Gary Cook

Microstructuring of lithium niobate using differential etch-rate between inverted and non-inverted ferroelectric domains

Single crystal samples of lithium niobate have been spatially patterned with photoresist, and subsequently domain inverted using electric field poling, to produce a range of two dimensional spatial domain structures. Differential etching has subsequently been carried out using mixtures of hydrofluoric and nitric acids, at a range of temperatures between room temperature and the boiling point. The structures produced show very smooth, well defined, deep features, which have a range of applications in optical ridge waveguides, alignment structures, V-grooves, and micro-tips. Details are given of the fabrication procedures, and examples of structures are shown.

Hybrid organic-inorganic photorefractives

The very high birefringence of liquid crystals makes them attractive for photorefractive applications. However, the drawbacks of using liquid crystals as photorefractives include a small phase shift between the optical and refractive index gratings, coarse grating spacings with narrow beam intersection angles, operation usually restricted to the Raman-Nath regime, a need to apply an external electric field, and, with most geometries, a need to tilt the cell at an angle to the grating k-vector. In this paper, we describe two-beam coupling with hybrid photorefractive cells comprising a nematic liquid crystal layer adjacent to inorganic photorefractive windows. In this arrangement, the underlying photorefractive properties are determined by the inorganic windows while the liquid crystal molecules amplify the overall refractive index modulation. Using this technique we have obtained Bragg matched liquid crystal gain coefficients of more than 1600 cm-1, grating periods of less than 300 nm and a wide range of beam intersection angles without the need to apply an external field.

Photovoltaic contribution to counter-propagating two-beam coupling in photorefractive lithium niobate

A direct observation of the role of the photovoltaic effect in counter-propagating two-beam coupling in photorefractive iron-doped lithium niobate has been made. Through the application of a strong external field we directly observe that the photovoltaic effect is the dominant mechanism for a counter-propagating two-beam optical limiting geometry. We find that the contribution to counter-propagating two-beam coupling gain coefficient from the photovoltaic effect is approximately five times greater than that arising from diffusion mechanisms alone.

Efficient high-gain laser amplification from a low-gain amplifier by use of self-imaging multipass geometry

We characterize a self-imaging multipass amplifier scheme that provides both high extraction efficiency and overall gain. A diode-pumped slab amplifier with a single-pass small-signal gain of 2.5 is used in a 16-pass mode to amplify an input pulse from 50 muJ to 50 mJ, extracting approximately 22% of the stored energy. A stimulated Brillouin-scattering phase-conjugate mirror provides isolation from amplified spontaneous emission, prevents gain depletion, and also ensures good beam quality. The system can be operated from 10 Hz to in excess of 450 Hz, with modest changes in the beam quality and energy. The scheme has the potential to be scaled to higher-energy and higher-power systems.

Optical limiting with lithium niobate

Fe:LiNbO3 in a simple focal plane geometry has demonstrated efficient optical limiting through two-beam coupling. The performance is largely independent of the total Fe concentration and the oxidation state of the Fe ions, providing the linear optical transmission of uncoated crystals is between 30% and 60%. Both the maximum change in optical density ((Delta) OD) and the speed improve with increasing pumping intensity, and neither the (Delta) OD or the speed have shown any signs of saturation for local cw pumping intensities up to 10 kW/cm2. Fe has been found to be the best dopant for LiNbO3, giving the widest spectral coverage and the greatest optical limiting. Optical limiting in Fe:LiNbO3 has been shown to be very much grater than predicted by simple diffusion theory. The reason for this is a higher optical gain than expected. It is suggested that this may be due to an enhancement of the space-charge field from a combination of hot diffusion with the photovoltaic effect. The standard two-beam coupling equations have been modified to include the effects of the dark conductivity. This has produced a theoretical intensity dependence on the (Delta) OD which closely follows the behavior observed in the laboratory. A further modification to the theory has also shown that the focusing lens f-number greatly affects the optical limiting characteristics of Fe:LiNbO3. A lens f-number of approximately 20 gives the best results.

