U.S. patent application number 09/896358 was filed with the patent office on 2002-03-28 for thermo-optic switch having fast rise-time.
This patent application is currently assigned to Gemfire Corporation. Invention is credited to Bischel, William K., Brinkman, Michael J., Huang, Lee L., Kowalczyk, Tony.
Application Number | 20020037129 09/896358 |
Document ID | / |
Family ID | 23457399 |
Filed Date | 2002-03-28 |
United States Patent
Application |
20020037129 |
Kind Code |
A1 |
Brinkman, Michael J. ; et
al. |
March 28, 2002 |
Thermo-optic switch having fast rise-time
Abstract
A thermo-optic switch is operated in a novel near-impulse mode
in which the drive pulse width is shorter than twice the diffusion
time of the switch. The drive pulse width is less than the rise
time of the steady-state optical response and also less than the
rise time-of the deflection efficiency response to the applied
drive pulse. The drive pulse can further include a sustaining
segment following the initial short pulse segment, if it is desired
to maintain the switch in an ON state for a longer period of time.
A number of additional techniques are described for further
reducing the response time of the switch. An array of thermo-optic
switches operated in this manner can form a display which, due to
the fast individual switch rise times, can operate at an overall
fast refresh rate.
Inventors: |
Brinkman, Michael J.;
(Redwood City, CA) ; Bischel, William K.; (Menlo
Park, CA) ; Kowalczyk, Tony; (Palo Alto, CA) ;
Huang, Lee L.; (Sunnyvale, CA) |
Correspondence
Address: |
HAYNES BEFFEL & WOLFELD LLP
P O BOX 366
HALF MOON BAY
CA
94019
US
|
Assignee: |
Gemfire Corporation
|
Family ID: |
23457399 |
Appl. No.: |
09/896358 |
Filed: |
June 29, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09896358 |
Jun 29, 2001 |
|
|
|
09369900 |
Aug 6, 1999 |
|
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Current U.S.
Class: |
385/16 |
Current CPC
Class: |
G02F 1/0121 20130101;
G02F 1/0147 20130101; G02F 1/011 20130101 |
Class at
Publication: |
385/16 |
International
Class: |
G02B 006/35 |
Claims
1. A method for controlling a first thermo-optic switch for
temporary deflection of optical energy out of a first optical path,
said first thermo-optic switch having a heater and further having a
deflection efficiency which changes in response to electrical
energy delivered to said heater, comprising the step of delivering
an electrical energy pulse to said heater, said pulse having a
pulse width which is shorter than 90% of the response time with
which said deflection efficiency reaches a maximum change after
said pulse begins being applied to said heater.
2. A method for controlling a first thermo-optic switch for
temporary deflection of optical energy out of a first optical path,
said first thermo-optic switch having a heater and further having a
deflection efficiency which changes in response to electrical
energy delivered to said heater, comprising the step of delivering
an electrical energy pulse to said heater, said pulse having a
pulse width which is measurably shorter than the response time with
which said deflection efficiency reaches a maximum change after
said pulse begins being applied to said heater.
3. A method according to claim 2, wherein the deflection efficiency
of said first thermo-optic switch increases in response to
electrical energy delivered to said heater.
4. A method for controlling a first thermo-optic switch for
temporary deflection of optical energy out of a first optical path,
said first thermo-optic switch having a heater and further having a
deflection efficiency which changes in response to electrical
energy delivered to said heater, said first thermo-optic switch
further having a diffusion time given by the length of time for the
deflection efficiency of said switch to reach a maximum change in
response to an impulse of electrical energy delivered to said
heater, comprising the step of delivering an electrical energy
pulse to said heater, said pulse having a pulse width which is
shorter than twice the diffusion time of said first thermo-optic
switch.
5. A method according to claim 4, wherein the deflection efficiency
of said first thermo-optic switch increases in response to
electrical energy delivered to said heater.
6. A method according to claim 5, wherein optical energy deflected
out of said first optical path is directed into a second optical
path.
7. A method according to claim 5, wherein optical energy deflected
out of said first optical path is directed toward a pixel of a
display.
8. A method according to claim 5, wherein said pulse is
rectangular.
9. A method according to claim 5, wherein said pulse has a pulse
rise time and a pulse fall time which is longer than said pulse
rise time.
10. A method according to claim 9, wherein said pulse has an
exponential decay.
11. A method according to claim 5, for controlling a plurality of
thermo-optic switches along said first optical path including said
first thermo-optic switch, each of said thermo-optic switches
having a respective heater and further having a respective
deflection efficiency which increases in response to electrical
energy delivered to the respective heater, each given one of said
thermo-optic switches further having a respective diffusion time
given by the length of time for the deflection efficiency of the
given switch to reach a maximum change in response to an impulse of
electrical energy delivered to the heater of the given switch,
comprising the step of delivering to each of said heaters in
sequence, a respective electrical energy pulse having a respective
pulse width which is shorter than twice the diffusion time of the
respective thermo-optic switch.
12. A method according to claim 11, wherein all of said pulse
widths are equal.
13. A method according to claim 5, wherein said electrical energy
pulse has an average amplitude, further comprising the step of,
immediately after said step of delivering an electrical energy
pulse to said heater, delivering sustaining energy to said heater
for a period of time which is longer than said pulse width, said
sustaining energy having an average amplitude which is smaller than
said average amplitude of said pulse.
14. A method according to claim 5, further comprising the step of
repeating, at constant time intervals, said step of delivering an
electrical energy pulse to said heater.
15. A method according to claim 5, wherein said heater comprises a
plurality of heater segments.
16. A method according to claim 15, wherein said heater segments
are connected in series.
17. A method according to claim 15, wherein said heater segments
are connected in parallel.
18. A method according to claim 5, for controlling a first
plurality of thermo-optic switches each disposed to temporarily
deflect optical energy out of a different respective optical path,
said first plurality of switches including said first thermo-optic
switch, each of said switches in said first plurality of switches
having a respective heater, comprising the step of delivering said
electrical energy pulse to each of said heaters.
19. A method according to claim 5, for controlling a plurality of
sets thermo-optic switches including said first thermo-optic
switch, each of said switches in said plurality of sets of switches
having a respective heater, wherein within each given one of said
sets, each of the switches is disposed to temporarily deflect
optical energy out of a different respective optical path,
comprising the steps of: delivering said electrical energy pulse to
the heaters of all of the switches in a first one of said sets and
not the heaters of the switches in a second one of said sets; and
subsequently delivering a subsequent electrical energy pulse to the
heaters of all of the switches in said second one of said sets.
20. A method according to claim 5, wherein the deflection
efficiency of said first thermo-optic switch is saturable, such
that the deflection efficiency change in response to electrical
energy delivered to said heater has a saturation value, and wherein
said maximum change in deflection efficiency of said first
thermo-optic switch is equal to said saturation value of the
deflection efficiency.
21. A method according to claim 5, wherein said first thermo-optic
switch comprises an optical waveguide having a core region and at
least a first cladding region, said first cladding region having
substantially greater electrical conductivity than said core
region, and wherein at least a portion of said heater comprises at
least a portion of said cladding region.
22. A method for controlling a first thermo-optic switch for
temporary deflection of optical energy out of a first optical path,
said first thermo-optic switch having a heater and further having a
deflection efficiency which changes in response to electrical
energy delivered to said heater, said first thermo-optic switch
further having an impulse response rise time given by the length of
time for the deflection efficiency of said switch to change from
10% to 90% of its maximum change in response to an impulse of
electrical energy delivered to said heater, comprising the step of
delivering an electrical energy pulse to said heater, said pulse
having a pulse width which is shorter than four times the impulse
response rise time of said first thermo-optic switch.
23. A method for controlling a first thermo-optic switch for
temporary deflection of optical energy out of a first optical path,
said first thermo-optic switch having a heater and further having a
deflection efficiency which changes in response to electrical
energy delivered to said heater, comprising the step of delivering
an electrical energy pulse to said heater, said pulse having a
pulse width which is shorter than 50 .mu.s.
24. A method for scanning a plurality of thermo-optic switches
along a first optical path, each of said switches having a
respective heater and deflecting optical energy out of said first
optical path in response to electrical energy delivered to said
heater, each of said switches having a respective deflection
efficiency, comprising the step of delivering a respective
electrical energy pulse to the heaters of each of said switches in
sequence, each i'th one of said pulses except a first one of said
pulses beginning at a respective time T.sub.i after the beginning
of the immediately preceding pulse, each i'th one of said pulses
having a respective pulse width which is sufficiently short to
cause the deflection efficiency of the respective switch to change
from 10% to 90% of its maximum change in response to the respective
pulse, within a time period of T.sub.(i+1)/2.
25. A method for scanning a plurality of thermo-optic switches
along a first optical path, each of said switches having a
respective heater and deflecting optical energy out of said first
optical path in response to electrical energy delivered to said
heater, each of said switches having a respective deflection
efficiency, comprising the step of delivering a respective
electrical energy pulse to the heaters of each of said switches in
sequence, each i'th one of said pulses except a first one of said
pulses beginning at a respective time T.sub.i after the beginning
of the immediately preceding pulse, each i'th one of said pulses
having a respective pulse width which is shorter than
T.sub.(i+1)/2.
26. A method for controlling a first thermo-optic switch for
temporary deflection of optical energy out of a first optical path,
said first thermo-optic switch having a heater and further having a
deflection efficiency which changes in response to electrical
energy delivered to said heater, comprising the steps of:
delivering initial electrical energy to said heater, said initial
electrical energy having an average amplitude; terminating delivery
of said initial electrical energy to said heater, said initial
electrical energy thereby having a delivery duration, said step of
terminating occurring before the change in deflection efficiency of
said first thermo-optic switch exceeds 90% of the maximum change in
deflection efficiency produced in response to said initial
electrical energy, for more than one-half said delivery duration;
and thereafter delivering sustaining electrical energy to said
heater over a sustaining time period immediately following said
step of terminating, said sustaining electrical energy having an
average amplitude which is lower than the average amplitude of said
initial electrical energy.
27. A method according to claim 26, wherein said first thermo-optic
switch further has a diffusion time given by the length of time for
the deflection efficiency of said switch to reach a maximum change
in response to an impulse of electrical energy delivered to said
heater, wherein said step of terminating delivery of said initial
electrical energy to said heater comprises the step of terminating
delivery of said initial electrical energy to said heater within a
time period after said initial electrical energy begins being
applied to said heater which is shorter than twice the diffusion
time of said first thermo-optic switch.
28. A method according to claim 26, wherein optical energy
deflected out of said first optical path is directed into a second
optical path.
29. A method according to claim 26, wherein optical energy
deflected out of said first optical path is directed toward a pixel
of a display.
30. A method according to claim 26, wherein said initial electrical
energy has a constant amplitude.
31. A method according to claim 26, wherein said initial electrical
energy has a rise time and a fall time which is longer than said
rise time.
32. A method according to claim 31, wherein said initial electrical
energy has an exponential decay.
33. A method according to claim 26, wherein the deflection
efficiency of said first thermo-optic switch increases in response
to electrical energy delivered to said heater.
34. A method according to claim 33, for controlling a plurality of
thermo-optic switches along said first optical path including said
first thermo-optic switch, each of said thermo-optic switches
having a respective heater and further having a respective
deflection efficiency which increases in response to electrical
energy delivered to the respective heater, comprising the steps of:
delivering to each of said heaters in sequence, respective initial
electrical energy having a respective average amplitude;
terminating delivery of each respective initial electrical energy
to the respective heater, the initial electrical energy delivered
to each of said heaters thereby having a respective delivery
duration, said step of terminating occurring with respect to the
heaters of each of said thermo-optic switches before the change in
deflection efficiency of the respective thermo-optic switch exceeds
90% of the maximum change in deflection efficiency produced in
response to said initial electrical energy delivered to the
respective switch, for more than one-half the respective delivery
duration; and thereafter delivering to each of said heaters
respective sustaining electrical energy over a respective
sustaining time period following the termination of the respective
initial electrical energy to the respective heater in said step of
terminating, each respective sustaining electrical energy having a
respective average amplitude which is lower than the average
amplitude of the respective initial electrical energy.
35. A method according to claim 33, wherein said sustaining
electrical energy has a constant amplitude over said sustaining
time period.
36. A method according to claim 33, wherein said sustaining
electrical energy has an amplitude which varies over said
sustaining period.
37. A method according to claim 36, wherein said step of delivering
sustaining electrical energy to said heater over a sustaining time
period following said step of terminating, comprises the step of
delivering a plurality of sustaining electrical energy pulses to
said heater over said sustaining time period.
38. A method according to claim 33, wherein said step of delivering
sustaining electrical energy to said heater over a sustaining time
period following said step of terminating, comprises the step of
delivering sufficient energy to said heater during said sustaining
time period to maintain the deflection efficiency change of said
first thermo-optic switch at least said maximum change, at least
until said sustaining time period ends.
39. A method according to claim 38, wherein the deflection
efficiency of said first thermo-optic switch saturates at a
deflection efficiency corresponding to a saturation deflection
efficiency change, and wherein said maximum change in deflection
efficiency produced in response to said initial electrical energy
is equal to said saturation deflection efficiency change.
40. A method according to claim 38, wherein the deflection
efficiency of said first thermo-optic switch saturates at a
deflection efficiency corresponding to a saturation deflection
efficiency change, and wherein said maximum change in deflection
efficiency produced in response to said initial electrical energy
is smaller than said saturation deflection efficiency change.
