Abstract:
An optical receiver comprises a package provided with an input window; a polarization-maintaining optical fiber fixable to the input window; a polarization beam splitter, disposed on the package, for inputting light outputted from the polarization-maintaining optical fiber and outputting first output light and second output light having respective polarization directions different from each other; a beam splitter, disposed on the package, for splitting the first output light; a first light-receiving element, optically coupled to the beam splitter, having two light-receiving parts corresponding to two kinds of the output light split by the beam splitter; and a second light-receiving element, disposed on the package, for receiving the second output light.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a Divisional of U.S. patent application Ser. No. 14/310,846, filed Jun. 20, 2014, which claims the benefit of Japanese Patent Application No. 2013-132042, filed Jun. 24, 2013. 
    
    
     TECHNICAL FIELD 
     The present invention relates to an optical receiver and an optical axis alignment method thereof. 
     BACKGROUND 
     Coherent optical communication systems have been known as high-speed, large-capacity optical communication systems. An optical receiver of a coherent optical communication system separates signal light inputted from a single-mode optical fiber into X-polarized light and Y-polarized light orthogonal to each other by using a polarization beam splitter (PBS). Thereafter, an optical hybrid makes the signal light interfere with local oscillator light (LO light) inputted from a polarization-maintaining optical fiber. Thereafter, a light-receiving element (e.g., photodiode (PD)) arranged dower stream of the optical hybrid converts an optical signal into an electric signal. 
     Known as a method for aligning an optical axis of the polarization-maintaining optical fiber is one comprising inserting a polarizer between the light-receiving element and the polarization-maintaining optical fiber and adjusting an angle of the polarization-maintaining optical fiber such that the light-receiving element attains the maximum output current in this state. For suppressing light reflection, an end face of the polarization-maintaining optical fiber may form a predetermined angle (e.g. about 8°) with a plane perpendicular to the optical axis. 
     SUMMARY 
     Conventional optical receivers have a problem that the angle of the polarization-maintaining optical fiber is hard to adjust accurately when aligning the optical axis thereof. 
     In view of the above-mentioned problem, it is an object of the present invention to provide an optical receiver and its optical axis alignment method which can increase the accuracy of optical axis alignment in the polarization-maintaining optical fiber. 
     The optical information processing unit in accordance with one aspect of the present, invention comprises a first optical input; a second optical input; a polarization beam splitter receives the first optical input, and outputs first and second output lights having polarization direction different from each other; an optical processing unit receives the first output light and the second optical input aligned with a polarization direction of the first output light, and produces an information signal from the first output light and the second optical input; an optical-electrical signal converter receives the second output light. 
     In the above-mentioned structure, the polarization beam splitter is inputting light outputted from a polarization-maintaining optical fiber. 
     In the above-mentioned structure, furthering another optical-electrical signal converter receives the first output light. 
     In the above-mentioned structure, optical-electrical signal converter is coupled directly to the polarization beam splitter with no optical element interposed therebetween. 
     In the above-mentioned structure, the polarization-maintaining optical fiber fixable to input window in a package, an end face on the package side of the polarization-maintaining optical fiber is tilted by an angle of at least 4 but not more than 12° with respect to an optical axis. 
     In the above-mentioned structure, the optical processing unit is an optical hybrid another optical-electrical signal converter photoelectrically convert the interference light outputted from the optical hybrid. 
     In the above-mentioned structure, the package provided with another input window, a no polarization-maintaining optical fiber fixable to the another input window. 
     In the above-mentioned structure, furthering the optical hybrid for inputting light outputted from the no polarization-maintaining optical fiber fixable. 
     In the above-mentioned structure, furthering a transimpedance amplifier, combining and amplifying paired positive and negative components of electric signals outputted from another optical-electrical signal convener. 
     In the above-mentioned structure, furthering an external local oscillator light is inputted into the polarization-maintaining optical fiber fixable. 
     The optical axis alignment method of an optical receiver in accordance with one aspect of the present invention is an optical axis alignment method of an optical receiver comprising a package provided with an input window; a polarization-maintaining optical fiber fixable to the input window; comprises a first step of adjusting an angle of the polarization-maintaining optical fiber according to results of detection by an optical-electrical signal converter; a second step of removing the polarization beam splitter and the optical-electrical signal converter from the package while keeping the angle of the polarization-maintaining optical fiber. 
