U.S. patent application number 11/765400 was filed with the patent office on 2008-07-03 for devices based on optical waveguides with adjustable bragg gratings.
This patent application is currently assigned to General Photonics Corporation, a Delaware corporation. Invention is credited to Xiaotian Steve Yao.
Application Number | 20080159692 11/765400 |
Document ID | / |
Family ID | 22847273 |
Filed Date | 2008-07-03 |
United States Patent
Application |
20080159692 |
Kind Code |
A1 |
Yao; Xiaotian Steve |
July 3, 2008 |
Devices Based on Optical Waveguides with Adjustable Bragg
Gratings
Abstract
A system for filtering light propagating in a waveguide is
described. The system utilizes an adjustable periodic grating which
induces mode coupling of predetermined frequencies of light
propagating in the waveguide.
Inventors: |
Yao; Xiaotian Steve;
(Diamond Bar, CA) |
Correspondence
Address: |
FISH & RICHARDSON, PC
P.O. BOX 1022
MINNEAPOLIS
MN
55440-1022
US
|
Assignee: |
General Photonics Corporation, a
Delaware corporation
|
Family ID: |
22847273 |
Appl. No.: |
11/765400 |
Filed: |
June 19, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10946665 |
Sep 21, 2004 |
7233720 |
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11765400 |
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10649179 |
Aug 26, 2003 |
6795616 |
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10946665 |
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09226030 |
Jan 6, 1999 |
6628861 |
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10649179 |
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Current U.S.
Class: |
385/28 ;
385/37 |
Current CPC
Class: |
G02B 6/2932 20130101;
G02B 6/2706 20130101; G02B 6/29322 20130101; G02B 6/276 20130101;
G02B 6/02195 20130101; G02B 6/266 20130101; G02B 6/2861 20130101;
G02B 6/02071 20130101; G02B 6/274 20130101; G02B 6/02047 20130101;
G02B 6/02061 20130101 |
Class at
Publication: |
385/28 ;
385/37 |
International
Class: |
G02B 6/26 20060101
G02B006/26; G02B 6/34 20060101 G02B006/34 |
Claims
1. A variable delay apparatus comprising: a waveguide segment; and
an adjustable periodic grating moveable along the waveguide segment
in the direction of light propagation in the waveguide, the
adjustable periodic grating inducing Bragg reflection in the
waveguide.
2. The variable delay apparatus of claim 1 further comprising: a
circulator to transmit light to the waveguide segment and to
receive light reflected within the waveguide segment.
3. The variable delay apparatus of claim 1 wherein the adjustable
periodic grating is moved by a screw.
4. A method of transferring a light signal from a first
polarization maintaining fiber to a second polarization maintaining
fiber comprising coupling the first polarization maintaining fiber
to the second polarization maintaining fiber at a joint to form a
combination polarization maintaining fiber; position a periodic
grating to induce mode coupling of one frequency of light in the
combination polarization maintaining fiber; adjusting pressure
between the periodic grating and the combination polarization
maintaining fiber to couple power between a fast mode and a slow
mode of the combination polarization maintaining fiber.
5. A tunable polarizer comprising: a polarization maintaining fiber
including a core and a cladding; and a periodic grating coupled to
the polarization maintaining fiber to couple a first polarization
mode of a first wavelength into the cladding.
6. The tunable apparatus of claim 5 wherein the periodic grating
presses against the polarization maintaining fiber to induce a
periodic change in a refractive index of the polarization
maintaining fiber to couple the first polarization mode of the
first wavelength into the cladding.
Description
[0001] This application is a continuation application and claims
the benefit of priority under 35 USC 120 of co-pending U.S.
application Ser. No. 10/946,665, filed Sep. 21, 2004, and issued as
U.S. Pat. No. 7,233,720 on Jun. 19, 2007. The U.S. application Ser.
No. 10/946,665 is a divisional of Ser. No. 10/649,179, filed Aug.
26, 2003 and issued as U.S. Pat. No. 6,795,616 on Sep. 21, 2004.
The U.S. application Ser. No. 10/649,179 is a divisional (and
claims the benefit of priority under 35 USC 120) of U.S.
application Ser. No. 09/226,030, filed Jan. 6, 1999 and issued as
U.S. Pat. No. 6,628,861 on Sep. 30, 2003. The disclosures of the
prior applications are considered part of (and are incorporated by
reference in) the disclosure of this application.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates generally to controlling light
propagating in a wave guide. More particularly, the present
invention relates to using gratings to cause mode coupling of light
propagating in a wave guide.
[0004] 2. Description of Related Art
[0005] Devices used in optical systems, such as in fiber optic
communication systems and sensing systems, often benefit from the
filtering or control of light propagating in a wave guide. Examples
of such devices include, but are not limited to, source lasers,
optical amplifiers, filters and other integrated-optical
components. One method of controlling and filtering light utilizes
diffraction gratings. Descriptions of such devices and how they
benefit from diffraction gratings are described in T. Erdogan and
V. Mizrahi, "Fiber Phase Gratings Reflect Advances in Lightwave
Technology," February 1994 edition of Laser Focus World.
[0006] There are three techniques typically used to create a
diffraction grating in a wave guide to induce mode coupling or
Bragg reflection. The most common method uses ultraviolet light to
induce a refractive index change in an optical fiber. A system for
producing a periodic refractive index change in the optical fiber
is illustrated in FIG. 1. In FIG. 1 a first beam 104 of coherent
ultraviolet "UV" light and a second beam 108 of coherent UV light
are directed at a photosensitive optical fiber 112. At the
intersection of the first beam 104 and the second beam 108, an
interference pattern 116 is generated. The refractive index of the
photosensitive optical fiber 112 changes with the intensity of the
UV exposure, thus an index grating with a periodicity determined by
the interference pattern 116 forms where the first coherent beam
104 and the second coherent beam 108 intersect.
[0007] A second technique for creating a grating in an optical
fiber involves etching a periodic pattern directly onto an optical
fiber. In one embodiment, a photomask is used to generate a
periodic pattern in a photolithographic process. An acid etch
etches the grating or periodic pattern into the optical fiber. Such
photomasks and etching are commonly used in semiconductor
processes.
[0008] A third technique to control light in a waveguide is used in
semiconductor waveguides. In one embodiment, a layered growth is
formed on the semiconductor wave guide to generate light reflection
in the wave guide.
[0009] The described techniques for creating a grating on or in a
wave guide are permanent. The gratings have a fixed periodicity at
a fixed location on the waveguide that cannot be easily changed.
Thus, a particular wave guide and grating combination will have a
predetermined transmission characteristic. In order to change the
characteristic, the entire wave guide segment containing the
grating is typically replaced with a wave guide segment having a
different transmission characteristic. Replacing wave guide
segments is a cumbersome process requiring that each end be
properly coupled to the light source and the light receiving
device.
[0010] Thus, an improved system and method to control light
propagating in a wave guide is needed.
SUMMARY OF THE INVENTION
[0011] The present invention relates to a method and apparatus of
controlling light transmitted in a wave guide. The apparatus uses a
holder to fix a wave guide in a fixed position relative to an
adjustable periodic grating. The periodic grating is movable to at
least two positions, in one position the periodic grating induces
mode coupling in the wave guide, and in the second position the
periodic grating does not induce mode coupling in the wave
guide.
