U.S. patent application number 09/727004 was filed with the patent office on 2003-02-13 for photothermal optical switch and variable attenuator.
Invention is credited to DeRosa, Michael E., Guermeur, Celine C., Loguov, Stephen L., Moroni, Marc, Vidiella, Guilhem M..
Application Number | 20030031402 09/727004 |
Document ID | / |
Family ID | 8242217 |
Filed Date | 2003-02-13 |
United States Patent
Application |
20030031402 |
Kind Code |
A1 |
DeRosa, Michael E. ; et
al. |
February 13, 2003 |
Photothermal optical switch and variable attenuator
Abstract
An optical device which utilizes a photothermal optical effect
to achieve switching or attenuation includes a waveguide defined by
a waveguide core and a surrounding cladding, wherein the polymer
waveguide core includes a region consisting of a photothermally
responsive material having an absorption coefficient at a switch
wavelength or attenuation wavelength that is higher than an
absorption coefficient at a signal wavelength. Switching devices
include an optical splitter circuit having a branch that includes
the photothermally responsive material, and either a multiplexer
for introducing light at the switch wavelength into the optical
circuit or a light source focused at the photothermally responsive
material. Attenuating devices include a Mach-Zehnder type
interferometer having a branch that includes the photothermally
responsive material and either a multiplexer for introducing light
at the attenuation wavelength into the optical circuit or a light
source focused at the photothermally responsive material.
Inventors: |
DeRosa, Michael E.; (Painted
Post, NY) ; Guermeur, Celine C.; (Chartrettes,
FR) ; Loguov, Stephen L.; (Corning, NY) ;
Moroni, Marc; (Melun, FR) ; Vidiella, Guilhem M.;
(Paris, FR) |
Correspondence
Address: |
CORNING INCORPORATED
SP-TI-3-1
CORNING
NY
14831
|
Family ID: |
8242217 |
Appl. No.: |
09/727004 |
Filed: |
November 30, 2000 |
Current U.S.
Class: |
385/16 ; 385/140;
385/45 |
Current CPC
Class: |
G02F 1/225 20130101;
G02F 1/061 20130101; G02F 1/3137 20130101; G02F 1/0126 20130101;
G02F 1/0147 20130101; G02F 2203/48 20130101; G02F 2202/022
20130101 |
Class at
Publication: |
385/16 ; 385/45;
385/140 |
International
Class: |
G02B 006/35; G02B
006/26 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 16, 1999 |
EP |
99403166.4 |
Claims
What is claimed is:
1. An optical switch for switching light at a switch wavelength as
opposed to a signal wavelength, the optical switch comprising: a
splitter having a waveguide defined by a core and a cladding
surrounding the core, the core including an input leg, a first
branch leg, and a second branch leg, the first branch leg and the
seconds branch leg joined at a junction whereby the light may be
propagated from the input leg through each of the first branch leg
and the second branch leg, at least the first branch leg including
a region fabricated from a material having an absorption
coefficient at the switch wavelength that is higher than an
absorption coefficient at the signal wavelength.
2. The optical switch of claim 1 wherein the material has an
absorption coefficient at the switching wavelength that is at least
about 100 times higher than the absorption coefficient at the
signal wavelength.
3. The optical switch of claim 2 wherein a portion of the core
comprises: a blend of a polymer and an added material which when
blended with the polymer imparts to the blend the ability to absorb
light at the switch wavelength while remaining generally
transparent to light at the signal wavelength.
4. The optical switch of claim 3 wherein the added material is
physically blended with the polymer.
5. The optical switch of claim 3 wherein the added material is
covalently bonded with the polymer.
6. The optical switch of claim 3 wherein the added material is
selected from a group consisting of quinone, indanthrenes, methine,
polymethines, phthalocyanines, naphthalocyanines, porphyrins,
organic dyes, a metal ion, or metal particles.
7. The optical switch of claim 6 wherein the organic dye is crystal
violet.
8. The optical switch of claim 6 wherein the metal ion is
Co(II).