     In the above-mentioned optical axis alignment method, furthering a polarization beam splitter for inputting light outputted from the polarization-maintaining optical fiber and outputting first output light as TE-polarized light and second output light as TM-polarized light; a third step of mounting a polarizer for inputting the TE-polarized light outputted from the polarization-maintaining optical fiber and another optical-electrical signal converter for receiving output light from the polarizer onto the package while keeping the angle of the polarization-maintaining optical fiber; and a fourth step of aligning optical axes of the polarization-maintaining optical fiber, polarizer, and another optical-electrical signal converter while keeping the angle of the polarization-maintaining optical fiber. 
     In the above-mentioned optical axis alignment method, the first step of a light being inputted in the polarization maintaining optical fiber, that has an external local oscillator light. 
     The optical axis alignment method of an optical receiver in accordance with another aspect of the present invention is an optical axis alignment method of an optical receiver comprising a package provided with first and second input windows; a polarization-maintaining optical fiber fixable to the second input window; comprises a first step of adjusting an angle of the polarization-maintaining optical fiber according to results of detection by the optical-electrical signal converter; a second step of removing the polarization-maintaining optical fiber from the second input window and fixing the polarization-maintaining optical fiber to the first input window while keeping the angle of the polarization-maintaining optical fiber; and a third step of aligning optical axes of the polarization-maintaining optical fiber, polarizer, and optical-electrical signal converter while keeping the angle of the polarization-maintaining optical fiber. 
     In the above-mentioned optical axis alignment method, the second step adjusts the angle of the polarization-maintaining optical fiber such that an amount of light received by the optical-electrical signal converter element is at least a first threshold while an amount of light received by another optical-electrical signal converter is not greater than a second threshold. 
     In the above-mentioned optical axis alignment method, furthering a polarization beam splitter for inputting light outputted from the polarization-maintaining optical fiber and outputting first output light as TE-polarized light and second output light as TM-polarized light; the TE-polarized light outputted from the polarization-maintaining optical fiber has an angle of polarization of 10° or less. 
     In the above-mentioned optical axis alignment method, the second step of moving the polarization-maintaining optical fiber from the second input window by sliding the polarization-maintaining optical fiber while keeping its angle. 
     In the above-mentioned optical axis alignment method, the first step of a light being inputted in the polarization-maintaining optical fiber, that has an external local oscillator light. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an overall block diagram of an optical receiver in accordance with first to third embodiments; 
         FIG. 2  is a schematic view illustrating, a process of optical axis alignment of the optical receiver in accordance with the first embodiment; 
         FIG. 3A  is an enlarged view of a junction part of a polarization-maintaining optical fiber in the optical receiver in accordance with the first embodiment; 
         FIG. 3B  is an enlarged view of the junction part of the polarization-maintaining optical fiber in the optical receiver in accordance with the first embodiment; 
         FIG. 4  is a flowchart illustrating an optical axis alignment method of the optical receiver in accordance with the first embodiment; 
         FIG. 5A  is a graph illustrating, changes in amounts of light received by light-receiving elements; 
         FIG. 5B  is a graph illustrating changes in amounts of light received by the light-receiving elements; 
         FIG. 6  is a schematic view illustrating an inner structure of an optical receiver in accordance with a comparative example; 
         FIG. 7  is a flowchart illustrating an optical axis alignment method of the optical receiver in accordance with the comparative example; 
         FIG. 8  is a schematic view illustrating an inner structure of an optical receiver in accordance with a second embodiment; 
         FIG. 9A  is a schematic view illustrating a process of optical axis alignment of the optical receiver in accordance with the second embodiment; 
         FIG. 9B  is a schematic view illustrating the process of optical axis alignment of the optical receiver in accordance with the second embodiment; 
         FIG. 10  is a flowchart illustrating an optical alignment method of the optical receiver in accordance with the second embodiment; 
         FIG. 11  is a schematic view (part 1) illustrating a process of optical axis alignment of the optical receiver in accordance with a third embodiment; 
         FIG. 12  is a schematic view (part 2) illustrating the process of optical axis alignment of the optical receiver in accordance with the third embodiment; and 
         FIG. 13  is a flowchart illustrating an optical alignment method of the optical receiver in accordance with the third embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     In the following, embodiments of the present invention will be explained. 
