Abstract:
A highly efficient electrooptic means and method of light modulation utilizing slow wave optical propagation is provided. A grating structure ( 35 ) integrated with a single mode optical waveguide ( 32 ) on an electrooptic substrate ( 30 ) induces coupling between forward- and reverse-propagating light waves. This contradirectional coupling leads to a reduction in the optical propagation speed in the forward direction. Electrodes ( 38   a  and  38   b ) are provided for applying an electric field to modulate the light propagating in the waveguide ( 32 ) via the linear electrooptic (Pockels) effect. In a preferred embodiment, a modulating radio frequency or microwave signal applied to the electrodes ( 38   a  and  38   b ) propagates in the same direction as the modulated light wave at substantially the same velocity.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     The U.S. Government has a paid-up license in this invention and the right in limited circumstances to require the patent owner to license others on reasonable terms, as provided for by the terms of Grant No. ECS-9522740 awarded by the National Science Foundation. 
    
    
     TECHNICAL FIELD OF THE INVENTION 
     This invention relates in general to the field of electrooptics, and in particular to electrooptic devices for the modulation of light. 
     BACKGROUND OF THE INVENTION 
     Interferometric electrooptic modulators fabricated in the substrate material lithium niobate (LN) are widely used in digital communication systems operating at 2.5 Gb/s and 10 Gb/s and in analog systems for cable television. Not only are modulator rise and fall times &lt;10 ps achieved with this technology, but interferometric designs provide the chirp free performance needed for long-distance transmission. These devices utilize a traveling wave (TW) configuration in which the modulating microwave signal propagates in a strip line or coplanar waveguide on the surface of the insulating substrate in the same direction as the modulated light wave, as described by G. K. Gopalakrishnan et al. in  Journal of Lightwave Technology , vol 12, pp. 1807-1818, 1994. Best performance for high speed or high bandwidth modulation is achieved if the velocity of the modulating radio frequency wave closely matches that of the modulated optical wave. Present practice for the highest bandwidths (&gt;&gt;1 GHz) is to use very thick (≈15-30 μm) electrodes to achieve velocity matching by increasing the microwave propagation speed to match that of the optical carrier. 
     In spite of recent commercial success, the present TW modulator technology still has some shortcomings. Electrical power required to drive the modulators at microwave frequencies is high (typically several hundred mW for a pi-radian phase retardation). This means that a medium power microwave amplification circuit is needed in each transmitter. In the case of analog transmission, the relatively low sensitivity of modulated power to applied voltage and the inherent nonlinearity in dependence of modulated power on applied voltage can adversely affect link dynamic range. Further, the requirement for very thick electrodes on the LN substrate substantially increases the fabrication cost of the modulator chip. 
     One approach to overcoming these shortcomings is to use a material which supports a stronger electrical/optical interaction. Ferroelectric materials such as strontium barium niobate (SBN) with much higher electrooptic coefficients than LN have been known for decades, and low-loss waveguides and GHz-bandwidth modulation have recently been demonstrated in such materials. However, it is well known that materials with such high electrooptic coefficients also have very large dielectric constants. This means that microwave propagation is very slow, so that prohibitively thick electrodes are needed for velocity matching by the conventional method. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, a method and apparatus are provided for the modulation of light which substantially eliminate or reduce disadvantages and problems associated with prior methods and apparatuses. 
     In particular, the present invention makes use of a grating structure integrated with the waveguide to induce slow wave optical propagation in optical waveguides. The grating structure induces contradirectional coupling of light in the waveguide whereby the light bounces back and forth in the waveguide as it propagates through it. This causes the transmitted light to emerge from the waveguide at a later time than would be the case if the grating structure were not present. For the purposes of this invention, the phenomenon whereby the forward propagating optical wave is slowed due to the integrated grating structure is identified as “slow wave optical propagation.” 
     In one embodiment of the present invention, a device for modulating the phase of a light wave is provided. In this embodiment a phase modulator comprises a single mode optical waveguide on a substrate of an electrooptic material. A grating structure integrated with the waveguide results in slow wave optical propagation. The grating structure is formed as a corrugation on the surface of the waveguide or as a refractive index variation in the waveguide material. Electrodes on the surface of the substrate are disposed to produce an electric field in the slow wave propagation region of the waveguide in response to a voltage V(t) applied across the electrodes. The electric field causes a change in the refractive index of the material in and near the optical waveguide, resulting in a modulation of the phase of the forward propagating light wave. In a preferred embodiment, the electrodes are disposed to form a transmission line, such that an applied voltage signal V(t) produces a traveling electromagnetic wave which propagates in the same direction as the incident light wave. The desired result is that the velocity of the modulated light wave matches that of the modulating electromagnetic wave. 
