Abstract:
According to an aspect of an embodiment, an optical device comprising: a first modulator for independently modulating first light having a first predetermined polarization mode; a second modulator for independently modulating second light having a second predetermined polarization mode; and a polarization beam coupler having a first port, a second port, a third port, and a fourth port; the polarization beam coupler for inputting the first light from the first modulator via the first port, inputting the second light from the second modulator via the second port, outputting the first light via the third port and inputting reflected and polarization converted light on the first light by a wave plate and a mirror, and outputting the first light having the converted polarization mode and the second light having the predetermined polarization mode via the fourth port.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is based upon and claims the benefits of priority from the prior Japanese Patent Application No. 2008-072330, filed on Mar. 19, 2008, the entire contents of which are incorporated herein by reference. 
       BACKGROUND 
       [0002]    (1) Field 
         [0003]    This disclosure relates to an optical device. This disclosure more particularly relates to an optical device with modulators used in optical communication. 
         [0004]    (2) Description of the Related Art 
         [0005]    In the field of optical communication, optical devices with a Mach-Zehnder interference optical modulator have been developing (see, for example, Japanese Unexamined Patent Publication No. 2008-46573). 
         [0006]    Such a modulator included in an optical device is formed by, for example, forming an optical waveguide in an electro-optic crystal substrate and locating electrodes near the optical waveguide. The optical waveguide of the modulator can broadly be divided into an input waveguide where light is input and propagated, a pair of modulation waveguides where the light propagated through the input waveguide is split and propagated, and an output waveguide where the light propagated through the pair of modulation waveguides is combined, propagated, and output. A signal electrode and an earth electrode are located over the pair of modulation waveguides. A method for locating lumped-constant type electrodes or traveling-wave type electrodes as electrodes of a modulator is known. For example, if traveling-wave type electrodes are located, then an end of the signal electrode and an end of the earth electrode are connected via a resistor and a microwave signal is applied from the input side. At this time the refractive index of each of the pair of modulation waveguides changes and the phase of light which is input to the input waveguide and which is propagated through the pair of modulation waveguides changes. Accordingly, intensity-modulated signal light is output from the output waveguide because of Mach-Zehnder interference. 
         [0007]    An optical sending apparatus for polarization multiplex communication which includes two modulators each having the above structure is proposed. The polarization modes of signal light output from the optical sending apparatus are, for example, TM mode and TE mode. That is to say, the polarization modes of signal light output from the optical sending apparatus are perpendicular to each other. For example, the following structure is proposed. In order to obtain signal light the polarization modes of which are perpendicular to each other, light output from one of the two modulators is made to pass through a transmission λ/2 plate. In addition, in order to multiplex the signal light the polarization modes of which are perpendicular to each other, a polarization beam coupler (PBC) is located on the end side of two output waveguides. 
         [0008]    In addition to a transmission plate, a reflection plate is proposed as a component, such as a λ/2 plate, for converting a polarization mode in the field of optical communication (see, for example, Japanese Unexamined Patent Publication No. 08-278422). 
         [0009]    In order to miniaturize the optical device including the above modulators, it is desirable that the two modulators and the PBC should be formed in one substrate for the purpose of forming them in one chip. Accordingly, it is necessary to locate a component, such as the λ/2 plate, for rotating polarization in the chip. 
         [0010]    For example, the method of forming a groove in a portion of an output waveguide included in one of the two modulators for cutting the output waveguide and of inserting the λ/2 plate into the groove is known as a method for locating the λ/2 plate in the chip. With this method, however, a plurality of optical device chips are formed on one wafer (electro-optic crystal substrate) and the wafer is cut into the plurality of optical device chips. After that, the above process must be performed on each chip. This requires a large number of steps. In addition, a production yield may deteriorate because of, for example, damage to a chip caused by the formation of the groove. Furthermore, if this method is adopted, a great optical loss occurs in the groove where the λ/2 plate is located, or there is variation in optical loss among chips. 
       SUMMARY 
       [0011]    The present invention was made under the background circumstances described above. An object of the present embodiment is to provide a high performance optical device with high reliability which can be fabricated easily and which is suitable for polarization multiplex communication. 
         [0012]    In order to achieve the above object, an optical device comprising a coupler formed on a substrate and having first and second ports and third and fourth ports opposite to the first and second ports, respectively, for outputting first-polarization-mode light input from one port to a port diagonally opposite to the one port and for outputting second-polarization-mode light input from one port to a port opposite to the one port, a first modulator formed on the substrate and including a first optical waveguide connected to the first port for modulating first-polarization-mode light input to the first optical waveguide and for outputting the modulated first-polarization-mode light to the first port, a second modulator formed on the substrate and including a second optical waveguide connected to the third port for modulating first-polarization-mode light input to the second optical waveguide and for outputting the modulated first-polarization-mode light to the third port, and a polarization mode conversion section located on one end of the substrate for converting the first-polarization-mode light which is input from the second modulator to the third port and which is output to the second port diagonally opposite to the third port into second-polarization-mode light and for returning the second-polarization-mode light to the second port is provided. 
         [0013]    The above and other objects, features and advantages will become apparent from the following description when taken in conjunction with the accompanying drawings which illustrate preferred embodiments by way of example. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  is a fragmentary schematic plan view showing an example of an optical device according to a first embodiment. 
           [0015]      FIG. 2  is a schematic sectional view taken along the line X-X of  FIG. 1 . 
           [0016]      FIG. 3  is a view for describing a PBC. 
           [0017]      FIG. 4  is a fragmentary schematic plan view showing an example of the conventional optical device. 
           [0018]      FIG. 5  is a fragmentary schematic plan view showing an example of an optical device according to a second embodiment. 
           [0019]      FIG. 6  is a fragmentary schematic plan view showing an example of an optical device having a return structure in which a mirror is used. 
           [0020]      FIG. 7  is a fragmentary schematic plan view showing an example of an optical device having a return structure in which an optical fiber is used. 
