Optical switch fabricated by a thin film process

An electro-optical switch is provided, that includes: a single mode optical waveguide having a thin ferroelectric oxide film for propagating a single mode of light; a coupler adjoining the single mode optical waveguide for coupling a part of the single mode of light from an optical fiber to the single mode optical waveguide; an electrically formed lens in the single mode optical waveguide for collimating the single mode of light from the coupler; and a switching module comprising another electrically formed lens in the single mode optical waveguide, for switching the single mode of light.

FIELD OF THE INVENTION

The invention relates generally to the field of optical switching devices, and in particular to an electro-optical (EO) switch having a thin ferroelectric oxide waveguide and operating on a single mode light signal.

BACKGROUND OF THE INVENTION

The increased use of the Internet has created the demand for higher data transfer rates, and greater bandwidth. One solution to meeting this demand is the use of all optical networks having fiber optic cables carrying data in the form of light signals routed by optical switches. As used herein an optical switch operates directly on a light signal during switching rather than first converting the light signal to an electrical signal, using a conventional electronic network switch for routing the signal, and then converting the switched electrical signal back to a light signal.

In optical networks there are two types of fiber optic cables: single mode and multi-mode. A single mode fiber is a glass fiber with a diameter of 8 to 10 microns or less that has one mode of transmission—i.e., only one light signal is propagated in the fiber. In a single mode fiber, only the lowest order mode propagates at the wavelength of interest, typically 1300 to 1320 nanometers (nm) or in the 1550 nm range. Single mode fiber allows for a high data transmission rate, e.g., above 10 gigabit per second (Gb/s), longer transmission distance, and the signal distortion and attenuation is small compared to a multi-mode fiber.

While single mode fiber optic cable is readily available, all optical single mode switches are still under development. One type of all optical single mode switch is a planar electro-optical (EO) switch having a slab waveguide. Light travelling in the waveguide can be bent, when an electric field is generated by electrodes, with a voltage difference, located on opposite sides of the waveguide. One material researched for use as the slab waveguide is a transparent ferroelectric oxide, for example, lithium niobate, barium titanate, lead zircornium titanate (PZT), lead lanthanum zirconium titanate (PLZT), and strontium barium niobate (SBN). When an electric field is applied across the transparent ferroelectric oxide, the refractive index changes depending on the strength of the electric field, and hence a light signal propagating in the ferroelectric oxide material can be bent.

Although an EO switch with a thin ferroelectric oxide waveguide core with a thickness about the diameter of a single mode fiber optic cable core, e.g., 8-10 microns, should satisfy the need for an optical switch in an all optical network, there are problems in achieving this thickness. There are three processes that have been investigated, the Metal-Organic Chemical Vapor Deposition (MOCVD) process, pulsed laser deposition (PLD), and the sol-gel process. Because the sol-gel process is much cheaper, producing a thin film core by a sol-gel process is preferred. Thus there has been much research into producing a waveguide core with about 8-10 microns in thickness using the sol-gel process on ferroelectric materials. However, there have been difficulties fabricating a thin ferroelectric oxide waveguide slab over about 1 micron in thickness for large size substrates, because when the sol-gel film reaches its critical thickness, the film cracks.

Even if a thin ferroelectric oxide waveguide core with a thickness over about 1 micron could be produced, there are still other problems in using the thin core in a switch. There would be coupling problems between the fiber optic cable and the thinner waveguide. In addition, the collimating lens is typically placed external to the waveguide, and hence causes significant reflection and coupling loss of the light signal.

Therefore, as the demand for high speed optical switches grows, there is a need for an electro-optical switch having a thin film ferroelectric oxide waveguide produced from the relatively inexpensive sol-gel process, that has less of the problems associated with conventional all optical switches, and that can provide switching for high data transmission rates using a single mode light signal.

SUMMARY OF THE INVENTION

The present invention provides for an electro-optical switch having a thin ferroelectric oxide film waveguide. The thickness of the core of the ferroelectric oxide film waveguide is chosen to allow propagation of a single mode light signal and ease in handling.

An embodiment of the present invention includes a single mode optical switch. The optical switch includes: a single mode planar optical waveguide having a thin ferroelectric oxide film for propagating a single mode of light; a coupler, e.g., grating or prism, adjoining the single mode planar optical waveguide for coupling a part of the single mode of light from an optical fiber to the single mode optical waveguide; an electrically formed lens in the single mode planar optical waveguide for collimating the single mode of light from the coupler; and a switching module comprising an electrically formed prism in the single mode planar optical waveguide, for switching the single mode of light.

