Abstract:
A Mach-Zehnder type semiconductor device includes electrodes for injecting carriers into first and second branch waveguides so as to change reflection indexes of the first and second branch waveguides. The device further includes electrodes which are placed above either one of the first or second branch waveguides or both of the first or second branch waveguides so as to remove the carriers. The Mach-Zehnder type semiconductor device can adjust a phase difference between first and second split light transmitted through the first and second branch waveguides, respectively, even if the lengths of the first and second branch waveguides deviate from designed values. The Mach-Zehnder type semiconductor device also can suppress optical loss of the first and second light, and generate outgoing light without deterioration.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a Mach-Zehnder type semiconductor device and a method of controlling the same. 
     2. Description of the Related Art 
     A Mach-Zehnder type optical modulator usable for an optical communication system and an optical information processing system is disclosed in, for example, “Japanese Patent Kokai No. 2000-28979” where a LiNbO 3  crystal is used as a material for the modulator. 
     A Mach-Zehnder type optical modulator using semiconductor materials is advantageous in miniaturization because the device length using semiconductor materials is one several tenth of that using the LiNbO 3  crystal, and also is advantageous because of its easiness for integration with other optical devices. 
     A conventional MZ type semiconductor device disclosed in, for example, “Claude Rolland et al., InGaAsP-based Mach-Zehnder modulators for high-speed transmission systems”, Optical Fiber Communication Conference 1998 (OFC&#39;98), p. 283-284. (a document D1) will now be described with reference to the accompanying drawings. 
       FIGS. 1A and 1B  represent schematic drawings which show a conventional MZ type semiconductor device.  FIG. 1A  is a plan view showing the conventional MZ type semiconductor device.  FIG. 1B  is a cross-section view showing a cross-section of the MZ type semiconductor device taken along A-A line in  FIG. 1A . 
     The conventional MZ type semiconductor device  100  disclosed in the document D 1  has a MZ (Mach-Zehnder) type modulator  110  and a DFB (Distributed Feedback) laser  120 , both of which are formed on a InP substrate  22  having n-type conductivity (an n-InP substrate) as a semiconductor substrate. A laser light emitted by the DFB laser  120  is incident to the MZ type modulator  110 . 
     The MZ type modulator  110  has an n-InP substrate  22 , a waveguide layer  30  formed on the n-InP substrate  22 , and a p-InP layer  26  formed on the waveguide layer  30 . It is to be noted that the p-InP layer  26  is not shown in  FIG. 1A  and a cap layer etc. for protecting the modulator is not also shown in  FIGS. 1A and 1B  for the sake of simplicity. 
     The waveguide layer  30  includes an entrance waveguide  32 , an optical splitter  33 , a first branch waveguide  34 , a second branch waveguide  36 , an optical coupler  37 , and an exit waveguide  38 . The entrance waveguide  32  guides propagation of the laser light emitted by the DFB laser  120  as an incident light to the optical splitter  33 . The optical splitter  33  connected to the entrance waveguide  32  splits the laser light transmitting through the entrance waveguide  32  into first and second split lights. The first branch waveguide  34 , which is connected to the optical splitter  33  and the optical coupler  37 , guides propagation of the first split light passing through the optical splitter  33  to the optical coupler  37 . The second branch waveguide  36 , which is connected to the optical splitter  33  and the optical coupler  37 , guides propagation of the second split light passing through the optical splitter  33  to the optical coupler  37 . The optical coupler  37  recombines the first and second split lights passing through the first branch waveguide  34  and the second branch waveguide  36 , respectively, so as to generate an outgoing light. The exit waveguide  38  connected to the optical coupler  37  guides propagation of the outgoing light passing through optical coupler  37 . The outgoing light passing through the exit waveguide  38  is emitted by the MZ type modulator  110 . 
     The MZ type modulator  110 , in which the first and second branch waveguides  34  and  36  have the same path length, will be described. It should be noted that the path length is defined as a geometrical length of the waveguide. The first and second split lights, which are split by the optical splitter  33 , transmit through the first and second branch waveguides  34  and  36 , respectively, and the two split lights are recombined by the optical coupler  37 . A phase difference between the first and second split lights recombined by the optical coupler  37  is 0 degrees. 
     A first modulating electrode  42  is provided above the first branch waveguide  34 . A refraction index of the first branch waveguide  34  which exists under the first modulating electrode  42  is changed in response to a voltage applied to the first modulating electrode  42 . A second modulating electrode.  44  is provided above the second branch waveguide  36  similarly to the first modulating electrode  42 . A refraction index of the second branch waveguide  36  under the second modulating electrode  44  is changed in response to a voltage applied to the second modulating electrode  44 . 
     A voltage is applied to either one of the first modulating electrode  42  or the second modulating electrode  44  or two different voltages are applied to the first modulating electrode  42  and the second modulating electrode  44 , respectively, thus causing an electric potential difference between the first modulating electrode  42  and the second modulating electrode  44 . Thereby, the reflection indexes of the first and second branch waveguides  34  and  36  are changed, the values of which are different from each other. Each optical length of the first branch waveguide  34  and the second branch waveguide  36 , through which the first split light and the second split light travel, respectively, is given by a product of the path length and the refraction index, as is well-known. If the refraction indexes of the first branch waveguide and the second branch waveguide are different from each other, a phase difference between the first split light and the second split light causes when the optical coupler  37  recombines the two lights. As a result, the optical intensity of the outgoing light, which is recombined with the first and second split lights by the optical coupler  37 , changes in accordance with the phase difference. 
