U.S. patent application number 10/720453 was filed with the patent office on 2004-07-15 for dielectric waveguide and method of making the same.
Invention is credited to Fuflyigin, Vladimir.
Application Number | 20040137168 10/720453 |
Document ID | / |
Family ID | 32397118 |
Filed Date | 2004-07-15 |
United States Patent
Application |
20040137168 |
Kind Code |
A1 |
Fuflyigin, Vladimir |
July 15, 2004 |
Dielectric waveguide and method of making the same
Abstract
In general, in one aspect, the invention features a method that
includes exposing a surface to a first gas composition under
conditions sufficient to deposit a layer of a first chalcogenide
glass on the surface, and exposing the layer of the first
chalcogenide glass to a second gas composition under conditions
sufficient to deposit a layer of a second glass on the layer of the
first chalcogenide glass, wherein the second glass is different
from the first chalcogenide glass.
Inventors: |
Fuflyigin, Vladimir;
(Medford, MA) |
Correspondence
Address: |
FISH & RICHARDSON PC
225 FRANKLIN ST
BOSTON
MA
02110
US
|
Family ID: |
32397118 |
Appl. No.: |
10/720453 |
Filed: |
November 24, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60428382 |
Nov 22, 2002 |
|
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|
60458645 |
Mar 28, 2003 |
|
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Current U.S.
Class: |
427/571 ;
427/237; 427/255.7 |
Current CPC
Class: |
G02B 6/02304 20130101;
C03B 37/01892 20130101; G02B 6/03688 20130101; C03B 37/0183
20130101; C03B 2203/42 20130101; C03B 2201/86 20130101; G02B
6/03638 20130101; C03B 2203/16 20130101; C03C 13/043 20130101; G02B
6/023 20130101; Y02P 40/57 20151101 |
Class at
Publication: |
427/571 ;
427/237; 427/255.7 |
International
Class: |
B05D 007/22 |
Claims
What is claimed is:
1. A method, comprising: exposing a surface to a first gas
composition under conditions sufficient to deposit a layer of a
first chalcogenide glass on the surface; and exposing the layer of
the first chalcogenide glass to a second gas composition under
conditions sufficient to deposit a layer of a second glass on the
layer of the first chalcogenide glass, wherein the second glass is
different from the first chalcogenide glass.
2. The method of claim 1, wherein exposing the surface to the first
gas composition comprises activating a plasma in the first gas
composition
3. The method of claim 2, wherein activating a plasma in the first
gas composition comprises exposing the gas to electromagnetic
radiation to activate the plasma.
4. The method of claim 3, wherein the electromagnetic radiation
comprises microwave radiation.
5. The method of claim 3, wherein the electromagnetic radiation
comprises radio frequency radiation.
6. The method of claim 1, wherein exposing the layer of the first
glass to the second gas composition comprises activating a plasma
in the second gas composition.
7. The method of claim 6, wherein activating a plasma in the second
gas composition comprises exposing the gas to electromagnetic
radiation to activate the plasma.
8. The method of claim 7, wherein the electromagnetic radiation
comprises microwave radiation.
9. The method of claim 7, wherein the electromagnetic radiation
comprises radio frequency radiation.
10. The method of claim 1, wherein the second gas composition is
different from the first gas composition.
11. The method of claim 1, wherein the first gas composition
comprises one or more halide compounds.
12. The method of claim 11, wherein the one or more halide
compounds comprises a chloride compound.
13. The method of claim 1, wherein the first gas composition
comprises a carrier gas.
14. The method of claim 13, wherein the carrier gas comprises
nitrogen.
15. The method of claim 13, wherein the carrier gas comprises a
noble gas.
16. The method of claim 15, wherein the noble gas is argon.
17. The method of claim 1, wherein the first gas composition
comprises a chalcogen.
18. The method of claim 1, wherein the first gas composition
pressure is between about 2 and 20 Torr.
19. The method of claim 1, wherein the second gas composition
comprises one or more halide compounds.
20. The method of claim 19, wherein the one or more halide
compounds comprises a chloride compound.
21. The method of claim 1, wherein the second gas composition
comprises a carrier gas.
22. The method of claim 21, wherein the carrier gas comprises
nitrogen.
23. The method of claim 21, wherein the carrier gas comprises a
noble gas.
24. The method of claim 23, wherein the noble gas is argon.
25. The method of claim 1, wherein the second gas composition
comprises a chalcogen.
26. The method of claim 1, wherein the second gas composition
comprises oxygen.
27. The method of claim 1, wherein the second gas composition
pressure is between about 2 and 20 Torr.
28. The method of claim 1, wherein the second glass is an oxide
glass.
29. The method of claim 1, wherein the second glass is a
chalcogenide glass.
30. The method of claim 1, wherein the surface is a surface of a
tube.
31. The method of claim 30, wherein the surface is an inner surface
of a tube.
32. The method of claim 30, wherein the tube comprises a glass.
33. The method of claim 32, wherein the glass is a silicate
glass.
34. The method of claim 32, wherein the tube comprises a
polymer.
35. The method of claim 1, wherein the surface is a planar
surface.
36. A method, comprising: introducing a first gas composition into
a tube, the first gas composition comprising a first compound that
is substantially inert with respect to a first material forming the
inner surface of the tube; and exposing the first gas composition
to conditions sufficient to change the first compound into a second
compound reactive with the first material and to deposit a layer of
a second material on the inner surface of the tube.
37. The method of claim 36, wherein exposing the first gas
composition to conditions sufficient to change the first compound
into a second compound comprises activating a plasma in the first
gas composition.
38. The method of claim 37, wherein activating the plasma comprises
exposing the first gas composition to electromagnetic
radiation.
39. The method of claim 38, wherein the electromagnetic radiation
comprises microwave radiation.
40. The method of claim 38, wherein the electromagnetic radiation
comprises radio frequency radiation.
41. The method of claim 36, wherein the first compound comprises
oxygen.
42. The method of claim 41, wherein the first compound is nitrous
oxide.
43. The method of claim 42, wherein the second compound is
oxygen.
44. The method of claim 38, wherein the first material is a
glass.
45. The method of claim 44, wherein the glass is a chalcogenide
glass.
46. The method of claim 36, further comprising exposing the layer
of the first material to a second gas composition under conditions
sufficient to deposit a layer of a second material on the layer of
the first material, wherein the second glass is different from the
first glass.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority to Provisional Patent
Application 60/428,382, entitled "HIGH POWER WAVEGUIDE," and filed
Nov. 22, 2002, and Provisional Patent Application 60/458,645,
entitled "PHOTONIC CRYSTAL FIBER," and filed Mar. 28, 2003, the
entire contents each of which are hereby incorporated by
reference.
BACKGROUND
[0002] This invention relates to the field of dielectric waveguides
and methods for making waveguides.
[0003] Waveguides play important roles in numerous industries. For
example, optical waveguides are widely used in telecommunications
networks, where fiber waveguides such as optical fibers are used to
carry information between different locations as optical signals.
Such waveguides substantially confine the optical signals to
propagation along a preferred path or paths. Other applications of
optical waveguides include imaging applications, such as in an
endoscope, and in optical detection.
[0004] The most prevalent type of fiber waveguide is an optical
fiber, which utilizes index guiding to confine an optical signal to
a preferred path. Such fibers include a core region extending along
a waveguide axis and a cladding region surrounding the core about
the waveguide axis and having a refractive index less than that of
the core region. Because of the index-contrast, optical rays
propagating substantially along the waveguide axis in the
higher-index core can undergo total internal reflection (TIR) from
the core-cladding interface. As a result, the optical fiber guides
one or more modes of electromagnetic (EM) radiation to propagate in
the core along the waveguide axis. The number of such guided modes
increases with core diameter. Notably, the index-guiding mechanism
precludes the presence of any cladding modes lying below the
lowest-frequency guided mode for a given wavevector parallel to the
waveguide axis. Almost all index-guided optical fibers in use
commercially are silica-based in which one or both of the core and
cladding are doped with impurities to produce the index contrast
and generate the core-cladding interface. For example, commonly
used silica optical fibers have indices of about 1.45 and index
contrasts ranging from about 0.2% to 3% for wavelengths in the
range of 1.5 .mu.m, depending on the application.
