System and method for providing temperature control for a thermally activated optical switch using constant total power

An optical switching device and a method of providing temperature control for the device utilize compensating thermal energy to maintain a consistent operating temperature. The optical switching device may be a thermally activated optical switch that routes optical signals using bubbles that are strategically created along optical paths within the device. The bubbles are created by thermal energy generated by switching heating elements. The compensating thermal energy may be generated by at least one compensating heating element or by at least one switching heating element that is not currently being used for optical switching, i.e., bubble creation. The compensating thermal energy is varied so that total thermal energy generated by the device is constant, which results in a consistent operating temperature. In a first embodiment, the device includes additional heating elements that are selectively activated to generate the compensating thermal energy. In a second embodiment, the compensating thermal energy is generated by all of the switching heating elements along one or more optical paths that are in a transmission mode. In a third embodiment, the device includes a single additional heating element that is supplied with a variable electrical power to generate the compensating thermal energy.

TECHNICAL FIELD
 The invention relates generally to optical switches and more particularly
 to a thermally activated optical switch.
 BACKGROUND ART
 Continuing innovations in the field of fiber optic technology have
 contributed to the increasing use of optical fibers in telecommunications
 and data communications networks. With the increased utilization of
 optical fibers, there is a need for efficient peripheral devices that
 assist in the transmission of data through these optical fibers, such as
 optical switches. An optical switch operates to selectively couple an
 optical fiber to one of two or more alternative optical fibers such that
 the two coupled optical fibers are in communication with each other.
 The coupling of the optical fibers performed by an optical switch can be
 effectuated through various techniques. One technique of interest utilizes
 micro-mirrors to selectively route optical signals from an input optical
 fiber to a selected output optical fiber. In the simplest implementation
 of the micro-mirror technique, the input optical fiber is aligned with one
 of two output optical fibers, such that when the micro-mirror is not
 placed in the optical path between these two aligned optical fibers, the
 two aligned optical fibers are in a communicating state. However, when the
 micro-mirror is interposed between the two aligned optical fibers, the
 micro-mirror steers, i.e., reflects, the optical signals from the input
 optical fiber to the other output optical fiber. The positioning of the
 micro-mirror in and out of the optical path between the two aligned
 optical fibers can be accomplished by using a micro-machined actuator that
 mechanically displaces the micro-mirror to a desired position.
 Another technique of interest utilizes thermally created bubbles, instead
 of micro-mirrors, to selectively route optical signals from input fibers
 to target output optical fibers. This technique is implemented in a
 thermally activated optical switch that is described in U.S. Pat. No.
 5,699,462 to Fouquet et al., which is assigned to the assignee of the
 present invention. A conventional thermally activated optical switch 10 is
 schematically illustrated in FIGS. 1 and 2. As shown in FIG. 1, the
 optical switch includes a waveguide chip 12, a heater chip 14 , and a
 metal substrate 16. The waveguide chip contains planar waveguides 18,
 shown in FIG. 2, that serve as media for transmission of optical signals.
 These waveguides form a matrix of optical paths. Optical paths 20, 22, 24
 and 26 facilitate lateral transmissions of optical signals, while optical
 paths 28, 30, 32 and 34 facilitate vertical transmissions of optical
 signals. The waveguide chip also contains a number of trenches 36, located
 at intersections of optical paths. Each trench is positioned so that an
 incoming optical signal from one of the optical paths 20-26 will impinge
 upon the trench at an angle of incidence greater than the critical angle
 of total internal reflection (TIR). When a trench is filled with a liquid
 having a refractive index generally matching that of the waveguides,
 optical signals propagating along the lateral optical path that extends
 across that trench will be transmitted through that trench. However, when
 a bubble is formed within the trench, the optical signals are reflected by
 the wall of the trench from the lateral optical path to a vertical optical
 path that intersects the lateral optical path at the location of the
 trench.
 The heater chip 14 of the optical switch 10 includes heating elements 38,
 i.e., resistors, and other electrical elements, such as transistors, to
 address individual resistors. For simplification, only the resistors are
 shown in FIG. 2. The heater chip is aligned with the waveguide chip 12 so
 that each resistor of the heater chip is positioned below a trench 36 of
 the waveguide chip, where two optical paths intersect. The resistors
 provide the thermal energy to create the bubbles within the trenches.
