Abstract:
A semiconductor waveguide includes a section containing free charge, either electrons or holes, which can be steered into or removed from the path of the beam under the control of electrical signals. The mobile charges come from a potential well which may be either filled or depleted under electrical control. When the well is filled, the charges speed the beam propagation, introducing a phase change. When the well is emptied the beam propagates with extra delay. The phase shifter allows very high speed modulation of the beam using low voltage and low power electronics. The device can be created using standard silicon processing techniques, and integrated with other optical components such as splitters and combiners to create amplitude modulators, attenuators and other optical devices.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     The present application is based on and claims priority from U.S. Provisional Patent Application No. 60/601,723, which was filed Aug. 16, 2004 and which is incorporated herein by reference in its entirety. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates to the field of high-speed optical communications, and in particular, to phase shifter devices that can vary the phase of optical signals in accordance with electrical signals.  
       BACKGROUND INFORMATION  
       [0003]     Data in optical communication is often encoded by modulation of the intensity of a beam of light. Such amplitude modulation can be achieved by combining a selectively delayed copy of a light beam with itself. When the copy of the light beam is subjected to a phase delay of π radians and combined with the original light beam, destructive interference occurs, yielding minimum output intensity. When the copy of the light beam is subjected to no phase delay, constructive interference occurs, yielding maximum output intensity. An optical phase-shifter device under the control of electrical signals can be used to selectively delay the light beam in accordance with a stream of data, thereby amplitude modulating the output light in accordance with the data.  
         [0004]     Various devices have been developed to modulate the intensity of a light beam. Lithium niobate (LiNbO 3 ) modulators can be fast and have reasonable voltage requirements, however, they are not polarization independent and do not lend themselves to integration of drive electronics and optical components. Integrated doped silica waveguides, also known as silicon optical bench components, offer polarization independence and a high degree of integration, however, their highest switching speeds are only in the 1 MHz range. Semiconductor modulators (InP or GaAs) can have a 40 GHz bandwidth; however, polarization independence and extensive integration of multiple channels and other components are not easily achieved with this technology. Silicon modulators consisting of silicon waveguides embedded in silica allow extensive integration; however, designs to date have had a rather low phase change per unit voltage and length, requiring either a high operating voltage or a large device. Many existing designs also dissipate a high degree of static power, for instance, P-I-N devices which have current flowing continuously through the device in order to maintain a steady concentration of carriers.  
       SUMMARY OF THE INVENTION  
       [0005]     In an exemplary embodiment of a device in accordance with the present invention, a semiconductor waveguide includes, in the path of the light beam, at least one region that can be filled or depleted of free charge carriers under the control of electrical signals to selectively subject the light beam to a delay. When the at least one region is filled with carriers, the carriers speed the beam propagation, thereby subjecting the light beam to a minimal delay. When the at least one region is emptied of free charge carriers, however, the light beam propagates more slowly and is thus phase-shifted with respect to the minimally delayed condition. The at least one region is formed by moderately doping one or more portions of the waveguide. The at least one region is depleted of free charge carriers when a reverse bias is applied to a PN junction proximate to the region. Removing the reverse bias allows the free charge carriers to re-fill the region. The free charge carriers can be selected to be electrons or holes.  
         [0006]     Advantageously, a phase-shifter device in accordance with the present invention can modulate the phase of an optical beam over a wide range, at high speeds, with low optical losses, and at low voltages and power consumption. Further advantageously, a device in accordance with the present invention can be readily designed to modulate orthogonally polarized beams of light at the same or different rates. The device of the present invention lends itself to integration with other components such as waveguides, splitters, combiners and integrated electronics and can be manufactured using practical, reliable and cost-effective manufacturing methods. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]      FIG. 1  is a cross-section of an exemplary embodiment of a phase-shifter device in accordance with the present invention.  
         [0008]      FIG. 2  is a plan view of the phase-shifter device of  FIG. 1 .  
         [0009]      FIGS. 3A and 3B  show simulation results illustrating the electrostatic potential across a central portion of an exemplary device of the present invention, without and with reverse bias applied, respectively; and  FIG. 3C  is a graph illustrating the speed of operation of an exemplary phase-shifter device of the present invention as predicted by a simulation thereof.  
