Piezoelectric optical switch device

[0002] 1. Field of the Invention

[0003] The present invention relates generally to piezoelectric optical switches, and particularly to planar Mach-Zehnder piezoelectric optical switches having low birefringence and a high extinction ratio.

[0004] 2. Technical Background

[0005] As the demand for bandwidth increases, so does the drive toward intelligent, low-cost, and dynamically-reconfigurable fiber optic networks. To bring this to pass, network designers are seeking ways to replace certain network functions that were traditionally performed in the electrical domain with solutions in the optical domain, as economics and system designs permit. Designers have recognized for quite some time that four port optical devices could find widespread application in fiber networks to provide fault tolerance, signal modulation, and signal routing. Integrated optical devices using either thermo-optical or electro-optical techniques are currently available. However, these devices have drawbacks due to high power consumption and low switching speeds.

[0006] Four-port piezoelectric optical devices are of particular interest because of their lower power consumption, reduced switching time and adaptability to mass production techniques, such as photolithography. One approach that has been considered involves an optical phase modulator fabricated by coating a fiber with a thick coaxial piezoelectric lead zirconate titanate film. This circular symmetric in-line fiber phase modulator provides phase modulation in a frequency range from 100 kHz to 25 MHz. Unfortunately, the efficiency of the device was poor as it exhibited high attenuation and low piezoelectricity because of difficulty of depositing a thick PZT film around an optical fiber.

[0007] In another approach that has been considered, a Mach-Zehnder fabricated from optical fibers was used to construct an optical switch. In this design, each optical fiber leg was positioned directly on a piezoelectric strip. This design also has several drawbacks. First, the piezoelectric strip required high voltage for commutation. Second, the positioning of the strip in relation to the fiber created asymmetrical stresses along the fiber axis that perturbed the polarization in the interferometer arms resulting in high birefringence and degraded cross-talk performance. As a consequence, polarized light is required when using this switch.

[0008] In yet another approach, a modulator was fabricated by laminating a piezoelectric strip on a planar waveguide device. The piezoelectric strip was formed by sandwiching a layer of piezoelectric material between a lower electrode and an upper electrode. The piezoelectric strip was then attached to the overclad of the device directly above the waveguide. However, when the piezoelectric strip was actuated, the resulting strain vector generated a strong birefringence effect that severely degraded the extinction ratio at the output of the device.

[0009] Thus, a need exists for a four port piezoelectric optical device having reduced birefringence characteristics, a high extinction ratio, lower power consumption, and reduced switching time. This switch must be cost effective, and its design suitable for mass production techniques.

[0010] The present invention is a four port piezoelectric optical switch that substantially solves the birefringence problem and addresses the other issues discussed above. In doing so, the piezoelectric switch of the present invention provides a high extinction ratio in addition to the lower power consumption and reduced switching time possible with piezoelectric switch technology. The planar design of the present invention is well suited for mass production techniques such as photolithography, and offers a promising low-cost solution for some of the signal routing and fault tolerance functionality needed to implement an intelligent fiber optic network.

[0011] One aspect of the present invention is an optical device for selectively directing a light signal in a direction of propagation, the optical device includes a propagation path for the light signal and an output. The optical device includes a piezoelectric element for directing the light signal into the output by creating a plurality of mutually orthogonal strain components in the optical device, wherein the piezoelectric element is disposed relative to the propagation path such that only a component of the plurality of mutually orthogonal strain components, aligned in the direction of propagation, may substantially exist in the propagation path.

[0012] In another aspect, the present invention is a Mach-Zehnder optical device for directing a light signal having a wavelength λ in a direction of propagation. The optical device includes: a first waveguide having a first propagation path, a refractive index n, a first length L₁, and a first output, wherein the light signal is propagated along the first propagation path; and a first piezoelectric rib for directing the light signal by creating a first plurality of mutually orthogonal strain components in the first waveguide, wherein the first piezoelectric rib is disposed on the first waveguide at a first offset from the first propagation path such that only a component of the first plurality of mutually orthogonal strain components that is aligned in the direction of propagation substantially may exist in the first propagation path.

[0013] In another aspect, the present invention is a method for directing a light signal having a wavelength λ, in a direction of propagation in an optical device including a first waveguide having a first propagation path, a refractive index n, a first length L₁, and a first output, wherein the light signal is propagated along the first propagation path. The method for directing a light signal includes the steps of: providing a first piezoelectric rib for generating a first plurality of mutually orthogonal strain components in the first waveguide, wherein the first piezoelectric rib is disposed on the first waveguide at a first offset from the first propagation path such that only a component of the first plurality of mutually orthogonal strain components that is aligned in the direction of propagation can substantially exist in the first propagation path; providing a second waveguide disposed adjacent to the first waveguide having a second propagation path, the refractive index n, a second length L₂, and a second output; and actuating the first piezoelectric rib to selectively deform the first waveguide, wherein a first waveguide deformation produces the first plurality of mutually orthogonal strain components in the first waveguide.

