Note: Descriptions are shown in the official language in which they were submitted.
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OPTICAL WAVEGUIDE GRATING
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 masks
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 along 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 recentlv been reported, but at the expense of either
requiring a
specially designed phase mask (publication reference 6 below), or with double
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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 5 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 grating
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 apodisation can be produced from a single phase mask by this
technique.
This contrasts with previous techniques where' either a respective phase mask
was
needed for each type of grating to be produced, or complex multiple exposure
techniques, with associated alignment and uniformity problems, had to be used.
Although, for example, planar waveguides could be used, preferably the
waveguide is an optical fibre. Because fibres tend to be lighter than phase
masks, it
is preferred that the fibre is displaced relative to a static phase mask (e.g.
by a
piezoelectric stage).
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Although various continuous or even non-continuous writing patterns could be
emplayed, it is preferred for ease of implementation that the writing light
beam is
longitudinally scanned along a portion of the waveguide. Again, for ease of
implementation, it is preferred that the writing light beam is scanned along
the portion
of waveguide with a substantially uniform velocity.
In order to provide a simple shift of the Bragg wavelength with respect to
that
provided by the phase mask, it is preferred that the relative position of the
mask and
waveguide is varied so that the relative linear displacement of the mask and
waveguide is linearly related to the distance along the waveguide of a
currently
exposed region of the waveguide. This can conveniently be achieved where the
writing scan is at a uniform velocity, by relatively displacing the mask and
waveguide
at a uniform velocity.
In order to provide a linearly chirped grating, it is preferred that the
relative
position of the mask and waveguide is varied so that the relative linear
displacement
of the mask and waveguide is linearly related to the square of the distance
along the
waveguide of a currently exposed region of the waveguide. Again, this can
conveniently be achieved wliere the writing scan is at a uniform velocity, by
relatively
displacing the mask and waveguide at a uniform acceleration.
In order to add apodisation to the grating, it is preferred that the relative
position of the mask and the waveguide is varied by an oscillatory dither
component.
This component can be superimposed onto other motion components such as those
described above. Similarly, the other various displacement components can be
superimposed or applied to adjacent portions of the waveguide.
Preferably the magnitude of the dither component varies along the length of
the grating. In particular, although modulated apodisation can be used to
create so-
called "superstructure" gratings, in one preferred embodiment the magnitude of
the
dither component increases with longitudinal distance from a central region of
the
grating. This can decrease the side-lobe reflection of the gratings.
AMENDED SHEET
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This invention also provides apparatus for fabricating an optical waveguide
grating, the apparatus comprising:
means for successively exposing a writing light beam through a mask onto
regions of a photosensitive optical waveguide, to generate corresponding
regions of
the grating; and
means for 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.
Preferred features of each aspect of the invention are equally applicable to
other aspects of the invention.
The invention will now be described by way of example with reference to the
accompanying drawings, throughout which like parts are referred to by like
references, and in which:
Figure 1 is a schematic diagram of an apparatus for fabricating an optical
waveguide grating;
Figure 2 is a schematic graph illustrating the reflection spectrum of a
prototype dual wavelength grating fabricated using the apparatus of Figure 1;
Figure 3 is a schematic graph illustrating the dependency of reflectivity on
wavelength shift for prototype gratings;
Figure 4 is a schematic graph illustrating the reflection spectrum for a 1
centimetre long uniform fibre grating produced using previously proposed
techniques;
Figures 5a and 5b are schematic graphs illustrating reflection spectra for a 1
centimetre long apodised grating and a 1 centimetre long uniform grating;
Figure 6 is a schematic graph illustrating the reflection spectrum of an
apodised chirped grating; and
Figure 7 is a schematic graph illustrating the dispersion characteristics of
the
apodised chirped grating of Figure 6.
Figure 1 is a schematic diagram of an apparatus for fabricating an optical
waveguide grating
An ultraviolet (LIV) writing beam 10 of 100 mW cw (milliwatt continuous
AMENDED SHEET
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wave) power at 244 nm (nanometre) wavelength is generated by a frequency-
doubled
argon laser (not shown). The writing beam 10 is steadily scanned by a scanning
mirror 20 (driven by a stepper motor - not shown) across a zero-order nulled
phase
mask 30, which in tum is positioned over a photosensitive optical fibre 40.
5 During the scanning process, the fibre 40 is slowly moved longitudinally
relative to the mask 30, causing a phase shift dependent on the fibre
displacement to
be added to the fibre grating being written.
In the present embodiment, the fibre movement is provided by mounting the
fibre on a piezo-electric transducer (PZT) stage 50 capable of providing a
longitudinal
displacement of up to about 20 Cm (micrometre). An example of a suitable PZT
device is available from Physik Instruments GmbH & Cb. In particular, a Physik
Instrument P-731-11 two-axis piezo scanning stage can be used in conjunction
with
a Physik Instrument P-864.11 low voltage piezo driver, a Physik Instrument E-
850.00
capacitive sensor board and a Physik Instrument E-802.00 piezo control module.
