Patent Publication Number: US-2015078702-A1

Title: System and Method for an Optical Phase Shifter

Description:
This application is a continuation of U.S. patent application Ser. No. 13/955,449 filed on Jul. 31, 2013, and entitled “System and Method for an Optical Phase Shifter,” which claims priority to U.S. Provisional Application Serial No. 61/827,400 filed on May 24, 2013, and entitled “Polarization Independent Waveguide Optical Phase Shifter,” both of which are hereby incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a system and method for photonics, and, in particular, to a system and method for an optical phase shifter. 
     BACKGROUND 
     In some photonic devices, shifting the phase of an optical signal is desirable. Optical phase shifting may be used in optical modulators, switches, sensors, multiplexers, demultiplexers, and other devices. When light propagates through a media, it travels an optical path length that depends on the effective index of refraction of the media. The optical phase may be adjusted when light propagates through a media having a desired optical path length to adjust the optical phase. 
     Optical devices may be integrated in a photonic integrated circuit (PIC) containing optical waveguides. Optical waveguides are light conduits that contain a slab, strip, or cylinder of a dielectric material surrounded by another dielectric material having a lower refractive index. The light propagates along, and is confined to, the higher refractive index material through total internal reflection. In a PIC, the core may be silicon, surrounded by a lower refractive index material, such as silicon dioxide, silicon nitride, silicon oxynitride, and/or air. The waveguides may be a single mode or multi-mode waveguide. In an example, a PIC operates at a telecommunications wavelength, such as 1550 nm or 1310 nm. The light may be coupled into, out of, or between optical waveguides. In a PIC, multiple photonic functions are integrated on a substrate, such as silicon-on-insulator (SOI). PICs are used for optical communications, and for other applications, such as biomedical application sand photonic computing. PICs may provide increased functionality, while being compact, and enabling higher performance than discrete optical devices. 
     SUMMARY 
     An embodiment optical phase shifter includes a first waveguide phase shifter and a second waveguide phase shifter. The optical phase shifter also includes a first polarization rotator optically coupled between the first waveguide phase shifter and the second waveguide phase shifter, where the first waveguide phase shifter, second waveguide phase shifter, and first polarization rotator are integrated on a single substrate. 
     An embodiment method includes phase shifting a first optical signal to produce a first phase shifted optical signal by a phase shifter and rotating a first polarization of the first phase shifted optical signal to produce a first rotated optical signal. The method also includes phase shifting the first rotated optical signal to produce a second phase shifted optical signal, where phase shifting the first optical signal, rotating the first polarization of the first phase shifted optical signal, and phase shifting the first rotated optical signals are performed on a photonic integrated circuit (PIC). 
     An embodiment Mach-Zehnder interferometer includes a first optical coupler and a first optical leg coupled to the first optical coupler. The first optical leg includes a first waveguide phase shifter and a second waveguide phase shifter. The first optical leg also includes a first polarization rotator coupled between the first waveguide phase shifter and the second waveguide phase shifter. Additionally, the Mach-Zehnder interferometer includes a second optical leg coupled to the first optical coupler and a second optical coupler optically coupled to the first optical leg and the second optical leg, where the first optical leg and the second optical leg are integrated on a single substrate. 
     The foregoing has outlined rather broadly the features of an embodiment of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of embodiments of the invention will be described hereinafter, which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiments disclosed may be readily utilized as a basis for modifying or designing other structures or processes for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawing, in which: 
         FIG. 1  illustrates an embodiment optical phase shifter; 
         FIG. 2  illustrates another embodiment optical phase shifter; 
         FIG. 3  illustrates a flowchart for an embodiment method of optical phase shifting; 
         FIG. 4  illustrates an embodiment Mach-Zehnder interferometer; 
         FIG. 5  illustrates another embodiment Mach-Zehnder interferometer; 
         FIG. 6  illustrates an additional embodiment Mach-Zehnder interferometer; 
         FIG. 7  illustrates another embodiment Mach-Zehnder interferometer; and 
         FIG. 8  illustrates a flowchart for an embodiment method of switching using a Mach-Zehnder interferometer. 
     
    
    
     Corresponding numerals and symbols in the different figures generally refer to corresponding parts unless otherwise indicated. The figures are drawn to clearly illustrate the relevant aspects of the embodiments and are not necessarily drawn to scale. 
     DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
     It should be understood at the outset that although an illustrative implementation of one or more embodiments are provided below, the disclosed systems and/or methods may be implemented using any number of techniques, whether currently known or in existence. The disclosure should in no way be limited to the illustrative implementations, drawings, and techniques illustrated below, including the exemplary designs and implementations illustrated and described herein, but may be modified within the scope of the appended claims along with their full scope of equivalents. 
     An optical signal may be viewed as a propagating oscillating electric field orthogonal to an oscillating magnetic field at an optical frequency. The polarization of the optical field is indicated by the direction of the electric field vector. Propagating light may be decomposed into transverse electric (TE) polarization and transverse magnetic (TM) polarization. For TE polarized light, the electrical fields are orthogonal to the plane of propagation. For TM polarized light, the magnetic field is orthogonal to the direction of propagation. 
     Many optical components are affected by the polarization of the optical signal. For example, polarization mode dispersion (PMD), polarization dependent loss (PDL), and polarization dependent wavelength characteristics (PDlambda) may occur, especially when a highly birefringent material is used. Silicon waveguides may have a high geometrical birefringence. Silicon is useful for PICs because of its high index of refraction and its compatibility with electronic integrated circuit fabrication methods. In a birefringent material, the refractive index depends on the polarization of an optical signal. The magnitude of a phase shift depends on the confinement factor and effective index, which differs for TE and TM modes in a waveguide. The phase shift is given by: 
     
       
         
           
             
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     where λ is the wavelength, Γ is the confinement factor, n eff  is the effective refractive index for the polarization, Δn is the refractive index change induced into the waveguide, L p  is the length of the device, and ΔL p  is the change in length of the device. 
     Large silicon waveguides may be polarization agnostic. However, such large waveguides have a large bend radius, leading to a low density of components. Also, such specially designed waveguides may be extremely sensitive to wavelength, dimensional parameter variations, and material parameter variations, so production may be problematic. It is desirable to use very fine waveguides for a high density PIC with a large refractive index contrast between the core and the cladding. This facilitates very small device sizes, but has a high birefringence. 
     When both TE and TM polarizations normally exist in an optical waveguide, a polarization diversity approach may be used. Polarization splitters split the optical signal to two separate paths based on polarization, with TE polarized light propagating along one path and TM polarized light propagating along the other path. Processing is applied to both paths in separate circuits to obtain similar effects. The outputs of the separate circuits are then combined. However, this approach leads to the device size more than doubling. Also, such networks may be susceptible to temperature gradients between the separate circuits. 
     In another example, the orientation of the TE and TM polarizations is exchanged at the midpoint of a semiconductor waveguide section. A gap is introduced into the waveguide at the midpoint, which leads to additional insertion losses. A discrete polarization rotating component is inserted into the gap for rotating both polarization orientations by ninety degrees. For example, a thin polyimide half wave-plate may be inserted into the gap. Alternatively, a polarization splitting grating coupler is inserted into the gap. Also, the assembly, with a micron tolerance, is costly. 
     The phase of an optical signal may be adjusted by an optical phase shifter which adjusts the optical phase by propagating the optical signal along a desired optical path length. A phase shifter may be used for a variety of optical components. For example, phase modulators, intensity modulators, photonic switches, multiplexers, arrayed waveguide gratings, and demultiplexers may include an optical phase shifter. 
       FIG. 1  illustrates optical phase shifter  100 . Initially, an optical signal enters optical phase shifter  100  at waveguide phase shifter  102 . The entering optical signal contains both a TE polarized component and a TM polarization component. The optical path length of waveguide phase shifter  102  is approximately equal to half of the average length for the desired phase shift for the TE polarization and the TM polarization. For example, waveguide phase shifter  102  may be from about 100 μm to about 10 mm in length. In one example, waveguide phase shifter  102  is passive. In another example, waveguide phase shifter  102  is active, and the optical path length may be adjusted by applying a voltage, current, stress, and/or heat. The TE polarization mode and the TM polarization mode experience different phase shifts in the passive waveguide phase shifter  102 , because waveguide phase shifter  102  is birefringent. The TE polarization mode and the TM polarization mode experience different phase shifts in the active waveguide phase shifter  102 , because waveguide phase shifter  102  has different confinement factors for the different polarizations. In one example, waveguide phase shifter  102  is made of silicon. In other examples, waveguide phase shifter  102  is made of InP, or other birefringent materials, such as InGaAsP and AlGaAs. 
