U.S. patent application number 15/226164 was filed with the patent office on 2018-02-08 for techniques for reducing polarization, wavelength and temperature dependent loss, and wavelength passband width in fiberoptic components.
The applicant listed for this patent is DiCon Fiberoptics, Inc.. Invention is credited to Chen-Wen Ho, Ho-Shang Lee, Min Chieh Lu, Yu-Sheng Yang.
Application Number | 20180039023 15/226164 |
Document ID | / |
Family ID | 61070095 |
Filed Date | 2018-02-08 |
United States Patent
Application |
20180039023 |
Kind Code |
A1 |
Lee; Ho-Shang ; et
al. |
February 8, 2018 |
Techniques for Reducing Polarization, Wavelength and Temperature
Dependent Loss, and Wavelength Passband Width in Fiberoptic
Components
Abstract
A pin hole or aperture is located or formed adjacent to the end
surface of one or more of the input ports or fibers, or adjacent to
one or more of the output ports or fibers, of a fiberoptic
component. The aperture allows light to enter (or exit) the core of
the associated fiber, and the non-transparent layer that surrounds
the aperture blocks light from entering or exiting the cladding
layer of the associated fiber. This blocking of the evanescent
field in the cladding layer serves to reduce the polarization,
wavelength, and temperature dependencies of the light coupling to
the output port(s) or fiber(s) of the optical component. It can
also reduce the passband width of the selected wavelength in
tunable optical filter applications. The non-transparent layer
surrounding the aperture can be made reflective, and light that is
reflected by the non-transparent layer can be used for optical
power monitoring.
Inventors: |
Lee; Ho-Shang; (El Sobrante,
CA) ; Lu; Min Chieh; (Kaohsiung City, TW) ;
Yang; Yu-Sheng; (Kaohsiung City, TW) ; Ho;
Chen-Wen; (Kaohsiung City, TW) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
DiCon Fiberoptics, Inc. |
Richmond |
CA |
US |
|
|
Family ID: |
61070095 |
Appl. No.: |
15/226164 |
Filed: |
August 2, 2016 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G02B 6/262 20130101;
G02B 6/266 20130101; G02B 6/29398 20130101; G02B 6/2793 20130101;
G02B 6/29397 20130101 |
International
Class: |
G02B 6/26 20060101
G02B006/26; G02B 6/136 20060101 G02B006/136; G02B 6/132 20060101
G02B006/132; G02B 6/028 20060101 G02B006/028; G02B 6/38 20060101
G02B006/38 |
Claims
1. An optical component, comprising: one or more optical
waveguides, including a first optical waveguide having an inner
core extending in a first direction that is radially surrounded by
an outer cladding along the first direction, the first optical
waveguide terminating in a first end and wherein the inner core has
a higher index of refraction than the index of refraction of the
outer cladding; and a non-transparent end structure covering the
first end of the first optical waveguide and having a transparent
aperture for at least a portion of inner core, wherein the
non-transparent end structure is reflective and the optical
component is configured to monitor at least a portion of incident
light reflected from the non-transparent end structure.
2. The optical component of claim 1, wherein the first optical
waveguide is an optical fiber.
3. The optical component of claim 2, further comprising: a ferrule
in which the optical fiber is embedded.
4. The optical component of claim 1, wherein the optical component
includes a substrate upon or within which the first optical
waveguide is formed.
5. (canceled)
6. The optical component of claim 1, wherein the end structure is
formed on the first end of the optical waveguide.
7. The optical component of claim 1, wherein the end structure is
formed on a plate, separate from the first end of the optical
waveguide.
8. The optical component of claim 1, wherein the inner core has a
uniform index of refraction.
9. The optical component of claim 1, wherein the inner core has a
non-uniform index of refraction.
10. The optical component of claim 1, wherein the transparent
aperture has an area that is less than the area of the inner core
on the first end, such that less than all of the inner core on the
first end is exposed by the transparent aperture.
11. The optical component of claim 1, wherein the transparent
aperture has an area that is larger than the area of the inner core
on the first end, such that all of the inner core and a portion of
the cladding is exposed by the transparent aperture.
12. The optical component of claim 1, wherein the transparent
aperture has an area that is substantially equal to the area of the
inner core on the first end, being aligned such that substantially
all of the inner core and none of the cladding is exposed by the
transparent aperture.
13. The optical component of claim 1, wherein the first end of the
first optical waveguide is angled at a non-right angle relative to
the first direction.
