Optoelectronic and photonic devices formed of materials which inhibit degradation and failure

Optoelectronic and photonic devices are formed by employing polymer materials that have a lower glass transition temperature (Tg) than the nominal operating temperature. By using such materials, the local or segmental mobility is increased so that local stress is eliminated or minimized on the polymer material, making performance more robust. The current invention involves use of a polymer in an optical device in an operating temperature range in the region above Tg, where the polymer segments between crosslinks are allowed local freedom of movement; however, large-scale movement of the material may be restricted by the crosslinked structure of the polymer material. The temperature operation point of a device constructed according to the invention is thus preferably distanced from both the viscoelastic region near Tg and from the glassy region below Tg; such that the device is operated in a region where viscoelastic effects do not significantly affect the materials system, and time-dependent responses of the polymer are minimized or eliminated. Device operation can thus achieve minimum degradation and show improved performance attributes.

BACKGROUND OF THE INVENTION
 This invention relates to optical or photonic components, more particularly
 to optoelectronic devices formed of polymers.
 Integrated optical devices (i.e., waveguides, switches, interconnects, and
 the like) are known which are constructed of polymer materials having a
 glass transition temperature (Tg) much higher than the operating
 temperature range of the device. The glass transition temperature is a
 range of temperatures over which significant local motion of the polymer
 backbone occurs. The Tg is usually defined as cooperative motion of about
 10 backbone units, or a viscosity of 10.sup.14 poise, or a second order
 phase transition in heat capacity. The temperature at which the change in
 slope occurs in the rate of change of volume with temperature is
 considered the glass transition temperature (Tg), or softening point. For
 a detailed description of viscoelasticity and characteristics illustrated
 by FIGS. 1 and 2, see G. B. McKenna, chapter 10, Comprehensive Polymer
 Science, Volume 2, Edited by C. Booth and C. Price, Permagon Press, Oxford
 (1989).
 Crosslinked materials manifest a glass transition when the molecular weight
 between crosslinks is significant enough to allow cooperative motion of
 the backbone units. Thus, a lightly crosslinked material will show a glass
 transition; while a highly crosslinked one may not.
 Below Tg, polymeric material is prevented from reaching equilibrium because
 of the limited amount of segmental motion. Thermodynamic (entropic)
 effects still drive change towards equilibrium, but if the temperature is
 far enough below Tg, those changes will occur at such a slow rate that it
 does not appear experimentally during the time scale of interest (in this
 case the time scale of observation).
 There are several reasons why materials having a high Tg have been chosen
 in the past, including compatibility with electronics processing and
 packaging, maintenance of orientation of chromophores incorporated within
 the material, and environmental robustness and performance stability. The
 use of high Tg materials (materials with a Tg higher than the operating
 temperature of the materials in a packaged device) ensures device
 operation in a region in which the local motion of the polymer segments is
 significantly restricted, and that the material operates in a glassy
 state. It was assumed in the earliest development of polymer films for
 optoelectronic devices that use of high Tg materials was a requirement.
 "For example, many of the first research EO polymers, whether guest-host
 or side chain, are based on thermo-plastic acrylate chemistry and exhibit
 glass transition temperatures.about.100-150.degree. C. This low Tg results
 in high polymer chain diffusion rates and a variation of at least 10% in
 the optical properties of the poled state over 5 years of operation at
 ambient temperature. This rapid change is the natural consequence of the
 dynamic processes by which glassy polymers, operating close to Tg, undergo
 physical aging and relaxation to reduce stress and minimize free volume.
 When higher operating temperatures are considered (125.degree. C.), the
 stability of the optical properties becomes even worse." (extracted from
 the review paper by R. Lytel et al., in Polymers for Lightwave and
 Integrated Optics, L. A. Hornak, ed., Marcel Dekker 1992 pp. 460).
 Higher glass transition materials developed for integrated optoelectronics
 include polyimide materials (glass transitions ranging from about
 250.degree. C. to well over 350.degree. C.) developed by Hoechst, DuPont,
 Amoco, and others, and polyquinolines (Tgs greater than 250.degree. C.)
 developed by Hitachi Chemical. The researchers were guided by the
 presumption that "The first priority for such waveguides should be high
 thermal stability to provide compatibility with high-performance
 electronics device fabrication. The fluorinated polyimides have a high
 glass transition temperature above 335.degree. C., and are thermally
 stable against the temperatures in IC fabrication processes involving
 soldering (.about.270.degree. C.)." (T. Matsuura et al., Elect. Lett. 29
 2107-2108 (1993)).
 The requirements for polymers used in thermo-optic switches are reported by
 R. Moosburger et al. (Proc. 21st Eur. Conf. On Opt. Comm.
 (ECOC95-Brussels) p. 1063-1066). "Low loss switches at a wavelength of 1.3
 .mu.m were fabricated with the commercially available and high temperature
 stable (Tg&gt;350.degree. C.) polymer CYCLOTENE.TM. . . . CYCLOTENE.TM. was
 chosen due to its low intrinsic optical loss, thermal stability in excess
 of 350.degree. C., low moisture uptake and excellent planarisation
 properties."
 The requirements for polymers for polymer passive optical interconnects are
 reported by DuPont for their Polyguide.TM. material system in R. T. Chen
 et al., SPIE Vol. 3005 (1997) p. 238-251, "High Tg and low coefficient of
 thermal expansion (CTE) polymers provide thermal-mechanical and
 environmental robustness and performance stability through their complete
 domination of the Polyguide.TM. packaged structure properties." DuPont
 uses cellulose acetate butyrate (CAB) materials as described in U.S. Pat.
 Nos. 5,292,620 and 5,098,804.
 In addition to the acrylate, polyimide, polyquinoline, benzocyclobutene and
 CAB materials systems mentioned above, other materials systems that have
 been used to make integrated optical devices include cardo-polymers (C. Wu
 et al., in Polymer for Second-Order Nonlinear Optics, ACS Symposium Series
 601, pp. 356-367, 1995), epoxy composites, (C. Olsen, et al., IEEE Phot.
 Tech. Lett. 4, pp. 145-148, 1992), polyalkylsilyne and polysilyne (T.
 Weidman et al., in Polymers for Lightwave and Integrated Optics, Op. Cit.
 pp. 195-205, 1992), polycarbonate and polystyrene (T. Kaino, in Polymers
 for Lightwave and Integrated Optics, Op. Cit., pp. 1-38, 1992), polyester
 (A. Nahata et al., Appl. Phys. Lett. 64, 3371, 1994), polysiloxane (M.
 Usui et al., J. Lightwave Technol. 14 2338, 1996), and silicone (T.
 Watanabe et al. J. Lightwave Technol. 16 1049-1055, 1998). Poly methyl
 methacrylate, polystyrene, and polycarbonate have also been used for
 polymer optical fibers (POFs). Polycarbonate is used as compact disc
 substrates, and is used in plastic eyeglass lenses, hard contact lenses,
 and related applications. Silicones are used in flexible contact lenses.
 Several researchers have designed optical switching devices using thermal
 effects in polymers. In addition to the research work of R. Moosburger,
 Op. Cit., one group has been trying to commercialize thermo-optic switches
 using a digital optical waveguide switch configuration (G. R. Mohlmann et
 al., SPIE Vol. 1560 Nonlinear Optical Properties of Organic Materials IV,
 pp. 426-433, 1991). In this work, a resistive heating element is deposited
 on a high glass transition temperature thermo-optic polymer stack that
 contains a waveguide y-branch splitter. Activation of a heater electrode
 produces a decrease in the refractive index under the activated electrode
 and results in light switching into the waveguide branch that is not
 activated.
 In work with polymers for thermo-optic integrated optical devices leading
 to the present invention, it has been observed that there are nonlinear
 responses due to the viscoelastic behavior of the materials. After
 repetitive switching of a thermo-optic device, for instance, the polymers
 begin to exhibit a local change in index of refraction where they were
 heated, disturbing the "off" state of the switch and its time response.
 The viscoelastic properties of a polymer determine the mechanical
 character of the material response to applied heat or other perturbation.
 These properties control the rate at which applied changes (such as heat,
 stress, acoustic excitation, etc.) produce time-dependent responses in the
 material properties (such as evolution of the index of refraction,
 mechanical strain, etc.). Any truly elastic contribution generally is
 linear and disappears after the applied change is removed. However,
 time-dependent elements of the material response are retained within the
 material after the removal of the applied change and may require minutes
 to eons for restoration. If the material response results in a degradation
 of the operating characteristics of a device, that degradation may
 accumulate over time and result in failure of the device to meet
 performance specifications.
 For optical devices used in communications, such behavior is undesirable
 because it can degrade the insertion loss, crosstalk immunity and other
 performance measures that are critical to the bit error rate of the
 system. Any such factor that changes with time is a problem for
 telecommunications applications, where reliability and reproducibility are
 essential, but where a broad range of environmental conditions may be
 encountered during a service lifetime. To enable effective thermo-optic
 switching devices, materials should not exhibit any such slow changes in
 optical properties.