The
scanning stage specified above has a resolution of about 1 nm and a positional
accuracy of about 0.05%. The device also incorporates a capacitive feedback
position
sensor, so the displacement of the PZT device can be controlled by
conventional
external control electronics, such as those specified above.
It will be appreciated that relative motion of the fibre and mask determines
the
phase shift applied to the grating. Accordingly, although in Figure 1 the
fibre moves
against a stationary mask, in alternative embodiments the mask could be moved
with
respect to a stationary fibre. Alternatively, both the fibre and the mask
could be
moved. In the present description, however, it will be assumed that the mask
is held
still and the fibre is moved as shown in Figure 1.
A uniform velocity of the fibre 40 in Figure 1 results in a simple shift of
the
Bragg wavelength. If 1o is the unshifted Bragg wavelength, and vf and vsc are
the
fibre and scanning beam velocities respectively, with vf << vsc (the case of
interest
here), the Bragg wavelength shift AA of the resulting grating from that
obtained with
the fibre position fixed with respect to the mask is given by A;L_;LovPJvs,.
Thus for
a shift of about 1 nm, the fibre has only to move at about 0.1% of the
scanning speed.
As one example of this technique, a dual wavelength grating was fabricated
= using a single uniform phase mask. A dual wavelength grating is one
comprising a
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portion having a first Bragg wavelength adjacent to a portion having a second
Bragg
wavelength.
Figure 2 shows the reflection spectrum of a prototype dual wavelength grating
written using a boron-germania photosensitive fibre. For this prototype, the
scanning
beam speed was 37,um/s, and the fibre speed was 0.01 ,um/s for the first half
of the
writing time, switching to -0.01 um/s for the second half of the writing time.
The
total length of the grating was 1 cm (centimetre). Figure 2 illustrates the
two
reflectivity peaks obtained by this technique.
For large wavelength shifts, the grating strength tends to decrease as the
index
modulation gets averaged or 'washed out' when the fibre moves too quickly
through
the interference pattern formed by the phase mask. In fact, the refractive
index
modulation An has the following dependence on vf:
An = sin(7tDvf/Av,)/(atDvt/Av,) = sin(Znef~rD A~./~.Zo)/(2ne~D 0 JL/~ Zo)
where D is the writing beam diameter, A is the fibre grating pitch and n~ff is
the effective refractive index (2neffA = ;Lo).
The above relation has been verified by writing weak (<20% reflectivity)
gratings with different wavelength shifts, and recording their reflectivities
R, which
will have a An 2 dependence. Figure 3 illustrates the experimental points
plotted
against the sinc2 (i.e. sinx/x) curve expected from the above formulae, and
shows that
the experimental reflection data fits well to the above relationship for the
(measured)
beam diameter D of about 350,um.
Also from the above equation, An vanishes when vf = Av,,/D, or AA = AZ/D
(where DA/A = A;L/;L,,,). Accordingly, the equation shows that theoretically
the
maximum achievable wavelength shift is thus only dependent on the beam
diameter
D. Physically, this condition simply corresponds to the case where a point in
the fibre
moves by one grating pitch during the time D/vsC that the scanning beam passes
over
it, resulting in a spatial averaging out of the index variation. On the other
hand, larger
wavelength excursions of up to several nm are achievable by simply reducing
the
writing beam diameter.
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Apodisation
Apart from shifting the Bragg resonance wavelength by uniformly moving the
fibre in a single direction, so--called 'pure' apodisation can also be applied
to the
' grating simply by longitudinally dithering the fibre back and forth as the
writing beam
is scanning.
= In one example, the magnitude of the dither was set to decrease linearly,
from
one half of the grating pitch at the grating ends, down to zero dither at the
centre of
the grating. This produces a cosinusoidal apodisation profile.
Using this technique for apodising the grating, since the average UV fluence
reaching the fibre is the same over the entire length of the grating, the
average
refractive index will be independent of position along the grating, and only
the
index modulation will vary, i.e. only An is modulated. This produces what is
referred
to as a 'pure' apodisation effect, which contrasts with an apodised grating
produced by
previous techniques in which the average index varies along the length of the
grating.
Figure 4 shows the reflection spectrum for a uniform grating, and Figures 5a
and 5b illustrate the corresponding spectrum obtained with the apodisation
present,
showing its effectiveness in reducing the side-lobe levels.
With apodisation, the reflectivity tends to be weaker (since the effective
grating
length is less), so for a fairer comparison, a uniform grating was written to
have the
same peak reflectivity and bandwidth as the apodised one. In Figure 5a, the
spectrum
of the apodised grating (thick curve) is superimposed for comparison over the
spectrum of the uniform grating of the same peak reflectivity and bandwidth.
Figure
5b illustrates the same apodised grating reflectivity, but without the
superimposition
of the uniform grating curve.
It can be seen that the side-lobes of the apodised spectrum are more than 25
dB (decibels) below the main peak, and 13 dB below those of the uniform
grating.