 Therefore, by selectively activating the resistors, any optical signals
 that were originally propagating through the lateral optical paths 20-26
 can be rerouted to the vertical optical paths 28-34. The heater chip is
 attached to the metal substrate 16 of the optical switch, as shown in FIG.
 1. The metal substrate contains a reservoir 40 of the refractive
 index-matching liquid. The reservoir is connected to the trenches of the
 waveguide chip by vias (not shown), which extend through the heater chip.
 In order to provide an optimized and consistent performance of the
 thermally activated optical switch 10, the heater chip 14 needs to be
 maintained at nearly constant and uniform temperature. Large temperature
 variations at the intersections of optical paths, or cross points, where
 the bubbles are created to reflect optical signals, cause increased
 optical losses and cross talk, as well as perturbations to the bubble
 behavior. Environmental changes aside, accurate and precise temperature
 control of an NxN thermally activated optical switch, where N is
 significantly large, is difficult because N resistors can be activated
 simultaneously. Therefore, the total heat load of the heater chip can
 change by as much as N times the power required at each cross point, which
 results in large temperature variations.
 An active temperature control device 42, shown in FIG. 1, can be utilized
 to try to control the temperature fluctuations within the optical switch
 10. However, for packaging reasons, the temperature control device is
 located at a significant distance from the heater chip 14, where the
 sudden heat load changes are generated. As shown in FIG. 1, the
 temperature control device is located below the optical switch. Thus, the
 temperature control device and the heater chip are separated by the metal
 substrate 16. This implies that (i) a thermal gradient will exist between
 the heater chip and the temperature control device and (ii) any change in
 the local temperature of the switch will be resolved on a time scale
 limited by the heat conduction between the heat generating resistors and
 the active temperature control device. The amplitude of the temperature
 fluctuations depends both on the power required for each resistor and on
 the thermal resistance of the path between the heat generating resistors
 and the temperature control device. Therefore, if the materials along the
 heat path have a low thermal diffusivity, the temperature control device
 will have a slow response time. It follows that on-chip temperature
 control will benefit from reducing the power requirements of the resistors
 and from devising packaging solutions that maximize the heat transfer
 between regions of heat production and heat removal.
 Although the above approaches will result in an improved on-chip
 temperature control for a thermally activated optical switch, additional
 improvement in temperature control is desired. Therefore, what is needed
 is a thermally activated optical switching device and a method for
 improving the temperature control of the switching device.
 SUMMARY OF THE INVENTION
 An optical switching device and a method of providing temperature control
 for the device utilize compensating thermal energy to maintain a
 consistent operating temperature. The optical switching device may be a
 thermally activated optical switch that routes optical signals using
 bubbles that are strategically created or manipulated along optical paths
 within the device.
 The bubbles are created by thermal energy generated by switching heating
 elements. The compensating thermal energy may be generated by at least one
 compensating heating element or by at least one switching heating element
 that is not currently being used for optical switching, i.e., bubble
 creation. The compensating thermal energy is varied so that total thermal
 energy generated by the device is constant, which results in a consistent
 operating temperature.
 The device includes a waveguide chip, a heater chip, a metal substrate and
 a control unit. The waveguide chip includes a number of waveguides that
 define intersecting optical paths. Thus, the device may be an N.times.M
 optical switching device, where N input optical paths intersect M output
 optical paths. The waveguide chip also includes trenches in which the
 bubbles are created. Each trench is positioned at an optical path
 intersection, so that optical signals along an input optical path are
 reflected to an output optical path at the intersection when a bubble is
 present in the trench. The waveguide chip may be composed of silica. The
 heater chip includes a number of switching heating elements, e.g.,
 switching resistors, and may include one or more compensating heating
 elements, e.g., compensating resistors. These switching and compensating
 resistors are controlled by the control unit. The control unit includes
 circuitry to selectively supply electrical power to these resistors. The
 heater chip also includes fluid fill-holes that extend through the heater
 chip. These fluid fill-holes supply the refractive index-matching liquid
 to the trenches from a reservoir in the metal substrate.