     In the case that the phase difference between the recombined first and second split lights is 180 degrees, the optical intensity of the outgoing light has a minimum, that is, the MZ type modulator  110  is turned off. In the case that the phase difference between the recombined first and second split lights is 0 degrees, the optical intensity of the outgoing light has a maximum, that is, the MZ type modulator  110  is turned on. 
     However, in the case that the first and second branch waveguides have the lengths different from each other, the modulation voltages required for turning on or off the MZ type modulator fluctuate corresponding to the path lengths of the first and second branch waveguides. For instance, if a light having a wavelength of 1.55 micro meters and the waveguide having a reflection index of 3.2, which is a typical value of a waveguide formed from semiconductor materials, are given, a path length of the waveguide corresponding to the phase difference of 180 degrees is 0.242 (=1.55/3.2/2) micro meters. In order to suppress fluctuations in the modulation voltages of the device, it is necessary to that a difference ΔL in the path length between the waveguides is reduced up to about one tenth of the path length of the waveguide corresponding to the phase difference of 180 degrees, that is, 0.0242 micro meters. A typical path length from the optical splitter to the optical coupler is longer than 600 micro meters. Thus, it is extremely difficult to form waveguides in which the difference in the path length between the waveguides is 0.0242 micro meters because the path lengths are designed to an accuracy of 0.04% and more of the difference in the path length between the waveguides. 
     The fluctuations in the phases of the two lights respectively transmitting through the two waveguides between which there is the difference in the path length are conventionally adjusted by applying a modulation voltage in addition to a direct current (DC) bias voltage to modulating electrodes. 
     The conventional phase adjustment effect will be described with reference to  FIGS. 2A ,  2 B, and  2 C.  FIGS. 2A ,  2 B, and  2 C are graphs showing optical properties to describe the phase adjustment effect, each representing a relation between optical loss in intensity of the outgoing light and modulation voltages applied to the first modulating electrode and the second modulating electrode. Each horizontal axis of  FIGS. 2A ,  2 B, and  2 C represents a first voltage V 1  (volts) applied to the first modulating electrode as a modulation voltage, while each vertical axis represents optical loss (dB) in intensity of the outgoing light for the conventional MZ type modulator. Curves I, II, III, and IV show optical loss profiles of the outgoing light emitted when the second voltages applied to the second modulating electrode are 0, −0.5, −1, and −1.5 volts, respectively. It should be noted that the path lengths of the first and second branch waveguides are so designed that the MZ type modulator is turned on when the first and second voltages V 1  and V 2  are 0 volts and turned off when the first and second voltages V 1  and V 2  are −4 and 0 volts, respectively. 
       FIG. 2A  shows a relation between optical loss of the outgoing light and modulating voltages applied to the first and second modulating electrodes for the MZ modulator in which path lengths of the first branch waveguide. and the second branch waveguide are formed as they are designed. The curvature I (V 2 =0 volts) has a minimum (−30 dB) at the first voltage V 1  of 0 volts, indicating that the MZ type optical modulator is turned off. On the other hand, the curvature I has a maximum (−10 dB) around at the first voltage of −4 volts, indicating that the MZ type optical modulator is turned on. As increasing the second voltage V 2 , the first voltage V 1  at which the MZ type modulator is turned off increases as denoted by an arrow V in  FIG. 2A . As described, the first voltage V 1  at which the MZ type optical modulator is turned off can be adjusted by changing the second voltage V 2 . 
     However, there is a problem as follows. As increasing the first and second voltages V 1  and V 2  applied to the first and second modulation terminals, respectively, the first and second split lights are likely to loss in intensity due to an electro-optical effect when transmitting through regions of the first and second waveguides where the first and second voltages are applied.  FIG. 3  is an optical characteristic for representing intensity loss of the outgoing light as a function of the first voltage V 1 . The vertical axis represents optical loss in intensity of the outgoing light (dB) and the horizontal axis represents the first voltage V 1  (volts).  FIG. 3  is a calculation result of the outgoing light as a function of the first voltage V 1  for the MZ type modulator whose device length is 600 micro meters. As increasing the negative first voltage V 1 , the loss in intensity gradually increases and drastically increases around at the first voltage of −4 volts. As increasing the phase difference, the optical loss in intensity of the light increases when the phase difference is adjusted. Thus, the optical loss in intensity of the light increases when the device is turned on. It is indicated from V 1  denoted in  FIG. 2A  that as increasing the absolute value of the second voltage V 2 , the loss of the light increases in intensity at the first voltage V 1  of −4 volts at which the modulator is turned on. 
       FIG. 2B  shows a relation between optical loss in intensity of the outgoing light and modulating voltages applied to the first and second modulating electrodes for the MZ type modulator in which path lengths of the first branch waveguide and the second branch waveguide are deviated from those designed values. The MZ type modulator is turned off at the second voltage V 2  of 0 volts (curve I) and the first voltage V 1  in a range of a positive voltage, that is, when the optical intensity loss has a minimum point (not shown in  FIG. 2B ). As decreasing the second voltage V 2 , the first voltage V 1  at which the MZ type modulator is turned off decreases. The MZ type modulator is turned off at the second voltage V 2  of −1.5 volts (curve IV) and the first voltage V 1  of about 0 volts. On the other hand, the MZ type modulator is turned on at the second voltage V 2  of −1.5 volts (curve IV) and the first voltage V 1  of about 4 volts. The MZ type modulator, to which the first and second voltages shown in  FIG. 2B  are applied, can be operated by setting the second voltage V 2  to a favorable value. 
