U.S. patent application number 09/919711 was filed with the patent office on 2002-02-14 for device and method for variable attenuation of an optical channel.
This patent application is currently assigned to Gemfire Corporation. Invention is credited to Bischel, William K., Kowalczyk, Tony C..
Application Number | 20020018636 09/919711 |
Document ID | / |
Family ID | 32233712 |
Filed Date | 2002-02-14 |
United States Patent
Application |
20020018636 |
Kind Code |
A1 |
Bischel, William K. ; et
al. |
February 14, 2002 |
Device and method for variable attenuation of an optical
channel
Abstract
A device for variable attenuation of an optical channel includes
an elongated core surrounded by a cladding. Optical energy
propagating along the longitudinal axis of the core is normally
confined thereto by the difference in refractive indices between
the core and cladding. The thermo-optic coefficients of the core
and cladding are closely matched such that waveguide confinement is
substantially invariant with respect to ambient temperature. The
device further includes a thermal source arranged above the core.
The thermal source is operable to generate a temperature gradient
of controllable magnitude along a vertical axis extending through
the core. The temperature gradient causes reduction of the local
refractive index within the core relative to surrounding regions of
the cladding such that a portion of the optical energy is deflected
away from the thermal source and extracted from the core.
Inventors: |
Bischel, William K.; (Menlo
Park, CA) ; Kowalczyk, Tony C.; (Palo Alto,
CA) |
Correspondence
Address: |
HAYNES BEFFEL & WOLFELD LLP
P O BOX 366
HALF MOON BAY
CA
94019
US
|
Assignee: |
Gemfire Corporation
|
Family ID: |
32233712 |
Appl. No.: |
09/919711 |
Filed: |
August 1, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09919711 |
Aug 1, 2001 |
|
|
|
09630891 |
Aug 2, 2000 |
|
|
|
Current U.S.
Class: |
385/140 ;
385/43 |
Current CPC
Class: |
H04J 14/0208 20130101;
G02F 1/0147 20130101; H04J 14/02 20130101; H04J 14/06 20130101;
G02F 1/065 20130101; G02F 2203/05 20130101; H04J 14/0206 20130101;
H04J 14/0201 20130101; G02B 27/283 20130101; G02F 1/011 20130101;
G02F 1/31 20130101; H04J 14/0209 20130101 |
Class at
Publication: |
385/140 ;
385/43 |
International
Class: |
G02B 006/26 |
Claims
What is claimed is:
1. An optical attenuator device selectively operable in a
non-actuated state and an actuated state, comprising: a waveguide
for guiding optical energy, the waveguide having an input section
coupled to an intermediate section, said intermediate section
having reduced confinement of the optical energy relative to said
input section; a thermal source, disposed above said intermediate
section, for generating a temperature gradient within a portion of
said intermediate section along a vertical axis thereof when said
device is in said actuated state, said temperature gradient being
sufficient to alter a refractive index profile within said
intermediate section such that a portion of said optical energy is
deflected downwardly and extracted from said intermediate
section.
2. The device of claim 1, wherein said intermediate section of said
waveguide comprises a core and a cladding bounding said core, said
core and cladding having matched thermo-optic coefficients.
3. The device of claim 2, wherein said core of said intermediate
section has at least one transverse dimension that is significantly
larger than a corresponding transverse dimension of a core of said
input section.
4. The device of claim 3, wherein said intermediate section is
coupled to said input section by an adiabatic taper.
5. The device of claim 1, wherein said waveguide further comprises
an output section optically coupled to said intermediate section,
said output section having increased confinement of the optical
energy relative to said intermediate section.
6. The device of claim 2, wherein said core is segmented.
7. The device of claim 2, wherein said core has a refractive index
that is less than a refractive index of a core of said input
section and greater than or equal to a refractive index of said
cladding.
8. The device of claim 7, wherein said refractive index of said
core of said intermediate section is equal to said refractive index
of said cladding.
9. The device of claim 8, wherein said core of said intermediate
section and said cladding are formed from the same material.
10. The device of claim 1, wherein said portion of said optical
energy extracted from said intermediate section is adjusted by
varying an electrical control signal applied to said thermal
source.
11. The device of claim 2, wherein said core and said cladding are
formed from polymeric materials.
12. A method for controllably removing optical energy from a
waveguide, comprising the steps of: (a) directing said optical
energy from an input section of said waveguide to an intermediate
section of said waveguide, said intermediate section having reduced
confinement of said optical energy relative to said input section;
and (b) generating a vertical temperature gradient within said
intermediate section sufficient to alter a refractive index profile
within said intermediate section such that a portion of said
optical energy is deflected downwardly and extracted from said
intermediate section.
13. An optical attenuator device selectively operable in an
actuated state and a non-actuated state, comprising: a core; a
lower cladding layer downwardly bounding said core; a first upper
cladding sublayer upwardly and laterally bounding said core,
wherein said core, said lower cladding layer and said first upper
cladding sublayer have matched thermo-optic coefficients; a second
upper cladding sublayer upwardly adjacent to said first upper
cladding sublayer and having a refractive index substantially lower
than the refractive index of said first upper cladding sublayer;
and a resistive heater positioned above said core, said resistive
heater being configured to generate a thermal gradient within said
core, when said attenuator device is in the actuated state, such
that the refractive index of a portion of said core is decreased
below the refractive index of a portion of said lower cladding
layer located downwardly adjacent to said core, causing a portion
of the optical energy traveling along said core to be deflected
downwardly and extracted from said core.
14. The device of claim 13, further comprising a substrate affixed
to said lower cladding layer.
15. The device of claim 14, further comprising an adhesion layer
interposed between said substrate and said lower cladding
layer.
16. The device of claim 15 wherein said adhesion layer has a
refractive index which is less than the refractive index of said
substrate and greater than or equal to the refractive index of said
lower cladding layer.
17. The device of claim 13, wherein said core, said lower cladding
layer, said first upper cladding sublayer, and said second upper
cladding sublayer all comprise polymeric materials.
18. The device of claim 13, wherein said portion of said optical
energy extracted from core is adjusted by varying an electrical
control signal applied to said resistive heater.
19. The device of claim 13, wherein said resistive heater is
capable of generating an average vertical thermal gradient within
said core of at least 0.53.degree. C./.mu.m.
20. The device of claim 13, wherein said resistive heater is
capable of generating an average vertical thermal gradient within
said core of at least 0.67.degree. C./.mu.m.
21. The device of claim 13, wherein said resistive heater is
positioned no more than 5 .mu.m above an upper boundary of said
core.
22. The device of claim 13, wherein the portion of optical energy
extracted from said core may be varied in a range between around 0%
to around 99.9%.
23. An optical attenuator selectively operable in an actuated and a
non-actuated state, comprising: a core bound by a cladding, said
core and said cladding having matched thermo-optic coefficients,
said cladding having an upper surface; a thermal source positioned
above said core, said thermal source being configured, when said
attenuator is in the actuated state, to generate a thermal gradient
within said core such that the refractive index of a portion of
said core is decreased below the refractive index of a portion of
said cladding located downwardly adjacent to said core, causing a
portion of optical energy traveling along said core to be deflected
downwardly and extracted from said core; and a cover plate affixed
to said upper surface of said cladding and being held in vertically
spaced apart relation with respect to said cladding.