[0012] The present application also describes other devices.
Examples of various devices include:
[0013] 1. A tunable apparatus for controlling light transmitted in
a wave guide comprising:
[0014] a waveguide holder; and
[0015] an adjustable periodic grating coupled to the waveguide
holder, the adjustable periodic grating moveable between a first
position and a second position.
[0016] 2. The tunable apparatus of claim 1 wherein when the
periodic grating is in the first position, the adjustable periodic
grating produces mode coupling from a first mode to a second mode
of at least one wavelength of light propagating in the
waveguide.
[0017] 3. The tunable apparatus of claim 2 wherein moving the
adjustable periodic grating to a second position prevents mode
coupling at the at least one wavelength of light.
[0018] 4. The tunable apparatus of claim 2 wherein in the first
position, an air gap separates the adjustable periodic grating and
the optic cable.
[0019] 5. The tunable apparatus of claim 2 wherein in the first
position, the adjustable periodic grating contacts the optic
cable.
[0020] 6. The tunable apparatus of claim 1 further comprising:
[0021] an index matching fluid between the waveguide and the
grating.
[0022] 7. The tunable apparatus of claim 1 wherein the distance
separating grooves on the adjustable periodic grating is between
0.1 um and 10 mm.
[0023] 8. The tunable apparatus of claim 2 wherein one wavelength
of light propagating in a forward direction that is coupled into a
backward direction is given by:
.lamda.=.LAMBDA./2n.sub.eff
wherein: .LAMBDA.=periodicity of the grating, and
[0024] n.sub.eff=the effective refractive index of the
waveguide.
[0025] 9. The tunable apparatus of claim 2 wherein one wavelength
of light propagating in a core of the waveguide that mode couples
into a cladding of the waveguide is determined by:
.lamda.=.LAMBDA.(n.sub.core-n.sub.cladding)
wherein: .LAMBDA.=periodicity of the grating
[0026] n.sub.core=the refractive index of a core of the waveguide,
and
[0027] n.sub.cladding=index of refraction of the cladding of the
waveguide.
[0028] 10. The tunable apparatus of claim 2 wherein one wavelength
of light coupled from a first polarization mode to a second
polarization mode is given by:
.lamda.=.LAMBDA.(n.sub.s-n.sub.f)
wherein: .LAMBDA.=periodicity of the grating
[0029] n.sub.s=effective refractive index of the first polarization
mode
[0030] n.sub.f=effective index of refraction of the second
polarization mode.
[0031] 11. The tunable apparatus of claim 2 wherein one wavelength
of light coupled from a first transversal mode to a second
transversal mode is given by:
.lamda.=.LAMBDA.(n.sub.1-n.sub.2)
wherein: .LAMBDA.=periodicity of the grating
[0032] n.sub.1=effective refractive index of the first transversal
mode
[0033] n.sub.f=effective index of refraction of the second
transversal mode.
[0034] 11(A). The tunable apparatus of claim 1 further comprising a
second periodic grating coupled to the waveguide holder.
[0035] 12. The tunable apparatus of claim 2 wherein the first
periodic grating produce mode coupling of a first frequency, the
tunable apparatus further comprising:
[0036] a second periodic grating coupled to the waveguide holder,
the second periodic grating producing mode coupling at a second
frequency, the adjustable periodic grating working in cooperation
with the second periodic grating to produce mode coupling at a
third frequency, the third frequency higher than either the first
frequency and the second frequency.
[0037] 13. The tunable apparatus of claim 1 further comprising:
[0038] a screw coupled to the periodic grating to move the periodic
grating between the first position and the second position.
[0039] 14. The tunable apparatus of claim 1 further comprising:
[0040] a spring coupled to the periodic grating to regulate a
pressure on the periodic grating and a waveguide in the waveguide
holder.
[0041] 15. The tunable apparatus of claim 1 further comprising:
[0042] a piezo-electric coupled to the periodic grating to move the
periodic grating between the first position and the second
position.
[0043] 16. The tunable apparatus of claim 1 wherein the waveguide
includes a cladding and a cladding reduced region, the periodic
grating in contact with the waveguide in the cladding reduced
region.
[0044] 17. The tunable apparatus of claim 1 wherein the waveguide
includes a cladding and a cladding reduced region, the periodic
grating oriented such that in the first position, the periodic
grating intersects an evanescent field of a waveguide mode in the
cladding reduced region.
[0045] 18. The tunable apparatus of claim 1 where the periodic
grating is a chirped grating.
[0046] 19. The tunable apparatus of claim 18 wherein a spacing of
grooves in the chirped grating varies quadratically.
[0047] 20. The tunable apparatus of claim 18 wherein a spacing of
grooves in the chirped grating varies linearly.
[0048] 21. The tunable apparatus of claim 1 wherein the periodic
grating is rotatable to an angle such that an effective grating
spacing to light propagating in the waveguide is a physical spacing
of the periodic grating divided by a cosine of the angle.
[0049] 22. A method of filtering at least one frequency of light
propagating in a waveguide comprising the acts of:
[0050] positioning a periodic grating to induce mode coupling of
one frequency of light in the waveguide;
[0051] determining the intensity of the one frequency of light
propagating in the waveguide; and
[0052] repositioning the periodic grating to change the intensity
of the mode coupling of the one frequency of light.
[0053] 23. The tunable apparatus of claim 22 wherein the waveguide
is an optical filter.
[0054] 24. The method of claim 22 wherein the act of repositioning
further comprises:
[0055] rotating a screw coupled to the periodic grating to
reposition the periodic grating.
[0056] 25. The method of claim 22 wherein the act of repositioning
further comprises:
[0057] changing an electric potential applied to a piezo-electric
to move the periodic grating.
[0058] 26. The method of claim 22 further comprising the act
of:
[0059] side polishing the optical fiber before positioning the
periodic grating.
[0060] 27. An optic equalizer comprising:
[0061] a waveguide to guide light;
[0062] a first adjustable periodic grating positioned at a first
region of the waveguide, the first adjustable periodic grating to
generate mode coupling at a first frequency;
[0063] a second adjustable periodic grating positioned at a second
region of the optical fiber, the second adjustable periodic grating
to generate mode coupling at a second frequency.
[0064] 28. The optic equalizer of claim 27 wherein the first
adjustable periodic grating is coupled to a screw to move the first
adjustable periodic grating between a first position and a second
position.
[0065] 29. The optic equalizer of claim 27 wherein the first
adjustable periodic grating is coupled to a piezo-electric to move
the first adjustable periodic grating between a first position and
a second position.
[0066] 30. The optic equalizer of claim 27 further comprising:
[0067] a third adjustable periodic grating positioned at a third
region of the optical fiber, the third adjustable periodic grating
to generate mode coupling at a third frequency, the combination of
the first adjustable grating, the second adjustable grating and the
third adjustable grating to adjust the spectral content of light
propagating in the waveguide at a fourth position in the
waveguide.
[0068] 31. A graphic equalizer comprising:
[0069] a waveguide;
[0070] a plurality of adjustable periodic gratings, each adjustable
periodic grating in the plurality of adjustable periodic gratings
independently adjustable to alter the intensity of a corresponding
frequency range of light propagating in the waveguide.
