Total internal reflection optical switch

An optical switch that is constructed on a substrate having first and second waveguides that intersect at a gap having a predetermined width. The first and second waveguides are positioned such that light traversing the first waveguide enters the second waveguide when the gap is filled with a liquid having a first index of refraction. The gap is part of a trench in the substrate having a first region that includes the gap and a second region adjacent to the first region. The second region has a width greater than the width of the first region. A liquid having the first index of refraction is disposed in the first region. The liquid generates a gas when heated to a predetermined temperature. A first heater is disposed in the first region for heating the liquid to the predetermined temperature thereby generating a gas bubble in the liquid at the gap. Light traversing the first waveguide is reflected by the gap when the gap is filled with a gas. To change the switch into the non-reflecting state, the bubble is displaced to the second region of the trench, in response to a control signal. The displacement mechanism can be constructed from a second heater having a portion thereof located in the first region between the first heater and the second region. The displacement mechanism can also be constructed from a mechanism that applies a pressure differential across the first region thereby causing the bubble to partially extend into the second region. A third waveguide having an end terminating on the trench can also be included in the optical switch. The third waveguide is positioned such that light traversing the first waveguide enters the third waveguide when the gap is not filled with liquid.

FIELD OF THE INVENTION
 The present invention relates to optical switches, and more particularly,
 to an improved cross-point switching element.
 BACKGROUND OF THE INVENTION
 Optical fibers provide significantly higher data rates than electronic
 paths. However, effective utilization of the greater bandwidth inherent in
 optical signal paths requires optical cross-connect switches. In a typical
 telecommunications environment, the switching of signals between optical
 fibers utilizes an electrical cross-connect switch. The optical signals
 are first converted to electrical signals. After the electrical signals
 have been switched, the signals are again converted back to optical
 signals that are transmitted via the optical fibers. To achieve high
 throughput, the electrical cross-connect switches utilize highly parallel,
 and highly costly, switching arrangements. However, even with such
 parallel architectures, the cross-connect switches remain a bottleneck.
 A number of optical cross-connect switches have been proposed; however,
 none of these have successfully filled the need for an inexpensive,
 reliable, optical cross-connect switch. One class of optical
 cross-connects depends on wavelength division multiplexing (WDM) to affect
 the switching. However, this type of system requires the optical signals
 being switched to have different wavelengths. In systems where the light
 signals are all at the same wavelength, this type of system requires the
 signals to be converted to the desired wavelength, switched, and then be
 re-converted to the original wavelength. This conversion process
 complicates the system and increases the cost.
 A second type of optical cross-connect utilizes total internal reflection
 (TIR) switching elements. A TIR element consists of a waveguide with a
 switchable boundary. Light strikes the boundary at an angle. In the first
 state, the boundary separates two regions having substantially different
 indices of refraction. In this state the light is reflected off of the
 boundary and thus changes direction. In the second state, the two regions
 separated by the boundary have the same index of refraction and the light
 continues in a straight line through the boundary. The magnitude of the
 change of direction depends on the difference in the index of refraction
 of the two regions. To obtain a large change in direction, the region
 behind the boundary must be switchable between an index of refraction
 equal to that of the waveguide and an index of refraction that differs
 markedly from that of the waveguide.
 One class of prior art TIR elements that provide a large change in index of
 refraction operates by mechanically changing the material behind the
 boundary. For example, U.S. Pat. No. 5,204,921, Kanai, et al. describes an
 optical cross-connect based on an array of crosspoints in a waveguide. A
 groove at each crosspoint, may be switched "on" or "off," depending upon
 whether the groove is filled with an index-matching oil. The
 index-matching oil has a refractive index close to that of the waveguides.
 An optical signal transmitted through a waveguide is transmitted through
 the crosspoint when the groove is filled with the matching oil, but the
 signal changes its direction at the crosspoint through total internal
 reflection when the groove is empty. To change the cross-point switching
 arrangement, grooves must be filled or emptied. In the system taught in
 this patent, a "robot" fills and empties the grooves. This type of switch
 is too slow for many applications of interest.
 A faster version of this type of TIR element is taught in U.S. Pat. No.
 5,699,462 which is hereby incorporated by reference. The TIR taught in
 this patent utilizes thermal activation to displace liquid from a gap at
 the intersection of a first optical waveguide and a second optical
 waveguide. In this type of TIR, a trench is cut through a waveguide. The
 trench is filled with an index-matching liquid. A bubble is generated at
 the cross-point by heating the index matching liquid with a localized
 heater. The bubble must be removed from the crosspoint to switch the
 cross-point from the reflecting to the transmitting state and thus change
 the direction of the output optical signal.