24. The optical attenuator of claim 23, wherein said cover plate is
affixed to said cladding by an adhesive applied to areas of said
cladding away from said thermal source such that said thermal
source is not contacted by either said adhesive or said cover
plate.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is a continuation-in-part of pending U.S.
application Ser. No. 09/630,891 entitled "Device and Method for
Variable Attenuation of an Optical Channel" by inventors Bischel et
al., filed Aug. 2, 2000. The two applications are commonly
assigned.
BACKGROUND
[0002] 1. Field of the Invention The invention relates generally to
optical communications, and more specifically to a variable
attenuator for removing optical energy from a waveguide in a
controllable manner.
[0003] 2. Description of the Prior Art
[0004] The telecommunication industry is making increasing use of
optical communication systems for high-bandwidth transmission of
voice, video, and data signals. In optical communication systems,
it is frequently necessary or desirable to precisely adjust optical
signal levels entering various system components. Adjustment of
optical signal levels is typically achieved by incorporating
variable optical attenuators into the optical circuits. In one
example, a variable optical attenuator may be employed to equalize
power levels among separate channels of optical circuits
implementing wavelength division multiplexing (WDM). Variable
optical attenuators may also be employed to avoid exposing optical
detectors to excessive signal levels, which may damage the
detectors and cause them to become inoperative.
[0005] Various references in the prior art disclose attenuators for
use in optical circuits. Examples of such attenuators include those
described in U.S. Pat. No. 4,762,383 to Yamashita et al. ("Two
Dimensional Light Beam Deflectors Utilizing Thermooptical Effect
and Method of Using Same"); U.S. Pat. No. 5,881,199 to Li ("Optical
Branching Device Integrated with Tunable Attenuators for System
Gain/Loss Equalization"), and; U.S. Pat. No. 5,966,493 to Wagoner
et al. ("Fiber Optic Attenuators and Attenuation Systems"). The
attenuators described in the foregoing references, as well as other
prior art attenuators, are known to suffer from operationally
significant problems or limitations. These problems include
sensitivity to ambient temperature, high power consumption, limited
or no adjustability, the occurrence of cross-talk between adjacent
channels, high coupling losses, bulkiness and slow responsiveness.
Further, some prior art attenuators have moving parts that tend to
degrade over time and have associated mechanical resonance.
[0006] Thus, there is a need in the optical communications art for
a variable optical attenuator which overcomes the problems
associated with prior art devices.
SUMMARY
[0007] In accordance with an embodiment of the invention, a
variable optical attenuator is provided having at least one
elongated core, a cladding surrounding the core, and a controllable
thermal source and a heat sink arranged on opposite sides of the
core and defining therebetween a first or vertical axis oriented
transversely to the longitudinal axis of the core. The core and
cladding collectively form a conventional waveguide structure,
which normally confines optical energy propagating along the
longitudinal axis of the core by virtue of the difference in
refractive indices between the core and cladding. The core and
cladding materials are preferably selected such that their
thermo-optic coefficients (i.e., dn/dT, where n is the local
refractive index and T is temperature) are closely matched within
the ambient temperature range of interest. Matching the
thermo-optic coefficients of the core and cladding ensures that the
waveguide confinement (a function of the difference between the
refractive indices of the core and cladding) is substantially
invariant with respect to ambient temperature, thereby obviating
the need to provide heating or cooling of the waveguide
package.
[0008] When attenuation of the optical energy propagating along the
core is desired, a control signal is applied to the thermal source,
which in turn causes a temperature gradient to be developed along
the first (vertical) axis extending between the thermal source and
the heat sink. The temperature gradient results in a "tilted" or
asymmetric refractive index profile within the core wherein the
refractive index of the core increases along the first axis from
the proximal core-cladding boundary (the boundary nearer to the
thermal source) to the distal core-cladding boundary (the boundary
more remote from the thermal source). Extraction of optical energy
from the waveguide occurs when the local refractive index at the
higher-temperature areas of the core (those adjacent the proximal
boundary) is depressed below that of the local refractive index of
the cladding immediately adjacent to the distal core-cladding
boundary. This condition causes at least a portion of the optical
energy propagating along the core to be transversely deflected in
the direction away from the thermal source (i.e., toward the heat
sink). The amount of optical energy extracted from the waveguide is
controlled by adjusting the signal (for example, a voltage) applied
to the thermal source. Extraction of optical energy may be
facilitated by providing a relatively weakly vertically confining
section in the waveguide and locating the thermal source above this
section.
[0009] The invention further encompasses an attenuation system,
incorporating a variable optical attenuator of the foregoing
description, in which a control circuit applies a variable signal
to the thermal source in accordance with a desired degree of
channel attenuation and with feedback information obtained by
monitoring the power level of optical energy traveling through the
core. The attenuator and attenuation system of the present
invention may be advantageously employed in any number of optical
circuit applications where it is necessary or desirable to control
optical power transmission within individual optical channels.
BRIEF DESCRIPTION OF THE FIGURES
[0010] In the accompanying drawings:
[0011] FIG. 1 is a schematic view of a device for variable
attenuation of an optical channel, in accordance with a general
embodiment of the invention;
[0012] FIG. 2 is a cross-sectional view of the FIG. 1 device;
[0013] FIG. 3 is a graph showing several exemplary vertical
refractive index profiles developed within the FIG. 1 device, each
profile corresponding to a different level of thermal source
actuation;
[0014] FIG. 4 is a graph showing the amount of attenuation achieved
within the FIG. 1 device versus thermal source actuation;
[0015] FIG. 5 is a schematic view of a more specific embodiment of
the FIG. 1 device;
[0016] FIG. 6 is a flowchart depicting the steps of a method for
constructing the FIG. 5 device;
[0017] FIGS. 7(a) and 7(b) depict alternate thermal source
geometries and arrangements for the FIG. 5 device;
[0018] FIG. 8 is a block diagram of an attenuation system, in
accordance with one embodiment of the invention;
[0019] FIGS. 9(a) and 9(b) are, respectively, top plan and elevated
side views symbolically depicting an alternative embodiment of the
FIG. 1 device, wherein a region of reduced optical confinement is
achieved by providing an intermediate waveguide section having
enlarged cross-sectional dimensions;
[0020] FIG. 10 is a symbolic view of a specific implementation of a
multi-channel attenuator device;
[0021] FIGS. 11(a) and 11(b) are, respectively, top plan and
cross-sectional views of an attenuator device having an attached
cover plate, a portion of the cover plate being cut away in FIG.
11(a) to reveal the arrangement of underlying structures; and
[0022] FIGS. 12(a) and 12(b) are, respectively, top plan and
elevated side views symbolically depicting an alternative
embodiment of the FIG. 1 device, wherein a region of reduced
optical confinement is achieved by providing an intermediate
waveguide section having a smaller refractive index.
