Note: Descriptions are shown in the official language in which they were submitted.
CA 02377208 2002-03-18
I
METHOD AND APPARATUS FOR RECORDING AN OPTICAL GRATING IN A
PHOTOSENSITIVE MEDIUM
FIELD OF THE INVENTION
s The present invention relates to the field of components for optical
telecommunications and more particularly concerns a method and a
corresponding apparatus for recording optical gratings in a photosensitive
medium
with an enhanced control of the characteristics of the grating.
lo BACKGROUND OF THE INVENTION
Phase masks are widely used for the fabrication of UV-induced fiber Bragg
gratings since their first reports (see for example K.O. Hill, B. Malo, F.
Bilodeau,
D.C. Johnson, and J. Albert, "Bragg gratings fabricated in monomode
photosensitive optical fiber by UV exposure through a phase mask" Appl. Phys.
Is Lett., pp.1035-1037 (1993); U.S. Patent No 5,367,588 (Hill et al.); D.Z.
Anderson,
V. Mizrahi, T. Erdogan, and A.E. White, "Production of in-fibre gratings using
a
diffractive optical element" Electron. Lett., pp.566-568 (1993); and U.S.
Patent No
5,327,515 (Anderson et al.). The use of such a diffractive element renders
easy
the mass production of fiber Bragg gratings as the mask acts somewhat as a
2o master replicated onto a large number of fiber Bragg gratings. However, a
typical
writing setup with a phase mask is not flexible and allows the fabrication of
only
one type of fiber Bragg gratings, that is, the one with the specifications
prescribed
by the phase mask.
The fiber is characterized by an effective index ne,~ that is modified by the
2s UV radiation. A fiber Bragg grating is mainly characterized by the period p
of the
index modulation in the core of the fiber, along its axis: The fiber Bragg
grating
reflects light having a wavelength ~,8 (the Bragg wavelength) given by:
~a(Z) - 2 P(z) ~~8(Z)
Where nerd is the slowly varying effective index of the fiber inside the
grating, z is
~o the position along the grating and the dependence of the parameters over z
indicates that both the period and the slowly varying effective index are not
necessarily uniform along the grating. There is an interest in the control of
the
,_
CA 02377208 2002-03-18
2
particular, it allows a fine control of the apadisation, that is the strength
of the
grating, along the fiber axis.
Several techniques based on a phase mask but with enhanced flexibility
have been proposed over the past few years. One of the most straightforward
way
lo to modify the grating period is by stretching the fiber, such as taught in
K.C. Byron,
and H.N. Rourke, "Fabrication of chirped fibre gratings by novel stretch and
write
technique" Electron. Lett., pp.60-61 (1995) and K. Sugden, 1. Bennion; A.
Molony,
and N.J. Copner, "Chirped gratings produced in photosensitive optical frbres
by
fibre deformation during exposure" Electron. Lett., pp.440-442 (1994). There
is
Is also suggested in Y. Painchaud, A. Chandonnet, and J. Lauzon, "Chirped
fibre
gratings produced by tilting the fibre" Electron. Lett., pp.171-172 (1995) and
U.S.
Patent No. 5,903,689 (PAINCHAUD et al.) to adjust the period by controlling
the
angles of both the phase mask and the fiiber with respect to the UV-beam axis.
Referring to U.S, patent no. 6;072,926 (COLE et al.) and M.J. Cole, W.H.
2o Loh, R.I. Laming, M.N. Zervas, and S. Barcelos, "Moving hbrelphase mask-
scanning beam technique for enhanced flexibility in producing fibre gratings
with
uniform phase mash' Electron. Lett., pp.1488-1490 (1995), it is known to
adjust
the period by moving the phase mask. For the fine tuning of the Bragg
wavelength,
Cole proposed a lateral displacement of the phase mask during a writing
process
Zs involving a scan of the UV beam. Excellent results have been obtained but
the
adjustment range is limited to about 1 nm. FIG. 1 (PRIOR ART) shows the limit
of -
the grating period adjustment when the UV beam diameter is 350 pm: the
reflectivity decreases as a function of the detuning which corresponds to a
decrease in the writing efficiency. The adjustment range increases as the UV
~o beam size decreases. Cole also proposed a displacement of the phase mask at
Bragg wavelength along a grating. This can be done by controlling the period
of
the grating along the fiber.
