Optical: systems and elements – Diffraction – From grating
Patent
1995-06-01
1997-12-09
Henry, Jon W.
Optical: systems and elements
Diffraction
From grating
359 9, 359 3, 359901, 385124, 430 3, G02B 518, G02B 300, G02B 634
Patent
active
056966288
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to the creation of optical components such as permanent diffraction gratings, volume holographs and distributed feedback mirrors in optical fibres where such components have a periodicity or quasi-periodicity in an optical property. The invention, in its various embodiments, can rely on periodic or quasi-periodic phase and/or amplitude effects to provide for example both phase gratings and amplitude gratings and phase holographs and amplitude holographs.
2. Discussion of Prior Art
At present, permanent diffraction gratings and holographs are created using techniques which are based on machining, etching or photographic processes. In current practice, non-permanent, ephemeral diffraction gratings are generated in transparent media using acoustic waves to produce cycles of compression and rarefaction which are accompanied by changes in refractive index. Cells in which acousto-optic diffraction gratings are created are known as "Bragg cells". All Bragg cells require a constant acoustic input from a suitable transducer to maintain the diffraction grating within the acoustic-optic medium.
Temporary diffraction gratings may be formed using acoustic waves by means of a suitable transducer which creates an acoustic standing wave in a chosen medium.
The acousto-optic grating created within a Bragg cell may be the result of a standing wave system produced by the acoustic transducer according to the boundary conditions within the cell or the acousto-optic grating may be the result of a travelling wave system. In the one case the grating nodes and antinodes are spatially stationary while, in the other, their location is time-dependent.
Present methods for creating distributed feedback mirrors (DFB) mirrors are "Coupled-Wave Theory of Distributed Feedback Lasers", J. Appl. Phys. 43(5) Electronics, QE27(8) 2025 (1991)! and multilayer laser mirrors deposited International (1987)!. There is no known method at present for creating a DFB mirror within a continuous length of optical waveguide or fibre.
SUMMARY OF THE INVENTION
The object of the present invention is to provide apparatus and methods for utilising the changes produced in media by acoustic or electric/waves and `freezing` these changes such that gratings, volume holographs and distributed feedback (DBF) mirrors persist after removal of the acoustic waves.
The invention provides in one form a method for manufacturing optical components having a periodic or quasi-periodic structure such as diffraction gratings, volume holographs or distributed feedback mirrors for optical fibres wherein: or electrical waves to produce acoustic or electrical waves in the medium; and in regions stressed by the presence of antinodes of the waves and the altered structure is at least partially retained on removal of the source of the waves.
In another form the invention provides a method for manufacturing optical components having a periodic or quasi-periodic structure such as diffraction gratings, volume holographs or DBF mirrors for optical fibres wherein: in regions of high stress produced by an acoustic wave in the medium; and produce acoustic waves in the medium such that the structure of the medium is altered in regions of the medium stressed by the presence of antinodes of the acoustic waves and the altered structure is at least partially retained on removal of the acoustic waves.
The formation of an acoustic or electrical standing wave in or the passage of such waves through a medium puts the medium under stress at the positions of the anti-modes of the waves. The inventors have realised that this variation in local stress, periodic in the case of a standing wave, can be utilised by careful processing of the medium such that the stress pattern is effectively remembered by the medium.
The method may make use of standing waves or travelling waves in the medium. Where the waves are such that the stress applied to the medium at the antinodes is sufficient to damage the material then no further processi
REFERENCES:
patent: 4400056 (1983-08-01), Cielo
patent: 4672012 (1987-06-01), Heitmann et al.
Gudzenko et al, "Residual changes . . . ", Sov. Tech. Phys. Lett., vol. 5, No. 11, p.580, Nov. 1979.
Findlay Ewan D.
Maitland, deceased Arthur
Sutton Philip
Henry Jon W.
The Secretary of State for Defence in Her Brittanic Majesty's Go
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