[0005] Drawing a fiber from a preform is the most commonly used
method for making fiber waveguides. A preform is a short rod (e.g.,
10 to 20 inches long) having the precise form and composition of
the desired fiber. The diameter of the preform, however, is much
larger than the fiber diameter (e.g., 100's to 1000's of times
larger). Typically, when drawing an optical fiber, the material
composition of a preform includes a single glass having varying
levels of one or more dopants provided in the preform core to
increase the core's refractive index relative to the cladding
refractive index. This ensures that the material forming the core
and cladding are rheologically and chemically similar to be drawn,
while still providing sufficient index contrast to support guided
modes in the core. To form the fiber from the preform a furnace
heats the preform to a temperature at which the glass viscosity is
sufficiently low (e.g., less than 10.sup.8 Poise) to draw fiber
from the preform. Upon drawing, the preform necks down to a fiber
that has the same cross-sectional composition and structure as the
preform. The diameter of the fiber is determined by the specific
rheological properties of the fiber and the rate at which it is
drawn.
[0006] Preforms can be made using many techniques known to those
skilled in the art, including modified chemical vapor deposition
(MCVD), outside vapor deposition (OVD), plasma activated chemical
vapor deposition (PCVD) and vapor axial deposition (VAD). Each
process typically involves depositing layers of vaporized raw
materials onto a wall of a pre-made tube or rod in the form of
soot. Each soot layer is fused shortly after deposition. This
results in a preform tube that is subsequently collapsed into a
solid rod, over jacketed, and then drawn into fiber.
[0007] Optical fibers applications can be limited by wavelength and
signal power. Preferably, fibers should be formed from materials
that have low absorption of energy at guided wavelengths and should
have minimal defects. Where absorption is high, it can reduce
signal strength to levels indistinguishable from noise for
transmission over long fibers. Even for relatively low absorption
materials, absorption by the core and/or cladding heats the fiber.
Defects can scatter guided radiation out of the core, which can
also lead to heating of the fiber. Above a certain power density,
this heating can irreparably damage the fiber. Accordingly, many
applications that utilize high power radiation sources use
apparatus other than optical fibers to guide the radiation from the
source to its destination.
SUMMARY
[0008] High power laser systems are disclosed. Such systems operate
at powers of at least about one Watt. In some cases, operational
intensity can be more than about 100 Watts, such as about a
kilowatt or more. These systems include dielectric waveguides for
delivering the laser beam to a target. The energy guided by the
waveguides can have extremely high power densities. For example,
the power density in some waveguides can be more than about
10.sup.6 W/cm.sup.2 (e.g., more than about 10.sup.8 W/cm.sup.2,
more than about 10.sup.10 W/cm.sup.2).
[0009] Suitable dielectric waveguides include fiber waveguides
capable of guiding high power electromagnetic energy, such as
certain photonic crystal fibers (e.g., certain Bragg fibers). Such
dielectric waveguides include one or more portions formed from a
chalcogenide glass. In some embodiments, the dielectric waveguides
can include two (or more) different chalcogenide glasses, where the
different chalcogenide glasses have different refractive indexes.
Note that the refractive index of a material refers to the
refractive index of a material at the wavelength at which the
waveguide is designed to guide light. Preferably, the different
glasses have similar thermomechanical properties and can be
co-drawn.
[0010] The portions of the waveguide are structural elements of the
waveguide that determine the optical properties of the waveguide
(e.g., structural elements that determine how the waveguide
confines an optical signal to a path). In preferred embodiments,
the fiber waveguide is a photonic crystal fiber, which includes a
core and a confinement region. The confinement region has a
refractive index variation that forms a bandgap and reflects light
within a certain range of frequencies, confining that light to the
core. One type of photonic crystal fiber is a Bragg fiber, in which
the confinement region can include multiple layers of different
composition that give rise to the index variation. In such cases,
each of the layers is considered a portion of the waveguide.
[0011] Photonic crystal waveguides can have hollow cores, which is
advantageous in high power applications because absorption of
guided energy by the core (and subsequent heating) is significantly
reduced compared to a solid core waveguide.
[0012] In some embodiments, the dielectric waveguides are
configured to guide electromagnetic energy at infrared wavelengths
(e.g., between about 1 micron and 15 microns, between about 5
microns and 12 microns, such as about 10.6 microns). The materials
forming the waveguides (e.g., chalcogenide glasses) may have
relatively low absorption at these wavelengths compared to other
materials, such as some other glasses. Thus, use of chalcogenide
glasses at these wavelengths can be advantageous because they may
have lower loss than similar waveguides formed from other materials
(e.g., polymers or oxide glasses), making them suitable for guiding
output energy from the high power laser to the target.
[0013] Methods for making dielectric waveguides are also disclosed.
In particular, chemical vapor deposition (CVD) methods suitable for
depositing layers of different materials in a deposition tube are
disclosed. These methods can be used, for example, to deposit
alternating layers of two different chalcogenide glasses in a
deposition tube or to deposit alternating layers of a chalcogenide
glass and an oxide glass. CVD methods can provide preforms that can
be drawn into fibers with low defect densities. Because defects
tend to scatter energy, which locally heats the fiber, low defect
density fiber is particularly desirable for high power density
transmission where excessive heating can be fatal to the fiber.
[0014] In general, in a first aspect, the invention features a
waveguide that includes a first portion extending along a waveguide
axis including a first chalcogenide glass, and a second portion
extending along the waveguide axis including a second chalcogenide
glass, wherein the second chalcogenide glass is different from the
first chalcogenide glass.
[0015] Embodiments of the waveguide can include one or more of the
following features and/or features of other aspects.
[0016] The first chalcogenide glass can have a different refractive
index than the second chalcogenide glass. The first chalcogenide
glass can include As and Se. For example, the first chalcogenide
glass can include As.sub.2Se.sub.3. In some embodiments, the first
chalcogenide glass can further include Pb, Sb, Bi, I, or Te. The
second chalcogenide glass can include As and S (e.g.,
As.sub.2S.sub.3), and/or P and S. The second chalcogenide glass can
include Ge or As.
[0017] The first chalcogenide glass can have a refractive index of
2.7 or more. The second chalcogenide glass has a refractive index
of 2.7 or less. The first chalcogenide glass can have a glass
transition temperature (T.sub.g) of about 180.degree. C. or more.
The second chalcogenide glass can have a T.sub.g of about
180.degree. C. or more.
[0018] The waveguide can have a loss coefficient less than about 2
dB/m for electromagnetic energy having a wavelength of about 10.6
microns. The waveguide can have a hollow core. The first portion
can surround a core (e.g., the hollow core). The second portion can
also surround the core. The second portion can surround the first
portion. The core can have a minimum cross-sectional dimension of
at least about 10.lambda. (e.g., about 20.lambda., 50.lambda.,
100.lambda.), where .lambda. is the wavelength of radiation guided
by the waveguide. The core can have a minimum cross-sectional
dimension of at least about 50 microns (e.g., at least about 100
microns, at least about 200 microns).
[0019] The waveguide can be a photonic crystal fiber, such as a
Bragg fiber. The photonic crystal fiber can include a confinement
region and the first and second portions are part of the
confinement region.
[0020] In general, in another aspect, the invention features a
method that includes providing a waveguide having a first portion
extending along a waveguide axis including a first chalcogenide
glass and a second portion extending along the waveguide axis, and
guiding electromagnetic energy from a first location to a second
location through the waveguide.
[0021] Embodiments of the method can include one or more of the
following features, and/or features of other aspects.
[0022] The second portion can include a second chalcogenide glass
different from the first chalcogenide glass. The electromagnetic
energy can have a wavelength of between about 2 microns and 15
microns. The electromagnetic energy can have an intensity of more
than about one Watt (e.g., more than about 5 Watts, 10 Watts, 50
Watts, 100 Watts, such as 1 kW or more).
[0023] The method can include coupling the electromagnetic energy
from a laser into the waveguide. The laser can be a CO.sub.2
laser.
[0024] The waveguide can be a photonic crystal fiber, such as a
Bragg fiber.
[0025] In general, in a further aspect, the invention features an
apparatus that includes a dielectric waveguide extending along an
axis and configured to guide electromagnetic radiation along the
axis, wherein the electromagnetic radiation has a power greater
than about 1 Watt.
[0026] Embodiments of the apparatus can include one or more of the
following features and/or features of other aspects.
