JP2008046105A - Apparatus and method of measuring stokes parameter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an apparatus and a method of measuring a Stokes parameter capable of highly accurate measurement. <P>SOLUTION: The apparatus includes a polarization diverging device including an optical element formed of a birefringent crystalline material for diverging signal light to be measured into a plurality of polarized beams by the optical element and adjusting a polarization state of one or more of the plurality of beams, and a light receiving part for photoelectrically converting an optic component of the signal light diverged and emitted from the polarization diverging device. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a Stokes parameter measuring apparatus and a Stokes parameter measuring method for measuring a polarization state of signal light or the like.

  The Stokes parameter is a parameter representing the polarization state. When measuring the Stokes parameters, for example, the incident signal light is branched into four parts using a branching means including a beam splitter, a half mirror, a filter, etc., and each signal light is differentiated by a phase element, a polarizing element, etc. The light component of each signal light is photoelectrically converted by the light receiving element, and the Stokes parameter is obtained by performing arithmetic processing on each photoelectrically converted electric component (see Patent Document 1). In addition, instead of the branching means composed of a beam splitter or the like, there is also a method of performing four branches at once using a quadrangular pyramid prism or the like (see Patent Document 2).

Japanese Patent Laid-Open No. 6-18332 Japanese Patent Laid-Open No. 2004-93549

  In the conventional Stokes parameter measuring apparatus as described above, the one that splits light using a beam splitter, a half mirror, etc., splits incident light due to interference, so PDL (Polarization dependent loss) occurs and high accuracy It was difficult to measure the Stokes parameters. In order to solve this problem, a method of branching light using a quadrangular pyramid prism has been invented. However, in this method, the branching ratio fluctuates unless the position of the beam with respect to the apex of the light prism is precisely matched. For this reason, if the positional relationship between the beam and the apex of the prism is shifted due to a temperature change, there is a problem that the measurement accuracy of the Stokes parameter is deteriorated. Furthermore, there is a possibility that light loss may occur at the edge part of the boundary between the surfaces of the prisms.

  The present invention has been made in view of the above, and an object of the present invention is to provide a Stokes parameter measurement device and a Stokes parameter measurement method capable of highly accurate measurement.

  In order to solve the above-described problems and achieve the object, a Stokes parameter measurement apparatus according to the present invention includes (a) an optical element formed of a birefringent crystal material, and the signal light to be measured is transmitted by the optical element. A polarization branching device that branches into a plurality of polarized light beams and adjusts the polarization state of one or more of the plurality of polarized light beams; and (b) an optical component of the signal light branched and emitted from the polarization branching device. And a light receiving portion that performs photoelectric conversion.

  According to the present invention, a polarization branching device including an optical element formed of a birefringent crystal material branches signal light to be measured into a plurality of polarized light beams by the optical element, and one of the plurality of polarized light beams. Since the polarization state is adjusted for one or more, the branching ratio can be adjusted by an optical element formed of a birefringent crystal material, and a plurality of linearly polarized light components can be obtained directly. At this time, the branching ratio by the optical element formed of the birefringent crystal material is unique to the optical element and is not easily affected by the arrangement and the like, so there is no need to adjust the branching ratio. In addition, according to the present invention, the use of a polarization branching device including an optical element formed of a birefringent crystal material can suppress the occurrence of substantial temperature fluctuations and can cope with the Stokes parameter with a small amount of light loss. A Stokes parameter measurement device that can obtain an optical component and has a small wavelength dependency of the branching ratio can be realized. As a result, it is possible to realize a Stokes parameter measurement device capable of high-precision measurement.

  In the Stokes parameter measurement device according to the present invention, in the above invention, the polarization branching device converts the signal light to be measured into the first direction polarized light and the second direction polarized light by (b1) the birefringent prism. A first branching portion that branches, and (b2) a plurality of birefringent prisms further splits the polarized light branched at the first branching portion in two stages, and a first group of polarized light beams corresponding to the first direction; A second group of polarized light beams corresponding to the second direction is generated, and any one or more polarized light beams of the first group and any one or more polarized light beams of the second group are combined in a predetermined combination. A second branching unit to be combined; and (b3) a polarization adjusting unit that adjusts a polarization state of one or more of the light beams combined in the second branching unit.

  According to the present invention, the optical component of the signal light is obtained from the polarized light beam branched by the first branching unit having the birefringent prism and the second branching unit having the plurality of birefringent prisms. The branching ratio can be adjusted by setting the axis, and a plurality of linearly polarized light components can be obtained directly. At this time, since the branching ratio by the first branching part and the second branching part is determined by the configuration of these birefringent prisms, it is not necessary to adjust the branching ratio. In addition, the temperature fluctuation of the branching ratio due to the first branching part and the second branching part is determined by the temperature fluctuation of the extinction ratio of the birefringent crystal, so that substantial temperature fluctuations can be suppressed. In addition, a polarization component can be obtained using a birefringent crystal having a high extinction ratio, and the loss is only the loss of the birefringent crystal whose surface is AR-coated, so that an optical component corresponding to the Stokes parameter can be obtained with a small amount of light loss. be able to. In addition, by branching light using a birefringent crystal, it is possible to realize a Stokes parameter measurement device that has a small wavelength dependency of the branching ratio and supports a wide wavelength range.

  In the Stokes parameter measurement device according to the present invention, in the above invention, the polarization adjusting unit combines the first polarized light beam of the first group and the first polarized light beam of the second group. A wave plate that performs a predetermined phase adjustment with respect to the light beam, a first polarizer that extracts a predetermined polarization component from the first combined light that has passed through the wave plate, a second polarized light beam and a second light beam in the first group And a second polarizer for extracting a predetermined polarization component from the second synthesized light obtained by synthesizing the second polarized light beam in the group. According to this invention, a circularly polarized light component can be obtained by the wave plate and the polarizer, and a polarized light component in a direction inclined with respect to the first and second directions can be obtained by the polarizer.

  The Stokes parameter measurement apparatus according to the present invention is the Stokes parameter measurement device according to the above invention, wherein the wave plate is a quarter wave plate, and the first combined light passes through the wave plate and the first polarizer, thereby A circularly polarized light component based on either one direction or the linearly polarized light in the second direction is generated, and the second combined light passes through the second polarizer, so that the first direction or the A 45 ° linearly polarized light component based on any one of the linearly polarized light in the second direction is generated. According to the present invention, the two light components corresponding to the Stokes parameters can be obtained directly by the polarization adjusting unit including the wave plate and the polarizer.

  In the Stokes parameter measurement device according to the present invention, in the above invention, the light receiving unit corresponds to the circularly polarized light component and the 45 ° linearly polarized light component obtained from the first and second combined lights emitted from the polarization adjusting unit. Of the obtained light intensity and the polarized light beams of the first group and the second group emitted from the second branching unit, the 0 ° linear polarization component and the 90 ° straight line obtained from the light beam not synthesized in the second branching unit The light intensity of the polarization component is individually photoelectrically converted. According to the present invention, each light component corresponding to the Stokes parameter can be directly photoelectrically converted.

