Optical waveguide grating

Details Optical waveguide grating Optical waveguide grating

1998-04-16

2000-06-06

Lee, John D.

Optical waveguides

With optical coupler

Input/output coupler

430290, G02B 634

Patent

active

060729260

DESCRIPTION:

BRIEF SUMMARY
This invention relates to optical waveguide gratings.
Optical waveguide gratings such as fibre Bragg gratings are recognised as key components for many fibre optic and laser systems, but ways for improving their characteristics and ease of fabrication continue to be a subject of considerable research interest. One recently proposed technique is the use of so-called phase casks for grating production (publication references 1 and 2 below). In this technique, a grating is imposed on a photosensitive optical waveguide by projecting writing optical radiation through the phase mask onto the waveguide.
The phase mask approach is attractive for it allows fibre gratings to be written with much relaxed tolerances on the coherence of the writing beam (in comparison with, for example, a two-beam interference technique), as well as providing greater repeatability than was previously possible. However, a major drawback has been that the grating wavelength and other characteristics are dictated by the period of the phase mask, and so separate masks are required for different wavelengths.
Considerable research has gone into making the phase mask approach more flexible, e.g. by incorporating a magnifying lens to alter the fibre Bragg wavelength (publication reference 3 below).
The introduction of a scanning writing beam was a further advance which enabled the fabrication of long fibre gratings without requiring a large beam magnification, as well as allowing more complex structures to be directly written by modulating the writing beam as it scans across the mask (publication references 4 and 5 below).
The ability to create more complex structures, such as apodised and/or controllably chirped gratings, is of great importance for many applications. While apodisation can be approximated by modulation of the scanning beam, this also introduces an accompanying variation in the average refractive index alone the grating length which in turn imparts an induced chirp to the grating which is often undesirable. `Pure` apodisation (i.e. apodisation without a variation in the average refractive index) has recently been reported, but at the expense of either requiring a specially designed phase mask (publication reference 6 below), or with double exposure to two different masks (publication reference 7 below).
Considerable effort has also gone into writing controllable chirp characteristics into the grating, via a double-exposure technique (publication reference 8 below), specially designed `step-chirp` phase masks (publication reference 9 below), or by straining the fibre (publication reference 10 below).
However, the techniques described above, which are intended to make the phase mask approach more flexible, in fact either increase dramatically the complexity of the process (by requiring multiple exposures) or still require an individual phase mask to be fabricated for each variation of the grating characteristics or pitch.
This invention provides a method of fabricating an optical waveguide mating in which a writing light beam is successively exposed through a mask onto regions of a photosensitive optical waveguide, to generate corresponding regions of the grating, the method comprising the step of: moving the mask and/or the waveguide so that the relative position of the mask with respect to the waveguide varies as different regions of the grating are generated.
Embodiments of the invention provide a simple technique which involves slowly moving the waveguide (e.g. fibre), or alternatively the phase mask (or both), as the writing beam is scanning, which is effective in overcoming or alleviating many of the limitations which are currently associated with phase masks. The approach can be used to produce multi-wavelength gratings, so-called `pure` apodisation, as well as a variety of dispersive structures such as distributed feedback (DFB) laser structures.
In other words, the previous inflexibility of the phase mask technique is alleviated by the invention. Gratings of different characteristics, Bragg wavelength, chirp or apodisati

REFERENCES:
patent: 5066133 (1991-11-01), Brienza
patent: 5104209 (1992-04-01), Hill et al.
patent: 5367588 (1994-11-01), Hill et al.
patent: 5619603 (1997-04-01), Epworth et al.
patent: 5652818 (1997-07-01), Byron
patent: 5837169 (1998-11-01), Rourke
patent: 5912999 (1999-06-01), Brennan, III et al.
R. Kashyap et al., "UV Written Reflection Grating Structures In Photosensitive Optical Fibres Using Phase-Shifted Phase Masks", Electronics Letters, vol. 30, No. 23, pp. 1977-1978, Nov. 10, 1994.
J. Martin et al., "Novel Writing Technique Of Long And Highly Reflective In-Fibre Gratings", Electronic Letters, vol. 30, No. 10, pp. 811-812, May 12, 1994.

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