DETAILED DESCRIPTION
[0023] The present invention generally provides a device and method
for variable attenuation of an optical channel wherein optical
energy is controllably extracted from a waveguide in a preferred
direction by generating a temperature gradient along an axis
transverse to the longitudinal axis of the waveguide (it should be
noted that the term "optical energy", as used herein, denotes
electromagnetic energy in general without limitation to specific
wavelengths or spectral windows). The device is constructed to
avoid problems associated with prior art devices, including
sensitivity to ambient temperature, thermal cross-talk between
adjacent optical channels, and high power consumption. Suitable
uses of the device include, without limitation, equalization of
optical power levels in input channels of a wavelength division
multiplexer device, span balancing, amplifier input balancing,
mux-demux balancing, and optical receiver protection.
[0024] FIGS. 1 and 2 depict elements of a variable optical
attenuator device (hereinafter "VOA") 100 in accordance with a
general embodiment of the invention. VOA 100 includes a
conventional waveguide structure having a core or confinement
region (hereinafter "core") 102 surrounded by a cladding region
(hereinafter "cladding") 104. Core 102 extends along a longitudinal
axis (indicated by arrow 105) and is oriented in substantially
parallel relation with an upper surface 106 of cladding 104 (the
terms "upper", "lower", "vertical" and "horizontal" are used herein
for the purpose of clarity, and are not intended to limit the
device to any particular orientation). Cladding 104 is
longitudinally coextensive with core 102, and may be constructed
from multiple layers (such as lower layer 108 and upper layer 110)
to facilitate fabrication of VOA 100, as will be discussed below in
connection with FIGS. 5 and 6. The cross-sectional size and shape
of core 102 may be selected to accommodate the mode (spatial
distribution of optical energy) at the input fiber (not shown in
FIG. 1) so as to minimize coupling losses.
[0025] VOA 100 further includes a thermal source 112 and a heat
sink 114 arranged in opposed relation about core 102, with thermal
source 112 contacting upper surface 106 of cladding 104 and heat
sink 114 contacting a lower surface 116 of cladding 104. As is
shown in FIG. 2, which depicts a cross-sectional view taken along
line 2-2 of FIG. 1, thermal source 112 and heat sink 114 define
therebetween a first or vertical axis (indicated by arrow 202)
transverse to the longitudinal axis of core 102. Thermal source 112
may comprise, for example, a thin film heater fabricated from a
segment of electrically resistive material, such as nichrome
(NiCr), across which a voltage signal is selectively applied, as
indicated in FIG. 1. Heat sink 114 may comprise a device substrate
fabricated from a glass or other material having a relatively high
thermal conductivity so as to efficiently remove heat from cladding
104. As will be discussed in greater detail below, thermal source
112 and heat sink 114 are configured to generate a controllable
temperature gradient in cladding 104 and core 102 along the
vertical (first) axis. This temperature gradient produces an
asymmetric spatially varying refractive index profile, which causes
at least a portion of the optical energy propagating along the
longitudinal axis of core 102 to be deflected in the direction of
heat sink and extracted from core 102.
[0026] It will be noted that VOA 100 is depicted as having only a
single waveguide structure. However, the invention contemplates
attenuator devices having an array of multiple waveguide structures
comprising a plurality of horizontally spaced cores (arranged
either in co-planar or vertically staggered relation) embedded in
the cladding. In such devices, each waveguide is associated with a
separate thermal source aligned therewith to enable independent
control of the attenuation of each optical channel. A specific
embodiment of the invention having plural independently controlled
waveguides is depicted in FIG. 5 and discussed below. It should
also be noted that although core 102 is shown as being fully
surrounded or circumscribed by cladding 104, alternative
embodiments wherein core 102 is substantially but not fully
surrounded by cladding 104 are within the scope of the
invention.
[0027] It will be further noted that alternative embodiments of the
invention may omit heat sink 114; however, omission of heat sink
114 may reduce overall VOA 100 performance.
[0028] Core 102 and cladding 104 are preferably fabricated from
thermo-optic materials (materials which exhibit a change in
refractive index in response to an increase or decrease in
temperature) having negative thermo-optic coefficients (dn/dT<0,
where n is the local refractive index and T is temperature),
although alternative embodiments may utilize a material for core
102 or cladding 104 having a positive thermo-optic coefficient,
such as glass. The materials of core 102 and cladding 104 will
preferably comprise polymers, although other materials having
suitable properties may be employed. To enable confinement of
optical energy traveling along core 102, cladding 104 is fabricated
from a material having a refractive index slightly lower (under
normal conditions) than the refractive index of core 102. In a
typical implementation of VOA 100, the difference .DELTA.n between
the refractive indices of the core and cladding will be
approximately 0.004-0.006. As is well known in the art, the
refractive index difference between core 102 and cladding 104
produces confinement of optical energy traveling along the
longitudinal axis of core 102 via multiple internal reflections at
the core-cladding boundaries.
[0029] A significant shortcoming of prior art switching, modulating
or attenuating devices based on the thermo-optic effect is that
such devices are known to be sensitive to variations in ambient
temperature, sometimes requiring carefully controlled cooling
and/or heating of the device package to ensure reliable operation.
In particular, waveguides constructed from core and cladding
materials having different thermo-optic coefficients (dn/dT) will
exhibit changes in .DELTA.n, and hence confinement, when the
waveguide temperature is uniformly raised or lowered. In order to
avoid sensitivity of VOA 100 to changes in the ambient temperature,
the materials for core 102 and cladding 104 are selected to have
substantially equal thermo-optic coefficients, at least within the
operating temperature range of interest; in this manner, the
refractive indices of core 102 and cladding 104 increase or
decrease by approximately equal amounts with a corresponding
decrease or increase in temperature, leaving the .DELTA.n and hence
the waveguide confinement substantially unchanged. As used herein,
the thermo-optic coefficients of core 102 and cladding 104 are
considered to be matched or substantially equal if they are
sufficiently close so as to maintain the insertion loss associated
with VOA 100 within acceptable limits over the full expected
operational temperature range (typically 5.degree. C. to 85.degree.
C.). In an exemplary implementation of VOA 100, insertion loss may
be limited to a maximum of 0.8 dB over an operational temperature
range of -5.degree. C. to 100.degree. C. by selecting core and
cladding materials having thermo-optic coefficients that differ
from each other by no more than ten percent (10%). Improved VOA
performance (e.g., lower insertion losses) may be achieved by more
closely matching the thermo-optic coefficients of the core and
cladding. In typical implementations of VOA 100, core 102 and
cladding 104 will have thermo-optic coefficients in the range of
-2.0*10.sup.-4/.degree. C. to -4.5*10.sup.-4/.degree. C., with a
preferred value of around -4.0*10.sup.-4/.degree. C.
[0030] Additional criteria for selection of materials for core 102
and cladding 104 include the following: (1) low and uniform
absorption losses across the optical energy spectrum of interest
(e.g., 1500-1610 nanometers for telecommunications applications);
(2) mechanical and optical property stability; (3) low glass
transition temperature (in the range of -40.degree. C. to
10.degree. C., with a preferred value of about 0.degree. C.), and;
(4) low optical birefringence. Examples of materials which
generally satisfy the foregoing criteria and hence which are
suitable for use in VOA 100 are discussed below in connection with
FIGS. 5 and 6.
