Title: Holey optical fibres
Abstract: An optical fiber structure having a holey fiber arranged in a holey outer support structure made up of holey tubes encased in a thin walled outer jacket. The holey fiber may have a solid core surrounded by a holey cladding having a plurality of rings of holes. With the invention it is possible to produce robust, coated and jacketed fibers with microstructured core features of micrometer size relatively easily using existing fiber fabrication technology. This improvement is a result of the outer holey structure which reduces the thermal mass of the supporting structure and makes it possible to reliably and controllably retain small hole features during the fiber fabrication process.
Patent Number: 6,968,107 Issued on 11/22/2005 to Belardi, et al.
| Inventors:
|
Belardi; Walter (Southampton, GB);
Furusawa; Kentaro (Southampton, GB);
Monro; Tanya (Southampton, GB);
Richardson; David (Southampton, GB);
Turner; Paul (Southampton, GB)
|
| Assignee:
|
University of Southampton (Hampshire, GB)
|
| Appl. No.:
|
344731 |
| Filed:
|
August 10, 2001 |
| PCT Filed:
|
August 10, 2001
|
| PCT NO:
|
PCT/GB01/03618
|
| 371 Date:
|
August 8, 2003
|
| 102(e) Date:
|
August 8, 2003
|
| PCT PUB.NO.:
|
WO02/16980 |
| PCT PUB. Date:
|
February 28, 2002 |
Foreign Application Priority Data
| Current U.S. Class: |
385/127; 385/125; 385/126; 65/385; 65/428 |
| Intern'l Class: |
G02B 006/02 |
| Field of Search: |
385/123-127,147
65/385,428,435,393
|
References Cited [Referenced By]
U.S. Patent Documents
| 5802236 | Sep., 1998 | DiGiovanni et al.
| |
| 6571045 | May., 2003 | Hasegawa et al.
| |
| 2003/0012535 | Jan., 2003 | Town.
| |
| 2004/0052484 | Mar., 2004 | Broeng et al.
| |
| Foreign Patent Documents |
| 0 810 453 | Dec., 1997 | EP.
| |
| WO 99/6490/3 | Dec., 1999 | WO.
| |
Other References
Birks, T. A., et al., "Full 2-D Photonic Bandgaps in Silica/Air Structures",
Electronics Letters, vol. 31, No. 22, pp. 1941-1943, (Oct. 26, 1995).
Bennett, P. J., et al., "Toward Practical Holey Fiber Technology: Fabrication,
Splicing, Modeling and Characterization", Optics Letters, vol. 24, No. 17, pp.
1203-1205, (Sep. 1, 1999).
Bennett, P. J. et al., "A Robust, Large Air Fill Fraction Holey Fibre", CLEO
'99, Baltimore, MD, May 23-28, 1999, IEEE, US, p. 293, (1999).
|
Primary Examiner: Assaf; Fayez G.
Attorney, Agent or Firm: Finnegan, Henderson, Farabow, Garrett & Dunner, L.L.P.
Parent Case Text
CROSS REFERENCE TO RELATED APPLICATIONS
This application is a national phase application based on PCT/GB01/03618, filed
Aug. 10, 2001, the content of which is incorporated herein by reference, and claims
the priority of European Patent application No. 00307090.1, filed Aug. 18, 2000,
the content of which is incorporated herein by reference, and claims the benefit
of U.S. Provisional Application No. 60/230,458, filed Sep. 6, 2000, the content
of which is incorporated herein by reference.
Claims
1. An optical fiber structure comprising a holey fiber arranged in a holey outer
support structure, wherein the holey fiber is contained by a tubular structure
and wherein the holey outer support structure comprises a plurality of tubular
structures arranged around the tubular structure.
2. An optical fiber structure according to claim 1, wherein the tubular structures
of the holey outer support structure have a lateral size at least five times greater
than a lateral size of holes in the holey fiber.
3. An optical fiber structure according to claim 1, wherein the tubular structures
of the holey outer support structure have a lateral size at least ten times greater
than a lateral size of holes in the holey fiber.
4. An optical fiber structure according to claim 1, wherein the holey fiber comprises
a solid or hollow core surrounded by a holey cladding.
