U.S. patent number RE39,865 [Application Number 09/342,707] was granted by the patent office on 2007-10-02 for method of fabricating bragg gratings using a silica glass phase grating mask and mask used by same.
This patent grant is currently assigned to Her Majesy in Right of Canada as Represented by the Minister of Communications, N/A. Invention is credited to Francois C. Bilodeau, Kenneth O. Hill, Derwyn C. Johnson, Bernard Y. Malo.
United States Patent |
RE39,865 |
Hill , et al. |
October 2, 2007 |
Method of fabricating Bragg gratings using a silica glass phase
grating mask and mask used by same
Abstract
An index grating is imprinted in the core of an optical fiber
using a specially designed silica glass phase grating mask. The
phase mask is held in close proximity to the optical fiber. Laser
irradiation of the phase mask with ultraviolet light at normal
incidence imprints (photoinduces) into the optical fiber core the
interference pattern created by the phase mask.
Inventors: |
Hill; Kenneth O. (Kanata,
CA), Malo; Bernard Y. (Gatineau, CA),
Bilodeau; Francois C. (Nepean, CA), Johnson; Derwyn
C. (Nepean, CA) |
Assignee: |
Her Majesy in Right of Canada as
Represented by the Minister of Communications (Ottawa,
CA)
N/A (N/A)
|
Family
ID: |
38535982 |
Appl.
No.: |
09/342,707 |
Filed: |
June 29, 1999 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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07811299 |
Dec 20, 1991 |
5216739 |
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07656462 |
Feb 19, 1991 |
5104209 |
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Reissue of: |
07969774 |
Oct 29, 1992 |
05367588 |
Nov 22, 1994 |
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Current U.S.
Class: |
385/37; 359/569;
359/900; 385/130; 385/147; 438/32; 385/14; 385/129; 359/573;
359/566 |
Current CPC
Class: |
Y02P
70/50 (20151101); Y02P 70/521 (20151101) |
Current International
Class: |
G02B
6/34 (20060101); G02B 6/12 (20060101); H01L
31/18 (20060101) |
Field of
Search: |
;385/14,37,129,130,131,147 ;359/558,559,562,566,569,571,573,576,900
;437/51 ;438/31,32 ;430/4 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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0175460 |
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Mar 1986 |
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EP |
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0220652 |
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May 1987 |
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EP |
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0271002 |
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Jun 1988 |
|
EP |
|
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Primary Examiner: Healy; Brian M.
Attorney, Agent or Firm: Antonelli, Terry, Stout &
Kraus, LLP.
Parent Case Text
.Iadd.CROSS REFERENCE TO RELATED APPLICATION
This application is a continuation-in-part application of U.S.
application Ser. No. 07/811,299, filed Dec. 20, 1991, now U.S. Pat.
No. 5,216,739 which is a continuation-in-part application of U.S.
application Ser. No. 07/656,462, filed Feb. 19, 1991, now U.S. Pat.
No. 5,104,209..Iaddend.
Claims
We claim:
1. A method of fabricating Bragg gratings in the interior of an
optical waveguide comprising disposing a silica glass phase grating
mask adjacent and parallel to a photosensitive optical waveguide
and applying a single collimating light beam through the mask to
said .Iadd.optical waveguide as a .Iaddend.medium.
2. A method as defined in claim 1 in which the mask has a surface
relief pattern selected to modulate by approximately .pi.+2 .pi.n
radians the phase of the light beam, wherein
.times..pi..function..times..lamda..pi..times..pi. ##EQU00003##
where A is the amplitude of the surface relief pattern, n=0,1,2,3,
.lamda. is the wavelength of the light used for writing
(photoinducing) an index change in the optical medium and
n.sub.silica is the refractive index of the silica used in the mask
at .lamda..
3. A method as defined in claim 2 in which the surface relief
pattern in cross-section is a square-wave.
4. A method as defined in claim 2 in which the surface relief
pattern in cross-section is a sine wave.
5. A method as defined in claim 1 in which the light beam is an
ultraviolet light beam.
6. A method as defined in claim 5 in which the light beam is a
laser beam.
7. A method as defined in claim 1 in which the light beam is
provided by a KrF excimer laser.
8. A method as defined in claim 1 in which the optical medium is an
optical fiber.
