U.S. patent application number 09/733357 was filed with the patent office on 2001-06-21 for optical routing/switching based on control of waveguide-ring resonator coupling6/023.
This patent application is currently assigned to CALIFORNIA INSTITUTE OF TECHNOLOGY. Invention is credited to Yariv, Amnon.
Application Number | 20010004411 09/733357 |
Document ID | / |
Family ID | 23805793 |
Filed Date | 2001-06-21 |
United States Patent
Application |
20010004411 |
Kind Code |
A1 |
Yariv, Amnon |
June 21, 2001 |
Optical routing/switching based on control of waveguide-ring
resonator coupling6/023
Abstract
An optical wave power control device and method enables signal
control, such as modulation transferring and switching, to be
effected with the application of very low power to a controller
which is in optical communication with a recirculating mode
resonator and an optical propagation element. The propagation
element is configured such that is in power communication with a
high Q volumetric resonator. Power of a chosen resonant wavelength
is coupled into said resonator, where it circulates with very low
loss and returns energy to the propagation element. By introducing
a control signal into the controller, the propagated power can be
varied between substantially full and substantially zero
amplitudes. Loss factors can be maintained such that said resonator
is overcoupled, i.e. parasitic losses are less than coupling
losses, and a critical coupling condition exists in which a small
swing in the controller causes a disproportionate change in the
optical output signal. The controller is preferably effectuated by
an interferometer in the optical path of said resonator and a
control signal, which can be an applied voltage, current or optical
signal.
Inventors: |
Yariv, Amnon; (Pasadena,
CA) |
Correspondence
Address: |
RIORDAN & MCKINZIE
300 SOUTH GRAND AVENUE
29TH FLOOR
LOS ANGELES
CA
90071
US
|
Assignee: |
CALIFORNIA INSTITUTE OF
TECHNOLOGY
|
Family ID: |
23805793 |
Appl. No.: |
09/733357 |
Filed: |
December 7, 2000 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
09733357 |
Dec 7, 2000 |
|
|
|
09454719 |
Dec 7, 1999 |
|
|
|
09733357 |
Dec 7, 2000 |
|
|
|
PCT/US99/28891 |
Dec 7, 1999 |
|
|
|
60170074 |
Dec 9, 1999 |
|
|
|
Current U.S.
Class: |
385/28 ; 385/15;
385/2; 385/40 |
Current CPC
Class: |
G02F 1/3132 20130101;
G02B 6/29391 20130101; B82Y 20/00 20130101; G02B 6/29341 20130101;
G02B 6/29352 20130101; G02F 1/011 20130101; H04J 14/0221 20130101;
G02F 1/01708 20130101; G02B 6/29395 20130101; G02B 2006/12142
20130101; G02B 2006/12145 20130101; G02B 6/12007 20130101; G02F
1/0118 20130101; G02B 6/29343 20130101; G02F 2203/48 20130101; G02F
1/225 20130101 |
Class at
Publication: |
385/28 ; 385/15;
385/40; 385/2 |
International
Class: |
G02B 006/26 |
Claims
What is claimed is:
1. An optical wave power control device for varying the transmitted
power at at least one optical frequency (i.e., optical carrier
wave) on at least one optical wave power transmission member,
comprising: an optical wave transmission member configured for
propagating optical power at at least one optical frequency; at
least one circulating mode resonator, said resonator disposed to
couple wave power from and/or to said transmission member; and, at
least one controller, each said controller in operative
relationship with a different one of said resonators so as to vary
the effect of said resonator on the optical power in said wave
transmission member in response to a control signal.
2. An optical power control device as set forth in claim 1 above,
wherein said controller includes a interferometer in the optical
path of said resonator, and said control signal is an external
signal.
3. An optical wave power control device as set forth in claim 2
above wherein said resonator and said transmission member introduce
losses such that said resonator is overcoupled and the loss induced
by said controller attains critical coupling.
4. An optical wave power control device as set forth in claim 1
above, wherein actuation of said interferometer by said control
signal maintains the loss per round trip of recirculating modes at
the resonant frequency in a critical coupling regime.
5. An optical power control device as set forth in claim 1 above,
wherein said resonator includes an enclosed cavity having an
equatorial periphery with a diameter of less than approximately
1000 microns, said cavity being optically coupled to said
transmission member and which circulates resonant modes
equatorially.
6. An optical power control device as set forth in claim 5 above,
wherein said resonator has a Q that is selected in accordance with
the desired wavelength and bandwidth of said transmission that is
being modified.
7. An optical power control device as set forth in claim 5 above
where the frequency separation of the modes of said resonator is
selected in accordance with the spectral extent spanned by the
frequencies propagating in said transmission member.
8. An optical power control device as set forth in claim 7 above
wherein the mode frequency separation of said resonator is greater
than 200 GHz.
9. An optical power control device as set forth in claim 6 above,
where the circumferential periphery diametral dimension of said
resonator is less than about 100 microns and has a Q of the order
of 20,000 in the 1550 nm telecommunications band.
10. An optical power control device as set forth in claim 1 above,
wherein said transmission member is a planar waveguide and said
resonator has the geometric shape of a ring.
11. An optical power control device as set forth in claim 1 above,
wherein the device operates as a modulator responsive to a given
optical frequency.
12. An optical power control device as set forth in claim 1 above,
wherein the device operates as a switch responsive to a given
optical frequency.
13. A control device as set forth in claim 1 above, wherein at
least one of said controllers includes an interferometer coupled to
the optical path of said resonator and responsive to a control
signal for variably modifying said resonator losses.
14. A control device as set forth in claim 13 above, wherein said
control signal is an externally applied differential voltage.
15. A control device as set forth in claim 13 above, wherein said
control signal is an externally applied differential current.
16. A control device as set forth in claim 13 above, wherein said
control signal is an externally applied optical signal.
17. A control device as set forth in claim 1 above, wherein said at
least one said controller includes an interferometer coupled to the
optical path of said resonator and responsive to a control signal
for variably modifying the coupling between said wave power
transmission member and said resonator.
18. A control device as set forth in claim 17 above, wherein said
control signal is an externally applied differential voltage.
19. A control device as set forth in claim 17 above, wherein said
control signal is an externally applied differential current.
20. A control device as set forth in claim 17 above, wherein said
control signal is an externally applied optical signal.
21. A control device as set forth in claim 1 above, wherein at
least one of said controllers includes one or more interferometers
in optical communication with at least one of said resonators, and
a controllable electrical field applied to the interferometers to
modify the power communicated from said resonator to said wave
transmission member.
22. A control device as set forth in claim 1 above, wherein at
least one of said controllers comprises one or more interferometers
in optical communication with at least one of said resonators, and
means for applying a controllable optical signal to the
interferometers to modify the power communicated from said
resonator to said wave transmission member.
23. A control device as set forth in claim 1 above, wherein the
frequency separation between the modes of said resonator is
selected in accordance with the spectral extent spanned by the
frequencies propagating in said transmission member.
24. A control device as set forth in claim 1 above, wherein the Q
of said resonator is established at a level determined by the
wavelength and data rate/signal bandwidth of said control
signal.
25. A control device as set forth in claim 1 above, wherein said
resonator has a Q of the order of 20,000 in the 1550 nm
telecommunications band, said resonator has a diameter of less than
100 microns, and said control signal has a data rate of the order
of 10 Gigabits per second.
26. A control device as set forth in claim 1 above, wherein said
optical wave transmission member propagates a number of different
frequencies and wherein the device includes a plurality of
resonators, each resonant at a different one of the propagated
frequencies and in coupling relation to said wave transmission
member, and a plurality of controllers, each disposed in relation
to a different one of said resonators and controlling said
resonator round trip loss thereat so as to vary power transmission
at a selected frequency.
27. An optical power control device in accordance with claim 1
above, wherein said controller introduces loss variations between
substantially full and substantially zero transmission such that at
least one optical frequency is switched on or off.
28. An optical power control device in accordance with claim 1
above, wherein said wave transmission member propagates a number of
different frequencies in a wavelength division multiplexed mode,
wherein the device includes multiple resonators, each resonant at a
different frequency, and wherein said controller selectively
switches (i.e., blocks or admits) frequencies out of the
multiplexed signals by varying transmission at each resonator.
29. An optical power control device in accordance with claim 1
above, wherein a plurality of resonators and associated controllers
are disposed in-line with said wave transmission member, each
resonator and associated controllers comprising a modulator
operating at a different optical frequency in a set of optical
frequencies, and further including a plurality of laser sources
that are in-line in said wave transmission member and transmitting
different frequencies of the set in a downstream direction on said
transmission member, and in which the modulator for each given
frequency is downstream of the laser source for that frequency.
30. An optical power control device in accordance with claim 29
wherein the device further includes optical pump means for the
lasers coupled into said transmission member.
31. An optical wave power control device as set forth in claim 1
above, and further comprising a second wave transmission member in
coupling relation to said resonator.
32. An optical wave power control device as set forth in claim 31
above, wherein said two transmission members couple to
substantially the same resonator modes, and where said resonator
round trip loss associated with said resonator to transmission
member couplings establishes near critical coupling between said
resonator modes and each said transmission member.
33. An optical power control device as set forth in claim 31 above,
including an interferometer in the optical path of said resonator
and responsive to an external control signal to vary the loss of
said resonator.
34. An optical power control device as set forth in claim 33 above,
wherein the external control signal includes applying an electric
current, voltage and/or an optical signal to said
interferometer.
35. An optical power control device as set forth in claim 1 above,
wherein the device includes a second transmission member optically
coupled to said resonator, and said resonator coupling with said
second member is varied by said controller.
36. An optical power control device as set forth in claim 35
wherein the second member is a waveguide and the means for varying
said power coupling includes an interferometer in the optical path
of said resonator.
37. An optical power control device as set forth claim 36 wherein
said interferometer is controlled by application of a variable
voltage.
38. An optical power control device as set forth claim 36 wherein
said interferometer is controlled by application of a variable
current.