 SUMMARY OF THE INVENTION
 According to the present invention optoelectronic and photonic devices are
 formed by employing polymer materials that have a lower glass transition
 temperature (Tg) than the nominal operating temperature. By using such
 materials, the local or segmental mobility is increased so that local
 stress is eliminated or minimized on the polymer material, making
 performance more robust.
 The current invention involves use of a polymer in an optical device in an
 operating temperature range in the region above Tg, where the polymer
 segments between crosslinks are allowed local freedom of movement;
 however, large-scale movement of the material may be restricted by the
 crosslinked structure of the polymer material. The temperature operation
 point of a device constructed according to the invention is thus
 preferably distanced from both the viscoelastic region near Tg and from
 the glassy region below Tg; such that the device is operated in a region
 where viscoelastic effects do not significantly affect the materials
 system, and time-dependent responses of the polymer are minimized or
 eliminated. Device operation can thus achieve minimum degradation and show
 improved performance attributes.
 This invention will be better understood upon reference to the following
 detailed description in connection with the accompanying drawings.

DESCRIPTION OF THE SPECIFIC EMBODIMENTS
 FIG. 1 illustrates schematically the change in volume as a function of
 temperature for an amorphous polymer material in general. Range A is the
 glassy range, range B is the rubbery (for a crosslinked material) or melt
 (non-crosslinked polymer) regime, and range C between range A and range B
 is the viscoelastic regime.
 According to the present invention, the region above the glass transition
 temperature (region B in FIG. 1) in the volume-temperature curve is
 utilized. In this region, the polymer segments are allowed local freedom
 of movement. Consequently, repetitive operation enables devices to
 function with minimal or negligible viscoelastic effects contributing to
 premature failure/degradation of performance. In the preferred embodiment,
 large-scale (bulk) movement is restricted by the polymer material's
 crosslinked structure.
 The motivation for operating in the region above the glass transition
 temperature (Tg) is to avoid the negative effects such as induced
 insertion loss associated with operating in the viscoelastic regime
 (described above). However, in applications such as integrated optics,
 localized heating must often be applied to microscopic regions to
 accomplish switching and other functions. Heating of the materials, even
 if the net temperature rise does not exceed Tg, has been found to cause
 long-lived changes in material properties such as index of refraction.
 These changes become quite pronounced after many cycles of applied heat
 pulses such as in an optical switch, for example. If the heating is
 localized, the changes in refractive index are localized, producing
 undesired optical effects such as increased insertion loss in devices.
 According to the invention, a new class of optical devices is disclosed
 with physical properties qualitatively different from that previously
 known wherein polymeric optical materials are employed which are
 characterized by a relationship between the Tg and the range of intended
 operating temperatures, and specifically wherein the operating
 temperatures are near or above the Tg of the optical materials. By
 operating devices near or above the Tg of the optical materials,
 viscoelastic contributions may be diminished or even removed. If the
 operating temperature is near (slightly below or at) Tg, the viscoelastic
 problems may be reduced, and if the operating temperature is above Tg,
 there should be no accumulation of degradative effects due to viscoelastic
 contributions. This new type of device can also show improved performance
 and allow a wider range of operation.
 In experiments to measure the glass transition temperature Tg, and the
 material properties related to Tg as a function of temperature, it has
 been observed that the rate of change of the temperature during the
 measurement changes the result. A Tg measured with a slow temperature ramp
 (Rate 2) is lower than a Tg measured with a fast ramp (Rate 1), as is
 illustrated in FIG. 2. The rate dependent Tg is sometimes called the
 effective Tg. For these purposes, Tg shall be assumed to be the value
 which is measured at a rate of 10.degree. C. per minute in a typical
 commercial DSC (differential scanning calorimetry) machine. However, the
 heating rate at which a thermo-optic switch of FIG. 3 is operated is much
 faster than 10.degree. C. per minute. The Tg that applies to the operation
 of the device is the effective Tg at the rate of operation of the device.
 In FIG. 2, the concept of a rate dependent Tg is illustrated. Examine first
 the top curve. The effective glass transition temperature,
 T.sub.g.sup.eff, is defined as the temperature at the intersection 11, 13
 of the slopes of the volume-temperature curve. In a real material
 (non-ideal) the break is not sharp, as is indicated by the dotted lines
 15, 17. The place at which the break is observed is a function of rate of
 change of the temperature; the curve of Rate 1 representing a faster
 heating or cooling rate; thus the break, or T.sub.g1.sup.eff, occurs at a
 higher temperature (i.e. if Rate 1&gt;Rate 2, T.sub.g1.sup.eff
 &gt;T.sub.g2.sup.eff). It is a well-established rule of thumb (see, for
 example, Materials Science of Polymers for Engineers by T. A. Osswald and
 G. Menges, Hanser Publishers, Munich 1995) that the glass transition
 temperature changes by roughly 3.degree. C. for every order of magnitude
 change in the rate of the temperature change (McKenna, 1989; or
 Viscoelastic Properties of Polymers by J. D. Ferry, 3rd Edition, Wiley,
 New York, 1980). The rate is a very important factor when comparing a
 thermo-optic device that may swing more than 10.degree. C. in one
 millisecond with a glass transition temperature measured by a DSC at
 1.degree. C./min. The difference in rate in this case is about six
 orders-of-magnitude. According to the rule of thumb, the effective Tg for
 such a thermo-optic device is about 18.degree. C. higher than the Tg
 measured in the same materials system with a DSC. A very fast thermo-optic
 switch might have an effective Tg 24.degree. C. or so higher than Tg. Even
 a slow-rise thermo-optic switch with a thermal swing of 3.degree. C. in
 100 milliseconds will still have an effective Tg about 9.degree. C. larger
 than the Tg as measured by the DSC at 1.degree. C. per minute.
 One preferred embodiment of the invention is a thermo-optically controlled
 optical polymer waveguide TIR (total internal reflection) device 100 as
 shown in FIG. 3. As a thermo-optic device, or a device that transports
 optical energy subject to control by thermal means, it functions as a
 switchable deflector of optical radiation. Transparent polymers are
 employed to guide the light, namely polymers in which optical radiation
 propagates with a predetermined minimal amount of attenuation at the
 intended operating wavelength.
 In FIG. 3 a multi-layer stack is constructed, namely several layers formed
 one on top of the next, in which an optically transparent polymer lower
 cladding layer 2 lies on a substrate 4. The lower cladding is preferably a
 polymer deposited by spin-coating. Alternatively, layer 2 can an inorganic
 or non-crosslinked organic material. Any deposition method known to those
 skilled in the art would appropriately be selected for deposition of
 alternative layers. A combination of lithographic definition of
 photoresist and RIE (reactive ion etching) processes as known in the prior
 art may be used to fabricate a trench 5 through the lower cladding layer
 2. A core layer 6, also spun on, lies above the lower cladding and fills
 the trench 5. The spinning process produces a film that tends to planarize
 the surface, filling the trench 5. A third optical layer, the upper
 cladding 10, is also spun on. As is well known in the art, the thickness
 of each layer is adjusted by selecting the spinning speed. The layer
 thicknesses are approximately 5 .mu.m, 1.2 .mu.m, and 1.4 .mu.m,
 respectively. As fully processed, the materials used in experimental
 construction of the three layers 4, 6, 10 provided indices of refraction
 of 1.488, 1.522, and 1.422, respectively. With a trench depth of about
 0.06 .mu.m and width of 6 .mu.m, single ode guiding at 980 nm was
 achieved.
 The materials used were: a Corning 1734 glass substrate 4, a Gelest UMS-992
 polyacrylate (Tg.about.45.degree. C.) lower cladding 2, a Norland Optical
 Adhesive 68 (Tg=35.degree. C.) core 6, and a Gelest UMS-182 polyacrylate
 (Tg below 0.degree. C.) upper cladding 10. All three polymer materials are
 crosslinked by a UV cure step as specified by the manufacturers. These
 materials were chosen to improve overall dimensional and chemical
 stability. However, no evidence was found of bulk dimensional or chemical
 instabilities in devices so constructed.
 A waveguide is any structure which permits the propagation of a wave along
 an optical path, throughout its length despite diffractive effects or
 curvature of the guide structure. Although the waveguide segment (a
 predetermined section of waveguide 12) shown in FIG. 3 is straight, the
 waveguide shape can easily be defined into much more complicated
 structures, if desired. By appropriately fabricating the mask used in
 defining the photoresist for the etch step, waveguide structures including
 curves, X- and Y-branches, parallel couplers can be incorporated. An
 optical waveguide is defined by a length of an extended bounding region of
 increased index of refraction relative to the surrounding medium. The
 strength of the guiding, or the confinement, of the wave depends on the
 wavelength, the index difference and the guide width. Stronger confinement
 leads generally to narrower modes. A waveguide may support either multiple
 optical modes or a single mode, depending on the strength of the
 confinement. In general, an optical mode is distinguished by its
 electro-magnetic field geometry in two dimensions by its polarization
 state, and by its wavelength. If the index of refraction change
 experienced by the optical mode is small enough (e.g. n=0.003) and the
 dimensions of the guide are narrow enough (e.g. 5.0 .mu.m), the waveguide
 will only contain a single transverse mode (the lowest order mode) over a
 range of wavelengths. For larger refractive index differences and/or
 larger waveguide physical dimensions, the number of optical modes
 increases.
