Abstract:
A semiconductor optical modulator includes a substrate, which has a first conductivity type, and a first electrode on a first main surface of the substrate. A first cladding layer having the first conductivity type, a transparent waveguide layer, a second cladding layer having the first conductivity type, an optical-absorption layer, and a third cladding layer having a second conductivity type, are sequentially laminated on a second main surface of the substrate. A ridge part is formed by removing a part of the third cladding layer and a part of the second cladding layer in a laminated direction. A second electrode on the ridge part is electrically connected to the third cladding layer.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority from Japanese Patent Application No. 2012-234010 filed on Oct. 23, 2012, the entire subject matter of which is incorporated herein by reference. 
       TECHNICAL FIELD 
       [0002]    This disclosure relates to an electro-absorption semiconductor optical modulator that is used in an optical transmitter for optical fiber communication and the like. 
       BACKGROUND 
       [0003]    As a light source of an optical transmitter for optical fiber communication for high speed/long distance, an optical modulator integrated semiconductor laser is useful in which a semiconductor laser and a semiconductor optical modulator are monolithically integrated. In an optical modulator unit of the optical modulator integrated semiconductor laser, an electro-absorption optical modulator is used. As a waveguide structure thereof, a high-mesa ridge type, where a core layer (optical waveguide layer) is provided at an inner side of a ridge, or a low-mesa ridge type, where a core layer is provided below a ridge is adopted (for example, refer to JP-A-2008-10484 (paragraphs [0038] to [0039] and FIG. 2 of JP-A-2008-10484)). 
       SUMMARY 
       [0004]    According to the electro-absorption optical modulator having the low-mesa ridge structure, a strong electric field is applied to the optical waveguide layer below the ridge by applying a negative voltage to an anode part. As a result, an optical-absorption coefficient of the optical waveguide layer is increased by the Quantum Confined Stark Effect, so that a light quenching operation is made. In this structure, since the optical waveguide layer also serves as an optical-absorption layer, the optical-absorption coefficient of the largest area of a light distribution is made to be largest. In general, the light has a property of propagating toward an area having a small optical-absorption coefficient while avoiding an area having a large optical-absorption coefficient. Accordingly, the unimodality of light that is propagated through the waveguide of the optical modulator breaks down, and then a shape of the laser light that is emitted from the optical modulator is not unimodal. 
         [0005]    In view of the above, this disclosure provides at least a semiconductor optical modulator where a shape of emitted laser light is unimodal. 
         [0006]    A semiconductor optical modulator of this disclosure includes: a substrate, which has a first conductivity type, and which includes a first electrode formed on a first main surface thereof; a first clad layer having the first conductivity type, a transparent waveguide layer, a second clad layer having the first conductivity type, an optical-absorption layer, and a third clad layer having a second conductivity type, which are sequentially laminated on a second main surface of the substrate from the substrate; a ridge part, which is formed by removing the third clad layer and a part of the second clad layer in a laminated direction, and a second electrode, which is formed on the ridge part and is connected to the third clad layer. 
         [0007]    According to this disclosure, since an optical-absorption area exists at an end of a light distribution, it is possible to obtain a semiconductor optical modulator where a shape of emitted laser light is unimodal. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    The foregoing and additional features and characteristics of this disclosure will become more apparent from the following detailed descriptions considered with the reference to the accompanying drawings, wherein: 
           [0009]      FIG. 1A  is a perspective view illustrating a semiconductor laser according to a first illustrative embodiment of this disclosure, and  FIG. 1B  illustrates a light distribution at a light emission point; 
           [0010]      FIG. 2  is a perspective view illustrating a semiconductor laser according to a second illustrative embodiment of this disclosure; 
           [0011]      FIG. 3  is a perspective view illustrating the semiconductor laser according to the second illustrative embodiment of this disclosure; 
           [0012]      FIG. 4  is a perspective view illustrating a semiconductor laser according to a third illustrative embodiment of this disclosure; 
           [0013]      FIG. 5  is a perspective view illustrating a semiconductor laser according to a fourth illustrative embodiment of this disclosure; 
           [0014]      FIG. 6  is a perspective view illustrating a semiconductor laser according to a fifth illustrative embodiment of this disclosure; 
           [0015]      FIG. 7  illustrates a relation between a horizontal/vertical transverse mode and an optical-absorption area of the background art; 
           [0016]      FIGS. 8A and 8B  illustrate relations between horizontal/vertical transverse modes and optical-absorption areas; and 
           [0017]      FIG. 9  is a perspective view illustrating a semiconductor optical modulator of the background art. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]    A semiconductor optical modulator according to illustrative embodiments of this disclosure will be described with reference to the drawings. The same or corresponding elements are denoted with the same reference numerals and the overlapping descriptions may be omitted. 