[0027] The electromagnetic radiation can have a wavelength greater
than about 2 microns (e.g., greater than about 5 microns). The
electromagnetic radiation can have a wavelength less than about 20
microns (e.g., less than about 15 microns). For example, the
electromagnetic radiation can have a wavelength between about 10
microns to 11 microns (e.g., about 10.6 microns).
[0028] The electromagnetic radiation can have a power greater than
about 5 Watts (e.g., greater than about 10 Watts, 50 Watts, 100
Watts, such as 1 kW or more).
[0029] The dielectric waveguide can include a first portion
extending along the waveguide axis including a first chalcogenide
glass. The dielectric waveguide can further include a second
portion extending along the waveguide axis, the second portion
having a different composition than the first portion. The second
portion can include an oxide glass or a chalcogenide glass. For
example, the second portion can include a second glass different
from the first chalcogenide glass.
[0030] The waveguide can be a photonic crystal fiber, such as a
Bragg fiber. The waveguide can have a hollow core.
[0031] In general, in another aspect, the invention features a
method that includes exposing a surface to a first gas composition
under conditions sufficient to deposit a layer of a first
chalcogenide glass on the surface, and exposing the layer of the
first chalcogenide glass to a second gas composition under
conditions sufficient to deposit a layer of a second glass on the
layer of the first chalcogenide glass, wherein the second glass is
different from the first chalcogenide glass.
[0032] Embodiments of the method can include one or more of the
following features and/or features of other aspects.
[0033] Exposing the surface to the first gas composition can
include activating a plasma in the first gas composition.
Activating the plasma can include exposing the gas to
electromagnetic radiation to activate the plasma (e.g., microwave
or radio frequency radiation).
[0034] Exposing the layer of the first chalcogenide glass to the
second gas composition can include activating a plasma in the
second gas composition, which can include exposing the second gas
composition to electromagnetic radiation to activate the plasma
(e.g., microwave or radio frequency radiation).
[0035] The second gas composition is typically different from the
first gas composition. The first gas composition can include one or
more halide compounds (e.g., one or more chloride compounds). The
first gas composition can include a carrier gas (e.g., nitrogen or
a noble gas, like argon). The first gas composition can include a
chalcogen. The first gas composition pressure can be between about
2 and 20 Torr.
[0036] The second gas composition can include one or more halide
compounds (e.g., chloride compounds). The second gas composition
can include a carrier gas (e.g., nitrogen or a noble gas, like
argon). The second gas composition can include a chalcogen.
Alternatively, or additionally, the second gas composition
comprises oxygen. The second gas composition pressure can be
between about 2 and 20 Torr.
[0037] The second glass can be an oxide glass or a chalcogenide
glass.
[0038] The surface can be a surface of a tube, e.g., an inner
surface of a tube. The tube can be a glass (e.g., an oxide glass,
such as a silicate glass) tube or a polymer tube. In some
embodiments, the surface is a planar surface.
[0039] In general, in a further aspect, the invention features a
method that includes introducing a first gas composition into a
tube, the first gas composition including a first compound that is
substantially inert with respect to a first material forming the
inner surface of the tube, and exposing the first gas composition
to conditions sufficient to change the first compound into a second
compound reactive with the first material and to deposit a layer of
a second material on the inner surface of the tube.
[0040] Embodiments of the method can include one or more of the
following features and/or features of other aspects.
[0041] Exposing the first gas composition to conditions sufficient
to change the first compound into a second compound can include
activating a plasma in the first gas composition. In some
embodiments, activating the plasma includes exposing the first gas
composition to electromagnetic radiation (e.g., microwave or radio
frequency radiation).
[0042] The first compound can include oxygen. For example, the
first compound can be nitrous oxide. The second compound can be
oxygen. The first material can be a glass, such as a chalcogenide
glass.
[0043] In some embodiments, the method further includes exposing
the layer of the first material to a second gas composition under
conditions sufficient to deposit a layer of a second material on
the layer of the first material, wherein the second glass is
different from the first glass. For example, the first glass can be
a chalcogenide glass and the second glass can be an oxide
glass.
[0044] Embodiments of the invention may include one or more of the
following advantages.
[0045] Waveguides disclosed herein can guide high intensity
electromagnetic radiation without sustaining damage due to heating.
These waveguides can exhibit low loss at guided wavelengths.
[0046] The CVD techniques disclosed herein may be used to deposit
layers of dissimilar materials (e.g., optically dissimilar) on a
substrate. In some embodiments, dissimilar materials can be
deposited without significant undesirable reactions occurring
between the gases used for depositing a second material and the
surface of the initially deposited material. In some embodiments,
the CVD process can deposit layers of optically dissimilar
materials that have similar thermomechanical properties, and can be
co-drawn. Waveguides formed using the CVD process can have low
defect densities (e.g., low impurity concentrations), and may thus
be particularly suitable for high power applications, where high
defect densities could result in significant heating (and
ultimately failure) of the waveguide.
[0047] Unless otherwise defined, all technical and scientific terms
used herein have the same meaning as commonly understood by one of
ordinary skill in the art to which this invention belongs. In case
of conflict, the present specification, including definitions, will
control. In addition, the materials, methods, and examples
disclosed herein are illustrative only and not intended to be
limiting.
[0048] Additional features, objects, and advantages of the
invention will be apparent from the following detailed description
and drawings, and from the claims.
DESCRIPTION OF DRAWINGS
[0049] FIG. 1 is a schematic diagram of a laser system
incorporating a photonic crystal fiber.
[0050] FIG. 2 is a cross-sectional view of an embodiment of a
photonic crystal fiber.
[0051] FIG. 3A is a plot showing modeled radiation loss of a
photonic crystal fiber as a function of wavelength.
[0052] FIG. 3B is a plot showing modeled absorption loss of the
photonic crystal fiber as a function of wavelength.
[0053] FIG. 4 is a schematic diagram of a chemical vapor deposition
(CVD) system.
[0054] FIG. 5 is a schematic diagram of a portion of the CVD system
shown in FIG. 4.
[0055] FIG. 6 is a schematic diagram of a laser system
incorporating a photonic crystal fiber.
[0056] Like reference symbols in the various drawings indicate like
elements.
DETAILED DESCRIPTION
[0057] Referring to FIG. 1, a laser system 100 includes a laser 110
and a photonic crystal fiber 120 for guiding electromagnetic (EM)
energy from the laser to a location 130 remote from the laser.
Radiation is coupled from laser 110 into fiber 120 using a coupler
140. Laser 110 can be continuous wave or pulsed. The distance
between laser 110 and location 130 can vary depending on the
specific application, and can be on the order of several meters or
more (e.g., more than about 10 m, 20 m, 50 m, 100 m).
[0058] Laser system 100 can operate at UV, visible, or infrared
(1R) wavelengths. In some embodiments, photonic crystal fiber 120
is configured to guide IR energy emitted by laser 110, and the
energy has a wavelength between about 0.7 microns and 20 microns
(e.g., between about 2 to 5 microns or between about 8 to 12
microns). In some embodiments, laser 110 is a CO.sub.2 laser and
the radiation has a wavelength of about 6.5 microns or 10.6
microns. Other examples of lasers which can emit IR energy include
Nd:YAG lasers (e.g., at 1.064 microns) Er:YAG lasers (e.g., at 2.94
microns), Er, Cr:YSGG (Erbium, Chromium doped Yttrium Scandium
Gallium Garnet) lasers (e.g., at 2.796 microns), Ho:YAG lasers
(e.g., at 2.1 microns), free electron lasers (e.g., in the 6 to 7
micron range), and quantum cascade lasers (e.g., in the 3 to 5
micron range.
[0059] The power emitted from laser 110 at the guided wavelength
can vary. Although the laser power can be relatively low, e.g., mW,
in many applications the laser system is operated at high powers.
For example, the laser output intensity can be more than about one
Watt (e.g., more than five Watts, 10 Watts, 20 Watts). In some
applications, the laser output energy can be more than about 100
Watts, such as several hundred Watts (e.g., about 200 Watts, 300
Watts, 500 Watts, 1 kilowatt).
[0060] For high power systems, the power density guided by fiber
120 can be extremely high. For example, power density in the fiber
can be more than about 10.sup.6 W/cm.sup.2, such as more than about
10.sup.7 W/cm.sup.2, 10.sup.8 W/cm.sup.2, 10.sup.9 W/cm.sup.2, or
10.sup.10 W/cm.sup.2.
