Patent Publication Number: US-10326253-B2

Title: Optical module implementing with optical source, optical modulator, and wavelength detector, and a method to assemble the same

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a Divisional of U.S. patent application Ser. No. 15/510,607, filed Mar. 10, 2017, which is a 371 National Phase of PCT/JP2015/005433, filed Oct. 28, 2015, which claims the benefit of Japanese Patent Application No. 2014-219585, filed Oct. 28, 2014. Japanese Patent Application No. 2014-236635, filed Nov. 21, 2014, Japanese Patent Application No. 2014-251138, filed Dec. 11, 2014, Japanese Patent Application No. 2015-006130, filed Jan. 15, 2015, and Japanese Patent Application No. 2015-008963, filed Jan. 20, 2015. 
    
    
     TECHNICAL FIELD 
     The present invention relates to an optical module that installs an optical source including a semiconductor laser diode (LD), an optical modulator, and a wavelength detector; and the invention further relates to a method of assembling the optical module. 
     BACKGROUND ART 
     An optical module that installs a wavelength tunable semiconductor laser diode (t-LD) and an optical modulator that modulates CW light emitted from the t-LD has been well known in the field. A Japanese patent application laid open No. 2009-146992 has disclosed such an optical module. The CW light output from the t-LD optically couples with the optical modulator via optical fibers. However, an optical fiber when it is bent with a large curvature causes a bent loss. Accordingly, when an optical transceiver with limited sizes in a housing thereof installs a t-LD and an optical modulator, techniques to compensate the bent loss caused in inner fibers is needed. 
     SUMMARY OF INVENTION 
     One aspect of the present application relates to a process of assembling an optical module that installs a laser unit, a modulator unit, and a detector unit within a housing. The laser unit includes a semiconductor laser diode (LD) having a front facet that outputs a first continuous wave (CW) beam and a rear face that outputs a second CW beam. The modulator unit modulates a first CW beam. The detector unit determines a wavelength of the second CW beam. The housing includes a first output port and a second output port. The process of the present application comprises steps of: (1) installing a first thermo-electric cooler (TEC), a second TEC, and a third TEC within the housing; (2) mounting the laser unit on the first TEC, the modulator unit on the second TEC, and the detector unit on the third TEC, respectively; (3) optically coupling one of the first CW beam with a first output port of the housing through the modulator unit and the second CW beam with a second output port of the housing through the detector unit; and (4) optically coupling another of the first CW beam with the first output port of the housing through the modulator unit and the second CW beam with the second output port of the housing through the detector unit. One of features of the process of the present application is that the step of coupling the first CW beam with the first output port includes steps of: (3-1) optically coupling the laser unit with the optical modulator through the input unit, and (3-2) optically coupling the modulator unit with the first output port through the output unit. Another feature of the present method is that the step of coupling the second CW beam with the second output port includes steps of: (4-1) optically coupling the detector unit with the laser unit and (4-2) optically coupling the detector unit with the second output port. 
     Another aspect of the present application relates to an optical module that comprises a wavelength tunable laser diode (t-LD) having a first facet and a second facet, an optical modulator, a wavelength detector, a housing, and first and second output ports. The t-LD outputs a first CW beam from the first facet and a second CW beam from the second facet. The optical modulator, which is primarily made of semiconductor materials, generates a first output beam by modulating the first CW beam. The wavelength detector, which may determine an oscillation wavelength of the t-LD, splits the second CW beam into a monitored beam and a second output beam. The housing, which includes a front wall, a rear wall, and two side walls connecting the front wall to the rear wall, encloses the t-LD, the optical modulator, and the wavelength detector in a space partitioned by the front wall, the rear wall, and the side walls. The first output port and the second output port, which are provided in the front wall, output the first output beam and the second output beam, respectively. One feature of the optical modulator of the present application is that the wavelength detector and the t-LD are arranged on an optical axis of the second output port along one of the side walls, but, the optical modulator is arranged on an optical axis of the first output port along another of the side walls. The optical modulator of the present application further provides a feature that the optical modulator has an input port, an output port, and a signal pad, where the input port is provided in a side of the optical modulator facing the one of the side walls of the housing, the output port is provided in a side of the optical modulator facing the front wall of the housing, and the signal pad is provided in a side of the optical modulator facing the rear wall, where the signal pad provides a signal containing high frequency components. 
     Still another aspect of the present application relates to an optical module. The optical module of the present aspect comprises an optical source, an optical modulator, and an input unit. The optical source, which is mounted on a first TEC as interposing a first carrier therebetween, generates a continuous wave (CW) beam, where the first carrier provides marks thereon. The optical modulator, which is mounted on a second TEC independent of the first TEC as interposing a base therebetween, modulates the CW beam. The input unit, which couples the CW beam with the optical modulator, is mounted on the base as interposing a second carrier therebetween, where the second carrier provides marks thereon. One feature of the present optical module is that the marks on the second carrier of the input unit are aligned with the marks on the first carrier of the optical source. 
     Still another aspect of the present application relates to an optical module. The optical module of the present aspect includes an optical source, an optical component, a housing, and a beam shifter. The optical source generates a beam accompanied with an optical axis. The optical component, which is optically coupled with the optical source, has another optical axis offset from the optical axis of the beam. The housing having a bottom disposes the optical source and the optical component on the bottom thereof. The beam shifter is interposed between the optical source and the optical component. A feature of the optical module of the present aspect is that the beam shifter aligns the optical axis of the beam measured from the bottom of the housing with the other optical axis of the optical component measured from the bottom of the housing. 
     Still another aspect of the present application relates to a method of assembling an optical module that provides an optical source, beam shifter, an optical component, a concentrating lens, and a housing. The optical source generates an optical beam. The optical component is optically coupled with the optical beam. The concentrating lens concentrates the optical beam on the optical component. The housing, which has a bottom, encloses the optical source, the concentrating lens, and the optical component therein. The method comprises steps of: (1) disposing the beam shifter between the optical source and the concentrating, where the beam shifter aligns an optical axis of the optical beam measured from the bottom of the housing to an optical axis of the optical component; and (2) coupling the optical beam output from the beam shifter with the optical component by aligning the concentrating lens. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a perspective view of an optical module according to an embodiment of the present invention; 
         FIG. 2  shows an inside of the optical module shown in  FIG. 1 ; 
         FIG. 3  shows a cross section of a wavelength tunable LD implemented within the optical module shown in  FIG. 1 ; 
         FIG. 4  is a plan view of a laser unit; 
         FIG. 5  is a plan view of an optical modulator; 
         FIG. 6  is a plan view of a detector unit; 
         FIG. 7  is a plan view of a modulator unit; 
         FIG. 8  is an exploded view of the modulator unit; 
         FIG. 9  is a plan view of the base of the modulator unit, where the base mounts the optical modulator through a carrier; 
         FIG. 10  is a plan view of a terminator unit mounted on the base shown in  FIG. 9 ; 
         FIG. 11  is a plan view of a bias unit mounted on the base shown in  FIG. 9 ; 
         FIG. 12  is a plan view of an input unit to guide the first CW light emitted from the laser unit shown in  FIG. 4  into the optical modulator shown in  FIG. 2 ; 
         FIG. 13A  schematically shows a ray tracing of the one lens system, and  FIG. 13B  shows a ray tracing of the two lens system; 
         FIG. 14A  to  FIG. 14F  show coupling tolerances of the one lens system ( FIGS. 14A and 14B ), coupling tolerances of the first lens in the two-lens system ( FIGS. 14C and 14D ), and coupling tolerances of the second lens in the two-lens system ( FIG. 14E  and  FIG. 14F ); 
         FIG. 15  is a plan view of a joint unit; 
         FIG. 16  is a plan view of an output unit; 
         FIG. 17  shows a cross section of the optical module taken along the optical axis extended from the second output port; 
         FIG. 18  shows a cross section of the optical module taken along the optical axis extended from the first output port; 
         FIG. 19  is a plan view of the arrangement along the wiring substrates, the laser unit, and the detector unit; 
         FIG. 20  is a plan view of the arrangement around the laser unit including two wiring substrates; 
         FIG. 21  is a perspective view of the laser unit and two wiring substrates; 
         FIG. 22  shows a flow chart of a process to assembly the optical module shown in  FIG. 1 ; 
         FIG. 23  shows a process to assemble the t-LD on the LD carrier; 
         FIG. 24  shows a process to assemble the optical modulator, the input unit, the joint unit, two bias units, two terminator units, and two PD sub-mount on the base of the modulator unit; 
         FIG. 25  shows a process to align the joint unit and the input unit on the base of the modulator unit with the laser unit; 
         FIG. 26  shows a process to assemble the output unit with the optical modulator; 
         FIG. 27  is a plan view showing a process to install the TECs, the wiring substrates, and the VOA substrate into the housing of the optical modulator; 
         FIG. 28  is a plan view showing a process to mount the LD carrier and the lens carriers on the first TEC, the base of the modulator unit, which mounts the input unit, the joint unit, the output unit, the bias units, the terminator units, and the PD sub-mounts thereon; 
         FIG. 29  shows a process to align the detector unit with the laser unit; 
         FIG. 30  shows a process to align the first collimating lens in a positon at which the output beam of the collimating lens becomes a collimated beam; 
         FIG. 31  shows a process to shift the optical axis of the laser unit so as to be aligned with the optical axis of the optical modulator; 
         FIG. 32  shows a process to align the beam splitter in the input unit; and 
         FIG. 33  shows a process to align the first lens and the second lens with the input port of the optical modulator. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Next, some preferred embodiments will be described as referring to drawings. In the description of the drawings, numerals or symbols same with or similar to each other will refer to elements similar to or same with each other without overlapping explanations. 
     (First Embodiment) 
       FIG. 1  is a perspective view of an optical module according to an embodiment of the present invention and  FIG. 2  shows an inside of the optical module shown in  FIG. 1 . The optical module  1  of the present embodiment may be implemented within an optical transceiver applicable to the optical coherent system. The optical coherent system utilizes the phase of light, in addition to the magnitude thereof, as one bit information. When the optical signals corresponding to the phase components of 0° and 90°, that is, when the coherent system multiplexes the optical signals each having the phase components of 0° and 90°, the system may transmit two-bits information at the same time. 
     The optical module  1  includes a laser unit  100 , a modulator unit  200 , and a detector unit  300  within a housing partitioned by a front wall  2 A, a rear wall  2 B, and two side walls,  2 C and  2 D, connecting the front wall  2 A to the rear wall  2 B. The laser unit  100  optically couples with both the modulator unit  200  and the detector unit  300 . Specifically, the optical module  1  outputs a modulation signal D 1  from the first output port  3   a,  where the modulation signal D 1  is obtained by modulating first continuous wave (CW) beam L 1  output from a wavelength tunable laser diode (t-LD)  10  implemented within the laser unit  100  by an optical modulator  20  installed in the modulator unit  200 . Concurrently with the first modulation signal D 1 , the optical module  1  may output another optical signal D 2  from the second output port  3   b,  where the optical signal D 2  is originated from the other CW beam L 2  output from the t-LD  10  to the detector unit  300  and divided in the detector unit  300 . The first CW beam L 1 , which is output from the t-LD  10  substantially in parallel to the optical axes of the output ports,  3   a  and  3   b,  toward the rear wall  3 B, enters the optical modulator  20  along the rear wall  2 B bent by substantially 90°. The other CW beam L 2 , which is emitted from the t-LD  10  substantially in parallel to the optical axes of the output ports,  3   a  and  3   b,  toward the front wall  2 A. 
     The optical module  1  of the present embodiment has a feature that the optical module  1  mounts the laser unit  100 , the modulator unit  200 , and the detector unit  300  on respective thermo-electric coolers (TECs) implemented in the housing independently. Moreover, the optical module  1  provides radio-frequency (RF) terminals  4  only in the rear wall  2 B, and DC terminals,  5   a  and  5   b,  in the respective side walls,  2 C and  2 D. Because the RF terminals  4  and the DC terminals,  5   a  and  5   b,  are independent in respective walls; the electrical control of the optical module  1  may be simplified and stabilized. 