An aspect of the present invention includes a thin film optical switch. The optical switch includes: a waveguide; a first electrode affixed to the waveguide; a second electrode affixed to the waveguide and positioned opposite the first electrode, where the first and second electrodes are used to form an electro-optic lens in the thin film core; and a substrate affixed to the second electrode. The waveguide includes: a thin film core having a first lead lanthanum zirconium titanate (PLZT) material for propagating a single mode of light, where the core has a thickness from 1 to 9 microns, inclusive; and a clad affixed to the core, where the clad includes a second lead lanthanum zirconium titanate (PLZT) material;

Another embodiment of the present invention includes a method for fabricating a single mode optical switch using a sol-gel process. A bottom clad layer is formed by repetitively spin coating and rapid thermal annealing a first lead lanthanum zirconium titanate (PLZT) material on a surface of a bottom electrode. Next, by repetitively spin coating and rapid thermal annealing a second PLZT material on the bottom clad layer, a core layer about 1 to 10 microns, inclusive, in thickness is formed. Lastly, a top clad layer is created on the core layer;

Another aspect of the present invention includes an optical switch. The optical switch includes: a single mode optical waveguide having a thin ferroelectric oxide film for propagating a single mode of light; a means for transferring a part of the single mode of light from an optical fiber to the single mode optical waveguide; a means for collimating the single mode of light in the single mode optical waveguide using a pair of electrodes; and a means for connecting a top electrode of the pair of electrodes to an external voltage source.

These and other embodiments, features, aspects and advantages of the invention will become better understood with regard to the following description, appended claims and accompanying drawings.

DETAILED DESCRIPTION OF THE INVENTION

In the following description, numerous specific details are set forth to provide a more thorough description of the specific embodiments of the invention. It is apparent, however, to one skilled in the art, that the invention may be practiced without all the specific details given below. In other instances, well known features have not been described in detail so as not to obscure the invention.

FIG. 1is a simplified side-view of part of a thin film electro-optical switch of an embodiment of the present invention. An optical waveguide having a top cladding112, a thin ferroelectric oxide core114, and a bottom cladding116is formed above a lanthanum aluminate (LaAlO3), magnesium oxide (MgO) or Strontium Titanate (SrTiO3) substrate118. A grating coupler120is located on one end of the thin ferroelectric oxide core114. The grating coupler120receives the signal mode light signal122from an optical fiber (not shown) and sends the signal mode light signal122to the thin ferroelectric oxide core114. The top electrodes125,127, and129above the top clad layer112work in conjunction with opposing blank bottom electrode130, below bottom clad layer116, to bend the light signal in the thin ferroelectric oxide core114, when needed. In another embodiment silicon (Si) is used as substrate118.

The core114of the single mode waveguide ofFIG. 1has a thickness that is dependant on the refraction indexes of the core114, i.e., n1, and clad (112,116), i.e., n2. The theoretical maximum thickness of a core for a single mode transmission only, i.e., “d,” is given by the equation:dλ<12⁢n12-n22[Equation⁢⁢1]

where λ is the wavelength of the light source. For a thickness equal to or greater than d, the waveguide can carry one or more modes. Thus, while for a transmission of a single mode of light, the thickness of the core can be greater than “d”, the core's thickness should be at or near to “d” in order to minimize the propagation of more than one mode. For example, if a 1310 nm light source is used with a barium titanate as core (refractive index of 2.4) and silica as clad (refractive index of 1.5), then the maximum thickness “d” of the barium titanate for a single mode wave propagation only is about 350 nm. This is a thickness that can be easily fabricated by the sol-gel process. However, this very thin core is too thin to use. A more practical thickness is a core about 1 micron or thicker. The above formula indicates as the core gets thicker, the difference between the core and clad refractive indexes gets smaller. Hence for a 1 micron thick core, the difference between the core and clad refractive indexes is about 0.1.

Table I shows the thickness of the ferroelectric oxide film (columns 3 and 4) for different refractive indexes of the clad (n2) and core (n1=2.4 and n1=2.5). The material given for the core, i.e., barium titanate and PLZT, are merely illustrative of materials that have indexes, n1=2.4 and n1=2.5, respectively, and other materials with these indexes may also be used. The first column gives an example of a material that could be used as a clad with the refractive index of the clad (n2) in the second column. For example, lithium niobate (LiNbO3) is an example of a clad that could be used for n2=2.3. Again the examples in the first column are merely illustrative and other materials with the same refractive indexes for n2 may also be used.