[0061] Fiber 120 can have relatively low losses at the guided
wavelength (e.g., less than about 10 dB/m, 5 dB/m, 2 dB/m, 1 dB/m,
0.5 dB/m, 0.2 dB/m). Due to the low loss, only a relatively small
amount of the guided energy is absorbed by the fiber, allowing the
fiber to guide high power radiation without substantial damage due
to heating.
[0062] Coupler 140 can be any coupler suitable for the wavelength
and intensity at which the laser system operates. One type of a
coupler is described by R. Nubling and J. Harrington in
"Hollow-waveguide delivery systems for high-power, industrial
CO.sub.2 lasers," Applied Optics, 34, No. 3, pp. 372-380 (1996).
Other examples of couplers include one or more focusing elements,
such as one or more lenses. Coupling efficiency can be high. For
example, coupler 140 can couple more than about 70% of the laser
output into a guided mode in the fiber (e.g., more than about 80%,
90%, 95%, 98%). Coupling efficiency refers to the ratio of power
guided away by the desired mode to the total power incident on the
fiber.
[0063] Optionally, system 100 includes a cooling apparatus 150
(e.g., a pump or compressor), which reduces heating of fiber 120
during operation. Cooling apparatus 150 can be an air-based system,
forcing air through a sheath 165, which surrounds other portions of
the fiber. Alternatively, cooling apparatus 150 can utilize a
liquid coolant (e.g., water), forcing a liquid through the sheath.
Cooling apparatus 150 may be particularly beneficial in
applications where the fiber guides energy at extremely high
intensities (e.g., several hundred Watts or kilowatts). For
example, the fiber may be maintained at temperatures within its
operational range at such high intensities.
[0064] Referring to FIG. 2, photonic crystal fiber 120 includes a
core 220 extending along a waveguide axis and a dielectric
confinement region 210 (e.g., a multilayer cladding) surrounding
the core. Confinement region 210 is surrounded by a support layer
250, which provides mechanical support for the confinement region.
Optionally, support layer 250 is surrounded by sheath 165. A space
265 exists between sheath 165 and fiber 120. As discussed
previously, a liquid or gas can be forced through the space between
the sheath and the cladding to cool the fiber during operation.
[0065] In the embodiment of FIG. 2, confinement region 210 is shown
to include alternating layers 230 and 240 of dielectric materials
having different refractive indices. One set of layers, e.g.,
layers 240, define a high-index set of layers having an index
n.sub.H and a thickness d.sub.H, and the other set of layers, e.g.,
layers 230, define a low-index set of layers having an index
n.sub.L and a thickness d.sub.L, where n.sub.H>n.sub.L (e.g.,
n.sub.H-n.sub.L can be greater than or equal to or greater than
0.01, 0.05, 0.1, 0.2, 0.5 or more). For convenience, only a few of
the dielectric confinement layers are shown in FIG. 1. In practice,
confinement region 210 may include many more layers (e.g., more
than about 15 layers, 20 layers, 0.30 layers, 40 layers, 50 layers,
such as 80 or more layers).
[0066] Although not illustrated in FIG. 2, fiber 120 may include
one or more additional layers between the confinement region and
the core. For example, the fiber may include one or more layers
selected to tailor the dispersion characteristics of the fiber.
Examples of such fibers are described in U.S. patent application
Ser. No. 10/057,440, entitled "PHOTONIC CRYSTAL OPTICAL WAVEGUIDES
HAVING TAILORED DISPERSION PROFILES," filed Jan. 25, 2002, and
having Pub. No. US-2002-0.176676-A1, the entire contents of which
are hereby incorporated by reference.
[0067] Layers 240 include a material having a high refractive
index, such as a chalcogenide glass. The high index material in
layers 240 can be rheologically compatible with the material
forming layers 230. The material in each of layers 240 can be the
same or different. Layers 230 include a material having a
refractive index lower than the high index material of adjacent
layers 240, and can include a second chalcogenide glass or an oxide
glass. In embodiments where layers 230 and 240 both include
chalcogenide glasses, the glasses are usually different. The
material in each of layers 230 can be the same or different.
Examples of high and low index materials are described below.
[0068] In the present embodiment, core 220 is hollow. Optionally,
the hollow core can be filled with a fluid, such as a gas (e.g.,
air, nitrogen, and/or a noble gas) or liquid (e.g., an isotropic
liquid or a liquid crystal). Alternatively, core 220 can include
any material or combination of materials that are rheologically
compatible with the materials forming confinement region 210. In
certain embodiments, core 220 can include one or more dopant
materials, such as those described in U.S. patent application Ser.
No. 10/121,452, entitled "HIGH INDEX-CONTRAST FIBER WAVEGUIDES AND
APPLICATIONS," filed Apr. 12, 2002 and now published under Pub. No.
US-2003-0044158-A1, the entire contents of which are hereby
incorporated by reference.
[0069] Photonic crystal fiber 120 has a circular cross-section,
with core 220 having a circular cross-section and region 210 (and
layers therein) having an annular cross-section. In other
embodiments, however, the waveguide and its constituent regions may
have different geometric cross-section such as a rectangular or a
hexagonal cross-section. Furthermore, as mentioned below, core and
confinement regions 220 and 210 may include multiple dielectric
materials having different refractive indices. In such cases, we
may refer to an "average refractive index" of a given region, which
refers to the sum of the weighted indices for the constituents of
the region, where each index is weighted by the fractional area in
the region of its constituent. The boundary between region 220 and
210, however, is defined by a change in index. The change may be
caused by the interface of two different dielectric materials or by
different dopant concentrations in the same dielectric material
(e.g., different dopant concentrations in silica).
[0070] Dielectric confinement region 210 guides EM radiation in a
first range of wavelengths to propagate in dielectric core 220
along the waveguide axis. The confinement mechanism is based on a
photonic crystal structure in region 210 that forms a bandgap
including the first range of wavelengths. Because the confinement
mechanism is not index-guiding, it is not necessary for the core to
have a higher index than that of the portion of the confinement
region immediately adjacent the core. To the contrary, core 220 may
have a lower average index than that of confinement region 210. For
example, core 220 may be air, some other gas, such as nitrogen, or
substantially evacuated. In such a case, EM radiation guided in the
core will have much smaller losses and much smaller nonlinear
interactions than EM radiation guided in a silica core, reflecting
the smaller absorption and nonlinear interaction constants of many
gases relative to silica or other such solid material. In
additional embodiments, for example, core 220 may include a porous
dielectric material to provide some structural support for the
surrounding confinement region while still defining a core that is
largely air. Accordingly, core 220 need not have a uniform index
profile.
[0071] The alternating layers 230 and 240 of confinement region 210
form what is known as a Bragg fiber. The alternating layers are
analogous to the alternating layers of a planar dielectric stack
reflector (which is also known as a Bragg mirror). The annular
layers of confinement region 210 and the alternating planar layers
of a dielectric stack reflector are both examples of a photonic
crystal structure. Photonic crystal structures are described
generally in Photonic Crystals by John D. Joannopoulos et al.
(Princeton University Press, Princeton N.J., 1995).
[0072] As used herein, a photonic crystal is a dielectric structure
with a refractive index modulation that produces a photonic bandgap
in the photonic crystal. A photonic bandgap, as used herein, is a
range of wavelengths (or inversely, frequencies) in which there are
no accessible extended (i.e., propagating, non-localized) states in
the dielectric structure. Typically the structure is a periodic
dielectric structure, but it may also include, e.g., more complex
"quasi-crystals." The bandgap can be used to confine, guide, and/or
localize light by combining the photonic crystal with "defect"
regions that deviate from the bandgap structure. Moreover, there
are accessible extended states for wavelengths both below and above
the gap, allowing light to be confined even in lower-index regions
(in contrast to index-guided TIR structures, such as those
described above). The term "accessible" states means those states
with which coupling is not already forbidden by some symmetry or
conservation law of the system. For example, in two-dimensional
systems, polarization is conserved, so only states of a similar
polarization need to be excluded from the bandgap. In a waveguide
with uniform cross-section (such as a typical fiber), the
wavevector .beta. is conserved, so only states with a given .beta.
need to be excluded from the bandgap to support photonic crystal
guided modes. Moreover, in a waveguide with cylindrical symmetry,
the "angular momentum" index m is conserved, so only modes with the
same m need to be excluded from the bandgap. In short, for
high-symmetry systems the requirements for photonic bandgaps are
considerably relaxed compared to "complete" bandgaps in which all
states, regardless of symmetry, are excluded.