5. An optical fiber structure according to claim 4, wherein the holey cladding
comprises cavities arranged in a plurality of rings concentrically about the core.
6. An optical fiber structure according to claim 5, wherein the number of rings
is two.
7. An optical fiber structure according to claim 5, wherein the number of rings
is three, four, five or six.
8. An optical fiber structure according to claim 1, wherein the tubular structures
of the holey outer support structure have lateral dimensions of between one fifth
and five times that of the holey fiber.
9. An optical fiber structure according to claim 1, wherein the tubular structures
of the holey outer support structure have lateral dimensions of between one half
and twice that of the holey fiber.
10. An optical fiber structure according to claim 1, wherein the holey outer
support structure further comprises an outer jacket surrounding the arrangement
of tubular structures.
11. An optical fiber preform comprising:
(a) a core rod;
(b) a plurality of cladding capillaries arranged around the core rod;
(c) an inner jacket containing the cladding capillaries; and
(d) a plurality of support capillaries arranged around the inner jacket.
12. An optical fiber preform according to claim 11, further comprising an outer
jacket containing the support capillaries.
13. An optical fiber preform according to claim 11, wherein the support capillaries
have lateral dimensions between one fifth and five times that of the inner jacket.
14. An optical fiber preform according to claim 11, wherein the support capillaries
have lateral dimensions between one half and twice that of the inner jacket.
15. An optical fiber preform according to claim 11, wherein the core rod is hollow.
16. An optical fiber preform according to claim 11, wherein the core rod is solid.
17. An optical fiber preform according to claim 11, wherein the cladding capillaries
are arranged in a plurality of rings concentrically about the core rod.
18. An optical fiber preform according to claim 17, wherein the number of rings
is two.
19. An optical fiber preform according to claim 17, wherein the number of rings
is three, four, five or six.
20. The optical fiber preform of claim 11, wherein the plurality of support capillaries
have a lateral size greater than a lateral size of the plurality of cladding capillaries.
21. A method of making a holey fiber preform comprising:
(a) arranging a core rod and a plurality of cladding capillaries within a first jacket;
(b) arranging the first jacket and a plurality of support capillaries to form
a tube assembly; and
(c) reducing the tube assembly to a preform.
22. A method according to claim 21, wherein the support capillaries are arranged
within a second jacket.
23. A method according to claim 21, wherein the support capillaries are arranged
within an outer jacket.
24. The method of claim 21, wherein the plurality of support capillaries have
a lateral size greater than a lateral size of the plurality of cladding capillaries.
25. A method of making a holey fiber comprising:
(a) making a holey fiber preform according to the method of claim 21; and
(b) drawing a holey fiber from the preform.
26. A method of guiding light along an optical fiber structure, the method comprising
using an optical fiber structure comprising a holey fiber arranged in a holey outer
support structure, wherein the holey fiber is contained by a tubular structure
and wherein the holey outer support structure comprises a plurality of tubular
structures arranged around the tubular structure, and arranging the light to have
a mode field extending in a cross-sectional plane through the holey fiber, wherein
the mode field is confined in the holey fiber so as to have less than one of 10%,
5%, 2%, 1%, 0.5% and 0.01% of its power extending beyond the holey fiber into the
holey outer support structure.
27. An optical fiber structure comprising a holey fiber arranged in a holey outer
support structure, wherein the holey fiber is surrounded by a first tubular structure
and wherein the holey outer support structure comprises a plurality of further
tubular structures arranged around the first tubular structure.
28. An optical fiber structure according to claim 27, wherein the further tubular
structures of the holey outer support structure have a lateral size at least five
times greater than a lateral size of holes in the holey fiber.
29. An optical fiber structure according to claim 27, wherein the further tubular
structures of the holey outer support structure have a lateral size at least ten
times greater than a lateral size of holes in the holey fiber.
30. An optical fiber structure according to claim 27, wherein the holey fiber
comprises a solid or hollow core surrounded by a holey cladding.
31. An optical fiber structure according to claim 30, wherein the holey cladding
comprises cavities arranged in a plurality of rings concentrically about the core.
32. An optical fiber structure according to claim 31, wherein the number of rings
is two.
33. An optical fiber structure according to claim 31, wherein the number of rings
is three, four, five or six.
