Abstract:
A semiconductor optical integrated circuit includes: a semiconductor substrate; a light reflecting portion and a gain region, formed on the semiconductor substrate; a first optical waveguide connecting between the reflecting portion and the gain region; and a second optical waveguide formed in conjunction with the first optical waveguide and having a larger optical absorptance than that of the first optical waveguide.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]    The present application claims the benefit of patent application No. 2003-79980 filed in Japan on Mar. 24, 2003, the subject matter of which is hereby incorporated herein by reference. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention The present invention relates to an optical integrated circuit and, more particularly, to an optical integrated circuit used for an optical fiber communication system and for an optical disk device.  
           [0003]    2. Description of the Related Art  
           [0004]    A conventional interference circuit such as a wavelength converter has a pair of gain regions which are formed parallel to each other on a semiconductor substrate. The gain region has an optical amplifier which amplifies an incident light by applying forward current between the electrodes, and performs an operation using nonlinear optical effect. At least two Y optical couplers are formed on the semiconductor substrate. The optical coupler and the gain region are connected by an optical waveguide.  
           [0005]    An incident light into a semiconductor optical amplifier circuit is demultiplexed by an optical coupler, transmitted through an optical waveguide to each gain region, and then amplified in the each gain region. The light amplified in the gain region is transmitted to another optical coupler and multiplexed, and then emitted from the circuit as an emitting light (IEEE PHOTONICS TECHNOLOGY LETTERS, Vol. 13, No. 6, JUNE, 2001, pp. 600-602 )  
           [0006]    When the length of the gain region of the circuit is relatively long and therefore the gain is large, an unintended laser oscillation (i.e., parasitic oscillation) is occurred by the reflection of spontaneous emission light generated in the gain region on the optical couplers provided on the opposite sides of the gain region. Therefore, a problem may occur that the laser oscillation introduces a noise into the laser signal. Where the optical coupler includes an embedded optical waveguide (e.g., 1.5 μm in width, 200 nm in thickness) having an InGaAsP core layer formed on an InP substrate, the residual reflectance is about one percent for the light of 1.5 μm in wave length, for example.  
           [0007]    The gain in the gain region of the circuit should be made smaller by shortening the length of the gain region, for example, to eliminate the parasitic oscillation, which results in that a higher amplification can not be obtained in the circuit.  
         SUMMARY OF THE INVENTION  
         [0008]    The object of the present invention is to obtain an optical integrated circuit having large optical gain in a gain region without generating parasitic oscillation.  
           [0009]    The present invention provides a semiconductor optical integrated circuit includes: a semiconductor substrate; a light reflecting portion and a gain region, formed on the semiconductor substrate; a first optical waveguide connecting between the reflecting portion and the gain region; and a second optical waveguide formed in conjunction with the first optical waveguide and having a larger optical absorptance than that of the first optical waveguide.  
           [0010]    As the optical integrated circuit has the second optical waveguide, the light reflected on the reflecting portion can be absorbed by the second optical waveguide. Hereby, the optical gain of the gain region can be enlarged without causing any parasitic oscillation by the reflected light entering into the gain region. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    [0011]FIG. 1 is a schematic top view of an optical integrated circuit according to the first embodiment of the present invention;  
         [0012]    [0012]FIG. 2 is a cross-sectional view of the optical integrated circuit according to the first embodiment of the present invention;  
         [0013]    [0013]FIG. 3 is a schematic top view of a semiconductor optical amplifier circuit according to the second embodiment of the present invention;  
         [0014]    [0014]FIG. 4 is a schematic top view of a semiconductor optical integrated circuit according to the third embodiment of the present invention;  
         [0015]    [0015]FIG. 5 is a schematic top view of a semiconductor. optical amplifier according to the fourth embodiment of the present invention;  
         [0016]    [0016]FIG. 6 is a schematic top view of another semiconductor optical amplifier according to the fourth embodiment of the present invention;  
         [0017]    [0017]FIG. 7 is a schematic top view of an optical integrated circuit according to the fifth embodiment of the present invention; and  
         [0018]    [0018]FIG. 8 is a cross-sectional view of the optical integrated circuit according to the fifth embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0019]    Embodiment 1  
         [0020]    [0020]FIG. 1 is a schematic top view of an optical integrated circuit of the first embodiment according to the present invention, generally denoted at  100 .  