           [0021]      FIG. 8  is a fragmentary schematic plan view showing an example of an optical device according to a third embodiment. 
           [0022]      FIG. 9  is a fragmentary schematic plan view showing an example of an optical device according to a fourth embodiment. 
           [0023]      FIG. 10  is a fragmentary schematic plan view showing an example of an optical device according to a fifth embodiment. 
           [0024]      FIG. 11  is a fragmentary schematic plan view showing an example of an optical device according to a sixth embodiment. 
           [0025]      FIG. 12  is a fragmentary schematic plan view showing an example of an optical device according to a seventh embodiment. 
           [0026]      FIG. 13  is a fragmentary schematic plan view showing an example of an optical device according to an eighth embodiment. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0027]    Embodiments will now be described in detail with reference to the drawings. 
         [0028]    A first embodiment will be described first. 
         [0029]      FIG. 1  is a fragmentary schematic plan view showing an example of an optical device according to a first embodiment.  FIG. 2  is a schematic sectional view taken along the line X-X of  FIG. 1 . 
         [0030]    An optical device  1  shown in  FIG. 1  includes two modulators  20  and  30  for modulating and outputting light input from both ends  2   a  and  2   b.    
         [0031]    An electro-optic crystal, such as lithium niobate (LiNbO 3 ) or lithium tantalate (LiTaO 2 ), is used as a substrate  2  in which the modulators  20  and  30  are formed. For example, the substrate  2  obtained by cutting (Z-cutting) LiNbO 3  parallel to a z-axis which is the direction of its crystallographic axis is used. The z-cut substrate  2  has a crystallographic axis by which the refractive index can efficiently be changed on the basis of an electro-optic effect in a direction perpendicular to its surface. 
         [0032]    An optical waveguide  21  of the modulator  20 , an optical waveguide  31  of the modulator  30 , a PBC  3 , and modulated light propagation waveguides  4  and  5  are formed on the above substrate  2 . 
         [0033]    The optical waveguides  21  and  31 , the PBC  3 , and the modulated light propagation waveguides  4  and  5  are formed on waveguide pattern formation regions of the substrate  2  by, for example, forming a metal film of titanium (Ti) or the like and performing thermal diffusion of titanium. The optical waveguides  21  and  31  may be formed by forming such a metal film and making a proton exchange in benzoic acid. 
         [0034]    The optical waveguide  21  of the modulator  20  has an input waveguide  21   a  where light input from the one end  2   a  of the optical device  1  is propagated, a branching waveguide  21   b  where the light propagated through the input waveguide  21   a  is split, and a pair of modulation waveguides  21   c  and  21   d  where the split light is propageted. The modulation waveguides  21   c  and  21   d  are formed so that they will be, for example, parallel straight lines. In addition, the optical waveguide  21  has a branching waveguide  21   e  where the light propagated through the modulation waveguides  21   c  and  21   d  is combined, and an output waveguide  21   f  where the combined light is propagated. 
         [0035]    Similarly, the optical waveguide  31  of the modulator  30  has an input waveguide  31   a  where light input from the other end  2   b  of the optical device  1  is propagated, a branching waveguide  31   b  where the light propagated through the input waveguide  31   a  is split, a pair of modulation waveguides  31   c  and  31   d  where the split light is propagated, a branching waveguide  31   e  where the light propagated through the modulation waveguides  31   c  and  31   d  is combined, and an output waveguide  31   f  where the combined light is propagated. 
         [0036]    The output waveguides  21   f  and  31   f  of the optical waveguides  21  and  31  each having the above structure are connected to ports A 1  and B 1 , respectively, of the 2×2 PBC  3  which are opposite to each other. The modulated light propagation waveguides  4  and  5  are connected to remaining ports A 2  and B 2 , respectively, of the PBC  3  which are opposite to each other. 
         [0037]      FIG. 3  is a view for describing the PBC. 
         [0038]    The PBC  3  has the four ports A 1 , A 2 , B 1 , and B 2  where light can be input or output, and a widened section  3   a . All of them can be formed on the substrate  2  in the same way that is used for forming the optical waveguides  21  and  31  and the modulated light propagation waveguides  4  and  5 . The output waveguides  21   f  and  31   f  are connected to the ports A 1  and B 1 , respectively, of the PBC  3  which are opposite to each other. The modulated light propagation waveguides  4  and  5  are connected to the ports A 2  and B 2 , respectively, of the PBC  3  which are opposite to each other. Width W and length L of the widened section  3   a  of the PBC  3  are set so that predetermined single-mode light (TM-mode light, in this example) input will pass through ports (A 1 -B 2  or A 2 -B 1 ) which are diagonally opposite to each other and so that another single-mode light (TE-mode light, in this example) input will pass through ports (A 1 -B 1  or A 2 -B 2 ) which are opposite to each other. 
         [0039]    The modulators  20  and  30  are located symmetrically on the port A 1  side and the port B 1  side, respectively, with the above PBC  3  between. 
         [0040]    The modulated light propagation waveguide  4  connected to the port A 2  of the PBC  3  is formed so that it will reach the one end  2   a  of the substrate  2 . A λ/4 plate  6  and a mirror  7  are located at the end of the modulated light propagation waveguide  4  as a polarization mode conversion section. In addition, the modulated light propagation waveguide  5  connected to the port B 2  of the PBC  3  is formed so that it will reach the other end  2   b  of the substrate  2 . 
         [0041]    For example, the λ/4 plate  6  may be attached to the end  2   a  with proper glue and the mirror  7  may be attached to the outside of the λ/4 plate  6 . Furthermore, a film which functions as the mirror  7  is formed on one side of a film which functions as the λ/4 plate  6  and the other side of the film which functions as the λ/4 plate  6  may be attached to the end  2   a.    
         [0042]    A buffer layer  8  is formed over an entire surface of the substrate  2  in which the optical waveguides  21  and  31 , the PBC  3 , and the modulated light propagation waveguides  4  and  5  are formed. For example, a silicon oxide (SiO 2 ) film with a thickness of about 0.2 to 2 μm is used as the buffer layer  8 . 
