Radiation imagery chemistry: process – composition – or product th – Imaging affecting physical property of radiation sensitive... – Making named article
Patent
1996-10-04
1998-11-03
Angebranndt, Martin
Radiation imagery chemistry: process, composition, or product th
Imaging affecting physical property of radiation sensitive...
Making named article
430945, 430394, 385 37, 372 96, 372102, G02B 610, G02B 2744
Patent
active
058306226
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
This invention relates to an optical grating, that is to a light transmitting device having periodic or aperiodic regions of differing refractive index in the direction of light propagation. The invention has particular application to optical fibres and is hereinafter described in the context of germanosilicate core fibres. However, it will be understood that the invention does have broader application, for example to planar waveguides and other light transmitting devices.
BACKGROUND ART
The first optical grating or so-called Bragg grating was made in 1978 using the standing wave pattern originating from two counter-propagating beams in a Gedoped core optical fibre. Since that time techniques have been developed to exploit the inherent photosensitivity of germanosilicate fibres, the photosensitivity being established by the bleaching of oxygen deficient centres by UV light to create the regions of differing refractive index. The refractive index change which is induced by the UV light arises from the creation of polarisable colour centres and structural rearrangement of the glass network.
Currently the most popular method of creating an optical grating involves so-called side-writing of a length of optical fibre using the holographic fringe pattern of two interfering UV beams. Side-writing of simple uniform periodic structures having a length in the order of about 1 cm with reflectivities approaching 100% and bandwidths greater than 1 nm has become an established practice, and attention is now shifting to the fabrication of complex grating structures for specific applications.
Side-writing of optical fibres by a recently developed point-by-point process has facilitated the fabrication of arbitrary phase gratings, and the employment of phase masks has been proposed for use in the side-writing of non-uniform gratings such as phase shifted and chirped gratings.
DISCLOSURE OF THE INVENTION
The present invention is directed to an alternative process for use in the creation of complex gratings and it stems from the discovery that grating characteristics may be modified to achieve a desired result by subjecting a grating structure to pre-processing or post-processing.
Thus, the present invention provides a method of forming an optical grating which comprises the steps of:
(a) exposing linearly spaced regions of a glass light transmitting device to optical irradiation, the regions being spaced apart in the direction of intended light propagation through the device, and
(b) either prior to or following exposure of the linearly spaced regions, exposing at least one concomitant region of the device to optical irradiation.
The optical irradiation used for exposing both the linearly spaced regions and the concomitant region(s) has an intensity, dose level and wavelength appropriate to cause a localised increase in the refractive index of the device in the exposed regions.
The invention also provides an optical device which comprises or incorporates a grating produced by the above defined method. A complex grating may be so produced, for example a .pi.-shifted distributed phase structure of the type required for a DFB laser.
The glass light transmitting device which is exposed to the optical irradiation preferably comprises an optical fibre and most preferably comprises a germanosilicate core optical fibre. However, any light transmitting device or any optical fibre having a core which exhibits photosensitivity may be employed. Thus, the present invention has application to the formation of a grating in any photosensitive light transmitting material within a waveguide. Germanium is recognised as being the leading photosensitive material and it is for this reason that the invention is most preferably directed to the formation of a grating within a germanosilicate core optical fibre. Co-doped optical fibres may also be employed, for example a germanium doped fibre core containing phosphorous and/or rare earths such as erbium and neodymium. The latter optical fibre has particular relevance in the dev
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Canning John
Sceats Mark
Angebranndt Martin
The University of Sydney
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