         [0010]      FIGS. 4A through 4D  schematically illustrate an exemplary process flow to create an exemplary phase-shifter device in accordance with the present invention.  
         [0011]      FIG. 5  shows a cross-section of a further exemplary embodiment of a phase-shifter device in accordance with the present invention.  
         [0012]      FIGS. 6A through 6C  show further exemplary embodiments of a phase-shifter device in accordance with the present invention having various depletion well configurations.  
         [0013]      FIG. 7  shows a cross-section of yet a further exemplary embodiment of a phase-shifter device in accordance with the present invention.  
         [0014]      FIG. 8  is a schematic representation of a typical modulator incorporating a phase-shifter device and an optional null phase-shifter. 
     
    
     DETAILED DESCRIPTION  
       [0015]     An exemplary embodiment of a phase shifting device  100  in accordance with the present invention is shown in cross section in  FIG. 1  and in plan view in  FIG. 2 . The exemplary device shown uses a silicon rib waveguide  110  on a buffer layer  120  of silica (SiO 2 ). A rib  105  is formed on the upper surface of the silicon rib waveguide  110 . As shown in  FIGS. 1 and 2 , the silicon rib waveguide  110  has an outer region  112  and a base region  113  that are heavily P-type doped surrounding an inner region  114  that is heavily N-type doped. The heavily P-doped outer region  112  extends down to the heavily P-doped base region  113  whereas the heavily N-doped inner region  114  extends partially down into the silicon waveguide  110 . Contacts are provided to the regions  112  and  114  to allow the application of a voltage there across, as described more fully below. To prevent leakage currents, the heavily doped regions  112  and  114  are preferably separated by a small gap as opposed to making direct contact with each other.  
         [0016]     As shown in  FIG. 1 , the heavily N-doped region  114  surrounds doped regions  115   a ,  115   b , which are separated by a core region  116  generally lying under the rib  105 . In the exemplary embodiment shown, the regions  115   a ,  115   b  are doped P-type to a moderate concentration, described more fully below. Unlike the heavily doped regions  12 ,  114 , the regions  115   a ,  115   b  are not provided with electrical contacts and thus do not make external electrical contact. The heavily N-doped region  114 , the moderately P-doped regions  115   a ,  115   b  and the core region  116  are located above a region  118 . The rib  105 , the core region  116 , and the region  118  are preferably undoped or very lightly doped. As shown in  FIG. 2 , the cross-section of the path of the light beam, also referred to as the “optical mode”  150  is primarily confined to the regions  115   a ,  115   b  and  116  and will likely extend partly up into the rib  105  and down into the region  118 . The optical mode  150  may also partly extend into the heavily doped regions to a limited extent. The shape of the optical mode  150  is determined primarily by the shape of the waveguide, including the width and height of the rib  105 . The speed at which the optical mode travels, however, will vary with the concentration of free carriers in the regions through which it travels.  
         [0017]     The PN junction between the P and N regions  112  and  114  allows carriers to be removed or added to the regions  115   a  and  115   b  located at least partially in the optical mode  150 . The moderately doped regions  115   a ,  115   b  on either side of the core region  116  provide potential wells where free charge carriers collect. With no reverse bias applied across the PN junction, these potential wells are full of carriers, in this embodiment, holes. As a result, a light beam passing through the device experiences a minimum of delay. When a reverse bias is applied, i.e., the N-doped region  114  is at a higher potential than the P-doped regions  112  and  113 , the carriers are depleted out of the regions  115   a ,  115   b . As a result, a light beam passing through the device is subjected to a greater delay and is thus phase shifted relative to the minimum delay condition.  
         [0018]     The optimal dimensions of the various features of the exemplary device of  FIGS. 1 and 2  will depend on a variety of considerations. For example, as discussed below, the widths of the well regions  115   a ,  115   b  will affect the reverse bias voltage required to deplete the regions of carriers. In an exemplary embodiment, the regions  115   a ,  115   b  are approximately 0.08 μm wide. The regions  114  and  115   a ,  115   b  are approximately 0.3 μm deep, with their bottoms approximately 0.7 μm above the silica layer  120 . The width of the rib  105  is approximately 0.5 μm and the length of the device  100  is approximately 1.0 mm. The optimal distance of the well regions  115   a ,  115   b  from the waveguide core is subject to competing considerations. The closer the wells approach the optical mode  150 , the more the tail of the optical mode will overlap with the heavy doping in regions  114 , and the more absorption loss there will be. The further the wells are removed from the core, the less influence they have on the speed of propagation. In an exemplary embodiment, the edge of each well  115   a ,  115   b  is within 0.1 μm of the proximal edge of the rib  105 , either inside or outside of the rib footprint. The vertical heights of the well regions  115   a ,  115   b  and the heavily doped regions  114  are chosen to be on the order of 0.1-1 μm to allow sufficient overlap with the optical mode  150  while permitting the buried p-type layer  113  to retain electrostatic control of the wells.  