     The optical output of waveguide phase shifter  102  proceeds to polarization rotator  104 , which rotates the polarization by ninety degrees. Thus, the TE polarization is converted to a TM polarization, and the TM polarization is converted to a TE polarization. Waveguide phase shifter  102  and polarization rotator  104  are integrated on a single substrate. 
     After the polarization has been rotated, the optical signal proceeds to waveguide phase shifter  106 . Waveguide phase shifter  106  is similar to waveguide phase shifter  102 . Thus, the optical path length and phase shift experienced by the TE polarization mode and the TM polarization mode are similar. In one example, the phase shift for waveguide phase shifter  102  is within π/16 of that of waveguide phase shifter  106 . In other examples the phase shift of waveguide phase shifter  102  and waveguide phase shifter  106  are within π/8, π/24, π/32, π/48, or π/64. The original TE polarized light passes through waveguide phase shifter  102  as TE polarized and through waveguide phase shifter  106  as TM polarized light. Conversely, the original TM polarized light passes through waveguide phase shifter  102  as TM polarized and through waveguide phase shifter  106  as TE polarized light. Waveguide phase shifter  102 , polarization rotator  104 , and waveguide phase shifter  106  are integrated on a single substrate. 
     After waveguide phase shifter  106 , there may be another ninety degree polarization rotator (not pictured). This additional phase rotator restores the optical signal to its original polarization state. The TE polarized light is converted to TM polarized light by the first polarization rotator, and back to TE polarized light by the second polarization rotator. Conversely, the TM polarized light is converted to TE polarized light by the first polarization rotator, and back to TM polarized light by the second polarization rotator. 
       FIG. 2  illustrates optical phase shifter  110 . Optical phase shifter  110  is disposed on substrate  118 , and may be a part of a PIC. Optical phase shifter  110  may be fabricated on silicon-on-insulator (SOI), where the waveguide layer is fabricated in the top silicon layer. 
     An input optical signal enters at waveguide phase shifter  112 . Waveguide phase shifter  112  phase shifts the input optical signal by approximately half the desired phase shift for the average of the TE and TM polarizations. The TE and TM polarizations are phase shifted by different amounts. In one example, waveguide phase shifter  112  is a thermo-optical phase shifter. In another example, waveguide phase shifter  112  is an electro-optical phase shifter. Alternatively, waveguide phase shifter  112  is a passive phase shifter. A passive waveguide phase shifter may require a slightly longer length of the waveguide than that of the input and output, or may rely on a stressed cladding for index modification. Examples of materials that may be used for active phase shifters include doped silicon, a heater on silicon, and lithium niobate. 
     Then, the phase shifted light is polarization rotated by ninety degrees by polarization rotator  114 . Thus, the TE polarization is transformed into a TM polarization, and the TM polarization is transformed to a TE polarization. Polarization rotator  114  is made of a highly birefringent material. The polarization rotator may be made from a birefringent crystal like lithium niobate. Alternatively, the polarization rotator is made from an asymmetrical waveguide, for example composed of silicon, or from an asymmetrical coupler. 
     Finally, the polarization rotated light proceeds to waveguide phase shifter  116 , which is similar to waveguide phase shifter  112 . After passing through waveguide phase shifter  112 , polarization rotator  114 , and waveguide phase shifter  116 , both polarizations experience a similar total phase shift. 
       FIG. 3  illustrates flowchart  250  for a method of phase shifting an optical signal. The TE polarization and the TM polarization are phase shifted by approximately the same amount. Initially, in step  252 , the optical input is received. The optical input may be received from another portion of a PIC. Alternatively, the optical signal is received from an external component or another source. The input optical signal has a TE polarization mode and a TM polarization mode. 
     Then, in step  254 , the light is phase shifted. For example, the light is phase shifted by approximately half of the average total desired phase shift for the TE polarization and the TM polarization. The phase shifting may be performed by an active or passive phase shifter. 
     Next, in step  256 , the polarization of the phase shifted light is rotated by ninety degrees. The TE polarization is converted to a TM polarization, and the TM polarization is converted to a TE polarization. 
     The polarization rotated light is then phase shifted, in step  258 , by a phase shifter similar to the one used in step  254 . Thus, both polarizations of light are phase shifted by the same total amount, because they experience one phase shift as TE polarized light and the other similar phase shift as TM polarized light. 