9. The optical switch of claim 11 wherein the metal particles have
an average particle size of from about 20 nm to about 100 nm.
10. The optical switch of claim 9 wherein the metal particles are
selected from a group consisting of copper, gold, silver, platinum,
or palladium.
11. The optical switch of claim 1 further comprising: a multiplexer
for combining the light at the signal wavelength and the light at
the switch wavelength to produce combined light, and propagating
the combined light through the input leg.
12. The optical switch of claim 1 further comprising: a laser diode
for emitting the light at the switch wavelength; and a lens for
focusing the light at the switch wavelength at the region of the
first branch leg fabricated from the material having the absorption
coefficient at the switch wavelength that is higher than the
absorption coefficient at the signal wavelength.
13. A variable optical attenuator for attenuating light at an
attenuation wavelength as opposed to a signal wavelength, the
variable optical attenuator comprising: a waveguide having a core
including a first input waveguide section, a second waveguide
section which branches from the first input waveguide section at a
first junction, a third waveguide section which branches from the
first input waveguide section at the first junction, and a fourth
output waveguide section joined to the second waveguide section and
the third waveguide section at a second junction, the second
waveguide section including a region fabricated from a material
which changes refractive index when exposed to the light at an
attenuation wavelength, but which is unresponsive to the light at a
signal wavelength.
14. The variable optical attenuator of claim 13 wherein the
material has an absorption coefficient at the switching wavelength
that is at least about 100 times higher than the absorption
coefficient at the signal wavelength.
15. The variable optical attenuator of claim 14 wherein a portion
of the core comprises: a blend of a polymer and an added material
which when blended with the polymer imparts to the blend the
ability to absorb light at the switch wavelength while remaining
generally transparent to light at the signal wavelength.
16. The variable optical attenuator of claim 15 wherein the added
material is physically blended with the polymer.
17. The variable optical attenuator of claim 15 wherein the added
material is covalently bonded with the polymer.
18. The variable optical attenuator of claim 15 wherein the added
material is selected from a group consisting of quinone,
indanthrenes, methine, polymethines, phthalocyanines,
naphthalocyanines, porphyrins, organic dyes, a metal ion, or metal
particles.
19. The variable optical attenuator of claim 18 wherein the organic
dye is crystal violet.
20. The variable optical attenuator of claim 18 wherein the metal
ion is Co(II).
21. The variable optical attenuator of claim 18 wherein the metal
particles have an average particle size of from about 20 nm to
about 100 nm.
22. The variable optical attenuator of claim 18 wherein the metal
particles are selected from a group consisting of copper, gold,
silver, platinum, or palladium.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] This invention relates generally to optical devices, and
more particularly to a thermorefractive switch or attenuator
utilizing temperature dependence of the refractive index of a
waveguide core to effect switching or attenuation in an optical
device.
[0003] 2. Technical Background
[0004] Optical switching devices are known which utilize materials
in which the refractive index of a polymer waveguide material can
be controlled through various phenomena such as by a second order
nonlinear electrooptic effect, a third order nonlinear optical Kerr
effect, a thermooptic effect, or an acoustooptic effect. Polymers
have been used in thermooptic switches because of their large
negative variations in the index of refraction as a function of
temperature (dn/dT), which are typically about
-3.times.10.sup.-4K.sup.-1. Stated differently, polymer materials
are useful in the fabrication of thermooptic switches because a
relatively small temperature change can effect a relatively large
change in the refractive index of the polymer.
[0005] Typical polymer based thermooptic switches use thin film
electrical strip heaters in contact with a planar polymer
waveguide. These heaters effect a change in the refractive index
thus causing thermooptic switching to occur. Fabrication of such
devices requires metal electrode deposition techniques and
integration of electronics into the planar optical devices which
increase the complexity and cost of the device. A further
disadvantage is that most electrically heated thermooptical devices
are limited to switching speeds of about one to about two
milliseconds due to thermal diffusion time lag.
