Abstract:
A variable-wavelength semiconductor laser device containing a tapered-stripe semiconductor laser amplifier and a wavelength selection unit. Light emitted from a back end surface of the tapered-stripe semiconductor laser amplifier is incident on the wavelength selection unit. The wavelength selection unit selects a portion of the light having a specific wavelength, and returns the selected portion to the tapered-stripe semiconductor laser amplifier. The tapered-stripe semiconductor laser amplifier amplifies the returned portion of the light to emit the amplified light from a front-end surface of the tapered-stripe semiconductor laser amplifier. The wavelength selection unit is constructed so that the specific wavelength can be changed according to strength of an electric field applied to the wavelength selection unit. An electric field applying unit is provided for applying the electric field to the wavelength selection unit. The wavelength selection unit may contain a fiber grating, a birefringent filter, or an optical waveguide element having therein a reflection grating. In this case, the specific wavelength selected by the wavelength selection unit can be changed by changing a refraction index and/or an effective pitch of the grating, which can be changed by adjusting the strength of the electric field.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a semiconductor laser device which emits laser light having an arbitrarily variable wavelength. In particular, the present invention relates to a variable-wavelength semiconductor laser device which contains a tapered-stripe semiconductor laser amplifier as a light source, where wavelength selection is performed on laser light emitted from the tapered-stripe semiconductor laser amplifier, and then the laser light having the selected wavelength is returned to the tapered-stripe semiconductor laser amplifier to be amplified therein. 
     2. Description of the Related Art 
     Various attempts have been made to obtain a high-power laser beam having a single wavelength by utilizing a semiconductor device. As one of such attempts, a semiconductor laser device is disclosed in Electronics Letters, vol. 29, No. 14 (1993), 1254-1255. 
     As illustrated in FIG. 7, the above semiconductor laser device contains a tapered-stripe semiconductor laser amplifier  1  as a light source, and laser light emitted from the back end surface la of the semiconductor laser device  1  is collimated by the lens  2  to be incident on the reflection grating  3 . Only the laser light  4  having a single wavelength is reflected by the reflection grating  3  to be returned to the tapered-stripe semiconductor laser amplifier  1 . Therefore, the wavelength of the laser light  4 F emitted from the front-end surface  1 b is locked to the single wavelength. Thus, the variable-wavelength semiconductor laser device of FIG. 7 can output a laser beam of a high output power (not less than 1 W) and high quality (close to a diffraction limit). 
     In the above semiconductor laser device, the selected wavelength can be changed by rotating the reflection grating  3  in the directions as indicated by the arrows A in FIG.  7 . Thus, the oscillation frequency can be changed. 
     In the semiconductor laser device in which the oscillation wavelength is selected by a reflection grating, the oscillation wavelength can be changed over a considerably wide range. However, there are drawbacks. That is, since the reflection grating is rotated by a mechanical driving means, downsizing of the device and precise tuning are difficult. In addition, the output is liable to become unstable, for example, due to misalignment of the constituents of the semiconductor laser device. 
     Japanese Unexamined Patent Publication No. 10(1998)-190105 proposes a semiconductor laser device in which a birefringent filter is provided as a wavelength selection means, instead of the above-mentioned reflection grating. 
     This provision is made in order to lower a threshold current for oscillation and increasing luminous efficiency. In addition, Applied Physics Letters, vol. 73, No. 5 (1998), 575-577 discloses a semiconductor laser device using a fiber grating as a wavelength selection means, instead of the above-mentioned reflection grating. In these semiconductor laser devices respectively using the birefringent filter and the fiber grating, the oscillation wavelength can be slightly changed by changing a driving current. 
     Since the semiconductor laser devices using a birefringent filter or fiber grating do not contain a mechanical driving means, the aforementioned drawbacks of the semiconductor laser device using the reflection grating do not exist in the semiconductor laser devices respectively using the birefringent filter and the fiber grating. However, the variable range of the oscillation wavelength by changing the driving current is very small, i.e., practically, the oscillation wavelength is not variable in the above semiconductor laser devices using the birefringent filter or the fiber grating. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to provide a variable-wavelength semiconductor laser device which contains a tapered-stripe semiconductor laser amplifier as a light source, emits laser light having a variable wavelength, and realizes downsizing, precise tuning of oscillation wavelength, and a stable output. 
