Note: Descriptions are shown in the official language in which they were submitted.
CA 02344880 2001-05-10
METHOD FOR SPATIALLY CONTROLLING THE PERIOD AND AMPLITUDE
OF BRAGG FILTERS
CROSS-REFERENCE TO RELATED APPLICATION
This application is a division of Canadian application no. 2.163.061 of
November 16, 1995.
FIELD OF THE INVENTION
The invention relates to a method for spatially controlling the period and
amplitude of
Bragg gratings in an optical medium written with electromagnetic radiation.
DESCRIPTION OF THE PRIOR ART
It is known in the art that UV light can be used transversally to permanently
increase
the refractive index of an optical fiber. Further, it is known that by
illuminating an optical
fiber with a UV-light interference pattern, a periodic index change is
produced in the core of
the optical fiber, and thus a strongly selective wavelength reflection filter
is obtained.
By using techniques such as interferometry, transverse holography or phase
masking
to obtain an interference pattern of the UV-light, a Bragg grating is
impressed in the core of
the optical fiber.
U.S. Patent 5,327,515 (Anderson et al.) describes a method for processing
optical
media in order to form gratings within them. The gratings are impressed by a
single actinic
beam through a phase mask having a given period. The beam may be at an angle A
with
respect to normal incidence (z-axis) on the phase mask. Furthermore, the
optical fiber itself
may be at an angle in the x- and y- axes. However, the author does not discuss
the effects of
using angles other than 0, when interference of orders ~l are used to produce
the periodic
index change in the core of the fiber.
This patent suggests that the amplitude of the fiber grating may be adjusted
by moving
the beam along the axis of the fiber. The disadvantage with this method is
that the phase may
drift slightly due to the movement of the beam, resulting in an uneven filter.
Reference may also be made to the following patents and articles: US 4,474,427
(Hill
CA 02344880 2001-05-10
2
et al.); US 4,725,110 (Glenn et al.); US 4,807,950 (Glenn et al.); US
5,363,239 (Mizrahi et
al.); US 5,351,321 (Snitzer et al.); US 5,367,588 (Hill et al.); US 5,384,884
(Kashyap et al.);
US 5,388,173 (Glenn); US 5,420,948 (Byron); HILL et al., «Photosensitivity in
Optical Fiber
Waveguides: Application to Reflection Filter Fabrication, Appl. Phys. Lett.,
Vol. 32, No. 10,
647-649, 15 May 1978; MELTZ et al., «Formation of Bragg Gratings in Optical
Fibers by
Transverse Holographic Method, Optics Letters, Vol. 14, No. 15, 823-825, 1
August 1989;
HILL et al., «Bragg Gratings Fabricated in Monomode Photosensitive Optical
Fiber by UV
Exposure Through a Phase Mask, Appl. Phys. Lett., Vol. 62, No. 10, 1035-1037,
8 March
1993; ANDERSON et al., «Production of In-Fibre gratings Using a Diffractive
Optical
Element, Electronics Letters, Vol. 29, No. 6, 566-568, 18 March 1993; KASHYAP
et al.,
«Wavelength Flattened Saturated Erbium Amplifier Using Multiple Side-Tap Bragg
Gratings, Electronics Letters, Vol. 29, No. 11, 1025-1026, 27 May 1993;
PROHASKA et al.,
«Magnification of Mask fabricated Fibre Bragg Gratings, Electronics Letters,
Vol. 29, No.
18, 1614-1615, 2 September 1993, MARTIN et al., «Novel Writing Technique of
Long and
Highly Reflective In-Fibre Gratings, Electronics Letters, Vol. 30, No. 10, 811-
812, 12 May
1994; FARRIES et al., «Very Broad Reflection Bandwidth (44 nm) Chirped Fibre
Gratings
and Narrow Bandpass Filters Produced by the Use of an Amplitude Mask,
Electronics
Letters, Vol. 30, No. 11, 891-892, 26 May 1994; KASHYAP et al., «Novel Method
of
Producing All Fibre Photoinduced Chirped Gratings, Electronics Letters, Vol.