9. A method as defined in claim 8 in which striations of the phase
mask grating are oriented orthogonal to or nearly orthogonal to the
axis of the fiber.
10. A method as defined in claim 8 in which striations of the phase
mask grating are oriented at an angle to the axis of the fiber.
11. A method as defined in claim 9 in which the light beam is
provided by a KrF excimer laser.
12. A method as defined in claim 11 in which the mask has a surface
relief pattern selected to modulate by approximately .pi.+2 .pi.n
n=0,1,2,3, radians the phase of the light beam, wherein
.times..pi..function..times..lamda..pi..times..pi. ##EQU00004##
n=0,1,2,3 where A is the amplitude of the surface relief pattern,
.lamda. is the wavelength of the light and n.sub.silica is the
refractive index of the silica material used to make the mask at
.lamda..
13. A method as defined in claim 12 in which the surface relief
pattern in cross-section is a square-wave.
14. A method as defined in claim 13, in which a large dimension of
the light beam cross-section is oriented parallel to striations of
the phase mask grating.
15. A method as defined in claim .[.8.]. .Iadd.10 .Iaddend.in which
the striations are chirped.
16. A method as defined in claim 9 in which the striations are
chirped.
17. A method as defined in claim 9 in which the phase mask contains
variations in either or both of pitch and amplitude of the
striations.
18. A method as defined in claim 11 in which the light beam is an
ultraviolet beam.
19. A method as defined in claim 1 including locating a refracting
lens between the mask and the optical medium prior to applying the
light beam.
20. A method as defined in claim 19 including placing an opaque
blocking means for the zero order light beam between the mask and
the lens prior to applying the light beam.
21. A method as defined in claim 19 including placing opaque
blocking means for the zero and second order light beams between
the mask and the lens prior to applying the light beam.
22. A method as defined in claim 1 further including a spatial
amplitude light filter for shaping the beam profile prior to
passing through the phase grating.
23. A method as defined in claim 22 in which the filter is coated
on a face of the mask opposite to a face containing the phase
grating.
24. A method as defined in claim 2, then repeatedly moving one of
the mask and medium relative to the other a distance corresponding
to the fringe pattern and applying said collimating light beam
through the mask to said medium, such that subsequent
photoimprinted gratings reflect in phase with previously
photoimprinted gratings.
25. A grating mask comprising a slab of silica glass having
parallel corrugations on a surface thereof forming a surface relief
pattern, the pattern containing variations in at least one of pitch
and amplitude of the corrugations.
26. A grating means comprising a slab of silica glass having
parallel corrugations on a surface thereof forming a surface relief
pattern, including a spacial amplitude light filter coated on a
surface of the slab opposite to the surface carrying said
pattern.
27. A grating as defined in claim 25 further including a spacial
amplitude light filter coated on a surface of the slab opposite to
the surface carrying said pattern.
28. A grating means comprising a slab of silica glass having
parallel corrugations on a surface thereof forming a surface relief
pattern, in which the corrugation are filled with transparent
material having an index of refraction different from that of the
silica glass.
29. A grating as defined in claim 28 in which the transparent
material is comprised of glass.
.Iadd.30. A method of fabricating Bragg gratings in the interior of
an optical waveguide comprising disposing a silica glass phase
grating mask adjacent and parallel to a photosensitive optical
waveguide and applying a single collimating light beam through the
phase grating to said optical waveguide as a medium, said phase
grating mask being a slab of silica glass having parallel
corrugations on a surface thereof forming a surface relief
pattern..Iaddend.
.Iadd.31. A method according to claim 30, wherein the surface
relief pattern contains variations in at least one of pitch and
amplitude of the corrugations..Iaddend.
.Iadd.32. A method of fabricating Bragg gratings in the interior of
an optical waveguide comprising disposing a silica glass phase
grating mask adjacent and a parallel to a photosensitive optical
waveguide and applying a single collimating light beam through the
phase grating mask to said optical waveguide as a medium, wherein
the phase grating mask is configured so as to substantially
suppress at least one portion thereof a zero-order diffracted light
beam of the light beam which passes through the phase grating
mask..Iaddend.
.Iadd.33. A method according to claim 32, wherein the zero-order
diffracted light beam is suppressed to less than 5% of the light
diffracted by the phase grating mask..Iaddend.