 Waveguides of this nature are commonly referred to as rib waveguides.
 Dimensions of the etched trench (rib depth and width) are carefully
 controlled along with the thickness of the core layer to control the
 number and shape of propagating modes. Preferably the waveguide is
 designed to support only a single lowest order mode, eliminating the
 complexities associated with higher order modes. Higher order modes have
 different propagation constants than lower order modes, and higher
 scattering loss, which can be problematic in some applications. In other
 applications where higher power is desired, higher order modes might be
 more beneficial.
 In a particular embodiment, a 100 nm layer of 80/20 NiCr is sputtered onto
 the top cladding layer and etched in its turn by standard lithographic
 means well known in the art, to form patterned structures such as the
 heater stripe 8. The control system 19, in this case a temperature control
 system (current source), controls the thermal excitation element which is
 the resistive heating element 8. The resistive element is oriented at an
 oblique angle (a few degrees) to the waveguide channel 5 beneath it. The
 control element supplies a sufficient amount of current to the heating
 element via an applicator electrode 9, fabricated as a thin gold layer
 over an enlarged area at the end of the heating element 8 such that the
 desired operating temperatures can be achieved. The resistive heating
 element 8 is an exciter since it produces the temperature change in the
 device in response to an applied current. The control system 19 forms an
 essential part of the exciter in the sense that it is the control system
 that generates and controls the current that leads to device operation.
 The increase of temperature achieved in the switch 100 as a function of
 time is essentially independent of external factors such as the device
 temperature, since the heat energy is applied during a short pulse; its
 time dependence is determined by its diffusion into the device. The
 resistive heating element 8 is an electrically conductive material such as
 a metal (in the preferred embodiment nickel-chromium) or other suitably
 conductive material that is deposited on the upper cladding. Deposition
 may also be achieved by chemical vapor deposition or other suitable
 technique for applying such materials. In the case of metallic electrodes,
 it may be best to incorporate an additional coating deposited below the
 electrode, to reduce the optical loss which occurs when a portion of the
 energy in the guided wave mode extends out to the metallic electrode.
 The length of the heating element, 800 .mu.m, is made to extend
 sufficiently before and beyond the region where the heating element passes
 over the waveguide so that activation of the heating element will produce
 temperature changes in the polymer that can be sensed by evanescent fields
 of the mode propagating in the waveguide.
 The width of the heating element, about 20 .mu.m, is selected to prevent or
 substantially reduce optical tunneling of optical radiation through the
 heated region of the waveguide. Optical tunneling is the coupling of light
 from a region of high refractive index through a region of lower
 refractive index to a region of higher refractive index. In general, the
 optical tunneling length will depend on the wavelength of the guided
 light, magnitude of the index change in the heated region, and length of
 the heated waveguide region.
 The return to an equilibrium temperature is accomplished using a cooling
 element. The cooling element may be any element that assists in the
 removal of thermal energy by either convection, conduction, or radiation
 (e.g. thermoelectric cooler, heat-sink, thermal pipe). The cooling element
 regulates the nominal operating temperature of an element attached to the
 thermo-optic device. In the preferred embodiment, we use a glass substrate
 as a cooling element because of the low heat load. Depending on the
 application, higher thermal conductivity substrates such as ceramic,
 silicon, or even diamond may be used, and active heat removal steps may be
 used such as Peltier-effect (TE) coolers, vapor wick coolers, or water
 cooled or forced air heat exchangers. The effect of these cooling elements
 is to provide a pathway for the removal of thermal energy so that the
 device may be operated continuously or intermittently as desired but still
 remain within an operating temperature.
 The operating temperature is the temperature of the polymer layer in the
 region traversed by the optical path, averaged over a time long compared
 to the optical response times to changes in the thermal excitation element
 but short compared to the times for environmental changes outside the
 device. The operating temperature is preferably controlled to within a
 desired range as determined by a sensor with a feedback loop to adjust the
 operation of a heater or cooler to maintain the desired temperature (for
 example, the minimum operating temperature) at the sensor as is well known
 in the art. The control loop may include feedforward to prepare for the
 effects of changes in pulse rate, etc. The minimum operating temperature
 is the lowest operating temperature allowed by the proper functioning of
 the device including any thermal control loop, when the ambient
 environment varies within the temperature, humidity, etc. values specified
 for device operation.
 When the heating element is activated, thermal energy from the heating
 element diffuses into the surrounding polymer layers and increases the
 temperature of the polymer while simultaneously lowering the refractive
 index of the heated polymer via the thermo-optic effect. Polymer regions
 closer to the heating element experience a larger increase in temperature
 as a result of absorbing more thermal energy per unit area from the
 heating element than regions further from the heating element. FIG. 4
 schematically shows a top view of the spatial variation in the refractive
 index in the polymer core layer during switch activation. As illustrated,
 region 22 for example, which is in proximity to the heating element (not
 shown) has a refractive index less than region 24 located further from the
 heating element.
 If the refractive index change of the heated polymer is large enough and
 the angle 14 between the heating element and the waveguide is sufficiently
 shallow, optical radiation propagating in the waveguide undergoes total
 internal reflection at the interface 20, called the TIR interface, and
 optical radiation illustrated as a beam 17 is deflected from the rib
 waveguide. The deflected radiation 17 is mostly optically confined
 vertically to the core layer 6, although it propagates within the planar
 waveguide formed by the core layer outside of the region defined by the
 trench 5. Light deflected from the waveguide via switch activation may be
 used, collected, or rerouted using gratings, mirrors, lenses, or by any of
 several other means known to those skilled in the art which route
 radiation in or out of the plane defined by the layer 6 (FIG. 3).
 The deflected optical radiation 17 can be used for any number of
 applications, for example optical beam routers, sensors and modulators. A
 plurality of heating elements can be placed along a single waveguide to
 deflect light out of the waveguide at any waveguide-heating element
 proximity. In addition, a single one or an array of heating elements can
 be placed above/below an array of waveguides depending upon the
 application in question.
 The optical throughput is measured as the optical power in the beam 18
 emerging from the waveguide after traversing the TIR switch 100. As a
 result of TIR reflection, throughput is decreased upon activation of the
 heating element. Because the reflected optical radiation of a rib
 waveguide TIR switch must overcome lateral waveguide confinement, rib
 waveguide TIR switches may not be as efficient as planar waveguide
 switches at the same level of excitation. (A planar waveguide switch is
 fabricated in the same way as described above in reference to FIG. 3, but
 without fabricating the trench; the input beam is confined in only one
 dimension, the dimension normal to the plane of the layer 6.)
 FIG. 5a shows a representation of the waveguide throughput 90 as a function
 of time. FIG. 5b illustrates that the switch is controlled by a current
 pulse 92, supplying maximum current to the switch at time t.sub.9 and
 continuing to do so until time t.sub.10 when it returns to its initial
 state. As shown in FIG. 5b, a control current pulse is turned on at time
 t.sub.9 and off at time t.sub.10, but the optical response (throughput) of
 the switch is not instantaneous. FIG. 5c shows the refractive index
 variation induced at a given depth below the heater element by the
 delivery of a single thermal energy pulse. The refractive index profile 94
 of the polymer material changes as a function of time as a result of the
 applied current pulse 92. When the refractive index discontinuity
 experienced by the optical mode rises toward and above the level required
 for total internal reflection (TIR), light is deflected from the
 discontinuity, and the throughput drops as shown in FIG. 5a. It can be
 seen that a predetermined time is required to allow the switch to respond
 to the heat that has been supplied to it by means of the current pulse,
 such that the index change will enable switching to occur at time t.sub.11
 to cause the throughput of the waveguide to fall from a value of T.sub.P
 to a value of T.sub.A. It can also be seen that a predetermined time is
 required to allow the switch to relax after the removal of heat, such that
 the index change of the polymer material enables the reflection to subside
 and the throughput of the waveguide to rise once again to (or
 substantially close to) its initial value T.sub.P at time t.sub.12. The
 polymer material as shown requires a longer time to respond to the removal
 of the heat supply and consequently a longer time for the index of
 refraction to return to its initial state. The time for the optical
 throughput to return close to equilibrium is known as the decay time. Here
 the decay time (t.sub.12 -t.sub.11) is longer than the width of the
 control pulse (t.sub.10 -t.sub.9).
 The condition of the switch at a time such that only a predetermined
 minimum quantity of optical radiation is deflected from the waveguide
 designed is the "off" state. When the switch is in an "off" state, light
 propagates the entire length of the waveguide without being substantially
 perturbed. This condition occurs prior to switch activation and after
 deactivation. In general the response of the material to the thermal
 energy delivered by the heating element is limited by the thermal velocity
 of the heat through the polymer. This means the observed switched light is
 delayed in time with respect to the flow of electrical current through the
 heating element, depending on the thermal constants of the polymer layers
 and physical thickness of the components of the multi-layer stack. When an
 initially activated switch is deactivated the optical response is retarded
 in time with respect to cessation of current flow through the heating
 element.