Table I shows that as the difference of refractive index between clad and core gets smaller the ferroelectric film gets thicker. When the difference of refractive index between clad and core (i.e., n1−n2) is 0.1, the thickness of the core for a single-mode symmetrical slab waveguide is about 1 μm, i.e., 1 micron, for both a Barium Titanate core and a PLZT core. If the difference of refractive indexes between core and clad (i.e., n1−n2) is only 0.01, the thickness of the core is 3 μm. For a 9 μm core, the difference of refractive indexes (i.e., n1−n2) is about 0.001. While a core of 9 microns is desirable, there are fabrication problems with a 0.001 refractive index difference. To get this small difference, impurities need to be doped into the core and clad. However, for a slab waveguide, it is very difficult to diffuse impurities into a material in a sandwich configuration, where the top clad and bottom clad need to have the same refractive index, unless the impurity concentration is controlled carefully before the waveguide material is deposited. Thus in one embodiment of the present invention a slab thickness from about 1 micron to less than about 9 microns, inclusive, is preferred.

In a preferred embodiment the core114is PLZT (lead lanthanum zirconium titanate) (8/65/35) and the top112and bottom116clads are PLZT (12/65/35). In another embodiment the core114is PLZT (12/40/60) and the top112and bottom116clads are PLZT (15/40/60). In yet another embodiment the core114is PLZT (8/40/60) and the top112and bottom116clads are PLZT (12/40/60). The formula of PLZT (x/y/1−y) is:
Pb1-xLax(ZryTi1−y)1-x/4O3[Equation 2]

The refractive indexes of PLZT (8/65/35) and (12/65/35) for two light wavelengths is given in Table 2 below. For a 1550 nm light source the difference of refractive indexes between the core and clad is (2.05-2.04)=0.01, which according to Equation 1 above should give a core thickness of about 3.8 microns. For a 635 nm light source the difference of refractive indexes between the core and clad is (2.1548-2.1478)=0.007, which according to Equation 1 above should give a core thickness of about 1.8 microns.

While in the preferred embodiment PLZT was used for the core and clad of the waveguide, other embodiments may include lithium niobate, barium titanate, strontium barium niobate, or lead zirconium titanate (PZT) for the core or clad or both.

Because the light signal comes from a fiber optic cable about 8 to 10 microns in diameter and for example, about 1310 nm or 1550 nm, in wavelength, the grating coupler120provides coupling from the fiber optic cable to the thin film ferroelectric oxide waveguide, when the waveguide is thinner than the fiber optic cable, for example, when the waveguide is about 1 to 7 microns thick. Although a grating coupler has an efficiency of about 80%, any significant coupling loss can be compensated for by using a laser pump at the other end of the switch to regenerate the switched light signal.

InFIG. 1the single mode light signal122comes from a single mode fiber optic cable (not shown) and enters the single mode thin film ferroelectric oxide waveguide114via grating coupler120. A voltage is applied across electrodes125and130to form a collimating lens from the electric field, which collimates the light signal in the waveguide coming from the grating coupler120. In another embodiment one, two, or three of the electrode pairs: (125,130), (127,130), or (129,130), or any combination thereof, may be used to collimate the light. The collimated light signal is then switched in a switch sub-module (not shown) using other electro-optical prisms (not shown) in the thin film ferroelectric oxide waveguide. In an alternative embodiment, the electrode pair (125,130) is used to form the collimating lens and the other two electrode pairs (127,130) and (129,130) are used in switching the collimated light signal in the switch sub-module.

FIG. 2is a simplified top-view of a thin film electro-optical switch210of an embodiment of the present invention. A first light signal from a first single mode fiber optic cable (not shown) enters the thin film ferroelectric oxide waveguide212via grating coupler214. Next, the first light signal is collimated by electro-optical collimating lens220, and the collimated light signal then propagates to switching sub-module224. The first light signal leaves switching sub-module224and then goes to either electro-optical diverging (concave) lens230or232. If the light signal goes to diverging lens230, the light signal exits the thin film electro-optical switch210and enters a second fiber optic cable (not shown). If the light signal goes to diverging lens232, the light signal exits the thin film electro-optical switch210and enters a third fiber optic cable (not shown). The grating coupler214is the top view of grating coupler120of FIG.1. The collimating lens220may be formed by the electrode pair (125,130) of FIG.1. The switching sub-module has a plurality of electrically formed lenses which can switch the collimated light from collimating lens220to diverging lens230or232.