         [0019]     In addition to the aforementioned dimensions, other considerations in designing a device in accordance with the present invention are the choice of free carriers and their concentration. Although the above-described embodiment uses holes as the free carriers, either electrons or holes can be used. Where electrons are designed to be the free carriers, the doping polarities of the various regions ( 112 ,  114 ,  115   a, b ) need to be reversed. At higher doping levels, the change in phase caused by depletion of the wells is more pronounced. Increasing the doping concentration, however, will require a higher voltage to achieve carrier depletion and will increase absorption losses in the waveguide. With increasing hole concentrations, the change in absorption is larger than the change in refractive index, while for electrons both parameters increase at about the same rate.  
         [0020]     The length of the waveguide required to achieve a phase shift of π radians is designated Lπ. This yields an overall absorption loss of ΔαLπ, with the absorption loss per length, Δα, depending on the free carrier concentration. Exemplary values for these parameters for a uniform free carrier profile across the waveguide optical mode are shown in Table I for holes and electrons as the free carriers.  
                                                     TABLE I                       Waveguide Doping   Holes       Electrons           ΔN (cm −3 )   Lπ (mm)   ΔαLπ(dB)   Lπ(mm)   ΔαLπ(dB)                                10 17     2.3   0.6   8.8   3.2       10 18     0.36   0.96   0.88   3.2       10 19     0.058   1.5   0.088   3.2                  
 
         [0021]     The values listed in Table I indicate that with holes as the free carriers, both the length required for a π radian phase change is shorter, and the absorption occurring in that length is smaller than with electrons. In addition, longer lengths and lower doping improve the trade-off between phase change and absorption for holes. Exemplary devices in accordance with the present invention can have a wide range of doping concentrations beyond those listed in Table I (e.g., 10 16  to 10 19  ions/cm 3 ).  
         [0022]     Potential wells doped at the above-indicated concentrations can be readily depleted using reasonable reverse-bias voltages, e.g., 40 volts or less. The voltage necessary to deplete a region of doping concentration N and width W is on the order of qNW 2 /2c, where c is the silicon dielectric constant and q is the electron charge. For example, for a doping concentration of 10 18  cm −3  and a width of 0.08 μm, the necessary voltage is approximately 5V.  
         [0023]      FIGS. 3A and 3B  show potential diagrams of a central portion of an exemplary device in accordance with the present invention, including the highly N-doped regions ( 114 ) and the potential wells ( 1115   a ,  1115   b ). The vertical axis is electrostatic potential and the two lateral axes are position left-to-right and front-to-back, with dimensions in microns. The bottom of the device is towards the front and the top of the device (rib area) is partially hidden from view. The diagram of  FIG. 3A  shows the potential when no bias is applied. The two N+ regions ( 114 ) correspond to the high potential areas to the left and right whereas the highly P-doped region ( 113 ) corresponds to the lowest potential area. The well regions ( 115   a ,  115   b ) correspond to the potential dips on either side of the center, inside and below the N+ regions ( 114 ). When reverse bias is applied, as illustrated in  FIG. 3B , the center area is raised relative to the back, the dips become flattened and the holes are forced out.  
         [0024]     A phase-shifter device in accordance with the present invention can be very fast. Carriers can reach the potential well in a time of approximately L/vsat, where vsat is the saturation velocity and L is the spacing between the regions  115   a ,  115   b  and the nearby heavily doped layer  113 . The time to fill or deplete the potential wells is therefore on the order of a few picoseconds.  