.Iadd.34. A method according to claim 32, wherein plus one and
minus one orders of the diffracted light beam are utilized for
fabricating the Bragg gratings in the interior of the optical
waveguide..Iaddend.
.Iadd.35. A method according to claim 32, wherein the Bragg
gratings are substantially permanently fabricated in the interior
of the optical waveguide..Iaddend.
.Iadd.36. A method according to claim 35, wherein the optical
waveguide is an optical fiber..Iaddend.
.Iadd.37. A method according to claim 32, wherein the phase grating
mask is configured so as to induce different phase in at least two
adjacent portions of the light beam which passes through the phase
grating mask..Iaddend.
.Iadd.38. A method according to claim 32, wherein the phase grating
mask has a surface relief pattern formed thereon..Iaddend.
.Iadd.39. A method according to claim 35, in which the surface
relief pattern in cross-section is a square-wave..Iaddend.
.Iadd.40. A method according to claim 35, in which the surface
relief pattern in cross-section is a sine wave..Iaddend.
.Iadd.41. A method according to claim 38, wherein the phase grating
mask has the surface relief pattern selected to modulate by
approximately .pi.+2 radians the phase of the light beam, wherein
.times..pi..function..times..lamda..pi..times..pi..times.
##EQU00005## where A is the amplitude of the surface relief
pattern, n=0, 1, 2, 3, .lamda. is the wavelength of the light used
for writing (photoinducing) an index change in the optical medium
and n.sub.silica is the refractive index of the silica used in the
mask at .lamda...Iaddend.
Description
FIELD OF THE INVENTION
This invention relates to optical media such as optical fibers, and
particularly to a method for fabricating Bragg
gratings-therein.
BACKGROUND TO THE INVENTION
Certain optical fiber waveguides exhibit the property of
photosensitivity which provides a practical means for photoinducing
permanent refractive index changes in the core of those fibers.
Photosensitivity is not restricted to fiber structures: it has also
been detected in several types of planar glass structures,
including, for example, silica-on-silicon and ion-implanted silica
waveguides devices.
The fabrication of optical waveguide devices such as intra-mode
retro-reflecting Bragg gratings, mode convertor gratings, and
rocking rotators have been achieved. The general approach for
making these devices is to photoinduce a refractive index grating
in the photosensitive core of the optical waveguide. The grating
consists of a periodic modulation of the core's refractive index
along the length of the waveguide. The period of the perturbation
is chosen to bridge the momentum (propagation constant) mismatch
between the two (normally bound) modes that the grating is designed
to couple. At the resonant wavelength of the structure,
phase-matched, efficient, power exchange between the coupled modes
is possible.
There are two basic methods used for photoinducing gratings in
photosensitive optical fiber waveguides: either by internal or by
external writing. Internal writing is usually a holographic process
where the modes to be coupled are launched as coherent bound modes
of the waveguide and are allowed to modify, by a two-photon
absorption process the refractive index of the waveguide core (i.e.
form the hologram). Subsequent launching of one mode "reconstructs"
the other. The activation wavelength for writing gratings
internally in Germanium-doped high-silica glass is in the visible
band (for example, at the 514.5 and 488.0 nm Argon-ion laser
wavelengths) with corresponding two-photon energy in the U.V. band.
External writing uses UV light directly (for germanium doped
high-silica fiber, UV light tuned to, or in the vicinity of, the
oxygen vacancy absorption band at 240 nm) incident from the side on
the optical waveguide. External writing can be accomplished
point-by-point, for mode convertor gratings, or using the
holographic interference of two coherent UV beams for Bragg
retro-reflectors.
Index gratings were first written in optical fibers using a
technique described by K.O. Hill et al and disclosed in U.S. Pat.
No. 4,474,427. The process requires launching into the core of a
Ge-doped fiber strand light having a wavelength in the visible
region. The light is reflected from the end of the fiber. The
forward propagating light interferes with the backward propagating
light to form a standing wave pattern with a period corresponding
to half the wavelength of the writing light. Through a
photosensitive effect in the fiber, a refractive index grating with
this period is written in the core of the fiber. With this
technique, only gratings can be fabricated which reflect light
having wavelengths close to the writing light.