These results were achieved by simply dithering the relative position of the
fibre and
phase mask, but are comparable to the results by Albert et. al. (publication
reference
6 below) achieved with a specialised variable diffraction efficiency phase
mask.
In another application, so-called "superstructure" gratings could be produced
in which the magnitude of the dither signal (and in turn, the apodisation) is
modulated
along the length of the grating, with a modulation period much larger than the
grating
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period.
Chirped Gratings
Instead of applying a constant velocity to the fibre to create a wavelength
shift,
=
it is also possible to produce chirped gratings by varying the speed of the
fibre
relative to the mask.
In one example, by simply ramping (linearly increasing or decreasing) the
velocity of the fibre during the scanning time, a linearly chirped grating can
be
produced. Figure 6 schematically illustrates the reflection spectrum of an
example
prototype 1.5 cm long apodised chirped grating made in this manner. The
bandwidth
of the grating of Figure 6 is 0.82nm and the peak reflectivity is about 40%.
Figure 7 is a schematic graph illustrating the dispersion characteristics of
the
apodised chirped grating of Figure 6. This graph demonstrates that the grating
has an
average time delay slope over bandwidth of about 170 pS/nm (picoseconds per
nanometre).
However, in addition to linearly chirped gratings, other nonlinear chirp
functions can be easily imposed on the grating simply by changing the velocity
profile
of the fibre during scanning. Indeed, in principle it is possible to
compensate for an
imperfect phase mask with this method and still produce good quality gratings,
provided the imperfections are characterised beforehand. Also, implementing
discrete
phase shifts for, say, distributed feedback (DFB) type laser structures, which
require
a phase discontinuity, multiple phase shifts or a chirped region within the
grating, can
be achieved just by shifting the fibre and/or phase mask by the desired amount
at the
appropriate time during scanning.
In summary, the moving fibre/phase mask-scanning beam technique for
producing gratings from a uniform phase mask imparts a considerable
flexibility to the
phase mask approach, enabling complex grating structures to be easily written
simply
by moving the fibre relative to the mask in the appropriate manner.
Multiwavelength
gratings, 'pure' apodisation and controlled chirp have all been successfully
demonstrated in prototype gratings.
The techniques described above can be combined, so that a chirped apodised
grating (such as that -described with respect to Figures 6 and 7) can be
produced by
superimposing a dither motion onto a steadily ramping velocity component. In
fact,
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any combination of wavelength shift, chirp and apodisation can be set up for a
particular phase mask, by combining the motion components appropriate to those
effects as described above.
In the above description, it has been assumed that the writing laser beam
scans
along the mask at a uniform velocity vsc. From this, it follows that a simple
wavelength shift requires a uniform mask/fibre displacement velocity, a chirp
requires
a steadily ramping mask/fibre displacement velocity and apodisation can be
achieved
by a symmetrical dither oscillation component (although asymmetrical dither
could be
used if desired). However, the skilled man will appreciate that the techniques
are
equally applicable if the exposure to the writing beam is not by a uniform
velocity
scan. For example, the writing beam exposure might be via a scanning sweep at
a
non-uniform velocity, or might even be via point exposures which do not form a
single continuous scanned path. In these cases, the correct relative
displacement path
of the mask and fibre can be derived routinely by the simple rule that the
phase shift
applied to the grating with respect to that provided by the mask at any time
during the
writing process is proportional to the diplacement of the fibre with respect
to the
mask.
Accordingly, embodiments of the invention provide a method of forming
optical waveguide gratings, such as in-fibre gratings. The gratings are formed
optically, with a phase mask being scanned by a writing laser beam to generate
the
grating pattern. The waveguide and phase mask are moved with respect to one
another during the writing process, to vary the grating properties along the
length of
the grating. Relative movement in a single direction provides a change of
grating
pitch, and so can be used to fabricate chirped or multi-wavelength gratings.
Bi-
directional dither alters the strength of the grating, and so can be used to
fabricate
apodised gratings.
The examples described above have referred primarily to optical fibres, but it
will be appreciated that the techniques are also applicable to other types of
waveguide
such as planar waveguides.
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PUBLICATION REFERENCES
1. K. O. Hill et. al., Appi. Phys. Lett., 62, 1993, pp 1035-1037.
2. D. Z. Anderson et. al., Electron. Lett., 29, 1993, pp. 566-567.
5 3. J. D. Prohaska et. al., Electron. Lett., 29, 1993, pp. 1614-1615.
4. J. Martin et. al., Electron. Lett., 30, 1994, pp. 811-812.
5. H. N. Rourke et. al., Electron. Lett., 30, 1994, pp. 1341-1342.
6. J. Albert et. al., Electron. Lett., 31, 1995, pp. 222-223.
7. B. Malo et. al., Electron. Lett., 31, 1995, pp. 223-224.
10 8. K. O. Hill et. al., Opt. Lett., 19, 1994, pp. 1314-1316.
9. R. Kashyap et. al., Electron. Lett., 30, 1994, pp. 996-997.
10. K. C. Byron et. al., Electron. Lett., 31, 1995, pp. 60-61.