[0006] It has been known that thermooptic effects can be induced in
polymers by a photothermal phenomena in which light absorbed by the
polymer is converted into heat which causes a change in refractive
index. The change in temperature in a material due to absorption at
steady state can be approximated by the following equation: 1 T = P
c r2 C
[0007] where .alpha. is the absorption coefficient, P is the steady
state power, .tau..sub.C is the characteristic decay time after the
power has been turned off, r is the spot size radius of the area of
the material which is irradiated with light having steady state
power P, .rho. is the density of the material, and C is the heat
capacity of the material. By exploiting photothermal effects,
active switches can be developed by inducing local refractive index
changes due to a finite amount of localized absorption of light by
the polymer. A known device utilizing a photothermal effect for
optical switching comprises a substrate of light absorbent
material, means defining a plurality of holes of pre-selected size
through the substrate, the holes being defined through the
substrate, and a liquid material disposed within the holes. The
liquid material has an index of refraction which is substantially
temperature dependent over a selected temperature range of
operation for the device. A disadvantage with this device is that
it uses a liquid material which could potentially escape from the
device, rendering the device inoperative. Accordingly, a solid
state device is preferable.
[0008] An optical switch comprising a waveguide having a polymeric
waveguide core including a region containing molecules which absorb
energy from a light source and thereby heat the core and change the
refractive index of the core are described in the patent
literature. The device includes a waveguide having an input region,
a Y-branch which splits light entering the input region into the
two separate branches, and a coupling region. One of the two
branches includes a means for changing the temperature to cause a
change in refractive index, which in turn results in a phase shift
between the light propagated through each of the two branches. When
the light enters the coupling region a predetermined transfer, or
switching, of light occurs from one leg to the other, with the
amount of the transfer depending upon the phase change.
SUMMARY OF THE INVENTION
[0009] The invention provides an optical switching device which is
capable of achieving submillisecond switching or attenuation for
regions that are irradiated on the order of the size of a single
mode waveguide mode field diameter.
[0010] In accordance with an aspect of this invention, an optical
switch is provided which includes an optical splitter having a
waveguide defined by a core and a surrounding cladding, wherein the
waveguide includes an input leg, and first and second branch legs.
The branch legs and the input leg are joined at a junction wherein
light may be propagated from the input leg through each of the
branch legs. The first branch leg includes at least a region
comprised of a material having an absorption coefficient at a
switch wavelength that is higher than an absorption coefficient at
a signal wavelength.
[0011] In accordance with another aspect of the invention, a
variable optical attenuator is provided. The variable attenuator
includes a waveguide defined by a core and a surrounding cladding,
wherein the core includes a first input waveguide section, a second
waveguide section branching from the first waveguide section at a
first junction, a third waveguide section branching from the first
waveguide section at the first junction, and a fourth output
waveguide section joined to the second and third waveguide sections
at a second junction. The second waveguide section includes a
region comprising a material which changes refractive index when
exposed to light at an attenuation wavelength, but which is
unresponsive to light at a signal wavelength.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] FIG. 1 is a block diagram of the testing equipment used for
demonstrating the fast switch decay times using the present
invention;
[0013] FIG. 2 is a graph demonstrating the fast switching response
of the device of the present invention shown in FIG. 1;
[0014] FIG. 3 is a schematic view of a 1.times.2 photothermal
switching device in which switching is achieved by irradiating a
local region of a polymer waveguide core with light at a switch
wavelength;
[0015] FIG. 4 is a schematic view of a 1.times.2 photothermal
switching device in which switching is achieved by propagating a
switch signal through a waveguide;
[0016] FIG. 5 is a schematic view of a photothermal variable
attenuator in which attenuation is achieved by propagating light at
the attenuating wavelength through the device; and
[0017] FIG. 6 is a schematic view of a photothermal variable
attenuator in which attenuation is achieved by directing light at
the attenuating wavelength at a photothermally responsive region of
the device.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0018] The invention relates to the generation of heat in a
thermooptical device through a light absorption mechanism. All or
part of the waveguide regions of planar optical devices may be
provided with a polymer waveguide core having an absorption
coefficient at a switch wavelength or an attenuation wavelength
that is higher than an absorption coefficient at a signal
wavelength. Preferably, the absorption coefficient at the switch
wavelength or attenuation wavelength is at least about 100 times
higher than the absorption coefficient at the signal wavelength
within at least a section of the polymer waveguide core. Those
regions may comprise a polymer inherently having such property, or
may comprise a polymer blended with a material which absorbs light
at the switch wavelength or attenuation wavelength and is
substantially transparent to light at the signal wavelength. The
polymer waveguide material which has an absorption coefficient at a
switch wavelength that is higher than an absorption coefficient at
a signal wavelength will hereafter be referred to as switching
material or attenuating material, depending on the application.
