Optical waveguides – With optical coupler – Input/output coupler
Utility Patent
1997-10-22
2001-01-02
Ullah, Akm E. (Department: 2874)
Optical waveguides
With optical coupler
Input/output coupler
Utility Patent
active
06169829
ABSTRACT:
This invention relates to chirped optical fibre gratings.
Optical fibre Bragg gratings are periodic index modulations impressed either in the cladding of an optical fibre (the low refractive index region to confine light) or the core of an optical fibre (the high refractive index region to guide light) or both. In order to imprint the grating, a suitable fibre usually has a photosensitive core, a photosensitive cladding, or both. A glass is photosensitive when its refractive index can be modified (usually, for these purposes, permanently) by optical radiation.
In silica based optical fibres, a core can be made photosensitive simply by incorporating certain amount of germanium, which also has the desired effect of raising the core refractive index to form a waveguide. The silica based cladding is normally transparent to the writing beams, giving easy access to the core from the side of the fibre.
In previous examples of this technique where only the core of a fibre is photosensitive, one imprinting technique involves exposing the side of the fibre to two coherent interfering optical beams. The grating pitch is constant along the length of the grating and can be adjusted conveniently by changing the angle between the two beams. The “writing” beam in this case can be optical radiation at a wavelength of about 240 nm (nanometres). Using a photosensitive fibre core provides a large spatial overlap between the index modulation and the guided optical transmission mode, since a large part of the optical power propagates in the core.
Another previously proposed method for writing fibre gratings is a technique using a phase mask. A phase mask is a silica plate with many parallel periodic grooves written on it, and an image of the periodic pattern is reproduced in the space behind the phase mask when optical radiation is directed onto the phase mask. A photosensitive fibre can be placed behind the phase mask for gratings to be imprinted in the photosensitive region of the fibre.
A significant feature of a non-chirped fibre grating is that it only reflects light at a certain resonant wavelength (Bragg wavelength) characteristic of the grating pitch. As a narrow band device, it has many applications such as reflectors for fibre lasers (particularly for single frequency fibre lasers), as band-stop filters, as band-pass filters, or in fibre sensors.
“Chirped” gratings are gratings for which the Bragg wavelength varies along the length of the grating. A chirped grating reflects light of different wavelengths at different positions along its length. This strong dispersive feature can be used to compensate for dispersion in an optical fibre link (publication references 1 and 2, below) and for optical pulse shaping. Chirped gratings can also be used as broad band reflectors in fibre and semiconductor lasers.
There are two main ways to make chirped gratings: these are (i) “post-chirping” a uniform grating, and (ii) introducing a chirp during grating writing.
As the Bragg wavelength (&lgr;
g
) is given by &lgr;
g
=2n
eff
&Lgr;, where n
eff
is the effective refractive index of the propagating optical mode and &Lgr; is the grating pitch, a grating can therefore be chirped by varying either the effective modal index or the grating pitch along its length.
In order to introduce a chirp into a grating while writing, there are several techniques which have been demonstrated, such as varying the effective modal index along the grating using a second exposure (reference 1, below), varying the grating pitch along the grating by bending the fibre (reference 3, below), varying the effective modal index along the grating by tapering the fibre core (reference 4, below), varying the grating pitch along the grating by using a chirped phase mask (reference 5, below), and varying the grating pitch along the grating by focusing the two interfering beams differently (reference 6, below). However, there are usually many difficulties in obtaining a controllable chirp apart from the technique using a chirped phase mask (reference 5, below), which in turn lacks flexibility.
A temperature (reference 7, below) or a strain gradient (reference 8, below) can be applied to post-chirp a uniform grating. The temperature or strain gradient varies both the effective modal index and the grating pitch along the length of the gratings. Good controllability has been demonstrated with a temperature gradient (reference 7, below), but a high temperature is required to obtain a large chirp (&Dgr;&lgr;
g
/&lgr;
g
=8.86×10
−6
&Dgr;T, where &Dgr;T is a change in temperature in degrees Celsius (°C.), so a 1 nm chirp at a wavelength of 1.55 &mgr;m (micrometre) requires a &Dgr;T of about 70° C.) and it is very difficult to obtain chirp profiles other than a linear chirp (i.e. a linear relationship between reflected wavelength and position along the grating). A good strain-chirped grating has been demonstrated with a cantilever arrangement to apply the strain gradient (reference 2, below), but the method is potentially polarisation sensitive due to the bend-induced birefringence. It is also difficult to produce a chirp other than a linear chirp and to package the chirped grating.
The article, “Fabrication of optical fibre probes for nanometer-scale dimensional metrology”, Review of Scientific Instruments, August 1994, Vol. 65, No. 8, pp 2538-2541 (H M Marchman et al) discloses an optical fibre etching bath comprising a beaker filled with etchant liquid.
This invention provides a method of manufacturing a chirped optical fibre grating, the method comprising the steps of:
etching the cladding of the optical fibre to vary the cross-sectional area of a portion of an optical fibre; and
impressing a grating of uniform pitch on the portion of the fibre;
in which the portion of the fibre is subjected to different respective longitudinal forces during the impressing step and during subsequent use of the grating.
In a method according to embodiments of the invention, a taper in the fibre cladding is produced by an etching process which in contrast to fusion tapering processes, leaves the fibre core relatively unaffected. A uniform grating is then impressed on the fibre core, under one of two force regimes:
(i) the grating is impressed while the fibre is subjected to a temporary force which is subsequently relaxed; or
(ii) the grating is impressed without a force applied to the fibre; a longitudinal force is then applied to the fibre in use.
In either case, the variation of local stress resulting from a uniform force is dependent upon the local cross-sectional area of the fibre (including cladding), so stress-related chirp can be induced by either of these methods without requiring a strain gradient to be externally imposed along the fibre.
The method is flexible, in that the grating properties can be varied by varying the fibre pre-tension, the fibre post-tension or the fibre taper profile, and yet controllability is not sacrificed. The method does not rely on techniques (such as cantilever bending) which can make the resultant grating polarisation-sensitive.
A large range of chirp characteristics can be obtained using this method, without needing any active control in the resulting device.
In one embodiment, the portion of the fibre is subjected to a greater longitudinal force during the impressing step than during subsequent use of the grating. In particular, it is preferred that the portion of the fibre is subjected to a substantially zero longitudinal force during subsequent use of the grating. This can lead to a strongly chirped grating since a possible conflict between a stress-induced chirp and a fibre lengthening induced chirp is avoided (because the fibre, in use, is not under stress). The grating can also be packaged in a substantially stress-free state. Although the applied longitudinal force could be a compression force, it is preferred that the portion of the fibre is subjected to a longitudinally stretching force at least during the impressing step.
In another embodiment, the portion of the fibre is subjected to
Cruz Jose Luis
Dong Liang
Ian Laming Richard
Reekie Laurence
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Pirelli Cavi E Sistemi S.p.A.
Ullah Akm E.
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