[0014] In another aspect, the present invention discloses a method of fabricating an optical device used for directing a light signal. The method of fabricating includes the steps of: forming a substrate; disposing a waveguide core layer on the substrate; forming a first waveguide from the waveguide core layer, wherein the first waveguide structure is characterized by a first propagation path, a refractive index n, a first length L₁, and a first axis, wherein the first axis is substantially perpendicular to the first length and the first propagation path; forming a second waveguide structure from the waveguide core layer, wherein the second waveguide structure is characterized by second propagation path, the refractive index n, a second length L₁, and a second axis parallel to the first axis; disposing a first piezoelectric rib on the first waveguide structure, wherein the first piezoelectric rib has a first rib axis which is substantially parallel to the first axis and separated from the first axis by an offset; and disposing a second piezoelectric rib on the second waveguide structure, wherein the second piezoelectric rib has a second rib axis which is substantially parallel to the second axis and separated from the second axis by the offset, wherein the offset is selected to minimize a birefringence in the optical device.

[0015] In another aspect, the present invention discloses a method for selectively directing a light signal into a first output or a second output of an optical device that includes at least one waveguide having at least one core connected to the first output, wherein the light signal propagates along the at least one waveguide in a direction of propagation, the method for selectively directing a light signal comprising the steps of: providing at least one piezoelectric element for switching the light signal from the first output into the second output by inducing a plurality of mutually orthogonal strain components in the at least one waveguide, the at least one piezoelectric element being disposed on the at least one waveguide in a predetermined position such that only a first component of the plurality of mutually orthogonal strain components substantially exists in the at least one core, wherein the first component is a strain component aligned to the direction of propagation; and actuating the at least one piezoelectric element to thereby generate a deformation in the at least one waveguide causing the plurality of mutually orthogonal strain components to be produced in the at least one waveguide.

[0016] Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawings.

[0017] It is to be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate various embodiments of the invention, and together with the description serve to explain the principles and operation of the invention.

[0018] FIG. 1 is a schematic of a piezoelectric optical switch according to a first embodiment of the present invention;

[0019] FIG. 2 is a detail view of the placement of a piezoelectric rib on a waveguide structure in accordance with the present invention;

[0020] FIG. 3 is a schematic of a piezoelectric optical switch according to a second embodiment of the present invention;

[0021] FIG. 4 is a chart showing the relationship between birefringence and the extinction ratio for the present invention;

[0022] FIG. 5 is a schematic of a piezoelectric optical switch according to an alternate embodiment of the present invention;

[0023] FIG. 6 is a detail view of an etched groove used to mechanically isolate the arms of a Mach-Zehnder to reduce cross-talk;

[0024] FIG. 7 is a schematic of a piezoelectric optical switch according to another alternate embodiment of the present invention;

[0025] FIG. 8 is a schematic of a piezoelectric optical switch according to another alternate embodiment of the present invention;

[0026] FIG. 9 is a schematic of a piezoelectric optical device featuring a variable attenuation controller according to yet another alternate embodiment of the present invention;

[0027] FIG. 10 is a schematic of a piezoelectric optical device featuring an optical modulator according to yet another alternate embodiment of the present invention; and

[0028] FIGS. 11A-Q are sequential diagrammatic views of the piezoelectric optical switch of the present invention in successive stages of fabrication.

[0029] Reference will now be made in detail to the present preferred embodiments of the invention, example of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. An exemplary embodiment of the piezoelectric optical switch of the present invention is shown in FIG. 1, and is designated generally throughout by reference numeral 10.

[0030] In accordance with the present invention, an optical device 10 for directing a light signal into a desired output includes a piezoelectric rib 40. Piezoelectric rib 40 directs the light signal by deforming the core of a at least one waveguide to thereby alter the optical path length. The deformation creates a three-dimensional strain vector having components in each dimension x, y, and z, of a cartesian coordinate system. The z-direction corresponds to the direction of propagation. By positioning the piezoelectric rib 40 on a waveguide at a predetermined offset position from the core, the strain components that are orthogonal to the direction of propagation, x and y, can be reduced to a negligible level. Since strains in these directions create bifringence, a reduction in these strains will also effect a reduction of the bifringence, as well. The only remaining strain is in the direction of propagation, and strain in the z-direction does not create bifringence. The elimination or reduction of bifringence is greatly desired because bifringence degrades the extinction ratio at the output of the optical device 10.

[0031] Thus, by solving the birefringence issue, the present invention provides an optical switch that has a high extinction ratio. Another benefit of the present invention is its low power consumption and fast switching time, due to the piezoelectric effect. In addition, the planar design of the present invention is well suited for mass production techniques such as photolithography and thus, offers a promising low-cost solution for some of the signal routing and fault tolerance functionality needed in implementing an intelligent fiber optic network.

[0032] As embodied herein, and depicted in FIG. 1, a schematic of a piezoelectric optical switch 10 according to a first embodiment of the present invention includes a planar directional coupler 100 formed by waveguide 20 and waveguide 30. A piezoelectric rib 40 is disposed on waveguide structure 20 at a predetermined offset distance “d” away from waveguide core 22 (see FIG. 2). Piezoelectric rib 40 has a length Lπ, a distance sufficiently long enough to produce a π radian phase difference between waveguide 20 and waveguide 30.