       FIG. 1  is an overall block diagram of the optical receiver in accordance with the first to third embodiments. This optical receiver  100  is an optical receiver used for coherent optical communications. The optical receiver  100  comprises an optical signal processing unit  20  for processing optical signals and an electric signal processing, unit  40 , disposed downstream of the optical signal processing unit  20 , for processing electric signals. 
     The optical signal processing unit  20  includes a polarization beam splitter (PBS)  22 , a beam splitter (BS)  24 , a polarization rotator  26 , optical hybrids(optical processing unit) 28   x ,  28   y , light-receiving units  30 , and amplifiers  32  (amplifiers  32   x ,  32   y  illustrated in  FIG. 2 ). The optical signal processing unit  20  further includes optical elements such as skew adjustment elements, lenses, mirrors, and polarizers (each illustrated in  FIG. 2 ) which are omitted in  FIG. 1 . The electric signal processor  40  includes an analog-digital converter (ADC)  42  and a digital signal processor (DSP)  44 . 
     The polarization beam splitter  22  splits signal light (SIGNAL) introduced by a single-mode optical fiber (SMF)  12  into X-polarized light and Y-polarized light which are orthogonal to each other. The X-polarized light enters the optical hybrid  28   y  on the Y side, while the Y-polarized light has its plane of polarization rotated 90° by the polarization rotator  26  so as to become X-polarized light, which then enters the optical hybrid  28   x  on the X side. For example, TM light and TE light may be used as the X-polarized light and Y-polarized light, respectively, or vice versa. 
     The beam splitter  24  splits local oscillator light (LO light) inputted by a polarization-maintaining optical fiber (PMF)  10  from an external local oscillator light source device  14  into X and Y sides. The local oscillator light (LO light) is preset to X-polarized light and enters the optical hybrids  28   x ,  28   y  on the X and Y sides. 
     Each of the optical hybrids  28   x ,  28   y  causes the incident signal light and local oscillator light to interfere with each other in the internal light circuit and outputs the resulting interference light from four ports. Each of the optical hybrids  28   x ,  28   y  may be constituted by a silica-based planar lightwave circuit (PLC), for example. The X-polarized signal light (SIGNAL) is combined with the local oscillator light (LO) in the optical hybrid  28   x  and then is split, into positive (p) and negative (n) in-phase I and quadrature Q components, which are outputted as four optical signals (X-Ip, X-In, X-Qp, X-Qn). Similarly, the Y-polarized signal light is combined with the local oscillator light (LO light) in the optical hybrid  28   y  and then is split into positive (p) and negative (n) in-phase I and quadrature Q components, which are outputted as four optical signals (Y-Ip, Y-in, Y-Qp, Y-Qn). 
     The light-receiving elements  30  photoelectrically convert the interference light(information signal) outputted from the optical hybrids  28   x ,  28   y  into analog electric signals. Each light-receiving element.  30  includes a photodiode (PD), for example. The amplifiers  32  combine and amplify paired positive and negative components of electric signals outputted from the light-receiving elements  30 . Each amplifier  32  includes a transimpedance amplifier (TIA), for example. The amplified electric signals are outputted from electric output terminals of the optical signal processing unit  20  and inputted to the electric signal processing unit  40 . 
     In the electric signal processor  40 , the analog-digital converters  42  convert the analog electric signals outputted from the optical signal processing unit  20  into digital signals by analog-digital conversion. The digital signal processor  44  subjects the converted digital signals to various kinds of signal processing such as signal demodulation. The foregoing processing can perform digital coherent communications. 
     First Embodiment 
       FIG. 2  is a schematic view illustrating an inner structure of the optical receiver in accordance with the first embodiment.  FIG. 2  illustrates only a structure corresponding, to the optical signal processing unit  20  while omitting the electric signal processing unit  40 . Members in common with  FIG. 1  are referred to with the same signs while omitting their overlapping descriptions. 
