Abstract:
An optical semiconductor device includes: semiconductor lasers; a wave coupling section multiplexing light output by the semiconductor lasers; an optical amplifying section amplifying output light of the wave coupling section; first optical waveguides respectively optically connecting respective semiconductor lasers to the wave coupling section; a light intensity lowering section located in each of the first optical waveguides and lower light intensity of reflected light that is reflected at a reflecting point located in the optical semiconductor device and that returns to the respective semiconductor lasers; to decrease line width of the light output by the semiconductor lasers.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a wavelength-variable optical semiconductor device used in optical communication systems. Specifically, the present invention relates to an optical semiconductor device wherein the increase of spectral line width due to the reflected light being reflected at the reflecting point present in the device and returning to the semiconductor laser can be inhibited. 
         [0003]    2. Background Art 
         [0004]    In the long distance communication system using relay by an optical amplifier, DWDM (Dense Wavelength Division Multiplexing) is used for increasing the transmission volume for one optical fiber. In this system, optical signals of about 80 different wavelengths are multiplexed in one fiber. At present, the development of wavelength-variable lasers that can oscillate at optional wavelengths from the used wavelength band has progressed, which has become the mainstream of the light source for long-distance optical transceivers. 
         [0005]    As a modem method, an IM-DD (Intensity Modulation-Direct Detection) system has been used in systems having the signal speed of up to 10 Gbit/s. In recently penetrating 40 Gbit/s system, phase modulation and differential detection methods are used. In the digital coherent system adopted in next-generation 100 Gbit/s systems, phase modulation systems are used. In the signal receiving side, a coherent detection system wherein local light and signal light are mixed to detect the intensity and phase information are used. 
         [0006]    In the conventional IM-DD system, since no phase information of the light is used, it is enough if the light source oscillates at a single wavelength, the phase noise causes no problems. However, in the digital coherent system, the phase noise of the signal light source and the local light source causes the deterioration of signal qualities. Although a spectrum line width is used as the indicator showing the size of the phase noise of the light source, it is required to narrow the spectrum line width for lowering the phase noise. 
         [0007]    As a method for realizing the wavelength-variable light source, an optical semiconductor device wherein a plurality of semiconductor lasers and optical amplifying sections are accumulated has been reported. In this method, any one of a plurality of semiconductor lasers arrayed in parallel is made to flash, and the output light thereof is output from a waveguide via a wave coupling section. By amplifying the output light in the optical amplifying section, light having a desired wavelength is output at a desired optical power. 
         [0008]    The spectrum line width Vo has generally the relationship shown in the following numerical expression 1. 
         [0000]        V   0 ∝( kL   DFB ) −2 ( L   DBF ) −1 (1+α2)  [Expression 1]
 
         [0000]    For realizing a narrow line width, it is desired to lengthen the laser length L DFB . Also in the above described optical semiconductor device wherein the semiconductor lasers and the optical amplifying sections are accumulated, it has been reported that the low line width of 1 MHz or below is realized by lengthening the laser length. However, there are causes to deteriorate the spectrum line width. In the case wherein a reflectivity on the front end surface is limited, the light reflected by the front end surface is again amplified by the optical amplifying section, and the reflected light returns to the semiconductor laser and causes adverse effects. 
         [0009]    When a reflectivity on the front end surface is made to be RO, the spectrum line width Δv when fed back is represented the following Numerical Expression 2 (for example, refer to IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, Vol. 15, No. 3, May/June 2009, pp. 514-520). 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       Δ 
                        
                       
                           
                       
                        
                       v 
                     
                     
                       v 
                       0 
                     
                   
                   = 
                   
                     1 
                     
                       
                         [ 
                         
                           1 
                           + 
                           
                             C 
                              
                             
                                 
                             
                              
                             sin 
                              
                             
                               { 
                               
                                 
                                   ω 
                                    
                                   
                                       
                                   
                                    
                                   τ 
                                 
                                 + 
                                 
                                   ∅ 
                                   c 
                                 
                                 + 
                                 
                                   arctan 
                                    
                                   
                                     ( 
                                     α 
                                     ) 
                                   
                                 
                               
                               } 
                             
                           
                         
                         ] 
                       
                       2 
                     
                   
                 
               
               
                 
                   [ 
                   
                     Expression 
                      
                     
                         
                     
                      
                     2 
                   
                   ] 
                 
               
             
           
         
       
     
         [0000]    Where, there is the relationship of the following numerical expressions 3 and 4. 
         [0000]    
       
         
           
             
               
                 
                   C 
                   = 
                   
                     
                       