[0033] As embodied herein, and depicted in FIG. 2, a detail view of the placement of piezoelectric rib 40 on waveguide structure 20 in accordance with the present invention is disclosed. A rectangular coordinate system is provided in FIG. 2 as a convenient means of describing element orientation and will be used throughout. Piezoelectric rib 40 includes an upper electrode 42 and a lower electrode 44. Electrodes 42 and 44 are connected to actuator 50. Piezoelectric rib 40 is disposed on overclad 24 of the waveguide structure 20 an offset distance “d” from the waveguide axis which bisects waveguide core 22. Waveguide structure 20 includes overclad 24 and core 22. Note that the direction of propagation is in the z-direction.

[0034] Waveguide structure 20 and waveguide structure 30 may be of any suitable well-known type, but there is shown by way of example a waveguide fabricated using silica glass with a refractive index of approximately 1.45. One of ordinary skill in the art will appreciate that polymers and other like materials may be used. The geometric shape of core 22 may be either square, rectangular, trapezoidal, or semi-circular. The dimensions of the core are dependent on the wavelength of the signal light and are designed to ensure that the waveguide is single mode at the signal wavelength. Core 22 is covered by overclad 24 having a thickness that is designed to confine the mode and limit propagation losses.

[0035] Piezoelectric rib 40 may be of any suitable well-known type, but there is shown by way of example, a layer of lead zirconate titanate(PZT) or zinc oxide(ZnO), having a thickness in an approximate range of between 3 μm to 300 μm, a width in an approximate range of between 20 μm to 300 μm, and a length in an approximate range of between 2 mm to 3 cm. The variation in the dimensions of the piezoelectric rib are dependent upon several factors, including the amount of phase shift piezoelectric rib 40 is required to produce. Piezoelectric rib 40 is produced by spin coating deposition of a PZT or ZnO sol-gel solution and annealing. A more detailed discussion of the dimensions and placement of piezoelectric rib 40 will be presented subsequently.

[0036] Actuator 50 may be of any suitable well-known type, but there is shown by way of example, a voltage source capable of supplying two discrete voltages to piezoelectric rib 40. The first discrete voltage is on the order of a few volts. The exact voltage depends upon the required phase shift. The second voltage level is approximately ground. As one of ordinary skill in the art will appreciate, Mach-Zehnders with perfect 3 dB couplers do not exist in practice. Thus, the actual voltage that actuator 50 supplies to piezoelectric rib 40 may include a bias voltage to compensate for the small phase variations generated by imperfections in the Mach-Zehnder. This “tuning” can done permanently by UV exposure of the waveguide to perfect the desired phase difference.

[0037] The network interface 60 allows optical device 10 to be adaptable to any network environment in terms of line levels and logic protocol. Network interface 60 may also be configured to send fault information back to the network.

[0038] The operation of optical device 10 according to the first embodiment of the present invention as depicted in FIGS. 1 and 2, is as follows. In a first actuation state, the network interface receives a command to direct the light signal to the output of waveguide 30. Network interface 60 drives actuator 50 and piezoelectric rib 40 is de-energized. A light signal is directed into directional coupler 100 as shown, and signal power is transferred into waveguide 30. In a second state, the network interface receives a command directing optical device 10 to direct all of the light signal into the output of waveguide 20. In response, network interface 60 drives actuator 50 to supply piezoelectric rib 40 with the appropriate voltage. Piezoelectric rib 40 expands and deforms waveguide structure 20 to induce a strain in waveguide 20. The induced strain caused by the deformation will cause the refractive index and the length of waveguide 20 to change. Both of these factors contribute to a change in the optical path length in waveguide 20. A π radian phase difference between waveguide 20 and waveguide 30 is established and light no longer couples into waveguide 30. As a result, optical device 10 is switched and the light signal exits device 10 from waveguide 20. As discussed above one of ordinary skill in the art will appreciate that the voltage amount depends on the amount of strain required to produce an index variation that will generate the desired phase difference.

[0039] The operating principles of the present invention that establish the relationship between the dimensions, power requirements, and positioning of piezoelectric rib 40 with respect to deformation, strain and the resulting phase shift induced in waveguides 20 and 30 are as follows. If E_inis the field of the input light signal, ΔΦ the phase difference between waveguides 20 and 30, and E_outthe field of the output signal, one has the following relationship: $\begin{array}{cc}{E}_{\mathrm{out}}=j\frac{E_{\mathrm{in}}}{2}\left(1+e^{\mathrm{j\Delta \Phi }}\right)& \left(1\right)\end{array}$

[0040] The transmission of Mach-Zehnder 100 is defined as the ratio of the output intensity over the input intensity. Thus, from equation (1) we have: $\begin{array}{cc}T=\frac{I_{\mathrm{out}}}{I_{\mathrm{in}}}=\frac{1+\cos \left(\mathrm{\Delta \Phi }\right)}{2}& \left(2\right)\end{array}$

[0041] The phase difference ΔΦ between waveguide 20 and 30 is expressed as: $\begin{array}{cc}\mathrm{\Delta \Phi }=\frac{2\pi d\left(\mathrm{nL}\right)}{\lambda }=\frac{2\pi L}{\lambda }\left(n\frac{\mathrm{dL}}{L}+\mathrm{dn}\right)& \left(3\right)\end{array}$

[0042] wherein λ is the wavelength, n is the effective index of the mode propagating in the device 10, and L is the length of waveguides 20 and 30 between coupler 112 and coupler 114. The term d(nL) is the difference of nL between waveguide 20 and waveguide 30.