     As illustrated in  FIG. 2 , a package  50  containing the optical signal processing, unit  20  is formed with two input windows  52 ,  54  for connecting and fixing optical cables. In the following, of the two input windows  52 ,  54 , one which fixes the polarization-maintaining optical fiber  10  and introduces the local oscillator light (LO light) will be referred to as first input window  52 , and the other which fixes the single-mode optical fiber  12  and introduces the signal light (SIGNAL) as second input window  54 . In addition to the structure explained with  FIG. 1 , skew adjustment elements  56   x ,  56   y , mirrors  58   x ,  58   y , lenses  60   a  to  60   d , and a light regulation unit  70  are arranged within the package  50 . The light regulation unit  70  is constituted by a polarization beam splitter  72  and a light-receiving element  74 (an optical-electrical signal converter). 
     The polarization-maintaining optical fiber  10  is fixed to the first input window  52 . The TE light emitted from the polarization-maintaining optical fiber  10  is converted into collimated light by a lens  86  within a lens holder  84  and then enters the polarization beam splitter  72  within the package  50 . The light having advanced straight through the polarization beam splitter  72  becomes TE light, which is inputted to the beam splitter  24 . The light reflected 90° by the polarization beam splitter  72  becomes TM light, which is inputted to the light-receiving element  74 . The TE light emitted from the polarization-maintaining optical fiber  10  may have an angle of polarization of 10° or less, for example. 
     The light having advanced straight through the beam splitter  24  passes through the skew adjustment element  56   x  and is condensed by the collimator lens  60   a , so as to enter the optical hybrid  28   x  on the X side. The structure indicated by signs  28   x ,  30   x  in the drawing is one integrating the optical hybrid  28   x  and light-receiving element  30   x  on the X side. 
     On the other hand, the light reflected 90° by the beam splitter  24  is reflected 90° again by the mirror  58   y  and then condensed by the collimator lens  60   c , so as to enter the optical hybrid  28   y  on the Y side. The structure indicated by signs  28   y ,  30   y  in the drawing is one integrating the optical hybrid  28   y  and light-receiving element  30   y  on the Y side. 
     The single-mode optical fiber  12  is fixed to the second input window  54 . The light emitted from the single-mode optical fiber  12  is converted into collimated light by a lens  83  within a lens holder  81  and then enters the polarization beam splitter  22  within the package  50 . The light entering the polarization beam splitter  22  has random polarization directions (indicated by Rand). The light having advanced straight through the polarization beam splitter  22  becomes TE light, which passes through the skew adjustment element  56   y  and then is condensed by the collimator lens  60   d , so as to enter the optical hybrid  28   y  on the Y side. 
     On the other hand, the light reflected 90° by the polarization beam splitter  22  once becomes TM light, but is converted into YE light by passing through the polarization rotator  26 . It is reflected 90° again by the mirror  58   x  and then condensed by the collimator lens  60   b , so as to enter the optical hybrid  28   x  on the X side. 
     Of the local oscillator light LO coming from the polarization-maintaining optical fiber  10 , the part entering the Y-side optical hybrid  28   y  has an optical path length longer than that of the part entering the X-side optical hybrid  28   x  by the distance between the beam splitter  24  and the mirror  58   y . Therefore, the skew adjustment element  56   x  arranged on the X side delays the phase of light on the X side such that the local oscillator light has the same optical path length on the X and Y sides. 
     Of the signal light coming from the single-mode optical fiber  12 , the part entering the X-side optical hybrid  28   x  has an optical path length longer than that of the part entering the Y-side optical hybrid  28   y  by the distance between the polarization beam splitter  22  and the mirror  58   x . Therefore, the skew adjustment element  56   y  arranged on the Y side delays the phase of light on the Y side such that the signal light has the same optical path length on the X and Y sides. As the skew adjustment elements  56   x ,  56   y , materials (e.g., glass) having a refractive index higher than that of air may be used. 
