COMPACT WAVELENGTH FILTER INTEGRATED TO A SINGLE-MODE OPTICAL FIBER

FIBRE OPTIQUE COMPACT INTEGRE A UNE FIBRE OPTIQUE MONOMODE

English Abstract

ABSTRACT OF THE DISCLOSURE
A wavelength filter is integrated to a
single-mode optical fiber capable of propagating a
light signal and comprising a longitudinal and opaque
outer jacket enveloping the fiber. This fiber is
stripped of its jacket on a given length thereof to
obtain a non-jacketed length of optical fiber. The
so produced non-jacketed fiber length is formed with
first and second concatenated biconical tapers
separated from each other by a given distance and
each having a given profile. By appropriately
selecting the distance separating the two tapers and
the given profile thereof, the filter will enable
transmission of a first, predetermined wavelength of
the propagated light signal while it will stop a
second, predetermined wavelength of the signal. A
method of integrating such a filter to a single-mode
optical fiber is also proposed.

Claims

WHAT IS CLAIMED IS:
1. A wavelength filter integrated to a
single-mode optical fiber capable of propagating a
light signal and comprising a longitudinal outer
jacket enveloping the optical fiber, the said fiber
being stripped of its jacket on a given length
thereof to form a non-jacketed length of optical
fiber, said non-jacketed fiber length being formed
with first and second concatenated biconical tapers
separated from each other by a given distance and
each having a given profile, wherein the said given
distance and the given profile of each biconical
taper can be chosen to enable transmission through
the filter of a first, predetermined wavelength of
the propagated light signal while stopping a second,
predetermined wavelength of said signal.
2. The wavelength filter of claim 1, wherein
said first and second biconical tapers have identical
profiles.
3. The wavelength filter of claim 1, in which
the said first and second biconical tapers have
different profiles.
4. The wavelength filter of claim 1, wherein a
non-jacketed section of said optical fiber is
interposed between the first and second biconical
tapers.
5. The wavelength filter of claim 4, in which
21

said given distance separating the first and second
tapers is given by the following relation:
.delta. .beta. L = (2n + 1) .pi.
where:
- L is said given distance separating the
first and second tapers;
- .delta..beta. is a difference between propagation
constants of two light propagation modes beating in
said non-jacketed section of optical fiber interposed
between the first and second biconical tapers; and
- n is an integer.
6. The wavelength filter of claim 4, in which
normalized light power ? (.lambda.) transmitted through the
said filter at a given wavelength .lambda. is approximated
by the following relation:
<IMG>
cos (.delta..beta.L)
where:
- T(.lambda.) is the normalized light power
transmitted through the first taper at the wavelength
.lambda.;
- T'(.lambda.) is the normalized light power
transmitted through the second taper at the
wavelength .lambda.;
22

- R(.lambda.) = 1 - T(.lambda.)
- R'(.lambda.) = 1 - T'(.lambda.)
- .delta..beta. is a difference between propagation
constants of two light propagation modes beating in
said non-jacketed section of optical fiber interposed
between the first and second biconical tapers: and
- L is said given distance separating said
first and second concatenated tapers.
7. The wavelength filter of claim 6, in which
the terms T(.lambda.) and T'(.lambda.) are given by an expression
of the form:
1 - Ro sin2 <IMG>
where:
- Ro is the amplitude of the spectral
response of the corresponding taper;
- ? is the period of the wavelength
response; and
- .lambda. p is a reference wavelength at which
light power transmitted through the first and second
tapers, respectively, is maximum.
8. A method of integrating a wavelength filter
to a single-mode optical fiber capable of propagating
a light signal and comprising a longitudinal outer
jacket enveloping the optical fiber, said method
comprising the steps of:
.
23

