Abstract. We present here a method to create spectrally addressable phase masks by encoding phase profiles into volume Bragg gratings, allowing these holographic elements to be used as phase masks at any wavelength capable of satisfying the Bragg condition of the hologram. Moreover, this approach enables the capability to encode and multiplex several phase masks into a single holographic element without cross-talk while maintaining a high diffraction efficiency. As examples, we demonstrate fiber mode conversion with near-theoretical conversion efficiency as well as simultaneous mode conversion and beam combining at wavelengths far from the original hologram recording wavelength. © The Authors. Published by SPIE under a Creative Commons Attribution 3.0 Unported License. Distribution or reproduction of this work in whole or in part requires full attribution of the original publication, including its DOI.

Permanent binary phase masks with planar surfaces and high tolerance to laser radiation are recorded in the volume of photo-thermo-refractive glass using the contact copying technique and binary amplitude master masks. Conversion of a Gaussian beam to higher order modes is shown.

We had preliminary evidence from histological preparations that the retina of the goldfish, Carassins auratits, does show a persistent retino-motor rhythm in prolonged darkness, and we felt that the conclusions of Wigger (1941) stood in need of verification. Wigger (1941) placed the fish in darkness at 1800 hr. and sampled two fish at 2-hour intervals during the first day and at 1200 hr. and 2000 hr. the second day. According to his graph, the cones elongated smoothly to their positions of maximal dark-adaptation by 2200 hr. the first night, contracted sharply to a new position by 2400 hr. and remained there until 0400 hr. Then they contracted sharply to a location intermediate between the light-and dark-adapted positions by 0600 hr. and remained approximately in that position to the end of the experiment.Wigger's results demonstrated that the rhythmic migration of cones in the goldfish retina did not persist in constant darkness after the first 12 hours. He did not distinguish between types of cones.It is well known that within a species the distances of migration of visual cells may vary considerably as do the thicknesses of the visual cell layers (Ali, 1963 ;Engstrom and Rosstrop, 1963 ) and that single and double cones migrate different distances (Walls, 1942;Nicol, 1965). In repeating these experiments on the goldfish, we followed the separate courses of migration of single and double cones. MATERIALS AND METHODSThe experiments involved 6 groups of goldfish (mean total length 6.6 0.3 cm.

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