JP2008249586A - Polarization analyzer - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an analyzer of simple constitution for a Stokes parameter, free from optical axis alignment, and allowing precise measurement. <P>SOLUTION: The analyzer includes a branching means 2 for branching a light beams 1 into at least four branch optical paths 3<SB>1</SB>-3<SB>4</SB>, a means 4 for carrying out prescribed photo-processing after generating the four branch optical paths, means 6<SB>1</SB>-6<SB>4</SB>for measuring respective light intensities from the four branch optical paths, and a means for calculating the Stokes parameter indicating a polarization state, based on the light intensities, and the branching means includes a diffraction grating 2<SB>g</SB>, and uses optical paths generated corresponding to different diffraction angles as the branch optical paths 3<SB>1</SB>-3<SB>4</SB>. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a polarization analyzer that can analyze the polarization state of laser light with high accuracy.

Development of high-speed optical digital transmission is underway, but especially at high speeds of 10 Gbit / sec or higher, transmission characteristics are affected by polarization mode dispersion that occurs in optical fibers, which has not been a problem at conventional transmission speeds. Is known to deteriorate. Polarization mode dispersion is a difference in group delay time between two orthogonal polarization modes due to deviation of the core portion of a single mode optical fiber from a perfect circle, lateral pressure on the fiber, etc., and they interfere with each other. It causes deterioration of transmission characteristics. Therefore, a measurement technique that can accurately evaluate the degree of polarization (DOP) in an optical fiber is important. Generally, evaluation is often performed using Stokes parameters (S ₀ , S ₁ , S ₂ , S ₃ ) composed of four parameters as DOP evaluation parameters. The Stokes parameter is widely used because the polarization state can be handled uniformly including completely polarized light (linearly polarized light, circularly polarized light, elliptically polarized light, etc.) and partial polarized light. ing. Specifically, the optical signal to be measured is divided into four parts, each is subjected to predetermined phase compensation processing and polarization filter processing, the light intensity after processing is measured, and predetermined calculation is performed based on this measured value. (See Non-Patent Document 1). Here, as a branching means for branching the optical signal to be measured into four, a method of branching a wavefront using an integrated unit of four prisms (Patent Document 1), a method of dividing an amplitude using a beam splitter (Patent Document) 2). In the former, four prisms are combined, a light beam is incident on the contact portion, and branches in four directions. In the latter, the light to be measured is sequentially branched into two by a plurality of beam splitters, and finally branched into four.
"Development of high-precision DOP measuring instrument" Furukawa Electric Times, No. 111 (refer to page 37 to page 38, Fig. 3, equations (1) to (3)) Japanese Patent Laid-Open No. 6-34444 (refer to reference numerals 11, 12, 13, and 14 in FIG. 4) JP-A-8-2011175 (see reference numerals 1, 2, and 3 in FIG. 1)

However, in the method using four prisms, the amount of light branched off due to fluctuations in the optical axis changes, so it is necessary to accurately align the optical axis with a predetermined position of the prism, which is applied to mechanical stability. There was a problem. In addition, the method using a beam splitter has a problem that it is difficult to obtain a highly accurate measurement result because a change in the ratio of polarization components and a change in phase occur in the transmission and reflection of light through the translucent plate. In addition, when it is used as a polarization monitor in consideration of application to polarization mode dispersion compensation, a branch for transmitting a signal is required in addition to the four branches for measurement. There was a problem that the optical system became more complicated.
Therefore, an object of the present invention is to provide a highly accurate ellipsometer having a branch circuit with a simple configuration and no optical axis alignment problem.

In order to achieve the above object, the ellipsometer according to the first aspect of the present invention includes, for example, a branching unit that branches the light beam (1) into at least four branching optical paths (3 _{1 to} 3 ₄ ) as shown in FIG. (2), means (4) for performing predetermined light processing after generating the four branched light paths, means (6 _{1 to} 6 ₄ ) for measuring the intensity of each light from the branched light paths, the light Means (9) for calculating a Stokes parameter indicating the polarization state from the intensity is provided, and the branching means includes a diffraction grating (2 _g ), and uses an optical path generated corresponding to a different diffraction angle as the branching optical path. .

Here, the predetermined optical processing is to obtain four parameters (S ₀ , S ₁ , S ₂ , S ₃ ) of Stokes parameters. For example, the polarization filter processing and the phase compensation processing are performed on the measured light. Specifically, it will be described later. The branching means (2) branches a branching optical path necessary for Stokes parameter measurement at a time, and is constituted by a single diffraction grating _2g or as shown in FIGS. 1 (b) and 1 (c). _g is configured as a component of the branching means. The diffraction angle is the angle between the optical axis that is deflected and emitted as a result of diffraction and the optical axis of the incident light, and the optical path that is generated corresponding to the diffraction angle is the light that is separated from the diffraction grating. Refers to each route.

  If comprised in this way, since many branch optical paths can be obtained at once, the number of parts can be reduced and the optical system of a measuring apparatus can be simplified. Further, since a diffraction grating is used as the branching means, the deviation of the optical axis does not matter. Furthermore, in the diffraction grating, the problem of deviation in the amplitude and phase of the polarization between the branched optical paths hardly occurs, so that highly accurate measurement can be performed.