         [0043]    Signal electrodes  9  and  10  and an earth electrode  11  each having a predetermined shape are formed on predetermined positions over the substrate  2  with the buffer layer  8  between. A signal source (not shown) which generates an electrical signal (modulation signal) for performing phase modulation in the modulation waveguides  21   c ,  21   d ,  31   c , and  31   d  is connected to the signal electrodes  9  and  10 . The earth electrode  11  has earth potential. If the z-cut substrate  2  is used, a change in refractive index by a z-direction electric field is used. Therefore, the signal electrodes  9  and  10  are located, for example, right over the modulation waveguides  21   c  and  31   c  of the modulators  20  and  30  respectively. In this case, the earth electrode  11  is located right over the modulation waveguides  21   d  and  31   d.    
         [0044]    The pattern shape of the signal electrodes  9  and  10  and the earth electrode  11  is not limited to those shown in  FIG. 1 . For example, the earth electrode  11  may have a pattern shape which does not cover the PBC  3  or the modulated light propagation waveguide  4  or  5 . As a result, light absorption by the earth electrode  11  is suppressed and propagation loss of light propagated through each waveguide under the earth electrode  11  can be reduced. 
         [0045]    When TM-mode light (CW (continuous wave) light) output from a semiconductor laser or the like is input to the input waveguides  21   a  and  31   a  of the modulators  20  and  30  included in the optical device  1  having the above structure, the input light is propagated through the optical waveguides  21  and  31 . 
         [0046]    In the modulator  20 , the TM-mode light input to the input waveguide  21   a  is split first in the branching waveguide  21   b  and is propagated to the modulation waveguides  21   c  and  21   d . By applying a predetermined modulation signal to the signal electrode  9  at this time, the refractive index of the modulation waveguides  21   c  and  21   d  changes. As a result, the phase of the TM-mode light propagated through the modulation waveguides  21   c  and  21   d  changes. A signal applied to the signal electrode  9  is controlled so that a predetermined difference in phase (zero or π, for example) will be obtained between the TM-mode light propagated through the modulation waveguides  21   c  and  21   d.    
         [0047]    The TM-mode light which is propagated through the modulation waveguides  21   c  and  21   d  and between which the predetermined difference in phase is obtained is combined in the branching waveguide  21   e , is intensity-modulated according to the difference in phase, and is propagated to the output waveguide  21   f . Then the TM-mode light after the modulation which is propagated through the output waveguide  21   f  is input to the port A 1  of the PBC  3 . In the PBC  3 , the TM-mode light input to the port A 1  is output to the port B 2  diagonally opposite to the port A 1 . The TM-mode light after the modulation output to the port B 2  is propagated through the modulated light propagation waveguide  5  and is output from the end  2   b  as TM-mode light. 
         [0048]    In the modulator  30 , on the other hand, the TM-mode light input to the input waveguide  31   a  is split in the branching waveguide  31   b  and is propagated to the modulation waveguides  31   c  and  31   d . By applying a predetermined modulation signal to the signal electrode  10  at this time, a predetermined difference in phase is obtained between the TM-mode light propagated through the modulation waveguides  31   c  and  31   d.    
         [0049]    The TM-mode light between which the predetermined difference in phase is obtained is combined in the branching waveguide  31   e , is intensity-modulated according to the difference in phase, and is propagated to the output waveguide  31   f . Then the TM-mode light after the modulation which is propagated through the output waveguide  31   f  is input to the port B 1  of the PBC  3 , and is output to the port A 2  diagonally opposite to the port B 1 . The TM-mode light after the modulation which is output to the port A 2  is propagated through the modulated light propagation waveguide  4 . 
         [0050]    When the TM-mode light after the modulation which is propagated through the modulated light propagation waveguide  4  reaches the end  2   a , the TM-mode light passes through the λ/4 plate  6  located on the end  2   a , is reflected from the mirror  7 , passes through the λ/4 plate  6  again, and is returned to the modulated light propagation waveguide  4 . That is to say, the TM-mode light after the modulation which is propagated through the modulated light propagation waveguide  4  passes through the λ/4 plate  6  twice. As a result, the TM-mode light after the modulation is converted into TE-mode light. 
         [0051]    After that, the TE-mode light is propagated through the modulated light propagation waveguide  4  and input to the port A 2  of the PBC  3 . In the PBC  3 , the TE-mode light input to the port A 2  is output to the port B 2  opposite to the port A 2 . The TE-mode light after the modulation which is output to the port B 2  is propagated through the modulated light propagation waveguide  5  and is output from the end  2   b  as TE-mode light. 
         [0052]    As a result, the TM-mode light after the modulation and the TE-mode light after the modulation are multiplexed and are output from the end  2   b.    
         [0053]    With the optical device  1  according to the first embodiment, as has been described, the TM-mode light after the modulation which is output from the two modulators  20  and  30  is input from the ports A 1  and B 1 , respectively, of the PBC 3  opposite to each other and is output to the port B 2  diagonally opposite to the port A 1  and the port A 2  diagonally opposite to the port B 1 , respectively. Then the TM-mode light output from the modulator  30  is converted into the TE-mode light by the λ/4 plate  6  and the mirror  7  located on the end  2   a , passes through the ports A 2  and B 2  of the PBC  3  opposite to each other, and is output from the end  2   b  together with the TM-mode light output from the other modulator  20 . With the optical device  1 , this polarization multiplexing function can be realized by one chip. 
         [0054]    An example of the conventional optical device will now be described. 
         [0055]      FIG. 4  is a fragmentary schematic plan view showing an example of the conventional optical device. Components in  FIG. 4  that are the same as or equivalent to those shown in  FIG. 1  are marked with the same symbols. 
