Abstract:
An optical module has an optical modulator configured to perform phase modulation on each of divided light components of an input light and output at least two phase-modulated signal lights, a semiconductor optical amplifier configured to amplify the phased-modulated signal lights in a same polarization mode, and a polarization multiplexer configured to convert the amplified signal lights into two orthogonally polarized signal lights and multiplex the orthogonally polarized signal lights.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2015-148663 filed Jul. 28, 2015, the contents of which are incorporated herein by reference in its entirety. 
       FIELD 
       [0002]    The disclosures herein relate to an optical module and an optical transmitter using the same. 
       BACKGROUND 
       [0003]    In recent years, to achieve high data transmission capacity in fiber-optic transmission systems, research studies for increasing a modulation rate and improving modulation schemes (including multi-level modulation and polarization division multiplexing) are being made. With dual polarization quadrature phase-shift keying (DP-QPSK) using a digital coherent technique, long distance fiber-optic transmission at 100 GB/s has been achieved. Polarization division multiplexing (PDM) permits two independent data signals to be transmitted at a time by combining two orthogonally polarized light signals. 
         [0004]      FIG. 1  illustrates a typical DP-QPSK modulator module  100 . The modulator module  100  includes a modulator chip  120  with four Mach-Zehnder (MZ) interferometers  121 - 124  arranged in parallel, a polarization rotator (PR)  125 , and a polarization beam combiner (PBC)  126 . A light beam emitted from a light source  105  such as a laser diode (LD) is input to the modulator chip  120  via a lens  101 . The input light is modulated under application of different electrical signals to the MZ interferometers  121 - 124 . The direction of polarization of the light component output from one of the MZ interferometer pairs is rotated by 90 degrees at the PR  125  with respect to the light component output from the other pair of the MZ interferometers. These light components are combined at the PBC  126 . The resultant signal is a multi-level modulated and polarization division multiplexed signal. The signal light output from the modulator module  100  is amplified by an erbium-doped fiber amplifier (EDFA)  127  and undergoes noise reduction at a bandpass filter (BPS)  128 . Then, the signal is output to a transmission path. 
         [0005]    With the configuration of  FIG. 1 , PDM and optical amplification are performed at separate components and therefore, the transmitter size becomes larger. To achieve a compact transmitter structure, the configuration of  FIG. 2  may be provided in which the function of optical amplification is incorporated, together with the PR  125  and PBC  126  used for PDM, in a modulator module  200 . In order to put the function of optical amplification into the modulator module  200 , a semiconductor optical amplifier (SOA)  227  is used in place of the EDFA  127 . The PBC  126 , the SOA  227  and the BPF  228  are optically coupled using lenses  204  and  206 . 
         [0006]    The publications listed below are also known. 
         [0007]    [Patent Document 1] Japanese Laid-open Patent Publication No. 2011-188213 
         [0008]    [Patent Document 2] Japanese Laid-open Patent Publication No. 2009-60461 
       SUMMARY 
       [0009]    According to an aspect of the embodiment, an optical module has 
         [0010]    an optical modulator configured to perform phase modulation on each of divided light components of an input light and output at least two phase-modulated signal lights, 
         [0011]    a semiconductor optical amplifier configured to amplify the phased-modulated signal lights in a same polarization mode, and 
         [0012]    a polarization multiplexer configured to convert the amplified signal lights into two orthogonally polarized signal lights and multiplex the orthogonally polarized signal lights. 
