WAVELENGTH FILTER INTEGRATED INTO AN OPTICAL WAVEGUIDE

FILTRE A LONGUEUR D'ONDE INTEGRE A UN GUIDE DE LUMIERE

English Abstract

ABSTRACT
The present invention relates to a wavelength
filter integrated into an optical waveguide and to a
method for manufacturing same. The optical waveguide
is constituted of a fiber of light transmitting
material on which is formed a plurality of biconical
tapered portions which constitute the wavelength
filter. The characteristics of the filter are governed
by the number of biconical tapers and their respective
profiles. The tapers are preferably formed by heating
locally and stretching a conventional monomode optical
fiber or by adding tapers in a monomode optical fiber,
such as by splicing.

Claims

The embodiments of the invention in which an
exclusive property or privilege is claimed are defined
as follows:
1. A wavelength filter integrated into an
optical waveguide, said wavelength filter having a
predetermined filtering characteristic, said optical
waveguide being constituted by a fiber of light
propagating material defining a light path, said
wavelength filter comprising a plurality of successive
concatenated biconical tapered portions on said fiber
along said light path, each biconical tapered portion
being formed on a length of a monomode optical fiber,
said biconical tapered portions having different
profiles such as to produce the desired filtering
characteristic.
2. A wavelength filter integrated into an
optical waveguide as defined in claim 1, wherein a
plurality of the biconical tapered portions of said
wavelength filter are integrally formed on a length of
a monomode optical fiber.
3. A wavelength filter integrated into an
optical waveguide as defined in claim 1, wherein each
biconical tapered portion is formed on an individual
22

section of a monomode optical fiber, the individual
sections being operatively connected to each other to
form said wavelength filter.
4. A wavelength filter integrated into an
optical waveguide as defined in claim 2, wherein at
least one of the biconical tapered portions of said
wavelength filter is formed on an individual section of
a monomode optical fiber, said section being
operatively connected to said length of a monomode
optical fiber.
5. A wavelength filter integrated into an
optical waveguide as defined in claim 1, wherein the
biconical tapered portions are formed by heating a
monomode optical fiber with a flame having a width
less than 2 mm up to the softening point of said
monomode optical fiber and by stretching said monomode
optical fiber.
6. A wavelength filter integrated into an
optical waveguide as defined in claim 5, wherein the
transmitted power of a given biconical tapered portion
is approximately:
t (.lambda.)= cos2(.pi.(.lambda.-.lambda.?)/2.DELTA.)
23

wherein:
- ? is the normalized transmitted power of
the biconical tapered portion along said light path;
- .lambda. is the wavelength of the light passing
through said filter;
- .lambda.o is a reference wavelength at which the
transmission of said given biconical tapered portion is
maximum; and
- .DELTA. is the half-period of the spectral
response of the said given biconical tapered portion.
7. A wavelength filter integrated into an
optical waveguide as defined in claim 6, wherein the
spectral response of said filter is approximately:
<IMG>
wherein:
T is the normalized transmitted power of said
filter;
24

.DELTA.i is the half-period of the spectral
response of the ith biconical tapered portion of said
filter; and
n is the number of biconical tapered
portions of said filter.
8. A wavelength filter integrated into an
optical waveguide as defined in claims 1, 2 or 3,
wherein said filter comprises four biconical tapered
portions.
9. A wavelength filter, as defined in claim 1,
wherein said optical waveguide comprises cladding-mode
stripper means.
10. A wavelength filter, as defined in claim 9,
wherein said cladding-mode stripper means include
jacket means at each end of each biconical tapered
portion.
11. A method for manufacturing a wavelength
filter integrated into an optical waveguide, said
wavelength filter having a predetermined filtering
characteristic, said method comprising the step of
forming a succession of concatenated biconical tapered
portions, said succession of biconical tapered portions