         [0025]     The energy per switching event is approximately VΔQ, where ΔQ is the total charge removed. The power consumed by an exemplary device with the aforementioned dimensions operating at 1 GHz and with a reverse-bias voltage of 5 V is therefore on the order of 25 mW. A feature of the device of the present invention is that the junctions are either in reverse or no bias. This differs from commonly found P-I-N devices which are operated in forward bias in the on-state, thereby causing significant current flow. With the device of the present invention, no current flows either in the on-state (reverse bias) or the off-state (no voltage), but only during the transition between the two states.  
         [0026]     The results of an electrical simulation of an exemplary phase shifter device are illustrated graphically in  FIG. 3C . In the simulation, a pulse of reverse bias is applied to the device, and then removed. The upper trace shows current flowing into the device at the rising edge of the pulse, whereas the lower trace shows current flowing out of the device at the falling edge of the pulse. The simulations support the aforementioned predicted results. Both on and off current transients are over in considerably less than 1 ns, and the energy per transition is approximately 10 pJ.  
         [0027]     The phase shift and absorption in Table I were determined for a uniform concentration of free carriers in the optical mode. In the device of  FIG. 1 , the light beam will not encounter a uniform carrier concentration. Rather, the carriers and changes in carrier concentration will tend to be localized in the wells and the induced change in refractive index and absorption loss are to some extent polarization dependent. There is a controlled, partial overlap of the potential wells ( 115   a ,  115   b ) with the waveguide core ( 116 ). The phase shift and absorption will be correspondingly higher in proportion to the amount of overlap of the free carrier well with the optical mode.  
         [0028]     The propagation of vertically and horizontally polarized light through the device of the present invention can be tailored to be the same or to be different by tailoring features of the waveguide and the doping. In the exemplary bilateral well structure of the embodiment of  FIG. 1 , horizontally polarized light will experience a larger phase shift than vertically polarized light. Thus the degree of well filling can be adjusted to cause a 180° phase shift for horizontally polarized light (and near complete cancellation when mixed with an undelayed split beam), while vertically polarized light will propagate with a smaller phase shift (and thus will not fully cancel when mixed with the a split beam). If a more symmetric well structure is used, e.g., with wells above and below as well as to the left and the right of the core (see, e.g.,  FIG. 6C ), both polarizations can be shifted by the same degree. By adjusting the degree of the overlap in the horizontal and vertical directions, the phase difference between horizontally and vertically polarized light can be adjusted at will.  
         [0029]     In the exemplary embodiment of  FIG. 1 , increasing the height of the rib  105  will generally tend to slow down the transverse magnetic (TM) mode more than the transverse electric (TE) mode for the exemplary waveguide dimensions discussed. In general, the TE and TM polarizations are impacted differently by changing the waveguide dimensions or shape: a wide waveguide that is not tall will slow down the TE polarization (higher TE effective optical index) and speed up the TM (lower TM optical index) whereas a narrow structure that is tall will have an opposite effect. An unstrained square waveguide will have the same effective index for the TM and TE polarizations. The rib structure follows this general rule, but since the structure is more complicated, the impact is a little less obvious. The overlap of the doped regions  115   a ,  115   b  with the optical mode  150  affects the change in the effective refractive index for each polarization as a function of the change in carrier concentration in the optical mode. By modifying the rib waveguide and doping profiles appropriately, the polarization dependence of the device response can be minimized or maximized.  
         [0030]     An exemplary method for forming a device  500  in accordance with the present invention will now be described with reference to  FIGS. 4A through 4D . In  FIG. 4A , a rib  505  is formed on the upper surface of a layer of silicon  510  arranged on an insulator substrate  520 , such as silica. The rib  505  is patterned using a hard mask  525  made of a dielectric material commonly used in silicon processing. The dielectric hard mask  525  is left in place after the rib  505  has been formed, and acts as a template for the subsequent formation of sidewalls, described below. The silicon layer  510  is undoped or very lightly doped except along its bottom, where a conductive layer  530  formed of a high concentration of P-type dopants is located. The dopants may be introduced by ion implantation or other methods familiar to those skilled in the art.  
         [0031]     A second dielectric is then deposited onto the device and etched. The material of the second dielectric is chosen so that it can be selectively etched without removing the first dielectric used in the hard mask  525 . For example, the first dielectric material might be silicon nitride (Si 3 N 4 ) and the second dielectric might be silicon dioxide (SiO 2 ). The etch is stopped when the second dielectric has been removed from the horizontal surfaces of the device but remains on the vertical sides of the rib  505 . As illustrated in  FIG. 4B , sidewalls  535  made of the second dielectric are thereby formed on either side of the rib  505 . The sidewalls  535  may or may not also extend at least partially up the vertical faces of the hard mask  525 .  
