Optical: systems and elements – Optical modulator – Light wave temporal modulation
Reexamination Certificate
2000-07-24
2001-10-16
Mack, Ricky (Department: 2873)
Optical: systems and elements
Optical modulator
Light wave temporal modulation
Reexamination Certificate
active
06304362
ABSTRACT:
The present invention relates to apparatus providing variable reflectivity to electromagnetic radiation, particularly at an interface between a body of predetermined material and a transparent region, where the interface provides a broad band reflecting surface externally to the body of material. In the context of this specification, the term “broad band reflecting surface” should be understood to mean a single surface which by itself reflects light specularly over at least one octave of optical wavelengths.
Variable reflection coefficients can be described theoretically as a result of optical non-linearity in the predetermined material, and more particularly as a non-linearity in at least one component of the dielectric tensor of the material.
The dielectric tensor is a multi dimensional expression defining how an electromagnetic radiation field propagates in and interacts with a medium. In the simple case of a so called Linear, Isotropic, Homogeneous (LIH) medium, the dielectric tensor reduces to the dielectric coefficient of the medium. However, to cover non LIH, and non-linear media, the tensor includes several components which may be functions of external excitation, in particular electromagnetic radiation. The dependence of the dielectric tensor on electromagnetic radiation is usually introduced by a multi dimensional expression using optical nonlinearity which relates the electromagnetic radiation fields with corresponding changes in the components of the dielectric tensor. From a knowledge of the dielectric tensor and its dependence on external stimulations the resulting change in phase, amplitude and polarisation of electromagnetic radiation interacting with a medium can be calculated both in terms of self induced changes and how the radiation effects the response of the medium to other optical beams of different polarisations, wavelengths and directions of propagation.
For further background information on the dielectric tensor and optical nonlinearities, reference may be made to “Electrodynamics of Continuous Media” by L. D. Landau, E. M. Lifshitz and L. P. Pitaevskii, chapters XI, XII, XIII published by Pergamon Press in 1984, “Nonlinear Optics” by R. Boyd, published by Academic, Press in 1992 and to “Linear and Non-Linear optics of Condensed Matter” by T. P. McLean published in Interaction of Radiation with Condensed Matter, Volume 1, published by International Atomic Energy Agency, Vienna 1997.
It has been proposed to use media exhibiting optical non-linearity in various devices in the field of optoelectronics. With continuing growth in the use of optical fibre communications, there is a continuing and increasing need for optical devices which can provide some of the functions of electronics using only optical signals. Various devices allowing the control of light with light are described in “Optical Bistability: Controlling Light with Light” by Hyatt M. Gibbs, published Academic Press Inc., 1985. All the devices described employ non-linear effects in transmission of light through a medium, particularly coefficients of absorption and refraction which depend on the applied light intensity. The use of optical nonlinearity within some form of optical interferometer is usually required to achieve a noticeable control of one light beam with another. In particular devices are described comprising of nonlinear materials used to fill the space in a Fabry-Perot resonator. When the resonator is tuned to an applied light wavelength, an optically bistable device can result. Other forms of interferometric switch are also possible. These include schemes based on nonlinear polarisation effects (i.e. Kerr gates), Sagnac and Mach-Zehnder interferometers. Most frequently these schemes are based on optical fibres, which due to the low nonlinearity of the fibres need to be physically long, and therefore are difficult to integrate into a small volume. They are also environmentally sensitive which severely limits their practical application. For more details see ‘Nonlinear fiber optics’ by G. P. Agrawal published by Academic Press in 1989. In all these schemes there is a difficulty in identifying suitable materials capable of providing the desired non-linear effects with response times, light power levels, operating optical bandwidth and associated optical losses at levels suitable for practical application.
Non-linear materials which show considerable optical nonlinearity are known. For example, liquid crystals and photochromic materials can also exhibit substantial nonlinearity. However the changes are relatively slow, of the order of 10 mSec or more.
Generally, no purely optical switching or control devices have yet been described which have found widespread commercial application. To succeed commercially, such devices would have to be small, fast and cheap, operate with low applied energy, be fully integrable with waveguide technology and preferably operate at or near room temperature.
As mentioned above, existing proposals for potentially practical optical switching or control devices have all been based on non-linear effects exhibited in transmission through essentially optically transparent media.
Metal-semiconductor phase transitions are known in a group of compounds of transition and rare-earth metals. Most attention has been directed at vanadium oxide which exhibits a change in solid phase from a semiconductor type to a metallic type at a transition temperature of about 67° C.
U.S. Pat. No. 4,283,113 discloses a device for optical switching using a vanadium oxide thin film on a transparent substrate. Films of thickness from 0.5 to 1 micron on a sapphire substrate are disclosed. Switching is accomplished by heating or cooling the device through the temperature of the phase transition. It is said that a transmission to reflection ratio of between 1 to 1,000 and 1 to 10,000 can be obtained at optical wavelengths of 10 microns.
Other devices employing vanadium oxide for temperature dependent optical effects are disclosed in U.S. Pat. Nos. 3,834,793 and Pat. No. 5,608,568.
The use of a film of vanadium oxide for Q-switching a laser resonator is disclosed in “Q-Switching of a Resonator by the Metal-Semiconductor Phase Transition”, A. A. Bugaev et al., Sov.J.Quantu Electron. II (Dec. 12, 1981) pp 1638-1639. An interference reflective structure is disclosed comprising a film of vanadium oxide deposited on a mirror with 100% reflection coefficient. It is reported that laser pulses induce a phase transition in the vanadium oxide film of the structure, so that use of the structure as one mirror of a resonator results in Q-switching.
Similar reflective structures are also disclosed in (a) “Self-Mode-Locking produced when a Mirror which undergoes a Metal-Semiconductor Phase Transition is used as the Modulator”, A. A. Bugaev et. al.; JETP Let, Vol.33, No. 12, Jun. 20, 1981, pp 629-632; and (b) “Sensitivity of Photoexcited Metal-Semiconductor Phase Transition in Vanadium Dioxide initiated by picosecond Pulses”, A. A. Bugaev et. al., JETP Let, Vol. 34, No. 8, Oct. 20, 1981, pp 430-433.
Optical non-linearity can also be exhibited by essentially optically opaque media. In such cases, the optical non-linearity is exhibited in the electromagnetic radiation reflected from a surface of the medium. At present nonlinear optical properties of opaque media are considerably less studied then those of transparent materials. Although nonlinear reflective effects are known in nonlinear optics, no suitable nonlinear material has been identified so far to insure their use in practical devices.
A very fast non-linear effect in reflected light has been reported in “Ultra Fast Non-Linearity of Metallic Indium Across the Liquid-Solid Transition”, by Zheludev et al, Ultra Fast Phenomena X, Proceedings of the Tenth International Conference, Del Coronado, Calif., May 28 to Jun. 1, 1996, page 461, published by Springer.
In this article, the cubic optical non-linearity of metallic indium was observed either side of melting point. The observations were made by measuring the Specular Inverse Faraday Effect. Backgrou
Dhanjal Sukhminder
Richardson David John
Zheludev Nikolay Ivanovich
Mack Ricky
University of Southampton
Westman Champlin & Kelly P.A.
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