[0073] Accordingly, the dielectric stack reflector is highly
reflective in the photonic bandgap because EM radiation cannot
propagate through the stack. Similarly, the annular layers in
confinement region 210 provide confinement because they are highly
reflective for incident rays in the bandgap. Strictly speaking, a
photonic crystal is only completely reflective in the bandgap when
the index modulation in the photonic crystal has an infinite
extent. Otherwise, incident radiation can "tunnel" through the
photonic crystal via an evanescent mode that couples propagating
modes on either side of the photonic crystal. In practice, however,
the rate of such tunneling decreases exponentially with photonic
crystal thickness (e.g., the number of alternating layers). It also
decreases with the magnitude of the index-contrast in the
confinement region.
[0074] Furthermore, a photonic bandgap may extend over only a
relatively small region of propagation vectors. For example, a
dielectric stack may be highly reflective for a normally incident
ray and yet only partially reflective for an obliquely incident
ray. A "complete photonic bandgap" is a bandgap that extends over
all possible wavevectors and all polarizations. Generally, a
complete photonic bandgap is only associated with a photonic
crystal having index modulations along three dimensions. However,
in the context of EM radiation incident on a photonic crystal from
an adjacent dielectric material, we can also define an
"omnidirectional photonic bandgap," which is a photonic bandgap for
all possible wavevectors and polarizations for which the adjacent
dielectric material supports propagating EM modes. Equivalently, an
omnidirectional photonic bandgap can be defined as a photonic band
gap for all EM modes above the light line, wherein the light line
defines the lowest frequency propagating mode supported by the
material adjacent the photonic crystal. For example, in air the
light line is approximately given by .omega.=c.beta., where .omega.
is the angular frequency of the radiation, .beta. is the
wavevector, and c is the speed of light. A description of an
omnidirectional planar reflector is disclosed in U.S. Pat. No.
6,130,780, the contents of which are incorporated herein by
reference. Furthermore, the use of alternating dielectric layers to
provide omnidirectional reflection (in a planar limit) for a
cylindrical waveguide geometry is disclosed in U.S. Pat. No.
6,463,200, entitled "OMNIDIRECTIONAL MULTILAYER DEVICE FOR ENHANCED
OPTICAL WAVEGUIDING," to Yoel Fink et al., the contents of which
are incorporated herein by reference.
[0075] When alternating layers 230 and 240 in confinement region
210 give rise to an omnidirectional bandgap with respect to core
220, the guided modes are strongly confined because, in principle,
any EM radiation incident on the confinement region from the core
is completely reflected. However, such complete reflection only
occurs when there are an infinite number of layers. For a finite
number of layers (e.g., about 20 layers), an omnidirectional
photonic bandgap may correspond to a reflection in a planar
geometry of at least 95% for all angles of incidence ranging from
0.degree. to 80.degree. and for all polarizations of EM radiation
having frequency in the omnidirectional bandgap. Furthermore, even
when photonic crystal fiber 120 has a confinement region with a
bandgap that is not omnidirectional, it may still support a
strongly guided mode, e.g., a mode with radiation losses of less
than 0.1 dB/km for a range of frequencies in the bandgap.
Generally, whether or not the bandgap is omnidirectional will
depend on the size of the bandgap produced by the alternating layer
(which generally scales with index-contrast of the two layers) and
the lowest-index constituent of the photonic crystal.
[0076] In additional embodiments, the dielectric confinement region
may include photonic crystal structures different from a multilayer
Bragg configuration. For example, rather than the Bragg
configuration, which is an example of a one-dimensionally periodic
photonic crystal (in the planar limit), the confinement region may
be selected to form, for example, a two-dimensionally periodic
photonic crystal (in the planar limit), such as an index modulation
corresponding to a honeycomb structure. See, for example, R. F.
Cregan et al., Science 285, p. 1537-1539, 1999. Furthermore, even
in a Bragg-like configuration, the high-index layers may vary in
index and thickness, and/or the low-index layers may vary in index
and thickness. The confinement region may also include a periodic
structure including more than two layers per period (e.g., three or
more layers per period). Moreover, the refractive index modulation
may vary continuously or discontinuously as a function of fiber
radius within the confinement region. In general, the confinement
region may be based on any index modulation that creates a photonic
bandgap.
[0077] In the present embodiment, multilayer structure 210 forms a
Bragg reflector because it has a periodic index variation with
respect to the radial axis. A suitable index variation is an
approximate quarter-wave condition. It is well-known that, for
normal incidence, a maximum band gap is obtained for a
"quarter-wave" stack in which each layer has equal optical
thickness .lambda./4, or equivalently d.sub.H/d.sub.L=n.sub.L/n.s-
ub.H, where d and n refer to the thickness and index, respectively,
of the high-index and low-index layers. These correspond to layers
240 and 230, respectively. Normal incidence corresponds to
.beta.=0. For a cylindrical waveguide, the desired modes typically
lie near the light line .omega.=c.beta. (in the large core radius
limit, the lowest-order modes are essentially plane waves
propagating along z-axis, i.e., the waveguide axis). In this case,
the quarter-wave condition becomes: 1 d H d L = n L 2 - 1 n H 2 -
1
[0078] Strictly speaking, this equation may not be exactly optimal
because the quarter-wave condition is modified by the cylindrical
geometry, which may require the optical thickness of each layer to
vary smoothly with its radial coordinate. Nonetheless, we find that
this equation provides an excellent guideline for optimizing many
desirable properties, especially for core radii larger than the
mid-bandgap wavelength.
[0079] Some embodiments of photonic crystal fibers are described in
U.S. patent application Ser. No. 10/057,258, entitled "LOW-LOSS
PHOTONIC CRYSTAL FIBER HAVING LARGE CORE RADIUS," to Steven G.
Johnson et al., filed Jan. 25, 2002 and published under Pub. No.
US-2002-0164137-A1, the entire contents of which are hereby
incorporated by reference.
[0080] The radius of core 220 can vary depending on the end-use
application of fiber 120. The core radius can depend on the
wavelength or wavelength range of the energy to be guided by the
fiber, and on whether the fiber is a single or multimode fiber. For
example, where the fiber is a single mode fiber for guiding visible
wavelengths (e.g., between about 400 nm and 800 nm) the core radius
can be in the sub-micron to several micron range (e.g., from about
0.5 .mu.m to 5 .mu.m). However, where the fiber is a multimode
fiber for guiding IR wavelengths (e.g., from about 2 .mu.m to 15
.mu.m, such as 10.6 .mu.m), the core radius can be in the tens to
thousands of microns range (e.g., from about 10 .mu.m to 2,000
.mu.m, such as 500 .mu.m to 1,000 .mu.m). The core radius can be
greater than about 5.lambda. (e.g., more than about 10.lambda.,
20.lambda., 50.lambda., 100.lambda.), where .lambda. is the
wavelength of the guided energy.
[0081] Two mechanisms by which energy can be lost from a guided
signal in a photonic crystal fiber are by absorption loss and
radiation loss. Absorption loss refers to loss due to material
absorption. Radiation loss refers to energy that leaks from the
fiber due to imperfect confinement. Both modes of loss can be
studied theoretically, for example, using transfer matrix methods
and perturbation theory. A discussion of transfer matrix methods
can be found in an article by P. Yeh et al., J. Opt. Soc. Am., 68,
p. 1196 (1978). A discussion of perturbation theory can found in an
article by M. Skorobogatiy et al., Optics Express, 10, p. 1227
(2002). Particularly, transfer matrix code finds propagation
constants .beta. for the "leaky" modes resonant in a photonic
crystal fiber structure. Imaginary parts of .beta.'s define the
modal radiation loss, thus Loss.sub.radiation.about.Im(.beta.).
Loss due to material absorption is calculated using perturbation
theory expansions, and in terms of the modal field overlap integral
it can be determined from 2 Loss absorption 2 0 .infin. r r ( E * E
) ,
[0082] where .omega. is the radiation frequency, r is the fiber
radius, .alpha. is bulk absorption of the material, and {right
arrow over (E)}.sub..beta. is an electric field vector.