 Switch Fidelity
 In order to understand the present invention, it is helpful to review
 certain properties of polymeric materials. In a linear system, the
 response of the system to an arbitrary input signal is given by the
 convolution of the input signal with the impulse response of the system.
 This system impulse response allows accurate prediction of system
 performance without having to measure the system response each time the
 input excitation may be changed. In a polymeric system where the input is
 from a thermal source, there are conventionally significant contributions
 from viscoelastic effects which can result in a change of the impulse
 response of the system, therefore modifying the system response to a
 specified input signal. In such a case, the actual system response is not
 equal to the response predicted based on a measurement of the system
 impulse response, and it is said that the fidelity of reproduction of the
 desired signal is impaired, or that the system response is distorted. The
 data illustrated in FIGS. 6 through 8 shows changes in the response due to
 viscoelastic behavior, and as explained hereinbelow is an indication of
 insertion loss. Specifically FIG. 6 shows optical transmission through a
 waveguide containing a 2-degree thermo-optic TIR switch when a thermal
 pulse of 200 pJ/.mu.m.sup.2 is applied to the heater stripe of dimensions
 16 .mu.m wide by 1300 .mu.m long and where the materials were Ablestick
 L4092 epoxy, Epoxy-lite R46 polyurethane, and Epo-tek UV0134 epoxy,
 arranged in a triple stack of thickness' 5.0 .mu.m, 1.2 .mu.m and 1.4
 .mu.m respectively, counting away from the substrate and operating at
 about room temperature. The heat pulse is 20 microseconds long beginning
 at 100 microseconds. Since the heat pulse is very short compared to the
 throughput response of the switch, the measured response is essentially
 equal to the impulse response of the system. At this energy level, the
 impulse response after 10 minutes of pulsing at 50 Hz (30,000 pulses) is
 the same as the impulse response after the first pulse. FIG. 6 therefore
 shows an example of a linear system with good fidelity and low distortion.
 FIG. 7, taken under the same conditions of FIG. 6 but with the higher
 thermal pulse energy density level of 350 pJ/.mu.m.sup.2, shows that the
 impulse response is degraded after 30,000 pulses. The waveguide
 transmission (seen prior to the switch response) is reduced to about 90%
 of its prior value (insertion loss of about 0.5 dB), and the fall time is
 degraded to a longer time. Therein the polymer material has been driven
 above a threshold for initiation of a strong viscoelastic response. The
 threshold in this waveguide stack therefore lies somewhere between 200
 pJ/.mu.m.sup.2 and 350 pJ/.mu.m.sup.2. As used herein, threshold means
 that for the quantity of interest, there is no substantial change below
 the threshold, but a change is observed above the threshold. As a result
 of the viscoelastic response of the material, the polymer near the switch
 heater stripe has acquired an index of refraction change or "set" which
 lasts for a time long compared to the time between switch pulses (20 ms).
 This index set turns the switch partially "on" where it had previously
 been completely "off", reflecting about 10% of the light out of the
 waveguide even in the "off" condition. In addition, the polymer decay time
 has been slowed by the viscoelastic response to the above-threshold
 excitation. FIG. 8 with an even higher excitation level of 480
 pJ/.mu.m.sup.2 shows an even more pronounced example of a response
 dominated by viscoelastic behavior. The insertion loss is now about 1.5
 dB, and the signal distortion shows a complex behavior involving both
 slower response time and multiple time responses.
 In the extreme case of FIG. 8, the multiple peaks present in the impulse
 response indicate that there will be additional frequency components
 introduced into the switch response to an arbitrary signal, compared to a
 device operating below the threshold as in FIG. 6. These additional
 frequency components introduce an undesired distortion into the switch
 response.
 According to the invention, the undesirable behaviors can be substantially
 reduced or eliminated by maintaining the temperature of the material above
 Tg, since the behaviors are tied to the viscoelastic response of the
 materials. The choices are to select optical waveguide materials with Tg
 below the operating temperature or to raise the operating temperature
 above the Tg of the materials.
 Other characteristics of the switch response regarding its fidelity (e.g.
 rise time t.sub.5, fall time t.sub.7, activation temperature, and switch
 dwell time t.sub.8, as illustrated in FIG. 5a, for example) may remain
 substantially unchanged after repeated cycles of operation under
 essentially similar operation conditions.
 Switch Insertion Loss
 According to the invention, use of one or more materials to fabricate the
 triple stack of FIG. 3 at a temperature above the Tg eliminates or reduces
 substantially several performance problems associated with the
 viscoelastic behavior of polymeric materials.
 At a temperature below the Tg or the effective Tg of the material, thermal
 excitation causes the polymer near the heating element to acquire a
 persistent refractive index change with respect to the switch cycle time.
 This unwanted refractive index change may have a variety of undesirable
 effects. This problem is due to time dependent segmental mobility. The
 thermal input energy excites the polymer chains away from their previous
 state. However, after a very short time, the chains reach a
 quasi-equilibrium (low mobility state) as the temperature drops, but in a
 potentially different configuration than that experienced previously. This
 change in chain configuration may lead to changes in density leading to
 changes in the index of refraction and other material properties. Large
 single pulses or multiple smaller pulses can cause significant changes in
 the index of refraction of the material. However, we have also found that
 there is a favorable change in the viscoelastic response of the material
 as the operating temperature is brought near or above Tg, so that the
 magnitude of the long time constant refractive index change is reduced (or
 eliminated, i.e. reduced so far that no effects are seen during the
 lifetime of the device). Some viscoelastic contributions are diminished
 above the glass transition temperature of crosslinked polymer materials.
 The additional loss observed in traversing an integrated optical device
 compared to an equal length of unperturbed waveguide is called the active
 insertion loss of a device. Specifically, referring to FIG. 4, when the
 switch is on, an input beam along axis 16 that is coupled into the
 waveguide channel 5 reflects off the TIR interface 20 and propagates out
 of the waveguide to form a deflected output beam along axis 17. When the
 switch is off, the input beam of axis 16 should propagate through the
 interface and continue along the waveguide to form an undeflected output
 beam along axis 18. Before switch activation, because the index difference
 at the TIR interface is low, the reflection in the off state is preferably
 very low. An "off" switch is preferably essentially invisible to light
 propagation in the waveguide, producing extremely low loss in the input
 guide. Low insertion loss is especially desirable when the input waveguide
 is a bus with many switches. The TIR switch region in the off-state may
 have negligible insertion loss when first fabricated, but the long time
 constant index of refraction change that occurs as a result of the thermal
 excitation can significantly increase the insertion loss.
 In one experiment, a core material is used having a Tg that is nearly
 120.degree. C. above the operating temperature. A TIR switch angled at 2
 degrees from the waveguide axis was fabricated from an Ablestick L4092
 epoxy lower cladding layer (Tg=53.degree. C.), an Epoxy-lite R46
 polyurethane core layer (Tg=150.degree. C.), and an Epo-tek UV0134 epoxy
 top cladding layer (Tg=148.degree. C.), of thicknesses of 5 .mu.m, 1.2
 .mu.m and 1.4 .mu.m, respectively, on a glass substrate. FIGS. 6-8 show
 the measured variation in throughput as a function of time for this switch
 activated with energies of 200, 350, and 480 pJ/.mu.m.sup.2, respectively.
 Specifically FIG. 6 shows the waveguide throughput for the first cycle of
 operation of a switch that is activated with an energy of 200
 pJ/.mu.m.sup.2 and the waveguide throughput after the same switch is
 cycled for 10 minutes at 50 Hz (30,000 pulses). After 10 minutes of
 pulsing, the response of the TIR-switched waveguide is substantially equal
 to its response during the first cycle of operation. From this data we
 conclude that this energy density is below the threshold of degradation
 resulting from viscoelastic response of the material.
 FIG. 7 shows the waveguide throughput of a TIR switched waveguide activated
 with an energy of 350 pJ/.mu.m.sup.2, a level at which the onset of
 degradation resulting from viscoelastic response occurs. After 10 minutes
 of cycling at 50 Hz the waveguide throughput, TP.sub.K, (measured
 approximately 100 .mu.sec prior to switch activation) decreased compared
 to the throughput measured prior to the first pulse, TP.sub.J. This
 difference in waveguide throughput is insertion loss which has been
 induced by thermal cycling. The additional loss is due to a long-lived
 change in index induced in the region of the heating element, that we
 attribute to the viscoelastic response of the polymer. FIG. 8 shows the
 waveguide throughput of a similar TIR switch that is activated with an
 even higher energy 480 pJ/.mu.m.sup.2 which consequently creates a larger
 insertion loss (.about.26% after 30,000 pulses).
