Patent Publication Number: US-2006018597-A1

Title: Liquid crystal grating coupling

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
      This application claims the benefit of Provisional Application No. 60/590,619, filed Jul. 23, 2004. 
    
    
     TECHNICAL FIELD  
      The present invention relates to a liquid crystal/grating coupling arrangement and, more particularly, to a tunable liquid crystal/grating arrangement for use in coupling free space optical signals into and out of a relatively thin SOI waveguiding layer of an SOI-based opto-electronic device structure.  
     BACKGROUND OF THE INVENTION  
      To meet the bandwidth requirements of current and future high speed communication applications, state-of-the-art telecommunication components and systems must provide a host of sophisticated signal processing and routing functions, in both the optical and electronic domains. As the complexity level increases, the integration of more functions and components within a single package becomes strategic in terms of meeting various system-level requirements, while also reducing the associated size and cost of the complete system. It has been recognized for some time that the integrated circuit devices, processes and techniques that revolutionized the electronics industry can be adapted to produce opto-electronic integrated circuits. In typical opto-electronic integrated circuits, light propagates through waveguides of high refractive index materials, such as, for example, silicon, gallium arsenide, indium phosphide, lithium niobate and the like. The use of these high index materials enables smaller device sizes, since a higher degree of mode confinement and smaller bend radii may be realized. While all transmitter, signal processing and receiver functions may be incorporated in a single opto-electronic circuit structure, the system may alternatively be constructed from a number of smaller, pre-packaged elements, referred to as “hybrid optoelectronic integration” or “multi-module opto-electronic integration”.  
      One issue associated with the use of opto-electronic integrated circuits is the problem of coupling light into and out of a planar waveguide structure, particularly when using a relatively thin (e.g., sub-micron thickness) waveguiding layer. An early attempt at developing a coupling arrangement for laboratory use is disclosed in U.S. Pat. No. 3,883,221, issued to William W. Rigrod on May 13, 1975. In particular, Rigrod discloses the use of a prism structure with a grating feature formed in one surface for coupling light into a thin-film (for example, GaAs) surface waveguide. The Rigrod structure is particularly configured to generate a first-order diffracted beam, where with appropriate beam steering an input optical signal may be coupled into the GaAs waveguide. The Rigrod prism element is specifically designed for laboratory use, as a way to perform non-destructive testing of multiple waveguides formed on various substrate structures, and is not intended for use as a “permanent” coupling arrangement. Moreover, the grating structure of Rigrod is found to be limited to diffracting a first-order mode of the light beam and is generally used for steering a particular wavelength input signal. As a result, the Rigrod structure remains incapable of efficiently coupling a relatively large range of wavelengths into a relatively thin surface waveguide layer.  
      Indeed, another aspect of advanced optical communication systems is the utilization of wavelength division multiplexing (WDM) to economically transmit large amounts of information between network nodes. The utilization of a plurality of different wavelengths to carry information signals from one point to another results in the need to either replicate the required system components for each wavelength (i.e., each system “tuned” to its own wavelength), or provide for wavelength insensitivity in the arrangement itself, including the input/output coupling structure.  
      A relatively new field of optics is based on the use of silicon as the integration platform, forming the necessary optical and electrical components on a common silicon substrate. The ability to couple a free space optical signal into and out of a planar waveguiding layer on a silicon substrate (particularly a sub-micron thick waveguiding layer) is a problem that is of current research. Two well-studied techniques, referred to as “butt coupling” and “end-fire coupling”, have traditionally been used to couple light from external sources into optical waveguides. Specifically, end faces are cleaved on the waveguides, and optical fibers (which may be lensed for focusing purposes) are aligned to the input and output waveguide facets. While these coupling methods are relatively wavelength-insensitive, the insertion loss associated with such an arrangement increases substantially as the waveguide thickness drops below 2.0 μm. For sub-micron thick waveguides, the dimensional mismatch between the input/output beams and the thickness of the waveguide results in an insertion loss that is unacceptable for most applications.  