                         R 
                         ext 
                       
                     
                      
                     
                       L 
                       ext 
                     
                      
                     
                       
                         P 
                         DFB 
                       
                       
                         
                           P 
                           av 
                         
                          
                         
                           L 
                           DFB 
                         
                       
                     
                      
                     
                       
                         K 
                         Z 
                       
                     
                      
                     
                       
                         1 
                         + 
                         
                           α 
                           2 
                         
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Expression 
                      
                     
                         
                     
                      
                     3 
                   
                   ] 
                 
               
             
             
               
                 
                   
                     R 
                     ext 
                   
                   = 
                   
                     
                       
                         R 
                         0 
                       
                        
                       
                         ( 
                         
                           
                             P 
                             SOA 
                           
                           
                             P 
                             DFB 
                           
                         
                         ) 
                       
                     
                     2 
                   
                 
               
               
                 
                   [ 
                   
                     Expression 
                      
                     
                         
                     
                      
                     4 
                   
                   ] 
                 
               
             
           
         
       
     
         [0000]    Where, v 0  represents the line width when C=0, i.e. R=0; τ represents the time required for one round trip of the oscillator exterior to LD. 
         [0010]    From the Numerical expression 2, when there is the feedback due to reflections, the spectra line width changes periodically, and becomes maximum when it is nearly 
         [0000]        C  sin {ωτ+φ c   +  arctan(α)}=−1
 
         [0000]    When change in the angular frequency for the current value applied to the laser is approximated as in the following Numerical expression 5, the line width changes periodically by the current values applied to the semiconductor laser as shown in  FIG. 3  ( a ) in IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, Vol. 15, No. 3, May/June 2009, pp. 514-520). 
         [0000]      ω−ω 0   =αI   2   DFB   +bI   DFB   [Expression 5]
 