[0043] As discussed above, when actuator 50 applies a voltage to piezoelectric rib 40, it expands or contracts, depending on the magnitude and polarity of the voltage. The expansion and contraction of piezoelectric rib 40 deforms waveguide 20 and causes a change in its refractive index and length. The index variation is related to strain by the following expression: $\begin{array}{cc}{\mathrm{dn}}_x=-\frac{n^3}{2}\left(p_{11}{ϵ}_x+p_{12}{ϵ}_y+p_{12}{ϵ}_z\right)& \left(4\right)\end{array}$ $\begin{array}{cc}{\mathrm{dn}}_y=-\frac{n^3}{2}\left(p_{12}{ϵ}_x+p_{11}{ϵ}_y+p_{12}{ϵ}_z\right)& \left(5\right)\end{array}$

[0044] wherein n_xis the refractive index for light polarized in the x-direction (see FIG. 3), n_yis the refractive index for light polarized in the y-direction, ε_x, ε_y, and ε_z=dL/L are mutually orthogonal strain components in the x, y, and z directions, respectively. The terms p₁₁and p₁₂are photoelastic coefficients and vary depending on the material used to fabricate the waveguide. The phase difference ΔΦ between waveguide 20 and 30 is generally different for the polarization components of the light signal in the x-direction and in the y-direction, thus: $\begin{array}{cc}{\mathrm{\Delta \Phi }}_x=\frac{2\pi L}{\lambda }\left(-\frac{n^3}{2}·p_{11}{ϵ}_x-\frac{n^3}{2}·p_{12}{ϵ}_y-\left[\frac{n^3}{2}{p}_{12}-n\right]{ϵ}_x\right)=\frac{2\pi L}{\lambda }{K}_x& \left(6\right)\\ {\mathrm{\Delta \Phi }}_y=\frac{2\pi L}{\lambda }\left(-\frac{n^3}{2}·p_{12}{ϵ}_x-\frac{n^3}{2}·p_{11}{ϵ}_y-\left[\frac{n^3}{2}{p}_{12}-n\right]{ϵ}_z\right)=\frac{2\pi L}{\lambda }{K}_y& \left(7\right)\end{array}$

[0045] From equations (6) and (7), the length of piezoelectric rib 40 required to produce a π radian phase difference can be calculated: $\begin{array}{cc}{L}_{\pi }=\frac{\lambda }{K_x+K_y}& \left(8\right)\end{array}$

[0046] Depending on the material used and the wavelength of the light signal, L_π has an approximate range of 2 mm and 3 cm. The acceptable range of widths and thicknesses of piezoelectric rib 40 are determined by comparing the strain in the direction of propagation ε_z, with PZT ribs having various widths and thicknesses, and waveguide structures having different overclad thicknesses. Thus, for acceptable results, the thickness of the piezoelectric rib 40 has an approximate range of 3 μm-300 μm and its width has an approximate range of 20 μm-300 μm. The depth of overclad depends on the signal wavelength and must be enough to confine the mode and limit propagation losses.

[0047] From equations (6) and (7) the polarization dependency of the phase difference due to the birefringence, is also evident. The main effect of the birefringence is to lower the extinction ratio. The extiction ratio refers to the ratio of light in the output of waveguide 20, for example, in the “ON” state versus the “OFF” state. Theoretically, there should be no light exiting waveguide 20 in the “OFF” state. Thus the extinction ratio is a measure of light leakage. Obviously, the output of waveguide 30 could also be used to take the measurement. If the extinction ratio is low, it means that there is an excessive amount of light leaking out of device 10 from the output of a waveguide that is supposed to be turned off. By lowering the birefringency of the device, one also improves the extinction ratio.

[0048] FIG. 3 is a chart showing the relationship between birefringence and the extinction ratio. One measure of birefringence is the Q-value. The top curve shows a non-birefringent Mach-Zehnder which corresponds to an infinite Q-value. The bottom curve shows a device having an extinction ratio of 10 dB which corresponds to a Q-value of about five. The Q-value must be higher than 16 to obtain a minimum extinction ratio of 20 dB. The Q-value is related to the difference in refractive indices for both polarizations by the expression: $\begin{array}{cc}Q=\frac{{\mathrm{dn}}_x+{\mathrm{dn}}_y}{{\mathrm{dn}}_x-{\mathrm{dn}}_y}& \left(9\right)\end{array}$

[0049] The parameters dn_xand dn_yare index variations produced by stresses induced by the piezoelectric rib 40. Equations (6) and (7) indicate that the change in the length of the waveguide dL/L and the index variation induced by the deformation compensates for strain in the direction of propagation, ε_z. Thus, birefringence can be significantly minimized by eliminating strain components ε_xand ε_yin the x and y-directions, respectively. This is accomplished by the present invention by disposing piezoelectric rib 40 at a predetermined offset distance from the central axis of core 22 (see FIG. 3) and the path of propagation. At the time of actuation, the geometric position of the piezoelectric rib acts to minimize strain components ε_xand ε_y; however, switching functionality is retained by using ε_zto vary the index and the length of the waveguide.

