Note: Descriptions are shown in the official language in which they were submitted.
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METHOD AND APPARATUS-FOR SPECTRALLY DESIGNING
ALL-FIBER FILTERS
BACKGROUND OF THE INVENTION
a) field of the invention
The present invention belongs to the field of photoinduced
gratings in optical media, and more particularly relates to a
method and an apparatus for photoinducing gratings having a
spatially variable and controllable diffraction efficiency and
average local index change.
b) brief description of prior art
It is now well established that Wavelength Division
Multiplexing (WDM) systems using Erbium Doped Fiber Amplifiers
(EDFA) will be the next enabling technology to access the huge
optical fiber bandwidth. In those type of systems, all-fiber
wavelength selective devices such as bandpass filters, gain
flattening filters for EDFAs, dispersion compensators, and
filters with any spectral shape and fine tuning of the nominal
wavelength of Bragg gratings will be required.
Photosensitivity in optical_ fiber can be used to fabricate
wavelength selective devices, since it allows to change the
refractive index in the core of the optical fiber. This is done
by illuminating the core with W light. Such refractive index
change is permanent and can be successfully used to .f_abricate
Bragg gratings to act as bandpass filters, chirped Bragg
gratings to make dispersion compensators, and spectrally
designed all-fiber filters.
Figure 1a to if (identified as "prior art") illustrate
various types of modulated refractive index changes and their
resulting reflectivity responses. It is well known that Bragg
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gratings having a uniform index modulation 7, as shown in
figure la, exhibit sidelobes 9 on both sides of the main
reflection peak 11 (figure 1b). Those sidelobes 9 are
undesirable because they induce crosstalk between adjacent
channels in WDM systems. It has been shown that those sidelobes
can be suppressed if the coupling efficiency varies spatially
along the grating length, as illustrate in figures lc and 1d.
This operation, called apodization, is ideally achieved by
photoimprinting an index change amplitude modulation 13 that
has a bell-like shape along the grating length. However, such
apodized gratings present a fine structure 15 on the short
wavelength side of their reflection response and lead to an
undesirable chirp of the Bragg wavelength. The variation of the
average index change causes the local Hragg wavelength at the
center of the grating to be longer than the local Bragg
wavelength at both ends; the grating then acts as a Fabry-Perot
cavity. In order to get rid of those short wavelength
resonances, the average refractive index has to be compensated
so that the Bragg resonance i.s uniform along the whole grating
length. Figures 1e and if show an example of the shape such a
refractive index change 17 might take, and the resulting
dispersion-free reflective response.
A number of techniques have been developed to produce
'gratings having a constant refractive index change. For
2.5 example, a double exposure method is disclosed in MALO, H. et
al.,"Apodised in-fiber Bragg grating reflectors photoimprinted
using a Phase Mask", Electronics Letters, vol 31, no 3, pp.
223-225 (1995). As implied by its name, this technique requires
that the optical medium be exposed to the writing light twice:
once to produce the modulated refractive index change, and
another time with a shadow mask to compensate for nonunifor_m
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variations in the average index. Phase masks with a locally
varying diffraction efficiency have also been developed (see
ALBERT, J. et al.,"Apodisation of the Spectral Response of
Fiber. Hragg Gratings using a Phase Mask with Variable
Diffraction Efficiency", Electronics Letters, vol 31_, no 3, pp
222-223 (1995)), and systems to move the fiber and phase mas)c
during the exposure have been proposed for pure apodization.
In one such system described in COLE, M.J. et al., "Moving
Fiber/Phase Mask-Scanning Heam Technique for Enhanced
Flexibility in Producing Fiber Gratings with Uniform Phase
Mask", a small dither is applied to the optical fiber while the
phase mask is kept fixed. If t:he magnitude of the dither is
half of a grating pitch, the net result is a DC index change.
If apodization is required, the magnitude of the dither is
changed accordingly along the exposed fiber length. However,
because the fiber is dithered, this writing system is very
sensitive to external perturbations.