         [0021]    As shown in FIG. 1, the optical integrated circuit  100  has a semiconductor substrate  20  made of InP, for example. Two gain regions  4  and  5  are formed on the semiconductor substrate  20  in a certain direction parallel to each other. First optical waveguides  3  are connected to the opposite ends of the gain region  4 , and a Y optical couplers  1  and  2  are respectively connected to the first optical waveguides  3  through second optical waveguide  6  or  8 . Furthermore,. first optical waveguides  3  are connected to the opposite ends of the gain region  5 , and a Y optical couplers  1  and  2  are respectively connected to the first optical waveguides  3  through second optical waveguide  7  or  9 .  
         [0022]    Each of the second optical waveguides  6 ,  7 ,  8  and  9  has a core layer of a semiconductor material having smaller energy gap than that of the first optical waveguide  3 . Consequently, each of the second optical waveguides  6 ,  7 ,  8  and  9  has a higher optical absorption characteristic than that of the first optical waveguide  3 , so that it absorbs a reflected light, which will be described hereinafter.  
         [0023]    [0023]FIG. 2 is a cross-sectional view of the first optical waveguide  3  taken on line I-I of FIG. 1. As shown in FIG. 2, an InGaAsP core layer  51  and an InP layer  52 , sandwiched between InP block layers  53  and  54 , are formed on an InP substrate  50 . An InP cladding layer  55  is formed on the InP layer  52 . A Ti/Au cathode electrode  56  is formed on the back surface of the InP substrate  50 . The front surface of the InP cladding layer  55  is covered with an insulating film  57  of SiO 2 . As described above, the first optical waveguide  3  is formed by cladding the InGaAsP core layer  51  between the InP substrate  50  and the InP layer  52 .  
         [0024]    Each second optical waveguides  6 ,  7 ,  8  and  9  has the similar cross-sectional structure to that of the first optical waveguide  3  except that the composition of the InGaAsP core layer defines smaller band gap than that of the first optical waveguide  3 .  
         [0025]    The cross-sectional structure of the gain regions  4  and  5  is similar to that of the first optical waveguide  3  except that a part of the insulating film  57  is removed and the anode electrode is provided instead.  
         [0026]    Each of the second optical waveguides  6 ,  7 ,  8  and  9  can be replaced by an optical amplifier having the same structure as that of the gain regions  4  and  5 . In this instance, the optical amplifier is driven with lower current so that the loss of the incident light is generated or the gain of the incident light, if any, becomes smaller. Alternatively, the anode and the cathode may be short-circuited by wire or the like.  
         [0027]    The operation of the optical integrated circuit  100  will now be briefly described. In the optical integrated circuit  100 , the gain region has gain A, the second optical waveguides  6  and  8  have optical absorptivity L, the Y optical coupler  1  has reflectance R 1 , and the Y optical coupler  2  has reflectance R 2 . The loop gain of the reflected light, which is reflected on the optical couplers  1  and  2  and then transmitted into the gain region  4 , should be smaller than 1to avoid the parasitic oscillation of the optical integrated circuit  100 . This means that the gain A, the optical absorptivity L, and the reflectance R 1  and R 2  need to fill the following equation 1 
           A·L·R   1 · R   2 &lt;1  (1) 
         [0028]    Consequently, in the optical integrated circuit.  100  including the second optical waveguides  6  and  8  having the optical absorptivity of L, the gain A of the gain region  4  becomes 1/L times as much as that of conventional circuit (L=1) without generating the parasitic oscillation.  
         [0029]    The optical absorptivity L is calculated by the following equation: L=exp(−α· 1 ), where α is an optical absorption coefficient and  1  is a length. Therefore, the optical absorptivity L of the second optical waveguides  6  and  8  is calculated with 0.3, where the absorption coefficient α a of the second optical waveguide is 20 cm −1  and the length of the second optical waveguide is 600 μm.  