14. The optical component of claim 1, further comprising: a second
optical waveguide terminating in a second end, the second end being
proximate to the first end of the first optical waveguide, wherein
the first and second optical waveguides are aligned such that light
transmitted from the second end of the second waveguide is incident
upon the first end of the first waveguide.
15. An optical component, comprising: one or more optical
waveguides, including a first optical waveguide having an inner
core extending in a first direction that is radially surrounded by
an outer cladding along the first direction, the first optical
waveguide terminating in a first end and wherein the inner core has
a higher index of refraction than the index of refraction of the
outer cladding; a non-transparent end structure covering the first
end of the first optical waveguide and having a transparent
aperture for at least a portion of inner core; and a second optical
waveguide terminating in a second end, the second end being
proximate to the first end of the first optical waveguide, wherein
the first and second optical waveguides are aligned such that light
transmitted from the second end of the second waveguide is incident
upon the first end of the first waveguide, wherein the
non-transparent end structure is reflective, and wherein the first
and second optical waveguides are further aligned such that at
least a portion of light transmitted from the second end of the
second optical waveguide that is incident upon the first end of the
first optical waveguide is reflected back onto the second end of
the second optical waveguide, and the optical component is
configured to monitor the light reflected back onto the second end
of the waveguide.
16-29. (canceled)
30. The optical component of claim 1, wherein the optical component
is further configured to determine the optical power of the at
least a portion of incident light reflected from the
non-transparent end structure.
31. The optical component of claim 1, further comprising: a
photo-detector, wherein the photodetector is configured to monitor
the at least a portion of incident light reflected from the
non-transparent end structure.
32. The optical component of claim 15, wherein the optical
component is further configured to determine the optical power of
the at least a portion light reflected back onto the second end of
the waveguide.
33. The optical component of claim 15, further comprising: a
photo-detector, wherein the photodetector is configured to monitor
the at least a portion of light reflected back onto the second end
of the waveguide.
34. An optical device, comprising: one or more optical waveguides,
including a first optical waveguide having an inner core extending
in a first direction that is radially surrounded by an outer
cladding along the first direction, the first optical waveguide
terminating in a first end and wherein the inner core has a higher
index of refraction than the index of refraction of the outer
cladding; a non-transparent end structure covering the first end of
the first optical waveguide and having a transparent aperture for
at least a portion of inner core, wherein the non-transparent end
structure is reflective; and a photo-detector configured such that
a portion of a beam of light incident on the end structure is
reflected thereon, wherein the photo-detector is further configured
to determine the optical power of the portion of the beam of
light.
35. The optical device of claim 34, wherein the first optical
waveguide is an optical fiber.
36. The optical device of claim 35, further comprising: a ferrule
in which the optical fiber is embedded.
37. The optical device of claim 34, wherein the end structure is
formed on the first end of the optical waveguide.
38. The optical device of claim 34, wherein the end structure is
formed on a plate, separate from the first end of the optical
waveguide.
39. The optical device of claim 34, wherein the inner core has a
uniform index of refraction.
40. The optical device of claim 34, wherein the inner core has a
non-uniform index of refraction.
41. The optical device of claim 34, wherein the transparent
aperture has an area that is less than the area of the inner core
on the first end, such that less than all of the inner core on the
first end is exposed by the transparent aperture.
42. The optical device of claim 34, wherein the transparent
aperture has an area that is larger than the area of the inner core
on the first end, such that all of the inner core and a portion of
the cladding is exposed by the transparent aperture.
43. The optical device of claim 34, wherein the transparent
aperture has an area that is substantially equal to the area of the
inner core on the first end, being aligned such that substantially
all of the inner core and none of the cladding is exposed by the
transparent aperture.
44. The optical device of claim 34, wherein the first end of the
first optical waveguide is angled at a non-right angle relative to
the first direction.
45. The optical device of claim 34, further comprising: a second
optical waveguide terminating in a second end, the second end being
proximate to the first end of the first optical waveguide, wherein
the first and second optical waveguides are aligned such that light
transmitted from the second end of the second waveguide is incident
upon the first end of the first waveguide.
Description
BACKGROUND
[0001] The following is related generally to optical or fiberoptic
components used in optical communication networks and, more
specifically, to reducing polarization, wavelength, and temperature
dependent loss in fiberoptic components.