     The modulator unit  200 , as described above, modulates the first CW beam L 1  in the phase thereof and outputs the phase-modulated optical signal. That is, the optical modulator  20  implemented within the modulator unit  200  divides the first CW beam L 1  into four beams, and modulates these four beams independently by four modulation signals provided through the RF terminals  4 , where two of four modulated signals output from the optical modulator  20  have phase components different by 90° from the rest of two modulated signals. The former two modulated signals are often called as I-components (In-phase component), while, the latter two modulated signals are called as Q-components (Quadrature component). One of I-components and one of Q-components are further modulated by the polarization thereof. That is, one of the I-components and one of the Q-components are rotated in the polarization thereof and multiplexed with the other of the I-components and the other of the Q-components. The optical module  1  may output the modulated signal D 1 , which multiplexes four optical signals, from the first output port  3   a  as the phase-polarization modulated signals, which is often called as the dual polarization quadrature phase shift keying (DP-QPSK). The optical module  1  may further output another optical signal D 2 , which is obtained by dividing the second CW beam L 2  output from the t-LD  10  by the detector unit  300 . One of the divided CW beam is used for determining the wavelength of the CW beam L 2 , and the rest is output from the second output port  3   b  as the output CW beam D 2 . 
     Next, details of the respective units,  100  to  300 , will be described. 
     Tunable Laser Diode (t-LD) 
       FIG. 3  shows a cross section of a wavelength tunable LD implemented within the optical module shown in  FIG. 1 . The t-LD  10  includes two semiconductor optical amplifiers SOAs,  10   a  and  10   d,  and a sampled grating distributed feedback (SG-DFB)  10   b  and a sampled grating distributed Bragg reflector (SG-DBR)  10   c,  where the latter two regions,  10   b  and  10   c,  may determine the emission wavelength of the t-LD  10 . These four regions are arranged along an optical axis of the t-LD  10 . The present t-LD  10  provides one facet  10 A in one of the SOA  10   a  to transmit the first CW beam L 1  and another facet  10 B in the other SOA  10   d  to transmit the second CW beam L 2 . 
     The SG-DFB  10   b  includes a sampled grating (SG)  18 , where the sampled grating  18  is featured by regions each including a plurality of gratings and separated by spaces without any gratings. The gratings in respective regions have a constant pitch and the spaces have a constant length along the optical axis. When the spaces have various lengths, the sampled grating may be called as the chirped-sampled grating. The SG-DFBs  10   b  includes gain regions,  12   a  to  12   c,  including the SG  18 , and modulation regions,  13   a  and  13   b,  also including the SG  18 . The gain regions,  12   a  to  12   c,  may be provided with carriers through electrodes  14   a  on a top surface of the device. On the other hand, the modulation regions,  13   a  and  13   b,  provides heaters,  15   a  and  15   b,  in the top surface thereof. A combination of the gain regions,  12   a  to  12   c,  and the modulation regions,  13   a  and  13   b,  the SG-DFB  10   b  may show the optical gain spectrum having a plurality of gain peaks reflecting the SG  18  in the SG-DFB  10   b.  Providing power to the heaters,  15   a  and  15   b,  that is, heating up or cooling down temperatures of the waveguide layers  19   b  beneath the heaters,  15   a  and  15   b,  optical characteristics of the modulation regions,  13   a  and  13   b,  may be modified, that is, wavelengths of the gain peaks inherently attributed to the SG-DFB  10   b  may be changed. 
     The CSG-DBR  10   c  of the present embodiment provides three sections,  16   a  to  16   c,  each having heaters,  17   a  to  17   c,  operable independently. Because the CSG-DBR  10   c  does not includes any gain regions, the CSG-DBR  10   c  inherently show reflection spectrum having a plurality of reflection peaks. Powering the heaters,  17   a  to  17   c,  to modify temperatures of the waveguide  19   b  beneath the heaters,  17   a  to  17   c,  the reflection peaks in the spectrum of the CSG-DBR  10   c  may be changed in the wavelengths and intervals thereof. At least one of the sections,  16   a  to  16   c,  has physical features distinguishable from those of the rest sections. In the present t-LD  10 , the section,  16   a  to  16   c,  provides optical lengths different from others. That is, the spaces without diffraction gratings have respective optical lengths different from others, which are called as the chirped-sampled diffraction Bragg reflector (CSG-DBR). The reason why the t-LD  10  of the present embodiment provides the CSG-DBR, not the SG-DBR, is that a range where the reflection peaks appears may be widened by modifying the temperatures of the waveguides in respective regions independently. Adjusting the power supplied to the heaters,  15   a  and  15   b,  in the SG-DFB  10   b  and the heaters,  17   a  to  17   c,  in the CSG-DBR  10   c,  one of the gain peaks attributed to the SG-DFB  10   b  matches with one of the reflection peaks attributed to the CSG-DBR  10   c.  Then, the SG-DFB  10   b  and the CSG-DBR  10   c  may form a cavity for the t-LD  10  and the t-LD may oscillate at the matched wavelength. This matched wavelength is optional by adjusting the power supplied to the heaters,  15   a  and  15   b,  and  17   a  to  17   c.    
     The first and second SOAs,  10   a  and  10   d,  may amplify an optical beam generated by the gain regions,  12   a  to  12   c,  and determined in the wavelength thereof by the optical coupling of the SG-DFB  10   b  with the CSG-DBR  10   c.  The optical gain of the SOAs,  10   a  and  10   d,  may be variable by injecting carries into the active layer  19   a  through the electrode  14   d  in the first SOA  10   a,  and carries into the other active layer  19   a  through the electrode  14   e  in the second OSA  10   d.  Thus, the amplitude of the first and second CW beam, L 1  and L 2 , are variable. The waveguide  19   b  in the modulation regions,  13   a  and  13   b,  in the SG-DFB  10   b  and that in the CSG-DBR  10   c  may be made of semiconductor material with energy band gap greater than that of the active layer  10   a  in the SOAs,  10   a  and  10   b,  and the gain regions,  12   a  to  12   c,  in the SG-DFB  10   b  to make the waveguide  19   b  substantially in transparent for the optical beam subject to the t-LD  10 . 
       FIG. 4  is a plan view of a laser unit  100 . The laser unit  100  includes a first thermo-electric cooler (TEC)  11  that mounts two collimating lenses,  110   a  and  110   b,  and the t-LD  10  through a base  100   a.  Specifically, the first TEC  11 , which includes a top plate  11   a,  a bottom plate  11   b,  and a plurality of thermo-electric converting elements, typically Peltier elements, may cause a temperature difference between two plates,  11   a  and  11   b,  depending on a magnitude and a direction of a current flowing in the Peltier elements. The bottom plate  11   b  has a size wider than that of the top plate  11   a.  That is, the bottom plate  11   b  has a portion exposed from the top plate  11   a,  and two posts,  11   c  and  11   d,  to supply the current to the Peltier elements on the bottom plate  11   b.  The temperature of the top plate  11   a  may be sensed by a thermistor  11   f  mounted on the top plate  11   a.    
     The base  100   a,  which has a size substantially same with that of the top plate  11   a  of the first TEC  11 , may be made of aluminum nitride (AlN) and mounts two collimating lenses,  110   a  and  110   b,  through respective lens carriers,  110 A and  110 B, and the t-LD  10  and the thermistor  11   f  through an LD carrier  100 A. These carriers,  100 A,  110 A and  110 B, may be also made of AlN but the LD carrier  100 A has a thickness greater than respective thicknesses of the lens carriers,  110 A and  110 B, to match the level of the optical axis of the t-LD  10  with those of the collimating lenses,  110   a  and  110   b.  The LD carrier  100 A provides interconnections  100   b  thereon to provide biases to the t-LD  10 . The t-LD  10  is necessary to be supplied with a bias to inject carriers into the gain regions,  12   a  to  12   c,  power to the heaters,  15   a  and  15   b,  in the SG-DFB  10   b,  power to the heaters,  17   a  to  17   c,  in the CSG-DBR  10   c,  biases to the SOAs,  10   a  to  10   d,  to secure the optical gains therein, and some grounds. The LD carrier  100 A requires the interconnections  100   b  to supply these biases and power to the t-LD  10 . 
     Optical Modulator 
       FIG. 5  is a plan view of an optical modulator. The optical modulator  20  may include a plurality of modulation elements, for example four (4) Mach-Zehnder (MZ) elements,  51  to  54 , are integrated on a semiconductor substrate made of indium phosphide (InP) in the present embodiment. The optical modulator  20  of the embodiment includes three 1:2 couplers,  50   a  to  50   c,  to distribute the CW beam L 1  entering from the input port  24  into four MZ elements,  51  to  54 . Specifically, the CW beam L 1  entering the input port  24  is bent substantially in a right angle along the waveguide and evenly divided into two partial beams by the first 1:2 coupler  50   a.  The respective partial beams are further evenly divided by the second and third 1:2 couplers,  50   b  and  50   c,  into four partial beams, and the four partial beams enter the MZ elements,  51  to  54 , respectively. Two 2:2 couplers,  50   d  and  50   e,  are provided in downstream of the MZ elements,  51  to  54 , to multiplex the modulated beam. 
     The explanation below concentrates on the first MZ element  51 . But, other MZ elements,  52  to  54 , may operate in the same manner with the first MZ element  51 . 
     The partial CW beam divided by the second 1:2 coupler  50   b  and entering the MZ element  51  is further evenly divided into two portions by the 1:2 coupler  51   a  each heading the arm waveguides,  51   h  and  51   i.  In the arm waveguides,  51   h  and  51   i,  in particular, within the functional region  51 M providing the modulating electrodes,  51   e  and  51   f,  and the ground electrode  51   g,  the divided beam are modulated in the phases thereof. After passing the functional region  51 M, the divided beam in the phases thereof are further modulated, or offset in the offset electrodes,  51   j  and  51   k.  Finally, the divided beams are combined by the 1:2 coupler  51   b  to be output from the MZ element  51 . 
     The operation of the functional region  51 M and the offset electrodes,  51   j  and  51   k,  will be described. The offset electrodes,  51   j  and  51   k  are statically pre-biased such that the optical beams propagating in the respective arm waveguides,  51   h  and  51   j,  have a phase difference of pi (π). For instance, the optical beam propagating in the one arm waveguide  51   h  is delayed by pi (π) with respect to the beam propagating in the other arm waveguide  51   j.  Then, one of the modulating electrodes  51   e  for the arm waveguide  51   h  is supplied with a bias to cause the phase delay of pi (π) but the other modulation electrode  51   f  is supplied with a bias causing no phase delay. The beam propagating in the arm waveguide  51   h  is caused by the phase delay of 2π by the modulation electrode  51   e  and the offset electrode  51   j ; but, the beam propagating in the other arm waveguide  51   i  shows no phase delay caused by the modulation electrode  51   f  and the offset electrode  51   k.  Combining two optical beams each propagating in the arm waveguides,  51   h  and  51   i ; the combined beam shows a phase delay of zero. The phase delay of 2π is equal to the phase delay of 0. 
     On the other hand, when the modulation electrode  51   e  is supplied with a bias causing no phase delay for the beam propagating in the arm waveguide  51   h  thereunder but the other modulation electrode  51   f  is supplied with a bias causing the phase delay of pi (π); the beam combined by the 2:1 coupler  51   b  has the phase delay of pi (π) because the former beam propagating in the arm waveguide  51   h  is delayed in the phase thereof by the static bias of the offset electrode  51   j.  Thus, the optical output of the MZ element  51  becomes CW beam whose phase is modulated between 0 and pi (π) but the amplitude thereof is kept substantially constant. The amplitude of the optical output strictly changes at the transitions of the phase. Referring to  FIG. 5 , the modulation signals provided to the modulation electrodes,  51   e  to  54   e  and  51   f  to  54   f,  are supplied from the pads,  41  to  44 , which are formed in one edge of the optical modulator  20 , through interconnections which are terminated at pads,  45   a  and  45   b,  provided in respective sides of the optical modulator  20  in downstream of the modulation electrodes,  51   e  to  54   e  and  51   f  to  54   f.  Also, the static biases supplied to the offset electrodes,  51   j  to  54   j  and  51   k  to  54   k,  are provided from the pads,  46   a  and  46   b,  formed in respective sides of the optical modulator  20 . 