[0031] The operation of VOA 100 may best be understood with
reference to FIG. 3, in which exemplary refractive index profiles
developed along the first or vertical axis are shown for varying
degrees of actuation of thermal source 112. The profiles are
plotted as the local refractive index versus distance d from
surface 106 along the vertical axis. A first refractive index
profile 302 corresponds to a normal or non-actuated condition
wherein thermal source 112 is not providing any heating, and
consequently the temperature along the vertical axis is
substantially uniform. Profile 302 consists of a flat region from
d=0 to d=D.sub.1, representative of a uniform local refractive
index across the upper portion of cladding 104; a sharp increase at
d=D.sub.1, representative of the increase in local refractive index
at the upper (proximal) cladding/core boundary; a second flat
region from D.sub.1 to D.sub.2 representative of a uniform local
refractive index across core 102; a sharp decrease at d=D.sub.2
representative of the decrease in local refractive index across the
lower (distal) cladding/core boundary; and, a third flat region
extending beyond D.sub.2 representative of a uniform local
refractive index across the lower portion of cladding 104. Because
the local refractive index across core 102 uniformly exceeds the
local refractive index of cladding 104 adjacent to the proximal and
distal core/cladding boundaries, optical energy traveling along
core 102 is confined thereto.
[0032] Second refractive index profile 304 corresponds to a "low
actuating" condition, wherein thermal source 112 is operated to
provide a relatively small degree of heating. Heat generated by
thermal source 112 (typically by resistive heating) flows along the
vertical axis from cladding upper surface 106 through the upper
portion of cladding 104, core 102 and lower portion of cladding 104
and into heat sink 114. The flow of thermal energy through the
cladding 104 and core 102, which is limited by the thermal
conductivities of the materials from which VOA 100 is formed,
creates a temperature gradient along the vertical axis, the
temperature being highest at a location immediately below thermal
source 122 and decreasing steadily in the downward direction. Owing
to the thermo-optic properties of core 102 and cladding 104, the
temperature gradient produces a reduction in local refractive
indices relative to the normal or non-actuating condition, the
reduction being greatest where the temperature is highest and
decreasing with increasing distance along the vertical axis. The
temperature gradient results in a tilting of the refractive index
profile of core 102, with the local refractive index increasing
along the vertical axis from the proximal core/cladding boundary to
the distal core/cladding boundary. This tilting is sufficient to
depress the local refractive index in the upper portion of core 102
(that portion adjacent to the proximal core/cladding boundary)
relative to the local refractive index of cladding 104 located
immediately adjacent to the distal core/cladding boundary. The
effect of the decreased local refractive index (relative to that of
cladding 104 adjacent to the distal boundary) in a portion of core
102 is to cause, via refractive processes, a portion of the optical
energy propagating along core 102 to be deflected downwardly out of
core 102 in the direction of heat sink 114. In this manner,
attenuation of the optical signal within core 102 is achieved. It
is noted that the asymmetry of refractive index profile 304
effectively prevents optical energy from leaking upwards toward
thermal source 112, as the local refractive index of the upper
portion of cladding 104 is uniformly lower than the local
refractive index within core 102. Instead, the optical energy is
preferentially extracted downwards toward heat sink 114 in the
direction indicated by arrow 202, into the higher local refractive
index of the lower portion of cladding 104.
[0033] Refractive index profiles 306 and 308 respectively
correspond to a "medium actuating" condition (where thermal source
112 is operated to provide a medium degree of heating) and a "high
actuating" condition (where thermal source 112 is operated to
provide a relatively high degree of heating). Increasing the amount
of heating provided by thermal source 112, as represented by the
progression from profile 304 to 306 and from profile 306 to 308,
creates a steeper temperature gradient and more pronounced tilting
of the refractive index profile of core 102. As the extent of
tilting of the refractive index profile becomes greater, the local
refractive index of a progressively larger portion of core 102 is
depressed relative to the local refractive index of cladding 104
adjacent to the distal core/cladding boundary, leading to
extraction of a larger portion of optical energy from core 102
through the distal core/cladding boundary and into the lower
portion of cladding 104. The amount of optical energy extracted
from core 102 may be generally expressed to be a function of the
difference .DELTA.n.sub.perturbation-.DELTA.n.sub.confine- ment,
where .DELTA.n.sub.perturbation is the difference between the local
refractive index of cladding 104 immediately adjacent to the distal
core/cladding boundary and the local refractive index of cladding
104 immediately adjacent to the proximal core/cladding boundary
(.DELTA.n.sub.perturbation characterizing the degree of refractive
index tilting arising from the presence of a temperature gradient),
and .DELTA.n.sub.confinement is the difference between the local
refractive index of core 102 and the local refractive index of
cladding 104 immediately adjacent to the distal core/cladding
boundary (.DELTA.n.sub.confinement characterizing the degree of
vertical confinement and being substantially insensitive to the
temperature gradient). In this manner, the attenuation of optical
energy propagating through core 102 may be controlled by adjusting
the operation of thermal source 112. Those skilled in the art will
appreciate that when thermal source 112 is operated to provide a
sufficiently high amount of heating, substantially all of the
optical energy will be extracted from core 102 via refraction and
diffraction.
[0034] High attenuation efficiencies (attenuation produced per unit
of power consumed) may be achieved by optimizing the design of VOA
100 (including adjustment of the device's dimensions and geometry
and selection of appropriate core and cladding materials) such that
large vertical temperature gradients can be developed within core
102. For a typical implementation of VOA 100 having a core 102
height of 7.5 .mu.m, an attenuation of 30 dB may be effected at
relatively low power consumption if the .DELTA.T (temperature
difference between the lower and upper boundaries of the core) is
greater than 4.degree. C. (representing an average vertical
temperature gradient within the core of about 0.53.degree.
C./.mu.m), and preferably greater than 5.degree. C. (representing
an average vertical temperature gradient within the core of about
0.67.degree. C./.mu.m).
[0035] It should be further noted that optical energy is not
deflected in the horizontal or lateral plane (indicated by arrow
204 in FIG. 2) because the heating provided by thermal source 112
does not substantially affect lateral waveguide confinement. The
temperature gradient along the lateral axis is significantly
smaller than the gradient along the vertical axis, and may be
controlled by adjusting the width of thermal source 112. A thermal
source having a width close to or exceeding the width of core 102
will produce a substantially uniform lateral temperature profile
across core 102 and portions of cladding immediately adjacent
thereto. This condition will result in refractive index decreases
of similar magnitude for horizontally aligned portions of the core
102 and cladding 104, thereby preserving horizontal waveguide
confinement. Minimization of the horizontal temperature gradient
may also be achieved by adjusting the geometry and configuration of
thermal source 112, such as by use of the "pedestal" configuration
discussed below in connection with FIG. 7(a), or by increasing the
depth of core upper layer 110.