 In a first embodiment, the heater chip includes N.times.M switching
 resistors. The heater chip also includes additional N resistors that
 function as compensating resistors. The compensating resistors are
 distributed throughout the heater chip to diffuse the source of heat
 generation and to reduce the likelihood of localized temperature
 gradients. In this embodiment, the control unit is configured to
 consistently activate N switching and/or compensating resistors,
 regardless of the optical switching configuration of the device. The
 compensating resistors generate the variable compensating thermal energy,
 so that the total thermal energy generated by the device remains constant
 for different switching configurations.
 In operation, X (0.ltoreq.X.ltoreq.N) switching resistors are selectively
 activated by the control unit to create bubbles in the trenches at which
 reflection is required for the current optical switching configuration. If
 X equals N, the thermal energy generated by the activated switching
 resistors is the desired total thermal energy. In this case, no
 compensating resistor needs to be activated. However, if X is less than N,
 the thermal energy generated by the activated resistors is less than the
 desired thermal energy. In this case, the control unit activates N-X
 number of compensating resistors, using the same electrical power
 delivered to each switching resistor for each compensating resistor. That
 is, for each column of switching resistors where no switching resistor is
 activated, a compensating resistor is instead activated. Since N resistors
 are activated for any optical switching configuration, the total thermal
 energy generated by the device is constant. Therefore, the operating
 temperature of the device will be more accurately and easily controlled.
 The total electrical power supplied to the switching resistors and/or the
 compensating resistors is the same for each optical switching
 configuration.
 In a second embodiment, the heater chip still includes N.times.M switching
 resistors, but does not include any compensating resistors. In this
 embodiment, the compensating thermal energy is generated by the switching
 resistors along each column in which no switching resistor is activated
 for optical switching. For each of these columns, the electrical power
 required for switching conditions is distributed to each resistor along
 the column. Thus, the cumulative thermal energy generated by the resistors
 on one of these columns would equal the thermal energy generated by a
 single activated resistor. At the same time, if N is sufficiently large,
 the power generated at each resistor will be small and below the minimum
 threshold to thermally activate a bubble. Consequently, the total thermal
 energy generated by the device is maintained at a constant value for
 different switching configurations, which equates to a consistent
 operating temperature. Again, the total electrical power supplied to the
 switching resistors remains the same for each optical switching
 configuration.
 In a third embodiment, the heater chip includes N.times.M switching
 resistors and a single large compensating resistor. In this embodiment,
 the control unit is configured to supply the compensating resistor with a
 variable electrical power to generate the compensating thermal energy. The
 amount of electrical power supplied to the compensating resistor would
 depend on the number of activated switching resistors for the current
 optical switching configuration, which determines the thermal energy
 generated by the activated switching resistors. Thus, the total thermal
 energy generated by the device will remain constant for different
 switching configurations, which provides the consistent operating
 temperature.

DETAILED DESCRIPTION
 Short term temperature fluctuations within a thermally activated optical
 switch, such as the optical switch of FIG. 1, are primarily due to the
 activation and deactivation of heating elements to form bubbles for
 selective routing of optical signals, where the number of activated
 heating elements can vary substantially during operation. For an N.times.M
 optical switch, the number of heating elements that may be activated for a
 given moment can be 0 to N, depending on the current optical coupling
 configuration of the switch. N is the typical maximum number of heating
 elements that are activated for a given moment, since only one heating
 element needs to be activated for each column of heating elements that
 corresponds to an input optical path of the switch. The total amount of
 thermal energy generated within the optical switch by the activated
 heating elements for a given moment depends on the number of activated
 heating elements times the power required by each heating element to
 create a bubble.
