Patent Publication Number: US-2013243366-A1

Title: Optical element and method of manufacture of optical element

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field 
     This invention relates to an optical element in which an optical fiber and a waveguide are coupled to each other, and to a method of manufacture of an optical element. 
     2. Background Art 
     In recent years, techniques have been developed for wavelength conversion using quasi-phase matching. Quasi-phase matching is performed using an element in which a polarization inversion structure is formed periodically in a ferroelectric crystal. Quasi-phase matching is performed by, for example, imparting a polarization inversion structure to a waveguide. A waveguide having a quasi-phase matching function has, for example, a ridge type structure. 
     For example, Japanese Patent Application Laid-open No. 2003-140214 discloses the following method of manufacture of a waveguide. First, a ferroelectric crystal having a polarization inversion structure is directly joined to a substrate. Then, a groove is formed on the periphery of the portion of the ferroelectric crystal which is to become the waveguide. By this means, a ridge type waveguide is fabricated. 
     Japanese Patent Application Laid-open No. 2011-75604 discloses the following method of manufacture of a waveguide. First, a ferroelectric crystal having a polarization inversion structure is joined to a substrate using an adhesive layer. Then, a groove is formed on the periphery of the portion of the ferroelectric crystal, which is to become the waveguide. By this means, a ridge type waveguide is fabricated. 
     Light incident on the waveguide is guided to the waveguide using an optical fiber. Hence it is necessary to join the optical fiber and the waveguide. When joining an optical fiber and a waveguide, it is desirable that the task efficiency when determining the relative positions of the optical fiber and waveguide be high. 
     The present invention is directed to overcoming or at least reducing the effects of one or more of the problems set forth above. 
     SUMMARY OF THE INVENTION 
     This invention was devised in the light of the above-mentioned circumstances. It provides an optical element and a method of manufacture of an optical element which enables easy determination of the relative positions of an optical fiber and a waveguide. 
     An optical element of this invention comprises an optical fiber and a ridge type waveguide having a convex-shaped cross-section. A waveguide mounting portion is formed in a portion of the optical fiber. The waveguide mounting portion is formed by cutting away a portion of the optical fiber in a direction of extension of the optical fiber at a cross-section passing through a core of the optical fiber. A first concave portion is formed in the waveguide mounting portion. The first concave portion is formed by removing the core of the optical fiber. The ridge portion of the waveguide is inserted into the first concave portion. 
     The following is a method of manufacture of an optical element of this invention. First, by cutting away an end face of an optical fiber in a direction of extension of the optical fiber at a cross-section passing through the core of the optical fiber, a waveguide mounting portion is formed. Next, by removing the exposed optical fiber core in the waveguide mounting portion, a concave portion is formed. Next, a ridge portion of a ridge type waveguide, having a convex-shaped cross-section, is inserted into the concave portion, and positioning between the optical fiber and the waveguide is performed. 
     By means of this invention, when joining an optical fiber and a waveguide, the relative positions of the optical fiber and waveguide can be determined easily. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing advantages and features of the invention will become apparent upon reference to the following detailed description and the accompanying drawings, of which: 
       The above-described object, as well as other objects, features and advantages will become clear from the preferred embodiments described below, and from the attached drawings. 
         FIG. 1  is a cross-sectional view showing the configuration of the optical element of a first embodiment; 
         FIG. 2  is a cross-sectional view showing the configuration of the optical element of the first embodiment; 
         FIG. 3  is a plane view of the optical element shown in  FIG. 1  and  FIG. 2 ; 
         FIGS. 4A to 4C  show cross-sectional views of a first example of a method of manufacture of a waveguide member; 
         FIGS. 5(   a ) to  5 ( c ) show cross-sectional views of a second example of a method of manufacture of a waveguide member; 
         FIGS. 6A and 6B  explain a method of manufacture of the optical element shown in  FIG. 1  to  FIG. 3 ; 
         FIG. 7  explains a method of manufacture of the optical element shown in  FIG. 1  to  FIG. 3 ; 
         FIGS. 8A and 8B  explain a method of manufacture of the optical element shown in  FIG. 1  to  FIG. 3 ; 
         FIGS. 9A and 9B  explain a method of manufacture of the optical element shown in  FIG. 1  to  FIG. 3 ; 
         FIG. 10  explains a method of manufacture of the optical element shown in  FIG. 1  to  FIG. 3 ; and 
         FIG. 11  is a cross-sectional view showing the configuration of the optical element of a second embodiment. 