 FIG. 9 is a replot of the insertion loss calculated from FIGS. 6-8, as a
 function of the switch energy density. As shown, prior to switch
 activation at a certain energy the insertion loss is negligible. Above a
 certain switch energy density M near 200 pJ/.mu.m.sup.2 the observed
 insertion loss increases with switch activation energy. The energy at
 which the observable insertion loss increases with switch activation is
 the onset or threshold M of degradation resulting from viscoelastic
 effects. The threshold of degradation resulting from viscoelastic response
 is related at least to the quantities of time, temperature, and energy.
 Trace K of FIG. 9 shows the measured insertion loss of a TIR switch
 incorporating the preferred, lower Tg polymer described above in reference
 to FIG. 3. The onset for the threshold of degradation resulting from
 viscoelastic effects occurs at a substantially higher energy N near 400
 pJ/.mu.m.sup.2. The higher threshold of FIG. 9 results in negligible
 changes in the index of refraction over an operating lifetime of a device
 operating at a point sufficiently below threshold such as 250
 pJ/.mu.m.sup.2. At this operating point, conventional devices made with
 high Tg materials will fail (i.e. show measurable changes in the index of
 refraction over an operating lifetime). We achieved this improvement in
 performance by reducing the Tg of the top cladding substantially below the
 operating temperature of the device, and by reducing the Tg of the core
 down to the neighborhood of the operating temperature. In our single pulse
 data, it should be noted that the operating temperature is room
 temperature, 23.degree. C. In our multiple pulse data, the operating
 temperature is elevated somewhat above room temperature, decreasing the
 time-dependent viscoelastic contribution to the observed response,
 reducing the long time constant change in the index of refraction. It is
 expected that there will be a temperature rise in the range of
 0-50.degree. C. above room temperature, for 50 Hz operation, with pulse
 energy densities from 200 pJ/.mu.m.sup.2 to 1000 pJ/.mu.m.sup.2. In the
 multipulse data, we are therefore operating the top cladding layer at
 least 33.degree. C. above its Tg. From the time dependence of our switch
 response our effective Tg is about 21.degree. C. above Tg, so the cladding
 layer is operating at least 11.degree. C. above its effective Tg. We are
 operating the core layer about 2.degree. C. below Tg and about 23.degree.
 C. below its effective Tg. The lower cladding is operated about 12.degree.
 C. below Tg and about 33.degree. C. below its effective Tg.
 The top cladding material experiences the highest temperature changes in
 the inventive device where it is directly adjacent the heater stripe. The
 core layer and the lower cladding layers experience lower temperature
 excursions because of thermal diffusion. For this reason, the Tg of the
 top cladding should be well below the operating temperature. Doing this
 results in the marked improvement represented in FIG. 9. Further
 improvements can be obtained by lowering the Tg of the core and the lower
 cladding materials. It is expected that long-time-constant index changes
 should be minimized or eliminated in the top cladding since the materials
 exhibit no or minimal viscoelastic response at the operating temperature
 is above Tg. The threshold observed in FIG. 9 is related to contributions
 from the core and/or bottom cladding layers. The best mode is to provide
 core and lower cladding materials having an effective Tg below the
 operating temperature of the device. Using the rule of thumb described
 below, if the operating temperature is kept 20.degree. C. above the Tg or
 the effective Tg, no viscoelastic effects are expected to appear.
 The glass transition temperature of the materials is preferably lowest in
 the upper cladding and highest in the lower cladding. This arrangement
 allows any stress (mechanical or fabrication related) or related
 perturbation generated by the thermal switching pulse to be readily
 transported through the stack and transported away from the region where
 switching occurs and light is guided. Energy is dissipated most
 efficiently in materials with high mobility (and low glass transition
 temperatures), thus as perturbations propagate through the stack, stress
 and other forces are driven toward the lower cladding.
 The viscoelastic regime (C) in FIG. 1, lies between the elastic (B) and the
 plastic (A) regimes. Viscoelasticity is defined as the deformation of a
 polymer specimen which is fully or partially reversible but
 time-dependent, and which associated with the distortion of polymer chains
 through activated local motion involving rotation around chemical bonds or
 related phenomena. Viscoelastic effects, usually observed in a temperature
 band near and below Tg, are demonstrated by a time-dependent response of
 the polymeric material. The materials are significantly influenced by the
 rate of straining or heating. For example, the longer the time to reach
 the final value of stress at a constant rate of stressing, the larger is
 the corresponding strain. The exact boundaries of the viscoelastic regime
 are poorly defined and application-dependent. A common rule of thumb is
 that viscoelastic effects are observed over common experimental time
 scales within a range of 20.degree. C. below to 20.degree. C. above the
 glass transition temperature. The exact range of temperatures is a
 function of the polymer chemistry, sample geometry, and the rate of change
 of the temperature during the experiment or the operation. Viscoelastic
 effects have been observed as far as 120.degree. C. below Tg. For a
 complete discussion, see the book by Ferry referenced earlier. For the
 purposes of this document, the term viscoelastic will encompass both
 linear and nonlinear responses of the material involving molecular motion.
 Since thermal excitations induce molecular motions, viscoelastic responses
 are of particular concern in thermo-optic devices.
 In the selection of materials for the construction of thermo optic devices,
 rate sensitivity should be observed as described in relation to FIG. 2.
 For a material to remain unaffected by viscoelastic contributions, its
 glass transition temperature would need to lie an additional amount lower
 than the Tg, for rapidly cycled devices. This effect is thus more
 significant the greater the rate or the shorter the active or "on" time
 under which the device operates. For example, a nanosecond pulse device
 would have about a ten order of magnitude rate effect, or a 30.degree. C.
 increase in the effective glass transition temperature compared to Tg. The
 terminology of a "bulk" or "large-scale (macroscopic)" glass transition
 temperature will be used to describe a glass transition temperature
 measured in a slow manner (such as dilatometry). This is the type of glass
 transition often found in handbooks and literature; usually, if no rate
 information is presented with the glass transition data, the implication
 is that the data was measured sufficiently slowly to reflect the bulk or
 equilibrium-like properties.
 If a device employs a polymer with a bulk or quasi-equilibrium glass
 transition temperature of 60.degree. C. and operated such that switching
 occurred on the microsecond time scale (seven orders of magnitude rate
 effect change), the effective glass transition would be about 80.degree.
 C. The measured dn/dT using a quasi-equilibrium method shows that dn/dT
 increases above about 60.degree. C. (as shown in FIG. 1) and thus one may
 conclude that a device operating above 60.degree. C. should show an
 enhanced thermo-optic effect. However, the device operating temperature
 would have to be raised above about 80.degree. C. to see this enhancement
 in a rapidly switched device.
 Operating a device above the glass transition temperature is potentially a
 problem. Non-crosslinked materials lose dimensional stability above Tg and
 thus flow. This problem may be resolved in practice by surrounding the
 material with rigid structures that contain the material, maintain its
 shape, and prevent it from flowing. Or, the problem may be resolved
 without use of surrounding structures by crosslinking the material such as
 in sol-gels, crosslinked polymers, etc. A crosslinked polymer is defined
 as a network formed by a multifunctional monomer/polymer. In a
 loosely-crosslinked material, local freedom of motion associated with
 small-scale motion of chain movement of chain segments is retained, but
 large-scale movement (flow) is prevented by the restraint of a diffuse
 network structure. The crosslinked network extending throughout the final
 article is stable to heat and cannot be made to flow or melt under
 conditions that linear polymers will flow or melt. Glass transitions as
 low as minus 100.degree. C. have been readily achieved in crosslinked
 systems; the presence of a glass transition indicates that the polymer
 chains retain moderate to high local mobility while the crosslinks prevent
 flow. By operating optical devices made by crosslinked polymer materials
 in this regime, the favorable viscoelastic behavior may be exploited
 without losing dimensional stability. The chemical stability of
 crosslinked materials is also generally enhanced over non-crosslinked
 materials. For example, lower solvent penetration minimizes solubility,
 and greater functionality limits residual reactive sites that could cause
 decomposition or degradation during use, and cycling materials leads to
 stable water and solvent absorption.
 Viscoelastic effects contribute to the degradation of optical switching
 devices by, for example, causing changes in optical throughput of the
 waveguide with time as described above, and/or affecting the rise, fall
 and dwell times of the switch. Viscoelastic effects can restrict the
 operating range of a device and limit both the application specifications
 and additionally limit the stability and lifetime of the device. In order
 to build and operate successful optical devices, viscoelastic effects must
 be minimized or eliminated under the device operating conditions.
 Viscoelastic effects that lead to permanent (or persistent) variation of
 the material properties of the core or cladding layers may also contribute
 to failure modes such as switch insertion loss. Other degradation
 mechanisms that need be considered include fatigue, creep and aging.
 Fatigue occurs in structures subjected to dynamic and fluctuating stresses
 (similar to those experienced in the repeated thermal cycling of the
 thermo-optic polymeric devices). The fatigue limit and fatigue life are
 greater for crosslinked polymers as compared to those that are not
 crosslinked. Both fatigue and creep (slow continuous deformation) are
 minimized or eliminated in elastic, crosslinked polymers.