39. An optical power control device as set forth claim 36 wherein
said interferometer is controlled by application of an optical
signal to said interferometer.
40. An optical power control device as set forth in claim 36 above,
wherein at least one said controller varies a property associated
with resonator round-trip loss associated with said resonator to
member coupling while other sources of resonator round-trip loss
are substantially fixed.
41. An optical power control device as set forth in claim 40 above,
wherein said round-trip loss is varied by varying said
resonator-to-member coupling amplitude K'.
42. An optical power control device as set forth in claim 41 above,
wherein the coupling amplitude is varied by electrooptic means.
43. An optical power control device as set forth in claim 41 above,
wherein the coupling amplitude is varied by optical means.
44. An optical power control device as set forth in claim 41
wherein the coupling amplitude is varied by application of a
variable voltage to an interferometer in the optical path of said
resonator.
45. An optical power control device as set forth in claim 41
wherein the coupling amplitude is varied by application of a
variable current to an interferometer in the optical path of said
resonator.
46. An optical power control device as set forth in claim 41
wherein the coupling amplitude is varied by application of an
optical signal to an interferometer in the optical path of said
resonator.
47. An optical power control device as set forth in claim 32 above,
wherein the property that is varied is said resonator loss,
.alpha..
48. An optical power control device as set forth in claim 47 above,
wherein said resonator loss is varied by electrooptic means.
49. An optical power control device as set forth in claim 47 above,
wherein said resonator loss is varied by optical means.
50. An optical power control device as set forth in claim 41
wherein said resonator loss is varied by application of an optical
signal to an interferometer in the optical path of said
resonator.
51. An optical power control device as set forth in claim 41
wherein said resonator loss is varied by application of a variable
voltage to an interferometer in the optical path of said
resonator.
52. An optical power control device as set forth in claim 41
wherein said resonator loss is varied by application of a variable
current to an interferometer in the optical path of said
resonator.
53. An optical power control device as set forth in claim 40 above,
wherein the device comprises means for varying the component of
round-trip negative resonator loss (optical gain) separately from
said resonator loss associated with the member couplings.
54. An optical power control device as set forth in claim 53 above,
wherein said resonator comprises means for providing optical
gain.
55. An optical power control device as set forth in claim 53 above,
wherein the optical gain induces overcoupling.
56. An optical power control device as set forth in claim 35 above
wherein the loss associated with said controller induces
undercoupling.
57. An optical wave power control device as set forth in claim 1
above, wherein the at least one resonator comprises at least two
resonators, each resonant at a like frequency and each disposed at
a different quadrant about the length of said wave transmission
member.
58. An optical power control device, comprising: a continuous
length of an optical waveguide arranged for transporting at least
one optical wave; at least one high Q optical wave recirculating
device in communication with the waveguide for exchanging wave
power therewith, and a wave power controller in optical
communication with the at least one circulating mode device for
varying the wave power returned to said waveguide from said optical
recirculating device, said wave power controller including at least
one interferometer in the optical path of said recirculating
device, and which interferometer is arranged to respond to a
control signal.
59. An optical power control device as set forth in claim 58 above,
wherein said control signal is a variable voltage which causes said
interferometer to vary said resonator loss, .alpha..
60. An optical power control device as set forth in claim 58 above,
wherein said control signal is a variable current which causes said
interferometer to vary said resonator loss, .alpha..
61. An optical power control device as set forth in claim 58 above,
wherein said control signal is an optical signal which causes said
interferometer to vary said resonator loss, .alpha..
62. An optical power control device as set forth in claim 58 above,
wherein said control signal is a variable voltage which causes said
interferometer to vary said resonator-to-member coupling amplitude
K'.
63. An optical power control device as set forth in claim 58 above,
wherein said control signal is a variable current which causes said
interferometer to vary said resonator-to-member coupling amplitude
K'.
64. An optical power control device as set forth in claim 58 above,
wherein said control signal is an optical signal which causes said
interferometer to vary said resonator-to-member coupling amplitude
K'.
65. An optical power control device as set forth in claim 58 above,
wherein said recirculating device is a member of the class of wave
power resonators characterized as whispering gallery mode devices
and comprising rings, and wherein said wave power propagating
member is of the class comprising optical fiber waveguides.
66. An optical power control device as set forth in claim 58 above,
wherein said recirculating device is a member of the class of wave
power resonators characterized as whispering gallery mode devices
and comprising rings, and wherein said wave power propagating
member is of the class comprising planar optical waveguides.
67. An optical power control device as set forth in claim 58 above,
wherein said wave power control varies said returned wave power
between said recirculating device and said propagating member
either from overcoupled to critically coupled or from critically
coupled to undercoupled conditions.
68. An optical wave transmission control for in-line variation of
power transmission on an optical waveguide, comprising: a low loss
optical wave power recirculating device having a periphery adjacent
to the optical waveguide in a relation to couple wave power
therefrom, said recirculating device also returning wave power to
the optical waveguide, and a variable coupling device operating
with said recirculating device for varying the power returned to
the optical waveguide from said recirculating device to vary power
transmission on the optical waveguide without introducing
discontinuities into the waveguide.
69. An optical wave transmission control as set forth in claim 68
wherein said variable coupling device introduces losses per
recirculation round trip to either establish critical coupling of a
previously overcoupled resonator or establish undercoupling of a
previously critically coupled resonator.
70. An optical wave transmission control as set forth in claim 68
above, wherein said variable coupling device interacts with said
recirculating wave to interfere with a portion of the recirculating
wave energy per round trip.
71. The method of modifying the power level of a mono-wavelength
signal in an optical waveguide comprising the steps of:
transferring a part of the power transmitted along the waveguide
into a whispering gallery mode resonant at the transmitted
wavelength; returning power to the optical waveguide from
circulating mode resonator; and, introducing a controllable loss in
the power of said resonator to modify the power level in the
transmitted signal in the waveguide.
72. A method as set forth in claim 71 above, wherein the intrinsic
losses in transferring power and in said resonator operation are
insufficient to extinguish the waveguide power level and the
controllable loss is varied in a range greater than the intrinsic
losses.
73. A method as set forth in claim 72 above, wherein the introduced
controllable loss is varied between a critical coupling level
wherein the waveguide transmitted power is at a minimum and a level
at which the waveguide transmitted power is substantially
unattenuated.
74. A method as set forth in claim 71 above, including the added
steps of: actuating an interferometer in the optical path of said
resonator by applying a variable voltage to said interferometer to
cause said controllable loss in power.
75. A method as set forth in claim 71 above, including the added
steps of: actuating an interferometer in the optical path of the
resonator by applying a variable current to said interferometer to
cause said controllable loss in power.
76. A method as set forth in claim 71 above, including the added
steps of: actuating an interferometer in the optical path of the
resonator by applying an optical signal to said interferometer to
cause said controllable loss in power.
77. A method as set forth in claim 74 above, wherein the waveguide
transmitted power is non-polarized and wherein the step of
introducing the controllable loss includes establishing at least
two resonators in which power circulates in planes that are
orthogonally disposed relative to each other.
78. A method of modulating or switching light at a single
wavelength along a continuous optical waveguide comprising the
steps of: propagating a guided part of the optical power along the
waveguide; transferring a portion of the power that is inside the
waveguide into a high Q recirculating path; returning power from
the recirculating path to the optical waveguide; and introducing
loss to the recirculating power in controlled fashion to modulate
the power propagated along the waveguide.
79. A method of modulating or switching light at a single
wavelength along a continuous optical waveguide as set forth in
claim 78 above, wherein said loss is controlled by an
interferometer in the optical path of said recirculating path.
80. A method of modulating or switching light at a single
wavelength along a continuous optical waveguide as set forth in
claim 79 above, wherein said interferometer is controlled by the
application of a variable voltage to said interferometer.
81. A method of modulating or switching light at a single
wavelength along a continuous optical waveguide as set forth in
claim 79 above, wherein said interferometer is controlled by the
application of a variable current to said interferometer.
82. A method of modulating or switching light at a single
wavelength along a continuous optical waveguide as set forth in
claim 79 above, wherein said interferometer is controlled by the
application of an optical signal to said interferometer.
83. A method as set forth in claim 80, above, wherein the optical
power transmitted comprises a single wavelength signal and wherein
the step of recirculating is resonant at that wavelength.
84. A method as set forth in claim 80 above, wherein the optical
power transmission comprises at least two different wavelength
signals and the interferometer controls at least one of the
different wavelength signals.
85. A modulator for use with an optical fiber transmission system,
comprising: an optical fiber; an optical resonator in communication
with said optical fiber for said transmission of optical power from
said fiber to said resonator and back again to said fiber, said
resonator being configured to be resonant and to generate internal
recirculating modes at the selected nominal frequency of the
optical power being transmitted by said waveguide; and, a loss
controller including an interferometer in the optical path of said
resonator, for introducing a loss as the modes recirculate to
thereby either establish critical coupling of a previously
overcoupled resonator or establish undercoupling of a previously
critically coupled resonator.
86. A modulator as set forth in claim 85 above, wherein said
resonator comprises a whispering gallery mode device.
87. A modulator as set forth in claim 85 above, wherein said
resonator comprises a ring, and wherein the Q of said resonator is
determined by sizing said resonator in accordance with the spectral
linewidth required for a data rate to be used in transmission.
88. A system for generating and controlling multiple optical
signals of different wavelengths on a single optical waveguide
capable of propagating multiple wavelengths within a chosen
bandwidth, comprising: an optical waveguide including at least two
in-waveguide optical power sources operating at different
wavelengths in the chosen bandwidth; at least two optical
resonators, each being resonant at a different one of the
wavelengths in the chosen bandwidth and each being disposed in
coupling relation to a different integral length of said optical
waveguide and coupled thereto; and, a control system optically
coupled to each of said resonators for controlling power loss
thereat, whereby propagated power at different wavelengths is
separately controlled in the single optical waveguide.