     The quadrature electrodes,  51   c  to  54   e,  in the function thereof will be described. The optical modulator  20  of the embodiment includes four (4) MZ elements,  51  to  54 . The two quadrature electrodes,  52   c  and  54   c,  are supplied with static biases such that the phases of the beams propagating thereunder cause a phase difference of π/2 with respect to the other beams propagating in the waveguides under the quadrature electrodes,  51   c  and  53   c,  which form the pairs with the respective quadrature waveguides,  52   c  and  54   c.  Accordingly, even after combining two optical beams each propagating in the waveguides under the quadrature electrodes,  51   c  and  52   c,    53   c  and  54   c,  the optical beams may be independently extracted. The optical beams output from the MZ elements,  51 M and  52 M, and those from the MZ elements,  53 M and  54 M, may be multiplexed with respect to the phases, one of the optical beams, subject to the MZ elements,  51 M and  53 M, are called as the I-component (In-phase), and the other is called as the Q-component (Quadrature). The optical modulator  20  may output two optical signals, M 2   b  and M 2   c,  each modulated in the phase, from respective output ports,  22   a  and  22   b,  and other two optical signals, M 2   a  and M 2   d,  from respective monitor ports,  25   a  and  25   b.    
     In the optical modulator  20  thus described, the modulation of the optical beams may be carried out by varying refractive index of the waveguide made of semiconductor materials in the functional regions,  51 M to  54 M. A semiconductor material shows a large electro-optical coupling efficiency, which is called as the Kerr effect, for the optical beam whose wavelength is slightly longer than a bandgap wavelength of the semiconductor material, which corresponds to the bandgap energy of the material. A larger Kerr efficiency means that a modulation signal with a smaller amplitude may cause the substantial modulation in optical characteristics of the semiconductor material. However, the bandgap wavelength of the semiconductor material has substantial temperature dependence, which results in a large variation of the modulation characteristic of the optical modulator  20 . The present optical module  1  mounts the optical modulator  20  on the second TEC  20  to compensate the temperature dependence of the modulation performance thereof. 
     Wavelength Detector 
       FIG. 6  is a plan view of a detector unit  300 . The detector unit  300  includes a first beam splitter (BS)  32   a,  an etalon filter  33 , a first monitor photodiode (m-PD)  34   a,  a second BS  32   b,  and a second m-PD  34   b,  where these components are mounted on a third TEC  31  through a carrier  300   a  made of AlN, where the carrier  300   a  will be referred as the third carrier. Two m-PDs,  34   a  and  34   b,  are mounted on the carrier  300   a  through respective PD sub-mounts,  34 A and  34 B. The third TEC  31 , similar to those first and second TEC,  11  and  21 , provides a top plate  31   a  and a bottom plate  31   b.  The bottom plate  31   b  is wider than the top plate  31   a  and the carrier  300   a,  and mounts two posts,  31   c  and  31   d,  in an area exposed from the top plate  31   a  and the carrier  300   a  in order to supply a current to Peltier elements mounted on the bottom plate  31   b.  The TEC  31  may compensate temperature characteristics of the etalon filter  33 . 
     The etalon filter  33 , which may be a parallel piped plate, shows a specific transmittance, in particular, periodical transmittance exhibiting strong wavelength dependence determined by a thickness of the parallel piped plate and refractive index of a material constituting the parallel piped plate. 
     The first BS  32   a  and the second BS  32   b  of the present detector unit  300  have a type of slab made of material substantially transparent to the second CW beam L 2 , typically, two BSs,  32   a  and  32   b,  may be a slab made of silica glass. The first BS  32   a  splits the second CW beam L 2 , which is output from the t-LD  10  and converted into a collimated beam by the second collimating lens  110   b,  into two beams. One of the split beam advances to the etalon filter  33 , while, the rest of the split beam goes to the second BS  32   b.  The present embodiment of the detector unit  300  sets a ratio of two beams to be 5:95, that is, about 5% of the second CW beam LS enters the etalon filter  33 , and the rest 95% goes to the second BS  32   b.  The former split beam transmitting through the etalon filter  33  enters the second m-PD  34   b.  The other split beam, which is bent by a right angle at the first BS  32   a,  goes to the second BS  32   b  and is split thereby into two beams. One of the split beams passing through the second BS  32   b  enters the first m-PD  34   a,  and the other beam, which is reflected in a right angle by the second BS  32   b,  is output from the optical module  1  as the second output D 2 . The split ratio of the second BS  32   b  is set to be also 5:95. Accordingly, the output beam D 2  has the magnitude of about 90% of that of the second CW beam L 2  entering the detector unit  300 . The residual of the second CW beam L 2  enters the first and second m-PDs,  34   a  and  34   b,  to determine the wavelength of the second CW beam L 2 . 
     The detector unit  300  may evaluate the transmittance of the etalon filter  33  by a ratio of the output of the second m-PD  34   b  to the output of the first m-PD  34   a.  Practical transmittance of the etalon filter  33  may be specified by the specification thereof, the ratio of the two outputs may determine the wavelength of the second CW beam L 2  by comparing this ratio with the specification of the etalon filter  33 . Moreover, controlling the biases supplied to the t-LD  10  and the temperature thereof by the first TEC  11  such that the ratio of the outputs of the two m-PDs,  32   a  and  32   b,  comes closer to the transmittance of the etalon filter  33  at a target wavelength, the emission wavelength of the t-LD  10  may coincide with the target wavelength. An etalon filter has been known as an optical device whose transmittance periodically varies against wavelengths. Accordingly, when the period of the periodic transmittance of the etalon filter matches with a span between nearest grids defined in the wavelength division multiplexing (WDM) system, which is 100, 50, and/or 25 GHz in the specification of the WDM system, the optical module  1  of the embodiment may easily set the emission wavelength to be equal to one of the grid wavelengths of the WDM system. 
     The temperature dependence of the periodic transmittance of the etalon filter  33  is far smaller than that of the emission wavelength of the t-LD  10 . However, the present optical module  1  provides the laser unit  100  and the detector unit  300  each independently providing TECs,  11  and  31 , because a temperature variation in a TEC in the laser unit slightly affects the transmittance of an etalon filter when the laser unit and the detector unit provide a common TEC. 
     Also, the output of the first m-PD  34   a,  which directly senses a portion of the second CW beam L 2 , namely, a portion of the optical beam not passing through the etalon filter  33 , may be served for controlling the output power of the t-LD  10 . That is, by feeding the output of the first m-PD  34   a  back to the bias, particularly, the injection current into the gain regions,  12   a  to  12   c,  in the SG-DFB  10   b,  the optical module  1  may control the magnitude of the second CW beam L 2  in a constant level, which may be called as the automatic power control (APC) of a t-LD  10 . 
     Modulator Unit 
       FIG. 7  is a plan view of a modulator unit  200 , and  FIG. 8  is an exploded view of the modulator unit  200 . The modulator unit  200  includes an input unit  210  and a joint unit  220  each couple the first CW beam L 1  output from the laser unit  100  with the optical modulator  20 . The input unit  210  includes a BS  61  and a lens system  63  that are mounted on the base  200   a  through another carrier  210   a,  which will be referred as the second carrier. The joint unit  220  includes a beam shifter  81  and an optical isolator  82  on the base  200   a  through another carrier  220   a,  which will be referred as the third carrier. The first CW beam L 1  output from the laser unit  100  enters the input port  24  of the optical modulator  20  after the beam shifter  81  compensates a level difference between the optical axis of the laser unit  100  and the input port  24  of the optical modulator  20 , and the optical isolator  82  cuts the backward beam from the optical modulator  20  to the t-LD  10 . The BS  61  may be a type of prism mirror shown in  FIG. 8  or parallel plate shown in  FIG. 7  each made of material substantially transparent for the first CW beam L 1 . The BS  61  reflects a part of the first CW beam L 1 , about 95% thereof, and transmits a rest 5% toward the m-PD  62   a  which is mounted on the base  200   a  through a PD sub-mount  62 A. 
     Base 
       FIG. 9  is a plan view of the base  200   a,  which may be made of AIN and has an L-shape with a cut  200   c  in a corner between a horizontal bar and a vertical bar of the L-character. Referring to  FIG. 8 , the second TEC  21 , similar to the aforementioned TECs,  11  and  31 , has a rectangular plane shape with posts,  21   c  and  21   d,  in a corner thereof facing the rear wall  2 B and the side wall  2 C of the housing  2 . The Peltier elements mounted on the bottom plate  21   b  of the second TEC  21  may be supplied with a current through the posts,  21   c  and  21   d.    
     The base  200   a,  which has a plane shape of an L-character, is mounted on the second TEC  21  in an area closer to a corner of the L-character. The base  200   a  has an area greater than the area of the top plate  21   a  of the second TEC  21 . That is, even when the base  200   a  is mounted on the second TEC  21 , periphery portions on the base  200   a  are not overlapped with the top plate  21   a  of the second TEC  21 . The base  200   a  provides a cut  200   c  in the corner of the L-character, through which two posts,  21   c  and  21   d,  of the second TEC  21  are exposed. The tops of the posts,  21   c  and  21   d,  project from the base  200   a,  that is, the top levels of the posts,  21   c  and  21   d,  are set higher than the primary surface of the base  200   a,  which enhances the productivity, or the wiring to the top of the posts,  21   c  and  21   d.    
     Two areas,  200 B and  200 C, in the base  200   a,  which correspond to end portions of respective bars of the L-shape, are not overlapped with the top plate  21   a  of the second TEC  21 . That is, the two areas,  200 B and  200 C, protrude from respective edges of the top plate  21   a  of the second TEC  21 . The latter area  200 C mounts the output unit  230 , while, the former area  200 B mounts the input unit  210  and the joint unit  220 . The joint unit  220  is set forward of the input unit  210 . 
     The base  200   a  has a size substantially equal to that of the optical modulator  20 . That is, base  200   a  has a lateral width substantially equal to a lateral width of the optical modulator  20  but narrower than a lateral width of the top plate  21   a  of the second TEC  21 . Mounting the base  200   a  on the top plate  21   a  of the second TEC  21  and the output unit  230  on the base  200   a,  the front edge of the second TEC  21  locates on a position of the second lens  73   b  in the output unit  230 , where the second lens  73   b  is set apart from the optical modulator  20  compared with the first lens  73   a.  Two m-PDs,  64   a  and  64   b,  are assembled on the base  200   a.  The m-PD  64   a  is mounted on the side of a sub-mount  64 A, and the m-PD  64   b  is mounted on the side of a sub-mount  64 B. The carrier  20   a  is located between m-PDs,  64   a  and  64   b  and between the sides of the sub-mount  64 A and the sub-mount  64 B. 
     The m-PDs,  64   a  and  64   b,  each have optically sensitive surfaces facing the optical modulator  20  to sense the monitor signals, M 2   a  and M 2   d,  output from the monitor ports,  25   a  and  25   b,  of the optical modulator  20 . The m-PDs,  62   a  and  62   b,  are assembled diagonally on respective sides of the optical modulator  20  corresponding to the positions of the monitor ports,  25   a  and  25   b.    
     Terminator Unit 
     The terminator units,  84   a  and  84   b,  are arranged in front sides of the m-PDs,  64   a  and  64   b,  so as to put the optical modulator  20  therebetween.  FIG. 10  is a plan view of one of the terminator units  84   b,  and the terminator units  84   a  has the same arrangement with those shown in  FIG. 10 . The terminator unit  84   a  includes terminators  85   b,  via-holes  85   c,  and die-capacitors  85   d  on a terminator carrier  84 B made of ceramics, typically, alumina (Al 2 O 3 ). The terminator  85   b  is a type of thin film resistor formed on the ceramic carrier  84 B with resistance of  1000 . The terminator  85   b  may terminate the interconnections  45   b  in the optical modulator  20  that transmit modulation signals to the MZ elements,  51 M to  54 M. The interconnections  45   b  shown in  FIG. 10  is divided into two groups each having three interconnections and corresponding to two MZ elements,  53 M and  54 M. Respective center interconnections in the respective groups are wired to the ground pad  85   f  and the respective two interconnections are wired to the signal pads  85   e.    
     The ceramic carrier  84 B in a top surface thereof provides interconnections  85   h,  while, a whole back surface thereof provides the ground pattern. The ground pad  85   f  provides a via-hole  85   c  in a center thereof and connected to the ground pattern of the back surface of the ceramic carrier  84 B. Interconnections  85   h  connected to the respective terminators  85   b  are connected to the ground pattern in the back surface of the carrier  84 B through respective die-capacitors  85   d  and the via-hole  85   c.  The interconnecting  85   h  common to respective terminators  85   b  may be externally biased. Thus, the interconnections  45   b  in the optical modulator  20  may be terminated in the AC mode through the terminators  85   b  as biased in the DC mode through the interconnections  85   h.  The terminator units,  84   a  and  84   b,  are mounted on the base  200   a  through respective carriers,  88 A and  88 B, provided commonly to the bias units,  86   a  and  86   b.    
