Abstract:
An optical device has a substrate, a polarization beam splitter, first and second pairs of optical waveguides, a transducer and a thin film. The polarization beam splitter is formed on the substrate and has input and output sides. The first and second pairs of optical waveguides are formed on the substrate to guide polarized optical signals. The first pair of optical waveguides meets at the input side of the polarization beam splitter, and the second pair of optical waveguides meets at the output side of the polarization beam splitter. The transducer is formed of a comb-tooth electrode on the substrate to excite a surface acoustic wave on the substrate and rotate the polarization of the optical signal. The thin film covers a portion of each waveguide of either the first or second pairs of optical waveguides. The thin film may be formed of silicon dioxide or indium dioxide, either with a metal oxide optionally added thereto. The speed of sound in the thin film is less than that in the substrate. The thin film is formed of a material which is transparent to the optical signal. The thin film has a refractive index smaller than that of the optical waveguides.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based up on and claims priority to Japanese Patent Application Number 09-216050 filed Aug. 11, 1997, the contents being incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an optical device in general and more particularly to an acousto-optic device that may be used in an optical transmission system to add, drop and modulate selected wavelengths. 
     2. Description of the Related Art 
     In recent years, progress to a highly sophisticated information society has generated a tremendous amounts of information and an optical communication system using an optical fiber has been introduced as a way of transmitting such information. With this optical communication system, the transmission capacity has been increased year by year with the realization of a high speed modulation rate. A modulation rate of Gb/s or higher has already been put into practical use. 
     However, the need for a transmission systems which can transmit a large amount of data, such as that from image information, is expected to increase in the future. Such a high capacity system may be now required to have the transmission capacity of one Tb/s or more. The current systems cannot satisfy the requirement for the above transmission capacity only by improving the modulation rate. Therefore, an optical wavelength multiplex transmission/communication system is considered indispensable, and there have been attempts in recent years to introduce such a system. 
     An important element for realizing optical wavelength multiplex communication is the optical wavelength filter. This filter can combine onto a single optical fiber light beams of different (perhaps many different) wavelengths respectively generated by different light sources and can branch light beams of the different wavelengths transmitted through the single optical fiber to respective different fibers and detectors. The filter is thus a key device of the optical wavelength multiplex transmission system. The filter is required to satisfy different requirements depending on the system in which it is used. For example, the filter should be able to work with different numbers of wavelengths, from several wavelengths to about 100 wavelengths. The filter should be able to work with different wavelengths interval, from 1 nm or less to several tens of nm. The filter should be extremely low cost for application to an access system. 
     There are several devices which utilize mutual interference between an acoustic wave and an optical beam. FIG. 1 is a perspective view of such a device, in which Ti metal is thermally diffused into an X-Y cut LiNbO 3  substrate  2  to form a channel waveguide  1 , and a flat waveguide. On the substrate  2 , this device has a waveguide lens  3  and a transducer  4  formed from a comb-tooth type electrode for exciting a surface acoustic wave (SAW). 
     In this device, a light beam is converted to a parallel light beam by the waveguide lens  3 . A SAW generates a refractive index grating from the photo acoustic effect of the SAW. The light beam is diffracted by this grating into different directions depending on the frequency of SAW. When this diffracted light beam is condensed by lens  5 , the diffracted light beam is focused to different points because the device functions as an optical deflector. 
     FIG. 2 is a top view of another example of a related art device utilizing the refractive index grating created by a SAW. This device, a collinear AO module with an inhomogeneous SAW waveguide, was present at the Photonics in Switching conference, at Sendai, Apr. 21-25, 1996. In this device, parallel optical waveguides  7 ,  8  are formed on a Y cut LiNbO 3  substrate  6  and a thin film  9  formed of Ta 2 O 5  is formed on the substrate as a SAW waveguide. In operation, even number mode light and odd number mode light are combined by the refractive index grating between the parallel optical waveguides  7 ,  8 . As before, the SAW creates the refractive index grating. A selected wavelength of a light beam incident to the optical waveguide  7  is switched to the optical waveguide  8 . The selected wavelength corresponds to the refractive index grating created by the SAW. In this device, the grating is weighted through a change in width a(z) and thickness h(z) of the thin film  9  which guides the SAW. The thin film  9  reduces a siderobe in the optical waveform. Moreover, a device in which weighting is realized by forming the SAW waveguide crossing the optical waveguide is also known. 
