U.S. patent application number 10/463862 was filed with the patent office on 2004-05-20 for method for reducing stimulated brillouin scattering in waveguide systems and devices.
This patent application is currently assigned to The Board of Trustees of the University of Illinois. Invention is credited to Dragic, Peter, Papen, George.
Application Number | 20040096170 10/463862 |
Document ID | / |
Family ID | 24559195 |
Filed Date | 2004-05-20 |
United States Patent
Application |
20040096170 |
Kind Code |
A1 |
Papen, George ; et
al. |
May 20, 2004 |
Method for reducing stimulated brillouin scattering in waveguide
systems and devices
Abstract
A waveguide configuration including a core having an index of
refraction and a shear velocity, a first cladding extending about
the core having a shear velocity which is less than that of the
core and an index of refraction which is less than the core, a
second cladding extending about the first cladding, the second
cladding having a shear velocity which is greater than that of the
first cladding, wherein an optical mode has an index of refraction
greater than that of the second cladding, and a buffer extending
about the second cladding.
Inventors: |
Papen, George; (San Diego,
CA) ; Dragic, Peter; (Champaign, IL) |
Correspondence
Address: |
GREER, BURNS & CRAIN
300 S WACKER DR
25TH FLOOR
CHICAGO
IL
60606
US
|
Assignee: |
The Board of Trustees of the
University of Illinois
|
Family ID: |
24559195 |
Appl. No.: |
10/463862 |
Filed: |
June 16, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10463862 |
Jun 16, 2003 |
|
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|
09638239 |
Aug 14, 2000 |
|
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6587623 |
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Current U.S.
Class: |
385/123 |
Current CPC
Class: |
G02B 6/02042 20130101;
G02B 6/03694 20130101; H04B 10/2537 20130101 |
Class at
Publication: |
385/123 |
International
Class: |
H01S 003/13; G02B
006/02; G02B 006/16 |
Claims
What is claimed is:
1. A waveguide configuration comprising: a core having an index of
refraction and a shear velocity; a first cladding extending about
the core having a shear velocity which is less than that of the
core and an index of refraction which is less than the core; a
second cladding extending about the first cladding, the second
cladding having a shear velocity which is greater than that of the
first cladding, wherein an optical mode has an index of refraction
greater than that of the second cladding; and a buffer extending
about the second cladding.
2. The waveguide configuration of claim 1 wherein a cross-sectional
configuration of each of the core, the first cladding and the
second cladding are substantially uniform along a length
thereof.
3. The waveguide configuration of claim 1 further comprising a
third cladding positioned between the second cladding and the
buffer, the third cladding having an index of refraction less than
that of each of the core, first cladding and second cladding.
4. The waveguide configuration of claim 1 wherein the first
cladding is configured to be doped with fluorine and the second
cladding is configured to be pure silica.
5. The waveguide configuration of claim 1 wherein the second
cladding has a shear velocity that is less than that of the core
and an index of refraction that is less than that of the core.
6. The waveguide configuration of claim 1 wherein the second
cladding has a shear velocity that is greater than that of the core
and an index of refraction that is greater than that of the
core.
7. The waveguide configuration of claim 1 wherein the second
cladding has a shear velocity that is approximately equal to that
of the core and an index of refraction that is approximately equal
to that of the core.
8. A laser delivery system, the delivery system having reduced
stimulated brillouin scattering effects, the system comprising: a
laser light source; a pump light source; a first core which guides
light output from the laser light source and the pump light source
and which is anti-guiding for acoustic waves; a second core which
confines the light output from the laser light source to the first
core and guides light output from the pump light source, the second
core guiding acoustic waves; a cladding which confines the light
output from the pump light source to the second core, the cladding
guiding acoustic waves.
9. The laser delivery system of claim 8 wherein the first core has
a shear velocity that is less than that of both the second core and
the cladding.
10. The laser delivery system of claim 8 wherein the cladding has a
shear velocity that is greater than that of the second core.
11. The laser delivery system of claim 8 further comprising a
buffer.
Description
RELATED APPLICATIONS
[0001] This application is a continuation of application Ser. No.
09/638,239, filed on Aug. 14, 2000.
FIELD OF THE INVENTION
[0002] The present invention concerns waveguide systems and
devices.