34. An optical fiber structure according to claim 27, wherein the further tubular
structures of the holey outer support structure have lateral dimensions of between
one fifth and five times that of the holey fiber.
35. An optical fiber structure according to claim 27, wherein the further tubular
structures of the holey outer support structure have lateral dimensions of between
one half and twice that of the holey fiber.
36. An optical fiber structure according to claim 27, wherein the holey outer
support structure further comprises an outer jacket surrounding the arrangement
of further tubular structures.
37. A method of guiding light along an optical fiber structure, the method comprising
using an optical fiber structure comprising a holey fiber arranged in a holey outer
support structure, wherein the holey fiber is surrounded by a first tubular structure
and wherein the holey outer support structure comprises a plurality of further
tubular structures arranged around the first tubular structure, and arranging the
light to have a mode field extending in a cross-sectional plane through the holey
fiber, wherein the mode field is confined in the holey fiber so as to have less
than one of 10%, 5%, 2%, 1%, 0.5% and 0.01% of its power extending beyond the holey
fiber into the holey outer support structure.
Description
BACKGROUND OF THE INVENTION
The invention relates to holey fibres and to a method of fabricating holey fibres
and holey fibre preforms.
A holey fibre is an optical fibre whose optical confinement mechanism and properties
are affected by an array of air holes defined by cavities that run lengthwise down
the fibre. Light can be guided within holey fibres by two distinct mechanisms.
First, with periodic arrangements of air holes, guidance can be obtained through
photonic band gap effects [
1]. Second, guidance can be obtained from volume
average refractive index effects. This second guidance mechanism does not rely
on periodicity of the air holes [
2].
Generally, a holey fibre has a solid core (FIG. 1A of the accompanying
drawings) or a hollow core (FIG. 1B of the accompanying drawings) surrounded by
a holey cladding region. The holey fibres illustrated have a hole structure characterised
by a hole diameter, d, and an interhole spacing, i.e. pitch, Λ.
A holey fibre structure is fabricated by stacking tubular capillaries in a hexagonal
close packed array within a larger tube that forms an outer jacket or casing containing
the capillaries. To form a solid core holey fibre as in FIG. 1A, one of the tubular
capillaries is removed from the stack and replaced with a solid rod of the same
outer dimensions. To form a hollow core holey fibre as in FIG. 1B, a number of
capillaries in the centre of the stack are removed. The fibre stack is then drawn
into a preform by a caning procedure and then placed in a fibre drawing tower and
drawn into fibre. The finished holey fibre structure is then characterised by an
inner core (solid or hollow) surrounded by a holey cladding. Fabrication of holey
fibres is discussed further in the literature [
3][
4].
To realise holey fibres for many applications, it is desirable to fabricate a
holey fibre with relatively small feature sizes, such as interhole spacing, i.e.
pitch, Λ˜1-2 microns. Fibres with such small hole feature sizes have
a number of interesting and unique properties such as anomalous dispersion at short
wavelength, high optical nonlinearity and the possibility for large evanescent
fields in air.
To satisfy the desire for small pitch, it is necessary to construct a preform
structure with relatively small capillaries. Because of the small size of the capillaries,
several hundred capillary elements are needed to provide a structure which is large
enough to handle conveniently during the fabrication stages of preform caning and
fibre drawing. Moreover, to be practical, the fabricated fibre needs to have an
outer diameter of about 80 microns or more. However, the large number of small
capillaries required to fulfil these requirements presents difficulties in the
fabrication and also results in a weak fibre.
An improvement is to stack the capillaries within an outer jacket which has a
relatively thick wall, as shown by FIG. 2 of the accompanying drawings which shows
a thick wall silica outer jacket
1 defining an inner cylindrical space in
which is placed two rings of silica cladding capillaries
2 which are arranged
concentrically about a centrally placed solid silica core
4. In the illustrated
example, the inner space of the outer jacket
1 is additionally sleeved by
a vycore tube
3. The dimensions included on the top of the figure are exemplary
preform dimensions in millimetres, whereas the dimensions at the bottom of the
figure are target fibre dimensions in microns. Use of a thick wall outer jacket
has the advantage of allowing the number of capillaries required to be greatly reduced.
