a cross-distorted diffraction grating known as an Image Multiplex (IMP(R)) grating, to measure the M2 value of laser beams. The advantage of the IMP(R) grating instrument lies in its ability to simultaneously image nine positions along the beam path. This enables beam propagation parameters to be calculated both for pulsed lasers and lasers with rapidly changing propagation characteristics. This is in contrast to the scanned technique recommended by the ISO, which is relatively slow and in practice can only be easily used with cw sources. The characterisation was accomplished by comparison of results from the IMP(R) grating device with those obtained using the accepted methodology described in the ISO 11146 series of standards through measurements conducted by the National Physical Laboratory. The scope of the work also included provision of a traceability route to international standards, and an uncertainty budget, to allow the intended user community to have confidence in measurements obtained when using the device, and to enable them to use it as part of their quality framework.

Results of atmospheric compensation using a wavefront curvature-based adaptive optics system

An adaptive optics system usually has three basic elements, a wavefront sensor, a deformable element, and a feedback scheme. Typically these components are a Shack-Hartmann sensor, a bimorph or segmented mirror, and a DSP solution for performing the necessary calculations. These components are expensive, and give rise to a complex optical and computational system. In this paper a novel implementation of an adaptive optics system will be discussed. The wavefront sensor is based on an IMP grating to measure the curvature of the incoming light. This sensor has been found to be robust to scintillation, so is applicable to horizontal propagation paths. An OKO technologies deformable mirror is used, and the feedback loop calculations run on a standard Pentium III computer using Windows 2000. Results from recent trials of the system correcting for errors over various horizontal propagation lengths will be shown. Additionally results using this system for laser beam propagation will also be discussed.

A new numerical model of optical communications in the maritime environment

QinetiQ, in association with Frazer-Nash Consultancy and Dstl, have developed a new numerical model of optical communications where part of the transmission is through water. The model (called Optical Communications Underwater Model or OCUM) finds the signal from air platform to sea platform (and vice versa) or between underwater platforms including the effects of scattering within the water, the refraction at the sea surface and transmission through cloud. The effects of scattering are found through Monte Carlo simulation before parameterizing the results to be used in subsequent calculations. The background light is also included from the sun to obtain the signal to noise ratio which is then used to find the analytical and numerical (via a message transmission simulation) bit error rate. This paper shows some of the details of the model and the approaches taken to obtain the transmission efficiency and performance. Some basic results will be presented to demonstrate the utility of the model.

Measurements of low-level atmospheric turbulence

Phase-diversity wavefront sensing has been implemented for the measurement of turbulence-distorted atmospheric wavefronts in applications of adaptive optics for essentially-horizontal propagation paths. The selected implementation of phase-diversity provides a wavefront sensor capable of estimating atmospheric distortions when observing extended scenes and provides a range-weighted sensing of the atmospheric distortions dependent on the angular region of the scene used for measurement. The data inversion, based on a Greens function analysis, is fast and robust enough for real-time implementation. For measurements of the atmospheric properties this wavefront sensor is being used with bright, compact sources to give high signal to noise measurements for integrated atmospheric effects along defined optical paths. The implementation used facilitates measurements of the atmospheric distortions along separate propagation paths. By simultaneous measurements along 3 separate paths a library of spatio-temporal atmospheric distortions and information about the isoplanicity of the distortions will be compiled for use in assessing applications of adaptive optics in horizontal propagation conditions. The principles of measurement, the details of implementation and some preliminary results will be described.

Optical propagation through low-level turbulence

Turbulence effects close to the air-ground interface may be expected to be non-Kolmogorov, even if that model is an adequate description of free-air turbulence effects. Direct measurements of the optical effects of propagation through the boundary layer are therefore required and are being undertaken as part of a program in which various potential applications of adaptive optics are being examined. The measurements are intended to characterize the spatio- temporal characteristics of optical wavefronts after propagation through the air-ground boundary layer. The objective in these measurements is to describe the level of performance that will be required in an adaptive system intended to mitigate the deleterious effects of atmospheric propagation on image formation and on other optical measurements. The principles of measurements and the preliminary results are presented.