[0071] 32. A method of using a graphic equalizer comprising:
[0072] receiving light in a waveguide;
[0073] determining the desired spectral content of light at a
position on the waveguide;
[0074] adjusting at least one periodic grating to cause mode
coupling of an undesired frequency of light propagating in the
waveguide.
[0075] 33. A variable delay apparatus comprising:
[0076] a circulator to receive a light beam at an input port;
[0077] a variable delay line coupled to a processing port of the
circulator, the circulator to propagate the light beam from the
input port to the variable delay line;
[0078] a plurality of adjustable gratings, each adjustable grating
movable between a corresponding first position which produces Bragg
reflection in the variable delay line and a second position which
does not produce Bragg reflection in the variable delay line, the
circulator receiving Bragg reflected signals at the processing port
and outputting the Bragg reflected signals at an output port of the
circulator.
[0079] 34. The apparatus of claim 33 further comprising:
[0080] a plurality of piezo-electrics, each piezo-electric in the
plurality of piezo-electrics coupled to a corresponding adjustable
grating to move the corresponding adjustable grating between the
first position and the second position.
[0081] 35. The apparatus of claim 34 further comprising:
[0082] a voltage source;
[0083] a plurality of switches, each switch in the plurality of
switches to couple the voltage source to a corresponding
piezo-electric in the plurality of piezo-electrics such that
switching a switch induces a corresponding piezo-electric to move a
corresponding adjustable grating.
[0084] 36. A variable delay apparatus comprising:
[0085] a waveguide segment; and
[0086] an adjustable periodic grating moveable along the waveguide
segment in the direction of light propagation in the waveguide, the
adjustable periodic grating inducing Bragg reflection in the
waveguide.
[0087] 37. The variable delay apparatus of claim 36 further
comprising:
[0088] a circulator to transmit light to the waveguide segment and
to receive light reflected within the waveguide segment.
[0089] 38. The variable delay apparatus of claim 36 wherein the
adjustable periodic grating is moved by a screw.
[0090] 39. A method of transferring a light signal from a first
polarization maintaining fiber to a second polarization maintaining
fiber comprising
[0091] coupling the first polarization maintaining fiber to the
second polarization maintaining fiber at a joint to form a
combination polarization maintaining fiber;
[0092] position a periodic grating to induce mode coupling of one
frequency of light in the combination polarization maintaining
fiber;
[0093] adjusting pressure between the periodic grating and the
combination polarization maintaining fiber to couple power between
a fast mode and a slow mode of the combination polarization
maintaining fiber.
[0094] 40. A tunable polarizer comprising:
[0095] a polarization maintaining fiber including a core and a
cladding; and
[0096] a periodic grating coupled to the polarization maintaining
fiber to couple a first polarization mode of a first wavelength
into the cladding.
[0097] 41. The tunable apparatus of claim 40 wherein the periodic
grating presses against the polarization maintaining fiber to
induce a periodic change in a refractive index of the polarization
maintaining fiber to couple the first polarization mode of the
first wavelength into the cladding.
[0098] 42. A variable optical attenuator comprising
[0099] a first polarization maintaining fiber;
[0100] a periodic grating to re-orient a polarization of a signal
propagating in the first polarization maintaining fiber, and
[0101] a polarizer to attenuate a portion of the signal output by
the periodic grating.
[0102] 43. The variable optical attenuator of claim 42 wherein the
polarizer further comprising:
[0103] a second polarization maintaining fiber including a cladding
and a core to receive the output of the first polarization
maintaining fiber; and
[0104] a second periodic grating to couple a first polarization
mode of a selected wavelength propagating in the core into the
cladding of the second polarization maintaining fiber.
[0105] 44. An add/drop filter comprising:
[0106] a periodic grating to reorient a polarization of a first
wavelength of light propagating in a polarization maintaining
fiber; and
[0107] a polarization beamsplitter having a first input port
coupled to the polarization maintaining fiber to direct the first
wavelength of light output from the polarization maintaining fiber
in a first direction and a second wavelength of light output from
the polarization maintaining fiber in a second direction.
[0108] 45. The add/drop filter of claim 44 wherein the polarization
beamsplitter further comprises a second input port to couple a
third wavelength of light into the second direction.
[0109] 46. An optical filer comprising:
[0110] a bimodal fiber;
[0111] a periodic grating to switch a portion of a signal
propagating in the bimodal fiber from a first mode to a second
mode; and
[0112] a bimodal coupler to separate the portion of the signal
propagating in the second mode from the portion of the signal
remaining in the first mode.
[0113] 47. An add/drop filter comprising:
[0114] a multimode fiber supporting at least two transversal
modes;
[0115] a transversal mode converter to convert predetermined
wavelengths of a first set of signals in the multimode fiber from a
first mode to a second mode; and
[0116] a bimodal coupler having a first input port coupled to the
multimode fiber to direct the predetermined wavelengths of the
first set of signals in a first direction and to direct other
wavelengths of the first set of signals in a second direction. 48.
The add/drop filter of claim 47 wherein the transversal mode
converter further comprises:
[0117] a periodic grating coupled to a waveguide carrying the set
of signals to induce a periodic change in an index of refraction of
the waveguide to convert the predetermined wavelengths of the set
of signals from the first mode to the second mode.
[0118] 49. The add/drop filter of claim 47 wherein the bimodal
coupler further comprising a second input port to couple a second
set of signals into the second direction.
[0119] 50. An optical buffer comprising:
[0120] a waveguide loop;
[0121] a bimodal coupler to output signals propagating in a first
mode from the waveguide loop, the bimodal coupler maintaining
signals in a second mode in the waveguide loop; and
[0122] a switchable mode converter including a grating to switch
the signal from the second mode to the first mode.
[0123] 51. A tunable apparatus for controlling light transmitted in
a waveguide comprising:
[0124] a waveguide holder;
[0125] a periodic grating; and
[0126] a coupling device to couple the waveguide holder to the
periodic grating, the periodic grating adjustable to change a
spacing between the waveguide holder and the periodic grating.
[0127] 52. The tunable apparatus of claim 51 further
comprising:
[0128] a waveguide in the waveguide holder wherein the coupling
device adjusts the periodic grating to induce a periodic change in
an index of refraction of the waveguide.
[0129] 53. A tunable apparatus for controlling light transmitted in
a waveguide comprising:
[0130] means for adjusting a pressure of a periodic grating against
a waveguide.
[0131] 54. The tunable apparatus of claim 53 wherein the adjusting
of the pressure induces a periodic change in an index of refraction
of the waveguide.
[0132] 55. The tunable apparatus of claim 53 wherein the means for
adjusting pressure includes a piezoelectric to move the periodic
grating.
[0133] 56. The tunable apparatus of claim 53 wherein the means for
adjusting pressure includes a screw to move the periodic
grating.
[0134] 57. A tunable apparatus for controlling light transmitted in
a waveguide comprising:
[0135] means for adjusting a spacing between a periodic grating and
a center point in a waveguide, the adjusting of the spacing to
induce mode coupling of a signal propagating in the waveguide.