Although the above-mentioned techniques usually produce
very good apodized gratings, they require some post-processing,
sophisticated phase mas)cs or elaborate setups, making them
unpractical for large-scale production. There is therefore a
need for a simple and flexible technique to photoinduce
apodized Bragg gratings or spectrally designed filters in
optical fibers or other types of optical waveguides, while
allowing to control at will the average index change over all
the W exposed region.
OBJECT AND SUMMARY OF THE INVENTION
Accordingly, an object of the present invention is to
provide a simple and flexible method to photoinduce a modulated
refractive index change in an optical medium.
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Another object of the present invention is to provide
such a method which produces a refractive index change having
both a variable intensity profile and a controlled average
value.
Yet another object of the present invention is to provide
such a method that does not require post-processing,
sophisticated phase mask or elaborate setups.
A further object of the present invention is to provide
an apparatus for implementing said method.
More particularly, the present invention provides a method
to photoinduce a modulated refractive index change in a length
of an optical medium. The optical medium is photosensitive and
has at least one waveguiding axis. The modulated refractive
index change has both a variable intensity profile and a
controlled average value along the waveguiding axis. The method
comprises steps of:
- performing consecutive writing steps, each of the
writing steps having an exposure time selected to generate the
controlled average value of the modulated refractive index
change, each writing step comprising substeps of:
a) exposing for the entire exposure time a segment
of the optical medium to a writing radiation beam having
an angle of incidence on the segment of the optical medium
that is generally perpendicular to the waveguiding axis,
the writing radiation beam also having an interference
pattern generated therein to produce the modulated
refractive index change; and
b) dithering the angle of. incidence of the writing
radiation beam for a fraction less than 1 of the exposure time to blur
the interference pattern and partially replace the
modulated refractive index change by a DC refractive index
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change, the fraction of the exposure time being selected
for each writing step to define the variable intensity
profile of the modulated refractive index change;
- translating the optical medium along the waveguiding
axis between each writing step to expose a different segment
of the optical medium at each writing step; and
- performing a sufficient number of writing steps and
translating steps to cover the length of the optical medium.
The present invention further provides a method to
photoinduce a modulated refractive index change in a length
of an optical medium, the optical medium being
photosensitive and having at least one waveguiding axis,
the modulated refractive index change having a variable
intensity profile and a controlled average value along the
waveguiding axis, the method comprising steps of:
- performing consecutive writing steps, each of the
writing steps having an exposure time selected to generate
the controlled average value of the modulated refractive
index change, each writing step comprising substeps of:
a) exposing for the entire exposure time a
segment of the optical medium to a writing radiation
beam having an angle of incidence on the segment of
the optical medium that is generally perpendicular to
the waveguiding axis, the writing radiation beam also
having an interference pattern generated therein to
produce the modulated refractive index change; and
b) dithering the angle of incidence of the
writing radiation beam for a fraction less than 1 of
the exposure time to blur the interference pattern and
partially replace the modulated refractive index
change by a DC refractive index change, the fraction
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of the exposure time being selected for each writing
step to define the variable intensity profile of the
modulated refractive index change;
- translating the optical medium along the
waveguiding axis between each writing step to expose a
different segment of the optical medium at each writing
step; and
- performing a sufficient number of writing steps and
translating steps to cover the length of the optical
medium;
wherein said substep a) comprises providing a phase
mask proximate the optical medium and generally parallel to
the waveguiding axis, the phase mask generating the
interference pattern; and
wherein each translating step comprises translating
the phase mask jointly with said optical medium.