[0050] The optimum range for the offset distance was determined by mapping the Q-value (inversely proportional to birefringence) as a function of the offset distance using a Mach-Zehnder having a PZT rib having length L_π, a width of approximately 100 μm, and a thickness of 20 μm. Under these conditions the optimal value for the offset distance in this configuration is approximately 100 μm. Generally speaking, the offset distance is approximately equal to λ/4n.

[0051] As embodied herein, and depicted in FIG. 4, a schematic of piezoelectric optical switch 10 according to a second embodiment of the present invention includes a planar Mach-Zehnder 100 formed by waveguide 20 and waveguide 30. A piezoelectric rib 40 is disposed on waveguide structure 20 an offset distance “d” away from the waveguide core 22 (see FIG. 2). Piezoelectric rib 40 has a length equaling L_π. Piezoelectric rib 40 is electrically connected to actuator 50. Actuator 50 is connected to network interface 60, which receives network commands and drives actuator 50 accordingly.

[0052] The operation of optical device 10 according to the second embodiment of the present invention as depicted in FIG. 4, is as follows. In a first actuation state, the network interface receives a command to direct the light signal to the output of waveguide 30. Network interface 60 drives actuator 50 and piezoelectric rib 40 is deenergized. A light signal is directed into Mach-Zehnder 100 as shown. Half of the light signal is coupled into waveguide 30 by 3 dB coupler 112. As one ordinary skill in the art will appreciate, a symmetric Mach-Zehnder with perfect 3 dB couplers operates in a cross state when no phase difference exits between waveguides 20 and 30 and the light signal will exit device 10 from the output of waveguide 30.

[0053] In a second state, the network interface receives a command directing the light signal to exit optical device 10 from the output of waveguide 20. In response, network interface 60 drives actuator 50 to supply piezoelectric rib 40 with the voltage necessary for inducing a π radian phase difference. Piezoelectric rib 40 expands and deforms waveguide structure 20 inducing a strain in waveguide 20. The induced strain caused by the deformation will cause the refractive index and the length of waveguide 20 to change. Both of these factors contribute to a change in the optical path length that the light signal follows when propagating in waveguide 20. One ordinary skill in the art will appreciate that the amount of voltage required depends on the amount of strain required to produce an index variation that will generate the desired phase difference. The phase difference determines the size of the electric field required to drive piezoelectric element 40. The voltage supplied to piezoelectric rib 40 establishes a π radian phase difference between waveguide 20 and waveguide 30. As a result optical device 10 is switched and the light signal exits device 10 from waveguide 20.

[0054] In a third embodiment of the invention, as embodied herein and as shown in FIG. 5, a schematic of piezoelectric Mach-Zehnder optical switch 10 includes a planar Mach-Zehnder 100 formed by waveguide 20 and waveguide 30. A piezoelectric rib 40 is disposed on waveguide structure 20 offset a distance “d” away from the waveguide 20. Another piezoelectric rib 70 is disposed on waveguide structure 30 also offset a distance “d.” Piezoelectric ribs 40 and 70 could also be disposed on the interior sides of waveguides 20 and 30, respectively. Note that FIG. 2 and the discussion of offset distance “d” applies to this embodiment as well as the first embodiment. Piezoelectric rib 40 is electrically connected to actuator 50 and piezoelectric rib 70 is electrically connected to actuator 52. Actuators 50 and 52 are driven in tandem by network interface 60.

[0055] It will be apparent to those of ordinary skill in the pertinent art that modifications and variations can be made to piezoelectric ribs 40 and 70 depending on the amount of phase shift each rib is required to provide. In the second embodiment of the present invention, the switching functionality is distributed between waveguides 20 and 30 by placing a second piezoelectric rib 70 on waveguide 30. As in the first embodiment, a total phase difference of π radians between waveguides 20 and waveguide 30 must be provided to switch the light signal into the output of waveguide 20. However, using piezoelectric rib 70 enables the use of a “push-pull” effect wherein piezoelectric rib 40 provides a positive phase shift and piezoelectric rib 70 provides a negative phase shift. Thus, piezoelectric rib 40 must provide a +π/2 radian phase shift and piezoelectric rib 70 must provide a −π/2 radian phase shift. Since the phase shift each piezoelectric rib is required to provide has been reduced from π radians to π/2 radians, the length of each rib can also be reduced by approximately a factor of two: $\begin{array}{cc}L\frac{\pi }{2}=\frac{\mathrm{\alpha \lambda }}{K_x+K_y}& \left(10\right)\end{array}$

[0056] where α≅0.5. To avoid problems associated with mechanical cross-talk, piezoelectric ribs 40 and 70 should be separated by a minimum distance of 500 μm. The recommended separation range is between 500 μm and 1000 μm. This is a trade off between the size of the device and cross-talk.