         [0030]    Consequently, if R 1  and R 2  are one percent, the parasitic oscillation occurs when the gain A exceeds 30,000, which is three times as much as the critical gain of 10,000 in the conventional optical integrated circuit.  
       Embodiment 2  
       [0031]    [0031]FIG. 3 is a schematic top view of a semiconductor optical amplifier circuit of the second embodiment according to the present invention, generally denoted at  200 . The numerals which are identical with those of FIG. 1 denote identical or corresponding components.  
         [0032]    The optical waveguides  3  of the semiconductor optical amplifier circuit  200  has diffraction gratings  10  and  11 , which are used in a distributed feedback semiconductor laser for instance, instead of the optical coupler  2  of the optical integrated circuit  100  described above. The diffraction gratings  10  and  11  are formed on the InGaAsP core layer of the optical waveguide,  3  across the InP layer of 100 nm in thickness, for instance. As shown in FIG. 3, the diffraction gratings.  10  and  11  are formed along the wave direction of the light and having a thickness (in the horizontal direction of FIG. 3) and a width (in the longitudinal direction of FIG. 3) of about 20 nm -80 nm and about 1.5 μm, respectively.  
         [0033]    The incident light is, partially reflected by the diffraction gratings  10  and  11  which are formed above the optical waveguide  3 . For instance, the incident light from the left end of the semiconductor optical amplifier circuit  200  is demultiplxed in the optical coupler  1 , and then amplified in the gain regions  4  and  5  and reaches the diffraction gratings  10  and  11 . The light reflected on the diffraction gratings  10  and  11  then reentering into the gain regions  4  and  5  causes the parasitic oscillation.  
         [0034]    In the embodiment 2, second optical waveguides  6  and  7  are formed between the optical coupler  1  and gain regions  4  and  5 , respectively. Furthermore, second optical waveguides  8  and  9  are formed between the gain regions  4  and  5  and the diffraction gratings  10  and  11 , respectively.  
         [0035]    Hereby, amount of the light reflected by the diffraction gratings  10  and  11  is reduced, so that no parasitic oscillation is generated in the gain regions  4  and  5 . Consequently, the optical gain in the gain regions  4  and  5  can be increased.  
       Embodiment 3  
       [0036]    [0036]FIG. 4 is a schematic top view of a semiconductor optical amplifier circuit of the third embodiment according to the present invention, generally denoted at  300 . The numerals which are identical with those of FIG. 1 denote identical or corresponding components.  
         [0037]    In the semiconductor optical amplifier circuit  300 , 2:1 MMI (Multimode Interferometric) type optical couplers  12  and  13 , for instance, are formed instead of the optical couplers  1  and  2  of the optical integrated circuit  100  described above. All other elements of the semiconductor optical amplifier circuit  300  are similar to those of the optical integrated circuit  100 .  
         [0038]    In the semiconductor optical amplifier circuit  300 , the light reflected on the MMI type optical couplers  12  and  13  is absorbed in the second optical waveguides  6  and  7  formed between the gain regions  4  and  5  and the optical couplers  12  and  13 . Hereby, no parasitic oscillation is generated in the gain regions  4  and  5 , so that the optical gain can be increased.  
       Embodiment 4  
       [0039]    [0039]FIG. 5 is a partial schematic view of a semiconductor optical amplifier of the fourth embodiment according to the present invention, generally denoted at  400 . In FIG. 5, the numerals which are identical with those of FIG. 1 denote identical or corresponding components. In the semiconductor optical amplifier  400 , the second optical waveguide  6  is formed between the end face  21  which the light passes and the gain region  4 .  