(a) stripping said optical fiber of its
jacket on a given length thereof to form a non-
jacketed length of optical fiber;
(b) heating said non-jacketed fiber length
at a first location thereof up to a point at which
the optical fiber becomes viscous;
(c) producing a longitudinal tension in
said fiber to stretch said heated first location and
thereby form a first biconical taper having a given
profile;
(d) heating said non-jacketed fiber length
at a second location thereof spaced apart from said
first location up to a point at which the optical
fiber becomes viscous; and
(e) producing a longitudinal tension in
said fiber to stretch said heated second location and
thereby form a second biconical taper also having a
given profile and separated from said first taper by
a given distance;
wherein the said given distance and
the given profile of each of said first and second
tapers can be adjusted to enable transmission through
the filter of a first, predetermined wavelength of
the propagated light signal while stopping a second,
predetermined wavelength of said signal.
9. The method of claim 8, further comprising
the step of etching the non-jacketed fiber length
24

with an acid before carrying out said steps (b) to
(e).
10. The method of claim 8, further comprising
the step of pre-tapering the non-jacketed fiber
length before carrying out said steps (b) to (e).
11. The method of claim 8, further comprising
the steps of:
propagating a monochromatic light signal
through the optical fiber; and
stopping stretching of said heated first
location after occurrence of a number N, N being an
integer, of oscillations in the amplitude of said
propagated monochromatic signal.
12. The method of claim 11, further comprising
the step of stopping stretching of said heated second
location after occurrence of a number N', N' being an
integer, of oscillations in the amplitude of said
propagated monochromatic signal.
13. The method of claim 12, wherein at least
one of the numbers N and N' is equal to 0.
14. The method of claim 12, wherein at least
one of the numbers N and N' is equal to 1.
15. The method of claim 12, in which at least
one of the numbers N and N' is equal to 2.

16. The method of claim 12, in which said
numbers N and N' are different.
17. The method of claim 12, in which said
numbers N and N' are equal.
18. The method of claim 8, further comprising
the step of tapering a non-jacketed section of said
optical fiber separating the first and second
biconical tapers.
19. The method of claim 8, further comprising
the steps of:
propagating a white light signal through
the optical fiber; and
analysing wavelength responses of the
filter after step (c) and after step (e) to determine
whether said filter has desired wavelength filtering
characteristics.
20. A wavelength filter integrated to a single-
mode optical fiber capable of propagating a light
signal and comprising a longitudinal outer jacket
enveloping the optical fiber, the said fiber being
stripped of its jacket on a given length thereof to
form a non-jacketed length of optical fiber, said
non-jacketed fiber length being formed with at least
two concatenated biconical tapers separated from each
other by a given distance and each having a given
profile, wherein the said given distance and the
given profile of each biconical taper can be chosen
to enable transmission through the filter of a first,
26

predetermined wavelength of the propagated light
signal while stopping a second, predetermined
wavelength of said signal.
27

Description

1 3 1 7495
COMPACT WAVELENGTN FILTER INTEGRATED TO A
SINGLE-MODE OPTICAL FIBER
BACKGROUND OF TNE INVENTION
l. Field of the invention:
The present invention relates to a compact
wavelenqth filter integrated to a single-mode optical
fiber, as well as to a method of manufacturing the
wavelength filter.
2. Brief description of the prior art:
Optical fibers are well-known in the art
and are extensively used in telecommunication and
control systems, and in sensing and medical
apparatuses. The advantages of the optical fibers
over the conventional copper conductors and coaxial
cables, when used in telecommunication systems, are
so significant that eventually the optical fibers
will replace the conventional conductors and cables
in many applications for the transmission of
information signals.
Optical fibers are waveguides capable of
propagating visible and/or infrared light. In order
to reduce dispersion of the light signals, single-
mode fibers are widely used and constitute the most

1317495
promising type of optical fibers for
telecommunication purposes. An advantage of the
single-mode optical fibers is their capacity to
carry light signals containing several wavelengths
simultaneously. However, many applications require
that only certain specific wavelengths be transmitted
by the fiber: an optical filter is then necessary to
eliminate the undesired wavelengths. Applications
such as demultiplexing of wavelengths division
multiplexed (WDM) light signals, that is the
separation of light signals of different wavelengths
transmitted by the same fiber, may require a
wavelength filter to isolate correctly the different
signals to be separated.
In standard WDM telecommunication systems,
two wavelengths are used instead of one to double the
data transmission capacity. However, special
components such as WDM fused couplers are required to
launch signals of different wavelengths, usually 1300
and 1550 nm, in a single fiber and to separate them
at the receiving end. Ideal WDM couplers would have
a coupling ratio of 100% at 1300 nm and of 0% at 1550
nm, or vice-versa. However, such ideal coupling
cannot be obtained in practice and the typical
isolation achieved is of approximately 17 dB, which
does not satisfy the telecommunication requirements
(isolation of the order of 35 dB is required).
Additional filtering devices are thus necessary to
better separate the demultiplexed signals. These
devices usually consist of bulky optics components.
Optical filters exist but they suffer from
many disadvantages such as poor performance, high