  The Stokes parameter measurement device according to the present invention is the above-described invention, wherein the Stokes parameter measurement device is connected to the light receiving unit, and the light receiving unit photoelectrically converts the circularly polarized light component, 45 ° linearly polarized light component, 0 ° linearly polarized light component, and 90 ° straight line. An arithmetic circuit unit for obtaining a Stokes parameter based on the light intensity of the polarization component is further provided. According to the present invention, it is possible to obtain a Stokes parameter obtained by performing a necessary process on the photodetection current value or the like.

  In the Stokes parameter measurement device according to the present invention, in the above invention, the birefringent prism provided in the first branching section is configured such that polarized light branched by the birefringent prism is incident on the birefringent prism. It is disposed so as to be incident again from a direction opposite to the direction, and also serves as one of the birefringent prisms of the second branching unit that generates and synthesizes the polarized light flux. According to this invention, the birefringent prism of the first branch portion can be used as a part of the second branch portion, and space saving can be achieved. Further, by performing branching / combining with the same prism, the optical paths of the components of the combined light can be matched with high accuracy, and PMD of the combined light beam can be easily compensated, and the Stokes parameter measurement accuracy is improved. be able to.

  In the Stokes parameter measurement device according to the present invention, in the above invention, the first branching unit also serving as the polarization combining unit includes a first surface on which the signal light to be measured is incident and a second surface on which the branched light is emitted. And the second branch part has first and second birefringent prisms facing the second surface of the first branch part, and the first branch part has a first branch part with respect to the first and second birefringent prisms. It has the reflective member arrange | positioned on the opposite side, It is characterized by the above-mentioned. According to this invention, the first branch portion and the second branch portion can be efficiently accommodated on the optical path folded back by the reflecting member.

  In the Stokes parameter measurement device according to the present invention, in the above invention, the optical axis of the birefringent prism constituting the first branch portion and the optical axes of the first and second birefringent prisms are from the optical axis direction. It is arranged so as to have a relative angle of 45 ° when viewed. According to this invention, by passing through the second branching section, two light beams that are polarized in the first direction as the first group of polarized light beams, and two light beams that are polarized in the second direction as the second group of polarized light beams, And the branching ratio of these light beams can be made equal.

  In the Stokes parameter measurement device according to the present invention, in the above invention, the birefringent prism constituting the first branch portion and the first and second birefringent prisms are formed of the same material. The polarization separation width of the birefringent prism constituting one branching portion is characterized by being √2 times the polarization separation width of the first and second birefringent prisms. According to the present invention, it is possible to align the pitches of the arrangements of the branched lights emitted through the second branching section together with the matrix.

  The Stokes parameter measurement apparatus according to the present invention is the Stokes parameter measurement device according to the above-described invention, wherein the second branching unit splits the polarized light branched by the first branching unit into first and second oblique directions; The polarized light branched by the first prism generates a first group of polarized light beams and a second group of polarized light beams, and combines one polarized light beam of the first group and one polarized light beam of the second group, respectively. And a second prism that forms two sets of combined light. According to the present invention, the birefringent prism provided in the first branch portion does not need to be used as a part of the second branch portion (means for splitting and synthesizing the polarized light), and the whole has a simple structure in series. It can be an optical system.

  In the Stokes parameter measurement method according to the present invention, (a) the step of branching the signal light to be measured into orthogonally polarized light in the first direction and polarized light in the second direction by the first branching unit having the birefringent prism. And (b) the second branching unit having a plurality of birefringent prisms further splits the polarized light branched at the first branching unit in two stages, and a first group of polarized light beams corresponding to the first direction; Generating a second group of polarized light fluxes corresponding to the second direction; and (c) any one or more polarized light fluxes of the first group and any one or more polarized light fluxes of the second group. A step of combining in a predetermined combination by the second branching unit, (d) a step of adjusting the polarization state by the polarization adjusting unit with respect to any one or more of the combined light beams, and (e) a second Light of signal light emitted from the branching unit and polarization adjusting unit Min based on the output of the light receiving unit for photoelectrically converting, characterized in that it comprises the step of determining the Stokes parameters serving light intensity component and 0 ° polarized light component and 45 ° polarization component and a circularly polarized light component.

  According to the present invention, the branching ratio can be adjusted by setting the optical axis of the birefringent prism, and a plurality of linearly polarized light components can be obtained directly. At this time, the branching ratio by the first branching part and the second branching part hardly fluctuates depending on the arrangement of the birefringent prisms constituting the first branching part and the second branching part. Occurrence can be suppressed. Furthermore, since the light is branched using a birefringent crystal having a high extinction ratio, no loss occurs at the time of branching, and the loss is only the loss of the birefringent crystal whose surface is AR-coated. Can be obtained. As a result, Stokes parameters can be measured with high accuracy.

  According to the present invention, the optical element includes an optical element formed of a birefringent crystal material, the signal light to be measured is branched into a plurality of polarized light beams by the optical element, and one or more of the plurality of polarized light beams are A polarization branching device that adjusts the polarization state and a light receiving unit that photoelectrically converts the light component of the signal light branched and emitted from the polarization branching device, so that there is no occurrence of PDL and the change in environmental temperature However, since the branching ratio can be maintained stably and the light component can be obtained with a small amount of light loss, it is possible to realize a Stokes parameter measuring apparatus capable of performing highly accurate measurement.

  Hereinafter, specific embodiments of a Stokes parameter measurement device and a Stokes parameter measurement method according to the present invention will be described with reference to the drawings. In addition, this invention is not limited by this embodiment.

[First Embodiment]
FIG. 1 is a block diagram showing a configuration of a Stokes parameter measurement apparatus according to the first embodiment of the present invention.

  As shown in FIG. 1, the Stokes parameter measuring apparatus 10 includes an input unit 2 that is an incident unit including a receptacle, a collimator lens, and the like, an ellipsometric optical unit 20 that is a polarization branching device, an electric circuit unit 30, and a GP. -It is comprised with the output part 3 provided with IB etc. The ellipsometric optical unit 20 includes an optical branching unit 21 and a polarization adjusting unit 22, and an electric circuit unit 30 includes a light receiving unit 31 including a photodiode, an A / D conversion circuit unit 32, and an arithmetic processing circuit unit. 33. The A / D conversion circuit unit 32 may be replaced with an analog output circuit unit as necessary. The arithmetic processing circuit unit 33 can be omitted, and when an analog output circuit unit is provided, the function can be incorporated in the analog output circuit unit.

  FIG. 2A is a diagram illustrating a configuration example of the light branching unit 21, the polarization adjusting unit 22, and the light receiving unit 31 illustrated in FIG. FIG. 2B is a conceptual diagram for explaining the roles of the light branching unit 21 and the polarization adjusting unit 22, and the polarization state of light in the cross section P1 when the light branching unit 21 is observed from the right side of the drawing. Etc. In order to explain the polarization direction and the like, an XYZ axis shown in FIG. 2A and an XY axis shown in FIG. 2B are defined.

