Patent Abstract:
A wavelength conversion device that converts a wavelength by second harmonic-wave generation and generates a laser beam, includes: a substrate having a plurality of electrodes; a semiconductor laser device mounted on the substrate and electrically connected to the plurality of electrodes; and a nonlinear optical element having an optical waveguide for guiding a laser beam emitted from the semiconductor laser device and for converting a wavelength of the laser beam. Here, the nonlinear optical element is mounted on the substrate so that the optical waveguide in the nonlinear optical element is located away from the center line of the substrate. Thereby, a small wavelength conversion device provided with a semiconductor laser device and a nonlinear optical element, which are mounted on the substrate in an integrated manner, can be obtained, and therefore an optical pickup unit in the optical disk employing this wavelength conversion device can be miniaturized.

Full Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a wavelength conversion device composed of a semiconductor laser device and a nonlinear optical element having a planar optical waveguide, which are mounted on a substrate in an integrated manner. 
     2. Related Background Art 
     In order to realize a high-density optical disk and a high-definition display, small short-wavelength light sources generating a laser beam in a blue range through violet range are desired. Techniques for obtaining a laser beam in this wavelength range include a second harmonic-wave generation method (hereinafter referred to as “SHG”) that employs a wavelength conversion device using a planar optical waveguide according to a quasi-phase-matching method, by which a wavelength of a semiconductor laser can be converted from 850 nm into 425 nm. 
     To miniaturize a short-wavelength light source according to this method, it is effective to mount a wavelength conversion device and a semiconductor laser device on a substrate in an integrated manner. 
     FIG. 15 shows one example of these small short-wavelength light sources, which is a wavelength conversion device disclosed in JP 2000-284135 A. 
     A semiconductor laser device  306  and a planar optical waveguide device  305  are mounted on a silicon substrate  300  in an integrated manner. The planar optical waveguide device  305  functions as a wavelength conversion device structured by forming a proton exchange planar optical waveguide  304  and a diffraction grating (not illustrated) with periodic domain inverted regions formed therein on an Mg doped LiNbO 3  substrate  302 . In addition, on the silicon substrate  300 , electrodes  307  electrically connected to the semiconductor laser device  306  are formed, and alignment keys  301  are formed at positions 10 μm away from the planar optical waveguide  304 . On each side of the planar optical waveguide  304 , alignment keys  303  are formed using a film made of the same material (e.g., Ta) and having the same thickness as those of the alignment keys  301 . Further, alignment keys  308  are formed on the semiconductor laser device  306  as well. 
     Here, the semiconductor laser device  306  is a Distributed Bragg Reflector (hereinafter, referred to as “DBR”) type semiconductor laser device. As shown in FIG. 15, the electrodes  307  are connected to each of a gain region and a wavelength control region, i.e., a DBR region (not illustrated) of the semiconductor laser device  306 . 
     The semiconductor laser device  306  and the planar optical waveguide device  305  are mounted onto the silicon substrate  300  in such a manner that a laser beam emitted from the semiconductor laser device  306  is guided through the optical waveguide  304  in the planar optical waveguide device  305 , and the alignment keys  308 ,  303 , and  301  are arranged at their predetermined positions. In this wavelength conversion device, a center line M 1 -M 2  of the silicon substrate  300 , a center line M 5 -M 6  of the semiconductor laser device  306 , and a center line M 3 -M 4  of the planar optical waveguide device  305  approximately coincide with one another. 
     In the future, to miniaturize an optical information processing system employing an optical disk and a display still more than present ones, an optical pick up unit included in an optical disk or the like needs to be made small. To this end, it becomes effective to make a wavelength conversion device smaller. 
     Meanwhile, when electrically driving such a wavelength conversion device, an oscillation wavelength of a laser beam emitted from the semiconductor laser device  306  needs to be controlled so as to maximize a conversion efficiency of the laser beam by the SHG. 
     In the wavelength conversion device shown in FIG. 15, by controlling a current applied to the electrode connected to the DBR region, among the electrodes  307 , a refractive index of the DBR region is varied so as to change a Bragg wavelength, whereby the oscillation wavelength is controlled. In this wavelength conversion device, however, a phase of the emitted laser beam cannot be controlled, because the semiconductor laser device  306  consists of only two regions, i.e., the gain region and the DBR region. Due to such a constraint, if a Bragg wavelength in the DBR region is varied by the passage of the electric current, then a so-called mode hoping would occur, where the oscillation wavelength changes discontinuously. In this case, it becomes difficult to control the oscillation wavelength, which might interfere with the operation of the device significantly. 
     SUMMARY OF THE INVENTION 
     Therefore, with the foregoing in mind, it is an object of the present invention to provide a small short-wavelength conversion device composed of a semiconductor laser device and an optical waveguide device, which are mounted on a substrate in an integrated manner. 
     To fulfill the above-stated object, a wavelength conversion device according to an embodiment of the present invention, which converts a wavelength by second harmonic-wave generation and generates a laser beam, includes: a substrate having a plurality of electrodes; a semiconductor laser device mounted on the substrate and electrically connected to the plurality of electrodes; and a nonlinear optical element having an optical waveguide for guiding a laser beam emitted from the semiconductor laser device and for converting a wavelength of the laser beam. Here, the nonlinear optical element is mounted on the substrate in such a manner that the optical waveguide in the nonlinear optical element is located away from the center line of the substrate. 
     To fulfill the above-stated object, a wavelength conversion device according to another embodiment of the present invention, which converts a wavelength by second harmonic-wave generation and generates a laser beam, includes: a substrate having a plurality of electrodes; a semiconductor laser device electrically connected to the plurality of electrodes; and a nonlinear optical element having an optical waveguide for guiding a laser beam emitted from the semiconductor laser device and for converting a wavelength of the laser beam. Here, the semiconductor laser device, the optical waveguide of the nonlinear optical element, and the plurality of electrodes are on approximately one line on the substrate. 
     It is another object of the present invention to provide a wavelength conversion device by which an oscillation wavelength of a laser beam emitted from a semiconductor laser device can be controlled with stability. 