The thick wall outer jacket approach has been demonstrated by other groups. However,
in the experience of the present inventors at least, it has proved difficult to
reliably and controllably retain small hole features during the fibre pulling stage
of the fabrication process when using such thick walled outer jacket structures.
It is believed that this problem is attributable to the relatively small ratio
of air to glass in the thick-walled structure, and to the relatively large thermal
mass of the glass of the outer jacket as the preform is melted in the drawing tower
furnace during the fibre drawing process.
It is therefore an aim of the invention to provide an improved method for fabricating
holey fibres with relatively small feature sizes.
SUMMARY OF THE INVENTION
According to a first aspect of the invention there is provided an optical
fibre structure comprising a holey fibre arranged in a holey outer support structure.
The holey outer support structure preferably has a lateral feature size at least
five or ten times greater than that of the holey fibre.
The holey fibre may have a solid or hollow core surrounded by a holey cladding
which may comprise cavities arranged in a plurality of rings concentrically about
the core, e.g. 2-6 or more rings.
The holey outer support structure is conveniently made of an arrangement of tubular
structures, each of roughly the same lateral dimensions as the holey fibre. The
lateral dimensions are preferably between one fifth and five times that of the
holey fibre, preferably between one half and twice that of the holey fibre. The
holey outer support structure may conveniently further comprise an outer jacket
surrounding the tubular structures.
An optical fibre structure embodying the invention possesses a microstructured
transverse cross section in which the microstructuring in the holey fibre itself
is on the scale of the wavelength of the light guided by the holey fibre, but is
on a considerably coarser scale within an outer holey structure supporting the
holey fibre (e.g. five times, ten times or a greater multiple of the wavelength).
With the invention it is possible to produce robust, coated and jacketed fibres
with microstructured core features relatively easily using existing fibre fabrication technology.
According to a second aspect of the invention there is provided an optical
fibre preform comprising: (a) a core rod; (b) a plurality of cladding capillaries
arranged around the core rod; (c) an inner jacket containing the cladding capillaries;
and (d) a plurality of support capillaries arranged around the inner jacket. The
preform may further comprise an outer jacket containing the support capillaries.
According to a third aspect of the invention there is provided a method
of making a holey fibre preform comprising: (a) arranging a core rod and a plurality
of cladding capillaries within a first jacket; (b) arranging the first jacket and
a plurality of support capillaries in a second jacket to form a tube assembly;
and (c) reducing the tube assembly to a preform. The support capillaries may be
arranged within an outer jacket.
According to a fourth aspect of the invention there is provided a method
of making a holey fibre comprising: (a) making a holey fibre preform according
to the method of the third aspect; and (b) drawing a holey fibre from the preform.
The support capillaries may be arranged within an outer jacket.
According to a fifth aspect of the invention there is provided a method
of guiding light along a holey fibre structure comprising a holey fibre arranged
in a holey outer support structure, the light having a mode field extending in
a cross-sectional plane through the holey fibre, wherein the mode field is mainly
confined in the holey fibre. In other words, the structure is designed so that
the holey outer support structure does not contribute in any significant way to
the optical guiding properties of the holey fibre contained within it. Preferably,
the mode field has less than one of 10%, 5%, 2%, 1%, 0.5% and 0.01% of its power
extending beyond the holey fibre into the holey outer support structure.
BRIEF DESCRIPTION OF THE DRAWINGS
For a better understanding of the invention and to show how the same may be carried
into effect reference is now made by way of example to the accompanying drawings
in which:
FIG. 1A is a schematic section of a solid core holey fibre;
FIG. 1B is a schematic section of a hollow core holey fibre;
FIG. 2 is a schematic diagram of a holey fibre preform according to a prior
art approach;
FIG. 3 is an end view of a holey fibre preform according to an embodiment of
the invention;
FIG. 4 is a cross-section of a holey fibre structure according to an example
of the invention;
FIG. 5 is an expanded view of the holey fibre region of the holey fibre structure
of FIG. 4; and
FIG. 6 is a cross-section of the holey fibre region of a holey fibre structure
according to another example of the invention.