     FIG. 3 is a top view of an optical waveguide device which extracts a light beam having a selected wavelength and executes modulation by rotating the main axis of the waveguide refractive index for the selected wavelength to thereby rotate the polarization of the selected wavelength. The selected wavelength corresponds to the frequency of the SAW generated in the device. Optical waveguides  11 ,  12  are formed by diffusing Ti in a X cut LiNbO 3  substrate  10  and creating deeply diffused regions  14  of Ti on both sides of a region  13  for guiding a SAW generated by a SAW transducer  15 . To generate the SAW transducer (IDT)  15  is provided with a radio frequency (RF) signal. 
     The TE (transverse electric) and TM (transverse magnetic) mode beams of an incident light beam are isolated by a crossing type polarization beam splitter (crossing type PBS)  16 , and thereby the TE mode beam is incident to the optical waveguide  12 , while the TM mode beam is incident to the optical waveguide  11 . In optical waveguide  11 , the light beam of a selected wavelength corresponding to the SAW is converted from the TM mode to the TE mode through rotation of polarization. In optical waveguide  12 , the TE mode beam of the selected wavelength is converted to a TM mode beam through rotation of the polarization. 
     In this example, the TM mode beams of non-selected, non-rotated wavelength light are output from the optical waveguide  11  to the non-selected beam side via a crossing type PBS  17 , while the TE mode beam of the selected wavelength is output from waveguide  11  to the selected beam side. In optical waveguide  12 , the TE mode beams of non-selected, non-rotated light are output to the non-selected beam side and the TM mode beam of the selected wavelength is output to the selected beam side. Thereby, the selected wavelength can be extracted and modulated using this optical waveguide device. In the FIG. 3 device, absorbing bodies  19  and  20  are SAW absorbing bodies for preventing the SAW from being reflected at end faces of the substrate. 
     In the FIG. 3 device, the SAW is propagated at a higher rate in the deeply diffused region  14  of Ti than in the substrate due to the influence of Ti. The SAW is thus trapped and propagated in the region  13  where the propagation rate of the SAW between deeply diffused regions  14  of Ti is rather low. Therefore regions  14  function as a SAW waveguide. 
     In the filter and switch of the optical waveguide shown in FIG. 1 or FIG. 2, where even and odd number modes are coupled, filtering and switching can be realized independently respectively for the TE mode beam and the TM mode beam. This can occur because the SAW is generated or formed with perfect symmetry, but the propagation constants of the TE mode beam and the TM mode beam in the optical waveguide formed on the plane are generally different. 
     Moreover, coupling between two adjacent optical waveguides is believed to depend on the polarization of the TE and TM mode beams. Therefore, filter and switch characteristics also depend on the polarization. This causes a problem in a device which is required to control a light beam having a desired polarization, which requirement may be present in an optical communication system. 
     To eliminate the polarization dependency, it has been proposed to use two pairs of the FIG.  1  and FIG. 2 devices to isolate the input beam by polarization. These two paris would correspond to the desired polarization. However, it is not practical to use two pairs of devices. On the other hand, in FIG. 3, the TE mode beam is converted to a TM mode beam and the TM mode beam to a TE mode beam by isolating polarization modes. Therefore, the dependence on polarization can be eliminated. However, to prepare the device, a time as long as several tens of hours is required to form the deeply diffused regions  14  of Ti Another problem with the FIG. 3 device is that regions  14  cannot be formed to cross the optical waveguides as shown in FIG.  2 . Regions  14  cannot be formed on the optical waveguides because deeply diffused Ti erases the optical waveguides, which may also be formed by Ti diffusion. Without formation on the optical waveguides, the siderobes in the waveform cannot be reduced. Moreover, in a SAW waveguide formed from deep diffused regions  14  of Ti, the trapping force for the SAW is weak, and therefore, the SAW waveguide is ineffective. 
     These three devices demonstrate that a device which operates with TE/TM mode conversion (FIG. 3) is known and devices having a thin film (FIGS. 1 or  2 ) are known. Although these devices have been known for some time, no one has ever been able to form a thin film on a TE/TM mode conversion device, as mentioned above. 