When the switching material or attenuating material, or at least a
portion thereof, is exposed to light at the switch wavelength or
attenuation wavelength, the exposed portion of the switching
material or attenuating material absorbs the light at the switch
wavelength or attenuation wavelength and converts it into heat,
thereby raising the temperature and lowering the refractive index
of the exposed portion of the switching material or attenuating
material. This photothermal optical effect allows much faster
switching and attenuating as compared with conventional devices
using thin film electrical strip heaters because heat is generated
in the waveguide core instead of being generated outside of the
waveguide and conducted through the cladding to the core.
[0019] The faster switching or attenuating is based on the
characteristic thermal propagation time (.tau..sub.char) given by
the equation: 2 char = a 2 4
[0020] where "a" is the radius of the irradiated zone, and .chi. is
the thermal diffusivity of the switching material or attenuating
material. Using a typical value for the thermal diffusivity of
polymers used for optical waveguide cores, the above equation
suggests that it is possible to achieve submillisecond switching
when the regions that are irradiated are on the order of the size
of a single mode waveguide mode field diameter.
[0021] The magnitude of the photothermal effect can be enhanced by
increasing the absorption of the polymer waveguide at a specified
switch wavelength or attenuation wavelength. In this manner, it is
possible to have a polymer waveguide that is transparent to the
signal wavelength (e.g. about 1300 nm or about 1550 nm) but can be
switched photothermally by a different switch wavelength or
attenuation wavelength (e.g. from about 700 to about 800 nm). The
polymer can be made to absorb more light at the switch wavelength
or attenuation wavelength by incorporation a chromophore that
absorbs light at the switch wavelength or attenuation wavelength
and is substantially transparent to the signal wavelength.
[0022] The test apparatus 10 shown in FIG. 1 was used to
demonstrate submillisecond switching of the refractive index of an
organic adhesive material 12 (switching material) at the end of a
single mode optical fiber 14. The apparatus demonstrated that it
was possible to achieve fast switch decay times due to the small
size of the irradiated dimensions provided by a single mode fiber.
A photothermal pump probe experiment was performed on adhesive 12.
Adhesive 12 was bonded to an end of a flat cleaved optical fiber
14. Optical fiber 14 was mounted perpendicular to a glass slide 16
so that adhesive 12 was between glass slide 16 and the end of fiber
14. The path length of adhesive 12 was approximately 100
micrometers and the mode field diameter at a wavelength of 1550 nm
was approximately 10 micrometers. A wavelength division multiplexer
(WDM) 18 was used to couple light having a wavelength of 633 nm
from a light source 20 with light having a wavelength of 1550 nm
from light source 22. The power of the 633 nm light was monitored
with a silicon photodiode detector located about 10 centimeters
from glass slide 16. As adhesive 12 absorbed light having a
wavelength of 1550 nm, the negative change in refractive index due
to a temperature increase caused the beam of light emitted from
glass slide 16 toward light detector 24 to defocus, thus causing
the power to drop. FIG. 2 shows the response of the 633 nm light
modulated by the 1550 nm light at 2 KHz. The thermal characteristic
decay time was measured to be about 240 microseconds.