BACKGROUND OF THE INVENTION
[0003] Waveguides direct the propagation of light in a controlled
fashion. A waveguide is therefore a fundamental component of
systems and devices which depend upon the controlled use of light.
The scale of waveguides in many modern devices is exemplified by
hair sized optical fibers used in communication systems. In
communications systems, such as telephone systems, the trend is
toward use of optical signals and away from use of electrical
signals. Practical reasons exist for the shift in focus to
optically driven systems. Unlike electrical signals, optical
signals are generally unaffected by electromagnetic fields created
by such things as power lines and lightning. These sources of
interference may create noise in electrical signals, but optical
signals are unaffected.
[0004] Information capacity of optical signals is also potentially
much larger than lower frequency electrical signals that are used
in wired electrical and wireless electromagnetic communication
systems. Generally, higher frequency signal carriers provide larger
information capacity than lower frequency signal carriers. This is
due to the wider bandwidth of the higher frequency signals. Another
important benefit of communicating with optical signals is the
aforementioned small size of optical fibers used as a transmission
medium. A typical fiber having hair sized dimensions is a suitable
replacement for bundles of copper wires having a much larger
diameter. As demands for information access become larger and
larger in modern times, the use of optical transmission systems
places less demand on space in the construction of underground,
above ground, and internal building communication systems.
[0005] Another important use of optical energy communicated through
a waveguide is in cutting, weapons, and other high power laser
technology. Laser light direct through a waveguide forms useful
lasers for cutting everything from machine parts to patients
undergoing delicate surgeries. Weapons technologies have focused on
laser light as potential bases for systems that track and destroy
projectiles, such as missiles, with the destruction being based
upon energy from laser light.
[0006] Common difficulties are encountered in the practical
implementation of such optical energy systems, however. Waveguides,
e.g., fibers, introduce losses. Losses limit the distance by which
the transmitter and receiver may be separated. These losses are
generally referred to as optical signal attenuation. Absorption of
signal light by the fiber acting as the transmission medium is one
factor causing attenuation. Other factors leading to attenuation
are the scattering of the signal light over a wider wavelength than
the original transmission and radiative losses, typically occurring
at bends in the optical fiber. Combination of these individual
losses leads to a total signal attenuation characteristic for a
particular optical transmission medium which is measured in
decibels per kilometer.
[0007] An effect called Stimulated Brillouin Scattering (SBS) has
been identified as a primary cause of scattering losses that limits
the effectiveness of waveguides. SBS is an interaction of optical
energy with acoustic energy. Optical energy guided into optical
waveguides, e.g., the core of an optical fiber, produces acoustic
energy. As is known in the art, once a certain amount of optical
power is directed into a waveguide from another optical source or
generated in the waveguide, the effect of SBS causes optical energy
to backscatter into the source. Typical waveguides, e.g., optical
fiber cable, are long (tens of meters) for the SBS interaction to
be efficient at low signal power, and SBS is known to affect
signals with spectral widths smaller than that of the SBS process.
This backscattering is undesirable in most, if not all,
applications.
[0008] Overcoming or reducing the SBS effect would therefore
significantly impact many optical waveguide systems and devices.
The ability to launch more power into an optical communication
fiber, for example, has the alternative advantages of reducing the
number of repeaters or, if the distance between repeaters is kept,
of providing higher information capacity. In the field of work
performing high power lasers, such as cutting lasers and weapons,
overcoming the SBS effect offers the potential to use small
semiconductor lasers. Though the semiconductor lasers have
advantages in the area of power consumption and compactness, they
have not yet found large application as work performing lasers due
to the overall limited power developed by the lasers. Improving
waveguide efficiencies would allow better use of the limited power
developed and allow combination of separate powers from multiple
lasers.
[0009] Currently, the highest brightness continuous-wave laser
sources are fiber lasers and fiber-amplified laser sources. To
realize, for example, a laser weapon, a high power laser for
cutting applications, a high power free-space communications laser,
a high power laser for tracking systems, or an earth-to-satellite
power delivery system, multiple fibers can be combined to achieve
required powers. However, the signal in each fiber must be coherent
(narrow spectral width) enough to allow for beam steering and field
shaping of the output of the fiber bundle over extended beam
propagation distances. The result is that high-power fiber
technology is limited by SBS to inadequate powers. Therefore,
overcoming the SBS problem in optical fibers will open the doorway
to a new generation of lasers and important applications. Thus,
there is a need for an improved method of limiting the SBS effect
in waveguides. It is an object of the invention to provide such an
improved method.