         [0013]    The object and advantages of the embodiment will be realized and attained by means of the elements and combinations particularly pointed out in the claims. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0014]      FIG. 1  is a schematic diagram of a typical modulator module; 
           [0015]      FIG. 2  is a schematic diagram of a modulator module in which an optical amplifier is incorporated; 
           [0016]      FIG. 3  is a schematic diagram of a modulator module according to the first embodiment: 
           [0017]      FIG. 4  is a schematic diagram of a modulator module according to the second embodiment; 
           [0018]      FIG. 5A  illustrates a fabrication process of the modulator chip illustrated in  FIG. 4 ; 
           [0019]      FIG. 5B  illustrates a fabrication process of the modulator chip illustrated in  FIG. 4 ; 
           [0020]      FIG. 5C  illustrates a fabrication process of the modulator chip illustrated in  FIG. 4 ; 
           [0021]      FIG. 6  is a modification of the modulator module illustrated in  FIG. 4 ; 
           [0022]      FIG. 7  is a schematic diagram of a modulator module according to the third embodiment; 
           [0023]      FIG. 8  is a schematic diagram of a modulator module according to the fourth embodiment; 
           [0024]      FIG. 9A  illustrates a fabrication process of the modulator chip illustrated in  FIG. 8   
           [0025]      FIG. 9B  illustrates a fabrication process of the modulator chip illustrated in  FIG. 8 ; 
           [0026]      FIG. 9C  illustrates a fabrication process of the modulator chip illustrated in  FIG. 8 ; 
           [0027]      FIG. 10  is a schematic diagram of a modulator module according to the fifth embodiment; 
           [0028]      FIG. 11  is a modification of the modulator module of the fifth embodiment; 
           [0029]      FIG. 12  is another modification of the modulator module of the fifth embodiment; and 
           [0030]      FIG. 13  is a schematic diagram of an optical transmitter using one of the modulator modules of the embodiments. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0031]    The configuration illustrated in  FIG. 2  has a problem in that the power levels of the output light differ between transverse electric (TE) polarized wave and transverse magnetic (TM) polarized wave because the gain of an SOA varies depending on the polarization mode. 
         [0032]    In view of this technical problem, the embodiments provide a compact optical modulator with reduced polarization dependency. This can be achieved by inserting an SOA before a polarization rotator and amplifying signal lights in the same polarization mode (e.g., TE mode) and then performing polarization division multiplexing. 
       First Embodiment 
       [0033]      FIG. 3  is a schematic diagram of a modulator module  1  according to the first embodiment. The modulator module  1  is one example of an optical modulator. The top figure of  FIG. 3  is a top view and the bottom figure of  FIG. 2  is a cross-sectional view along the light propagation direction. The modulator module  1  has a modulator chip  10 , an SOA  25  and an SOA  26  which serve as an optical amplifier, and a polarization multiplexer  30 . The SOAs  25  and  26  are arranged between the modulator chip  10  and the polarization multiplexer  30 . In other words, the SOAs  25  and  26  are provided before a polarization rotator (hereinafter abbreviated as “PR”)  27 . 
         [0034]    The modulator chip  10  is formed of a silicon substrate  41  and has four MZ interferometers  21 - 24  formed by parallel waveguide pairs  31   a  to  31   d . In the first embodiment, the modulator chip  10  forms an optical modulator  20 . A n/2 radian phase difference is added between light waves travelling through the MZ interferometers  21  and  22 . Similarly, a n/2 radian phase difference is added between light waves travelling through the MZ interferometers  23  and  24 . In the figure, phase shifters for applying the n/2 radian phase difference are omitted for the convenience of illustration. Electrodes  33  are provided to the parallel waveguide pairs  31   a ,  31   b ,  31   c  and  31   d  that form the MZ interferometers  21 ,  22 ,  23  and  24 , respectively. The electrode  33  is, for example, a coplanar electrode including a signal electrode and a ground electrode. 
         [0035]    In operations, a continuous wave output from an LD  15  is input to an optical waveguide  31  of the modulator chip  10  by a lens  11 . The input light is in, for example, TE mode with its electric field parallel to the chip surface. The MZ interferometers  21  to  24  are driven by drive signals supplied externally. When driving the modulator chip  10  at a high rate, the ends of the signal electrode and the ground electrode are terminated by matched resistance to from a travelling-wave electrode and a microwave (electrical signal) is applied from the input side. In an electro-optic modulator using an electro-optic crystal such as lithium niobate (LN) or lithium tantalate (LT), the index of refractions of the parallel waveguide pairs  31   a  to  31   d  of the MZ interferometers  21 - 24  change due to the electric field of the applied electrical signals. In a semiconductor modulator, the carrier density of each of the parallel waveguide pairs  31   a - 31   d  changes due to the applied electric field and the light absorbance changes (electro-absorption effect). As a result, the signal light subjected to phase modulation between 0 radians and n radians is output at a high contrast by MZ interference. 