      To improve the insertion loss associated with wavelength-insensitive coupling into relatively thin waveguides, a variety of tapered structures that gradually reduce the beam size from its large external value to a dimension that is more closely matched to the waveguide have been proposed. Some examples include tapers that neck down in one or two dimensions from the external beam to the waveguide, and an “inverse taper” (or “nanotaper”) that has a narrow tip (on the order of 100 nm, for example) coupling to the external beam, with the taper increasing laterally in dimension until it matches the width of the waveguide. Of these examples, only the inverse taper has been successfully used to couple an appreciable amount of light into sub-micron thick waveguides. However, the inverse taper arrangement suffers from a number of drawbacks including, for example, a rapid increase in insertion loss with sub-micron misalignments and the need for additional waveguide structures to be formed prior to the tip of the inverse taper if the end of the tip is not coincident with the edge of the input facet.  
      Indeed, these various prior art techniques are require access to an “edge”/end face of the silicon substrate to provide optical coupling. As optoelectronic circuits begin to increase in complexity, the ability to always allow for such coupling rapidly diminishes, requiring an arrangement that permits coupling into the waveguide at virtually any location across the substrate surface. Our previous co-pending applications, particularly Ser. Nos. 10/668,947, 10/720,372 and 10/935,146, disclose the use of a prism coupler to evanescently couple a free space optical signal into a sub-micron thick silicon waveguiding layer (hereinafter referred to as an “SOI layer”). In most of the embodiments disclosed in these applications, there is a need to precisely control the incident angle of an input beam at the prism facet so as to allow for an adequate amount of the signal to be coupled into the waveguide. Various types of evanescent coupling layers, including layers with a tapered geometry and/or an embedded grating structure (see, for example, our co-pending application Ser. No. 10/935,1146) are used to allow for multiple wavelengths to be coupled into and out of the SOI layer, as well as to allow for some latitude of the incident angle at the prism facet surface.  
      A need remains in the art, however, for a coupling arrangement that is “tunable”, and thus controllable with feedback so that maximum coupling efficiency between a free-space optical signal and an SOI structure can be achieved.  
     SUMMARY OF THE INVENTION  
      The need remaining in the prior art is addressed by the present invention, which relates to a liquid crystal/grating coupling arrangement and, more particularly, to a particular liquid crystal/grating arrangement for use in coupling free space optical signals into and out of a relatively thin SOI waveguiding layer of an SOI-based opto-electronic device structure.  
      In accordance with the present invention, a layered multi-substrate arrangement is used to achieve a “tunable” coupling angle for coupling a free space optical signal into and out of a waveguiding SOI layer. The arrangement comprises a first silicon substrate including at least one diffractive optical element (DOE), (such as a grating) and an associated layer of tunable liquid crystal. A change in an electrical signal (or other type of control signal, such as magnetic field) applied to the liquid crystal material results in a change in its effective refractive index, which, in combination with the deflection associated with the one or more diffractive optical elements, changes the coupling angle into the SOI layer.  
      It is an aspect of the present invention that by adjusting the signal applied to the liquid crystal material, the coupling angle may likewise be adjusted until an optimum amount of coupling is achieved.  
      It is a further aspect of the present invention that the various diffractive optical elements may include collimators, reflectors, polarization beam splitters, deflectors, and the like, so as to allow for the coupling arrangement of the present invention to be used with various types of free space input signals (e.g., polarized, unpolarized, collimated, diverging, etc.).  
      Other and further embodiments and aspects of the present invention will become apparent during the course of the following discussion and by reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      Referring now to the drawings,  
       FIG. 1  illustrates a first tunable coupling arrangement formed in accordance with the present invention;  
       FIG. 2  contains a side view of an exemplary tunable optical coupling arrangement of the present invention, which in this case utilizes a tunable grating structure;  
       FIG. 3  is a top view of the embodiment of  FIG. 2 ;  
       FIG. 4  illustrates another embodiment of the present invention, suitable for use with a diverging input beam;  
       FIG. 5  illustrates yet another embodiment of the present invention, suitable for use with input signals operating at multiple wavelengths; and  
       FIG. 6  contains a series of plots that illustrate the improvement in insertion loss that may be achieved by using a liquid crystal “tunable” coupling arrangement in accordance with the present invention 
    
