Abstract:
Systems and methods for controlling and measuring modes possessing even and odd symmetry in a slow-light photonic crystal waveguide. An example device comprises a photonic crystal waveguide having modes possessing even and odd symmetry, and a Mach-Zehnder coupler comprising two waveguide branches one of which has a phase adjuster. Another example device, which can be used as an optical isolator, comprises two Mach-Zehnder couplers, and a photonic crystal waveguide comprising an electro-optic modulator therein. A method of measuring a group index of a mode with odd symmetry comprises: coupling light into a photonic crystal waveguide through a Mach-Zehnder coupler with a mixed even/odd symmetry, measuring insertion loss of the combined light signal after passing through the photonic crystal waveguide, determining the spacings of adjacent peaks or valleys from the insertion loss versus wavelength plot, and using the spacings to determine the group index of the odd symmetry mode.

Description:
RELATED APPLICATION AND CLAIM OF PRIORITY 
     This application claims the priority benefit of U.S. Provisional Application No. 61/672,948 entitled “Systems and methods for controlling and measuring modes possessing even and odd symmetry in a photonic crystal waveguide,” and filed Jul. 18, 2012, the content of which is hereby incorporated by reference in its entirety. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     This application was developed with U.S. Government support under Grant No. FA-9550-08-1-0394, which was awarded by the Air Force Office of Scientific Research. The U.S. Government has certain rights in this application. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Statement of the Technical Field 
     The present invention relates to optical waveguides, and more particularly to Photonic Crystal Waveguides (“PCWs”) and optical isolators. 
     2. Description of the Related Art 
     PCWs can modify light propagation and dispersion characteristics through their periodic structures, thus have important applications in communications and sensing. Particularly, the slow light effect in a PCW can significantly enhance light-matter interaction, as demonstrated in significant reduction of interaction lengths for PCW based modulators and switches. To date, most of the PCW research has been focused on the TE-like mode with even symmetry. However, a PCW often has an odd TE-like mode inside the photonic bandgap exhibiting the slow light effect as well. This odd mode can potentially open up the opportunities for mode-symmetry-based novel devices, such as one-way waveguides that exploit indirect interband photonic transitions between even and odd modes. The slow light effect in PCWs can help reduce the interaction length for such transitions, enabling ultra-compact devices. To utilize this odd mode in any devices, it is crucial to control its excitation symmetry and understand its slow light spectral characteristics. Normally, this odd mode does not exhibit itself evidently in the PCW transmission spectrum because its odd symmetry prohibits its excitation by the fundamental even mode of a conventional waveguide typically used at input. Symmetry-breaking structure imperfections sometimes may induce some coupling to this odd mode, causing a decrease of PCW transmission in the odd mode band. But such imperfection-induced coupling between this odd mode and other modes is not controllable, and therefore is not useful in practice. 
     SUMMARY 
     The invention concerns systems and methods for controlling and measuring modes possessing even and odd symmetry in a photonic crystal waveguide, and designing devices based on modes possessing even and odd symmetry in such a waveguide. The primary object of the invention is to provide mode-symmetry control devices with shorter interaction lengths through slow light photonic crystal waveguides. Another object of the invention is to reduce power consumption of mode-symmetry control devices, including one type of optical isolators. Another object of the invention is to present a method for measuring spectral characteristics and a group velocity of an odd mode in a photonic crystal waveguide and using such information in subsequent device design. Other objects and advantages of the present invention will become apparent from the following descriptions, taken in connection with the accompanying drawings, wherein, by way of illustration and example, an embodiment of the present invention is disclosed. 
     In accordance with a preferred embodiment of the present invention, a device for controlling the mode symmetry of a photonic crystal waveguide comprises: a photonic crystal waveguide having a waveguide core along which light is guided and a Mach-Zehnder coupler comprising two waveguide branches one of which has a phase adjuster. 
     In accordance with another preferred embodiment of another aspect of the present invention, an optical isolator comprises: a photonic crystal waveguide having one or more cores, a first Mach-Zehnder coupler at a first end of the photonic crystal waveguide, a second Mach-Zehnder coupler at a second end of the photonic crystal waveguide, and an electro-optic modulator formed in the photonic crystal waveguide comprising at least one of a diode and a capacitor. 