       SUMMARY OF THE INVENTION 
       [0011]    In reality, the end-face reflectivity cannot be 0, but there is always a limited reflectivity. Therefore, the spectra line widths of a conventional optical semiconductor device wherein semiconductor lasers and optical amplifying sections are accumulated changes periodically depending upon the current values of the semiconductor lasers, and at times, increase causing problems on the system may be caused. Furthermore, when modulators or the like are further accumulated, reflection may occur from each part or the like to constitute the modulators, and similarly, the increase of the spectra line width may be caused. 
         [0012]    As long as such a limited reflectivity of a front end surface or a reflection point present in the device is present, the reflected light returns to the semiconductor laser after the reflected light is amplified in the optical amplifying section. Therefore, there was a problem wherein the increase of the spectra line width is caused depending on the driving current conditions of the semiconductor laser. 
         [0013]    In view of the above-described problems, an object of the present invention is to provide an optical semiconductor device which can inhibit the increase of the spectra line width by the reflected light. 
         [0014]    According to the present invention, an optical semiconductor device includes: a plurality of semiconductor lasers; a wave coupling section multiplexing output light of the plurality of the semiconductor lasers; an optical amplifying section amplifying output light of the wave coupling section; a first optical waveguide respectively optically connecting the plurality of semiconductor lasers to the wave coupling section; a second optical waveguide optically connecting the wave coupling section to the optical amplifying section; a third optical waveguide optically connected to an output of the optical amplifying section; and a phase regulator provided in at least one of the first, second, and third optical waveguides, and regulating a phase of reflected light that is reflected at a reflecting point present in the optical semiconductor device and returns to the plurality of semiconductor lasers. The phase regulator adjusts the phase of the reflected light so as to decrease line width of the output light of the plurality of semiconductor lasers. 
         [0015]    The present invention makes it possible to inhibit the increase of the spectra line width by the reflected light. 
         [0016]    Other and further objects, features and advantages of the invention will appear more fully from the following description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIG. 1  is a top view showing an optical semiconductor device according to the first embodiment of the present invention. 
           [0018]      FIG. 2  is a top view showing a modified example 1 of the optical semiconductor device according to the first embodiment of the present invention. 
           [0019]      FIG. 3  is a top view showing a modified example 2. 
           [0020]      FIG. 4  is a top view showing an optical semiconductor device according to the second embodiment of the present invention. 
           [0021]      FIG. 5  is a top view showing the modified example of the optical semiconductor device according to the second embodiment of the present invention. 
           [0022]      FIG. 6  is a top view showing an optical modulator according to the third embodiment of the present invention. 
           [0023]      FIG. 7  is a top view showing the modified example of the optical semiconductor device according to the third embodiment of the present invention. 
           [0024]      FIG. 8  is a top view showing an optical semiconductor device according to the fourth embodiment of the present invention. 
           [0025]      FIG. 9  is a top view showing the modified example of the optical semiconductor device according to the fourth embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0026]    An optical semiconductor device according to the embodiments of the present invention will be described with reference to the drawings. The same components will be denoted by the same symbols, and the repeated description thereof may be omitted. 
       First Embodiment 
       [0027]      FIG. 1  is a top view showing an optical semiconductor device according to the first embodiment of the present invention. On an InP substrate  1 , a plurality of semiconductor lasers  2 , a wave coupling section  3 , an optical amplifying section  4 , optical waveguides  5   a ,  5   b ,  5   c , and  5   d , and a phase regulator  6  are accumulated. A plurality of semiconductor lasers  2  are the DFB-LD (Distributed Feedback Laser Diode) array. The wave coupling section  3  is an MMI coupler (Multi-Mode Interference). The optical amplifying section  4  is an SOA (Semiconductor Optical Amplifier). A control section  7  controls the bias applied to the phase regulator  6  and controls the phase regulator  6 . 
         [0028]    The wave coupling section  3  multiplexes the output light of a plurality of the semiconductor lasers  2 . The optical amplifying section  4  amplifies the output light of the wave coupling section  3 . The optical waveguide  5   a  is optically connected to the input side of the semiconductor lasers  2 . A plurality of optical waveguides  5   b  respectively optically connect a plurality of semiconductor lasers  2  to the wave coupling section  3 . The optical waveguide  5   c  optically connects the wave coupling section  3  to the optical amplifying section  4 . The optical waveguide  5   d  is optically connected to the output of the optical amplifying section  4 . The phase regulator  6  is provided in the optical waveguide  5   d , and specifically, an electrode to which a bias is applied to the upper portion of the optical waveguide  5   d  is provided. The phase regulator  6  regulates the phase of the light that is reflected at reflecting points present in the device and returns to a plurality of semiconductor lasers  2 . 
         [0029]    When the control section  7  applies a forward bias or a reverse bias to the phase regulator  6 , by the carrier plasma effect in the forward bias applying time, by the quantum confined Stark effect or the like in the reverse bias applying time, the reflectivity of the optical waveguide  5   d  is varied, and the light path length is varied. Therefore, T in the numerical expression 2 is varied, and the term of sin in the numerical expression 2 can be optimized (where sin {ωτ+φ c +arctan (α)}=1). As described above, by adjusting the bias applied to the phase regulator  6 , the spectrum line width Av can be minimized. In addition, in the numerical expression 2, although the front end surface is assumed as the reflecting point, the feedback from the reflecting point other than the front end surface can be also expressed by a similar numerical expression by replacing RO of the numerical expression 4 with the reflectivity of the reflecting point. 
         [0030]    Then, the control section  7  adjusts the bias applied to the phase regulator  6 , and makes the phase regulator  6  adjust the phase of the reflected light so as to decrease the line width of the output light of a plurality of semiconductor lasers  2 . Thereby, the increase of the spectra line width by the reflected light can be inhibited. 
