16.18 Electro-Optics and Magneto-Optics

The objective of electro-optics and magneto-optics is to use the dependence of optical properties on electrical or magnetic fields to make useful devices.

Electro-optics - The refractive index of transparent materials is the amount the speed of light is slowed in the material, and this phenomenon is responsible for underwater objects appearing to be at a different distance. If this, or the optical dielectric constant, can be modified by an electric field, it can be used to process light beams to achieve signal processing in an analogous manner to the way electrical signals are processed by electrical circuitry.  Such materials are able to act as modulators, that add signals to optical beams, and they can also be used to transpose infrared light to visible.

Towards the end of the 1960’s improvements in hot pressing technology enabled the fabrication of fully dense (no pores) ceramic bodies such as PLZT with a high degree of transparency (around 65% over the range 0.5 to 6.5 micron wavelength). These materials can be relatively easily tailored for the particular optical effect desired, giving significant potential for a range of applications.

As illustrations, barium sodium niobate is used for electro-optic modulators and deflectors; potassium dihydrogen phosphate is soluble but properly packaged is used for optical shutters for cameras and tuneable filters; PLZT is used for image storage, displays, colour filters and was used in goggles (these, and cockpit viewing windows, were used by military pilots in the event of a nuclear explosion that can cut the light level by 1000 in 100 microseconds).

Fast reaction goggles - source Steen
Hartou collection

Electro-optic ceramics such as lithium niobate can be used to switch light between one fibre optic and another in communications systems. Because of this it has been used in integrated optical circuits, but PLZT has more potential in thin film form for surface optical wave devices, as it has higher electro-optic coefficients. It can be deposited as a film 0.35 microns thick on to a sapphire substrate, and optical waveguides and devices are etched into the PLZT forming switches and phase shifters. It is now possible to combine optical, electro-optical and electronic components on a single substrate. However it is even better to keep all the processing optical rather than convert in and out of electrical signals, so the potential in the future is to use electro-optic ceramics in a completely optical computer.

If an electric field is applied to an electro-optical ceramic that has polarised light passing through it, the polarisation is rotated, so such a device can be used to measure an electric field. This enables measurement of very high voltage and current within a totally insulating system.

Integrated optical current and temperature
sensor, and sensor attached to power cable
- source Airak

Magneto-Optics - In a similar way, if the properties of a transparent material are modified by a magnetic field it is described as magneto-optic. For example, polarised light is rotated when it passes through such a material – known as the “Faraday Effect” or when it is reflected off the material – known as the “Kerr Effect”.

The most significant application of magneto-optics is associated with storing data on tape. A magnetically aligned thin ferrite film, such as bismuth iron garnet, around 0.1 microns thick has the data written on it with an intense focussed laser beam around 5 microns in diameter. This raises the temperature of a small area on the tape above the Curie temperature, which changes its magnetisation on cooling. Reading the information is by a low power polarised laser beam, measuring the polarisation rotation by the Faraday or Kerr effects. The advantage of this technique is the virtual elimination of wear. By the early 1990’s discs storing 600 MB were available.