     Optionally, in step  260 , the light is again polarization rotated by ninety degrees. This restores the light to its original polarization, for applications that need the original polarization. 
     Finally, in step  262 , the output light is transmitted. This may be done, for example, to another part of a PIC, another optical device, or externally coupled. 
     An embodiment optical phase shifter may improve polarization dependent loss performance. Also, an embodiment reduces the total insertion loss, facilitating the construction of large optical switches. An embodiment may lower thermal dependence due to inherent thermal compensation. In an embodiment, manufacturability is improved. For example, an embodiment may be fabricated on a wafer scale, for example in a complementary metal oxide semiconductor (CMOS) silicon wafer environment. Additionally, in an embodiment, there is high power efficiency due to a reduced component count. An embodiment facilitates increased flexibility and scope of applications for PICs, because both TE and TM polarizations are processed in the same optical circuit. 
       FIG. 4  illustrates Mach-Zehnder interferometer (MZI)  190  containing a polarization insensitive phase shifter. A Mach-Zehnder interferometer may be used for switching in telecommunications, for example for high speed dense wavelength division multiplexing (DWDM). Incoming light enters input  191  or input  193 , and proceeds to coupler  192 , where it is split. Half of the optical signal is coupled to leg  194  and half of the optical signal is coupled to leg  196 . The optical signals from legs  194  and  196  are combined by coupler  206 , where it is output in output  208  or output  209 . The output depends on the relative optical path lengths of leg  196  and leg  194 . When the optical path lengths are the same, or have a difference in phase shift of a multiple of 2n, between leg  194  and leg  196 , there is complete constructive interference in leg  209 . However, if the path lengths have a relative phase shift of −π, π, 3π, etc., there is complete destructive interference in leg  209 . For intermediate relative phase shifts, there is an intermediate interference. If the optical path lengths are varied, for example by introducing a variable phase shift into one or both legs, Mach-Zehnder interferometer  190  may be used as an optical switch. Mach-Zehnder interferometer  190  is integrated on a single substrate, for example on a PIC. 
     Leg  194  of Mach-Zehnder interferometer  190  contains a polarization insensitive phase shifter. An optical signal propagating in leg  194  initially is phase shifted by waveguide phase shifter  198 , which shifts the optical phase by approximately half of the total desired phase shift. Then, the polarization is rotated by ninety degrees by polarization rotator  200 . Next, the optical signal is phase shifted by waveguide phase shifter  202 , similar to waveguide phase shifter  198 . The TE and TM polarizations are phase shifted by the same amount, because both polarizations experience one phase shift as TE polarized light and the other phase shift as TM polarized light. Finally, the polarization is phase shifted by ninety degrees by polarization rotator  204 . The second polarization rotator returns the optical output to its original polarization. This may be used if coupler  192  and coupler  206  are polarization sensitive. If the polarizations experienced by coupler  192  and coupler  206  are different, and they are polarization sensitive, the coupling effects will be different, and there will be noise, preventing complete destructive interference and complete constructive interference. To switch Mach-Zehnder interferometer  190 , waveguide phase shifter  198  and waveguide phase shifter  202  may be adjusted, for example by applying a current, voltage, stressed cladding, or heat, to alter the phase shift between the optical signal propagating along leg  194  and the optical signal propagating along leg  196 . 
       FIG. 5  illustrates Mach-Zehnder interferometer  240 , a mirrored Mach-Zehnder interferometer containing a polarization insensitive phase shifter in both legs. Mach-Zehnder interferometer  240  is integrated on a single substrate, such as an SOI substrate. As with Mach-Zehnder interferometer  190 , an optical signal enters Mach-Zehnder interferometer  240  at input  212  or  214 , and is split by coupler  216 . Half the light is coupled to leg  242 , and half the light is coupled to leg  244 . The optical signals from the two legs are combined by coupler  222 , and output to output  224  and/or output  226 . Leg  242  contains waveguide phase shifter  228 , polarization rotator  230 , waveguide phase shifter  232 , and polarization rotator  246 . Leg  244  contains waveguide phase shifter  234 , polarization rotator  236 , waveguide phase shifter  238 , and polarization rotator  248 . One leg may contain active phase shifters while the other leg contains passive phase shifters, both legs may contain active phase shifters, or both legs may contain passive phase shifters. Incorporating phase shifters in both legs helps the legs have similar losses. Different losses in the two legs may lead to more crosstalk and may prevent complete constructive interference and complete destructive interference. 