     To fulfill the above-stated object, a wavelength conversion device according to an embodiment of the present invention, which converts a wavelength by second harmonic-wave generation and generates a laser beam, includes: a substrate having a plurality of electrodes; a semiconductor laser device mounted on the substrate and including three regions of a gain region, a phase control region, and a wavelength control region; and a nonlinear optical element mounted on the substrate and for converting a wavelength of a laser beam emitted from the semiconductor laser device. Here, the plurality of electrodes include a first electrode group formed corresponding to the three regions and a second electrode group for carrying out wire-bonding with an external power source, the three regions of the semiconductor laser device are connected electrically to the respective electrodes in the first electrode group, and the first electrode group further is connected to the respective electrodes in the second electrode group via wires, and a wire among the wires, which is connected between the phase control region and the wavelength control region of the semiconductor laser device, has a portion functioning as a resistor. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a plan view of a wavelength conversion device according to Embodiment 1; 
     FIG. 2 is a plan view of a wavelength conversion device according to Embodiment 2; 
     FIG. 3 is a plan view of a wavelength conversion device according to Embodiment 3; 
     FIG. 4 is a plan view of a wavelength conversion device according to Embodiment 4; 
     FIG. 5 is a plan view of a wavelength conversion device according to Embodiment 5; 
     FIG. 6 is a plan view of a wavelength conversion device according to Embodiment 6; 
     FIG. 7 is a plan view of a wavelength conversion device according to Embodiment 7; 
     FIG. 8 is a plan view of a wavelength conversion device according to Embodiment 8; 
     FIG. 9 is a plan view of a wavelength conversion device according to Embodiment 9; 
     FIG. 10 is a plan view of a wavelength conversion device according to Embodiment 10; 
     FIG. 11 is a plan view of a wavelength conversion device according to Embodiment 11; 
     FIG. 12 is a plan view of a wavelength conversion device according to Embodiment 12; 
     FIG. 13 is a graph showing the relationship among oscillation longitudinal mode orders, and amount of current fed into a phase control region and a DBR region; 
     FIG. 14 is a circuit diagram showing a state where the phase control region and the DBR region of the semiconductor laser device are driven at the same voltage; and 
     FIG. 15 is a plan view of a wavelength conversion device according to the prior art. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following describes embodiments of the present invention, with reference to the drawings. 
     [Embodiment 1] 
     FIG. 1 is a plan view of a wavelength conversion device according to this embodiment. On a silicon substrate (3 mm in width, 15 mm in length), electrodes  1 ,  2 , and  3  are formed by patterning, and a DBR laser element  4  (0.3 mm in width, 1.2 mm in length) and a nonlinear optical element  7  (2.8 mm in width, 10 mm in length) are mounted in an integrated manner. Numeral  6  denotes an optical waveguide, and  8  denotes a diffraction grating formed in the optical waveguide  6 . Line  100  is a center line of the optical waveguide  6 . Line M 1   a -M 2   a  is a center line of the width direction of the silicon substrate  5 , and line M 5   a -M 6   a  is a center line of the width direction of the nonlinear optical element  7 . Hereinafter, in the wavelength conversion devices according to the present invention, the direction perpendicular to the optical waveguide is referred to as the width direction, while the direction parallel to the optical waveguide is referred to as the longitudinal direction. 
     The DBR laser element  4  is made up of three regions including a gain region that adjusts an output power of a laser beam emitted therefrom, a phase control region that changes a phase of the laser beam, and a DBR region that feeds back a laser beam with an oscillation wavelength into a cavity. Note here that these regions referred to in this embodiment or later in this specification have the functions as stated above. 
     With respect to these three regions, electrodes that are electrically isolated from one another are formed (not illustrated). The DBR laser element  4  is mounted on the silicon substrate  5  in a junction down manner where a surface with the p-n junction faces to the side of the silicon substrate  5 , and electrodes corresponding to the gain region, the phase control region, and the DBR region are bonded to the electrodes  1 ,  2 , and  3  on the silicon substrate  5 , respectively. Also, wire-bonding regions are formed in each of the electrodes for carrying out wiring with an external power source so as to electrically drive the gain region, the phase control region, and the DBR region. 
     In this way, the gain region, the phase control region, and the DBR region of the DBR laser element  4  are connected electrically to the electrodes  1 ,  2 , and  3 , respectively. In this state, by feeding an electrical signal to each of the electrodes, an oscillation wavelength of the laser beam emitted from the DBR laser element  4  can be varied. The oscillation wavelength of the laser beam emitted from the DBR laser element  4  is set at 820 nm, and the beam is oscillated in the single longitudinal mode. 
     The nonlinear optical element  7  is made of LiNbO 3 , and the optical waveguide  6  having the diffraction grating  8  is formed therein. The nonlinear optical element  7  is fixed onto the silicon substrate  5  at a predetermined position with an adhesive such as a UV curing agent. 
     The diffraction grating  8  is formed by inverting a polarization of LiNbO 3  crystals with the application of an external electric field. The optical waveguide  6  is positioned within 3 μm of the DBR laser element  4  so as to introduce the laser beam emitted from the DBR laser element  4  securely. 
     When guiding the laser beam through the optical waveguide  6 , a second harmonic-wave generated beam (hereinafter, referred to as “SHG beam”) with a wavelength of 410 nm generated in the nonlinear optical element  7  due to a diffraction by the diffraction grating  8  and the laser beam with an oscillation wavelength of 820 nm are quasi-phase matched. Thereby, an SHG beam having a high output power can be obtained. In addition, by controlling the oscillation wavelength of the laser beam emitted from the DBR laser element  4 , a conversion efficiency into the SHG beam can be improved. 
     In this embodiment, as shown in FIG. 1, the nonlinear optical element  7  is mounted on the silicon substrate  5  in such a manner that the center line  100  of its optical waveguide  6  is 1.0 mm away from the center line M 1   a -M 2   a  of the silicon substrate  5 . In this way, in this embodiment, the optical waveguide  6  does not necessarily need to be formed on the center line M 5   a -M 6   a  of the nonlinear optical element  7 . 