SUMMARY OF THE INVENTION
[0010] Those and other needs and objects of the invention are met
or exceeded by the present method for reducing SBS in waveguides.
The method of the invention controls the acoustic waves produced to
be guided away from the portion of the waveguide which guides the
light. The method of the invention results in novel systems and
devices in which SBS effects are reduced and system efficiencies
are increased.
[0011] In a preferred single clad optical fiber of the invention,
cladding around the waveguide core of the fiber is set to guide the
acoustic waves generated by the light which is guided in the core.
Thus, acoustic waves are guided into the cladding. A substantial
reduction in the SBS effect is then realized in the core that
guides light.
[0012] The method of the invention is applicable to single clad
optical waveguides, such as optical fibers, as well as dual clad
optical fibers and other waveguides. A preferred dual clad (a.k.a.
dual core) waveguide structure permits realization of a pump laser
system having reduced SBS effect in the core used for guiding
transmitted light and allows the light in the core to be pumped
(amplified). A second core guides acoustic waves outside the core
used for guiding transmitted light, and also guides pump light
which amplifies the light transmitted in the core for light
transmission. Pumping may also be assisted by cladding the second
"pump" core with a cladding that is anti-guiding for light and
guiding for acoustic waves.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] Other objects, features and advantages of the invention will
be apparent from the detailed description and the drawings, of
which:
[0014] FIG. 1a is the optical index profile for a conventional
single clad optical fiber;
[0015] FIG. 1b is the optical index profile for a conventional
double clad optical fiber;
[0016] FIG. 2a is the shear velocity profile for acoustic modes in
a frequently used conventional Ge doped core silica single clad
optical fiber;
[0017] FIG. 2b is the shear velocity profile for acoustic modes in
a less common optical fiber;
[0018] FIG. 2c is the shear velocity profile for a conventional
dual clad (a.k.a dual core) fiber;
[0019] FIG. 3a illustrates the shear velocity profile for an
optical fiber produced by the method of the invention which
corresponds to the type of fiber having the shear velocity profile
in FIG. 2a;
[0020] FIG. 3b illustrates the shear velocity profile for an
optical fiber produced by the method of the invention which
corresponds to the type of fiber having the shear velocity profile
in FIG. 2b;
[0021] FIG. 3c illustrates an optical fiber produced by the method
of the invention corresponding to the profile in FIG. 2c.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0022] The present invention is generally a method for reducing SBS
effect in an optical waveguide. The primary principal of the
invention is that acoustic energy produced in a core by the light
signal guided by it is guided into material around the core. Thus,
for example, in a single clad optical fiber, the method sets the
cladding around the core to guide acoustic waves. The cladding
guides acoustic waves outside the core and the SBS effect is
reduced in the core. The method of the invention is applicable to
various waveguide structures having a core, i.e., a section for
light guiding, and cladding, i.e., a section for light containment
within the core, and systems and devices using such waveguides. The
discussion herein is primarily directed to preferred single and
dual clad optical fibers, but artisans will appreciate the broader
applicability of the invention to other devices and systems using
waveguides.
[0023] The basic acoustic guiding properties of a waveguide are
determined by the acoustic V-number of the waveguide. The acoustic
"V" number is given by: 1 V = a v s 1 - ( 1 - v s1 2 v s2 2 ) 1 /
2
[0024] where the v's are shear velocities and a is the radius. The
wave is guided if v.sub.waveguide<v.sub.clad. The same
principles apply to a cladding surrounding a waveguide. If material
surrounding the cladding has a shear velocity greater than the
cladding, then acoustic waves are guided by the cladding. In the
case of an optical fiber, the buffer material coated onto the
cladding to improve the fiber's mechanical properties is usually
much softer (and thus less dense) than glass, and thus the cladding
does not guide acoustic waves in current fibers.