       FIG. 3A  is a schematic view of a junction part between the polarization-maintaining optical fiber  10  and the package  50 , while  FIG. 3B  is a diagram enlarging the leading, end of the polarization-maintaining optical fiber  10 . In  FIG. 3A , the optical axis direction of the polarization-maintaining optical fiber  10  is defined as Z direction, a direction perpendicular to the Z direction and parallel to the bottom face of the package  50  as X direction, and a direction (from the front side to rear side of the paper) perpendicular to the X and Z directions as Y direction. On the other hand, θ is the angle of rotation about the Z axis, and φ is the angle of rotation about the X axis. 
     As illustrated in  FIG. 3A , the polarization-maintaining optical fiber  10  comprises a ferrule  80 , a ferrule holder  82 , and the lens holder  84 . The lens holder  84  contains the lens  86  for converting the light emitted from the polarization-maintaining optical fiber  10  into collimated light. As illustrated in  FIG. 3B , the ferrule  80  is constituted by an inner ceramic part  80   a  covering a coated fiber  88  of the polarization-maintaining optical fiber  10  and an outer metal part  80   b  covering the ceramic part  80   a . The leading end part of the ferrule  80  is contained in the ferrule holder  82 . An end face  11  of the leading end part of the polarization-maintaining optical fiber  10  is tilted with respect to a plane perpendicular to an optical axis L. The tilt angle α may be at least 4° but not more than 12°, for example. 
     Here, the ferrule  80  has degrees of freedom of adjustment in the Z, θ, and φ axes. The ferrule holder  82  and lens holder  84  have degrees of freedom of adjustment in the X and Y axes. Thus, the polarization-maintaining optical fiber  10  has various degrees of freedom of adjustment for fixing the same, whereby angular shifts may occur in the process of aligning the optical axis of the polarization-maintaining optical fiber  10 . 
       FIG. 4  is a flowchart illustrating the optical axis alignment method of the optical receiver in accordance with the first embodiment. First, laser light is let into the package  50  from the polarization-maintaining optical fiber  10  (step S 10 ). Here, as illustrated in  FIG. 3 , the polarization beam splitter  72  disposed upstream of the beam splitter  24  is constructed such as to transmit TE light therethrough but reflect TM light by 90°. In the output light outputted from the polarization beam splitter  72 , TE light and TM light correspond to the first output light and second output light, respectively, in this embodiment. The TE light as the first output light is received by the downstream light-receiving elements  30   x ,  30   y , while the TM light as the second output light is received by the downstream light-receiving element  74 . In this embodiment, the light-receiving elements  30   x ,  30   y  receiving the first output light TE correspond to the first light-receiving element, while the light-receiving element  74  receiving the second output light TM corresponds to the second light-receiving element. 
     Next, as illustrated in  FIG. 4 , the position and angle of the polarization-maintaining optical fiber  10  are adjusted according to the results of detection by the first light-receiving elements  30   x ,  30   y  and second light-receiving element  74  (step S 12 ). 
       FIGS. 5A and 5B  are graphs illustrating changes in amounts of light received by the light-receiving elements.  FIG. 5A  illustrates the relationship between angular shift and PD current for each of the first and second light-receiving elements and their total. The PD current is a current outputted from the light-receiving element and increases as the amount of received light is greater.  FIG. 5B  illustrates the relationship between optical axis shift and PD current for the amount of light received by each light-receiving element. 
     In the first light-receiving element receiving the TE light, as illustrated in  FIG. 5A , the PD current value becomes greater and smaller as the angular shift is smaller and greater, respectively. In the second light-receiving element receiving the TM light, on the other hand, the PD current value becomes smaller and greater as the angular shift is smaller and greater, respectively. The total of the PD current values in the first and second light-receiving elements is always constant. 
     In view of the foregoing, it is preferred for the angle of the polarization-maintaining, optical fiber  10  to be adjusted such as to maximize and minimize the PD currents (received light amounts) in the first and second light-receiving elements, respectively, thereby making the angular shift as small as possible. The angle of the polarization-maintaining optical fiber  10  may be adjusted such that the PD currents received light amounts) in the first and second light-receiving elements become at least a first threshold and not greater than a second threshold, respectively. The first and second thresholds may be predetermined fixed values or variable values which are changed as appropriate. 
