A.R.D. Somervell

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.

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.

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.

Aberration correction using a multi-segment mirror with feedback interferometry

A novel real-time aberration correction system using feedback interferometry and a multi-segment mirror for the wavefront corrector is described. The result is a simple and high-speed adaptive optics system demonstrating that feedback interferometry may potentially be an alternative method to conventional aberration correction techniques. Its high-speed processing and simplicity makes real-time correction practical. Our results show that the system can respond up to 37 Hz for sinusoidal phase variations with amplitude of one wavelength and improves Strehl ratio significantly.

Fast, accurate measurement of path difference with white light

Abstract An interferometric system is described for measuring phase distributions with white light at a speed limited by the CCD frame rate using a simple phase shifting algorithm where the phase is recovered from quadrature fringe sets. The technique used is almost immune to vibration, and highly accurate. A phase measurement system is implemented using a polarizing Michelson-type interferometer together with geometric phase modulation. The measured phase distributions enabled the recovery of path differences with an accuracy of better than about λ m /70 where λ m is the mean wavelength.

Low-cost segmented mirrors for aberration correction in small aperture systems

Abstract Two low-cost 3×3 and 4×4 segmented mirrors built for near-real time adaptive optics are described. The construction and performance characteristics of the mirrors are discussed and it is shown that they have the potential to provide very good correction of atmospheric aberration despite their low number of segments. Their use in an adaptive optics system is described.

All-optical image transmission through a multimode dielectric waveguide

We show that images can be transmitted directly through optical waveguides by matching image pixels to waveguide modes. We describe how this can be achieved using a dielectric slab waveguide and present initial experimental results showing successful image transmission.

All-optical binary phase encoding/decoding for image transmission through apertures smaller than the Rayleigh limit

We describe a novel scheme for all-optical image encoding/decoding, for transmission via a serial link. In our method, the pixels of the coherent input image are encoded using orthogonal binary phase codes and the sum of the resultant optical fields transmitted via the link. At the receiving end, the field is divided spatially into output pixels, and each output pixel has impressed upon it the conjugate of one of the input pixel phase codes. The output pixel fields are then combined interferometrically with a reference beam and the resultant intensity fluctuations averaged over the phase code sequence, to recover the original input image information.

Practical issues of feedback interferometry for adaptive optics

The idea of feedback interferometry for aberration correction was proposed by the author. Conventional adaptive optics (AO) systems usually use separate subsystems to perform wavefront sensing and correction, which involve large matrix manipulation. In a feedback interferometer, the intensity of the interference pattern is used to control the wavefront corrector, thus combining the two subsystems together. To develop a practical system for use in real turbulent environments, there are several issues that need to be considered. There are concerns in the control algorithm as well when try to use the system in practice. Scintillation and frequency response are the major issues. The frequency response determines how fast the aberration that the system can track and correct. Atmospheric induced aberration might have frequency components as high as 200 Hz.