     The above object is accomplished by the present invention, which provides a variable-wavelength semiconductor laser device containing a tapered-stripe semiconductor laser amplifier which emits first light from a first end surface thereof, receives second light from the first surface, and emits third light from a second surface thereof; a wavelength selection unit which receives the first light, selects a wavelength from among a plurality of wavelengths included in the first light according to strength of an electric field applied to the wavelength selection unit, and returns to the tapered-stripe semiconductor laser amplifier a portion of the first light having the selected wavelength as the second light; and an electric field applying unit which applies the electric field to the wavelength selection unit. 
     According to the present invention, the selected wavelength can be changed by varying the strength of the electric field applied to the wavelength selection unit, and therefore the selected wavelength can be precisely tuned by appropriately adjusting the strength of the electric field applied to the wavelength selection unit. In addition, since the variable-wavelength semiconductor laser device according to the present invention contains no mechanical driving means for changing the selected wavelength, the size of the device can be reduced compared with the conventional device which uses the mechanical driving means, and a stable output can be obtained. 
     (i) In the above variable-wavelength semiconductor laser device according to present invention, the wavelength selection unit may include a fiber grating which contains a core and a plurality of refractive index variation portions formed in the core at regular intervals, and reflects and diffracts the first light to determine the selected wavelength and return the second light to the tapered-stripe semiconductor laser amplifier. In this case, the electric field applying unit may include a pair of electrodes which are formed so that the plurality of refractive index variation portions are located between the pair of electrodes. In addition, the electric field applying unit may include a unit for applying the electric field between the pair of electrodes. 
     In the variable-wavelength semiconductor laser device described in the above paragraph (i), when an electric field is applied to the above plurality of refractive index variation portions in the fiber grating, the plurality of refractive index variation portions generate heat to change the volume of the plurality of refractive index variation portions. At this time, refractive indexes of the plurality of refractive index variation portions also change due to a thermooptic (TO) effect, and thus an effective grating constant (i.e., an effective pitch of the fiber grating) changes. Therefore, the effective grating constant, the selected wavelength, and the oscillation wavelength can be arbitrarily changed by changing the strength of the electric field applied to the plurality of refractive index variation portions in the fiber grating. 
     (ii) In the above variable-wavelength semiconductor laser device according to present invention, the wavelength selection unit may include a birefringent filter containing at least one birefringent element made of a material exhibiting an electrooptic (EO) effect, and a mirror which reflects the first light after the first light has passed through the birefringent filter. In this case, the electric field applying unit may include a pair of electrodes which are formed so that portions of the birefringent filter through which the first light passes are located between the pair of electrodes, and a unit for applying the electric field between the pair of electrodes. 
     In the variable-wavelength semiconductor laser device described in the above paragraph (ii), when an electric field is applied to the at least one birefringent element made of the material exhibiting the electrooptic (EO) effect, the refractive index of the at least one birefringent element changes due to the electrooptic (EO) effect. Therefore, the refractive index of the at least one birefringent element, the selected wavelength, and the oscillation wavelength can be arbitrarily changed by varying the strength of the electric field applied to the at least one birefringent element. 
     (iii) In the variable-wavelength semiconductor laser device described in the above paragraph (ii), the material of which the at least one birefringent element is made may be LiNb x Ta 1−x O 3  (0≦x≦1), or doped with at least one of MgO and ZnO. When the birefringent element has a c axis, the pair of electrodes may be separated from each other in the direction of the c axis of the birefringent element. 
     (iv) In the above variable-wavelength semiconductor laser device according to present invention, the wavelength selection unit may include an optical waveguide element which contains a substrate showing an electrooptic effect, an optical waveguide being formed in the substrate and having therein a reflection grating which reflects and diffracts the first light to determine the selected wavelength and return the second light to the tapered-stripe semiconductor laser amplifier. In this case, the electric field applying unit may include a pair of electrodes which are formed so that the reflection grating is located between the pair of electrodes, and a unit for applying the electric field between the pair of electrodes. 
     In the variable-wavelength semiconductor laser device described in the above paragraph (iv), when an electric field is applied to the above reflection grating formed in the optical waveguide, an effective grating constant (i.e., an effective pitch of the reflection grating) changes due to the electrooptic (EO) effect. Therefore, the effective grating constant, the selected wavelength, and the oscillation wavelength can be arbitrarily changed by changing the strength of the electric field applied to the reflection grating in the optical waveguide. 