     In the case that the second voltage of −1.5 volts is applied, optical intensity loss of light transmitting through at a region of the waveguide where the second voltage is applied is increased due to the electro-optical effect. Therefore, the loss in intensity of the outgoing light produced when the MZ modulator is turned on is larger than that shown in  FIG. 2A . 
       FIG. 2C  shows a relation between optical loss in intensity of the outgoing light and the modulation voltage applied to the first and second modulation electrodes for the MZ type modulator in which a pass difference between the two waveguides is longer than that of the case shown in  FIG. 2B . It is required that the second voltage V 2  is in a range of −3 to −4 volts to modify a phase difference (not shown in  FIG. 2C ). The loss in intensity of the outgoing light is so large that the MZ type modulator is not for practical use. 
     As described above, the MZ type modulator where the phase difference between the first and second split lights is large beyond necessity is impractical. 
     Inventors applying for this application of the present invention devote themselves to an investigation for a MZ modulator that can solve the above-mentioned problems and found that the phase of the two lights can be adjusted by applying a voltage added to a positive bias voltage as a modulation voltage. 
     A phase adjusting effect will be now described with reference to  FIG. 4 .  FIG. 4  is a graph showing a first voltage V 1  versus a second voltage V 2  for the MZ type semiconductor device  100  shown in  FIG. 1A . The first and second voltages V 1  and V 2  are applied to the first and second modulating electrodes  42  and  44 , respectively. Vertical and horizontal axes of the graph represent the first voltage V 1  (volts) at which the MZ type semiconductor device  100  is turned off and the second voltage V 2  (volts). 
     The first voltage V 1  at which the MZ type modulator  100  is turned off is −4.7 volts when the second voltage V 2  of −3 volts is applied. As increasing the second voltage V 2 , that is, as the second voltage V 2  approaching to 0 volts, the first voltage V 1  at which the MZ type modulator  100  is turned off gradually approaches to 0 volts. As further increasing the second voltage V 2 , the first voltage V 1  at which the MZ type modulator  100  is turned off suddenly increases around at the second voltage V 2  of 0.5 volts. This phenomenon originates in a plasma effect caused by the carriers injected into the waveguide layer. 
     In general, a change of reflection index originating in the plasma effect is larger than that originating in the electro-optical effect. Therefore, if the positive bias voltage is applied, the first voltage V 1  at which the MZ type modulator becomes an off state changes more largely, indicating that the MZ type modulator has a high phase adjustment function. Furthermore, a light losses in intensity due to the electro-optical effect in the case that a negative bias voltage is applied, whereas a light does not loss in intensity due to the electro-optical effect in the case that the positive bias voltage is applied. As a result, the phase adjustment can be efficiently performed. 
     However, there is a possibility that the carriers injected into the waveguide by applying the positive bias voltage leak from a region where the positive bias voltage was applied, thus causing deterioration in an eye pattern of the outgoing light. 
       FIGS. 5A and 5B  are drawings for describing a carrier injection.  FIG. 5A  shows the carrier injection schematically. A horizontal axis of  FIG. 5B  represents a position in the direction of a light and a horizontal axis of  FIG. 5B  represents an electric potential V.  FIGS. 5A and 5B  shows the carrier injection for the conventional MZ type semiconductor device described with reference to  FIG. 1A  similar to  FIG. 4  in the case that the positive bias voltage is applied to the first modulating electrode  42 . By applying the positive bias voltage to the first modulating electrode  42 , carriers are injected into a region of the waveguide layer  30  which exists under the first modulating electrode  42 . The carriers diffuse out a region of the first branch waveguide  34  which exists under the first modulating electrode  42 . If the carriers are modulated under the second modulation voltage applied to the second modulating electrode  44 , an eye pattern of the outgoing light will be disturbed. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a Mach-Zehnder type semiconductor device that can adjust a phase difference even if lengths of two waveguides are deviated from those designed values and suppress loss in intensity of a light and not to deteriorate an eye pattern. 
     According to a first aspect of the present invention, there is a provided a Mach-Zehnder type semiconductor device including a waveguide layer having a first conductive type formed on one of principle surfaces of a semiconductor substrate, a semiconducting layer having a second conductive type formed on the waveguide layer, and a common electrode formed on the other of principle surfaces of the semiconductor substrate. 
     The waveguide layer includes an entrance waveguide, an optical splitter, a first branch waveguide, a second branch waveguide, an optical coupler, an exit waveguide. The entrance waveguide guides propagation of an incident light entered from outside of the Mach-Zehnder type semiconductor device. The optical splitter connected to the entrance waveguide splits the incident light into first and second split lights. The first branch waveguide connected to the optical splitter guides propagation of the first split light. The second branch waveguide connected to the optical splitter guides propagation of the second split light. The optical coupler connected to the first and second branch waveguides recombines the first and second split lights so as to produce an output light. The exit waveguide connected to the optical coupler guides propagation of the outgoing light so as to emit the output light from the Mach-Zehnder type semiconductor device. 