Light-induced frustration of etching in Fe-doped LiNbO3

Abstract We report the results of light-induced frustration of the normal etching behaviour observed when LiNbO 3 is immersed in a solution of HF and HNO 3 acids. Light of wavelength 488 nm, from an air-cooled 100 mW Ar ion laser, is incident on the rear surface (+ z -face) of a thin Fe-doped LiNbO 3 sample, whose front face (− z -face) is in contact with the etchant solution. At power densities of >100 W cm −2 , etching is suppressed through light-induced charge migration. Below this power density, partial suppression occurs, leading to submicron scale features, whose orientation follows the crystal symmetry.

Optical Limiting with Lithium Niobate

Iron doped lithium niobate (Fe:LiNbO 3 ) in a simple focal plane geometry has demonstrated efficient optical limiting through two-beam coupling. The performance is largely independent of the total Fe concentration and the oxidation state of the Fe ions, providing the linear optical transmission of uncoated crystals is between 30% and 60%. Fe has been found to be the best dopant for LiNbO 3 , giving the widest spectral coverage and the greatest optical limiting. Optical limiting in Fe:LiNbO 3 has been shown to be very much greater than predicted by simple diffusion theory. The reason for this is a higher optical gain than expected. It is suggested that this may be due to an enhancement of the space-charge field arising from the photovoltaic effect. The standard two-beam coupling equations have been modified to include the effects of the dark conductivity. This has produced a theoretical intensity dependence on the ΔOD which closely follows the behaviour observed in the laboratory. A further modification to the theory has also shown that the focusing lens f-number greatly affects the optical limiting characteristics of Fe:LiNbO 3 . A lens f-number of approximately 20 gives the best results.

Developing photorefractive fibers for optical limiting

Fe:LiNbO3 in a simple focal plane geometry has already demonstrated its potential as an efficient optical limiter of low power continuous wave visible lasers. However, widespread use of this material is hampered by a severe reduction in performance in fast optical systems. Coherent bundles of photorefractive optical fibers, used in place of bulk crystalline material, will allow efficient optical limiting in fast optical systems. Owing to the crystalline nature of the photorefractive media, the fabrication of large numbers of high quality photorefractive fibers is extremely challenging. This paper describes some of the options available, including novel composite photorefractive materials.

Developing photorefractive glass composites

The production of a transparent photorefractive glass composite would offer a useful alternative to bulk crystal materials. We aim to produce such a material by incorporating single domain photorefractive Fe:LiNbO3 particles into a refractive index matched glass host. This glass host is also required to be chemically compatible with the photorefractive material. This compatibility will ensure that the Fe:LiNbO3 particles added to the host glass will remain in the intended crystalline phase and not simply dissolve in the glass. Due to the high refractive index of the Fe:LiNbO3 (no equals 2.35 532 nm), producing a chemically compatible and refractive index matched glass host is technically challenging. By examining common Tellurite, Bismuthate, and Gallate glasses as a starting point and then developing new and hybrid glasses, we have succeeded in producing a chemically compatible glass host and also a refractive index matched glass host. We have produced preliminary glass composite samples which contain a large amount of Fe:LiNbO3. We are currently able to retain nearly 90% of the incorporated Fe:LiNbO3 in the correct crystalline phase, a substantial improvement over previous work conducted in this area in recent years. In this paper we present our progress and findings in this area.

Transversely excited liquid crystal cells

The integration of photorefractive liquid crystal beam coupling devices into optical systems is often hampered by the need to tilt the liquid crystal cells to high angles of incidence in order to obtain efficient beam coupling. Owing to poor charge diffusion in most liquid crystal systems, charge migration depends mainly on an externally applied drift field. Conventional cells, with electrodes applied to the surfaces of the windows, therefore need to be tilted with respect to the incident light to enable a component of the applied electric field to appear along the direction of the optical grating k-vector. This paper reports on an alternative design in which the electric field is applied transversely, enabling devices to be presented at normal incidence to the system optical propagation direction. We demonstrate the optical gain from transversely excited homeotropic liquid crystal cells is very similar to that obtainable with conventional homeotropic cells, with the added unexpected advantage of an order of magnitude increase in speed.