Optical waveguides – With optical coupler – Input/output coupler
Reexamination Certificate
2002-09-05
2004-12-21
Ullah, Akm Enayet (Department: 2874)
Optical waveguides
With optical coupler
Input/output coupler
C385S010000
Reexamination Certificate
active
06834144
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a gain flattening optical filter, to an optical amplifier comprising such an optical filter and to a method for manufacturing such an optical filter.
2. Technical Background
In this description, reference is 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.
Typically, the optical fibres used for telecommunications have the core doped with germanium to alter the refractive index. Doping with germanium induces a photosensitivity property to the UV radiation, which can be used to locally modify the refractive index through UV illumination, in such a way as to obtain a Bragg grating in the optical fibre.
As known, an optical fibre Bragg grating is a portion of fibre that 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 a predetermined wavelength &lgr;
B
, known as Bragg wavelength. The Bragg wavelength &lgr;
B
is related to the effective refractive index n
eff
and to the grating period &Lgr;(z) (both typically being function of coordinate z along the fibre axis) by the following Bragg phase-matching condition (see, for example, international patent application WO 99/31537):
&lgr;
B
=2
n
eff
&Lgr;(
z
) (1)
By selectively reflecting a predetermined wavelength band, an optical fibre Bragg diffraction grating may be interposed in an optical fibre transmission line to filter a multi-wavelength optical signal.
The pattern of the refractive index n along axis z of the fibre can be expressed as follows:
n
(
z
)=
n
0
(
z
)+&Dgr;
n
(
z
)sin(2&pgr;
z/&Lgr;
(
z
)) (2)
where n
0
(z) is the local mean value of the refractive index (hereinafter also referred to as “mean refractive index”) and &Dgr;n(z) represents the local envelope of the refractive index modulation (shortly referred to as “refractive index envelope”, or alternatively as “refractive index modulation amplitude”). More precisely, &Dgr;n(z) defines, for each position z along the fibre, the distance between the upper and the lower envelope lines of the refractive index representative curve. For example, when the upper ad the lower envelope lines are straight lines, their distance and, therefore, the refractive index envelope &Dgr;n(z), are constant. 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.
A known method for writing periodic refractive index lines in the fibre comprises directing a UV writing beam onto the fibre through a periodic phase-mask facing the fibre, so as to illuminate the fibre with a predetermined UV fringe pattern. The pitch of the lines or fringes of the interference patter projected onto the fibre is half that of (i.e. twice as close as that of) the lines physically present (e.g. etched) in the phase mask. For example, if the phase mask has a “physical” pitch of 1 &mgr;m, the lines projected onto the fibre have a pitch of 0.5 &mgr;m.
A different technique for writing periodic refractive index lines makes use of a holographic arrangement for generating an appropriate UV fringe pattern on the fibre lateral surface.
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. 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 (centered at a predetermined wavelength) in a multi-channel optical transmission system. In this case, in fact, the secondary lobes of the reflection spectrum introduce an undesired attenuation in the transmission channels adjacent that to be filtered.
In apodised gratings, the term &Dgr;n(z) is suitably modulated in order to have a reduction of secondary lobes. Such a grating can thus be advantageously used for filtering a channel in a multi-channel system, since it reduces 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, and the chirping may be referred to as “amplitude chirping” or “pitch chirping”, respectively. Due to the variability of n
0
(z) or &Lgr;(z), and due to the fact that—according to what stated above—the Bragg wavelength is proportional to the product between n
0
(z) and &Lgr;(z), chirped gratings have a relatively broad reflection band.
FIGS. 1
a
,
1
b
and
1
c
respectively show the qualitative pattern of the refractive index in the case the term n
0
(z) is modulated, the qualitative pattern of the refractive index in the case the term &Lgr;(z) is modulated (for example, with a continuous variation form about 500 nm to about 502 nm), and the typical reflection spectrum of a chirped grating. As it can be noted from
FIG. 1
c
, the reflection spectrum shows a peak that is relatively broad.
Pitch chirping is predominantly used, as it offers broader grating bandwidths and relative ease of production. The chirp can be incorporated in to the fibre during the fabrication process (“intrinsic chirp”) or can be obtained by applying an external perturbation to a fibre already including a non-chirped grating (“extrinsic chirp”).
Intrinsic chirp can be introduced in different ways, for example by using a non-uniform period phase-mask, by subjecting the filter to strain of temperature gradients during the writing process, by writing gratings on pre-strained fibres or in fibre tapers, by curving the fibre in a standard phase-mask set-up, by tilting the fibre with respect to a phase-mask, or by interfering wavefronts of dissimilar curvatures in a holographic arrangement. These methods of writing broad-bandwidth gratings, which require very good mechanical stability and spatial coherence properties of the writing beam, suffer from the disadvantage of allowing a limited choice of filter spectral response.
To form an extrinsic chirp, external perturbations such as strain gradients or temperature gradients can be used. This external perturbation can also be used to vary the chirp so as to tune the filter spectral response. U.S. Pat. No. 6,169,831, in the name of Lucent Technologies, for example, teaches how to use a temperature gradient or a strain gradient as an extrinsic gradient for this purpose. These devices have, however, the drawback that relatively large external gradients perturbations are required to obtain a suitable range of chirping, and such perturbations may have a negative impact on the reliability of the fibre.
It is known to use chirped gratings for compensating the chromatic dispersion in a WDM transmission system.
WO 98/08120, in the name of PIRELLI CAVI E SISTEMI S.P.A., tackling the problem of chromatic dispersion, proposes a technique (defined “Continuous Fibre Grating Technique”) to produce a fibre grating suitable to compensate said dispersion. According to this technique, a fibre, exposed through a mask to a UV radiation periodically time modulated, is continuously translated along its axis by a translation stage, so that subsequent exposures produce overlapped fringes. Arbitrary phase profiles and in particular a linear chirp can be built up by inducing phase shifts along the grating as it is fabricated.
WO 98/08120 also refers to a previously developed technique, described in U.S. Pat. No. 6,072,926 (Cole et al.), wherein a phase mask is scanned by a writing laser beam to generate the grating pattern. The fibre and the phase mask are moved with respect to one another during the writing process,
Belmonte Michele
Tormen Maurizio
Avanex Corporation
Moser, Patterson & Sheridan L.L.P.
Ullah Akm Enayet
LandOfFree
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