         [0056]    With an optical device  200  shown in  FIG. 4 , TM-mode light (CW light) is input to the side of one end  2   b  of both modulators  20  and  30  located in parallel. A groove  201  is formed midway along an output waveguide  31   f  of the modulator  30  and a λ/2 plate  202  is located in the groove  201 . A PBC  3  is located at the next stage. An output waveguide  21   f  of the modulator  20  and the output waveguide  31   f  of the modulator  30  are connected to ports A 1  and A 2 , respectively, of the PBC  3 . 
         [0057]    The TM-mode light input to an input waveguide  21   a  of the modulator  20  is split in a branching waveguide  21   b . A signal electrode  9  and an earth electrode  11  are used for obtaining a predetermined difference in phase between the TM-mode light propagated through modulation waveguides  21   c  and  21   d . Then the TM-mode light is combined in a branching waveguide  21   e  and the intensity-modulated TM-mode light is output from the output waveguide  21   f . The TM-mode light output from the modulator  20  is input to the port A 1  of the PBC  3  and is output from a port B 2  diagonally opposite to the port A 1 . 
         [0058]    Similarly, the TM-mode light input to an input waveguide  31   a  of the modulator  30  is modulated and is output from the output waveguide  31   f  as TM-mode light. The TM-mode light output from the modulator  30  passes through the λ/2 plate  202  located in the groove  201  once. As a result, the TM-mode light is converted into TE-mode light, is input to the port A 2  of the PBC  3 , and is output from the port B 2  opposite to the port A 2 . 
         [0059]    The TM-mode light after the modulation and the TE-mode light after the modulation can also be multiplexed and output by the use of the optical device  200  having the above structure. 
         [0060]    In this case, however, it is necessary to form the groove  201  which has certain width and depth and into which the λ/2 plate  202  can be inserted midway along the output waveguide  31   f  in each optical device  200  chip by, for example, cutting after separating a wafer (electro-optic crystal substrate) into individual chips. It is not necessarily easy to form the groove  201  which has predetermined width and depth in this way in an area of the substrate  2 . When the groove  201  is formed, the substrate  2  may be damaged. In addition, a layer of air with certain thickness where the λ/2 plate  202  can be inserted or glue or the like for fixing the λ/2 plate  202  in the groove  201  exists between the end of the output waveguide  31   f  and the λ/2 plate  202 . Accordingly, loss of propagated light cannot be neglected. Furthermore, there may be variation in optical loss among individual optical devices  200 . 
         [0061]    With the above optical device  1  according to the first embodiment, unlike the optical device  200 , there is no need to form the groove  201  in each chip, and the λ/4 plate  6  and the mirror  7  can be located on the end  2   a . Therefore, it is comparatively easy to form the optical device  1 , and loss of propagated light can be reduced effectively. For example, the λ/4 plate  6  and the mirror  7  can be fixed in the following way. A plurality of optical device  1  chips are formed on one wafer and the wafer is cut into the plurality of optical device  1  chips. Then the end  2   a  where the modulated light propagation waveguide  4  is exposed is formed. The λ/4 plate  6  and the mirror  7  are fixed onto a portion of the end  2   a  where the modulated light propagation waveguide  4  is exposed by the use of glue or the like. 
         [0062]    A second embodiment will now be described. 
         [0063]      FIG. 5  is a fragmentary schematic plan view showing an example of an optical device according to a second embodiment. Components in  FIG. 5  that are the same as or equivalent to those shown in  FIG. 1  are marked with the same symbols. 
         [0064]    With an optical device  40  shown in  FIG. 5 , TM-mode light (CW light) is input from a semiconductor laser or the like to the side of an end  2   b  of both modulators  20  and  30  located in parallel. An optical waveguide  21  has an input waveguide  21   a , a branching waveguide  21   b , modulation waveguides  21   c  and  21   d , a branching waveguide  21   e , and an output waveguide  21   f . Similarly, an optical waveguide  31  has an input waveguide  31   a , a branching waveguide  31   b , modulation waveguides  31   c  and  31   d , a branching waveguide  31   e , and an output waveguide  31   f.    
         [0065]    In addition, the optical device  40  has a bent waveguide  21   g  connected to the output waveguide  21   f  of the modulator  20  as part of the optical waveguide  21 . TM-mode light output from the output waveguide  21   f  is returned to the side of the end  2   b  by the bent waveguide  21   g . The bent waveguide  21   g  can be formed by thermal diffusion of metal, a proton exchange, or the like. This is the same with the other waveguides. A comparatively shallow groove  41  the depth of which is almost the same as that of, for example, the bent waveguide  21   g  is formed beside the bent waveguide  21   g  in a substrate  2 . By forming the groove  41 , the bent waveguide  21   g  the curvature of which is obtuse and in which a propagation loss is small can be obtained. The bent waveguide  21   g  having the above structure is connected to a port A 1  of a PBC  3 . 
         [0066]    The output waveguide  31   f  of the other modulator  30  is connected to a port B 1  of the PBC  3  opposite to the port A 1 . In addition, a modulated light propagation waveguide  4  which reaches an end  2   a  is connected to a port A 2 . A λ/4 plate  6  and a mirror  7  are located on, for example, an entire surface of the end  2   a . A modulated light propagation waveguide  5  which reaches the end  2   b  is connected to a port B 2 . 
         [0067]    A buffer layer (not shown) is formed over an entire surface of the substrate  2  in which the optical waveguides  21  and  31 , the PBC  3 , and the modulated light propagation waveguides  4  and  5  are formed. Signal electrodes  9  and  10  are located right over the modulation waveguides  21   c  and  31   c , respectively, with the buffer layer between and an earth electrode  11  is located right over the modulation waveguides  21   d  and  31   d.    
         [0068]    The modulated light propagation waveguide  5  which extends from the port B 2  of the PBC  3  to the end  2   b  is comparatively long, so propagation loss of light may occur. Absorption by an electrode located over the modulated light propagation waveguide  5  has a great influence on optical propagation loss. Accordingly, with the optical device  40  there are regions over the modulated light propagation waveguide  5  where the earth electrode  11  is not formed. As a result, the influence of such absorption is reduced. 