An improvement on this technique for writing grating has been
disclosed by Glenn et al in U.S. Pat. No. 4,807,950. In that
process, the gratings are produced in the fiber by illuminating the
fiber from the side with a coherent ultraviolet radiation having
245 nm wavelength. By using a two beam technique, an interference
pattern is set up along the length of the fiber. The period of the
pattern is controlled by controlling the angle between the
interfering beams. Thus index gratings can be written in the fiber
which will reflect light at much longer wavelengths.
A further improvement on the above-noted methods for writing
gratings in optical fibers is the point-by-point writing technique
which is disclosed in U.S. Pat. No. 5,104,209. In this patent, a
point-by-point technique for writing gratings in fibers is
disclosed in which each index perturbation in the grating is
photoinduced individually through a slit-mask.
The principal drawbacks of the grating fabrication technique
described in the first patent is that only gratings with a period
similar to that of one half the wavelength of the writing light can
be made. The second patent discloses a method of writing gratings
with a different pitch. However, the technique requires an
ultraviolet laser source with a high degree of spatial and temporal
coherence. Such laser sources are research lasers that are
expensive, have low writing efficiencies and are not suitable for
use in a manufacturing environment. Furthermore, the technique does
not provide full flexibility in the writing of apodized Bragg
reflectors or chirped Bragg reflectors.
The point-by-point writing method is an effective technique for
writing the coarse period gratings needed in spatial and
polarization mode converters. However, this technique is not
practical for writing Bragg gratings. In the case of Bragg
gratings, the writing of each index perturbation individually
requires high accuracy in the translation of the optical fiber in
front of the slit. A more serious drawback, is the serial manner
for writing the index perturbations forming the Bragg grating. That
writing process needs a very long exposure time to fabricate a
single Bragg reflector. U.S. Pat. No. 5,104,209 proposes to
overcome this problem by the use of slit-masks to permit the
writing of several index perturbations in a single operation.
SUMMARY OF THE INVENTION
In the present invention the index grating is imprinted in the core
of the optical fiber using a specially designed silica glass phase
grating mask. The phase mask is held in close proximity to the
optical fiber. Laser irradiation of the phase mask with ultraviolet
light at normal incidence imprints (photoinduces) into the optical
fiber core the interference pattern created by the phase mask.
The present invention improves on the point-by-point writing
technique by using a novel slit-mask for printing Bragg gratings in
optical fibers and planar optical waveguides. The method is a
non-holographic technique for writing Bragg retro-reflectors and is
particularly applicable to photosensitive optical fiber, but the
method applies as well to planar waveguide structures.
In accordance with an embodiment of the invention, a method of
fabricating Bragg gratings in an optical medium is comprised of
disposing a silica glass phase grating mask adjacent and parallel
to a photosensitive optical medium and applying a collimated light
beam through the mask to the medium.
In accordance with another embodiment a phase grating slit-mask is
used to modulate spatially the phase of a UV beam (for example,
from an excimer laser) with pitch .lamda. ##EQU00001## where
.lamda..sub.Bragg is the desired resonant wavelength for
retro-reflective intra-mode coupling in the fiber and
n.sub.Effective is the effective index of the coupled modes at
.lamda..sub.Bragg.
In accordance with another embodiment, a grating mask is comprised
of a slab of silica glass having parallel corrugations on a surface
thereof forming a surface relief pattern.
BRIEF INTRODUCTION TO THE DRAWINGS
A better understanding of the invention will be obtained by
reference to the detailed description below, in conjunction with
the following drawings, in which:
FIG. 1 is a diagram of photolithographic apparatus for
photo-imprinting a refractive index Bragg grating in a
photosensitive optical fiber waveguide,
FIGS. 2, 3, 4 and illustrate additional diagrams of
photolithographic apparatus for photo-imprinting a Bragg grating in
an optical fiber, and
FIG. 6 is a graph of spectral response of a Bragg grating
fabricated with a UV laser source and using phase-mask
photolithography.
DETAILED DESCRIPTION OF THE INVENTION
A phase grating slit-mask 1 is used in a precision
photolithographic apparatus and is placed in contact, or
near-contact, with an optical fiber 3, its grating striations 5 (as
illustrated in magnification 6 of the mask) directed normal or near
normal to the fiber axis. A UV light beam 7 from a suitable laser,
a KrF excimer laser (249 nm) in a successful prototype is passed
through the mask 1 by which it is phase modulated spatially and is
diffracted to form an interference pattern 9A laterally (Bragg
grating pitch) and along the incident laser beam direction 9B
(Talbot pitch) as illustrated in magnification 11 of the core of
the fiber.