The present invention also provides a method to
photoinduce a modulated refractive index change in a length
of an optical medium, the optical medium being
photosensitive and having at least one waveguiding axis,
the modulated refractive index change having a variable
intensity profile and a controlled average value along the
wavegiding axis, the method comprising steps of:
- performing consecutive writing steps, each of the
writing steps having an exposure time selected to generate
the controlled average value of the modulated refractive
index change, each writing step comprising substeps of:
a) exposing for the entire exposure time a
segment of the optical medium to a writing radiation
beam having an angle of incidence on the segment of
the optical medium that is generally perpendicular to
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the waveguiding axis, the writing radiation beam also
having an interference pattern generated therein to
produce the modulated refractive index change; and
b) dithering the angle of incidence of the
writing radiation beam for a fraction less than 1 of
the exposure time to blur the interference pattern and
partially replace the modulated refractive index
change by a DC refractive index change, the fraction
of the exposure time being selected for each writing
step to define the variable intensity profile of the
modulated refractive index change;
- translating the optical medium along the
waveguiding axis between each writing step to expose a
different segment of the optical medium at each writing
step; and
- performing a sufficient number of writing steps and
translating steps to cover the length of the optical
medium;
wherein substep b) of each writing step comprises
providing a mirror in a path of the writing radiation beam
and dithering said mirror to generate the dithering of the
angle of incidence of the writing radiation beam.
The present invention also provides an apparatus for
photoinducing a modulated refractive index change in a length
of an optical medium, the optical medium being photosensitive
and having at least one waveguiding axis. The modulated
refractive index change has a variable intensity profile and
a controlled average value along the waveguiding axis.
The apparatus comprises an optical source for producing
a writing radiation beam, the writing radiation beam being
directed on the optical medium at an angle of incidence
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generally perpendicular to the waveguiding axis. The apparatus
additionally comprises modulating means for generating an
interference pattern in the writing radiation beam. The
interference pattern generates the modulated refractive index
change in the optical medium. Controllable dithering means are
also comprised and include a mirror disposed in a path of
the optical writing beam, for dithering the angle of
incidence of the writing radiation beam. The dithering
blurs the interference pattern and partially replaces the
modulated refractive index change by a DC refractive index
change. The apparatus further comprises controllable
translating means, for translating the optical medium along
the waveguiding axis.
Advantageously, the present invention may be used to
produce a number of wavelength selective devices, such as
bandpass filters, dispersion compensators or spectrally
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designed all-fiber filters. No post-processing needs to be
performed, and sophisticated ptrase masks or elaborate setups
are not necessary. The method described herein rrray be fully
automated, and produces pure apodization.
The present invention and its advantages will be better
understood upon reading the following non restrictive
description of a preferred embodiment thereof, made with
reference to the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
Figures la to if (prior art) are graphic representations
of modulated refractive index changes and the corresponding
reflectivity response of the medium; figures la and 1b concern
a uniform index change, figures lc and 1d an apodized index
change, and 1e and if an apodized index change of uniform
average value;
figure 2 is a graphic representation of the variation of
the minimum deviation angle of the dithering with respect to
the distance between the phase mask and the optical fiber, when
use is made of an apparatus according to the present invention;
figure 3 is a schematic view or an apparatus used to
photoinduce a modulated refractive index change in an optical
fiber according to the present invention;
figure 4a is a graphic representation of the transmission
of a Bragg grating fabricated according to prior art; figure
4b is the same for a grating fabricated according to the
present invention;
figure 5 is a schematic view of an apparatus comprising
an optical medium wherein a refractive index change according
to the present invention may be applied; and
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figures 6a and 6b are graphic representations of the
response of a gain flattening filter fabricated according to
the present invention, before and after packaging and thermal
annealing.
DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION
According to the present invention, a method to
photoinduce a modulated refractive index change in a length of
an optical medium is provided. In the preferred embodiment
described hereinafter, optical fiber is the optical medium of
choice, but it should be understood that the present method
could be applied to photoimprint a refractive index change in
other types of waveguiding medium, as long as it exhibits
photosensitive characteristics.
The present method allows to produce a modulated
refractive index change having both a variable intensity
profile and a controlled average value along the waveguiding
axis of the optical medium. The method comprises performing a
series of consecutive writing steps, each having a
predetermined exposure time,. In. a preferred embodiment of the
invention wherein a constant average value of the modulated
refractive index change is desired, the chosen exposure time
is the same for all the writing steps. In this manner, the
amount of writing radiation incident on the optical medium is
also the same for each writing step, which ensures that the
average value of the modulated refractive index change is
constant throughout the exposed region.