         [0069]    In this example, the λ/4 plate  6  and the mirror  7  are located on the entire surface of the end  2   a . However, the λ/4 plate  6  and the mirror  7  may be located on a portion of the end  2   a  so that they will cover the modulated light propagation waveguide  4 . In this case, one wafer is cut into a plurality of optical device  40  chips and the λ/4 plate  6  and the mirror  7  are fixed to a predetermined portion of each chip. If the λ/4 plate  6  and the mirror  7  are located on the entire surface of the end  2   a  as shown in  FIG. 5 , then the λ/4 plate  6  and the mirror  7  are fixed to the ends  2   a  of the plurality of chips before cutting the wafer. After that, the wafer should be cut into the plurality of chips each including the λ/4 plate  6  and the mirror  7 . 
         [0070]    With the optical device  40  having the above structure, TM-mode light input from the side of the end  2   b  to the input waveguide  21   a  of the modulator  20  is split in the branching waveguide  21   b . The signal electrode  9  and the earth electrode  11  are used for obtaining a predetermined difference in phase between the split TM-mode light propagated through the modulation waveguides  21   c  and  21   d . Then the split TM-mode light is combined in the branching waveguide  21   e  and is propagated through the output waveguide  21   f . The TM-mode light after the modulation is returned to the side of the end  2   b  by the bent waveguide  21   g , is input to the port A 1  of the PBC  3 , and is output from the port B 2  diagonally opposite to the port A 1 . Then the TM-mode light is propagated through the modulated light propagation waveguide  5  and is output from the end  2   b.    
         [0071]    Similarly, TM-mode light input from the side of the end  2   b  to the input waveguide  31   a  of the modulator  30  is modulated. The TM-mode light after modulation which is propagated through the output waveguide  31   f  is input to the port B 1  of the PBC  3  and is output from the port A 2  diagonally opposite to the port B 1 . Then the TM-mode light is propagated through the modulated light propagation waveguide  4 . After the TM-mode light reaches the end  2   a , the TM-mode light passes through the λ/4 plate  6  located on the end  2   a , is reflected from the mirror  7 , and passes through the λ/4 plate  6  again. As a result, the TM-mode light is converted into TE-mode light. Then the TE-mode light is propagated through the modulated light propagation waveguide  4 , is input to the port A 2  of the PBC  3 , is output to the port B 2  opposite to the port A 2 , is propagated through the modulated light propagation waveguide  5 , and is output from the end  2   b.    
         [0072]    As a result, the TM-mode light after the modulation and the TE-mode light after the modulation are multiplexed and are output from the end  2   b . With the above optical device  40 , the two modulators  20  and  30  are located in parallel and output from the one modulator  20  is returned. By doing so, necessary components can be formed on one chip and miniaturization can be realized. 
         [0073]    In this example, the bent waveguide  21   g  is used as a means for returning light output from the modulator  20 . However, a mirror or an optical fiber may be used. 
         [0074]      FIG. 6  is a fragmentary schematic plan view showing an example of an optical device having a return structure in which a mirror is used.  FIG. 7  is a fragmentary schematic plan view showing an example of an optical device having a return structure in which an optical fiber is used. 
         [0075]    With an optical device  40   a  shown in  FIG. 6 , for example, a mirror  42  is located on a portion of an end  2   a . Output waveguides  21   fa  and  21   fb  for inputting or outputting light are formed so that TM-mode light output from a modulator  20  is input to the mirror  42  at a constant angle and so that the TM-mode light is reflected from the mirror  42  at the constant angle. By doing so, the TM-mode light output from the modulator  20  can be returned. 
         [0076]    Furthermore, with an optical device  40   b  shown in  FIG. 7 , output waveguides  21   fc  and  21   fd  which reach an end  2   a  are formed and an optical fiber  43  is connected to the output waveguides  21   fc  and  21   fd . By doing so, TM-mode light output from a modulator  20  can be returned. 
         [0077]    With the optical devices  40   a  and  40   b , a λ/4 plate  6  and a mirror  7  should be formed on a portion of the end  2   a  where a modulated light propagation waveguide  4  is exposed. 
         [0078]    With the above optical devices  40 ,  40   a , and  40   b , the earth electrode  11  is not formed on an area  2   c  over the modulated light propagation waveguide  5  in order to reduce optical propagation loss caused by the absorption of light by an electrode. 
         [0079]    If the area  2   c  is not secured, the refractive index of the modulated light propagation waveguide  5  should be made lower than that of the other waveguides. By doing so, the absorption of light by the earth electrode  11  can be reduced and optical propagation loss can be reduced. If the refractive index of the modulated light propagation waveguide  5  is lowered, a light trapping effect in the modulated light propagation waveguide  5  is weakened and a region through which light is propagated can be moved in the direction of the depth of the substrate  2 . As a result, light becomes distant from the earth electrode  11  and absorption is suppressed. 
         [0080]    In addition, if the width of the modulated light propagation waveguide  5  is made narrower than that of the other waveguides, a light trapping effect in the modulated light propagation waveguide  5  is also weakened. As a result, a region through which light is propagated is moved in the direction of the depth of the substrate  2 . Therefore, the absorption of light by the earth electrode  11  can be suppressed. 
         [0081]    Moreover, by thickening the buffer layer formed between the modulated light propagation waveguide  5  and the earth electrode  11  or lowering the refractive index of the buffer layer, the effect of trapping light propagated through the modulated light propagation waveguide  5  in the substrate  2  by the buffer layer can be enhanced and the absorption of light by the earth electrode  11  can be suppressed. 
         [0082]    Not only the method for securing the area over the modulated light propagation waveguide  5  where the earth electrode  11  is not formed but also this method for suppressing the absorption of light by the earth electrode  11  is also applicable to the above optical device  1  according to the first embodiment. 
         [0083]    A third embodiment will now be described. 
         [0084]      FIG. 8  is a fragmentary schematic plan view showing an example of an optical device according to a third embodiment. Components in  FIG. 8  that are the same as or equivalent to those shown in  FIG. 1  or  5  are marked with the same symbols. 
