Note: Descriptions are shown in the official language in which they were submitted.
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OPTICAL ATTENUATOR AND METHOD OF MAKING SAME
Field of the Invention
This invention relates to an optical attenuator and more particularly to a
compact, tunable, wavelength independent all-fiber optical attenuator produced
by creating a mode-field expanded section within or optically coupled (for
example by
fusion splicing) with a single-mode fiber and, eventually, bending said
structure to
induce, adjust or control optical loss, which makes it possible to achieve a
wavelength
1 o independent attenuator with the desired value of attenuation. The method
of making such
attenuator is also part of the invention.
Background of the Invention
In optical fiber communication systems, optical detectors are calibrated to
function
linearly in a given range of optical power. However, depending on the quality
of the
different connections and components in the system and on the power budget
allowed in
the design, the optical power available at the detector is often greater than
the desirable
upper limit of operation of the detector. An optical attenuator can be used to
solve this
2o problem. In other applications, optical attenuators can be used to balance
the optical
power between several lines of a system. They are also useful in calibrating
systems at
different power levels. In system design, they can be used in planning future
development
of a network by increasing the power budget available in case new connections
are made.
The attenuators used in systems often have a fixed value of attenuation, and
are usually
packaged in a small robust case. The attenuation value normally covers a range
from 1
dB to 25 dB or more. Attenuators used in testing equipment or procedure may
have
adjustable values. In both cases, attenuators that have a direct connection to
the optical
fiber have an advantage. A compact attenuator can also be easily integrated in
a patch
3o cord or cable, or even in a connector casing. Another important property is
to have a very
small wavelength dependence in the wavelength window or windows of operation.
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Furthermore, it is often very important for the attenuator to have a very low
back
reflection in order not to perturb the optical fiber system.
In the prior art, various types of attenuators have been described and the
following are examples of prior patents in this area:
U.S. Pat. No. 4,529,262 teaches an inline single-mode fiber attenuator which
requires
special polarization preserving fibers.
to U.S. Pat. No. 4,697,869 discloses a variable attenuator is described
comprising a bend
mechanism with different radii.
U.S. Pat. No. 4,884,859 teaches an all-fiber optical attenuator made by
heating a part of
an optical fiber and applying a tension and/or twist to form an optical
attenuator area
having fine cracks in the heated part of the optical fiber. This attenuator is
found not to be
practicable in certain instances.
In U.S. Pat. No. 5,311,614 a continuously variable fiber optic attenuator is
disclosed which is produced by bending at least a portion of an optical fiber
which is
2o supported by a resilient support member.
In U.S. Pat. No. 5,321,790 of Jun. 14, 1994 teaches an inline type optical
attenuator
which is formed by heating a portion of a single continuous optical fiber and
then
physically deforming said portion in the axial direction while maintaining it
in a heated
state. This attenuator cannot be used over a wide range of wavelengths.
U.S. Patent No. 5,694,512 incorporated herein by reference, discloses an all-
fiber or
inline type of optical attenuator which is formed by heating and then drawing
a portion
of a single mode continuous fibre which is then used as an attenuator by
applying a small
3o bend to the drawn narrowed section. Although this invention may be useful
in some
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instances where polarization dependent loss is not a consideration, this
solution is found
to be somewhat polarization dependent.
The instant invention provides, in a sense, an opposite solution to the '512
patent which
narrows the core of the fibre. In the instant invention the core or mode field
is expanded
in a section where a controlled bend will be applied to achieve attenuation.
And, finally, UK patent application GB 2128766 A in the name of Harding et al,
published May 2, 1984 discloses a single-mode optical fibre attenuator which
provides a
to fixed an non-variable attenuation of light passing therethrough. The
attenuator is based
on creating a splice joint which is lossy, i.e. where light will leak out.
Alternatively, it is
suggested that suitable heat can be applied to the fibre other than a splice
joint to provide
an attenuator.
In nearly direct contrast to this, the instant invention provides a thermally
expanded
portion of the optical fibre, which exhibits nearly no loss when the fibre is
unbent. Hence,
the system operates as if there was no attenuator in-line in under normal
circumstances
when the bend radius of the fibre is greater than 30 cm. In practice it is
possible to make a
TEC region with and attenuation less than 0.1 dB. Furthermore, to attain a
useful device
2o as is defined in the claims of this invention, the optical fibre should not
be heated to the
melting point as is done for example during fusion splicing; by ensuring that
the fibre is
not overheated in this manner, a very low attenuation can be maintained i.e.
substantially
about or less than 1 or 2 dB within a straight region of this fibre. However,
when such
carefully fabricated expanded core optical fibre is bent sufficiently or at a
plurality of
locations, significant attenuation is attainable. In a preferred embodiment
less than 0.5 dB
of attenuation is attained within the straight region of the expanded core
fibre.