     In addition, the nonlinear optical element  7  is mounted so that the center line M 1   a -M 2   a  of the silicon substrate  5  coincides with the center line M 5   a -M 6   a  of the nonlinear optical element  7  in FIG.  1 . However, these center lines do not necessarily coincide with each other. 
     Furthermore, the end of the optical waveguide  6  at the side of the nonlinear optical element  7  where the SHG beam is emitted is located at least 5 μm beyond the edge of the silicon substrate  5 . This construction prevents the SHG beam from being reflected from the silicon substrate  5  and scattered, and therefore a favorable image can be obtained in the far field for the SHG beam emitted from the nonlinear optical element  7 . 
     According to this embodiment, since the nonlinear optical element  7  is mounted on the substrate in such a manner that its optical waveguide  6  is located away from the center line M 1   a -M 2   a  of the silicon substrate  5 , the width of the wavelength conversion device can be narrowed to 5 μmm or less, and therefore a small wavelength conversion device having approximately the same size as the nonlinear optical element  7  can be realized. 
     [Embodiment 2] 
     FIG. 2 is a plan view of a wavelength conversion device according to this embodiment. On a silicon substrate  5  (2 mm in width, 6 mm in length), electrodes  1 ,  2 , and  3  are formed by patterning, and a DBR laser element  4  (0.3 mm in width, 1.2 mm in length) and a nonlinear optical element  7  (2.8 mm in width, 10 mm in length) are mounted in an integrated manner. Numeral  6  denotes an optical waveguide, and  8  denotes a diffraction grating formed in the optical waveguide  6 . Line  100  is a center line of the optical waveguide  6 . Line M 1   b -M 2   b  is a center line of the width direction of the silicon substrate  5 , and line M 5   b -M 6   b  is a center line of the width direction of the nonlinear optical element  7 . In this way, the construction of the wavelength conversion device in this embodiment is similar to that of the wavelength conversion device according to Embodiment 1, except that the silicon substrate  5  is miniaturized so that a length of a region where the DBR laser element  4  is mounted on the silicon substrate  5  is 3 mm along the longitudinal direction of the silicon substrate  5 , and the nonlinear optical element  7  is mounted on the substrate so that the center line  100  of its optical wavelength  6  is 0.7 mm away from the center line of the silicon substrate  5 . That is, in this embodiment also, the nonlinear optical element  7  is positioned within 3 μm of the DBR laser element  4  so as to introduce the laser beam emitted from the DBR laser element  4  securely, and the end of the optical waveguide  6  at the side where the SHG beam is emitted is located at least 5 μm beyond the edge of the silicon substrate  5 . Therefore, their explanations will be omitted. 
     According to this embodiment, the same effects as in Embodiment 1 can be obtained. In addition, by reducing the length of the silicon substrate  5 , the region where the nonlinear optical element  7  is mounted on the silicon substrate  5  is narrowed. Therefore, distortion generated due to the contact between the nonlinear optical element  7  and the silicon substrate  5  can be reduced, and a conversion efficiency from the laser beam emitted from the DBR laser element  4  into the SHG beam can be improved. 
     [Embodiment 3] 
     FIG. 3 is a plan view of a wavelength conversion device according to this embodiment. On a silicon substrate  5  (3.0 mm in width, 12 mm in length), electrodes  9 ,  10 , and  11  are formed by patterning, and a DBR laser element  4  (0.3 mm in width, 1.2 mm in length) and a nonlinear optical element  7  (2.8 mm in width, 10 mm in length) are mounted in an integrated manner. Numeral  6  denotes an optical waveguide, and  8  denotes a diffraction grating formed in the optical waveguide  6 . Line  100  is a center line of the optical waveguide  6 . Line M 1   c -M 2   c  is a center line of the width direction of the silicon substrate  5 , and line M 5   c -M 6   c  is a center line of the width direction of the nonlinear optical element  7 . 
     The DBR laser element  4  is made up of three regions including a gain region, a phase control region, and a DBR region. 
     With respect to these three regions, electrodes that are electrically isolated from one another are formed (not illustrated). The DBR laser element  4  is mounted on the silicon substrate  5  in a junction down manner where a surface with the p-n junction faces to the side of the silicon substrate  5 , and electrodes corresponding to the gain region, the phase control region, and the DBR region are bonded to regions  9   b,    10   b , and  11   b  of the electrodes  9 ,  10 , and  11 , respectively. 
     Also, wire-bonding regions  9   a ,  10   a , and  11   a  are formed in each of the electrodes  9 ,  10 , and  11  for carrying out wiring with an external power source so as to electrically drive the gain region, the phase control region, and the DBR region of the DBR laser element  4 . Here, widths of portions formed between these wire-bonding regions and regions  9   b ,  10   b , and  11   b  (hereinafter, refered to as “connection regions”, which are connected to the respective electrodes formed in the three regions in the DBR laser element  4 ) are narrower than those of the wire-bonding regions and the connection regions. In this way, by partially narrowing the width of each of the electrodes formed on the silicon substrate  5 , the parasitic capacitance of these electrodes can be reduced. 
     As stated above, the construction of the wavelength conversion device according to this embodiment is similar to that of the wavelength conversion device according to Embodiment 1, except that the width of each electrode formed on the silicon substrate  5  is narrowed in part, and the nonlinear optical element  7  is mounted on the substrate so that the center line  100  of its optical wavelength  6  is 1.0 mm away from the center line of the silicon substrate  5 . That is, in this embodiment also, the nonlinear optical element  7  is positioned within 3 μm of the DBR laser element  4 , and the end of the optical waveguide  6  at the side where the SHG beam is emitted is located at least 5 μm beyond the edge of the silicon substrate  5 . Therefore, their explanations will be omitted. 
     According to this embodiment, the same effects as in Embodiment 1 can be obtained. In addition, by partially narrowing the width of each electrode formed on the silicon substrate  5 , the parasitic capacitance of these electrodes can be reduced, and therefore an electrical modulation frequency of the DBR laser element  4  can be increased. 