constituting an optical waveguide defining a light
path, each biconical tapered portion being formed on a
section of a monomode optical fiber by heating locally
and controllably stretching said section of a monomode
optical fiber to form a biconical tapered portion
having a certain profile, the biconical tapered
portions having different profiles such as to produce
the predetermined filtering characteristic of said
wavelength filter.
12. A method as defined in claim 11 wherein a
plurality of biconical tapered portions of said
wavelength filter are formed integrally on a length of
a monomode optical fiber.
13. A method as defined in claim 11 wherein each
biconical tapered portion is formed on an individual
section of a monomode optical fiber, then the
individual sections being operatively connected to each
other to form said wavelength filter and said optical
waveguide.
14. A method as defined in claim 12, wherein at
least one biconical tapered portion of said wavelength
filter is formed on an individual section of a monomode
optical fiber, said individual section being
operatively connected to said length of a monomode
26

optical fiber.
15. A method as defined in claim 11, further
comprising the steps of:
- illuminating one end of said section of a
monomode optical fiber when said section is heated and
stretched; and
- monitoring the output light signal at an
opposite end of said section during the stretching
thereof, said output light signal being of oscillatory
nature during the stretching of said section.
16. A method as defined in claim 15, wherein
during the stretching of said section the oscillations
of said output light signal being counted.
17. A method as defined in claim 16, wherein the
stretching of said section is interrupted when a
predetermined number of oscillations has been reached.
18. A method as defined in claim 11, wherein a
biconical tapered portion of said wavelength filter is
formed by stretching a section of a monomode optical
fiber a predetermined length.
19. A method as defined in claim 16 wherein the
27

number of oscillations for the ith biconical tapered
portion of said wavelength filter is determined from
the relation:
Ni = (1/2) i-1N1
wherein N1 is the number of oscillations corresponding
to the longest biconical tapered portion of said
wavelength filter.
20. A method as defined in claim 15, wherein said
filter is tuned at a certain wavelength, said section
being illuminated with a light source emitting at said
certain wavelength.
21. A method as defined in claim 15, wherein said
filter is tuned at a certain wavelength, said section
being illuminated with a light source emitting at a
different wavelength.
28

Description

34Z82
-- 1 --
The present invention relates to a wavelength
filter integrated into an optical waveguide and to a
method for manufacturing same.
Optical fibers are well known and are
extensively used in communications, control systems,
sensing or medical devices. The advantages that
optical fibers possess, for communication purposes,
over conventional copper wire conductors and coaxial
cables are such that eventually the optical fiber will
replace them in many applications for transmission of
information signals.
Optical fibers are waveguides that can
support visible or infrared light. In order to reduce
the dispersion of signals, monomode fibers are used and
are now the most promising type of fiber in
communications. As waveguides, monomode optical fibers
can carry several wavelengths. However, many
applications require that only certain specific
wavelengths be transmitted by the fiber, requiring a
filter to eliminate the undesired wavelengths.
Applications such as the wavelength demultiplexing of
signals, that is the separation of signals at different
wavelengths transmitted in the same fiber may need a
wavelength filter, in particular, a narrow passband
filter. Such a filter may also be used to lock the

frequency of a laser or of a laser diode, or to
construct a frequency amplitude converter.
Optical filters exist but they suffer from
many disadvantages such as poor performance, high
complexity, high manufacturing costs, etc., and they
are not usually an inherent part of the optical
transmission media.
Therefore, an object of the present invention
is an improved wavelength filter integrated into an
optical waveguide.
Another object of the invention is an
improved method for manufacturing a wavelength filter
integrated into an optical waveguide.
The term "light" as used herein is intended
to encompass both visible light as well as light in
other parts of the spectrum than visible light which
may propagate in the wavelength filter integrate~ into
an optical waveguide, according to this invention.
The wavelength filter integrated into an
optical waveguide (hereinafter "wavelength filter"),
according to this invention, is fabricated from a
monomode optical fiber comprising a core made of light