 Thermal history is an important parameter in determining viscoelastic and
 thermomechanical behavior. For example, in quenching amorphous polymers
 from above Tg, the free volume or local mobility is increased, which
 facilitates relaxation and recovery. Annealing the polymer below Tg
 decreases free volume and enthalpy, increasing the yield stress and
 decreasing fracture toughness. (This phenomenon, known as aging, is well
 described in the polymer research literature; see for example Physical
 Aging of Polymers by John M. Hutchinson, Prog. Polym. Sci., Vol. 20,
 703-760, 1995). Aging refers to changes in the polymer properties with
 time, including embrittlement, changes in index, changes in density, and
 other factors that will cause optical device degradation.
 Other factors which cause performance degradation include mechanical stress
 relaxation and processing induced residual stresses which can cause
 refractive index changes in the material that may degrade device
 performance efficiency, e.g. switch efficiency. When polymer films are
 laid down onto substrates, the deposition processes may induce stresses in
 the film which remain to a degree as residual stresses after completion of
 all the process steps involved in fabricating a part. These stresses
 should be different in the direction in the plane of the surface of the
 substrate, as compared to in the direction normal to the plane of the
 substrate. Since stresses generally produce a change in the optical index
 of refraction, such differential stresses produce slightly different index
 values for TE and TM optical polarization (in the plane and normal to the
 plane, respectively). As a result, the polymer film is birefringent. By
 operating a device above the Tg of one of more of the films, this
 birefringence is minimized. Above the Tg, the polymer chains acquire a
 degree of freedom of motion (limited by their viscoelastic properties, the
 amount of allowed motion depending upon the properties of the polymer such
 as the chain rigidity and the crosslink density) which allows the material
 to relax under the applied strain. The relaxation effectively reduces the
 birefringence. A reduction in birefringence is desirable for many optical
 devices.
 Gratings, which are discussed in more detail later, are particularly
 sensitive to birefringence because the two polarizations which may be
 propagating in the waveguide that transits the grating experience
 different index of refraction. The resonant frequency of the grating. (The
 highest peak of the grating spectrum) depends on the index of refraction,
 so gratings fabricated in birefringent films will exhibit a frequency
 dependence that is different for the two polarizations. Operating such
 devices above the Tg to exploit the high mobility relaxation therefore
 significantly improves their performance characteristics (reducing their
 polarization dependence).
 In general, the benefits of using a optical material system with at least
 one crosslinked transparent polymer with an effective glass transition
 below the operating temperature may be exemplified in part as follows: By
 operating above the viscoelastic regime, thermal cycling will not lead to
 time-dependent responses such as increased cycle time and switch insertion
 loss resulting from thermally-induced materials changes such as density
 drift, index of refraction changes, volumetric evolution, and thermal
 stress build-up. Additionally, the device may be operated over a
 significantly broader range of application temperatures/service
 temperatures without fatigue, embrittlement, cracking, and crazing. This
 enhances the device performance and commercial viability of a given device
 technology. The reproducibility of the information obtained from a device
 as embodied in this invention is also enhanced, since time-dependent
 effects are minimized or eliminated.
 As indicated above, viscoelastic effects can restrict the operating
 temperature range of a device, and limit both the application
 specifications and the stability and lifetime of the device. If
 viscoelastic effects on all time scales of interest to the device during
 operation and use can be avoided, it will provide a time-independent
 device which can be reproducible, stable, and robust to operation. The
 present invention addresses the need to provide optoelectronic and
 photonic devices that are less affected by viscoelastic effects.
 Degradation in material properties from viscoelastic effects may lead to a
 variety of failure mechanisms. Viscoelastic effects are the result of time
 dependent rearrangements of the polymer segments which are long on the
 time scale of the perturbation applied. In order to compete effectively in
 the marketplace advances in both performance and reliability must be
 achieved. Degradation in material properties from viscoelastic effects
 include failure mechanisms relating to changes in density, volume, thermal
 (thermal conductivity, coefficient of thermal expansion), mechanical
 (stress relaxation, modulus), electrical (dielectric constant), magnetic
 (susceptibility), optical (index of refraction, loss), chemical (solvent
 stability, environmental stability) and processing (residual stress,
 manufacturability) characteristics. Note that the degradation resulting
 from viscoelastic effects listed above may occur independently,
 sequentially, or in combination whether or not they are observed over the
 time scales of measurement.
 The device fabricated in this embodiment using the materials described
 above will have a multiplicity of benefits that can be obtained by
 exploiting the viscoelastic properties of the materials above Tg. These
 benefits include but are not limited to, an enhanced thermo-optic
 coefficient, improved switching efficiency, reduced energy consumption,
 faster switch response time, improved cycle time, extended operational
 lifetimes and switch fidelity, reduced creep, linearity of index of
 refraction as a function of temperature, and reduced birefringence.
 Enhanced Thermo-optic Coefficient
 Larger dn/dT values, specifically for TIR switches, enable lower operating
 temperatures to be utilized. Therefore to exploit lower design
 temperatures, it is desirable to fabricate devices using polymers with
 larger thermo-optic coefficients. The vertical axis of FIG. 1 is related
 by a multiplicative constant to the index of refraction of the material.
 It follows that larger values of dn/dT can be obtained by operating above
 Tg. We measured dn/dt values of several polymers as a function of Tg. In
 Table 1 below, values of dn/dT are listed for several polymers, which
 results have been obtained by either the inventors or were reported in R.
 S. Moshrefzadeh. J. Lightwave Technol., Vol. 10, April 1992, pp. 423-425.
 Polystyrene (PS), poly(methyl methacrylate) (PMMA), polycarbonate (PC),
 polyimide (PI) and polyurethane are high Tg (Tg&gt;100.degree. C.) linear
 polymers (thermoplastics), Norland 61, Norland 68, are crosslinked epoxies
 with Tgs of about 100.degree. C. and 35.degree. C. respectively. We have
 observed thermo-optic coefficients that are two to three times higher in
 lightly crosslinked lower Tg materials as compared to higher Tg linear and
 crosslinked materials.
 TABLE 1
 Material dn/dT (.times. 10.sup.-4) [1/.degree. C.] Tg
 [.degree. C.]
 Polyimide -1.5 250
 Polyurethane -1.4 150
 PC/MA -1.3 130
 PS -0.83 100
 PMMA -1.1 100
 Norland 61 -2.6 80
 Norland 68 -3.1 35
 Using the Norland crosslinked polymers, higher values of dn/dT were
 obtained because of enhanced local mobility of the polymer chains at the
 operating condition; the lower glass transition implies higher mobility
 for this experiment. The data in Table 1 suggests that further
 enhancements in the thermo-optic coefficients may be realized by further
 reducing the Tg of the polymer below the operating temperature.
 Switching Efficiency
 Using materials from the family described above, increased switching
 efficiency may be obtained compared to similar devices fabricated using
 high Tg materials operated under identical conditions (same wavelength,
 switch energy, etc.). The increased switching efficiency results from the
 lower switch activation energy required to induce the same refractive
 index difference at the TIR interface.
 In the above example the TIR switch is designed to operate in a temperature
 range such that a predetermined minimum quantity of optical radiation is
 deflected from the waveguide, depending upon application and field of use.
 Switching efficiency is determined by first measuring the waveguide
 throughput, Tp, before device activation and then during switch
 activation, T.sub.A The switch efficiency is calculated using the
 expression eff=1-T.sub.A /T.sub.P. Switching efficiency refers to the
 maximum amount of optical radiation deflected from the waveguide when a
 switch is activated under repetitive pulsing at 50 Hz compared to the
 throughput of the waveguide when the switch is in the "off" state.
 Table 2 lists the results of switch efficiency measurements on devices
 containing thermo-optic TIR switches that were operated at temperatures
 near 23.degree. C. Device 1 was a 2-degree thermo-optic switch comprising
 the high Tg materials set described above (Epo-tek/Epoxy-lite/Ablestick)
 on a glass substrate. Device 2 is a preferred embodiment fabricated with
 lower Tg materials (Gelest/Norland/Gelest) on glass with nominally the
 same switch geometry and layer thicknesses. In all measurements,
 essentially similar TIR switches were activated with a current pulse that
 delivered 200 pJ/.mu.m.sup.2 of energy to the heating element and the
 switch efficiency was measured as described earlier. As Table 2 shows,
 devices that incorporated our lower Tg material system had much improved
 switch efficiencies. The switch efficiency increased from near 0% to
 approximately 80% when the thermo-optic coefficient of the core layer was
 changed from -1.4.times.10.sup.-4 [1/C] to -3.3.times.10.sup.-4 [1/C].
 These results show that the switch efficiency can be improved by operating
 the device near or above the glass transition temperature of the polymers
 used in the optical waveguide.
