U.S. patent application number 15/282465 was filed with the patent office on 2017-01-19 for phase and amplitude control for optical fiber output.
The applicant listed for this patent is The Government of the United States of America, as represented by the Secretary of the Navy, The Government of the United States of America, as represented by the Secretary of the Navy. Invention is credited to Ishwar D. Aggarwal, Catalin M. Florea, Rafael R. Gattass, Jasbinder S. Sanghera.
Application Number | 20170017035 15/282465 |
Document ID | / |
Family ID | 54017166 |
Filed Date | 2017-01-19 |
United States Patent
Application |
20170017035 |
Kind Code |
A1 |
Sanghera; Jasbinder S. ; et
al. |
January 19, 2017 |
Phase and Amplitude Control for Optical Fiber Output
Abstract
A method for shaping an output light beam from an optical fiber
by controlling the phase and amplitude of the beam by producing
beam shaping elements on an exit facet of the optical fiber by
direct surface texturing of the exit facet, where a controlled
phase difference is achieved across the fiber cross-section over a
predefined pattern. The optical fiber can be a single mode fiber or
a multi-mode fiber. Either a binary or a complex phase difference
can be achieved. Also disclosed is the related system for shaping
an output light beam from an optical fiber.
Inventors: |
Sanghera; Jasbinder S.;
(Ashburn, VA) ; Florea; Catalin M.; (Washington,
DC) ; Gattass; Rafael R.; (Washington, DC) ;
Aggarwal; Ishwar D.; (Waxhaw, NC) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
The Government of the United States of America, as represented by
the Secretary of the Navy |
Arlington |
VA |
US |
|
|
Family ID: |
54017166 |
Appl. No.: |
15/282465 |
Filed: |
September 30, 2016 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
14210480 |
Mar 14, 2014 |
9507090 |
|
|
15282465 |
|
|
|
|
61786656 |
Mar 15, 2013 |
|
|
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G02B 27/0927 20130101;
G02B 1/005 20130101; G02B 6/262 20130101; G02B 6/02295
20130101 |
International
Class: |
G02B 6/26 20060101
G02B006/26; G02B 6/02 20060101 G02B006/02; G02B 27/09 20060101
G02B027/09 |
Claims
1. A system for shaping an output light beam, the system
comprising: an optical fiber configured to transmit the output
light beam; and an exit facet of the optical fiber, wherein the
exit facet comprises a surface textured according to a texturing
pattern designed to initiate a static phase difference between a
first portion of the output light beam and a second portion of the
output light beam such that the textured surface has a plurality of
varying depths in the exit facet.
2. The system of claim 1, wherein the optical fiber comprises
chalcogenide, fluoride, or tellurite.
3. The system of claim 1, wherein the optical fiber comprises a
solid core photonic crystal fiber.
4. The system of claim 1, wherein the optical fiber is a single
mode fiber.
5. The system of claim 1, wherein the optical fiber is a multi-mode
fiber.
6. The system of claim 1, wherein no additional material is
attached to or deposited on the exit facet of the optical fiber to
initiate the static phase difference.
7. The system of claim 1, wherein the surface is stamped to form
the textured surface.
8. The system of claim 1, wherein the exit facet comprises a
plurality of beam shaping elements formed by the textured
surface.
9. The system of claim 1, wherein the textured surface comprises
multiple steps created in a spiral pattern, and wherein the output
light beam is in a shape of a ring with no light in the center.
10. The system of claim 1, wherein the texturing pattern is a
periodic texturing pattern.
11. The system of claim 1, wherein the texturing pattern includes
an array of circular symmetric lines.
12. The system of claim 1, wherein the texturing pattern includes
an array of non-circular symmetric lines.
13. A optical fiber for shaping an output light beam transmitted
through the optical fiber, the optical fiber comprising: an exit
facet; a first step formed on a surface of a first portion of the
exit facet according to a texturing pattern designed to initiate a
static phase difference between a first portion of the output light
beam and a second portion of the output light beam; and a second
step formed on the surface of a second portion of the exit facet
according to the texturing pattern, wherein a first depth of the
first step in the surface of the exit facet is different than a
second depth of the second step in the surface of the exit
facet.
14. The optical fiber of claim 13, wherein the static phase
difference between the first portion of the output light beam and
the second portion of the output light beam is a .pi. phase
shift.
15. The optical fiber of claim 14, wherein a difference between the
first depth and the second depth is determined according to the
equation d=.lamda./(2(n-1)), wherein n represents an effective
index of the fundamental mode, wherein .lamda. represents an
operating wavelength, and wherein d represents the difference
between the first depth and the second depth.