     (v) In the variable-wavelength semiconductor laser device described in the above paragraph (iv), the substrate may be LiNb x Ta 1−x O 3  (0≦x≦1), or doped with at least one of MgO and ZnO. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a plan view of the variable-wavelength semiconductor laser device as the first embodiment of the present invention. 
     FIG. 2 is an enlarged view of the tapered-stripe semiconductor laser amplifier in the variable-wavelength semiconductor laser device as the first embodiment of the present invention. 
     FIG. 3 is an enlarged perspective view of the fiber grating in the variable-wavelength semiconductor laser device as the first embodiment of the present invention. 
     FIG. 4 is a plan view of the variable-wavelength semiconductor laser device as the second embodiment of the present invention. 
     FIG. 5 is a diagram illustrating an arrangement of a pair of electrodes in the birefringent element in the variable-wavelength semiconductor laser device as the second embodiment of the present invention. 
     FIG. 6 is a plan view of the variable-wavelength semiconductor laser device as the third embodiment of the present invention. 
     FIG. 7 is a plan view of an example of the conventional variable-wavelength semiconductor laser devices. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Embodiments of the present invention are explained in detail below with reference to drawings. 
     First Embodiment 
     FIG. 1 is a plan view of the variable-wavelength semiconductor laser device as the first embodiment of the present invention. The variable-wavelength semiconductor laser device in FIG. 1 contains a tapered-stripe semiconductor laser amplifier  10  having a tapered stripe structure  10   a , a reflection-type fiber grating  20  which is arranged so that a light beam emitted from the back end surface  10   b  of the tapered-stripe semiconductor laser amplifier  10  is incident on the core  20   b  of the reflection-type fiber grating  20 , a pair of electrodes  31  and  32  formed on the external surface of the reflection-type fiber grating  20 , and an electric field applying circuit  40  which applies an electric field between the pair of electrodes  31  and  32 . 
     FIG. 2 is an enlarged view of the tapered-stripe semiconductor laser amplifier in the variable-wavelength semiconductor laser device in FIG.  1 . As illustrated in FIG. 2, the tapered-stripe semiconductor laser amplifier  10  contains the tapered stripe structure  10   a . In an example, the width Wi of the tapered stripe structure  10   a  at the back end surface  10   b  is 4 μm, the width Wo of the tapered stripe structure  10   a  at the front end surface  10   c  is 360 μm, and the length L is 1.5 mm. For example, the tapered-stripe semiconductor laser amplifier  10  contains an n-type GaAs substrate (doped with Si of 2×10 18  cm −3 ), an n-type GaAs buffer layer (doped with Si of 1×10 18  cm −3  and 0.5 μm thick), an n-type Al 0.5 Ga 0.5 As cladding layer (doped with Si of 1×10 18  cm −3  and 2.5 μm thick), an n-type Al 0.25 Ga 0.75 As optical waveguide layer (undoped and 0.05 μm thick), an n-type Al 0.05 Ga 0.95 As quantum well layer (undoped and 8 nm thick), an n-type Al 0.25 Ga 0.75 As optical waveguide layer (undoped and 0.05 μm thick), a p-type Al 0.5 Ga 0.5 As cladding layer (doped with Zn of 1×10 18  cm −3  and 2 μm thick), a p-type GaAs cap layer (doped with Zn of 5×10 18  cm −3  and 0.3 μm thick), formed in this order by the low pressure MOCVD technique. 
     For example, the tapered stripe structure  10   a  may be formed as follows. First, an SiO 2  film is formed on the above cap layer by the plasma CVD technique. Next, a portion, corresponding to the tapered stripe structure  10   a , of the SiO 2  film is removed by photolithography and etching. Then, an AuZn/Au p-electrode and an AuGe/Ni/Au n-electrode are formed as ohmic electrodes. 
     In addition, low reflectance coating is laid on both the end surfaces  10   b  and  10   c  of the tapered-stripe semiconductor laser amplifier  10  so that the reflectance viewed from inside of the tapered-stripe semiconductor laser amplifier  10  is not more than 0.5%. Thus, the tapered-stripe semiconductor laser amplifier  10  is formed as a traveling wave amplifier. 