1~34282
-- 3
transmitting material surrounded by a layer of
cladding, also made from a light transmitting material.
The cladding layer is received in a jacket made
typically of opaque plastic material.
The wavelength filter is constituted by a
plurality of successive concatenated biconical tapered
portions (hereinafter ~'tapers~) along the light path
defined by the monomode fiber, and formed at spaced
locations thereon.
Each taper is a constriction or narrowing of
the monomode optical fiber and comprises three
constituent elements, namely an elongated beating
region which is connected to the monomode optical fiber
by two conical coupling zones. In the beating region,
light is essentially guided by the cladding surrounded
by air. As opposed to the monomode core-cladding
guide, this cladding-air guide is highly multimode. In
the conical coupling zones, a power transfer or
coupling takes place from the HEll core-mode to the
HEl1 and HE12 cladding-modes and vice-versa. In the
central quasi-cylindrical region, only beating between
these excited cladding-modes occurs. The transmission
of the taper depends on the shape and size of the
coupling zones and the beating region, thus on the
profile of the taper. The profile of the taper is the

- lZ84282
-- 4
the diameter of the monomode optical fiber, in the
taper region, as a function of the length.
In combination, the characteristics of the
tapers, namely their respective profiles and their
number determine the characteristics of the filter.
A taper may be formed on the monomode fiber
by heating locally the fiber, by using preferably a
flame having a width of 2 mm or less, up to the point
at which the fiber becomes viscous and then stretching
the fiber by applying a small tension along the central
axis thereof. The heat is then removed allowing the
fiber to cool.
As an alternative, the tapers may be
manufactured independently on short sections of a
monomode optical fiber and to construct the wavelength
filter, the tapers are joined together, by using a
known technique, such as splicing.
It is plain that the two options may be
combined and a wavelength filter may be manufactured by
integrally forming some of the tapers on a monomode
optical fiber and by adding the remaining tapers in the
fiber, such as by splicing.

12842~32
Provided the flame used to manufacture the
taper is narrow (~2 mm), the following relation may be
used to approximate the transmitted power of an abrupt
taper formed on a monomode optical fiber:
t( ~) = cos2(~ o)/2 ~ )
where:
t is the normalized transmitted power;
1~
~ O is a reference wavelength at which the
taper transmission is maximum;
~ is the half period of the spectral
lS response of the taper.
The spectral response of the wavelength
filter is equal to the product of the spectral
responses of each of the tapers. If the transrnission
of the tapers is chosen to be maximum at the same
reference wavelength ~ O, the spectral response of the
wavelength filter may be approximated by:
T(~ ) = ~ cos ( ~ o)/2 ~ i)
i=l
where:

~Z8428Z
T is the normalized transmitted power of the
wavelength filter;
n is the number of tapers which constitute
the filter; and
~ i is the half period of the spectral
response of the i taper.
Thus, the characteristics of the wavelength
filter, according to the present invention, depend on
the number of tapers as well as on the half period ~
of each taper. ~ of one taper depends on the taper
profile. During the manufacture of the wavelength
filter, one end of a monomode optical fiber is
illuminated and the output signal at the opposite end
of the fiber is monitored. During the stretching, the
output signal oscillates. The present inventors have
found that ~ , the half period of the spectral
0 response of a taper is approximately equal to ~ ,
2N
being the wavelength of the light source used to
illuminate the monomode fiber, and N being the number
of oscillations counted during the stretching of the
monomode fiber. Thus, for each taper, the profile
which gives the desired ~ may be obtained by counting
the number of oscillations during the stretching and

~8~28Z
-- 7
interrupting the tapering process when a predetermined
number of oscillations has been reached.
The light source for illuminating the optical
monomode fiber during the manufacture of the wavelength
filter should preferably emit at the same wavelength at
which the wavelength filter is tuned, which is ~ O.
In certain cases, however, it may be desirable to use a
light source which emits at a different wavelength.
0 This may be achieved by using the A = ~ relation to
2N
effectively predict the number of oscillations N which
would be counted at different wavelengths.
For a mass production of the wavelength
filter, according to the present invention, it may very
well be envisaged to form each taper on the optical
fiber simply by stretching the fiber a predetermined
length instead of counting the number of oscillations
of a light signal in the fiber. This method would be
well suited for a highly automated production and may
be used only if the characteristics of the
manufacturing setup (i.e. the flame size, ternperature,
etc.) are reproducible with precision.
Therefore, the present invention comprises in
a most general aspect a wavelength filter integrated