       FIG. 6  illustrates Mach-Zehnder interferometer  150 , a mirrored Mach-Zehnder interferometer. An optical signal enters at input  122 , and is split by coupler  124 . The light is split into two legs. In one leg, the light passes through waveguide phase shifter  126 , polarization rotator  130 , waveguide phase shifter  134 , and polarization rotator  152 . In the other leg, light passes through waveguide phase shifter  128 , polarization rotator  132 , waveguide phase shifter  136 , and polarization rotator  154 . The optical signals are then combined by coupler  138  to output  140 . In one example, Mach-Zehnder interferometer  150  is a push-pull configuration, where the light experiences a π/2 phase shift in one leg and a −π/2 phase shift in the other leg, leading to complete destructive interference. As long as the phase difference between the two legs is odd multiples of π (e.g., ±π, ±3π, etc.). 
       FIG. 7  illustrates Mach-Zehnder interferometer  120 , a mirrored Mach-Zehnder interferometer, where each leg contains only one polarization rotator. Mach-Zehnder interferometer  120  may be used when coupler  124  and coupler  138  are polarization insensitive. An optical signal enters in input  122 , and is split by coupler  124 . One leg contains waveguide phase shifter  126 , polarization rotator  130 , and waveguide phase shifter  134 , while the other leg contains waveguide phase shifter  128 , polarization rotator  132 , and waveguide phase shifter  136 . The optical signals from the two legs are combined in coupler  138  to output  140 . 
       FIG. 8  illustrates flowchart  160  for a method of switching optical signals using a Mach-Zehnder interferometer. Initially, in step  162 , an optical input signal is received. The optical input signal may be received from another portion of a PIC, another optical component, or from an external source. 
     Then, in step  164 , the optical input signal is split. One portion of the optical input signal goes to a first leg of a Mach-Zehnder interferometer, and proceeds to step  166 . The other portion of the optical input signal goes to a second leg of the Mach-Zehnder interferometer, and proceeds to step  174 . In one example, only one leg contains a phase shifter. In another example, both legs contain phase shifters. 
     In step  166  and step  174 , the light in the two legs experiences a phase shift. The phase shift in the two legs may be the same, or it may be different. The phase shift in one or both legs may be adjustable. A phase shift may be adjusted by applying a voltage, current, stressed cladding, or heat to the phase shifter. 
     Next, in step  168  and step  176 , the polarizations of the optical signals are in both legs are rotated by ninety degrees, exchanging the TE polarization and the TM polarization. 
     After rotating the polarizations, the optical signals are phase shifted again in step  170  and step  178 . The phase shift achieved by step  170  is very close to the phase shift achieved by step  166  Likewise, the phase shift achieved by step  178  is very close to the phase shift achieved by step  174 . Thus, the TE and TM polarizations are phase shifted by the same amount. 
     Optionally, the polarizations of the optical signals are rotated by an additional ninety degrees in step  172  and step  180  to return the light to its original polarization, so the polarization at the input coupler is the same as the polarization at the output coupler. 
     The optical signals from the two legs are combined in step  182 . Depending on the relative phase shifts between optical signals propagating along the two legs, there may be complete destructive interference, complete constructive interference, or an intermediate amount of interference. 
     Finally, in step  184 , the optical output is transmitted, for example to another portion of a PIC, another optical component, or externally. 
     While several embodiments have been provided in the present disclosure, it should be understood that the disclosed systems and methods might be embodied in many other specific forms without departing from the spirit or scope of the present disclosure. The present examples are to be considered as illustrative and not restrictive, and the intention is not to be limited to the details given herein. For example, the various elements or components may be combined or integrated in another system or certain features may be omitted, or not implemented. 
     In addition, techniques, systems, subsystems, and methods described and illustrated in the various embodiments as discrete or separate may be combined or integrated with other systems, modules, techniques, or methods without departing from the scope of the present disclosure. Other items shown or discussed as coupled or directly coupled or communicating with each other may be indirectly coupled or communicating through some interface, device, or intermediate component whether electrically, mechanically, or otherwise. Other examples of changes, substitutions, and alterations are ascertainable by one skilled in the art and could be made without departing from the spirit and scope disclosed herein.