     Letting θ(°) be the angular shift from the TE-polarized light, the transmittance of optical power is cos 2 θ and sin 2 θ for rectilinear light and reflected light, respectively. When θ=5, the transmittance of optical power is 0.99 and 0.01 for the rectilinear light. (cos 2 θ) and reflected light (sin 2 θ), respectively. When θ=10, the transmittance of optical power is 0.97 and 0.03 for the rectilinear light (cos 2 θ) and reflected light (sin 2 θ), respectively. When θ=20, the transmittance of optical power is 0.88 and 0.12 for the rectilinear light (cos 2 θ) and reflected light (sin 2 θ), respectively. 
     In the graph of  FIG. 5B , on the other hand, each of the first and second light-receiving elements and their total lowers the PD current value as the amount of optical axis shift is greater. Therefore, when correcting the optical axis shift, it is preferred for adjustment to be done such that both of the PD currents (received light amounts) in the first and second light-receiving elements become the maximum values, thereby making the optical axis shift as small as possible. 
     Referring to  FIG. 4 , after the adjustment of the position and angle of the polarization-maintaining optical fiber  10  is completed, each member is fixed by YAG welding while keeping the position and angle (step S 14 ). Here, parts  76   a  to  76   f  illustrated in  FIG. 3A  are fixed by the YAG welding. The ferrule  80  and the ferrule holder  82  are fixed at the parts  76   a ,  76   b , the ferrule holder  82  and the lens holder  84  at the parts  76   c ,  76   d , and the lens holder  84  and the package  50  at the parts  76   e ,  76   f . The forgoing fixing deprives the ferrule  80 , ferrule holder  82 , and lens holder  84  of degrees of freedom of adjustment, whereby the polarization-maintaining optical fiber  10  can be fixed at the predetermined angle and position. 
     The optical axis alignment of the polarization-maintaining optical fiber  10  in an optical receiver in accordance with a comparative example will now be studied. 
       FIG. 6  is a schematic view illustrating an inner structure of the optical receiver in accordance with the comparative example. In the comparative example, a polarizer  160  is arranged in place of the light regulation unit  70  in the first embodiment in front of the beam splitter  24 . The polarizer  160  is set such as to cut off light other than the TE-polarized component inputted from the polarization-maintaining optical fiber  10 . The optical receiver in accordance with the comparative example is equipped with the first light-receiving elements  30   x ,  30   y  for monitoring the TE light, but not the second light-receiving element (light-receiving element  74  of  FIG. 2 ) for monitoring the TM light. The remaining structure, is common with the first embodiment ( FIG. 2 ) and thus will not be explained in detail. 
       FIG. 7  is a flowchart illustrating the optical axis alignment method of the optical receiver in accordance with the comparative example. First, as in the first embodiment, laser light is let into the package  50  from the polarization-maintaining optical fiber  10  (step S 20 ). Subsequently, while monitoring the light-receiving elements, the position and angle of the polarization-maintaining optical fiber  10  are adjusted according to the results of detection by the monitoring (step S 22 ). After the adjustment of the position and angle of the polarization-maintaining optical fiber  10  is completed, each member is fixed by YAG welding while keeping the position and angle (step S 24 ) as in the first embodiment. 
     When the position or angle of the polarization-maintaining optical fiber  10  shifts in the comparative example, light other than the TE-polarized component is cut off by the polarizer  160 , whereby optical losses occur in the light-receiving elements  30   x ,  30   y  according to the magnitude of the angular shift. Specifically, letting θ(°) be the angular shift from the TE-polarized light, the transmittance of optical power is cos 2 θ. When θ=5, the transmittance of optical power is cos 2 θ=0.99. When θ=10, the transmittance of optical power is cos 2 θ=0.97. When θ=20, the transmittance of optical power is cos 2 θ=0.88. Thus, monitoring the light-receiving elements  30   x ,  30   y  makes it possible to detect shifts in the position or angle of the polarization-maintaining optical fiber. Adjusting, the position and angle of the polarization-maintaining optical fiber  10  such as to maximize the received light amounts (PD currents) in the light-receiving elements  30   x ,  30   y  can maximize the amount of TE light incident on the optical hybrid  28   x.    