[0002] Fiberoptic components such as Variable Optical Attenuators
(VOAs), optical switches, and tunable optical filters are widely
deployed in optical networks, typically in the 1550 nm or 1310 nm
wavelength windows, as well as other wavelength ranges. In
wavelength-division-multiplexed optical networks where multiple
wavelengths are used, so that multiple channels of information can
be transmitted or carried on a single fiber, Variable Optical
Attenuators are used at various points in the network, to manage
the optical power of the multiple optical signals or wavelengths.
Optical switches are used to redirect or re-route signals that are
transmitted or carried on fibers, by establishing connections
between fibers. Tunable optical filters are used to select specific
wavelengths or wavelength ranges, and may also be used to scan
multiple wavelengths in channel or fiber monitoring
applications.
[0003] Optical beam-steering technologies of various kinds are
often used to implement fiberoptic components such as VOAs, optical
switches, and tunable optical filters. For example, MEMS
(Micro-Electro-Mechanical Systems) tilting mirrors are often used
to steer light from one or more input ports or fibers of a
fiberoptic component, towards one or more output ports or fibers.
In a MEMS-based VOA, the beam is steered toward an output port, and
the degree of alignment of the beam to the output port determines
the amount of attenuation. In a MEMS-based optical switch, the
intent is usually to have minimal insertion loss, as the beam is
steered to the desired output port. Similarly, in a tunable optical
filter, the intent is usually to have minimal insertion loss of the
selected wavelength or wavelength range, as it is steered to the
output port. Also, in the case of some tunable optical filters, the
coupling of light to the output port and the geometry of the
optical path, serve to determine the shape and width of the
selected wavelength's passband.
[0004] In fiberoptic components that make use of beam-steering, the
coupling of light from the one or more input ports or fibers, to
the one or more output ports or fibers, depends on many factors,
including the configuration and design of optical elements in the
path between the input and output ports, as well as the coupling of
the steered beam to the output port(s) or fiber(s). The loss
through the fiberoptic component may be dependent on the
polarization of the input light, wavelength, and even the ambient
temperature. In the case of tunable optical filter components, the
coupling of light to the output port(s) or fiber(s) may also
determine the shape and width of the filter's passband. The
reduction of polarization dependent loss (PDL), wavelength
dependent loss (WDL), and temperature dependent loss (TDL) has
great value to the designers and implementers of fiberoptic
networks. Similarly, improvements to the passband characteristics
of tunable optical filter components, such as providing greater
isolation of adjacent wavelength channels, also has great
value.
[0005] In many of the optical network applications of Variable
Optical Attenuators, as well other fiberoptic components, it is
often necessary or desirable to monitor the optical power of the
signal, either on the input side of the component, or (more
typically) on the output side. For this reason, it is common
practice to use an optical tap and an optical power detector, at
either the input or output of an optical component or function. The
optical tap splits off a small portion of the optical signal. The
split-off optical signal is then directed to an optical detector
device, which converts the optical power to an electrical signal,
from which the optical power of the signal can be determined. The
remainder of the optical signal (the portion that was not split off
and directed to the detector circuit) is than passed on to the rest
of the network. The portion of the optical power that was split off
by the optical splitter, or tap, represents a source of insertion
loss to the desired/intended optical signal. Consequently, optical
systems could benefit from improvements in providing a tap function
for monitoring purposes.
SUMMARY
[0006] An optical component has one or more optical waveguides,
including a first optical waveguide having an inner core extending
in a first direction that is radially surrounded by an outer
cladding along the first direction, the first optical waveguide
terminating in a first end. The inner core has a higher index of
refraction than the index of refraction of the outer cladding. A
non-transparent end structure covers the first end of the first
optical waveguide and has a transparent aperture for at least a
portion of inner core.
[0007] A ferrule structure for an optical fiber includes one or
more through-holes for the embedding of a corresponding one or more
optical fibers that are inserted into a first end of the ferrule
structure. The ferrule structure also includes an end plate
covering a second end of a first of the through-holes, the end
plate having a non-transparent outer surface with a central
transparent aperture.
[0008] In a method of forming an optical component, a first end of
an optical waveguide is coated with a photoresist material. The
optical waveguide has an inner core extending in a first direction
that is radially surrounded by an outer cladding along the first
direction, where the optical waveguide terminates at the first end.