         [0031]      FIG. 2  is a top view showing a modified example 1 of the optical semiconductor device according to the first embodiment of the present invention.  FIG. 3  is a top view showing a modified example 2. In addition to the configuration in the first embodiment, an optical modulator  8  is optically connected to the output of the optical amplifying section  4 . In the modified example 1, the phase regulator  6  is provided between the optical amplifying section  4  and the optical modulator  8 . In the modified example 2, the phase regulator  6  is provided in the optical waveguide  5   e  in the output side of the optical modulator  8 . In these cases, an effect similar to the effect of the first embodiment can also be obtained. 
         [0032]    In this time, the optical waveguide  5   a  can be omitted. The layer constructions of the optical waveguides  5   a ,  5   b ,  5   c , and  5   d  can be identical to the semiconductor laser  2  or the optical amplifying section  4 , or can be butt-jointed waveguides having different construction and configuration. The optical modulator  8  can be a plurality of optical modulators connected in series. 
       Second Embodiment 
       [0033]      FIG. 4  is a top view showing an optical semiconductor device according to the second embodiment of the present invention. The phase regulator  6  is provided in the optical waveguide  5   c . In this case also, an effect similar to that in the first embodiment can be obtained. 
         [0034]      FIG. 5  is a top view showing the modified example of the optical semiconductor device according to the second embodiment of the present invention. In addition to the constitution of the second embodiment, the optical modulator  8  is optically connected to the output of the optical amplifying section  4 . In this case also, an effect similar to that in the first embodiment can be obtained. 
       Third Embodiment 
       [0035]      FIG. 6  is a top view showing an optical modulator according to the third embodiment of the present invention. A plurality of phase regulators  6  are respectively provided in a plurality of optical waveguides  5   b . In this case also, an effect similar to that in the first embodiment can be obtained. 
         [0036]      FIG. 7  is a top view showing the modified example of the optical semiconductor device according to the third embodiment of the present invention. In addition to the configuration of the third embodiment, the optical modulator  8  is optically connected to the output of the optical amplifying section  4 . In this case also, an effect similar to that in the first embodiment can be obtained. 
         [0037]    In first to third embodiments, although the phase regulators  6  are respectively provided in the optical waveguides  5   d ,  5   c , and  5   b , the present invention is not limited thereto, but the phase regulator  6  is not limited thereto, but the phase regulator  6  may be provided in at least one of the optical wave guides  5   b ,  5   c , and  5   d.    
       Fourth Embodiment 
       [0038]      FIG. 8  is a top view showing an optical semiconductor device according to the fourth embodiment of the present invention. In place of the phase regulator  6  to adjust the phase of the reflected light, a light intensity lowering section  9  to lower the light intensity of the reflected light is provided in the optical waveguide  5   b.    
         [0039]    The layer configuration of the light intensity lowering section  9  is identical to the layer configuration of the phase regulator  6 . The control section  7  supplies a larger bias to the light intensity lowering section  9  than to the phase regulator  6 , and positively generates light absorption. When light absorption occurs in the light intensity lowering section  9 , the intensity of the light inputted from the semiconductor lasers  2  to the optical amplifying section  4  is lowered. However, in the optical amplifying section  4 , if the input reaches a constant value or more, the saturation of the gain occurs. Using this characteristic, by adjusting current value to the semiconductor lasers  2  so that the power of light after passing through the light intensity lowering section  9  is in the region of the gain saturation of the optical amplifying section  4 , the effect of the loss by the light intensity lowering section  9 , the effect of the loss by the intensity lowering section  9  can be ignored. 
         [0040]    On the other hand, the reflected light from the reflection point in the device on the front end surface or on the side nearer to the front end surface than the optical amplifying section  4  is amplified by the optical amplifying section  4 , and returns to the semiconductor laser  2 . Before this, the light intensity lowering section  9  lowers the light intensity of the reflected light. Therefore, the effect of the reflected light on the semiconductor laser  2  is weakened, and the line width of output light of a plurality of semiconductor lasers  2  is decreased. Therefore, the control section  7  adjusts the bias applied to the light intensity lowering section  9 , and makes the light intensity lowering section  9  lower the light intensity of the reflected light so that the line width of the output light. Thereby, the increase of the spectrum line width by the reflected light can be inhibited. 
         [0041]    In addition, by reversed biasing the light intensity lowering section  9 , the loss of the reflected light occurs, and at the same time, change in the phase also occurs. Therefore, since the effect of the third embodiment can also be obtained, the periodical change of the line width observed in the numerical expression 2 occurs. As the bias point, in addition to the effect by the above-described absorption, the bias point to be the most suitable in the points of the phase must be searched. Furthermore, the optical waveguide  5   a  or the optical modulator  8  can be omitted. 
         [0042]      FIG. 9  is a top view showing the modified example of the optical semiconductor device according to the fourth embodiment of the present invention. In addition to the configuration of the fourth embodiment, phase regulators  6  are respectively provided in the optical waveguides  5   c ,  5   d , and  5   e . The phase regulators  6  can also be provided in only one or two of the optical waveguides  5   c ,  5   d , and  5   e . The control section  7  adjusts the bias applied to these phase regulators  6 , and makes the phase regulators  6  adjust the phase of the reflected light so that the line width of the output light of a plurality of semiconductor lasers  2  is decreased. 
         [0043]    Furthermore, in the first to fourth embodiments, an electrical resistor can be provided on the electrode of the phase regulator  6  or the light intensity lowering section  9  to make the resistor produce heat as a heater. Specifically, a forward/reverse bias is not applied to the phase regulator  6  and the light intensity lowering section  9  to change the reflectivity, but their temperatures are varied to change the reflectivity. In this case also, the same effects of the above-described first to fourth embodiments can be obtained. 
         [0044]    Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described. 
         [0045]    The entire disclosure of Japanese Patent Application No. 2012-030779, filed on Feb. 15, 2012, including specification, claims, drawings, and summary, on which the Convention priority of the present application is based, is incorporated herein by reference in its entirety.