         [0085]    With an optical device  40   c  shown in  FIG. 8 , patterns of signal electrodes  9  and  10  are formed so that input sections  9   a  and  10   a  of the signal electrodes  9  and  10  where a modulation signal sent from a signal source is input can be located on the same side. The optical device  40   c  differs from the above optical device  40  according to the second embodiment in this respect. By adopting this structure, two connectors optically connected to the input sections  9   a  and  10   a  from which modulation signals are input can be located at the same side of the optical device  40   c . As a result, a package including the optical device  40   c  can be miniaturized. 
         [0086]    By the way, when TM-mode light after modulation and TE-mode light after modulation are multiplexed and are output from an end  2   b , it is necessary in some cases to output the TM-mode light and the TE-mode light without any delay between them. Such cases can be dealt with by adjusting the length of a waveguide or an electrode. 
         [0087]    Descriptions will now be given with the optical device  40   c  shown in  FIG. 8  as an example. It is assumed that the distance from the end  2   b  from which the TM-mode light is input to the end of an output waveguide  21   f  (or to the beginning or end of a bent waveguide  21   g  or the beginning of a modulated light propagation waveguide  4 ) is La, that the distance from the beginning of the modulated light propagation waveguide  4  to the end of the modulated light propagation waveguide  4  (end  2   a ) is Lb, that the distance between the input sections  9   a  and  10   a  of modulators  20  and  30  is Lc, that the distance from the input section  10   a  of the modulator  30  to the end of the output waveguide  21   f  (or to the beginning or end of the bent waveguide  21   g  or the beginning of the modulated light propagation waveguide  4 ) is Ld, that the distance between modulation waveguides  21   c  and  31   c  of the modulators  20  and  30  (between the signal electrodes  9  and  10 ) is Le, that the radius of curvature of the bent waveguide  21   g  is R, that the refractive indexes of the TM-mode light and the TE-mode light are Ne and No respectively, and that the effective refractive index of a modulation signal is Nm. 
         [0088]    Optical length for the TM-mode light from the modulation to the output in the modulator  20  is given by 
         [0000]      (Lc+Ld+πR+La) 
         [0089]    Optical length for the TM-mode light and the TE-mode light from the modulation to the output in the modulator  30  is given by 
         [0000]      Ne(Ld+Lb+(1+No/Ne)(Lb+La) 
         [0090]    The difference in optical length between modulation signals is given by 
         [0000]      NmLe 
         [0091]    To avoid delay between the TM-mode light and the TE-mode light for, for example, a distance of Lb, the length of the modulated light propagation waveguide  4  should be set so that 
         [0000]        Lb =( Lc+πR+LeNm/Ne−LaNo/Ne )/(2+ No/Ne ) 
         [0092]    A fourth embodiment will now be described. 
         [0093]      FIG. 9  is a fragmentary schematic plan view showing an example of an optical device according to a fourth embodiment. Components in  FIG. 9  that are the same as or equivalent to those shown in  FIG. 1  or  5  are marked with the same symbols. 
         [0094]    With an optical device  50  shown in  FIG. 9 , a λ/4 plate  6  and a mirror  7  are located on one side end and multiplexed TM-mode light and TE-mode light are output from the other side end. 
         [0095]    With the optical device  50 , an output waveguide  21   f  of a modulator  20  is connected to a bent waveguide  21   h  and the bent waveguide  21   h  is connected to a port A 1  of a PBC  3 . An output waveguide  31   f  of a modulator  30  is connected to a bent waveguide  31   h  and the bent waveguide  31   h  is connected to a port B 1  of the PBC  3  opposite to the port A 1 . A shallow groove  51  the depth of which is almost the same as that of the bent waveguide  21   h  is formed beside the bent waveguide  21   h . Similarly, a shallow groove  52  the depth of which is almost the same as that of the bent waveguide  31   h  is formed beside the bent waveguide  31   h . A modulated light propagation waveguide  4  connected to a port A 2  of the PBC  3  reaches the one side end of the optical device  50  and a modulated light propagation waveguide  5  connected to a port B 2  of the PBC  3  reaches the other side end of the optical device  50 . Signal electrodes  9  and  10  and an earth electrode  11  are located over the above structure with a buffer layer between. 
         [0096]    TM-mode light modulated by the modulator  20  passes through the PBC  3 , is propagated through the modulated light propagation waveguide  5 , and is output. TM-mode light modulated by the modulator  30  passes through the PBC  3 , is propagated to the λ/4 plate  6  and the mirror  7 , is converted into TE-mode light, passes through the PBC  3 , is propagated through the modulated light propagation waveguide  5 , and is output. 
         [0097]    With the optical device  50 , an electrode is not located over the modulated light propagation waveguide  5 . In addition, compared with, for example, the above optical device  40 , the length of the modulated light propagation waveguide  5  is short. As a result, the optical device  50  can be fabricated as one chip and be miniaturized. Moreover, the absorption of the light propagated through the modulated light propagation waveguide  5  by an electrode can be suppressed and optical propagation loss can be reduced. 
         [0098]    In this example, the λ/4 plate  6  and the mirror  7  are located on a portion of the one side end where the modulated light propagation waveguide  4  is exposed. However, the λ/4 plate  6  and the mirror  7  may be located on the whole of the one side end. 
         [0099]    In addition, for example, if the λ/4 plate  6  and the mirror  7  in particular are located on the whole of the one side end of the optical device  50  according to the fourth embodiment, input sections from which modulation signals are input may be located on the same side. This is the same with the above optical device  40   c  according to the third embodiment. 
         [0100]    A fifth embodiment will now be described. 
         [0101]      FIG. 10  is a fragmentary schematic plan view showing an example of an optical device according to a fifth embodiment. Components in  FIG. 10  that are the same as or equivalent to those shown in  FIG. 1  or  5  are marked with the same symbols. 