       First Illustrative Embodiment 
       [0019]      FIG. 1A  is a perspective view illustrating an optical modulator integrated semiconductor laser according to a first illustrative embodiment of this disclosure. In  FIG. 1A , a reference numerals  1  indicates an n electrode made of Ti/Pt/Au, a reference numerals  2  indicates a substrate made of n-type InP, a reference numerals  3  indicates a first clad layer made of n-type InP, a reference numerals  4  indicates a transparent waveguide layer made of Multi Quantum Well (MQW), a reference numerals  5  indicates a second clad layer made of n-type InP, a reference numerals  6  indicates an optical-absorption layer made of Multi Quantum Well (MQW), a reference numerals  7  indicates a third clad layer made of p-type InP, a reference numerals  8  indicates a ridge part, a reference numerals  9  indicates a channel part, a reference numerals  10  indicates a pedestal part, a reference numerals  11  indicates an insulation film made of SiO 2 , and a reference numerals  12  indicates a p electrode made of Ti/Pt/Au. The Multi Quantum Well is an InGaAsP-MQW in which an undoped InGaAsP well layer and an undoped InGaAsP barrier layer are alternately stacked. However, this disclosure is not limited thereto. For example, AlGaInAs-MQW and the like may be also used. In the meantime, the semiconductor laser is formed at the rear of the optical modulator in the drawing (not shown) so that it is close to the optical modulator. 
         [0020]      FIG. 1B  shows a light distribution  15  at a light emission point  13  from which a laser light  14  is emitted. The light distribution  15  at the light emission point  13  is referred to as a near-field image and has an elliptical shape as shown. The near-field image is evaluated with being divided in a horizontal direction (X direction) and a vertical direction (Y direction), which are respectively referred to as a horizontal transverse mode  16  and a vertical transverse mode  17 . 
         [0021]    For comparison,  FIG. 9  shows a perspective view illustrating an optical modulator of the background art. In  FIG. 9 , a reference numeral  103  indicates a clad layer made of n-type InP, a reference numeral  104  indicates an optical-absorption layer made of Multi Quantum Well (MQW) and a reference numeral  105  indicates a clad layer made of p-type InP. 
         [0022]    In the optical modulator of this disclosure, the transparent waveguide layer  4  is provided at the position of the optical-absorption layer  104  of the optical modulator of the background art, and the transparent waveguide layer  4  is sandwiched between the n-type semiconductor layers. Also, the optical-absorption layer  6  is positioned above the transparent waveguide layer  4  and is sandwiched between the n-type and p-type semiconductor layers (the second clad layer  5  and the third clad layer  7 ). 
         [0023]    In order to manufacture the optical modulator of this illustrative embodiment, the first clad layer  3 , the transparent waveguide layer  4 , the second clad layer  5 , the optical-absorption layer  6  and the third clad layer  7  are laminated and grown on the n-type InP substrate  2  by a MOCVD method. Then, the channel  9  is etched to form the ridge part  8  and the pedestal part  9  by a wet etching and the like. Subsequently, the insulation film  11 , the n electrode  1  and the p electrode  12  are formed to manufacture the optical modulator. 
