Abstract:
A first optical waveguide for guiding light is formed on a substrate. A surface acoustic wave transducer for generating a surface acoustic wave which propagates along a direction in which the first optical wave guide guides the light at least partial region of the first optical waveguide, is formed on the substrate. A wavelength variable laser oscillator can be formed which is easy to control and has a fast response speed.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
         [0001]    This application is based on Japanese Patent Application No. 2002-074869, filed on Mar. 18, 2002, the entire contents of which are incorporated herein by reference.  
         BACKGROUND OF THE INVENTION  
         [0002]    A. FIELD OF THE INVENTION  
           [0003]    The present invention relates to a wavelength controllable optical device, and more particularly to a wavelength controllable optical device suitable for application to a wavelength variable laser oscillator.  
           [0004]    B. DESCRIPTION OF THE RELATED ART  
           [0005]    A sampled grating laser oscillator, a vertical cavity surface emitting laser (VCSEL) integrated with a micro electro-mechanical systems (MEMS) mirror, and the like are known as wavelength variable laser oscillators.  
           [0006]    [0006]FIG. 3A is a schematic cross sectional view of a sampled grating laser oscillator. An active layer  100  is sandwiched between a lower clad layer  101  and an upper clad layer  102 . Along a light propagation direction, a first reflector region  110 , a gain region  111 , a phase adjusting region  112  and a second reflector region  113  are defined.  
           [0007]    In the first and second reflector regions  110  and  113 , gratings  115  and  116  are formed at the interface between the active region  100  and lower clad layer  101 . The gratings  115  and  116  have the structure that a distributed Bragg reflector (DBR) is cut off at a constant pitch, and peaks of the reflection spectrum are disposed at regular intervals.  
           [0008]    The DBR cutting off period of the grating  115  of the first reflector region  110  is different from that of the grating  116  of the second reflector region  113 . The pitches of peaks of the reflection spectra of the gratings  115  and  116  are therefore different. One of a plurality of peaks of the reflection spectrum of the first reflector region  110  is superposed upon one of the peaks of the reflection spectrum of the second reflector region  113 . The laser oscillator oscillates at the wavelength where the two peaks are superposed.  
           [0009]    Carriers are injected from a common electrode  120  into the active region  100  via the lower clad layer  101 . Electrodes  121 ,  122 ,  123  and  124  are formed on the surfaces of the first reflector region  110 , gain region  111 , phase adjusting region  112  and second reflector mirror region  113 , respectively of the upper clad layer  102 . As carriers are injected from the electrode  121  on the surface of the first reflector region  110  into the active layer  100 , the peak position of the reflection spectrum of the first reflector region  110  is shifted. Of the peaks of the reflection spectra of the first and second reflector regions  110  and  113 , peaks are superposed which are different from the peaks superposed when carriers are not injected. Therefore, the oscillation wavelength of the laser oscillator changes.  
           [0010]    A change in the oscillation wavelength has a discontinuity corresponding to about the pitches between a plurality of peaks of the reflection spectra of the first and second reflector regions  110  and  113 . As current is injected from the electrode  123  on the surface of the phase adjusting region  112  into the active layer  100 , a refractive index of the phase adjusting region  112  changes so that an effective optical resonator length changes. Fine adjustment of the oscillation wavelength of the optical resonator is therefore possible.  
           [0011]    [0011]FIG. 3B is a schematic cross sectional view of a MEMS mirror integrated VCSEL. A recess is formed in the back surface layer of a semiconductor substrate  130  and an exciting laser oscillator  139  is mounted on the bottom surface of the recess (on the upper surface of the recess as viewed in FIG. 3B). On the principal surface of the semiconductor substrate  130 , an active layer  131  and a cap layer  132  are stacked in this order.  
           [0012]    An electrode  133  is formed on the surface of the cap layer  132 . An opening  133   a  is formed through the electrode  133  above the exciting laser oscillator  139 . A mirror holder  135  is mounted on the electrode  133  via a spacer  134 . The mirror holder  135  is disposed above the electrode  133  at a position spaced apart from the electrode  133  by a predetermined distance. An opening  135   a  is formed through the mirror holder  135  at the position corresponding to the opening  133   a . A mirror  138  covers the opening  135   a . The mirror  138  and exciting laser oscillator  139  constitute an optical resonator.  
           [0013]    As a d.c. voltage is applied across the electrode  133  and mirror holder  135 , the distance between the mirror holder  135  and electrode  133  is shortened by a Coulomb force. Since the mirror  138  is displaced toward the semiconductor substrate  130  side, an optical resonator length is shortened. In this manner, by changing the optical resonator length, the oscillation wavelength can be changed.  
           [0014]    The oscillation wavelength of the sampled grating laser oscillator can be changed almost continuously by shifting the peak position of the reflection spectrum of the reflector region  110  or  113  and changing the refractive index of the phase adjusting region  112 . However, adjustment becomes complicated because the refractive indices of both one reflector region  110  and the phase adjusting region  112  are to be adjusted.  
           [0015]    The MEMS mirror integrated VCSEL utilizes a mechanical structure for displacing the mirror. A response speed is therefore low in the order of several ms.  