Each writing step comprises a first substep a) of
exposing for the entire exposure time a segment of the optical
medium to a writing radiation beam. The writing radiation beam
has an angle of incidence on the segment of the optical medium
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generally perpendicular to the wavegui.ding axis, and has an
interference pattern generated therein to produce the modulated
refractive index change. This interference pattern is generally
generated by a phase mask disposed in the path of the optical
writing beam, proximate the optical medium. Phase masks are
fairly common tools in the manufacture of Hragg gratings in
optical fiber. They are usually designed to maximise the
contrast of the -1 and -i-1 diffraction orders which, when
superposed, generate interference fringes constituting the
required interference pattern. For this condition to be
realized, the depth of the grooves of the phase mask must be
chosen to correspond to an optical path difference of n radians
at the wavelength of the writing beam. It is possible to modify
the groove~depth or width or the period of the mask grating to
locally change the fringe contrast and therefore the properties
of the photoinduced refractive index change, but, for the
present invention, no sophisticated design is necessary; a
uniform phase mask is sufficient to realize the method
described herein.
Each writing step comprises a second substep b) of
dithering the angle of incidence of the writing radiation beam
for a fraction of the exposure time. The effect of this
dithering is to blur the interference pattern photoinduce in
the optical medium. The net result is to partially replace the
modulated refractive index change by a DC refractive index
change. In this manner, by choosing an appropriate fraction of
the exposure time during which t:he writing beam is dithered for
each writing step, any profile of both the intensity of the
modulated refractive index change and its average value may be
defined independently. Of course, the fraction of the exposure
time may have any value between 0 and 7., depending on the
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desired result of each independent step. To generate an
apodized Bragg grating, each st=ep is given the same exposure
time to keep the average index change constant and a bell-like
shape is preferably given to the intensity profile by a
judicious choice of the fraction of this time during which the
writing beam is dithered. Various other forms may be given to
tlne intensity profile in view of the desired filtering effect
of the resulting structure.
The method according to the present invention also
comprises translating the optical medium along the waveguiding
axis between each writing step, to each time expose a different
segment of the optical medium. Long gratings can therefore be
manufactured in the optical medium. If a phase mask is provided
to generate the interference pattern, then the method
preferably comprises translating the phase mask jointly with
the optical medium.
The method finally comprises performing a sufficient
number of writing steps and translating steps to cover the
length of the optical medium.
The dithering of the writing radiation beam of substep b)
may be simply realized by providing a mirror in the path of the
writing beam, and giving this mirror a small oscillatory
movement when required. Preferably, the writing beam is given
a movement equal to or greater than a minimum deviation angle
6nlin. For a grating photoinduced in an optical fiber with the
help of a phase mask, emin may be def fined in degrees by the
equation:
1018 U d
8 - f _ i+ a
mjn 9wn n n
o i z
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where A is a pitch of the phase mask, n0, n1 and n2 are
respectively refractive indices of the phase mask, the air and
the fiber cladding, dl is the distance between the phase mask
and the fiber and d2 is the radius of the fiber cladding. A
5 dithering according to this deviation angle allows to
completely blur the fringes of the interference pattern,
therefore generating a DC component in the index change in the
fiber. For standard communication fiber and a phase mask pitch
of 1 Vim, appropriate for a writing radiation beam as for
l0 example produced by a Q-switched Nd:YAG laser quadrupled at 266
nm, the minimum deviation angle with respect to the distance
dl between the phase mask and the fiber is shown in figure 2.
For a phase mask held in contact with the fiber (dl=0), this
angle is about ~0.23°. Of course, the deviation angle of the
1~~ dithering should not be increased unnecessarily to avoid
exposure of a segment of fiber adjacent to the one targeted by
the current writing step.
The present invention concerns both the above-mentioned
method and an apparatus 19 to perform it. An example of such
an apparatus 19 is shown in figure 3. The optical medium in
which a modulated refractive index change is to be induced is
an optical fiber 21, having a core 23 and a fiber cladding 25.