     FIG. 3 is an enlarged perspective view of the fiber grating in the variable-wavelength semiconductor laser device in FIG.  1 . As illustrated in FIG. 3, the reflection-type fiber grating  20  is an optical fiber comprising the above-mentioned core  20   b  and a cladding  20   a  which surrounds the core  20   b  and has a greater refractive index than the core  20   b . In the core  20   b , a plurality of refractive index variation portions  20   c  are formed at regular intervals. In an example, the outside diameter of the cladding  20   a  is 125 μm, and the diameter of the core  20   b  is about 10 μm. For example, in a core of an optical fiber for use in communication having the above dimension, the plurality of refractive index variation portions  20   c  are formed by generating two-beam interference fringes of excimer laser light (in the ultraviolet region) to change (increase) refractive indexes of portions which are exposed to the two-beam interference light. When the core  20   b  is doped with germanium dioxide, it is considered that the above change of the refractive index is caused by chemical change of germanium dioxide due to the exposure to the ultraviolet light. 
     The electrodes  31  and  32  are formed on the external surface of the cladding  20   a  so that the plurality of refractive index variation portions  20   c  are located between the electrodes  31  and  32 . The electric field applying circuit  40  includes a closed circuit  41  containing a direct current power supply  42  and a variable resistor  43  connected in series. The electrodes  31  and  32  are connected to both ends of the closed circuit  41 , respectively. 
     The operations of the variable-wavelength semiconductor laser device as the first embodiment of the present invention are explained in detail below. 
     Light beam  11  emitted from the back end surface  10   b  of the tapered-stripe semiconductor laser amplifier  10  is incident on an end surface of the reflection-type fiber grating  20  to enter the core  20   b  and propagate therethrough. The plurality of refractive index variation portions  20   c  formed in the core  20   b  realize a grating along the direction of the propagation of the light beam  11 . This grating reflects only a portion of the light beam  11  which has a specific wavelength corresponding to the pitch of the grating (grating constant) to feedback the reflected portion to the tapered-stripe semiconductor laser amplifier  10 . 
     Although the light beam  11  originally emitted from the back end surface  10   a  of the tapered-stripe semiconductor laser amplifier  10  includes a wavelength range of 800 to 820 nm, the portion of the light beam  11  reflected by the reflection-type fiber grating  20  is light having a single wavelength and a half width of about 0.1 nm, due to the function of the wavelength selection in the reflection-type fiber grating  20 , where the single wavelength is in the above wavelength range of the original light beam  11 . Thus, the oscillation wavelength of the tapered-stripe semiconductor laser amplifier  10  is simplified to the single wavelength. This single-wavelength light is amplified while the light beam travels in the forward direction through the tapered-stripe semiconductor laser amplifier  10 , and the amplified light  11 F is emitted from the front-end surface  10   c  of the tapered-stripe semiconductor laser amplifier  10 . 
     Next, explanations are provided on the change of the oscillation wavelength. 
     When a voltage is applied from the direct current power supply  42  to the electrodes  31  and  32 , an electric field is applied to the plurality of refractive index variation portions  20   c  in the reflection-type fiber grating  20 , where the plurality of refractive index variation portions  20   c  are located between the electrodes  31  and  32 . The strength of the electric field can be continuously changed by manipulating the variable resistor  43 . 
     When the electric field is applied to the plurality of refractive index variation portions  20   c , the plurality of refractive index variation portions  20   c  generate heat. The volume of the plurality of refractive index variation portions  20   c  is changed due to the heat, and the refractive indexes of the plurality of refractive index variation portions  20   c  are also changed due to the thermooptic effect. Thus, the effective pitch of grating is changed. Therefore, when the strength of the electric field is changed by manipulating the variable resistor  43 , the effective pitch of the grating is changed, and thus the wavelength selected by the reflection-type fiber grating  20 , i.e., the oscillation wavelength, is changed. That is, the oscillation wavelength can be changed arbitrarily by manipulating the variable resistor  43 . 
     As described above, the oscillation wavelength in the variable-wavelength semiconductor laser device as the first embodiment can be changed without the provision of the mechanical driving means. Therefore, the size of the variable-wavelength semiconductor laser device as the first embodiment can be reduced to a smaller size than that of the variable-wavelength semiconductor laser device which uses the mechanical driving means for changing the oscillation wavelength. In addition, the oscillation wavelength of the variable-wavelength semiconductor laser device as the first embodiment can be precisely tuned, and a stable output can be obtained. 
     Second Embodiment 
     FIG. 4 shows the construction of the variable-wavelength semiconductor laser device as the second embodiment of the present invention, and FIG. 5 shows an arrangement of a pair of electrodes in the birefringent element in the variable-wavelength semiconductor laser device in FIG.  4 . In FIGS. 1,  4 , and  5 , elements bearing the same reference numbers function in the same manner. 