[0025] Such a fiber is represented by its optical index profile in
FIG. 1a. The FIG. 1a fiber is a single mode fiber that is used
extensively in the telecommunications industry. The refraction
index for a core 10 exceeds that of a cladding 12 and a buffer 14,
while the index of refraction for the buffer 14 exceeds that of the
cladding 12. A typical dual clad optical fiber is represented in
FIG. 1b, and includes an additional outer cladding 16 having an
index of refraction less than the cladding 12 (the cladding 12 is
also referred to as a second core). The invention is applicable to
these and other waveguide structures. The FIG. 1b type of fiber can
also be modified by the invention to provide optical guiding, but
reduce SBS by controlling the distribution of the acoustic
power.
[0026] Most telecommunication fiber uses Ge-doped core and a pure
silica cladding. For these materials, n.sub.core>n.sub.clad,
v.sub.core<v.sub.clad and thus the fiber core guides both the
optical wave and the acoustic wave. Neither the optical nor the
acoustic wave is guided by the cladding because
n.sub.clad<n.sub.buffer and v.sub.clad>v.sub.buffer.
[0027] The core guides the acoustic modes with the lowest order
mode being dominant because of the overlap of this field with
single mode optical field. In particular, the cladding does not
guide acoustic modes. Other acoustic modes are excited within the
fiber, but most are evanescent. Resonant enhancement of SBS process
occurs due to coupling of acoustic energy into the guided sound
waves in the core. Shear velocity profiles for conventional fibers
are shown in FIGS. 2a (single mode), 2b(single mode), and 2c (dual
clad). The method of the invention sets the cladding properties to
guide acoustic modes, and results in the new shear velocity
profiles respectively shown in FIGS. 3a (single mode), 3b (single
mode) and 3c (dual clad). An additional outer cladding layer 20 is
used to help create the modified shear velocity profiles. The outer
cladding layer 20 may be a separate layer from the cladding 12, or
it may be realized by doping an outer portion of the cladding
12.
[0028] This guiding of acoustic modes by the cladding reduces the
SBS in two ways. Coupling acoustic energy out of the core into the
cladding modes faster than the SBS interaction time results in a
significant amount of power being carried in the cladding and less
in the core, thereby increasing the SBS threshold. A second
independent effect is that the total number of acoustic modes
increases. The total acoustic power is then distributed over more
modes. These modes interfere producing an acoustic speckle pattern
within the fiber. The spatial variation or contrast of this speckle
pattern is what causes the light to backscatter. Making the
cladding guide the acoustic energy implies that more acoustic modes
propagate. The speckle pattern from the increased number of modes
has less variation than without the clad guiding. This reduced
contrast reduces the ability of the total acoustic field to
backscatter the light and thus increases the SBS threshold.
[0029] The reduction of SBS by generating an acoustic speckle
pattern comprised of many modes is supported by two experimental
facts. The first is that the SBS threshold in a fiber is higher
than bulk. This is somewhat counter intuitive, since the total
acoustic power that overlaps the optical field in a fiber should be
larger than in bulk due to the guiding properties of the fiber and
there should be a resonant enhancement of the SBS in the fiber.
However, if this additional acoustic power is distributed among
many modes such that the total contrast of the speckle pattern is
reduced, the net result is a higher SBS threshold. This higher
threshold is seen experimentally. The second fact is that the SBS
threshold appears to increase in double clad fibers relative to
single clad fibers. Again, this increase can be explained by the
fact that the double clad structure can support more acoustic modes
than the single clad structure.
[0030] The method of the invention sets material properties to
determine the acoustic and optic guiding and anti-guiding nature of
core and cladding. To realize an acoustically guiding cladding, the
condition v.sub.clad<v.sub.outside clad must be met. Dopants
which increase the shear velocity while also increasing the index
of refraction required to maintain the optical index profile may be
used to form cladding that is acoustic guiding and light
anti-guiding in accordance with the invention. Aluminum is one
exemplary dopant for a fiber of the conventional types shown in
FIGS. 1a and 1b. It can be incorporated into the outer part of the
cladding to increase the shear velocity, resulting in the shear
velocity profile produced by the invention and shown in FIG. 3a.
This increase of the index on the outer edge of the fiber cladding
is a simple additional diffusion step that can be done during the
manufacturing stage of the fiber pre-form. Alternately, the
increase of the index might be the result of an additional Aluminum
doped layer.