 If the bubble contains noncondensable gases (such as air), it takes too
 long (minutes) to collapse when the heater is turned off. This is not
 acceptable for most applications which require a faster cycle time. Such a
 gas bubble can be removed by applying a force to the bubble to move it out
 of the optical path, to one side.
 The bubble can also be moved to another section of the trench by increasing
 the pressure on one side of the bubble. Such pressure increases can be
 accomplished by heating the fluid on one side of the cross-point or by
 physically displacing the fluid on one side of the cross-point so as to
 push or pull the bubble away from the cross-point. If the walls of the
 trench are parallel to one another, the displacement must be sufficient to
 move the entire bubble out of the cross-point area. Such large
 displacements require relatively long times or expensive hardware.
 Broadly, it is the object of the present invention to provide an improved
 cross-point for use in cross-connect switches and the like.
 It is a further object of the present invention to provide a cross-point in
 which the bubble clearing time is shorter than in prior art cross-point
 switches.
 These and other objects of the present invention will become apparent to
 those skilled in the art from the following detailed description of the
 invention and the accompanying drawings.
 SUMMARY OF THE INVENTION
 The present invention is an optical switch that is constructed on a
 substrate having first and second waveguides that intersect at a gap
 having a predetermined width. The first and second waveguides are
 positioned such that light traversing the first waveguide enters the
 second waveguide when the gap is filled with a liquid having a first index
 of refraction. The gap is part of a trench in the substrate having a first
 region that includes the gap and a second region adjacent to the first
 region. The first region has parallel walls. The second region has a width
 greater than the width of the first region. Light traversing the first
 waveguide is reflected by the gap when the gap is filled with a gas. A
 liquid having the first index of refraction is disposed in the first
 region. The liquid generates a gas when heated to a predetermined
 temperature. A first heater is disposed in the first region for heating
 the liquid to the predetermined temperature thereby generating a gas
 bubble in the liquid at the gap. A displacement mechanism causes the gas
 bubble in the first region to extend partially into the second region in
 response to a control signal. The displacement mechanism can be
 constructed from a second heater having a portion thereof located in the
 first region between the first heater and the second region. The
 displacement mechanism can also be constructed from a mechanism that
 applies a pressure differential across the first region thereby causing
 the bubble to partially extend into the second region. A third waveguide
 having an end terminating on the trench can also be included in the
 optical switch. The third waveguide is positioned such that light
 traversing the first waveguide enters the third waveguide when the gap is
 not filled with liquid.

DETAILED DESCRIPTION OF THE INVENTION
 The present invention may be more easily understood with reference to FIGS.
 1 and 2, which are top views of a prior art cross-point switching element
 10 having two states. Switching element 10 is constructed from three
 waveguides 11-13 that are fabricated in a planar lightwave circuit on top
 of a substrate. The substrate is preferably a silica, but other materials,
 such as silicon, may be used. The waveguides are defined by two cladding
 layers and a core layer. To simplify the drawing, the individual layers
 have been omitted. The fabrication of such waveguides in silica is well
 known to the art, and hence will not be discussed in detail here. For
 example, Hitachi Cable and Photonic Integration Research, Inc. in
 Columbus, Ohio have demonstrated waveguides in SiO.sub.2 on silica and
 silicon substrates. The core is primarily SiO.sub.2 doped with another
 material, such as Ge or TiO.sub.2. The cladding material is SiO.sub.2,
 doped with another material such as B.sub.2 O.sub.3 and/or P.sub.2
 0.sub.5. Because the core material has a refractive index that is
 different from the refractive index of the cladding layers, optical
 signals will be guided along waveguides 11-13.
 A trench 14 is etched through the waveguide and preferably into the silicon
 substrate. Trench 14 is positioned such that a light signal travelling
 down waveguide 11 will be reflected into waveguide 13 if the index of
 refraction of the material filling trench 14 is substantially different
 from the index of refraction of the waveguides as shown in FIG. 1. This
 state of the switching element will be referred to as the "reflecting"
 state. If, however, the intersection of the trench and the waveguides is
 filled with a material having an index of refraction that matches that of
 the core of the waveguides, the light signal will pass through trench 14
 and exit via waveguide 12 as shown in FIG. 2. This state of the switching
 element will be referred to as the "non-reflecting" state.