     In order to successfully form a SAW waveguide, the sound velocity within the guide, where the acoustic waves are to propagate, must be less than that at peripheral portion thereof. For this purpose, the waveguide must intuitively be formed with a material assuring lower sound velocity. However, the speed of sound is as high as about 6000 m/s in material, such as SiO 2  and Al 2 O 3 , which is often used for a buffer layer. It is intuitively not thought to be possible to form a higher speed film on such a buffer layer material. Moreover, sound velocity in LiNbO 3  is different, to a large extent, depending on the propagation direction (about 3500 to 4000 m/s) and a material assuring the adequate sound velocity for the selected direction is required. Moreover, if a film is to be formed on the optical waveguide, the material used for thin film must be transparent and have a refractive index which is smaller than that of LiNbO 3 . 
     In addition, it is necessary to form, in the LiNbO 3  substrate, a SAW having an amplitude sufficient for executing the TE/TM mode conversion. It is considerably difficult and requires a large amount of work to design a thin film having a material, thickness and width satisfying the various conditions. Moreover, in performing TE/TM mode conversion in a substrate, when films of different materials are formed on the surface of substrate, the surface conditions change and this is feared to create problems effecting TE-TM mode conversion. Therefore, films of different materials have not yet been attempted. 
     With the reason explained above, it has been thought that a device for guiding a SAW with a thin film to perform TE/TM mode conversion cannot be realized. It is apparent that a device having a waveguide formed in the z-axis direction, as in FIG.  1  and FIG. 2, cannot realize the TE/TM mode conversion performed by the device of FIG.  3 . 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide an acousto-optic device which can add and drop light of a specific wavelength. 
     It is another object of the present invention to provide an acousto-optic device which is not dependent on polarization. 
     It is a further object of the present invention to provide an acousto-optic device which does not require a long period of diffusion to fabricate. 
     It is a still further object of the present invention to provide an acousto-optic device which propagates light having a reduced siderobe. 
     These and other objects are accomplished by providing an optical device having a substrate, a polarization beam splitter, first and second pairs of optical waveguides, a transducer and a thin film. The polarization beam splitter is formed on the substrate and has input and output sides. The first and second pairs of optical waveguides are formed on the substrate to guide polarized optical signals. The first pair of optical waveguides meets at the input side of the polarization beam splitter, and the second pair of optical waveguides meets at the output side of the polarization beam splitter. The transducer is formed of a comb-tooth electrode on the substrate to excite a surface acoustic wave on the substrate and rotate the polarization of the optical signal. The thin film covers a portion of each waveguide of either the first or second pairs of optical waveguides. The thin film may be formed of silicon dioxide or indium dioxide, either with a metal oxide optionally added thereto. The speed of sound in the thin film is less than that in the substrate. 
     This speed of sound means the speed around the film (mainly, the speed under the film). The thin film is formed of a material which is transparent to the optical signal. The thin film has a refractive index smaller than that of the optical waveguides. 
     The SAW waveguide, of the present invention which can effectively guide a SAW, can be simply formed by providing a transparent thin film of SiO 2 to a thickness of about 1 μm or less on an LiNbO 3  substrate. The thin film forms a ridge type structure in approximately the same direction as the optical waveguide formed within the substrate. This device propagates the light beam approximately in either the X or Y direction. 
     The SAW is thought to be trapped because of the additional mass on the substrate from the film, the change of surface conditions and the trapping effect of the ridge type structure at the surface of the LiNbO 3 . The SAW is not necessarily thought to be trapped by the different sound velocity in the thin film. 
     In the SAW guide formed as above, rotation of the crystal axis is sufficient for TE/TM mode conversion. 
     Moreover, by properly choosing the thickness, width and material of the film, the trapping strength of the SAW waveguide can be enhanced to a large extent compared with that of the related art device, and the strength and velocity of the SAW can be controlled to have the same distribution in both the width and traveling directions. The present invention provides wide flexibility in design and realizes sophisticated functions of TE/TM mode conversion in a SAW device. 
     With the optical waveguide device of the present invention, it is possible to realize a wavelength filter which is tunable to control the target wavelength. The present invention is indispensable and can be used as an add drop multiplexer (ADM), an optical cross connect and an optical exchange in an optical transmission system. 