The slit-mask preferably is comprised of a one dimensional
surface-relief structure as shown at 6 fabricated in a high quality
fused silica flat transparent to the KrF excimer laser radiation.
The shape of the periodic surface-relief pattern of the phase mask
preferably approximates a square wave in profile, as shown at 6.
The amplitude of the periodic surface-relief pattern is chosen to
modulate by .pi.+2.pi.n radians (n=0,1,2,3, . . . ) the phase of
the UV light beam. In a successful prototype of the phase mask for
a KrF excimer laser beam, the amplitude A of the surface relief
pattern is given by .times..pi..function..times..lamda..pi.
##EQU00002## where .lamda. is the wavelength of the light used for
writing (photoinducing) an index charge in the optical medium, and
n.sub.silica is the refractive index of the silica used in forming
the mask. This choice of surface-relief-grating amplitude results
in a grating diffraction pattern for the design wavelength that
nulls the zero-order diffracted (through) beam. In practice, the
zero-order beam 13 has been suppressed to less than 5% of the light
diffracted by the mask. The principal beams 15 exiting our mask are
the diverging plus-one and minus-one orders each of which contained
typically more than 35% of the diffracted light.
The mask perturbations need not be shaped to a square wave. For
example, zero-order nulled surface-relief phase-grating masks
having perturbations with a sinusoid shape would be equally useful
in our application.
To manufacture Bragg gratings with a length longer than the phase
mask, a stop and repeat process can be used. In this process, the
mask (or fiber) is translated precisely a distance corresponding to
the fringe pattern length such that subsequent photoimprinted
gratings reflect in phase with previously photoimprinted
grating.
It is worth noting that the principal period of the mask's
diffraction pattern is independent of wavelength. Therefore, in
principle, it is possible to write a Bragg grating with a
collimated broadband source, as long as the waveguide core is not
located too far away from the phase mask during writing and the
combined spectral width of the source and the photosensitive band
of the waveguide material is not broader than the nulling bandwidth
for the zero-order diffracted beam.
For photoimprinting Bragg reflectors, the preferred placement of
the phase grating striations is normal to the fiber axis. Tilted
index gratings can be photoimprinted by placing the phase grating
striations at an angle to the fiber axis. Such index grating
structures are useful for coupling core guided light out the fiber
and into free space.
It should be noted that the preferred embodiment has the grating
striations of the surface relief phase grating face the fiber. This
configuration is not necessary for the phase grating to phase
modulate the UV beam. A configuration with the striations facing
away from the fiber functions will also work.
Phase gratings in which the phase modulation is produced by a
refractive index modulation rather than a surface relief modulation
will function as well. For example, the striations 5 of the mask
shown in magnification 6 of FIG. 1 can be filled with glass
material having a different index of refraction than the
surrounding regions, producing alternating refractive indexes.
In order to test the performance of the photolithographic apparatus
we have described for photo-imprinting a refractive Bragg grating
in a photosensitive optical fiber waveguide, we selected two
optical fibers known to be highly photosensitive. The first was an
Andrew Corporation standard D-type polarization-maintaining fiber
optimized for 1300 nm (cut-off=960 nm, beat length L.sub.B=1.02 cm
at 1292 nm, core/cladding .DELTA.n=0.031 and elliptical core size
1.5.times.3.mu.m) but which nonetheless exhibits sufficiently low
loss at the Bragg resonant wavelength of 1531 nm that we use in our
experiments. This fiber has a core that is highly doped with
Germanium in comparison with standard telecommunications fiber;
Germanium-doped fibers are usually photosensitive. In particular,
Andrew D-type fiber has been reported to be strongly photosensitive
(we measured unsaturated photoinduced refractive index changes of
the order of 6.times.10.sup.-4), an attribute linked to its
relatively high Germanium dopant concentration. The second fiber
was obtained from AT&T Bell Laboratories and was specially
formulated to be strongly photosensitive. Our experiments confirmed
that the AT&T fiber was more photosensitive than the D-type
fiber for the conditions prevalent during our experiments.