BRIEF DESCRIPTION OF THE DRAWINGS
[0136] The advantages of the present invention will become more
readily apparent to those ordinarily skilled in the art after
reviewing the following detailed description and accompanying
drawings wherein:
[0137] FIG. 1 illustrates a prior art system for creating a
periodic grating in a wave guide.
[0138] FIGS. 2A, 2B, illustrate two embodiments of the invention to
cause mode coupling in a waveguide.
[0139] FIGS. 2C, 2D, 2E, 2F and 2G illustrate alternative groove
patterns which may be used for the periodic grating.
[0140] FIGS. 3A and 3B are a graphs which plot light intensity as a
function of wavelength output by one embodiment of the present
invention.
[0141] FIGS. 4A and 4B illustrate cross-sectional views of a
tunable apparatus used in one embodiment of the present
invention.
[0142] FIGS. 5A and 5B illustrate use of a piezo-electric to move
the periodic grating in one embodiment of the present
invention.
[0143] FIGS. 6A and 6B illustrate an optical fiber for use as a
waveguide in one embodiment of the present invention.
[0144] FIG. 7A illustrates an apparatus to rotate a periodic
grating with respect to the direction of light propagation in one
embodiment of the invention.
[0145] FIGS. 8A, 8B and 8C illustrate alternative embodiments of
implementing an optical equalizer implemented using a plurality of
adjustable periodic gratings.
[0146] FIG. 9 is a graph illustrating an example of the output of
an optic equalizer.
[0147] FIG. 10 illustrates using a plurality of adjustable periodic
gratings to create a variable delay line.
[0148] FIGS. 11A and 11B illustrate a variable delay line crated
using one adjustable periodic grating.
[0149] FIGS. 12A, 12B, 12C, and FIG. 12D illustrate using a grating
induced polarization mode converter to connect two polarization
maintaining fibers.
[0150] FIG. 13A illustrates wavelength selective polarization mode
conversion.
[0151] FIG. 13B illustrates one embodiment of an add/drop filter
including a wavelength selective polarization mode converter and a
polarization beamsplitter.
[0152] FIG. 14A illustrates using a grating induced polarization
mode converter with a fiber polarizer to form a variable
attenuator.
[0153] FIG. 14B illustrates making a variable attenuator with a
grating induced fiber polarizer and a grating induced polarization
mode converter.
[0154] FIG. 14C illustrates making a modulator with a grating
induced polarization mode converter and a polarization
beamsplitter.
[0155] FIG. 15A and FIG. 15B illustrate a variable attenuator and a
modulator made from a grating induced transversal mode converter
and a bimodal coupler.
[0156] FIG. 16A and FIG. 16B illustrate the operation of a bimodal
coupler.
[0157] FIG. 17 shows an add/drop filter made from a grating induced
transversal mode converter and a bimodal coupler.
[0158] FIG. 18 illustrates an optical recirculating delay line
including a grating induced transversal mode converter and a
bimodal coupler.
DETAILED DESCRIPTION OF THE INVENTION
[0159] The following invention describes a method and apparatus of
using an external grating to cause mode coupling in a wave guide.
In the following invention, a number of terms will be used which
are herein defined. A wave guide holder as used in this application
is any device which holds a wave guide such that relative position,
distance, or pressure between the waveguide and a periodic grating
can be adjusted. In one embodiment, the waveguide holder holds the
waveguide in a fixed position while the position of the external
grating is adjusted. In an alternative embodiment, the waveguide
holder is adjusted to move the waveguide to different positions
with respect to the external grating. Examples of wave guide
holders includes, but is not limited to, a block, a substrate of a
semiconductor wave guide, the insulation surrounding a wave guide,
or other apparatus which can be used to grip or prevent unwanted
movement of the wave guide relative to a periodic grating.
Furthermore, although the term periodic will be used throughout
this application, the term "periodic grating" will be defined to
include chirped gratings in which the periodicity of the grating is
not constant across the surface of the grating. Finally, the term
"mode coupling" will be defined to include the coupling of light in
the fiber between different transversal modes (such as L.sub.01 and
L.sub.11 modes), between counter propagation modes such as the
forward and backward propagating modes (e.g. light propagating in
the forward and backward directions), between a core mode and a
cladding mode (e.g., light confined in the fiber core or leaked
into the cladding), and between polarization modes (e.g., in
birefringent fibers where the light signal polarized along the slow
axis is coupled to the fast axis). The term "adjustable grating"
will be defined to include movable gratings as well as gratings
which are fixed in position, but are coupled to a waveguide holder
which may be repositioned to move a waveguide with respect to the
fixed position of the grating.
[0160] In the following application, techniques for changing the
distance or pressure between grating and waveguide will be
described. The described techniques will include using screws,
piezo-electrics, and springs. However, other devices may be used,
such as magnets and electromagnetic actuators. Likewise, the
specification will describe implementing the invention with a fiber
optic cable although other wave guides may be used. It is
understood that the following detailed description will include
these specifics to illustrate the preferred embodiments and also to
enable one of ordinary skill in the art to implement the invention,
however, these specifics should not be interpreted to limit the
invention to only the embodiments described herein as other
embodiments which would be obvious to one of ordinary skill in the
art are also possible.
[0161] In one embodiment of the invention, an external periodic
grating is positioned to effect an internal refractive index (or
phase) grating that influences the light signal propagating inside
the waveguide. In the illustrated example FIG. 2A, the wave guide
is an optical fiber 204. However, it is to be understood that the
wave guide does not have to be an optical fiber and may include
semiconductor wave guides and other media for channeling light.
[0162] One method of using an external grating to create an
internal index grating is to press the external grating against the
waveguide, as shown in FIG. 2a. When the external grating 212 is
pressed against the fiber (waveguide) 204, an index grating grating
will be generated in the waveguide via the photoelastic effect.
Such an effect has been successfully used by the inventor to induce
birefringence in fibers and hence to control the polarization
states of light. (U.S. Pat. No. 5,561,726 by X. Steve Yao hereby
incorporated by reference).
[0163] In FIG. 2A, the optical fiber 204 is pressed between a
holder, which in one embodiment includes a flat block 208 and a
periodic grating 212. In the illustrated embodiment of FIG. 2A the
periodic grating 212 is formed in a grooved block 216. Movement of
the grooved block 216 against the optical fiber 204 which is held
in position by the holder or flat block 208 causes a periodic
refractive index change inside the fiber with a periodicity defined
by the external grating. This periodic index change in turn causes
mode coupling inside the optical fiber 204. Changing the pressure
of the periodic grating 212 against the optical fiber 204 which is
held in position by holder or flat block 208 causes certain wave
lengths of light propagating in the optical fiber 204 in one
propagating mode to couple to a different mode. The different mode
includes, but not limits to a transversal mode, a cladding mode, a
polarization mode, and a counter-propagation mode. The changing
pressure allows "tuning" (adjusting of the light propagation
characteristics) of the optical fiber 204. In particular, changing
the pressure alters the coupling strength or the amount of coupling
between the two modes.