30, No. 12,
996-998, 9 June 1994; PAINCHAUD et al., «Chirped Fibre Crratings Produced by
Tilting the
Fibre, Electronics Letters, Vol. 31, No. 3, 171-172, 2 February 1995; and COLE
et al.,
«Moving Fibre/Phase Mask-Scanning Beam Technique for Enhanced Flexibility in
Producing
Fibre Gratings with Uniform Phase Mask, Electronics Letters, Vol. 31, No. 17,
1488-1490,
17 August 1995.
SUMMARY OF THE INVENTION
The original application no. 2.163.061 broadly describes and claims an
improved
method for spatially controlling the period and amplitude of Bragg filters in
an optical medium
which is simpler, more flexible and more suitable for mass production.
The present divisional application has as its object to provide an improved
method for
producing linearly-chirped Bragg gratings in an optical medium where the
magnitude of the
CA 02344880 2001-05-10
3
index change varies as a function of the longitudinal axis of the optical
medium.
In accordance with the invention, this object is achieved with a method for
impinging gratings
in an optical medium that is sensitive to at least some wavelengths of
electromagnetic
radiation, said medium having a longitudinal axis, said method comprising the
steps of:
- providing a chirped optical phase mask of variable period;
- laying said chirped phase mask close to said optical medium; and
- impinging a single beam of electromagnetic radiation on said chirped phase
mask such that an amount of said radiation is diffracted, thereby resulting in
an interference
pattern having an amplitude and a period varying linearly along said
longitudinal axis that is
impinged into said optical medium;
the improvement wherein:
- a slit is inserted between said single beam and said chirped phase mask; and
- said slit is slid in a direction perpendicular to said single beam at a
variable
velocity,
whereby said amount of radiation that is impinged into said optical medium may
be
adjusted, thereby modifying said amplitude of said interference pattern along
said optical
medium.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention and its advantages will be more easily understood after
the
reading of the following non-restrictive description of the preferred
embodiments thereof,
made with reference to the following drawings where:
Figure 1 is a schematic representation of the first preferred embodiment of
the
invention;
Figure 2 is a schematic representation of the second preferred embodiment of
the
invention;
Figure 3 is a schematic representation of the third preferred embodiment of
the
invention;
Figure 4 shows the diffraction efficiency as a function of the angle of
incidence for 0
and ~ 1 orders for TE and TM polarizations, for a binary phase mask having a
period of 1.06
microns and a duty cycle of 0.5,
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Figure 5 is a schematic representation of a blazed grating;
Figure 6 shows the reflection spectrum of various gratings of different Bragg
wavelength obtained using the first preferred embodiment of the invention;
Figure 7 show the reflection spectrum of a 17-nm bandwidth chirped fiber
grating
obtained using the second preferred embodiment of the invention; and
Figure 8 shows a fitted (solid line) and target (broken line) transmission
spectrum of
a Bragg grating obtained using the third preferred embodiment of the
invention.
DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION
The following description will be made with reference to glass optical fibers,
but it
should be understood that it is applicable to any optical medium that is
sensitive to at least
some wavelength of electromagnetic radiation.
In a first preferred embodiment shown in Figure 1, a phase mask 3 of period A
is laid
close to an optical fiber 1 having a longitudinal axis 5. A single beam 7 of
electromagnetic
radiation, preferably ultra-violet light (hereinafter denoted by «UV»), having
a wavelength ~,
and a width D, is directed on the phase mask 3 such that the radiation is
diffracted by the
phase mask 3. This results in an interference pattern (not shown) having a
period P being
impinged into the optical fiber 1.