1 31 7495
complexity, high manufacturing cost, bulkiness, high
loss, etc, and in most of the cases they are not an
integrated part of the optical transmission media
(fiber).
As an example, Canadian patent application
number 517,920 filed on September 10, 1986, in the
name of Suzanne Lacroix and François Gonthier,
proposes a wavelength filter integrated to a single-
mode optical fiber. Such a filter comprises severaltapers formed onto the fiber which comprises
conventionally an opaque outer jacket. To form each
taper, a given length of the outer jacket is removed
from the fiber, the latter fiber is then heated
locally to the point at which it becomes viscous, and
the so heated fiber is stretched along its axis. To
heat locally the fiber, a very small heat source such
as a small flame is used.
Each taper modifies the light signal
transmitted through the single-mode fiber, and the
plurality of tapers formed in series onto the optical
fiber will perform the function of a wavelength
filter. By changing the profile of the tapers, as
well as the number of such tapers on the fiber,
control of the characteristics of the wavelength
filter is enabled.
The integrated wavelength filter of
Canadian patent application number 517,920 presents
the drawback of not being compact. Also, in the
infrared region of the light spectrum, the technique
involved cannot be applied as its efficiency reduces.

13174q5
Compact wavelength filters capable of being
integrated to an optical fiber and eventually in WDM
couplers packages are therefore sought to solve this
problem.
In the present disclosure and in the
appended claims, the term "light" is intended to
encompass both visible and invisible light including
of course infrared light, and which may propagate in
the wavelength filter in accordance with the present
invention integrated to an optical fiber.
OBJECTS OF THE INVENTION
An object of the present invention is
therefore to eliminate the above discussed drawbacks
of the prior art wavelength filters.
Another object of the invention is to
provide a compact wavelength filter integrated to a
single-mode optical fiber and which can be realized
with fibers of different index profiles (matched-
cladding or depressed-cladding).
A further object of the invention is a
compact wavelength filter which is almost temperature
independent and which is also independent of the
polarization of the light.
A still further object of the invention is
a compact wavelength filter offering good isolation
and low loss over a wide wavelength bandwidth.

1317495
SUMMARY OF THE INVENTION
The present invention is more specifically
concerned with a wavelength filter integrated to a
single-mode optical fiber capable of propagating a
light signal and comprising a longitudinal outer
jacket enveloping the optical fiber. The fiber is
stripped of its jacket on a given length thereof to
form a non-jacketed length of optical fiber, which
non-jacketed fiber length being formed with at least
two concatenated biconical tapers separated from each
other by a given distance and each having a given
profile. The distance separating the two tapers and
the profile of each of these tapers can be chosen to
e~able transmission through the filter of a first,
predetermined wavelength of the propagated light
signal while stopping a second, predetermined
wavelength of the latter signal.
The invention is further concerned with a
method of integrating a wavelength filter to a
single-mode optical fiber capable of propagating a
light signal and comprising a longitudinal outer
jacket enveloping the optical fiber. This method
comprises the steps of:
a) stripping the optical fiber of its
jacket on a given length thereof to form a non-
jacketed length of optical fiber;
b) heating the non-jacketed fiber length