         [0036]    A modulated light L 1  produced by combining the light beams from the MZ interferometers  21  and  22  and a modulated light L 2  produced by combining the light beams from the MZ interferometers  23  and  24  are output from the modulator chip  10 . The modulated lights L 1  and L 2  are both in TE mode, and each of the modulated lights L 1  and L 2  contains an in-phase component and a quadrature component. The modulated light L 1  is focused into the SOA  25  by a lens  12 . The modulated light L 2  is focused into the SOA  26  by a lens  13 . The SOA  25  and the SOA  26  may have obliquely inclined input surfaces  25   a  and  26   a  and output surfaces  25   b  and  26   b , respectively. The input surfaces  25   a  and  26   a  and the output surfaces  25   b  and  26   b  of the SOA  25  and  26  are not necessarily perpendicular to the light propagation axis. By using the input/output surfaces not completely perpendicular to the light propagation axis but with a certain degree of inclination, reflection is prevented and noise is reduced. The SOA  25  and the SOA  26  amplify the power levels of the input lights under injection of electric currents. Because the signal lights are amplified in the same polarization mode at the SOA  25  and the SOA  26 , respectively, there are little variations in gain caused due to polarization difference even if the SOA  25  and the SOA  26  themselves have polarization dependent gain characteristics. 
         [0037]    The signal lights output from the SOA  25  and the SOA  26  are input via the lens  15  and the lens  17 , respectively, to the polarization multiplexer  30 . The polarization multiplexer  30  has a PR  27 , a polarization beam combiner (hereinafter abbreviated as “PBC”)  28 , and a bandpass filter (hereinafter abbreviated as “BPF”)  29 . 
         [0038]    The PR  27  rotates the polarization axis of one of the amplified modulated lights L 1  and L 2  by  90  degrees. In the example of  FIG. 3 , the polarization axis of the light L 1  output from the SOA  25  is rotated. Consequently, the polarization mode of the light L 1  is converted to TM mode. On the other hand, the light L 2  output from the SOA  26  remains in TE mode. By combining the TM polarized light L 1  and the TE polarized light L 2  at the PBC  28 , signal lights with mutually orthogonal polarizations are multiplexed. The combined light output from the PBC  28  is focused into the BPF  29  by a lens  18  and noise is reduced. The output of the polarization multiplexer  30  is the output of the modulator module  1 , and the signal light is output to an optical path such as an optical fiber through a lens  19 . 
         [0039]    With the configuration of  FIG. 3 , the modulator module  1  performs dual-polarization phase modulation and optical amplification in a compact structure, while reducing polarization dependency of SOA gain. The SOA  25  and the SOA  26  provided to the respective channels (for signal lights L 1  and L 2 ) can be independently controlled. Even if polarization dependency may be produced due to difference in optical loss among the optical modulator  20 , the PR  27  and the PBC  28 , such polarization dependency can be corrected. The PR  27  is of an arbitrary type including an liquid crystal 
         [0040]    PR, a half-wave plate, a combination of liquid crystal and a quarter-wave plate, a fiber optic polarizer, and so on. 
         [0041]    The polarization multiplexer  30  may be formed in a silicon chip. In this case, the SOA  25  and the SOA  26  may be optically coupled with the silicon waveguides on the polarization multiplexer  30  via spot size converters. 