41. A method according to claim 26, wherein said step of delivering
sustaining electrical energy to said heater over a sustaining time
period following said step of terminating, comprises the step of
delivering energy to said heater during said sustaining time period
with an amplitude profile which maintains the deflection efficiency
change of said first thermo-optic switch at said maximum change, at
least until said sustaining time period ends.
42. A method according to claim 26, wherein said initial electrical
energy is delivered over an initial time period, and wherein said
sustaining time period is longer than said initial time period.
43. A method according to claim 26, further comprising the step of
charging an energy storage element prior to said step of delivering
initial electrical energy to said heater, and wherein said step of
delivering initial electrical energy to said heater comprises the
step of delivering energy from said energy storage element at least
partly to said heater.
44. A method according to claim 43, wherein said step of delivering
sustaining electrical energy to said heater over a sustaining time
period following said step of terminating comprises the step of
connecting a constant amplitude energy source to said heater during
said sustaining time period.
45. A method according to claim 26, further comprising the step of
repeating, at constant time intervals, said steps of delivering
initial electrical energy to said heater, terminating delivery of
said initial electrical energy to said heater, and delivering
sustaining electrical energy to said heater.
46. Thermo-optic switch apparatus, comprising: an optical waveguide
having a core region and at least a first cladding region, said
first cladding region having substantially greater electrical
conductivity than said core region; and at least first and second
electrodes electrically connected to respective different
connection points on said first cladding region.
47. Apparatus according to claim 46, further comprising a second
cladding region on the opposite side of said core region from said
first cladding region.
48. Apparatus according to claim 46, wherein said core region
comprises a core layer and said first cladding region comprises a
first cladding layer.
49. Apparatus according to claim 48, comprising a substrate and a
second cladding layer supported by said substrate, said core layer
overlying said second cladding layer and said first cladding layer
overlying said core layer.
50. Apparatus according to claim 46, wherein said first and second
electrodes are electrically connected to respective first and
second connection points on said first cladding region, a line
passing through said first and second connection points crossing a
first optical path in said core region diagonally.
51. Apparatus according to claim 46, wherein said first cladding
region comprises a conductive polymer.
52. Apparatus according to claim 46, further comprising a current
source connected across said first and second electrodes.
53. Thermo-optic switch apparatus, comprising: an optical waveguide
having a core region and at least a first cladding region, said
core region having substantially greater electrical conductivity
than said first cladding region; and at least first and second
electrodes electrically connected to respective different
connection points in said core region.
54. Thermo-optic switch apparatus comprising, in combination: a
core region; first and second cladding regions on opposite sides of
said core region; and first and second heating elements on opposite
sides of said first and second cladding regions.
55. Apparatus according to claim 54, further comprising at least
one electrical conductor interconnecting said first and second
heating elements.
56. Apparatus according to claim 55, wherein said at least one
electrical conductor interconnects said first and second heating
elements in series.
57. Apparatus according to claim 55, wherein said at least one
electrical conductor interconnects said first and second heating
elements in parallel.
58. Apparatus according to claim 54, wherein said core region and
said first and second cladding regions comprise respective core and
first and second cladding layers of a waveguide.
59. Apparatus according to claim 54, for use with optical energy
propagating along a first optical path defined at least in part by
said core region and said first and second cladding regions,
wherein said first and second heating elements are shaped and
oriented so as to induce, in response to electrical energy applied
to said heating elements, a total-internal-reflection index of
refraction boundary across said first optical path.
60. Apparatus according to claim 54, wherein said core region and
said first and second cladding regions at least in part define a
first optical path, said first thermo-optic switch apparatus having
a deflection efficiency which changes in response to electrical
energy delivered to said heater elements, further comprising a
pulse generator which delivers to said heater elements an
electrical energy pulse having a pulse width which is shorter than
90% of the response time with which said deflection efficiency
reaches a maximum change after said pulse begins being applied to
said heater elements.
61. Apparatus according to claim 54, wherein said core region and
said first and second cladding regions at least in part define a
first optical path, said first thermo-optic switch apparatus having
a deflection efficiency which changes in response to electrical
energy delivered to said heater elements, said first thermo-optic
switch apparatus further having a diffusion time given by the
length of time for the deflection efficiency change of said switch
to reach a maximum change in response to an impulse of electrical
energy delivered to said heater, further comprising a pulse
generator which delivers to said heater elements an electrical
energy pulse having a pulse width which is shorter than twice the
diffusion time of said first thermo-optic switch apparatus.
62. Thermo-optic switching apparatus comprising: a first
thermo-optic switch disposed along a first optical path, said first
thermo-optic switch having a heater and further having a deflection
efficiency which changes in response to electrical energy delivered
to said heater; and a driver which delivers an electrical energy
pulse to said heater, said pulse having a pulse width which is
shorter than 90% of the response time with which said deflection
efficiency reaches a maximum change after said pulse begins being
applied to said heater.
63. Thermo-optic switching apparatus comprising: a first
thermo-optic switch disposed along a first optical path, said first
thermo-optic switch having a heater and further having a deflection
efficiency which changes in response to electrical energy delivered
to said heater; and a driver which delivers an electrical energy
pulse to said heater, said pulse having a pulse width which is
measurably shorter than the response time with which said
deflection efficiency reaches a maximum change after said pulse
begins being applied to said heater.
64. Apparatus according to claim 63, wherein the deflection
efficiency of said first thermo-optic switch increases in response
to electrical energy delivered to said heater.
65. Thermo-optic switching apparatus comprising: a first
thermo-optic switch disposed along a first optical path, said first
thermo-optic switch having a heater and further having a deflection
efficiency which changes in response to electrical energy delivered
to said heater, said first thermo-optic switch further having a
diffusion time given by the length of time for the deflection
efficiency of said switch to reach a maximum change in response to
an impulse of electrical energy delivered to said heater; and a
driver which delivers an electrical energy pulse to said heater,
said pulse having a pulse width which is shorter than twice the
diffusion time of said first thermo-optic switch.
66. Apparatus according to claim 65, wherein the deflection
efficiency of said first thermo-optic switch increases in response
to electrical energy delivered to said heater.
67. Apparatus according to claim 65, further comprising a display
pixel, wherein optical energy deflected out of said first optical
path is directed toward said display pixel.
68. A method according to claim 65, wherein said pulse delivered by
said driver has a pulse rise time and a pulse fall time which is
longer than said pulse rise time.
69. Apparatus according to claim 65, wherein said driver comprises:
a pulse generator which outputs a plurality of said electrical
energy pulses in sequence; and a first electrical switch which
couples only a first proper subset of said pulses to said
heater.
70. Apparatus according to claim 69, further comprising a second
thermo-optic switch disposed along said first optical path, said
second thermo-optic switch having a heater, wherein said driver
further comprises a second electrical switch which couples only a
second proper subset of said pulses to the heater of said second
thermo-optic switch, the pulses in said second subset alternating
with the pulses in said first subset.
71. Apparatus according to claim 70, further comprising a third
thermo-optic switch disposed along said first optical path, said
third thermo-optic switch having a heater, wherein said driver
further comprises a third electrical switch which couples only a
third proper subset of said pulses to the heater of said third
thermo-optic switch, the pulses in said first, second and third
subsets following each other in round robin sequence.
72. A method according to claim 65, wherein said heater comprises a
plurality of heater segments.
73. Thermo-optic switching apparatus comprising: a first
thermo-optic switch disposed along a first optical path, said first
thermo-optic switch having a heater and further having a deflection
efficiency which changes in response to electrical energy delivered
to said heater, said first thermo-optic switch further having an
impulse response rise time given by the length of time for the
deflection efficiency of said switch to change from 10% to 90% of
its maximum change in response to an impulse of electrical energy
delivered to said heater; and a driver which delivers an electrical
energy pulse to said heater, said pulse having a pulse width which
is shorter than four times the impulse response rise time of said
first thermo-optic switch.
74. Thermo-optic switching apparatus comprising: a first
thermo-optic switch disposed along a first optical path, said first
thermo-optic switch having a heater and further having a deflection
efficiency which changes in response to electrical energy delivered
to said heater; and a driver which delivers an electrical energy
pulse to said heater, said pulse having a pulse width which is
shorter than 50 .mu.s.
75. Display apparatus comprising: a first plurality of thermo-optic
switches along a first optical path, each of said switches in said
first plurality of switches having a respective heater and
deflecting optical energy out of said first optical path in
response to electrical energy delivered to said heater, each of
said switches in said first plurality of switches having a
respective deflection efficiency; and a driver which delivers
electrical energy pulses to the heaters of selected ones of said
switches in sequence, each i'th one of said pulses except a first
one of said pulses beginning at a respective time T.sub.i after the
beginning of the immediately preceding pulse, each i'th one of said
pulses having a respective pulse width which is sufficiently short
to cause the deflection efficiency change of the respective switch
to increase from 10% to 90% of its maximum change in response to
the respective pulse, within a time period of T.sub.(i+1)/2.
76. Apparatus according to claim 75, wherein optical energy
deflected out of said first optical path by each of the switches in
said first plurality of switches is directed toward a respective
pixel of said display.
77. Apparatus according to claim 75, wherein each of said pulses is
rectangular.
78. Apparatus according to claim 75, wherein the pulses delivered
to one of the switches in said first plurality of switches has
pulses has a pulse rise time and a pulse fall time which is longer
than said pulse rise time.
79. Apparatus according to claim 78, wherein said pulses delivered
to said one of the switches in said first plurality of switches an
exponential decay.
80. Apparatus according to claim 75, wherein each of said
electrical energy pulses has a respective average amplitude,
wherein immediately after said driver finishes delivering the
electrical energy pulse to each given one of said heaters, said
driver further delivers sustaining energy to the given heater, the
sustaining energy delivered to each given one of said heaters
lasting for a respective period of time which is longer than the
width of the respective electrical energy pulse delivered to the
given heater and having an average amplitude which is smaller than
said average amplitude of the respective electrical energy pulse
delivered to the given heater.
81. Apparatus according to claim 75, wherein each of said heaters
comprises a plurality of heater segments.
82. Apparatus according to claim 75, further comprising a second
plurality of thermo-optic switches along a second optical path,
each of the switches in said second plurality of switches having a
respective heater; and electrical connections among the heaters in
said first and second pluralities of switches such that electrical
energy delivered to each heater in said first plurality of switches
is also delivered to a respective corresponding heater in said
second plurality of switches.
83. Display apparatus comprising: a first plurality of thermo-optic
switches along a first optical path, each of said switches in said
first plurality of switches having a respective heater and
deflecting optical energy out of said first optical path in
response to electrical energy delivered to said heater, each of
said switches in said first plurality of switches having a
respective deflection efficiency; and a driver which delivers
electrical energy pulses to the heaters of selected ones of said
switches in sequence, each i'th one of said pulses except a first
one of said pulses beginning at a respective time T.sub.i after the
beginning of the immediately preceding pulse, each i'th one of said
pulses having a respective pulse width which is shorter than
T.sub.(i+1)/2.
84. A method for controlling a first thermo-optic switch for
temporary deflection of optical energy out of a first optical path,
comprising the steps of: charging an energy storage element; and
thereafter delivering thermal energy to said switch in response to
energy released from said energy storage element.
85. A method according to claim 84, wherein said first thermo-optic
switch has a heater and further has a deflection efficiency which
changes in response to electrical energy delivered to said heater,
wherein said step of delivering thermal energy to said switch
comprises the step of discharging electrical energy from said
energy storage element at least partly into said heater.
86. A method according to claim 85, wherein said energy storage
element comprises an inductance.
87. A method according to claim 85, wherein said step of charging
said energy storage device comprises the step of charging said
energy storage device beginning at a predetermined time period
prior to said step of discharging and continuing to said step of
discharging.
Description
BACKGROUND
[0001] 1. Field of the Invention
[0002] The invention relates to thermo-optic switches, and more
particularly to methods and structures to achieve fast switching
rise times in thermo-optic switches, primarily but not exclusively
for display applications.
[0003] 2. References
[0004] The following references are incorporated herein by
reference:
[0005] U.S. Pat. No. 4,635,082 to Domoto et al.
[0006] U.S. Pat. No. 5,544,268 to Bischel et al.
[0007] M. B. J. Diemeer et al., "Polymeric optical waveguide switch
using the thermo-optic effect", Journal of Lightwave Technology,
vol. 7, No. 3, March 1989, pp. 449-453.
[0008] Haruna et al., "Thermo-optic effect in LiNbO3 for light
deflection and switching," Electronics Letters, vol. 17, No. 22,
Oct. 29, 1981, pp. 842-844.
[0009] Y. Hida et al., "Polymer waveguide thermo-optic switch with
low electric power consumption at 1.3 .mu.m", IEEE Photonics
Technology Letters, vol. 5, No. 7, July 1993, pp. 782-784.
[0010] C. C. Lee et al, "2.times.2 single-mode zero-gap
directional-coupler thermo-optic waveguide switch on glass,"
Applied Optics, vol. 33, No. 30, Oct. 20, 1994, pp. 7016-7022.
[0011] Y. J. Min et al., "Transient thermal study of semiconductor
devices", IEEE Transactions of Components, Hybrids, and
Manufacturing Technology, vol. 13, No. 4, December 1990.
[0012] H. Nishihara et al., Optical Integrated Circuits, New York:
McGraw-Hill, 1989.