DETAILED DESCRIPTION
In order to get around the problem of applying a relatively thick-walled jacket,
of large thermal mass, to a relatively fine microstructured inner cane, an approach
has been adopted in the embodiments described below that may be viewed as replacing
the thick wall outer jacket of the prior art approach of FIG. 2 with a combination
of an outer thin wall jacket and an inner stack of relatively large capillaries.
A microstructured inner cane containing the core and holey cladding is then supported
by the larger scale capillaries.
FIG. 3 is an end view of a holey fibre preform according to this approach. The
preform comprises an inner cane
14 containing the elements that will form
the holey fibre after fibre drawing. Although not clearly evident from this figure,
the central region comprises a solid core rod surrounded by a plurality of small
capillaries arranged around the core rod, which ultimately form the holey cladding
of the fibre. The rod and capillaries are retained in an inner jacket which forms
the outer surface of the inner cane. The small cladding capillaries are arranged
in one or more rings concentrically about the core rod. Generally at least two
rings of cladding capillaries will be needed for most holey fibre applications.
In fact, two is a preferred number, since it represents the smallest number of
rings for providing the optical properties desired in many applications. The number
of rings may be greater, e.g. three, four, five, six or more, but it should be
borne in mind that very large numbers of capillaries will present fabrication difficulties,
as described further above in relation to the prior art.
The inner cane
14 is supported by a plurality of relatively large-scale
support capillaries
12 arranged around the inner cane. The support capillaries
are retained in a relatively thin outer jacket
10. In an alternative embodiment,
the outer jacket could be dispensed with and the support capillaries fused together
at the top and bottom prior to pulling to hold them together. As can be seen from
the figure, the support capillaries have an outside diameter approximately the
same as the outside diameter of the inner cane
14, so that the inner cane
can be arranged with the support capillaries in a hexagonal close packed array.
More generally, it is convenient for the support capillaries to be of the same
order of magnitude of lateral dimension as the inner cane. Preferably the support
capillaries have lateral dimensions of between one fifth and five times that of
the inner cane, more especially between one half and twice that of the inner jacket.
The capillaries can be made in a variety of ways. Typically, the starting point
for the capillaries is a large-scale tube. The large-scale tubes can be produced
by: extrusion, milling and drilling, polishing, piercing, spin/rotational casting,
other casting methods (e.g. built-in casting), compression moulding, direct bonding
etc. The tubes are then caned down using a fibre draw tower to the dimensions required
for the preform assembly.
With this preform design, the thermal mass of the supporting structure used
to bulk out the central region of the holey fibre is significantly reduced in comparison
to a thick-wall outer jacket used in the prior art. It is thus easier to pull the
preform and to retain the desired form of microstructure within the vicinity of
the central holey fibre region.
The completed preform is then ready for the next main stage of fibre drawing.
For drawing, the preform is placed in a fibre drawing tower. Fibre drawing is performed
by the controlled heating and/or cooling of the silica or other glass through a
viscosity range of around 10
6 poise. It is useful to monitor the diameter
and tension of the fibre as it is being drawn and use the data thus acquired in
an automatic feedback loop to control the preform feed speed, the fibre draw speed
and/or other parameters related to the furnace in order to yield a uniform fibre diameter.
A principal component of the drawing tower used to pull the preform into fibre
form is a heat source, which may be a graphite resistance heater or a radio-frequency
(RF) furnace.
It is critical to control the fibre drawing temperature, and hence the glass
viscosity,
so that two criteria are met. First, the fibre drawing temperature must soften
the glass to provide a viscosity for which the glass can deform and stretch into
a fibre without crystallisation. Second, the softening of the glass must not be
so great that the crucial internal structure, i.e. the holes, collapse and flow
together. Cooling is provided above and below the furnace's hot zone. The cooling
keeps the glass outside the hot zone cooled to below its crystallisation temperature.
FIG. 4 is a cross-section of a holey fibre structure according to an example
of the invention which has been drawn from a preform generally of the kind illustrated
in FIG. 3.
It is evident that the basic structure of the preform has been retained in the
drawn holey fibre structure. Namely, the drawn holey fibre structure comprises
a holey fibre
20 arranged in a holey outer support structure. The holey
outer support structure comprises an arrangement of tubular structures
22
laterally bounded by a relatively thin wall outer jacket
24 of outer diameter
approximately equal to 250 microns. The outer dimensions is preferably at least
80 microns. A preferred range of outer dimensions is 80 microns to between 1-5
mm. The internal structure of the holey fibre at the centre of the structure is
just visible in FIG. 4, but is better seen in the enlarged view of FIG. 5.