         [0040]    Generally, the end face, which the light passes, of an optical integrated circuit is coated with a dielectric multilayer film, alumina for example, so that the reflectance of the light (incident light and/or emitting light) on the end face is decreased. However, it is difficult to make the reflectance smaller over the large wavelength range. As shown in FIG. 5, the optical waveguide  3  inclines from the normal line of the end face  21  and is connected to the end face  21 , so that the reflectance substantially becomes smaller. In this structure, however, the reflectance is still about ten percent of that of the structure in which the optical waveguide  3  is connected perpendicularly to the end face  21 .  
         [0041]    In the semiconductor optical amplifier  400 , the second optical waveguide  6  is formed between the end face  21  and the gain region  4  to absorb the residual reflected light. Consequently, the optical gain in the gain region  4  can be larger without generating the parasitic oscillation in the gain region  4 .  
         [0042]    [0042]FIG. 6 is a partial schematic view of another semiconductor optical amplifier of the fourth embodiment according to the present invention, generally denoted at  500 . In FIG. 6, the numerals which are identical with those of FIG. 1 denote identical or corresponding components. In the semiconductor optical amplifier  500 , the second optical waveguide  6  is formed between the end face  22  through which the light is emitted and the gain region  4 .  
         [0043]    Generally, the S/N ratio of the semiconductor optical amplifier is deteriorated by a spontaneous emission light generated in the optical amplifier. In the semiconductor optical amplifier  500 , the second optical waveguide  6  is formed between the end face  22  and the gain region  4  to absorb the reflected light on the end face  22 , so that the parasitic oscillation is not generated and the noise caused by the spontaneous emission light can be attenuated. Consequently, the Noise Figure (NF) of the optical amplifier  500  can be smaller.  
       Embodiment 5  
       [0044]    [0044]FIG. 7 is a perspective view of a hybrid type optical integrated circuit of the fifth embodiment according to the present invention, generally denoted at  600 . FIG. 8 is a cross-sectional view of the circuit taken on line VII-VII of FIG. 7.  
         [0045]    The optical integrated circuit  600  has a substrate  107  made of silicon or quartz glass, for example. Semiconductor optical amplifiers  101  and  102  are fixed on the substrate  107  to amplify light, and semiconductor optical amplifiers  103 ,  104 ,  105  and  106  are also fixed on the substrate  107  to absorb light. These semiconductor optical amplifiers  101 - 106  are made separately from the substrate  107 , and then is attached on the substrate  107 . For example, the semiconductor optical amplifiers  101 - 106  are die bonded on the surface of the substrate  107  so that each surface of the amplifiers  101 - 106  having pn junction is contacted with the surface of the substrate  107  (junction down). Optical waveguides  108  are formed on the surface of the substrate  107  and are connected with the amplifiers  101 - 106 . As shown in FIG. 8, the optical waveguide  108  essentially consists of core layer  108   b  made of silicon for instance, into which the upper and lower sides are inserted in the cladding layers  108   a  and  108   c , and is formed on the surface of the substrate  107 .  
         [0046]    In the hybrid type optical integrated circuit as well as a monolithic type semiconductor optical amplifier circuit, the spontaneous emission light generated in the semiconductor optical amplifier for amplifying the light is reflected on the optical coupler, and then enter into the semiconductor optical amplifier again, so that the parasitic oscillation is generated in the semiconductor optical amplifier.  
         [0047]    In the optical integrated circuit  600  according to the fifth embodiment of the present invention, each of the semiconductor optical amplifiers  103 - 106  for absorbing the light is formed between the semiconductor optical amplifier  101  or  102  for amplifying the light and the optical coupler, so that the semiconductor optical amplifiers  103 - 106  absorb the reflected light from the optical coupler. The semiconductor optical amplifiers  103 - 106  for absorbing the light are driven with lower current so that the loss of the reflected light is generated or the gain of the reflected light, if any, becomes smaller.  
         [0048]    Each of the semiconductor optical amplifiers  103 - 106  may be replaced by an optical waveguides which absorb the reflected light from the optical coupler.  
         [0049]    By using this structure, the optical gain of the. hybrid type optical integrated circuit  600  can be enlarged without generating the parasitic oscillation in the semiconductor optical amplifiers  101  and  102  for amplifying the light.