     In the construction of FIG. 4, a light beam  11  emitted from the back end surface  10   a  of the tapered-stripe semiconductor laser amplifier  10  is collimated by the collimator lens  53 , and passes through the birefringent filter  50 . Then, the light beam  11  is collected by the condenser lens  54 , and focused on the mirror  55 . The light beam  11  is reflected by the mirror  55 , and retraces the above path to be returned to the tapered-stripe semiconductor laser amplifier  10 . The wavelength of the returned light beam is simplified to a single wavelength by the birefringent filter  50 , and thus the oscillation wavelength of the tapered-stripe semiconductor laser amplifier  10  is also simplified to this wavelength. The light beam having this wavelength is amplified while the light beam travels through the tapered-stripe semiconductor laser amplifier  10  in the forward direction. Then, the amplified light  11 F is emitted from the front-end surface  10   c  of the tapered-stripe semiconductor laser amplifier  10 . 
     Next, explanations are provided on the operation of selecting a wavelength by the birefringent filter  50 . 
     The birefringent filter  50  in FIG. 4 contains two birefringent elements  51  and  52 , each realizing a half-wave plate for a single wavelength. In this example, the single wavelength is 810 nm. The birefringent elements  51  and  52  are arranged so that the light beam  11  from the tapered-stripe semiconductor laser amplifier  10  is incident on the birefringent element  51  at the Brewster&#39;s angle (polarizing angle). Therefore, transmittance of a p-polarization component of the light beam  11  having the single wavelength at the boundary between the air and the birefringent element  51  is 100%, while transmittances of other polarization components of the light beam  11  at the boundary between the air and the birefringent element  51  is less than 100%, e.g., transmittance of an s-polarization component of the light beam  11  at the boundary between the air and the birefringent element  51  is 30%. Transmittances of the respective polarization components are the same as above at the other boundaries: the other boundary between the air and the birefringent element  51  and two boundaries between the air and the birefringent element  52 . 
     Thus, during a round trip between the back end surface  10   b  and the mirror  55 , the light beam  11  is incident on the above boundaries eight times at the Brewster&#39;s angle. Since the p-polarization component of the light beam having the wavelength of 810 nm maintains the p-polarization state after the p-polarization component is incident on (and passes through) any of the above boundaries, transmittance of the p-polarization component of the light beam  11  through the round trip between the tapered-stripe semiconductor laser amplifier  10  and the mirror  55  is close to 100%, e.g., about 99%. However, when p-polarization components of the light beam  11  having other wavelengths are incident on any of the above boundaries, the orientation of the linear polarization is changed, i.e., the p-polarization states of the components of the light beam  11  having the wavelengths other than the above single wavelength (810 nm) are not maintained. Therefore, when a linearly polarized light beam is incident on the birefringent filter  50  in the p-polarized mode, only the p-polarization component of the above single wavelength is returned to the tapered-stripe semiconductor laser amplifier  10  after the round trip between the tapered-stripe semiconductor laser amplifier  10  and the mirror  55 . That is, the single wavelength is selected by the birefringent filter  50 , which functions as a Lyot filter. 
     Next, explanations are provided on the change of the oscillation wavelength. 
     The birefringent elements  51  and  52  are made of LiNbO 3  which is doped with MgO. This material exhibits an electrooptic (EO) effect in addition to birefringence. A pair of electrodes  56  and  57  are formed on the birefringent element  52 , as illustrated in FIG.  5 . The electrodes  56  and  57  are separated in the direction of the c-axis of the birefringent element  52  so that the portion of the birefringent element  52  through which the light beam  11  passes is located between the electrodes  56  and  57 . The electric field applying circuit  40 , which is similar to the electric field applying circuit  40  in FIG. 1, is connected to the electrodes  56  and  57 . 
     When a voltage is applied from the direct current power supply  42  to the electrodes  56  and  57 , an electric field is applied to the portion of the birefringent element  52  through which the light beam  11  passes. The strength of the electric field can be continuously changed by manipulating the variable resistor  43 . 
     When the electric field is applied to the portion of the birefringent element  52  through which the light beam  11  passes, the refractive index of the portion of the birefringent element  52  through which the light beam  11  passes is changed due to the electrooptic effect. Therefore, when the strength of the electric field is changed by manipulating the variable resistor  43 , the refractive index of the portion of the birefringent element  52  through which the light beam  11  passes is changed, and thus the wavelength selected by the birefringent filter  50 , i.e., the oscillation wavelength, is changed. That is, the oscillation wavelength can be changed arbitrarily by manipulating the variable resistor  43 . 
