astronomical filters
spectroscopic filters
holographic applications
holographic data storage
wavelength division multiplexing

In 1966 Ashkin and coworkers first described the index modulation produced in LiNbO3 during illumination, at that time called ¡°optical damage,¡± as a photoinduced change in the refractive index, now called the ¡°photorefractive effect.¡± Staebler and Amodei pioneered the use of irondoping and reduction to enhance the photorefractive properties of LiNbO3. Staebler also found that protons in lithium niobate become mobile at elevated temperatures and can be used to compensate and ¡°fix¡± photorefractive charge distributions in the form of an ionic charge distribution. Today, lithium niobate is still one of the most preferred photorefractive materials due to its material and photorefractive properties. LiNbO3:Fe grown by Deltronic Crystal has excellent photorefractive response in the visible near an absorption peak centered at ¥ë=470nm.


 

To write photorefractive gratings in LiNbO3:Fe, typically a few mW/cm2 are used for write light intensities. This helps one avoid extremely high photovoltaic fields and optical damage. Grating erasure is accomplished with uniform UV illumination or by heating the crystal to ~200¡É. Photorefractive grating storage times in LiNbO3:Fe can be on the order of months to many years. Fixing of these gratings with compensating protons typically takes place when the crystal is heated to ~150¡É. Cooling to room temperature fixes the ionic charge distribution. UV illumination reveals the protonic grating.

The top figure represents the reflection geometry typically used for narrow band optical filters. In this configuration, the reflection wavelength bandwidth is inversely proportional to the length of the crystal. The bandwidth can be as narrow as 0.01nm for a 7-mm long crystal.

Shown in the bottom figure is a LiNbO3 crystal and the light beam orientation used to form holograms for holographic data storage. An index grating ¥Än is created by the photorefractive effect to store data. The index modulation is ¥Än=-(1/2)n3r13E1cos(kgz) where n is the bulk index of refraction, r13 is the electrooptic coefficient, E1 is the space-charge field directed along the Z axis for a light polarization along the Y-axis, and the grating wave vector is kg=(4¥ðn/¥ë)sin¥È.

LiNbO3:Fe can have a strong photovoltaic effect primarily along the Z-axis. The strength of this effect is proportional to the light intensity. It can produce an internal field of a magnitude sufficient to create index changes on the order of 10-3.