1 31 7495
at a first location thereof up to a point at which
the optical fiber becomes viscous;
c) producing a longitudinal tension in
the fiber to stretch the heated first location and
thereby form a first biconical taper havinq a given
profile;
d) heating the non-jacketed fiber length
at a second location thereof spaced apart from the
first location up to a point at which the optical
fiber becomes viscous; and
e) producing a longitudinal tension in
the fiber to stretch the heated second location and
thereby form a second biconical taper also having a
given profile and separated from the first taper by a
given distance;
Again, during the integrating process, the
distance separating the two tapers and the profile of
these tapers can be adjusted to enable transmission
through the resulting filter of a first,
predetermined wavelength of the propagated light
signal while stopping a second, predetermined
wavelength of the latter signal.
The method in accordance with the invention
enables low cost production of high performance
wavelength filters from optical fibers having
different index profiles (matched-cladding or
depressed-cladding). Such filters have a very simple
structure and offer good isolation (about 15 dB) and
low loss (less than 1 dB) over a wide wavelength

--` ` 1 3t 7495
bandwidth (typically 40 nm). They are compact ;
their length is shorter than l cm. They are almost
temperature independent and are also independent of
the polarization of the light.
The objects, advantages and other features
of the present invention will become more apparent
upon reading at the following non restrictive
description of a preferred embodiment thereof, given
by way of example only with reference to the
accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
In the appended drawings:
Figure l, which is labelled as prior art,
provides a schematic illustration of the structure of
a conventional, single-mode optical fiber;
Figure 2 illustrates the structure of a
tapered fiber;
Figure 3 illustrates the structure of a
wavelength filter in accordance with the present
invention integrated to a single-mode optical fiber
such as that shown in Figure l;
Figures 4 and 5, are graphs representing
theoretical wavelength responses of example filters
in accordance with the present invention;
Figure 6 shows a possible setup which can

1317495
be used in the manufacture of the wavelength filter
according to the present invention;
Figure 7 is a graph showing an example of
oscillations in the amplitude of a monochromatic
light signal propagated in a single-mode fiber as a
taper of the wavelength filter is produced; and
Figures 8 and 9 are graphs representing
experimental wavelength responses of integrated
filters according to the invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
A conventional single-mode optical fiber 1
is illustrated in Figure 1 and comprises a
longitudinal core 2 made of transparent material such
as germanium doped silica, a longitudinal cladding 3
surrounding the core 2 and made of transparent
material such as pure silica, and a longitudinal
jacket 4 made of opaque plastic material surrounding
the cladding 3 and acting as a cladding-mode
stripper.
Optical fiber 1 is well known in the art
and is commonly used in telecommunications and
control systems. There exists essentially two types
of single-mode fibers commonly used in the infrared:
the matched-cladding one and the depressed-cladding
one. These two types of optical fiber are well known
to those skilled in the art.
The wavelength filter in accordance with
the present invention comprises two concatenated

1317495
biconical tapers 8 and 9 (Figure 3) along the path of
light defined by the single-mode fiber 1. As
illustrated in detail in Figure 2, each taper 8, 9 is
composed of two conical regions 11 and 12 separated
by a central region 13. It can be seen from Figure 3
that the tapers 8 and 9 are formed at spaced apart
locations on the fiber 1. More specifically, they
are separated by a length "L". A non-altered section
of fiber 10 is therefore present between the two
tapers 8 and 9. The fiber 1 is stripped of its
jacket 4 in the region of the tapers 8 and 9 and
section 10 of fiber.
Single-mode fibers such as 1 (Figure 1) are
so designed that only one mode, namely the mode HEll,
can propagate in the core-cladding guide, i.e. due
only to guidance provided by the index of the core
being higher than that of the surrounding cladding.
This is a core-mode. However, if the jacket 4 is
removed and the fiber surrounded by air, modes of
higher order such as HE12 can propagate due to
additional guidance provided by the cladding-air
interface. The latter modes are referred to as
cladding-modes which obviously can propagate in the
cylindrical section 10.
Considering the light signal propagating
from the left to the right, the conical regions 11
and 12 of the taper 8 constitute coupling regions
where the power of the core-mode HEll is transferred
to the cladding-modes HElm, but mainly HE12, having
the same circular symmetry as the core-mode and vice-
versa for the conical regions 11 and 12 of the taper
9. In the central region 13 of each taper 8, 9 the