 The fundamental idea of the present invention is to operate a thermally
 activated optical switch at a constant thermal power, i.e., a fixed
 electrical power, so that the optical switch can be maintained at a
 substantially consistent operating temperature. For an N.times.M optical
 switch, this fixed electrical power would be N times the required power
 for activating a heating element. Since most of the optical coupling
 configurations of the switch would require the activation of fewer than N
 heating elements, the amount of electrical power used by these heating
 elements would be less than the fixed electrical power. Consequently, the
 thermal energy generated by the activated heating elements would yield an
 operating temperature for the optical chip that is less than the desired
 temperature. However, the desired operating temperature can be achieved by
 distributing the remaining electrical power, i.e., the fixed electrical
 power minus the power used by the activated heating elements, to those
 heating elements that are not being activated and/or other heat-producing
 elements to generate a compensating thermal energy. The compensating
 thermal energy will then raise the temperature of the switch to that of
 the desired operating temperature. In this manner, the optical switch can
 be maintained at a consistent operating temperature. Described below are
 three embodiments of the invention that utilize this idea of generating a
 compensating thermal energy to maintain the consistent operating
 temperature.
 With reference to FIGS. 3, and 4, a thermally activated optical switch 46
 in accordance with a first embodiment of the invention is shown. The
 optical switch includes a waveguide chip 48, a heater chip 50, and a metal
 substrate 52. The waveguide chip is preferably made of silica. The
 waveguide chip contains sixteen planar waveguides 54 and twenty-four
 intermediate waveguides 56 that define intersecting optical paths 58, 60,
 62, 64, 66, 68, 70 and 72, as illustrated in FIG. 4. Although the optical
 switch is shown as a 4.times.4 switch, the optical switch may be
 configured as an N.times.M switch, where N and M are integers. The optical
 switch is attached to four sets of optical fibers 74, 76, 78 and 80, with
 each set including four optical fibers. Typically, the operations of the
 optical switch that are of greatest concern are the operations that
 determine the optical coupling of the optical fibers 76A, 76B, 76C and 76D
 with the optical fibers 78A, 78B, 78C and 78D. The remaining optical
 fibers may be used for add and drop operations or to connect to other
 switches to form a larger switching matrix. The waveguide chip also
 contains trenches 82 that can be filled with the refractive index-matching
 liquid and that enable the formation of a bubble at each intersection of
 optical paths. As previously described with reference to FIG. 1, the
 optical coupling of the waveguides depends on the presence of the
 refractive index-matching liquid or a bubble at a waveguide intersection.
 The heater chip 50 of the optical switch 46 includes sixteen switching
 heating elements 84 and four compensating heating elements 86 that are
 formed on a substrate, which may be made of silicon. Both types of heating
 elements are resistors that are designed so that each resistor generates
 the same amount of thermal energy when activated by a predefined voltage
 or current. The switching resistors are used to create bubbles within the
 trenches 82 of the waveguide chip 48, while the compensating resistors are
 used to generate a variable compensating thermal energy to maintain the
 consistent operating temperature. The manner in which the compensating
 resistors are utilized for temperature control will be described below. As
 illustrated in FIG. 4, each switching heating element 84 is aligned with
 an optical path intersection, so that a bubble can be formed in the trench
 at that intersection. The compensating resistors 86 are positioned at
 various locations throughout the heater chip. The exact locations of the
 compensating resistors are not critical to the invention, but preferably
 promote uniform heat distribution. The compensating resistors are shown
 FIG. 4 to illustrate one possible distribution of the compensating
 resistors on the heater chip 50. The heater chip further includes
 electrical traces (not shown) and other electrical components (not shown),
 such as transistors, that allow selective activation of the switching and
 compensating resistors. The heater chip also includes a number of fluid
 feed-holes 88 that extend completely through the heater chip. These fluid
 feed-holes supply the refractive index-matching liquid to the trenches of
 the waveguide chip from a reservoir 90 of refractive index-matching
 liquid, which is located in the metal substrate 52.
 The electrical components of the heater chip 50, including the switching
 and compensating resistors 84 and 86, are connected to a control unit 92.
 The control unit includes circuitry to drive the electrical components of
 the heater chip. The control unit may be an off-chip unit that is
 connected to the heater chip by electrical leads. However, the control
 unit may be partially or completely fabricated on the heater chip. The
 control unit operates to selectively activate one or more switching
 resistors 84, so that the optical switch 46 is in a particular switching
 configuration. In addition, the control unit selectively activates one or
 more compensating resistors 86, depending on the number of switching
 resistors that are activated.