When the fiber 1 is not parallel to the phase mask 3 but rather forms a tilt
angle a with
the phase mask 3 and the UV beam 7 is incident on the phase mask 3 at an
incidence angle cp,
the period P of the interference pattern along the axis 5 of the optical fiber
1 is given by the
following expression (assuming that orders other than ~ 1 are negligible):
n n _,
[1] P= (I+-sin9sin/.3tana)
2 cos a
where 8 = (A, + 6Z)/2
p=~-ez+e
8, = arcsin(7~/A + since) - cp
6z = aresin(~,/A - since) + cp
CA 02344880 2001-05-10
For small angles a and cp, equation [ 1 ] reduces to:
[2] p- ~ 1+ ~z-a~
nJ~
Thus, for a tilt angle a different from 0, the period P of the interference
pattern can be
modified by changing the incidence angle cp of the UV beam 7 or the angle a.
This result can
be used to precisely control the period of periodic Bragg gratings. Preferably
for this purpose,
the tilt angle a is equal to the incidence angle cp, by having the beam 7
preferably directed
toward the fiber 1 at normal incidence with respect to the axis 5 of the fiber
1. Thus, the
optical fiber 1 is fixed with respect to the UV beam 7 and only the phase mask
3 is rotated in
order to control the period P of the grating. From equation [2], valid for
small angles, the
relationship between the wavelength at which the optical fiber 1 reflects (the
Bragg
wavelength, denoted hereinafter by ~,B) and the incidence angle cp is given
by:
[3] ~,N=nn(1-Y~Pzl2)
where y = 2(1 - ~,Z/n2)-~~' - 1
and n is the core index of the fiber 1,
so that by adjusting the incidence angle cp, the Bragg wavelength ~.B can be
controlled.
Since the method involves the use of a phase mask 3 out of normal incidence,
the
dependence of the diffraction efficiencies on the angle of incidence cp must
be considered. As
can be seen in Figure 4, the diffraction efficiencies of 0 and ~ 1 orders as a
function of the
incidence angle cp for both TE and TM polarizations remain almost identical
for incidence
angles cp smaller than 20°. Thus, the method must be limited to
incidence angles cp between
0° and 20°, which allows for a very good flexibility. Curves of
Figure 4 were calculated
considering a typical binary phase mask having a period of 1.06 microns and a
duty cycle of
0.5.
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Furthermore, the method also involves irradiation of the fiber 1 by an
interference
pattern with the fringes tilted at a certain angle with respect to the fiber
axis 5, resulting in a
blazed angle 8, shown in Figure 5, given by:
[4] S = [3 - a,
where (3 is defined above.
It is important that the blazed angle 8 remains small in order to avoid a
coupling of the
propagating light outside of the fiber 1. The condition that must be met is:
[5] 8 < ? arcsinC NA.
n
where N.A. is the numerical aperture and n is the core index of the fiber 1.
For example, in a typical telecommunications fiber, n = 1.45 and N.A. = 0.12,
so that
8 must remain below 2.4°.
According to the first preferred embodiment of the invention, the period P of
the
grating is adjusted only by rotating the phase mask 3, so that a = c~. Since
[3 is approximately
equal to cp, the condition a = cp leads to an almost unblazed grating. More
precisely, for cp =
20°, the blazed angle 8 is 0.8". For example, for a grating made in the
1550 nm region and an
incidence angle cp between 0" and 20°, which is sufficient for most
applications, the blazed
angle 8 does not exceed 0.8°.
Experimental tests have shown that the Bragg wavelength ~,B may be tuned with
this
method. Figure 6 presents the reflection spectrum of a fiber 1 in which nine
gratings of
different periods were written side by side by rotating the phase mask 3. As
can be seen from
Figure 6, the result shows a tuning of the Bragg wavelength i~a by 22 nm over
the 10° range
of the incidence angle cp. This is in accordance with the theoretical
prediction of 25 nm from
equation [3].
Thus, the Bragg wavelength ~,~ can be precisely tuned by simply rotating the
phase
mask 3 in a simple manner which is appropriate for mass production.