     
    
    
     DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS 
     Below, embodiments of the invention are explained using the drawings. In all of the drawings, the same constituent elements are assigned the same symbols, and explanations are omitted as appropriate. 
     First Embodiment 
       FIG. 1  and  FIG. 2  are cross-sectional views showing the configuration of the optical element of a first embodiment.  FIG. 3  is a plane view of the optical element shown in  FIG. 1  and  FIG. 2 .  FIG. 1  is a cross-sectional view at A-A′ in  FIG. 3 , and  FIG. 2  is a cross-sectional view at B-B′ in  FIG. 3 . 
     This optical element comprises optical fiber  100  and ridge type waveguide  220 . Waveguide mounting portion  102  is formed in one portion of optical fiber  100 . Waveguide mounting portion  102  is formed by cutting away a portion of optical fiber  100  in the direction of extension (the right-left direction in the figure) of optical fiber  100  at a cross-section passing through core  120  of optical fiber  100 . First concave portion  122  ( FIG. 2 ) is formed in waveguide mounting portion  102 . First concave portion  122  is formed by removing core  120  of optical fiber  100 . As shown in  FIG. 2 , ridge type waveguide  220  is inserted into first concave portion  122 . Waveguide  220  has a convex-shaped cross-section. Waveguide  220  is, for example, formed by stacking two layers with different refractive indices, and forming grooves on both sides of the guiding portion of one of the layers. In this optical element, optical fiber  100  causes light to be incident on waveguide  220  in some cases (incidence portion), and guides light emitted from waveguide  220  to the outside in some cases (emission portion). Below, the optical element is explained in detail. 
     As shown in  FIG. 1  to  FIG. 3 , the end portion of optical fiber  100  is exposed from covering film  130 . Waveguide mounting portion  102  is provided at this exposed portion. Specifically, waveguide mounting portion  102  is provided at the end portion of optical fiber  100 . Waveguide mounting portion  102  is formed by cutting away the end portion of waveguide mounting portion  102  in the direction of extension of optical fiber  100  at a cross-section passing through core  120 . On waveguide mounting portion  102 , concave portion  104  is formed in the portion opposing the end portion of waveguide member  200 . The role of concave portion  104  is explained when explaining a method of manufacture of the optical element. 
     Core  120  of optical fiber  100  has a refractive index different from that of the periphery due to the addition of an additive (for example, Ge). Because an additive is added to core  120 , the etching selection ratio is different, under specific etching conditions, than for other portions of optical fiber  100 . 
     Waveguide member  200  has a structure in which waveguide  220  is provided on ridge formation face  202  of substrate  210 . The cross-sectional shape of ridge type waveguide  220  is, for example, square (rectangular), but may be semicircular or trapezoidal. Substrate  210  is formed from a material with a refractive index lower than that of waveguide  220 , such as for example LiNbO 3  in a fixed ratio (stoichiometric composition). As shown in  FIG. 2 , the width of substrate  210  is wider than the diameter of optical fiber  100 . Waveguide  220  is formed from a ferroelectric crystal. However, waveguide  220  may be formed from another material, such as quartz glass, silicon, or a compound semiconductor. The width of waveguide  220  is smaller than the diameter of core  120 . On substrate  210 , concave portions  212  are formed on both sides of waveguide  220 . Concave portions  212  extend along waveguide  220 . In plane view, the side faces of concave portions  212  on the sides opposite waveguide  220  are positioned further outside than optical fiber  100 . Consequently, substrate  210  does not make contact with optical fiber  100 . 
     The ferroelectric crystal forming waveguide  220  has a periodic polarization inversion structure. Consequently the optical element of this embodiment functions as a wavelength-converting device. The ferroelectric crystal forming waveguide  220  is, for example, LiNbO 3  with Mg added, but other materials may be used. 
     As shown in  FIG. 2 , optical fiber  100  and waveguide member  200  are mutually fixed by fixing member  300 . Fixing member  300  has second concave portion  304  in fixing face  302 , which holds the fiber. Second concave portion  304  is a groove, which is V-shaped in cross-section, and optical fiber  100  is inserted into second concave portion  304 . The cross-sectional shape of second concave portion  304  is an isosceles triangle, such as, for example, a right isosceles triangle. However, the cross-sectional shape of second concave portion  304  is not limited to such shapes. Of fixing face  302 , the portions positioned on both sides of second concave portion  304  are joined with ridge formation face  202  of substrate  210 . Fixing member  300  is for example quartz glass, but a ceramic or resin may also be used. 