     [Embodiment 4] 
     FIG. 4 is a plan view of a wavelength conversion device according to this embodiment. On a silicon substrate  5  (2.0 mm in width, 6.0 mm in length), electrodes  9 ,  10 , and  11  are formed by patterning, and a DBR laser element  4  (0.3 mm in width, 1.2 mm in length) and a nonlinear optical element  7  (2.8 mm in width, 10 mm in length) are mounted in an integrated manner. Numeral  6  denotes an optical waveguide, and  8  denotes a diffraction grating formed in the optical waveguide  6 . Line  100  is a center line of the optical waveguide  6 . Line M 1   d -M 2   d  is a center line of the width direction of the silicon substrate  5 , and line M 5   d -M 6   d  is a center line of the width direction of the nonlinear optical element  7 . 
     The DBR laser element  4  is made up of three regions including a gain region, a phase control region, and a DBR region. 
     With respect to these three regions, electrodes that are electrically isolated from one another are formed (not illustrated). The DBR laser element  4  is mounted on the silicon substrate  5  in a junction down manner where a surface with the p-n junction faces to the side of the silicon substrate  5 , and electrodes corresponding to the gain region, the phase control region, and the DBR region are bonded to regions  9   b ,  10   b , and  11   b  of the electrodes  9 ,  10 , and  11 , respectively. 
     Also, wire-bonding regions  9   a ,  10   a , and  11   a  are formed in each of the electrodes  9 ,  10 , and  11  for carrying out wiring with an external power source so as to electrically drive the gain region, the phase control region, and the DBR region of the DBR laser element  4 . Here, widths of portions formed between these wire-bonding regions and regions  9   b ,  10   b , and  11   b  (hereinafter, referred to as “connection regions”, which are connected to the respective electrodes formed in the three regions n the DBR laser element  4 ) are narrower than those of the wire-bonding regions and the connection regions. In this way, by partially narrowing the width of each of the electrodes formed on the silicon substrate  5 , the parasitic capacitance of these electrodes can be reduced. As stated above, the construction of the wavelength conversion device according to this embodiment is similar to that of the wavelength conversion device according to Embodiment 1, except that the silicon substrate  5  is miniaturized so that a length of a region where the DBR laser element  4  is mounted on the silicon substrate  5  is 3 mm along the longitudinal direction of the silicon substrate  5 , the nonlinear optical element  7  is mounted on the substrate so that the center line  100  of its optical wavelength  6  is 0.7 mm away from the center line of the silicon substrate  5 , and the width of each electrode formed on the silicon substrate  5  is narrowed in part. That is, in this embodiment also, the nonlinear optical element  7  is positioned within 3 μm of the DBR laser element  4 , and the end of the optical waveguide  6  at the side where the SHG beam is emitted is located at least 5 μm beyond the edge of the silicon substrate  5 . Therefore, their explanations will be omitted. 
     According to this embodiment, the same effects as in Embodiment 1 can be obtained. In addition, by reducing the length of the silicon substrate  5 , the region where the optical element  7  is mounted on the silicon substrate  5  is narrowed. Therefore, distortion generated in the optical waveguide  6  due to the contact between the nonlinear optical element  7  and the silicon substrate  5  can be reduced, and a conversion efficiency from the laser beam emitted from the DBR laser element  4  into the SHG beam can be improved. Furthermore, by partially narrowing the width of each electrode formed on the silicon substrate  5 , the parasitic capacitance of these electrodes can be reduced, and therefore an electrical modulation frequency of the DBR laser element  4  can be increased. 
     [Embodiment 5] 
     FIG. 5 is a plan view of a wavelength conversion device according to this embodiment. On a silicon substrate  112  (3 mm in width, 15 mm in length), electrodes  101 ,  102 ,  103 ,  104 ,  105 , and  106  are formed by patterning, and a DBR laser element  107  (0.3 mm in width, 1.2 mm in length) and a nonlinear optical element  115  (2.8 mm in width, 10 mm in length) are mounted in an integrated manner. Numeral  110  denotes an optical waveguide, and  111  denotes a diffraction grating formed in the optical waveguide  110 . Line  100  is a center line of the optical waveguide  110 . 
     The DBR laser element  107  is made up of three regions including a gain region, a phase control region, and a DBR region. 
     With respect to these three regions, electrodes that are electrically isolated from one another are formed (not illustrated). The DBR laser element  107  is mounted on the silicon substrate  112  in a junction down manner where a surface with the p-n junction faces to the side of the silicon substrate  112 , and electrodes corresponding to the gain region, the phase control region, and the DBR region are bonded to the electrodes  101 ,  102 , and  103  (hereinafter called “connection electrodes”), respectively. 
     Electrodes  104 ,  105 , and  106  are wire-bonding electrodes for carrying out wiring with an external power source so as to electrically drive the gain region, the phase control region, and the DBR region, respectively. Then, these wire-bonding electrodes and the connection electrodes formed corresponding to the respective regions in the DBR laser element  107  are connected with each other by wires  13   a ,  13   b , and  13   c , respectively. In this state, by feeding an electrical signal to each of the connection electrodes, an oscillation wavelength of the laser beam emitted from the DBR laser element  107  can be varied. The oscillation wavelength of the laser beam emitted from the DBR laser element  107  is set at 820 nm, and the beam is oscillated in the single longitudinal mode. 
     The nonlinear optical element  115  is made of LiNbO 3 , and the optical waveguide  110  having the diffraction grating  111  is formed therein. The nonlinear optical element  115  is fixed onto the silicon substrate  112  at a predetermined position with an adhesive such as a UV curing agent. 
     The diffraction grating  111  is formed by inverting a polarization of LiNbO 3  crystals with the application of an external electric field. The optical waveguide  110  is positioned within 3 μm of the DBR laser element  107  so as to introduce the laser beam emitted from the DBR laser element  107  securely. 