         [0102]    An optical device  60  shown in  FIG. 10  includes four modulators  71 ,  72 ,  81 , and  82  and has a multistage structure. The structure of each of the modulators  71 ,  72 ,  81 , and  82  is the same as that of the above modulator  20  or  30 . 
         [0103]    In this example, an output side of the modulator  71  is connected to an input side of the modulator  81  via a bent waveguide  62  beside which a groove  61  is formed. An output side of the modulator  72  is connected to an input side of the modulator  82  via a bent waveguide  64  beside which a groove  63  is formed. An output side of the modulator  82  is connected to a port A 1  of a PBC  3  via a bent waveguide  66  beside which a groove  65  is formed. An output side of the modulator  81  is connected to a port B 1  of the PBC  3  opposite to the port A 1 . A modulated light propagation waveguide  4  which reaches an end  2   a  is connected to a port A 2  of the PBC  3 . A λ/4 plate  6  and a mirror  7  are located on a portion of the end  2   a . A modulated light propagation waveguide  5  which reaches an end  2   b  is connected to a port B 2  of the PBC  3 . A signal electrode  67  and an earth electrode  68  are located over the above structure with a buffer layer between. 
         [0104]    With the optical device  60  having the above structure, the λ/4 plate  6  and the mirror  7  are located on the end  2   a  from which TM-mode light is input to the modulators  71  and  72  at a first stage. Therefore, the λ/4 plate  6  and the mirror  7  are formed on a portion of the end  2   a  where the modulated light propagation waveguide  4  is exposed. By adopting this structure, the miniaturized optical device  60  which includes the modulators  71 ,  72 ,  81 , and  82  located at multiple stages and which is fabricated as one chip is realized. 
         [0105]    A sixth embodiment will now be described. 
         [0106]      FIG. 11  is a fragmentary schematic plan view showing an example of an optical device according to a sixth embodiment. Components in  FIG. 11  that are the same as or equivalent to those shown in  FIG. 1  or  5  are marked with the same symbols. 
         [0107]    With an optical device  90  shown in  FIG. 11 , input waveguides  21   a  and  31   a  of two modulators  20  and  30  from which TM-mode light output from a semiconductor laser or the like is input are considered as a branching waveguide  91 . TM-mode light is split in the branching waveguide  91  and is input to the modulators  20  and  30 . The optical device  90  also includes a PBC  92  for preventing TE-mode light obtained by making a conversion by the use of a λ/4 plate  6  and a mirror  7  from returning to the modulator  30 . The optical device  90  differs from the above optical device  40   c  according to the third embodiment shown in  FIG. 8  in these respects. 
         [0108]    With the optical device  90  having the above structure, TM-mode light input from an end  2   b  may, for example, equally be split by the branching waveguide  91  and be input to the modulators  20  and  30 . If TM-mode light and TE-mode light ultimately output from a modulated light propagation waveguide  5  differ in intensity by the influence of, for example, a loss which occurs at the time of being propagated or passing through the λ/4 plate  6 , a branching ratio of the branching waveguide  91  may be set so that they will be equal in intensity. In this case, the TM-mode light is split at the set branching ratio in the branching waveguide  91  and is input to the modulators  20  and  30 . 
         [0109]    For example, the PBC  92  is located so as to connect an output waveguide  31   f  of the modulator  30  to a port B 1   a  and so as to connect a port A 2   a  diagonally opposite to the port B 1   a  to a port B 1  of an other PBC  3 . Remaining ports A 1   a  and B 2   a  of the PBC  92  are made open. 
         [0110]    The TM-mode light modulated by the modulator  30  is output from the port B 1   a  to the port A 2   a  of the PBC  92 , is output from the port B 1  to a port A 2  of the PBC  3 , is propagated through a modulated light propagation waveguide  4 , and is converted into TE-mode light by the λ/4 plate  6  and the mirror  7 . The TE-mode light is output from the port A 2  of the PBC  3  to a port B 2  opposite to the port A 2 . However, part of the TE-mode light may be output to the port B 1  diagonally opposite to the port A 2 , depending on a branching ratio of the PBC  3 . The TE-mode light output to the port B 1  of the PBC  3  is input to the port A 2   a  of the PBC  92  and is output to the port B 2   a  opposite to the port A 2   a . If the TE-mode light returns to the modulator  30 , the TE-mode light is input to the semiconductor laser or the like which outputs the TM-mode light input to the optical device  90 , and the operation of the semiconductor laser or the like is badly affected. With the optical device  90 , however, it is difficult to output the TE-mode light which is input to the port A 2   a  to the port B 1   a  of the PBC  92  diagonally opposite to the port A 2   a . This prevents the operation of the semiconductor laser or the like from being badly affected. 
         [0111]    In this example, the two PBCs  3  and  92  are used for preventing the TE-mode light from returning to the modulator  30 . However, it is a matter of course that three or more PBCs may be used for preventing the TE-mode light from returning to the modulator  30 . 
         [0112]    In addition, the branching waveguide  91  described in the sixth embodiment is also applicable to the above optical devices  40 ,  40   a , and  40   b  according to the second embodiment, the above optical device  50  according to the fourth embodiment, and the above optical device  60  according to the fifth embodiment. 
         [0113]    Furthermore, the method of using a plurality of PBCs for preventing the TE-mode light from returning to the modulator  30  described in the sixth is also applicable to the above optical device  1  according to the first embodiment, the above optical devices  40 ,  40   a , and  40   b  according to the second embodiment, the above optical device  50  according to the fourth embodiment, and the above optical device  60  according to the fifth embodiment. 
         [0114]    A seventh embodiment will now be described. 
         [0115]      FIG. 12  is a fragmentary schematic plan view showing an example of an optical device according to a seventh embodiment. Components in  FIG. 12  that are the same as or equivalent to those shown in  FIG. 1  or  5  are marked with the same symbols. 