Translating a UV-beam along the phase mask is a convenient way to
achieve long gratings (J. Martin, and F. Ouellette, "Navel writing technique
of long
s and highly reflective in-fibre gratings" Electron. Letf., pp.911-812
(1994)). In
variable velocity for the adjustment of a chirp in the grating period.
CA 02377208 2002-03-18
3
On another hand, Prohaska, described in U.S. patent no. 5,351,321
(SNITZEft) and J.D. Prohaska, E. Snitzer, S. Rishton, and V. Boegli,
"Magnitlcafion of mask fabricated fibre Bragg gratings" Electron. Lett.,
pp.1614-
1615 (1993) a technique for controlling the period of a Bragg grating over a
large
s range (several nanometers) by using a magnifying lens along the UV beam
axis.
The right side of F1G. 2 (PR10R ART) shows the interference fringes at the
output
of a phase mask when a convergent UV beam is incident at the input surface. By
placing a fiber at a distance q from the output surface of the phase mask, a
grating
will be photo-imprinted having a period p given by:
p - 2 ~M~ (2)
where M - 1- q , (3)
z~.
is the magnification factor, A is the phase mask period, q is the distance
between
the output surface of the phase mask and the fiber core and zf is the distance
between the output surface of the phase mask and the focal plane, that is the
is plane where the beam would be focalized. The distance zf also corresponds
to the
radius of curvature of the wavefront at the phase mask.
Oppositely, the left side of FIG. 2 {PRIOR ART) illustrates the interference
fringes at the output of a phase mask when a collimated beam is incident. In
this
case, the period of the grating is independent of the distance between the
phase
2o mask and the fiber.
The technique described by Prohaska allows an adjustment of the Bragg
wavelength over a large range (several nanometers). However, the optical
characteristics of the resulting grating are degraded: the photo-induced
grating is
slanted (blazed) in a spatially-dependent manner. Such a slanted fringes
inside
2s the grating causes a spatial dependence of the diffraction efficiency and
increases
significantly the polarization dependent loss and polarization mode
dispersion.
Another drawback is the need for uncommonly large lenses when a long grating
is
to be photo-induced, making the method more costly and unpractical.
CA 02377208 2002-03-18
4
There is therefore a need for a fabrication techniques for Bragg gratings or
the like alleviating the above mentioned drawbacks of the prior art.
OBJECTS AND SUMMARY OF THE INVENTION
s Accordingly, it is an object of the present invention to provide a method of
recording optical gratings in a photosensitive medium that is versatile and
commercially practical.
It is another object of the present invention to provide an apparatus adapted
to carry out such a method.
io It is a preferable object of the invention to provide such a method that
enables the recording of long gratings over a large wavelength range.
It is another preferable object of the invention to provide a method and
apparatus for recording superimposed grating components in a photosensitive
medium.
is Accordingly, the present invention provides a method for recording an
optical grating along a waveguiding axis in a photosensitive medium. The
method
includes:
a) providing a phase mask proximate the photosensitive medium along the
waveguiding axis;
zo b) projecting a light beam through a portion of the phase mask to generate
a Light
beam with a modulated intensity profile. The light beam with a modulated
intensity profile impinges on the photosensitive medium to locally record
therein a portion of the optical grating, having a characteristic period;
c) moving the light beam along the waveguiding axis of the photosensitive
2s medium to successively record portions of the optical grating therealong;
and
d) concurrently to the moving of the light beam:
i) moving the phase mask in a direction parallel to the moving of the light
beam. The moving of the phase mask is adjusted relative to the moving of the
light beam to locally tune the characteristic period of each portion of the
optical
3o grating; and
CA 02377208 2002-03-18
ii} providing a curvature in the light beam wavefront along the direction of
the waveguiding axis, this curvature having a wavefront radius of curvature at
a
phase mask plane selected to generally optimize an efficiency of the recording
of the optical grating for this characteristic period.