     A modulation electrode is provided above the first branch waveguide, for changing a refraction index of the first branch waveguide by applying a modulated voltage across the modulation electrode and the common electrode. A phase adjusting electrode group having a first negative bias electrode, a positive bias electrode, and a second negative electrode, which are provided above the second branch waveguide in sequence in the direction of propagation of the second split light and separated from each other is provided. The positive bias electrode changes a refraction index of the second branch waveguide by applying a positive bias voltage across the positive bias electrode and the common electrode so as to inject carriers to the second branch waveguide. The first and second bias electrodes remove the carriers from the second branch waveguide to the semiconducting layer by applying a first negative bias voltage across the first bias electrode and the common electrode and a second negative bias voltage across the second bias electrode and the common electrode. 
     According to a second aspect of the present invention, there is a provided a method of controlling the Mach-Zehnder type semiconductor device according to the first aspect of the present invention. The method includes the following steps. A continuous light is entered into the entrance waveguide. A first voltage which is given by a sum of a modulated voltage having a low level indicating an OFF state and a negative DC bias voltage is applied to the modulation electrode. A positive bias voltage is applied to the positive bias electrode so as to minimize optical intensity of the outgoing light emitted from the exit waveguide of the Mach-Zehnder type semiconductor device. First and second bias voltages which are smaller than the negative DC bias voltage are applied to the first and second negative bias electrodes, respectively. 
     The Mach-Zehnder type semiconductor device of the present invention includes the modulating electrode on the first branch waveguide and the phase adjusting electrode group on the second branch waveguide. The phase adjusting electrode group has the first negative bias electrode, the positive bias electrode, and the second negative bias electrode to which the negative bias voltage is applied. By applying the positive bias voltage between the positive bias electrode and the common electrode which is connected to the earth potential, carriers are injected into a region of the second branch waveguide which is sandwiched by the positive bias electrode and the common electrode. As a result, plasma effect of the carries causes a change of reflection index of the region of the second branch waveguide, so that the phase of the second split light passing through the region of the second branch waveguide can be modified. 
     The positive bias electrode is provided between the first negative bias electrode and the second negative bias electrode. The first and second negative bias voltages are applied across the common electrode and the first and second negative bias electrodes, respectively. Thereby, the carriers injected into regions of the second branch waveguide which are sandwiched by the common electrode and the first and second negative bias electrodes, respectively, can be transferred from the second branch waveguide to the semiconducting layer. As a result, deterioration of an eye pattern originating in the carriers injected by applying the positive bias voltage can be prevented. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a schematic plan view showing a conventional MZ type semiconductor device; 
         FIG. 1B  is a schematic cross-sectional view showing the conventional MZ type semiconductor device taken along A-A line in  FIG. 1A ; 
         FIG. 2A  is a graph showing an optical characteristic for the conventional MZ type semiconductor device; 
         FIG. 2B  is a graph showing an optical characteristic for the conventional MZ type semiconductor device; 
         FIG. 2C  is a graph showing an optical characteristic for the conventional MZ type semiconductor device; 
         FIG. 3  is a graph showing a voltage dependence of optical intensity for the conventional MZ type semiconductor device; 
         FIG. 4  is a graph showing an optical characteristic for describing a phase adjustment effect of the conventional MZ type semiconductor device; 
         FIG. 5A  is a schematic drawing representing a carrier injection by applying a positive bias voltage for the conventional MZ type semiconductor device; 
         FIG. 5B  is a schematic drawing representing a carrier injection by applying a positive bias voltage for the conventional MZ type semiconductor device; 
         FIG. 6A  is a schematic plan view showing a MZ type semiconductor device which is a first embodiment of the present invention; 
         FIG. 6B  is a schematic cross-sectional view showing the MZ type semiconductor device taken along A-A line in  FIG. 6A ; 
         FIG. 7A  is a schematic explanation drawing representing a carrier injection for the MZ type semiconductor device shown in  FIG. 6A ; 
         FIG. 7B  is a schematic explanation drawing representing a carrier injection for the MZ type semiconductor device shown in  FIG. 6A ; 
         FIG. 8A  is a graph showing an eye pattern of a light emitted by the MZ type semiconductor device shown in  FIG. 6A ; 
         FIG. 8B  is a graph showing an eye pattern of a light emitted by the MZ type semiconductor device shown in  FIG. 6A ; 
         FIG. 9  is a schematic plan view showing a MZ type semiconductor device which is a second embodiment of the present invention; 
         FIG. 10  is a schematic plan view showing a MZ type semiconductor device which is a third embodiment of the present invention; and 
         FIG. 11  is a schematic plan view showing a MZ type semiconductor device which is a fourth embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiments of the present invention will be now described with reference to drawings. Each of the drawings schematically illustrates only shapes of, sizes of, and arrangement of components to the extent of which the present invention can be understood. Materials and numerical conditions of the components are preferred examples. The present invention is not limited to the following embodiments. 
     First Embodiment 
     A configuration of a MZ type semiconductor device which is a first embodiment of the present invention will be described with reference to  FIGS. 6A and 6B .  FIGS. 6A and 6B  are schematic drawings for describing the MZ type semiconductor device of the first embodiment.  FIG. 6A  is a schematic plan view showing the MZ type semiconductor device which is the first embodiment of the present invention.  FIG. 6B  is a schematic cross-sectional view showing the MZ type semiconductor device taken along A-A line in  FIG. 6A . 