16. The optical fiber of claim 13, further comprising: a plurality
of steps, including the first step and the second step, formed in a
spiral pattern on the surface of the exit facet according to the
texturing pattern, wherein the output light beam is in the shape of
a ring with no light in the center.
17. The optical fiber of claim 13, wherein the texturing pattern is
a periodic texturing pattern.
18. A optical fiber for shaping an output light beam transmitted
through the optical fiber, the optical fiber comprising: an exit
facet; a first step formed on a surface of a first portion of the
exit facet according to a texturing pattern designed to initiate a
static phase difference between a first portion of the output light
beam and a second portion of the output light beam; and a second
step formed on the surface of a second portion of the exit facet
according to the texturing pattern, wherein a first depth of the
first step in the surface of the exit facet is different than a
second depth of the second step in the surface of the exit facet,
and wherein the first step and the second step are formed such
that, after passing through the exit facet, the first portion of
the output light beam and the second portion of the output light
beam differ in phase according to the static phase difference.
19. The optical fiber of claim 18, wherein a light beam with a
substantially uniform phase is transmitted through an entire core
of the optical fiber to produce the output light beam.
20. The optical fiber of claim 18, further comprising: a plurality
of steps, including the first step and the second step, formed in a
spiral pattern on the surface of the exit facet according to the
texturing pattern, wherein the output light beam is in the shape of
a ring with no light in the center.
Description
PRIORITY CLAIM
[0001] The present application is a non-provisional application
claiming the benefit of U.S. Provisional Application No.
61/786,656, filed on Mar. 15, 2013 by Jasbinder S. Sanghera et al.,
entitled "Phase and Amplitude Control for Optical Fiber Output,"
the entire contents of which is incorporated herein by
reference.
BACKGROUND OF THE INVENTION
[0002] Field of the Invention
[0003] The present invention relates generally to optical fiber
outputs and, more specifically, to controlling the phase and
amplitude of a light beam profile exiting an optical fiber.
[0004] Description of the Prior Art
[0005] A typical optical system will transmit, reflect, refract or
otherwise modify the propagation of light or its salient properties
such as phase, amplitude or polarization. In particular, an optical
fiber will present at the cross-section of the output aperture a
beam of light characterized by a certain amplitude (intensity) and
phase distribution. The very familiar situation is that of the
light propagation through a single-mode fiber which will have at
the output a profile close to that of a Gaussian beam. The
intensity is highest at the center and then it decreases as radius
increases. The Gaussian beams are important because they maintain a
Gaussian intensity profile at any location along the beam axis,
even after passing through lenses (ignoring lens aberrations). The
phase profile of such a beam is also very simple, usually linear or
quadratic (described by a polynomial). The quadratic case is
important as it is implying convergence or divergence of the beam
(change in the beam radius).
[0006] There are however many situations when a Gaussian beam is
not desirable. Particle trapping and ultra high-resolution
fluorescence microscopy are achieved using beams that have a ring
or doughnut shape (no light in the center). Flat top beams, where
the intensity is constant over most of the cross-section, are also
of interest when uniform illumination and efficient focusing are
required such as in material laser processing. Most of the work is
done in bulk, with light beams manipulated by macro optics
(gratings, phase plates etc.).
[0007] Beam shaping can be implemented through different
techniques: use of apertures, use of a combination of various
optical elements, such as micro-lens arrays, or through
manipulation of the near field which results in the desired changes
in the far field. This last method, requiring modification in the
near field of the beam phase rather than amplitude, is easy to
implement. It can be achieved by placing a phase mask in the beam
path. It also provides the desired profile with minimal loss in
total energy. In very few cases direct beam manipulation was
performed at the output of an optical fiber.
[0008] Beam shaping has been researched intensively and a variety
of patents have provided a multitude of approaches. For example,
U.S. Pat. No. 8,031,414 (2011), U.S. Pat. No. 8,016,449 (2011), and
U.S. Pat. No. 7,593,615 (2010) provide for instructive reading with
respect to various means of beam shaping (all covering refractive
methods using external lenses, diffusers, waveguides or other
optical elements). Prior art discussing the idea of creating a
phase mask-like structure directly on the fiber end is extremely
limited. Existing approaches require deposition of photosensitive
material on the fiber end, material in which the surface structure
is to be created.
BRIEF SUMMARY OF THE INVENTION
[0009] The aforementioned problems are overcome in the present
invention which provides a method for shaping an output light beam
from an optical fiber by controlling the phase and amplitude of the
beam by producing beam shaping elements on an exit facet of the
optical fiber by direct surface texturing of the exit facet, where
a controlled phase difference is achieved across the fiber
cross-section over a predefined pattern. The optical fiber can be a
single mode fiber or a multi-mode fiber. Either a binary or a
complex phase difference can be achieved. Also disclosed is the
related system for shaping an output light beam from an optical
fiber.
