Optical: systems and elements – Diffraction – From grating
Patent
1994-06-15
1997-09-09
Dzierzynski, Paul M.
Optical: systems and elements
Diffraction
From grating
359569, 372 96, 372102, 372 20, G02B 518, H01S 308, H01S 310
Patent
active
056662246
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to optical gratings.
2. Related Art
An optical grating can be considered to be a sequence of grating lines. The lines modify the reflection and transmission characteristics of an optical transmission medium to which the grating is applied so allowing the characteristics to be tailored, to a greater or lesser degree, to a desired application. For example, an optical grating is used in a distributed feedback laser (DFB) to control the wavelength at which the laser is able to lase. In another application, an optical grating is used to control the transmission characteristics of an optical waveguide, for example an optical fiber.
An article titled "D-Fiber Grating Reflection Filters", P Yennadhiou and S. A. Cassidy, OFC 90 (1990) describes a D-fiber mounted on a flat substrate to expose the optical field in the fiber core. A holographically formed grating was placed on top of the substrate to give a periodic sequence of changes to the effective refractive index seen by the electric field. The changes in refractive index caused by the grating are very small but at each change in index there is a small amount of light reflected back down the fiber. At a certain resonant wavelength these small reflections build up through constructive interference to provide a large reflection whose magnitude is determined by the length of the grating and the size of the refractive index change. For a periodic grating with an arbitrary index profile this resonance occurs where the grating period is an integer multiple of half the wavelength, .lambda./2, divided by the mean effective index n.sub.0. In the special case when the index profile is a sequence of discrete jumps, the resonance only arises when the period is a odd multiple of .lambda./(2n.sub.0).
At wavelengths around the exact resonance, the reflection has a characteristics "sin (.lambda.)/.lambda." wavelength response profile of a finite-sized grating. The width of response peak is roughly inversely proportional to the grating length unless the reflectivity is very high. (see FIGS. 1(a) and 1(b)). When the peak reflectivity is high then multiple reflections become important and the reflection profile no longer narrows with increasing grating length. Instead the response flattens at around 100% reflectivity near the peak with very strong side lobes in the vicinity of the peak (see FIGS. 2(a) and 2(b)).
This characteristic profile is very difficult to change with conventional design methods. In particular, if the periodic change in effective refractive index is fixed by the material properties, then it is not possible to adjust the width of the wavelength response independently of the peak reflection. Nor is it possible by explicit design to remove the side lobe structure of smaller resonances on either side of the peak (although minor errors in the exact periodicity in the grating will often wash these out in practice).
Requirements have emerged which need reflection profiles that differ qualitatively from known prior art gratings. The first is to obtain a reflection profile that is flat over a comparatively large wavelength range (greater than about 1 nm wide) but with no side lobe reflections in the immediate neighborhood of this range. The peak reflection in this case is not important but it needs to be at least 10%. Such an optical grating could be positioned with an optical fiber network so that the connection with a central control could be checked by monitoring the reflections from an interrogation signal sent from the control center. The wavelength of the peak reflection would then be used to label the position of the grating and hence the integrity of the network could be checked at several places. A wide reflection is needed because the wavelength of the interrogation laser could not be accurately specified unless very expensive components were used. The side lobes need to be suppressed to prevent interference between different gratings in the network.
The second requirement is for a
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Cassidy Stephen Anthony
McKee Paul Francis
Wilkinson Mark Robert
Wood David Charles
British Telecommunications public limited company
Chang Audrey
Dzierzynski Paul M.
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