     The present invention can focus on a single wavelength and act as a switch for that wavelength, thereby enabling modulation of that wavelength. Not only can the present invention simultaneously switch and modulate different selected wavelengths, but it can also function as an ASE light elimination filter to eliminate an ASE beam generated by an optical amplifier, thereby improving receiver sensitivity even after transmission over a long transmission line. 
     Because the filter of the present invention is tunable, can be mass produced, and can be produced for a low cost, the present invention is applicable to a wide range of optical transmission and processing systems. The present invention can enable combining and branching filters, provide switching and modulating functions, and improve wavelength characteristics. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described in more detail in connection with the attached drawings wherein: 
     FIG. 1 is a perspective view of a first optical device of the related art; 
     FIG. 2 is a perspective view of a second optical device of the related art; 
     FIG. 3 is a top view of a third optical device of the related art; 
     FIG. 4 a  is a top view of an optical device according to a first preferred embodiment of the present invention; 
     FIG. 4 b  is a cross-sectional view of the device of FIG. 4 a,  taken along line IVB-IVB in FIG. 4 a;    
     FIG. 5 is a representation of a waveform resulting from the device shown in FIGS. 4 a  and  4   b;    
     FIG. 6 shows a plot of a SAW intensity distribution across a width of the thin film shown in FIGS. 4 a  and  4   b;    
     FIG. 7 is a perspective view of an optical device according to a second preferred embodiment of the present invention; 
     FIG. 8 is a perspective view of an optical device according to a third preferred embodiment of the present invention; 
     FIG. 9 is a top view of an optical device according to a fourth preferred embodiment of the present invention; 
     FIG. 10 is a representation of a waveform resulting from the device shown in FIG. 9; 
     FIG. 11 is a top view of a first modification to the device shown in FIG. 9; 
     FIG. 12 is a top view of a second modification to the device shown in FIG. 9; 
     FIG. 13 a  is a top view of a third modification to the device shown in FIG. 9; 
     FIG. 13 b  is a cross-sectional view of the device shown in FIG. 13 a  taken through line XIIIB—XIIIB in FIG. 13 a.    
     FIG. 14 a  is a top view of an alternative to the device shown in FIGS. 13 a  and  13   b;    
     FIG. 14 b  is a cross-sectional view taken along line XIVB—XIVB in FIG. 14 a;    
     FIG. 15 is a perspective view of an optical device according to a fifth preferred embodiment of the present invention; 
     FIG. 16 is a top view of an optical device according to a sixth preferred embodiment of the present invention; 
     FIG. 17 is a chart showing how film hardness and weight effect SAW propagation rate; 
     FIG. 18 a  is a cross sectional view of the device shown in FIG. 2 taken through an optical waveguide, annotated to show power distribution of a SAW therein; 
     FIG. 18 b  is a cross sectional view of the device shown in FIGS. 4 a  and  4   b  taken though an optical waveguide, annotated to show power distribution of a SAW therein; and 
     FIG. 19 is a top view of an optical device according to a seventh preferred embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The preferred embodiments of the present invention will be explained with reference to the accompanying drawings in which like reference indicators represent like elements. 
     FIG. 4 a  is a top view of an optical device according to a first preferred embodiment of the present invention. Optical waveguides  11 ,  12  are formed extending in the y axis direction on an X-cut LiNbO 3  substrate  10 . A transducer  15  formed of a comb-tooth electrode is formed on the substrate  10  to generate a SAW. 
     A SAW absorbing body  19  formed of a resist is formed between the optical input side of the LiNbO 3  substrate  10  and the transducer  15 . Moreover, toward an optical output side of the substrate  10  from the substrate  15 , a thin film  18  is formed on the substrate to serve as a SAW waveguide. The thin film  18  is an SiO 2  film which is transparent optically and has a refractive index lower than that of the LiNbO 3  substrate  10 . In addition, a second SAW absorbing body  20  formed of a resist is provided between an optical output side of the substrate  10  and the thin film  18 . 
     A light beam incident on an upper side waveguide (shown toward the top in this figure) is isolated into a TE mode beam and a TM mode beam by the crossing type PBS  16 . These beams are propagated in parallel through the parallel waveguides formed under the thin film  18 , and then these beams are combined again at the crossing PBS  17 . From there, non-selected wavelengths are emitted from an upper side output waveguide  11 ′. A selected wavelength corresponding to the frequency of the SAW is subjected to mode conversion within the waveguides  11 ,  12  and is extracted from a lower side output waveguide  12 ′. 