     In the MZ type semiconductor device of the first embodiment, a semiconductor substrate having a first conductive type is described as an n-type semiconductor substrate, and a semiconducting layer having a second conductive type is described as a p-type semiconductor layer. It is to be noted that the embodiment is not limited to the configuration. Even if a p-type semiconductor substrate and an n-type semiconducting layer are employed as a semiconductor substrate having the first conductive type and a semiconductor layer having the second conductive type, respectively, effects will be obtained similarly to the first embodiment. 
     A MZ type semiconductor device  10  includes an n-InP substrate  22  prepared as an n-type semiconductor substrate, a waveguide layer  30 , and a p-InP layer  26  prepared as a p-type semiconductor substrate. The waveguide layer  30  and the p-InP layer  26  are sequentially formed on one of principle surfaces of the n-InP substrate  22 , that is, an upper surface of then-InP substrate  22 . The waveguide layer  30  has, for example, a multiple quantum well (MQW: Multi-Quantum Well) structure fabricated with semiconducting compound such as InGaAsP system. It is to be noted that the semiconducting compounds are not limited to the above-mentioned example. The semiconducting compound such as InAlAs system etc. may be used according to a wave-length of incident light etc. The p-InP layer  26  is omitted in  FIG. 6A  for simplicity. In addition, a cap layer for protecting the device is omitted in  FIGS. 6A and 6B . 
     The waveguide layer  30  has an entrance waveguide  32 , an optical splitter  33 , a first branch waveguide  34 , a second branch waveguide  36 , an optical coupler  37 , and an exit waveguide  38 . The entrance waveguide  32  guides propagation of an incident light to the optical splitter  33 . The incident light is irradiated from outside of MZ type semiconductor device  10 . 
     The optical splitter  33 , which is connected to the entrance waveguide  32 , splits the incident light transmitting through the entrance waveguide  32  into first and second split lights. 
     The first branch waveguide  34 , which is connected to the optical splitter  33  and the optical coupler  37 , guides propagation of the first split light split by the optical splitter  33  to the optical coupler  37 . 
     The second branch waveguide  36 , which is connected to the optical splitter  33  and the optical coupler  37 , guides propagation of the second split light split by the optical splitter  33  to the optical coupler  37 . 
     The optical coupler  37  recombines the first split light and the second split light, whereby to generate an outgoing light. 
     The exit waveguide  38 , which is connected to the optical coupler  37 , guides propagation of the outgoing light. The outgoing light transmitting through the exit waveguide  38  is emitted by the MZ type semiconductor device  10 . 
     The MZ type semiconductor device  10  further includes a modulating electrode  40 , a first negative bias electrode  55 , a positive bias electrode  53 , and a second negative bias electrode  57 . The modulating electrode  40  is formed above the first branch waveguide  34 . The first negative bias electrode  55 , the positive bias electrode  53 , and the second negative bias electrode  57  are formed above the second branch waveguide  36 , arranged in sequence in the direction of propagation of the second split light, and separated with each other. 
     The common electrode  46  is provided on the other of principle surfaces of the n-InP substrate  22  which is revert to the one of principle surfaces of the n-InP substrate  22  on which the waveguide layer  30  is formed. The common electrode  46  is connected to, for example, an earth potential. 
     The modulating electrode  40  has a function of changing a refraction index in a region of the first branch waveguide  34  which is sandwiched by the modulating electrode  40  and the common electrode  46  by applying a modulation voltage across both electrodes  40  and  46 . The positive bias electrode  53  has a function of changing a refraction index in a region of the second branch waveguide  36  which is sandwiched by the positive bias electrode  53  and the common electrode  46 . By applying a positive bias voltage across the positive bias electrode  53  and the common electrode  46 , carriers are injected into the region of the second branch waveguide  36 . Thus, the refraction index in the region of the second branch waveguide  36  can be changed. First and second negative bias voltages are applied across the common electrode  46  and the first and second negative bias electrodes  55  and  57 , respectively, and thus the carriers doped into the second waveguide layer  36  are removed into the p-InP layer  26 . Therefore, each of the first and second negative bias electrodes  55  and  57  has a function of removing the carriers injected into the second waveguide layer  36  to the p-InP layer  26 . The first negative bias electrode  55 , the positive bias electrode  53 , and the second negative bias electrode  57  compose a phase adjusting electrode group  50 . Magnitude of the voltage applied to these electrodes  40 ,  53 ,  55 , and  57  are different from each other, which are dependent on lengths etc. of these electrodes  40 ,  53 ,  55 , and  57 . The detailed will be described later. 
     The waveguide of the MZ type modulator provided on the semiconductor substrate may be arbitrarily and suitably formed by use of a conventional method. A method of fabricating the waveguide on the semiconductor substrate is omitted here. A ridge-type waveguide is shown in  FIG. 6A . The waveguide structure is not limited to the ridge-type waveguide. The waveguide may have a semiconductor buried structure (BH: Buried Heterostructure). 
     An operation of the MZ type semiconductor device  10  of the first embodiment will be now described with reference to  FIGS. 7A ,  7 B,  8 A, and  8 B. 