  The optical branching unit 21 into which the signal light SL1 from the input unit 2 is incident includes a first optical system 25 configured by a single birefringent prism made of a birefringent crystal, two birefringent prisms made of a birefringent crystal, etc. And a second optical system 26 configured by: Here, the first optical system 25 is a portion that functions as a first branching portion, and is composed of a birefringent prism 25a whose XZ section has a parallelogram shape in order to suppress interference between crystal optical surfaces and return loss. . Note that the XZ cross section of the birefringent prism 25a may not be a parallelogram, and may be a rectangle whose optical surface is inclined with respect to the X axis. The birefringent prism 25a is cut out from a birefringent crystal such as rutile, TiO2, YVO4, or LN. The optical axis OA11 of the birefringent prism 25a is parallel to the XZ plane and has a constant inclination angle with respect to the X axis. This inclination angle is around 45 ° in order to obtain a large separation width between the two polarized waves. For example, when rutile is used, the inclination angle is 47.8 °.

  Here, the signal light SL1 as an input incident on the first surface 25i of the birefringent prism 25a is branched into an extraordinary ray oscillating in the X direction and an ordinary ray oscillating in the Y direction. That is, the birefringent prism 25a is an optical member that also serves as a branch element and a polarizer. As shown in a cross section P3 in FIG. 2B, the signal light SL1 including various polarized lights is converted into the signal light SL1 in FIG. As shown in the cross section P4, the light is branched into a polarization component SL11 in the X direction that is polarized light in the first direction and a polarization component SL12 in the Y direction that is polarized light in the second direction. At this time, the branching ratio between the polarization component SL11 and the polarization component SL12 depends on the extinction ratio of the birefringent prism 25a, and is a fixed value of approximately 1: 1. Note that deviation from an ideal branching ratio caused by extinction ratio, crystal characteristics, and the like can be compensated by performing appropriate calibration in the arithmetic processing circuit unit 33 described later. In addition, by appropriately shaping the cross-sectional shape of the birefringent prism 25a, the polarization components SL11 and SL12 that are two light beams emitted from the second surface 25o are both precisely parallel to the signal light SL1. Yes.

  The second optical system 26 includes a rectangular first birefringent prism 26a having an inclined XZ section, a right-angle prism 26b, a rectangular second birefringent prism 26c having an inclined XZ section, and a half-wave plate 26d. Prepare. The first and second birefringent prisms 26a and 26c do not have to be rectangular with the XZ cross section inclined, and may be parallelograms like the birefringent prism 25a. Unlike the case of the first optical system 25, the optical axis OA21 of the first birefringent prism 26a has an inclination angle of 45 ° with respect to the X axis and the Y axis. The first birefringent prism 26a is also cut out from a birefringent crystal such as rutile, TiO2, YVO4, and LN.

  Here, the pair of signal lights SL11 and SL12 incident on the incident surface 26i of the first birefringent prism 26a is an extraordinary ray that vibrates in the direction of the optical axis OA21 and an ordinary ray that vibrates in the direction orthogonal to the optical axis OA21. Branch off. That is, the first tilted polarization components SL21 and SL22 that are polarized in a direction inclined 45 ° counterclockwise with respect to the X axis from the signal light SL11 and SL12 that have passed through the first optical system 25, and 45 ° tilted clockwise. The second inclined polarized light components SL23 and SL24, which are polarized light in the selected direction, are generated (see the upper half of the cross section P5a). At this time, the branching ratio between the first inclined polarization components SL21 and SL22 and the second inclined polarization components SL23 and SL24 is a relative angle between the optical axis OA11 of the birefringent prism 25a and the optical axis OA21 of the first birefringent prism 26a, Depending on the extinction ratio of the first birefringent prism 26a and the like, this configuration has a fixed value of approximately 1: 1, but may not necessarily be 1: 1. Further, an error from the design branching ratio is caused by the processing accuracy and placement accuracy of the birefringent crystal, but by performing appropriate calibration in the arithmetic processing circuit unit 33 described later, the error from the ideal branching ratio can be obtained. The bias can be compensated. The first and second inclined polarized light components SL21 to SL24, which are four light beams emitted from the exit surface 26o, are precisely formed into the signal light SL1 by appropriately shaping the cross-sectional shape of the first birefringent prism 26a. The light flux is parallel.

  Next, the right-angle prism 26b is a triangular prism whose XZ cross section is a right-angled isosceles triangle and extends in the Y direction, and has an inclined surface 26j facing the first optical system 25 and other two surfaces that totally reflect light. The inclined surface 26j is inclined twice with respect to the optical axis in order to suppress return loss. The right-angle prism 26b reflects the light traveling in the + Z direction twice by two surfaces having the same side length, thereby shifting the light beam traveling backward in the −Z direction while shifting it in the −X direction by a predetermined interval. it can. Here, according to Fresnel's law, when light is totally reflected by the right-angle prism 26b, different phase shifts occur between the horizontal component and the vertical component with respect to the normal line of the total reflection surface.

  Therefore, in the first embodiment, the half-wave plate 26d having the optical axis OA27 in the direction inclined 22.5 ° counterclockwise with respect to the X axis or the Y axis is used as the first and second birefringent prisms. 26a, 26c and the right-angle prism 26b. Then, before the first and second inclined polarized light components SL21 to SL24 enter the right-angle prism 26b, the polarized light is transmitted through the half-wave plate 26d to rotate the polarization direction, and is polarized to the polarized light in the X direction or the Y direction, respectively. The state is converted into polarization components SL21a to SL24a. (Refer to the upper half of the cross section P5b) Further, after passing through the right-angle prism 26b, the polarized light components SL21a to SL24a are reconverted into the first and second inclined polarized light components SL21 to SL24 by being transmitted again through the half-wave plate 26d. As a result, the first and second inclined polarization components SL21 to SL24 traveling in the + Z direction are shifted by a predetermined interval in the −X direction while maintaining the polarization state and the phase relationship, and the light flux travels backward in the −Z direction. (See the lower half of the cross section P5a).

  Next, the second birefringent prism 26c has the same outer shape as the first birefringent prism 26a, and the optical axis OA22 of the second birefringent prism 26c is 45 ° with respect to the X axis and the Y axis. It has an inclination angle. In the first embodiment, the same birefringent prism 26a as that of the first birefringent prism 26a is disposed by being rotated by 180 ° around the Z axis and used as the second birefringent prism 26c. That is, the number of parts can be reduced by making the first and second birefringent prisms 26a and 26c a common crystal.