 The metal substrate 52 of the optical switch 46 includes the reservoir 90
 of index-matching liquid. Preferably, the metal substrate is made of a
 material that has a high thermal diffusivity, so that the active
 temperature control device 42 can regulate the temperature of the optical
 switch. The temperature control device may be a solid state thermoelectric
 converter. The temperature control device is thermally coupled to the
 optical switch 46.
 In operation, X switching resistors 84 are activated by the control unit 92
 to create bubbles in selected trenches in accordance with a current
 switching configuration of the optical switch. The activation of a
 switching resistor involves supplying a predefined amount of electrical
 power, P, to generate a bubble within a trench aligned with that switching
 resistor. For the optical switch 46 of FIGS. 3 and 4, X is an integer from
 zero to four, since the optical switch can selectively route optical
 signals from the four input optical fibers 76A, 76B, 76C and 76D to the
 output optical fibers 78A, 78B, 78C and 78D. If X equals N, the
 compensating resistors 86 are not activated. However, if X is less than N,
 N-X compensating resistors are also activated by the control unit 92.
 Therefore, the number of switching resistors and compensating resistors
 that are activated is N, regardless of the switching configuration of the
 switch 46. For a subsequent switching configuration of the switch, the
 number of switching resistors and the number of compensating resistors
 that are activated may each change, but the total number of activated
 resistors, switching and/or compensating, remains the same. Since the
 total number of activated resistors is the same for different
 configurations, the total thermal energy generated by the activated
 resistors is substantially the same for every possible switching
 configuration. Consequently, the total electrical power supplied to the
 switching and/or resistors is constant. Thus, the operating temperature of
 the optical switch can be maintained at a consistent temperature.
 When one or more compensating resistors 86A, 86B, 86C and 86D are activated
 to generate the compensating thermal energy, the selected compensating
 resistors should be those that are in close proximity to the optical paths
 that are in a transmission mode. That is, compensating resistors are
 activated near one or more input optical paths 58, 60, 62 and 64 for which
 no switching resistor 84 is activated, so that optical signals are
 transmitted to fibers 80A, 80B, 80C and 80D without being reflected by a
 bubble. In contrast, if the optical path 58, 60, 62 or 64 is in a
 reflection mode, one of the switching resistors along that optical path is
 activated to create a bubble to reflect the optical signals on that input
 optical path to one of the output optical paths 66, 68, 70 and 72. As an
 example, the compensating resistor 86A may be fired when the optical path
 58 is in the transmission mode. Similarly, the compensating resistors 86B,
 86C and 86D may be individually fired when the optical paths 60, 62 or 64,
 respectively, are in the transmission mode. In this manner, the
 compensating thermal energy generated by the compensating resistors is
 distributed on the heater chip 50 to reduce the likelihood of creating a
 localized heat spot on the waveguide chip 48.
 Turning to FIG. 5, a thermally activated optical switch 94 in accordance
 with a second embodiment of the invention is shown. The thermally
 activated optical switch 94 includes all of the components of the
 thermally activated optical switch 46 of FIGS. 3 and 4, excluding the
 compensating resistors 86A, 86B, 86C and 86D. In addition, the optical
 switch 94 includes a control unit 96 that replaces the control unit 92 of
 the optical switch 46. The control unit 96 is similar to the control unit
 94. However, the control unit 96 is configured to use some of the
 switching resistors 84 that are not being activated for a switching
 function to generate the compensating thermal energy. The operation of the
 control unit 94 will be further described below. The compensating thermal
 energy and the thermal energy generated by the activated resistors result
 in the desired total thermal energy for maintaining the consistent
 operating temperature of the optical switch 94.
 In operation, X switching resistors 84 are selectively activated, if
 necessary, by the control unit 96 to create bubbles in the trenches that
 are aligned with the activated resistors for the current optical coupling
 configuration of the optical switch. If X equals N (the number of input
 optical fibers, e.g., 76A, 76B, 76C and 76D, attached to the optical
 switch 94), the thermal energy generated by the activated switching
 resistors is the desired total thermal energy. Thus, there is no need to
 generate any compensating thermal energy in this switching configuration.