         [0024]    In the below, operations are described. The laser light emitted from the semiconductor laser is incident (not shown) onto the transparent waveguide layer  4  from the rear of  FIG. 1A  and propagates in a z direction from the transparent waveguide layer  4  serving as a core layer. When a negative voltage is applied to the p electrode  12 , the optical-absorption layer  6  sandwiched between the n-type and p-type semiconductor layers (the second clad layer  5  and the third clad layer  7 ) is applied with an electric field and an optical-absorption coefficient is thus increased, so that the optical-absorption layer  6  absorbs the laser light. Since the transparent waveguide layer  4  is sandwiched between the n-type semiconductor layers (the first clad layer  3  and the second clad layer  5 ), the transparent waveguide layer  4  is not applied with an electric field, so that it does not absorb the laser light. 
         [0025]    Meanwhile, as shown in  FIG. 8A , a center of the vertical transverse mode  17  is in the transparent waveguide layer  4 , and an optical-absorption area  18  (optical-absorption layer  6 ) exists at an end of the vertical transverse mode  17 . Therefore, the unimodality of the light distribution  18  scarcely breaks down, so that a shape of the emitted laser light  14  is not degraded. 
         [0026]    On the other hand, according to the optical modulator of the background art, as shown in  FIG. 7 , the centers of the horizontal transverse mode  16  and vertical transverse mode  17  are in the optical-absorption area  18  (optical-absorption layer  104 ), and thus the optical-absorption coefficient is large. Accordingly, the light intends to propagate towards both sides having smaller optical-absorption coefficients while avoiding the area having the larger optical-absorption coefficient. Thereby, the unimodality of the light distribution breaks down, so that a shape of the emitted laser light  14  is degraded. 
         [0027]    According to this illustrative embodiment, since the optical-absorption area exists at the end of the light distribution propagating through the optical waveguide, it is possible to implement a light quenching operation without breaking down the unimodality of the light distribution  15 . Therefore, it is possible to obtain the optical modulator where the shape of the emitted laser light  14  is kept unimodal. 
       Second Illustrative Embodiment 
       [0028]      FIG. 2  is a perspective view illustrating an optical modulator according to a second illustrative embodiment. In  FIG. 2 , a reference numeral  21  indicates a clad layer made of n-type InP, a reference numeral  26  indicates an optical-absorption layer made of Multi Quantum Well and a reference numeral  22  indicates a clad layer made of p-type InP. Also, a reference numeral  23  indicates a buried layer made of undoped InP, a reference numeral  24  indicates a transparent waveguide layer and a reference numeral  25  indicates a clad layer made of p-type InP. 
         [0029]    In the second illustrative embodiment, the optical-absorption layer  26  is provided in the clad layer below the transparent waveguide  24  and is sandwiched between the n-type semiconductor (clad layer  21 ) and the p-type semiconductor (clad layer  22 ). 
         [0030]    In order to manufacture the optical modulator of this illustrative embodiment, the n-type InP clad layer  21 , the MQW optical-absorption layer  26  and the p-type InP clad layer  22  are laminated and grown on the n-type InP substrate  2  by the MOCVD method. Then, a ridge stripe pattern is formed by a wet etching and the like and the undoped InP buried layer  23  is buried and grown at both sides of the ridge stripe. Subsequently, the transparent waveguide layer  24  and the p-type InP clad layer  25  are laminated and grown by the MOCVD, and then the ridge part  8  is formed by the same method as the first illustrative embodiment. 
         [0031]    Also in the optical modulator of this illustrative embodiment, the same effects as the first illustrative embodiment are obtained. Also, since a capacitance is reduced by the buried layer  23 , it is possible to obtain the optical modulator having excellent high-speed responsiveness. 
         [0032]    Meanwhile, in this illustrative embodiment, the buried layer  23  is used. However, as shown in  FIG. 3 , a configuration where the buried layer  23  is not provided may be also used. 