     When guiding the laser beam through the optical waveguide  110 , an SHG beam with a wavelength of 410 nm generated due to a diffraction by the diffraction grating  111  and the laser beam with an oscillation wavelength of 820 nm are quasi-phase matched. Thereby, an SHG beam having a high output power can be obtained. In addition, by controlling the oscillation wavelength of the laser beam emitted from the DBR laser element  107 , a conversion efficiency from the laser beam into the SHG beam can be improved. 
     In this embodiment, the DBR laser element  107 , the optical waveguide  110  of the nonlinear optical element  115 , and the electrodes  101  through  106  are arranged on the line  100  on the silicon substrate  112 . 
     Furthermore, the end of the optical waveguide  110  in the nonlinear optical element  115  at the side where the SHG beam is emitted is located at least 5 μm beyond the edge of the silicon substrate  112 . This construction prevents the SHG beam from being reflected from the silicon substrate  112  and scattered, and therefore a favorable image can be obtained in the far field for the SHG beam emitted from the nonlinear optical element  115 . 
     According to this embodiment, since the DBR laser element  107 , the optical waveguide  110  of the nonlinear optical element  115 , and the electrodes  101  through  106  are arranged on the line  100  on the silicon substrate  112 , the width of the wavelength conversion device can be narrowed to 5 mm or less, and therefore a wavelength conversion device having a width approximately the same as the width of the nonlinear optical element  115  can be obtained. 
     [Embodiment 6] 
     FIG. 6 is a plan view of a wavelength conversion device according to this embodiment. The construction of the wavelength conversion device according to this embodiment is similar to that of the wavelength conversion device according to Embodiment 5, except that the length of the silicon substrate  112  is made 6 mm, which is less than half the length of the silicon substrate  112  in Embodiment 5 (15 mm), and that a length of a region where the DBR laser element  107  is mounted on the silicon substrate  112  is 3 mm along the longitudinal direction of the silicon substrate  112 . That is, in this embodiment also, the nonlinear optical element  115  is positioned within 3 μm of the DBR laser element  107 . Therefore, their explanations will be omitted. 
     According to this embodiment, the same effects as in Embodiment 5 can be obtained. In addition, by reducing the length of the silicon substrate  112 , the silicon substrate  112  can be miniaturized, and therefore the wavelength conversion device can be miniaturized and the cost can be reduced. Furthermore, by narrowing the region where the optical element  115  is mounted on the silicon substrate  112 , distortion of the optical waveguide  110  generated due to the contact between the nonlinear optical element  115  and the silicon substrate  112  can be reduced, and a conversion efficiency from the laser beam emitted from the DBR laser element  107  into the SHG beam can be improved. 
     [Embodiment 7] 
     FIG. 7 is a plan view of a wavelength conversion device according to this embodiment. The construction of the wavelength conversion device according to this embodiment is similar to that of the wavelength conversion device according to Embodiment 6, except that the width of the silicon substrate  112  is made to be 2.5 mm, which is narrowed by 0.5 mm versus that in Embodiment 6 (3 mm). That is, in this embodiment also, a length of a region where the DBR laser element  107  is mounted on the silicon substrate  112  is 3 mm along the longitudinal direction of the silicon substrate  112 , and the nonlinear optical element  115  is positioned within 3 μm of the DBR laser element  107 . Therefore, their explanations will be omitted. 
     According to this embodiment, the same effects as in Embodiment 6 can be obtained. In addition, the silicon substrate  112  further can be miniaturized, and therefore the wavelength conversion device can be miniaturized and the cost can be reduced. 
     [Embodiment 8] 
     FIG. 8 is a plan view of a wavelength conversion device according to this embodiment. On a silicon substrate  112  (3 mm in width, 15 mm in length), electrodes  101 ,  102 ,  103 ,  104 ,  105 , and  106  are formed by patterning, and a DBR laser element  107  (0.3 mm in width, 1.2 mm in length) and a nonlinear optical element  115  (1.5 mm in width, 10 mm in length) are mounted in an integrated manner. Numeral  110  denotes an optical waveguide, and  111  denotes a diffraction grating formed in the optical waveguide  110 . Line  100  is a center line of the optical waveguide  110 . Line M 100 -M 200  is a center line of the width direction of the silicon substrate  112  and line M 500 -M 600  is a center line of the width direction of the nonlinear optical element  115 . 
     In this embodiment, the DBR laser element  107 , the optical waveguide  110  of the nonlinear optical element  115 , and the electrodes  101  through  106  are arranged on the line  100  on the silicon substrate  112 , and connection wires  13   a ,  13   b , and  13   c  are connected to the electrodes  101  through  103  at the same side thereof so as to extend in the longitudinal direction of the silicon substrate  112 . Also, the width of the silicon substrate  112  is made to be 2 mm, which is narrowed by 0.5 mm versus that in Embodiment 7 (2.5 mm). 
     In addition, the nonlinear optical element  115  is mounted on the silicon substrate  112  in such a manner that the center line  100  of its optical waveguide  110  is 0.5 mm away from the center line M 100 -M 200  of the silicon substrate  112  and 0.3 mm away from the center line M 500 -M 600  of the nonlinear optical element  115 . Thereby, the width of the nonlinear optical element  115  is narrowed further to 1.5 mm from 2.0 mm. 
     In the same manner as in Embodiment 7, a length of a region where the DBR laser element  107  is mounted on the silicon substrate  112  is 3 mm along the longitudinal direction of the silicon substrate  112 , and the nonlinear optical element  115  is positioned within 3 μm of the DBR laser element  107 . 
     According to this embodiment, the same effects as in Embodiment 7 can be obtained. In addition, with the construction where the connection wires  13   a ,  13   b , and  13   c  are connected to the electrodes  101  through  103  at the same side thereof on the silicon substrate  112 , and the nonlinear optical element  115  is mounted on the silicon substrate  112  in such a manner that the optical waveguide  110  is away from the center line M 100 -M 200  of the silicon substrate  112  and away from the center line of the nonlinear optical element  115 , regions where any components and wires are not formed on the silicon substrate  112  can be reduced, and the width of the silicon substrate  112  can be narrowed, and therefore a wavelength conversion device using the same can be miniaturized. 