       Second Embodiment 
       [0042]      FIG. 4  is a schematic diagram of a modulator module  2 A which is one example of an optical modulator. In the first embodiment, separate optical components are arranged in the modulator module  1  and optically coupled using lenses. In the second embodiment, a modulator module  2 A is formed of a single modulator chip  40 . The modulator chip  40  has an optical modulator  20 , an SOA  25  and an SOA  26  that serve as an optical amplifier, and a polarization multiplexer  30 . The configuration of the optical modulator  20  is the same as that of the modulator chip  10  illustrated in  FIG. 3  and four MZ interferometers  21 - 24  are arranged in parallel. The polarization multiplexer  30  has variable attenuators (VATs)  42  and  43  arranged at the input end, in addition to the PR  47 , PBC  48  and BPF  49 . 
         [0043]    When forming the modulator chip  40  using the silicon substrate  41 , the material of the SOA  25  and the SOA  26  may be different from that of the modulator chip  40 . The SOA  25  and the SOA  26  are generally formed of a material other than silicon, such as a compound semiconductor or an organic material. When different materials are used between the modulator chip  40  and the SOAs  25  and  26 , the SOA  25  and the SOA  26  fabricated separately from the modulator chip  40  are embedded in the substrate (i.e., the silicon substrate  41  in  FIG. 4 ) of the modulator chip  40 . The active layers  25 Q and  26 Q of the SOA  25  and SOA  26  (only the active layer  25 Q is illustrated in the cross-sectional view of  FIG. 4 ) are aligned and optically coupled with the silicon waveguides extending from the optical modulator  20  and the silicon waveguides of the polarization multiplexer  30 . 
         [0044]      FIG. 5A  through  FIG. 5C  illustrate a fabrication process of the modulator chip  40 . In  FIG. 5A , optical waveguides  31  including the MZ interferometers  21 - 24 , the VATs  42  and  43 , the PR  47 , the PBC  48  and the BPF  49  are formed on the silicon substrate  41 . The optical waveguides  31  may be of a rib type or a silicon photonic nanowire. The PR  47  is formed by processing the core of a corresponding portion of the optical waveguide  31  into a shape with refractive index anisotropy with respect to the propagating light wave. 
         [0045]    In  FIG. 5B , a recess  45  for accommodating the SOA  25  and the SOA  26  is formed in the silicon substrate  41  by etching or cutting. In  FIG. 5C , the SOAs  25  and  26  are placed in the recess  45  and bonded. The silicon cores of the optical waveguides  31  formed in the silicon substrate  41  are optically coupled with the active layers  25 Q and  26 Q of the SOA  25  and the SOA  26  at aligned positions. Spot size converters may be formed in the optical waveguides  31  at positions facing the input planes and the output planes of the SOA  25  and the SOA  26 . 
         [0046]    With this configuration, the modulator module  2 A can be downsized. The SOA  25  and the SOA  26  are arranged before the PR  47 . The modulated lights L 1  and L 2  output from the optical modulator  20  in the same polarization mode (e.g., TE mode) are amplified at the SOA  25  and the SOA  26 , respectively, and then polarization division multiplexing is performed on the two signal lights. Influence from polarization dependency of the SOA  25  and the SOA  26  can be avoided. The VAT  42  and the VAT  43  are formed in the modulator chip  40  by silicon photonics technology. Polarization dependent loss that may be produced due to difference in optical loss among the optical modulator  20 , the PR  47  and the PBC  48  can be corrected by the VAT  42  and the VAT  43 , and the input power or injected current for the SOA  25  and the SOA  26  can be made constant. 
         [0047]      FIG. 6  illustrates a modulator module  2 B, which is a modification of the modulator module  2 A. In the modulator module  2 B, an SOA  55  with two channels in a single chip is embedded in the modulator chip  50 . A recess (see  FIG. 5B ) formed in the substrate of the modulator chip  50  accommodates a single chip SOA. The SOA  55  has two active layers (waveguides)  56   a  and  56   b  that are independent from each other. Each of the active layers  56   a  and  56   b  is optically connected to a corresponding one of the two optical waveguides  31  extending from the optical modulator  20 . The light L 1  travelling through one of the optical waveguides  31  is input to the active layer  56   a  and amplified. The light L 2  travelling through the other optical waveguide  31  is input to the active layer  56   b  and amplified. This configuration is advantageous because the number of components or chips mounted in the modulator chip  50  is reduced and the assembling cost can be reduced. The modulator module  2 B also has a downsizing effect and polarization dependency reduction effect as in the structure illustrated in  FIG. 4 . 