[0031] Fibers of the conventional type exhibiting the shear
velocity profile in FIG. 2b have a different structure in which the
core does not guide acoustic waves. In typical realization of the
conventional FIG. 2b fiber, the core is pure silica, but the
cladding is Fluorine doped. The optical properties are the same
n.sub.core>n.sub.clad, but the acoustic guiding properties of
the core are reversed v.sub.core>v.sub.clad compared to the
conventional fiber structure shown in FIG. 1a. The cladding does
not guide acoustic energy because v.sub.clad>v.sub.buffer. The
SBS threshold should be similar to bulk because no guiding occurs.
The ability of this structure to reduce SBS relative to the first
structure considered depends on the magnitude of the radiated
acoustic power within the SBS interaction time. To increase the SBS
threshold in accordance with the invention, the waveguide
properties must be changed so that the cladding guides acoustic
waves, as shown in FIG. 3b. In order for this structure to be more
efficient than the FIG. 3a structure, the acoustic power must
radiate from the core before the optical field interacts with it.
In other words, the guided cladding acoustic modes must be
established before the SBS interaction between the acoustic and
optical fields takes place. If this occurs, the modified FIG. 3b
structure should work better than the FIG. 3a structure because the
core in FIG. 3b does not trap acoustic energy.
[0032] The FIG. 3b structure may be achieved, for example, by using
a pure silica outer cladding with an increased index and increased
shear velocity relative to the Fluorine doped cladding. Almost all
of the acoustic energy is coupled into cladding modes and the
resultant many mode speckle pattern will not produce efficient SBS
in the core.
[0033] Double cladding fiber has an additional optical guiding
structure that is used in high power fiber sources to guide the
pump light. Double clad fibers are also called double core fibers.
A conventional velocity profile for a double clad fiber is shown in
FIG. 2c, and the corresponding ideal shear velocity profile for a
double clad fiber modified by the method of the invention is shown
in FIG. 3c.
[0034] The FIG. 3c modification utilizes an outer cladding material
with a shear velocity greater (and an optical index lower) than the
inner cladding (also called a second core). Resonant acoustic modes
excited in cladding will draw acoustic energy out of the core
acoustic mode. The resulting speckle pattern will not be as
efficient in backscattering the light. We again note that there is
evidence that this effect occurs in double fiber. This is
consistent with our explanation that the additional cladding
structure allows for more acoustic modes to propagate relative to
single clad fiber reducing the contrast of the total speckle
pattern of the acoustic field and thus increasing the SBS
threshold. The proposed mechanism that is the basis for the present
invention may explain this effect and allow us to control and
enhance this reduction mechanism resulting in significantly
(.about.10.times.) higher power fiber sources.
[0035] This leads to the FIG. 3c dual clad (dual core) structure.
The surrounding cladding 12 is acoustic guiding and anti-light
guiding for light modes of the core 10 but guiding for pump light
modes that can be used for optical amplification of the core light
modes. The outer cladding 20 is acoustic mode guiding and
anti-light guiding. In this arrangement with the method of the
invention applied, SBS will be reduced in the core 10. A lesser
reduction is realized in the cladding 12 (second core), however,
the band of pump light need not be narrow and the cladding/second
core 12 therefore provides an excellent structure for amplifying
the signal in the core 10. A practical application to a fiber, for
example, might therefore use the core 10 for light signal
transmission, and the cladding/second core 12 for pumping
light.
[0036] There are two basic unknowns in the present method that may
limit its utility:
[0037] 1. The quantitative relationship of overlap between sound
wave and optical field and the magnitude of the SBS.
[0038] 2. What is the magnitude of the time constant for the
coupling of acoustic energy into guided acoustic
(.about.a/v.sub.s2) modes relative to that of the SBS interaction
time constant?
[0039] These issues will affect the magnitude of the reduction, but
should not affect the basic application of the SBS reduction of the
invention. Optimizations may be made in recognition of these
principles without departing from the scope of the invention.
[0040] While various embodiments of the present invention have been
shown and described, it should be understood that other
modifications, substitutions and alternatives are apparent to one
of ordinary skill in the art. Such modifications, substitutions and
alternatives can be made without departing from the spirit and
scope of the invention, which should be determined from the
appended claims.
[0041] Various features of the invention are set forth in the
appended claims.
* * * * *