s In the alternative, the present invention provides another method for
recording an optical grating along a waveguiding axis in a photosensitive
medium,
the method comprising:
a) providing a phase mask proximate to the photosensitive medium along the
waveguiding axis;
to b) projecting a light beam through a portion of said phase mask to generate
a light
beam with a modulated intensity profile, said light beam with a modulated
intensity profile impinging on the photosensitive medium to locally record
therein a portion of the optical grating having a characteristic period;
c) moving the light beam along the waveguiding axis of the photosensitive
is medium to successively record portions of the optical grating therea(ong;
and
d) concurrently to said moving of the light beam:
l) moving the photosensitive medium in a direction parallel to the moving of
the light beam, said moving of the photosensitive medium being adjusted
relative to the moving of the light beam to locally tune the characteristic
period
20 of each portion of the optical grating; and
ii) providing a wavefront curvature in said light beam along the direction of
the waveguiding axis, said curvature having a wavefront radius of curvature in
a plane of the phase mask selected to generally optimize an efficiency of the
recording of the optical grating for said characteristic period.
2s In accordance with another aspect of the invention, there is also provided
an apparatus for recording an optical grating along a waveguiding axis in a
photosensitive medium. The apparatus includes a phase mask provided proximate
the photosensitive medium along the waveguiding axis. A light source is also
provided, generating a fight beam for projection through a portion of the
phase
~o mask to generate a (fight beam with a modulated intensity profile. The
light beam
CA 02377208 2002-03-18
6
with a modulated intensity profile impinges on the photosensitive medium to
locally
record therein a portion of the optical grating having a characteristic
period.
light beam moving means are provided for moving the light beam along the
waveguiding axis of the photosensitive medium to successively record portions
of
s the optical grating therealong, and phase mask moving means are also
included
for moving the phase mask in a direction parallel to the moving of the light
beam
and concurrently thereto. The moving of the phase mask is adjusted relative to
the
moving of the light beam to locally tune the characteristic period of each
portion of
the optical grating:
to The apparatus finally includes curvature means for providing a curvature in
the light beam along the direction of the waveguiding axis. The curvature has
a
wavefront radius of curvature in a plane of the phase mask selected to
generally
optimize an efficiency of the recording of the optical grating for the
characteristic
period of each portion of the optical grating.
is Finally, in accordance with a preferred embodiment of the invention, there
is
provided a method for recording an optical grating along a waveguiding axis in
a
photosensitive medium, the optical grating comprising a plurality of
superimposed
grating components each having a characteristic period profile, the method
comprising:
2o a) providing a phase mask proximate the photosensitive medium along the
waveguiding axis;
b) for each of the superimposed grating component:
l) projecting a light beam through a portion of said phase mask to generate a
light beam with a modulated intensity profile, said fight beam with a
modulated
25 intensity profile impinging on the photosensitive medium to locally record
therein a portion of the optical grating component having a characteristic
period;
ii) moving the light beam along the waveguiding axis of the photosensitive
medium to successively record portions of the optical grating component
~o therealong; and
iii) concurrently to said moving of the light beam:
CA 02377208 2002-03-18
7
1 ) moving the phase mask in a direction parallel to the moving of the
light beam, said moving of the phase mask being adjusted relative to
the moving of the light beam to locally tune the characteristic period
of each portion of the optical grating component; and
s 2) providing a curvature in said light beam along the direction of the
waveguiding axis, said curvature having a wavefront radius ofi
curvature in a plane of the phase mask selected to generally
optimize an efficiency of the recording of the optical grating for said
characteristic period.
Fa Advantageously, the present invention allows to locally control the period
of
an optical grating over a large range (about ~2% of the nominal period) white
keeping the optical quality of the grating unaffected. Other techniques have
been
proposed but result in a degradation of the optical perFormances.
