Photorefractivity: Insights and Applications

If you are interested in finding out more about the scientific field of photorefractivity and its potential applications, you have come to the right place. Here, Dr. Russell Kurtz, Chief Scientist at RAN Science & Technology, and well-known expert on photorefractivity and the fast photorefractive effect, shares his research and insights into this exciting technology.

The science behind photorefractivity

Photorefractivity is a nonlinear optical effect. That means its strength is not related to the optical field amplitude, like lenses, mirrors, and prisms. Instead it’s a multiple of the intensity of the light (technical note: light intensity is linearly proportional to the square of the optical field amplitude).

Photorefractivity is seen in crystals and other materials.  The most common photorefractive materials are crystals, such as barium titanate (BaTiO3), lithium niobate (LiNbO3), and strontium barium niobate (SrBaNb2O6).  But it is also seen in liquids, such as carbon disulfide (CS2).  I am not aware of any photorefractive gases, but it’s not impossible.  Most photorefractive crystals are semiconductors, and the article I wrote gives the technical reasons for this.  Under the right conditions, however, a large number of materials can be photorefractive.

Photorefractivity is a response to light that causes a change in the material’s refractive index.  The refractive index controls the phase of the light that passes through the material; this is why a straw put into a glass of water seems to bend.

What makes photorefractivity so useful?

What makes photorefractivity so useful is that the changes in the refractive index of the material last for a long time after the light that caused the changes is turned off.  There are a lot of things you can do with the refractive index change.  By controlling the light that causes it, you can make the photorefractive material hold a very complex pattern of refractive index.  The simplest complex pattern is to just add a grating onto the refractive index.  This type of pattern is called a phase grating, and you can buy permanent phase gratings at optical supply stores.  These gratings are expensive, and the grating made with photorefractivity has a number of advantages over them; the most obvious is that you can change the photorefractive grating just by changing something about the light that makes it.

A photorefractive system can amplify the "signal" while rejecting the "noise"

Two more interesting facts about the phase grating are:  (1) if two optical beams cross inside it, power can be transferred from one beam to the other; and (2) this amplification effect only works over a very narrow optical bandwidth.  The photorefractive material, then, can be use as an optical amplifier.  The amplification bandwidth is less than 10% of the bandwidth of a typical “narrowband” optical amplifier.  The photorefractive system, then, can be designed to amplify the “signal” (at the same wavelength as the optical beams used to set up the photorefractivity) while rejecting “noise” (the background).  It is not only an optical amplifier, it is also a method of improving signal-to-noise ratio (SNR).  In our experiments, a small signal was amplified by a factor of 34, while the SNR was increased by a factor of 30.

The optical amplification, and SNR improvements, would be enough to make photorefractivity very important in optical communication, long-range measurements, and so on, but there is a downside that correlates with the ease of setting up the optical amplifier.  The difficulty is that photorefractivity is slow; it can take several seconds to start, or to change, the photorefractive phase grating.  For frequency modulation (FM), then, the photorefractive amplifier is not very useful; the bandwidth of a typical photorefractive amplifier might be 100 Hz or less, while an FM radio signal is broadcast at a frequency more than a million times higher than this.

The "fast" photorefractive effect has a lot of technical applications

That is part of the reason the fast photorefractive effect can be important. The bandwidth of a normal fast photorefractive amplifier is a thousand times greater than is needed to measure an FM radio signal -- but the fast photorefractive effect works only on phase modulation (PM), not FM.

Although it is not likely to be useful for FM radio, the fast photorefractive effect has a lot of technical applications. One of the most obvious, because it requires measurement of a PM signal, is vibrometry -- measuring the vibration of something from a distance. This can be used to study the health of a car, an airplane, or a satellite -- changes in how they vibrate can indicate a change in their condition. In some cases it can be used like sonar -- to recognize an engine or a satellite by its vibration pattern. A fast photorefractive amplifier can even be used to measure the speed of a car--like a police radar gun--or to help determine whether or not a car or truck has something packed into it.

In summary

So we can say that photorefractivity is a low-noise optical amplifier, and it’s good for measuring vibration and speed from a long distance.