         [0032]     After the sidewalls  535  are formed, P-type dopant ions are implanted, to a moderate dose, to create P-type regions  540 . Preferably, the dopant ion concentration in the resultant P-type regions  540  is on the order of 10 17  to 10 18  ions/cm 3 . The dielectric hard mask  525  and the sidewalls  535  exclude the P-type dopant ions from the core of the waveguide, with the thickness of the sidewalls  535  dictating the lateral positions of the P-type regions  540  relative to the rib  505 .  
         [0033]     Portions of the regions  540  adjacent to the waveguide core correspond to the aforementioned uncontacted P-doped regions  115   a ,  115   b  of the exemplary device of  FIG. 2 . Due to lateral scatter of the dopant implantation as well as dopant diffusion, the regions  540  (and thus  115   a ,  115   b ) may spread sidewise, possibly as far as the area under the rib  105 . The width of the sidewalls  535  and the dopant implantation and annealing recipes can be adjusted to give the desired overlap of the doped regions  540  with the optical mode. The widths of the sidewalls  535  can be controlled to within a small fraction of the film thickness using the above-described sidewall formation procedure. This is much better than lithographic tolerances and allows excellent lateral position control. With this technique, the un-contacted doped regions of free carriers (regions  115   a ,  115   b , in  FIG. 2 ) can be created with widths of approximately 0.1 μm and with control better than approximately 0.01 μm, without requiring lithographic features finer than approximately 0.5 μm.  
         [0034]     The above-described sidewall formation procedure is repeated, with the second dielectric being applied to a greater thickness than in the first sidewall formation procedure. The second dielectric is then etched away from the horizontal surfaces of the device, and as shown in  FIG. 4C , a second set of sidewalls  545  is formed, the sidewalls  545  being wider than the first sidewalls  535 . The sidewalls  545  can be formed over the sidewalls  535  or the sidewalls  535  can be removed before the sidewalls  545  are formed.  
         [0035]     An implant mask  550  is then applied and N-type dopant ions are implanted at a high concentration to form N-doped regions  555  (corresponding to region  114  in  FIG. 1 ). The larger thickness of the sidewalls  545  causes the resultant N-type regions  555  to be formed further from the waveguide core, thereby minimizing static losses in the waveguide core.  
         [0036]     As shown in  FIG. 4D , a further implant mask  560  is then applied and P-type dopant ions are implanted at a high concentration to form regions  565 , which extend down to the buried conductive P-type layer  530 . The result is a device having a structure as illustrated in  FIG. 1 .  
         [0037]     The device can then be completed using well known techniques. Dielectric layers can be used to cover the device and via holes created in the dielectric where metal wiring is needed to contact the highly doped N-type and P-type layers. Metal layers are deposited and patterned to create the necessary wiring, using standard processing methods.  
         [0038]     As shown in  FIG. 2 , the exemplary phase-shifter device  100  of the present invention can be implemented over a portion of a length of semiconductor waveguide  101  by selectively doping the waveguide to form the device  100 . A variety of configurations can thus be formed, including, for example, multiple phase-shifter devices arranged in series along a length of semiconductor waveguide, or multiple parallel waveguides with phase-shifters on a single silicon layer.  
         [0039]     Although the present invention has been shown and described with respect to a particular process flow, it should be understood that many other similar process flows may be used to achieve substantially the same structure, and may be used without departing from the scope of the present invention.  
         [0040]     Although exemplary embodiments of the present invention have been shown and described with respect to a silicon waveguide and silicon-compatible dielectrics and metals, it should be understood that any semiconductor material system might be used, without departing from the scope of the present invention.  
         [0041]     Moreover, although the present invention has been shown and described with respect to a silicon-on-insulator (SOI) substrate embodiment, a conventional silicon substrate may also be used, without departing from the scope of the present invention. The substrate should be covered with a dielectric material, and then conventional dielectric patterning and selective epitaxial lateral overgrowth (ELO) processes may be used to create single-crystal silicon waveguides above the dielectric layer. Such an embodiment is illustrated in  FIG. 5 .  