     The waveguides  11 ,  11 ′,  12  and  12 ′ are respectively formed by the thermal diffusion of a Ti metal film at 1000° C. for 10 hours. The diffusion is done to provide waveguides having a depth of about 0.1 μm and a width of about 7 μm, for example. On the other hand, the transducer  15  is formed of 15 pairs of comb-tooth electrodes having a length of 140 μm and a period of about 20 μm. When a high frequency (perhaps radio frequency) power of about 175 MHz is applied to the transducer electrodes, a SAW is generated in the surface of the LiNbO 3  substrate  10  and the light beam having a wavelength of about 1.54 μm corresponding to the 175 MHz SAW frequency is subjected to TE/TM mode conversion. 
     The SAW generated by the transducer  15  would be immediately diffused if a device for guiding the SAW was not provided. The thin film  18  acting as a SAW waveguide is formed of an SiO 2  film. The thin film  18  has a width of 120 μm, a thickness of 0.5 μm and a length of 30 mm. FIG. 4 b  is a cross-sectional view of the device shown in FIG. 4 a,  taken along line IVb-IVB in FIG. 4 a.    
     FIG. 5 is a representation of a waveform resulting from the device shown in FIGS. 4 a  and  b.  The waveform shown in FIG. 5 is that from optical waveguide  12 ′. FIG. 5 shows a half width value of 0.82 μm and this corresponds with the result obtained from a theoretical calculation. 
     Moreover, when the thin film  18  is positioned as shown to evaluate the half width value and the power, no attenuation of the SAW was observed in the thin film  18  used for a SAW waveguide. 
     FIG. 6 shows a plot of SAW intensity distribution across a width of the thin film shown in FIGS. 4 a  and  4   b.  For FIG. 6, the thin film  18  had a width of 180 μm. 
     As can be seen, the intensity shifted from center, little by little, and a typical Gaussian distribution was observed. FIG. 6 confirms that the typical SAW of the basic mode is propagated, and that the thin film  18  functions as a SAW waveguide. Evaluation of many samples has confirmed that these samples operate as the waveguides with widths from 30 to 250 μm. 
     When indium oxide is added to SiO 2  for the material of the thin film  18 , the trapping effect for the SAW is increased. An excellent SAW waveguide can also be formed when 100% indium oxide is used as the thin film material. However, in this case some of the light beam is absorbed by the thin film  18 . The same effect can also be obtained when a metal oxide is added to a thin film formed mainly of indium oxide. It appears that a wide range of materials can be used for the thin film  18  because the trapping effect is caused by the additional mass on the substrate, the change in surface conditions from the film and the SAW trapping effect of the film itself (perhaps caused by the ridge type structure at the surface of the LiNbO 3  substrate  10 ). That is, the trapping effect is not necessarily the result of a different sound velocity in the thin film. 
     FIG. 17 is a chart showing how film hardness and weight effect SAW propagation rate. As can be seen, when the thin film having a ridge shape is harder, the propagation rate is faster. When the thin film has a reduced mass (weight), the SAW propagation rate is faster. 
     FIG. 18 a  is a cross sectional view of the device shown in FIG. 2 taken through an optical waveguide, annotated to show power distribution of a SAW therein. FIG. 18 b  is a cross sectional view of the device shown in FIGS. 4 a  and  4   b  taken though an optical waveguide, annotated to show power distribution of a SAW therein. As can be seen in FIG. 18 a,  the SAW is propagated in the thin film  9  itself due to the hardness of the thin film  9 . As can be seen in FIG. 18 b,  the SAW is propagated in the substrate, and this is thought to be the result of the differing mass and surface conditions. Based on FIG. 18 b,  it appears that the SAW is trapped within the substrate, and this fact has been shown experimentally. 
     In the first preferred embodiment shown in FIG. 1, an X cut LiNbO 3  substrate is used because it provides the maximum SAW excitation efficiency. However, a Z cut LiNbO 3  substrate can also generate the TE/TM mode conversion. When a Z cut LiNbO 3  substrate is used, TE/TM mode conversion is generated on either the X axis or Y axis, in the direction of light beam propagating through the optical waveguide. 