[0013] 3. Description of Related Art
[0014] Referring to FIG. 1A, guided wave devices typically consist
of an optical path defined by at least a core 115 and a cladding
110/120 that confines the optical path in two dimensions. The core
layer 115 is adjacent to one or more cladding materials 110/120
that have a lower refractive index than the core. In the
illustration shown, the substrate itself forms a lower cladding 120
for confinement normal to the plane of the surface, while either
air or a material deposited on the core forms an upper cladding 110
to complete the confinement normal to the plane. In some glassy or
crystalline materials, the core 115 of the waveguide can be formed
by diffusion of an ion into a substrate, raising the index of
refraction. In this case, both the core layer 115 and lower
cladding 120 are part of the substrate. In other materials such as
polymers, the core and cladding are typically deposited in layers,
with a core layer 115 surrounded by lower 120 and upper 110
cladding layers to provide confinement for the waveguide normal to
the plane. Confinement in the second dimension, the plane of the
substrate, can be provided by either a difference in thickness or
refractive index of a portion 135 of the core layer 115. Optical
waveguides may have many forms, such as channel waveguides
described above, planar waveguides and optical fiber waveguides for
example.
[0015] Thermo-optic ("TO") switches may be formed using any
waveguide forms including but not limited to those mentioned above.
TO switches operate on the principle of a thermally-induced change
in index of refraction of the optical path at a switch location.
Thermo-optic devices are useful for many applications because of
polarization insensitivity, the availability of low-loss
thermo-optically active materials, and the absence of charging
affects associated with EO devices.
[0016] As illustrated in FIG. 1A, a conventional TO device 100
typically includes a resistive heater 105 which, by injecting
thermal energy through a top cladding layer 110 into the core 115,
increases the temperature in the core and changes its refractive
index, forming an index-modified region 125. The index-modified
region acts as a switch, causing the light propagating along 130 to
be diverted from the waveguide. The resistive heater 105 is shown
symbolically in the figure and the switch could be any optical
switch known in the art including, but not limited to, Mach-Zehnder
interferometers, directional couplers, two-mode interferometers,
and total internal reflection (TIR) devices. The switch is
activated by applying a control signal, such as a voltage or
current, to the resistive heater 105.
[0017] The prior art discloses two different regimes of operation
for therno-optic switches: one regime in which the electrical power
is applied continuously to the heater so that the deflection
efficiency of the switch approaches a constant steady-state value
during application of the electrical power (sometimes referred to
herein as "regime I" or a "steady-state regime"), and a second
regime in which electrical power is applied in a drive pulse that
ends before a steady-state deflection efficiency is reached
(sometimes referred to herein as "regime II" or an "overdrive
regime"), such that the response time of the device is
approximately equal to the drive pulse width.
[0018] For the purpose of clarity, we specifically define a device
to be operating in the steady state regime when the change in
deflection efficiency of the device exceeds 90% of the maximum
deflection efficiency change that occurs as a result of a specific
control pulse for at least one-half the length of the control
pulse. Contrarily, a device is specifically operating in the
overdriving regime when the change in deflection efficiency of the
device exceeds 90% of the maximum deflection efficiency change that
occurs as a result of a specific control pulse for less than
one-half the length of the control pulse, and is not otherwise
operating in a third regime, the "near-impulse response regime,"
which is defined elsewhere in this document.
[0019] FIG. 1B illustrates the amplitude of the control signal over
time for a switch operated in the steady-state regime. FIG. 1C
illustrates the resulting deflection efficiency response of the
switch. As shown in FIG. 1B, in steady-state operation of the
switch, the control signal, for example a voltage or current, is
applied to the resistive heater 105 of the TO device 100, causing
the heater to inject thermal energy into to optical path, thereby
increasing the temperature of the material in the optical path 130
near the resistive heater 105, forming an index-modified region
125. During steady-state excitation shown in FIG. 1C, the
temperature of the core 115, as well as the low power deflection
efficiency of the device, asymptotically approaches a steady-state
maximum value. The deflection efficiency of a device is defined
herein as the percentage of optical energy that was originally in
the optical path 130 that is diverted from the optical path 130 as
a result of switch activation. With reference to deflection
efficiency, low power implies non-saturation of the deflection
efficiency response; i.e., the index of refraction does not exceed
the critical index of the device during the pulse so that the shape
of the deflection efficiency response is similar to that of the
index response. Once the device reaches steady-state, the
deflection efficiency and thermally-induced refractive index do not
change until the control signal changes. Typical switch rise and
fall times reported for switches operated in the steady-state
regime in a polymer material system are on the order of 0.5-9
ms.
[0020] In the second regime (II) of operation for thermo-optic
devices disclosed in the prior art has been referred to as
("overdriving"), an electrical energy pulse applied to the optical
heater ends before a steady state optical response is reached. FIG.
2A illustrates a control signal operating a TO switch in the
overdrive regime, and FIG. 2B illustrates the deflection efficiency
response. Referring to FIG. 2B, the deflection efficiency of the
device operated in this regime continues to increase during the
entire time that the electrical drive pulse shown in FIG. 2A is
applied. The deflection efficiency never saturates so that the
device never reaches a steady state; thus, the response time from
the start of the drive pulse to the peak deflection efficiency is
approximately equal to the pulse width. The thermo-optic response
to heat pulses in this regime has been analyzed by several authors,
and Nishihara et al disclose an approximate expression to calculate
the transient surface temperature for pulsed operation in Optical
Integrated Circuits, New York: McGraw-Hill, 1989. Typical switch
response times reported for thermo-optic switches operated in the
overdrive regime are on the order of 75-200 .mu.s in polymer
material systems.
[0021] Some applications, such as fiber-optic routers for
communications signals and optical displays, require faster rise
times than can be obtained with the prior art operating in the
first two regimes. Commonly assigned Bischel et al. U.S. Pat. No.
5,544,268 for "Display Panel with Electrically-Controlled
Waveguide-Routing", describes two-dimensional addressable
electro-optical switch arrays used to provide flat panel video
displays. In these devices, fast switch rise times are required in
order to sequence through an entire row of switches at a rate
appropriate for display applications. By incorporating the
invention described herein, faster responses can be achieved
compared to methods discussed in literature.
SUMMARY OF THE INVENTION
[0022] It is an object of this invention to provide a method for
minimizing the rise time of a thermo-optic (TO) device while
simultaneously maximizing the device lifetime. Roughly described,
this object is achieved in part by operating the device in a third
regime (III) of operation not before disclosed. In this regime,
sometimes referred to herein as the near-impulse response regime,
the drive pulse width is reduced to a value that is less than two
times the diffusion time of the switch so that the drive pulse has
a width that is less than the rise time of the steady state optical
response and less than the rise time of the deflection efficiency
response of the applied over-drive pulse. By comparison, the width
of the drive pulse in the second regime is approximately equal to
the response time of the deflection efficiency to the applied drive
pulse. Various other techniques can also be used to help reduce the
switch rise time. In another embodiment of this invention, a
sustaining pulse is combined with an initial pulse in regimes I or
II, in order to extend the ON time of the device while maintaining
a fast rise time and maximizing the device lifetime.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] The invention will be described with respect to particular
embodiments thereof, and reference will be made to the drawings, in
which:
[0024] FIG. 1A symbolically illustrates a guided wave thermo-optic
(TO) switch, with isothermal contours showing the temperature
distribution during operation of the switch.
[0025] FIG. 1B is a graphical representation of an electrical
energy control signal for driving the TO switch shown in FIG. 1A,
in accordance with the steady-state regime.
[0026] FIG. 1C shows the deflection efficiency response of the TO
switch shown in FIG. 1A, in response to the FIG. 1B electrical
energy control signal, in accordance with the steady-state
regime.
[0027] FIG. 2A is a graphical representation of an electrical
energy control signal for driving the TO switch shown in FIG. 1A,
in accordance with the overdrive regime.
[0028] FIG. 2B is a graph of the deflection efficiency response of
the FIG. 1A TO switch, in response to the FIG. 2A electrical energy
control signal, in accordance with the overdrive regime.
[0029] FIG. 3A is a graphical representation of an near-impulse
control signal applied to the switch in FIG. 1A.
[0030] FIG. 3B is a graph of the deflection efficiency response of
the FIG. 1A TO switch, in response to the FIG. 3A electrical energy
control signal.
[0031] FIG. 3C is a graph of the deflection efficiency response of
the FIG. 1A TO switch, as a function of the pulse width of the
control signal for a TO device.
[0032] FIG. 4 symbolically illustrates a TO switch in accordance
with aspects of the present invention.
[0033] FIG. 5 symbolically illustrates a second embodiment of a TO
switch in accordance with aspects of the present invention.
[0034] FIG. 6A is a graphical representation of an electrical drive
pulse in accordance with aspects of the present invention.
[0035] FIG. 6B is a graphical representation of the
thermally-induced refractive index response expected from the FIG.
6A control pulse.
[0036] FIG. 6C is a graphical representation of deflection
efficiency response of a saturated TO switch expected from the FIG.
6A control pulse and corresponding to the refractive index response
in FIG. 6B.
[0037] FIGS. 7A and 7B illustrate different embodiments of TO
switches incorporating a cladding layer which also acts as an
electrically conductive layer.
[0038] FIG. 7C illustrate an embodiment of a TO switch in which the
at least a portion of waveguide core functions as a heating element
also.
[0039] FIGS. 8A, 8D & 8F show a series of graphs of excitation
conditions along a common time base.
[0040] FIG. 8B shows the index of refraction response corresponding
to the excitation conditions shown in FIG. 8A.
[0041] FIGS. 8C, 8E and 8G show the thermo-optic deflection
efficiency responses produced, using an electrical drive shown in
FIGS. 8A, 8D and 8F.
[0042] FIG. 8H shows the deflection efficiency response measured
for a device driven with several different sustaining pulse widths,
using an electrical drive similar to that shown in FIG. 8A.
[0043] FIG. 9 illustrates a portion of a visual display device
which employs a matrix of TO switches incorporating aspects of the
invention.
[0044] FIGS. 10A and 10C are schematics of electrical drive
circuits used to generate the electrical drive pulse signals
according to aspects of the invention.
[0045] FIG. 10B is a schematic of a electrical switch array
connected to the electrical drive circuit of FIG. 10A or 10C.
[0046] FIG. 11 shows voltages and electrical current flow in
certain circuit components in FIG. 10A in the generation of the
drive pulse shown in FIG. 8D.
DETAILED DESCRIPTION
[0047] A. Overview
[0048] An objective of the present invention is the creation of a
very fast rise time in a thermo-optic (TO) switch device in part
with the use of a specially-designed electrical drive signal, in
accordance with the near-impulse regime (III) of operation for
thermo-optic switches. In this regime, the drive pulse width is
reduced to a value that is less than two times the diffusion time,
as defined below. For a given pulse energy, this regime of
operation produces the fastest rise time without stressing the
device with high temperatures, thereby maximizing the device
lifetime.
[0049] As used herein, the deflection efficiency of a device is the
efficiency of deflection of optical energy out of a first optical
path. In a two-port device, such as a modulator, the deflected
energy is absorbed or otherwise lost outside the first optical
path. In a three-port switch, the deflected energy may be
redirected into a second optical path or to an application
structure during operation of the switch. During pulsed driving of
switches in regimes II or III, the change in deflection efficiency
has a distinct maximum, hereby referred to as the maximum
deflection efficiency change, which may be less than 100%
efficiency.
[0050] The response time of a device is the time from the start of
a drive pulse to the time at which the maximum deflection
efficiency change occurs, in response to that drive pulse. The
response time is clearly defined for devices operating in regimes
II and III, although not clearly defined for devices operating in
regime I, as the maximum efficiency change does not occur as a
specific point in time.
[0051] The diffusion time of a device is the deflection efficiency
response time of a device to an impulse drive (e.g. a delta
function). Although the response time for a device depends on the
length of the drive pulse, the diffusion time does not. In
determining the diffusion time, the switch should be driven at a
low enough energy so that the maximum deflection efficiency change
can easily be determined and the switch is not operating in
saturation; i.e., the index of refraction does not exceed the
critical index of the device during the pulse so that the shape of
the deflection efficiency response is similar to that of the index
response.
[0052] The rise time of the switch is the length of time required
for the switch optical deflection efficiency to change from 10% to
90% of the maximum efficiency change. As the prior art is not
consistent in the distinction between usage of "rise time" and
"response time," the definitions described herein are used in
describing the prior art.
[0053] In the near-impulse regime (II), the drive pulse, by
definition, has a width that is less than twice the diffusion time.
Alternatively, regime III may be approximated by a drive pulse with
a width that is measurably less than the response time (e.g., less
than 90% of the response time) to the drive pulse, or by a drive
pulse that is less than four times the rise time of the deflection
efficiency change resulting from the drive pulse. By comparison,
the width of the applied drive pulse in the second regime is
approximately equal to the response time of the deflection
efficiency to the applied drive pulse. In the second (overdriving)
regime (II), the response time of the deflection efficiency
response varies linearly with the drive pulse width, while in the
third (near-impulse) regime, the rise time of the deflection
efficiency response approaches a constant value (the impulse
response rise time) with decreasing pulse width.
[0054] The 3-layer TO device shown in FIG. 1A has at least two
ports, an input port and an output port. In the case of a single
output port, the TO device is a modulator. Other embodiments
include two or more output ports, and the device controllably
deflects optical energy from a first optical path including the
first output port, to a second optical path including a second
output port. The term "switch", as used herein, is intended to
cover both devices having one output port as well as devices having
two or more output ports.
[0055] With reference to a typical 3-layer TO device shown in FIG.