     A first voltage V 1  is applied to the modulating electrode  40 . The first voltage V 1  is given by a sum of a modulation voltage for modulating the first split light and a DC bias voltage. The modulation voltage is supplied as, for example, a RZ (RZ: Return to Zero) signal having a low (L) level indicating an ON state and a high (H) level indicating an OFF state. The DC bias voltage is 0 volts or a negative DC voltage. 
     When the first voltage V 1  is applied to the modulating electrode  40 , the refraction index in a region of the first branch waveguide  34  which is under the modulating electrode  40  is changed in response to the modulation voltage having the H or L levels, and thus the first split light is modulated. 
     A positive bias voltage is applied to the positive bias electrode  53 . 
     By applying a positive bias voltage to the positive bias electrode  53 , carriers are injected into a region of the second branch waveguide  36  which is sandwiched by the positive bias electrode  53  and the common electrode  46 . A plasma effect of the carriers injected into the second branch waveguide  36  causes a change of refraction index in the second branch waveguide  36 . By using the change of refraction index in the second branch waveguide  36  due to the plasma effect, an optical length of the second branch waveguide  36  can be changed, so that the second split light can be adjusted in phase. 
     On the other hand, first and second negative bias voltages are applied to the first negative bias electrode  55  and the second negative bias electrode  57 , respectively, according to the MZ type semiconductor device  10  of the first embodiment of the present invention. A reason for applying these negative bias voltages is that deterioration in an eye pattern of the outgoing light, which is attributed to carriers injected into the second branch waveguide  36  by applying the positive bias voltage to the positive bias electrode  53 , is prevented. 
       FIGS. 7A and 7B  are drawings for describing operations of the MZ type semiconductor device  10  according to the first embodiment of the present invention.  FIG. 7A  schematically shows the operation of the MZ type semiconductor device  10  of the first embodiment. A horizontal axis of  FIG. 7B  represents a position (arbitrary unit) in the direction of propagation of the light and a vertical axis represents an electrical potential V (arbitrary unit).  FIGS. 7A and 7B  show behaviors of carriers injected into the second branch waveguide  36 . The carriers are injected by applying the first and the second negative bias voltages across the common electrode  46  and the first and second negative bias electrodes  55  and  57 , respectively, and by applying the positive bias voltage across the positive bias electrode  53  and the common electrode  46 . The carriers injected by applying the first and the second negative bias voltages is are not removed from the waveguide layer  30  to the semiconducting layer  26  which is formed above the waveguide layer  30 . And also, the injected carriers are not diffuse out of a region of the waveguide layer  30  which exists under the phase adjusting electrode group  50 . 
       FIGS. 8A and 8B  show eye patterns of the outgoing lights for the case that the positive bias voltage is applied. The horizontal axis represents time (arbitrary units) and the vertical axis represents intensity of the outgoing light (arbitrary units).  FIG. 8A  is an eye pattern of the outgoing light which is obtained when the negative bias voltages having sufficient magnitudes are not applied to the first negative bias electrode  55  and the second negative bias electrode  57 .  FIG. 8B  is an eye pattern of the outgoing light which is obtained when the negative bias voltages having sufficient magnitudes are applied to the first negative bias electrode  55  and the second negative bias electrode  57 . 
     It is measured from the eye pattern shown in  FIG. 8A  that the carriers injected into the waveguide layer  30  are not sufficiently removed to the semiconducting layer  26  because the negative bias voltages are not sufficiently applied to the first negative bias electrode  55  and the second negative bias electrode  57 . On the other hand, it is measured from the eye pattern shown in  FIG. 8B  that the carriers injected into the waveguide layer  30  are sufficiently removed to the semiconducting layer  26  because the negative bias voltages are sufficiently applied to the first negative bias electrode  55  and the second negative bias electrode  57 . It is understood that an aperture of the eye pattern shown in  FIG. 8B  is larger compared to that shown in  FIG. 8A . 
     As described above, the MZ type semiconductor device  10  according to the first embodiment of the present invention includes the modulating electrode  40  provided above the first branch waveguide  34  and further includes the phase adjusting electrode group  50  having the first negative bias electrode  55 , positive bias electrode  53 , and the second negative bias electrode  57 , which is provided above the second branch waveguide  36 . 
     By applying the positive bias voltage across the positive bias electrode  53  and the common electrode  46 , carriers are injected into a region of the second branch waveguide  36  which exists between the positive bias electrode  53  and the common electrode  46 . The reflection index in the region of the second branch waveguide  36  is changed, which originates in the plasma effect of the carriers, so that the second split light transmitting through the second branch waveguide  36  can be adjusted in phase. 
     By applying the first and the second negative bias voltages to the first negative bias electrode  55  and the second negative bias electrode  57  respectively, both of which sandwich the positive bias electrode  53 , the carriers injected into the second branch waveguide  36  can be removed out of the second branch waveguide  36 . Thus, deterioration in the eye pattern originating in the carriers injected by applying the voltage of positive bias can be prevented. 
     A method of controlling the Mach-Zehnder type semiconductor device of the present invention, that is, a method of configuring the first negative bias voltage, the positive bias voltage, and the second negative bias voltage, includes the following steps. 
     First of all, a continuous light emitted by a semiconductor laser etc. is incident into the entrance waveguide  32 . 
     A first voltage, which is given by a sum of a modulation voltage having an L level indicating an OFF state and a negative DC bias voltage, is applied to the modulating electrode  40 . It should be noted that the negative DC bias voltage is not positive and may be zero volts. 