The inner core has a higher index of refraction than the index of
refraction of the outer cladding. Light is subsequently transmitted
through the optical waveguide to thereby expose at least a portion
of the photoresist material. Non-exposed portions of the
photoresist material are removed from the first end of the optical
waveguide. A non-transparent coating is deposited over the first
end of the optical waveguide, including the exposed portion of the
photoresist material. The exposed portion of the photoresist,
including the non-transparent coating deposited over the exposed
portion of the photoresist, is subsequently removed to thereby form
an aperture in the non-transparent coating.
[0009] Various aspects, advantages, features and embodiments are
included in the following description of exemplary examples
thereof, which description should be taken in conjunction with the
accompanying drawings. All patents, patent applications, articles,
other publications, documents and things referenced herein are
hereby incorporated herein by this reference in their entirety for
all purposes. To the extent of any inconsistency or conflict in the
definition or use of terms between any of the incorporated
publications, documents or things and the present application,
those of the present application shall prevail.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] FIG. 1A illustrates the electric field power distribution of
the fundamental mode in a single mode fiber.
[0011] FIG. 1B shows three examples for cross-section geometries of
planar waveguides.
[0012] FIG. 2 illustrates the electric field power distribution for
an optical beam that is launched into a single mode fiber, with a
lateral offset from the center of the fiber core.
[0013] FIG. 3A illustrates an embodiment in which an opaque surface
with an aperture is used to largely block light from entering or
exiting the cladding layer of an optical fiber.
[0014] FIG. 3B illustrates one of the methods in which a pin hole
or aperture can be imposed on a fiber end surface.
[0015] FIG. 4 shows the electric field power distributions of the
fundamental modes of two different wavelengths.
[0016] FIGS. 5A and 5B show fiber misalignment in the lateral and
angular directions, respectively.
[0017] FIG. 6 illustrates a generalized fiberoptic component that
has optical elements that are located between a group of input
fibers and a group of output fibers.
[0018] FIG. 7 shows a wavelength spectrum or passband that has been
selected, and is being carried or transmitted in an optical
fiber.
[0019] FIGS. 8A-8D illustrate a process for creating a pin hole or
aperture directly on the end surface of the core of an optical
fiber, using UV lithography.
[0020] FIG. 9 shows an embodiment in which the non-transparent area
surrounding the pin hole or aperture is designed to reflect a
portion of the light, back towards the input.
DETAILED DESCRIPTION
[0021] The output of a fiberoptic component is typically dependent
on the polarization and wavelength of the input light or optical
signal, and may also depend on the ambient temperature, and other
parameters. The techniques presented here relate to methods for
reducing polarization, wavelength, and temperature dependent loss
in fiberoptic components. These techniques can also be used to
reduce the wavelength passband width in some types of fiberoptic
components, and have application in optical power monitoring.
[0022] More specifically, a pin hole or aperture can be located or
formed adjacent to the end surface of one or more of the input
ports or fibers, or adjacent to one or more of the output ports or
fibers, of a fiberoptic component. The pin hole or aperture allows
light to enter (or exit) the core of the associated fiber, and the
non-transparent layer that surrounds the pin hole or aperture
blocks light from entering or exiting the cladding layer of the
associated fiber. This blocking of the evanescent field in the
cladding layer serves to reduce the polarization, wavelength, and
temperature dependencies of the light coupling to the output
port(s) or fiber(s) of the optical component. It can also reduce
the passband width of the selected wavelength in tunable optical
filter applications. The non-transparent layer surrounding the
pin-hole or aperture can be made reflective (such as a metallic or
other reflective material, such as a reflective dielectric), and
the light that is reflected by the non-transparent layer can be
used for optical power monitoring.
[0023] As shown in FIG. 1A, an optical fiber 130, such as is used
in optical communication networks, includes a transparent core 101
surrounded by a transparent cladding layer 102 that has a
refraction index n2 that is lower than the refraction index n1 of
the core 101. A majority of the light that passes down the fiber is
confined in the core 101 by total internal reflection, occurring at
the interface 103 of the core 101 and cladding 102, and the
remaining small portion of light, referred to as an evanescent
field, and indicated by 105, penetrates into the cladding layer 102
and decays out along the radial direction r. If the core of the
fiber carries only one propagation mode, then the fiber is called a
single-mode (SM) fiber. The fiber is called multi-mode (MM) fiber
if it carries more than one propagation mode. In the following
description, single-mode fiber is assumed in the drawings and
description, for illustrative purposes. However, the techniques are
applicable to multi-mode fibers as well, with similar physics.