     However, as illustrated in  FIGS. 5A and 5B , the PD current in the first light-receiving element may be lowered by any of angular and optical axis shifts. Therefore, the comparative example equipped with no second light-receiving element is hard to determine whether the decrease in PD current is caused by an angular shift or optical axis shift. As a result, even if the polarization-maintaining optical fiber is fixed at a location where the PD currents of the light-receiving elements are maximized, a large angular shift (e.g., 20°) may occur, which makes it hard to align optical axes accurately. 
     When the end face  11  of the polarization-maintaining optical fiber  10  is tilted as illustrated in  FIG. 3B  in particular, light is emitted from the polarization-maintaining optical fiber  10  in a direction oblique to the optical axis L. Therefore, when the polarization-maintaining optical fiber  10  rotates, the light emission direction changes greatly, whereby the optical axis alignment becomes more difficult. 
     By contrast, the optical receiver in accordance with this embodiment can accurately calculate the magnitude of angular shift from the output ratio of PD currents in the first and second light-receiving elements. As a result, the angular shift at the time of adjustment can be reduced. For example, the angular shift, which is up to about 20° in the comparative example, can be suppressed to 10° or less in this embodiment. In terms of optical loss, it can greatly be improved from 12% in the comparative example to about 3% in this embodiment. 
     In view of the foregoing, the optical receiver and its optical axis alignment method in accordance with this embodiment can enhance the accuracy in optical axis alignment of the polarization-maintaining optical fiber  10 . 
     Second Embodiment 
       FIG. 8  is a schematic view illustrating an inner structure of the optical receiver in accordance with the second embodiment. In the second embodiment, a polarizer  60  is arranged in place of the light regulation unit  70  in the first embodiment in front of the beam splitter  24 . The polarizer  60  is set such as to cut off light other than the TE-polarized component inputted from the polarization-maintaining optical fiber  10 . The remaining structure is common with the first embodiment ( FIG. 2 ) and thus will not be explained in detail. 
       FIGS. 9A and 9B  are schematic views illustrating the process of optical axis alignment of the optical receiver in accordance with the second embodiment.  FIG. 9A  is a schematic view illustrating the state at the time of starting the optical axis alignment, in which a light regulation unit  90  is provided within the package  50 . The light regulation unit  90  is constituted by a polarization beam splitter  92  and two light-receiving elements  94 ,  96 . The polarization beam splitter  92  is set such as to allow TE light to advance straight therethrough and reflect TM light by 90°. In the output light from the polarization beam splitter  92 , the TE light as the first output light is inputted to the first light-receiving element.  94 , while the TM light as the second output light is inputted to the second light-receiving element  96 . The first and second light-receiving elements  94 ,  96  are arranged equidistantly from the polarization beam splitter  92 . 
       FIG. 9B  is a schematic view illustrating the state at the end of optical axis alignment. In  FIG. 9B , a dummy element with the light regulation unit  90  in  FIG. 9A  is removed and replaced with an actual element illustrated in  FIG. 8 . The other actual elements are not depicted in  FIG. 9B . 
       FIG. 10  is a flowchart illustrating the optical axis alignment method of the optical receiver in accordance with the second embodiment. By the time of starting the optical axis alignment, the dummy element with the light regulation unit  90  has already been arranged. First, laser light is let into the package  50  from the polarization-maintaining optical fiber  10  (step S 30 ). Subsequently, while monitoring the first and second light-receiving elements  94 ,  96 , the position and angle of the polarization-maintaining optical fiber  10  are adjusted according to the results of detection by the first and second light-receiving elements  94 ,  96  (step S 32 ). These are the same as with the optical axis alignment method of the optical receiver in accordance with the first embodiment. 
     Next, the dummy element with the light regulation unit  90  is removed (step S 34 ). Thereafter, the dummy element is replaced with the actual element (step S 36 ). The steps S 34  and S 36  keep the angle of the polarization-maintaining optical fiber  10  determined by the step S 32 . After the mounting of the actual element is completed, optical axes of the actual elements of the polarization-maintaining optical fiber  10  are aligned (step S 38 ). At this time, the position of the polarization-maintaining optical fiber  10  or the positions or angles of the other optical elements may be changed, but the angle of the polarization-maintaining optical fiber  10  is kept unchanged. 