     In another embodiment of the present invention, a device for modulating the intensity of a light wave is provided. In this embodiment an intensity modulator comprises a Mach Zehnder waveguide interferometer on a substrate of an electrooptic material. The interferometer consists of an input single mode optical waveguide section, a branching waveguide region whereby the input waveguide diverges into two parallel waveguide sections, and a second branching waveguide region whereby the two parallel waveguide sections converge to form an output single mode waveguide section. Grating structures integrated with the two parallel waveguide sections between the branches induce slow wave optical propagation. Electrodes on the surface of the substrate are disposed to produce electric fields in these slow wave propagation regions in response to a voltage V(t) applied across the electrodes. These electric fields are of substantially the same magnitude but opposite in sign in the two waveguide sections which support slow wave optical propagation. The electric fields cause changes in the refractive index of the material in and near the slow wave propagation regions, resulting in a modulation of the phase of the forward propagating light waves in each. The phase modulation is substantially the same in magnitude but opposite in sign in the two slow wave optical propagation regions. A light wave coupled into the input optical waveguide section and propagates through the Mach Zehnder waveguide interferometer experiences intensity modulation in response to the voltage V(t) due to optical interference of the phase modulated light waves in the waveguide sections which support slow wave optical propagation. In a preferred embodiment, the electrodes are disposed such that an input voltage signal applied to them propagates in the same direction as the incident light wave in the slow wave optical propagation regions. The desired result is that the speed of the phase modulated light wave in the slow wave optical propagation regions matches that of the modulating electromagnetic wave. Furthermore, in cases where the modulator is used in the transmission of analog signals, the grating structure is designed to produce a dependence of modulated optical power on applied voltage which is substantially more linear than the sinusoidal dependence characteristic of conventional Mach-Zehnder modulators. 
     An important advantage of the present invention is that slowing the speed of optical propagation in the electrical-optical interaction region enables velocity matching between the modulated light wave and a modulating radio frequency or microwave signal in materials in which the microwave propagation speed is normally much lower than the optical propagation speed. It is well known in the art that velocity matching is required for very high frequency modulation performance. Conventional high frequency modulators in lithium niobate (LN) substrates achieve velocity matching by speeding the microwave propagation through the use of very thick (15-30 μm) electrodes, as described by K. Noguchi et al. in  Electronics Letters , vol. 34, pp. 661-663, 1998. In the present invention, velocity matching, as disclosed above and more fully hereafter, is achieved with thin (&lt;1 μm) electrodes, which are much simpler and less expensive to produce than the very thick electrodes. 
     Another advantage of the present invention is the ability to achieve velocity matching in materials with very high dielectric constants, such as strontium barium niobate (SBN). The conventional approach to velocity matching using thick electrodes is not practical in such materials, because electrode thicknesses &gt;100 μm would be required. Manufacturing of such thick electrodes would either be impossible or prohibitively expensive. Materials such as SBN are preferred for modulator application because they possess very large electrooptic coefficients, generally designated r ij  coefficients in the art. For example, r 33  in the SBN compositions known as SBN:60 and SBN:75 are 420 pm/V and 1400 pm/V, respectively, vs. 30.8 pm/V in LN. Large electrooptic coefficients make it possible to achieve a given modulator performance with unprecedented low applied voltage and low electrical power supplied to the device. 
     Yet another advantage of the present invention is the enhancement of modulation efficiency due to the fact that optical propagation is slower than in conventional electrooptic modulators. Since the light is present for a longer time in the electrical-optical modulation region in the slow wave modulator of the present invention, a much larger phase shift is achieved in response to a given driving voltage V(t). 
     A further advantage of the present invention, particularly in systems for transmission of analog signals, is the improved linearity of dependence of modulated optical power on applied voltage which may be achieved through appropriate design of the aforementioned integrated grating structure. 
