1988-05-11
1990-07-03
Sikes, William L.
G02B 626, G02B 642
Patent
active
049385577
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The invention relates to an optical device for example for use as an optical amplifier or optical switch.
BACKGROUND AND SUMMARY OF THE INVENTION
There is an increasing requirement in the field of optical communications and optical logic circuitry to develop simple optical components to achieve functions such as switching, logic operations, amplification and the like.
In accordance with one aspect of the present invention, an optical device comprises an optical waveguide formed from an assembly of optically conductive media having different refractive indices, at least one of which is non-linear, as hereinafter defined whereby an optical signal with a single mode is guided or not guided along the waveguide in accordance with the intensity of the signal; and coupling means for coupling at least two optical signals into an input end of the waveguide.
In accordance with a second aspect of the present invention, a method of operating such an optical device comprises injecting a first optical signal into the waveguide, the intensity of the first signal being such that the signal is substantially cut-off; and injecting a second optical signal into the waveguide with an intensity such that the resultant of the first and second signals has an intensity sufficient to be guided by the waveguide.
This invention use of the properties of inhomogeneous media having a non-linear refractive index such that the waveguide switches from a guiding to a non-guiding condition depending upon the intensity of the total incident light. This self-guiding property can be used in a number of different applications.
Preferably, the waveguide comprises at least three nested media, the two inner media having refractive indices in the guiding condition respectively greater and less than the refractive index of the outer medium, the refractive index of at least one of the inner media being non-linear.
Typically, the waveguide comprises a core, an inner cladding surrounding the core, and an outer cladding surrounding the inner cladding.
For example, a typical optical fibre has a core with a refractive index higher than that of the outer cladding, and an inner cladding with a refractive index lower than the outer cladding.
An example of a suitable optical fibre is a fibre having a W profile which has a fundamental mode cut-off which can be moved to the desired wavelength by suitable design. Silica has a self-focussing Kerr coefficient which would require powers of about 100 kW. Other materials which could be used include doped silica and silica W capilliaries containing highly non-linear organic materials.
By "non-linear" we mean that the refractive index of the medium (n.sub.1) varies with intensity. Typically, this will be according to the formula: Kerr coefficient, and .vertline.E.vertline..sup.2 is the intensity of the incident light.
Typically, .vertline.E.vertline..sup.2 n.sub.2 is very much less n.sub.0 and thus the non-linear effect is only apparent at high intensities. The invention is concerned with waveguides where the intensity of the incident light and the Kerr coefficient are such that n.sub.0 is preferably not more than two to three orders of magnitude greater than n.sub.2 .vertline.E.vertline..sup.2.
It should be understood that the refractive index n.sub.2 may increase or decrease with intensity.
In a homogeneous medium the non-linear refractive index of the waveguide will cause a transverse optical field to disperse at low intensities but to collapse to a self-focussing singularity at high intensities. In other media, a self-defocussing non-linearity will occur where the refractive index decreases with intensity. The type of non-linearity depends upon the sign of the Kerr coefficient. In an inhomogeneous medium it is possible for guiding to occur before catastrophic self focussing occurs.
The device is operated in the following way for a waveguide with a self-focussing non-linearity; if a waveguide with a self-defocussing non-linearity is used then the words in brackets apply. At a frequency near
REFERENCES:
patent: 4515429 (1985-05-01), Smith et al.
Blow; "Nonlinear Effects in Optical Fibers and Devices"; IEEE Conference; May 1986.
British Telecommunication public limited company
Sikes William L.
Wise Robert E.
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