FIG. 5 is a magnified view of the centre region of the holey fibre structure
shown in FIG. 4. The holey fibre comprises a solid core
32 surrounded by
a cladding
30 comprising hole rings generally concentrically arranged about
the core. It will be understood that the holes will not form perfect circles around
the core owing to the realities of the drawing process. The term concentric is
thus not to be construed with any geometric rigour in this document. The cladding
is in turn surrounded by the remnant
28 of the outer jacket of the preform.
In other embodiments of the invention, the core could be hollow instead of solid,
for example for photonic crystal fibre.
As well as the holey fibre of FIG. 5, a range of other similarly capillary-supported
holey fibres of various dimensions have been pulled. By contrast, the inventors
attempts to produce fibres with a thick outer jacket, according to the prior art
approach described above with reference to FIG. 2, have been tended to result in
loss of structural integrity of the core.
The large change in lateral feature size between the holey fibre on the one hand
and the support tubes on the other hand is apparent. The support capillaries preferably
have an outside diameter at least five or ten times greater than that of the holey
fibre
20.
In FIG. 4 it can be seen that the holey fibre
20 has an outside diameter
somewhat smaller than that of the support capillaries
22. Generally, these
two lateral dimensions will be comparable. Specifically, it is preferred that the
tubular support structures
22 have lateral dimensions of between one fifth
and five times that of the holey fibre, more especially between one half and twice
that of the holey fibre.
FIG. 6 is a cross-section of the central region of another holey fibre structure
according to the invention. In this example, a larger number of cladding capillaries
were used in the preform to form a larger number of generally concentric hole rings
in the cladding. Otherwise the example of FIG. 6 will be understood from the previous description.
Although the above examples uses tubes as a basis for the holey fibre preform,
it will be understood that other shapes could be used either in the holey support
structure or for the holey cladding part of the structure. It is sufficient that
the holey outer support structure and holey cladding have a sufficient number of
gaps or cavities to provide the desired properties. It will also be understood
that the hole arrangement in the support structure will generally have no bearing
on the optical properties of the fibre, since the fibre waveguide modes will usually
have no significant power outside the holey cladding. Periodic or aperiodic arrangements
may be used. It will also be understood that the holes in the cladding need not
be periodic, unless the fibre is intended to have photonic crystal effects.
Holey fibre structures according to the invention may find application in many
of the areas previously proposed to be of interest for holey fibres.
One application is sensing. It has been proposed that a fluid, i.e. gas or liquid,
is present in the fibre cavities. A property of the fluid is then sensed by its
effect on that part of the optical mode, generally an evanescent wave part, which
propagates in the holey cladding region.
Another application suggested for holey fibres is for low-loss telecommunication
fibre. Propagation losses may be reduced in a holey fibre, by virtue of the lower
losses associated with the holes relative to the glass regions of the fibre. More
fundamentally, a holey fibre with a photonic band gap could reduce losses through
photonic crystal effects.
Some specific applications of interest are:
- 1) transport of high power optical beams (low optical non-linearity fibre);
- 2) low-loss optical fibre for transmission systems;
- 3) optical sensors (gas detection, liquid composition, medical);
- 4) atom optics;
- 5) optical manipulation of microscopic particles;
- 6) particle separation (by mass, induced polarisability, electric dipole moment);
- 7) Raman lasers;
- 8) non-linear optical devices;
- 9) referencing of a laser to specific gas absorption lines:
- 10) metrology; and
- 11) dispersion compensation in transmission systems (holey fibre structures
embodying the invention can be made to exhibit high dispersion).
REFERENCES
1. T A Birks et al: Electronic Letters, vol. 31, pages 1941-1943 (1995)
2. U.S. Pat. No. 5,802,236: DiGiovanni et al: Lucent Technologies Inc.
3. P J Bennett et al: Optics Letters, vol. 24, pages 1203-1205 (1999)
4. P J Bennett et al: CLEO '99, CWF64, page 293
*