  The second birefringent prism 26c is also cut out from a birefringent crystal such as rutile, TiO2, YVO4, and LN, and adjusts the arrangement of the first and second inclined polarization components SL21 to SL24 incident on the incident surface 26i. It has a function. That is, of the first and second inclined SL21 to SL24 incident on the incident surface 26i, the first inclined polarization components SL21 and SL22 that are extraordinary rays oscillating in the direction of the optical axis OA22 are included in the second birefringent prism 26c. The position is shifted, and the second inclined polarized light components SL23 and SL24 are emitted from the emission surface 26o in a state in which the positions in the X-axis direction coincide with each other (see the lower half of the cross section P4). Note that the first and second inclined polarized light components SL21 to SL24, which are four light beams emitted from the exit surface 26o, are both precise with respect to the signal light SL1 by appropriately shaping the cross-sectional shape of the second birefringent prism 26c. The light flux is parallel to

  Next, the first and second inclined polarization components SL21 to SL24 emitted from the second birefringent prism 26c, that is, emitted from the second optical system 26, are provided in the birefringent prism 25a of the first optical system 25. The light enters the second surface 25o and exits from the first surface 25i. The birefringent prism 25a functions as a first branching portion for the signal light SL1 incident from the forward direction, but is second for the first and second inclined polarization components SL21 to SL24 incident from the reverse direction. It functions as a branching synthesis element and a polarizer in the branching section.

  That is, as shown in the lower half of the cross section P3 in FIG. 2B, the first inclined polarization component SL21 is caused to vibrate along the optical axis OA11 of the birefringent prism 25a by the birefringent prism 25a. The light is branched into the polarization component SL31 and the polarization component SL32 in the Y-axis direction having vibration orthogonal to the optical axis OA11 so as to be separated in the X direction. Similarly, the first inclined polarization component SL22 is branched so as to be separated into an X-axis polarization component SL33 and a Y-axis polarization component SL34 in the X direction. Similarly, the second inclined polarization component SL23 is branched so as to be separated into the X-axis direction polarization component SL35 and the Y-axis direction polarization component SL36 in the X direction. Similarly, the second inclined polarization component SL24 is branched so as to be separated into the X-axis direction polarization component SL37 and the Y-axis direction polarization component SL38 in the X direction. That is, the polarization components SL31, SL33, SL35, and SL37 in the X-axis direction corresponding to the first direction are branched and generated as the first group of polarized light beams, and the Y-axis corresponding to the second direction as the second group of polarized light beams. Directional polarization components SL32, SL34, SL36, and SL38 are branched and generated.

  Note that the branching ratio between the polarization components SL31, SL33, SL35, and SL37 in the X-axis direction and the polarization components SL32, SL34, SL36, and SL38 in the Y-axis direction is the optical axis OA11 of the birefringent prism 25a and the second birefringent prism. 26c depends on the relative angle of the optical axis OA22 to the optical axis OA22 and the extinction ratio of the birefringent prism 25a. In the present embodiment, it is a fixed value of approximately 1: 1, but it does not necessarily have to be 1: 1. . Similarly to the above, the error from the design branch ratio can be compensated for deviation from the ideal branch ratio by performing appropriate calibration in the arithmetic processing circuit unit 33 described later.

  At this time, the amount by which the polarization components SL31 and SL35 are shifted relative to the polarization components SL32 and SL36 by the birefringence prism 25a in the X-axis direction, and the polarization components SL33 and SL37 are relative to the polarization components SL34 and SL38. The amount of relative shift in the X-axis direction is equal to the amount of relative shift of the X-polarized component SL11 in the X-axis direction with respect to the Y-polarized component SL12. Furthermore, even if it passes through the first and second birefringent prisms 26a and 26c, the relative interval in the X-axis direction between the first inclined polarization components SL21 and SL22 and the X-axis between the second inclined polarization components SL23 and SL24. The relative distance between the directions is maintained to be equal to the relative distance between the polarization components SL11 and SL12 in the X-axis direction. Therefore, on the first surface 25i of the birefringent prism 25a, the X-direction polarization component SL31, which is the first group of one-polarized light flux, and the Y-direction polarization component SL34, which is the second group of one-polarized light flux, are combined and completely superimposed. In this state, it is emitted as the second combined light. In addition, on the first surface 25i, the X-direction polarization component SL35, which is the first group of one-polarized light flux, and the Y-direction polarization component SL38, which is the second group of one-polarized light flux, are combined and completely superimposed. One synthetic light is emitted (see the lower half of the cross section P3).

  Here, the X-direction polarization component SL31, which is a first polarized light beam, the Y-direction polarization component SL34, which is a second polarized light beam, the X-direction polarization component SL35, which is a first polarized light beam, and the second group. The polarization component SL38 in the Y direction, which is one polarized light beam of the group, is transmitted through the birefringent prism 25a in a reciprocating manner and is in a polarization state parallel to each other when transmitted through the birefringent prisms 26a and 26c. While passing through 21, the same optical path length is transmitted. In other words, the first combined light and the second combined light are obtained by recombining the X-polarized component and the Y-polarized component of the signal light SL1 once branched without relative phase shift, and branched from the signal light SL1. It can be regarded as equivalent to light.

  Here, this phase shift amount needs to be precisely controlled. That is, if the X-polarized component and the Y-polarized component of the signal light SL1 are branched by the birefringent prism 25a and then combined again by another birefringent prism, the length and arrangement of these birefringent prisms are highly accurate. Adjustment is necessary. When this accuracy is low, it is conceivable that a temperature variation of the phase shift amount between the orthogonal polarizations constituting the combined light occurs, and as a result, the measurement accuracy of the Stokes parameter measurement device deteriorates with temperature. However, in the first embodiment, the change of the phase shift amount due to the temperature variation can be easily suppressed by performing both the branching and the synthesis of the signal light SL1 by the single birefringent prism 25a.

  The polarization components SL33 and SL37 and the polarization components SL32 and SL36 can be said to be lights obtained by extracting the X-direction polarization component and the Y-direction polarization component from the light branched from the signal light SL1, respectively. The branching ratio of these branched lights depends on the relative angle of each birefringent prism, the extinction ratio of the birefringent prism, etc., as described above, and is less affected by changes in the environmental temperature. In addition, even if the positional relationship between the optical element and the beam is slightly moved due to a change in environmental temperature, the branching ratio is not affected. Therefore, the Stokes parameter measurement apparatus 10 according to the first embodiment can realize stable measurement regardless of the environmental temperature.

  Further, the length (polarization separation width) in the Z direction of the birefringent prism 25a is √2 times the length (polarization separation width) in the Z direction of the first and second birefringence prisms 26a and 26c. The relative angle of the optical axes of the birefringent prisms 25a, 26a, and 26c is 45 °. As a result, the interval between the first and second inclined polarization components SL21 and SL23 emitted from the second birefringent prism 26c in the Y-axis direction is the interval between the first inclined polarization components SL21 and SL22 in the X-axis direction or the polarization component SL31. , SL32 and the like in the X-axis direction. Therefore, the polarization component SL33 emitted from the first surface 25i of the birefringent prism 25a, the polarization component SL37, the polarization components SL31 and SL34 constituting the second combined light, and the polarization component SL35 constituting the first composite light. SL38, polarization component SL32, and polarization component SL36 are in a state of being arranged at an equal pitch in the X-axis and Y-axis directions.

