Optical waveguide device

Details Optical waveguide device Optical waveguide device

1998-01-30

2000-12-12

Scott, Jr., Leon

Optical waveguides

Planar optical waveguide

G02B 610

Patent

active

061609447

DESCRIPTION:

BRIEF SUMMARY
This invention relates to optical waveguide devices.
Certain types of glass used for fabricating optical waveguides (such as optical fibres) and waveguide devices (such as Bragg gratings) are known to have a substantially permanent refractive index change when exposed to optical radiation. These types of glass are known as photosensitive (or photorefractive) glass, and the photosensitive properties are often induced by doping the glass with one or more dopant materials. Therefore, the term "photosensitive" is used here in the established context within the field of optical devices, namely that the refractive index of a photosensitive glass may be modified by exposure to optical radiation, with the modified refractive index material still being a glass.
The radiation used to initiate the refractive index change is normally in the ultraviolet (UV) or visible ranges; however, it can also be infrared.
One example of a photosensitive glass is germanium-doped silica glass. (The term "silica" glass is used in the conventional way to describe glasses which may contain one or more dopants but which are chemically compatible with glasses formed of substantially pure silica, which can be drawn into fibres, and which are typically (but not exclusively) formed by a chemical vapour deposition (CVD) process, a "sol-gell" process in which a liquid organo-silicate is dried to form the glass, or possibly by an initial CVD process to produce a base glass followed by a solution doping process. Accordingly, the term encompasses many families of doped glasses which are based on silica. Here, the term is used to describe glasses in which the largest single component is silica). In one use, periodic optical radiation patterns can be imprinted into such a glass to form refractive index gratings, which can then selectively reflect/diffract light at a wavelength/angle determined by the period and incident angle, if the light is directed onto the gratings. One of the examples is a reflection grating in an optical waveguide which has a periodically-varying index pattern located along its axis, and which selectively reflects light at a wavelength equal to 2n.DELTA. (n is the effective refractive index of the guided optical mode and .DELTA. is the period of the index modulation).
Waveguide gratings of this type have many applications. They can be used in wavelength-division-multiplexed systems as channel add-and-drop filters, as narrow-band reflectors for fibre lasers, as transmission filters, as optical strain/temperature sensors, or as modal couplers. Chirped gratings have been used for dispersion compensation in optical fibre links, for pulse compression and broadening.
The periodic optical radiation pattern for imprinting gratings into the glass can be generated in many ways. It can be easily produced by two interfering optical beams (publication reference 1 below). The angle between the two beams can be changed to adjust the period of the gratings. Another method is to direct the beam from a radiation source perpendicularly through a phase mask (reference 2 below). The phase mask consists of many periodic grooves on a silica substrate, and the image of the periodic pattern on the phase mask is then formed in space behind the phase mask. A third method is point-to-point writing with a tightly focused beam moving in steps, and this method is particularly useful for writing gratings with large periods (larger than tens of micrometers (.mu.ms)), for which the previous method using interfering beams is rendered impossible because of the small angle between the beams required for such large periods.
In a waveguide, light is largely confined within a high-index region (hereinafter called the core) and a lower index region surrounds the core (hereinafter called the cladding). For waveguide gratings, the photosensitive glass can form either the core or the cladding or both. Preferably the core glass consists of the photosensitive glass since most of the light is confined within the core, and therefore there is a large overlap between the gr

REFERENCES:
patent: 5349473 (1994-09-01), Kurasawa et al.
G.M. Williams et al., Permanent Photowritten Optical Gratings in Irradiated Silicate Glasses, Apr. 1, 1992, pp. 532-534.
E.G. Behrens et al., Characteristics of Laser-induced Gratings in Pr3+--and Eu3+--Doped Silicate Glasses, Aug. 1990, pp. 1437-1444.
G.R. Atkins et al., Photodarkening in Tb3+--Doped Phosphosilicate and Germanosilicate Optical Fibers, Jun. 15, 1994, pp. 874-876.
L. Dong et al., Enhanced Photosensitivity in Tin-Codoped Germanosilicate Optical Fibers, Sep. 1995, pp. 1048-1050.
L. Dong et al., Strong Photosensitive Gratings in Tin-Doped Phosphosilicate Optical Fibers, Oct. 1, 1995, pp. 1982-1984.

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