     The outgoing light emitted from the exit waveguide  38  of the MZ type semiconductor device  10  is minimized in optical intensity by applying the positive bias voltage to the positive bias electrode while monitoring the optical intensity of the outgoing light by use of a conventional measuring instrument. 
     The first negative bias voltage and the second negative bias voltage having small magnitudes are applied to the first negative bias electrode  55  and the second negative bias electrode  57 , respectively. The first negative bias voltage and the second negative bias voltage is configured so as not to cause the deterioration in the eye pattern while monitoring the eye patterns as described with reference to  FIGS. 8A and 8B . It is preferable that the first negative bias voltage and the second negative bias voltage are smaller than the negative DC bias voltage which is added to the first voltage applied to the modulation electrode  40 . 
     As will be described later, in the case that the first and the second negative bias electrodes are shorter in length than the positive bias electrode, a change in the optical length originating in the applied first negative bias voltage and the second negative bias voltage can be disregarded. When the change in the optical length originating in the applied the first and second negative bias voltages can not be disregarded, the positive bias voltage may be adjusted after the first negative bias voltage and the second negative bias voltage are applied. 
     When a MZ type semiconductor device is manufactured, it is preferable that the device length L is shorted as much as possible. That is, it is preferable that the positive bias electrode and the first and second negative bias electrodes are formed shortly in the direction of propagation of the lights. 
     On the other hand, the positive bias voltage applied to the positive bias electrode  53  is in inverse proportion to the length of positive bias electrode  53 . The applied positive bias voltage decreases as increasing in length of the positive bias electrode  53  and increases as decreasing in length of the positive bias electrode  53 . In order to decrease the positive bias voltage, it is necessary to increase in length of the positive bias electrode  53 . 
     If the positive bias electrode  53  having a length of about 100 micro meters is formed for example, the phase adjustment effect which is described with reference to  FIGS. 7A and 7B  is achieved. It is, therefore, preferable that the length of positive bias electrode  53  is about 100 micro meters. In this case, the MZ type semiconductor device can be driven under the positive bias voltage of 3.3 volts or less and with a power supply shared with other optical device. 
     The positive bias voltage applied to the positive bias electrode  53  is adjusted as follows. The DC bias voltage, which is added to the first voltage applied to the modulation electrode  40 , may be adjusted according to need of the semiconductor device of the present invention. For instance, the DC bias voltage which is added to the first voltage V 1  may be 0 volts and a negative voltage of about −1 volts. 
     On the other hand, the lengths of the first negative bias electrode  55  and the second negative bias electrode  57  are so defined that the carriers injected into the second branch waveguide  36  do not extend over a region of the second branch waveguide  36  which exists under the first negative bias electrode  55  and the second negative bias electrode  57  and do not leak out a region of the second branch waveguide  36  which exists under the phase adjusting electrode group  50 . It is preferable that each length of the first and second negative bias electrodes  55  and  57  corresponds to a diffusion length of the carriers, and in this case, is 5 to 10 micro meters. The leakage of the carriers can be prevented if the lengths of first and second negative bias electrodes  55  and  57  are in a range of the diffusion length of the carriers, so that deterioration in the eye pattern can be prevented. In addition, it is preferable that gaps between the positive bias electrode  53  and the first negative bias electrode  55  and between the positive bias electrode  53  and the second negative bias electrode  57  are about 10 micro meters. 
     In the case, the first negative bias voltage and the second negative bias voltage which are applied to the first negative bias electrode  55  and the second negative bias electrode  57 , respectively, are negative. It is preferable that absolute values of the first and second negative bias voltages are larger than that of the DC bias voltage added to the first voltage V 1 . Although the first negative bias voltage and the second negative bias voltage are not necessarily equivalent to each other, it will be easy to apply the voltages if both of them are equivalent. 
     The numeric values described above are merely exemplifications, and thus the first embodiment of the present invention is not limited to these numeric values. The lengths of the electrodes and the gap sizes between the electrodes may be selected according to a demanded device length and usable bias voltages etc. 
     Second Embodiment 
     A MZ type semiconductor device which is a second embodiment of the present invention will be now described with reference to  FIG. 9 .  FIG. 9  is a schematic plan view showing the second embodiment of the present invention. 
     The MZ type semiconductor device  11  shown in  FIG. 9  has a first modulating electrode  42  and a second modulating electrode  44  which are not provided with the MZ type semiconductor device  10  shown in  FIG. 6A . The first modulating electrode  42 , through which a modulation voltage is applied to a first branch waveguide  34 , is provided above a first branch waveguide  34 . The second modulating electrode  44 , through which the modulation voltage is applied to a second branch waveguide  36 , is provided above a second branch waveguide  36 . Other components excluding these modulating electrodes  42  and  44  in  FIG. 9  are not be described here because they are similar to that shown in  FIG. 6A . 
     By providing the modulating electrodes with the first and second branch waveguides, first and second split lights can be modulated by an inverse phase modulation voltage. By applying the inverse phase modulation voltage, modulation voltages can be decreased and transmission properties of the first and second branch waveguides can be improved. 