89. A system as set forth in claim 88 wherein said control system
includes an interferometer, said interferometer having a variable
voltage applied thereto to control said optical power in said
waveguide for at least one of the wavelengths in said optical
waveguide.
90. A system as set forth in claim 88 wherein said control system
includes an interferometer, said interferometer having a variable
current applied thereto to control said optical power in said
waveguide for at least one of the wavelengths in said optical
waveguide.
91. A system as set forth in claim 88 wherein said control system
includes an interferometer, said interferometer having an optical
signal applied thereto to control said optical power in said
waveguide for at least one of the wavelengths in said optical
waveguide.
92. A system as set forth in claim 88 wherein said control system
includes an interferometer in the optical path of each of said
resonators, said interferometer having a variable voltage applied
thereto to independently control said optical power in said
waveguide at each of the wavelengths in said optical waveguide.
93. A system as set forth in claim 88 wherein said control system
includes an interferometer in the optical path of each of said
resonators, said interferometer having a variable current applied
thereto to independently control said optical power in said
waveguide at each of the wavelengths in said optical waveguide.
94. A system as set forth in claim 88 wherein said control system
includes an interferometer in the optical path of each of said
resonators, said interferometer having an optical signal applied
thereto to independently control said optical power in said
waveguide at each of the wavelengths in said optical waveguide.
95. In an optical system for introducing a variable optical power
transmission in an optical waveguide in communication with an
optical wave resonator, the improvement comprising: including an
interferometer responsive to an external stimulus in the optical
path of said resonator for controlling the coupling between the
waveguide and said resonator, K'.
96. The improvement set forth in claim 95 wherein said external
stimulus is a variable voltage.
97. The improvement set forth in claim 95 wherein said external
stimulus is a variable current.
98. The improvement set forth in claim 95 wherein said external
stimulus is an optical signal.
99. A. In an optical system for introducing a variable optical
power transmission in an optical waveguide in communication with an
optical wave resonator, the improvement comprising: including an
interferometer responsive to an external stimulus in the optical
path of said resonator for controlling the internal loss of said
resonator, .alpha..
100. The improvement set forth in claim 99 wherein said external
stimulus is a variable voltage.
101. The improvement set forth in claim 99 wherein said external
stimulus is a variable current.
102. The improvement set forth in claim 99 wherein said external
stimulus is an optical signal.
103. An optical wave power control device for varying the
transmitted power at at least one optical frequency (i.e., optical
carrier wave) on an optical wave power transmission member
comprising: An optical wave transmission member configured for
propagating and guiding optical power at at least one optical
frequency; At least one optical wave resonator disposed in coupling
relation to said transmission member, positioned to couple wave
power from and to said member, and in frequency resonance with a
selected optical wave propagating on said transmission member; At
least one controller, each in operative relationship to a different
one of said resonators, for varying a property of said respective
resonator such that the optical wave transmitted in said wave
transmission member is varied in power level.
104. An optical wave power control device as set forth in claim 103
above, wherein said property that is varied is the component of
round-trip resonator loss that is distinct from said resonator loss
associated with said member coupling.
105. An optical wave power control device as set forth in claim 104
above, wherein said loss is varied by varying the internal loss of
said resonator.
106. An optical wave power control device as set forth in claim
104, wherein said loss is varied by an interferometer which is
actuated by applying a differential voltage to said
interferometer.
107. An optical wave power control device as set forth in claim
104, wherein said loss is varied by an interferometer which is
actuated by applying a differential current to said
interferometer.
108. An optical wave power control device as set forth in claim
104, wherein said loss is varied by an interferometer which is
actuated by applying an optical signal to said interferometer.
109. An optical wave power control device as set forth in claim 104
above, wherein said loss is varied by varying the coupling of
resonator mode power into another member or structure.
110. A variable coupling as set forth in claim 109 wherein the
structure is an interferometer in the optical path of said
resonator, whose phase matching is varied to control power
coupling.
111. A waveguide as set forth in claim 110 wherein said phase
matching is varied by applying a Voltage to said
interferometer.
112. A waveguide as set forth in claim 110 wherein said phase
matching is varied by applying a current to said
interferometer.
113. A waveguide as set forth in claim 110 wherein said phase
matching is varied by applying an optical signal to said
interferometer.
114. An optical wave power control device as set forth in claim 103
above, wherein said property that is varied is the component of
round-trip resonator loss associated with said resonator to member
coupling and with other sources of resonator round-trip loss
fixed.
115. An optical wave power control device as set forth in claim 114
above, wherein said loss is varied by varying said
resonator-to-member coupling amplitude K'.
116. An optical wave power control device as set forth in claim 103
above, wherein said property that is varied is the component of
round-trip negative resonator loss (optical gain) that is distinct
from said resonator loss associated with said member coupling.
117. An optical wave power control device as set forth in claim 116
wherein said optical gain is provided by said resonator.
118. An optical wave power control device as set forth in claim 104
above wherein said resonator and member introduce components of
round-trip resonator loss such that said resonator is over-coupled
and the loss associated with said controller induces critical
coupling.
119. An optical wave power control device as set forth in claim 104
above wherein said resonator and member introduce components of
round-trip resonator loss such that said resonator is critically
coupled and the loss associated with said controller induces under
coupling.
120. An optical wave power control device as set forth in claim 114
above wherein said resonator and member introduce components of
round-trip resonator loss such that said resonator is over-coupled
and the loss associated with said controller is varied to induce
critical coupling.
121. An optical wave power control device as set forth in claim 114
above wherein said resonator and member introduce components of
round-trip resonator loss such that said resonator is critically
coupled and the loss associated with said controller is varied to
induce under coupling.
122. An optical wave power control device as set forth in claim 116
above wherein said resonator and member introduce components of
round-trip resonator loss such that said resonator is critically
coupled and the optical gain associated with said controller
induces over coupling.
123. An optical wave power control device as set forth in claim 114
above wherein said resonator and member introduce components of
round-trip resonator loss such that said resonator is under coupled
and the optical gain associated with said controller induces
critical coupling.
124. An optical power control device as set forth in claim 103
above, wherein said optical wave transmission member propagates a
number of different frequencies and wherein said device includes a
plurality of resonators, each resonant at a different one of said
propagated frequencies and in coupling relation to said wave
transmission member, and a plurality of controllers, each disposed
in relation to a different one of said resonators and controlling a
property of said resonator thereat so as to vary power transmission
at a selected frequency.
125. An optical power control device in accordance with claim 103
above, wherein a plurality of resonators and associated controllers
are disposed in-line with said wave transmission member, each
resonator and associated controllers comprising a modulator
operating at a different optical frequency in a set of optical
frequencies, and further including a plurality of laser sources
that are in-line in said wave transmission member and transmitting
different frequencies of the set in a downstream direction on said
transmission member, and in which said modulator for each given
frequency is downstream of said laser source for that
frequency.
126. A control device as set forth in claim 21 above, wherein the
frequency separation between the modes of said resonator is
selected in accordance with the spectral extent spanned by the
frequencies propagating in said transmission member.
127. A control device as set forth in claim 22 above, wherein the
frequency separation between the modes of said resonator is
selected in accordance with the spectral extent spanned by the
frequencies propagating in said transmission member.
128. A control device as set forth in claim 21 above, wherein the Q
of said resonator is established at a level determined by the
wavelength and data rate/signal bandwidth of said control
signal.
129. A control device as set forth in claim 22 above, wherein the Q
of said resonator is established at a level determined by the
wavelength and data rate/signal bandwidth of said control
signal.
130. A control device as set forth in claim 21 above, wherein said
resonator has a Q of the order of 20,000 in the 1550 nm
telecommunications band, said resonator has a diameter of less than
100 microns, and said control signal has a data rate of the order
of 10 Gigabits per second.
131. A control device as set forth in claim 22 above, wherein said
resonator has a Q of the order of 20,000 in the 1550 nm
telecommunications band, said resonator has a diameter of less than
100 microns, and said control signal has a data rate of the order
of 10 Gigabits per second.
132. A method as set forth in claim 75 above, wherein the waveguide
transmitted power is non-polarized and wherein the step of
introducing the controllable loss includes establishing at least
two resonators in which power circulates in planes that are
orthogonally disposed relative to each other.
133. A method as set forth in claim 76 above, wherein the waveguide
transmitted power is non-polarized and wherein the step of
introducing the controllable loss includes establishing at least
two resonators in which power circulates in planes that are
orthogonally disposed relative to each other.
134. A method as set forth in claim 81 above, wherein the optical
power transmitted comprises a single wavelength signal and wherein
the step of recirculating is resonant at that wavelength.
135. A method as set forth in claim 82, above, wherein the optical
power transmitted comprises a single wavelength signal and wherein
the step of recirculating is resonant at that wavelength.
136. A method as set forth in claim 81 above, wherein the optical
power transmission comprises at least two different wavelength
signals and the interferometer controls at least one of the
different wavelength signals.
137. A method as set forth in claim 82 above, wherein the optical
power transmission comprises at least two different wavelength
signals and the interferometer controls at least one of the
different wavelength signals.
Description
[0001] This application is a continuation of and claims priority on
U.S. patent application Ser. No. 09/454,719, filed on Dec. 7, 1999,
entitled "Resonant Optical Wave Power Control Devices and Methods"
and PCT Application No. PCT/US 99/28891, of the same date and
having the same title. This application also claims priority on
U.S. provisional application No. 60/170,074, filed Dec. 9, 1999,
and entitled, "Optical Routing/Switching Based on Control of
Waveguide-Ring Resonator Coupling". The disclosure of each of the
forgoing is incorporated by reference herein as if set forth in
full hereat.
FIELD OF THE INVENTION
[0002] This invention relates to optical wave power control devices
and methods, and more particularly to systems, devices and methods
for modulating and switching signals transmitted in optical
waveguides.