In a second preferred embodiment of the invention shown in Figure 2, the above-
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mentioned method comprises an additional step, where a lens 9, having a focal
length f and
placed at a distance d from the phase mask 3 is used to generate a
distribution of incidence
angles cp on the phase mask 3 and produce a linearly-chirped Bragg grating, as
shown in
Figure 2.
Using the lens 9 produces an interference pattern for which the period P(x)
varies
linearly along the axis 5 of the fiber 1, given by the following expression:
~6~ Pox) - 2 1 _ f.x d ~7
where r~ = 1 + 1 1--
2 2 A' ,
Consequently, the grating reflects light of different wavelengths at different
grating portions
following:
[7] ~,H (x) = 2 h P(x)
For a grating of length L. the spectral broadening associated with the chirp
is then given by:
~g~ Q~-2nALar~_2nADcxr~
.f - d .f
CA 02344880 2001-05-10
g
In this second preferred embodiment, the lens 9 is used to generate a
distribution of incidence
angles cp on the phase mask 3, resulting in a distribution of blazed angles 8.
In order to avoid
any coupling of the propagating light outside the fiber 1, the following
condition must be
satisfied:
[9] arctan D < 1 arcsin N~A.
2,f 2 ( n
For example, for a standard telecommunications fiber where n = 1.45, N.A. =
0.12, using a UV
beam 7 of width D = 10 mm, the smallest focal length f of the lens 9 that can
be used is 120
mm. Using that focal length in combination with a tilt angle a of 1 °
leads to a grating having
a bandwidth A~, of 4.5 nm.
In experimental tests, a divergent lens 9 having a focal length f of 50 mm was
placed
at a distance d of 20 mm from the phase mask 3 and a fiber 1 having a large
numerical
aperture was tilted at an angle a of 1.6°. A 14 mm-long Bragg filter
was obtained, having the
reflection spectrum shown in Figure 7. The bandwidth 07~ of this chirped
grating is 17 nm,
which is in accordance with the theoretical prediction of 17.3 nm from
equations [6] and [7].
This method for obtaining a chirped grating is flexible and can be used for
mass
production.
In a third preferred embodiment of the invention shown in Figure 3, the method
described above comprises the additional step of inserting a moving slit 11
between the UV
beam 7 and the lens 9. Such a setup allows the amplitude of the Bragg grating
to be adjusted
in a controlled manner along the fiber axis 5. Moving the slit 11 at a
variable velocity along
the phase mask 3 allows a variation of the amount of UV radiation received by
different
portions of the fiber.
Since the grating is chirped, as discussed in the second preferred embodiment,
there
is a relationship between the position along the fiber l and the wavelength at
which the grating
reflects. By controlling the grating amplitude along the fiber axis 5, the
transmission of the
fiber 1 can be controlled as a function of the wavelength. This kind of filter
may be used to
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adjust the transmission spectrum of a fiber device. For example, the gain
spectrum of an
optical amplifier may be flattened using this method.
This method of profiling the grating by using a moving slit 11 can also be
used in
combination with other chirped grating fabrication techniques to obtain the
same result. For
example, profiling a chirped fiber grating may be achieved by using a moving
slit and a
chirped phase mask.
Experimental tests using the third preferred embodiment, as shown in Figure 3,
yielded
the transmission spectrum shown in Figure 8. Using the transmission spectrum
of a 15 nm
bandwidth fiber grating, the dose of radiation was adjusted as a function of
the position along
the fiber axis 5 in order to fit the targeted transmission spectrum of Figure
8. The result shows
that a profiling of the transmission is possible with this method.
This method has the advantage that the beam is fixed with respect to the fiber
1 since
only the slit 11 is moved along the axis 5, so that the phase of the beam 7
does not drift. Thus,
such filters can be easily mass produced with a fair degree of accuracy.
Although the present invention has been explained hereinabove by way of a
preferred
embodiment thereof, it should be pointed out that any modifications to this
preferred
embodiment within the scope of the appended claims is not deemed to alter or
change the
nature and scope of the present invention.