[0036] FIG. 4 depicts the relationship between the amount of
actuation, expressed as the power consumed by thermal source 112
and the resultant attenuation of optical energy propagating along
core 102, expressed in decibels, measured in connection with a
exemplary implementation of VOA 100. The amount of attenuation is
shown to increase monotonically with increasing thermal source
actuation, with the attenuation-power consumption relationship
being quasi-linear over a substantial portion of the depicted power
consumption range. This relationship between attenuation and
thermal source actuation is representative of the ability of VOA
100 to provide precise control of the optical power over an
extended range of attenuation (0-30 dB) by simply adjusting the
power supplied to thermal source 112. Those skilled in the art will
also observe that high degrees of attenuation may be achieved with
relatively low power consumption. For example, an attenuation of 15
dB requires only about four milliwatts (4 mW) of power. In other
typical implementations of VOA 100, an attenuation of 10 dB may be
achieved at a thermal source power consumption of less than 5.0 mW
(preferably around 3.5 mW), an attenuation of 20 dB may be achieved
at a thermal source power consumption of less than 10.0 mW
(preferably around 6.5 mW), and an attenuation of 30 dB may be
achieved at a thermal source power consumption of less than 15.0 mW
(preferably around 10.0 mW).
[0037] The response time of VOA 100 (the time it takes VOA 100 to
go from a non-actuated state to an actuated state providing a
desired amount of attenuation and vice-versa) will typically be
less than 10 milliseconds. The actual response time will of course
depend on a variety of design and performance parameters. In a
preferred implementation of VOA 100, rise times (the time it takes
to attain a desired actuated state from an unactuated state) are
around 3.1, 1.4, 0.55 and 0.27 milliseconds for (respectively)
actuated states providing 5, 10, 20, and 30 dB of attenuation.
Corresponding fall times (time from an actuated state to an
unactuated state) for the preferred implementation are around 1.26,
1.61, 2.5, and 4.2 milliseconds.
[0038] Examination of the attenuation-power consumption curve of
FIG. 4 reveals a plateau region beginning at a power consumption of
approximately 4.5 mW. It has been postulated that the decrease in
the attenuation efficiency represented by the plateau region
results from reflection of a portion of the deflected optical
energy from the cladding/substrate boundary, the reflected light
being re-coupled into core 102. It may be possible to reduce this
effect through optimization of the geometry and materials of VOA
100. Specifically, reflection from the cladding/substrate boundary
may be reduced or eliminated by matching the refractive indices of
the cladding and substrate materials, or alternatively, by
interposing a relatively thick adhesion layer between the cladding
and substrate, the refractive index of the adhesion layer being
matched to that of the cladding. Another possible solution to the
above-described "re-coupling" problem involves roughening the
surface of the substrate contacting the cladding, thereby
randomizing the angle of reflection of the reflected optical
energy, which in turn reduces the fraction of the reflected optical
energy re-coupled into core 102. Yet another solution to the
re-coupling problem consists of disposing a set of longitudinally
spaced "reflection blockers" at or proximal to the
cladding/substrate boundary. The reflection blockers comprise
vertically oriented specular reflectors which avoid re-coupling
into core 102 by redirecting optical energy such that it is not
reflected from the cladding/substrate boundary and/or by
redirecting optical energy reflected at the cladding/substrate
boundary such that it is not re-coupled into core 102.
[0039] The fabrication of a specific embodiment of the invention
will now be discussed in connection with FIGS. 5 and 6. FIG. 5
depicts a VOA 500 closely similar in many respects to VOA 100 of
FIGS. 1 and 2. In contrast to FIG. 1, however, which depicts only a
single waveguide, VOA 500 is provided with an array of four
waveguides comprising elongated thermo-optic cores (individually
and collectively denoted as 502) set in thermo-optic cladding 504,
each waveguide carrying a separate optical signal. Cores 502 are
arranged in co-planar, mutually parallel relation with typical
horizontal spacing of 250 .mu.m between adjacent cores. Each core
502 is substantially aligned and in thermal association with a
thermal source (individually and collectively denoted as 506)
uniquely corresponding thereto, each thermal source 506 being
operable to generate a temperature gradient in the associated core
502 along a vertical or first axis extending between thermal source
506 and substrate 508 (which serves as a heat sink) which in turn
causes at least a portion of the optical energy traveling along
core 502 to be deflected downwardly in the direction of substrate
508 and thereby extracted from core 502.
[0040] In a preferred implementation, substrate 508 is formed from
a commercially available glass material such as Corning.RTM. 1734
flat glass, but may alternatively be formed from any suitable
material having sufficiently high thermal conductivity to rapidly
conduct heat away from cladding 504. The thickness of substrate 508
will be chosen in view of mechanical, thermal and processing
requirements. Typical substrate thickness for VOA 500 will be about
1000 .mu.m.
[0041] In the first step 602 of the VOA fabrication method depicted
by the FIG. 6 flowchart, an adhesion layer is deposited on
substrate 508 to promote adhesion between substrate 508 and a lower
cladding layer 510. The adhesion layer will typically comprise a
commercially available substance such as AZ 4330 positive
photoresist, and is preferably deposited to a thickness of about 3
.mu.m.
[0042] Next, in step 604, planar lower cladding layer 510 and a
planar core layer are sequentially deposited on substrate 508.
Lower cladding layer 510 will preferably comprise Norland Optical
Adhesive 68 (NOA 68, available from Norland Products of New
Brunswick, N.J.), which has a refractive index of 1.534 and a
thermo-optic coefficient of 3.3.times.10.sup.-4, although other
optically transparent polymeric or non-polymeric materials having
suitable properties may be substituted. A standard polymer layer
deposition procedure may be employed for deposition of lower
cladding layer 510, comprising viscosity adjustment prior to
deposition, spinning, and curing with ultraviolet light (which
cross links the polymer chains and creates a robust layer). Other
methods which may be utilized to deposit lower cladding layer 510
include meniscus, extrusion, spray, dip, evaporation coating, or
sputtering. Lower cladding layer 510 may be deposited to a
preferred thickness of about 9.5 .mu.m.
[0043] A core layer is then deposited on top of lower cladding
layer 510. The core layer will preferably be formed from a material
having the same polymer structure as lower cladding layer 510
(e.g., NOA 68) to which a refractive index-raising additive has
been added. The additive will preferably raise the refractive index
of the core layer by about 0.006 relative to lower cladding layer
510, yielding a core layer refractive index of 1.540 for the NOA 68
example. Because the basic chemical structures of the core layer
and lower cladding layer 510 are generally the same, their
thermo-optic coefficients are matched, thereby making the
refractive index difference substantially invariant with ambient
temperature. The core layer is preferably deposited by the standard
deposition techniques alluded to above to a thickness of about 6.4
.mu.m.
[0044] In the third step 606, aluminum is sputtered and patterned
via lithography to form a reactive ion etching (RIE) mask for
waveguide fabrication. The RIE mask features, which define cores
502, will preferably comprise parallel lines having a width of
about 6.4 .mu.m which run along the entire length of the mask. As
referenced above, the lateral core spacing will typically be about
250 .mu.m. The unmasked portions of the core layer (representing
the regions between adjacent cores 502, are then removed by RIE,
step 608. Other wet and dry etching techniques known in the art may
be used in place of the RIE technique for defining cores 502.
[0045] The cross-sectional dimensions and refractive index of cores
502 will preferably be selected to produce single mode operation in
the optical energy wavelengths of interest. In the foregoing
example, cores 502 will have a square cross-section with a width
and height of 6.4 .mu.m, which produces single mode operation for
optical energy having a wavelength of 1.55 .mu.m, which is a
typical wavelength used for telecommunications applications.