 The angle at which waveguides 11 and 13 intersect trench 14 depends on the
 difference in the index of refraction between the waveguide material and
 the material used to create the reflecting state in the trench. The angles
 of incidence of the waveguides and the position of the trench are chosen
 such that light incident on the trench wall from waveguide 11 is totally
 reflected into waveguide 13. This angle is typically between 53 and 75
 degrees with respect to the normal direction of the trench wall.
 When the trench is filled with the index matching material, light traveling
 down a fourth waveguide 19 will pass into waveguide 13. Waveguide 19 is
 used to construct cross-connect switches utilizing a two-dimensional array
 of cross-point switching elements. An array of this type is typically
 constructed as a plurality of rows and columns of cross-point switching
 elements. The rows and columns are connected via row and column
 waveguides. The cross-connect switch connects signals input on the row
 waveguides to the column waveguides. The specific switching pattern
 depends on the states of the switching elements.
 In these simple cross-connect switches, at any given time, there is at most
 one switching element in each column that switches light from a row
 waveguide into a column waveguide. The light switched into the column
 waveguide is transmitted to the end of the column through switching
 elements that are in the non-reflecting state. Waveguide 19 allows light
 switched by a switching element above element 10 in the array to be
 transmitted to the next switching element in the column below it so that
 the light can eventually exit from the last switching element in the
 column.
 As noted above, the index matching material may be displaced from the
 intersection by forming a bubble 15 at the intersection with the aid of a
 heating element 16. Small heating elements suitable for this function are
 well known in the ink jet printing arts, and hence, will not be discussed
 in detail here. The heating element is preferably located below the
 waveguides to assure that light crossing the trench is not intercepted by
 the heating element. The bubble can be generated by vaporizing the index
 matching liquid or by releasing a gas dissolved in the liquid.
 The bubble may be removed by allowing it to collapse or by moving it to one
 side as shown in FIG. 2. Moving the bubble to one side requires that the
 bubble be positively displaced by a distance of at least the length of the
 bubble. Such positive displacements pose technical problems.
 The present invention reduces the amount of positive displacement needed to
 remove the bubble, and hence, avoids this problem. Refer now to FIGS. 3-5.
 FIG. 3 is a top view of a cross-point trench 100 according to the present
 invention. FIG. 4 is a side view of trench 100. Trench 100 replaces trench
 14 shown in FIGS. 1 and 2. FIG. 5 is a top view of a switching element 101
 utilizing trench 100. FIG. 5 illustrates the reflection of a light signal
 from the trench wall when a bubble 117 is present. To simplify the
 drawings, the waveguides shown at 121-123 in FIG. 5 have been omitted from
 FIGS. 3 and 4. Trench 100 is preferably etched in a substrate 180 and
 includes a gap section 113 having parallel walls that either reflect a
 light signal as shown in FIG. 5 or allow the light signal to pass through
 the trench when section 113 is filled with an index matching material. The
 portion of the trench on either side of section 113 is flared as shown at
 112 and 114. A heating element 116 is located on the bottom of trench 100.
 In the preferred embodiment of the present invention, the walls 111 and
 115 of the trench on either side of the flared region are parallel to one
 another; however, other geometries can be utilized without departing from
 the teachings of the present invention.
 Refer now to FIGS. 6 and 7 which illustrate the manner in which a bubble in
 region 113 is displaced. The present invention is based on the observation
 that a bubble formed in region 113 that does not extend beyond 113 will
 remain within region 113 as shown in FIG. 5. However, if the bubble is
 displaced slightly to one side so that a portion of the bubble enters one
 of the flared regions as shown at 128 in FIG. 6, the bubble will be drawn
 into the flared region by the surface tension of the bubble until the
 bubble is entirely within the flared region or region beyond the flared
 region as shown in FIG. 7 at 129. Once the bubble is displaced into the
 flared region, the trench will be transparent to the light and the
 cross-point will have been switched. The bubble will then collapse without
 further aid. If the cross-point must be switched back to the reflective
 state, a new bubble can be introduced in region 113 by re-activating
 heating element 116.