Other aspects and advantages of the present invention will be better
is understood upon reading a preferred embodiments thereof with reference to
the
appended drawings.
BRIEF DESCRIPT10N OF THE DRAWINGS
FIG. 1 (PRIOR ART) shows the reflectivity as a function of wavelength far
ao an optical grating made using the technique described by Cofe et al.
FIGS. 2A and 2B (PRIOR ART) illustrate the interference fringes at the
output of a phase mask when the incident light beam is collimated (FIG. 2A)
and
convergent (F1G. 2B).
FIGs. 3A and 3B illustrate the index modulation in two successive portions
2s of a photosensitive medium when the light beam is convergent (FIG. 3A) and
when the light beam has a curvature .(FIG. 3B).
FIG. 4A is a side view of an apparatus according to a preferred embodiment
of the invention; FIG. 4B is a perspective side view of a portion of the
apparatus of
FIG. 4A.
CA 02377208 2002-03-18
g
FIGs. 5A, 5B and 5C are graphs respectively showing the reflectivity the
group delay and the dispersion of superimposed gratings fabricated using a
preferred embodiment of the invention.
s DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION
The present invention advantageously allows to improve the range of the
phase mask moving technique disclosed by Cole et al. where the phase mask (or
photosensitive medium) is displaced parallel to the scanning of the fight
beam.
In accordance with a first preferred embodiment of the invention, there is
io provided a method for recording an optical grating along a waveguiding axis
in a
photosensitive medium. Preferably, the photosensitive medium is a length of
optical fiber, but any other type of waveguide where a refractive index
grating
needs to be written would be equally covered by the present invention. The
expression "optical grating" refers generally to a periodic (or nearly
periodic)
is refractive index change in an optical medium. It can be of any length
allowed by
the geometry of the system, and if desired it may be chirped, apodised, etc.
The method first involves providing a phase mask proximate the
photosensitive medium, along its waveguiding axis. The phase mask preferably
has a constant period along its length but a chirped mask could also be used,
with
2o appropriate modifications to the other parameters of the system. By the
expression
"proximate the photosensitive medium" it is meant that the photosensitive
medium
is positioned within the diffraction field of a fight beam modulated by the
phase
mask. Preferably, the phase mask extends in parallel to the waveguiding axis
of
the photosensitive medium, but the two could alternatively form a small angle
to
25 induce a chirp in the photoinduced optical grating.
The next step of the method of the present invention involves projecting a
light beam through a portion of the phase mask, to generate a light beam with
a
modulated intensity profile. The light beam with a modulated intensity profile
impinges on the photosensitive medium to locally record therein a portion of
the
~o optical grating. This portion of the optical grating has a characteristic
period p(z), z
CA 02377208 2002-03-18
9
representing the position of this particular portion of the grating along the
waveguiding axis. The light beam is preferably UV radiation produced by a
laser.
The light beam is then moved along the waveguiding axis, to successively
record portions of the optical grating therealong at different positions z.
s Concurrently to this movement of the light beam, the phase mask is also
moved in
a parallel direction, in accordance with the technique of Cole et al. The
movement
of the phase mask is adjusted relative to the moving of the light beam to
locally
tune the characteristic period of each portion of the optical grating which is
being
photoinduced in the photosensitive medium. A translation of the phase mask in
the
Io same direction as the movement of the sight beam will increase the local
characteristic period of the grating, whereas moving the two in opposite
directions
shortens the grating's period.
As explained above, the tunability of the grating period with the Cole et al.
technique is limited by the range of the reflectivity peak seen on FIG. 1. To
is alleviate this drawback, the present invention provides a curvature in the
light
beam wavefront, and the wavefront radius of curvature at the phase mask plane
is
selected to generally optimize the efficiency of the recording of the optical
grating
for a given characteristic period. This essentially allows to a large
wavelength shift
in the efficiency curve of FIG. 'I to align the reflectivity peak with the
desired
ao wavelength.
The principle of the present invention may be understood by the following
reasoning.