~2~4X8Z
into an optical waveguide, the wavelength filter having
a predetermined filtering characteristic, the optical
waveguide being constituted by a fiber of light
propagating material defining a light path, the
5 wavelength filter comprising a plurality of successive
concatenated biconical tapered portions on the fiber
along the light path, each biconical tapered portion
being formed on a length of a monomode optical fiber,
the biconical tapered portions having different
profiles such as to produce the desired filtering
characteristic.
This invention further encompasses a method
for rnanufacturing a wavelength filter integrated into
lS an optical waveguide, the wavelength filter having a
predetermined filtering characteristic, the method
comprising the step of forming a succession of
concatenated biconical tapered portions, the succession
of biconical tapered portions constituting an optical
waveguide defining a light path, each biconical tapered
portion being formed on a section of a monomode optical
fiber by heating locally and controllably stretching
the section of a monomode optical fiber to form a
biconical tapered portion having a certain profile, the
biconical tapered portions having different profiles
such as to produce the predetermined filtering
characteristic of the wavelength filter.

12842132
Preferred embodiments of the present
invention will now be described with reference to the
annexed drawings in which:
Figure 1 is a perspective view of a
commercially available monomode optical fiber;
Figure 2 is a perspective view of a monomode
optical fiber on which a taper has been formed;
Figure 3 is a diagram illustrating the
transmitted power of a taper as a function of
elongation;
Figure 4:
a) is a diagram illustrating the transmitted
power through a taper as a function of the wavelength;
b~ is the normalization curve of the system;
Figure 5 is an experimental plot of the
inverse half period in the spectral response of a taper
versus N the number of power oscillations counted
during the elongation of the taper;
Figure 6 is a diagram illustrating the

3428Z
-- 10 --
theoretical response of a wavelength filter with four
tapers;
Figure 7 is a schematical view of a setup for
manufacturing a wavelength filter, according to the
present invention;
Figure 8:
a) is a diagram of the spectral response of
the wavelength filter shown in Figure 7;
b) is the normalization curve of the system;
and
Figure 9 is a theoretical diagram of the
inverse half-period in the spectral response of a
taper, with respect to N, the number of power
oscillations counted during the stretching of the
taper, at two different wavelengths.
The monomode optical fiber 10 illustrated in
Figure 1 comprises a core 12 made of transparent
material such as germanium dopped silica, surrounded by
a cladding layer 14 made of transparent material such
as pure silica. A jacket 16 of opaque plastic material
is mounted on cladding layer 14 and acts as a cladding-
mode stripper.

l2s42a2
Optical fiber 10 is well known in the art and
is commonly used in communications and control systems.
As an example, Newport Corporation supplies such fibers
which, typically, have a core and a cladding diameters
s of 3.6 ~m and 127 ,um respectively, and a second mode
cut off wavelength of 578 nm.
The wavelength filter, according to the
present invention, is fabricated from the monomode
optical fiber 10 by forming on the fiber 10, or by
serially connecting a plurality of successive tapers.
Figure 2 illustrates a taper 18 which comprises a thin
and approximately cylindrical beating zone 21 connected
to the fiber 10 by two conical coupling zones 19 which
taper toward the beating zone 21.
The taper 18 modifies the light signal
passing through monomode fiber 10, and a plurality of
such tapers will perform the function of a wavelength
filter. By changing the profile of the tapers, as well
as the number of tapers on the fiber 10, the
characteristics of the wavelength filter may be
controlled.
The taper 18 is constructed by heating
locally the fiber 10 up to the point at which it
becomes viscous and, by stretching the fiber along its