The loss from the fibre described in the '766 UK patent application is a
function of
distortions in the core region of the fiber caused by the fusion or heating
process which
3o cause light to be scattered out of the core of the fiber even when the
fiber is held straight,
wherein the desired loss attained by bending the otherwise nearly lossless
expanded core
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optical fibre in accordance with the teaching of this invention is attributed
to a controlled
misalignment of a tensed fiber-to-fiber coupling system. In this system the
TEC region of
the fiber can be thought of as two adjacent'/4 pitch GRIN lenses which serve
to first
collimate the light in the fiber to an expanded beam at the mid-point of the
TEC region
and then refocus the light into the downstream fiber section. Attenuation is
created by
bending the TEC region which can be thought of as equivalent to misaligning
two quarter
pitch GRIN lenses so that they no longer focus the light into the downstream
fiber with
low loss.
1 o Summary of the Invention
In accordance with the invention, there is provided a compact, tunable, all-
fiber optical
attenuator comprising:
a single mode optical fibre having a portion with a core diameter d 1 and
having a portion
I 5 with a core diameter of d2 where d2 is substantially greater than d 1;
means for controllably imparting at least a small bend to the portion of the
fibre having a
core diameter d2 to provide a controlled amount of attenuation.
In accordance with the invention there is further provided, a compact,
tunable, all-fiber
20 optical attenuator comprising:
a single mode optical fibre having mode field diameter d, and having one or
more
sections with a mode field diameter of d2 where d2 is substantially greater
than d,;
a mechanism for controllably imparting at least a small bend to the portion of
the fibre
having a core diameter d2 to provide a controlled amount of attenuation.
In accordance with another aspect of the invention there is provided a method
of
attenuating a beam of light in a controlled and wavelength independent manner
comprising the steps of:
launching the beam of light into a portion of optical fibre having a thermally
expanded
core (TEC); and,
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in a controlled manner, applying at least a small bend to the portion of TEC
optical fibre
to achieve a desired amount of attenuation.
According to the present invention there is provided an all-fiber attenuator
which is
compact, tunable, wavelength independent over a range of several hundred
nanometers
and has negligible back reflection. Depending on the final packaging, it can
be either
fixed and be very stable with the environment, or have an adjustable value. It
is made by
creating a TEC structure on a single-mode fiber and bending said TEC structure
to adjust
or control the optical loss of the attenuator. One or a plurality of expanded
mode field
1 o sections may be created. The expanded mode field section or sections of
the fiber is/are
formed by heating the fiber without subjecting it to any physical distortion
such as
pulling, pushing, twisting or the like. Another application using an expanded
mode field
fibre is described in the a patent in the name of one of the applicant's
entitled A Method
of Reducing Unwanted Effects Associated with High Power Density at an Optical
Connector Coupling, now issued as U.S. Patent No. 5,594,825 issued January 14,
1997.
In the preferred embodiments of the invention, since there is no discontinuity
in the fiber,
this attenuator has negligible back reflection. Its typical size is smaller
than 4 mm long,
and the bend radius, when bending is performed, is normally in the range of 1
cm to 10
cm.
The attenuator of the present invention is fabricated by creating one or more
expanded mode field sections on the single-mode fiber, preferably by
approaching a
small heat source, such as a micro-torch, to the fiber for a period of time of
about 30
minutes. Once the heat source is removed, the mode field of the single mode
fibre is
expanded from ~lOpm to ~30-SO~m; the level of attenuation can be adjusted or
controlled by bending the expanded mode field section or sections. One way the
bend
can be achieved is by compressing the structure.
3o The present invention has small levels of polarization dependent loss (PDL)
since the
attenuation occurs as a result of a portion of the light propagating within
the expanded
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mode field section, not coupling into its destination single mode fibre, after
the TEC or
expanded mode field section. The result is an almost wavelength independent
attenuated
spectral response with little PDL.
Advantageously, the attenuator described hereafter, in accordance with this
invention, is
suitable and capable of attenuating high power optical signals, without
resulting in
damage to the attenuator.
Brief Description of the Drawings
to
Exemplary embodiments of the invention will now be described in conjunction
with the drawings in which:
Fig. 1 is a side view of a optical fiber having a thermally expanded mode
field diameter
as is known in the prior art;
Fig. 2 is a side view of an optical fibre having a section with a mode field
expanded core
wherein that section is shown to be held in a bent position by a positioning
device;
Fig. 3 is a block diagram depicting the optical fibre attenuator with feedback
control
circuitry; and,
Fig. 4 is a side view of an attenuator in accordance with an embodiment of the
invention
2o wherein a plurality of sections having mode field expanded cores are
utilized, to reduce
PDL.