         SUMMARY OF THE INVENTION  
         [0016]    An object of this invention is to provide a wavelength controllable optical device capable of configuring a wavelength variable laser oscillator which is easy to control and has a fast response speed.  
           [0017]    According to one aspect of the present invention, there is provided a wavelength controllable optical device, comprising: a first optical waveguide for guiding light, the first optical waveguide being formed on a substrate; and a surface acoustic wave transducer for generating a surface acoustic wave which propagates along a direction of guiding the light toward at least some region of the first optical waveguide, the surface acoustic wave transducer being formed on the substrate.  
           [0018]    According to another aspect of the present invention, there is provided a light control method comprising steps of: propagating light in a first optical waveguide of a wavelength controllable optical device comprising: the first optical waveguide for guiding light, the first optical waveguide being formed on a substrate; and a surface acoustic wave transducer for generating a surface acoustic wave which propagates along a direction of guiding the light toward at least some region of the first optical waveguide, the surface acoustic wave transducer being formed on the substrate; and applying an a.c. voltage to the surface acoustic wave transducer to excite a surface acoustic wave.  
           [0019]    A refractive index grating whose refractive index changes periodically is formed in the first optical waveguide because of the influence of a surface acoustic wave. This refractive index grating causes the light propagation characteristics of the first optical waveguide to have a wavelength dependency.  
           [0020]    As above, by utilizing the interaction between an optical waveguide and a surface acoustic wave, the propagation characteristics of the optical waveguide can be made to have a wavelength dependency. By utilizing these characteristics, a wavelength variable laser oscillator can be manufactured. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]    [0021]FIGS. 1A and 1B are a schematic plan view and a schematic cross sectional view of a wavelength variable laser oscillator according to a first embodiment of the invention.  
         [0022]    [0022]FIG. 2 is a schematic plan view of a wavelength variable laser oscillator according to a second embodiment of the invention.  
         [0023]    [0023]FIGS. 3A and 3B are cross sectional views of conventional wavelength variable laser oscillators. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0024]    A plan view of a wavelength variable laser oscillator according to a first embodiment of the invention is shown in FIG. 1A. On the surface of a substrate  1  of n-type InP, an optical waveguide  10  of InGaAsP is formed. The optical waveguide  10  is constituted of a linear first region  10   a , a linear second region  10   b  and a linear third region  10   c  and has a width of about 3 μm. The first and third regions  10   a  and  10   c  extend in an X-axis direction (in a crosswise direction in FIG. 1A) and the second region  10   b  extends in a Y-axis direction (in a lengthwise direction in FIG. 1A). The first to third regions  10   a  to  10   c  are smoothly coupled in this order and have a plan shape like a crank bent at right angles. The lengths of the first to third regions  10   a  to  10   c  are 10 mm, 2 mm and 4 mm, respectively.  
         [0025]    A high reflective film is formed on the end face of the optical waveguide  10  on the third region  10   c  side and a low reflective film is formed on the end face of the optical waveguide  10  on the first region  10   a  side. The reflective films on both end faces of the optical waveguide  10  constitute an optical resonator.  
         [0026]    A surface acoustic wave transducer  20  is disposed along a straight line drawn by extending the first region  10   a  toward the connection side of the second region  10   b . The surface acoustic wave transducer  20  includes a pair of comb electrodes  21   a  and  21   b  meshed with each other. Teeth of the comb electrodes  21   a  and  21   b  are disposed along the X-axis direction at an equal pitch of 40 μm. The width of each tooth is 20 μm. An a.c. power source applies a voltage having a predetermined frequency across the comb electrodes  21   a  and  21   b.    
         [0027]    A cross sectional view taken along one-dot chain line B 1 -B 1  shown in FIG. 1A is shown in FIG. 1B. On the principal surface of an n-type InP substrate  1 , a lower clad layer  2  of n-type InP having a thickness of 2 μm, an active layer  3  of InGaAsP having a thickness of 100 nm, an upper clad layer  4  of p-type InP having a thickness of 2 μm and a contact layer  5  of p-type InGaAsP having a thickness of 400 nm are laminated in this order. These layers are formed, for example, by metal organic chemical vapor deposition (MO-CVD).  
         [0028]    This lamination structure is mesa-etched to the intermediate depth of the lower clad layer  2  to leave a ridge-like structure having a plan shape corresponding to the optical waveguide  10  shown in FIG. 1A. This etching can be performed, for example, by dry-etching using CF 4  by using a silicon oxide film as a mask.  
         [0029]    In the etched region, a first buried layer  6  of p-type InP having a thickness of 1.2 μm and a second buried layer  7  of n-type InP having a thickness of 2.3 μm are buried. The buried layers  6  and  7  may be formed by MOCVD. InP can be grown only in the etched region by covering the upper surface of the ridge structure with an insulating film of silicon oxide or the like used as the mesa-etching mask.  
         [0030]    The InGaAsP active layer  3 , whose upper, lower and side wall surfaces are surrounded by InP, constitutes the optical waveguide  10  shown in FIG. 1A.  