         [0048]    The modulator chip  40  and the modulator chip  50  may be formed of a silicon-on-insulator (SOI) wafer, or alternately, an oxide layer may be formed on a silicon substrate  41 . In the latter case, a silicon layer is epitaxially grown on the oxide layer and patterned into the optical waveguides  31 . 
       Third Embodiment 
       [0049]      FIG. 7  illustrates a modulator module  3  according to the third embodiment, which module is one example of an optical module. In the second embodiment, the optical modulator  20  and the polarization multiplexer  30  are formed of silicon and SOA(s) are amounted on the silicon platform. It may be difficult for a silicon modulator to achieve a wideband optical modulation scheme simultaneously with low loss and low drive voltage. Then, in the third embodiment, the optical modular is formed of a compound semiconductor. The modulator module  3  has a silicon chip  70 . The silicon chip has a modulator chip  60  formed of a compound semiconductor and embedded in the silicon substrate  71 , SOAs  25  and  26  embedded in the silicon substrate  71 , and a polarization multiplexer  30  formed on the silicon substrate  71 . The polarization multiplexer  30  includes a VAT  42 , a VAT  43 , a PR  47 , a PBC  48 , and a BPF  49  as in the second embodiment. 
         [0050]    The modulator chip  60  forms an optical modulator  20 . The modulator chip  60  has four MZ interferometers  61   a  through  61   d  arranged in parallel. The MZ interferometers  61   a  to  61   d  may be formed by optical waveguides  61  having a core of multilayer quantum well (MQW) of InGaAlAs surrounded by an InP clad layer, for example. 
         [0051]    Signal light L 1  produced by combining the light waves travelling through the MZ interferometers  61   a  and  61   b  is amplified by the SOA  25 . Signal light L 2  produced by combining the light waves travelling through the MZ interferometers  61   c  and  61   d  is amplified by the SOA  26 . The configuration of the polarization multiplexer  30  arranged after the SOAs  25  and  26  in the light propagation direction is the same as those illustrated in  FIG. 4  and  FIG. 6 . The power of the signal light L 1  amplified at the SOA  25  is adjusted by the VAT  42 . The polarization of the power-adjusted signal light L 1  is rotated by 90 degrees at the PR  47  and converted into TM mode. The TM polarized signal light L 1  and the TE polarized signal light L 2  are multiplexed at the PBC  48 , and noise is reduced at the BPF  49 . 
         [0052]    By mounting the modulator chip  60  made of a compound semiconductor on the silicon platform, wideband optical modulation can be achieved. Although in  FIG. 7  the SOA  25  and the SOA  26  are embedded separately in the silicon substrate  71 , the optical modulator  20  and the SOAs  25  and  26  may be formed monolithically on a single chip. In this case, the modulator module  3  is further downsized. The SOA  25  and the SOA  26  may be formed in a single chip with two independent channels as illustrated in  FIG. 6 . 
       Fourth Embodiment 
       [0053]      FIG. 8  illustrates a modulator module  4  according to the fourth embodiment, which module is one example of an optical module. In the previous (third) embodiment, the modulator chip  60  is formed of a compound semiconductor. The modulation characteristic of the compound semiconductor modulator chip  60  is satisfactory. On the other hand, the light confinement effect of the optical waveguide  61  made of a compound semiconductor material is smaller compared with a silicon nanowire core. It is difficult for the compound semiconductor waveguide to reduce the bending radius at the branched portions  65  indicated by the dashed circles in  FIG. 7  from the viewpoint of reducing bending loss. Then, in the fourth embodiment, cross-interaction part of the optical modulator  20  is formed of a compound semiconductor, while the branched portions (including combined portions)  65  are formed of silicon. 