[0024] For a single-mode (SM) fiber, the index n.sub.1 in the core
101 can be either uniform (in which case it can be referred to as
step-index) or non-uniform (for example, graded index fiber, having
a maximum index of refraction at the core center). Similarly, the
index n.sub.2 of the cladding layer 102 can either be a uniform
index or have a distribution. The electric field distribution and
its corresponding power distribution (proportional to the square of
the electric field) of the fundamental mode in a single-mode fiber,
plotted as intensity I versus radial distance, is indicated by 110.
Inset drawing 120 shows the perspective view of the power
distribution, as emitted from a fiber end surface 107. The
propagation mode can carry its electric field in any transverse
polarization direction, which can be generally decomposed into two
orthogonal directions, represented by E.sub.x and E.sub.y, as
indicated by 121 and 122, respectively. The core diameter of SM
fibers used in optical communications is typically 9 micro-meters
and the cladding diameter is typically 125 micro-meters.
[0025] The techniques described here also extend to other forms of
optical or photonic waveguides, in addition to optical fiber.
Optical or photonic waveguides can be formed in or on substrates,
using a variety of materials and fabrication processes. Devices
using optical or photonic waveguides are sometimes referred to as
photonic integrated circuits (PICs) or photonic lightwave circuits
(PLCs). Common materials used for optical or photonic waveguides
include silicon and silica. The fabrication processes are similar
to semiconductor fabrication processes, and include etching,
deposition, oxidation, lithography, etc. Similar to optical fiber,
optical or photonic waveguide structures have a core of relatively
higher index of refraction, surrounded by cladding material with
relatively lower index of refraction. The cross-sectional shape of
the core may be rectangular, or square, or any number of shapes.
The cladding that surrounds the core may also have different shapes
and configurations. FIG. 1B gives some examples shown in
cross-section, oriented such that the light would run into or out
of the page. In each of these examples, the lower cladding is
formed over the substrate, upon which the core is formed, either as
a slab, a ridge shaped slab, or a channel-shaped core, over which
an upper cladding is in turn formed. Most optical or photonic
waveguides are single-mode (SM), although it is also possible to
have multi-mode (MM) waveguides. Although the following description
and figures are based on, or assume, the use of optical fiber, it
will be understood that the techniques are also applicable to
optical or photonic waveguide structures, with cores of varying
shape. Similarly, the techniques could be applied to other fiber
types, such as multicore fiber, where, depending on the embodiment,
the end structure could have a separate aperture for each core, or
more than one core could share a common aperture.
[0026] FIG. 2 shows a beam, having electric field distribution 201,
located in a plane at Z=-.di-elect cons. in air or vacuum
(.di-elect cons. is a small distance). (Note that in FIG. 2, the
positive direction of Z is towards the left of the figure.) The
polarization of the beam is in either the X or Y direction. The
beam is launched to the fiber end surface 212, located in a plane
at Z=0, but the beam is substantially offset (laterally) from the
fiber core center 215. This sort of lateral beam shift is typical
of many types of fiberoptic components that use various forms of
beam-steering technologies, such as Variable Optical Attenuators
(VOAs). One example of lateral beam shift in a VOA application is
shown in the product web site of DiCon Fiberoptics, Inc. Another
example of lateral beam shift in shown in U.S. patent application
Ser. No. 15/184,722. Part of the optical power in beam 201 is
coupled to the fundamental propagation mode 220 of the single-mode
fiber, and the rest is coupled into the fiber cladding, and then
leaked out of the fiber 210. As the lateral offset from the core
center increases, the optical power that is coupled to the core of
fiber 210 decreases. The coupling efficiency is also dependent on
the polarization state of the input beam 201, as well as the stress
distribution inside the cladding layer, due to differences in the
electric field matching at the interface (in accordance with
electromagnetics theory) for different polarization states. The
difference in the coupling efficiency that results from
polarization is called polarization dependent loss (PDL).