     In accordance with another preferred embodiment of another aspect of the present invention, a method for measuring the spectral characteristics and a group velocity of an odd mode comprises: coupling light into a photonic crystal waveguide through a Mach-Zehnder coupler with a mixed even/odd symmetry, measuring insertion loss of the combined light signal after passing through the photonic crystal waveguide in a range of wavelengths, creating a graph showing the relationship of the insertion loss and a wavelength of light; determining the spacings of adjacent peaks or valleys of a line in the graph, determining a group index based on the values, and using the group index values in subsequent device design of a functional waveguide. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments will be described with reference to the following drawing figures, in which like numerals represent like items throughout the figures, and in which: 
         FIG. 1(   a ) is a schematic illustration of an exemplary system that is useful for understanding the present invention. 
         FIG. 1(   b ) is a graph showing a relationship between a coupling efficiency of an exemplary system and the wavelength of light. 
         FIG. 2  is a schematic illustration of an exemplary system that is useful for understanding the present invention. 
         FIG. 3  is a flow diagram of an exemplary method for designing a PCW. 
         FIG. 4(   a ) is a band diagram for r=0.325a. 
         FIG. 4(   b ) is a graph showing a variation of the band edge and cutoff of even and odd TE-like modes with hole radius. 
         FIG. 5(   a ) is a graph showing a relationship between an insertion loss of an odd-mode Mach-Zehnder coupler and a plurality of different wavelengths. 
         FIG. 5(   b ) is a graph showing a first relationship between an insertion loss of an even-mode without a Mach-Zehnder coupler and the wavelength of light, and a second relationship between an insertion loss of a mixed even/odd mode Mach-Zehnder coupler and the wavelength of light. 
         FIG. 5(   c ) is a graph showing the difference of the group indices, Δn g , obtained from the mixed even/odd mode spectrum of  FIG. 5(   b ). 
         FIG. 5(   d ) is a graph showing a Fourier transform of the transmission spectrum of a directly coupled PCW. 
     
    
    
     DETAILED DESCRIPTION 
     It will be readily understood that the components of the embodiments as generally described herein and illustrated in the appended figures could be arranged and designed in a wide variety of different configurations. Thus, the following more detailed description of various embodiments, as represented in the figures, is not intended to limit the scope of the present disclosure, but is merely representative of various embodiments. While the various aspects of the embodiments are presented in drawings, the drawings are not necessarily drawn to scale unless specifically indicated. 
     The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by this detailed description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope. 
     Reference throughout this specification to features, advantages, or similar language does not imply that all of the features and advantages that may be realized with the present invention should be or are in any single embodiment of the invention. Rather, language referring to the features and advantages is understood to mean that a specific feature, advantage, or characteristic described in connection with an embodiment is included in at least one embodiment of the present invention. Thus, discussions of the features and advantages, and similar language, throughout the specification may, but do not necessarily, refer to the same embodiment. 
     Furthermore, the described features, advantages and characteristics of the invention may be combined in any suitable manner in one or more embodiments. One skilled in the relevant art will recognize, in light of the description herein, that the invention can be practiced without one or more of the specific features or advantages of a particular embodiment. In other instances, additional features and advantages may be recognized in certain embodiments that may not be present in all embodiments of the invention. 
     Reference throughout this specification to “one embodiment”, “an embodiment”, or similar language means that a particular feature, structure, or characteristic described in connection with the indicated embodiment is included in at least one embodiment of the present invention. Thus, the phrases “in one embodiment”, “in an embodiment”, and similar language throughout this specification may, but do not necessarily, all refer to the same embodiment. 
     The present invention concerns systems and methods for controlling the excitation symmetry for a high purity transmission of an odd TE-like mode in a PCW and measuring the characteristics of this mode. An exemplary system  100  is provided in  FIG. 1 . As shown in  FIG. 1 , the system  100  comprises a Mach-Zehnder Coupler (“MZC”)  102  and a PCW  104 . PCWs are well known in the art, and therefore will not be described herein. Still, it should be noted that the PCW may comprise silicon with an array of apertures  106  formed therein. The array  106  facilitates the confinement of light in a solid middle region  108 . The solid middle region  108  can be formed of silicon. 