[0083] Based on theoretical and/or empirical investigations,
photonic crystal fibers, such as fiber 120, can be designed to
minimize one or both mode of loss. Guided modes can be classified
as one of three types: pure transverse electric (TE); pure
transverse magnetic (TM); and mixed modes. Loss often depends on
the type of mode. For example, TE modes can exhibit lower radiation
and absorption losses than TM/mixed modes. Accordingly, the fiber
can be optimized for guiding a mode that experiences low radiation
and/or absorption loss. Alternatively, or additionally, the fiber
can be optimized for a mode that is well matched to the mode of
laser 110. For example, the fiber can be optimized for guiding the
HE.sub.11 (mixed) mode, which is well matched to the TEM.sub.00
mode of a laser. Being "well matched" refers to efficient coupling
between the mode of the laser and the guided mode of the fiber.
[0084] Radiation loss can be reduced by adding layers to the
confinement region of fiber 120, increasing the index contrast
between the high and low index layers, increasing the core radius
and/or lowering the intrinsic absorption losses of the first few
layers by selecting materials with low absorption at the guided
wavelengths. For example, at wavelengths of about 3 microns,
chalcogenide glasses exhibit an absorption coefficient of about 4
dB/m compared to many polymers which have an absorption coefficient
of about 10.sup.5 dB/m in that wavelength range. Similarly, at 10.6
microns, chalcogenide glasses have an absorption coefficient of
about 10 dB/m compared to 10.sup.5 dB/m for many polymers. Thus,
using chalcogenide glasses instead of polymers can reduce losses in
some cases. However, polymers, like oxide glasses, can provide
lower index materials than chalcogenide glasses.
[0085] As an example, consider a photonic crystal fiber having a
core radius R.sub.i=500 .mu.m, the confinement region materials
have indices of n.sub.l=2.3 and n.sub.h=2.7, with a bi-layer
thickness, d=2.3 .mu.m. The corresponding thickness of the low
index and high index layers are 1.3 .mu.m and 1.0 .mu.m,
respectively. For the purposes of this example, the intrinsic bulk
absorption loss of high/low index materials is taken to be 10 dB/m.
The support layer (R.sub.c=1500 .mu.m) is assumed to have
absorption loss of 10.sup.5 dB/m, typical of polymers. The
confinement region has 55 layers, thus R.sub.m=563 .mu.m.
[0086] At .lambda.=10.6 .mu.m, a theoretical model indicates that
these structural parameters define a fiber radiation loss of 24
dB/km (with a radiation loss decreasing by about an order of
magnitude with every 30 layers added to the confinement region),
and a material absorption loss in the confinement region of 0.23
dB/km. Adding 60 more layers to the confinement region reduces
radiation loss, which then becomes comparable to the material
absorption loss in the mirror. These results are summarized in FIG.
3A and FIG. 3B, which respectively show the dependence of the
radiation and absorption losses on the operating wavelength.
[0087] In contrast, consider a fiber having a similar structure,
except where the low index and high index materials have refractive
indices of n.sub.l=1.5 and n.sub.h=2.8, with a bi-layer thickness
of d=2.82 .mu.m (the bi-layer refers to a high index and low index
layer pair). These refractive index values are representative of a
polymer low index material and a chalcogenide glass high index
material. The corresponding layer thicknesses are 1.97 .mu.m and
0.84 .mu.m for the low and high index layers, respectively. The
intrinsic bulk absorption loss of high index material is 10.sup.5
dB/m. The support layer (R.sub.c=1500 .mu.m) and low index material
are assumed to have absorption loss of 10.sup.5 dB/m, typical of
polymers. In this example, the confinement region is assumed to
have 35 layers (17.5 bi-layers), thus R.sub.m=549 .mu.m.
[0088] At .lambda.=10.6 .mu.m, these structural parameters define a
fiber radiation loss of 1.09 dB/km (with a radiation loss
decreasing by an order of magnitude with every 4 bi-layers added),
and a material absorption loss, in the mirror, of 320 dB/km, where
power dissipation loss will be dominated by material absorption in
the first few polymer layers of the confinement region.
[0089] Accordingly, in some embodiments, the low index material can
be selected to have low absorption loss in the first few layers of
the confinement region, and higher relative absorption loss in
outer layers. The index contrast can be higher in the outer layers
compared the inner layers. For example, the confinement region can
have low index layers that include a chalcogenide glass in layers
close to the core, but include a polymer or oxide glass in layers
further from the core. The high index layers can include a
chalcogenide glass throughout.
[0090] As discussed previously, materials can be selected for the
confinement region to provide advantageous optical properties
(e.g., low absorption with appropriate indices of refraction at the
guided wavelength(s)). However, the materials should also be
compatible with the processes used to manufacture the fiber. In
some embodiments, the high and low index materials (e.g., the first
and second chalcogenide glasses) should preferably be compatible
for co-drawing. Criteria for co-drawing compatibility are provided
in aforementioned U.S. patent application Ser. No. 10/121,452,
entitled "HIGH INDEX-CONTRAST FIBER WAVEGUIDES AND APPLICATIONS."
In addition, the high and low index materials should preferably be
sufficiently stable with respect to crystallization, phase
separation, chemical attack and unwanted reactions for the
conditions (e.g., environmental conditions such as temperature,
humidity, and ambient gas environment) under which the fiber is
formed, deployed, and used.
[0091] As mentioned in the foregoing description of fiber 120,
layers 240 and 230 can include a first and second chalcogenide
glass, respectively (e.g., glasses containing a chalcogen element,
such as sulphur, selenium, and/or tellurium). In addition to a
chalcogen element, chalcogenide glasses may include one or more of
the following elements: boron, aluminum, silicon, phosphorus,
sulfur, gallium, germanium, arsenic, indium, tin, antimony,
thallium, lead, bismuth, cadmium, lanthanum and the halides
(fluorine, chlorine, bromide, iodine).
[0092] Chalcogenide glasses can be binary or ternary glasses, e.g.,
As--S, As--Se, Ge--S, Ge--Se, As--Te, Sb--Se, As--S--Se, S--Se--Te,
As--Se--Te, As--S--Te, Ge--S--Te, Ge--Se--Te, Ge--S--Se,
As--Ge--Se, As--Ge--Te, As--Se--Pb, As--S--Tl, As--Se--Tl,
As--Te--Tl, As--Se--Ga, Ga--La--S, Ge--Sb--Se or complex,
multi-component glasses based on these elements such as
As--Ga--Ge--S, Pb--Ga--Ge--S, etc. The ratio of each element in a
chalcogenide glass can be varied.
[0093] The amount of the first chalcogenide glass in the high index
material can vary. Typically, the high index material includes at
least about 50% by weight of the first chalcogenide glass (e.g., at
least 70%, 80%, 90%, 95%, 98%, 99%). The high index material can be
substantially exclusively chalcogenide glass (i.e., about 100%
chalcogenide glass). In some embodiments, in addition to the first
chalcogenide glasses, the high index material can include one or
more additional chalcogenide glasses, heavy metal oxide glasses,
amorphous alloys, or combinations thereof.
[0094] In some embodiments, the high index material is a
chalcogenide glass including As and Se. For example, the high index
material can include As.sub.2Se.sub.3. As.sub.2Se.sub.3 has a glass
transition temperature (T.sub.g) of about 180.degree. C. and a
thermal expansion coefficient (TEC) of about
24.times.10.sup.-6/.degree. C. At 10.6 .mu.m, As.sub.2Se.sub.3 has
a refractive index of 2.7775, as measured by Hartouni and coworkers
and described in Proc. SPIE, 505, 11 (1984), and an absorption
coefficient, .alpha., of 5.8 dB/m, as measured by Voigt and Linke
and described in "Physics and Applications of Non-Crystalline
Semiconductors in Optoelectronics," Ed. A. Andriesh and M.
Bertolotti, NATO ASI Series, 3. High Technology, Vol. 36, p. 155
(1996). Both of these references are hereby incorporated by
reference in their entirety.
[0095] The first chalcogenide glass can include As.sub.2Se.sub.3
and one or more other elements. Examples of other elements that can
be included are In, Sn, Sb, Te, I, Tl, Pb, and/or Bi. The index of
the first chalcogenide glass can be greater than the refractive
index of As.sub.2Se.sub.3. For example, chalcogenide glasses
including Sb and/or Te in addition to As.sub.2Se.sub.3 can increase
the refractive index of the chalcogenide glass above the refractive
index of As.sub.2Se.sub.3. The refractive index of the first
chalcogenide glass in these embodiments can be greater than about
2.8 (e.g., more than 2.9, such as about 3.0 or more).