    
     DETAILED DESCRIPTION  
      Materials classified as liquid crystals are typically liquid at high temperatures and solid at low temperatures, but in the intermediate temperature range they display properties of both. The essential feature of a liquid crystal is the long, rod-like molecular structure. The molecules will align in the presence of an electric field, where the alignment is the result of the anisotropic dielectric constant (refractive index) characteristic of liquid crystals. In accordance with the present invention, these attributes of liquid crystals are capitalized on to provide an input/output coupling arrangement that is “tunable” so as to accommodate for different input wavelengths, different incident angles, and the like.  
      In particular, most liquid crystal materials exhibit a refractive index in the range of 1.3 to 2.0. It has been found that a change in the index value on the order of 0.2 (i.e., Δn=0.2) will correspond to providing a coupling angle control on the order of 6.5°. In most cases, the refractive index of a liquid crystal material is a function of the wavelength of the propagating signal, the ambient temperature, and the voltage applied to the liquid crystal. The voltage control may vary over the range of, for example, 0.1-1.0 volts/μm.  
      As will be evident in the following figures, the use of a diffractive optical element (DOE), which in one form may comprise a grating, in conjunction with a “tunable” liquid crystal material allows for a free space optical beam to be re-directed into a coupling angle most efficient for coupling into a planar waveguiding layer. In one case, a DOE is configured to perform a polarization splitting function. That is, the DOE (or grating) functions to split a single mode optical input beam into its polarized TE x  and TE y  components. The TE x  optical beam component will thereafter propagate in both the y and z directions, with the TE y  optical beam propagating in the x and z directions. These beams will then be evanescently coupled into the SOI layer, where the coupling angle of the optical beam with respect to the surface of the SOI layer must be mode matched. The coupling angle is a critical factor, which is dependent upon a number of different variables: the input signal wavelength, the indexes of refraction of the various substrates, the thickness of the SOI layer (as well as its index of refraction), and the thickness and refractive index of the evanescent coupling layer.  
       FIG. 1  illustrates a first tunable coupling arrangement  10  formed in accordance with the present invention. In this particular example, the input signal is a single mode, TE y  polarized optical beam component that has been collimated, thus providing a plane wavefront normal to the surface of arrangement  10  (although other input angles of incidence may be used). The ultimate goal in each of the coupling arrangements of the present invention is to couple a free space propagating optical beam into a relatively thin (e.g., sub-micron) silicon waveguiding layer with as little insertion loss as possible. The coupling needs to be controlled/tuned as a function of changes in angle of incidence, propagating wavelength(s), types of materials used, thicknesses of the various layers, etc. As will become evident below, use of a liquid crystal material as part of the coupling regime allows for a degree of tunability in the coupling arrangement so as to optimize the optical coupling in light of conventional manufacturing tolerances.  
      Referring to  FIG. 1  in detail, arrangement  10  is seen to comprise an SOI structure  12  including a silicon substrate  14 , a buried oxide layer  16  and a silicon surface waveguiding layer  18  (hereinafter referred to as “SOI layer  18 ”). An evanescent coupling layer  20  (comprising, for example, silicon dioxide, silicon nitride or some other material having a refractive index less than silicon) is disposed over SOI layer  18  and functions to promote the coupling of the free space optical signal into the SOI layer, as fully discussed in our co-pending applications hereby incorporated by reference. In accordance with the present invention, coupling arrangement  10  provides the desired tunable coupling through the use of a layer of tunable optical material  22 , such as a layer of liquid crystal material, disposed over a silicon substrate  24  including a diffractive optical element (DOE)  26 , in this case a grating. The grating can be formed, as is well-known in the art, by etching top surface  28  of silicon substrate  24 . The grating is defined by its period, Λ, and its “order” (defined by m), where the grating is designed to improve the diffraction efficiency in the desired order. For the purposes of the present invention, it is presumed that a “first-order” grating is utilized, where m=1. The following formula, associated with the grating structure, is used to define the angle Φ m , the coupling angle providing maximum coupling (i.e., lowest insertion loss) of a free space optical signal into SOI layer  18 : 
 