[0164] In another embodiment of the invention, periodic grating 212
does not have to be in actual contact with optical fiber 204 to
cause mode coupling. It is sufficient that periodic grating 212 is
positioned within an evanescent field of the light propagating in
optical fiber 204. The distance the evanescent field extends from
the core of optical fiber 204 is typically fairly small, on the
order of sub microns and microns. Therefore, the cladding of the
fiber needs to be reduced or removed to allow positioning of
grating 212 close to the core and provide a sufficiently strong
influence on the light signal. Moving periodic grating 212 in and
out of the evanescent field causes light in optical fiber 204 to
couple from one mode to another, and hence change the spectrum,
polarization, or signal strength of the light. Index matching
fluids or gels may also be applied between the periodic grating 212
and optical fiber 204 to enhance the mode coupling. Evanescent
fields and the effective distance of the evanescent field from the
surface of a fiber are well understood in the art.
[0165] Depending on the position of periodic grating 212 relative
to waveguide 204, three effects may be achieved. At a first
position, the periodic grating is far enough away spatially from
the optical fiber 204 that it does not cause a perturbation of the
field of the signal propagating in the optical fiber. At a second
position the periodic grating is within the evanescent field
produced by the signal in the fiber. As a result, the light signal
inside the fiber is influenced by the grating 212 and couples
inside fiber from one mode to another. In a third position, the
periodic grating 212 contacts optical fiber 204. The pressure of
the periodic grating on optical fiber 204 also creates a periodic
change in the index of refraction, n, of the fiber via the
photoelastic effect. Thus the light signal inside the fiber is
influenced by both the periodic grating 212 itself and by the
pressure induced grating inside fiber, resulting in possibly
stronger mode coupling, including Bragg reflection of light
propagating in the optical fiber. Therefore, the amount of mode
coupling can also be controlled by the position of the periodic
grating 212.
[0166] As previously indicated, there are many types of mode
coupling that can occur in an optical fiber. The periodicity, or
the grating spacing of the fiber, and the optical wavelength of the
propagating light determine the type of the mode coupling. For
example, when .lamda. is the wavelength of light field in the fiber
(or waveguide), n.sub.c is the effective index of refraction of the
fiber core, n.sub.c1 is the effective index of the cladding,
.LAMBDA. is the grating spacing, then the condition for coupling
between a forward and backward propagating modes, or Bragg
reflection, is:
.LAMBDA.=.lamda./2n.sub.c (1)
[0167] For coupling light from a fiber core to a fiber cladding,
the grating spacing is determined by:
.LAMBDA.=/(n.sub.c-n.sub.c1) (2)
[0168] For mode coupling in a birefringent fiber between the fast
mode (the polarization of light is along the fast axis of the
fiber) and the slow mode (the polarization of light is along the
slow axis of the fiber), the grating spacing is:
.LAMBDA.=.lamda./(n.sub.s-n.sub.f) (3)
where n.sub.s is the effective refractive index of the slow axis
and n.sub.f is the index of the fast axis.
[0169] For mode coupling in a multimode fiber between two
transversal modes, the grating spacing is:
.LAMBDA.=.lamda./(n.sub.1-n.sub.2) (4)
where n.sub.1 is the effective refractive index of mode 1 and
n.sub.2 is the index of mode 2.
[0170] Conversely, when a grating period is given, the wavelength
of the light signal that may influenced by the grating can be
calculated using Eq. (1) to Eq. (4).
[0171] These principals of Bragg reflection and mode coupling are
well understood in the art and are described in Yariv's Optical
Waves & Crystal, pages 405 to 503.
[0172] Because the amount of mode coupling can be controlled by
controlling the pressure of the grating on the fiber or the
position of the grating, tunable devices for controlling the light
signal inside the fiber can be realized. FIG. 3 illustrates the
result of a tunable wavelength selective variable attenuator based
on the signal coupling between the core and cladding modes. Such a
device is important in wavelength division multiple (WDM) access
systems where the strength of the different wavelength channels
need to be precisely controlled. In FIG. 3, the transmission
characteristic of light through the waveguide at predetermined
wavelengths is plotted. In the experiment, a 2 cm long external
grating was pressed onto a standard communication fiber with a
removed buffer. The grating was designed to coupled the light
signal of around 1310 nm inside the core of the fiber into the
fiber cladding. Once the signal coupled into the cladding, it will
be strongly attenuated because the cladding has extremely high loss
compared with the core. In one example, the index difference
between the core and the cladding is 0.25.times.10.sup.-2, a
suitable grating spacing may be 524 um as determined using Eq. (2).
Each curve 312, 316, 320 represents the output of the waveguide at
a particular pressure of the periodic grating against the
waveguide. At high pressure, curve 312 of FIG. 3 shows that a
significant portion of the light is coupled out and attenuated. At
intermediate pressure, substantially less light at a given
frequency is attenuated, as illustrated by curve 316. At low
pressure, curve 320 shows that very little light is attenuated.
Thus by altering the pressure of periodic grating 212 against
optical fiber 204, the amount of light reflected can be adjusted or
"tuned." Therefore, a narrow bandwidth variable attenuator is
created.
[0173] FIG. 2B illustrates an alternate embodiment of the invention
which allows altering the periodicity of the grating. In FIG. 2B,
the holder 224 which holds the optical fiber 220 such that the
fiber does not move away from grooved block 228 is also grooved
with a second set of periodic grating 232. Thus, both holder 224
and grooved block 228 contain corresponding periodic gratings 232
and 236. By moving the first periodic grating 232 with respect to a
second periodic grating 236 in the direction shown by arrow 240,
the effective periodicity of the periodic grating combination can
be adjusted. When an offset distance 244 is zero, the first grating
232 and second grating 236 coincide or are aligned, the effective
periodicity of the two gratings is equal to the periodicity of the
first grating 232. However, when offset distance 244 is a maximum,
where maximum is defined to be when offset distance 244 is equal to
one-half of the periodicity of a grating, the effective periodicity
seen by the optical wave propagating fiber 220 is twice the
periodicity of first periodic grating 232.
[0174] FIG. 2C, FIG. 2D, FIG. 2E and FIG. 2F illustrate different
patterns which may be used for periodic grating 212, 236, to 236.
In FIG. 2C a rectangular periodic grating is illustrated for use in
the grooved block 216. FIG. 2D illustrates the use of a trapezoidal
254 periodic grating in a grooved block 258 as illustrates in FIG.
2D. FIG. 2E illustrates a sinusoidal periodic grating 262 in a
grooved block 266. FIG. 2F illustrates a triangular periodic
grating 270 in a grooved block 274. The grooved blocks illustrated
in FIG. 2C, FIG. 2D, FIG. 2E, and FIG. 2F illustrate examples of
groove patterns which may be used in the devices shown in FIG. 2A
and FIG. 2B. The periodic gratings illustrated in FIG. 2C through
2F are for example only, other periodic structures may be used as
is understood by those of ordinary skill in the art.
[0175] FIG. 2G illustrates a chirped periodic grating 278 in a
grooved block 282. Although chirped grating 278 does not have
uniformly spaced groove peaks, for purposes of this application,
chirped grating 278 is defined to be one type of periodic gratings.