The use of a wavefront curvature in the light beam, that is making the light
beam convergent or divergent, allows to modify the Bragg wavelength depending
2s on the position of the photosensitive medium with respect to the phase
mask, as
illustrated in FIG. 2B. However, if the UV beam is translated during the
writing
process, a grating having a period p=AI2 will be obtained regardless of the
wavefront radius of curvature. This is because the effect of the wavefront
curvature is reduced to a loss of efficiency similar to the one obtained by a
vibration of the phase mask. This can be understood as follow: an optical
grating is
to be written using a phase mask having N fringes over a length L. By scanning
a
CA 02377208 2002-03-18
to
light beam over the whole phase mask, the photosensitive medium will be
illuminated over the same length L and will contain 2111 fringes. The period
of the
resulting grating depends only on the phase mask characteristics and do not
depend on the wavefront curvature of the scanned light beam.
s However, If the phase mask is properly displaced during the scan of a fight
beam having a curvature in its wavefront, a Bragg wavelength adjustment can be
obtained over a large range. The range of Bragg wavelength adjustment can be
as
large as the one obtained with the Prohaska technique, but without the adverse
efifects of slanted fringes.
to FIGs. 3A and 3B illustrates the index modulation induced inside two
portions of a photosensitive medium during a step-by-step scan. When the Light
beam is collimated (FIG. 3A), the phase in the index modulation is the same in
the
two portions. When the UV beam has a curvature in its wavefront (FIG. 3B), a
phase shift exists between the two portions. This phase shift depends on the
is wavefront curvature, on the laser beam displacement and on the distance
between
the phase mask and the waveguiding axis of the photosensitive medium according
to:
a _ a~ . an C~ _ M
This phase shift between portions is however eliminated by properly
2o coordinating the movements of the phase mask as the light beam. The
required
displacement ~xm of the phase mask for eliminating the phase shift is
~:Cm . ~Z Ct . .M~
This displacement of the phase mask, in combination with the use of a
wavefront curvature in the light beam therefore allows a strong increase in
the
?s adjustment range of the Bragg wavelength with respect to the technique
disclosed
by Cole et al.
In the method according to the present invention, it is the displacement of
the phase mask which determines the period of the grating. The wavefront
curvature of the .light beam is then adjusted in order for t>1e writing
process to be
CA 02377208 2002-03-18
11
efficient. The effect of the wavefront curvature can be viewed as a wavelength
shift
in the efficiency curve shown in FIG. 1. A disagreement between the phase mask
displacement and the light beam wavefront curvature causes a loss of writing
efficiency according to the graph shown in FIG. 1. Thus, the adjustment of the
s wavefront curvature is not extremely critical, as long as the resulting
recording
efficiency remains within the acceptable range.
In accordance with a first preferred embodiment of the invention, the
moving of the light beam and of the phase mask are both done step by step.
Therefore, between the recording of each consecutive grating portions the
light
to beam undergoes a displacement Oz and the phase mask is displaced by a
quantity nlxm. If, for a given grating portion, the nominal period of the
optical grating
po (corresponding to a Bragg wavelength x,80) is the one obtained without any
phase mask displacement, the period shift op from this nominal value, and the
corresponding shift in the Bragg wavelength of the grating portion, is given
by:
1s ~' -_ _nR 0/ZB = L~Xm (6)
Po y1r a'BO ~ - ~nr ~
where n9 and nee are the group and effective indices of the waveguiding
axis respectively.
The required wavefront radius of curvature of the light beam at the phase
mask plane for the writing to be efficient is:
20 zf
r<r
where q is the distance between the phase mask and the waveguiding axis
of the photosensitive medium. The approximate nature of this relationship
denotes
the tolerance of the Cole technique as seen in FIG. 1. This radius of
curvature
corresponds to a magnification given by Equation (3).