BACKGROUND OF THE INVENTION
[0003] The present invention describes, among other things, a
switch/router which can be used to control the flow of optical
power. The switch can dump and dissipate the total power incident
in an optical waveguide at a specific wavelength into a resonator
thus reducing the transmitted power in the waveguide to zero. It
can alternatively, with a very small (compared to conventional
switches) control signal, let the power move on substantially
unmolested or it can switch/route the power from the initial
waveguide through the resonator to a second waveguide. This can
therefore provide the main building block in a microelectronic,
possibly monolithic, switchyard for optical signals of different
wavelengths which arrive at the subject device and are then
redistributed to a multitude of output guides.
[0004] In the now rapidly expanding technology relating to the use
of optical waveguides, a number of discrete devices and subsystems
have been developed to modulate, route or otherwise control,
optical beams that are at specific wavelengths. The approaches
heretofore used, however, have not fully overcome one or more
problems inherent in the requirements imposed by modern systems.
Present day communication systems increasingly use individual
waveguides to carry densely wavelength multiplexed optical beams,
and modulate the beams at very high digital data rates or with
wideband analog data, or both.
[0005] For example, it is known how to modulate the power of a
monofrequency laser source, typically a semiconductor laser. Using
such a source, one must accept a limited modulation bandwidth
because of constraints on the rate at which the laser can be turned
on and off. In addition, this type of modulation introduces
chirping, or spreading of the bandwidth of the signal from the
monofrequency laser, so that dispersion variations with wavelength
in signals that are transmitted in optical waveguides over a
substantial distance place an inherent limit on that distance. This
approach does have the advantage, as compared to some other
systems, of modulating at the source, so that continuity in the
waveguide structure can be preserved. However, semiconductor lasers
that are modulated have historically been coupled to optical
waveguides by means which introduce problems with yield,
reliability and cost. Consequently, the limitations mentioned above
are such that long distance transmission systems have previously
tended to employ external modulators.
[0006] The two forms of external modulators that are currently
employed are monolithic waveguide devices. A widely used lithium
niobate modulator of this type is based on a Mach-Zhender
interferometer and is being employed in long distance transmission
systems and other applications because it creates clean waveforms
at the highest data rates and produces a minimal amount of
chirping. As a monolithic waveguide device, it must be coupled at
its input and output to an optical fiber, which requires costly
packaging and assembly but even so introduces a substantial
mismatch between the chip waveguide and the optical fiber
waveguide, thus entailing losses in the range of about 5 db.
Furthermore, it is polarization sensitive and must be actively
temperature stabilized to compensate for the thermal drift
characteristics of the interferometer.
[0007] A second waveguide device, more recently introduced, is also
a monolithic on-chip device using an electro-absorption effect.
This modulator is fabricated integrally with a semiconductor laser,
requiring sophisticated and costly fabrication technology that
inevitably decreases the yield of the overall laser device. In
addition, such a device is subject to chirping, which places a
limitation on high (10 gigabit/sec and higher) modulation rates.
The integral laser/modulator chip must be coupled to optical
fiber--again adding cost to manufacturing.
[0008] There are a number of other patents of recent interest which
disclose variants on the monolithic device structure, but all
require a matching technique to be used to function with an optical
waveguide. Mention of signal modulation is made in at least two
patents which often employ dielectric microcavities for
recirculating electromagnetic wave energy at optical wavelengths.
"Whispering gallery mode" (WGM) structures, which comprise
microresonators of generally spherical, ring, or disc-like
configuration, are of dielectric material, e.g. glass or silica.
They are essentially totally internally reflective and support
internal modes at frequencies determined by size and other factors,
with very low losses, and therefore high Q. They are being
investigated for use in a number of different optical
configurations. U.S. Pat. No. 5,343,490 to McCall, for example,
discloses a closed loop WGM system configured as a thin element,
described as "an active material element of thickness
characteristically of a maximum of a half wavelength . . . " (Col.
1, lines 62-63). Disks are described that have thicknesses in the
range of 1,000-1,500 .ANG. and have at least one optically active
layer, sandwiched between thicker barrier layers. The optically
active material may be InGaAs and the barrier layers InGaAsP
material, for example. Fabricated into a microcavity using
photolithographic techniques, the structure is described as having
multiple potential functions. These comprise optically pumped
single quantum well to multiple quantum well structures and various
two port and three port devices which may function as, for example,
detectors, data amplifiers, and current meters. This disclosure,
the substance of which is incorporated herein by reference,
mentions in passing, as at Col. 6, lines 3-23, that the output may
be modulated or unmodulated, but apart from general statements
(e.g., "delicate destructive phase interference" in terms of
canceling an unmodulated output) there is no teaching as to how
modulation, much less high speed modulation could be effected.
[0009] A somewhat related approach is described in the "Photonic
Wire Microcavity Light Emitting Devices" application of Ho, et al.
in U.S. Pat. No. 5,878,070. The inventors also describe a WGM
microcavity with a gain medium of InGaAs sandwiched between InGaAsP
layers of submicron thickness, but closely surround a ring of this
optically active structure with an arc of lower refractive index
waveguide material in a general U-shape, the side arms of which may
be tapered (FIG. 9). With this arrangement, there is resonant
photon tunneling from the active material of the gain cavity to the
output-coupled waveguide, which serves as the core of the
structure. The possibility of modulation, by varying the pumping
power of the active medium section, is also suggested, (Col. 15,
lines 54-58) with no specific implementation being described. The
disclosure of the Ho '070 patent also is incorporated herein by
reference, as the control inventions of the present application may
also have applicability thereto.
[0010] In addition to the rapidly increasing use of fiber optic
systems, there is constant evolution toward denser wavelength
division multiplexing and higher data rates per channel. This in
turn means that factors such as spectral bandwidth, frequency
stability, compactness and reproducibility are of added importance,
and place added requirements on any new approach.
[0011] A device and method of efficiently controlling the coupling
between a waveguide and a circulating mode resonator coupled to one
or more waveguides will enable a significant expansion in the use
of such structures as an active or passive component in optical
circuits. It is evident that such a device can be used as a
modulator, an on-off switch, or a switchable bandpass filter, where
required for specific applications. Preferably, for complex
switching and routing systems having many channels, units using the
same concepts can be fabricated using microlithographic or
micromachining techniques.
[0012] The following references, each of which is incorporated by
reference, describe the matters related to the area of the present
invention: A. F. Levi, R. E. Slusher, S. L. McCall, J. L. Glass, S.
J. Pearson, and R. A. Logan, "Directional Light Coupling from
Microdisk Lasers," Appl. Phys. Lett. 62, 561- 563 (1993); B. E.
Little, "Ultra Compact Si--SiO2 Microring Resonator Optical Channel
Dropping Filters," Opt. Lett. 23, 1570 (1998); Ming Cai, Oskar
Painter, and Kerry J. Vahala, "Observation of Critical Coupling in
a Fiber Taper to a Silica-Microsphere Whispering Gallery Mode
System," Phys. Rev. Lett. 85, 74-76 (2000); Amnon Yariv, "Universal
Relations for Coupling of Optical Power Between Microresonators and
Dielectric Waveguides," Elect. Lett. 36, 32 (2000); Amnon Yariv and
Pochi Yeh, "Optical Waves in Crystals," J. Wiley and Sons 1984 p.
187; and, John E. Heebner and Robert W. Boyd, "Enhanced All-Optical
Switching by Use of a Nonlinear Fiber Ring Resonator," Opt. Lett.
24, 847-849 (1999).
SUMMARY OF THE INVENTION
[0013] The present invention provides for, inter alia, control of
any circulating mode resonator which is coupled to one or more
waveguides which permits the resonator to act as a switch, a
modulator, a transfer gate or any other type of function where
efficient control of the resonator losses and/or the coupling
between the resonator and one or more waveguides can be employed to
advantageous uses.
[0014] The objectives of the invention are met by a power transfer
structure and method of operation which variably attenuate
(modulates) or completely block (switches off) the power propagated
in a section of an optical waveguide. An optical waveguide can be
coupled to an adjacent circulating mode resonator in which wave
energy of a resonant mode recirculates with power accumulation
before return to the waveguide. In a first possible mode of
operation, the optical losses upon one round trip in the resonator
are such that resonator to waveguide coupling losses are greater
than other resonator losses. This is referred to as an over-coupled
condition, under which condition the resonator minimally attenuates
resonant optical power incident from the waveguide resulting in
maximal waveguide transmission. By increasing the resonator loss
per round trip (with resonator to waveguide coupling loss fixed) to
bring it into balance with resonator to waveguide coupling loss,
the condition goes from one of over-coupling to critical-coupling,
a condition in which waveguide power transmission is zero. The
transmission along the waveguide is thereby modulated from
essentially unity to essentially zero. This requires a very small
change in the round-trip loss induced by the method and/or
structure of the present control element in a first mode of
operation. As disclosed herein, the controller element can include
an interferometer which is in the optical path of the resonator
itself.
[0015] A second mode of operation, between a critical-coupling
condition and an undercoupled condition, is enabled by the present
invention and can also be used to effect signal modulation. In this
second mode of operation, round-trip resonator to waveguide
coupling loss is in balance with resonator losses before increase
of the resonator loss by the control element. In this condition
waveguide transmission is zero as described above. By increase of
the resonator loss beyond the condition of balance a condition of
under-coupling is obtained in which waveguide transmission is
restored to a value approaching unity transmission. A controller to
modify the loss as disclosed herein can include an interferometer
which is in the optical path of the resonator itself.