1317495
light signal is essentially guided by the cladding
surrounded by the ambient air. The region 13 is
highly multimode and cladding-modes excited in the
first conical region 11 propagate therein without
coupling, thereby accumulating phase differences
between them. The coupling in the second conical
region 12 depends on these accumulated phase
differences. Thus, core transmission of light power
through a taper such as 8 or 9 depends on the length
and shape of their central region 13. The light
power that is not recovered in the core-mode after
passage of the light signal through to the filter is
lost in the cladding-modes which are excited in the
filter (tapers 8 and 9 and central section 10) and
which cannot be re-excited in the fiber 1, the latter
being capable of supporting only one core-mode,
namely the mode HEll-
Each taper 8, 9 is a constriction or20 narrowing of the single-mode optical fiber 1. As
will be described in more detail in the following
description, each taper may be formed on the fiber 1
by heating locally the fiber, using for example a
flame, up to a point at which the optical fiber
becomes viscous. The fiber is then stretched by
applying a small tension along the longitudinal axis
thereof. The heat is then removed allowing the fiber
to cool.
Assuming that only two modes (HEll, HE12)
are involved in the tapered region, the following
relation can be used to approximate core transmission
of light power through a taper formed on a single-
mode fiber:

1 3 1 7495
11
T (A) = 1 - Ro sin2 [ ~ p) ] (1)
where:
- T (~) is the normalized transmitted
light power at the wavelength ~ ;
- ~ is the wavelength of the light
signal;
- Ro is the amplitude of the spectral
response of the taper;
- ~ is the period of the spectral
response; and
- ~ p is a reference wavelength at
which the power transmission of the taper is maximum.
If it is considered that there is no loss,
the complementary power transmission, that is light
power transmission by the cladding-modes, mainly
HE12, can be approximated by the following relation:
R (~) = 1 - T (A) = Ro sin2 [~ p)] (2)
Knowing the relation approximating the core
and complementary power transmissions through each
taper 8 and 9, one can approximate the global
transmission of light power through the two
concatenated tapers 8 and 9 and the intermediate,
stripped cylindrical section 10 illustrated in Figure

1 3 1 7 4 9 5
3. More specifically, this global transmission can
be approximated by the following relation:
~ = TT' + RR' + 2~ TT'RR'lcos (~ ~ L) (3)
where:
- T is the core transmission of the
first taper 8;
- T' is the core transmission of the
second taper 9;
- R is the complementary transmission of
the first taper 8, i.e. transmission in the cladding-
mode HE12;
- R' is the complementary transmission
of the second taper 9;
- ~ p is the difference between the
propagation constants of the two modes HE11 and HEl2
beating in the intermediate stripped cylindrical
section 10 of length L. The dependence of the term
~p on the wavelength ~ is determined by the index
profile of the fiber in the section 10. The product
6~ L is therefore representative of the accumulated
phase differences in the stripped fiber section 10,
that is the product of the difference between the
propagation constants of the two modes HEll and HE12
propagating in the section 10 by the length L of the
latter section.

1 31 74q5
13
Of courseO the terms ~ , T, T', R and R'
are function of the wavelength ~ .
Assuming there is no loss, one can further
determine the two following relations from the above
relation (2):
R (~) + T ( ~ ) = l (4)
R' (~) + T' ( ~ ) = 1 (5)
As evidenced hereinabove, the
characteristics of the wavelength filter of Figures 2
and 3 depends on the following parameters:
1) the core transmission of light power T
and T' of the tapers 8 and 9 determined by the index
profile of the optical fiber and the geometric
profile of these tapers; and
2) the length L of the intermediate
section 10 and also the difference ~ ~ between the
propagation constants of the modes HEl1 and HE12,
which difference depends on the index profile of the
fiber in the section 10.
The global power transmission
given by the relation 3, can be regarded as an
interferometric pattern having a fringe contrast
given by the following relation:
C (A) = 2 ~R R' T T' (6)
R R' + T T'