[0046] Next, in step 610 the RIE etch mask is removed and an upper
cladding layer 512 is deposited on top of lower cladding layer 510
and cores 502, thereby forming buried channel waveguides. The
material of upper cladding layer 512 may be identical to that of
lower cladding layer 510 (e.g., NOA 68 for the example discussed
above). Deposition of upper cladding layer 512 may be accomplished
by the same standard polymer deposition technique used for
deposition of lower cladding layer 510 and the core layer. Upper
cladding layer 512 is preferably deposited to a thickness of about
10 .mu.m.
[0047] In alternative embodiments of VOA 500 upper cladding layer
512 may be fabricated from two or more sub-layers in order to
optimize various properties of upper cladding layer 512. For
example, upper cladding layer 512 may comprise a upper sub-layer of
a first material and a lower sub-layer of a second material, the
first material selected to have a low refractive index in order to
provide more effective optical isolation of the mode from thermal
source 506, and the second material selected to provide good
confinement of the mode. In this manner, it may be possible to
construct VOA 500 with a thinner overall upper cladding layer 512
thickness (which advantageously increases the thermal gradient
obtainable within core 102) and/or to improve other mechanical or
optical properties of VOA 500.
[0048] It is noted that other polymeric material sets may be
employed to fabricate the core and cladding layers in lieu of the
NOA-68 material set described above. Non-limiting examples of
polymer material sets suitable for fabrication of VOA 500 include
the following: fluorinated acrylates, siloxane-based polymers, and
polyimides.
[0049] Next, NiCr (used to form thermal sources 506) and gold
layers (used to form electrical contact pads, which are not
depicted in the figures) are sequentially deposited by sputtering
on top of upper cladding layer 512 to form a two-layer metal
structure, step 612. The NiCr layer will be deposited to a
thickness sufficient to achieve the desired electrical resistance
of thermal sources 506. For thermal sources 506 having preferred
dimensions of 12 .mu.m wide by 300 .mu.m length, the target
electrical resistance is 300 .OMEGA..
[0050] In step 614, gold electrical contact pads, for electrically
connecting thermal sources to lines carrying the thermal source
control signals, are defined and formed using conventional
photolithography techniques. Briefly, photoresist is deposited on
the gold layer by spinning and exposed through a contact pad mask.
The exposed regions of the gold layer are subsequently removed
using a gold etch solution, thereby defining the contact pad.
Typical dimensions of the contact pads are 200 .mu.m by 200
.mu.m.
[0051] Thermal sources 506 are then formed using conventional
photolithography techniques, step 616. Photoresist is deposited on
the NiCr layer and exposed through a thermal source mask. The
thermal source mask is preferably positioned to align the thermal
source features with the underlying cores 502, such that the
resultant thermal sources 506 (i.e., thin film heaters) have their
longitudinal axes centered above and parallel to the longitudinal
axes of corresponding cores 502. The exposed regions of the NiCr
layer are then removed using a NiCr etch solution to thereby define
thermal sources 506. Typical dimensions for thermal sources 506
will 6-20 .mu.m width by 150-450 .mu.m length, with the preferred
width and length being around 12 .mu.m and 300 .mu.m,
respectively.
[0052] It is to be appreciated that, in a typical production
environment, multiple VOA 500 devices may be formed simultaneously
on a common substrate. In step 618, individual VOA 500 devices are
separated by dicing the common substrate. End faces 514 are then
polished to enable coupling of optical energy in and out of cores
502.
[0053] While specific techniques for electrically connecting
thermal sources 506 to a control signal source have not been
described herein, those skilled in the art will recognize that
various well-known methods and structures may be employed for this
purpose.
[0054] FIGS. 7(a) and 7(b) are fragmentary cross-sectional views
depicting alternative geometries and configurations of the thermal
source. In FIG. 7(a), a thermal source comprising a thin film
heater 702 is positioned atop a pedestal region 704 extending
upwardly from upper surface 706 of cladding 708 and is in
substantial vertical alignment with core 710. It is believed that
the geometry depicted in FIG. 7(a) will tailor the flow of thermal
energy from heater 702 to the heat sink (not shown) such that the
temperature gradients produced within core 710 and surrounding
cladding 708 are maximized along the vertical axis and minimized
along the horizontal axis, which in turn will increase device
efficiency (in terms of power consumed per unit of attenuation).
Pedestal 704 may be formed, for example, by using heater 702 as a
mask while etching the upper margins of cladding 708 by the
required amount.
[0055] FIG. 7(b) depicts another alternative configuration of the
thermal source wherein two horizontally spaced thin film heaters
720 and 722 are employed in place of a single heater. Thin film
heaters 720 and 722 are positioned in contact with upper surface
706 of cladding 708 and are horizontally offset by equal distances
from the centerline of core 710. Heaters 720 and 722 are preferably
interconnected such that each heater received the same thermal
source control signal (e.g., an identical voltage). This
arrangement offers the advantage of locating heaters 720 and 722 a
greater overall distance from core 710, thus further reducing
absorption of the optical mode by the thermal source.
[0056] It will be realized by those skilled in the art that other
geometries, arrangements, and sizes of thin film heaters may be
utilized to optimize various operational and design aspects of the
VOA. For example, multiple heaters may be placed over a waveguide
core to increase the maximum achievable attenuation.
[0057] According to yet another embodiment of the invention, a VOA
(not depicted) is provided wherein each waveguide has two or more
longitudinally spaced thermal source zones, each zone being
associated with a thin film heater or other thermal source. At the
thermal source zone located upstream in the optical path, a portion
of the optical energy refracted by the thermo-optic effect is
reflected upwardly at the distal core/cladding boundary and remains
within the core. The longitudinal spacing and size of the thermal
source(s) located downstream in the optical path is selected such
that the downstream thermal source acts to increase the vertical
momentum component of the previously reflected wavefront, thereby
avoiding cancellation occurring between the reflected and
diffracted wavefronts at the downstream thermal source zone(s) and
increasing the attenuation efficiency of the VOA.
[0058] FIG. 8 schematically depicts an attenuation system 800 in
accordance with the present invention. Attenuation system 800
includes a VOA 802 of similar description to VOA 500 described
above in connection with FIGS. 5 and 6. VOA 802 is provided with a
plurality of waveguides individually comprising an elongated core
embedded in a cladding. Each waveguide has in operational
association therewith a thermal source (individually and
collectively denoted as 804) for generating a thermal gradient
across the core and surrounding cladding to enable the controlled
extraction of optical energy in the direction away from thermal
source 804 in the manner described above. To enable measurement of
the amount of optical energy (i.e., an optical power level) in the
core at a location downstream of the thermal source, each waveguide
is optically coupled to a conventional optical sensor or detector
(individually and collectively denoted as 808) configured to
measure the optical power level within the core and to responsively
generate an output signal representative of the measured power
level. Coupling of the waveguides to the sensor 808 array (each
sensor 808 uniquely corresponding to a waveguide) may be achieved
using optical taps or similar expedient well known in the art.
Sensors 808 may comprise, for example, an integrated germanium
detector array formed on a separate platform 809 and attached to
VOA device 802 using conventional alignment and attachment
techniques.