[0057] As embodied herein, and depicted in FIG. 6, an alternate embodiment of the present invention may include an etched groove 80 between waveguide 20 and waveguide 30. The etched groove 80 is provided to reduce the mechanical cross-talk by isolating arms 20 and 30 of the Mach-Zehnder 100.

[0058] The operating principles of the present invention that establish the width, thickness, power requirements, and positioning of piezoelectric ribs 40 and 70 with respect to the strain and phase shift induced in waveguides 20 and 30 are essentially identical to those discussed above with respect to the first embodiment depicted in FIGS. 1 and 2.

[0059] The operation of optical device 10 according to the third embodiment of the present invention as depicted in FIG. 5 is as follows. In a first actuation state, the needs of the network require that the light signal directed into the output of waveguide 30. Half of the light signal entering Mach-Zehnder 100 is coupled into waveguide 30 by 3 dB coupler 112. A symmetric Mach-Zehnder with perfect 3 dB couplers 112 and 114 will operate in a cross state when no phase difference exits between waveguides 20 and 30 and the light signal will the output of waveguide 30. Thus, upon processing the network command, network interface 60 drives actuators 50 and 52 and the voltage supplied to piezoelectric ribs 40 and 70 drops to approximately zero volts. As discussed above and as one of ordinary skill in the art will appreciate, Mach-Zehnders with perfect 3 dB couplers do not exist in practice. At each switch state, actuators 50 and 52 supply piezoelectric ribs 40 and 70, respectively, with the nominal voltages plus small bias voltages to compensate for the small phase variations generated by the imperfections of the Mach-Zehnder.

[0060] In a second actuation state, the network interface is commanded to direct the light signal into the output of waveguide 20. Network interface 60 drives actuators 50 and 52 accordingly. Actuator 50 supplies a positive voltage to piezoelectric rib 40 and actuator 52 supplies a negative voltage of approximately the same magnitude to piezoelectric rib 70. Piezoelectric rib 40 will expand when deforming waveguide structure 20. Piezoelectric rib 70 will contract when deforming waveguide structure 30. The deformations strain waveguides 20 and 30 and thereby change their path lengths. The path length variation in waveguide 20 results in approximately a +π/2 radian phase shift whereas the path length variation in waveguide 30 yields approximately a −π/2 radian phase shift. Thus, a π radian phase difference, or an odd multiple of π radions, between waveguide 20 and waveguide 30 is established and the light signal is directed into the output of waveguide 20. Of course the polarities of the voltages could be reversed to yield the same results. However, the voltages must have opposite polarities.

[0061] In another alternative embodiment of the present invention, as embodied herein and as shown in FIG. 7, piezoelectric rib 40 consists of an outer piezoelectric strip 46 disposed on an exterior side of waveguide 20, and inner piezoelectric strip 48 disposed on an interior side of waveguide 20. Piezoelectric rib 70 consists of outer piezoelectric strip 72 disposed on an exterior side of waveguide 30, and inner piezoelectric strip 74 disposed on an interior side of waveguide 30. Inner piezoelectric strip 48 and inner piezoelectric strip 74 are separated by a minimum of 500 μm, the distance being within the recommended separation range between 500 μm and 1000 μm, as discussed above and shown in FIG. 7. This is a trade off between cross-talk and device size. The etched groove shown in FIG. 6 could also be used in this embodiment. Actuator 50 is connected to piezoelectric strips 46 and 48, supplying them with identical voltages. Actuator 52 is connected to piezoelectric strips 72 and 74, supplying them with identical voltages. Network interface 60 is connected to actuator 50 and actuator 52, and it drives then in tandem.

[0062] It will be apparent to those of ordinary skill in the pertinent art that modifications and variations can be made to piezoelectric ribs 40 and 70 of the present invention depending on the amount of phase shift each is required to provide. By placing piezoelectric strips 46, 48, 72 and 74 on both sides of their respective waveguides 20 and 30, the length of the piezoelectric ribs can be reduced by a factor of two with respect to the second embodiment and a factor of four with respect to the first embodiment. Thus, in equation (10), α≅0.25.

[0063] With the exception of the variations discussed above, optical switch 10 in FIG. 7 operates in the same way as the embodiment depicted in FIG. 5 and thus, a description of its operation will not be repeated.

[0064] In yet another alternative embodiment, as embodied herein and as shown in FIG. 8, a schematic of piezoelectric optical switch 10 includes Mach-Zehnder 100 formed by waveguide 20 and waveguide 30. Piezoelectric rib 40 is disposed on waveguide structure 20 with an offset distance from the waveguide core 22. Another piezoelectric rib 70 is disposed on waveguide structure 30 also disposed an offset distance from the core. The discussion of offset distance with respect to FIG. 2 applies to this embodiment, as well. Piezoelectric rib 40 is electrically connected to actuator 50. Piezoelectric rib 70 is electrically connected to actuator 52. Actuators 50 and 52 are connected to, and driven in tandem by, network interface 60.