     The MZC  102  is generally configured to receive light from an input waveguide (not shown). This light has a field pattern with an even symmetry, as shown in block  110 . Throughout this specification, the field pattern refers to one electric field component (E x ) of light unless specified otherwise. The MZC  102  converts the field pattern of the light to an odd symmetry, as shown by block  112 . This conversion is achieved using branching arms  114 ,  116  and a phase modifier  118 . The phase modifier  118  can include a passive phase modifier as shown in  FIG. 1  or an active phase modifier. The passive phase modifier may be implemented by increasing the total length of one of the branching arms  114 ,  116  by an amount so as to provide a 180 degree phase difference between light signals traveling therethrough. The active phase modifier can be implemented by disposing electrical contacts adjacent to one of the branching arms  114 ,  116 . In this scenario, the application of a voltage to the electrical contacts causes a phase of a light signal traveling through the branching arm  116  to change such that it is 180 degrees out of phase with the light signal traveling through the other branching arm  114 . The two phase offset light signals are then combined in an end portion  120  of the MZC  102 . The combined light signal has a field pattern with an odd symmetry as shown by block  112 . 
     An output fixture may be attached to the output of the PCW  104  for further manipulation of the mode symmetry. In some embodiments, another MZC  200  may be coupled to an output  202  of the PCW  104 . This MZC  200  converts the field pattern of the light signal output from the PCW  104  from an odd symmetry to an even symmetry. This conversion is achieved in the same or substantially similar manner as the conversion provided by the MZC  102 . Notably, the structures shown in  FIG. 1  and  FIG. 2  may be used in various applications. Such applications include, but are not limited to, one way waveguide applications and isolator applications. 
     In some embodiments, a modulator may be formed in the PCW  104  to dynamically modulating the symmetry of mode therein. In one embodiment, the modulator may comprise one or more p-n diodes. In another embodiment, it may comprise at least one of a p-i-n diode, and a metal-oxide-semiconductor (MOS) capacitor. 
     In some embodiments, a photonic crystal waveguide may have two or more missing rows of holes. In one embodiment, the missing rows of holes may be immediately next to each other to form a single wide line-defect photonic crystal waveguide. In another embodiment, the missing rows of holes may be separated by one or several rows of holes, forming two waveguide cores that are coupled. In the latter embodiment, a supermode of even or odd symmetry can still exist across two waveguide cores. 
     In some embodiments, two or more apparatuses of this invention may be cascaded to improve the performance characteristics. In one embodiment, the output fixture of the first apparatus is coupled to the input of the second apparatus. Cascaded apparatuses are particularly useful in those embodiments where each apparatus is designed to be an optical isolator. The optical isolation may be increased as the number of cascading apparatus increase. For example, a single optical isolator formed based upon the teaching of this invention may have an optical isolation of 8 dB. Two cascaded optical isolators may achieve an overall optical isolation of 15 dB or more. 
     Referring now to  FIG. 3 , there is provided an exemplary method  300  for designing the PCW  104  based on operational characteristics of the MZC  102 . As shown in  FIG. 3 , the method begins with step  302  and continues with step  304 . In step  304 , a MZC  102  is obtained. As noted above, the MZC  102  comprises two branching arms  114 ,  116  and a phase modifier  118 . Next, in step  306 , a light signal is input into the MZC  102 . This light signal has a field pattern with an even symmetry. The phase modifier  118  is then used to modify a phase of the light signal as it travels through one of the branching arms, as shown by step  308 . In a next step  310 , the light signals which traveled through the branching arms  114 ,  116  are combined so as to obtain a combined light signal. The combined light signal has a field pattern with a mixed even/odd symmetry. 
     The insertion loss of the combined light signal is then measured in a range of wavelengths, as shown by step  312 . A graph showing the relationship between the measured values obtained in step  312  and the wavelength of light is created in step  314 . After this graph is created, step  316  is performed where values specifying spacings between adjacent peaks and/or valleys of a line connecting plotted points within the graph are determined. These values are then used in step  318  to determine a group index. Group index is well known in the art, and therefore will not be described herein. The group index may be used as a parameter for designing a functional waveguide, such as PCW  104 , a passive silicon waveguide or a waveguide with a modulator. 
     Example 1 
     The following EXAMPLE 1 is provided for purposes of explaining in more detail the present invention. The present invention is not limited to the contents of the following discussion discussed under the heading EXAMPLE 1. 
     Consider a W1 PCW formed on a Silicon-On-Insulator (“SOI”) wafer by removing a row of air holes in a hexagonal lattice with lattice constant a=400 nm, hole radius r=0.325a, and Si slab thickness t=260 nm. The band diagram in  FIG. 4(   a ) is calculated by 3D plane wave expansion. Below the lightline (for the oxide bottom cladding), the even TE-like mode has a flat dispersion relation with group index n g &gt;50, and a narrow bandwidth (&lt;4 nm). In contrast, below the lightline, the odd TE-like mode has a much wider bandwidth ˜20 nm with n g  down to ˜15. Such a moderate n g  range is favorable for many applications as various types of losses are reduced at lower n g . Furthermore, the dispersion relation of the TM-like guided mode usually crosses that of the even mode, as seen in  FIG. 4(   a ). But the TM-like mode does not cross the odd mode in the region below the lightline in  FIG. 4(   a ). For εα/2πc=0.28˜0.286, only the odd mode is below the lightline. 