[0016] Both the first and second modes of operation can also be
realized using negative optical loss (or optical gain), however,
the sense in which the optical gain is applied is opposite to that
for positive optical loss. For example, in the first mode of
operation, the losses would be such that a condition of critical
coupling exists prior to application of the optical gain. The
control element would then apply optical gain to achieve a
condition of over-coupling, thereby modulating the transmission
from essentially zero to essentially unity. These third and fourth
modes of operation parallel the first and second modes of operation
in that variation between conditions of over coupling and critical
coupling (mode 1 and mode 3) or between conditions critical
coupling and under coupling (mode 2 and mode 4) is used to modulate
waveguide transmission. However, in these modes of operation, the
resonator to waveguide coupling loss is varied (as opposed to being
held fixed) while the other resonator losses are held fixed. The
control element in these cases effects a variation in the resonator
to waveguide coupling loss. Otherwise, the principle of operation
is essentially the same as that for modes 1 and 2. As disclosed
herein, such a controller can include an interferometer which is in
the optical path of the resonator itself. So long as the controlled
parameters are maintained in the critical coupling regime as
described herein to effectuate control between the critical
coupling and either an undercoupled or overcoupled state, the
controller and/or methods of the present invention will provide the
benefits described herein.
[0017] Since the combined elements are very small and frequency
specific a number of units can be used in combination with separate
controls for dense wavelength division multiplexing. Switching
systems and multiple modulation arrangements, with or without
in-fiber signal sources or amplifiers, can be arrayed as needed for
particular applications.
[0018] The resonator element is a circulating type of resonator of
the types known to those skilled in the art. Although a preferred
embodiment of the present invention is described more fully below
in connection with a ring resonator, the structure and/or methods
described herein are also applicable to other configurations
described herein or known elsewhere in the art.
[0019] Both theory and practice establish that the effective range
of loss control that is to be observed need vary only between an
overcoupled condition in which transmission is unity, or only
slightly less, and a critical coupling condition in which
transmission is attenuated by in excess of 90%. Because this
results, in real terms, from only a small change in applied loss by
a loss control mechanism, this approach is therefore preferred to
operation between a critical condition and an undercoupled
condition and to operation in which criticality is fixed while
resonant frequency is varied. In the latter cases different dynamic
ranges must be recognized as to both control and power. It is an
important feature of the preferred embodiment of the controller
described herein and claimed below that to complete the round trip
around the resonator, the light passes through an externally
controlled multi-port optical interferometer.
[0020] The modulator is polarization sensitive, which is typically
not of importance when it can be placed close to a source laser
which provides a polarized output. Where it is desired to provide
polarization insensitivity, two resonators, such as rings or silica
microspheres, can be disposed in orthogonal positions relative to
the central axis of the fiber. The geometry of the resonator
itself, as well as the material used, can be varied as long as the
desired Q value and resonator modal frequency separation is
maintained. Thus various geometries, including ring shapes, among
others, are known and may be employed in this application.
[0021] To utilize the concepts for concurrent modulation of
different wavelength signals multiplied on the same waveguide, it
is merely required to dispose a series of resonator/loss controller
combinations along one section of the waveguide. Each resonator is
responsive only to its own chosen wavelength and the wavelengths
are separately modulated with minimal cross-talk. In-fiber laser
sources, such as DFB fiber lasers, can also be employed in the
series, adding optical pumping in co-directional or
counter-directional relation. The integration of multiple
resonator-based modulators in a wavelength division multiplex
system provides a wavelength addressable transmission system.
[0022] For concurrent modulation and for wavelength specific
modulation of one co-propagated wave with other waves, an
appropriate frequency separation between adjacent resonances is
established to prevent unintended interference effects. Further the
adjacent modal frequency separations within resonators, which
support multiple modes at different frequencies, are arranged to
exceed the total bandwidth of a frequency range of interest.
Resonator geometries are adaptable to meet these requirements.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] A better understanding of the invention may be had by
reference to the following description, taken in conjunction with
the accompanying, in which:
[0024] FIG. 1a is a simplified block diagram and perspective
representation of an all fiber optical wave control device in
accordance with the disclosure contained herein;
[0025] FIG. 1b illustrates the generic geometry for a
waveguide-ring resonator coupling;
[0026] FIG. 2 is a fragmentary and idealized representation of a
tapered optical fiber and microsphere with a controllable loss
element which may be utilized in the arrangement of FIG. 1a;
[0027] FIG. 3 is a simplified representation of the cross section
of an optical absorber that may be utilized as a loss element in
the transducer of FIG. 2;
[0028] FIG. 4 is a fragmentary depiction of the interaction between
fields of electromagnetic wave energy in the example of FIGS. 1a
and 2;
[0029] FIG. 5 is a graph of the relation between waveguide
transmission and resonator amplitude attenuation per round trip (a
measure of round trip resonator loss) for calculated values;
[0030] FIG. 6 is a graph of transmission values in relation to
modal linewidth derived experimentally and confirming the
calculated values of FIG. 5;
[0031] FIG. 7 is a generalized view of a first alternative
arrangement for control of resonator loss;
[0032] FIG. 8 is a generalized view of a second alternative
combination for control of resonator loss;
[0033] FIG. 9 is a modification in which two optical waveguides
interact with a single resonator and in turn with each other;
[0034] FIG. 10 is a schematic representation of field amplitudes
and coupling coefficients in modeling a resonance-based control
system;
[0035] FIG. 11 is a simplified representation of a system for
varying waveguide transmission by shifting the frequency of
resonance modes;
[0036] FIG. 12 is a graph showing the relation between transmission
drop and resonance mode center frequency shift;
[0037] FIG. 13 is a fragmentary perspective view of a modulator in
accordance with the disclosure herein employing a planar waveguide
and a disc resonator;
[0038] FIG. 14 is an example of how multiple modulators can be used
with a common optical waveguide;
[0039] FIG. 15 depicts a system in which multiple resonators
interact with two waveguides;
[0040] FIG. 16 is an example of an all-fiber source and modulator
system;
[0041] FIG. 17 is a generalized example of a polarization
insensitive optical modulator or switch;
[0042] FIG. 18 shows the universal transmission plot for the
configuration of FIG. 1b;
[0043] FIG. 19 is a diagram of a composite interferometer for
achieving voltage (or light) control of the coupling between a
waveguide and a ring resonator;
[0044] FIG. 20 illustrates a directional coupler with
electronically controlled phase mismatch
(.beta..sub.1-.beta..sub.2) used as a coupling element;
[0045] FIG. 21 plots waveguide power transmission against frequency
(.theta.=.omega.L/c) with internal loss factor .alpha. as a
parameter and where .vertline.t.vertline.=0.9998; and
[0046] FIG. 22 is a diagram of an alternative embodiment of a
controller utilizing multiple waveguides.
DETAILED DESCRIPTION OF THE INVENTION
[0047] The present invention is directed to controlling the effect
of one or more circulating mode resonators on the flow of optical
power in one or more optically coupled waveguides by interposing an
externally controllable multi-port interferometers, preferably
monolithically constructed with such resonator, in the optical path
of the resonator in order to vary the resonator loss and/or the
coupling between the waveguides and the resonators. The structures
and methods of the present invention can be used to vary the
waveguide transmission from essentially unity to essentially zero
by introducing a differential phase shift (".crclbar.") between the
arms of the interferometer by applying a signal (e.g. voltage,
current or another light beam) to or through the interferometer.
This provides the ability to modulate, switch, route and/or
otherwise control the flow of power in the waveguide
a.sub.1.fwdarw.b.sub.1 (as illustrated in FIG. 1b) with
dramatically less switching signal power; indeed, at a level of
values smaller that 10.sup.-2 Volts. Because of the universal
application of the fundamental relationships developed herein
between a waveguide and a circulating mode (i.e. "ring") resonator,
the present invention can be employed independent of the details of
the coupling or the resonator.
[0048] In current practical use, the resonator will need to
accommodate a data rate in the range of 1 to 10 Gb/sec, for a 1550
nm signal. This is merely for example, however, and the present
invention is not limited to any particular wavelength or signal. In
general, the spectral width of the resonator mode should be larger
or equal to twice the width of the desired information bandwidth.
The spectral width of the measured power transmission is related to
the resonator quality factor ("Q") as follows: 1 Q = V O / V
Equation (1)
[0049] where the half width at half maximum is .DELTA.V, and the
resonator center line frequency is V.sub.O. For a resonance having
a typical telecommunications wavelength of 1550 nm and a data rate
of 10 Gbits/sec (5 GHz bandwidth with NRZ format) the required
optical bandwidth will be approximately 10 GHz, and the Q should be
19000 or less. To be consistent with the preferred operation mode
in the over-coupled to critically coupled range, Q should be
decreased and hence spectral linewidth increased by either reducing
the round-trip propagation time within the resonator (i.e., reduce
resonator size) or by increasing the resonator to wave guide
coupling loss.
[0050] Coupling loss can be controlled by varying the losses in the
resonator or by or by varying the coupling coefficient between the
resonator 20 and the waveguide 12. This can be accomplished by
interposing an interferometer into the optical path of the
resonator 20 or in the optical path between the waveguide 12 and
the resonator 20. Alternatively, depending on the geometry of the
modulator, coupling loss can be increased either by increasing the
spatial overlap of resonator modes with the field exterior to the
fiber waist, by improving phase matching conditions between the
resonator modes and the taper modes, or both.
[0051] The controllable loss device 22 can be derived from a class
of electrically or optically variable controllers. Controlling the
optical loss by interposing an interferometer into the optical path
of the resonator 20 or in the optical path between the waveguide 12
and the resonator 20 as illustrated in FIG. 19 is one preferred
embodiment. The geometry of this embodiment is illustrated in FIGS.
1b and 22.
[0052] A waveguide 12 and a ring resonator 200 both enter and
emerge from a coupling region 202 where power exchange takes place.
This exchange is describable in terms of universal relations which
are independent of the specific embodiment as described in Yariv,
"Universal Relations for Coupling of Optical Power Between
Microresonators and Dielectric Waveguides," Elect. Lett. 36, 32
(2000), the contents or which are specifically incorporated herein
in full by reference. Some key results of that analysis, helpful to
more fully understand this embodiment, are set forth below.