The UV source we used in our experiments was an unmodified Lumonics
KrF excimer laser operated at 249 mn, with a beam cross-section
0.7.times.2 cm.sup.2, pulse duration 12 nsec and pulse repetition
rate of 50 Hz. The unfocused energy density per pulse was 100 mJ
per cm.sup.2. Such a laser produces a low coherence beam when no
provision is made either to injection-lock or to filter spatially
the beam. The only optimization we undertook in preparation for
writing a Bragg grating was to place the phase mask grating with
its striations parallel to the long dimension of the beam
cross-section, because we determined experimentally that the
spatial coherence is better for its placement than for an
orthogonal placement.
Successful writing with low coherence beams was an important test
of the performance of the present photolithographic Bragg grating
photo-imprinting method. An advantage of Bragg grating
photolithography is it provides a means for the fabrication of
several devices in parallel, permits the use of proven,
high-fluence, industrial laser sources and simplifies manufacturing
alignment procedures.
The phase grating that we used in the successful prototype method
was a surface-relief device manufactured on a high optical quality
fused silica flat. The period of the grating was approximately 1060
nm with 249 nm zero-order diffracted beam nulled below 5% and 37%
of transmitted light in each of the plus- and minus-one diffracted
beams. The grating measured 1 mm square. The period of the Bragg
grating photo-imprinted with the flat was 530 nm resulting in an
estimated high-fused-silica-waveguide (refractive index=1.46) Bragg
resonance wavelength of 530.times.2.times.1.46=1549 nm. We observed
experimentally a resonance at 1531 nm.
The fluence level per pulse used for Bragg grating exposure was
increased from 100 to 200 mJ/cm.sup.2 by mild focusing of the
excimer laser beam, using a cylindrical lens aligned with cylinder
axis parallel to the fiber. Typical exposure times are a few
minutes long with fluence levels of 1 joule/cm.sup.2 per pulse and
pulse rates of 50 ppc. The photoinduced Bragg grating reflectivity
increases rapidly at the beginning of the exposure process and then
saturates subsequently at a value related to the level of the
incident fluence per pulse. A larger fluence level per pulse
increases, to some limit, the saturated reflectivity of the ensuing
Bragg grating. However, above a certain fluence level, a peak in
reflectivity is obtained, and further photoexposure results in
decreased reflectivity and at the same time the shape of the
wavelength response of the Bragg grating changes significantly
developing, for example, a notch at the center wavelength of the
response.
It should be noted that a lens or lenses can be used for increasing
the fluence level. For example, an embodiment is shown in FIG.
2.
A light source 19 is focussed by a cylindrical lens 21 to the core
23 of optical fiber 25, through phase grating slit mask 27, which
has its face 29 containing the grating striations facing the
fiber.
In another embodiment of the invention shown in FIG. 3, a spatial
amplitude or filter 37 is disposed in front of the phase mask 27.
The spatial amplitude filter 37 modifies the intensity profile 39
of the UV beam so it varies along the length of the phase grating
in a predetermined way. The profile 39 indicates for example a UV
beam with gaussian intensity profile. Illumination of the phase
grating with the UV beam 19 results as before in an interface
pattern. The envelope of the interference fringes however has the
same intensity profile along the length the fiber as the intensity
profile of the UV beam. The regions of high intensity result in a
larger photoinduced refractive index change than the regions of low
intensity. Thus an index grating can be formed in a fiber whose
coupling strength varies in a predetermined way along the fiber
length.
In another embodiment of the invention shown in FIG. 4, a phase
grating 41 is used in which the grating period varies along the
length of the grating in a predetermined manner, to form a chirped
grating. Irradiation of the chirped phase grating with UV light 19
photoimprints an index grating (reflector) in the fiber core 23
that is also chirped. The spectral response of the chirped Bragg
reflector is broader than that of the Bragg reflector resonant at a
single wavelength.
Simultaneous use of the two embodiments shown in FIGS. 3 and 4
modulates spatially the phase and amplitude of the UV beam allowing
independent control respectively of the resonant frequency and the
strength of the resonance along the length of the Bragg reflector.
This capability permits the synthesis of useful spectral response
characteristics such as an apodized Bragg reflector.
FIG. 5 illustrates an embodiment of the invention in which the lens
is located between the mask 27 and the optical fiber 25. An opaque
blocker 33 is located between the mask and the lens to block the
zero order light beam 35. Opaque beam blockers 36 are also located
between the mask and the lens to block light diffracted into beams
higher than the first order. The plus one and minus one orders of
the light beam are passed through the lens.