1A, an electrical drive signal (also called a control signal) is
applied to a resistive heater 105 adjacent to a cladding layer
110.
[0056] In an embodiment of the present invention, the electrical
drive/control signal is applied over a short period of time
compared to twice the diffusion time of the thermal energy into the
core layer 115 of the TO switch device. In order to make the
diffusion time a measurable parameter, it is defined in terms of
the deflection efficiency response change, the change in the
efficiency of deflection out of a first optical path in response to
a drive signal.
[0057] FIG. 3A illustrates a near impulse drive pulse and FIG. 3B
illustrates the deflection efficiency response of a TO switch in
response to the impulse. Referring to FIG. 3B, the diffusion time
(ED) 305 of a TO switch is defined to be the length of time for the
efficiency to reach its maximum change 310 when driven by an
impulse 320 of electrical energy as shown in FIG. 3A. An impulse
signal 320 is an idealistic electrical pulse having a finite energy
but a temporal pulse width that approaches zero and therefore a
pulse amplitude that approaches infinity. The diffusion time is
measured from the start time of the impulse drive pulse. In one
embodiment of a TO switch, the diffusion time is on the order of 18
.mu.s. The rise time 315 of the impulse response, which is measured
from the time the deflection efficiency reaches 10% of its maximum
change to the time that it reaches 90% of its maximum change, is on
the order of 8 .mu.s. Note that both the diffusion time and the
impulse response rise time make reference to the "maximum change"
in deflection efficiency. This is the maximum change that the
switches will achieve in response to the applied pulse, and may be
less than the maximum deflection efficiency change that the device
can ever achieve. Note also that the diffusion time, as well as the
impulse response rise time, are parameters of the switch structure
and do not depend on the applied drive signal for a nonsaturated
device.
[0058] The impulse response of a device, for example, the
deflection efficiency response of the switch, corresponds to the
shortest possible rise time for a given energy. FIG. 3C is a plot
of the deflection efficiency response time of a sample switch as a
function of applied pulse width. It can be seen that the response
time of the device approaches a constant value as the applied pulse
width approaches zero (an impulse drive). The impulse response time
of the deflection efficiency response for a TO device can therefore
be estimated by driving the resistive heater with successively
shorter constant-energy pulses, plotting the optical rise time, and
extrapolating to the rise time for a zero pulse width. An impulse
drive function can therefore be approximated by a pulse width that
has essentially the same deflection efficiency response as a true
impulse drive pulse. For example, the response time of the impulse
response shown in FIG. 3C is 35 .mu.s. Since the response time for
the device tested does not decrease significantly for a pulse
widths less than 5 .mu.s, a pulse width of 5 .mu.s or less can be
used to approximate an electrical drive impulse for the purposes of
determining diffusion time (.tau..sub.D). The time from the start
of this approximate impulse function to the peak efficiency of the
TO device may therefore be taken as the diffusion time of the
device. The impulse response rise time of the device can be
determined in a similar manner, by measuring the time required for
the deflection efficiency to increase from 10% to 90% of the
maximum value in response to the electrical drive impulse.
[0059] Although the fastest possible rise time for a TO switch
device operating at a given energy is created by applying an
impulse electrical drive signal, this is not practical in a real
device. For drive pulse widths that are less than twice the
diffusion time (regime IfI), many of the advantages of an impulse
drive can be achieved. Therefore, the preferred drive pulse width
for the device measured in FIG. 3C is a pulse of 36 .mu.s or less.
In the regime of operation discussed, the refractive index
gradients in the optical path never reach a steady-state condition.
For this reason, such operation is referred to as dynamic. Although
the short pulse width of the electrical drive pulse is necessary
for a fast rise time, the optical switching efficiency induced by
such a pulse is necessarily transient.
[0060] Faster rise times can generally be achieved for a particular
pulse width by increasing the drive energy. For devices that can
withstand the higher temperatures and thermal gradients associated
with higher energy densities, this approach may be attractive,
particularly for devices that saturate.
[0061] In the dynamic operation of a thermo-optic device, whether
the deflection efficiency is increasing or decreasing with time
depends on the balance of thermal energy flow into the optical path
(from the resistive heater) and away from the optical path
(primarily into the substrate). In response to the end of a drive
pulse, the thermal energy flow from the resistive heater also
terminates simultaneously; however, there is an inherent delay time
(Td) before the temperature in the optical path is affected. This
nonzero delay time (Td) results from the nature of the diffusion
process, in which thermal energy flow depends on thermal gradients;
even though energy flow from the heater stops, a substantial amount
of energy is stored in the volume between the heater and the
optical path. Referring to FIG. 3C, the response time
asymptotically approaches a linear function as the drive pulse
width increases into regime II. The asymptote for the response time
in FIG. 3C is equal to the response time plus the delay time, which
in this case is equal to 10 .mu.s. Thus in regime II, the response
time only approximately equals the control pulse width; more
precisely, the response time equals the control pulse width plus
the delay time. It is understood that references to this
approximate relationship imply the more precise description defined
herein.
[0062] The electrical drive pulse is not constrained to be a
rectangular pulse with a constant amplitude. In fact, a
varying-amplitude pulse may be preferred for some applications, and
may be decreasing or have a dip during the pulse. The width of
varying-amplitude drive pulses is defined to be the full-width at
half-maximum (FW of the power drive pulse, or the FWHI of the
square of the control pulse. Drive pulses may have a dip that
extends below the half-maximum level, providing that the length of
time that the pulse amplitude is less than the half-maximum level
is less than twice the diffusion time of the device. For varying
amplitude pulses, an average power amplitude of the drive pulse is
defined as the root-mean-square (RMS) value of the control pulse,
for example a current or voltage, during the pulse width interval
defined here. The pulse width and average amplitude are defined for
pulses in all of the drive regimes (I, II, or III). The average
amplitude of the pulse during its width is set such that the pulse
energy achieves a desired deflection efficiency, preferably greater
than 90%. In polymer devices with dimensions of 10 .mu.m.times.300
.mu.m as described herein for example, pulse energies less than 3
.mu.J are typical.
[0063] B. General Structure
[0064] FIG. 4 illustrates a switch structure which may be used to
implement the invention. As shown, a device 400 consists of an
optical stack and an electrical layer that creates switching in the
stack. The optical stack includes a lower cladding layer 420 on a
substrate 445, a core layer 415, and a top cladding layer 410. The
electrical layer includes a resistive heater 405 that injects a
thermal pulse into the stack, causing switching to occur, and a
pair of conductors 450 that deliver electrical current to the
resistive heater. The resistive heater, together with the portion
of the waveguide stack adjacent the heater that has an increased
temperature and altered refractive index when the thermal pulse is
injected, forms the thermo-optic switch. Light enters the device
through a first optical path 430, which is preferably defined by a
channel waveguide 425. It should be noted that the application of a
change in temperature to some materials (an increase or decrease of
heat) may alter the refractive index by either causing the
refractive index to increase or decrease. This alteration of
refractive index is dictated not only by the choice of material,
but by the architecture of the device in question. Hence in some
devices, the application of heat may turn off the switch, and the
removal of the heat may cause deflection to occur. As used herein,
the change in deflection efficiency produced in response to a drive
signal is always expressed as a positive number. If the deflection
efficiency of a given TO switch increases in response to a drive
signal, then the deflection efficiency change represents the amount
by which the defection efficiency increases. If the deflection
efficiency of a given TO switch decreases in response to a drive
signal, then the deflection efficiency change represents the amount
by which the deflection efficiency decreases.
[0065] Returning to FIG. 4, at the thermo-optic switch, a second
optical path 440, illustrated in the same plane as the core,
directs light away from the first optical path 430. When the switch
is off, light travels along the first optical path 425 to the
output 435 of the device; when the switch is on, light in the first
optical path 425 is diverted to the second optical path 440. The
portion of light that is diverted from the first optical path 425
to the second optical path 440 is defined as the deflection
efficiency of the switch. Since the switch is preferably operated
in regime III, the application of an electrical drive signal to the
switch will turn the device on, although the switch will not
necessarily be on during that application. In a similar manner, the
termination of an electrical drive signal will cause a device to
turn off, although the switch may be on and even increasing in
efficiency after the electrical drive signal has terminated.
[0066] In the embodiment illustrated in FIG. 4, light is confined
within the core layer 415 of an optical polymer stack, which is
comprised of a lower cladding 420, the core layer 415, and a top
cladding 410 deposited on a substrate 445. The substrate 445 may be
any material of sufficient thickness to support the cladding, and
may consist of polymer, glass, or a semiconductor. The lower
cladding 420, optionally, may be the same as the substrate 445 and
thus would not require deposition. The core layer 415 has a
refractive index that is larger than both the top and lower
claddings, forming a planar waveguide that confines the optical
beam within the core layer 415. The thickness of the core layer 415
is set so that the waveguide preferably supports only a single
mode, and the thickness of the top cladding layer 410 is set so
that resistive heater 405 or any other materials deposited above
the top cladding layer do not cause appreciable optical loss in the
device.
[0067] The substrate 445 may comprise polymer, glass, ceramic,
metal or other material with mechanical support stability that
allows subsequent layers to be fabricated on the substrate. For
applications requiring flexibility, polymer substrates may be
preferable to rigid substrates. In one embodiment, the substrate is
composed of transparent polymer The lower cladding 520, core 525,
and top cladding 530 layers are then sequentially deposited onto
the substrate through a succession of spin coating and curing steps
that include but are not limited to UV or thermally-induced
hardening, solvent removal, and vacuum drying. Depending on the
material, other means of fabricating optical stacks can be used
including wet and dry roll coating or lamination. The materials are
fully cured and minimally interacting such that deposition of
subsequent layers does not destroy or alter the optical,
mechanical, or other properties of adjacent layers. The resistive
heater is deposited and patterned to the desired shape using
standard photolithography techniques such as sputtering and wet or
dry etching or laser ablation.
[0068] Layer thicknesses for the top cladding 410 and core 415 less
than 2 .mu.m are desirable in order to minimize the diffusion time
from the resistive heater 405 to the core 415, thereby also
minimizing the device rise time. Because of the need for a thin
core layer 415, the refractive index difference between the core
and cladding layers is preferably at least 0.02. Similarly, the
desire for a thin top cladding layer puts an additional preference
on the difference in refractive index between the core and top
cladding to be at least 0.05 to prevent absorption or scattering of
the optical mode from the resistive heater elements.
[0069] In addition to the index requirements, optimization of the
stack for fast switching puts additional constraints on the
materials, particularly for the top cladding 410. In particular,
the rise time is approximately inversely proportional to the
thermal diffusivity .kappa.=K/.rho.Cp. Therefore, a lower heat
capacity Cp, a lower density .rho., and a higher thermal
conductivity (K) lead to faster device rise times. With other
device parameters, such as top cladding thickness, held constant, a
material with the maximum .kappa. will minimize the rise time of a
TO device.
[0070] When devices are driven with short pulses as described
herein, the temperatures of the top resistive heater 405 and the
top cladding 410 become much hotter than when driven with longer
pulses associated with the steady-state and overdrive regimes.
Thus, the top cladding 410 and resistive heater 405 must be able to
withstand short, high temperature, thermal pulses without
degradation of the device. The resistive heater 405 must adhere
well to the top cladding 410 without delaminating, and the
resistive heater film should be formed in a manner that minimizes
stress in the metal layer. The top cladding 410 must be able to
expand upon application of the thermal pulse without cracking or
damaging the resistive heater 405.
[0071] One way to accomplish this is to use stack materials that
comprise polymers such that the device is maintained and operated
at temperatures above the glass transition (Tg) of at least one of
the stack components. For applications where the operating
temperatures are close to room temperature (23.degree. C.), it may
be desirable to use polymers with Tg's less than 0.degree. C. When
such polymers are used and operated sufficiently above their Tg
they operate in a plastic regime and as such undergo minimized
stress induced index anisotropies, aging, embrittlement,
density/volume changes, and index changes resulting from repetitive
switch excitation. It will be noted that these degradation
mechanisms can occur independently or in combination, and lead to
premature device failure resulting from increased optical insertion
loss, delamination, or cracking.
[0072] In this embodiment it is preferential for the layers closest
to the resistive heater 405, specifically the upper cladding layer
410 and preferably also the core 415, to be composed of optically
transparent cross linked or cross linkable homopolymers and/or
copolymers of monomers from the classes of urethane, siloxanes,
acrylates, fluoroelastomers, alkenes, dienes, ayrlates,
methyacrylics, methacrylic acid esters, vinyl ethers, vinyl esters,
oxides, and esters or perhaps other polymers that possess
tailorable Tg's, optical transparency, and cross linking, with the
addition of appropriate cross linking agents as required. In
addition, one or more stack components may comprise non-cross
linked polymers.
[0073] Placing the resistive heater adjacent to the lower cladding
is a variation in the structure that produces essentially the same
design. However, this embodiment changes the thermal flow patterns.
If the resistive heater for a polymer device is deposited directly
on a higher thermal conductivity substrate, most of the thermal
energy flow will be towards the substrate, increasing the drive
energy requirements for the switch to reach a given efficiency. For
this embodiment, the heater would preferably be deposited on a low
thermal conductivity substrate (such as Mylar or Kapton), or on a
low thermal conductivity layer (such as another polymer layer)
covering a higher thermal conductivity material (such as glass).
This approach will reduce the required energy to drive the switch
to a given efficiency, although still requiring about twice the
energy of a switch having a resistive heater that is superposed
only covered by a gas (such as nitrogen or air) or another low
thermal conductivity layer on one side.