         [0054]    The modulator module  4  has a silicon chip  70 . A compound semiconductor substrate  80  and the SOAs  25  and  26  are embedded in the silicon substrate  71  of the silicon chip  70 . On the silicon substrate  71  are formed a branched part  72 , a combined part  73  and a polarization multiplexer  30 . The polarization multiplexer  30  includes a VAT  42 , a VAT  43 , a PR  47 , a PBC  48 , and a BPF  49  as in the second and third embodiments. 
         [0055]    Four pairs  85   a  to  85   d  of parallel waveguides are formed on the compound semiconductor substrate  80 . The parallel waveguide pairs  85   a  to  85   d  are formed by optical waveguides  85 . The optical waveguides  85  are formed of a material with energy band gap smaller than the compound semiconductor substrate  80  and with refractive index greater than the compound semiconductor substrate  80 . The parallel waveguide pairs  85   a  through  85   d  are optically coupled to the optical waveguide  61  formed in the branched part  72  of the silicon substrate  71  at the input side of the compound semiconductor substrate  80 . The parallel waveguide pairs  85   a  through  85   d  are optically coupled to the optical waveguide  61  formed in the combined part  73  of the silicon substrate  71  at the output side of the compound semiconductor substrate  80 . When coupled to the silicon waveguides  61  of the branched part  72  and the combined part  73 , each of the parallel waveguide pairs  85   a  to  85   d  forms a MZ interferometer. The parallel waveguide pairs  85   a  to  85   d  modulate the optical phases of light beams travelling thought the parallel waveguides upon application of electrical signals (i.e., electric fields). In the sense that the electric field and light wave interact with each other, the parallel waveguide pairs  85   a  to  85   d  form a section that may be called a cross-interaction part. 
         [0056]    The branched part  72 , the four parallel waveguide pairs  85   a  to  85   d  (namely, the cross-interaction part) formed on the compound semiconductor substrate  80 , and the combined part  73  form an optical modulator  20 . This configuration can reduce polarization dependency with a compact module structure. 
         [0057]    The signal lights L 1  and L 2  output in the same polarization mode (e.g., TE mode) from the combined part  73  undergo optical amplification at the SOAs  25  and  26  and attenuation adjustment at the VATs  42  and  43 . Then one of the signal lights L 1  and L 2  is subjected to polarization rotation at the PR  47 . The orthogonally polarized signal lights are multiplexed at the PBC  48 . 
         [0058]      FIG. 9A  through  FIG. 9C  illustrate a fabrication process of the silicon chip  70  illustrated in  FIG. 8 . In  FIG. 9A , optical waveguides  61  including the branched part  72 , combined part  73 , the VATs  42  and  43 , the PR  47 , the PBC  48  and the BPF  49  are formed on the silicon substrate  71 . The optical waveguides  61  may be of a rib type or a silicon photonic nanowire. 
         [0059]    In  FIG. 9B , a recess  75   a  for accommodating the compound semiconductor substrate  80  and a recess  75   b  for accommodating the SOA  25  and the SOA  26  are formed in the silicon substrate  71  by etching or cutting. In  FIG. 9C , the compound semiconductor substrate  80  on which the parallel waveguide pairs  85   a  to  85   d  are formed by optical waveguides  85  in advance is bonded to the recess  75   a . The SOAs  25  and  26  are placed and bonded in the recess  75   b . The cores of the parallel waveguide pairs  85   a  to  85   d  are aligned with the silicon cores of the optical waveguides  61  formed in the branched part  72  and the combined part  73 . Spot size converters may be formed in the optical waveguides  61  at positions facing the input planes and the output planes of the SOA  25  and the SOA  26 . 
       Fifth Embodiment 
       [0060]      FIG. 10  illustrates a modulator module  5 A according to the fifth embodiment. In the fifth embodiment, a photo-detector for monitoring modulated light is provided to the modulator module  5 A. The modulator module  5 A has a modulator chip  95  and a silicon chip  90 A. An optical modulator  20  is formed in the modulator chip  95  by an arbitrary type of modulator, such as LN modulator, silicon (Si) modulator, or a compound semiconductor modulator. The silicon chip  90 A performs both optical amplification and polarization division multiplexing. 