[0065] Actuator 50 and actuator 52 may be of any suitable well-known type, but there is shown by way of example, a voltage source capable of supplying three discrete voltages to piezoelectric rib 40 and piezoelectric rib 70. This embodiment uses a “push-pull” effect similar to the technique discussed above with respect to an earlier embodiment. Commutation is effected by driving piezoelectric rib 40 and piezoelectric rib 70 with voltages having opposite polarities. Thus, the voltage sources operate in tandem such that actuator 52 supplies −V volts when actuator 50 supplies +V volts. When actuator 50 supplies −V volts, actuator 52 is supplying +V volts. When actuator 50 is at approximately ground, so is actuator 52. As discussed above, the nominal voltage V, is dependent on a variety of factors, such as the desired phase difference and size of the piezoelectric rib. It will be apparent to one of ordinary skill in the pertinent art, that multiple voltage combinations may be used to split the light signal between waveguides 20 and 30 as desired.

[0066] It will be apparent to those of ordinary skill in the pertinent art that modifications and variations can be made to the present invention depending on the amount of phase shift each rib is required to provide. In FIG. 8, waveguide 20 is shorter than waveguide 30 by a distance ΔL=L₂−L₁, which is approximately 250 μm when λ=1.55 μm and n≅1.5. This path length difference between waveguide 20 and waveguide 30 establishes a π/2 radian phase difference between waveguide 20 and waveguide 30. Thus, in order to obtain either π radian phase shift or zero phase shift between waveguides 20 and 30, each of piezoelectric elements 40 and 70 are only required to produce a π/4 radian phase shift. Because the phase shift piezoelectric rib 40 and piezoelectric rib 70 must provide has been reduced from π radians to π/4 radians, the length can also be reduced by approximately a factor of four. Thus, equation (10) can be used to calculate the lengths L_(π/4), of piezoelectric rib 40 and piezoelectric rib 70, where α≅0.25. One of ordinary skill in the art will also recognize that this embodiment can be implemented using one piezoelectric rib or four piezoelectric ribs.

[0067] It will be apparent to those of ordinary skill in the pertinent art that modifications and variations can be made to the present invention depicted in FIG. 8. Instead of designing the path length difference to provide a π/2 radian phase difference, the path length difference can be designed to provide a permanent π radian phase difference. In this case, when the piezoelectric ribs are not actuated, optical device 10 is in the bar state rather than in a cross-state. This design is of interest when it is more probable that the switch will be used in the bar state rather than the cross-state.

[0068] As discussed above with respect to an earlier embodiment, to avoid problems associated with mechanical cross-talk, piezoelectric ribs 40 and 70 should be separated by a minimum distance of 500 μm. The recommended separation range is between 500 μm and 1000 μm. As discussed above, the separation range is a trade-off between cross-talk and device-size. The etched groove shown in FIG. 6 can also be used in this embodiment.

[0069] The operation of optical device 10 according to the invention as depicted in FIG. 8 is as follows. In a first actuation state, the network commands optical device 10 to direct the light signal into the output of waveguide 20. Network interface 60 drives actuators 50 and 52 accordingly. Actuator 50 supplies a positive predetermined voltage to piezoelectric rib 40. Actuator 52 applies a negative voltage of the same magnitude to piezoelectric rib 70. Piezoelectric rib 40 deforms waveguide 20 and approximately a π/4 radians phase shift is generated. Piezoelectric rib 70 deforms waveguide 30 and approximately a −π/4 radians phase shift is generated. As one of ordinary skill in the art will recognize, the actual phase shifts are dependent upon the inherent imperfections in the MZ1. The phase variation may be slightly different on each one. The requirement is that a total phase difference of π radians is established between waveguide 20 and waveguide 30. Upon doing so, optical device 10 is commutated and the light signal exits the device from the output of waveguide 20.

[0070] In a second actuation state shown in FIG. 8, actuator 50 and 52 supply approximately zero volts to their respective piezoelectric ribs, 40 and 70. As discussed above, the asymmetric Mach-Zehnder in FIG. 8 is fabricated having an inherent phase difference of approximately π/2 radians between waveguide 20 and waveguide 30. Thus, when piezoelectric ribs 40 and 70 are not deforming waveguides 20 and 30, respectively, the inherent π/2 radian phase difference causes the light signal to be equally split between the outputs of waveguides 20 and 30. In this state, the optical device 10 is a 3 dB splitter.

[0071] In a third actuation state, the network interface 60 is commanded to direct the light signal into the output of waveguide 30. Network interface 60 drives actuator 50 to supply piezoelectric rib 40 with a negative voltage. In similar manner, actuator 52 supplies piezoelectric rib 70 with a positive voltage of approximately the same magnitude. Piezoelectric rib 40 deforms waveguide 20 and generates approximately a −π/4 radian phase shift. Piezoelectric rib 70 deforms waveguide 30 and approximately a +π/4 radians phase shift is generated. In this actuation state, the phase shifts generated by piezoelectric ribs 40 and 70 cancel the inherent π/2 phase difference between waveguide 20 and waveguide 30 caused by their path length difference. Thus, no phase difference exists between waveguide 20 and waveguide 30 and the light signal is directed into the output of waveguide 30 as commanded.

[0072] In yet another alternative embodiment, as embodied herein and as shown in FIG. 9, a schematic of piezoelectric variable attenuator 10 includes a Mach-Zehnder 100 formed by waveguide 20 and waveguide 30. Piezoelectric rib 40 is disposed on waveguide structure 20 an offset distance from the waveguide core 22. The discussion of the offset distance with respect to FIG. 2 equally applies to this embodiment, as well. Piezoelectric rib 40 is electrically connected to actuator 50.