[0096] Some elements that can be added to As.sub.2Se.sub.3 to
increase the refractive index of the first chalcogenide glass can
change the thermomechanical properties of the first chalcogenide
glass from the thermomechanical properties of As.sub.2Se.sub.3. The
thermomechanical properties include phase transition temperatures,
such as T.sub.g, and other parameters such as the glass's TEC. For
example, iodine may increase the refractive index of the first
chalcogenide glass, but can reduce T.sub.g. In such cases, one or
more additional compounds may be added to the first chalcogenide
glass to mitigate the effects of the index-raising element on the
glasses thermomechanical properties. On example of an element that
can reduce such thermomechanical effects is Ge. In embodiments, the
second chalcogenide glass can have a T.sub.g of more than about
180.degree. C. (e.g., about 200.degree. C., 220.degree. C.,
250.degree. C. or more).
[0097] The amount of additional compounds added to As.sub.2Se.sub.3
in the first chalcogenide glass can vary. Typically, the amount of
various elements in the first chalcogenide glass is determined
empirically according to the specifics of the photonic crystal
fiber. For example, where the fiber design requires the first
chalcogenide glass to have specific refractive index, an amount of
an index-raising element sufficient to provide the desired index is
added. Preferably, the amount of any index-raising element included
will be sufficiently small to not substantially affect the
stability of the glass (e.g., to prevent phase separation of the
glass components). In some embodiments, the amount of
As.sub.2Se.sub.3 in the first chalcogenide glass can be more than
about 80% molar (e.g., more than about 90%, 95%, 99%) and the
amount of one or more additional elements can be less than about
20% molar (e.g., less than about 10%, 5%, 1%).
[0098] The amount of the second chalcogenide glass in the low index
material can vary. Typically, the low index material includes at
least about 50% by weight of the second chalcogenide glass (e.g.,
at least 70%, 80%, 90%, 95%, 98%, 99%). The low index material can
be substantially exclusively chalcogenide glass (i.e., about 100%
chalcogenide glass). In some embodiments, in addition to the second
chalcogenide glasses, the high index material can include one or
more additional chalcogenide glasses, heavy metal oxide glasses,
amorphous alloys, or combinations thereof.
[0099] In some embodiments, the low index material is a
chalcogenide glass including As and Se. For example, the high index
material can include As.sub.2Se.sub.3.
[0100] The second chalcogenide glass can include As.sub.2Se.sub.3
and one or more other elements. Examples of other elements that can
be included are B, F, Al, Si, P, S, and/or Ge. In these
embodiments, the index of the second chalcogenide glass can be less
than the refractive index of As.sub.2Se.sub.3. For example,
chalcogenide glasses including P and/or S in addition to
As.sub.2Se.sub.3 can reduce the refractive index of the
chalcogenide glass below the refractive index of As.sub.2Se.sub.3.
The refractive index of the second chalcogenide glass in these
embodiments can be less than about 2.7 (e.g., less than 2.5, such
as about 2.0 or less).
[0101] Some elements that can be added to As.sub.2Se.sub.3 to
reduce the refractive index of the second chalcogenide glass can
change the thermomechanical properties of the first chalcogenide
glass from the thermomechanical properties of As.sub.2Se.sub.3. For
example, Si may reduce the refractive index of the second
chalcogenide glass, and can increase T.sub.g. In some such cases,
one or more additional compounds may be added to the second
chalcogenide glass to mitigate the effects of the index-reducing
element to ensure the low index material is compatible with the
high index material. In embodiments, the second chalcogenide glass
can have a T.sub.g of more than about 180.degree. C. (e.g., about
200.degree. C., 220.degree. C., 250.degree. C. or more).
[0102] The amount of additional compounds added to As.sub.2Se.sub.3
in the second chalcogenide glass can vary. Typically, the amount of
various elements in the second chalcogenide glass is determined
empirically according to the specifics of the photonic crystal
fiber. For example, where the fiber design requires the second
chalcogenide glass to have specific refractive index, an amount of
an index-reducing element sufficient to provide the desired index
is added. Preferably, the amount of any index-reducing elements
included will be sufficiently small to not substantially affect the
stability of the glass (e.g., to prevent phase separation of the
glass components). In some embodiments, the amount of
As.sub.2Se.sub.3 in the second chalcogenide glass can be more than
about 80% molar (e.g., more than about 90%, 95%, 99%) and the
amount of one or more additional elements can be less than about
20% molar (e.g., less than about 10%, 5%, 1%).
[0103] In some embodiments, the second chalcogenide glass can
include As.sub.2S.sub.3, GePS, and/or AsPS. The composition of the
second chalcogenide glass including As.sub.2S.sub.3, GePS, and/or
AsPS can be manipulated to obtain a desired refractive index and/or
thermomechanical properties as described for As.sub.2Se.sub.3
above.
[0104] The first and/or second chalcogenide glasses can have
relatively low loss at a wavelength of interest compared to some
non-chalcogenide glasses and/or some polymers (e.g., PES). For
example, at 10.6 microns, the first and/or second chalcogenide
glasses can have a loss co-efficient of less than about 1,000 dB/m.
More preferably, the first and/or second chalcogenide glasses can
have a loss coefficient of less than about 50 dB/m, such as less
than about 20 dB/m, 10 dB/m or less. In contrast, polymers such as
PES can have a loss co-efficient of 10,000 dB/m or more.
[0105] In order for dielectric waveguides to function reliably at
high power densities, they should have low defect densities. In
photonic crystal fibers, such as those described herein, defects
include delamination between layers, cracking, or other structural
defects, and material defects, such as impurities. Selecting
materials with matched thermomechanical properties can reduce the
occurrence of defects. One way to form preforms of these materials
with high purity is to use CVD.
[0106] In embodiments where CVD is used, the high and low index
materials (e.g., the first and second chalcogenide glasses) should
be compatible with this process. To be compatible with CVD,
precursors for the compounds from which solid deposits can be
formed should be available for forming the high and low index
materials.
[0107] Referring to FIG. 4, during the CVD process, a CVD system
500 is used to deposit layers of different materials on the inner
surface of a deposition tube 501. CVD system 500 includes a gas
source 510, a gas manifold 520, and a lathe 530 on which deposition
tube 501 is mounted. The material the system deposits in tube 501
forms in a chemical reaction between gases supplied to tube 501 by
gas source 510 via manifold 520. System 500 also includes a
microwave source 550, which excites a plasma in the gas within the
tube, causing the gases to react and deposit material on the tube
surface. A furnace 540 heats tube 501 to a desired temperature
during the deposition process. System 500 also includes tubes 570
that transport gases from gas source 510 to manifold 520. Valves
580 modulate the flow of gases from gas source 510 to manifold 520.
The gases mix inside manifold 520 before being transported to
deposition tube 501 via a pipe 590. The deposition process is
controlled by an electronic controller 560 (e.g., a system
including a processor for executing instructions, such as a
computer).
[0108] Referring also to FIG. 5, microwave source 550 includes a
resonator enclosing a segment of deposition tube 501. During
operation, the resonator couples microwave energy from a waveguide
into gas (e.g., vapor) within tube 501. Typically, this energy has
a frequency in the range of about 1 to about 40 GHz. For example,
the energy can have a frequency of about 5 to 15 GHz, such as about
12.5 GHz. The energy generates a local non-isothermal low-pressure
plasma region 610 within the tube. Gas flowing through the
deposition tube is deflected by plasma region 610 to the space
between plasma region 610 and tube 501, as indicated by arrows 620
and 630. Gasses proximate to the plasma react with each other,
forming a layer of material one the inner surface of tube 501
adjacent plasma 610. Preferably, microwave energy is transferred
without substantial energy loss to the tube itself, and microwave
energy is coupled directly into the activated plasma inside the
tube.
[0109] During operation, system 500 translates microwave source 550
back and forth along the axis of tube 501, exciting plasma in the
portion of the tube adjacent the source. Each pass of microwave
source 500 relative to the tube results in a layer of material
being deposited within the tube. The microwave source 550 can be
translated as many times as necessary to provide the desired
thickness of material with in the tube.
[0110] Furnace 540 heats the tube surface to a temperature
sufficient to ensure that deposited materials diffuse to form a
consolidated layer. For this reason, the temperature depends upon
the type of material being deposited. For many materials, the tube
is heated to a temperature between about 80.degree. C. and
250.degree. C., such as about 100.degree. C. The tube temperature
is kept below a temperature that would cause any substantial
adverse reaction in the deposited layer. For example, chalcogenide
glasses may oxidize at temperatures above 250.degree.