     Second Preferred Embodiment 
     FIG. 7 is a perspective view of an optical device according to a second preferred embodiment of the present invention. In FIG. 7, a transparent film  22  having a refractive index smaller than that of the optical waveguides  11 ,  12  is formed on a portion of the LiNbO 3  substrate to cover a portion of the optical waveguides  11 ,  12 . That is, the thin film  22  covers the portion of the substrate toward an output and, from the transducer  15 . A metal film  21  is formed on a portion of the transparent film  22 , in a shape of a SAW waveguide. The transparent thin film  22  may be an SiO 2  film having a thickness of 0.4 μm. The metal film  21  may be a gold metal film having a thickness of 0.3 μm. With the device shown in FIG. 7, a high quality SAW waveguide is obtained. 
     The metal film  21  has a lower sound velocity than the LiNbO 3  substrate and provides additional mass on the substrate. Referring to FIG. 4 b,  a ridge structure is formed with the thin film  18 . In the second preferred embodiment, there is no ridge structure in the thin film  22 , and thus, there is no uneven distortion caused by stress between the LiNbO 3  substrate  10  and the thin film  22  at an edge portion of the thin film  22 . That is, in FIG. 7, thin film  22  extends to the edges (substantially the entire width) of the substrate  10 . 
     Moreover, gold may be used as the metal film  21 , and gold is rather soft. Therefore, if gold is used, little stress is caused. As mentioned above, when the thin film is patterned as shown in FIGS. 4 a  and  4   b,  there is stress at an edge portion thereof. Patterning can also decrease the trapping efficiency, and where no patterning is used, both of these problems are alleviated. 
     The Third Preferred Embodiment 
     FIG. 8 is a perspective view of an optical device according to a third preferred embodiment of the present invention. In the third preferred embodiment, like the second preferred embodiment, the transparent thin film  22  does not have a ridge structure to thereby avoid stress generating from patterning within film  22 . However, a thin film portion  23  having an increased thickness is allowed to remain. The thickness of the thin film  22  is 0.3 μm and the height of the thin film portion  23  above the thin film  22  is 0.5 μm. It has been confirmed that the device shown in FIG. 8 serves as an excellent SAW waveguide. 
     The Fourth Preferred Embodiment 
     FIG. 9 is a top view of an optical device according to a fourth preferred embodiment of the present invention. In FIG.  9 . the thin film  18  is formed to cross the optical waveguides  11 ,  12 . For the material of the thin film  18 , 60 wt % InSn with SiO 2  added was used. The width of the thin film  18  was 120 μm. The thickness of the thin film was 0.4 μm and the angle with respect to the waveguides  11 ,  12  was 0.5 degrees. With the device shown in FIG. 9, TE/TM mode conversion effectuates a Gaussian type weighting in the traveling direction of the optical waveguide to realize a reduced siderobe in the resulting waveform. FIG. 10 is a representation of the waveform resulting from the device shown in FIG.  9 . The weighting of the FIG. 9 device is only possible with the thin film  18  of the present invention. 
     FIG. 11 shows a first modification to the device shown in FIG.  9 . As can be seen in FIG. 11, the width of a thin film  27  acting as a SAW waveguide is changed to be greater at a middle portion than at end portions. Alternatively, the thickness can be varied. The device shown in FIG. 11 also exhibits an excellent intensity weighing following a Gaussian distribution. 
     FIG. 12 is a top view of a second modification to the device shown in FIG.  9 . In FIG. 12, a thin film  26  forming a SAW waveguide has a curved center line, whereas the device shown in FIG. 9 has a straight center line. The device shown in FIG. 12 exhibits a better SAW intensity distribution than the device shown in FIG.  9 . 
     FIG. 13 a  shows a third modification of the device shown in FIG.  9 . FIG. 13 b  is a cross-sectional view of the device shown in FIG. 13 a  taken through line XIIIB—XIIIB in FIG.  13 A. In FIG. 13 a,  a SAW waveguide is realized by having a multilevel ridge  25 . In the multilevel ridge  25 , the lowest layer is longest and is a transparent thin film. A metal layer (different material than the lowest layer) is patterned on the lowest layer. The metal layer may be the top layer, having a length less than that of an intermediate transparent thin film, which in turn has a length less than that of the lowest layer. The lowest and intermediate layers may be formed at the same time. 