     Combining these attributes, the present invention provides a means and method of modulating light at very high speeds with order-of-magnitude reduction over prior art in both voltage and electrical power required to achieve a desired modulation performance. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects, features and advantages of the present invention will become more fully apparent from the following detailed description of the preferred embodiment, the appended claims and the accompanying drawings, wherein like reference numerals represent like parts, in which: 
     FIG. 1 is a simplified schematic of a conventional prior art electrooptic phase modulator; 
     FIG. 2 is a simplified schematic of a conventional prior art traveling wave electrooptic intensity modulator; 
     FIG. 3 is a top view of a slow wave optical propagation region in accordance with the present invention; 
     FIG. 4 is a side view of a corrugated grating reflector, a multiplicity of which can be integrated with an optical waveguide to produce slow wave optical propagation in accordance with the present invention; 
     FIG. 5 is a top view of a traveling wave electrooptic phase modulator using slow wave optical propagation apparatus in accordance with the present invention; and 
     FIG. 6 is a top view of a traveling wave electrooptic intensity modulator using slow wave optical propagation apparatus in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The preferred embodiment of the present invention is illustrated by way of example in FIGS. 3-6. With reference to the drawings, FIG. 1 illustrates a conventional prior art electrooptic phase modulator  5 . A single mode optical waveguide  12  is formed on a substrate  10  of a material which exhibits the linear (Pockels) electrooptic effect. Waveguide  12  is positioned to receive an input light wave  11 . Metal electrodes  14   a  and  14   b  are formed on the surface of the substrate  10 . A dynamic voltage 13 V(t) applied between the electrodes  14   a  and  14   b  generates an electric field in and around the optical waveguide  12 . The phase of light propagating through the waveguide  12  is modulated in proportion to V(t)  13  via the linear electrooptic (Pockels) effect. The modulating voltage signal applied between the electrodes  14   a  and  14   b  propagates in the same direction as the modulated, transmitted light wave  15 . In common practice, the substrate  10  is a crystal of lithium niobate (LN), the waveguide  12  is produced by diffusing titanium atoms into the substrate  10 , and the electrodes  14   a  and  14   b  are made of gold. 
     FIG. 2 illustrates a conventional, prior art, traveling wave electrooptic intensity modulator  16 . A single mode optical waveguide  22  is formed on a substrate  20  of a material which exhibits the linear (Pockels) electrooptic effect. Waveguide  22  is positioned to receive an input light wave  21 . The waveguide is configured as a Mach Zehnder interferometer consisting of an input straight waveguide section  19 , a branch  23  whereby the input straight waveguide section  19  diverges to form two parallel waveguide sections  25  and  27 , and a second converging branch  29  whereby the two parallel waveguide sections  25  and  27  converge to form an output waveguide section  17 . Metal electrodes  24   a ,  24   b , and  24   c  are formed on the surface of the substrate  20 . A voltage V(t) applied between the center electrode  24   b  and the two outer electrodes  24   a  and  24   c  generates an electric field in and around the two parallel optical waveguide sections  25  and  27 . The magnitude of the electric field is substantially the same in the vicinity of the two parallel waveguide sections  25  and  27 , but the sign (direction) of the electric field is opposite. The electric fields cause changes in the refractive index of the material  20  in and near the parallel waveguide sections  25  and  27  via the linear electrooptic (Pockels) effect, resulting in a modulation of the phase of the forward propagating waves. The phase modulation is substantially the same in magnitude but opposite in sign in the two parallel waveguide sections  25  and  27 . An incident light wave  21  which is coupled into the input optical waveguide  22  and propagates through the Mach Zehnder interferometer experiences intensity modulation in the transmitted light wave  33  in response to the voltage V(t) due to optical interference of the phase modulated light waves. An input voltage signal applied to the electrodes  24   a ,  24   b  and  24   c  produces a traveling electromagnetic wave which propagates in the same direction as the incident light wave in the parallel waveguide sections  25  and  27 . Desirably, the speed of the phase modulated light wave in the parallel waveguide sections  25  and  27  matches that of the modulating electromagnetic wave. In common practice, the substrate is a crystal of lithium niobate (LN), the waveguide  22  is produced by diffusing titanium atoms into the substrate, and the electrodes  24   a ,  24   b  and  24   c  are made of gold. 
     FIG. 3 illustrates a slow wave propagation region  34  in an optical waveguide  32  in accordance with the present invention. A single mode optical waveguide  32  is formed on a substrate  30  of a material which exhibits the linear electrooptic (Pockels) effect. Waveguide  32  is positioned to receive an incident light wave  11 . Preferably, the velocity of microwave propagation in the substrate material is lower than the velocity of optical propagation in the waveguide  32 . Suitable substrate materials  30  include lithium niobate (LN), lithium tantalate (LT), and strontium barium niobate (SBN), all three materials being of monocrystalline form. Waveguides  32  in LN and LT can be formed, for example, by in diffusion of titanium or nickel, and in SBN by indiffuision of sulfur or zinc. Alternatively, waveguides  32  can be formed in all three substrate materials  30  by the effect of a strain-inducing surface film as described by O. Eknoyan et al. in  Applied Physics Letters , Vol. 60, pp. 407-409, 1992. Any other suitable materials and waveguides now known or hereafter developed may be used as well. 