[0074] A waveguide is any structure which permits the propagation
of optical energy throughout its length despite diffractive
effects, and possibly curvature of the guide structure. An optical
waveguide is defined by an extended region of increased index of
refraction relative to the surrounding medium. An optical channel
waveguide 425 defines the optical path 430 within the plane of the
substrate and may be formed by any definition process found in the
art, such as a rib, index, or strip confinement. In an embodiment
fabricated in a polymer material system, the channel waveguide may
be fabricated by any technique found in the art, including rib
confinement with trench or top hat shapes, index-confinement, or
strip confinement. Rib confinement corresponds to a thicker core
layer in the region of the waveguide, possible methods of
fabrication include, but are not limited to dry etching, wet
etching, laser ablation, selective layer growth, or swelling. Index
confinement techniques include photo definition and diffusion. In
alternate material systems such as lithium niobate, diffusion
processes such as annealed proton exchange (APE) or titanium in
diffusion may be used. Strip confined structures consist of index
differentials in a cladding layer adjacent to the core. For
example, GaAs strip waveguides are made by employing layers of GaAs
and AlGaAs of a lower refractive index.
[0075] It should be noted that some of the channel waveguide
fabrication processes, such as diffusion in lithium niobate, serve
to confine the mode in two dimensions. These channel waveguides can
be fabricated within a separately-fabricated planar waveguide as
described according to the preferred embodiment. However, an
alternate embodiment uses the two-dimensional confinement
waveguides fabricated directly in the substrate, without a
separately-fabricated planar waveguide. If the core material is
restricted entirely to the channel waveguide then the core layer
thickness is the depth of the index perturbation that forms the
waveguide. Trench structures fabricated in the substrate may also
be used.
[0076] The width and effective index difference between the channel
waveguide and the surrounding area are set so that preferably only
a single lateral mode is supported in the channel. For waveguides
characterized by abrupt index discontinuities such as those formed
by rib-, trench- or strip-confinement, the mode cutoff for
different modes can be calculated using the effective index method,
as described by Nishihara et al in Optical Integrated Circuits, New
York: McGraw-Hill, 1989.
[0077] For more complicated structures having graded-index
profiles, a numerical calculation of the mode cutoffs is preferable
using one of a number of commercially available software packages.
However, while single mode operation is preferred to minimize the
complexity of the device, this invention does not require it.
[0078] In general, the resistive heater 405 (FIG. 4) may be formed
by any material that can be deposited in a thin enough layer to
meet both the heat storage and resistance requirements of the
device. In addition, the bulk electrical resistivity of the
resistive heater material must also be significantly smaller,
typically by at least an order of magnitude, than that of the top
cladding layer 410, throughout the desired thermal operating range
of the device. For example, the resistive heater could consist of
metals such as aluminum, nickel, chromium, gold, titanium, copper,
etc., conductive paint, conductive epoxy, semiconducting material,
or optically transparent materials such as oxides of indium and
tin.
[0079] The resistive heater 405 (FIG. 4) in one embodiment is
formed of a resistive material like NiCr that has been deposited on
the top cladding 410 using standard techniques such as sputter or
evaporative metal deposition. The resistive heater 405 is patterned
to the desired shape using standard processes such as
photolithography, wet or dry etching, or laser ablation.
[0080] Low resistance conductor elements 450 are connected to the
ends of the resistive heater 405. Preferably, the conductor
material has a lower resistivity than the resistive heater
material, and may for example be gold, silver, copper, or aluminum,
deposited and patterned by any of the techniques known in the art.
The conductor dimensions should be of sufficient size that the heat
generated in the conductor due to the activating current flow is
significantly lower than that generated in the resistive heater.
Typically, a connector 460 is used to make electrical contact
between the conductors 450 and the electrical drive circuitry
455.
[0081] The resistive heater width, which for example might be in
the range of 3 to 15 .mu.m, is set by a tradeoff between a number
of conflicting desires. The desire for a low power device makes a
narrower resistive heater preferable. However, lithography becomes
more complex and expensive at narrow widths. In addition, narrower
widths tend to require the resistive heater to be heated to a
higher temperature in order to maintain a given switch efficiency,
potentially decreasing the device lifetime.
[0082] An upper constraint on the thickness of the resistive heater
is set by the heat storage in the resistive heater material. A
large heat storage capacity in the resistive heater will cause the
temperature at the surface of the device to be low, which implies a
low rate of heat flow through the top cladding layer 410. In order
to prevent the resistive heater dimensions from significantly
impacting the rise time of the device, the heat stored in the
resistive heater 405 should be significantly less than the heat
stored in the top cladding 410 at a given temperature:
.rho..sub.ECP.sub.EX.sub.E<0.5 .rho..sub.TCCp.sub.TCX.sub.TC
(eq. 1)
[0083] where .rho..sub.E is the density of the resistive heater,
Cp.sub.E is the heat capacity of the resistive heater, X.sub.E is
the thickness of the resistive heater, .rho..sub.TC is the density
of the top cladding, Cp.sub.TC is the heat capacity of the
resistive heater, and X.sub.TC is the thickness of the top
cladding. For typical polymer materials used in the preferred
embodiment, .rho..sub.TC=1.0 g/cm.sup.3 and Cp.sub.TC=1.8
J/g.degree. C., giving a volume heat storage capacity of 1.8
J/cm.sup.3.degree. C. in the top cladding. For a typical NiCr
composition, .rho..sub.E=8.0 g/cm.sup.3 and Cp.sub.E=0.45
J/g.degree. C., giving a volume heat storage capacity of 3.6
J/cm.sup.3.degree. C. in the resistive heater. For a top cladding
thickness X.sub.TC less than 2 .mu.m, the NiCr resistive heater
thickness therefore is preferably less than 0.5 .mu.m. Although
this constraint optimizes the device, it should not be construed as
a design requirement.
[0084] The choice of the resistive heater material is constrained
by the desired operation voltage and current for the switch.
Assuming a TIR switch that requires a drive energy density of 100
pJ/.mu.m.sup.2 to achieve efficient switching, a switch width of 10
.mu.m, a length of 460 .mu.m, and a drive pulse width of 10 .mu.s,
a peak drive power of 46 mW might be used. In order to operate this
device at a 5 volt drive, a switch resistance of 540 .OMEGA. is
required. For the switch dimensions assumed, this device resistance
implies a sheet resistance Ps of 12 .OMEGA. per square. For a NiCr
resistivity of 1.8 .OMEGA..mu.m, a thickness of 0.15 .mu.m meets
the target, which is consistent with the maximum thickness
determined previously.
[0085] The switch length depends on the type of optical device. In
one embodiment, the switch is of the total internal reflecting
(TIR) type, in which the electrode is laid out at an angle to the
waveguide as described by Bischel et al in U.S. Pat. No. 5,544,268.
In this device, the electrode length L is at least long enough to
bisect the entire waveguide mode full width 2.omega..sub.o:
L.gtoreq.2.omega..sub.i/sin(.theta.) (eq. 2)
[0086] where .theta. is the angle of incidence of the switch with
the waveguide. As an example, a mode width of 4 .mu.m and an angle
of incidence of 1.degree. require the electrode length to be at
least 460 .mu.m.
[0087] FIG. 4 symbolically illustrates a TIR switch device. In this
device, the first optical path 430 is defined by a channel
waveguide 425 as described herein. The second optical path 440 is
not defined by a channel waveguide per se but a region in which the
beam deflected by the switch travels toward an output, confined at
least to the core layer by the planar waveguide structure. The
output might be an out-of-plane mirror that reflects the beam out
of the core, the edge of the substrate, or a pit filled with an
absorbing material such as a pixel in a display. An output close
enough to the switch to prevent significant divergence of the
switched beam traveling along the second optical path does not
require a waveguide to confine the second optical path. In
particular, a waveguide that intersects the waveguide forming the
first optical path is not required. This is an important aspect of
devices having a large number of switches and therefore requiring a
low loss per switch, as described in Bischel et al. Alternatively,
some devices may utilize a single- or multi-mode confining
structure along the second optical path in order to maintain a
specific mode size at the output. The confining structure can be
created by the same process that forms the channel waveguide on the
first optical path, or it may be formed by a separate process step
that creates a different index change to confine the beam.
[0088] An alternate embodiment for the fast TO switch is a
directional coupler or two mode interferometer, such as that
described by C. C. Lee et al, "2.times.2 single-mode zero-gap
directional-coupler thermo-optic waveguide switch on glass,"
Applied Optics, vol. 33, No. 30, Oct. 20, 1994, pp. 7016-7022. The
required electrode length for such a device is determined by the
desired drive voltage. Typically, directional coupler lengths range
from a few millimeters to over a centimeter.
[0089] Other embodiments for the fast TO switch include digital
optical switch, Mach-Zehnder modulators, waveguide interrupters,
and other optical switches that rely on a thermally-induced change
in index.
[0090] C. Two-sided Resistive Heater
[0091] FIG. 5 shows another embodiment of a switch device. It
comprises a substrate 510, a lower electrode 550 that is adjacent
to the lower cladding layer 520 and the substrate, a core layer
525, an upper cladding 530, and an upper electrode 535.
[0092] The structure is formed by depositing a resistive metal of a
desired thickness (typically about 0.1 .mu.m for metals such as
Ni:Cr) by evaporation or sputtering on the substrate (or on a low
thermal conductivity layer overlying the substrate). The substrate
preferably comprises of polymer, polymer-coated glass, or other
thermally insulating material. It is preferable for the thermal
conductivity of the substrate or thermally insulating material to
be equal to or less than that of the lower cladding so that thermal
energy preferentially travels into the optical stack during switch
excitation. Glass, ceramic, and other rigid substrates will also
suffice but are preferably coated with a polymer layer to act as a
thermal diffusion barrier. In some applications the substrate may
serve as the lower cladding layer. The resistive heater is
-patterned to the desired shape using standard photolithographic
techniques such as etching or laser ablation. In the preferred
embodiment the optical stack is composed of layers of transparent
polymer materials. The lower cladding 520, core 525, and top
cladding 530 layers of thickness 1.5 mm, 1.0 mm, and 1.5 mm
respectively, are sequentially deposited onto the
electrode-patterned substrate through for instance a succession of
coating and curing steps as previously described. Depending on the
material, other means of fabricating optical stacks can be used
including spinning, wet and dry roll coating, or lamination. The
materials are fully cured or minimally interacting such that
deposition of subsequent layers do not destroy or alter the
optical, mechanical, or other properties of adjacent layers. A
metal layer is deposited on the top cladding layer 530 and
patterned to the desired resistive heater 535 shape, aligned to the
structures located below the waveguide.
[0093] The top and bottom resistive heaters 535, 550 are connected
to electrical drive circuitry 560 that allows nearly simultaneous
(or simultaneous) electrical excitation of the resistive heaters.
The drive circuitry is connected to contact pads 540 located at the
ends of the resistive heaters that allow individual excitation of
the resistive heaters 535, 550. In addition, the contact pads 540
enhance the mechanical stability of an otherwise thin (.about.0.1
.mu.m) resistive heater. Note that in general, there is no
requirement that the contact pads 540 be located at "ends" of the
resistive heater material, only that they be located at different
positions on the resistive heater material so that current flow
through the heater material will create the desired thermal
patterns in the optical stack. Connection of the lower resistive
heater 550 to the drive electronics is accomplished by selectively
etching, ablating, or dissolving the optical stack material from
regions near the contact pads 540. The resulting access holes 570
to the lower-contact pads are typically referred to as electrical
vias. In some applications it may be preferable to electrically
connect top and bottom resistive heaters. This type of connection
may be accomplished by depositing electrical bus lines that connect
resistive heater ends serially or in parallel.
[0094] When the resistive heaters of FIG. 5 are excited with
electrical energy, the core layer 525 experiences an increase in
temperature. Compared to single-resistive heater device
architectures (as in FIG. 4), a dual-resistive heater architecture
may produce a higher temperature in the core layer upon switch
activation, without generating excessively high temperatures at the
heater-cladding boundary that would otherwise cause device failure
resulting from resistive heater fracture or delamination.
Specifically, the dual resistive electrode device shown in FIG. 5
will produce a higher temperature in the core when each resistive
heater is energized with electrical energy E (total energy 2E) than
a essentially similar single-heater device excited with a total
energy E. In practice, if equal amounts of electrical energy are
used to excite top and bottom resistive heaters, the top resistive
heater 535 will transfer more thermal energy into the stack than
the bottom resistive heater 550 because the thermal energy from the
lower resistive heater 550 will also travel into the substrate 510.
The ratio of thermal energy deposited into the stack compared to
the substrate is approximately equal to the ratio of thermal
conductivities (K.sub.stack/K.sub.sub) of the stack and substrate
510. Alternatively, additional electrical energy can be supplied to
the lower resistive heater 550 to compensate for the loss of
thermal energy into the substrate 510.
[0095] As previously described, a response time less than 50 .mu.s
can be achieved with a single-sided resistive heater. The
alternative approach of a two-sided heater reduces the peak
temperature in the polymer cladding layers, reducing the risk of
premature failure mechanisms including resistive heater
delamination, material embrittlement, and cracking of either metal
or cladding layer. A second consequence of the dual resistive
heater architecture is lower peak temperature in the cladding
layers and a reduced (vertical) thermal gradient in the region
between the resistive heaters which may relieve stress that causes
premature aging of the optical stack materials. Reduction of these
degradation mechanisms can extend device lifetime. Further benefits
relating to device performance and longevity may arise from a more
uniform thermal gradient across the stack layers. At a minimum, the
dual electrode architecture allows lower temperatures at the
resistive heater-cladding interface without compromising switch
speed. As used herein, if electrical energy is said to be applied
to "the heater", such language will be understood to refer to the
application of electrical energy to all heaters or heater segments
in the switch structure.