     Further, as shown in  FIG. 1  and  FIG. 3 , the optical element comprises pressing member  400 . Pressing member  400  together with fixing member  300  sandwiches and holds optical fiber  100 . Pressing member  400  is formed from material similar to that of fixing member  300 . 
       FIG. 4  shows cross-sectional views of a first example of a method of manufacture of waveguide member  200 . First, a polarization inversion structure is formed in a ferroelectric crystal  222 . Next, as shown in  FIG. 4A , ferroelectric crystal  222  is fixed on substrate  210 . This fixing method is, for example, direct joining. In this case, heating is applied in a state in which the ferroelectric crystal  222  is pressed against substrate  210 . Substrate  210  and ferroelectric crystal  222  may also be fixed using an adhesive. In this case, after applying the adhesive to the face of ferroelectric crystal  222  which is to be joined to substrate  210 , ferroelectric crystal  222  is pressed against substrate  210 . In place of an adhesive, a low-melting point glass may be used. 
     Next, as shown in  FIG. 4B , the thickness of ferroelectric crystal  222  is reduced to the required thickness. The method for reducing the thickness of ferroelectric crystal  222  may be mechanical polishing, or may be dry etching, or a method may be used in which ferroelectric crystal  222  is cut from a side face using a dicing saw. Faces of ferroelectric crystal  222  which are to be coupled with other optical members (for example, optical fiber  100 ) are mirror-polished. 
     Next, as shown in  FIG. 4C , concave portions  212  are formed using a dicing saw and dry etching. By this means, waveguide  220  is formed. 
       FIG. 5  shows cross-sectional views of a second example of a method of manufacture of waveguide member  200 . First, as shown in  FIG. 5A , ferroelectric crystal  222  is prepared. Then, a polarization inversion structure is formed in ferroelectric crystal  222 . 
     Next, as shown in  FIG. 5B , the refractive index is changed in a region of ferroelectric crystal  222  which is to become substrate  210 . By this means, substrate  210  is formed. Substrate  210  is formed by, for example, subjecting ferroelectric crystal  222  to proton exchange treatment. Proton exchange treatment is performed by, for example, annealing ferroelectric crystal  222  in a state in which the face of ferroelectric crystal  222  which is to become substrate  210  is held in contact with benzoic acid or another acid. 
     Next, as shown in  FIG. 5C , concave portions  212  are formed using a dicing saw and dry etching. By this means waveguide  220  is formed. 
       FIG. 6  to  FIG. 10  explain a method of manufacture of the optical element shown in  FIG. 1  to  FIG. 3 . Of these,  FIG. 6A ,  FIG. 6B ,  FIG. 7 ,  FIG. 8A , and  FIG. 9A  correspond to the cross-section at A-A′ in  FIG. 3 .  FIG. 8B  and  FIG. 9B  correspond to the cross-section at B-B′ in  FIG. 3 .  FIG. 10  is a plane view of optical fiber  100 , fixing member  300  and pressing member  400  shown in  FIG. 9 . 
     As shown in  FIG. 6A , covering film  130  is removed from an end portion of optical fiber  100 . The end portion of optical fiber  100  is inserted into second concave portion  304  (see  FIG. 2 ) of fixing member  300 , and then pressing member  400  is fixed against fixing member  300 . By this means optical fiber  100  is fixed between fixing member  300  and pressing member  400 . In this state, the end of optical fiber  100  protrudes from between fixing member  300  and pressing member  400 . 
     Next, as shown in  FIG. 6B , the portion of optical fiber  100  which protrudes from between fixing member  300  and pressing member  400  is removed by polishing or similar means. By this means, the end face of optical fiber  100 , the end face of fixing member  300 , and the end face of pressing member  400  are flush. 
     Next, as shown in  FIG. 7 , a dicing saw is introduced from pressing member  400  into optical fiber  100  in a direction perpendicular to the direction of extension of optical fiber  100 . By this means, concave portion  104  is formed. The bottom portion of concave portion  104  is positioned within optical fiber  100  and lower than core  120 . It is preferable that the abrasive of the dicing saw used to form concave portion  104  be sufficiently fine that the side faces of concave portion  104  are mirror surfaces. 
     Next, as shown in  FIG. 8A  and  FIG. 8B , the portion positioned on the end portion side of concave portion  104  among the upper half of optical fiber  100  and pressing member  400  is removed by dicing and polishing. By this means, waveguide mounting portion  102  is formed. Waveguide mounting portion  102  has a planar shape. However, in this stage, a portion of core  120 , for example approximately half, remains. 