[0010] The present invention provides a method of controlling the
amplitude and phase of the output beam from an optical fiber. The
purpose of this invention is to shape the output beam from an
optical fiber in terms of phase and amplitude using surface relief
structures integrated directly into the fiber facet. Direct
modification of the fiber end allows for control of amplitude,
phase and direction of the light beam profile exiting the optical
fiber with direct implications in laser processing, optical
trapping, super high-resolution fluorescence microscopy, optical
switching etc.
[0011] The present invention allows for optical performance across
a very broad wavelength range and across a wide range of materials.
It provides for a cheap implementation requiring, for example, a
single master with the negative of the structure of interest. That
master can then be used to create the desired surface structure in
multiple fibers without loss of quality from one fiber to another.
The direct alternative technique to the method of the present
invention is the use of external phase masks. However, these add to
the complexity and the cost of the technique while reducing the
ruggedness.
[0012] These and other features and advantages of the invention, as
well as the invention itself, will become better understood by
reference to the following detailed description, appended claims,
and accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] FIG. 1 shows a .pi. phase change created over the central
portion of the beam up to the 1e-2 intensity boundary. Output field
intensity shown is after Fourier transform.
[0014] FIG. 2 shows a .pi. phase change created across 50% of the
beam. Output field intensity shown is after Fourier transform.
[0015] FIG. 3 shows that two types of gratings (2D linear and
circular) on the fiber end facet will yield different light output
profiles.
[0016] FIG. 4 shows a 2D linear grating stamped on the end face of
a 22 .mu.m core fiber of a low mode count As.sub.2S.sub.3 fiber (6
modes at wavelength of 4.8 .mu.m).
[0017] FIG. 5 is an illustration of a surface structure that
creates a 2.pi. phase change along the cross-section of the fiber
end in a total of 8 steps.
[0018] FIG. 6 is an example of a commercially-available substrate
with a surface structure that creates a 2.pi. phase change along
the cross-section of a laser beam.
[0019] FIG. 7 is an example of a multi-mode chalcogenide fiber
stamped with a 2D pattern.
DETAILED DESCRIPTION OF THE INVENTION
[0020] The present invention provides a novel method and system for
beam shaping through the modification of the near field directly at
the exit facet of the optical fiber. This is achieved through
direct surface texturing of the fiber facet, which allows for
controlled phase change across the beam diameter. This approach is
very different from other methods because it does not require an
extraneous material to be attached or deposited to the fiber end,
which makes the present approach more robust and simpler to
implement.
[0021] This type of surface texturing requires in certain
situations nanometer-level control of the fiber facet structures,
as will be made clear in the examples. The phase change will
provide the required near field transformation without the need of
external phase plates, thereby reducing system complexity and
enhancing ruggedness. The surface texturing can be performed by
stamping the fiber end onto typical substrates such as silicon
wafers or fused silica plates which have the appropriate patterns
built in. US Patent Publication 20110033156 (2010) discloses a
technique for surface microstructuring of optical fiber ends with
intent of reducing the reflection loss occurring at the fiber-air
interface.
[0022] In one embodiment, the facet of a single-mode chalcogenide,
fluoride, silica, silicate, germanate, tellurite or any other
optical fiber is modified such that a certain binary phase
difference can be achieved in a controlled manner across the fiber
cross-section over a predefined pattern.
EXAMPLE 1
[0023] A single-mode fiber end-surface is modified with a circular
step of depth d in the core region. The width of the step should
match a certain portion of the diameter of the output beam. The
depth of the step is determined by the desired phase change and the
operating wavelength .lamda..
[0024] For single-mode fibers we require a .pi. phase change in the
central portion of the beam, up to the 1e.sup.-2 (13.5%) diameter
of the beam, with respect to the remaining beam. The output beam is
converted to a sinc function whose Fourier transform (as given by
lens or in the far field) is a flat top profile. The situation is
illustrated in FIG. 1.
[0025] For this case, the required depth (d) of the surface relief
is given by Equation (1), where n is the effective refractive index
of the mode:
d = .lamda. 2 ( n - 1 ) ( 1 ) ##EQU00001##
[0026] In particular, consider a typical single-mode
As.sub.2S.sub.3 fiber with a 1e.sup.-2 diameter of about 6 .mu.m
and a cladding size of 170 .mu.m. The effective index of the
fundamental mode is n=2.404 as determined from fiber Bragg gratings
data (Florea et al., "Fiber Bragg gratings in As.sub.2S.sub.3
fibers obtained using a 0/-1 phase mask," Opt. Mat., 31, 942-944
(2009), the entire contents of which is incorporated herein by
reference). For operation at .lamda.=1.55 .mu.m, one needs a
surface relief depth d=552 nm. Other chalcogenides can also be
considered, such as As.sub.2Se.sub.3, with the operating wavelength
changed to accommodate the transmission window of the material.