         [0031]    A piezoelectric film  30  of zinc oxide (ZnO) having a thickness of 0.2 μm is formed on the second buried layer  7 . For example, the piezoelectric film  30  can be formed by sputtering using a zinc oxide target. An opening  30   a  is formed through the piezoelectric film  30 , exposing the upper surface of the contact layer  5 .  
         [0032]    The comb electrodes  21   a  and  21   b  are formed on the surface of the piezoelectric film  30  and an electrode  25  is formed on the bottom of the opening  30   a . For example, the electrodes  21   a ,  21   b  and  25  are made of a lamination of Ti/Pt/Au. The electrode  25  is in ohmic contact with the contact layer  5 .  
         [0033]    Next, the operation principle of the wavelength variable laser oscillator shown in FIGS. 1A and 1B will be described. As an a.c voltage is applied across the comb electrodes  21   a  and  21   b , a surface acoustic wave is excited in the piezoelectric film  30 . The excited surface acoustic wave propagates along the X-axis direction shown in FIG. 1A. The first region  10   a  of the optical waveguide  10  is influenced by the surface acoustic wave so that a refractive index grating is formed whose refractive index changes periodically along the light propagation direction. The pitch of the refractive index grating is equal to the wavelength of the surface acoustic wave.  
         [0034]    The refractive index grating functions in a manner similar to the grating of a distributed feedback (DFB) type laser oscillator so that light having the wavelength corresponding to the pitch of the refractive index grating is excited with a priority over other wavelengths. As the frequency of the a.c. voltage applied across the comb electrodes  21   a  and  21   b  is changed, the wavelength of the surface acoustic wave is also changed. By changing the frequency of the a.c. voltage, the oscillation frequency of the laser oscillator can be changed.  
         [0035]    Generally, if the pitch of teeth of the comb electrodes  21   a  and  21   b  is made equal to a half wavelength of a surface acoustic wave to be excited, the surface acoustic wave can be excited most efficiently. In the first embodiment, although the pitch of teeth is fixed, a surface acoustic wave can be excited efficiently to a sufficient level if a deviation between the half wavelength of a surface acoustic wave and the pitch of teeth is about 8%.  
         [0036]    A depth down to which a surface acoustic wave has influence is about 10 μm. In the first embodiment, a depth from the surface of the piezoelectric film  30  to the bottom of the active layer  3  is 2.7 μm so that the active layer  3  can be influenced sufficiently by a surface acoustic wave.  
         [0037]    The frequency of the a.c. voltage of the a.c. power source  22  used in the first embodiment is about 50 MHz. It is easy to form a variable frequency driver circuit in such a frequency band. A time required for a surface acoustic wave to propagate through the first region  10   a  of the optical waveguide  10  is about 1 μs. The wavelength can therefore be changed at as high response speed as about 1 μs.  
         [0038]    A schematic plan view of a wavelength variable laser oscillator according to a second embodiment is shown in FIG. 2. The structures of a semiconductor substrate  1 , an optical waveguide  10  and a surface acoustic wave transducer  20  are similar to those of the wavelength variable laser oscillator of the first embodiment shown in FIG. 1A.  
         [0039]    In the second embodiment, at the side of the first region  10   a  of the optical waveguide  10 , another optical waveguide  11  is disposed. A distance between the optical waveguides  10  and  11  is about 1 μm and the two optical waveguides constitute a directional coupler  12 . The optical waveguide  11  is formed by the same processes as those of forming the optical waveguide  10 , and the lamination structures of the two optical waveguides are the same.  
         [0040]    A surface acoustic wave excited by the surface acoustic wave transducer  20  propagates through the first region  10   a  of the optical waveguide  10  and through the region where the optical waveguide  11  is disposed.  
         [0041]    A directional coupler  12  functions as a band-pass filter. The laser oscillator oscillates at the wavelength in the band-pass range of the band pass filter. The wavelength in the band-pass range of the band pass filter changes with the wavelength of a surface acoustic wave. Therefore, by changing the frequency of the a.c. voltage of the a.c power source  22  shown in FIG. 2, the oscillation frequency of the laser oscillator can be changed.  
         [0042]    A wavelength variable laser oscillator of the second embodiment was manufactured and the output characteristics thereof were evaluated. The wavelength was able to change in the range of 1530 to 1560 nm and the side band suppression ratio was 30 dB.  
         [0043]    In the above embodiments, although a laser oscillator of an InGaAsP series has been described, other compound semiconductors may be used as the materials of the active layer and clad layers. Although ZnO is used as the material of the piezoelectric film, other piezoelectric materials may also be used. In the above embodiments, a laser oscillator utilizing an interaction between an optical waveguide and a surface acoustic wave has been described. This interaction between an optical waveguide and a surface acoustic wave may be used not only for a laser oscillator but also for other devices which provide a wavelength dependency of the propagation characteristics of light propagating in an optical waveguide. For example, by utilizing this technique, a wavelength variable distributed Bragg reflector can be obtained.  
         [0044]    The present invention has been described in connection with the preferred embodiments. The invention is not limited only to the above embodiments. It is apparent that various modifications, improvements, combinations, and the like can be made by those skilled in the art.