     Next, as shown in  FIG. 9A  and  FIG. 9B  and in the plane view of  FIG. 10 , core  120  is removed by etching. By this means, first concave portion  122  is formed. The etching liquid used contains, for example, HF. However, core  120  may be removed by dry etching. 
     Thereafter, waveguide member  200  is placed on waveguide mounting portion  102 . At this time, in a state in which waveguide  220  of waveguide member  200  is inserted into first concave portion  122 , the angle of waveguide member  200  with respect to optical fiber  100  is adjusted, and the optical axes of waveguide  220  and optical fiber  100  are made to coincide. At this time, the end face of substrate  210  may be brought into contact with the face of optical fiber  100 , which was a side face of concave portion  104 . Then, ridge formation face  202  of substrate  210  and fixing face  302  of fixing member  300  are fixed using adhesive. In this way, the optical element shown in  FIG. 1  to  FIG. 3  is formed. 
     In the above embodiment, by removing the core  120  of optical fiber  100 , first concave portion  122  is formed. And, by inserting waveguide  220  of waveguide member  200  into first concave portion  122 , the relative positions of optical fiber  100  and waveguide member  200  are adjusted. Hence the relative positions of optical fiber  100  and waveguide  220  can easily be determined. When positioning waveguide  220 , damage to ridge type waveguide  220  can be suppressed. Further, optical element manufacturing processes do not become complex. 
     Optical fiber  100  is inserted into second concave portion  304  formed in fixing face  302  of fixing member  300 . Further, ridge formation face  202  of waveguide member  200  is fixed on fixing face  302  of fixing member  300 . Hence after fabrication of the optical element of this embodiment, application of force to waveguide  220  of waveguide member  200  and damage to waveguide  220  can be suppressed. 
     Second Embodiment 
       FIG. 11  is a cross-sectional view showing the optical element of a second embodiment, and corresponds to  FIG. 2  (B-B′ cross-section) in the first embodiment. The optical element of this embodiment has a configuration in which a plurality of optical fibers  100  is connected to different waveguides  220 . 
     The plurality of waveguides  220  is formed in one waveguide member  200 . The structure and method of manufacture of each of waveguides  220  are as described in the first embodiment. 
     The plurality of optical fibers  100  are held by a single fixing member  300 . In fixing face  302  of fixing member  300  are formed a plurality of second concave portions  304 . Into each of the plurality of second concave portions  304  is inserted an optical fiber  100 . 
     In this embodiment also, advantageous results similar to those of the first embodiment can be obtained. Further, optical fibers  100  and waveguides  220  can be configured in an array easily and inexpensively. Further, upon configuration in an array, damage to ridge type waveguides  220  can be suppressed. 
     Example 
     Waveguide member  200  was fabricated using the method shown in  FIG. 5 . LiNbO 3  with Mg added was used in waveguide  220 , and quartz glass was used in substrate  210 . Concave portions  212  were formed by dicing. A polarization inversion structure was formed in waveguide  220 . This polarization inversion structure was provided with a period to perform wavelength conversion by SHG (second harmonic generation) of infrared light (wavelength 1064 nm). 
     A single mode optical fiber was used as optical fiber  100 . More specifically, as optical fiber  100 , a polarization maintaining optical fiber with a cutoff wavelength of 980 nm was used. First concave portion  122  was formed by wetting optical fiber  100  for 15 minutes with a 10% HF aqueous solution. 
     Further, an ultraviolet light-hardening adhesive was used to fix waveguide member  200  and fixing member  300 . 
     An optical element formed in this way satisfactorily performed wavelength conversion of infrared light by means of SHG. Hence this optical element demonstrated that use is possible as a wavelength conversion device for a laser light source device. 
     In the above, embodiments of the invention have been explained referring to the drawings, but the embodiments are merely examples of the invention, and various configurations other than the above can be adopted. 
     Thus, an optical element in which an optical fiber and a waveguide are coupled to each other, and a method of manufacture of an optical element have been described according to the present invention. Many modifications and variations may be made to the techniques and structures described and illustrated herein without departing from the spirit and scope of the invention. Accordingly, it should be understood that the methods and devices described herein are illustrative only and are not limiting upon the scope of the invention. 
     This application claims priority on the basis of Japanese Patent Application No. 2011-221858, filed on 6 Oct. 2011, the entire disclosure of which is herein incorporated by reference.