Kerr Cell - the birth of optoelectronics (1870s)
The Rev. John Kerr, M.A., LL.D., F.R.S. (1824-1907) was almost exactly contemporary with William Thomson (Lord Kelvin) and from being a student in natural Philosophy in 1846, when Thomson became Professor, to their deaths in 1907 they were very close friends. Although entering the University as a divinity student, and subsequently being ordained a minister of the Free Church of Scotland, his brilliance in mathematics and physics made Kerr a valued colleague of Thomson's. In 1864, after his publication of a pamphlet The Metric System: its Prospects in this Country and a book Rational Mechanics, the University conferred on him the honorary degree of doctor of laws. This was perhaps partly as recognition of the assistance he gave to Thomson during the institution of the first physics laboratory for students housed in the disused wine-cellar of the Old College, in 1848.
In 1857 Kerr became lecturer in mathematics and physical science at the Free Church Training College in Glasgow. His first publication on what came to be known as the Kerr electro-optic effect came in 1875. This effect, for which Faraday has searched in vain some 40 years before, is the rotation of the plane of polarisation of light in passing through an optical medium across which an electric potential is applied. His first results were for solid glass; these were followed by results using liquids in cells. In the following year he published details of another effect, the magneto-optic effect, using an electro-magnet. The magnetic effect showed that a rotation of the plane of polarisation of light occured on reflection from the polished pole of a magnet.
The collection of Kerr cells in the Hunterian Museum include many large shaped blocks of glass which represent a considerable technique in the drilling and shaping. This work was done for Kerr by a German glass worker who has settled in Glasgow.
The original solid Kerr cell is a simple slab of glass measuring 6 ins. by 3 ins. by 3/4 in. thick. Two holes are drilled along the long axis of the slab, leaving about 1/2 in. of solid glass between them. Kerr applied the output from an induction coil to two metal probes inserted in the holes, to observe this effect.
from Kelvin's Instruments and the Kelvin Museum by G. Green and J. T. Lloyd
The schematic more clearly demonstrates the experiment. The polarisers are aligned perpendicular to one another so normally no light is transmitted. When a high-tension voltage is applied across the electrodes, birefringence is induced in the glass; different phase velocities are obtained for the horizontal and vertical components of the polarisation. Thus when the components are recombined after traversing the glass block, elliptically polarised light is now obtained. This has a component which is transmitted by the analyser.
John Kerr LL.D. (1875): XL. A new relation between electricity and light: Dielectrified media birefringent , Philosophical Magazine Series 4, 50:332, 337-348. http://www.tandfonline.com/doi/pdf/10.1080/14786447508641302
The Rev. John Kerr, F.R.S., Inventor of the Kerr Cell by Robert C. Gray, Nature 136, 245 (1935). http://www.nature.com/nature/journal/v136/n3433/pdf/136245a0.pdf
This process where the change in refractive index is proportional to the square of the electric field is called the Kerr effect. (An analogous process where the change is proportional to the electric field is called Pockel's effect and only occurs in anisotropic media.) The experiment forms the basis of an electro-optic modulator. We are currently exploiting similar effects in optical waveguides to modulate and control the polarisation of light at high-speed and low-power.