~284282
- 12 -
axis. As a heat source, it is preferable to use a
narrow flame, however, it may be envisaged to use other
heat sources.
The variations of the transmitted power or
the spectral response of a light signal in monomode
fiber 10, during the tapering, is illustrated in the
diagram of Figure 3. N, which is the number of
oscillations counted during the stretching process can
be easily determined from the power versus elongation
diagram.
Provided the flame used to manufacture the
taper is narrow (~ 2 mm), one can obtain the wavelength
response illustrated in Figure 4. Figure 4a
illustrates the response of the taper 18 with respect
to ~ , the wavelength of the electromagnetic
excitation. Figure 4b is the normalization curve of
the system. It may be observed that the response of
taper 18 is not exactly sinusoidal which is due to a
perturbation, indicating the presence of a third mode
(HE13) in the taper 18.
By neglecting the perturbation effect, the
following relation may be used to approximate the
spectral response of an abrupt taper, to an
electromagnetic radiation having a wavelength ~ :

lZ84Z~32
- 13 -
t ~ ~ ) = cos2 ( ~(~ - ~ o)/2 ~ )
where:
t is the transmitted power of the taper;
~ O is a reference wavelength at which the
taper transmission is maximum;
~ is the half period of the oscillation
which occurs in the beating region 21 of taper 18.
The inverse half period of the wavelength
response is approximately a linear function of N, the
number of oscillations encountered while tapering the
fiber, as can be seen in Figure 5. Thus, the period of
the spectral response decreases as N increases, that is
as the elongation increases.
The spectral response of a series of
concatenated tapers on fiber lO is equal to the
product of the spectral responses of each of the
tapers. If the transmission of the tapers are chosen
to be maximum at the same reference wavelength ~ O, the
spectral response of the series of concatenated tapers
may be approximated by:

lZ~3~2E~2
_ 14
T (~ ) = ~ cos (~ (~ -~ o)/2 ~ i)
i=l
where:
T is the transmitted power of the ith taper;
~ i is the half period of the oscillation
which occurs in the beating region of the ith taper;
and
n is the number of tapers of the wavelength
filter.
Choosing ~ i=2i 1~ 1 gives a series that when
n tends to infinity, is equal to sinc2{~ ~ ~)/ ~ 1}
which represents a narrow band filter centered on ~ O
with a half-power width~ which is proportional to
~ 0.89 ~ 1) which is the smallest ~ in the
series. Since 1/ ~ is proportional to N, the series
can be obtained with Ni=(1/2)i 1N1 , the width of the
filter being determined by N1 which corresponds to the
taper with the greatest number of oscillations.
Practically, the choice of N1 is limited by
Nmax which is the number of oscillations recorded
during the stretching of the fiber 10 before the taper

~12:84~
breaks. Since the series cannot be infinite, the
spectrum will exhibit periodic peaks, the distance
between the peaks depending on the taper with the
largest spectrum period, in other words, the smallest
N.
The theoretical spectral response of a
wavelength filter with four tapers, is shown on the
diagram of Figure 6. Each of the periodic peaks A, B
and C, respectively, has a half power width of 0.89 ~ l
and the wavelength filter may be assimilated to a
passband filter over a bandwith which includes only one
of the peaks. For example, for applications were the
operating bandwith is restricted between the peaks A
and C, the filter will perform as a bandpass filter,
eliminating, or strongly attenuating all the
frequencies in the operating bandwith except a narrow
band centered on ~o.
~ eferring to Figure 7, a wavelength filter 23
is constructed by forming on an optical monomode fiber
10 four consecutive tapers, as follows. A white light
source 20 is coupled at one end of fiber 10 and the
opposite end of the fiber is placed in the entry slot
of a grating monochromator 22, known in the art, and
equipped with a photomultiplier. The wavelength of the
filter, in other words, the wavelength which will be

lX842~3~
- 16 -
the least attenuated by the filter is selected
arbitrarily at 766 nm.
To achieve a good selectivity, the number of
tapers is established to four. The taper with the
greatest number of oscillations, designated N1 is fixed
at 32.
By using N1 = 3Z, the formula
Ni=(1/2) N1 gives
N2 = 16
N3 =
The jacket of the optical fiber is stripped
at the location where the first taper is to be formed.
The fiber 10 is then heated locally by using a narrow
flame, having preferably a width of 2 mm or less, to a
temperature up to its softening point and it is
stretched while monitoring the output signal by the
monochromator. When the number of oscillations reaches
32, the stretching is interrupted and the fiber is
allowed to cool. The same steps are followed for
forming the three other tapers on the fiber except that
the number of oscillations is different for each taper.
Alternatively, the wavelength filter 23 may