[0027] Mathematically, the coupling efficiency .eta..sub.c is equal
to:
.intg..intg..sub.Z.gtoreq.0E.sub.a(x,y)E.sub.q(x,y)dS (integrated
over the plane surface for Z.gtoreq.0),
where E.sub.a is the normalized amplitude of the electric field
distribution of the input beam for Z.gtoreq.0, and E.sub.q is the
normalized fundamental mode 220, which is a Gaussian beam. When the
input beam 201 enters the fiber end surface 212, the E.sub.a
distribution for Z.gtoreq.0 is slightly dependent on the
polarization state of the incident beam and the stress distribution
inside the cladding layer, even though the amplitude distribution
of the electric field at Z=-.di-elect cons. is the same for all
polarization states, due to differences in the electric field
matching at the interface 203, for different polarization
states.
[0028] The techniques presented here present a method for reducing
polarization dependent loss (PDL), as well as wavelength dependent
loss (WDL) and temperature dependent loss (TDL) due to thermal
expansion and contraction changing the alignment of elements. The
techniques can also be used to reduce the wavelength passband width
of fiberoptic components, by covering up the cladding layer that is
adjacent to the fiber core at the fiber end surface with a
non-transparent (or opaque) layer, which may consist of one or more
sub-layers. In FIG. 3A, 302 indicates the interface of the fiber
core 301 and cladding 303. The shaded area 305 denotes the portion
of the cladding area that is covered by a non-transparent material,
such as a metal. Thus, a pin hole opening or aperture 306 is
created on the fiber end surface 320. The pin hole can be
substantially the same size as, slightly larger than, or smaller
than the fiber core 302.
[0029] In the right-hand portion of FIG. 3A, an input beam 308,
having an electric field distribution denoted by 310, is incident
onto the end surface of an optical fiber, identical in structure to
the fiber shown in the left-hand portion of the figure. (The labels
shown on the left-hand figure also apply to the right-hand figure.)
As shown in FIG. 3A, the input beam 308 is offset laterally from
the center of the fiber core. Thus, a majority portion of the input
optical power, denoted in the figure as area B, is blocked by the
non-transparent material, and only the remaining portion denoted as
area A is able to enter (or exit) the optical fiber. Because the
optical power of portion A is coupled to the fiber core 301
directly, and the optical power of portion B is blocked by the
non-transparent layer outside pin hole 306, the electric field
matching at the interface 302 is eliminated. Thus, the influence of
polarization on the coupling of portion A into the fundamental
propagation mode of the fiber is substantially reduced or limited,
in comparison to the case in which there is no pin hole and no
non-transparent area. In short, the pin hole or aperture, and the
surrounding non-transparent layer, serve to reduce or limit the
polarization dependency of the power coupling of an input beam to
an SM fiber. Experimental results verify that applying a pin hole
or aperture with a diameter that is close to the size of the fiber
core can reduce PDL by an order of magnitude, or more.
[0030] The end structure of the pin hole or aperture on the fiber
end surface can be created using UV lithography (as explained
later, and shown in FIG. 8), or by attaching a pin hole plate 325
closely to the fiber end 317 as shown in FIG. 3B, as well as
through the use of other metal (or other material) deposition
methods. In alternate embodiments, a ferrule structure can include
a pin hole end plate at the end of the through-hole, into which a
fiber can then be embedded. These methods are all within the scope
of the present discussion, as long as the pin hole or aperture with
surrounding non-transparent layer is closely proximate to a fiber
end, to inhibit optical power being coupled into or propagating in
the cladding layer. In FIG. 3B, a metallic or other non-transparent
layer 327 with a pin hole opening 329 is first printed or deposited
on a transparent plate 325 using photo-lithography and chemical
etching. Then the pin hole of the plate is aligned with the core
322 of the fiber 320, and the plate 325 is tightly fixed against
the fiber end surface 327, as indicated by assembly 330.
[0031] As explained above, an SM fiber carries a fundamental mode,
whose evanescent field spreads out into the cladding layer. The
longer the wavelength, the more spread-out it is. As shown in FIG.
4, the amplitudes of two electric fields at a longer wavelength
.lamda..sub.L and a shorter wavelength .lamda..sub.s are indicated
by the two curves 401 and 402, respectively. In FIG. 4, d
represents the core diameter, and the pin hole or aperture diameter
is D. The so-called mode field diameter (MFD) is the diameter at
which the amplitude of the field decays to 1/e of the peak
amplitude that is located at the core center (e is the mathematical
constant that is the base of the natural logarithm, sometimes
called Euler's number, and is approximately equal to 2.718). If the
fundamental mode of an SM fiber propagates toward a fiber end that
has a pin hole or aperture of diameter D, such that the pin hole is
lined up with the fiber core and is imposed on the fiber end
surface, only light that is within the pin hole is able to exit the
fiber end. The optical power of the evanescent field in the fiber
cladding is blocked by the non-transparent layer or film that
surrounds the pin hole. This results in the total power exiting the
fiber being less wavelength dependent, compared to the case without
the pin hole or surrounding non-transparent layer. This exiting
power may then pass other optical components, before reaching an
output port or a photo-detector, at which point a power measurement
of the light would show less wavelength dependence. Similarly, the
reduced beam size caused by a pin hole and non-transparent layer,
either in the input fiber(s) or output fiber(s), results in reduced
loss variation from temperature-induced optical coupling changes in
the optical elements that lie along the optical path.