[0053] If the coupling is limited only to waves traveling in one
sense and if the total powers entering and leaving the coupling
region 202 are equal (i.e., a zero loss case), then the coupling
can be described by means of two constants k and t and a unitary
scattering matrix 2 b 1 b 2 = t * - t * a 1 a 2 (A1) t 2 + 2 = 1
(A2)
[0054] Equations (A1) and (A2) are supplemented by the circulation
condition in the ring 200 3 a 2 = b 2 i (A3)
[0055] where .alpha. and .theta., real numbers, give respectively,
the loss (or gain) and the phase shift per circulation. The above
equations are solved to yield the transmission factor for the input
waveguide. 4 b 1 a 1 2 = 2 + t 2 - 2 t cos 1 + 2 t 2 - 2 t cos
(A4)
[0056] In the above, power normalization is used so that
.vertline..alpha..sub.i.vertline..sup.2,
.vertline.b.sub.i.vertline..sup.- 2 are the respective traveling
wave powers. It is possible to assume, without a loss of
generality, that the incident power
.vertline..alpha..sub.1.vertline..sup.2 to be unity. At resonance
.theta.=m2.pi., m an integer, and 5 b 1 2 = ( - t ) 2 ( 1 - t ) 2
(A5)
[0057] This simple universal relation, plotted in FIG. 18, has two
very important features. First, the transmitted power
.vertline.b.sub.1.vertli- ne..sup.2 is zero at a value of coupling
.alpha.=t, "critical coupling". Second, for high Q resonators
(.alpha..ltoreq.1) the portion of the curve to the right of the
critical coupling point is extremely steep. "Small" changes in a
for a given t, or vice versa, can control the transmitted power,
.vertline.b.sub.1.vertline..sup.2, between unity and zero. The
ability to control .alpha. and/or t provides a basis for a
switching and/or routing technology. Done sufficiently rapidly, the
device will act as an optical modulator.
[0058] The first of the proposed coupling control schemes is
illustrated in FIG. 19. It incorporates a Mach-Zhender
Interferometer (MZI) 110 sandwiched between two 3 dB couplers 112,
114 (collectively, the "composite" interferometer, "CI" 116) into
the ring resonator 200. The MZI 110 introduces a differential phase
shift .DELTA..phi. between its two arms 120, 122. This is
illustrated in FIG. 22 as part of an embodiment that utilizes
multiple waveguides, but is equally applicable to a configuration
that only utilizes a single waveguide 12. Using the same wave
designation .alpha..sub.i, b.sub.i (i=1, 2) as in FIG. 1b, Eqs. A4,
A5 describe the CI 116 as: 6 b 1 b 2 = t * - t * a 1 a 2 = - i cos
2 - i sin 2 - i sin 2 - i cos 2 a 1 a 2 (A6)
[0059] so that 7 t = - i cos 2 , = - i sin 2 (A7)
[0060] The form of the matrix on the right side of Eq. (A6) follows
from a multiplication of the three Jones matrices of its individual
components.
[0061] Critical coupling control, and in general coupling control,
are thus achieved by controlling .DELTA..phi.. If .DELTA..phi. is
zero, .vertline.t.vertline.=1, k=0 and
.vertline.b.sub.1.vertline.=.vertline..a- lpha..sub.1.vertline.
i.e., unity transmission. When .DELTA..phi.=.pi., t=0,
.vertline.k.vertline.=1.
[0062] Using (Eq. A7) in (Eq. A4) produces a transmission
expression: 8 P out P in = b 1 a 1 2 = 2 + cos 2 2 - 2 cos 2 cos 1
+ 2 + cos 2 2 - 2 cos 2 cos (A8)
[0063] One implemental of this derivation is to employ an electric
or optical signal to vary .DELTA..phi.. A preferred embodiment of
such an implementation is to place an interferometer in the optical
path of the resonator 200 to achieve this control. If the two arms
120, 122 of the MZI 110 consist of an electrooptic material, the
differential phase shift, .DELTA..phi., is proportional to the
applied voltage V. 9 = V V
[0064] where V.sub..pi. is the voltage causing a differential phase
shift .DELTA..phi.=.pi. in the MZI 110. Using the last relation and
Eq. (A7) in Eq. (A5) produces an expression for the transmission at
resonance (.theta.=m2.pi., m=1, 2, 3 . . . ): 10 P out P in = ( -
cos V 2 V ) 2 ( 1 - cos V 2 V ) 2 (A9)
[0065] The power transmission in the "through" waveguide 12 can
thus be controlled via the applied voltage. When V=0 the
transmission is unity. When V/V.sub..pi.=2/90 cos.sup.-1 .alpha.
critical coupling occurs and the transmission is zero. For
.alpha..apprxeq.1 (high Q ring resonator) the voltage V.sub.c
needed to turn the transmission off (from a transmission of unity
at V=0) is 11 V c V ~ 1 - 2 (A10)
[0066] For a value of .alpha.=0.999,
V.sub.c/V.sub..pi..apprxeq.{fraction (1/32 )}. Since conventional
electrooptic modulators require V.sub.c.apprxeq.V.pi. this
embodiment of the perfect invention provides a reduction of nearly
two orders of magnitude. Using present day materials, control of
the optical power in a waveguide by the structure and methods of
the present invention can be achieved by the application of
voltages measured in tenths of millivolts. In lieu of applying
voltage, similar control can be obtained by applying a current or
by applying an optical signal to one arm of the MZI 116 to induce a
controlled .DELTA..phi., again with a substantial reduction in
control power over previous devices.
[0067] A second scheme for coupling control involves uses a very
weak directional coupler 118 as shown in FIG. 20. In this
embodiment, the coupling matrix is described in (A11) by [Eq. A6]
as: 12 t * - t * = cos K 2 + 2 L - i sin K 2 + 2 L K 2 + 2 - iK sin
K 2 + 2 L K 2 + 2 - iK sin K 2 + 2 L K 2 + 2 - cos K 2 + 2 - i sin
K 2 + 2 L K 2 + 2 (A11)
[0068] where K is the coupling coefficient of the directional
coupler 118 and .delta..ident..beta..sub.2-B.sub.1 is the
propagation constant mismatch of the directional coupler waveguides
12. Here too, it is possible to control the coupling conditions so
as to operate over the steep part of FIG. 18 by varying the
mismatch parameter .delta. electrooptically. (For example by
applying a voltage to the guides comprising the directional coupler
which will be fabricated from a material possessing an electrooptic
coefficient.)
[0069] There exist many biasing conditions which can be used with
this second scheme to operate in this steep part of FIG. 18. One
example would be to bias the directional coupler 118 initially with
KL=.pi., .delta.=0 in which case t=1 and the transmission
.vertline.b.sub.1.vertline..sup.2=- 1. A voltage can then be
applied such that: 13 = ( 2 L 2 ) 1 2 ( 1 - 2 ) 1 4 (A12)
[0070] This will cause the coupling 118 to go "critical" resulting
in zero transmission. The voltage needed to satisfy Eq. (A12),
which is the voltage needed to go from "on" to "off" is: 14 V c V ~
( 1 - 2 ) 1 4 (A13)
[0071] For a value of .alpha.=0.999, V.sub.c/V.sub..pi.=0.211.
[0072] The control of .delta. can be achieved, instead of
electrooptically, by injecting an optical signal into one arm of
the MZI 110 of FIG. 19 or the coupler 118 of FIG. 20 and utilizing
the Kerr effect. The reductions in the ratio V.sub.c/V.sub..pi.
will be reflected in similar reductions in the switching light
intensity compared to nonresonant geometries.
[0073] A plot of the transmission as given by Eq. A4 as a function
of optical frequency .omega. (or equivalently .theta.=.omega.nL/c
where L is the optical path) is shown in FIG. 21. The critical
coupling, at .alpha.=.vertline.t.vertline., is shown in the lower
solid trace. Note the net transmission gain for values of .alpha.,
such that 15 1 < < 1 t (A14)
[0074] The condition .alpha..vertline.t.vertline.=1 corresponds to
infinite transmission i.e., to laser oscillation since it implies a
finite output, .vertline.b.sub.1.vertline..sup.2, for a zero input
.vertline..alpha..sub.1.vertline..sup.2.
[0075] It is believed that a preferred realization of this
modulator, switch or router will be in a III-V or II-VI
semiconductor material configuration compatible with GaInAsP
lasers. The device of FIG. 1b (or FIG. 19) can be coupled
monolithically to a III-V or II-VI laser via dielectric waveguides.
Also, the ring resonator 200 can be current pumped to obtain gain
and make up for optical losses.
[0076] Multiple controllers of the type described above can be
employed in a single waveguide to modulate, switch or route optical
power which exists at specific frequencies and/or phase
orientations. Similarly, the method and structure at the above
described preferred embodiments can be utilized to alternatively
connect the optical power between multiple waveguides. As is
illustrated in FIG. 22, the MZI 116 can be used to alternatively
couple multiple waveguides (here 3 waveguides are shown as 12, 130
and 140) based upon the application of a control signal (e.g.
voltage, current or an optical signal). As described below as will
be understood by those skilled in the art, numerous additional
implementations of this structure and/or method can be made without
departing from the scope or spirit of the invention as described
herein. While the implementations described below are directed to
an embodiment of a modulator/switch which utilizes a tapered fiber
and a microsphere resonator, it will be understood by those skilled
in the art that such configurations and/or combinations are
applicable to the above described embodiment of the present
invention.
[0077] In an alternative geometry, the resonator 20 can be attached
directly to the waist region 14 of a tapered fiber for positional
stability. A controllable loss transducer 22 in close juxtaposition
to the opposite of a resonator 20, here illustrated as a silica
microsphere, from the waist region 14 is driven by a modulating
signal source 24 to control the absorption of wave power
circulating within and about the resonator 20, thus adding a loss
factor per round trip. If the control is analog between limits,
then the waveguide power signal is modulated. If the loss control
is varied between conditions of maximum and zero transmission, then
the unit functions as an on-off switch or as a digital modulator.