       Third Illustrative Embodiment 
       [0033]      FIG. 4  is a perspective view illustrating an optical modulator according to a third illustrative embodiment. In  FIG. 4 , a reference numeral  33  indicates a clad layer made of n-type InP, a reference numeral  34  indicates a transparent waveguide layer made of Multi Quantum Well (MQW), a reference numeral  35  indicates a clad layer made of p-type InP and a reference numeral  36  indicates a p electrode, respectively. 
         [0034]    The optical modulator of this illustrative embodiment has a configuration where the arrangement of the p electrode  12  is changed in the modulator having a structure shown in  FIG. 9 . 
         [0035]    In the below, operations are described. The laser light emitted from the semiconductor laser is incident into the transparent waveguide layer  34  and propagates in the transparent waveguide layer  34  serving as a core layer. When a negative voltage is applied to the p electrode  36 , the transparent waveguide layer  34  sandwiched between the n-type and p-type semiconductor layers (the clad layer  33  and the clad layer  35 ) is applied with an electric field and an optical-absorption coefficient is thus increased, so that the laser light is absorbed. At this time, the electric field is mainly applied to the transparent waveguide layer  34  just below the channel  9  and is not applied to the transparent waveguide layer  34  just below the ridge, so that the absorption area  18  is eccentrically distributed in the transparent waveguide layer  34  just below the channel  9 . Therefore, as shown in FIG.  8 B, the light is not absorbed at the center of the horizontal transverse mode  16 , and the optical-absorption area  18  exists at both ends of the horizontal transverse mode  16 . Accordingly, the unimodality of the light distribution  15  scarcely breaks down, so that the shape of the emitted laser light  14  is not degraded. 
       Fourth Illustrative Embodiment 
       [0036]      FIG. 5  is a perspective view illustrating an optical modulator according to a fourth illustrative embodiment. In  FIG. 5 , a reference numeral  37  indicates a p electrode, and an arrangement of p electrode  37  is changed the arrangement of the p electrode  36  in the third illustrative embodiment. 
         [0037]    In the optical modulator of this illustrative embodiment, the electric field is mainly applied to the transparent waveguide layer  34  just below the pedestal  10  and is not applied to the transparent waveguide layer  34  just below the ridge, the absorption area  18  is eccentrically distributed in the transparent waveguide layer  34  just below the pedestal  10 . Therefore, as shown in  FIG. 8B , the light is not absorbed at the center of the horizontal transverse mode  16 , and the optical-absorption area  18  exists at both ends of the horizontal transverse mode  16 . Accordingly, the unimodality of the light distribution  15  scarcely breaks down, so that the shape of the emitted laser light  14  is not degraded. 
       Fifth Illustrative Embodiment 
       [0038]      FIG. 6  is a perspective view illustrating an optical modulator according to a fifth illustrative embodiment.  FIG. 6  shows a configuration where the p electrode  12  is added to the optical modulator of  FIG. 5 . Instead of the configuration of  FIG. 6 , the p electrode  12  may be added to the optical modulator having the configuration of  FIG. 4 . The same effects as the first illustrative embodiment are obtained, and an effect of increasing the optical-absorption area to thus shorten a length of the optical modulator is also obtained. 
         [0039]    Also, by independently controlling voltages to be applied to the three p electrodes, it is possible to obtain an effect of controlling the shape of the emitted laser light and an emission direction thereof. 
         [0040]    In the above illustrative embodiments, the optical modulator integrated semiconductor laser has been exemplified. However, even when a single laser and a single semiconductor optical modulator are used, the same effects are obtained. 
         [0041]    Although the n-type substrate has been exemplified, a p-type substrate may be also used. In this case, the conductivity types of the n-type and p-type may be reversed each other. Although the InP-based material has been exemplified as the semiconductor material, the other materials may be also used. 
         [0042]    The configuration where the p electrode and the clad layer are directly connected has been illustrated. However, when the p electrode and the clad layer are connected with a contact layer being sandwiched between the p electrode and the clad layer, it is possible to form an ohmic electrode more securely.