     Bias Unit 
     Two bias units,  86   a  and  86   b,  are arranged in side by side to the terminator units,  84   a  and  84   b,  on the common carriers,  88 A and  88 B, and sandwiches the optical modulator  20  therebetween.  FIG. 11  is a plan view showing one of the bias units  86   b,  but the other of the bias units  86   a  has the same arrangement with those shown in  FIG. 11 . The optical modulator  20  provides two offset electrodes,  51   j  and  51   k,  for the X-polarization and the 0° signal, those,  52   j  and  52   k,  for the X-polarization and 90° signal, those,  53   j  and  53   k,  for the Y-polarization and 0° signal, and those,  54   j  and  54   k,  for the Y-polarization and 90° signal. In addition, the optical modulator  20  further provides the quadrature electrodes,  51   c  and  52   c,  for the X-polarization and those,  53   c  and  54   c,  for the Y-polarization. Thus, the optical modulator  20  is necessary to be supplied with twelve (12) biases. One of the bias units  86   a  provides 6 biases for the X-polarization and the other of the bias units  86   b  supplies 6 biases for the Y-polarization. 
     The bias unit  86   b,  as shown in  FIG. 11 , provides six (6) die-capacitors  87   a  and some interconnections  87   b.  The six biases are supplied to respective electrodes  45   b  in the optical modulator  20  through the interconnections  87   b  and the die-capacitors  87   a.  Some of the interconnections  87   b  are served for grounding the back surfaces of the die-capacitors  87   a.    
     Input Unit 
       FIG. 12  is a plan view of the input unit  210 . The input unit  210  is mounted on the base  200   a  in a portion  200 D, which extends from a center portion  200 A thereof, through a carrier  210   a  made of AlN, which will be referred as the second carrier. That is, the input unit  210  positions on the base  200   a  in an end of the vertical bar of the L-character. 
     The input unit  210  includes, in addition to the carrier  210   a,  the input lens system  63  including the first lens  63   a  and the second lens  63   b,  and a beam splitter (BS)  61 . The first CW beam L 1  generated in the laser unit  100  is bent by a right angle by the BS  61  to couple the input port  24  of the optical modulator  20  through the input lens system  63 . 
     The input unit  210 , as described above, has the two-lens system  63  including the first lens  63   a  closer to the optical modulator  20  and the second lens  63   b.    
       FIG. 13A  and  FIG. 13B  compare the two-lens system ( FIG. 13B ) with a conventional one-lens system ( FIG. 13A ). The one-lens system of  FIG. 13A  and the two-lens system of  FIG. 13B  both assume the aspheric lenses in which a curvature of the optical beam incoming surface is different from a curvature of the optical beam outgoing surface. Specifically, the one lens system assumes an aspheric lens  63  with an optical beam incoming surface and an optical beam outgoing surface both having spherical surfaces but the curvatures thereof are different from each other, while, the first lens  63   a  in the two-lens system shown in  FIG. 13B  has an arrangement similar to that of the lens  63  in the one-lens system but the second lens  63   b  in the two-lens system has a plane surface for the incoming optical beam. 
     Also, the first lens  63   a  in the two-lens system has a thickness different from the lens  63  in the one-lens system. For instance, the lens  63  in  FIG. 13A  has a thickness W 1  of 0.84 mm, but the first lens  63   a  and the second lens  63   b  in  FIG. 13B  have thicknesses, W 5  and W 3 , of 0.7 mm and 0.65 mm, respectively. A distance W 2  from the beam outgoing surface  63 B of the lens  63  to the input port  24  is set to be 0.25 mm or a distance W 6  from the beam outgoing surface  63   a B to the input port  24  is also 0.25 mm. The distance W 4  from the beam outgoing surface  63   b B of the second lens  63   b  to the beam incoming surface  63   a A in  FIG. 13B  is 0.5 mm. For such lens systems, focal lengths at which the optical coupling efficiency of the CW beams to the input port  24  of the optical modulator become 645 μm. 
       FIG. 14A  to  FIG. 14F  show alignment tolerances of the lens in the one-lens system and those in the two-lens system, where vertical scales are normalized with respect to the maximum coupling efficiency.  FIG. 14A  shows a coupling tolerance of the lens  63  in the one-lens system for a deviation from the position at which the maximum coupling efficiency is obtained, along the x-direction, namely perpendicular to the optical axis of the lens.  FIG. 14B  also shows a coupling tolerance of the lens  63  for the deviation from the position along the optical axis, z-direction.  FIG. 14C  and  FIG. 14D  show the tolerances of the coupling efficiency of the first lens  63   a  in the two-lens system along the x- and z-directions, and  FIGS. 14E and 14F  also show the coupling tolerances for the second lens  63   b  in the two-lens system along the x- and z-directions. 
     The lenses,  63 ,  63   a  and  63   b,  are fixed at respective positions where the maximum coupling efficiency against the input port  24  is realized by adhesive material typically ultraviolet curable resin. However, solidification of such resin inevitably shrinks through curing, which causes positional deviations of the lenses and degrades the coupling efficiency. Assuming that 20% reduction in the coupling efficiency is acceptable, the lens  63  in the one-lens system and the first lens  63   a  in the two-lens system show tolerances along the x-direction of 1.04 and 0.97 μm, respectively. These values are comparable to the shrinkage of the adhesive resin. Accordingly, in the one-lens system, even the lens  63  is aligned in the position at which the maximum coupling efficiency is realized, this maximum coupling efficiency may not secured after the solidification of the adhesive resin, and, no means are left to compensate the degraded coupling efficiency. 
     On the other hand, the second lens  63   b  in the two-lens system shows the alignment tolerances far greater than those of the lens  63  in the one-lens system and the first lens  63   a.  In particular, the second lens  63   b  shows a large tolerance, about two figures greater than that of the first lens  63   a,  along the z-direction. Even when the second lens  63   b  deviates from the designed position by 230 μm, the degradation of the coupling efficiency may be set within −0.5 dB. For the tolerance along the x-direction, the second lens  63   b  shows a greater tolerance, several times greater than that of the first lens  63   a,  and that of the lens  63  in the one-lens system. Accordingly, the two-lens system may securely recover or compensate by the second lens  63   b  the coupling efficiency degraded by the shrinkage of the adhesive resin for the first lens  63   a.  The adhesive resin for the second lens  63   b  also shrinks during the solidification thereof. However, the shrinkage with the second lens  63   b  is negligibly smaller compared with the large positional tolerance acceptable for the second lens  63   b.    
     The carrier  210   a  further mounts the m-PD  64  via the PD sub-mount  64 A, four interconnections  63   c  along a side  210   b  facing the joint unit  220  to carry the sensed signals output from the m-PDs,  64   a  and  64   b,  other two interconnections  63   d  along one side  210   c  to carry another sensed signal output from the m-PD  62   a.  Two of the four interconnections  63   c  are for the first m-PD  64   a,  and the other two interconnections  63   c  are for the second m-PD  64   b  mounted in another side of the optical modulator  20 . Wiring from the PD sub-mount  64 B for the m-PD  64   b  in the other side to the PD sub-mount  64 A across the optical modulator  20  and further wiring the PD sub-mount  64 A to the interconnections  63   c,  the sensed signal output from the m-PD  64   b  may be carried to the DC terminals  5   a  in the side wall  2 C. The m-PD  62   a  mounted behind the BS  61  may sense the magnitude of the first CW beam L 1  entering the optical modulator  20 . The BS  61  splits the CW beam L 1  by a ratio of 5:95, that is, 5% of the CW beam L 1  passes the BS  61 , and the rest 95% thereof is reflected by the BS  61  toward the lens system  63 . The sensed signal output from the m-PD  62  may be carried on the interconnections  63   d  provided along the side  210   c  of the carrier  210   a,  and wire-bonded to the DC terminals  5   a  in the side wall  2 C. Feeding the sensed signal of the m-PD  62   a  to the bias supplied to the SOA  10   a  in the t-LD  10 , the first CW beam L 1  entering the optical modulator  20  may be kept in the magnitude thereof in constant. The arrangement of the wirings thus described may enable the sensed signals output from the m-PDs,  62   a  to  64   b,  to be extracted from the DC terminals  5   a  in one side wall  2 C, even when the m-PD  62   b  is placed in the side of the other side wall  2 D. Moreover, the interconnections  63   c  on the carrier  210   a  may not interfere with the optical axis of the first CW beam L 1  connecting the laser unit  100  to the BS  61 . 
     Joint Unit 
       FIG. 15  is a plan view of the joint unit  220 . The joint unit  220 , similar to the input unit  210 , provides a carrier  220   a  with a rectangular shape, which will be referred as the third carrier, is mounted on the area  200 B of the base  200   a  and upstream the input unit  210  closer to the laser unit  100 , where the area  200 B extends from the center area  200 A overlapping with the top plate  21   a  of the second TEC  21 . The joint unit  220  includes a beam shifter  81 , an optical isolator  82 , and some interconnections  220   d  on the top surface of the carrier  220   a.  The interconnections  220   d  are wired between the beam shifter  81  and the optical isolator  82 . 
     The beam shifter  81  compensates a vertical discrepancy between the optical axis of the laser unit  100  and the input port  24  of the optical modulator  20 . The laser unit  100  and the modulator unit  200  are mounted on respective TECs,  11  and  21 , independent to each other. This arrangement often cause an offset between the optical axes of components in the laser unit  100  and those in the modulator unit  200  within a range of allowable tolerances in physical dimensions of those components. Also, even in the modulator unit  200 , the coupling unit  220 , the input unit  210 , and the optical modulator  20  are mounted on the base  200   a  via respective carriers,  20   a,    210   a,  and  220   a,  independent to each other. Accordingly, vertical discrepancies between optical axes of components, namely, the optical isolator  82 , the BS  61 , the lens system  63 , and the optical modulator  20  are often encountered. Adhesive resin to fix the BS  61  and the lens system  63  on the carrier  210   a  may adjust the vertical discrepancies of the optical axes. However, when the offset between the optical axes of the laser unit  100  and the input port  24  of the optical modulator  20  becomes large, or exceeds an allowable limit, the resin in thicknesses thereof may not compensate those discrepancies in the optical axes. The lens system  63  is impossible to lower the top level of the carrier  210   a,  and thicker adhesive resin for the lens system  63  may degrade the reliability of the fixation. 
     The beam shifter  81  of the embodiment may compensate the offset between the optical axis of the laser unit  100  and that of the optical modulator  20 . The beam shifter  81  is a rectangular block with a beam incoming surface and a beam outgoing surface extending in parallel to each other and made of material transparent to the first CW beam L 1 . Setting the beam shifter  81  on the carrier  220   a  as vertically inclining against the top surface of the carrier  220   a,  the optical axis of the first CW beam L 1  may translate vertically. The beam shifter  81  is also set on the carrier  220   a  inclined horizontally so as to prevent the first CW beam L 1  back to the laser unit  100 . 
     The interconnections  220   d  are wired between the beam shifter  81  and the optical isolator  82  between one side facing the terminator unit  84   a  and the bias unit  86   a  to another side facing the side wall  2 C so as to avoid the beam shifter  81 . Similar to the interconnections  63   c  on the input unit  210 , the interconnections  220   d  on the joint unit  220  may not interfere with the optical axis of the first CW beam L 1 . The terminator unit  84   a  and the bias unit  86   a  are electrically connected to the DC terminals  5   a  in the side wall  2 C through the interconnections  220   d.  Although the interconnections  220   d  shown in  FIG. 15  avoid the beam shifter  81 , the interconnections  220   d  may intersect the beam shifter  81 , that is, the interconnections  220   d  may run beneath the beam shifter  81 . Because signals are carried on the interconnections  220   d  substantially in the DC mode, the quality of those signals is not affected by an environment of the wiring. 
     Output Unit 
       FIG. 16  is a plan view of the output unit  230 . The output unit  230  includes the output lens system  73  comprising two first lenses  73   a  and two second lenses  73   b.  The output lens system  73  converts two modulated beams, M 2   b  and M 2   c,  output from the optical modulator  20  into collimated beams, multiplexes the collimated two beams, and outputs the multiplexed beam to the first output port  3   a  as the output beam D 1 . The output unit  230  further includes a skew adjuster  74 , two optical isolators,  75   a  and  75   b,  a polarization beam combiner (PBC) unit  76 , and a variable optical attenuator (VOA)  77 . The skew adjuster  74  may compensate a difference of optical paths from the optical modulator  20  to the PBC unit  76  for the respective modulated beams, M 2   b  and M 2   c.  The PBC unit  76  includes a reflector  76   a  and a PBC element each made of multi-layered optical films. 