[0032] FIGS. 5A and 5B show examples of two fibers that are in
lateral mis-alignment and angular mis-alignment, respectively, such
as may occur in an optical device or system. In FIG. 5A, an input
fiber 501 carrying optical power is coupled to an output fiber 502,
with a substantial mis-alignment in the relative lateral positions
of the two fiber cores 503 and 504. Both insertion loss (IL) and
polarization dependent loss (PDL) are high for this coupling fiber
pair. However, if a pin hole or aperture is imposed on either or
both of the fiber end surfaces 507 and 508, of fibers 501 and 502,
respectively, then the polarization dependent loss of this coupling
fiber pair can be reduced significantly. This provides substantial
performance advantages for optical components and optical system
design, which desire stable optical output regardless of the
polarization state of the transmitted optical signal. The penalty
for imposing a pin hole in this example is that the insertion loss
of the fiber coupling will be increased slightly. In FIG. 5B, the
two fibers 511 and 512 are in severe angular mis-alignment.
Similarly, a pin hole imposed on either or both of the fiber end
surfaces 515 and 516, of fibers 511 and 512, respectively, can
significantly reduce PDL.
[0033] FIG. 6 illustrates a generalized optical component having
one or more optical fibers as a group (only two fibers are shown in
the figure) comprising an input port 601, and one or more optical
fibers as a group (only two fibers are shown in the figure)
comprising an output port 602. The fibers in the input and output
ports may be embedded in fiber ferrules 603 and 604, respectively,
for optical alignment, positioning, and fixing in place. A
generalized assembly of optical elements 610 is positioned between
input port 601 and output port 602. Because the transmission
characteristics (such as index birefringence) of many optical
elements are dependent on the polarization state of the light (at
least to some extent), the optical beam that impinges on the output
port also depends somewhat on the polarization state. If a pin hole
and surrounding non-transparent layer is imposed on one or more of
the input and output fibers, the optical power coupling from the
input to the output fibers can be made less polarization dependent.
In FIG. 6, all of the fibers are shown with an end structure.
[0034] For spectral-selective (or wavelength-selective) optical
components, as described in U.S. Pat. No. 7,899,330 and U.S. patent
application Ser. No. 15/081,294, the output port/fiber carries a
wavelength spectrum as shown by plot 701 in FIG. 7. Due to
wavelength cross-talk, diffraction effects, and scattering caused
by the optical elements in the optical path, the selected spectrum
(or wavelength) of such an optical component may have limited
wavelength isolation, typically about 25 dB, outside of its FWHM
(full width at half maximum) passband. This is represented by the
bottom portion 705, of spectral curve 701. Maximizing wavelength
isolation is highly desirable for optical system design, as it
increases the effective signal-to-noise ratio of the optical
signals. By imposing a pin hole with surrounding non-transparent
layer onto the input fibers, output fibers, or both, the wavelength
"noise" on the output ports can be reduced, as indicated by plot
702.
[0035] FIGS. 8A-8D illustrates some of the stages for an exemplary
process for creating a pin hole or aperture, with surrounding
non-transparent layer, on the fiber end surface. At FIG. 8A, an
optical fiber 801, with fiber core 802 and cladding layer 803, is
embedded in a through-hole (or bore or passage) a fiber ferrule 804
(not shown to actual scale), and its end surface 806 is well
polished either perpendicular or being slanted with a small angle
with respect to the optical axis of the fiber core 802. A
photoresist material 809 is then coated on the end surface 806, as
shown in FIG. 8B. As indicated in FIG. 8C, UV light, typically with
wavelengths ranging from about 230 nm to about 400 nm, is then
launched into the fiber core 802 from the other fiber end 815. The
UV light is then carried by the fiber core 802 toward the
photoresist layer, causing the photoresist material to cure or
solidify, in the area above or on top of the fiber core 802. By
controlling the UV light intensity and exposure time, the size of
the area of cured or solidified photoresist can be tuned. By
washing away the un-exposed photoresist, a circular photoresist
island 820 is left remaining on top of the fiber core 802.