1 31 7495
14
The fringe contrast C ( ~ ) is determined
only by the two tapers 8 and 9, whereas the phase
thereof depends on the length L of the intermediate
cylindrical section 10.
Thanks to the parameters involved, it is
possible to synthesize a large number of possible
wavelength responses for the filter of Figure 3. As
an example, one can realize a filter with ~ l-pass/
A 2-stop or ~2-pass/ ~l-stop responses in which A 1
and ~ 2 are two predetermined wavslengths; for
example the wavelength Al is 1300 nm and A2 1550
nm in conventional WDM systems.
In order to realize a passband filter at
the wavelength A p (e.g. A p = 1300 nm) one has to
choose R ( ~ p) = R' ( ~ p) = 0. This condition
ensures a maximum transmission of light power through
the filter at the wavelength A p. To stop the
wavelength ~ s, one must realize simultaneously the
maximum contrast condition C ( ~ s) = 1, and the
destructive interference condition. The maximum
contrast condition is obtained when the tapers have
complementary power transmissions:
T (~s) = 1 - T' (~ s) = R' (~ s) (7)
If the tapers 8 and 9 are identical, all
the transmission and reflection coefficients are
equal: T ( ~ s) = T' ( ~ s) = R ( ~ s) = R' ( ~ s)
= 1/2-

1317495
If experimentally the transmissions are notperfectly complementary, the maximum contrast may
also be obtained by inducing some loss in the
cladding-modes of the fiber section 10 until minimum
transmission is obtained.
The destructive interference condition is
given by the relation:
~p L = ( 2 n + 1 ) 1~ (8)
in which n is an integer.
It should be noted that the condition of
maximum power transmission at the wavelength A p also
makes the fringe contrast C ( ~ p) = 0 allowing a
large passband width. To realize a large stopband
width, one must choose the shortest length L possible
fulfilling the destructive interference condition
(relation (8)), i.e. one must set the integer n = 0.
With this additional condition, one can enlarge the
undesired fringe period (period of the oscillations
in the light power transmitted through the filter
between the wavelength ~p and ~ s) and thus broaden
the stop-band width. The condition n = 0 determines
the length L, taking of course into consideration the
characteristics of the optical fiber.
Knowing the fiber index profile, one can
calculate ~ ~ and derive theoretical power
transmissions ~ (~) through the filter. Examples of
wavelength responses of such theoretical filters are
given in Figures 4 and 5. More specifically, Figure
4 represents the theoretical wavelength response of a

1 31 7495
16
1300 nm-pass/1550 nm-stop filter according to the
invention, while Figure 5 shows the theoretical
wavelength response of a 1300 nm-stop/1550 nm-pass
filter.
A possible setup used for manufacturing and
testing the integrated wavelength filter is shown in
Figure 6. More specifically, the manufacturing
procedure comprises the following steps:
(a) first of all, the optical fiber 1 is
stripped of its jacket 4 a few centimeters long to
form a non-jacketed length of optical fiber;
(b) the optical fiber 1 is connected
between a monochromator 16 and a light detector 18.
A source 14 emitting white light is connected to the
monochromator 16 and a computerized data acquisition
system 17 is connected to both the monochromator 16
and detector 18 to receive information therefrom;
(c) sweeping a wavelength bandwidth with
the monochromator 16 and recording in the system 17
the wavelength response of the non altered fiber 1
whereby the data acquisition system 17 can establish
a normalization curve used as reference;
(d) preparing, if necessary, the fiber 1
by etching the same with hydrofluoric acid and/or
pre-tapering it using a large flame, with a typical
width of 5 mm. Indeed, one can manipulate the fiber
index profile by etching or pre-tapering the fiber 1
before forming thereon the two tapers of the filter.
Such preparation of the fiber 1 can facilitate