     One of the output lens systems  73  collimates the modulated beam M 2   c  toward the first output port  3   a,  while, the other of the output lens systems  73  also collimates the other modulated beam M 2   b  toward the mirror  76   a  in the PBC unit  76 . The output lens systems  73  each include the first lens  73   a  set closer to the optical modulator  20  and the second lens  73   b  set closer to the PBC unit  76 . The two modulated beams, M 2   b  and M 2   c,  are each collimated by the respective lens system  73 . 
     One of the modulated beams M 2   c  is collimated by the output lens system  73  and enters the PBC unit  76  as passing through the skew adjuster  74  and the optical isolator  75   b.  The other modulated beam M 2   b  is also collimated by the output lens system  73  and enters the PBC unit  76  as passing through the optical isolator  75   a.  The skew adjuster  74  may compensate the optical path difference of the two modulated beams, M 2   b  and M 2   c.  That is, the modulated beam M 2   b  comes the PBC element  76   b  running on an extra path from the mirror  76   a  to the PBC element  76   b  compared with the other modulator beam M 2   c  that directly comes straight to the PBC element  76   b  from the optical modulator  20 . The skew adjuster  74 , by being set intermediate of the optical path for the modulated beam M 2   c,  may compensate the optical length of this extra path. The skew adjuster  74  of the embodiment may be a block made of material transparent for the first CW beam, silicon (Si) in the present embodiment, and set slightly inclined with respect to the optical axis of the modulated beam M 2   c  to prevent the optical beam reflected thereby from coming back to the optical modulator  20 . 
     The modulated beams, M 2   b  and M 2   c,  inherently have the polarization reflecting that of the first CW beam entering the optical modulator L 1 , because the optical modulator  20  includes no components to rotate the polarization of the incident beam. Accordingly, two modulated beams, M 2   b  and M 2   c,  have the polarization identical to each other. Two optical isolators,  75   a  and  75   b,  may rotate the polarization of the incident beams, M 2   b  and M 2   c,  independently, that is, the optical isolators,  75   a  and  75   b,  may set a difference of 90° in the polarization between two outgoing beams. For instance, setting a half-wave plate (λ/2-plate), which may rotate the polarization of incident beam by 90°, only in one of the optical isolates, two modulated beams, M 2   b  and M 2   c,  output from the optical isolators,  75   a  and  75   b,  may show the polarization status different by 90° to each other. The modulated beams, M 2   b  and M 2   c,  enter the PBC element  76   b  as maintaining the polarization status thereof. 
     The PBC element  76   b  includes multi-layered optical films and shows a peculiar property depending on the polarization of the incoming beam. For instance, the PBC element  76   b  may show large reflectance, equivalently small transmittance, for the incident beam having the polarization within the incident plane while large transmittance, equivalently small reflectance, for the incident beam with the polarization perpendicular to the incident plane, where the incident plane may be formed by the optical axis of the incident beam and the normal of the incident surface of the PBC element  76   b.  Setting the polarization direction of the modulated beam M 2   c  in perpendicular to the incident plane for the PBC element  76   b,  but that of the other modulated beam M 2   b  in parallel to the incident plane, the former modulated beam M 2   c  in almost all portion thereof may transmit the PBC element  76   b,  and the latter modulated beam M 2   b  in almost all portion thereof may be reflected by the PBC element  76   b.  Thus, the two modulated beams, M 2   b  and M 2   c,  may be effectively multiplexed, e.g., polarization-multiplexed, by the PBC element  76   b  by rotating the polarization of one of the modulated beam M 2   b  by 90° by the optical isolator  75   a.  The PBC unit  76  outputs thus multiplexed beam to the VOA  77 . 
     The two optical isolators,  75   a  and  75   b,  are the type of polarization dependent isolator. By setting a magnet, not shown in figures, for inducing magnetic fields commonly to the isolators,  75   a  and  75   b,  the embodiment is implemented with the integrated optical isolator  75 . Moreover, the description above concentrates on an arrangement where only the optical isolator  75   a  provides the λ/2-plate in the output thereof. However, an alternative may be applicable where one of the optical isolators  75   a  sets the crystallographic axis thereof in −22.5° but the other isolator  75   b  sets the crystallographic axis in 22.5° with respect to the polarization direction of the modulated beams, M 2   b  and M 2   c.  Then, the modulated beams, M 2   b  and M 2   c,  output from respective optical isolators,  75   a  and  75   b,  have the respective polarization directions thereof perpendicular to each other. 
     Thus, the arrangement, where two first lenses  73   a  and two second lenses  73   b,  the skew adjuster  74 , the optical isolator  75 , and the PBC unit  76 , are mounted on the base  200   a  via the carrier  230   a  made of A1N, which will be referred as the fourth carrier, may simplify the optical alignment for those components with respect to the modulated beams, M 2   b  and M 2   c,  output from the optical modulator  20 . Because those optical components on the carrier  230   a  inherently have dull temperature characteristics, it is unnecessary to control temperatures of those components by the second TEC  21  in the modulator unit  200 . Accordingly, the area  200 C of the base  200   a,  where the carrier  230   a  is mounted, is overhung from the area  200 A overlapping with the top plate  21   a  of the TEC  20  and leaves a wide space under the carrier  230   a.  The optical module  1  of the present invention installs two wiring substrates,  90   a  and  90   b,  to carry signals from the DC terminals  5   b  in the side wall  2 D of the housing  2  to the laser unit  100  installed in the side of the other side wall  2 C. 
     The reason to set the VOA  77  downstream the PBC unit is, when the optical module  1  is installed within an optical transceiver having functions to transmit an optical signal and to receiver another optical signal concurrently, a situation is probably encountered where only the function of the signal transmission is killed as leaving the function of the signal reception. In such a case, only the second output D 2  of the optical module  1  is required. When the biases supplied to the t-LD  10  is cut to stop the operation thereof, the second output D 2  also disappears. The VOA  77  set in the path for the first optical output D 1  may interrupt the operation only of the signal transmission. 
     When a VOA is set in upstream of the optical modulator  20 , the function to stop the signal transmission may be realized. However, this arrangement fully suspends the input of the first CW beam L 1  to the optical modulator  20 . The optical modulator  20  is necessary to adjust the biases supplied to the offset electrodes and the quadrature electrodes using the first CW beam L 1  to adjust the phases of two optical outputs, M 2   b  and M 2   c.  Such adjustments may be carried out for the modulated signals, M 2   a  and M 2   d,  output from the monitor ports,  25   a  and  25   b,  even when the modulated beams, M 2   b  and M 2   c,  are suspended. 
     The optical module  1  sets the m-PD  79   a  in downstream of the VOA  77 . The m-PD  79   a,  which is mounted in a side of the PD sub-mount  79 A, senses a portion of the optical output D 1  split by the BS  78 . The m-PD  79   a,  the PD sub-mount  79 A, and the BS  78  are mounted on the VOA carrier  77 A, which is placed on the bottom of the housing  2  independent of the carrier  230   a.  The output of the m-PD  79   a  is used for detecting the degradation of elements integrated within the optical modulator  20  and the excessive output power of the optical module  1 . 
     As shown in  FIG. 16 , the output unit  230  is also implemented with the two-lens system  73  for respective modulated beams, M 2   b  and M 2   c.  The field pattern of the modulated beams, M 2   b  and M 2   c,  is usually deviated from a true round reflecting the cross section of the waveguide in the optical modulator  20 . Such a distorted beam usually degrades the coupling efficiency against an optical fiber with a circular field pattern. The two-lens system of the present optical module  1  may suppress the reduction of the coupling efficiency between the optical beam with a distorted filed pattern and an optical medium with a circular cross section. In an alternative, the optical module  1  may set a beam shaper downstream the PBC unit  76  to modify the field pattern of the output beam D 1 . 
       FIG. 17  shows a cross section of the optical module  1 , which is taken along the optical axes of the first CW beam L 1  and the second CW beam L 2 . Referring to  FIG. 17 , the carrier  220   a  of the joint unit  220  does not interfere with the optical axis of the first CW beam L 1  coming from the t-LD  10  to the BS  61 . Referring to  FIG. 15 , the joint unit  220  provides the interconnections  220   d  that carries the biases from the DC terminals  5   a  in the side wall  2 C to the pads  46   a  on the optical modulator  20  connected to the offset electrodes,  51   j  to  52   k,  and the quadrature electrodes,  51   c  and  52   c.  When the pads  46   a  are directly wire-bonded to the DC terminals  5   c,  not only the bonding wires lengthen but sometimes interrupt the optical axis of the first CW beam L 1 . The optical module  1  of the embodiment avoids the optical axis by the interconnections  220   d  on the joint unit  220   a.  That is, the pads  46   a  on the optical modulator  20  are first wired to the ends of the interconnections  220   d,  and further wired in the other ends thereof to the DC terminals  5   a.  Thus, the optical axis of the first CW beam L 1  does not interfere with members except for the beam shifter  81  and the optical isolator  82 . 
     Also, the carrier  210   a  of the input unit  210  mounts the m-PD  64   a via the PD sub-mount  64 A. The m-PD  64   a  optically couples with the monitor port  25   a.  The biases supplied to the offset electrodes,  51   j  to  52   k,  and the quadrature electrodes,  51   c  and  52   c  may be determined based on the output of the m-PD  64   a.  The interconnections  63   c  on the carrier  210   a  that carries the output of the m-PD  64   a  to the DC terminal  5   a  also does not interfere with the optical axis of the first CW beam L 1 . 
     The area A 3  of the base  200   a  mounts the terminator unit  84   a  in addition to the bias unit  86   a.  The terminator unit  84   a  provides four terminators  85   b  and two capacitors  85   d.  The terminators  85   b  terminate the interconnections,  41  and  42 , carrying the modulation signals to the MZ elements,  51 M and  52 M. The modulation signals provided to the respective MZ elements,  51 M to  54 M, have magnitudes of about 1 Vp-p. The terminators  85   b  with impedance of 50Ω for such modulation signals each consume the power of 20 mW. Accordingly, the optical modulator  20  of the embodiment sets the terminators externally to suppress the power consumption thereof. However, bonding wires from the optical modulator  20  to the terminators  85   b  are necessary to be short as possible, the terminator units,  84   a  and  84   b,  are set immediate to the optical modulator  20 . 
     The area B 1  of the base  200   a  mounts the other m-PD  64   b  via the PD sub-mount  64 B for the MZ elements,  53 M and  54 M, and the area B 2  mounts the other terminator unit  84   b  and the other bias unit  86   b,  where the arrangements of those units,  84   b  and  86   b,  are same with those aforementioned units,  84   a  and  86   a.    
     As described, the optical modulator  20  is mounted on the base  200   a,  and the base  200   a  is mounted on the top plate  21   a  of the second TEC  21 . An optical modulator like the present embodiment inherently shows dull temperature dependence of characteristics thereof. However, the optical coupling between the optical modulator  20 , the input unit  210 , the joint unit  220 , and the output unit  230  may be varied depending on the temperature, which is generally called as the tracking error. Accordingly, the present optical module  1  mounts those units,  210 ,  220 , and  230 , commonly on the base  200   a,  and the base  200   a  is set on the second TEC  21  to suppress the tracking error. However, the temperature dependence of the optical coupling of those units,  210 ,  220 , and  230 , are far smaller than that of the t-LD  10 . Accordingly, the base  200   a  of the present embodiment mounts those units,  210 ,  220 , and  230  on the areas,  200 B and  200 C, not overlapping with the TEC  21 . 
       FIG. 18  schematically shows a cross section of the optical module  1  taken along the optical axis of the first output port  3   a.  The output unit  230  is mounted on the area A 3  of the base  200   a  projecting from the second TEC  21  via the carrier  230   a,  which forms a space under the output unit  230 . The optical module  1  installs two wiring substrates,  90   a  and  90   b,  in this space to supply the biases from the DC terminals  5 B in the side wall  2 D to the t-LD  10 . 