1284;~:~2
- 17 -
be contructed by forming each of the tapers
independently, on a short section of a monomode fiber
and by joining the tapers serially.
S It is plain that both options may be combined
and the wavelength filter, according to this invention,
may also be constructed by forming some of the tapers
of the filter directly on the monomode optical fiber,
while adding the remaining tapers in the monomode
optical fiber, such as by splicing.
In order to obtain a good performance of the
wavelength filter, according to the present invention,
it is necessary to provide a cladding-mode stripper at
each end of each taper on the optical fiber. As an
example, a suitable cladding-mode stripper may be
obtained by leaving the jacket of the fiber at such
locations.
Figure 8 illustrates the response of the
wavelength filter 23. The expected 766 nm peak is
present and has a width of 5.8 nm with a transmission
of 50%. The power losses are due to the inherent loss
of each taper and to the difficulty of controlling
precisely the stretching process, in other words,
stopping exactly on a maximum in the transmitted power.
The perturbation due to the presence of a third mode
can also contribute to losses in each taper up to 15%.

~8~:8~
- 18 -
It might be observed that throughout the studied range
(300 nm) there is an almost total extinction of all the
non-desired peaks, even the one predicted by the model
(corresponding to 598 nm) The residual peaks can
always be suppressed by adding the appropriate taper
(which is not necessarily in the series ex: adding a
taper with N= 24 reduces the side lobes). To broaden
the range of utility of the filter, it is always
possible to bend the fiber to eliminate the
transmission of greater wavelengths.
The above described method for manufacturing
wavelength filters necessitates a light source emitting
at a wavelength at which the filter is to be tuned.
This may be a disadvantage in certain cases and it may
be desirable to use a light source emitting at a
certain wavelength to manufacture a large variety of
filters tuned at different wavelengths.
For example, one might want to construct a
wavelength filter at ~ F = 900 nm which requires that
the longest taper be of a period ~ = 12 nm. If only
a HeNe (Helium/Neon) laser is available which emits at
~ L = 633 nm, the following reasoning may be
followed.
Since, as stated earlier

lZ~gL;i~;~:
2N
the number of oscillations NF which are necessary to
obtain ~ = 12 nm, when the stretching of the taper is
performed when the optical fiber is illuminated with a
light source at ~ F = 900 nm, is:
N = ~ = 900 = 37,5,
F
2 ~ 24
To obtain a maximum in the transmission at
~ F~ NF must be an integer. Therefore, the closest
value to choose for NF in order to obtain a maximum in
the transmission at ~ F = 900 nm is 37 or 38.
lS
By choosing arbitrarily 37, one will have
F = 900 = 12.16 nm.
2NF 2 x 37
The value for NL, the number of oscillations to be
counted during the stretching of the taper while it is
illuminated with the laser emitting at 633, nm may then
be determined:
NL = ~ = 633 = 26,03.
2 ~ 2x12,16

~:ZB~ZBZ
- 20 -
By interrupting the stretching after 26.03
oscillations, a transmission having a maximum at 900 nm
will be obtained with a ~ = 12.16 nm.
5Figure 9 illustrates the relation between
and N, for two wavelengths ~ = 633 nm and
~ = 9oO nm, respectively. At each wavelength,
there is a linear relation between ~ 1 and N, more
specifically, ~ = 2N.
The other tapers of the filter may then be
constructed, using the Ni = 1/2 1Nl series, but with
N1= 26,03,
15For a mass production of the wavelength
filter, according to the present invention, it may very
well 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 a
Zlight signal 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 light
25signal in the fiber. However, it should be observed
that the heat source, used to soften the fiber prior
the stretching, must have highly reproducible

~Z8~8~
- 21 -
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 would have non
reproducible characteristics, tapers having the same
length, may not have the same response.
It should be understood that the scope of the
present invention is not intended to be limited to the
specific preferred embodiments illustrated in the
drawings and described above.

Representative Drawing