17
simultaneous fulfillment of all the optimum
conditions and accordingly can facilitate the
fabrication of the integrated filter:
(e) selecting with the monochromator 16
the passband center wavelength ~ p; one can
alternatively use a monochromatic source emiting a
light beam at the wavelength A p;
(f) making the first taper 8 by locally
heating the non-jacketed length of fiber 1 (e.g. with
a microtorch flame of width typically 0.5 mm which
must be set according to the fiber used for
experimentation and to the desired taper
transmission) until the fiber becomes viscous, and by
stretching it longitudinally by means of a
longitudinal tension as shown at 15 in Figure 6. The
fiber is tapered until oscillations appear in the
transmission vs elongation curve of Figure 7, which
curve can be displayed by the system 17. The
elongation is stopped at a maximum, that is for a
number N of oscillations observed in the curve of
Figure 7 at the wavelength ~ p. Typically N is an
integer and is equal to 0, 1 or 2. N = 0 corresponds
to the region 5 of the curve of Figure 7, where the
transmitted power reduces for the first time.
Indeed, as illustrated in Figure 7, the optical power
transmitted from the source 14 to the detector 18
through the monochromator 16 varies as the fiber 1 is
tapered, and oscillations such as those shown in the
latter Figure are produced. N represents the number
of these oscillations observed during the tapering
operation;

1 ~ 74q5
18
(g) recording the wavelength response of
the first taper 8 in the system 17 and normalizing it
to determine whether the passband of the latter taper
is centered on the wavelength A p;
(h) moving the heat source (e.g. flame)
away from the fiber 1 and then along the fiber to
place it at the appropriate distance from the first
taper 8, this distance being typically < 3 mm;
(i) making the second taper 9 with N equal
to 0, 1 or 2 by heating locally the non jacketed
length of fiber until it becomes viscous and by
producing a longitudinal tension in the fiber to
stretch the same. The second taper 9 may be
identical to or different from the first one (N can
be equal or different from that of the taper 8);
(j) recording the wavelength response of
the so formed filter and normalizing it;
(k) analysing the wavelength response
recorded in step (j) and checking if the zero power
transmission, i.e. the destructive interference and
the maximum contrast conditions are realized at the
wavelength ~ 8. If not, one has to adjust those
conditions: the destructive interference is achieved
by setting the monochromator at the wavelength A S
and tapering the non jacketed fiber section 10
between the two tapers 8 and 9 until minimum power
transmission is obtained.
After the correct manufacturing parameters
have been established for a given type of fiber, it

13174~5
19
will be apparent to those skilled in the art that the
manufacturing procedure can be greatly simplified.
As an example, laser diodes at the pass and stop
wavelengths can be used instead of the white light
S source 14 and monochromator 16 to determine maximum
and minimum power transmissions. This enables a more
precise monitoring and thus a more precise adjustment
for a better performance of the filter. Also, for a
mass production of the wavelength filter according to
the present invention, it may be envisaged to produce
each taper by stretching the optical fiber a
predetermined length, instead of counting for each
taper the number of oscillations of the amplitude of
a light signal propagated in the fiber. This method,
well suited for a highly automated production would
require a device that can elongate the optical fiber
with precision, and has an important advantage in
that it obviates the use of an equipment to monitor
the propagated light signal. However, it should be
noted that the heat source, used to soften the fiber
prior the stretching, must have highly reproducible
characteristics, such as the flame size, temperature
etc., in order to produce tapers having identical
responses from one production run to another.
Generally speaking, if the manufacturing setup has
non reproducible characteristics, tapers having the
same length, may not have the same wavelength
response.
Typical experimental results obtained are
shown in Figures 8 and 9, for a matched-cladding
fiber using the pre-tapering technique. A symmetric
configuration has been chosen to realize the 1300 nm-
pass/ 1550 nm-stop filter of Figure 8, i.e. the two

~ 31 ~4q5
tapers 8 and 9 are identical with N = 1 at the
wavelength ~ p = 1300 nm. Concerning the 1550 nm-
pass/1300 nm-stop filter of Figure 9, it has been
formed with two different tapers 8 and 9 with N = 1
and N = 2, respectively, at the wavelength ~ p = 1550
nm. The total length of these two filters is shorter
than 1 centimeter, whereby very compact filters can
be integrated to a single-mode optical fiber in
accordance with the present invention. Such filters
offer good isolation (about 15 dB) and low loss (less
than 1 dB) over a wide wavelength bandwidth
(typically 40nm).
Although the present invention has been
described hereinabove by way of a preferred
embodiment thereof, such an embodiment can be
modified at will, within the scope of the appended
claims, without departing from the spirit and nature
of the present invention.

Representative Drawing