       FIGS. 19 and 20  are plan views of the arrangement around the wiring substrates,  90   a  and  90   b,  and  FIG. 21  is a perspective view of the wiring between the wiring substrates,  90   a  and  90   b,  and the laser unit  100 . As already described, the t-LD  10  of the present embodiment is necessary to be biased in the electrodes,  14   a  to  14   e,  to inject carriers into two SOAs,  10   a  and  10   d ; in the heaters,  15   a  and  15   b,  in the SG-DFB  10   b,  and the heaters,  17   a  to  17   c,  in the CSG-DBR  10   c ; in two heater grounds; and in the signal ground, where total of ten (10) electrodes are necessary to be supplied with respective biases. In a case where these electrodes are biased from the DC terminals  5   a  arranged in the side wall  2 C closer to the laser unit  100  compared with the other side wall  2 D, the DC terminals  5   a  in the number thereof occasionally becomes insufficient when the detector unit  300  and the modulator unit  200  are also biased from the DC terminals  5   a.  On the other hand, the other side wall  2 D along the modulator unit  200  leaves spares of the DC terminals  5   b  not connected to anywhere. Accordingly, the optical module  1  supplies the biases to the t-LD  10  from the DC terminals  5   b  in the side wall  2 D via the wiring substrates,  90   a  and  90   b.    
     As shown in  FIGS. 20 and 21 , the LD carrier  100 A mounts the t-LD  10  and the thermistor  11   f  thereon. Two wires W 1  extracted from the thermistor  11   f  are connected to the DC terminals  5   a  in the closer side wall  2 C. The other wires W 2  are extracted to the DC terminals  5   b  in the other side wall  2 D through the wiring substrates,  90   a  and  90   b.  The wiring substrate  90   a  closer to the second TEC  21  has a thickness greater than that of the other wiring substrate  90   b  because of a room to wire the respective substrates,  90   a  and  90   b.  That is, the wiring for the wiring substrate  90   a  is necessary to be done in a space between the base  200   a  and the first TEC  11 . On the other hand, the wiring to the other substrate  90   b  may be done in a space between the carrier  300   a  of the detector unit  300  and the lens carrier  100 B, which is relatively wider than the former space. Thus, a space relatively wider is left for the wiring to the wiring substrate  90   b.    
     The carrier  300   a  of the detector unit  300  and the lens carrier  110 B on the base  100   a  of the laser unit  100 , where they sandwich the wiring substrate  90   b  therebetween, have relatively thinner thicknesses to mount the BSs,  32   a  and  32   b,  and the collimating lens  110   b  thereon. On the other hand, the other wiring substrate  90   a  which locates next to the LD carrier  100 A with a thickens thereof greater than a thickness of the lens carrier  110 B to align the level of the optical axis of the t-LD  10  and that of the collimating lens  110   b  with each other, which means that the top of the t-LD  10  is higher than the top of the lens carrier  110 B and that the wiring substrate  90   a  is necessary to have a thickness thereof to reduce the difference in the top level between the t-LD  10  and that of the wiring substrate  90   a.    
     Second Embodiment 
       FIG. 22  shows a flow chart of a process of assembling the optical module of the first embodiment. Next, the process of assembling the optical module  1  will be described. 
     S 1 : Assembling of Laser Unit 
     The process first assembles the laser unit  100  independent of the optical module  1 . The t-LD  10  and the thermistor  11   f  are mounted on metal patterns on the LD carrier  100 A by a conventional die-mount process using eutectic solder of gold tin (AuSn).  FIG. 23  is a perspective view of the t-LD  10  mounted on the LD carrier  100 A. The t-LD  10  is mounted on a metal pattern provided on the top of the LD carrier  100 A and wire-bonded from electrodes corresponding to the metal patterns  100   b.  After the wire-boding, the t-LD  10  may be tested in the DC mode, such as the I-L characteristic of the t-LD  10 , and so on, by probing the metal patterns  100   b.  When the DC test fmds any failures in a t-LD  10 ; such t-LD  10  is extracted from the subsequent production. 
     S 2 : Assembling Modulator Unit 
       FIG. 24  is a perspective view of the modulator unit  200 . The process of assembling the modulator unit  200  is also carried out independent of the assembly of the optical module  1 . Specifically, the optical modulator  20  is mounted in a center area  200 A of the base  200   a  as shown in  FIG. 9  via the modulator carrier  20   a ; then, the terminator unit  84   a  and the bias unit  86   a,  and the terminator unit  84   b  and the bias unit  86   b  are mounted in the areas, A 3  and B 2 , of the sides of the optical modulator  20  on the base  200   a,  respectively. These areas, A 3  and B 2  put the optical modulator  20  therebetween. The terminator units,  84   a  and  84   b,  solder two chip capacitors  85   d  in advance to be mounted on the base  200   a.  The terminators  85   b,  which are the type of thin film resistor, are formed concurrently with the formation of the interconnections  85   h  on the terminator units,  84   a  and  84   b.  Although the optical module  1  of the embodiment uses the chip capacitors  85   d,  the optical module  1  may mount the die-capacitors on the terminator units,  84   a  and  84   b.  For the bias units,  86   a  and  86   b,  the die-capacitors  87   a  are soldered on the bias units,  86   a  and  86   b,  in advance to be mounted on the base  200   a.  The terminator unit  84   a  and the bias unit  86   a,  and the terminator unit  84   b  and the bias unit  86   b  are mounted on the base  200   a  via the carriers,  88 A and  88 B, respectively, where these carriers,  88 A and  88 B have thicknesses such that the terminator units,  84   a  and  84   b,  and the bias unit,  86   a  and  86   b,  in respective top levels become substantially comparable with the top level of the optical modulator  20 . 
     In the process S 2  above, the carrier  20   a  is first soldered with the base  200   a  by a eutectic solder, and the optical modulator  20  is next soldered on the carrier  20   a  also by a eutectic solder. Subsequently, the carrier  210   a  for the input unit  210 , which may be referred as the second carrier, the carrier  220   a  for the joint unit  220 , which may be referred as the third carrier, the carries,  88 A and  88 B, commonly for the terminator units,  84   a  and  84   b,  and the bias units,  86   a  and  86   b,  the carrier  66 A for mounting the m-PD  64   b  via the PD sub-mount  64 B are also soldered in respective areas on the base  200   a.  The carrier  66 A mounts a thermistor  66  thereon. Accordingly, the carrier  66 A may be called as the thermistor carrier. At the process for soldering the input carrier  210   a  on the base  200   a,  a rough alignment of the carrier  210   a  is carried out. 
     Specifically, referring to  FIG. 25 , the carrier  210   a  includes a side  210   b,  which faces the side  220   c  of the joint unit  220 , with marks,  210   e  to  210   g,  linearly extending inward from the edge of the carrier  210   a.  The center mark  210   f  substantially aligns with the optical axis of the first CW light L 1  coming from the laser unit  100  to the BS  61 . The side marks,  210   e  and  210   g,  have distances equal to each other. Aligning the marks,  210   e  to  210   g,  with the marks,  220   e  to  220   g,  on the carrier  220   a  of the joint unit  220 , the input unit  210  may be roughly aligned with the joint unit  220  only by the visual inspection. 
     The optical modulator  20  also provides marks,  20   c  and  20   d,  along the edge  20   b  facing the input unit  210 . The former mark  20   c  corresponds to the input port  24 , while, the latter mark  20   d  indicates the monitor port  25   a.  These marks,  20   c  and  20   d,  have a shape of an isosceles divided into two part by a line evenly dividing a corner constituting the isosceles sides. However, the shapes of those marks,  210   e  to  210   g,    220   e  to  220   g,  and  20   c  to  20   d,  are optional. 
     Using those alignment marks, the rough alignment of the input port  24  of the optical modulator  20  with the carrier  210   a,  and that between the carrier  210   a  of the input unit  210  and the carrier  220   a  of the joint unit  220  may be carried out only by the visual inspection. For the alignment of the m-PD  64   a  with the monitor port  25   a,  because of a large sensitive surface of the m-PD  64   a,  only the rough alignment by the visual inspection may achieve an optical coupling efficiency between the m-PD  64   a  and the monitor port  25   a  with practically acceptable level. 
     Referring to  FIG. 26 , the carrier  230   a  of the output unit  230 , which may be referred as the fourth carrier, provides in a side  230   d  facing the optical modulator  20  two marks,  230   b  and  230   c.  Similarly, the optical modulator  20  provides two marks,  20   e  and  20   f,  in a side facing the output unit  230 . The mark  230   b  in the carrier  230   a  aligns with the mark  20   b  of the optical modulator  20  and also with the optical axis of the modulated beam M 2   b.  The mark  230   c  aligns with the mark  20   e  and corresponds to the optical axis of the modulated light M 2   c.    
     The carrier  230   a  also provides three marks,  230   e  to  230   g,  in a side  230   h  facing the VOA carrier  77 A. The BS carrier  78 A also provides three marks,  78   e  to  78   g , in a side  78   a  facing the carrier  230   a.  These marks,  78   e  to  78   g , in the BS carrier  78 A align with the marks,  230   e  to  230   g,  in the carrier  230   a  of the output unit  230 . The two modulated beams, M 2   b  and M 2   c,  output from the optical modulator  20  are multiplexed as passing through the BS  78 . Thus, the rough alignment of the carrier  230   a  with the optical modulator  20  and the BS carrier  78   a  with the carrier  230   a  of the output unit  230  may be easily performed only by the visual inspection of those marks. 
     The process of assembling the optical module  1  of the present embodiment omits fine alignments for the BS  78  and the m-PD  79   a  to be mounted on the BS carrier  78 A. Only the visual inspection of those marks,  78   e  to  78   g , and  230   e  to  230   g,  for the BS  78  and the m-PD  29   a  may align the output unit  230  with the optical modulator  20  and the BS. 
     After mounting those carriers,  210   a,    220   a,  and  230   a  on the base  200   a,  the pads on the optical modulator  20  are wire-bonded to the interconnections on respective carriers. Specifically, the pads,  45   a  and  45   b,  on the optical modulator  20  are wire-bonded to the terminators  85   b  on the terminator units,  84   a  and  84   b ; the interconnections  85   h  on the terminator units,  84   a  and  84   b,  are wire-bonded to the interconnections on the carrier  220   a  of the joint unit  220 ; the pads,  46   a  and  46   b,  on the optical modulator  20  are also wire-bonded to the die capacitors  87   a  on the bias units,  86   a  and  86   b ; the die capacitors  87   a  are wire-bonded to the interconnections  87   b  on the bias units,  86   a  and  86   b ; and the interconnections  87   b  on the bias units are wire-bonded to the interconnections  220   d  on the carrier  220   a  of the joint unit  220 . 
     The embodiment thus described, the terminator unit  84  and the bias unit  86   a  are commonly mounted on the carrier  88 A, and the terminator unit  84   b  and the bias unit  86   b  are also commonly mounted on the carrier  88 B. However, the carriers,  88 A and  88 B, may be divided into two parts, one of which mounts the terminator units,  84   a  and  84   b,  and the other mount the bias units,  86   a  and  86   b.  Further, the terminator unit  84   a  and the bias unit  86   a  disposed in the side of the side wall  2 C of the housing may have a substrate common to those units,  84   a  and  86   a.  Similarly, the terminator unit  84   b  and the bias unit  86   b  arranged along the side wall  2 D may have a substrate common to each units,  84   b  and  86   b.  Because the bias units,  86   a  and  86   b,  and the terminator units,  84   a  and  84   b,  in portions outside of the terminators  85   b  process DC signals; respective common substrates do not degrade or affect the operation of the optical modulator  20 , rather, the assembly of the bias units and the terminator units may be simplified. 
     Assembling Detector Unit 
     The process mounts the thermistor  31   f,  two m-PDs,  34   a  and  34   b,  as interposing respective PD sub-mounts,  34 A and  34 B, on the carrier  300   a,  in the outside of the housing  2 . Those components are fixed on respective metal patterns by eutectic solder. As already described, the m-PDs,  34   a  and  34   b,  have wide optical sensitive areas with diameters thereof greater than several scores of micron-meters; accordingly, the m-PDs,  34   a  and  34   b,  are unnecessary to be actively aligned with the t-LD  10 . The etalon filter  33  is also mounted on the carrier  300   a  in this process. 