  Next, the polarization adjusting unit 22 includes a phase element 22a made of a ¼ wavelength plate or the like, and a polarizing element 22b made of a transmissive polarizing plate. Here, if the polarization components SL33 and SL37, which are linearly polarized components oscillating in the X direction, of the light polarized and separated by the birefringent prism 25a are set as 0 ° polarization components, the phase element 22a as a wave plate is used. The principal axis orientation of the polarizing element corresponds to an azimuth angle orthogonal to that (that is, the Y-axis direction), and the principal axis orientation of the polarizing element 22b as a polarizer, ie, the polarization axis, is based on a polarization component of 0 °. This corresponds to a 45 ° polarization azimuth angle (that is, a direction inclined by 45 ° with respect to the X axis: −45 ° when viewed from the prism side). The principal axis directions of the phase element 22a and the polarizing element 22b may be a combination of 45 ° and 0 °. The phase element 22a is disposed on the optical path of the polarization components SL35 and SL38 constituting the first combined light (see the cross section P3), and the polarization element 22b is the polarization components SL31 and SL34 constituting the second combined light. And the polarization components SL35 and SL38 constituting the first combined light (see cross section P2). As a result, the first combined light that has passed through the phase element 22a and the polarization element 22b becomes a polarization component SL42 obtained by extracting the right circular polarization component of the signal light SL1. Further, the second synthesized light that has passed through the polarizing element 22b becomes a polarized light component SL41 that is obtained by extracting a component related to the polarization azimuth angle of 45 ° by the polarizing element 22b.

Next, FIG. 3 is a block diagram for explaining the light receiving unit 31 in the electric circuit unit 30 of the Stokes parameter measuring apparatus 10 shown in FIG. The light receiving unit 31 is configured by arranging six photodiodes PD1 to PD6, which are photoelectric conversion elements, at an equal pitch in accordance with the arrangement of the polarization components transmitted through the light branching unit 21 and the polarization adjusting unit 22. The second photodiodes PD1 and PD2 are connected in parallel, and their outputs I ₁₁ and I ₁₂ are added and output as a current value I ₁ . The fifth and sixth photodiodes PD5 and PD6 are also connected in parallel, and their outputs −I ₁₁ and −I ₁₂ are added and output as a current value I ₁ ′. Here, although the first and second photodiodes PD1 and PD2 are connected in parallel, only the output of one of the photodiodes can be used. The same applies to the fifth and sixth photodiodes PD5 and PD6. The output from the third photodiode PD3 is output as a current value I _2, the output of the fourth photodiode PD4 is output as a current value I _3.

  Here, as shown in FIG. 2B, the polarization component SL33 is directly incident on the first photodiode PD1, and the polarization component SL37 is similarly incident on the second photodiode PD2. Similarly, the polarization component SL32 is directly incident on the diode PD5, and the polarization component SL36 is similarly incident on the sixth photodiode PD6. The polarization component SL41 obtained from the second combined light is incident on the third photodiode PD3, and the polarization component SL42 obtained from the first combined light is incident on the fourth photodiode PD4. That is, the polarization component of the polarization azimuth angle of 0 ° of the signal light SL1 is divided and incident on the first and second photodiodes PD1 and PD2, respectively, and the signal is input to the fifth and sixth photodiodes PD5 and PD6. The polarized light components of 90 ° polarization azimuth angle of the light SL1 are divided and incident respectively. On the other hand, the polarization component of the 45 ° polarization azimuth angle of the signal light SL1 is incident on the third photodiode PD3, and the right circular polarization component of the signal light SL1 is incident on the fourth photodiode PD4.

In general, the Stokes parameter is obtained by measuring the power of signal light and the power of 0 ° · 45 ° · circularly polarized light component of the signal light. In the first embodiment, the signal output I ₁ = I ₁₁ + I _{12 obtained} by adding the first and second photodiodes PD1 and PD2, and the signal output I ₁ ′ obtained by adding the fifth and sixth photodiodes PD5 and PD6. The sum of = −I ₁₁ + (− I ₁₂ ) corresponds to the intensity of signal light, which is one of the Stokes parameters. The signal output I ₁ = I ₁₁ + I _{12 obtained} by adding the first and second photodiodes PD1 and PD2 is _one of the Stokes parameters and corresponds to the intensity of the polarization component having the polarization azimuth angle of 0 °. . Further, the signal output I ₂ of the third photodiode PD3 corresponds to the intensity of the polarization component having the polarization azimuth angle of 45 °, and the signal output I ₃ of the fourth photodiode PD4 corresponds to the intensity of the right circular polarization component. ing.

Next, the A / D conversion circuit unit 32 includes the signal output I _{1 of} the first and second photodiodes PD1 and PD2, the signal output I ₂ of the third photodiode PD3, and the signal output I of the fourth photodiode PD4. ₃ and the signal output I ₁ ′ of the fifth and sixth photodiodes PD5 and PD6 are converted into digital data.

Next, the arithmetic processing circuit unit 33 performs addition and coefficient processing on the outputs I ₁ , I ₂ , I ₃ , and I ₁ ′, which are signals digitized by the A / D conversion circuit unit 32, to obtain a final result. It is an arithmetic circuit part which calculates Stokes parameters. At this time, as described above, compensation calibration can be performed in consideration of the branching ratios of the birefringent prisms 25a, 26a, and 26c. In the first embodiment, the splitting of the optical power and the separation of the polarization component are performed at the same time. However, as can be understood from the above description, it is assumed that the polarization component is extracted after the signal light is virtually branched into four. Can do. Wherein _{I 1, I 2, I 3} , ' the power after 4 branches corresponding to _{I 1in, I 2in, I 3in} , I 1in' I 1 as branching the ratio between power and I _1in 'after each branch When the ratio I _ir = I _i / I ₁ ′, the Stokes parameter is given by the equation (1) as a normalized value. The branching ratio of the four polarization components is arbitrary, but is 2: 1: 1: 2 in the configuration of FIG.

S ₀ = 1
S _i = {2 · I _i / (I ₀ · I _ir )} − 1 (i = 1, 2, 3) (1)
Here, I ₀ = (I ₁ / I _1r ) + I ₁ ′

  Specifically, as shown in FIG. 4, an input light source α capable of emitting completely polarized light is prepared, and the polarization state of the completely polarized light from the input light source α is changed via the polarization controller F. The Stokes parameter measuring device 10 is input.

Approximately, the branching ratio can be measured by the following two methods.
(1) The ratio of the maximum values of I ₁ , I ₂ , I ₃ , I ₁ ′ is the branching ratio I _ir .
(2) I _1r = 1 That is, 'on the assumption that the branching ratio corresponding to is 1, I ₀ = I ₁ + I _1' I ₁ and I ₁ from seeking I ₀ as, I ₂ and I ₃ The branch ratio of I ₀ to I ₀ is measured in real time, and I _ir = (I _i / I ₀ ) _max (i = 2, 3) is taken as the branch ratio.

  An accurate branching ratio parameter can be obtained by performing minimum value and maximum value fitting by using the data measured as described above, for example, by a method disclosed in Japanese Patent Application Laid-Open No. 2006-28497. In addition, the above method can correct the deviation of each Stokes parameter component from orthogonality, and can perform measurement with higher accuracy. Further, the degree of polarization DOP is calculated as equation (2).