C.-300.degree. C. Thus, for these glasses, the tube surface is
maintained below these temperatures. Lower process temperatures can
also reduce mechanical stress in the deposited layers, reducing the
possibility of fracture and/or delamination in the multilayer
structure. The tube surface temperature may be varied between
depositing layers of different materials therein.
[0111] Controller 560 controls numerous parameters associated with
the deposition process to provide a layer of material having the
desired thickness and material properties (e.g., composition,
density, homogeneity and/or layer morphology). These parameters
include surface temperature, gas pressure, gas composition,
microwave energy, and microwave frequency. The effects of the
parameters on deposition rate and material properties are typically
interrelated. For example, changes in gas pressure and/or gas
composition can affect the deposition rate by providing more or
less of one or more reactant gases to the tube. Variations in
microwave energy and/or frequency can vary the deposition rate by
changing the temperature of the tube surface.
[0112] Due to its shape, plasma region 610 is often referred to as
a plasma "ball." The shape and size of the plasma ball is related
to the plasma mode excited by the radiation and can be affected by
gas pressure, the shape of the cavity, the gas composition, and/or
the ionization potential of the gas. For example, under otherwise
equivalent conditions, the size of a plasma ball formed in nitrogen
is typically smaller than a plasma ball formed in argon. Because
the gas phase reaction of component gases occurs proximate to the
plasma ball, the shape and size of the plasma ball can be selected
to control the tube area over which deposition occurs. In many
embodiments, where the deposition tube is cylindrical, the T.sub.01
plasma mode is desirable.
[0113] Initially, a first gas composition is used to produce a
layer of a first material. After depositing the first material but
prior to depositing the second material the tube is purged of
residual reactive gases. Typically, the system flows an inert gas
(i.e., inert with respect to the layer of material just deposited
in the tube and with residual gases in the tube) through the tube
for a time sufficient to purge substantially all of the first gas
composition from the tube. Examples of inert gases include nitrogen
and noble gases, such as argon. The system can monitor the
composition of gas purged from the tube to establish when the
concentration of the first gas composition in the tube is
sufficiently small to be negligible.
[0114] The first and second gas compositions include component
gases that react upon heating by the plasma to form the first and
second materials, respectively. The type and relative concentration
of component gases are selected based on the desired composition of
the materials. In embodiments where either of the materials are a
chalcogenide glass, at least one of the respective component gases
includes a chalcogen element. In embodiments where either of the
materials is an oxide glass, the respective gas composition
includes oxygen (e.g., as oxygen gas or the gas of an oxygen
containing compound). In each gas composition, one or more of the
components can be a halide (e.g., a chloride) gas or a hydride gas.
Examples of chlorides include SiCl.sub.4, BCl.sub.3, POCl.sub.3,
PCl.sub.3, GeCl.sub.4, SeCl.sub.2, AsCl.sub.3, and S.sub.2Cl.sub.2.
Examples of hydrides include H.sub.2Se, GeH.sub.4, H.sub.2S,
H.sub.2Te, AsH.sub.3, and PH.sub.3. In some embodiments, chlorides
may be preferred over hydrides, especially where hydrogen and/or
oxygen can contaminate the deposited material. Such contamination
may occur where decomposition of the component gas is incomplete
and/or due to the presence of water and/or oxygen.
[0115] During the deposition of a layer of the first or second
material, the relative concentration of component gases can remain
the same or vary. Where a homogeneous layer is desired, the
relative concentration of component gases is substantially
constant. However, where variations in composition are desired
through the layer, the relative concentration of component gases
can vary during deposition of the layer. For example, where a
refractive index gradient through the layer is desired, the
relative concentration of component gases can be varied during
deposition of the layer.
[0116] The first and/or second gas compositions can also include a
carrier gas, which is inert with respect to the other component
gases. A carrier gas can be used to adjust the pressure of the
first gas composition without affecting the relative concentration
of the component gases. Carrier gases are selected based on the
composition of the component gases. Examples of carrier gases
include nitrogen and noble gases, such as argon, and mixtures
thereof.
[0117] The ratio of carrier gas to component (reactant) gas(es) in
a gas composition may vary as desired. Typically, the ratio of
carrier to component gas(es) is between about 1:10.sup.-4 and
1:10.sup.-1. The relative amount of component gas(es) to carrier
gas can affect the deposition rate and the morphology of the
deposited material.
[0118] In some embodiments, the first deposited layer may adversely
react with a compound or element forming the subsequent layer while
that element or compound is in the form of a gas. An adverse
reaction introduces impurities into the preform, which can be
detrimental to fiber performance. For example, where an oxide glass
is being deposited onto a layer of a chalcogenide glass, gaseous
oxygen can oxidize the chalcogenide glass. In such instances, an
inert component gas containing the reactive element or compound can
be chosen for the gas composition to reduce (e.g., mitigate) any
adverse reaction between the gas and the previously deposited layer
(or tube). An example of a gas that can be used to provide oxygen
when depositing an oxide glass on a chalcogenide (or other
oxidizable glass) is nitrous oxide. In some embodiments, the
relative concentration of the reactive gas (e.g., oxygen) can be
increased once a thin layer of material (e.g., oxide glass) has
been deposited on the previous layer.
[0119] Material may be deposited at relatively high rates. For
example, the deposition rate may be about 1 .mu.m/min or more
(e.g., more than about 5 .mu.m/min, 8 .mu.m/min, 10 .mu.m/min).
[0120] In general, tube 501 can be formed from any material. Where
the tube forms part of the final drawn fiber, the tube should be
formed from a material that can be co-drawn with material deposited
within the tube. In some embodiments, tube 501 is formed from a
glass or a polymer. Examples of suitable glasses include
silica-based glasses. Examples of suitable polymers include
polysulfones, fluoropolymers (e.g., Teflon.RTM.), polyethylene and
their derivatives.
[0121] Although microwave radiation is used to excite plasma in
system 100, other forms of EM radiation can also be used. For
example, radio frequency radiation (e.g., with frequencies less
than about 10.sup.9 Hz) can be used to excite plasma in the tube.
Furthermore, in some embodiments, plasma can be excited thermally
alternatively or additionally to using EM radiation.
[0122] To make a preform for a photonic crystal fiber, additional
layers of material can be deposited on the layer of the second
material. In some embodiments, the sequential deposition of layers
of the first and second materials is repeated multiple times (e.g.,
twice, three time, four times, or more). Alternatively, the
composition of, e.g., a third layer may differ from the composition
of the first layer. For example, to make a preform for a low loss
photonic crystal fiber, materials with high index contrast (e.g.,
layers of a chalcogenide glass and an oxide glass) can be deposited
initially, followed by layers of materials with low absorption
(e.g., two different chalcogenide glasses). In some embodiments,
many layers can be deposited (e.g., more than about 10 layers, such
as 20 or more layers).
[0123] The thickness of each layer may vary as desired. Generally,
the deposited layer thickness will depend on the desired structure
of the waveguide and draw ratio to be used in subsequent fiber
drawing. The thickness of alternating layers may be the same or
different. In some embodiments, layers are formed that have the
same optical thickness. Deposited layer thickness is typically
between about 0.1 nm and 500 .mu.m.
[0124] Although the CVD methods described herein are with reference
to photonic crystal fibers, they can also be used to make other
types of waveguides (e.g., TIR optical fibers).
[0125] Referring to FIG. 6, in some embodiments, system 100 may be
modified to simultaneously provide output energy from laser 110 at
multiple locations. Modified system 700 includes a number of
couplers 710, which couple energy guided in waveguide 120 into
other waveguides 720. Each waveguide 720 can deliver laser energy
to a different location remote from laser 110. Waveguides 720 can
be the same or different as waveguide 120. For example, waveguides
720 can be photonic crystal fibers or some other type of waveguide
(e.g., TIR fiber). The intensity of laser energy coupled into each
waveguide 720 can be the same or different. Where each waveguide's
output is used in similar applications, the intensity delivered by
each waveguide can be the same. However, where applications are
different, the delivered intensity can vary accordingly.
[0126] It will be understood that various modifications to the
foregoing embodiments may be made without departing from the spirit
and scope of the invention. Accordingly, other embodiments are
within the scope of the following claims.
* * * * *