     S 4 : Assembling Optical Module 
     S 4   a : Installing Three TECs 
       FIG. 27  is a plan view showing a process of installing three TECs,  11  to  31 , within the housing  2 . The VOA carrier  77 A, that mounts the VOA  77  in advance to the installation thereof, and two wiring boards,  90   a  and  90   b,  are concurrently installed within the housing  2 . A conventional technique of the die-bonding is applied to the installation of those devices. As shown in  FIG. 27 , the bottom plates,  11   b  to  21   b,  of the respective TECs,  11  to  31 , prepare posts in areas exposed from the respective top plates,  11   a  to  31   a,  to supply the driving currents to the Peltier elements. Those posts are wire-bonded to the DC terminals,  5   a  and  5   b,  in the respective side walls,  2 C and  2 D, after the installation of the TECs,  11  to  31 . 
     S 4   b : Mounting Laser Unit and Modulator Unit on Respective TECs 
     The step S 4   b  mounts the base  100   a  of the laser unit  100 , which is assembled in the step S 1 , and the base  200   a  of the modulator unit  200 , which mounts various units thereon in the step S 2 , on the respective TECs,  11  and  21 . 
       FIG. 28  is a plan view showing the process which the laser unit  100 , the modulator unit  200 , and the detector unit  300  are mounted on the respective TECs,  11  to  31 , in the housing  2 . Optical components required for the active alignment are not implemented therewith. Specifically, the LD carrier  100 A that mounts the t-LD  10  by the first eutectic solder in advance to the present step is mounted on the base  100   a  of the laser unit  100  by the second eutectic solder whose melting point is lower than that of the first eutectic solder. In the present embodiment, the first eutectic solder is made of SnAgCuBi with a melting point of about 240 C.°. Concurrently with the installation of the LD carrier  100 A on the base  100   a,  two lens carriers,  110 A and HOB, are set on the base  100   a.  Referring to  FIG. 25  again, the LD carrier  100 A provides two marks,  112   e  and  112   g , and the lens carrier  110 A provides marks,  111   e  to  111   g.  Aligning the marks,  111   e  to  111   g,  on the lens carrier  110 A with the marks,  112   e  and  112   g , on the LD carrier  100 A only by visual inspection, the lens carrier  110 A may be roughly aligned with the LD carrier  100 A. Referring to  FIG. 29 , the other lens carrier  110 B may be also mounted on the base  100   a  by aligning marks,  114   e  to  114   g,  on the lens carrier  110 B with marks,  113   e  to  113   g , on the LD carrier  100 A in the side opposite to that appearing in  FIG. 25 . The two lens carriers,  110 A and  110 B, are mounted on the base  100   a  but the collimating lenses,  110   a  and  110   b,  are not placed on respective positions on the lens carriers,  110 A and 110 B. The base  100   a  thus mounting the LD carrier  100 A and the two lens carriers,  110 A and  110 B, is to be set on the TEC  11 . 
     The base  200   a  of the modulator unit  200 , which mounts the various units including the input unit  210  and the joint unit  220 , is also fixed on the second TEC  21  by an eutectic solder. Referring to  FIG. 25  again, the carrier  220   a  of the joint unit  220  provides the marks,  221   e  to  221   g,  in a side  221   c  facing the laser unit  100 . On the other hand, the lens carrier  110 A of the laser unit  100  also provides marks,  110   e  to  110   g,  in a side facing the joint unit  220 . Aligning these marks,  221   e  to  221   g,  with the marks,  110   e  to  110   g ; the modulator unit  200  may be roughly aligned with the laser unit  100 . The rough alignment using these marks described above may simplify the fine alignment subsequently carried out for lenses and so on. Positions, where the lenses are to be mounted, provide indices on the respective carriers. However, when the respective carriers are largely misaligned, the fine alignment sometimes becomes unable, because even the components to be finely aligned is set on the indices, substantial optical coupling efficiency could not be obtained. The alignment process inevitably begins a step to find a position at which substantial coupling efficiency is realized. 
     S 4   c : Mounting Detector Unit on TEC 
     The process next installs the carrier  300   a  of the detector unit  300  onto the third TEC  31 , where the carrier  300   a  assembles the thermistor  31   f,  two m-PDs,  34   a  and  34   b,  and the etalon filter  33  thereon. Referring to  FIG. 29 , the lens carrier  110 B which is assembled on the LD carrier  100 A in the aforementioned process provides marks,  115   e  to  115   f,  in a side opposite to that facing the LD carrier  100 A. The carrier  300   a  of the detector unit  300  is mounted on the third TEC  31  such that marks,  311   e  to  311   f,  on the carrier  300   a  are visually aligned with the marks,  115   e  to  115   g,  on the lens carrier  110 B. Thus, the detector unit  300  is roughly aligned with the laser unit  100 . 
     S 5 : Optical Alignment 
     S 5   a : Alignment of Input Unit 
     The process finally assembles optical components that are required for active alignment. The step S 5   a  first aligns the input unit  210  of the modulator unit  200  with the laser unit  100  in step S 5   a ( a ). Specifically, the first collimating lens  110   a  in the laser unit  100  is necessary to be set in a position where an optical beam output from the first collimating lens  110   a  becomes a collimated beam. Referring to  FIG. 30 , the process first sets a special tool  91   d  on a position to which the beam shifter  81  is placed as practically activating the t-LD  10  to emit the dispersive light therefrom. The special tool  91   d,  which provides two mirrors fixed in parallel to each other and making an angle of 45° with respect to the optical axis, guides the first CW light L 1  output from the t-LD  10  outside of the housing  2  by the parallel translation. Checking the collimation of the first CW light L 1  by an optical detector set apart from the housing  2 , where the optical detector is set apart about one (1) meter from the optical module in the present embodiment, as sliding the first collimating lens  110   a  along the optical axis thereof, the first collimating lens  110   a  is fixed in a position where the output beam becomes a collimated beam. 
     Then, removing the special tool  91   d  and setting the beam shifter  81  on the carrier  220   a  of the joint unit  220 , the process may compensate the offset between the optical axis of the CW light L 1  of the laser unit  100  and that of the modulator unit  200 . Referring to  FIG. 31 , the first TEC  11  mounts the t-LD  10  and the collimating lens  110   a  thereon, while, the modulator unit  200  mounts the optical modulator  20  on the second TEC  21  through the base  200   a independent of the first TEC  11 . Accordingly, the optical axis of the t-LD  10  and that of the optical modulator  20  are usually not aligned in the levels thereof, namely, offset each other. The optical coupling system mounted on the input unit  210 , that is, the BS  61  and the two-lens system  63 , may compensate this discrepancy of the optical axes. However, it would be hard enough to compensate the discrepancy solely by the BS  61 . A rotation angle, an elevation angle, and/or a depression angle are required to align the BS  61 . Moreover, it would be physically impossible for the two-lens system to lower them beyond the top of the carrier  210   a.  Also, when the lenses,  63   a  and  63   b,  are set apart from the carrier  210   a  beyond a designed distance, the resin that fixes the lenses,  63   a  and  63   b,  degrades the reliability thereof. Accordingly, the beam shifter  81  of the embodiment compensates the offset in the optical axes between the laser unit  100  and the modulator unit  200 . The beam shifter  81  of the present embodiment may be a parallel-piped block made of material transparent to the CW beam L 1  and may offset the optical axis of the incident beam by setting the incident surface thereof inclined to the optical axis of the incident beam. 
       FIG. 31  schematically shows a process of aligning the beam shifter  81  at step S 5   a ( b ). The process measures the level of the first CW light L 1  output from the collimating lens  110   a  and that of the input port  24  of the optical modulator  20  in advance to a process of assembling the beam shifter  81 . The former level may be measured concurrently with the process of forming the collimated beam in the output of the first collimating lens  110   a.  From two evaluated values above described, the inclined angle of the beam shifter  81  may be estimated from the following equation:
 
Δ d=t ×sin θ×(1−cos θ)/√( n   2 −sin 2  θ),
 
where Δd, t, n, and θ are the offset between two optical axes, a thickness of the beam shifter  81 , refractive index of the material constituting the beam shifter  81  and an angle to be inclined for the beam shifter  81 , respectively. Evaluating the angle θ from the equation above, the beam shifter  81  is passively set so as to make the angle θ with respect to the carrier  210   a  without any active alignment.
 
       FIG. 32  schematically shows a process of setting the BS  61  at step S 5   a ( c ). The process first aligns the angle of the BS  61  in 45° against the side wall  2 C of the housing  2  using an optical source  91   e,  a power monitor  91   m,  a 3 dB coupler  91   s,  and an auto-collimator  91   a.  Specifically, setting the side wall  2 C of the housing to be an optical reference plane, the auto-collimator  91   a  is set so as to make an angle of 45° with respect to the side wall  2 C. During the preparation of the auto-collimator  91   a,  the optical beam coming from and reflected to the auto-collimator  91   a  passes above the housing  2 . Then, the BS  61  is first aligned in the rotation angle thereof in the space outside of the housing  2  such that the optical beam reflected by the back surface of the BS  61  and detected by the power monitor  91   m  through the auto-collimator  91   a  becomes a maximum. Moving the BS  61  down into the housing  2  as keeping the angle with respect to the side wall  2 C, the BS  61  is next adjusted in longitudinal and lateral positions thereof. That is, sliding the BS  61  longitudinally along the optical axis of the beam shifter  81 , namely, that of the laser unit  100 , and laterally along the optical axis of the input port  24  of the optical modulator  20 , a position of the BS  61  is found, at which the monitored beam is detected by the m-PD  64   a  and/or the m-PD  64   b  through the optical modulator  20 . In this step, two lenses,  63   a  and  63   b,  are uninstalled yet and the m-PDs,  64   a  and  64   b,  are practically activated. Because the light output from the t-LD  10  is already collimated by the first collimating lens  110   a,  the determination of the maximum of the monitored beam, that is, the position of the BS  61 , may be accomplished. 
     Among optical components set between the collimating lens  110   a and the input port  24  of the optical modulator  20 , the beam shifter  81 , the BS  61 , and the two lenses,  63   a  and  63   b,  may shift the optical axis. The optical alignment in the present embodiment, only the two lenses,  63   a  and  63   b,  are actively aligned in positions thereof to get the maximum coupling efficiency. Other components, namely, the beam shifter  81  and the BS  61 , have functions to roughly align the collimated beam L 1  in a position from which the fine alignment for the two lenses,  63   a  and  63   b,  becomes possible. 
     The process of aligning the first lens  63   a  at step S 5   a ( d ), places the first lens  63   a  in a designed positon but yet fixed there. Then, as practically activating the t-LD  10  and guiding the optical beam output from the first lens  63   a  to the optical modulator  20 . Sensing the monitored beam, M 2   a  or M 2   d,  by the m-PDs,  64   a  or  64   b,  the position of the first lens  63  is evaluated at which the sensed monitored beam becomes a maximum. Because no biases are supplied to the optical modulator  20 , two m-PDs,  64   a  and  64   b,  may sense the respective monitored beams, M 2   a  and M 2   d.  Subsequent to the evaluation of the desired position, the first lens  63   a  is fixed at a position slightly apart from the evaluated position along the optical axis of the input port  24 . An ultraviolet curable resin used for the fixation of the first lens  63   a  usually shrinks during the curing by several micron-meters, which may misalign the position of the first lens  63   a.  The second lens  63   b  may compensate this misalignment of the first lens  63   a.    
     The second lens  63   b  may be aligned as sensing the monitored beam, M 2   a  or M 2   d,  through the optical modulator  20 . Specifically, the second lens  63   b  is slid from the center of the designed position along longitudinally, laterally, and vertically as sensing the monitored beam, M 2   a  or M 2   d,  and is fixed by also an ultraviolet curable resin at the position at which the sensed monitored beam, M 2   a  or M 2   d,  becomes a maximum. Although the ultraviolet resin also shrinks during the curing, which causes deviations from the desirable position determined above, the second lens  63   b  has positional tolerance far greater than that of the first lens  63   a.  The first lens  63   a  has the tolerance only of sub-micron meters, while, the second lens  63   b  has the positional tolerance thereof far greater, two or three scores greater than that of the first lens  63   a.  Accordingly, the shrink of the ultraviolet curable resin during the curing is substantially negligible for the second lens  63   b.  Thus, the optical active alignment of the input unit  210  is completed. 