[0096] D. Driving the Device
[0097] FIG. 6A illustrates an electrical drive pulse 610 beginning
at a time To and having a width 620 less than twice the diffusion
time 625 of a device (for example, that shown in FIG. 4). FIG. 6B
illustrates the resulting refractive index change 630 in the
material when the drive pulse of FIG. 6A is applied to the
resistive heater of the device. This refractive index change has a
peak np at a time period 635 following To due to the finite width
620 of the drive pulse 610, and then decreases back to zero.
Because of the narrowness of drive pulse 610, the time period 635
approaches the diffusion time 625 of the device.
[0098] FIG. 6C illustrates the deflection efficiency response 640
of the switch. It can be seen that the deflection efficiency in
this example rises to a maximum efficiency 670, remains there for a
time, then falls off asymptotically to a small value. The rise time
of the switch is defined herein to be the length of time required
for the switch optical efficiency to change from 10% to 90% of the
maximum efficiency change. In the example of FIG. 6C, since the
optical efficiency increases in response to the applied control
signal, and since the optical efficiency was zero (or very small)
prior to activation of the control signal, the rise time is
approximately equal to the length of time 650 required for the
switch optical efficiency to change from 10% to 90% of the maximum
efficiency 670. The fall time 660 of the switch is defined to be
the length of time required for the switch optical efficiency to
change from 90% to 10% of the maximum efficiency 670. The ON time
680 is also defined in terms of the maximum efficiency, equaling
the length of time that the switch efficiency is greater than 90%
of the maximum value 670. Note again that the maximum efficiency
670 may or may not equal or be near to 100% in different
embodiments.
[0099] Because of the nonlinear relationship between the deflection
efficiency response and the induced refractive index change, the
rise time of the deflection efficiency response is not necessarily
the same as that of the refractive index change. In particular,
increasing the energy in the drive pulse does not change the rise
time of the refractive index change but does increase the peak
index change and duration, and may decrease the rise time of the
deflection efficiency response, due to saturation. However, due to
the above non-linear relationship, the rise time for the deflection
efficiency response is generally faster than that of the refractive
index response.
[0100] The rise time of a device operated in regimes II or III is
generally significantly less than the fall time. For example,
although the device producing the plot of FIG. 3C may produce rise
times consistently less than 20 .mu.s, the fall time of the same
device might exceed 50 .mu.s. This phenomenon is believed to result
from the broadening and cooling of the thermal profile after the
applied pulse. Broader profiles have a lower thermal gradient,
giving rise to a smaller heat flow and therefore a slower change in
the thermal profile with time. Device rise times can be
significantly improved using the techniques described herein, but
the same techniques may not significantly impact the fall time. In
one embodiment, a fast fall time is not required. In other
embodiments, the fall time can be minimized with the use of a high
thermal diffusivity substrate and a thin lower cladding. Examples
of potential substrates, in order of increasing thermal diffusivity
are polymer, glass, semiconductor, and metal. By placing the same
restriction on the lower cladding as the top cladding, i.e., its
index is at least 0.05 less than the core, its thickness can be
reduced to less than 2 .mu.m. It should be noted that use of a thin
lower cladding and a high-diffusivity substrate may decrease the
peak index change, particularly for pulse widths in regimes I and
II for a given pulse width, effectively requiring more energy to
drive the device to a given efficiency. For applications requiring
only a fast rise time, a thicker lower cladding and lower thermal
diffusivity substrate are therefore preferred.
[0101] Dynamic operation is naturally suited to devices with
saturable deflection efficiency responses. For reference, FIG. 6B
shows the refractive index deviation response of the switch to the
electrical pulse shown in FIG. 6A. For a TIR device such as that
shown in FIG. 4, the deflection efficiency increases as a function
of the refractive index change in the switch, until the refractive
index change is larger than a critical value nc. Above nc, the
induced refractive index change diverts the maximal amount of
optical energy from the first optical path 425 to the second
optical path 430. At refractive index changes greater than this
value, the deflection efficiency response does not increase further
(FIG. 6C).
[0102] Although easier to realize useful operation in a saturable
response device, regime III operation for driving the TO switch is
extendible to devices with periodic responses with refractive
index, such as mode-interference-based TO switch devices employed
as directional couplers. TIR and directional coupler devices are
disclosed in numerous prior art documents, e.g., Nishihara et al.,
Optical Integrated Circuits, New York: McGraw-Hill, 1989.
[0103] The TO switch device is driven by an electrical drive
circuit that provides a control pulse adequately in advance of the
desired ON time, in order to synchronize the switch ON time with an
optical data stream traveling along the optical path. The
electrical drive circuit is electrically connected to the conductor
elements on the device with an electrical connection technique
known in the art.
[0104] E. Electrode Loss Optimization
[0105] Any optical beam propagating along the first optical path
through the switch will experience loss, and in practical devices,
it is desirable that this loss experienced be less than some
maximum value. The optical properties of the resistive heater
element (the refractive index and optical attenuation constant), as
well as the properties of the core and cladding materials,
contribute to the insertion loss of the switch. In general, the
optical attenuation constant for the resistive heater is higher
than that of the core and cladding layers, so that the amplitude of
the evanescent tail of the optical beam confined by the waveguide
layers, desirably, is small at the lossy resistive heater. A low
loss material, such as silver or indium tin oxide (ITO), may be
used as the resistive heater material. However, the mechanical and
electrical properties of these materials are not optimal, so
resistive metals such as NiCr are preferably used.
[0106] The thicknesses of the core and top cladding can be chosen
to tailor the amplitude of the tail of the optical beam and,
thereby, to maintain the switch loss at an acceptable level. For
applications requiring a large number of switches along an optical
path, this level may need to be as low as 0.001 dB/switch.
Preferably, the optical layers support only a single mode in the
waveguide, in which case the core layer thickness is first chosen
to be at (or near) the maximum value that supports a single
waveguide mode, as described by Nishihara et al in Optical
Integrated Circuits, New York: McGraw-Hill, 1989. The optical mode
tail amplitude decreases exponentially in the top cladding layer
away from the core, with a decay constant that depends on the
wavelength of the light, the core thickness and the refractive
indices of the core and cladding layers. The cladding layer
thickness is chosen so that the tail amplitude at the heater
element is low enough to produce a switch insertion loss that is
less than the allowed level. The thickness is not made larger than
necessary, as both the required electrical drive energy and the
diffusion time increase with increasing top cladding thickness.
[0107] FIG. 7 illustrates another aspect of the invention in which
an electrically conductive polymer cladding layer, with electrical
conductivity nominally between 1.0 and 2000 Ohm-cm, functions
simultaneously as an optical buffer layer (optical cladding) and a
heating element. Referring to FIG. 7, the conductive polymer 710 is
deposited onto the core layer 720 to a nominal thickness of 1.0
.mu.m. The thickness is selected to achieve the desired resistance
for a given switch area and polymer resistivity. The resistive
heater may have electrodes connected to the two ends along its
length (FIG. 7A) or across its width wig. 7B). The metal layer 740
is deposited on the conductive polymer 710 and patterned to control
the location of current flow though the conductive polymer 710. The
metal electrodes are made sufficiently thick such that they do not
act as significant resistive heating elements themselves when
energized. A current source 730 is connected to the electrodes 740
such that current flows between the electrodes 740 and through the
polymer 710. This design produces a TO switch with response times
faster than similar devices with resistive heaters in a separate
layer different from the top cladding because the heating element,
in this case the conductive polymer cladding layer, is closer to
the middle of the core layer 720. For example, in the embodiment
shown in FIG. 4, the upper resistive heater 405 is separated from
the center of the core 415 by approximately 2.0 .mu.m. In a similar
structure, FIG. 7, comprising a conductive cladding layer 710, the
separation between the center of the conductive cladding layer and
the center of core layer is only about 1.0 .mu.m.
[0108] Referring to FIG. 7A, a strip of conducting polymer intended
for use as a heating element with the dimensions 1.0 .mu.m.times.10
.mu.m.times.300 .mu.m and resistivity of 0.1 Ohm-cm has a
resistance of 3.times.10.sup.4 Ohm along its 300 .mu.m length. In
order to confine the flow of current along the length of the
switch, the electrically conducting polymer is patterned by a
process such as reactive ion etching, laser ablation, or wet
chemical processing, for example by lithographically patterning the
conductive polymer material to the dimensions of the desired
resistive heater. In this case, a second top cladding layer 715 may
be deposited to at least partially planarize the region in between
the resistive heaters. This second top cladding material could be
the same non-electrically-conductive materials used to create the
top cladding 410 shown in FIG. 4.
[0109] Referring to FIG. 7B, the conductive polymer may also remain
in a continuous layer, with the electrode placed adjacent to each
side of the resistive heater rather than each end. In this case,
the dimensions of the conductive polymer heating element are
defined by the volume of polymer through which current flows and
not necessarily the physical dimensions of the polymer layer. The
strip of conducting polymer with the dimensions 1.0 .mu.m.times.10
.mu.m.times.300 .mu.m and resistivity of 0.1 Ohm-cm has a
resistance of 33 Ohm across its 10 .mu.m width.
[0110] In yet another aspect of this invention an electrically
conductive transparent polymer functions as a waveguide core and
heating element simultaneously, as illustrated in FIG. 7C. In this
embodiment an optical buffer or cladding layer 750 with electrical
conductivity substantially less than the conductive polymer core
material is deposited on the substrate 755 by spin-coating to a
thickness of a few microns. The electrically conductive polymer
core layer is then preferably spin-coated to a nominal thickness of
1.0 .mu.m on the lower cladding layer. The conductive polymer is
patterned, for example using a combination of photolithography and
reactive ion etching processes to form channels or waveguides of
conductive polymer 760. The dimensions of the core layer are
preferably selected to achieve a desired resistance for a
predetermined switch length, illustrated in FIG. 7C by 770.
[0111] Electrical contact to the resistive heater 770 is preferably
made with a conductive metal such as Au, Ag, NiCr or oxide such as
indium-tin-oxide, which is deposited for example by evaporation or
sputtering onto the core and lower cladding structure, and
patterned using a combination of photolithography and wet-etching
to form electrical bus lines 765 that are ultimately used to
deliver current to portions of electrically conductive polymer core
layer. It is preferable for the electrical bus lines to be
sufficiently thick and wide to prevent resistive heating of the bus
lines during switch operation. The electrical connecter may consist
of two materials, one (such an indium tin oxide) having a lower
optical loss that is adjacent to the core layer, and a second
having high electrical conductivity to minimize electrical power
loss in delivering the electrical drive power to the resistive
heater. A non-electrically conducting top cladding material (not
shown) may be deposited on the stack at a thickness to provide
optical isolation and mechanical protection.
[0112] When energized, the control element 790 delivers current via
the electrical connections 780 to the electrical bus lines 765,
thus causing current to flow through a portion of the conductive
polymer waveguide core (resistive heater, 770) located between the
electrical bus lines. The resistive heater portion 770 experiences
a change in temperature upon electrical excitation and optical
energy propagating in the waveguide core experiences a change in
refractive index upon traveling through the activated core
region.
[0113] An alternate embodiment of the device illustrated in FIG. 7C
may be fabricated by depositing an electrical conductor layer above
the top cladding layer. In this case, at predetermined regions in
proximity to the core layer, top cladding material is selectively
removed using laser ablation or reactive ion etching to form
electrical access vias through which the electrical conductors
connect to the electrically conductive core layer. It is preferable
to fill the vias with a separate electrically conductive material,
for example another polymer that is nearly index matched to the
cladding layer to minimize perturbative effects (scattering and
absorption) caused by an electrode-waveguide proximity as discussed
earlier.
[0114] These designs produce thermo-optic switches with response
times faster than similar devices that require transport of thermal
energy through a cladding or other layer to produce a desired
response (for example FIG. 4), since the heat is generated directly
in the optical path.
[0115] Conductive polymers such as polyaniline, polythiophene,
polypyrrole or other polymers with conductivity between 1.0 and
2000 Ohm-cm are examples of transparent conducting polymers that
may potentially be suitable for this application because they may
allow devices to be powered with reasonable voltages. For example,
if 1000 switches each having a resistance of 30K-Ohm were driven in
parallel, the resistance per switch column would be 30 Ohm/column.
Applying a voltage of 54V to the electrode would result in 100 W of
peak power for the column. It will be noted that material
properties of a polymer such as refractive index can be tuned for a
desired application by blending conductive polymer into a
nonconductive polymer host to obtain a combination of target
conductivity and refractive index values. Increasing the
concentration of conducting polymer will increase the conductivity
of the polymer blend. Other benefits of this conductive polymer
approach include better stack adhesion resulting from reduced
coefficient of thermal expansion mis-match between the conductive
polymer and core polymer layer (compared to a resistive
heater-polymer interface) and potentially reduced optical insertion
loss in device architectures where the activating electrode is no
longer in proximity to the waveguide. Note that the next layer
below the electrically conductive polymer layer, whether it be the
core layer 720 or some intermediate layer, should be electrically
insulating (or should at least be so much less conductive than the
electrically conductive polymer as to render the resistive heating
effect of any electric current in this layer negligible).