2s According to a second preferred embodiment of the invention, the moving of
the light beam and the phase mask may be done continuously. In such a
continuous scanning process, in any point along the grating, the local value
of the
nominal grating period po(z) (and Bragg wavelength ~,eo(z)) is modified by a
c
CA 02377208 2002-03-18
12
quantity dp(z) (O~.B(z)) by displacing the phase mask by a quantity dxm(z)
according to:
~P(Z) ~g ~~a (Z) _ dx~» (z) v~~f (z)
Po (z) ~ tz~~ ~.ao (Z) dz -dx"~ (z) y v(z) -vht (Z) ~ (8)
where dz is the translation of the UV beam, vm(z) is the velocity of the
s phase mask at position z and v(z) is the velocity of the light beam
translation at
position z. The wavefront radius of curvature mus# be such that at each point
along
the grating, the following relationship holds:
Ivl (z) '.. 1 + ~i~ (Z) _ 1 + yn~ (z)
dZ - dX,yn (Z) v(Z) - vm ~Z)
and therefore zf is adjusted so that:
to Z,f ~ q. yrn(Z)-y(Z) . (10)
vin (Z)
Advantageously, the method of the present invention allows to tailor the
characteristic period profile of the recorded grating by simply controlling
two
parameters, that is the relative translation of the phase mask and light beam
and
the wavefront radius of curvature of the fight beam. The present invention may
is also be combined with any other appropriate techniques that may further
provide
an adjustment of the characteristics of an optical grating, such as stretching
the
fiber, varying the scanning speed, etc.
In accordance with another' embodiment of the present invention, the
method above could be modified so that the photosensitive medium is moved
2o instead of. the phase mask, since the desired effect on the period depends
on the
relative displacement between these ~o components, It will be readily
understood
that to obtain the same effect, the photosensitive medium should be translated
in
the opposite direction as the phase mask would have been.
Now referring to FIGs. 4A and 4B, there is show an apparatus 10 for
2s recording on optical grating along a waveguiding axis 12 of a
photosensitive
medium 14, according to a preferred embodiment of the invention.
The apparatus first includes a phase mask 16 provided proximate the
photosensitive medium 14 along the waveguiding axis 12. A light source 18,
CA 02377208 2002-03-18
13
preferably a UV laser source, is also provided and generates a light beam 20
which is directed to project through a portion of the phase mask 16. This in
turn
generates a light beam with a modulated intensity profile which impinges on
the
photosensitive medium 14 to locally record therein a portion of the optical
grating
s having a characteristic period.
Means for moving the Light beam 20 along the waveguiding axis 12 of the
photosensitive medium 14, to successively record portions of the optical
grating
therealang, are further provided. In the present embodiment, these moving
means
include a 45° mirror 22 disposed to redirect the light beam 20 from the
light source
18 towards the phase mask 16, this mirror being mounted on a translation stage
24.
Similarly, means for moving the phase mask 16 in a direction parallel to the
moving of the light beam 20 and concurrently thereto are provided. Preferably,
this
is embodied by a second translation stage 26 on which the phase mask 16 is
is mounted. The relative movements of the phase mask 16 and the light beam 20
are
adjusted to locally tune the characteristic period of . each portion of the
optical
grating, in accordance with the technique described above.
Appropriate optical components are further provided to give the light beam
a wavefront curvature along the direction of the waveguiding axis. The
ao wavefront radius of curvature, in the plane of the phase mask, is selected
to
generally optimize the efficiency of the recording of the optical grating, as
also
explained above.