     Systematic simulations show that as the hole radius increases, the odd-mode bandedge moves up faster than the TM cutoff, as shown in  FIG. 4(   b ). For a sufficiently large r, the TM cutoff is below the odd mode bandedge; thus the two modes do not cross each other below the lightline, helping avoid their inter-coupling due to asymmetric top and bottom claddings. However, as r increases, the transmission bandwidth bounded by the bandedge and the cutoff decreases for both the even and odd modes, as shown in  FIG. 4(   b ). Hence, this work focuses on the intermediate r case shown in  FIG. 4(   a ), which shows a sufficient clearance between the odd mode bandedge and the TM cutoff, and a sufficiently wide bandwidth. 
     Excitation of this odd PCW mode is usually deterred by the opposite symmetry of the fundamental even mode of a Si waveguide. To solve this problem, a two-step approach is employed. First, a MZC whose two arms have a phase difference of π is utilized to transform mode symmetry and excite an odd mode in a wide (multimode) Si wire waveguide; then this odd mode is coupled to the odd mode of the PCW. To create π phase difference in this odd-mode MZC, its two arms can be designed to have a length difference of (Δl) π =λ/2n eff , where n eff  is the effective index of the Si waveguide. A Finite Difference Time-Domain (“FDTD”) simulation confirmed that such a MZC produces an odd mode in a wide output waveguide, as shown in  FIG. 1(   a ). The input and output waveguide widths are 400 nm and 700 nm respectively. The coupling between the odd mode of a Si wire waveguide (700 nm wide) and that of the PCW is also simulated. Simulation results in  FIG. 1(   b ) show coupling efficiencies up to ˜84% (˜0.75 dB) for the odd mode. The field pattern in  FIG. 1(   b ) left inset confirms that the coupled PCW mode is an odd mode. The fundamental even mode of an Si wire waveguide couples into the PCW with inconsequential change of coupling efficiency for the spectral range in  FIG. 1(   b ). The field pattern in  FIG. 1(   b ) right inset indicates that the coupled mode has even symmetry. Indeed, this mode is an even TE-like mode above the lightline. The E x  field has been shown in  FIGS. 1(   a )- 1 ( b ) for direct comparison with the modes of the conventional Si waveguide, whose TE-modes are commonly visualized by E x  (note E x  and H z  have the same symmetry with respect to x). 
     The photonic crystal waveguide structure is fabricated on an SOI wafer with a 2 μm buried oxide layer and a 260 nm top Si layer according to the parameters used in  FIG. 4(   a ). The structure is patterned by a JEOL JBX-6300FS high-resolution e-beam lithography system, operating at 100 keV, on a 100 nm thick layer of ZEP 520A e-beam resist. Then the pattern is transferred to the Si layer by an Oxford Plasmalab 100 ICP etcher.  FIG. 2  is a Scanning Electron Microscope (“SEM”) image of the fabricated structure. Two MZCs with a 10 μm bending radius are connected through 700 nm-wide Si waveguides of 1 μm lengths to both ends of the PCW. 
     To measure transmission spectra, light from a super-luminescent LED with a spectral range of about 80 nm is coupled to the TE mode of Si access waveguides (tapered to 4 μm at chip edges) via lensed fibers. A polarizer is used at the output end to block TM polarization. The PCW insertion loss is measured with reference to an Si wire waveguide.  FIG. 5(   a ) shows the spectrum of a PCW with odd-mode MZCs. A substantial transmission bandwidth is observed, approximately 22 nm at 10 dB below the peak. The contrast between the transmitted mode and background is &gt;20 dB. The peak insertion loss is about ˜4 dB. Separate measurements show that each MZC contributes ˜1 dB. Thus, the loss due to the PCW is estimated ˜2 dB. For comparison, the spectrum of a directly coupled PCW without MZCs is shown in  FIG. 5(   b ). The transmission is due to the leaky even TE-like mode as simulated in  FIG. 1(   b ).  FIG. 5(   b ) also shows the PCW transmission with MZCs whose two arms have a length difference Δl deliberately designed to be 50% greater than (Δl) π . Such a mixed-mode MZC offers a symmetry configuration that can excite a mixture of even and odd modes according to I ± ∞(½)[1±cos(2πn eff l/λ)]. As such, the background transmission due to the even mode rises. In the odd mode band, the mixed-mode spectrum oscillates strongly due to the beating of two modes.  FIGS. 5(   a )- 5 ( b ) illustrate that distinctive spectral signatures can be observed with controlled excitation symmetries. 