 However, if X is less than N, there are one or more input optical paths
 58, 60, 62 and 64 that are in the transmission mode. For each of these
 optical paths, the control unit 96 distributes the power P among all of
 the switching resistors along that optical path. As stated above, P is the
 predefined amount of electrical power that is supplied to a single
 switching resistor 84 to generate a bubble within a corresponding trench
 82. The control unit may be configured to simultaneously apply power equal
 to P/M to each switching resistor on an input optical path that is in the
 transmission mode. Alternatively, the control unit may be configured to
 cycle the electrical power P through each switching resistor on that input
 optical path by sequentially addressing the resistors. Since each
 switching resistor will be supplied with an electrical power less than the
 power P, these switching resistors will not generate enough thermal energy
 to create bubbles. Thus, the transmission mode of the optical path will
 not be compromised.
 In any switching configuration, the total electrical power supplied to the
 switching resistors 84 of the optical switch 94 is the power P times N. If
 all of the input optical paths 58, 60, 62 and 64 are in the reflection
 mode, N switching resistors will be activated, one for each optical path.
 Thus, the total electrical power supplied to the activated resistors will
 be P.times.N. However, if one or more optical paths are in the
 transmission mode, the power P is distributed among all of the resistors
 along those optical paths. Consequently, the total electrical power
 supplied to the switching resistors still remains P.times.N. As a result,
 the total thermal energy generated within the optical switch 94 is fixed,
 regardless of the switching configuration of the optical switch. When the
 switching configuration changes, the total thermal energy generated for
 that configuration will be the same as the previous configuration.
 Therefore, the operating temperature of the optical switch, which depends
 on the generated thermal energy, will remain consistent.
 With reference to FIG. 6, a thermally activated optical switch 98 in
 accordance with a third embodiment of the invention is shown. In this
 figure, the thermally activated optical switch 98 is illustrated with the
 waveguide chip 48 removed. Although not shown, the waveguide chip includes
 the waveguides, the intermediate waveguides, and the trenches described
 above. Also not illustrated is the fact that the heater chip includes the
 fluid fill-holes that connect the fluid reservoir in the metal substrate
 with trenches of the waveguide chip. However, these features of the
 optical switch 98 are identical to those of previous embodiments. The
 heater chip also includes the switching resistors 84. However, in this
 embodiment, the heater chip further includes a large serpentine
 compensating resistor 100. In a preferred design, the large compensating
 resistor is fabricated within the heater chip, below the surface on which
 the switching resistors are located. The material and shape of the
 compensating resistor can be chosen to generate a desired range of thermal
 energy and to spread as uniformly as possible the generated thermal energy
 throughout the heater chip. The compensating resistor and the switching
 resistors of the heater chip are electrically connected to a control unit
 102. The control unit includes circuitry to selectively drive the
 switching resistors. In addition, the circuitry of the control unit is
 configured to selectively activate the compensating resistor with variable
 electrical power, which would depend on the required compensating thermal
 energy that is to be generated by the compensating resistor. The control
 unit may be an off-chip device that is connected to the heater chip by
 electrical leads. However, the control unit may be partially or completely
 fabricated on the heater chip.
 In operation, X switching resistors 84 are selectively activated by the
 control unit 102 to create bubbles in the trenches that are aligned with
 the activated resistors, so as to achieve the current switching
 configuration of the optical switch 98. If X equal N, the thermal energy
 generated by the activated switching resistors is the desired total
 thermal energy. Thus, no compensating thermal energy needs to be
 generated. However, if X is less than N, the thermal energy generated by
 the activated resistors is less than the desired thermal energy. In this
 case, the control unit supplies the compensating resistor 100 with
 electrical power to generate a compensating thermal energy that equals the
 difference between the desired total thermal energy and the thermal energy
 generated by the activated switching resistors. As a result, the total
 thermal energy generated within the optical switch 98 is fixed, regardless
 of the switching configuration of the optical switch. As the thermal
 energy generated by the switching resistors changes for different
 switching configurations, the compensating resistor is driven by the
 control unit to generate a compensating thermal energy that will result in
 the desired total thermal energy. Therefore, the operating temperature of
 the optical switch 98 will remain consistent. The design of the
 compensating resistor may be chosen such that the total electrical power
 supplied to the switching resistors and/or the compensating resistor will
 always equal the power P times N, regardless of the switching
 configuration of the optical switch. In alternative arrangements, the
 optical switch 98 may include additional compensating resistors that are
 supplied with a variable electrical power to generate the desired
 compensating thermal energy.