[0116] It will be appreciated, therefore, that either the core
layer or a cladding layer can double as a resistive heating element
of a TO switch. It will be appreciated further that for some
embodiments, both the core and cladding of an optical waveguide can
double as resistive heating elements.
[0117] F. Sustaining Pulse Segment
[0118] For applications such as displays that require switching of
bursts of optical information within the response time of the
switch, the short optical switch response associated with a narrow
electrical drive pulse may be adequate. However, many applications
require both a fast rise time and a longer ON time than can be
achieved with a short drive pulse. It is therefore another aspect
of this invention to achieve such a response by combining an
initial short drive pulse segment with a sustaining pulse segment,
preferably of lower average amplitude than the short drive pulse
segment. The peak temperature of a device driven in this manner is
significantly lower than if the full amplitude of the initial pulse
were maintained, thereby maximizing the lifetime of the device.
FIG. 8A illustrates such a drive signal. For the signal shape
shown, the initial pulse segment is 810 and the sustaining pulse
segment is 820. As the term is used herein, the sustaining pulse
segment begins immediately at the end of the initial pulse segment
and may last for any length of time, although the drive signal may
initially be zero immediately after the initial pulse segment
ends.
[0119] A typical index response for the drive pulse in FIG. 8A is
shown in FIG. 8B. As can be seen, the index response may contain
dips because of the non-optimal drive pulse shape. For a device
operated for saturation in the ON state such as, for example, a TIR
switch, the deflection efficiency response dips may be lessened in
amplitude, as shown in FIG. 8C. If the dip in the index response
does not go below the critical index value that saturates the
deflection efficiency, then no dip in the deflection efficiency
will be observed. If the dip does go below the critical index
value, then the deflection efficiency response will have dips
similar to those shown in FIG. 8B.
[0120] This segmented-pulse drive technique (which may include a
combination of pulses) is most advantageous for initial pulse
segment widths in the near-impulse regime (III); however,
significant advantages in device speed can also be achieved from
short initial drive pulse segments that are somewhat longer, in the
overdriving regime (II). FIG. 8H illustrates the deflection
efficiency of a sample TIR TO switch driven with such a
segmented-pulse technique. X, Y and Z are deflection efficiency
response curves for three different sustaining pulse widths: 40
.mu.s for X, 100 .mu.s for Y, and 200 .mu.s for Z. In all three
cases, the initial pulse segment has a width of 10 .mu.s. In order
to minimize a dip after the initial pulse, it is desirable to have
a steadily-decreasing thermal energy addition from the short
initial pulse segment, such as by lengthening the decay time of the
initial pulse segment. FIG. 8D, for example, shows a drive pulse
having an initial segment 830 which, after a quick rise, decays
exponentially to the start of the sustaining segment 840. This
pulse has a fall time defined herein to be the length of time for
the instantaneous amplitude of the pulse segment to fall from 90%
to 10% of the difference between its maximum during the initial
pulse segment and the RMS average amplitude during the sustaining
pulse segment. The overshoot and temporal variations in switch
efficiency can be minimized with a drive pulse that smoothly decays
from an initial amplitude to a steady-state amplitude, as shown in
FIG. 8D, producing the deflection efficiency response shown in FIG.
8E. In order to achieve an optimally flat switch efficiency
response, the decay constant should be approximately equal to the
steady-state rise time associated with the sustaining pulse
segment, and the ratio of the initial amplitude to the steady state
amplitude adjusted to flatten the deflection efficiency response.
As before, the control pulse can be divided into an initial pulse
that creates a fast rise time and a sustaining pulse that creates a
long ON time. The separation between the two pulse segments is the
point at which the amplitude of the pulse is midway between the
initial amplitude and the sustaining pulse amplitude. The pulse
width of the initial pulse segment still preferably has a width
short enough to cause the device to operate in regime III.
Alternately, a non-exponentially decaying electrical control pulse
may be used to maintain the device deflection efficiency (or
refractive index response) at a constant level throughout the
sustaining pulse period.
[0121] In an alternative embodiment, the sustaining function can be
achieved with a periodically-varying amplitude. For example, as
shown in FIG. 8F, the sustaining pulse segment 860 may consist of a
series of pulses (which might be thought of for the purposes of
this description as "sub-pulses") that are individually
approximately the same shape and amplitude as the short initial
pulse segment 850, but appropriately spaced to reduce the variation
in the refractive index response during the sustaining pulse to an
acceptable level (FIG. 8G). The sustaining drive pulse has an
average power amplitude 865 that is maintained during the pulse in
order to achieve a specific deflection efficiency. In a case in
which the sustaining function is accomplished with one or more
"sub-pulses", the "sustaining pulse segment", as that term is used
herein, is intended to end together with the last time in a given
switching episode that the drive signal returns to zero or to some
other non-switched value. As above, the "sustaining pulse segment"
is considered to begin immediately after the initial pulse segment
even where, as in FIG. 8F, the drive signal is zero immediately
after the initial pulse segment ends.
[0122] G. Fast Thermo-optic Switch Operating in a Display
[0123] FIG. 9 illustrates a portion of a visual display device
according to features of the present invention, which employs a
matrix of optically energized display elements, shown here as a
3.times.3 array of discrete segments arranged in columns and rows,
with each row of display elements and respective diverting devices
disposed along an optical path. In this embodiment, the array has a
set of three optical paths, paths 904, 924 and 944. Each optical
path is capable of allowing energy from a respective optical energy
source 902, 922 or 942 to be propagated along the optical path when
the optical diverting devices in the path are in their full-off
states. In the embodiment illustrated, the optical paths are
defined by respective optical waveguides which have their own
independent optical energy sources 902, 922 and 942. However, in an
alternate embodiment, the optical paths may be supplied by a single
optical energy source.
[0124] The diverting devices serve to divert energy from the
optical path and thus depending upon the nature of the application
may compromise diverters, deflectors, diffractors, directional
couplers, Mach-Zehnder interferometers, refractors, reflectors,
switches, switched absorbers or switched detectors, and are not
limited to the examples of diverting devices described above. In an
embodiment of the invention, the visual display may comprise two or
more layers of these matrix arrays, thus forming a three
dimensional visual display.
[0125] In this arrangement, the optical energy is propagated from
an upstream position, for example an energy source 902, to a
downstream position along the optical path 904, through each of the
diverting devices 906, 908 and 910 in sequence. In this manner,
energy can be diverted to application structures 912, 914 and 916
respectively, for example, the display elements or pixels of a
display. In order to facilitate the asserting of turn-on and
turn-off signals for the diverting devices, a controller 918 is
provided which controls each of the diverting devices or switches
906, 908 and 910, such that each can be switched on at a
predetermined time in a predetermined sequence. In an embodiment,
the circuitry can be implemented in a single control logic to
provide such operation. Preferably, each column of switches is
connected together so that one switch in each waveguide is
activated simultaneously. Preferably also, the controller 918
modulates the sources 902, 922 and 942 with image data in a manner
that is timed properly relative to ON times of the optical
switches.
[0126] The controller 918 asserts the appropriate control signal to
the switch in question, specifying that the switch should turn
itself "on", and the switches, in response to the turn-on signal,
divert energy from the primary optical path at a finite time
thereafter. Depending upon the type of switch utilized, the
assertion of the control signals may be carried on separate
conductors, such that the assertion of one of such control signals
may involve increasing the voltage or current on the appropriate
conductor from a low level to a high level, or vice-versa. In an
embodiment in which the turn-on and turn-off control signals are
carried on the same conductor, the assertion of one of such control
signals may involve increasing the voltage or current on that
conductor from a low level to a high level, while assertion of the
other of such control signals may involve decreasing the voltage or
current on that conductor from a high level to a low level. In the
latter case, assertion of the turn-on control signal is synonymous
with negation of the turn-off control signal, and vice-versa.
[0127] In the arrangement shown, assuming that each device has a
faster rise time compared to its fall time, the controller 918
switches the devices on in upstream sequence, allowing downstream
switches to turn off gradually and harmlessly as each more upstream
switch is turned on.
[0128] By using the techniques described herein, the controller 918
controls the diverting devices 906, 908 and 910 in a manner which
optimizes the turn-on speed that is-attainable by the individual
switching devices. The desired drive pulse widths depend on the
frame rate, the number of columns in the display, and the blanking
time per frame. Specifically, the drive pulse widths are preferably
less than one-half the column time, where the column time is
defined to be: 1 Column Time = ( 1 frame rate - blanking time per
frame ) No. columns scanned per frame
[0129] Stated another way, if pulses are delivered to the various
columns in sequence, each i'th one of the pulses beginning at a
respective time interval T.sub.i after the beginning of the
immediately preceding pulse, then the pulse width of the drive
pulses should be sufficiently short that the switch rise time is
less than T.sub.(i+1)/2. This can be achieved with pulse widths
that are shorter than T.sub.(i+1)/2. Preferably the switches are
driven in regime III of operation in accordance with the invention
in order to achieve the fastest possible rise time for the
switches, thus minimizing cross talk between switches and
maximizing the available ON time.
[0130] Thus, for a display with 200 columns scanned and operating
at a 50 Hz frame rate, with a blanking time per frame of 2000
.mu.s, the drive pulse width is therefore preferably less than 45
.mu.s. A short initial pulse segment meeting this constraint may be
combined with a sustaining pulse segment, in order to achieve a
uniform deflection efficiency during the available time for
switching a particular column. Also, short drive pulses that are
approximately equal in duration to the column time may be used
without a sustaining pulse, if either the rise time of the
deflection efficiency is sufficiently less that one-half the column
time such that the device is operating in the steady state regime
or the diffusion time is greater than the column time (in which
case the display proceeds to the next column before the sustaining
pulse can have an impact).
[0131] For example, in an embodiment in which the rise time is
shorter than the fall time, the controller 918 asserts control
signals to the switches 906, 908 and 910 by turning on the switches
sequentially in a direction that is opposed to the direction of
propagation of energy. That is, switch 910 is switched on before
908, and 906 is switched on after 908. When switch 910 is turned
on, the energy that is propagating along 904 travels through
switches 906 and 908 and is eventually diverted into the
application structure 916 by optical switch 910, while it is in the
ON state and prior switches are in the off state. Without waiting
until switch 910 is turned completely off, that is, without waiting
for the fall time to pass, the controller sends the appropriate
instruction for device 908 to be turned on. Hence, optical switches
908 and 910 both may be effectively in their on-states, but not
necessarily both in the full-on state, at the same time. When this
instruction has been carried out, energy propagating along optical
path 904 travels through switch 906 and is diverted into the
application structure 914 by switch 908, and does not reach switch
910. In this case, it is assumed that in the full-on state, 100
percent of the optical energy is diverted. In reality, a small
amount of the energy may be propagated along to diverting device
910. In this manner, one does not have to wait until a switch has
reached its off state before switching the next switch on, and thus
a fast scanning speed/rate can be attained. Once all of the
switches have been switched in this manner and sufficient time has
elapsed that all switches in the row including switch 906 is in its
off state, energy can once again propagate along optical path 904
all the way to switch 910, the sequence may then begin again,
starting with the switch at the end farthest from the energy source
902. Depending upon the fall time of the optical diverting devices
used, it may be necessary to not only wait for sufficient time to
elapse for the switch 906 to turn off, but also for sufficient time
to permit substantial energy propagation along the optical path
904, such that most of the switches along the path are in the
full-off state. Switching this particular architecture in this
sequence allows the scanning speed of the diverting devices to be
faster than that which is attained if the switching operation is
initiated in a sequence from switch 906 closest to the energy
source 902, to switch 910 farthest from the energy source. This
architecture and the associated switching method also enables
sequential switching to occur without waiting for each previously
scanned switch to return to its off state.
[0132] In another embodiment, the controller may be able to assert
the turning on of the next switch at the same time as asserting the
turning off or the sustaining pulse of the current switch. In this
manner, the next switch reaches its on state before the previous
switch reaches its off state, and efficient energy transfer is
maintained. The timing required (including drive pulse widths and
delays between driving successive switches) to obtain such
efficiency is dependent upon the relative rise and fall times
associated with the switching arrangement in question, the switch's
ability to divert energy when between the predetermined full-on and
full-off states, and the requirements imposed by the application in
question. In this particular case, the scanning direction is
considered to be in the reverse direction of the energy
propagation, and is determined by the direction in which the
assertion of the turn-on signals for the switches is carried
out.
[0133] H. Electrical TO Switch Pulse Shape and Drive Circuit
[0134] In order to achieve a sustained switch efficiency with a
fast rise time, a special circuit design which generates a control
signal such as that shown in FIG. 6D can be used. Preferably, the
control signal is periodically replicated for applications such as
a display. FIG. 10A shows a pulse generator circuit 1000 that
produces such a pulsed signal. The components shown in the figure
are intended to be schematic rather than specific components. The
circuit contains an energy storage device 1015, an electrical
switch 1030 that controls the state of the energy storage device
1015, a control input line 1005, and an electrical output line
1025. A thermo-optical drive circuit 1020 is connected between the
electrical output line 1025 and the energy return path 1010. The
electrical switch may be a bipolar transistor, a field effect
transistor, relay, SCR, or any other electrical switch with a turn
on time that is less that the desired charging time for the energy
storage device. A voltage is applied across the control input line
1005 and ground 1010. Miscellaneous additional components such as
resistors that control current flows, time constants, and
electrical overshoots are not shown in the figure. The energy
storage device 1015 is preferably an inductor; however, other
energy storage devices may also be used, for example, electrical
storage devices such as capacitors, transmission lines and tank
circuits. In the case of a capacitor storage device, the circuit is
connected differently than shown in the figure, according to known
switched capacitor circuit techniques. The value of the inductor is
chosen to achieve a particular time constant L/R, where R is the
load resistance in the thermo-optic drive circuit 1020. The optimal
time constant is chosen according to the desired optical response
of the optical heater as discussed previously.