DOP = (S ₁ ² + S ₂ ² + S ₃ ² ) ^1/2 / S ₀ (2)

  Therefore, if the arithmetic processing circuit unit 33 has the arithmetic function of the expression (2), the Stokes parameter measuring apparatus 10 can be operated as a DOP monitor. In this way, the Stokes parameter and the DOP value can be calculated. According to this method, the light branching in the polarization analysis optical unit 20 that is a polarization branching device is the processing accuracy of the birefringent prisms 25a, 26a, and 26c. Because it is determined by the placement accuracy, it is not easily affected by temperature. As a result, the separation and synthesis of light is stable and highly accurate, and the PDL and the like in the polarization analysis optical unit 20 can be structurally suppressed to a small size, so that a more accurate Stokes parameter measurement device, that is, an optical polarization analyzer is configured. it can.

  The arrangement pitch of the six photodiodes PD1 to PD6 can be changed according to the material of the birefringent prisms 25a, 26a, and 26c, the length in the Z direction, and the optical axis direction. It is also possible to change the structure of the birefringent prisms 25a, 26a, and 26c according to the arrangement of the photodiodes PD1 to PD6. If the birefringent prisms 25a, 26a, and 26c are downsized to reduce the arrangement pitch of the photodiodes PD1 to PD6, the Stokes parameter measuring apparatus 10 can be downsized and inexpensive, but the signal light SL1. Is likely to be kicked by the phase element 22a, the polarizing element 22b, and the photodiodes PD1 to PD6. Therefore, in the first embodiment, in order to ensure the tolerance of the positional accuracy of each element, an appropriate optical system is provided in the input unit 2 to reduce the beam size by condensing the signal light SL1, and the photodiode PD1. -The spot size incident on PD6 or the like is reduced.

  In the first embodiment, the incident / exit surfaces 25i, 25o, 26i, and 26o of the birefringent prisms 25a, 26a, and 26c are inclined with respect to the optical axis, that is, the Z-axis. This prevents return light and interference. Therefore, these inclination angles can be changed as appropriate according to the specifications of the apparatus. The phase element 22a, the polarizing element 22b, and the photodiodes PD1 to PD6 are also arranged in a state where they are not inclined in the drawing, but can be arranged so as to be appropriately inclined with respect to the optical axis.

[Modification of First Embodiment]
Hereinafter, various modifications of the first embodiment will be described. Parts that are not particularly described are the same as in the Stokes parameter measurement apparatus 10 of the first embodiment, and redundant description is omitted.

[Modification 1]
FIG. 5 is a diagram for explaining the arrangement of the right-angle prism and the half-wave plate provided in the optical branching section of the Stokes parameter measurement device according to the first modification of the first embodiment. As shown in FIG. 5, in the first modification, the half-wave plate 26d in the first embodiment is interposed between right-angle prisms 26ba and 26bb similar to the right-angle prism 26b in the first embodiment. It is arranged. Even in such a configuration, similarly to the first embodiment, the first and second inclined polarization components SL21 to SL24 traveling in the + Z direction are maintained at a predetermined interval in the −X direction while maintaining the polarization state and the phase relationship. The light beam can be made to travel backward in the −Z direction while being shifted.

[Modification 2]
FIG. 6A is a diagram illustrating a part of the optical branching unit of the Stokes parameter measurement apparatus according to the second modification of the first embodiment, and FIG. 6B is a conceptual diagram illustrating the role of the optical branching unit. is there. As shown in FIG. 6, in the second modification, instead of using the first and second birefringent prisms 26a and 26c as in the first embodiment, the height of the second birefringent prism 26c is increased. The first and second inclined polarization components SL21 to SL24 pass through the second birefringent prism 26c when traveling in the + Z direction and the −Z direction. Furthermore, instead of the half-wave plate 26d in the first embodiment, a half-wave plate 26e and a half-wave plate 26f are arranged. Here, the half-wave plate 26e has an optical axis OA 26e in a direction inclined 22.5 ° counterclockwise with respect to the X-axis or Y-axis, and the half-wave plate 26f Alternatively, it has the optical axis OA26f in a direction inclined 22.5 ° clockwise with respect to the Y axis. Even in such a configuration, similarly to the first embodiment, the first and second inclined polarization components SL21 to SL24 traveling in the + Z direction are maintained at a predetermined interval in the −X direction while maintaining the polarization state and the phase relationship. The light beam can be made to travel backward in the −Z direction while being shifted.

[Modification 3]
FIG. 7A is a diagram illustrating a part of the optical branching unit of the Stokes parameter measurement device according to the third modification of the first embodiment, and FIG. 7B is a conceptual diagram illustrating the role of the optical branching unit. is there. As shown in FIG. 7, in the third modification, the half-wave plate 26d in the first embodiment is disposed adjacent to the birefringent prism 25a. At the same time, the first birefringent prism 26a in the first embodiment is replaced with a birefringent prism 26g made of the same material as the birefringent prism 25a, and the second birefringent prism 26c is omitted. The birefringent prism 26g has an optical axis OA21g that is parallel to the YZ plane and has an inclination angle of 47.8 ° with respect to the Y axis. Even in such a configuration, similarly to the first embodiment, the first and second inclined polarization components SL21 to SL24 traveling in the + Z direction are maintained at a predetermined interval in the −X direction while maintaining the polarization state and the phase relationship. The light beam can be made to travel backward in the −Z direction while being shifted.

[Second Embodiment]
Hereinafter, the Stokes parameter measurement device according to the second embodiment will be described. The Stokes parameter measurement device according to the present embodiment is a further modification of the Stokes parameter measurement device 10 according to the first embodiment. The parts that are not particularly described are the same as the Stokes parameter measurement device 10 according to the first embodiment. Duplicate explanation is omitted.

  FIG. 8A corresponds to FIG. 2A, and shows one configuration example of the light branching unit 121, the polarization adjusting unit 122, and the light receiving unit 131 constituting the Stokes parameter measurement device according to the second embodiment. Show. FIG. 8B is a conceptual diagram for explaining the roles of the light branching unit 121 and the polarization adjusting unit 122.

  The light branching unit 121 includes a first optical system 125 configured by a single birefringent prism and a second optical system 126 configured by two birefringent prisms. Here, the first optical system 125 is a portion that functions as a first branching portion, and includes a birefringent prism 125a whose XZ section has a parallelogram shape. The second optical system 126 is a portion that functions as a second branching portion, and includes birefringent prisms 126a and 126b whose XZ cross section has a parallelogram shape. The birefringent prism 125a has the same optical axis OA11 and the like as the birefringent prism 25a of the first embodiment, but only allows the signal light SL1 to enter from one side. The birefringent prism 126a has the same optical axis OA21 and the like as the birefringent prism 26a of the first embodiment, but its length is doubled. Further, the birefringent prism 126b has the same optical axis OA11 and the like as the birefringent prism 125a provided in the first optical system 125.