         [0061]    A modulator that performs optical phase modulation may be structured so as to output a monitor light used for phase adjustment, in addition to the modulated signal light. In this embodiment, monitoring photo-detectors  91  and  92  are provided in the silicon chip  90 A. 
         [0062]    The optical waveguide  96  formed on the modulator chip  95  is branched to form four MZ interferometers (or four pairs of parallel waveguides)  96   a  through  96   d . The combined light from the MZ interferometers  96   a  and  96   b  is input as a signal light L 1  to the SOA  25  mounted on the silicon chip  90 A. The combined light from the MZ interferometers  96   c  and  96   d  is input as a signal light L 2  to the SOA  26  mounted on the silicon chip  90 A. The SOA  25  and the SOA  26  are formed of a material different from that of the silicon chip  90 A and embedded in the silicon chip  90 A as has been explained in connection with  FIG. 5  and  FIG. 9 . The signal lights L 1  and L 2  input to the SOA  25  and the SOA  26  are in the same polarization mode (e.g., in TE mode. 
         [0063]    One of the two optical waveguides of a branch waveguide  96   e  extending from the combined part of the MZ interferometers  96   a  and  96   b  is optically coupled to an optical waveguide  89   a  formed in the silicon chip  90 A. The monitor light is received at the PD  91  through the optical waveguide  89   a . One of the two optical waveguides of a branch waveguide  96   f  extending from the combined part of the MZ interferometers  96   c  and  96   d  is optically coupled to an optical waveguide  89   b  formed in the silicon chip  90 A. The monitor light is received at the PD  92  through the optical waveguide  89   b . Based upon the monitoring results at the PD  91  and the PD  92 , the quantities of phase adjustment at the SOA  25  and the SOA  26  are controlled so as to make the optical phases of the signal light L 1  and the signal light L 2  consistent with each other. 
         [0064]    The VAT  42 , VAT  43 , PR  47 , PBC  48  and BPF  49  arranged after the SOAs  25  and  26  are the same as those described in the second through fourth embodiments. At the PR  46 , the direction of polarization of the amplified signal light L 1  is rotated and converted into TM-mode signal light, while the amplified signal light L 2  remains in the TE mode. The TM-mode signal light L 1  and the TE-mode signal light L 2  are multiplexed at the PBC  48 . 
         [0065]    With this configuration, a PD carrier used in a conventional structure is eliminated and a compact modulator module  5 A is achieved. Besides, the optical phases of the orthogonally polarized waves can be made consistent with each other. 
         [0066]      FIG. 11  illustrates a modulator module  5 B, which is a modification of the modulator module  5 A. The modulator module  5 B has a modulator chip  95  and a silicon chip  90 B in a module case. The modulator chip  95  is the same as that illustrated in  FIG. 10 . SOAs  25  and  26  are embedded in the silicon chip  90 B and PD  91  and  92  are arranged on the silicon chip  90 B. 
         [0067]    The silicon chip  90 B has BPFs  93  and  94  provided between the PR  47  and the SOAs  25  and  26 . When a bandpass filter is formed by a diffraction grating, optical loss varies depending on polarization mode of the incident light. In the example of  FIG. 11 , the BPF  93  is inserted between the SOA  25  and the VAT  42 , and the BPF  94  is inserted between the SOA  26  and the VAT  43 . The signal lights L 1  and L 2  are amplified in the same polarization mode (e.g., TE mode) and noise is removed. Polarization rotation is performed on one of the amplified and noise-reduced signal lights L 1  and L 2  at the PR  47 , and polarization division multiplexing is performed at the PBC  48 . 
         [0068]    This configuration can achieve a compact modulator module  5 A with less influence of polarization dependency. 