     Alignment of Output Unit 
     The process next assembles the output unit  230  of the modulator unit  200 . Because the input unit  210  accompanied with the laser unit  100  and the joint unit  220  is already aligned with the optical modulator  20 , the first CW light L 1  is practically input to the input port  24  and two output beams, M 2   b  and M 2   d,  are output from the output ports,  22   a  and  22   b,  by adjusting the biases to the offset electrodes,  51   j  to  54   j  and  51   k  to  54   k,  and the quadrature electrodes,  51   c  to  54   c.  Setting the special tool  91   d  at a position where the second lens  73   b  is to be placed, the first lens  73   a  is positioned such that the optical beam output from the first lens  73   a  becomes a collimated beam. Then, the first lens  73   a  is fixed in a position slightly closer to the optical modulator  20  (step S 5   b ( a )). Accordingly, the optical beam output from the first lens  73   a  becomes a dispersive beam. 
     In an alternative, the optical modulator  20  is set such that only one of the output ports, for instance, the output port  22   a,  generates the modulated beam M 2   b  by adjusting the biases supplied to the electrodes,  51   j  to  54   j,    51   k  to  54   k,  and  51   c  to  54   c.  The first lens  73   a  is aligned in a position thereof such that, as detecting the optical beam output from the first lens  73   a  at a far point through a window set in the first output port  3   a,  and an initial position of the first lens  73   a  is determined such that the output beam becomes a collimated beam. The first lens  73   a  is fixed in a point slightly closer to the optical modulator from the initial position along the optical axis of the first lens  73   a.  Because the PBC unit  76  is assembled on the carrier  300   a,  the output beam M 2   b  output from the output port  22   a,  which is offset from the optical axis of the first output port  3   a,  may be detected through the first output port  3   a  as passing through the PBC unit  76 . The other first lens  73   a  optically coupled with the output port  22   d  of the optical modulator  20  may be similarly aligned with the optical modulator  20  and fixed on the carrier  230   a.    
     The process (S 5   b ( b )) of aligning the second lens  73   b  will be described. The process first sets a dummy port on the first output port  3   a  of the housing  2 . The dummy port, which emulates the coupling unit  6  practically provided on the output ports,  3   a  and  3   b,  includes a coupling fiber and a concentrating lens that concentrates an optical beam entering therein onto the coupling fiber. An optical beam coupled to the coupling fiber may be detected from another end of the coupling fiber. 
     The process first aligns the second lens  73   b  to be set for the output beam M 2   b  output from the port  22   a.  Adjusting the biases supplied to the optical modulator  20 , the process sets the optical modulator  20  in a status at which only the output beam M 2   b  is output from the port  22   b  by eliminating the other beam M 2   c.  Sliding the second lens  73   b  in a plane in parallel to the carrier  230   a,  the initial position of the second lens  73   b  is evaluated at which the optical power detected through the coupling fiber in the dummy port becomes a maximum. Subsequently, procedures same as above described are performed for the other second lens  73   b.  That is, adjusting the biases supplied to the optical modulator  20 , the procedure sets the optical modulator  20  in the status where only the output beam M 2   c  is output from the port  22   b  by eliminating the other output beam M 2   b.  Then, adjusting the position of the second lens  73   b  for the other output beam M 2   c  and evaluating the position at which the maximum coupling efficiency is obtained for the coupling fiber by detecting the output power through the coupling fiber in the dummy port. Comparing the maximum output power obtained for the output beam M 2   b  with the maximum output power obtained for the other output beam M 2   c,  the output beam by which a greater output power is obtained is called as the primary beam, while, the other output beam showing a smaller output power is called as the subsidiary beam. The procedure then adjust the position of the second lens  73   b  of the primary beam such that the output detected through the dummy port becomes equal to the output power for the subsidiary beam. The second lens  73   b  for the primary beam is fixed thereat. The second lens  73   b  for the subsidiary beam is fixed at a position the output power detected through the coupling fiber becomes a maximum. Thus, two beams, i.e., the primary beam and the subsidiary beam, may couple with the dummy port in the same coupling coefficient, which is carried out in step S 5   b ( b ). 
     When the maximum output power for the subsidiary beam exceeds a designed power, which is primarily defined by the eye-softer for laser light, the second lens for the primary beam is positioned such that the output power detected through the dummy port becomes equal to the designed maximum and the second lens  73   b  for the subsidiary beam is also positioned such that the output power detected through the dummy port becomes equal to the designed maximum. 
     Finally, removing the dummy port from the output port  3   a  and setting the coupling unit  6  onto the first output port  3   a,  the alignment of the coupling unit  6  may be carried out as follows: that is, releasing the biases supplied to the optical modulator  20 , the two beams, M 2   b  and M 2   c,  output from the output ports,  22   a  and  22   b,  couple the coupling unit  6 . The coupling unit  6  is aligned such that the output power detected through the coupling fiber in the coupling unit  6  becomes a maximum. The coupling unit has a function to move the coupling fiber in a plane perpendicular to the optical axis thereof and in parallel to the optical axis. Accordingly, moving the coupling fiber relative to the concentrating lens in the coupling unit, the maximum coupling efficiency may be evaluated. 
     In an alternative, similar to the modified alignment procedures for the second lenses  73   b  described above, only one of the output beams, M 2   b  and M 2   c,  is coupled with the coupling unit  6  by adjusting the biases supplied to the optical modulator  20 , and the position of the coupling fiber relative to the collimating lens in the coupling unit is aligned such that the output power detected through the coupling fiber becomes equal to that obtained in the alignment process for the second lens  73   b.  When the coupling unit  6  is once aligned for the one of the output beams, M 2   b  and M 2   c ; the other of the output beams, M 2   b  and M 2   c,  may be automatically obtained because the second lens  73   b  for the other output beam is aligned such that the output power detected through the coupling fiber is equal to the one for the other output beam. 
     The reason why the second lenses  73   b  are independently adjusted in the positions thereof such that the output power detected through the coupling unit  6  becomes equal to each other is that the two output beams, M 2   b  and M 2   c,  have respective polarizations perpendicular to each other and each containing transmitting information of 0° and 90° independent to each other. Accordingly, when the output power of the two beams, M 2   b  and M 2   c,  show a large difference, the error rate contained within the transmission information drastically increases. 
     S 5   c : Alignment of Detector Unit 
     Before the alignment of the detector unit  300 , the process first aligns the second collimating lens  110   b  mounted on the base  100   a  of the laser unit  100  through the lens carrier  110 B. The procedure first activates the t-LD  10  and sets the special tool  91   d,  which is used in the alignment of the other collimating lens  110   a,  at a position where the first BS  32   a  is to be placed. The tool  91   d  carries the second CW light L 2  output from the back facet  10 B of the t-LD  10  out of the housing  2 . Similar to the alignment of the first collimating lens  110   a,  as monitoring the second CW light L 2  at a point apart from the housing  2 , and the process aligns the second collimating lens  110   b  in the point where the monitored CW light L 2  becomes a collimated beam. Finally, the second collimating lens  110   b  is fixed thereat by curing ultraviolet curable resin. 
     Then, the process aligns two BSs,  32   a  and  32   b.  First, as monitoring the second CW light L 2  by the first m-PD  34   a,  the first BS  32   a  is slid from a designed position along a direction in parallel to the optical axis of the second CW light L 2  output from the second collimating lens  110   b.  The first BS  32   a  is fixed at the position, slightly apart from a temporal position along the optical axis of the second CW light L 2 , at which the second CW light L 2  monitored by the first m-PD  34   a  becomes a maximum. The reason why the first BS  32   a  is slightly slid is that the second CW light L 2  reflected by the first BS  32   a  and entering the second m-PD  34   b  is refracted by the second BS  32   b.  The m-PD  34   a  is set at a position slightly offset from the optical axis of the second CW light L 2  because the second CW light L 2  passing through the first BS  32   a  and the etalon filter  33  is refracted thereby. During the alignment of the first BS  32  above, the process does not rotate the BS  32  because the second CW light L 2  is converted into a collimated beam having a relatively large field diameter. The second BS  32   b  is aligned as follows: the process first sets a dummy port, which has the same arrangement with that of the aforementioned dummy port utilized in the alignment process for the output unit  230  of the modulator unit  200 , on the second output port  3   b  of the housing  2 . The second BS  32   b  is aligned such that the optical beam reflected by the second BS  32   b  and detected through the coupling fiber in the dummy port becomes a maximum. 
     The optical module  1  may replace the BSs,  32   a  and  32   b,  of the parallel plate type with those of the prism type. A BS of the prism type sticks two optical prisms and has a cubic plane shape. The optical alignment of the BS of the prism type may be accomplished by the same procedures with those above described for the parallel plate type. That is, without performing the rotational alignment of the prism BS, the first and second BSs are aligned as sliding parallel and perpendicular to the optical axis of the second CW light L 2  output from the second collimating lens  110   b  to find respective positions at which the optical power detected through the dummy port becomes a maximum. A BS with the prism type inherently has a medium split ratio of about 10:90; that is, 10% of the incident beam may transmit the BS, and the rest 90% thereof may be reflected. Accordingly, the optical output power available at the second output port  3   b  is reduced to 80% of the optical beam just output from the t-LD  10 . On the other hand, a BS with the parallel plate type shows a split ratio of about 5:95, 5% of the incident beam transmits but the rest 95% is reflected. Accordingly, the optical output power available at the second output port  3   b  becomes 90% of that of the optical beam just output from the back face  10 B of the t-LD  10 , which is about 10% greater than that available for the BSs for the prism type. The dummy port set on the second output port  3   b  is replaced by the coupling unit having the arrangements same with those of the coupling unit as aligning the coupling unit on the second output port  3   b  so as to recover the optical coupling efficiency between the second BS  32   b  with the coupling port. 
     S 6 : RF Wiring 
     Finally, the process of assembling the optical module  1  performs the wiring from the RF terminals  4  in the rear wall  2 B to the signal pads,  41  to  44 , on the optical modulator  20 . However, the wiring for the RF pads,  41  to  44 , may be carried out concurrently with the wiring for the DC terminals,  5   a  and  5   b.  Ceiling the housing  2 , the process of assembling the optical module  1  is completed. 
     Modification 
     The process thus described has an order to assemble respective units,  100  to  300 , from the laser unit  100 , the input unit  210 , the output unit  230 , and the detector unit  300 . However, the process is not restricted to this order. The alignment of the detector unit  300  may be carried out just after the alignment of the laser unit  100  before the process of aligning the modulator unit  200 . Only the limited order is that the alignment of the input unit  210  is necessary to be done before the alignment of the output unit  230 , because the latter alignment uses the optical beams, M 2   b  and M 2   c,  output from the optical modulator  20 , and these beams, M 2   b  and M 2   c,  derive from the first CW light provided from the input unit  210 . 
     The optical module  1 , as described, installs the laser unit  100 , the modulator unit  200 , and the detector unit  300  within one housing  2 , which results in a complex arrangement within the housing  2 . However, an optical coherent transceiver implementing the optical module  1  of the invention may simplify the arrangement thereof. Such a coherent optical transceiver is at least unnecessary to install an optical source independently. Also, the optical alignment process between the units becomes unnecessary when the coherent optical transceiver installs the optical module  1  of the invention. 
     The optical module  1  thus described provides TECs,  11  to  31 , independent for the laser unit  100 , the modulator unit  200 , and the detector unit  300 . Accordingly, the respective units,  100  to  300 , may be precisely controlled in temperatures thereof depending on calorific amounts of respective units,  100  to  300 . The emission wavelength of the t-LD  10  may be precisely controlled independent of the temperatures of the optical modulator  20  and that of the detector unit  300 . The optical modulator  20  may be optionally controlled in the operation thereof. The detector unit  300  may precisely determine the emission wavelength of the t-LD  10 . 
     The optical alignment of the collimating lenses,  110   a  and  110   b,  utilizes the special tool  91   d  that takes the optical beams output from the t-LD  10  out of the housing  2 , which enables to determine the positions of the collimating lenses at which the optical beams output from the respective lenses,  110   a  and  110   b,  become collimated beams. Also, the input unit  210  provides the two-lens system to couple the first CW light L 1  with the input port  24  of the optical modulator  20 . The two-lens system may compensate the deviation inherently caused during the solidification of the ultraviolet curable resin. 
     The optical modulator  20  of the embodiment provides the monitor ports,  25   a  and  25   b,  that output the monitored beams, M 2   a  and M 2   d,  respectively, which are split from the output beams, M 2   b  and M 2   c.  Accordingly, the monitored beams, M 2   a  and M 2   d,  may be served for the active alignment of the optical components in the input unit  210 . 
     In the foregoing detailed description, the method and module of the present invention have been described with reference to specific exemplary embodiments thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the present invention. The present specification and figures are accordingly to be regarded as illustrative rather than restrictive.