     [Embodiment 9] 
     FIG. 9 is a plan view of a wavelength conversion device according to this embodiment. On a silicon substrate  212  (3 mm in width, 15 mm in length), electrodes  201 ,  202 ,  203 ,  204 , and  205  are formed by patterning, and a DBR laser element  207  (0.3 mm in width, 1.2 mm in length) and a nonlinear optical element  215  (2.8 mm in width, 10 mm in length) are mounted in an integrated manner. Numeral  210  denotes an optical waveguide, and  211  denotes a diffraction grating formed in the optical waveguide  210 . Line  100  is a center line of the optical waveguide  210 . 
     The DBR laser element  207  is made up of three regions including a gain region, a phase control region, and a DBR region. 
     With respect to these three regions of the DBR laser element  207 , electrodes that are electrically isolated from one another are formed (not illustrated). The DBR laser element  207  is mounted on the silicon substrate  212  in a junction down manner where a surface with the p-n junction faces to the side of the silicon substrate  212 , and electrodes corresponding to the gain region, the phase control region, and the DBR region are bonded to the electrodes  201 ,  202 , and  203  (connection electrodes), respectively. 
     Electrodes  204  and  205  are wire-bonding electrodes for carrying out wiring with an external power source so as to electrically drive the gain region, the phase control region, and the DBR region, respectively. Then, these wire-bonding electrodes and the connection electrodes formed corresponding to the respective regions in the DBR laser element  207  are connected with each other by wires  214  and  213 . Here, the wire  214  is connected to the gain region, and the wire  213  is connected to the phase control region and the DBR region. The wire  214  is made of metal, and the wire  213  is made of p-type polysilicon with a resistor  213   a  formed at a portion thereof. 
     In this state, by feeding an electrical signal to each of the connection electrodes, an oscillation wavelength of the laser beam emitted from the DBR laser element  207  can be varied. By varying a voltage applied across the electrode  205 , a current fed into the gain region in the DBR laser element  207  can be controlled, and thus an output power of the laser beam can be controlled. The oscillation wavelength of the laser beam emitted from the DBR laser element  207  is set at 820 nm, and the beam is oscillated in the single longitudinal mode. 
     The nonlinear optical element  215  is made of LiNbO 3 , and the optical waveguide  210  having the diffraction grating  211  is formed therein. The nonlinear optical element  215  is fixed onto the silicon substrate  212  at a predetermined position with an adhesive such as a UV curing agent. 
     The diffraction grating  211  is formed by inverting a polarization of LiNbO 3  crystals with the application of an external electric field. The optical waveguide  210  is positioned within 3 μm of the DBR laser element  207  so as to introduce the laser beam emitted from the DBR laser element  207  securely. 
     When guiding the laser beam through the optical waveguide  210 , an SHG beam with a wavelength of 410 nm generated due to a diffraction by the diffraction grating  211  and the laser beam with an oscillation wavelength of 820 nm are quasi-phase matched. Thereby, an SHG beam having a high output power can be obtained. In addition, by controlling the oscillation wavelength of the laser beam emitted from the DBR laser element  207 , a conversion efficiency from the laser beam into the SHG beam can be improved. 
     In this embodiment, the DBR laser element  207 , the optical waveguide  210  of the nonlinear optical element  215 , and the electrodes  201  through  205  are arranged on the line  100  on the silicon substrate  212 . With this construction, the width of the wavelength conversion device can be narrowed to 5 mm or less, and a small wavelength conversion device having approximately the same width as the nonlinear optical element  215  can be realized. 
     By providing the DBR laser element  207  with the phase control region, in addition to the gain region and the DBR region, so-called mode hopping can be prevented, and thus the oscillation wavelength can be controlled continuously. Unlike the gain region, the phase control region is a region from which gain is not obtained by the passage of electric current. In addition, the phase control region does not have a wavelength selectivity, because it is not provided with a diffraction grating as in the DBR region. When passing electric current through the phase control region, an effective refractive index in the optical waveguide within the region varies, and therefore a phase of the laser beam at a resonant state can be changed. 
     FIG. 13 is a graph showing a relationship among oscillation longitudinal mode orders, amount of current fed into the phase control region and the DBR region in an AlGaAs class laser element. When injecting an electric current into the DBR region, the effective refractive index is increased, and the Bragg wavelength is shifted to the long wavelength side. Therefore, the oscillation longitudinal mode order mode-hops from the N-th to N−1-th, i.e., to the lower order. Meanwhile, when injecting an electric current into the phase control region, the effective refractive index is increased, and the effective cavity length is increased. Therefore, the oscillation longitudinal mode order mode-hops from the N-th to N+1-th, i.e., to the higher order. 
     Consequently, as shown by the broken line in FIG. 13 where a ratio of the current injected into the DBR region to that into the phase control region is kept constant, when injecting an electrical current into the DBR region, the Bragg wavelength is shifted to the long wavelength side, and the oscillation wavelength whose mode gain is the highest is shifted to the long wavelength side. When injecting an electric current into the phase control region, the effective refractive index in this region is increased, and the effective resonator length is increased. Therefore, even when the oscillation wavelength shifts to the longer wavelength side, the oscillation at the same N-th longitudinal mode can be kept in the same phase state, and thus mode hopping can be prevented. 
     FIG. 14 is a circuit diagram showing a state where resistors are connected in series to each of the phase control region  401   a  and the DBR region  401   b  in the semiconductor laser device  401  and the respective regions are driven with the same bias voltage applied by the power source  404 . In such a state, current injected into the phase control region and the DBR region has the relationship as represented by the following formula (1). 
     
       
           I   DBR =( R   2   +R   DBR )/( R   1   +R   PHASE )× I   PHASE   (1)  
       
     
     Here, I PHASE  and I DBR  are currents injected into the phase control region and the DBR region, respectively. R 1  and R PHASE  are a value of differential resistance of the phase control region (constant value) and a value of resistance of the resistor  402  connected to the phase control region, respectively, while R 2  and R DBR  are a value of differential resistance of the DBR region (constant value) and a value of resistance of the resistor  403  connected to the DBR region, respectively. 