         [0069]    The modulator chip  95  used in the modulator module  5 A ( FIG. 10 ) and/or the modulator module  5 B ( FIG. 11 ) may have a structure illustrated in  FIG. 8  with a cross-interaction part formed of a compound semiconductor substrate and branch and combined part formed of a silicon substrate. 
         [0070]      FIG. 12  illustrates a modulator module  5 C, which is another modification of the modulator module  5 A. The modulator module  5 C has a modulator chip  97  and a silicon chip  90 C. Similar to the structures in  FIG. 10  and  FIG. 11 , the SOAs  25  and  26  and the PDs  91  and  92  are mounted on the silicon chip  90 C. In  FIG. 12 , branch waveguides  89   c  and  89   d  for extracting monitor light are formed on the silicon chip  90 C. 
         [0071]    When individual difference in gain and/or loss is not negligible among the SOAs  25  and  26 , the BPFs  93  and  94  and the VATs  42  and  43 , a tap (or a branch)  99  is provided after the element with a large individual difference. Branched light components are received at the PD  91  and the PD  92  to monitor the power levels of the signal lights L 1  and L 2 . In the example of  FIG. 12 , branch waveguides  89   c  and  89   d  are extended from the optical waveguides  89  after the VTA  42  and VTA  43  toward the PD  91  and PD  92 , respectively. Because the branch waveguides  89   c  and  89   d  are formed in the silicon chip  90 C, it is unnecessary to provide a branch waveguide in the modulator chip  97  for extracting monitor light. The modulator chip  97  has four parallel MZ interferometers  89   a  through  98   d  formed by optical waveguides  98  that are branched and combined at predetermined positions. For the modulator chip  97 , the modulator chip  10  of the first embodiment (in  FIG. 3 ) or the modulator chip  60  of the third embodiment (in  FIG. 7 ) may be used. 
         [0072]    Because the modulator chip  97  does not have monitoring branch waveguides, the modulated signal lights output from the modulator chip  97  can be incident onto the SOA  25  and the SOA  26  via a lens  87  and a lens  88 , respectively. The modulated signal lights can be optically coupled directly to the SOA  25  and the SOA  26  without using silicon waveguides, and consequently insertion loss can be reduced. 
         [0073]    The structural elements of the modulator modules of the first through fifth embodiments can be combined appropriately with each other. For example, in each of the embodiments, the cross-interaction part of the optical modulator  20  may be formed of a compound semiconductor as in the fourth embodiment. BPFs may be arranged before the PR  27  (or PR  48 ) in each of the embodiments to reduce noise in the same polarization mode prior to polarization rotation. 
         [0074]      FIG. 13  illustrates an optical transmitter  6  that uses any one of the modulator modules  1  to  5 C of the first through fifth embodiments. The optical transmitter  6  has a modulator module  1  (or any one of modulator modules  2  through  5 C), a light source (such as an LD)  15 , a data generating circuit  7  and a driver  8 . 
         [0075]    Electrical signals generated by the data generating circuit  7  are converted into high-speed drive signals by the driver  8  and applied to signal electrodes of the respective MZ interferometers of the optical modulator. From the view point of reducing the driving voltage, a pair of drive signals with opposite phases (or polarities) may be applied to each of the MZ interferometers. The light beam input from the light source  15  to the modulator module  1  is phase-modulated by the high-speed drive signals. The modulator module  1  (or any one of the modulator modules  2  through  5 C) has abilities of optical modulation, optical amplification, and polarization division multiplexing. Phase-modulated two signal lights are amplified in the same polarization mode and then converted into orthogonally polarized signal lights for polarization division multiplexing. Thus, a DP-QPSK optical signal is output from the modulator module  1  (or any one of the modulator modules  2  through  5 C) to a transmission path  9 . 
         [0076]    Any one of the modulator modules  1  to  5 C has a compact structure and reduced polarization dependency. Accordingly, the optical transmitter  6  is made compact and can output optical signals modulated at the optimum modulation factor. 
         [0077]    All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although the embodiment(s) of the present inventions have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.