Again, the coupling loss can be controlled by interposing an
interferometer into the optical path of the resonator 20 or in the
optical path between the waveguide 12 and the resonator 20.
[0078] The tapered sections, 15, 16 and intermediate waist region
14 of the waveguide may be provided, as is known, by stretching the
waveguide under controllable tension as it is softened by one or
more fixed or movable heat sources (e.g., torches). Commercially
available machines can be used for this purpose in production
environments. The consequent reduction in diameter of about one or
more orders of magnitude reduces the central core in the
core/cladding structure of the optical fiber to vestigial size and
function, such that the core no longer serves to propagate the
majority of the wave energy. Instead, without significant loss, the
wave power in the full diameter fiber transitions into the waist
region, where power is confined both within the attenuated cladding
material and within a field emanating into the surrounding
environment as depicted in fragmentary form in FIG. 4. After
propagating through the waist region 14, exterior wave power is
recaptured in the diverging tapered region 16 and is again
propagated with low loss within the outgoing fiber section 18.
[0079] The high Q resonator 20 in this example is coupled to the
externally guided power about the waist region 14 of the waveguide.
That is, at all times there is a coupling interaction from the
principal fiber into the interior of the resonator 20 via the
resonator periphery, as shown in FIG. 4. The resonator 20
additively recirculates the energy with low loss in the "whispering
gallery mode", returning a part of the power to the waveguide at
the waist 14. There is also coupling to the controllable loss
transducer 22 during each round trip. When a resonance exists at
the chosen wavelength, the resonator 20 functions with effectively
total internal reflection and with minimal internal attenuation and
radiative losses. However, the emanating portion of the wave power
is still confined and guided, so it is presented for coupling back
into the waveguide waist 14. Extremely high Q values (as much as 8
billion have been observed) exist in this whispering gallery mode,
seemingly first explicated by Rayleigh in an article entitled "The
Problem of the Whispering Gallery" in 1912. The phenomenon has
since been investigated both theoretically (as in an article by M.
L. Gorodetsky, et al. in Optics Letters 21, 453 (1996)) and in
various implementations, as shown in the McCall and Ho patents
referenced above. Different WGM devices have been disclosed and
investigated in the literature, including discs, rings, polygons,
oblate and prolate spheroids. Furthermore, concentricity or
approximate concentricity may in some instances not be necessary,
since the WGM effect can exist in non-concentric boundary
structures such as ellipses or race-track structures.
[0080] Another embodiment includes a quantum well structure having
controllable properties of photon absorption is also suitable,
because the transducer 22 can comprise a plurality of layers
disposed on or near a part of the circumference of the resonator
20, with layers comprising both active material (e.g., InGaAs,
numbered 22') and buffer layers (InGaAsP numbered 22"), so as to
vary the photon absorption within a range controlled by an
electrical signal. Such structures are described in detail in both
the McCall and Ho et al patents referenced above.
[0081] Other available approaches to provide material absorption of
the optical waves are based, for example, on the use of
semiconductor materials having band gaps which are either (1)
larger than the energy of the signal wave photon energy or (2)
smaller than the signal photon energy. In either case, as seen in
FIG. 7, the semiconductor could be deposited as a layer 30 on a
part of the resonator 32 or situated near the resonator, and
irradiated by an optical source such as a laser 36. In the former
example, optical pumping from the laser 36 generates carriers in
the semiconductor layer 30, which causes free carrier absorption of
the optical wave thereby taking the resonator from an over-coupled
to a critically coupled condition (assuming preferred operation)
and reducing modulator transmission. While the modulation rate is
determined by the carrier lifetime, this parameter can be shortened
by introduction of defects into the semiconductor.
[0082] In the latter case, optical pumping from the laser 36
generates carriers which cause band-filling-induced reduction of
the optical absorption. In this case the modulator characteristic
would be designed for maximum extinction (critical coupling) when
there is no optical pumping; which is advantageous since the
highest extinction can be "designed" into the device during
manufacture. The wave power coupling relationship thus becomes over
coupled as optical pumping is applied, and output transmission
increases. As above, modulation rate is determined by carrier
lifetime. In each of these examples, carriers can be generated in
the semiconductors and the modulation (or switching) can result, by
the use of electrical or optical excitation.
[0083] A different effect using a semiconductor layer 40 on or near
a resonator 42 can also be understood by reference to FIG. 8. Here
a small parallel plate capacitor 44 spans the resonator 42 and
applies a variable field, which can be modulated at a high rate, to
the semiconductor layer. In this example the energy gap is selected
to be close to but slightly larger than the signal photon energy.
The resonator is initially overcoupled and hence wave power
transmission in the waveguide 46 is maximum. To increase absorption
an electric field is applied to the semiconductor layer 40 via the
capacitor 44, and by way of the Franz-Keldish effect an increase in
absorption is experienced by the wave in the resonator 42, thereby
taking the resonator to the critical condition. This in turn
decreases transmission from the optical waveguide 46 coupling to
the resonator 42, and can be applied to modulate (or switch) power
in the waveguide 46.
[0084] The variation of loss can be effected in other ways,
including using a resonator of variable loss material, by varying
relative positions of resonator and fiber, or by introducing an
element that couples power from the resonator into another
structure such as a second waveguide. For the case of coupling to a
second waveguide, the coupling loss might feasiblely be varied by
varying the phase matching condition to the second waveguide as,
for example could be done using an electro-optic material. The
relatively slow variations achievable with mechanical devices or
temperature variations may be fully acceptable as loss control
elements for some applications.
[0085] A double optical waveguide combination with a common
resonator 50 is shown in FIG. 9, to which reference is now made.
The narrow waist sections 52, 53 and the two optical fiber
waveguides 55,56 are shown, but it should be understood that input
sources and output circuits (not shown) can be arranged to utilize
the bi-directional properties of the waveguides 55, 56 and
resonator 50. Both waveguides 55,56 are coupled to the resonator 50
as is a loss transducer 58 which is varied by a control source 59
in the critical coupling range as previously described. The
coupling is such that the waist sections 52, 53 couple to
essentially the same modes of the resonator 50 thereby enabling
resonant power transfer from one waveguide to the other under the
control of the loss transducer 58. When this coupling is
symmetrical with respect to the two waist sections 52,53 and when
the associated resonator to waveguide coupling losses exceed other
resonator losses, then the resonator 50 is critically coupled to
each waveguide and nearly complete power transfer from one
waveguide to the other is possible on resonance. This power
transfer is spoiled and the resonator 50 under coupled when
resonator loss is increased substantially by the loss transducer
58. In this case, the power transfer is interrupted and resonant
power in either waist 52, 53 proceeds with near unity transmission
to respective waveguide outputs 55, 56. In this way the device
functions as a wavelength addressable 2.times.2 switch in which
signals can be controllably redirected. In all instances wavelength
multiplexed signals out of resonance with the modes in the
resonator 50 are passed through transparently from input side to
output side. The loss transducer element in this 2.times.2
configuration would be essentially the same as that described for
the modulator (1.times.1 switch) except that the 2.times.2 switch
operates nominally in the critical to under-coupled regime.
Bandwidth, modal frequency separation, and other design issues
concerning the resonator structure would also be the same as those
for the modulator. Similar functionality can be achieved with the
structure and method described above using an interferometer, and
as illustrated in FIG. 22.
[0086] The coupling and control principles described herein differ
substantially and uniquely from prior studies and disclosure as to
WGM devices. From these it is known that an evanescent coupling
exists, for example, between an optical beam directed into a prism
and reflected internally off one face at a point at which a WGM
microsphere is externally positioned. The prism will evanescently
couple a portion of its wave energy into a recirculating path
within the microsphere if the frequency is at one of the resonant
modes of the microsphere. It is also known that input optical waves
are transmitted out at essentially undiminished power, except for a
minimum in the resonance range. A similar effect exists for the
combination of a dielectric WGM resonator adjacent a tapered
optical fiber waveguide, as has been shown.
[0087] However, the ability to employ the recirculating resonant
modes and the coupling effects requires understanding and proper
use of a number of controlling conditions. Varying the transmitted
power output between substantially full transmission and
substantially zero transmission, whether in modulation or
switching, requires understanding and control of a number of
parameters, including the sources of resonator loss. The sources of
loss experienced by the circulating wave are varied and distinct,
and include:
[0088] (1) Loss associated with the portion of the WGM field that
is intentionally coupled from the microsphere back into the
taper.
[0089] (2) Distributed loss associated with the intrinsic
properties of the microsphere such as optical absorption in the
microsphere material, surface imperfections and surface
contamination. With careful material selection and processing,
however, pure silica microspheres or discs having smooth surfaces
can be prepared that introduce only very low distributed loss.
[0090] (3) Parasitic losses, such as any arising from unintended
coupling of optical power into modes that are not returned to the
fiber waveguide, e.g. radiation modes. By observation, these are
found to be very low if proper conditions are observed for
coupling.
[0091] (4) Loss that is intentionally introduced into the sphere
(that is not associated with the coupling to the waveguide taper)
to induce modulation or switching.
[0092] If the only source of loss is coupling loss [Eq. (1) above],
conservation of energy dictates that power from input to output
will be 100% transmitted. Since past development and practical
results show the that non-coupling losses [Eqs. (2), (3) above] can
be made small, they can be ignored in the following analytical
model depicted graphically in FIG. 5 and based upon the following
set of coupled linear equations for the complex field amplitude,
using the quantities defined symbolically in FIG. 10:
[0093] Four-port scattering equations: 16 E st = KE i + t ' E si
Equation (2) E t = K ' E si _ + tE i Equation (3)
[0094] Round trip propagation condition in sphere: 17 E si = E st
where = kC Equation (4)
[0095] In equation (4), .alpha. gives the resonator amplitude
attenuation per round trip associated with one round trip of
propagation in the sphere, .crclbar. is the phase associated with
that propagation, k is the propagation constant of the excited
mode, and C is the sphere circumference. Additionally, in equation
(4), K, K' are the amplitude coupling coefficients from the
waveguide to the resonator and vice versa and depend on the device
parameters including resonator waveguide field overlaps and phase
matching, while t, t' are the four-port transmission amplitudes on
the waveguide side and the resonator side (not to be confused with
modulator transmission). This model makes it possible to calculate
the maximum transmission attenuation as a function of a loss from
an unspecified source other than loss factors inherent in the
microsphere/waveguide system. The curve in FIG. 5 shows the results
of a calculation that assumes numerical values for the coefficient
in the model that are consistent with measured Q's in tapered
fiber-microsphere system tests. These values are only illustrative.