Chirped grating 278 includes a series of grooves 286, 290, 294, the
spacing of the grooves with respect to adjacent grooves can be
defined by a mathematical function of a position along the grating
block 282. In one embodiment of the chirped grating, the spacing of
grooves 286, 290 and 294 increases linearly across chirped grating
278. In an alternate embodiment, spacing of grooves 286, 290, 294
increases in a quadratic function across chirped grating 278. By
altering the grating spacing across chirped grating block 282, the
spectrum of the the light signal which undergoes mode coupling
within the optical fiber 204 can be increased, thereby increasing
the bandwidth of light affected by the grating. In the case of
Bragg reflection, a chirped grating can also be used to compensate
dispersion of the light signal (F. Ouellette, "All fiber filter for
efficient dispersion compensation," Optics Letters, Vol. 16, No. 5,
pp. 303-305) and hence increase the fibers transmission rate.
[0176] FIG. 4A and FIG. 4B show cross-sectional views of an
apparatus 400 used to press a periodic grating 404 against an
optical fiber 408. In FIG. 4A, grooves cut into a grooved block 412
form the periodic grating 404. A holder 416 which in the embodiment
shown in FIG. 4A includes a flat surface 420 that supports an
opposite side of the optical fiber 408. The holder 416 and the
grooved block 412 containing the period grating 404 interact to
clamp the fiber between the grooved block 412 and flat surface
420.
[0177] A spring 424 presses the grooved block 412 to keep a
constant pressure on the optical fiber 408. Spring 424 typically
has a spring constant K such that the force applied to the grooved
blocks is equal to F=KX where X is the distance by which the spring
is compressed.
[0178] The pressure applied by the spring is adjusted by changing
the compression of spring 424. In one embodiment, the pressure on
spring 424 is controlled by a screw 428. Threads 432 on the screw
interlock with threads 436 in the holder 420 such that rotation of
screw 428 moves the screw in and out of holder 420. Rotation of
screw 428 such that additional pressure is applied to spring 424
causes grooved block 412 to press harder against optical fiber 408
resulting in a greater change in the index of refraction of fiber
408 and more intense mode coupling of the predetermined wavelength
of light. In one embodiment of the invention, when screw 424 is
rotated outward, a tip 440 of the screw attaches to a portion of
the grooved block 412 lifting the grooved block away from the
optical fiber 408 to prevent mode coupling of light transmitted in
optical fiber 408.
[0179] Other methods for moving the periodic grating towards and
away from the optical fiber 408 may be implemented. For example,
FIG. 5A and FIG. 5B illustrate using a piezo electric acurator to
move the grooved block 412 towards and away from optical fiber 408.
FIG. 5A illustrates a piezo electric stack 508. A power source such
as voltage source 504 is connected to a stack 508 of piezo electric
elements 512, 516, 520. Altering the voltage applied across the
stack 508 changes the displacement of stack 508. When the piezo
electric stack 508 is substituted for spring 424 of FIG. 4A, the
grooved block 412 can be moved towards or away from the optical
fiber 408 by adjusting voltage source 504.
[0180] Two methods of moving the grooved block have been
illustrated in FIG. 4B and FIG. 5B. The first method uses a
mechanical spring and screw arrangement structure while a second
method uses a piezo electric device. Other methods of moving a
grooved block are available to one of ordinary skill in the art.
These methods may include but are not limited to lever
arrangements, and other mechanical, electro-mechanical, magnetic,
or electromagnetic devices suitable for moving an object over small
distances. In the remaining description, various embodiments of
this invention will be described using primarily a spring and
screw, although it is understood that piezo electric stacks may be
substituted for the screw spring arrangements as well as other
mechanical and electro-mechanical devices.
[0181] In one embodiment of the invention, the waveguide used is an
optical fiber which includes an optical fiber core surrounded by a
cladding. When a clad fiber is used, a portion of the cladding may
be reduced as illustrated in FIGS. 6A and 6B to improve the
effectiveness of the periodic grating. FIG. 6A illustrates a side
polished fiber where a portion of the cladding 604 has been
polished away to create a flat surface 608. A periodic grating
positioned against the side polished flat surface 608 is in close
proximity to the fiber core 612 such that a light signal
propagating down the fiber core 612 is strongly influenced by the
grating and undergoes mode coupling.
[0182] FIG. 6B illustrates one method of side polishing a clad
optical fiber. In FIG. 6B, fiber 408 is placed in a substrate 616
which holds the fiber steady. The fiber 408 is then polished to
create a flat surface 608 approximately level with a top surface
620 of the substrate 616. In one embodiment of the invention, the
substrate 616 can subsequently be used as the fiber holder to hold
the fiber steady while the periodic grating is applied to flat
surface 608 of fiber 408.
[0183] The preceding description describes a basic tunable
apparatus in which the periodic grating is moved in a direction
perpendicular to the direction of light propagating down a
waveguide such as an optic cable. By adjusting the orientation or
periodicity of a grating in the tunable apparatus or by
repositioning the tunable apparatus, various devices can be
made.
[0184] In one embodiment of the invention, the periodic grating is
rotatable. Rotating the periodicity of the grating changes the
effective periodicity of the gratings as illustrated in FIG. 7A. In
FIG. 7A periodic grating 704 is rotated with respect to the
direction of light propagation in waveguide 708 by an angle
.theta.. Screw mechanism 712 is used to apply pressure to one end
of the periodic grating which rotates periodic grating 716. It is
recognized that a piezo electric or other device can be substituted
for screw 712. The effective grating spacing of the rotated grating
is equal to the spacing of the grating divided by the cosine of the
angle .theta.:
.LAMBDA.'=.LAMBDA./cos .theta. (5)
Changes in an effective grating also changes the wavelength of the
light signal that undergoes mode coupling according to equations
(1) to (4). When the grating spacing is selected such that light
inside a fiber core either undergoes the Bragg reflection as
defined in Eq. (1) or is coupled out into fiber cladding as defined
by equation (2) and illustrated in FIG. 3, a wavelength tunable
variable attenuator (WTVA) is created. By adjusting both the angle
.theta. and the position (or pressure) of the grating against the
fiber, one is able to selectively attenuate a signal of any
wavelength by a variable amount. Such a wavelength tunable variable
attenuator is extremely useful for wavelength division multiple
(WDM) access systems to equalize optical powers in different
channels.
[0185] Several such wavelength tunable variable attenuators
cascaded together can operate as an optical spectrum equalizer.
FIGS. 8A, 8B and 8C illustrate various embodiments of such an
optical equalizer using a plurality of tunable gratings. In FIG.
8A, a fiber holder 808 or casing holds a waveguide 804 steady. A
plurality of moving devices such as screws 812, 816, 820, 824 moves
a corresponding grooved block. A cross-section of one embodiment of
an optical equalizer is illustrated in FIG. 8B. Each screw and
spring arrangement corresponds to a corresponding grooved block.
For example, screw 812 corresponds to grooved block 828, screw 816
corresponds to grooved block 832, screw 820 corresponds to grooved
block 836, and screw 824 corresponds to grooved block 840. Each
grooved block has a grating with a different periodicity. A user
selects what frequencies or wavelengths of light to filter and then
selects a grooved block with a periodicity which will induce mode
coupling (including core/cladding coupling and Bragg reflection) at
the selected wavelength. The user then adjusts the selected grooved
block to press against waveguide 804 and induce mode coupling at
the desired wavelengths. By selecting and positioning periodic
gratings with predetermined periods against fiber 804, a user
selects which wavelengths of light to couple out and thus
filter.