     FIG. 14 a  is a top view of an alternative device to the device shown in FIGS. 13 a  and  13   b.  FIG. 14 b  is a cross-sectional view taken along line XIVB—XIVB in FIG. 14 a.  In FIGS. 14 a  and  14   b,  the thin film has a multilevel ridge  28 , similar to the multilevel ridge  25  shown in FIGS. 13 a  and  13   b.  However, in FIGS. 14 a  and  14   b,  the upper part of the multilevel ridge  28  has a rhombus shape. That is, the metal film has a center portion with a width decreasing extending away from the center portion, in a length direction. The upper part may be formed of metal (material different than the lower part). The lower part may have a square shape. In the travelling direction, the upper part is not as long as the lower part. With the device shown in FIGS. 14 a  and  14   b,  an excellent SAW intensity distribution can be realized. 
     Fifth Preferred Embodiment 
     FIG. 15 is a perspective view of an optical device according to a fifth preferred embodiment of the present invention. In the device shown in FIG. 15, two thin films  24 ,  25  cross at a center of the LiNbO 3  substrate. Two transducer units  15 - 1 ,  15 - 2  are provided in the vicinity of the thin films  24 ,  25 . The thin films  25 ,  25  respectively guide the SAWs produced by the transducer units  15 - 1 ,  15 - 2 . The device shown in FIG. 15 can generate a SAW with a high efficiency, over a wide range of frequencies, and can realize small changes in wavelength. 
     In the device shown in FIG. 15, the center areas of the thin films  24 ,  25  overlap to form a single SAW waveguide, and the ends branch to form a plurality of SAW waveguides. The overlapping area of the thin films  24 ,  25  generates a large loss in SAW when the waveguides are combined, but a characteristic not depending on the incident direction can be realized at the center area. Moreover, in the case of FIG. 15, since the waveguide has the symmetrical structure, the light beams travelling in the vicinity of the branched ends are equally influenced by the SAW. Moreover, in this fifth preferred embodiment, the transducer is formed of two transducer units, and this can alleviate heat generation and mutual interference between SAWs. Such structure can easily be realized using the SAW waveguide of the present invention. 
     Sixth Preferred Embodiment 
     FIG. 16 is a top view of an optical device according to a sixth preferred embodiment of the present invention. FIG. 16 shows an embodiment where the SAW waveguide (thin film  18 ) is formed on only one optical waveguide  11 . This structure can rotate the polarization of an optical signal guided by the optical waveguide  11  and perform TE/TM mode conversion. The thin film  18  guides and traps the SAW. The thin film  18  may be configured as shown in any one of FIGS. 7-9, and  11 - 15 . 
     Seventh Preferred Embodiment 
     FIG. 19 is a top view of an optical device according to a seventh preferred embodiment of the present invention. In FIG. 19, a waveform multiplexed optical signal transmitted from an optical fiber  31  as the transmission line is input, a SAW corresponding to a selected wavelength λ of the optical signal is generated by the transducer  15 , and the SAW generated by the transducer  15  is trapped in the substrate by the thin film  18 . With the SAW, the polarization components for the selected wavelength λ are rotated in the optical waveguides. Thereby, the selected wavelength λ can be extracted from the optical fiber  32 . The device shown in FIG. 19 can also be used provide a modulated signal of the selected wavelength λ at optical fiber  32 . Modulation is done by intermittently outputting the SAW from the transducer  15 . 
     According to the preferred embodiments of the present invention, a transducer excites a surface acoustic wave using the piezo effect. An optical waveguide propagates a light beam almost in the X or Y direction and is formed in the surface of an LiNbO 3  substrate. A waveguide is formed on the substrate for guiding the surface acoustic wave. The axial direction of the elliptic body having the refractive index is inclined by the surface acoustic wave. The device can rotate polarization and perform TE/TM mode conversion. An effective SAW waveguide can be realized by forming a thin film on the LiNbO 3  substrate. There is a wide degree of freedom in controlling filter characteristics. Filter characteristics can be controlled by varying film material, width, thickness and shape, using a multilevel ridge and/or using a pair of crossing thin films. 
     While the invention has been described in connection with the preferred embodiments, it will be understood that modifications within the principles outlined above will be evident to those skilled in the art. Thus, the invention is not limited to the preferred embodiments, but is intended to encompass such modifications.