 TABLE 2
 Switch efficiency at 200
 Core Layer Tg pJ/.mu.m.sup.2
 Device 1 Epoxy-Lite R46 +150.degree. C. .about.0%
 Device 2 Norland 68 +35.degree. C. 80%
 Although we used room temperature devices, the same effect of using a using
 a lower Tg/higher mobility polymer can be achieved with higher Tg
 materials by heating the device to operate at a nominal operating
 temperature that equals or exceeds the Tg or the effective Tg.
 Switch Energy Consumption
 The device fabricated in this embodiment requires the control element to
 deliver less electrical energy to the switch element since larger
 thermo-optic coefficients enable lower operation temperatures to achieve
 the same or perhaps better switch efficiency than similar devices
 fabricated using higher Tg materials. To illustrate this point further we
 tested Device 1 and Device 2 as described above, by measuring the amount
 of electrical energy that produced a predetermined switch efficiency of
 .about.80% in each of the devices. Table 3 lists the electrical energy
 supplied to the TIR heating element to achieve nearly 80% switch
 efficiency. The data in Table 3 indicates that devices incorporating
 material layers with larger thermo-optic coefficients required less
 electrical energy to achieve similar switch efficiency than devices
 comprised of higher Tg materials. Again, these results show that the
 switch energy consumption can be reduced by operating the device near or
 above the glass transition temperature of the polymers used in the optical
 waveguide.
 TABLE 3
 Energy for 80% switch
 Core Layer Tg efficiency
 Device 1 Epoxy-Lite R46 +150.degree. C. &gt;450 pJ/.mu.m.sup.2
 Device 2 Norland 68 +35.degree. C. 200 pJ/.mu.m.sup.2
 Switch Cycle Time
 The device fabricated in this embodiment produces a faster switch for a
 given heating rate since lower minimum operating temperatures are
 necessary to achieve the refractive index differential to achieve TIR
 switch activation. FIG. 10 shows temperature responses for two different
 polymer TIR switches. Trace A illustrates the temperature response of a
 device incorporating a high mobility/lower Tg/large dn/dT polymer that is
 operated at a temperature to achieve TIR switch activation. The switch
 reaches the activation temperature, T.sub.A, enabling TIR switching to
 occur at a time t.sub.1. After the switch has been deactivated the
 temperature returns to equilibrium, a value T.sub.E, close to its original
 temperature at a time t.sub.2. The switch "cycle time" for this high
 mobility/lower Tg/larger dn/dT polymer switch is (t.sub.2 -t.sub.0).
 Trace B illustrates a device incorporating a higher Tg polymer switch that
 is operated at a temperature to achieve TIR. After a larger application of
 thermal energy than for the switch of Trace A, the switch of Trace B
 reaches the activation temperature, T.sub.B, enabling TIR switching to
 occur at a time t.sub.3, later than the time t.sub.1. After the switch has
 been deactivated, the temperature returns to equilibrium, T.sub.E, a
 temperature close to its original temperature at a time t.sub.4 and
 consequently the refractive index of the polymer material reverts to its
 equilibrium state. The switch cycle time for this higher Tg polymer switch
 is (t.sub.4 -t.sub.0). Note that it takes longer to return to a
 temperature near equilibrium from a higher temperature than it does from a
 lower temperature, thus increasing the switch cycle time. The switch cycle
 time can be improved by operating the device near or above the glass
 transition temperature of the polymers used in the optical waveguide.
 Note that the benefits described above may occur independently,
 sequentially, or in combination whether or not they are observable in a
 specific device.
 Many variations in implementation apply to this invention. Most
 importantly, any material known in the art with a glass transition
 temperature may be used for the waveguide materials, including urethanes,
 siloxanes, acrylates, fluoroelastomers, alkenes, dienes, ayrlates,
 methyacrylics, methacrylic acid esters, vinyl ethers, vinyl esters,
 oxides, and esters or perhaps other polymers that possess tailorable Tg's,
 and optical transparency. These materials may be combined with other
 materials known in the art including glass, polymer, semiconductor,
 sol-gel, aero-gel, and/or metal, to form the desired waveguiding
 structure, provided that at least one of the materials in the waveguiding
 structure (i.e. traversed by at least an evanescent field of optical
 radiation) is a polymer operated above Tg.
 Other types of waveguiding structures known in the art can be used,
 including ridge waveguides fabricated into the core rather than the lower
 cladding, patterned waveguides formed from four-layer (or more) stacks,
 cladding-loaded waveguides, buried waveguides, diffused waveguides,
 photodefined waveguides, bleached or poled waveguides, serial grafted
 guiding structures, etc., provided that a local index enhancement is
 produced within the boundaries of the desired guided mode pattern. The
 local index enhancement may by symmetric or asymmetric relative to the
 center of the waveguide, and different combinations of refractive indexes
 may be used as is known in the art. Patterning techniques known in the art
 that can be used include wet etching, in- or out-diffusion, liftoff, laser
 ablation, focused ion beam processing, etc. Coating techniques known in
 the art that can be used include spinning, extrusion, slot-die,
 evaporation or vapor phase deposition, meniscus coating, lamination, etc.
 Substrates may be chosen from among many known in the art including glass,
 silicon, metal, semiconductor, polymer, etc.
 Other resistive films known in the art may also be chosen, including NiCr,
 WSi, SiN, other metals and compounds, and various other forms of silicon
 such as amorphous silicon, and all these films may be doped with other
 species to improve their properties, provided that the resistivity
 obtained with the film is adequate for heating the waveguide in the
 thermo-optic region. The resistive film pattern may or may not include
 electrode structures made of other materials such as conductive polymers,
 metals including Al, Cu, Pd, solder, etc., but these connection structures
 are preferably made of a high conductivity material that enhances the
 connection process to the external electronic leads that should be
 connected to the control element with low contact resistance.
 Other switch elements (including Y-branch switches, crossing waveguides,
 parallel couplers, gratings, electro-optic and electro-strictive devices,
 etc.) could be used in place of the TIR switch. It will be apparent to
 those of ordinary skill in the art that certain modifications well known
 in the art will be required to enable the alternative devices to operate
 as desired. For example, in an electro-optic grating which requires the
 use of an electro-optic polymer layer as compared to the thermo-optic
 polymer layer in the example above, the control element would be in the
 form of a voltage supply. Supplying voltage to an electrode placed over
 the waveguide in a similar fashion to the resistive heating element
 described above creates an electric field in the electro-optic polymer
 layer, and changes its refractive index through the electro-optic effect.
 Ultimately switch activation will cause the deflection of light from the
 waveguide as in the previous example. However, double crosslinking of the
 chromophores will be desirable to maintain their orientation when
 operating the materials above their Tg to exploit the favorable
 viscoelastic properties. In some applications it may be advantageous to
 deposit additional layers (e.g. for heaters, for hermetic layers, opaque
 layers, etc.) as device and material requirements necessitate.
 The TIR switch is an example of a controller that controls the propagation
 of optical radiation in a transparent material. Other examples include
 Mach-Zehnder modulators, Y-branch splitters, gratings, parallel couplers,
 and many others including in general thermo-optic, electro-optic, and
 acousto-optic devices and devices actuated by applied stress or strain.
 These alternatives may be combined with any of the devices or
 implementations of our invention described herein, repeated units may be
 fabricated, and parts of one device described here many be integrated with
 all or parts of other devices described here, or known in the prior art.
 Mach-Zehnder Modulator
 An illustration of a thermo-optic Mach-Zehnder modulator is shown in FIG.
 11. This figure shows a three-dimensional rendering of a multi-layer stack
 comprised of a lower cladding layer 32, a crosslinked polymer waveguide
 core layer 34, into which a waveguiding structure has been defined by one
 of many means described earlier, and a crosslinked polymer top cladding
 layer 36. The core layer contains input and output waveguides, 38 and 40
 respectively, input and output y-branches, 42 and 44 respectively, bias
 and a signal waveguides, arms 46 and 48. Located over the bias and signal
 waveguides on top of the multi-layer stack are two resistive heating
 elements, one of which serves as a bias heating element 50 and the other
 as the modulating heating element 52. There are control elements 56 and 54
 to individually supply current to the bias heating element and modulating
 heating element respectively.
 In this optical device, light enters through an input waveguide 38 where it
 is then split in the input y-branch and propagates into the bias and the
 signal waveguides. In the absence of any control current to the heating
 elements, light propagating in bias and signal waveguides are recombined
 at the output y-branch and interfere constructively or destructively
 according to the relative phases and finally exit the device through the
 output waveguide 40.
 The control current supplied to the bias heating element is adjusted to
 change the temperature Tbias and hence the steady state refractive index
 of the polymer in the proximity of the bias heating element n1(Tbias). The
 refractive index change caused by the thermo-optic effect changes the
 optical path length of the light in proximity to the bias heating element
 such that the optical phase difference between the two arms of the
 interferometer is nearly +/-.pi./4 and a half-maximum optical intensity is
 observed at the output waveguide. A modulated control current is then
 applied to the modulating heating element. Since the device is biased at
 the half-maximum intensity location, subsequent device output will be
 proportional to the applied driving current for small modulation currents.
 Changes in the control current will result in time dependent optical
 response.