Fig. 5 is a side view of an embodiment similar to that of Fig. 4, wherein a
plurality of
sections having mode field expanded cores are utilized, to reduce PDL.
Detailed Description
"Since the core diameter of fiber is not always precisely measurable, due to
diffusion
effects, reference is usually made to the "mode field diameter" or "MFD", and
the kind of
fiber with an expanding core may be termed "expanded MFD" fiber. Such a fiber
is
3o disclosed in a reference entitled "Beam Expanding Fiber Using Thermal
Diffusion of the
Dopant" in Journal of Lightwave Technology. Vol. 8, No. 8 August 1990. The
beam
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expanding fiber of the above reference has a core whose index of refraction is
determined
by the dopant e.g., Ge, that is thermally diffused so that a spot size of the
fundamental
mode, which corresponds to the mode-field diameter of the optical fiber, is
partially
expanded. Fibers produced by such methods are known as "Thermally-diffused
Expanded
Core" or TEC fibers. For convenience, the term "expanding core fiber" will be
used to
include all such beam expanding type fibers. This method has been
conventionally used
because of its practicality.
The enlarged mode filed diameter (MFD) of thermally-diffused expanded core
(TEC)
optical fibre is obtained by diffusing the dopant in the core. The manufacture
of TEC
fibres using a reproducible fabrication technique with a MFD of 40 pm while
preserving
the outer diameter, is described in a paper entitled Fabrication of an
Expanded Core Fiber
Having MFD of 40 p,m While Preserving the Outer Diameter, IEEE Photonics
Technology Letters, Vol 6, No. 7 July 1994, by Osamu Hanaizumi, Yoshizo
Aizawa,
Hiroaki Minamide, and Shojiro Kawakami. Ge-doped silica single-mode fibers
(SMFs)
are used in the experiment. The heat treatment of the fiber is performed by a
electric
cylindrical furnace, as schematically shown in Fig. 1. In the Hanaizumi paper.
Fibers are
put into a silica vacuum tube after stripping off the protective-coating
layer. The silica
tube is set perpendicularly. In this paper it is said that diffusion at high
temperature and
2o for a long time is required for realizing larger MFD.
In another paper entitled High-performance lensless in-line filters by Yimin
Wang, Takashi Sato, Junichiro Minowa, and Haruki Kataoka published in Applied
Opticss Vol. 34, No. 4, 1 February, 1995 the process of Fabricating TEC fibres
is well
described.
Referring now to Fig. 1, a conventional thermally expanded core (TEC) optical
fiber 10 is shown. The fiber 10 is typical single mode fiber having a core
diameter 12
throughout most of its length 10 Vim; through the application of heat by
flame, by a
3o resistive heater, or by conduction, a 3 mm to 5 mm portion 14 of the fiber
end is
expanded to have a mode field diameter (MFD) of between 20 to 50 Vim.
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Turning now to Fig. 2, an arrangement is shown, wherein the optical fibre is
bent about
two posts 22a and 22b when a force is applied on the TEC section 14 by an
adjustable
screw-like macrobending member 20. Of course a motor and control circuitry
(now
shown) is adapted to turn the screw to achieve a desired amount of
attenuation. The
control circuitry is responsive to an optical tap monitor so that a desired
amount of
attenuation can be maintained. This is shown schematically in Fig. 3.
In Fig. 3 a tap is provided by a filter 32 which provides a 1% tap signal to a
detector 36.
1o The detector 36 provides a electrical signal proportional to the power of
the optical signal
launched into an input end of the optical fibre 10. This electrical signal is
provided to a
control circuit for example in the form of a suitably programmed
microcontroller 37
which provides a control signal to the motor 38. This feedback loop allows
constant
monitoring and adjustment of the attenuator.
Referring now to Fig. 4, an attenuator is shown wherein a plurality of
sections 40a, 40b,
40c, 40d are shown having thermally expanded cores. Conveniently, these
sections can
be one contiguous section of mode field expanded fibre or alternatively as is
shown in
Fig. 5, can be separate spaced sections. A form shaped to conform with the
shape shown
2o in Fig. 4 can be used having two similar sides to press in the fibre (by
varying amounts)
to form the sine like length of fibre to achieve a controlled attenuation.
Of course in a more simple embodiment, the optical fibre shown in Fig. 1 can
be bent a
predetermined or suitable amount and then can be held in place with an
adhesive to
provide a fixed attenuator.
Numerous other embodiments may be envisaged, without departing from the spirit
and
scope of the invention.
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