[0059] The sensor output signals are conveyed as input to control
circuitry 810, which is configured to determine, for each
waveguide, the amount of attenuation required to meet a set of
predetermined conditions (e.g., equalization of optical power
levels among the several waveguides). Control circuitry 810 is
further configured to apply a thermal source control signal to each
thermal source 804, the signal being of appropriate strength to
effect the required amount of attenuation. As is known in the art,
the thermal source control signals applied to thermal sources 804
may be determined by control circuitry 810 in accordance with
pre-established input-output relationships, and may be adjusted on
a continuous basis to reflect variations in the downstream optical
power, as measured by sensors 808.
[0060] VOA 802 may be coupled to input optical fibers 812 (which
carry the input optical signals) and output optical fibers 814
(which carry the post-attenuation output optical signals) using any
one of a variety of coupling techniques. Input and output optical
fibers 812 and 814 will typically comprise standard commercially
available fibers, such as the SMF-28.TM. single mode fiber
available from Corning Incorporated of Corning, N.Y. Coupling of
fibers 812 and 814 to VOA 802 may be accomplished by using a
silicon submount (not shown) having V-grooves for receiving and
positioning the fiber ends. The submount will preferably include
means for first positioning and attaching thereto a set of guide
fibers, which are in turn employed to precisely align the submount
with VOA 802. Following alignment and attachment of VOA 802 to the
submount, input and output optical fibers 812 and 814 are attached
to the submount/VOA 802 assembly using the V-grooves formed in the
submount to align the fibers with the VOA 802 waveguides. Input and
output coupling losses may be reduced by pigtailing the optical
fiber ends and filling the gaps between the fibers and the
waveguides using a UV-curing polymer or other suitable material
having an index substantially equal to the index of the waveguide
core material.
[0061] VOA performance may be improved by providing a region of
reduced vertical confinement in the VOA waveguide(s) and locating
the thermal source above this relatively weakly confining region.
The weakly confining portion of the waveguide may be created, for
example, by appropriately varying the transverse dimensions or
cross-sectional geometry of the waveguide core, and/or by
segmentation of the core. FIGS. 9(a) and 9(b) depict a VOA 900
according to an alternative embodiment of the present invention,
wherein a region of reduced confinement is produced by expanding
one or both of the transverse dimensions of a portion of core 902.
A waveguide defined by core 902 and surrounding cladding 904
includes an input section 906 and output section 908, in which core
902 has a relatively small cross-sectional area, and an
intermediate or weakly confining section 910, in which one or more
of the transverse dimensions of the core (i.e., height and width)
are expanded to produce a broadened and/or deepened waveguide. In a
typical implementation of the FIG. 9 embodiment, core 902 has
transverse dimensions of 7.5 .mu.m high by 7.5 .mu.m wide in input
section 906, as compared to 10.0 .mu.m high by 10.0 .mu.m wide in
intermediate section 910. Core 902 and cladding 904 are preferably
fabricated from polymeric materials having matched thermo-optic
coefficients, such as the material set described above in
connection with FIG. 5. While FIG. 9 depicts both transverse
dimensions of core 902 as being relatively enlarged in intermediate
section 910, other implementations of the FIG. 9 embodiment may
utilize a core having one constant transverse dimension and one
enlarged transverse dimension. A substrate (not shown in FIG.9) may
be attached to the lower major surface of cladding 904 to provide
mechanical stability and to serve as a thermal heat sink.
[0062] Intermediate section 908 is optically coupled to input
section 904 and output section 906 by (respectively) tapers 912 and
914. Tapers 912 and 914 are preferably designed as adiabatic tapers
to reduce losses associated with expansion and contraction of the
optical mode, and may be fabricated by photolithography and etching
processes known in the art. It is noted that the typical sizes and
aspect ratios of tapers 912 and 914 cannot be accurately depicted
in the figures due to space constraints.
[0063] A thin film heater 916 positioned above reduced-confinement
section 910 is operative to generate a vertical thermal gradient in
core 902 and surrounding cladding 904, causing a portion of the
optical energy propagating through core 902 to be deflected in the
downward direction and thereby extracted from the waveguide, in a
manner similar to that described above in connection with FIG. 1.
Heater 916 preferably has its longitudinal centerline aligned with
that of core 902 and has a width sufficient to avoid the generation
of large lateral (horizontal) thermal gradients within core
902.
[0064] As is discussed above in connection with FIG. 3, the amount
of optical energy extracted from core 902 for a given amount of
refractive index perturbation induced by heater 916 is negatively
correlated with the degree of mode confinement. Because the
cross-sectional area of core 902 increases within intermediate
section 910, its effective confinement will decrease, thus allowing
optical energy to be more easily deflected from core 902. Reduction
of vertical confinement within intermediate section 910 thereby
facilitates desired extraction of optical energy from core 902 and
allows a specified amount of signal attenuation to be accomplished
within a shorter distance and/or using reduced heater power
consumption relative to that required in a more strongly vertically
confining waveguide.
[0065] A relatively weakly confining section may alternatively be
created in the waveguide by segmentation of a portion of the
waveguide. The segmentation of the core may be accomplished by
employing suitable fabrication techniques known in the art. The
thermal source is located above the segmented portion of the core,
and the reduced vertical confinement thereby facilitates extraction
of optical energy as described above.
[0066] FIGS. 12(a) and 12(b) symbolically depict a VOA 1200
according to yet another alternative embodiment of the present
invention. In this embodiment, a region of reduced confinement is
produced by providing an intermediate section 1210 in core 1202
that has a reduced refractive index. Specifically, the refractive
index n.sub.int of intermediate section 1210 is in the range
n.sub.clad.ltoreq.n.sub.int<n.sub.input, where n.sub.input is
the refractive index of core input section 1206 and n.sub.clad is
the refractive index of cladding 1204. In a preferred
implementation of the FIG. 12 embodiment, intermediate section 1210
is formed from the same material as cladding 1204, and is
constructed by removing a portion of core 1202 and backfilling the
removed portion with cladding material. Intermediate section 1210
is optically coupled to input section 1206 and to an output section
1208, which will typically be formed of the same material as input
section 1206 to provide good confinement of the optical mode. A
substrate (not shown in FIG. 12) may be attached to the lower major
surface of cladding 1204 to provide mechanical stability and to
serve as a thermal heat sink.
[0067] When actuated, thin film heater 1216 generates a vertical
thermal gradient in intermediate section 1210 and surrounding
cladding 1204, causing a portion of the optical energy propagating
through intermediate section 1210 to be extracted from the
waveguide, in a manner similar to that described above. Again,
reduction of the confinement of the optical mode within
intermediate section 1210 (due to the relatively small refractive
index difference n.sub.int-n.sub.clad) facilitates the controlled
extraction of optical energy from the waveguide and allows a
specified amount of signal attenuation to be accomplished within a
shorter distance and/or using reduced heater power consumption
relative to that required for a more strongly vertically confining
waveguide. The FIG. 12 embodiment offers the further advantage of
improved polarization dependent loss (PDL) relative to a VOA having
a uniformly strongly confining waveguide.