Similarly, a fourth single mode fiber optic cable (not shown) is connected to the thin film ferroelectric oxide waveguide212via grating coupler216. A second light signal entering the waveguide212via grating coupler216is collimated by electro-optical collimating lens222and the collimated light signal then propagates to switching sub-module224. The second light signal leaves switching sub-module224and then goes to either diverging (concave) lens230or232. From the diverging lens, e.g., lens230or232, the light signal exits the thin film electro-optical switch210and enters the second or third fiber optic cable (not shown).

FIG. 3is a structural side view of a thin ferroelectric oxide film switch of another embodiment of the present invention. The switch inFIG. 3has some parts similar toFIG. 1, and those parts are given the same numbers as labels. Note the fiber optic cable having core352, and clad350, is not part of the switch. The substrate118has deposited on it a blank bottom electrode314. The bottom clad116is deposited on top of the bottom electrode314. The ferroelectric oxide film114is deposited on top of the bottom clad116to form the core114of the waveguide. The top clad112is then formed above the core114, and the grating coupler120is created on top of core114. Top electrodes326-1to326-5are formed above clad112. A flexible substrate332has a plurality of through holes formed in it. In each through hole is a via, e.g.340. A through-hole contact is formed using this via, e.g.,340, and a top metal contact, e.g.,344, and a bottom metal contact, e.g.,342. The flexible substrate332having the vias and metal contacts, is connected to the top electrodes326-1to326-5, by z-connection solder joints, e.g.346. For example, the bottom contact342of via340is soldered using z-connection solder joint346to top electrode326-1. The top metal contact344is connected to the external voltage source to produce an electric field between top electrode326-1and bottom electrode314, which is connected to the same external voltage source. The structure330having top metal contact344, via340, bottom contact342, z-connection solder joint346, and top electrode326-1, is similarly repeated for each of the remaining top electrodes326-2to326-5.

InFIG. 3the single mode light signal360comes from a first cylindrical single mode fiber optic cable (not shown) and is coupled by grating coupler120into the ferroelectric oxide film114. A light signal from the grating coupler120passes through an electrically formed collimator lens produced by, for example, a voltage on the electrode pair (326-1and314). The collimated light signal362is switched by the electric fields in the ferroelectric oxide film114produced by the switching sub-module, having the electrode pairs, e.g., (326-2and314), (326-3and314), and (326-4and314). The collimated light signal362, after being switched, passes through a converging lens, e.g., converging lens230inFIG. 2, formed by electrode pair, e.g., (326-5,314). Alternately, the collimated light signal362, after being switched, passes through a converging lens formed by another electrode pair (not shown), e.g., converging lens232in FIG.2. Next, the light signal364leaves the ferroelectric oxide film114and enters a second cylindrical single mode fiber optic cable having core352and clad350. As the light signal364goes from a thin core film114to a thicker core film352, direct edge coupling can be used with very little light loss.

Some of the advantages of the thin film electro-optical switch ofFIG. 3are: that the complicated and precise process of aligning the first fiber optic cable's thicker core (not shown) to the switch's thin waveguide core114via an intermediary lens is not needed, because of use of the grating coupler120; secondly, a collimating lens embedded inside the waveguide core114reduces significantly the reflection and coupling losses which occur when an external collimating lens is used instead of an internal one; and lastly, the use of a single mode thin ferroelectric oxide material in an electro-optical switch provides a fast response time for giga-bit per second optical networks.

FIGS. 4-1to4-9illustrate the process of making an optical switch similar toFIG. 3of an embodiment of the present invention. Where applicable the labels inFIG. 3are used. A sol-gel process is used, which has three basic stages: forming a particulate suspension, i.e., a “sol”; depositing the liquid on a substrate to form a gel, e.g., spin coating; and firing to form a desired thin film. The process starts with a 3 inch lanthanum aluminate substrate118. In other embodiments other substrates such as magnesium oxide (MgO) or strontium titanate (SrTiO3) may be used as substrate118.FIG. 4-1shows a lanthanum nickel oxide blank bottom electrode layer314that is formed by spin coating a lanthanum nickel oxide sol-gel precursor on top of substrate118and then heating the film in an infrared (IR) furnace, i.e., rapid thermal annealing (RTA).