         [0116]    With an optical device  100  shown in  FIG. 12 , input waveguides  21   a  and  31   a  of two modulators  20  and  30  from which TM-mode light output from a semiconductor laser or the like is input are considered as a branching waveguide  101 . An isolator  102  is located before the branching waveguide  101 . The optical device  100  differs from the above optical device  40   c  according to the third embodiment shown in  FIG. 8  in these respects. A branching ratio of the branching waveguide  101  of the optical device  100  can be set. This is the same with the branching waveguide  91  described in the sixth embodiment. 
         [0117]    Even if the TM-mode light modulated by the modulator  20  returns to the modulator  30  via a PBC  3  in the optical device  100  or even if the TM-mode light modulated by the modulator  30  returns to the modulator  20  via the PBC  3  in the optical device  100 , the isolator  102  prevents the TM-mode light from inputting to the semiconductor laser or the like. 
         [0118]    The branching waveguide  101  and the isolator  102  are also applicable to the above optical device  1  according to the first embodiment, the above optical devices  40 ,  40   a , and  40   b  according to the second embodiment, the above optical device  50  according to the fourth embodiment, and the above optical device  60  according to the fifth embodiment. 
         [0119]    In addition, the PBC  92  described in the above sixth embodiment may be located before the PBC  3  on the output side of the modulator  30  in the optical device  100 . As a result, the isolator  102  prevents the TM-mode light from returning to the modulator  20  or  30  and a PBC  92  prevents TE-mode light from returning to the modulator  30 . 
         [0120]    An eighth embodiment will now be described. 
         [0121]      FIG. 13  is a fragmentary schematic plan view showing an example of an optical device according to an eighth embodiment. Components in  FIG. 13  that are the same as or equivalent to those shown in  FIG. 1  or  5  are marked with the same symbols. 
         [0122]    With an optical device  110  shown in  FIG. 13 , TM-mode light is input from an end  2   b  to a modulator  20  and TM-mode light is input from an end  2   a  to a modulator  30 . A PBC  111  is located on the output side of the modulator  20  and a PBC  112  is located on the output side of the modulator  30 . 
         [0123]    An output waveguide  21   f  of the modulator  20  is connected to a port B 1   b  of the PBC  111  and a modulated light propagation waveguide  4   a  which reaches the end  2   a  is connected to a port A 2   b  diagonally opposite to the port B 1   b . A λ/4 plate  6  and a mirror  7  are located on a portion of the end  2   a  where the modulated light propagation waveguide  4   a  is exposed. A modulated light propagation waveguide  4   b  is connected to a port B 2   b  of the PBC  111  and is connected to a port A 1   c  of the other PBC  112 . An output waveguide  31   f  of the modulator  30  is connected to a port A 2   c  of the PBC  112 . With the optical device  110 , an area over the modulated light propagation waveguide  4   b  where an earth electrode  11  is not formed may be secured. 
         [0124]    With the optical device  110  having the above structure, TM-mode light is input first from a semiconductor laser or the like to the modulators  20  and  30 . The TM-mode light modulated by the modulator  20  is input to the port B 1   b  of the PBC  111 , is output to the port A 2   b  diagonally opposite to the port B 1   b , is propagated through the modulated light propagation waveguide  4   a , and is converted into TE-mode light by the use of the λ/4 plate  6  and the mirror  7 . The TE-mode light is output from the port A 2   b  of the PBC  111  to the port B 2   b  opposite to the port A 2   b , is propagated through the modulated light propagation waveguide  4   b , is input to the port A 1   c  of the PBC  112 , and is output from a port B 1   c  opposite to the port A 1   c . On the other hand, the TM-mode light modulated by the modulator  30  is input to the port A 2   c  of the PBC  112  and is output to the port B 1   c  diagonally opposite to the port A 2   c . The TM-mode light and the TE-mode light is multiplexed and is output from the port B 1   c  of the PBC  112  in this way. 
         [0125]    The optical device  110  effectively prevents the TM-mode light modulated by the modulator  20  or  30  from returning and prevents this TM-mode light from inputting to the semiconductor laser or the like. 
         [0126]    With the above-mentioned optical device  1  and the like, certain TM-mode light input is finally output as multiplexed TM-mode light and TE-mode light. In this case, the multiplexed TM-mode light and TE-mode light may differ in propagation loss because of a difference in propagation path. Accordingly, in order to make propagation losses which occur along both propagation paths equal, one of the following methods can be adopted. A portion which differs from the rest in width may be formed on, for example, an input waveguide  21   a  or  31   a  or an output waveguide  21   f  or  31   f  to increase or decrease a propagation loss which occurs along one propagation path. A shallow groove may be formed beside a portion of a waveguide on one propagation path to control an optical propagation loss. The depth of the groove is almost the same as that of the waveguide. A bent waveguide may be formed as a portion of a waveguide on one propagation path to control an optical propagation loss. A metal film may be formed over a waveguide on one propagation path with a buffer layer between to control an optical propagation loss. The metal film absorbs light. 
         [0127]    In the above descriptions, the case where the z-cut substrate  2  is used is taken as an example. However, the above techniques are also applicable to the case where an x-cut substrate is used. In this case, it is necessary to properly change the pattern shape and arrangement of the signal electrodes  9  and  10 , the earth electrode  11 , and the like. 
         [0128]    The above-mentioned techniques are also applicable to optical devices in which various modulation systems, such as a return to zero (RZ) modulation system, a differential quadrature phase shift keying (DQPSK) modulation system, and a RZ-DQPSK modulation system are employed. 
         [0129]    As has been described in the foregoing, two modulators and one or more PBCs are arranged in one chip and a polarization mode conversion section is located on an end of the chip. By doing so, a miniature high performance optical device with high reliability which can be fabricated comparatively easily and which has a polarization multiplexing function is realized. In addition, various optical communication units in which such optical devices are used and which have an optical communication function can be realized. 
         [0130]    The foregoing is considered as illustrative only of the principles. Further, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and applications shown and described, and accordingly, all suitable modifications and equivalents may be regarded as falling within the scope of the invention in the appended claims and their equivalents.