[0023] In FIG. 3, there is shown a 1.times.2 photothermal switch 30
including a substrate 32 on which is formed a waveguide comprising
waveguide cladding 34 and a waveguide core 36. Although illustrated
schematically in FIG. 3, it is well known in the art that cladding
34 completely surrounds or envelops waveguide core 36. Typically by
first depositing a layer of undercladding material on substrate 32,
forming a patterned waveguide core 36 on the undercladding, and
overcoating the patterned waveguide core 36 so that core 36 is
surrounded by cladding material having an index of refraction less
than the index of refraction of waveguide core 36. Waveguide core
36 is made of a polymeric material that is transparent to a signal
wavelength (typically a signal having a wavelength of about 1300,
about 1310 or from about 1530 to about 1570 nm), but which absorbs
light at a switch wavelength (e.g. 700-800 nm). It should be
understood that the expressions "transparent" and "absorbent" as
used to describe the ability of a material to propagate and absorb
light are relative terms. Likewise, it is to be understood that the
expressions "photothermally responsive", "photothermally
unresponsive" and the like are also relative terms. That is
transparent materials may absorb some light, and absorbing
materials may absorb only a small fraction of an impinging light
and allow the remaining light to propagate through the absorbent
material. However, the polymeric switching material preferably has
at least a two order of magnitude higher absorption coefficient at
the switch wavelength than at the signal wavelength. The high
absorption coefficient at the switch wavelength can be inherent in
the polymeric structure or can be incorporated into the switching
material by blending a suitable polymer with a material which when
blended with the polymer absorbs light at the switch wavelength and
is substantially transparent to light at the signal wavelength. The
light absorbing material can for example be a linear absorbing dye
that is either doped or covalently attached to the polymeric
structure. Examples of organic dyes that can be used include
quinone, indanthrenes, methine and polymethines, phthalocyanines,
naphthalocyanines, and porphyrins. Device 30 is a Y splitter having
waveguide core 36 which includes an input leg 36A and branch output
legs 36B and 36C which branch or split away from input leg 36A at
junction 36C. One or both of branch legs 36B and 36C may include a
region 38 in which the waveguide core is comprised of a polymeric
material having an absorption coefficient at a switch wavelength
that is at least about 100 times higher than an absorption
coefficient at a signal wavelength (i.e., switching material). For
example, region 38 may comprise a polymer blend containing a dye
that absorbs light at the switch wavelength and is transparent to
light at the signal wavelength. The switching material region is
located adjacent junction 36C. A laser diode 40 and a microlens 42
are mounted on substrate 32 to focus light at the switch wavelength
on region 38 (switching material) of waveguide core 36. When light
at the switch wavelength is focused on region 38 from laser diode
40, the refractive index of region 38 is reduced as the temperature
of region 38 increases. When the refractive index is low enough to
cause total internal reflection (i.e., when the refractive index of
the switching material at region 38 is about the same as the
refractive index of the cladding), a signal beam propagated through
waveguide 36 in the direction indicated by arrow 43 is directed
exclusively through branch leg 36C, with substantially none of the
light being directed through branch leg 36B. The switch signal from
laser diode 40 is focused on an area of switching material in
region 38 having a diameter of approximately 8 to 10 microns.
Although the switching material is localized in area 38 of the
illustrated embodiment (i.e., is focused on a small spot), and
because the photothermal effect occurs only at the spot illuminated
by light at the switch wavelength, the entire waveguide 36 may be
comprised of switching material. However, in this case care should
be taken to ensure that the light at the switch wavelength is
focused on region 38 and not on input leg 36A. This may be
preferred to simplify fabrication when the switching material is
relatively inexpensive. A region 39, similar to region 38, may be
provided in leg 36C, and laser diode 40' and microlens 42' may be
provided to allow light to be exclusively propagated through leg
36B when diode 40 is off and diode 40' is on.