In the illustrated embodiment, the light beam 20 is modified previously to
impinging on the phase mask 16 by an optical system composed of lenses L7, L2
2s and Lf. Lens Lf focuses the light beam 20 along the waveguiding axis of the
photosensitive medium and therefore does not influence the wavefront curvature
of the beam in this axis. Lenses L7 and L2 decrease the width of the light
beam 20
and allow, via a longitudinal displacement of lens L7, an adjustment of the
wavefront curvature. Distance p is the one between the phase mask and the
~o waveguiding axis, I is the distance between lens L2 and the waveguiding
axis, D,
D' and D" are respectively the beam widths at the system input, at lens L2 and
at
CA 02377208 2002-03-18
14
the waveguiding axis, and f~ and f2 are the focal lengths of lenses L7 and L2
respectively. Distance d between lenses L7 and L2 has a nominal value, before
adjustment of the wavefront radius of curvature, f~+f2; with this choice, the
light
beam 20 incident on the phase mask 16 is collimated and the period of the
grating
s corresponds to the nominal period po. it is convenient to write d as:
d - .ft + ,fz _ s ( 11 )
so that s varies around 0 for adjusting the Local characteristic period of the
grating. Using this setup, the relationship between the desired change of the
characteristic period ~p and fihe required position s of lens L1 can be
written as:
dp ~_ gs (12)
1 o pa .fl + ~~l - .fz
The light beam width at lens L2 and at the.waveguiding axis is respectively
given by:
~,-D fz -s
f. (13)
D'~-ID .fz _ s 1_ 11 ~14)
t f f2
is It is convenient to chose I=f2. Then, the light beam width at the
waveguiding
axis is independent of the wavefront curvature and fihe grating period depends
linearly on the position of lens L1, which is therefore preferably mounted on
another translation stage 28.
In accordance with another particularly advantageous embodiment of the
ao present invention, there is provided a method for recording an optical
grating
having a plurality of superimposed grating components each having a different
characteristic period profile in a photosensitive medium. Such a superimposed
grating is for example useful for compensation of chromatic dispersion as for
disclosed in assignee's simultaneously filed co-pending application entitled
2s "OPTICAL STRUCTURE FOR THE COMPENSATION OF CHROMATIC
DISPERSION IN A LIGHT SIGNAL". The method according to this embodiment
includes as before a first step of providing a phase mask proximate the
CA 02377208 2002-03-18
photosensitive medium along the waveguiding axis. Then, for each of the
superimposed grating components, the steps of the method described above are
performed, that is:
i) projecting a light beam through a portion of the phase mask to generate a
s light beam with a modulated intensity prof;le, the light beam with a
modulated
intensity profile impinging on the photosensitive medium to locally record
therein a portion of the optical grating component having a characteristic
period;
ii) moving the light beam along the waveguiding axis of the photosensitive
to medium to successively record portions of the optical grating component
therealong; and
iii) concurrently to the moving of the light beam:
1 ) moving the phase mask in a direction parallel to the moving of the
light beam, this moving of the phase mask being adjusted relative to
is the moving of the light beam to locally tune the characteristic period
of each portion of the optical grating component; and
2) providing a curvature in the light beam along the direction of the
waveguiding axis, the wavefront radius of curvature in the plane of
the phase mask being selected to generally optimize the efficiency of
2o the recording of the optical grating for said characteristic period.
Alternatively; the photosensitive medium is translated instead of the phase
mask. It is understood that al! of the above described variants of the method
according to the previous emboc(iments may be equally applied to the current
embodiment.
2s FIGs. 5A and 5B show the measurements of the reflectivity and group delay
obtained for a mufti-channel fiber Bragg grating made of superimposed grating
components using the method of the present invention and the apparatus of FIG.
4A. For each of the channels, the central wavelength and the dispersion of the
grating are adjusted by a proper control of the position of lens L7 and the
phase
~o mask displacement. FIG. 5C shows the dispersion measured en each channel in
comparison with the targeted values., It is clear from this example that the
present
CA 02377208 2002-03-18
16
invention allows a precise and broad adjustment of both the Bragg wavelength
and
the chirp of the grating.
In summary, the present invention advantageously allows the adjustment of
the Bragg wavelength over a large range, while avoiding slanted fringes inside
the
s grating. Some of the advantages of the invention as described in the
embodiments
above are:
~ a smaller number of phase masks is required for fabricating any grating
within
a specified period range. Phase masks are typically expensive;
~ Mass production is easier because of less frequent phase mask changes;
to ~ It is made practical to make superimposed gratings having different Bragg
wavelengths. This operation was previously made possible by stretching of the
fiber, but this technique is applicable over a more limited wavelength, range.
Of course, numerous modifications could be made to the embodiments
described above without departing from the scope of the invention as defined
in
is the appended claims.