Light is to be guided inside the fiber 21 according to a
waveguiding axis 27.
The apparatus 19 first comprises an optical source 29,
which produces a writing radiation beam 31. The holographic
writing process requiring a coherent beam of light, the source
29 is preferably a laser source. The writing radiation beam 31
is directed on the fiber 21 at an angle of incidence generally
perpendicular to the waveguiding axis 27, as shown in figure
3.
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Modulating means are also provided in the apparatus 19,
for generating an interference pattern in the writing radiation
beam 31. This interference pattern generates the modulated
refractive index change in the optical fiber 21. In the
preferred embodiment of figure 3, the modulation means comprise
a phase mask 33 in proximity of the fiber 21, and oriented
generally parallel to the waveguiding axis 27. As explained
above, a simple uniform phase mask is appropriate for use in
the present apparatus.
l0 The apparatus 19 according to the present invention
additionally comprises controllable dithering means for
dithering the angle of incidence of the writing radiation beam
31 on the optical fiber 21. t~Tith the phase mask 33 or other
modulation means in the path of the writing radiation beam 31,
the effect of the dithering of angle of incidence of the beam
31 is to blur the interference pattern, and partially replace
the modulated refractive index change by a DC refractive index
change. In this manner, the amount of radiation incident on the
optical fiber 21 may be controlled independently of the
modulation pattern.
The controllable dithering means preferably comprises a
mirror 35, disposed in a path of the optical writing beam 31.
This mirror 35 is preferably mounted on a galvanometer 37,
which is modulated when the dithering is required. A motorized
rotation stage or any other device allowing a controlled
oscillatory movement may also be used. Additional optical
elements may be provided as a matter of course, for example a
cylindrical lens 39 to focus the writing radiation beam and a
slit 41 to limit the beam width.
The apparatus 19 further comprises controllable
translating means, for translating the optical fiber 21 along
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the waveguiding axis 27. In thi:~ manner, the optical fiber 19
may be exposed to the writing radiation beam 31 only a segment
at a time. When a phase mask is comprised, as is the case in
the embodiment of figure 3, this phase mask 33 is preferably
translatable by the translating means jointly with the optical
fiber 21.
Ae a demonstration of the present invention, two apodized
fiber gratings were realized in optical fiber, using
respectively a prior art technique and the above-described
method and apparatus. For both gratings, a Q-switched Nd:YAG
laser quadrupled at 266 nm was used as an optical source,
producing 7 ns pulses of 2 mJ at a 5 Hz repetition rate. The
interference pattern was generated by a uniform phase mask
having a pitch of about 1 ~m he:ld at a distance of 50 ~.cm from
the fiber. IIl both cases the optical fiber was of a standard
type used in telecommunications, having a cladding radius of
62.5 ~Cm. It has a high Ge02 content and was previously
sensitized by high pressure hydrogen loading.
The prior art grating was written by translating step-by
step the assembly formed by the optical fiber and the uniform
phase mask in front of the W writing beam. For each step, the
exposure time was set in order to have an index change
amplitude modulation having a bell-like shape. The total light
intensity incident on the fiber is therefore different from one
step to the other. The total length of the grating is 2.75 mm,
written with 125 ~m long steps.
The second grating was photoinduced using the same laser
source and phase mask, but this time the method according to
the present invention was applied. Each writing step was
performed for the same exposure time of 1 minute. A W mirror
mounted on a galvanometer wag provided in the path of the
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writing beam, and this mirror wa.s dithered for an appropriate
fraction of the exposure time at each step to give the index
change amplitude modulation a bell-like shape. The dithering
deviation angle was of about t0.1°, which is sufficient to
completely blur the interference fringes as suggested by figure
2 for d1~50~m. Each step was once again 125 ~m long, and the
resulting grating is also of 2.'75 mm.