[0073] Actuator 50 may be of any suitable well known type, but there is shown by way of example, a variable voltage source for dynamically varying the voltage over a continuous range of voltages. One of ordinary skill in the art will appreciate that the power level of the light signal in the output of either waveguide 20 or 30 is dynamically controlled in proportion to voltage level supplied by actuator 50. Thus, variable attenuator 10 is implemented by varying the voltage over a continuous range.

[0074] The operation of variable attenuator 10 according to the invention as depicted in FIG. 9 is as follows. As discussed with respect to the first embodiment, if piezoelectric rib 40 is de-energized, the light signal propagating in a symmetric Mach-Zehnder 100 will be directed into the output of waveguide 30. When a command is received ordering that the light output from waveguide 30 be attenuated to a certain level, network interface 60 interprets the command and translates it into a voltage level within the range provided by actuator 50. Actuator 50 supplies piezoelectric rib 40 with the voltage level as ordered. In response, piezoelectric rib 40 expands and deforms waveguide structure 20, causing the refractive index and the length of waveguide 20 to change. Thus, a portion of the light signal is diverted from the output of waveguide 30 and redirected into the output of waveguide 20. As the voltage is increased more of the signal is diverted from waveguide 30 and is thereby attenuated. When actuator 50 supplies the predetermined voltage to piezoelectric rib 40 a π radian phase difference between waveguide 20 and waveguide 30 is established. In this state, the output from waveguide 30 is completely attenuated. Thus, the voltage supplied by actuator 50 is proportional to the attenuation amount.

[0075] In yet another alternative embodiment, as embodied herein and as shown in FIG. 10, a schematic of piezoelectric tunable filter 10 includes a Mach-Zehnder 100 formed by waveguide 20 and waveguide 30. Note that waveguide 20 is shorter than waveguide 30 by a distance ΔL=L₂−L₁, which is approximately 200 μm. Piezoelectric rib 40 is disposed on waveguide structure 20 an offset distance from the waveguide core 22. The discussion of the offset distance with respect to FIG. 2 equally applies to this embodiment, as well. Piezoelectric rib 40 is electrically connected to actuator 50.

[0076] The operation of tunable filter 10 according to the invention as depicted in FIG. 10 is as follows. The phase variation between the two arms is given by the following equation: $\begin{array}{cc}\mathrm{\Delta \Phi }=\frac{2\pi }{\lambda }n\Delta L& \left(11\right)\end{array}$

[0077] Since the refractive index n is wavelength dependent, the product nΔL is also wavelength dependent. For a large ΔL, a large phase difference may be obtained between different wavelengths. For example, in a first actuation state wherein piezoelectric rib 40 is not actuated, there is no phase difference for light at λ₁=1554.5 nm and a π phase difference exists for light at λ₂=1558.5 nm. Thus, λ₁won't be interferred with, whereas λ₂will experience destructive interference. In a second actuation state, piezoelectric rib 40 induces a π phase difference between waveguide 20 and waveguide 30. Because of the wavelength dependency discussed above, the attenuation at the different wavelengths will change and λ₂won't be interferred with and λ₁will be destroyed by destructive interference.

[0078] FIGS. 11A-Q are sequential diagrammatic views of the piezoelectric optical switch of the present invention in successive stages of fabrication. In FIG. 11A substrate 100 is formed. Substrate 100 may be of any suitable well known type, but there is shown by way of example a substrate formed of silicon glass. FIG. 11B shows buffer layer 112 being deposited on substrate 100. Buffer layer 112 may be of any suitable well known type, but there is shown by way of example a layer formed of silica glass. FIG. 11C shows core layer 114 being deposited on buffer layer 112. Core layer 114 may be of any suitable well known type, but there is shown by way of example a layer formed of silica glass having a refractive index n, higher than that of the buffer layer 112. One of ordinary skill in the pertinent art will appreciate that the fabrication steps described in FIGS. 11A-11C can also be realized using polymers, copolymers, monomers or other suitable materials. FIGS. 11D and 11H show the photolithographic process of forming waveguide structure 20 and waveguide structure 30. Mask 116 is positioned over core layer 114 and the pattern of waveguide structures 20 and 30 are transferred to the core layer 114 by illumination of the mask. The etching process shown in FIG. 11G removes excess core material. In FIG. 11H overclad layer 24 is deposited over waveguide structures 20 and 30. FIGS. 11I-11N show piezoelectric rib 40 being formed on waveguide structure 20. A layer of PZT or ZnO is deposited on bottom electrode 44. The dimensions of the PZT rib will vary within the ranges provided in the discussion above. In FIG. 11P pigtails 18 are connected to waveguides 20 and 30 to provide optical connectivity. Finally, in FIG. 11Q, the piezoelectric rib electrodes are wired to a connector disposed in the packaging optical unit 118.

[0079] It will be apparent to those skilled in the art that various modifications and variations can be made to the present invention without departing from the spirit and scope of the invention. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.