The horizontal axis gives the amplitude attenuation per round trip,
".alpha.", induced by the unspecified loss, where .alpha.=1
corresponds to no additional loss. At .alpha.=1 there is therefore
unity transmission of resonant wave power.
[0096] The effect of introducing added loss, as seen in FIG. 5,
where increasing coupling loss is to the left on the horizontal
axis, is to increase attenuation until there is zero power
transmitted. At this point added loss per round trip is the sole
cause, in this model, of the total drop in attenuation, and is
achieved in the example used for FIG. 5 at an .alpha. of only about
0.9997. Such a condition, known in microwave theory as "critical
coupling", thus requires only a minute amount of added loss to
induce a large swing in the transmitted waveguide power.
Modification of the state of the recirculating resonator in this
manner thus provides the basis for the exemplifications of the
invention. Moreover, the resonant modes provide precise frequency
selectivity.
[0097] The calculated model results shown in the curve in FIG. 5
are fully confirmed by experimental measurements of a tapered
optical fiber/microsphere modulator, as shown in FIG. 6. These
measurements were made with an approximately 3 micron waist fiber
diameter and an approximately 300 micron diameter microsphere,
adjacent to which a moveable microprobe was variably positioned to
introduce incrementally controlled coupling loss. Due to the nature
of the study, the horizontal axis is related to linewidth instead
of .alpha., and the curve is reversed but the proof of critical
coupling is clear. Significantly, critical coupling exists over a
very small a variation, and the total loss at .alpha.=1.000 is
observed to be small. This is also meaningful in other respects,
because it shows that distributed losses and parasitic losses in
the measured structure are not only low, but less than tapered
fiber to microsphere coupling losses. Thus an "overcoupled"
condition naturally exists when there is no intentionally added
loss. The experimental work empirically demonstrates further that
the characteristics of the model for added coupling loss are
reliable.
[0098] As described earlier, operation of an optical modulator or
switch of any of the embodiments described herein can be posited
where an undercoupled condition exists, but would entail greater
spreads in attenuation values, and likely be subject to lower
dynamic ranges, and may require more power. Modulation from the
critical coupling part into the overcoupled regime is preferable in
an optimized configuration because the needed attenuation is so
small that the loss control transducer or device can be minute and
minimally invasive to the resonator modes. In addition, power
consumption is minimized in this mode of operation. Depending on
whether the attenuator is non-absorbing or absorbing in the absence
of a control signal, the modulator or switch will be inverting or
non-inverting, respectively.
[0099] An alternative approach to modulation/switching is based
upon varying the optical path length of the dielectric resonator
itself under fixed resonator loss and coupling conditions necessary
to obtain critical coupling. Referring now to FIGS. 11 and 12, this
effect varies waveguide transmission loss by shifting the resonant
frequency of a resonator 60 toward or away from the transmitted
optical wave frequency. In the example shown, the surface of the
resonator 60 is coated with a polymer material 62 which varies in
refractive index depending on the electric field applied by an
associated electrode pair 64, 65. The electric field is controlled
by a signal source 66 so as to vary the coating 62 refractively,
which in turn causes the resonant frequency of the resonator 60 to
shift. In consequence, as seen in FIG. 11, a given optic wave
frequency V.sub.L from a laser source remains constant but the WGM
line center frequency Vo for maximum resonance shifts, causing a
degree of extinction of the transmitted optical wave that varies
with the degree of shift. In this example, the resonator 60 is
designed to provide full extinction at full coincidence (critical
coupling), between V.sub.L and Vo in FIG. 12
[0100] The WGM resonant frequency can also be modulated in other
ways. For example, the material of the resonator can be chosen to
vary in refractive index under optical or electrical excitation.
Temperature variations can also be used in cases where modulation
rates are very low.
[0101] Microlithographic fabrication techniques suitable for making
optical waveguides and microresonators are now available that are
based upon a number of different principles. As evidenced by the
McCall and Ho et al patents referenced above, electro-optic WGM
structures using layers of materials form controllable
electro-optical devices with variable absorption (or gain)
characteristics. As seen in FIG. 13, a narrow planar waveguide 70
comparable in waveguiding properties to a tapered optical fiber is
built on a substrate 72 in evanescent coupling relation to the edge
of a WGM disc 74, also built upon the substrate 72. A loss control
element that is responsive to electrical signals or optical pumping
could also be added on the substrate 72 adjacent the disc 74.
Alternatively, the dielectric constant of the disc 74 could be
changed to vary the resonant modes in the disc 74, as discussed
above. For this purpose an area 76 of the substrate 72 is provided
under and in contact with the disc 74, to shift the dielectric
constant on the disc 74 in response to a control source 78 of
modulating or switching signals. Microlithographic elements can be
reliably made on a production basis, and with precise positioning
of multiple elements can satisfy the packaging needs of complex
DWDM systems. Since they can be serially coupled on a substrate, a
substantial number of couplings to transmission fibers are not
required.
[0102] There are many systems configurations in which multiple
frequencies must be separately modulated or switched, and a
multi-modulator combination of any of the above embodiments could
be used. An example of such a combination using the tapered waist
80 of a single optical fiber 82 is shown in FIG. 14. Each modulator
resonator 84a, 84b, 84c, 84d is resonant at a different frequency
corresponding to one in the WDM signals on the fiber 82, is
disposed as part of a spaced series along the waist 80. Each
modulator resonator 84a-d is separately modulated (or switched on
and off) by a different loss control, 86a-d respectively, the
system provides separate but non-interfering variation of the WDM
components. It will be recognized that these waist regions need not
be shared but can be at different positions along the length of a
fiber transmission line. In the example of FIG. 15, the same idea
is extended into a combination with the double tapered waveguide
concept of FIG. 9. Because the two-spaced apart waveguide waists
52', 53' each interact with the different modulator resonators
84a'-d'; and can interact with each other as previously described
such greater versatility in system design becomes feasible.
[0103] The potential for WDM applications described immediately
above is expandable to include active elements, such as tandem
fiber lasers (e.g. DFB fiber lasers) in series with multiple
resonator based modulators to form an all-fiber multi-wavelength
system of modulators and sources. Referring now to FIG. 16, a fiber
with tapered sections (not shown) each including a controlled
microcavity modulator 90 and responsive to a selected wavelength,
.lambda..sub.1, .lambda..sub.2, .lambda..sub.3 . . .
.lambda..sub.n-1, .lambda..sub.n disposed along an optical fiber 92
is alternated with in fiber DFB lasers 94, operating at like
wavelengths. This creates a wavelength division multiplexed source
having N channels. If N is not too large a single optical pump
diode 96 can be used to pump the laser 94 in a counter-directional
fashion, as shown (or in a co-directional fashion). While the
modulators and fiber lasers are shown as alternating, they can also
be arranged in serial sets, since they do not generate interfering
signals in any event.
[0104] WGM resonators are resonant at a number of frequencies, and
the separation to be established between them is dependent in part
on the requirements of any associated multi-frequency system. Thus
the frequency separation between resonances must be sufficiently
large to prevent unintended modulation of waves co-propagated with
the wave to be modulated. In a WDM system, the separation should
encompass the bandwidth of all channels on the optical waveguide.
For example, in a WDM system using 16 channels with 100 GHz channel
separation a resonator modulator would need to have a modal
frequency separation exceeding approximately 1.5 THz of bandwidth.
Greater numbers of co-propagating waves on a WDM waveguide would
necessarily require greater model frequency separation. Such
considerations affect resonator selection, as in the geometry of
the microcavity. For example, to meet such separation requirements
oblate spheroidal, disc and ring geometries would be preferable to
microspheres.
[0105] The value of a completely in-line multiplexing system will
be evident to those skilled in the art. Given that the frequency
selectivity of the modulators combines with their transparency to
all other signals, and that all components are of sizes of the
order of microns, simplicity, freedom from mismatch and compactness
are all achieved concurrently.
[0106] The transmission function of a WGM microcavity resonator is
polarization dependent, because of the orientation needed for
electromagnetic mode recirculation about the equator of the
microcavity. Normally this is not of concern because the resonator
can be placed in proper relation close to a laser source, which
emits predominantly polarized optical waves. In systems where this
is not feasible or other factors affect polarization, an
arrangement such as that in FIG. 17 can be used. A tapered optical
fiber 100 with a narrow waist region as previously described coacts
with two resonators 102, 103, here microspheres, which are
orthogonally separated about the circumference of the fiber 100.
Each is associated with a different loss transducer 104, 105
properly oriented, that as varied by a loss control 108. Separate
loss controls may be employed in some situations. Regardless of the
vectorial direction or arbitrary state of polarization, this
arrangement modulates or switches the optical wave energy as in the
previous examples.
[0107] It will be appreciated that a substantial number of other
expedients are made possible because of the capability for
frequency selective power control afforded by the concepts of this
invention. For example, where input optical power is itself
modulated the power transduction at the resonator can be made to
function as a detector. This means that the input optical waves in
a WDM signal can be selectively converted to electrical signals
without discontinuity being introduced into the optical
transmission line.
[0108] It will also be recognized that optical gain (negative loss)
instead of loss can be used to vary critical coupling in the
modulator (see also discussion in summary section).
[0109] While there have been described above various forms and
modifications, it will be appreciated that the invention is not
limited thereto but encompasses all variations and expedients
within the scope of the appended claims.
* * * * *