     A grating structure  35  integrated with a portion of single mode waveguide  32  forms a slow wave propagation region  34  of length L. The grating structure  35  consists of a multiplicity of grating reflectors  36  with a center-to-center spacing ΔL. Reflectances of grating reflectors  36  are designated R j , j=1,2, . . . , N, with N the number of reflectors  36 . In the illustration of FIG. 3 the value of N is 4, but it is understood that the present invention can be implemented with other numbers of reflectors  36 , so long as N≧2. 
     Metal electrodes  38   a  and  38   b  are formed on the surface of the substrate  30 . A dynamic voltage V(t) applied between the electrodes  38   a  and  38   b  generates an electric field in and around the optical waveguide  32  in the slow wave propagation region  34 . The phase of light propagating through the waveguide is modulated in proportion to V(t) via the linear electrooptic (Pockels) effect. The modulating voltage waveform V(t) propagates in the same direction as the modulated light wave. 
     One preferred embodiment for realizing grating reflectors  36  is the corrugated grating illustrated in side view in FIG.  4 . Waveguide  32  is formed on substrate  30 , as heretofore described. The medium  31  above and adjacent to the waveguide  32  is air. A grating reflector  36  of extent L R  is formed by a multiplicity of corrugations  37  etched into the waveguide  32  and substrate  30 . The corrugations  37  can be formed, for example, by plasma etching, reactive ion etching, ion milling, or chemical etching, with a mask to define the material to be etched (corrugations) or by any method now known or hereafter developed. Light propagating in waveguide  32  is reflected at the dielectric interfaces formed by the corrugations  37 . For example, it is well known that the reflectance R for a single such interface, defined as the ratio of reflected optical power to incident optical power, is given by the Fresnel formula              R   =                n   1     -     n   2           n   1     +     n   2              2             (   1   )                                
     with n 1  and n 2  the refractive indices of the media adjacent to the interface. For example, if n 1 =2.15, approximately the effective refractive index of a waveguide mode in LN or SBN, and n 2 =1.00, the refractive index of air, then R=0.133. Furthermore, with two such interfaces, as would be formed by a single corrugation  37 , the reflectance would be almost 4 times this value (0.133×4=0.53), provided that the width of the corrugation is λ/4 with λ the optical wavelength. Thus, for an optical wavelength of 1.5 μm, the total length L R  of a grating reflector  36  needed to provide a reflectance near 50% is only 0.38 μm. Multiple reflectors  36  with quarter wave spacing will give correspondingly higher reflectance values. Thus, it is evident that very short grating reflectors  36 , with length of the order of 1 μm, can give very high reflectances (&gt;90%). 
     As an alternative to the deep corrugations  37  illustrated in FIG. 4, grating reflectors  36  for realization of the slow wave propagation region  34  could consist of shallow corrugations which extend only part way through the waveguide  32 . Another alternative is to use refractive index variations in the waveguide material and adjacent substrate material to form the grating reflectors  36 . Such gratings can be written holographically in materials such as LN and SBN by the photorefractive effect through the interference of laser beams in the waveguide  32  and substrate materials  30 . Preferably, the wavelength of the interfering laser beams is in the ultraviolet region of the spectrum. 
     It is desirable to minimize the optical power reflected from the slow wave optical propagation region  34 , since this reflected power constitutes an optical loss for the modulator. This is accomplished by the appropriate choice of the spacing ΔL of the grating reflectors  36 . In particular, if the round trip optical phase change Δφ between reflectors  36 , given by is equal to (2m+1)π             Δφ   =       4      π                   n   1        Δ                 L     λ             (   2   )                                
     radians, with m an integer, n 1  the refractive index of the optical waveguide mode, and π the wavelength of the incident (unmodulated) light wave  11 , then reflective contributions from adjacent mirrors will interfere destructively, thereby tending to minimize the optical power reflected for the slow wave optical propagation region  34 . Combining this condition with eq. (2) yields as the                             Δ                 L     =         (       2      m     +   1     )        λ       4        n   1                   (   3   )                                
     condition on ΔL to minimize the reflectance from the slow wave region  34 . For example, if λ=1.5 μm, n 1 =2.15, and m=2000, then it is calculated from eq. (3) that ΔL=697.84 μm. 