[0024] As another alternative, rather than focusing a beam of light
at the switch wavelength at the waveguide core, light at the switch
wavelength may be propagated through the waveguide of an optical
switching device along with light at the signal wavelength using a
wavelength division multiplexer as shown in FIG. 4. Optical
switching device 50 shown in FIG. 4 includes a waveguide core 52
including an input leg 52A and two branch legs 52B and 52C which
branch or split away from input leg 52A at junction 52C. An optical
fiber 54 is used for directing light at a signal wavelength from a
light source 56 to an input port on a wavelength division
multiplexer 58, and an optical fiber 60 is used for directing light
at a switch wavelength from light source 62 to a second input port
on wavelength division multiplexer 58. The signals are combined by
wavelength division multiplexer 58 and are propagated through main
leg 52A from an output port of wavelength division multiplexer 58
in the direction generally indicated by arrow 64. Branch leg 52B
includes a region 66 comprised of a switching material having an
absorption coefficient at the switch wavelength that is higher than
the absorption coefficient at the signal wavelength. When light at
the switch wavelength from light source 62 is not directed into
wave division multiplexer 58, light at the signal wavelength from
light source 56 is split between branch legs 52B and 52C. However,
when the switch light from light source 62 is directed into the
optical circuit, the light at the switch wavelength is absorbed by
the switching material at region 66 of branch leg 52B of waveguide
core 52, and the material at region 66 is heated causing the index
of refraction to change, thus preventing light from being
propagated through branch leg 52B, such that all of the light at
the signal wavelength propagates through branch leg 52C (i.e.,
region 66 behaves as part of the cladding).
[0025] As a specific example, a 1 mmol concentration of
phthalocyanine dye in the switching material regions 38, 66 of the
waveguide cores in the devices shown in FIGS. 3 and 4 would have an
absorption coefficient of about 150 cm.sup.-1 at 698 nm. Thus, it
would be possible to induce a refractive index change of about
5.times.10.sup.-3 with a 0.5 mW laser diode focused on a 10
micrometer diameter spot on the switching material for the
embodiment shown in FIG. 3, or coupled into the fiber as shown in
the embodiment of FIG. 4.
[0026] In accordance with another aspect of the invention, a
variable attenuator employing the attenuating material of this
invention is shown in FIG. 5. Variable attenuator 67 includes a
wave division multiplexer 58', a signal source 56', a waveguide 54
(such as an optical fiber) for propagating light to an input on
wave division multiplexer 58, an attenuation light source 62', a
waveguide 60' (such as an optical fiber) for propagating light from
the attenuation light source to an input on wave division
multiplexer 58', and an interferometer waveguide 68A-68F including
a region 70 comprising a photothermally responsive attenuation
material which changes refractive index upon exposure to light at
an attenuation wavelength. The attenuating material at region 70
may be identical to the switching material at region 38.
Interferometer 68 has a Mach-Zehnder geometry comprising a first
input waveguide section 68A, a second waveguide section 68B which
branches from the first waveguide section at first junction 68C, a
third waveguide section 68D which also branches from first input
waveguide section 68A at first junction 68C, and a fourth output
waveguide section 68E joined to the second waveguide section 68B
and the third waveguide section 68D at a second junction 68F. When
light at an attenuation wavelength is emitted from attenuation
light source 62', the region 70 absorbs the light at the
attenuation wavelength, which heats and changes the refractive
index of region 70. This change in the refractive index of region
70 causes light at the signal wavelength to propagate through
region 70 at a different speed. As a result, there is a phase shift
of the light propagating through section 68B relative to light
propagating through section 68D. When light propagating through
section 68B and 68D are recombined at junction 68F, destructive
and/or constructive interference can result in light propagated
through waveguide section 68E having the same amplitude or
intensity as light propagated through input waveguide 68A when the
phase shift is zero (assuming negligible losses through the
waveguides), essentially no light propagating through output
waveguide 68E when the phase shift is 180.degree. (or .pi.
radians), or any amplitude or intensity inbetween, depending on the
amount of the phase shift. The phase shift, and, therefore, the
amount of attenuation, can be controlled depending on the power or
intensity of the light at the attenuation wavelength which is
emitted from attenuation light source 62'.