The advantage of this embodiment is that only the first order
diffracted beams are used in forming the interference fringes and
high contrast fringes are obtained. A further advantage is that the
lens can be used for reducing the image size of the fringe pattern.
Thus lenses that produce different image reductions will
photoimprint Bragg gratings with different resonant frequencies
from the same phase grating. Also, the pitch of the phase grating
can be longer thus relaxing the difficulty in manufacturing the
phase grating. Finally, the lens provide a means for increasing the
fluence levels on the irradiated optical core.
FIG. 6 illustrates a spectral response graph 17 of a Bragg grating
photo-imprinted through the above-described phase mask in the
embodiment of FIG. 1 into the Andrew Corp. D-type fiber. The 249 nm
KrF excimer laser irradiated the fiber for 20 minutes with 100
mJ/cm.sup.2 pulses at 50 pps. A peak reflectivity of 16% was
achieved in a grating estimated to be approximately 0.95 mm long,
which is calculated assuming a uniform in-fiber grating and using
the 0.85 nm spectral width of the response, the 530 nm pitch of the
Bragg grating and the 1531 nm Bragg resonance wavelength. The
sidebands in the spectral response are clearly visible, suggesting
that the grating is substantially uniform along its entire length.
From the grating reflectivity data we calculated (in the tightly
bound mode limit) the amplitude for the refractive index modulation
to be 2.2.times.10.sup.-4. This value compares favourably with the
average refractive index change which we determined to be
6.times.10.sup.-4 from the shift in Bragg grating resonance during
photo-exposure and knowledge of the fiber's effective index
dispersion at 1531 nm. Ideally, we expect the apparent modulation
depth to be the same or larger than the average index change when
we expose fibers to maximum-contrast grating-diffraction patterns.
The depth-to-average-index change ratio is influenced by the
following intrinsic factors: non-linearities in the photosensitive
response of the fiber, the less-than-perfect nulling of the
zero-order beam, the presence of higher-order diffracted beams down
stream from the mask and the low coherence of the laser source. It
is also influenced by fiber/mask alignment during fabrication: the
reflectivity of the photoinduced Bragg grating will be reduced if
the grating is tilted with respect to the fiber axis. The low
reflectivity from tilting translates to a reduced apparent depth of
modulation of the refractive index. Tilting does not affect the
increase in the average refractive index due to photoexposure.
A Bragg grating spectral response similar to FIG. 6 was observed
for a Bragg grating written under similar conditions but using the
special AT&T fiber. In that case the peak reflectivity reached
25%.
In comparison to other methods of making in-fiber Bragg gratings,
the technique of photolithographic imprinting through the phase
mask as described herein offers much flexibility for modifying the
pitch and the strength of the Bragg grating coupling coefficient,
.kappa.(z), as a function of distance z along the waveguide axis.
Intricate variations in pitch can be written into the phase mask
during its fabrication e.g. under computer control; a spatial
amplitude mask can be used as well to grade the strength of the
coupling coefficient. Simultaneous use of these two techniques to
modulate spatially the phase and amplitude of the UV writing beam
allows independent control respectively of the resonant frequency
and the strength of resonance along the waveguide Bragg grating
written with the mask, making the synthesis of some useful spectral
responses possible.
The surface-relief phase grating masks used have been found to
tolerate fluence levels per pulse of 1 J/cm.sup.2 without damage.
Because fused quartz has a damage threshold per pulse of about 5
J/cm.sup.2 under KrF excimer laser irradiation, it seems likely
that these phase masks can tolerate even higher fluence levels.
Using a fluence level per pulse of 1 J/cm.sup.2 we photoimprinted a
Bragg grating with 30% reflectivity in Andrew D-fiber after a 5
minute 50 Hz exposure.
The present invention thus provides a simple method for fabricating
high-quality Bragg gratings in photosensitive optical waveguides,
using low coherence lasers suitable for industrial environments.
The combination of phase mask photo-imprinting with single-pulse
writing of in-fiber Bragg gratings could yield high-performance,
low-cost devices.
A person understanding this invention may now conceive of
alternative structures and embodiments or variations of the above.
All of those which fall within the scope of the claims appended
hereto are considered to be part of the present invention.
* * * * *