     It is preferable that a phase adjusting electrode group  50  is arranged between a second modulating electrode  44  and an optical splitter  33 , both of which are formed above the second branch waveguide  36 . A reason is that the second split light modulated by the second modulating electrode  44  is not influenced by a fluctuation of refraction index in a region of a waveguide  30  above which the phase adjusting electrode group  50  is provided. The fluctuation of refraction index originates in carrier injected into the waveguide  30 . 
     In the case that a plurality of modulation electrodes are provided as the MZ type semiconductor device of the second embodiment, it is preferable that the first and second negative bias voltages are negative bias voltages, absolute values of which are larger than either one of the largest absolute values of DC bias voltages applied to the first modulating electrode or the second modulating electrode. 
     Third Embodiment 
     A MZ type semiconductor device that is a third embodiment will be described with reference to  FIG. 10 .  FIG. 10  is a schematic plan view showing the MZ type semiconductor device of the third embodiment. 
     A MZ type semiconductor device  12  of the third embodiment includes a modulating electrode  40 , a first phase adjusting electrode group  51  as a phase adjusting electrode group, and a second phase adjusting electrode group  52  as a phase adjusting electrode group. The modulating electrode  40  and the first phase adjusting electrode group  51  are provided above a first branch waveguide  34 . The second phase adjusting electrode group  52  is provided above a second branch waveguide  36 . 
     Excluding the modulating electrode  40  and the first phase adjusting electrode group  51 , both of which are formed above the first branch waveguide  34 , the MZ type semiconductor device  12  of the third embodiment are designed similarly to the MZ type semiconductor device  10  of the first embodiment. Details of the components excluding the modulating electrode  40  and the first phase adjusting electrode group  51  are omitted here. 
     The modulating electrode  40  is used for applying a modulation voltage to the first branch waveguide  34 . The first phase adjusting electrode group  51  are configured similarly to the phase adjusting electrode group (denoted by  50  in  FIG. 6A ) which is described with reference to  FIG. 6A . That is, the first phase adjusting electrode group  51  has a first negative bias electrode  55   a  to which a first negative bias voltage is applied, a first positive bias electrode  53   a  to which a first positive bias voltage is applied, and a second negative bias electrode  57   a  to which a second negative bias voltage is applied, which are arranged in sequence in the direction of propagation of a first split light. The second phase adjusting electrode group  52  having components similar to the first phase adjusting electrode group  51  is provided with a third negative bias electrode  55   b  to which a third negative bias voltage is applied, a second positive bias electrode  53   b  to which a second positive bias voltage applied, and a fourth negative bias electrode  57   b  to which a fourth negative bias voltage is applied, which are arranged in the direction of propagation of a second split light. 
     Both of the first branch waveguide  34  and the second branch waveguide  36  have the phase adjustment electrode groups, so that a positive bias voltage may be applied to either one of the first branch waveguide  34  or the second branch waveguide  36 . The positive bias voltage may be applied to, for example, either one of the branch waveguides having a length longer than the other. Therefore, phases of the first and second split lights can be adjusted by applying a smaller bias voltage compared to the case that the positive bias voltage are applied to both of the first branch waveguide  34  and the second branch waveguide  36 . 
     If the positive bias voltage is applied to either one of the first positive bias electrode  53   a  or the second positive bias electrode  53   b , a phase adjusting electrode group having the positive bias electrode to which the positive bias voltage is not applied may not be applied to a negative bias voltage. For example, if the positive bias voltage is applied to only the first positive bias electrode  53   a  and is not applied to the second positive bias electrode  53   b , the third and fourth bias electrodes  55   b  and  57   b , both of which are included by the second phase adjusting electrode group  52 , may not be applied to the negative bias voltages. 
     Fourth Embodiment 
     A MZ type semiconductor device which is a fourth embodiment of the present invention will be now described with reference to  FIG. 11 .  FIG. 11  is a schematic plan view showing the MZ type semiconductor of the fourth embodiment of the present invention. 
     A MZ type semiconductor device  13  shown in  FIG. 11  includes a first modulating electrode  42  and a second modulating electrode  44 , both of which is not provided with the MZ type semiconductor device of the third embodiment. Other components shown in  FIG. 11  are similar to that of the third embodiment shown in  FIG. 10 . The first modulating electrode  42  is provided above a first branch waveguide  34  as a modulating electrode to which a modulation voltage is applied. The second modulating electrode  44  is provided above a second branch waveguide as a modulating electrode to which a modulation voltage is applied. Details of the components excluding the first modulating electrode  42  and the second modulating electrode  44  are omitted here. 
     If a plurality of the modulating electrodes are provided as the MZ type semiconductor device of the fourth embodiment, it is preferable that first to fourth negative bias voltages are negative bias voltages, absolute values of which are larger than either the largest one of absolute values of DC bias voltages applied to the first modulating electrode or the second modulating electrode. 
     By providing the modulating electrodes with the first and second branch waveguides, first and second split lights can be modulated by an inverse phase modulation voltage. By applying the inverse phase modulation voltage, modulation voltages can be decreased and transmission properties of the first and second branch waveguides can be further improved. 
     It is preferable that each of the phase adjusting electrode groups is so arranged that it is provided on the entrance side, that is, between the modulating electrode and the optical splitter because each of the first and second split lights is not influenced by fluctuations of a refraction index due to carriers injected around at a region of the waveguide layer above which the phase adjusting electrode group is provided. 
     This application is based on Japanese Patent Application No. 2006-072775 which is hereby incorporated.