[0027] In FIG. 6, there is shown an alternative variable attenuator
72. Variable attenuator 72 includes a Mach-Zehnder type
interferometer similar to that shown in FIG. 5, including input
waveguide section 68A', branch waveguide section 68B' branching
from input waveguide section 68A' at junction 68C', branch
waveguide section 68D also branching from input waveguide section
68A' at junction 68C', and output waveguide section 68E' joined to
branches 68B' and 68D' at junction 68F'. Branch waveguide section
68B' includes a region 70' comprising an attenuating material which
is photothermally responsive to light at an attenuating wavelength,
but is not responsive to light at a signal wavelength. Variable
attenuator 72 operates in substantially the same manner as variable
attenuator 68, except rather than propagating light at both the
signal wavelength and the attenuating wavelength through the
Mach-Zehnder type waveguide, as with the embodiment of FIG. 5, only
the signal is propagated through the waveguide of device 72 shown
in FIG. 62 while light at the attenuating wavelength is focused on
region 70' from an external laser diode 40' by a lens 42'.
[0028] The switching materials, such as those used in regions 38,
66 and 69 of the embodiments shown in FIGS. 3, 4, 5 and 6
respectively, can be made by blending a polymer which is suitable
for forming a polymer waveguide core with a material that, when
blended with the polymer, absorbs light at the switch wavelength,
but is substantially transparent to light at the signal wavelength.
Examples of suitable polymers include copolymers containing
fluorinated monomers (such as those selected from vinylic, acrylic,
methacrylic and/or allylic monomers), with specific examples
including copolymers made from about 15% to about 70% by weight of
pentafluorostyrene and from about 30% to about 85% by weight
trifluoroethylmethacrylate.
[0029] In order to have a good switching efficiency, the absorbent
should exhibit strong absorption at a photodiode wavelength
sufficiently far from the signal wavelength (e.g., 1300 nm or 1550
nm) to limit losses due to the absorption tail. In the theoretical
case of a cylindrical core in which the heat is homogeneously
generated, it can be shown (see table below) that the requirement
.alpha..sub.maxL=0.5-1.3 has to be met (in the case of polymers or
hybrid sol-gel media) in order to obtain a phase shift of .pi. for
an input control power P.degree. in the range of 10-20 mW
(.alpha..sub.max is the maximum absorption coefficient and L the
distance over which the signal is phase-shifted).
[0030] For a digital geometry (DOS), an index variation of at least
10.sup.-3 is commonly required to achieve the commutation between
the arms in the splitting zone of length L ranging from 1 to 2 mm.
By using a material with a dn/dT of 2.times.10.sup.-4 and a thermal
conductivity (.chi.) of 0.15 w/(mK) (i.e., reasonable values for
polymers or hybrid sol-gels), the required absorption coefficient
will be in the range of 500-1000 m.sup.-1 provided that the input
control power is large enough (P.degree.>30 mW).
[0031] Table I summarizes the criteria and requirements for each
design. The requirements concern the input control power delivered
by a visible source at a wavelength .lambda..sub.max for which the
absorption coefficient is maximal. From this last value and taking
into account the requirement of propagating losses (absorption at
the waveguiding wavelengths .alpha..sub.wg=.alpha..sub.1.3-1.6.mu.m
lower than 0.1 dB/cm or 2.3 m.sup.-1) the minimum absorption ratio
.alpha..sub.max/.alpha..sub- .wg required is determined.
1TABLE 1 Mach-Zehnder DOS Core physical properties dn/dT =
2.10.sup.-4 (polymers, hybrid sol-gels) .chi. = 0.15 W/(m.K)
Criterions .DELTA..PHI.(L) = .pi. .DELTA.n(L) = 10.sup.-3 -
5.10.sup.-3 L.about.1 cm L = 1-2 mm (splitting zone length)
Requirements -P.degree.: input control power (visible diode) at
.lambda..sub.max P.degree. = 10-20 mW P.degree. = 30-300 mw (**)
-.alpha..sub.max: visible maximal absorption at .lambda..sub.max
-.alpha..sub.max.L.about.0.5-1.3 -.alpha..sub.max.about.0.5-1.3
cm.sup.-1 -.alpha..sub.max.about.5-10 cm.sup.-1 1 2 (*) The minimum
value is a function of the input control power P.degree.. These two
values are related respectively to P.degree. = 20 mW and P.degree.