Figures 4a and 4b show the transmission characteristics
for the respective resulting gratings. For the prior art method
1.0 as evident from figure 4a, the grating has a transmission peak
43 having full width at half maximum of 1.1 nm, agreeing with
the predicted value in these conditions. As expected, an
undesirable fine structure 45 is apparent on the short
wavelength side, associated with the non-uniform average value
1.5 of the index change. On figure 4b, it can be seen that the
grating photoinduced according to the present invention has a
smaller transmission peak 47 of full width at half maximum
equal to 0.71 nm, also in agreement with predicted values. The
features of figure 4a associated with the non-uniform average
20 value of the index change have almost vanished. Those results
clearly demonstrate that the present invention allows to
greatly improve the frequency response of Bragg gratings.
To show the flexibility of the method according to the
present invention, it was used to spectrally design gain
25 flattening filters which could be incorporated in the middle
of a two-stages EDFA, such as shown in figure 5. It comprises
a gain flattening filter 49 as designed with the present
technique, photoinduced in an optical fiber between the two
stages 53 and 55 of the Erbium-doped fiber amplifier. Both
30 fiber sections 53 and 55 are pumped by appropriate pumping
sources 57 and 59. Isolators 61, 63 and 65 are provided. This
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amplifier configuration can be designed to optimize
simultaneously the gain, noise figure and saturation output
power even if a lossy element is inserted between the two
stages.
In accordance with the present invention, the filter 49
is written by translating discretely a phase mask and optical
fiber assembly in front of a W writing beam, with translation
steps of 400 Vim. For this particular application, a chirped
phase mask is used. With this phase mask, a particular position
corresponds to a particular wavelength photoinduced. The
writing source was the same as previously described. For each
step, the total exposure time was of 30 seconds and when a DC
index change was required, proportional dithering of the
writing beam was performed. The same amount of radiation thus
reaches the fiber core at all times so that the average index
is kept constant all along the grating length. The linearly
chirped phase mask used has a period of 1.0659 ~Cm and a chirp
of 11.6 nm/cm. The fiber wherein the grating was photoimpri.nted
has a high Ge02 content and was previously sensitized by high
pressure hydrogen loading at 1.00 atm for 10 days . The total
length of the grating is about 2 cm.
The shape of the target filter is presented in figure 6a,
and takes into account the facts that the obtained filter will
be subsequently packaged and thermally annealed. This shape has
been obtained by considering the gain of the EDFA in a certain
WDM configuration, which includes such factors as the number
of channels, input power and the channel spectral distribution.
The grating was written using a multilayer approach, that
is with multiple passes. This approach is advantageous to
control in real-time the writing process. Figure 6a shows the
obtained filter before packaging and thermal annealing were
CA 02230200 1998-03-27
performed. This filter is accurate within 10.25 d8 from 1530
to 1565 rm. The small ripples observed in the experimental data
are caused by the step-by-step writing process which creates
small discontinuities along the grating length. In a chain of
5 cascade optical amplifiers, those ripples will not cause any
dispersion penalty because no correlation is observed from one
grating to another.
Figure 6b shows the same filter after packaging and
thermal annealing for 8 hours at 100°C. As can be seen, the
10 packaging and annealing processes can be well controlled so
that the obtained filter is still )cept within ~0.25 dB of the
targeted values from 1530 to 1565 rm.
As amply illustrated hereinabove, the method according to
the present invention ie a simple and flexible technique to
15 spectrally design fiber Bragg gratings. This technique does not
require post-processing, sophisticated phase mask or elaborate
setups, and can be fully automated. Pure apodization without
any Fabry-Perot effects may be achieved. Moreover, if a means
to broaden the frequency response of the grating is used, such
as a linearly chirped phase mask, all-fiber filters with any
spectral shape can be designed. In the above-presented example
of such a filter, the wavelength range covered was only limited
by the length of the available linearly chirped phase mask.
Finally, with minor modifications, the present method could
also be implemented with other types of fiber grating writing
techniques, such as two-beam interferometric exposure.
Of course, numerous modifications could be made to the
preferred embodiment disclosed hereinabove without departing
from the scope of the invention as defined in the appended
claims.
