Optical waveguides – With optical coupler – Input/output coupler
Reexamination Certificate
2001-03-29
2003-04-22
Kim, Robert H. (Department: 2882)
Optical waveguides
With optical coupler
Input/output coupler
C438S032000
Reexamination Certificate
active
06553163
ABSTRACT:
The present invention relates to a method and apparatus for creating a Bragg grating in a waveguide, in particular an apodised Bragg grating.
In this description, reference shall be made to optical fibres, but this reference shall be intended as a matter of example only and not as a limitation, since the technology described is equally applicable also to integrated optical waveguides.
BACKGROUND OF THE INVENTION
Typically, the optical fibres used for telecommunications are doped with germanium, which induces a photosensitivity property to the UV radiation. To write a Bragg grating in an optical fibre, this property is used to locally modify the refractive index through UV illumination.
As known, an optical fibre Bragg diffraction grating is a portion of fibre which has, in its core, an essentially periodic longitudinal modulation of the refractive index. Said structure has the property of back reflecting the light passing through it in a wavelength band centered around the Bragg wavelength. The Bragg wavelength, as known (for example, from the relation 3.3 of the text “Fiber Bragg Gratings”, Andreas Othonos, Kyriacos Kalli, Artech House, Boston/London, 1999), can be expressed as follows:
&lgr;
B
=2·
n
eff
·&Lgr; (1)
where n
eff
is the effective refractive index and &Lgr; is the spatial period of the diffraction grating.
Moreover, as known (for example, from the relation 3.4 of the above text “Fiber Bragg Gratings” ), the pattern of the refractive index n along axis z of the fibre core can be expressed by the following relation (wherein all of the possible dependencies from variable z are shown):
n
(
z
)=
n
0
(
z
)+&Dgr;
n
(
z
) sin(2
&pgr;z
/&Lgr;(
z
)) (2)
where n
0
(z) is the mean local value of the refractive index and &Dgr;n(z) represents the local envelope of the refractive index. The effective refractive index n
eff
is proportional to the mean refractive index n
0
(z) through a term defining the confinement factor (typically indicated with &Ggr;) of the fundamental mode of the fibre.
On the basis of the pattern of the refractive index, uniform gratings, so-called “chirped” gratings and apodised gratings are known.
In uniform gratings, the terms n
0
(z), &Dgr;n(z) and &Lgr;(z) are constant, as shown in
FIG. 1
a
, wherein there is represented the typical pattern of the refractive index n (normalised to 1) as a function of the z coordinate (expressed in arbitrary units). Moreover, as shown in
FIG. 1
b
, the reflection spectrum of a uniform grating typically exhibits a central peak at the Bragg wavelength, and a plurality of secondary lobes. Said secondary lobes can be disadvantageous in some applications, for example when the Bragg grating is used to filter a channel (at a respective wavelength) in a multi-channel optical transmission system. In this case, in fact, the secondary lobes of the reflection spectrum introduce an undesired attenuation into the transmission channels adjacent that to be filtered.
In apodised gratings, the pattern of the refractive index n(z) is of the type qualitatively shown in
FIG. 2
a
(wherein n is normalised to 1 and z is expressed in arbitrary units). As it can be noted, the term &Dgr;n(z) is suitably modulated in order to have a reduction of the above-mentioned secondary lobes. A typical pattern of the reflection spectrum of an apodised grating is illustrated in
FIG. 2
b
. The reduction of the secondary lobes around the main reflection peak is evident. Such a grating can thus be advantageously used for filtering a channel in a multi-channel system, reducing the above-mentioned problem of the attenuation of the channels adjacent that filtered.
In chirped gratings, either of the terms n
0
(z) and &Lgr;(z) is variable. Due to this variability, and due to the fact that—according to what said before—the Bragg wavelength is proportional to the product between n
0
(z) and &Lgr;, said gratings have a broader reflection band with respect to uniform gratings.
FIGS. 3
a
,
3
b
and
3
c
respectively show the qualitative pattern of the refractive index in the case the term n
0
(z) is modulated, and in the case the term &Lgr;(z) is modulated (for example, with a continuous variation from about 500 nm to about 502 nm), as well as the typical reflection spectrum of a chirped grating. As it can be noted from the spectrum of
FIG. 3
c
, the reflection peak is significantly broadened. Such a grating can thus be used as wideband reflection filter or, more typically, as a device for compensating the chromatic dispersion. If, in addition, also the term &Dgr;n(z) is modulated, the grating will be of the apodised chirped type.
The international patent application WO 00/29884 in the name of The University of Sidney describes an arrangement for writing a grating in a photosensitive optical fibre. A UV light from a UV light source impinges on an aperture mask which is in the form of a series of spaced apart lines, the lines being opaque to UV lights. The UV light passes between the gaps in the aperture mask and is imaged by a lens having a focal length. The fibre is placed near the focal point and the aperture mask is imaged on the photosensitive fibre so as to form a grating structure. The position of the fibre can be moved forward and backwards so as to alter the periodicity of the grating (i.e. the image size)
Various techniques for writing an apodised Bragg grating are known. According to these techniques, the fibre is exposed to suitably shaped UV interference fringes so as to obtain a corresponding pattern of the refractive index, in particular of the local envelope &Dgr;n(z).
The current techniques can substantially be divided into two categories: that of interferometric techniques, and that of phase masks.
Interferometric techniques essentially consist in splitting a UV beam into two components, and in causing them to impinge onto the fibre at a predetermined relative angle so as to generate the interference fringes that induce the desired variation of the refractive index. These techniques are very versatile because by changing the relative angle between the two components, it is possible to change the grating parameters, in particular its period.
Nevertheless, interferometric techniques are not very suitable for mass production since the writing set-up is particularly sensitive to external agents (temperature, vibrations, etc.); thus, it requires several interventions for re-aligning the components. Therefore, their application is essentially limited to the research field.
Examples of interferometric techniques are provided, for example, in the article by Fröhlich and Kashyap, “
Two methods of apodisation of fibre-Bragg-gratings”, Optics Communications,
157, 1998, 273-281.
Phase-mask techniques are generally deemed as more suitable for large-scale production due to a high repeatability, a lower susceptibility to mechanical vibrations, and to the fact that UW beams require a shorter coherence length.
A phase mask is a quartz substrate on a face of which there is, along a main direction, a series of rectilinear ridges parallel to one another, which define, in section, a substantially square-wave pattern. Typically, said ridges are equally spaced and of equal height in the case of a uniform mask, with variable pitch in the case of a chirped mask, and with variable height in the case of an apodised mask.
For writing the grating, the phase mask is usually arranged in front of the portion of fibre concerned, oriented so that its main direction (as defined above) is parallel to the fibre axis. When passed through by the UV radiation, the phase mask generates, at the output, interference fringes with a substantially sinusoidal pattern and with a period &Lgr; equal to half the period &Lgr;
m
of the ridges of the mask itself. More in detail, at the output of the phase mask there are different orders associated to respective angles according to the following relation:
sin
θ
m
=
m
λ
Λ
(
3
)
The above-mentioned fringes are generated starting from the +1
Corning Incorporated
Kim Robert H.
Oliver Kevin A.
Shout Svetlana
Suchecki Krystyna
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