Patent
1989-05-01
1991-06-11
Ullah, Akm
350 9619, G02B 626, G02B 642
Patent
active
050227343
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
This invention relates to optical waveguides, and, amongst other things, to a method of modifying the core of an optical fibre, and an optical fibre so modified.
Field of the Invention
The high damage threshold of single-mode silica fibre (>10 GW cm.sup.-2) has allowed the use of optical fibres for several nonlinear effects such as Raman amplication soliton generation, ultrafast optical gates and second harmonic generation. The first experimental observation of self-focusing in multimode fibres using high-peak power pico-second pulses has been reported, but no damage to the fibre was observed. Optical damage mechanisms involve stimulated Brilluion scattering or dielectric breakdown at high field strengths using Q-switched and mode-locked lasers. Damage to optical fibres usually occurs at the launch end, where there is a finite probability of encountering contamination which then absorbs the laser energy causing the end to melt, through intense stimulated scattering process or at end fractures. Optical damage of fibres is of great concern to designers of power delivery systems in medicine and industry, and of high bit-rate non-linear devices. Power delivery systems, too, generally suffer end damage, and care needs to be taken in the safe design and operation of the fibre cables Baldeck P. L, Raccah F. & Alfano R. R., Opt. Letts., 12(8), August 1987.
SUMMARY OF THE INVENTION
According to a first aspect of the present invention a method of modifying the optical structure of a waveguide comprises launching optical power into the waveguide and raising the temperature of a portion of the waveguide, the optical power and temperature rise being sufficient to initiate and sustain the propagation of a structural modification along the waveguide towards the source of the optical power.
In this context, the term "optical" is intended to refer to that part of the electromagnetic spectrum which is generally known as the visible region together with those parts of the infrared and ultraviolet regions at each end of the visible region which are capable of being transmitted by dielectric optical waveguides such as optical fibres.
Once initiated in the heated portion of the waveguide, the optical power delivered to that portion eventually recreates at a point near the optical source the conditions necessary for local absorption of power from the laser. There is then a large local heating at the new location which similarly causes absorption at a point yet nearer the optical source. The enhanced local absorption in the waveguide is associated with damage to the waveguide and a consequent local modification to the optical properties of the waveguide, in particular its transmissivity. The propagation of the local modifications continue as long as sufficient optical power continues to be launched into the waveguide.
The progress of the propagation can be followed by eye when the waveguide is an optical fibre since the region where the optical power is being absorbed at a given time emits intense light through the cladding. The region of localised light emission travels along the fibre towards the optical source as fibre core is progressively modified. It requires low peak powers (0.5 watts, 3 MWcm.sup.-2) is silica mono-mode fibres, and once initiated, propagates unimpeded towards the source with devastating results for the transmission medium.
The temperature of the waveguide can be raised in a variety of ways to initiate the propagation modification. An external heat source can be applied, for example by applying a fusion splicer normally used for splicing optical fibres together. Alternatively an absorbing material can be placed in the optical field for example by placing a substrate with a metal film on it into contact with the fibre at a polished fibre half-coupler block (it is thought the optical field vaporises the metal film causing local heating of the fibre) or by placing the end of an optical fibre against an absorbent material so the optical power is absorbed to create local hea
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patent: 4725110 (1989-02-01), Glenn et al.
patent: 4807950 (1989-02-01), Glenn et al.
"Multimode CW Nd:YAG Laser Beam Transmission through the Fiber Optic Delivery System" by Jankiewicz SPIE vol. 670 Optical Fibers and their Applications IV (1986).
Optics Letters, vol. 12, No. 8, Aug. 1987, Optical Society Of America, P. L. Baldeck et al.: "Observation of Self-Focusing in Optical Fibers with Picosecond Pulses", pp. 588-589.
Physical Review Letters, vol. 27, No. 14, 4 Oct. 1971, C. R. Giuliano: "Observations of Moving Self-Foci in Sapphire", pp. 905-908.
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Soviet Journal of Quantum Electronics, vol. 4, No. 8, Feb. 1975, American Institute of Physics, (New York, U.S.), Yu. K. Danileiko et al.: "Role of Absorbing Defects in the Mechanism of Laser Damage of Real Transparent Dielectrics", pp. 1005-1008.
British Telecommunications public limited company
Ullah Akm
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