Optical: systems and elements – Diffraction – From grating
Reexamination Certificate
1998-03-09
2001-10-23
Spyrou, Cassandra (Department: 2872)
Optical: systems and elements
Diffraction
From grating
C359S008000, C359S015000, C359S016000, C359S558000
Reexamination Certificate
active
06307679
ABSTRACT:
FIELD OF THE INVENTION
1. Background
This invention relates to a method of recording an apodised refractive index grating in a photosensitive optical medium and has particular but not exclusive application to forming gratings in optical fibres.
2. Related Art
It is known that the refractive index of an optical fibre can be altered by exposing it to high intensity light. Germanium doped fibre exhibits a photosensitivity in this manner and the effect can be used to form a so-called refractive index grating in the fibre. Reference is directed K. O. Hill et al, “Photosensitivity in Optical Waveguides: Application to reflection filter fabrication”, Appl. Phys. Lett., Vol 32, no. 10, 647 (1978). The grating can be produced by forming an optical interference pattern with two interfering beams, and exposing the optical fibre to the interference pattern, so as to record a grating in the fibre.
The interference pattern may be formed by directing an optical beam longitudinally through the fibre and reflecting it back along its path through the fibre, so as to produce a standing wave pattern, which becomes recorded in the fibre due to its photosensitivity. In an alternative method, beams derived from a coherent source are directed transversely of the length of the fibre, so as to interfere with one another and produce an interference pattern externally of the fibre, which becomes recorded in the fibre as a result of its photosensitivity. A block for producing an external interference pattern for this purpose is described in EP-A-0523084.
Another way of forming the grating is to use a phase mask in which the desired amplitude pattern has been recorded holographically as a mask pattern. The phase mask is placed adjacent to the fibre and the illuminated laser light, so as to expose the fibre to the holographic pattern. Reference is directed to K. O. Hill et al “Bragg grating fabricated in monomode photosensitive fiber by u.v. exposure through a phase mask” Appl. Phys. Lett. Vol. 62, No. 10, 1035 (1993).
For a general review of refractive index gratings, reference is directed to “Photosensitive Optical Fibres: Devices and Applications” R. Kashyap, Optical Fiber Technology 1, 17-34 (1994).
Also, reference is directed to U.S. Pat. No. 4,474,427 to Hill and PCT/GB91/01968 (WO92/08999) which disclose the formation of more than one refractive index grating pattern in a common optical fibre.
Refractive index gratings, which operate as Bragg gratings, have many applications in optical data communication systems as discussed by Kashyap, supra, and in particular can be used as wavelength filters. It is well known that the large bandwidth offered by an optical fibre can be used to transmit data at a number of different wavelengths, for example by wavelength division multiplexing (WDM). It has been proposed to use refractive index gratings to separate information from adjacent WDM channels. Conventionally, optical telecommunication networks transmit data in channels centred on 1.3 &mgr;m and 1.5 &mgr;m. In either of these wavelength regions, a Bragg grating can be used to reflect out a narrow wavelength channel of the order of 1 nm or less, in order to permit WDM demultiplexing. A series of gratings can be provided to select individual closely spaced channels. The gratings exhibit a main wavelength peak centred on the wavelength of the channel to be filtered, but each grating also exhibits a series of side lobes at harmonics of the wavelength peak, which produce reflection in adjacent channels, resulting in cross-talk. As a result, it has proved necessary to apodise the Bragg gratings so as to suppress the effect of the side lobes and reduce the cross-talk.
Prior apodisation techniques will now be discussed. Referring to
FIG. 1
, this shows a conventional method of forming a refractive index grating in an optical fibre, in which light from a laser source
1
is fed through a beam spitter
2
in order to form coherent beams
3
,
4
, which are directed by a mirror arrangement
5
,
6
so as to interfere with one another in region
7
adjacent to an optical fibre
8
which exhibits photosensitivity at the wavelength of operation of the laser
1
. The result is an optical interference pattern, which is recorded in the fibre as a result of its photosensitivity. The result of the recording is shown in FIG.
2
. The spatially periodic intensity of the interference pattern produces a corresponding pattern of refractive index variations along the length of the fibre, which in
FIG. 2
are schematically shown as refractive index regions n
1
and n
2
. These regions act as a reflection grating in a manner well known per se. The grating has a wavelength dependent reflection characteristic with a main lobe centred at a particular wavelength depending upon the periodic spacing of the refractive index regions n
1
, n
2
, together with a series of side lobes at harmonics of the centre wavelength. The reflection wavelength &lgr;
Bragg
is given by
&lgr;
Bragg
=2&Lgr;n
eff
/N
where A is the period of diffraction pattern and n
eff
is the effective refractive index of the waveguide. N is an integer.
Referring to
FIG. 2
b
which shows the variation in refractive index recorded in the fibre, the spatially periodic function has an envelope
10
which in the simple example shown in
FIG. 2
b
is theoretically flat for an infinitely long grating. This is shown again in
FIG. 3
a
, with the periodic function omitted. The overall refractive index exhibited by the fibre n
eff
is at a reduced value
14
as shown in
FIG. 3
a
. The corresponding spectral characteristic for the grating, i.e. the response in the wavelength domain, is shown in
FIG. 3
b
and it can be seen that the grating exhibits a main lobe
11
and a series of side lobes on either side of the main lobe. When the grating is used as an optical filter e.g. in a WDM demultiplexer, the spacing of the grating pattern is chosen so that the main lobe
11
corresponds to the centre wavelength of the WDM channel, but a problem arises in that the side lobes extend into adjacent wavelength channels for the WDM system, particularly when the channels are closely spaced in wavelength. The side lobes thus will produce reflection in the adjacent channels and result in cross-talk.
Apodisation suppresses the effect of the side lobes. This has been achieved hitherto in a number of different ways. Referring to
FIG. 1
, the grating pattern formed in the region
7
will not in fact have a constant amplitude along its length and as a result, the refractive index pattern recorded in the fibre does not in practice have a flat envelope
10
as shown in
FIG. 3
a
. Actually, the beams
3
,
4
have an approximately Gaussian amplitude spread across their physical width, with the result that the envelope
10
in practice has a shape more like that shown in
FIG. 4
a
. It can be shown that suppression of the side lobes will be achieved if the envelope
10
has a shape which tapers from a central region towards its opposite ends, for example in accordance with the function cos
2
z along the length z of the recorded grating. In the past, this has been attempted by modifying the amplitude distribution across the width of the beams
3
,
4
. The corresponding spectral response of the filter is shown in
FIG. 4
b
, from which it can be seen that the effect of side lobes is suppressed.
For gratings recorded in a phase mask, apodisation has been achieved by varying the intensity of the pattern across the mask, or by selective destruction of the phase pattern recorded in the mask. Reference is directed to “Apodised in-fibre Bragg grating reflectors photoimprinted using a phase mask”, B. Malo et al Electronics Letters Feb. 2, 1995, Vol 31, No. 3, pp 223-225; and also to “Apodisation of the spectral response of fibre Bragg gratings using a phase mask with variable diffraction efficiency”, J. Albert et al, Electronics Letters, Feb. 2, 1995, Vol 31, No. 3 pp 222-223.
However, a problem with all of these prior techniques is that the side lobes are not suppressed completely, due to the fact that
British Telecommunications public limited company
Nixon & Vanderhye P.C.
Spyrou Cassandra
Winstedt Jennifer
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