 The optimum performance of this device under repetitive cycling of the
 modulating current requires a polymer material that returns to equilibrium
 or near equilibrium when the modulating control current is turned-off and
 minimal drift of the refractive index of the polymer near the bias heating
 element. If the material properties of the polymer, for example the
 refractive index, density, or volume in proximity to the heating elements
 evolve with time, the required bias temperature to achieve .pi./4 optical
 phase shift will differ from the originally designed temperature. When
 operating such devices below the Tg of the optical materials as in the
 prior art, differential index changes can build up that unbalance the
 phase of the two beams in the output 40 and device performance will
 degrade. This degradation may be partially compensated by changing the
 bias temperature controlled by the heating and control elements 50 and 56,
 but in practice a drift in the bias temperature usually requires
 additional hybrid feedback or tracking electronics. For device simplicity
 and cost concerns, it is desirable to have devices that function normally
 without additional control electronics.
 FIG. 12 illustrates how changes in the bias temperature affect the
 intensity of the output light. The figure shows the output signal
 intensity as a function of bias waveguide temperature (Tbias) plotted as a
 solid line and indicates the temperature at which the interferometer is
 originally biased at T1. If the material properties change due to
 viscoelastic material response, the optical response of the device will
 also change so that a different temperature, T2, is now required to attain
 the same .pi./4 phase shift (dotted line on the figure). A device designed
 to operate with a bias temperature T1 no longer functions as intended.
 Furthermore, if the guide properties of either arm of the interferometer
 change with respect to the other (as by changes in density due to the
 viscoelastic response), the splitting of light at the input y-branch will
 be unbalanced and the contrast ratio of the interferometer will decrease
 in time.
 For the device to operate with negligible decrease in contrast ratio and at
 the temperature intended without additional control electronics, it is
 desirable to utilize materials with negligible viscoelastic response. A
 device comprised of lower Tg material would be less effected by
 viscoelastic effects and as such would function more reliably than devices
 comprised of materials exhibiting observable viscoelastic responses.
 Y-branch Splitter
 FIG. 13 shows a top view of a three-layer stack comprising a lower cladding
 layer 60, a crosslinked polymer waveguide core layer 62 (into which
 waveguide structure has been defined by means described earlier), and a
 crosslinked polymer top cladding layer 64. The core layer contains an
 input waveguide 66, and two output waveguides 68 and 70, with an angle of
 separation 72 between them. Located on the stack are two resistive heating
 elements 74, 76 which lie approximately over the output waveguides and
 have nearly the same width. Each switching element is powered by a current
 supply 80, 82 so that either electrode can be individually activated. The
 figure also shows a waveguide branch where the single input waveguide
 splits into the two output waveguides at 78. The heating elements are
 offset from the branching section to allow a gradual heating (as viewed
 along the axis of one branch of the waveguide as compared to the other
 (deactivated side).
 A heating element increases the temperature of the polymer material near
 it, and lowers the effective refractive index of output waveguide under
 the activated heating element compared to the unheated output waveguide as
 a result of the thermo-optic effect. Light will preferentially couple into
 the output waveguide with the higher effective refractive index as is well
 known in the art. Such a design produces an adiabatic thermal heating of a
 region in proximity to the activated heating element. Without any current
 applied, light entering the branch from the input waveguide is split
 between output waveguides.
 Such devices that operate at temperatures near or below the effective Tg
 are inherently susceptible to changes in material properties from
 viscoelastic effects. For example, consider the case of a permanent change
 of the refractive index in polymer material of one of the output
 waveguides compared to the other as a result of viscoelastic effects. If
 the refractive index of polymer material under the heating element of
 either waveguide evolves with repeated switch operation, failure in the
 form of preferential routing of light into the waveguide with a higher
 refractive index will occur, even in the absence of a control current to
 the heating element.
 FIG. 14 illustrates the Y-branch degradation mode of a splitter utilizing
 high Tg polymer material(s). The figure shows an example of the optical
 power in each output waveguide after the completion of a given number of
 operation cycles of switch 76. Initially, the Y-branch equally distributes
 power into both output waveguides, by design. As the number of cycles
 increases, viscoelastic effects cause a long time constant refractive
 index change, and the branching symmetry is broken. Eventually a state may
 be reached when the splitting of light into the output waveguides in
 highly asymmetric when neither heating element is activated, and the
 device no longer functions as the desired EDB splitter in the off-state.
 We have shown the evolution to be linear, but the detailed temporal form
 of the throughput change in a given application depends on both the
 materials used, the pattern of arrival of switching control signals.
 As stated earlier, if a substantially permanent index of refraction change
 occurs in the polymer material under a heating element (76), light will
 preferentially route into the waveguide with the higher refractive index
 (68). In order to route light into output waveguide (70) a higher current
 would be required to overcome the preferential routing caused by damage
 (the degradation in material properties) to output waveguide 70. If the
 cycle is repeated, excess damage will be incurred in each cycle. The
 failure mechanisms described above will be reduced or eliminated if the
 device is fabricated using lower Tg polymer materials enabling operation
 above the Tg of the polymer material(s).
 Thermo-optic Grating Devices
 In practical devices, it is desirable for the device to respond linearly to
 the application of a control signal. This property is desirable because it
 simplifies device electronics that control and monitor performance
 compared to systems that possess a nonlinear response which then require
 complex algorithms to relate device control signals to device response. In
 addition, response linearity allows uncomplicated adjustment, tuning, and
 control of device operation because signal and response are related by a
 simple derivative relationship and device performance can be predicted if
 the control signal is known.
 Thermo-optic devices operated in the spirit of this invention, comprise
 materials with Tg below the operating temperature of the device and
 therefore naturally operate in a regime where the refractive index of the
 polymer reasonably changes linearly with temperature (see region B of FIG.
 1).
 In contrast, devices comprised of materials with Tg above the operating
 temperature will experience a change in the slope of the thermo-optic
 coefficient as the temperature of the device is raised above Tg. This
 change in slope produces a nonlinear response of the index of refraction
 to the applied control signal (temperature). Note that devices operated at
 temperatures well below Tg (see region A of FIG. 1) also exhibit a linear
 relationship between refractive index and temperature, but these devices
 operate with a lower thermo-optic coefficient than devices operating at a
 temperature above Tg, and they experience the unfavorable viscoelastic
 effects described above such as long-time constant change in the index of
 refraction which may unbalance a device or increase its insertion loss.
 Elements capable of being regulated to attain a desired temperature or
 index of refraction include devices such as gratings as shown in FIG. 15.
 Polymer thermo-optic grating devices may be used as optical filters,
 add/drop multiplexers, or more generally as thermo-optically tunable Bragg
 gratings. Desirable properties include long-term stability of index of
 refraction, a large material thermo-optic coefficient, linearity of
 response as a finction of temperature, and lack of birefringence. All of
 these properties are uniquely obtained with optical polymer waveguide
 materials operated above their Tg, and preferably above their effective
 Tg.
 Consider a Bragg grating formed by fabricating a polymer multi-layer stack
 consisting of a lower cladding 94, core 100, and upper cladding 92, on a
 substrate 96. The core layer contains a waveguide (as described earlier)
 where the optical mode in the waveguide 102 now overlaps a region
 containing a grating 104. The grating may be fabricated by one of several
 methods known in the art including etching, ablation, molding, embossing,
 lamination, e-beam writing, holographic exposure, etc., provided that the
 process provides adequate modulation of the index of refraction with the
 desired periodicity. The grating period (typically on the order of the
 wavelength of light) is selected to achieve Bragg reflection for at least
 a predetermined wavelength of light 98 propagating in or coupled into the
 waveguide. Light of wavelength satisfying the Bragg condition is reflected
 or coupled into another path. In a preferred embodiment, the grating
 retro-reflects light in the waveguide.
 The Bragg waveguide reflector can be made thermally tunable by fabricating
 a heating electrode 106 on the device in proximity to the grating element.
 When a control element 110 delivers current to the heating element the
 temperature of the polymer (grating) in proximity to the heater will
 change as a result of the thermo-optic effect. The refractive index change
 of the grating affects the wavelength of light that satisfies the Bragg
 condition so that a different wavelength is now Bragg reflected in the
 waveguide. If the process is repeated at another temperature another
 wavelength will then satisfy the Bragg reflection condition. In this
 manner the device is tunable because a temperature can be selected to
 achieve Bragg reflection at many predetermined wavelengths. It should be
 noted that this device is usually operated in a steady state temperature
 condition so that a single wavelength will satisfy the Bragg reflection
 condition over a given time interval. A linear temperature change of the
 polymer material comprising the grating in this invention then produces a
 linear response of the resonant wavelength of the grating with respect to
 temperature, thus providing linear tunability. In addition, grating
 devices in this invention will have wide resonant wavelength tuning
 capability (bandwidth) because of enhanced thermo-optic coefficients.
 The invention has now been explained with reference to specific
 embodiments. Other embodiments will be apparent to those of ordinary skill
 in the art. Therefore it is not intended that the invention be limited,
 except as indicated by the appended claims, which form part of the
 invention description.