         [0042]     In the embodiment of  FIG. 5 , a silicon substrate  610  is covered with a layer  620  of silica. A hole is formed in the silica and a silicon rib  605  is grown up through the hole and out laterally  625 , through the process of epitaxial lateral overgrowth (ELO). Chemical-mechanical polishing can be used to planarize the top surface of the silicon waveguide as needed. Heavily P-doped regions  612  and heavily N-doped regions  614 , which correspond to the regions  112  and  114  of the embodiment of  FIG. 1 , are formed in the silicon layer  625  through the usual processes of ion implantation and annealing. P-doped regions  615   a ,  615   b  provide the wells of free carriers which are depleted upon reverse bias of the heavily doped P and N regions,  612  and  614 . The optical mode is shown as  650 .  
         [0043]     As with the other embodiments, electrons can be used instead of holes as the free carriers and the doping polarities of the device reversed. Furthermore, although the present invention has been shown and described with respect to a single crystal silicon waveguide, a polysilicon or amorphous silicon waveguide may also be used without departing from the scope of the present invention.  
         [0044]     Although the present invention has been shown and described with respect to a particular doping profile with two wells (e.g.,  115   a ,  115   b ) on either side of a waveguide core, it should be understood that any doping profile which permits a partial or complete overlap of a potential well with the waveguide, where the potential well may be emptied or filled under the control of external electrodes, may be used without departing from the scope of the present invention. For example, a single central well overlapping the waveguide core may also be used. Such an embodiment is illustrated in  FIG. 6A , wherein a single central well  715  substantially overlaps the waveguide core.  FIG. 6B  illustrates yet another exemplary embodiment of a phase-shifting device having a single central well  715  that is T-shaped, with the vertical portion (or tongue) of the “T” overlapping the waveguide core.  
         [0045]      FIG. 6C  illustrates an exemplary embodiment with a four-well profile, with wells  715   a  and  715   b  to the left and right of the waveguide core and wells  715   c  and  715   d  above and below the waveguide core.  
         [0046]     An exemplary device having an N-well profile, where N&gt;2, in which the wells surround the waveguide core, is within the scope of the present invention.  
         [0047]      FIG. 7  illustrates a modification of the embodiment of  FIG. 1  in which a gap is provided in the heavily P-doped bottom region  113 . The gap reduces the absorption of the light beam passing through the waveguide while minimally impacting the transport of free carriers to and from the potential wells  115   a  and  115   b . In the exemplary device of  FIG. 7 , the regions  115   a  and  115   b  are approximately 0.08 μm wide. The thickness of the silicon layer  110  (without the rib  105 ) is approximately 0.2 μm. The depth of the regions  114 ,  115   a  and  115   b  is approximately 0.08 μm.  
         [0048]     A phase-shifter device of the present invention may be combined with other elements to create a modulator.  FIG. 8  shows a block diagram of a typical modulator  10  using a phase-shifter device  12  in one arm of a beam that has been split. A splitter  14  splits an input beam into two arms, one of which has the phase-shifter device  12  in line therewith, which selectively subjects its respective portion of the beam to a phase shift. The other arm does not include a phase-shifting device and nominally introduces no phase shift. The phase-shifted and un-shifted beams of the two arms are combined by a combiner  16  and output from the modulator  10 . If the beam in the phase-shifted arm is shifted by π radians, it will interfere destructively with the beam in the other arm and no or little output will result. If the beam is not shifted, it will interfere constructively, and a nearly complete replication of the input beam will appear at the output, apart from inevitable losses due to coupling at the waveguides and propagation losses in the waveguides and phase-shifter device.  
         [0049]     Optionally, a null phase-shifter  22  can be included in the un-shifted arm of the modulator  10 . The null phase-shifter  22  is preferably a duplicate of the phase shifter  12  but it is in an operating state in which the delay through it is minimized. As such, when the phase-shifter  12  is not activated to introduce a delay, the two arms will experience similar same propagation delays and losses, thereby maximizing constructive re-combination. It is understood that the above-described embodiments are illustrative of only a few of the possible specific embodiments which can represent applications of the invention. Numerous and varied other arrangements can be made by those skilled in the art without departing from the spirit and scope of the invention.