[0135] During operation of the device, a control signal generated
by a controller 1045 is applied to the control line 1035 connected
to the electrical switch 1030 that controls the state of the energy
storage device 1015. When the electrical switch is closed
(conducting), current begins to flow through the energy storage
device 1015, thus charging the device.
[0136] The term "charging" corresponds to a storing of energy in
the element 1015 which increases with time (1=VtAL, where t is the
charging time). The storage process is nearly a conservative one,
in which the only energy losses are due to minor imperfections in
the components. This charging concept is important because it is of
operational significance that an inductor charged with a voltage
(or a capacitor charged with a current) does not degrade system
efficiency: all energy is productively utilized. The energy stored
is (LI.sup.2)/2 and (CV.sup.2)/2 for inductors and capacitors
respectively. It is also significant that the charging does not
require any "infinite" currents or voltages. During charging, the
value of the input voltage (Vcc) appears at the output line 1025.
An energy flow controller 1041 such as a diode allows the voltage
at the bottom end of the energy storage device 1015 to be reduced
below the value on the output line 1025. When the energy storage
device 1015 has adequate stored energy to produce the desired
output pulse amplitude, the electrical switch 1030 is opened
(rendered non-conducting) and current flows from the energy storage
device 1015 through the energy flow controller 1041 to the output
line 1025. In the case of an inductor (and some other storage
elements) the stored energy must be immediately released when the
charging period ends to avoid energy losses. Current flows from the
energy storage device 1015 to the thermo-optical drive circuit 1020
as long as the product of the current and load resistance is
greater than the control input line 1005. An energy flow controller
1040 such as a diode allows the voltage on the output line 1025 to
be greater than the input DC voltage line 1005. The net effect of
the drive circuit 1000 is to produce a DC voltage at the output
line 1025, with a voltage spike from the energy storage device 1015
which decays while it discharges. The amplitude of the voltage
spike is controlled by the value (e.g. inductance) of the energy
storage device 1015, the length-of time that the electrical switch
1030 is closed between pulses, the charge voltage, switch impedance
and the applied voltage. This approach enables the use of a low
voltage supply to deliver a large amount of energy in a short
amount of time. The peak voltage delivered by this circuit can be
significantly larger than the supply voltage, depending on the
charging time, inductance, the series resistance of the inductor
circuit, and the load resistance.
[0137] The input DC voltage line 1005 creates the sustaining pulse
segment. The same circuit can be used to generate only the spike
without the sustaining pulse segment if the current flow controller
1040 is replaced with an open circuit. Thus, essentially the same
pulse generator circuit 1000 can be used to generate either a
fast-rise electrical drive pulse for switch operation in regime III
or a two step drive pulse that consists of a fast-rise pulse
segment and a sustaining pulse segment.
[0138] The periodic pulsed signal can be created in other ways as
well. For example, the pulse shown in FIG. 8A could be generated by
a switching circuit with two or more different voltage levels, such
as that shown in FIG. 10C Note that the sustaining supply can be
connected using a diode instead of a switch. In this example, two
voltage sources 1005, 1006 set the two voltage levels 810, 820 for
the generated pulse. Two electrical switches 1031, 1032 connect the
input voltage to the output control line 1025. A control signal
from the controller 1045 is applied to control line 1036 to drive
electrical switch 1031 to generate the first part of the pulse,
while a second control signal is applied to the control line 1037
to drive electrical switch 1032 to generate the sustaining portion
of the drive pulse. The control signals are synchronized such that
the electrical switch 1032 turns on after 1031 turns on and
preferably at the same time that electrical switch 1031 turns off,
and electrical switch 1032 turns off after electrical switch 1031
turns off. Additional voltage sources and switches may be added to
approximate the idealized drive pulse 830 shown in FIG. 8A.
[0139] There are several alternate means by which the pulses shown
in FIG. 8A and 8C may be created. One such way is with a digital
pulse synthesizer that drives an amplifier. The digital pulse
synthesizer might consist of memory that stores two or more digital
amplitude values, a controller that sequentially addresses the
amplitude values, a D/A converter that converts the digital signals
to a variable output voltage, and a power amplifier that converts
the low power analog signal into an output signal capable of
supplying the desired output voltage to drive the TO switches.
Another way of creating such pulses is by making a multilevel pulse
by switching the primary windings of a transformer, each with a
different turns ratio to produce a different drive level.
[0140] The pulsed drive signal shown in FIG. 8F could also be
generated using a circuit similar to that shown if FIG. 10C, but
with only a single voltage source 1005, a single electrical switch
1031, and a single control signal 1036. In this case, the control
signal 1036 turns on the electrical switch 1031 multiple times at
predetermined intervals, with each successive pulse occurring
sufficiently before the switch efficiency falls below an
unacceptable level.
[0141] As a further alternate means for approximating an idealized
drive signal such as that shown in FIG. 8A, the amplitude of the
drive pulse may be set by a feedback loop. For example, an
amplifier circuit can generate the correct output voltage 1025 to
maintain the optical switch 1020 at a particular deflection
efficiency. When the drive pulse begins to be applied, the output
voltage 1025 goes to a maximum level; once the deflection
efficiency reaches a target value, the output voltage decreases.
Note that unless the varying voltage drive comes from the natural
decay of an LR or RC circuit there will be energy lost. A variable
output voltage power supply is not conservative with energy, but
rather throws energy away in order to produce the proper output
level. An LR derived pulse is energy conservative because no energy
(except minor losses due to small component imperfections) is
thrown away as waste heat.
[0142] Feedback may also be used to create the pulsed drive signal
shown in FIG. 8F. In order to turn on the switch initially, a first
drive pulse with a particular width is applied to the device,
causing the deflection efficiency to achieve a desired maximum
level. Once the deflection efficiency falls below a specific
minimum level, a second drive pulse with a particular level is
triggered. Multiple drive pulses continue until it is desired to
turn the TO device off The particular pulse widths may be
predetermined or may also be set by a feedback loop. In the latter
case, each drive pulse terminates when the deflection efficiency
achieves the desired maximum level. Numerous other circuits can be
used to generate the desired drive pulse shape.
[0143] FIG. 10B shows symbolically an electrical switch array
circuit 1050 that is preferably used as the thermo-optic drive
circuit 1020 in FIG. 10A or 10C. The output control line 1025 of
the pulse generator circuit 1000 is connected to the input control
line 1055 of the electrical switch array circuit. An electrical
control signal generated by controller 1045 is connected to the
switch control circuit input 1061 for electrical switch 1071. When
electrical switch 1071 is closed, the voltage between the input
control line 1055 and ground 1010 is applied across thermo-optic
switch heater element 1091; when electrical switch 1071 is open,
current does not flow to the heater element 1091 and the power
delivered to that heater element is zero.
[0144] Additional electrical switches 1072 and 1073 may also be
connected to additional thermo-optic switches 1092 and 1093.
Similarly, electrical control signals may be connected to switch
control circuit inputs 1062 and 1063 for electrical switches 1072
and 1073. These electrical switches deliver power to the
thermo-optic switch heater elements 1092 and 1093 when opened and
closed as described above for electrical switch 1071.
[0145] The drive signal for electrical switch 1071 is synchronized
with the drive signal for electrical switch 1030. The relationship
between these drive signals in shown in FIG. 11. The drive signal
1110 is applied to the control line 1035 for electrical switch
1030. The drive signal consists of a series of pulses, during each
of which the energy storage device 1015 is charged, and after each
of which the energy storage device 1015 is discharged, providing
the higher voltage decaying spike 1121. The circuit can be driven
with a series of pulses 1111, 1112, 1113 to produce a series of
spikes 1121, 1122, 1123. If the current flow controller 1040 is not
an open circuit, then the baseline voltage 1125 is approximately
equal to the supply voltage applied to the control input line
1005.
[0146] In the electrical switch array circuit, a control signal
1130 is applied to the control line input 1061. A pulse 1031 in the
control signal connects the input control line 1055 of the
electrical switch array circuit to the output 1081. This pulse is
timed to select the higher voltage decaying spike 1121 to generate
a drive pulse 1141 for the thermo-optic switch. This means that the
turn off of the control pulse 1131 applied to the pulse generator
circuit 1000 is approximately coincident the turn on of the control
pulse 1131 applied to the electrical switch array circuit 1050. It
should be noted that this approximation, it is preferred that the
turn on of the control pulse 1131 applied to the array circuit 1050
occur before the turn off of the control pulse 1131 applied to the
generator circuit 1000--i.e., a "make before break" relationship.
If a sustaining pulse segment is desired, then the pulse 1131
remains on for a desired time period after the signal 1120 decays
to voltage level 1125. Otherwise, it can be turned off when the
signal 1120 decays to voltage level 1125. A second pulse 1132 in a
second drive signal 1135 can be used to select a second higher
voltage decaying spike 1122 to generate a second drive pulse 1142
for a second thermo-optic switch. Selection of different pulses is
shown. Alternatively, the same pulse 1121 may instead be selected
to be applied to both outputs 1081 and 1082.
[0147] This electrical switch array circuit may be used to drive an
array of thermo-optic switches in the manner described above. For
example, the switch array may be part of a display consisting of a
two dimensional array of thermo-optic switches as described in
Bischel et at. U.S. Pat. No. 5,544,268 for "Display Panel with
Electrically-Controlled Waveguide-Routing". The switches may be
connected in columns so that an entire column of the display is
activated at the same time. In this case, heaters represented by
switches 1091, 1092, 1093 are columns of thermo-optic switch
heaters in the display, rather than individual heaters. Only a
single pulse generator circuit 1000 is required to generate the
pulses required to generate the decaying pulses required for the
entire display. The control signals 1130, 1135 are preferably timed
to sequentially select the series of pulses generated by pulse
generator circuit 1000 to sequentially drive the columns in the
display, thus scanning all of the columns across the display.
[0148] It will be appreciated that if one were to view the pulses
applied to only two of the optical switches along one of the
optical paths in the display, one would observe one subset of the
pulses from the pulse generating circuit being applied to one of
the optical switches and a different subset of the pulses from the
pulse generating circuit being applied to the other of the optical
switches. In fact, one would observe pulses being applied to these
two optical switches alternatingly. Further, if one were to view
the pulses applied to three of the optical switches along one of
the optical paths in the display, one would observe pulses from the
pulse generating circuit being applied in a round robin sequence to
the three optical switches. Specifically in an upstream scanning
embodiment, one would observe the pulses being applied to these
three optical switches in a repetitive upstream sequence.
Similarly, if one were to view the pulses applied to four of the
optical switches along one of the optical paths in the display, one
would observe pulses from the pulse generating circuit being
applied in a round robin sequence to the four optical switches,
again more specifically in a repetitive upstream sequence, and so
on.
[0149] Although only one pulse generator circuit 1000 is required,
in some cases it may be desirable to have more than one. For
example, the time between activation of thermo-optic switches or
display columns may be less than the desired charging time in the
control pulse 1111 plus the decay time of the voltage spike 1121.
In order to achieve a longer charging time, subsequent drive pulses
1141, 1142 may be connected to separate pulse generator circuits.
In the case of two pulse generator circuits, voltage spikes for the
even columns in the display may be generated by the first pulse
generator circuit, and voltage spikes for the odd columns in the
display may be generated by the second pulse generator circuit. For
a display with a large number of columns, it may be desirable to
have multiple pulse generator circuits, each connected to every
third column, every fourth column, etc. As additional pulse
generator circuits are added, the current demand on the power
supply is smoothed. This means that the average current from the
power supply becomes more nearly the peak current and, with this
smaller variation in current levels, power supply filtering and EMI
problems are reduced. Therefore, multiple pulse generator circuits
will help to reduce highly variable loading on the power supply,
decreasing the risk for supply instability and RF radiation.
[0150] As used herein, a given signal, event or value is
"responsive" to a predecessor signal, event or value if the
predecessor signal, event or value influenced the given signal,
event or value. If there is an intervening processing element, step
or time period, the given signal, event or value can still be
"responsive" to the predecessor signal, event or value. If the
intervening processing element or step combines more than one
signal, event or value, the signal output of the processing element
or step is considered "responsive" to each of the signal, event or
value inputs. If the given signal, event or value is the same as
the predecessor signal, event or value, this is merely a degenerate
case in which the given signal, event or value is still considered
to be "responsive" to the predecessor signal, event or value.
[0151] The foregoing description of preferred embodiments of the
present invention has been provided for the purposes of
illustration and description. It is not intended to be exhaustive
or to limit the invention to the precise forms disclosed.
Obviously, many modifications and variations will be apparent to
practitioners skilled in this art. In particular, and without
limitation, any and all variations described, suggested or
incorporated by reference in the Background section of this patent
application are specifically incorporated by reference into the
description herein of embodiments of the invention. The embodiments
described herein were chosen and described in order to best explain
the principles of the invention and its practical application,
thereby enabling others skilled in the art to understand the
invention for various embodiments and with various modifications as
are suited to the particular use contemplated. It is intended that
the scope of the invention be defined by the following claims and
their equivalents.
* * * * *