L. Young

Criterion for Bragg and Raman-Nath diffraction regimes

The idea is well entrenched in the literature that thin phase gratings (whether holographic or acoustically induced) should exhibit Raman-Nath behavior (and thus give several diffracted waves), and that thick phase gratings should show Bragg behavior (one diffracted beam and that only for Bragg angle incidence). The parameter Q of Klein and Cook, which is a normalized measure of grating thickness, has been extensively used as a criterion for deciding which regime will apply. It is perhaps not generally realized that Q is not a reliable parameter for this purpose but requires, as indeed Klein and Cook noted, a limitation on grating strength. This limitation is a matter of practical concern. For example, we have observed Raman-Nath behavior with Fe-doped LiNbO(3) even for very large values of Q. The purpose of the present paper is to note that a parameter rho (first defined by Nath) is an effective replacement for Q, since rho is reliable and Q is not. rho is defined as lambda(0)(2)/Lambda(2)n(0)n(1), where lambda(0) is the vacuum wavelength of the light, Lambda is the grating spacing, n(0) is the mean refractive index, and n(1) is the amplitude of the sinusoidal modulation of the refractive index. The grating thickness does not enter rho, so the terms thin and thick are, strictly speaking, irrelevant to the question of which regime is operative. However, thin enough gratings will tend to operate in the Raman-Nath regime because the index modulation must be large for a thin grating to produce appreciable diffraction.

Theory of formation of phase holograms in lithium niobate

The production of refractive‐index gratings by photoexcitation of electrons from traps followed by drift or diffusion and then retrapping is treated without the assumption made by Amodei that the migration or diffusion length is small compared to the grating wavelength. It is shown that the efficiency of hologram writing increases with increasing diffusion or migration length up to a certain limit. The spatial shift between the refractive‐index grating and the light‐intensity pattern which produces it is different according to whether drift or diffusion is operative only for short drift lengths.

Mach-Zehnder interferometer tuning with Ta2O5 film loading

Unwanted path length variations cause integrated optical Mach-Zehnder modulators to have random intrinsic phase and directional couplers to have uncertain coupling lengths. The feasibility of tuning these devices by modifying the propagation constant with Ta2O5 loading of Ti:LiNbO3 waveguides was studied. The intrinsic phase of a Mach-Zehnder modulator was adjusted by this method.

Reading and optical erasure of holograms stored by the photorefractive effect in lithium niobate

The reading and optical erasure (on and off the Bragg angle) of holograms stored in lithium niobate and similar materials by the photorefractive effect have been modeled using numerical methods for the case of a single trapping level and with the further restrictions of uniformly and completely illuminated crystals, short transport lengths, and constant applied voltage. For the first time, the model allowed simultaneously for finite dark conductivity, feedback effects due to space charge on the redistribution of photoliberated electrons, writing of a new hologram by interference between the reconstructed and the reading waves (causing enhancement or disenhancement of the original hologram according to the conditions), and nonuniformity of the hologram through the thickness of the crystal. The model generates all the reported types of behavior, and it is believed to be accurate enough for a meaningful comparison with experiment. Excluded are those effects due to the large scale field associated with the envelope of the illumination when this is nonuniform.

Luminescence due to iron centers in lithium niobate

A luminescence band at 770 nm due to iron centers has been observed in lithium niobate crystals treated at 520u2009°C in lithium carbonate. Without this treatment only a broad background luminescence was observed. The main effect of the treatment is to reduce iron centers to the Fe2+ state but some evidence was found that destruction of shallow traps is also involved in increasing the luminescence.

MECHANISM OF IONIC CONDUCTION AT HIGH FIELDS

Capacitance measurements have been used to distinguish between the effective field and ionic avalanche models for the autocatalytic buildup of ionic current on applying a constant high field to tantalum pentoxide films.

Space charge fields: a fringe technique for observing such fields during hologram writing in LiNbO(3).

An interference fringe method is reported that allows rapid monitoring of the large scale (nonsinusoidal) space charge field which develops during hologram writing with nonuniform light intensity in LiNbO(3). This large scale field is distinct from the sinusoidal space charge fields that constitute the hologram. The large scale field has been shown elsewhere to be important in that it affects the efficiency of hologram writing and leads to history effects. Using the fringe method, a dielectric breakdown process associated with the large scale field was observed which may have been responsible for some effects reported by Carlsen in his work on computer applications.