Depending on the UV light intensity and exposure time, the circular
photoresist island may be slightly larger or slightly smaller than
the fiber core diameter. In FIG. 8D, a thin metal film or a layer
(or layers) of some non-transparent material 825 is deposited onto
the fiber ferrule end surface 806, typically by using some form of
physical or chemical vapor deposition process. The photoresist
island 820 is then removed by solvents, such that a pin hole
opening or aperture 828 is created in the non-transparent film or
layer 825, right on top of the fiber core. The size and placement
of the pin hole opening or aperture 828 is directly related the
size and placement of the photoresist island 820. The size of the
photoresist island 820 in turn depends on the exposed UV dose,
while the placement of the photoresist island 820 is self-aligned
to the fiber core 802. Thus, by controlling the exposure dose, the
pin hole opening size of the end structure can be tuned and
controlled. Normally, having a pin hole opening size that is either
slightly larger or slightly smaller than the fiber core size, is
desired for enhancing the stability of the output optical power
without significantly sacrificing power transmission, as described
above.
[0036] In another embodiment, a fiber with a pin hole or aperture
on its end surface can be used to reflect optical power that is
contained in the evanescent field of the fundamental mode, as shown
in FIG. 9. A fiber 902 has its end surface 907 coated with a
non-transparent layer 905, that contains a pin hole 906, located
over the fiber core 903. The fiber end surface 907 may be slanted
by a small angle, with respect to the optical axis of the fiber
core 903. The other fiber 901 has one end surface 909 that is well
polished, and may also be slanted with a small angle. Fiber end
surface 909 may be coated with an anti-reflection coating. The
slanted fiber end face(s) and anti-reflection coating are intended
to avoid back-reflection of the optical signal, as well as the
formation of an unintended optical cavity between end surface 905
and end surface 909. The two fibers 902 and 901 may be positioned
proximate to one another with a small air gap between them, or they
may be physically butted against each other, or they may be fused
together using a fusion splicing machine, to form a fiber pair
assembly, as indicated by 950 in the bottom portion of FIG. 9.
[0037] As shown in the bottom portion of FIG. 9, the fundamental
mode 951 of fiber 901 propagates along fiber core 908 toward the
interface 952, and its power around the mode center 954 is
transmitted through the pin hole or aperture, and eventually turns
into the fundamental mode of fiber 902, with smaller amplitude, as
represented by 957. However, its evanescent field 955 is reflected
by the non-transparent (and reflective) layer 905, back to fiber
901. Although most (or even all) of the optical power of reflected
evanescent field 955 is coupled into the cladding layer of fiber
901, a portion of the reflected evanescent field will eventually be
partially coupled into the core of fiber 901, as fundamental mode
960, with a much smaller amplitude. The reflected optical power
960, exiting the other end 962 of fiber 901, can be used for
monitoring the optical power level carried by the incoming
fundamental mode 951 in the fiber 901, or it can be used for other
optical processing. The fiber end 962 can be slanted by a small
angle (although this is not shown in FIG. 9) with respect to the
optical axis of fiber core 908, such that the reflected power 960
exits fiber 901 into free space with a small angle (also with
respect to the optical axis of the fiber core 908), for easy
detection by an optical receiver or photo-detector. The transmitted
(output) optical power 957, at the output end 961 of fiber 902, has
the benefits of low polarization, temperature, and wavelength
dependence, as described above. It should be noted that the
transmission of light in fiber pair 950 is optically bidirectional.
Fiber 902 may be used as the input fiber, and fiber 901 may be used
as the output fiber, as long as the non-transparent layer 905 is
reflective on both sides.
[0038] The foregoing detailed description has been presented for
purposes of illustration and description. It is not intended to be
exhaustive or to limit the invention to the precise form disclosed.
Many modifications and variations are possible in light of the
above teaching. The described embodiments were chosen in order to
best explain the principles involved and their practical
application, to thereby enable others skilled in the art to best
utilize the various embodiments and with various modifications as
are suited to the particular use contemplated. It is intended that
the scope of the invention be defined by the claims appended
hereto.
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