EXAMPLE 2
[0027] Another particular case is that of a modified fiber
end-surface where half of the beam output aperture experiences a it
phase shift with respect to the other half, as illustrated in FIG.
2. The depth of the step on the fiber surface is given by Equation
(1) as well.
[0028] In another embodiment, the facet of a single-mode
chalcogenide, fluoride, silica, silicate, germanate, tellurite or
any other optical fiber is modified such that a certain complex
(non binary) phase difference can be achieved in a controlled
manner across the fiber cross-section over a predefined
pattern.
EXAMPLE 3
[0029] A single-mode fiber end-surface is modified with a grating
of period L in the core region (FIG. 3). A variety of gratings
(circular, blazed etc.) are possible. The type, period and depth of
the grating should be adjusted to provide the desired diffraction
for the light beam exiting the fiber. A variety of gratings and
situations can be considered such as to manipulate the amplitude
and direction of the resulting output beams. FIG. 4 shows a 2D
linear grating stamped on the end face of a 22 .mu.m core fiber of
a low mode count As.sub.2S.sub.3 fiber (6 modes at wavelength of
4.8 .mu.m).
EXAMPLE 4
[0030] A 2.pi. phase change is achieved by a finite number of steps
created in a spiral pattern across the fiber end facet, around the
center of the cross-section. This surface structure will create an
output beam in the shape of a ring or doughnut, with no light in
the center. The situation where the 2.pi. phase change is created
by a total of 8 steps is illustrated in FIG. 5.
[0031] The thickness of each step is easily calculated from the
requirement that the phase change occurring at each step be exactly
2.pi./8 and it is given by Equation (2):
d = .lamda. 8 ( n - 1 ) ( 2 ) ##EQU00002##
[0032] In the case of a typical single-mode As.sub.2S.sub.3 fiber
with an effective index of the fundamental mode of n=2.404 and for
operation at .lamda.=1.55 .mu.m one needs a step thickness d=138
nm. The control of the thickness is important but easily
implemented given the advanced state of art of the fabrication
techniques involved.
[0033] An extension of Example 4 is that of a spiral that has a
very large number of steps or that achieves the 2.pi. phase change
in a continuous fashion rather than step-wise fashion. This surface
structure will also create an output beam in the shape of a ring or
doughnut, with no light in the center. This is essentially similar
to a vortex phase plate, which is commercially available and which
is illustrated in FIG. 6.
[0034] In another embodiment, the facet of a multi-mode
chalcogenide, fluoride, silica, silicate, germanate, tellurite or
any other optical fiber is modified such that a certain binary
phase difference can be achieved in a controlled manner across the
fiber cross-section over a predefined pattern. Of great interest is
the situation of low-mode number fibers where phase change can be
used as a modal filter.
EXAMPLE 5
[0035] A multimode As.sub.2S.sub.3 fiber has been stamped with a
macroscopic 2D array of holes and imaged in reflection mode with
white light as shown in FIG. 7.
[0036] In another embodiment, the facet of a multi-mode
chalcogenide, fluoride, silica, silicate, germanate, tellurite or
any other optical fiber is modified such that a certain complex
(non binary) phase difference can be achieved in a controlled
manner across the fiber cross-section over a predefined pattern. Of
interest is the situation of low-mode number fibers where phase
change can be used as a modal filter.
[0037] In another embodiment, the facet of a solid-core photonic
crystal fiber is modified such that a certain binary phase
difference can be achieved in a controlled manner across the fiber
cross-section over a predefined pattern. The photonic crystal fiber
can be made of chalcogenide, fluoride, silica, silicate, germanate,
tellurite or any suitable material.
[0038] In another embodiment, the facet of a solid-core photonic
crystal fiber is modified such that a certain complex (non binary)
phase difference can be achieved in a controlled manner across the
fiber cross-section over a predefined pattern. The photonic crystal
fiber can be made of chalcogenide, fluoride, silica, silicate,
germanate, tellurite or any suitable material.
[0039] The above descriptions are those of the preferred
embodiments of the invention. Various modifications and variations
are possible in light of the above teachings without departing from
the spirit and broader aspects of the invention. It is therefore to
be understood that the claimed invention may be practiced otherwise
than as specifically described. Any references to claim elements in
the singular, for example, using the articles "a," "an," "the," or
"said," is not to be construed as limiting the element to the
singular.
* * * * *