sin(Φ m )= mλ   n0 /(Λ n   1 )+sin(Φ 1 )( n   0   /n   1 ), 
 
 where n 0  is the refractive index of liquid crystal material  22 , λ nO  is equal to λ/n 0 , which is the wavelength of the beam propagating through liquid crystal material  22 , λ is the vacuum wavelength of the propagating beam, n 1  is the index of refraction for silicon substrate  24 , and Φ 1  is the input angle of incidence. In the case where the input beam is directed at liquid crystal material  22  at a direction normal to its surface, this relation simplifies to: 
 
sin(Φ m )= mλ /(Λ n   1   n   0 ). 
 
 Therefore, for the case where m=1 (i.e., a first-order grating is used), the grating formula becomes: 
 
sin(Φ 1 )=λ n0 /(Λ n   1   n   0 ). 
 
      By adjusting an electrical (or magnetic) field applied to liquid crystal material  22 , its refractive index (n 0 ) will change, thus modifying the angle at which the TE y  polarized signal intercepts grating  26 . This controlled change in the refractive index of liquid crystal material  22  results in a change of Φ m  that is also controlled. Therefore, by monitoring the insertion loss along SOI layer  18 , the refractive index of liquid crystal material layer  22  can be adjusted until the optimum value of Φ m  is achieved.  
      While the embodiment of  FIG. 1  requires the use of a polarized input beam, it is also possible to provide tunable input coupling for an unpolarized beam in accordance with the present invention, as shown in  FIGS. 2 and 3 .  FIG. 2  contains a side view, and  FIG. 3 a  top view, of an exemplary tunable optical coupling arrangement  30  of the present invention, which in this case utilizes a tunable grating structure  32 , where grating structure  32  includes an internal crystal material layer, as known in the art and described in U.S. Pat. No. 6,587,180 issued to X. Wang et al. on Jul. 1, 2003. In this particular embodiment of the present invention, grating structure  32  functions to split the polarization of the incoming beam into its orthogonal polarization states (TE x  and TE y ), as well as to deflect the beam(s) through a predetermined angle. Referring to  FIG. 3 , both the TE x  and TE y  polarization states are clearly shown. A control element  36  is also shown in  FIG. 3 , where control element  36  is used to adjust the electrical signal applied to grating structure  32  and thus modify the deflection angles of the propagating beams, where deflection angle α, associated with the TE y  polarized signal, is illustrated in  FIG. 2 . Therefore, by using control element  36  to adjust deflection angle α, maximum optical coupling (and, therefore, least insertion loss) into SOI layer  18  may be achieved.  
      It is possible to utilize a diverging optical beam (such as normally exiting an optical fiber) as an input signal to an embodiment of the present invention, where a particularly-configured diffractive optical element is first used to collimate the diverging beam.  FIG. 4  illustrates one such embodiment, where coupling arrangement  52  is formed to utilize silicon substrate  24  and liquid crystal material  22  (and its associated control layers  42 - 1 ,  42 - 2 , which comprise an optically transparent electrically conductive material, such as indium tin oxide) in combination with an additional diffractive optical element layer  52  formed to include at least a collimating input grating  54 . In this case, therefore, an incoming diverging beam (unpolarized) first passes through collimator grating  54  to produce a collimated signal. The collimated signal propagates through the thickness of layer  52  and then impinges a polarization beam splitting grating  56 , disposed along bottom surface  58  of layer  52 . In this particular example, polarization beam splitting grating  56  functions to reflect the TE y  beam (which remains collimated), and diffract the TE x  beam through a first angle into underlying liquid crystal material layer  22 . The reflected TE y  beam is shown as intercepting a beam-deflecting grating  60  formed on the top surface of layer  52 , where beam-deflecting grating  60  then re-directs the TEy beam back through layer  52  and into liquid crystal material layer  22 .  