  The birefringent prism 125a is an optical member that also serves as a branching element and a polarizer. From the signal light SL1 shown in the cross section P1 in FIG. 8B, the first direction as shown in the cross section P2 in FIG. 5B. X-polarized light component SL11 that is the polarized light of the second direction, and Y-polarized light component SL12 that is the polarized light in the second direction. At this time, the branching ratio between the X-polarized component SL11 and the Y-polarized component SL12 is approximately 1: 1 and is a fixed value.

  The birefringent prism 126a is a first prism unit for branching the polarization component with respect to the 45 ° direction, and branches the signal lights SL11 and SL12 emitted from the birefringence prism 125a to produce first inclined polarization components SL21 and SL22. And second inclined polarization components SL23 and SL24 are generated (see cross section P3). At this time, the branching ratio between the first inclined polarization components SL21 and SL22 and the second inclined polarization components SL23 and SL24 is approximately 1: 1 and is a fixed value.

  The birefringent prism 126b is a first prism unit that also serves as a branching element and a combining element. By the birefringent prism 126b, the polarization component SL21 is branched into the polarization component SL31 and the polarization component SL32 so as to be separated in the X direction as shown in the cross section P4. Similarly, the polarization component SL22 is branched to be separated into the polarization component SL33 and the polarization component SL34 in the X direction. Similarly, the polarization component SL23 is branched so as to be separated into the polarization component SL35 and the polarization component SL36 in the X direction, and the polarization component SL24 is branched so as to be separated into the polarization component SL37 and the polarization component SL38. The

  At this time, the X-direction polarization component SL33 and the Y-direction polarization component SL32 are completely superimposed on the exit surface of the birefringent prism 126b, and are emitted as the first combined light. The X-direction polarization component SL37 and the Y-direction The polarized light component SL36 is emitted as the second combined light in a state of being completely superimposed. Further, the Y-axis of the polarization components SL31, SL32, SL33, SL34 as the first group of polarized light beams emitted from the second birefringent prism 126b and the polarization components SL35, SL36, SL37, SL38 as the second group of polarized light beams. The interval in the direction is equal to the interval in the X-axis direction of the polarization components SL31 to SL38. Therefore, the polarization component SL31 emitted from the birefringent prism 126b, the polarization component SL35, the polarization components SL32 and SL33 constituting the first combined light, the polarization components SL36 and SL37 constituting the second combined light, and the polarization component SL34 and polarization component SL38 are arranged at equal pitches in the X-axis and Y-axis directions.

  Further, in the polarization adjusting unit 22, the phase element 22a is disposed on the optical path of the polarization components SL32 and SL33 that constitute the first combined light, and the polarization element 22b is the polarization component SL32 that constitutes the first synthesized light. It arrange | positions on the optical path of SL33 and the polarization components SL36 and SL37 which comprise 2nd synthetic | combination light. As a result, the first synthesized light that has passed through the phase element 22a and the polarization element 22b becomes the polarization component SL42 from which the right circular polarization component has been extracted. Further, the second synthesized light that has passed through the polarizing element 22b becomes a polarized light component SL41 that is obtained by extracting a component related to the polarization azimuth angle of 45 ° by the polarizing element 22b.

  Further, in the light receiving unit 131, as shown in FIG. 8B, the polarization component SL31 is directly incident on the first photodiode PD1, and the polarization component SL35 is similarly incident on the second photodiode PD2. Similarly, the polarization component SL34 is directly incident on the fifth photodiode PD5, and the polarization component SL38 is similarly incident on the sixth photodiode PD6. The polarization component SL42 obtained from the first combined light is incident on the third photodiode PD3, and the polarization component SL41 obtained from the second combined light is incident on the fourth photodiode PD4. That is, the polarization component of the polarization azimuth angle of 0 ° of the signal light SL1 is divided and incident on the first and second photodiodes PD1 and PD2, respectively, and the signal is input to the fifth and sixth photodiodes PD5 and PD6. The polarized light components of 90 ° polarization azimuth angle of the light SL1 are divided and incident respectively. On the other hand, the circularly polarized component of the signal light SL1 is incident on the third photodiode PD3, and the polarized component having a 45 ° polarization azimuth angle of the signal light SL1 is incident on the fourth photodiode PD4.

Here, the signal output I ₁ = I ₁₁ + I _{12 obtained} by adding the first and second photodiodes PD1 and PD2, and the signal output I ₁ ′ = −I ₁₁ + obtained by adding the fifth and sixth photodiodes PD5 and PD6. The sum of (−I ₁₂ ) is the total current value I ₀ corresponding to the signal intensity, which is one of the Stokes parameters. The signal output I ₁ = I ₁₁ + I _{12 obtained} by adding the first and second photodiodes PD1 and PD2 is _one of the Stokes parameters and corresponds to the intensity of the polarization component having the polarization azimuth angle of 0 °. . The signal output I ₃ of the third photodiode PD3 corresponds to the intensity of the right circular polarization component, and the signal output I ₂ of the fourth photodiode PD4 corresponds to the intensity of the polarization component having a polarization azimuth angle of 45 °. . As described above, the Stokes parameters S _{0 to} S ₃ can be obtained from the current values I ₀ , I ₁ , I ₂ , and I ₃ (see the above formula (1)).

  In the case of the Stokes parameter measurement device of the second embodiment, it is necessary to improve the PMD (Polarization mode dispersion) accuracy generated by the birefringent prism 125a and the birefringent prism 126b, but the structure is simple as long as the optical path is not folded back. It will be something. Moreover, since there are few transmissive optical surfaces compared with 1st Embodiment, light quantity loss can be reduced.

[Third Embodiment]
Hereinafter, a Stokes parameter measurement apparatus according to the third embodiment will be described. The Stokes parameter measurement device of this embodiment is a further modification of the Stokes parameter measurement device 10 of the first embodiment.

  FIG. 9 corresponds to FIG. 2A and shows a configuration example of the light branching unit 221, the polarization adjusting unit 22, and the light receiving unit 31 constituting the Stokes parameter measurement device according to the third embodiment. In the third embodiment, the optical branching unit 221 is a series type.

  The light branching unit 221 includes a pair of birefringent prisms 25a and 25a and a pair of birefringent prisms 26a and 26c sandwiched therebetween. Here, the birefringent prisms 25a, 25a, 26a, and 26c are the same as those in the first embodiment, but the signal light is made to travel straight except for the right-angle prism 26b, so that the birefringent prism 25a in FIG. It is divided into two. Also in the case of the Stokes parameter measurement device of the third embodiment, the structure is simple and the light amount loss on the transmission optical surface can be reduced, as in the second embodiment.

  Although the present invention has been described with reference to the embodiments, the present invention is not limited to the above-described embodiments, and can be implemented in various modes without departing from the gist thereof.

  For example, the configuration of the polarization adjustment unit 22 shown in FIG. 2A is merely an example, and the arrangement location of the phase element 22a and the polarization element 22b and the direction of the optical axis can be changed as appropriate. It is sufficient to change the processing contents in the unit 33.