     As an alternative to discrete grating reflectors  36 , it is evident that a continuous grating can be integrated with a single mode waveguide  32  to provide a slow wave propagation region  34 . Such a continuous grating, preferably formed as shallow corrugations  37  on the waveguide surface, or as a photorefractive holographic grating in the material  30  of the waveguide  32  and substrate, could subtend the entire length of the slow wave propagation region  34  with minimal or no gaps in the grating pattern. 
     FIG. 5 illustrates a phase modulator  41  incorporating the slow wave optical propagation of the present invention. A single mode optical waveguide  32  is formed on a substrate  30  of a material which exhibits the linear (Pockels) electrooptic effect. Waveguide  32  is positioned to receive an input light wave  11 . A slow wave optical propagation region  34  is formed by a grating structure (as discussed above) integrated with the optical waveguide  32 . Metal electrodes  38   a  and  38   b  are formed on the surface of the substrate  30 . A dynamic voltage V(t) applied between the electrodes  38   a  and  38   b  generates an electric field in and around the optical waveguide  32 . The phase of light propagating through the waveguide  32  and exiting therefrom is modulated in proportion to V(t) via the linear electrooptic (Pockels) effect. The modulating voltage signal applied between the electrodes  38   a  and  38   b  propagates in the same direction as the modulated light wave. By way of the present invention, the speed of the phase modulated light wave in the slow wave optical propagation region matches that of the modulating electromagnetic wave. The substrate  30  can be a crystal of LN or SBN; the waveguide  32  can be produced by diffusing titanium or nickel atoms into the substrate in the case of LN, or by diff-using sulfur or zinc atoms into the substrate in the case of SBN, or by a strain-inducing film on the surface of the substrate for either LN or SBN; and the electrodes can be made of gold. Obviously, any other substrate  30  and waveguide  32  now know or hereafter developed that are suitable may be utilized. 
     FIG. 6 illustrates an electrooptic intensity modulator  51  incorporating slow wave optical propagation of the present invention. A single mode optical waveguide  42  is formed on a substrate  40  of a material which exhibits the linear (Pockels) electrooptic effect. Waveguide  42  is positioned to receive an input, incident, light wave  11 . The waveguide  42  is configured as a Mach Zehnder interferometer consisting of an input straight waveguide section  19 , a branch  23  whereby the input straight waveguide section  19  diverges to form two parallel waveguide sections  25  and  27 , and a second branch  29  whereby the two parallel waveguide sections  25  and  27  converge to form an output waveguide section  17 . Slow wave optical propagation regions  48   a  and  48   b  are formed by a grating structure  53  integrated with the two parallel sections  25  and  27  of optical waveguide  42 . Metal electrodes  44   a ,  44   b , and  44   c  are formed on the surface of the substrate  40 . A voltage V(t) applied between the center electrode  44   b  and the two outer electrodes  44   a  and  44   c  generates an electric field in and around the two parallel optical waveguide sections  25  and  27 . The magnitude of the electric field is substantially the same in the vicinity of the two parallel waveguide sections  25  and  27 , but the sign (direction) of the electric field is opposite. The electric fields cause changes in the refractive index of the material in and near the parallel waveguide sections  25  and  27  via the linear electrooptic (Pockels) effect, resulting in a modulation of the phase of the forward propagating waves. The phase modulation is substantially the same in magnitude but opposite in sign in the two parallel waveguide sections  25  and  27 . A light wave  11  which is coupled into the input optical waveguide  42  and propagates through the Mach Zehnder interferometer as transmitted light wave  33  experiences intensity modulation in response to the voltage V(t) due to optical interference of the phase modulated light waves. An input voltage signal applied to the electrodes  44   a ,  44   b  and  44   c  produces a traveling electromagnetic wave which propagates in the same direction as the incident light wave  11  in the parallel waveguide sections  25  and  27 . In accordance with the invention, the speed of the phase modulated light wave in the slow wave optical propagation regions  48   a  and  48   b  matches that of the modulating electromagnetic wave. The substrate  40  can be a crystal of LN or SBN; the waveguide  42  can produced by diffusing titanium or nickel atoms into the substrate  40  in the case of LN, or by diffusing sulfur or zinc atoms into the substrate  40  in the case of SBN, or by a strain-inducing film on the surface of the substrate  40  for either LN or SBN; and the electrodes  44   a ,  44   b  and  44  can be made of gold or aluminum. Again, any now known or hereafter developed substrate, waveguide, and/or electrodes suitable for the invention disclosure herein may be omitted. 
     While the present invention has been disclosed in connection with the preferred embodiment thereof, it should be understood that there may be other embodiments which fall within the spirit and scope of the invention as defined by the following claims.