After the bottom electrode314is created, the optical waveguide having bottom clad116, core114, and top clad112is created. The refractive index of the core is slightly larger than the refractive index of the clad layers, e.g., Table 2 above.FIG. 4-2shows a PLZT (12/65/35) bottom clad layer116formed by using repetitive spin coating and rapid thermal annealing on top of blank bottom electrode layer314. A layer of about 0.35 microns to 0.5 microns is formed by each spin coat and rapid thermal annealing cycle, until a bottom clad116thickness of about 1 to 4 microns is reached.FIG. 4-3shows about a 2 to 7 micron PLZT (8/65/35) core layer114formed by using repetitive spin coating and rapid thermal annealing on top of bottom clad layer116. Next a tungsten (W) or gold (Au) metal layer is deposited on top of core layer114. Photo lithography is used to etch the metal from area412.FIG. 4-4shows the etched metal area412and the remaining metal layer segments410-1and410-2. These metal layer segments410-1and410-2cover the grating coupler (or prism coupler) areas.FIG. 4-5shows about a 1 to 4 micron PLZT (12/65/35) top clad layer112formed by using repetitive spin coating and rapid thermal annealing on top of core114in the etched metal area412. In other embodiments the core is any thickness equal to or greater than 1 micron, for example, about 1 to 9 microns inclusive. In addition the clads may be any value equal to greater than 1 micron and is not limited to the ranges 1 to 4 microns.

FIG. 4-6shows the structure ofFIG. 4-5after the metal areas410-1and410-2have been removed by etching. Next by seed metal deposition, photo lithography, and electroplating metal, the top electrodes, e.g.,326-1,326-2,326-3,326-4, etc., are formed on top of top clad layer112. The top electrodes and the bottom blank electrode are used form with their electric fields, for example, deflectors (such as prisms) or collimators or electro-optic other lenses.

In the preferred embodiment the core/clad pair included PLZT (8/65/35) for the core114and PLZT (12/65/35) for the clad layers112and116. In another embodiment the core/clad pair includes PLZT (12/40/60) for the core and PLZT (15/40/60) for the clad layers. In yet another embodiment the core/clad pair includes PLZT (8/40/60) for the core and PLZT (12/40/60) for the clad layers. In a further embodiment PLZT (7/62/38) is used for the core.

FIG. 4-8shows grating couplers420and422over the core114in areas414-1and414-2, respectively. The grating couplers are formed by coating and imaging a dielectric, such as photoepoxy, over areas414-1and414-2.

FIG. 4-10shows the thin film switch ofFIG. 4-9with prism couplers424and426in place of grating couplers420and422, respectively. The prisms are diced out of a substrate and then fixed by external fixtures over areas414-1and414-2.

In another embodiment, the structure430can be built-up directly on the top electrodes, e.g.,326-1,326-2,326-3, and326-4. The process starts withFIG. 4-7.FIG. 5-1to5-4show the rest of the process of building a thin film switch with a built-up circuit.

FIG. 5-1shows forming the dielectric structure332on top of the top electrodes, e.g.,326-1,326-2,326-3, and326-4. The dielectric may be photoimagible epoxy or polyimide and is formed on the top electrodes by a lithography process. When an ordinary dielectric is used, the through holes, e.g.,438-1,438-2,438-3, and438-4, aligned over the top electrodes, e.g.,326-1,326-2,326-3, and326-4, respectively, are created by a reactive ion etching (RIE) plasma process.FIG. 5-2shows the metal filled through holes, e.g., via340, and top contacts, e.g.,344, that have been added to the dielectric332. Thus an external voltage is connected to the top electrodes, e.g.,326-1, by top contact344and via340.

FIG. 5-3shows grating couplers440and442over the core114in areas414-1and414-2, respectively. The grating couplers are formed by coating and imaging a dielectric, such as photoepoxy, over areas414-1and414-2.FIG. 5-4shows the thin film switch ofFIG. 5-3with prism couplers444and446in place of grating couplers440and442, respectively. The prisms are diced out of a substrate and then fixed by external fixtures over areas414-1and414-2.

The specification and drawings are provided for illustrative purposes. It will be evident that additions, subtractions, deletions, and other modifications and changes may be made thereunto without departing from the broader spirit and scope of the invention as set forth in the claims.