T.H. Barnes

High resolution adaptive optics using an interference phase loop

Abstract The Interference Phase Loop is an interferometer with feedback of the output intensity to a phase modulator in one of the inteferometer arms. When a spatial optical phase modulator is used, the modulator phase distribution approximates to the conjugate of the spatial phase distribution in the interferometer, and so the device has potential for application in adaptive optics. We have implemented an interference phase loop using a high-resolution liquid crystal spatial light modulator (PAL-SLM) and a common path radial shearing interferometer. We show how the system can be analysed in simple terms, derive some design and operating criteria, and demonstrate diffraction-limited real-time aberration correction of arbitrary input wavefronts using the system. Because of the nature of the interferometer used, our system has the potential to operate in white light.

The Application of Phase-only Filters to Optical Interconnects and Pattern Recognition

Abstract We have recently developed a new type of liquid-crystal spatial light-modulator which acts as a phase-only spatial light-modulator for coherent light. The use of this device as a flexible filter in the Fourier plane of an optical correlation system is described, and it is shown how it may also be used as a flexible interconnect capable of simultaneously providing continuous amplitude and binary phase modulation.

Direct image transmission through a multi-mode square optical fiber

Abstract We explore the potential for direct image transmission through a square optical fiber. We show that when an image is sampled appropriately and its optical Fourier transform imaged on the end of a square fiber with perfectly reflecting walls, the components in the Fourier transform excite corresponding fiber modes. Specifically, even–even fiber modes carry information from one pixel only, while the odd modes carry information from neighboring pixels and give rise to cross-talk. When the odd modes are suppressed, the image can be perfectly recovered at the end of the fiber by a second optical Fourier transform. We suggest a method of suppressing the odd modes. In our system, dispersion of the mode phase velocity gives rise to different arrival times for the information in different image pixels, but has little or no effect on the output intensity distribution. We show that the square shape of the fiber is critical in forming the output image and confirm our theoretical predictions by computer simulation.

Multiple-beam lateral shear interferometry for optical testing

Lateral shear interferometry is used to obtain the lateral aberrations of a lens. The zeroth-order fringe in an interferogram obtained from a wedge-plate lateral shear interferometer, however, directly displays the lateral aberration curve of a test lens. Nevertheless, the intensity distribution, is cosinusoidal. Multiple-beam interferometry results in sharpened fringes; hence the multiple-beam wedge-plate shear interferometer displays the lateral aberration curve of a lens sharply, provided the shear is small. For large shear, some new artifacts appear in the interferogram, which are also explained.

Unambiguous measurement of surface profile using a Sagnac interferometer with phase feedback

The fringe profiles from a two beam interferometer with phase feedback are almost unambiguous over a range of 2π or more. This allows phase and therefore surface profile to be determined directly from output intensity of an interferometer. In this paper we describe how feedback can be applied to a polarisation Sagnac common path interferometer using a parallel aligned nematic liquid crystal spatial light modulator. Initial experiments have shown that the device is capable of measuring profiles to accuracies greater than one tenth of a wavelength.

Characterization of dye-doped media for real-time holography

Abstract Studies of saturable dyes used as holographic media where saturation gives rise to both amplitude and phase holograms are presented. Using a simple absorption model together with the Kramers-Kronig relation, a theory for the prediction of holographic diffraction efficiencies over a range of read-wavelengths was developed. Results of experimental tests are presented as support for the model.

Optical Sine transformation and image transmission by using square optical waveguide

Abstract In this paper a system able to perform all optically the discrete-Sine transformation is discussed. Similar to that proposed previously [C.Y. Wu, A.R.D. Somervell, T.H. Barnes, Opt. Commun. 157 (1998) 17–22], the new system is also composed of a piece of perfectly reflecting square-optical fiber (waveguide) and lens systems but with the input image being directly projected on to the front face of the fiber. At the other end of the fiber the field is transformed by a Fourier lens and projected on to a screen. Theoretical analysis shows that the electric field on the output screen represents exactly the discrete-Sine transform of the input image when a spherically symmetrical phase term is compensated and the original image can be restored by sending the Sine transformed field back through the same system. Phase compensation methods are discussed. The analysis is confirmed by a numerical simulation.

Path difference measurement using phase stepping with white light

Phase stepping algorithms are commonly used with narrow bandwidth sources for fast accurate measurement of interferometer phase differences. We show that path difference can also be measured using phase stepping with a wide bandwidth source. The errors resulting from the spectral characteristics of the source are discussed and shown to be relatively small, allowing measurement of path difference to typically 0.1 μm or better. White-light phase stepping is demonstrated experimentally with results which closely match those predicted theoretically.

Feedback interferometry with membrane mirror for adaptive optics

Feedback interferometers can be used for many optical measurements as well as aberration correction. The choice of the phase modulator in the interferometer has major influence on the operation of the feedback system. Conventional wisdom is that independent actuator movement is a basic requirement for the modulator in order to allow the feedback system to operate in stable fashion and that membrane mirrors are not suitable. We demonstrate the operation of a feedback interferometer which uses a membrane mirror as the phase modulator. We show that even with the strong coupling introduced by the influence function of the membrane mirror, the feedback system can still operate normally. Our interferometer is based on a radial shearing polarizing Sagnac interferometer. The current system is able to correct static aberration with significant improvement on Strehl ratio.

Phase measurement accuracy of feedback interferometers

Feedback interferometers are described with specific reference to potential applications in micro-machines. A theoretical analysis is developed to determine the linearity, stability, and noise performance of this type of interferometer. The theoretical analysis was tested using a prototype high-precision feedback interferometer which showed that, at a feedback loop gain which enabled the system to track 6 fringes, the linearity of the interferometer was better than λ/100 and single phase measurements could be made with an accuracy of λ/80.