     Therefore, as shown by Formula (1), by varying the values of R PHASE  and R DBR  connected to the phase control region and the DBR region, a ratio between the current I DBR  and I PHASE  (i.e., (R 2 +R DBR )/(R 1 +R PHASE ), hereinafter, referred to as a ratio between currents) can be controlled. 
     In this embodiment, as shown in FIG. 9, the resistor  213   a  is formed at a portion of the wire connected between the phase control region and the DBR region in the semiconductor laser device. Assuming that the resistor  213   a  has a value of resistance represented by R, the ratio between currents becomes I DBR /I PHASE =(R 2 +R)/(R 1 +R). Therefore, by adjusting the value R of the resistor  213   a  so that the oscillation longitudinal mode orders does not generate mode-hopping, the oscillation wavelength of the laser beam emitted from the semiconductor laser device can be varied continuously. Note here that the value of R preferably is set within a range between 10 −3  Ω·cm and 10 6  Ω·cm. 
     According to this embodiment, by providing a portion of the wire connected between the phase control region and the DBR region with a function as a resistor and adjusting the value of the resistor, the oscillation wavelength of the laser beam emitted from the semiconductor laser device can be controlled with stability. 
     [Embodiment 10] 
     FIG. 10 is a plan view of a wavelength conversion device according to this embodiment. The construction of the wavelength conversion device is similar to that of the wavelength conversion device according to Embodiment 9, except that the length of the silicon substrate  212  in the longitudinal direction is made to be 6 mm, which is a half or less of the length in Embodiment 9 (15 mm), the length of a region where the DBR laser element  207  is mounted on the silicon substrate  212  in the longitudinal direction of the silicon substrate  212  is 3 mm, and the width of the silicon substrate  212  is made to be 2 mm, which is narrowed by 1 mm from the width of the silicon substrate  212  in Embodiment 9 (3 mm). That is, in this embodiment also, the nonlinear optical element  215  is positioned within 3 μm of the DBR laser element  207 . Therefore, their explanations will be omitted. 
     According to this embodiment, the same effects as in Embodiment 9 can be obtained. In addition, by reducing the length of the silicon substrate  212 , the silicon substrate  212  can be miniaturized, and therefore the wavelength conversion device can be miniaturized and the cost can be reduced. Furthermore, by narrowing the region where the optical element  215  is mounted on the silicon substrate  212 , distortion of the optical waveguide  210  generated due to the contact between the nonlinear optical element  215  and the silicon substrate  212  can be reduced, and a conversion efficiency from the laser beam emitted from the DBR laser element  207  into the SHG beam can be improved. 
     [Embodiment 11] 
     FIG. 11 is a plan view of a wavelength conversion device according to this embodiment. On a silicon substrate  212  (3.2 mm in width, 11.5 mm in length), electrodes  221 ,  222 ,  223 ,  224 , and  225  are formed by patterning, and a DBR laser element  227  (0.3 mm in width, 1.2 mm in length) and a nonlinear optical element  215  (3 mm in width, 10 mm in length) are mounted in an integrated manner. Numeral  210  denotes an optical waveguide, and  211  denotes a diffraction grating formed in the optical waveguide  210 . Line  100  is a center line of the optical waveguide  210 . Line M 10   a -M 20   a  is a center line of the width direction of the silicon substrate  212  and line M 50   a -M 60   a  is a center line of the width direction of the nonlinear optical element  215 . 
     The DBR laser element  227  is made up of three regions including a gain region, a phase control region, and a DBR region. 
     With respect to these three regions, electrodes that are electrically isolated from one another are formed (not illustrated). The DBR laser element  227  is mounted on the silicon substrate  212  in a junction down manner where a surface with the p-n junction faces to the side of the silicon substrate  212 , and electrodes corresponding to the gain region, the phase control region, and the DBR region are bonded to the electrodes  221 ,  222 , and  223  (connection electrodes), respectively. 
     Electrodes  224  and  225  are wire-bonding electrodes for carrying out wiring with an external power source so as to electrically drive the gain region, the phase control region, and the DBR region, respectively. Then, a wire  214  is connected between the electrodes  221  and  224 , and a wire  213  is connected among the electrodes  222  and  223 , and  225 . The wire  214  is made of metal, and the wire  213  is made of p-type polysilicon with a resistor  213   a  formed at a portion thereof. 
     In this state, by feeding an electrical signal to each of the connection electrodes, an oscillation wavelength of the laser beam emitted from the DBR laser element  227  can be varied. By varying a voltage applied across the electrode  224 , a current fed into the gain region in the DBR laser element  227  can be controlled, and thus an output power of the laser beam can be controlled. The oscillation wavelength of the laser beam emitted from the DBR laser element  227  is set at 820 nm, and the light is oscillated in the single longitudinal mode. 
     The nonlinear optical element  215  is made of LiNbO 3 , and the optical waveguide  210  having the diffraction grating  211  is formed therein. The nonlinear optical element  215  is fixed onto the silicon substrate  212  at a predetermined position with an adhesive such as a UV curing agent. 
     The diffraction grating  211  is formed by inverting a polarization of LiNbO 3  crystals with the application of an external electric field. The optical waveguide  210  is positioned within 3 μm of the DBR laser element  227  so as to introduce the laser beam emitted from the DBR laser element  227  securely. 
     When guiding the laser beam through the optical waveguide  210 , an SHG beam with a wavelength of 410 nm generated due to a diffraction by the diffraction grating  211  and the laser beam with an oscillation wavelength of 820 nm are quasi-phase matched. Thereby, an SHG beam having a high output power can be obtained. In addition, by controlling the oscillation wavelength of the laser beam emitted from the DBR laser element  227 , a conversion efficiency from the laser beam into the SHG beam can be improved. 
     In this embodiment, as shown in FIG. 11, the nonlinear optical element  215  is mounted on the substrate in such a manner that the center line  100  of its optical waveguide  210  is located 1.0 mm away from the center line M 10   a -M 20   a  of the silicon substrate  212 . In this way, the optical waveguide  210  does not necessarily need to be formed on the center line M 50   a -M 60   a  of the nonlinear optical element  215 . 