 Refer now to FIG. 8, which is a top view of another embodiment of a
 cross-point trench 130 according to the present invention. To simplify the
 following discussion, those features of trench 130 that serve the same
 functions as features of trench 100 shown in FIGS. 3 and 4 have been given
 the same numeric designations. Trench 130 differs from trench 100 in that
 a second heating element shown at 131 has been introduced on the bottom of
 the trench at a location that is partially in flared region 112. Heating
 element 131 is used to destabilize a bubble generated by heating element
 116 in region 113 such as the bubble shown at 137. When heating element
 131 is activated, bubble 137 is enlarged in the area over heating element
 131. The new bubble now extends into the flared region of the trench. As
 noted above, such a bubble is automatically drawn into the flared region
 and out of region 113 by the mechanism described above with reference to
 FIGS. 6 and 7. It should be noted that the heater shown in FIG. 8 can also
 be placed on the other end of region 113.
 A bubble in region 113 can also be displaced sufficiently to cause it to
 leave region 113 by generating a pressure differential across region 113.
 Refer now to FIGS. 9 and 10 which are side and top views, respectively, of
 another embodiment of a trench according to the present invention. To
 simplify the following discussion, those features of trench 130 that serve
 the same functions as features of trench 100 shown in FIGS. 3 and 4 have
 been given the same numeric designations. Trench 150 includes two
 diaphragms, shown at 141 and 142 that can be deformed to alter the
 pressure in trench 150. In the embodiment shown in FIGS. 9 and 10, the
 diaphragms are operated in a "push-pull" manner such that the pressure on
 one side of region 113 is increased while the region on the other side is
 decreased. This pressure differential is sufficient to shift the location
 of bubble 147 sufficiently into region 114 to cause the bubble to leave
 region 113.
 The required displacement is much smaller than in systems requiring
 displacement to completely move the bubble out of region 113. Diaphragm
 designs of the type utilized in ink jet printers may be utilized for this
 purpose. While the embodiment shown in FIGS. 9 and 10 utilizes diaphragms
 that are placed over the top of the trench, it will be obvious to those
 skilled in the art from the preceding discussion that any device that
 alters the pressure or volume on at least one side of region 113 may be
 utilized, for example piezoelectric transducers or micromechanical
 devices. For example, a heating element 153 may be placed in region 115
 with sufficient power to generate a bubble that alters the pressure on the
 side of region 113 having the heater. After the bubble in region 113 has
 been dislodged, the pressure inducing bubble is allowed to collapse.
 The length of region 113 is preferably chosen to be sufficiently large to
 accommodate the entire light signal passing through region 113 when the
 switching element is transparent. Due to the nature of optical propagation
 in planar lightwave circuits, this region must be slightly larger than the
 waveguide cores that terminate on each side of region 113.
 In the preferred embodiment of the present invention, the trench walls on
 each side of region 113 are parallel to one another, that is, the
 waveguide segments 11 and 12 are collinear and segments 19 and 13 are
 collinear". In principle, only the wall that reflects the light signal
 when the switch is in the reflective state needs to be planar. However, if
 a liquid must be used whose refractive index does not exactly match that
 of the waveguide, the optical signal refracts as it enters the liquid when
 the trench is in the transparent state and again as it exits. This
 refraction leads to a net lateral translation. The exit waveguide of the
 switching element can be moved to accommodate this translation. If the
 trench walls are parallel at the entry and exit points, the original
 waveguide pitch and angle can be maintained despite these translations.
 However, it the entry and exit walls are not parallel, the exit waveguide
 angle differs from the entrance waveguide angle and the pitch is
 distorted. These changes complicate the optical interconnections required
 when constructing a cross-point switch having many switching elements,
 leading to increased cost.
 The above-described embodiments of the present invention have three
 waveguides; however, embodiments having only two waveguides can also be
 constructed. Referring to FIG. 1, either waveguide 12 or waveguide 13 can
 be replaced by a light absorbing medium. In such an embodiment, the
 optical switch has a first state that transmits the light signal from the
 input waveguide to the remaining output waveguide, and a second state in
 which the light signal is absorbed. The embodiment in which output
 waveguide 13 is eliminated is particularly useful in constructing N:1
 optical multiplexers.
 The above-described embodiments of the present invention have utilized a
 flared region to each side of the gap in the trench. However, the flared
 region can be eliminated provided the region on the side of the gap to
 which the bubble is to be displaced is larger than the gap. Refer now to
 FIG. 11, which is a top view of a trench 300 that utilizes such an
 alternative configuration. Trench 300 connects waveguides 321 to 322 when
 bubble 317 is present in the trench and connects waveguides 321 and 323
 when the trench is filled with an index matching liquid.
 Various modifications to the present invention will become apparent to
 those skilled in the art from the foregoing description and accompanying
 drawings. Accordingly, the present invention is to be limited solely by
 the scope of the following claims.