[0186] FIG. 8C illustrates the optic equalizer illustrated in FIG.
8B where the screws 812, 816, 820, 824 and corresponding springs
have been replaced with a plurality of voltage sources 844, 848,
852, 856 and corresponding piezo electric stacks 860, 864, 868, 872
of piezo electric elements. By adjusting the voltage of the voltage
sources and thereby changing the dimensions of the corresponding
piezo electric stack, each periodic grating with its corresponding
periodicity can be moved away from or towards fiber 804 to induce
mode coupling of light at the desired frequencies.
[0187] FIG. 9 is a graph illustrating an example output of an
optical equalizer for flattening the output spectrum of a WDM
system. FIG. 9 plots the intensity of light on a Y-axis 904 with
respect to the wavelength of light which is plotted on X-axis 908.
In the example, the "output" of the optical equalizer is defined to
be the light which undergoes Bragg reflection or other types of
mode coupling. In the example, three peaks 912, 916, 920 are the
spectral "bumps" of the original signal input to the equalizer. At
the output, each peak is removed by a corresponding grating with a
properly adjusted angle .theta. and pressure (or position) via a
proper type of mode coupling. The removal of the highest peak 912
requires more mode coupling, corresponding to higher pressure of a
grating against a fiber in core/cladding coupling (or closer
positioning of a grating against a fiber in the case of Bragg
reflection). Removal of lower peaks requires a weaker mode coupling
and therefore less pressure.
[0188] FIG. 10 illustrates using a plurality of tunable
apparatuses, each apparatus including adjustable periodic gratings
to create a variable delay line via grating induced Bragg
reflection. In the variable delay line of FIG. 10, an input 1008 of
a circulator 1004 receives an incoming signal. Light entering input
1008 exits the circulator 1004 at an input-output port 1012 into a
delay unit 1016. Delay unit 1016 includes a plurality of periodic
gratings 1018, 1020, 1022, 1024. In one embodiment of the
invention, each periodic grating 1018, 1020, 1022, 1024 has the
same periodicity. A switch such as switches 1026, 1028, 1030, 1034
couples a voltage source 1140 to the corresponding periodic grating
1018, 1020, 1022, 1024. Closing a switch, such as switch 1026,
moves a corresponding periodic grating 1018 towards a section of
the waveguide in delay unit 1016.
[0189] By selecting one grating in the plurality of gratings 1018,
1020, 1022, 1024 to move against the section of waveguide 1044, a
variable delay is created. Moving a periodic grating positioned far
away from circulator 1012 against the section of waveguide 1044
results in a long delay because light must travel from the
circulator to the grating and then return to the circulator 1012
before being output. Moving a periodic grating such as grating 1018
positioned close to the circulator 1012, against the section of
waveguide results in shorter delays because the light has to travel
only a short distance before being reflected back to the
circulator. The delayed signal re-enters circulator 1004 through
input-output port 1012 and exits the circulator from output port
1052. In typical use of the invention, only one switch in the
plurality of switches is closed creating one reflected signal with
a predetermined delayed time.
[0190] When the range of delays is not large, a simple delay
circuit as illustrated in FIGS. 11A and 11B may be used. In the
illustrated embodiment, a circulator such as circulator 1104 may be
used. An input port 1108 of circulator 1104 receives an input
signal. A delay unit 1116 processes the signal between the time the
signal is input and output by the circulator 1130. Delay unit 1116
includes a periodic grating which is adjustable in a lateral
direction 1120. A screw 1124 moves the periodic grating 1122 in
lateral direction 1120 to create variable delays in the delay line
segment 1112. Moving the periodic grating 1122 in a lateral
direction 1120 closer to the circulator results in shorter delays
while moving the grating 1122 in a lateral direction 1120 away from
circulator 1104 results in longer delays. The delayed signal along
delay unit 1112 returns to circulator 1104 through input-output
port 1108 and is output through output port 1130.
[0191] FIG. 12A illustrates using an adapter 1204 to connect two
polarization maintaining (PM) fibers 1208 and 1218. The apparatus
of FIG. 12A uses the relationship defined by equation (3) to rotate
the polarization in polarization maintaining (PM) fibers. As
described in U.S. Pat. No. 5,561,726 entitled "Apparatus and method
for connecting polarization sensitive devices", traditional methods
of interconnecting two PM fibers involves precision alignment of
the fiber axes. However, using the device shown in FIG. 12A, one
can simplify the cumbersome fiber axis alignment procedure. In the
example illustrated in FIG. 12B, a polarization state 1248 of light
propagating in PM fiber 1208 at the connector ferrule 1210 is
aligned with a slow axis 1240 of PM fiber 1208. However, as
illustrated in FIG. 12C, slow axis 1244 of receiving fiber 1218 is
not aligned with slow axis 1240 at an input to a second connector
ferrule 1212.
[0192] For light in receiving fiber 1218 to polarize along the slow
(or fast) axis, polarization mode converter 1216 presses an
external grating with a grating spacing 1238 defined by Eq. (3)
against fiber 1218 to cause coupling between the two polarization
modes. Rotating screw 1288 adjusts the pressure of the external
grating against fiber 1218 until a substantial portion of the power
polarized along the fast axis is coupled into the slow axis, or
vise versa. Consequently, polarization mode converter 1216 aligns
polarization state 1250 with slow axis 1244 of receiving PM fiber
1218. In alternate embodiments of the invention, a mechanical
splice or a fusion splice may be substituted for fiber connectors
1268 and 1278.
[0193] Because polarization mode coupling is wavelength dependent,
signals of a selected wavelength may be coupled into one
polarization while signals of a second wavelength remain in an
original polarization state. FIG. 13A illustrates using
polarization mode coupling to fabricate a wavelength division
multiplexer (WDM). In FIG. 13A, a first signal with wavelength
.lamda..sub.1 and a second signal with wavelength .lamda..sub.2
propagate in PM fiber 1388. Both signals have polarization states
1300 and 1302 oriented along the slow axis of PM fiber 1388. When a
grating 1328 with a spacing 1338 defined by
.LAMBDA..sub.1=.lamda..sub.1/(n.sub.s-n.sub.f) (6)
is applied against PM fiber 1388 with sufficient pressure, the
first signal with wavelength .lamda..sub.1 is coupled into a fast
axis while the second signal with wavelength .lamda..sub.2 remains
oriented along the slow axis. By combining the wavelength selective
polarization mode converter 1216 with a polarization beamsplitter
(PBS) 1310, a wavelength division demultiplexer is created to
separate the signals of two wavelengths.
[0194] FIG. 13B illustrates using polarization mode coupling to
fabricate an add/drop filter. In FIG. 13B, the signal with
wavelength .lamda..sub.1 is output from port 1318 of PBS 1310,
while the signal with a wavelength .lamda..sub.2 continues through
port 1316 of PBS 1310. In addition, a third signal with a
wavelength .lamda.'.sub.1 entering port 1320 of PBS 1310 is added
to .lamda..sub.2 one aspect of the devices illustrated in FIG. 13A
and FIG. 13B is that the WDM and add/drop filter is switchable, a
feature that is extremely useful in WDM networks.