     The mode-beating pattern of the mixed-mode spectrum contains important information of the odd mode. The beating period is related to the group indices of even and odd modes through Δλ=λ 2 /(n g,odd −n g,even )L, where L is the PCW length. Simulation indicates that n g,even  is virtually a constant (˜5) in the odd mode band. Thus, the chirped beating periods are due to the dispersion of n g,odd . Δn g =n g,odd −n g,even  was calculated from the mixed-mode spectrum and plotted it in  FIG. 5(   c ). The peak-spacing and valley-spacing of the spectrum give two sets of Δn g , data, plotted by circles and crosses, respectively. The peak-spacing and valley-spacing agree with each other as expected. The Δn g  value obtained from two adjacent peaks (valleys) is assigned to the mid-point wavelength in-between. Further, n g,even =4.9 is obtained in  FIG. 5(   d ) through the Fourier transform of the transmission spectrum of another directly coupled PCW with more obvious spectral ripples. The Fourier frequency f 2  is just the inverse of the spectral oscillation period δλ, thus n g,even =f λ ×λ 2 /2L. Based on  FIGS. 5(   c )- 5 ( d ), n g,odd =Δn g +n g,even  is in the range of 14˜29. The F-P oscillation amplitude in  FIG. 5(   a ) is relatively weak. In contrast, the mode-beating amplitude of the mixed-mode spectrum in  FIG. 5(   b ) is much higher and more robust against noise, which facilitates the evaluation of n g,odd . Also as shown in  FIG. 5(   a ), the background transmission increases discernibly beyond 1430 nm due to the dispersive effect in the odd-mode MZC, which modifies the phase shift difference between the two arms as λ deviates far from the designed value (1390 nm). The TM-like mode (guided for λ&gt;1.45 μm) may also contribute to the background at long wavelengths. However, these effects are much weaker for 1380-1415 nm. 
     Although this example focuses on PCWs on an SOI chip, the MZC and the mode-beating based n g,odd  measurement method can be adapted to the cases of air-bridge or oxide-covered PCWs, and coupled-cavity PCWs. The odd mode wavelength can also be shifted to ˜1550 nm or other values (depending on specific applications) by changing the lattice constant. In an SOI PCW, there is some coupling between the TE-like guided modes and the TM-like photonic crystal bulk modes due to asymmetric top/bottom claddings. Prior work on the even mode has demonstrated that reducing n g  can reduce the loss due to such coupling. This odd mode has a much lower n g ˜14 than the normal even mode (n g ˜50) below the lightline. This helps to reduce the coupling to the TM-like bulk modes. For many PCW devices operating at a short length &lt;80 μm, the propagation loss of the odd mode is expected to be reasonable. Lastly, the understanding of the slow light and mode-beating characteristics of this odd mode, and the controlled excitation and n g,odd  characterization schemes developed here can facilitate the development of mode-symmetry based novel devices, such as one-way waveguides that involve active transition and passive conversion between even and odd modes. Slow light can help reduce device interaction length. The odd-mode MZC of the present invention is focused on transforming mode symmetry to attain high purity odd-mode; and the mixed-mode MZC offers a symmetry configuration for coherent mixing of even and odd modes, which enables n g,odd  measurement through slow-light mode beating. 
     In summary, the present invention provides a novel approach to controlling an excitation symmetry for an odd TE-like mode in a PCW. An odd-mode MZC is utilized to selectively excite the odd mode with a contrast &gt;20 dB over the background. Assisted by a mixed-mode MZC, slow-light mode beating is observed and is utilized to measure the group index of this odd mode. 
     While various embodiments of the present invention have been described above, it should be understood that they have been presented by way of example only, and not limitation. Numerous changes to the disclosed embodiments can be made in accordance with the disclosure herein without departing from the spirit or scope of the invention. Thus, the breadth and scope of the present invention should not be limited by any of the above described embodiments. Rather, the scope of the invention should be defined in accordance with the following claims and their equivalents.