     As described above, the oscillation wavelength of the variable-wavelength semiconductor laser device as the second embodiment can be changed without the provision of the mechanical driving means. Therefore, the size of the variable-wavelength semiconductor laser device as the second embodiment can be reduced to a smaller size than that of the conventional variable-wavelength semiconductor laser device which uses the mechanical driving means for changing the oscillation wavelength. In addition, the oscillation wavelength of the variable-wavelength semiconductor laser device as the second embodiment can be precisely tuned, and a stable output can be obtained. 
     Third Embodiment 
     FIG. 6 is a plan view of the variable-wavelength semiconductor laser device as the third embodiment of the present invention. In the third embodiment, an optical waveguide element  60  is provided as a wavelength selection element. As illustrated in FIG. 6, the optical waveguide element  60  is constructed by forming an optical waveguide  62  in the substrate  61 , and a reflection grating  63  in the optical waveguide  62 . The substrate  60  is made of a material exhibiting the electrooptic effect. In this example, LiNbO 3  doped with MgO is used. 
     In the variable-wavelength semiconductor laser device in FIG. 6, the light beam  11  emitted from the back end surface  10   a  of the tapered-stripe semiconductor laser amplifier  10  is collimated by the collimator lens  53 , and collected by the condenser lens  54  to be focused on an end surface of the optical waveguide  62 . The light beam  11  enters the optical waveguide  62 , and propagates through the optical waveguide  62  in a guided mode. At this time, only a portion of the light beam  11  having a specific wavelength is reflected and diffracted by the reflection grating  63 . The portion of the light beam  11  reflected and diffracted by the reflection grating  63  retraces the above path to be returned to the tapered-stripe semiconductor laser amplifier  10 . 
     The wavelength of the returned light beam is simplified to a single wavelength by the reflection grating  63 , and thus the oscillation wavelength of the tapered-stripe semiconductor laser amplifier  10  is also simplified to this wavelength. A light beam  11 F having this wavelength is amplified while the light beam travels through the tapered-stripe semiconductor laser amplifier  10  in the forward direction. The amplified light beam  11 F is emitted from the front-end surface  10   c  of the tapered-stripe semiconductor laser amplifier  10 . 
     Next, explanations are provided on the change of the oscillation wavelength. 
     A pair of electrodes  65  and  66  are formed on the substrate  61  of the optical waveguide element  60  so that the reflection grating  63  in the optical waveguide  62  is located between the electrodes  56  and  57 , as illustrated in FIG.  6 . The electric field applying circuit  40 , which is similar to the electric field applying circuit  40  in FIG. 1, is connected to the electrodes  65  and  66 . 
     When a voltage is applied from the direct current power supply  42  to the electrodes  65  and  66 , an electric field is applied to the reflection grating  63  in the optical waveguide  62 . The strength of the electric field can be continuously changed by manipulating the variable resistor  43 . 
     When the electric field is applied to the reflection grating  63  in the optical waveguide  62 , the refractive index of the reflection grating  63  in the optical waveguide  62  is changed due to the electrooptic effect. Therefore, when the strength of the electric field is changed by manipulating the variable resistor  43 , the refractive index of the reflection grating  63  in the optical waveguide  62  is changed, and thus the effective pitch of the reflection grating  63  is changed. Therefore, when the strength of the electric field is changed by manipulating the variable resistor  43 , the effective pitch of the reflection grating  63  is changed, and thus the wavelength selected by the reflection grating  63 , i.e., the oscillation wavelength, is changed. That is, the oscillation wavelength can be changed arbitrarily by manipulating the variable resistor  43 . 
     As described above, the oscillation wavelength of the variable-wavelength semiconductor laser device as the third embodiment can be changed without the provision of the mechanical driving means. Therefore, the size of the variable-wavelength semiconductor laser device as the third embodiment can be reduced to a smaller size than that of the variable-wavelength semiconductor laser device which uses the mechanical driving means for changing the oscillation wavelength. In addition, the oscillation wavelength of the variable-wavelength semiconductor laser device as the third embodiment can be precisely tuned, and a stable output can be obtained. 
     In addition, all of the contents of the Japanese Patent Application No. 11(1999)-61648 are incorporated into this specification by reference.