[0068] FIG. 10 symbolically depicts a multi-channel VOA 1000
constructed in accordance with another, more specific embodiment of
the invention. VOA 1000 is substantially similar in its structure
and mode of operation to VOA 500 of FIG. 5; however, VOA 1000
employs a more complex waveguide structure wherein the upper
cladding layer is formed from upper and lower sub-layers 1008 and
1010 having separately optimized properties. Beginning with the
lowermost layer of VOA 1000 and proceeding upwardly, an adhesion
layer 1004 is deposited (typically by spinning) on top of substrate
1002 to enable uniform adhesion between substrate 1002 and lower
cladding layer 1006 and prevent de-lamination. Adhesion layer 1004
will preferably have a refractive index that is less than that of
substrate 1002 but greater than or equal to the of lower cladding
1006 in order to prevent adhesion layer 1004 from serving as a
waveguide for light removed from cores 1012. In an exemplary
implementation of VOA 1000, adhesion layer will be formed from a
polymer having a refractive index of about 1.514, whereas substrate
1002 and lower cladding layer 1006, which vertically bound adhesion
layer 1004, will have respective refractive indices of 1.52 and
1.514 (or less). Adhesion layer 1004 will have a thickness of
approximately 1 .mu.m.
[0069] Lower cladding layer 1006 is preferably formed from a
polymeric material deposited to a typical thickness of 30 .mu.m on
top of adhesion layer 1004. As described above in connection with
FIG. 5, lower cladding layer 1006 has a thermo-optic coefficient
matched to that of the core 1012 material in order to minimize
device sensitivity to changes in ambient temperature. Cores 1012
may be formed by depositing a core layer of suitable thickness on
top of lower cladding 1006, and then defining the lateral
boundaries of each core 1012 by a subsequent RIE step, as described
above in connection with FIG. 6. The resultant cores 1012 will each
have a rectangular cross-section with exemplary dimensions of 7.5
.mu.m wide by 7.5 82 m high. In order to provide confinement of the
optical mode, cores 1012 have a refractive index that is greater
(at a given temperature) than the refractive index of the adjacent
cladding layers. In one example, cores 1012 have a refractive index
of 1.52, which exceeds that of the adjacent cladding layers by
approximately 0.005.
[0070] The upper cladding of VOA 100 comprises a lower or first
sublayer 1010 and an upper or second sublayer 1008 deposited
sequentially on top of lower cladding layer 1006 and cores 1012.
Lower sublayer 1010 will typically have a thickness of about 10.5
.mu.m and extend to approximately 3 .mu.m above the top of cores
1012. Lower sublayer 1010 may be deposited in a two-step process to
provide better sublayer planarity. The lower sublayer 1010 material
is selected to have a thermo-optic coefficient matched to that of
core 1012 material in order to avoid or minimize operational
sensitivity to ambient temperature, in the manner discussed in
greater detail above. Lower sublayer 1010 may, for example, be
formed from the same polymeric material as is used to form lower
cladding layer 1006 (which, as stated above, has a typical
refractive index of 1.514).
[0071] Upper sublayer 1008 may be deposited on top of lower
sublayer 1010 by spinning or other conventional technique. Upper
sublayer 1008 is preferably formed from a polymer material having a
relatively low refractive index to optically isolate the optical
energy propagating along cores 1012 from heaters 1014 and prevent
undesirable absorption of the optical modes. In an exemplary
implementation of the FIG. 10 embodiment, upper sublayer 1008 has a
thickness of about 3 .mu.m and a refractive index of 1.48; however,
other polymer materials having lower refractive indices may be
preferable in certain implementations. Since upper sublayer 1008
does not bound cores 1012, their respective thermo-optic
coefficients need not be matched. Following deposition of upper
sublayer 1008, thin film heaters 1014, positioned above cores 1012,
may then be formed on the upper surface of upper sublayer using the
photolithography techniques described above or by any suitable
alternatives. The vertical distance between each thin film heater
1014 and the upper (proximal) boundary of the associated core 1012
will typically be in the range of 4.0-8.0 .mu.m, with a preferred
value of 5.0 .mu.m
[0072] FIGS. 11(a) and 11(b) depict a VOA 1100 to which a cover
plate 1102 has been attached, thereby forming a quasi-hermetic
package for VOA 1100. A portion of cover plate 1102 has been cut
away in FIG. 11(a) to reveal underlying structures so that the
features and purpose of this embodiment may be more easily
comprehended. As shown in FIG. 11(b), which depicts a vertical
cross-section taken through the midpoint of heaters 1108, VOA 1100
generally comprises a polymer stack 1104 deposited on top of
substrate 1106 and a set of heaters 1108 arranged on the upper
major surface 1110 of polymer stack 1104. Polymer stack 1104 will
include a core region bounded by a set of cladding layers, and may
be formed in accordance with any of the embodiments described
above. Cover plate 1102 serves to protect the upper major surface
of the polymer stack 1104 from contact with corrosive, reactive, or
otherwise deleterious components in the ambient atmosphere and
thereby prolong VOA 1100 operational lifetime. Cover plate 1102 may
consist of a glass or other suitable material and will have planar
dimensions which are generally coextensive with those of VOA 1100.
Typical cover plate 1102 thickness will be approximately 1000
.mu.m.
[0073] Cover plate 1002 is preferably affixed to the upper major
surface of polymer stack 1104 by means of a thixotropic adhesive
1112, such as a conventional epoxy. Adhesive 1112 may be applied to
the upper major surface of polymer stack 1104 and/or the facing
(lower) surface of cover plate by stencil printing or other
technique known in the art, and curing of the adhesive may be
accomplished thermally, by exposure to UV radiation, or by another
method appropriate to the selected adhesive. As depicted in FIG.
11(b), the cured adhesive 1112 acts as a stand-off structure such
that cover plate 1102 and polymer stack 1104 are held in slightly
vertically spaced apart relation. The vertical distance between the
facing surfaces of the two components will typically be about 150
.mu.m.
[0074] Adhesive 1112 is preferably not applied in areas above and
immediately adjacent to heaters 1108, thereby creating a void 1116
surrounding the heaters. This arrangement (i.e., avoiding thermal
contact between heaters 1108 and cover plate 1102 and/or adhesive
1112) minimizes the undesirable conduction of heat generated by
heaters 1108 upwardly into cover plate and adhesive 1112. By
minimizing such upward conduction, thermal energy generated by
heaters 1108 is directed primarily downwardly into polymer stack
1104, which allows steep thermal gradients to be developed within
the waveguides with relatively small heater 1108 power consumption.
In this manner, VOA 1100 will possess a high attenuation efficiency
(i.e., amount of attenuation effected per unit of power
supplied).
[0075] It will be recognized by those skilled in the art that,
while the invention has been described above in terms of preferred
embodiments, it is not limited thereto. Various features and
aspects of the above described invention may be used individually
or jointly. Further, although the invention has been described in
the context of its implementation in a particular environment and
for particular applications, e.g., optical communications, those
skilled in the art will recognize that its usefulness is not
limited thereto and that the present invention can be beneficially
utilized in any number of environments and implementations.
Accordingly, the claims set forth below should be construed in view
of the full breadth and spirit of the invention as disclosed
herein.
* * * * *