      Referring to  FIG. 4 , both the TE x  and TE y  beams propagate through liquid crystal material layer  22 , encountering a pair of beam-deflection gratings  62 ,  64  that are utilized to re-direct both polarizations into an angle appropriate for coupling into SOI layer  18 . As with the embodiments described above, by modifying the refractive index of liquid crystal material layer  22 , the orientation of the signals impinging gratings  62 ,  64  will be changed, thus allowing for the coupling angle into SOI layer  18  to be adjusted.  
      The refractive index of liquid crystal materials is known to be a function of the wavelength of the signal passing through the material, the temperature of the material, and the voltage applied to the material. In general, most liquid crystal materials exhibit a decrease in refractive index as the propagating wavelength increases. However, for wideband optical coupling applications, the decrease in refractive index as the propagating wavelength increases, in association with the negative dispersion associated with a DOE grating, will reduce the optical coupling bandwidth if a compensating liquid crystal material is not used. Therefore, in order to provide optimum coupling into an SOI layer, the opposite effect is required; that is, as the wavelength increases the need for a material with a greater refractive index (not lesser) is desired.  FIG. 5  illustrates an exemplary tunable coupling arrangement  70  of the present invention, suitable for use with multiple wavelengths, where the “negative dispersion” (i.e., decrease in refractive index of the liquid crystal material) is compensated for to provide the desired tunability.  
      Referring to  FIG. 5 , arrangement  70  is illustrated as including a DOE/grating layer  72  including a collimating grating  74  formed on its top surface  76 . Thus, as with embodiments described above, a diverging (and unpolarized) input signal will be transformed into a collimated beam as it passes through collimating grating  74  and thereafter propagates through the thickness of grating layer  72 . In accordance with this particular embodiment of the present invention, a beam-deflecting grating  78  is formed on bottom surface  80  of layer  72  and configured to deflect the collimated beam at an angle opposite in sign to what is required for later coupling into SOI layer  18  (see  FIG. 5 ). Therefore, this coupling direction creates “positive dispersion”, which has the effect of compensating for the dispersion associated with the remaining components of the coupling arrangement. As the input wavelength changes, therefore, the refractive index of liquid crystal material layer  22  can be adjusted to create differing degrees of positive dispersion and thus provide relatively low insertion loss into SOI layer  18  over a range of different operating wavelengths.  
       FIG. 6  contains a series of plots that illustrate the improvement in insertion loss that may be achieved by using a liquid crystal “tunable” coupling arrangement in accordance with the present invention. Curve A illustrates the usual insertion loss associated with a conventional prism coupling arrangement including a planar (oxide) evanescent coupling layer. As shown, a loss on the order of 8 dB can be expected with such an arrangement over the nominal operating range of 1530-1570 nm. The use of a tapered evanescent coupling layer is shown to improve the insertion loss, as shown by curve B. In theory, the use of a tunable liquid crystal material in an embodiment such as illustrated in  FIG. 5 , can essentially eliminate the insertion loss associated with wavelength sensitivity, as illustrated by curve C in  FIG. 6 .  
      It is to be understood that the above-described embodiments of the present invention are exemplary only, and should not be considered to define or limit the scope of the present invention. Indeed, the present invention is most properly defined by the claims appended hereto.