It is a block diagram which shows the structure of the Stokes parameter measuring apparatus which concerns on 1st Embodiment of this invention. It is a figure explaining the role of one structural example of an optical branching part of FIG. 1, a phase compensation part, and a light-receiving part, and an optical branching part and a polarization adjustment part. It is a figure explaining the specific structure of the light-receiving part of FIG. It is a figure explaining the method of performing compensation calibration in the measuring device of FIG. It is a figure explaining arrangement | positioning of the right angle prism and 1/2 wavelength plate with which the optical branching part of the Stokes parameter measurement apparatus which concerns on the modification 1 of 1st Embodiment was equipped. It is a figure explaining the role of a part of optical branch part of the Stokes parameter measuring device concerning the modification 2 of 1st Embodiment, and an optical branch part. It is a figure explaining the role of a part of optical branch part of the Stokes parameter measuring device which concerns on the modification 3 of 1st Embodiment, and an optical branch part. It is a figure explaining one structural example, such as an optical branching part, and the role of an optical branching part etc. in the measuring device concerning a 2nd embodiment. It is a figure explaining the optical branching part etc. in the measuring device concerning a 3rd embodiment.

Explanation of symbols

DESCRIPTION OF SYMBOLS 2 Input part 3 Output part 10 Stokes parameter measurement apparatus 20 Polarization analysis optical part 21 Optical branch part 22 Polarization adjustment part 22a Phase element 22b Polarization element 25 1st optical system 25a Birefringent prism 26 2nd optical system 26a 1st birefringent prism 26b Right angle prism 26c Second birefringent prism 26d 1/2 wavelength plate 30 Electric circuit unit 31 Light receiving unit 32 A / D conversion circuit unit 33 Arithmetic processing circuit unit OA11, OA21, OA22 Optical axes PD1 to PD6 Photodiode SL1 Signal light SL11 , SL12, SL21 to SL24, SL31 to SL38 Polarized component SL41, SL42 Combined light

Claims (12)

Including an optical element formed of a birefringent crystal material, the signal light to be measured is branched into a plurality of polarized light beams by the optical element, and a polarization state is adjusted for one or more of the plurality of polarized light beams A polarization splitter;
A Stokes parameter measurement device comprising: a light receiving unit that photoelectrically converts an optical component of signal light branched and emitted from the polarization branching device.   The polarization branching device includes a first branching unit that splits signal light to be measured into polarized light in a first direction and polarized light in a second direction by a birefringent prism, and the first branching unit by a plurality of birefringent prisms. The polarized light branched in step 2 is further branched in two stages to generate a first group of polarized light beams corresponding to the first direction and a second group of polarized light beams corresponding to the second direction, and A second branching unit that combines any one or more polarized light beams of one group and any one or more polarized light beams of the second group in a predetermined combination, and a light beam combined by the second branching unit The Stokes parameter measurement device according to claim 1, further comprising: a polarization adjustment unit that adjusts a polarization state with respect to one or more of them.   The polarization adjusting unit includes a wave plate that performs a predetermined phase adjustment on the first combined light obtained by combining the first polarized light beam of the first group and the first polarized light beam of the second group, A first polarizer that extracts a predetermined polarization component from the first combined light that has passed through the wave plate, and a second polarized light beam of the first group and a second polarized light beam of the second group are combined. The Stokes parameter measurement device according to claim 2, further comprising: a second polarizer that extracts a predetermined polarization component from the second synthesized light.   The wave plate is a quarter wave plate, and the first synthesized light passes through the wave plate and the first polarizer, so that either the first direction or the second direction of linearly polarized light is used as a reference. When the circularly polarized component is generated and the second synthesized light passes through the second polarizer, the linearly polarized light in the first direction or the second direction is used as a reference. The Stokes parameter measuring device according to claim 3, wherein a linearly polarized light component is generated.   The light receiving unit includes light intensity obtained corresponding to the circularly polarized light component and the 45 ° linearly polarized light component obtained from the first and second combined lights emitted from the polarization adjusting unit, and the second branching unit. The light intensity of the 0 ° linearly polarized light component and the 90 ° linearly polarized light component obtained from the polarized light beams not synthesized in the second branching portion among the polarized light beams of the first group and the second group emitted from The Stokes parameter measurement device according to claim 4, wherein the photoelectric conversion is performed individually.   A Stokes parameter is obtained based on the light intensity of the circularly polarized component, the 45 ° linearly polarized component, the 0 ° linearly polarized component, and the 90 ° linearly polarized component that are connected to the light receiving unit and photoelectrically converted by the light receiving unit. The Stokes parameter measurement device according to claim 5, further comprising an arithmetic circuit unit.   The birefringent prism provided in the first branching portion is disposed so that the polarized light branched by the birefringent prism is incident again on the birefringent prism from a direction opposite to the incident direction of the signal light. The Stokes parameter measurement device according to any one of claims 2 to 6, wherein the Stokes parameter measurement device also serves as one of the birefringent prisms of the second branching unit that generates and combines light beams.   The first branch portion has a first surface on which signal light to be measured is incident and a second surface on which the branched light is emitted, and the second branch portion is on the second surface of the first branch portion. The first and second birefringent prisms facing each other, and a reflecting member disposed on the opposite side of the first branch portion with respect to the first and second birefringent prisms. 8. The Stokes parameter measuring device according to 7.   The optical axis of the birefringent prism constituting the first branching portion and the optical axes of the first and second birefringent prisms are arranged so as to have a relative angle of 45 ° when viewed from the optical axis direction. The Stokes parameter measurement device according to claim 8, wherein   The birefringent prism constituting the first branch portion and the first and second birefringent prisms are formed of the same material, and the polarization of the birefringent prism constituting the first branch portion. The Stokes parameter measuring device according to claim 9, wherein the separation width is √2 times the polarization separation width of the first and second birefringent prisms.   The second branching unit includes a first prism unit that splits the polarized light branched by the first branching unit in first and second oblique directions, and the polarized light branched by the first prism of the first group. A second light beam that generates a polarized light beam and a polarized light beam of the second group, and forms two sets of combined light, each of which combines the polarized light beam of the first group and the polarized light beam of the second group. The Stokes parameter measurement device according to claim 2, further comprising a prism. Branching the signal light to be measured into polarized light in a first direction and polarized light in a second direction by a first branching unit having a birefringent prism;
The second branching unit having a plurality of birefringent prisms further splits the polarized light branched by the first branching unit in two stages, and a first group of polarized light beams corresponding to the first direction, and the second Generating a second group of polarized light beams corresponding to the direction;
Combining any one or more polarized light beams of the first group and any one or more polarized light beams of the second group in a predetermined combination by the second branching unit;
A step of adjusting a polarization state by a polarization adjusting unit with respect to any one or more of the combined light fluxes;
Based on the output of the light receiving unit that photoelectrically converts the light component of the signal light branched and emitted from the second branching unit and the polarization adjusting unit, the light intensity component, the 0 ° polarization component, and the 45 ° polarization component that are Stokes parameters Determining a circularly polarized component and
A method for measuring a Stokes parameter, comprising:

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