 A method of providing temperature control for an N.times.M thermally
 activated optical switch in accordance with the invention will be
 described with reference to a flow diagram of FIG. 7. During step 104, X
 number of switching resistors are selectively supplied with electrical
 power to configure the optical switch in a particular switching state,
 where 0.ltoreq.X.ltoreq.N. As a result, these switching resistors generate
 a switching thermal energy. The switching thermal energy depends on the
 number of switching resistors that are activated and the electrical power
 that is supplied to each switching resistor. In a preferred method, the
 switching resistors are supplied with a predefined amount of electrical
 power to generate bubbles within corresponding trenches in a waveguide
 chip of the optical switch. Thus, in this preferred method, the switching
 thermal energy would only depend on the number of switching resistors that
 are activated. Next, during step 106, it is determined whether X equals N.
 If X equals N, the method comes to an end. However, if X is less than N,
 the method proceeds to step 108. During step 108, a compensating thermal
 energy is generated to produce a desired total thermal energy, where the
 total thermal energy equals the sum of the switching thermal energy and
 the compensating thermal energy. Depending on the switching thermal
 energy, the compensating thermal energy is varied to produce the desired
 thermal energy, regardless of the current optical switching configuration
 of the optical switch. Since the total thermal energy remains fixed with
 different switching configurations, the operating temperature of the
 optical switch is maintained at a substantially consistent temperature.
 In a first embodiment, the compensating thermal energy is generated by one
 or more compensating resistors. In this embodiment, the compensating
 resistors are structurally identical to the switching resistors. The
 compensating resistors are positioned to minimize any interference with
 the switching operation. Preferably, the compensating resistors are
 distributed throughout a heater chip of the optical switch, on which the
 switching and compensating resistors are formed. When X switching
 resistors are activated, N-X compensating resistors are activated to
 generate the compensating thermal energy, where N is the number of input
 optical paths of the optical switch. Thus, for any switching configuration
 of the optical switch, N resistors are activated. Consequently, the
 desired total thermal energy is generated for different switching
 configurations to maintain the optical switch at a substantially
 consistent operating temperature.
 In a second embodiment, the compensating thermal energy is generated by all
 of the switching resistors along one or more columns of resistors that are
 not being used for optical switching, i.e., the input optical paths of the
 switch that are in the transmission mode. In this embodiment, for each
 column, an electrical power P is supplied to that column. The power P is
 the required electrical power for a switching resistor to generate a
 bubble within a corresponding trench. If that column is being used for
 optical switching, one of the switching resistors on that column is
 supplied with the power P, activating that switching resistor. However, if
 that column is not being used for optical switching, the power P is
 distributed to each switching resistor on that column. Thus, each
 switching resistor is supplied with electrical power equal to P/M. Since
 each of these switching resistors is supplied with power less than P,
 these resistors are not activated, i.e., they do not cause bubbles to be
 created within the corresponding trenches. As a result, the total thermal
 energy generated by the activated resistors and the resistors that are
 supplied with P/M power will be fixed for different optical switching
 configurations. Consequently, the optical switch will be maintained at a
 consistent operating temperature.
 In a third embodiment, the compensating thermal energy is generated by a
 single large compensating resistor. In this embodiment, the single
 compensating resistor is supplied with a variable electrical power to
 generate a thermal energy that equals the difference between the desired
 total thermal energy and the switching thermal energy generated by the
 activated switching resistors. Thus, the desired total thermal energy can
 be generated for different switching configurations of the optical switch,
 which results in a consistent operating temperature.