= 10 mW (current visible LD power). (**) These two values are
deduced from the condition P.degree. >
2.72.DELTA.n(L).L.4.pi..chi./(dn/dT) applied to the criterion
extreme values of .DELTA.n and L. (***) Deduced from the optimal
condition -.alpha..sub.max.about.1/L (1 mm < L < 2 mm).
[0032] Organic dyes can be incorporated into polymeric or mainly
polymeric waveguides. For example, an absorption coefficient of 720
cm.sup.-1 and below 0.4 cm.sup.-1 can be obtained at 556 nm and
1550 nm respectively with Crystal Violet embedded in a partially
fluorinated polymer.
2 Absorption ratio Resonance position Material .sup..alpha.res
.sup..alpha.1.55 .mu.m (*) (.lambda..sub.max) Crystal Violet
>1800 556 nm embedded in partially fluorinated polymer (*)The
absorption ratio is equal to the equivalent optical density
ratio.
[0033] Crystal Violet
[0034] Other dyes can be used to shift the .lambda..sub.max to
higher wavelengths, to match the emission wavelength of commercial
light emitting diodes (LEDs) or laser diodes (LDS).
[0035] Metallic ions can be incorporated into organic or inorganic
waveguides. The absorbance of cobalt (II) at 600 nm is about 2.2.
Co(II) could be introduced at the same concentration in a glass
that fulfills the transparency requirement at the waveguiding
wavelength (.alpha..sub.max/.alpha..sub.wg>320). This value is
in the appropriate range for Mach-Zehnder or DOS switches.
[0036] The surface plasmon resonance taking place in small metallic
particles leads to a huge increase of the effective cross section
around the resonance wavelength. This wavelength is situated in the
visible spectrum for most metals in silica-like, polymeric or
composite surrounding media. The width of the resonance is about 50
to 100 nm for small particles (.about.20-100 nm average particle
size). It is possible to use this selective absorption in the
visible region (at the resonance) to create local heating and
induce an index of refraction change. Different metals have been
analyzed. Table II gives the main characteristics.
3TABLE II Maximal absorption(*) Particle diameter for Resonance
position Metal ratio .sup..alpha.res/.sup..alpha.1.5- 5 .mu.m
.sup..alpha.res/.sup..alpha.1.55 .mu.m >60 (.sup..lambda.max) Cu
120-325 <100 nm 380-610 nm Au 170-920 <100 nm 520-614 nm Ag
155-2300 <100 nm 410-534 nm Pt 60-80 <30 nm 280-300 nm Pd
60-130 <60 nm 280-330 nm (*)The absorption ratio decreases with
increasing particles diameter.
[0037] The following conclusion can be drawn from these
figures:
[0038] The maximal absorption ratio is obtained for small particles
as long as they behave as a conductor.
[0039] Silver shows the best performance. However, a trade-off has
to be made between maximum absorption ratio and the resonance
spectral position (.lambda..sub.max). The latter has to match with
currently available LEDs or LDs. For this reason, gold is a better
candidate.
[0040] The invention has many advantages over conventional switch
designs. In conventional switch designs, the heating is provided by
heaters placed above the guides and cladding. The heat has to
diffuse through the cladding before it can warm the waveguide core,
which changes the refractive index of the core and then induces
switching. In this invention, heating is internal to the waveguide
core, with no delay due to heat conduction through the cladding. As
a result, a decrease of switching time to less than 1 .mu.s can be
realized. Moreover, the switching power may be lowered by as much
as two orders of magnitude. In usual thermooptical switches the
power required is about 500 mW. In this invention the power
required is only in the 10-50 mW range for light induced
thermooptical switches.
[0041] It will become apparent to those skilled in the art that
various modifications and adaptations can be made to the present
invention without departing from the spirit and scope of this
invention. Thus, it is intended that the present invention cover
the modifications and adaptations of this invention, provided that
they come within the scope of the appended claims and their
equivalents.
* * * * *