     The optical receiver and its optical axis alignment method in accordance with the second embodiment, adjust the angle of the polarization-maintaining optical fiber  10  by using the dummy element at the time of assembling and replace the dummy element with the actual element while keeping the angle. This can suppress the angular shift in the polarization-maintaining optical fiber  10  and improve the accuracy in optical axis alignment as with the first embodiment. 
     The structure in accordance with the second embodiment finally removes the dummy light regulation unit including the polarization beam splitter  92  and thus can make the number of components smaller than that in the first embodiment, thereby cutting cost down. In the structure in accordance with the second embodiment, the first and second light-receiving elements  94 ,  96  are arranged equidistantly from the polarization beam splitter  92 . This can reduce the difference in light-receiving sensitivity between two light-receiving elements  94 ,  96 , thereby making it possible to detect the angular shift of the polarization-maintaining optical fiber  10  more accurately. 
     Third Embodiment 
       FIGS. 11 and 12  are schematic views illustrating the process of optical axis alignment of the optical receiver in accordance with the third embodiment.  FIGS. 11 and 12  illustrate respective states at the start and end of optical axis alignment. The inner structure of the package  50  in  FIGS. 11 and 12  is common with the second embodiment ( FIG. 8 ). 
     As illustrated in  FIG. 11 , the polarization-maintaining optical fiber  10  is fixed to the second input window  54  at the time of starting the optical axis alignment. The second input window  54  is the one to which the single-mode optical fiber  12  is supposed to be fixed. TE light coming from the polarization-maintaining optical fiber  10  is inputted to the polarization beam splitter  22 . As mentioned above, the polarization beam splitter  22  is set such as to allow TE light to advance straight therethrough and reflect TM light by 90°. Therefore, the TE light as the first output light from the polarization beam splitter  22  is inputted to the light-receiving element  30   y  through the skew adjustment element  56   y  and lens  60   d . In this embodiment, the light-receiving element  30   y  functions as the first light-receiving element for receiving the first output light. 
     The TM light as the second output light from the polarization beam splitter  22  is inputted to the light-receiving element  30   x  by way of the polarization rotator  26 , mirror  58   x , and lens  60   b . In this embodiment, the light-receiving element  30   x  functions as the second light-receiving element for receiving, the second output light. 
       FIG. 13  is a flowchart illustrating the optical axis alignment method of the optical receiver in accordance with the third embodiment. First, laser light is let in from the polarization-maintaining optical fiber  10  fixed to the second input window  54  (step S 40 ). Subsequently, the first and second light-receiving elements  30   x ,  30   y  are monitored, and the angle of the polarization-maintaining optical fiber  10  is adjusted according to the results of detection by the monitoring (step S 42 ). 
     Next, as illustrated in  FIG. 12 , the polarization-maintaining, optical fiber  10  is removed from the second input window  54  and fixed to the first input window  52  to which it is supposed to be fixed (step S 44 ). At this time, the polarization-maintaining optical fiber  10  is moved by sliding while keeping its angle. After completing the movement of the polarization-maintaining optical fiber  10 , optical axes of the actual element of the polarization-maintaining, optical fiber  10  are aligned (step S 46 ). At this time, the position of the polarization-maintaining optical fiber  10  or the positions or angles of the other optical elements may be changed, but the angle of the polarization-maintaining optical fiber  10  is kept unchanged. 
     The optical receiver and its optical axis alignment method in accordance with the third embodiment can monitor the TE tight and TM light by utilizing the polarization beam splitter  22  and light-receiving elements  30   x ,  30   y , which constitute an optical system on the signal light side. This can suppress the angular shift of the polarization-maintaining optical fiber  10  and improve the accuracy in optical axis alignment as with the first embodiment. 
     The structure in accordance with the third embodiment does not use constituents such as the light regulation unit  70  in the first embodiment and the dummy light regulation unit  90  in the second embodiment and thus can thither cut the manufacturing cost down as compared with the first and second embodiments. 
     While the first to third embodiments set forth TE light, as an example of the local oscillator light (LO) introduced from the polarization-maintaining optical fiber  10 , TM light can be used in place of the TE light. 
     While embodiments of the present invention are explained in detail in the foregoing, the present invention is not limited to such specific embodiments but can be modified and altered within the scope of the gist thereof set forth in claims.