[0195] Gratings with selected grating spacing corresponding to a
set of selected wavelengths may be used to reorient the
polarization of the selected wavelengths from a first polarization
mode to a second polarization mode. Combining a set of polarization
mode converters with a polarization beamsplitter allows separation
of the selected wavelength channels from unselected channels. Such
wavelength selective polarization mode conversion can be used to
double the channel spacing of a WDM system. Wavelength selective
polarization mode conversion may also be used in a fiber gyro
system.
[0196] Core/cladding mode coupling in a PM fiber can also be used
to fabricate a polarizer. Because the indices of refraction are
different for a mode polarized along the slow axis and a second
mode polarized along the fast axis, core/cladding coupling may
occur for one polarization mode and not a second mode. For example,
when a grating spacing is chosen such that
.LAMBDA.=.lamda./(n.sub.cs-n.sub.c1), (7)
where n.sub.cs is the effective index of the guided mode polarized
along the slow axis, n.sub.c1 is the effective index of the
cladding, and .lamda. is the wavelength of the propagating light,
the mode polarized along the slow axis will be coupled into the
fiber cladding and be attenuated. The signal polarized along the
fast axis will remain in the core and unaffected. Therefore, a
polarizer is created without the light exiting the fiber. By
adjusting pressure of the grating against the fiber, the extinction
ratio of the polarizer can be controlled to produce a polarization
dependent variable attenuator.
[0197] A fiber switch or a variable attenuator may be formed by
combining a grating induced polarization mode converter with a
fiber polarizer. FIG. 14A illustrates a polarization mode converter
1416 connected to a fiber polarizer 1458. Adjusting the pressure of
a grating against a section of fiber rotates the polarization of a
signal propagating in the fiber. In the illustrated embodiment,
polarizer 1458 allows only one polarization to pass, thereby
reducing the power of the signal propagating in the fiber. A fiber
optic modulator/switch can be realized by replacing adjustment
screw 1488 with a piezo-electric actuator 1498 controlled by an
electrical source 1486, as illustrated in FIG. 14C. FIG. 14B
illustrates one embodiment of the invention in which polarizer 1458
of FIG. 14A has been replaced with an all fiber polarizer 1450. All
fiber polarizer 1450 includes a grating with a spacing 1468 defined
by Eq. (7) coupled to a section of fiber 1488. In FIG. 14C, a
polarization beamsplitter 1490 replaces polarizers 1458 and 1450 in
FIGS. 14A and 14B to make a variable polarization separator or a
fiber optic modulator/switch with two complimentary output
ports.
[0198] A transversal mode converter conforming to the relationship
described in Eq. (4) is especially useful in an optical fiber which
supports two transversal modes (bimodal fiber). In practice, any
single mode fiber can be used as a bimodal fiber when the
wavelength of a light signal in the fiber is below the cutoff
wavelength of the fiber. In the following description, several
devices which can be made using the wavelength tunability and
coupling strength tunability of the invention described in FIGS. 4,
5 and 7 will be described.
[0199] FIG. 15A and FIG. 15B illustrate a fiber optic modulator, a
switch, and a variable attenuator formed by coupling the output of
the transversal mode converter 1580 with a bimodal coupler
1520.
[0200] FIG. 16A illustrates a bimodal coupler which separates two
modes 1608 and 1610 in a bimodal fiber 1602 into different fibers
1604 and 1606. The bimodal coupler can also be used to transfer two
signals 1612 and 1614 from two different fibers 1616 and 1618 into
two different transversal modes in a bimodal fiber 1620, as shown
in FIG. 16B. A biconic fused coupler technique or side polished
coupler technique commonly used in manufacturing fiber optic
couplers and WDMs as understood by those of skill in the art can be
used to make a bimodal fiber coupler. Positioning two bimodal
fibers or one bimodal and one single mode fiber together in close
proximity, as shown in FIG. 16A, induces mode coupling between the
two fibers. Different propagation constants of the two modes
results in different coupling strengths of each mode. For a
properly selected coupling strength (determined by the distance
between the two fibers, the propagation constant of the mode, and
coupling length), one mode will be completely (or near completely)
coupled into the other fiber and the remaining mode will remain in
the original fiber.
[0201] FIG. 15A illustrates adjusting a pressure on grating 1528
with a groove spacing defined by Eq. (4) against bimodal fiber 1588
to control mode coupling. Changing the amount of mode coupling
changes the output from port 1522 and/or port 1524 of bimodal
coupler 1520, resulting in variable attenuation of the output
signal. FIG. 15B illustrates using an electrical actuator 1598 to
control the mode converter 1580 such that the variable attenuator
operates as a switch or a modulator.
[0202] Because the transverse mode converter is highly wavelength
selective, the devices illustrated in FIGS. 15A and 15B can also be
used as a wavelength division multiplexer/demultiplexer or add/drop
filter. As shown in FIG. 17, multiple signals of different
wavelengths propagating in a first mode of bimodal fiber 1788 exit
bimodal coupler 1720 from a first port 1724. Activating transversal
mode converter 1780 for wavelength .lamda..sub.i converts
wavelength .lamda..sub.i signals in a first mode to a second mode.
Signals in the second mode exit coupler 1720 from a second output
port 1722. In one embodiment of the invention, signals in second
output port 1722 are removed (or dropped) from the system. To add a
signal with a wavelength .lamda.'.sub.i, the added signal is input
into second input port 1712 of bimodal coupler 1720. A second mode
converter may be used to ensure that the added signal is in the
second mode. The bimodal coupler 1720 combines the added signal
with other propagating signals in port 1724 which may be connected
to a system bus (not shown).
[0203] FIG. 18 illustrates using the mode converter 1880 and
bimodal coupler 1820 combination as a recirculating optical delay
line. An input optical pulse 1808 in a first mode, "mode 1", is
coupled into a bimodal fiber loop 1888 via bimodal coupler 1820.
The pulse remains in mode 1 and exits loop 1888 from port 1822 of
coupler 1820 after propagating around the loop once when mode
converter 1880 is in an off state. However, when mode converter
1880 is activated to convert the pulse from mode 1 into a second
mode, mode 2, the pulse does not exit the loop and instead
propagates around the loop until the mode converter is activated
again to convert the pulse back to the first mode whereupon the
optical pulse exits the loop from port 1822 of coupler 1820. By
controlling the mode converter as illustrated, an optical pulse may
be delayed for a controlled period of time. Such recirculating
optical delay lines are useful as a memory buffer in optical
networks and in optical computers.
[0204] A similar recirculating optical delay line can also be made
by replacing transversal mode converter 1880 of FIG. 18 with a
polarization mode converter and replacing the bimodal coupler with
a polarization beamsplitter.
[0205] While the Applicant has described various embodiments of the
tunable apparatus which involves moving a periodic grating towards
and away from a waveguide to induce mode coupling at certain
predetermined wavelengths and various devices which can be built
from such a tunable apparatus including a variable delay line, an
optical equalizer, a wavelength division multiplexer, an add/drop
filter, a polarization converter, and a wavelength selective
variable attenuator, other embodiments and uses may be apparent to
one of ordinary skill in the art. Thus, the invention should not be
limited to merely the embodiments described in the preceding
specification. The limitations of the application are specifically
claimed in the Claims which follow.
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