     In addition, although the nonlinear optical element is mounted on the substrate in such a manner that the center line M 10   a -M 20   a  of the silicon substrate  212  coincides with the center line M 50   a -M 60   a  of the nonlinear optical element  215 , these center lines do not necessarily need to be aligned. 
     Furthermore, the end of the optical waveguide  210  at the side of the optical waveguide  210  where the SHG beam is emitted is located at least 5 μm beyond the edge of the silicon substrate  212 . This construction prevents the SHG beam from being reflected from the silicon substrate  212  and scattered, and therefore a favorable image can be obtained in the far field for the SHG beam emitted from the nonlinear optical element  215 . 
     In this embodiment, a resistor  213   a  is formed at a portion of the wire connected between the phase control region and the DBR region of the semiconductor laser device. Due to the same principle as in Embodiment 9, by controlling the value R of the resistor  213   a  so that the oscillation longitudinal mode orders do not generate mode-hopping, the oscillation wavelength of the laser beam emitted from the semiconductor laser device can be varied continuously. Note here that the value of R preferably is set within a range between 10 −3  Ω·cm and 10 6  Ω·cm. 
     According to this embodiment, by providing a portion of the wire connected between the phase control region and the DBR region with a function as a resistor and controlling the value of the resistor, the oscillation wavelength of the laser beam emitted from the semiconductor laser device can be controlled with stability. 
     In addition, since the nonlinear optical element  215  is mounted on the substrate in such a manner that its optical waveguide  210  is located away from the center line M 10   a -M 20   a  of the silicon substrate  212 , the width of the wavelength conversion device can be narrowed to 5 mm or less, and therefore a small wavelength conversion device having approximately the same size as the nonlinear optical element  215  can be realized. As a result, the silicon substrate  212  further can be miniaturized, and therefore the wavelength conversion device can be miniaturized and the cost can be reduced. 
     [Embodiment 12] 
     FIG. 12 is a plan view of a wavelength conversion device according to this embodiment. On a silicon substrate  212  (2.0 mm in width, 6 mm in length), electrodes  221 ,  222 ,  223 ,  224 , and  225  are formed by patterning, and a DBR laser element  227  (0.3 mm in width, 1.2 mm in length) and a nonlinear optical element  215  (2.8 mm in width, 10 mm in length) are mounted in an integrated manner. Numeral  210  denotes an optical waveguide, and  211  denotes a diffraction grating formed in the optical waveguide  210 . Line  100  is a center line of the optical waveguide  210 . Line M 10   b -M 20   b  is a center line of the width direction of the silicon substrate  212  and line M 50   b -M 60   b  is a center line of the width direction of the nonlinear optical element  215 . In this way, the construction of the wavelength conversion device in this embodiment is similar to that of the wavelength conversion device according to Embodiment 11, except that the silicon substrate  212  is miniaturized so that a length of a region where the DBR laser element  227  is mounted on the silicon substrate  212  is 3 mm along the longitudinal direction of the silicon substrate  212 , and the nonlinear optical element  215  is mounted on the substrate so that the center line  100  of its optical wavelength  210  is 0.7 mm away from the center line of the silicon substrate  212 . That is, in this embodiment also, the nonlinear optical element  215  is arranged within 3 μm from the DBR laser element  227  so as to securely introduce the laser beam emitted from the DBR laser element  227 . Therefore, their explanations will be omitted. 
     According to this embodiment, the same effects as in Embodiment 11 can be obtained. In addition, by reducing the length of the silicon substrate  212 , the region where the nonlinear optical element is mounted on the silicon substrate  212  is narrowed. Therefore, distortion generated due to the contact between the nonlinear optical element  215  and the silicon substrate  212  can be reduced, and a conversion efficiency from the laser beam emitted from the DBR laser element  227  into the SHG beam can be improved. 
     In the above-stated embodiments, the substrate is made of silicon. However, instead of silicon, materials such as SiC or AlN may be used. With these materials, thermal dissipation of the device can be improved, the operational current of the semiconductor laser device can be decreased, and the operational temperature range of the semiconductor laser device can be broadened. Alternatively, resin such as plastic may be used. If using a resin substrate, an electrical wiring pattern can be integrated on the substrate. As a result, a more light-weight, miniaturized, and low-cost wavelength conversion device can be obtained. 
     In the above-stated embodiments, the nonlinear optical elements are made of LiNbO 3 . Instead, materials such as LiTaO 3 , KTiOPO 4 , and KNbO 3  may be used. 
     In the above-stated embodiments, DBR laser elements are used as the semiconductor laser device. Instead, multielectrode driven type laser elements such as a multielectrode semiconductor laser device capable of a Fabry-Perot mode oscillation, a multielectrode Distributed Feedback (abbreviated as “DFB”) type laser element, a multielectrode bistable semiconductor laser element, and a pulse laser may be used. With these elements, the time dependency of the output power of the SHG beam can be lessened. Alternatively, instead of the DBR laser element, laser elements whose wavelength can be controlled may be used. 
     In the above-stated embodiments, the semiconductor laser devices have three regions. However, insofar as the oscillation wavelength of the laser beam emitted therefrom can be controlled adequately, semiconductor laser devices having two regions or four or more regions may be used. 
     If optical components such as a lens, birefringence material, prism, mirror, and an optical modulator may be integrated as the integrated components, in addition to the semiconductor laser device and the nonlinear optical element, a small wavelength conversion device can be obtained. 
     Furthermore, the wires connecting components may be integrated on the silicon substrate directly. In the case of the substrate made of silicon, instead of metal, polycrystal silicon, p-type silicon, and n-type silicon can be used as a material of the wire. 
     The invention may be embodied in other forms without departing from the spirit or essential characteristics thereof. The embodiments disclosed in this application are to be considered in all respects as illustrative and not limiting. The scope of the invention is indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are intended to be embraced therein.

Technology Classification (CPC): 7