Double-pass Technique Research Articles

Reconstruction of the point-spread function of the human eye from two double-pass retinal images by phase-retrieval algorithms

In the double-pass technique used to measure the optical performance of the eye, the double-pass image is the cross correlation of the input spread function with the output spread function [J. Opt. Soc. Am. A 12, 195 (1995)]. When entrance and exit pupil sizes are equal, the information on the point-spread function is lost from the double-pass image, although the modulation transfer function of the eye is obtained. A modification of the double-pass technique that uses unequal-sized entrance and exit pupils allows a low-resolution version of the ocular point-spread function to be recorded [J. Opt. Soc. Am. A 12, 2358 (1995)]. We propose the combined use of these two double-pass measurements as input in a phase-retrieval procedure to reconstruct the ocular point-spread function. We use an adapted version of the iterative Fourier-transform algorithm consisting of two steps. In the first step, error-reduction iterations with expanding weighting functions in the Fourier domain yield an estimation of the phase that serves as an initial guess for the second step, which consists of cycles of hybrid input-output iterations. We tested the robustness and limitations of the retrieval algorithm by using simulated data with and without noise. We then applied the procedure to reconstruct the point-spread function from actual measurements of double-pass retinal images in the living eye.

Double pass technique for fabricatingcompact optical fibre rocking filters

By using the photosensitivity of Ge-doped birefringent fibres, efficient narrowband rocking filters have been fabricated that, to the authors' knowledge, are the shortest in length per unit beat length to date. This has been achieved by using a point-by-point external writing process in which a UV beam periodically exposes sections of an elliptically-cored fibre. Alternate sections are exposed from opposite sides of the fibre, effectively doubling the coupling strength per beat length. Alignment with the fibre axes is assisted by use of a keyed optical fibre.

Xenon-dimer-laser net-gain measurements

The gain of the vacuum-ultraviolet xenon-dimer laser was evaluated by employing a double-pass technique with spectral filtering of the probe radiation. At the wavelength center of the continuum-emission spectrum (λ0 ?1720 Å), the net-gain coefficient at pressures less than 1.1×104 Torr was found to be g0= (1.05±0.2) ×10−25Ps cm−1, where Ps is the spontaneous-emisssion intensity of the continuum-emission band in units of photons/sec cm3. By comparison of laser-output waveforms with temporally resolved net-gain measurements, it is shown that the laser intensity is limited by an unidentified time-dependent loss mechanism that is either foreign to the excited medium or that is driven by the laser radiation. While we do not know the source of this mechanism, our observations show that this time-dependent loss mechanism is not associated with the lasing states of the xenon dimer. At very high pressures (≳1.3×104 Torr) the effects of photoscattering by the background xenon were found to reduce the value of g0.