Further, the ellipsometer according to the invention of claim 2 is the first branch optical path (3 ₁ ) of the branch optical paths (3 _{1 to} 3 ₄ ) according to claim 1, for example, as shown in FIG. _{Includes a} quarter-wave plate (12) and a 45 ° polarizer (11 _45B ), and a 45 ° polarizer (11 _45A ) is branched into the second branched light path (3 ₂ ) of the branched light paths. The third branch optical path (3 ₃ ) of the optical paths includes a 0 ° polarizer (11 ₀ ).

  Here, the quarter wavelength plate changes the phase difference of the orthogonal polarization components by 90 °, and the 0 ° polarizer and the 45 ° polarizer transmit only the polarization components of 0 ° and 45 °, respectively. Is. It should be noted that the 45 ° polarizer does not matter which direction is ± 45 ° with respect to the reference polarized light (0 °).

  With this configuration, many branching optical paths can be obtained at one time, so that the number of parts of the branching unit can be reduced and the optical system of the measuring apparatus can be simplified. Further, since a diffraction grating is used as the branching means, the deviation of the optical axis is not a problem. Furthermore, in the diffraction grating, the problem of deviation in the amplitude and phase of the polarization between the branched optical paths hardly occurs, so that highly accurate measurement can be performed.

Further, the ellipsometer according to the invention of claim 3 is the ellipsometer of claim 2, for example, as shown in FIG. 4, wherein the branching means (2) is a single diffraction grating (2 _g ). It is composed.

  If comprised in this way, since many branch optical paths can be obtained at once, the number of parts can be reduced and the optical system of a measuring apparatus can be simplified. Further, since a diffraction grating is used as the branching means, the deviation of the optical axis does not matter. Furthermore, in the diffraction grating, the problem of deviation in the amplitude and phase of the polarization between the branched optical paths hardly occurs, so that highly accurate measurement can be performed.

In addition, the ellipsometer according to the invention of claim 4 is characterized in that, as shown in FIGS. 5 and 6, for example, the first branch light path (3 ₁ ) of the branch light paths is 1 / A four-wave plate (12) and a 45 ° polarizer (11 _45B ) are provided, and a second branch optical path (3 ₂ ) of the branch optical paths is provided with a 45 ° polarizer (11 _45A ), and the branch means ( In 2), light from one branched optical path (3 ₆ ) other than the first branched optical path (3 ₁ ) and the second branched optical path (3 ₂ ) is converted to 0 ° polarization component and 90 ° polarized light. It has orthogonal polarization separation means (2 _s ) that separates the components into two branched optical paths (3 ₃ , 3 ₄ ).

  With this configuration, a large number of branches can be performed at a time with a small number of branching means, and a 0 ° polarizer is not necessary, so that the optical system of the measuring apparatus can be simplified. Further, since a diffraction grating is used as the branching means, the deviation of the optical axis is not a problem. Furthermore, in the diffraction grating, the problem of the deviation of the polarization component between the branched optical paths hardly occurs, so that highly accurate measurement can be performed.

Further, the ellipsometer according to the invention of claim 5 is the ellipsometer according to claim 5, for example, as shown in FIG. 6, wherein the branching means is a single diffraction grating (2 _g ), and Consists of the orthogonal polarization separation means (2 _s )

  If comprised in this way, since a number of parts can be reduced, an ellipsometer can be simplified.

The polarization analysis device of the invention according to claim 6, for example, as shown in FIG. 8, the ellipsometer according to any one of claims 2 to 5, wherein the first branch light path (3 ₁ ) And the 45 ° polarizer (11 _45A , 11 _45B ) provided in the _second branch optical path (3 ₂ ) are integrally configured (11 ₄₅ ).

  If comprised in this way, since a number of parts can be reduced, an ellipsometer can be simplified.

Further, the ellipsometer according to the invention of claim 7 is the ellipsometer according to any one of claims 1 to 6, for example, as shown in FIG. 9, wherein the branched optical path (3 _{1 to} 3 ₄ ) Is adjusted by the efficiency of branching into the branching optical path. The branching efficiency means that the light passes through each branch optical path (3 _{1 to} 3 ₄ ) with respect to the light intensity incident on the branching means (2) and reaches each polarizer (11) (no polarizer is inserted). For the branched optical path, it is the ratio of the light intensity of the means for measuring the light intensity (photoelectric conversion means) (when reaching 6 ₄ ). The branching efficiency includes the branching efficiency of the branching means (2) and the loss of each branch optical path (3 _{1 to} 3 ₄ ) including the polarizer (11) and the quarter wavelength plate (12). May be determined. The adjustment may be performed at the stage of the optical signal as in the optical level adjustment means (23), or may be performed at the stage of the electrical signal after obtaining the measurement data as in the level compensation means (24). Furthermore, the calculation may be performed by the data processing unit (9).

  If comprised in this way, the difference of the branching efficiency of branching optical path (3) including the loss of polarizer (11), quarter wave plate (12), etc., and the conversion efficiency of photoelectric conversion means (6) It is also possible to adjust the error due to the difference between the two and obtain a highly accurate measurement result.

In addition, the ellipsometer according to the invention of claim 8 is, for example, as shown in FIG. 10, the ellipsometer according to any one of claims 1 to 7, other than the branched optical path used for ellipsometry. The light obtained by the branched optical path (branched optical path 3 _{0 in} FIG. 10) is used as signal light. As the branching optical path other than the branching optical path, the branching optical path obtained by the diffraction grating can be arbitrarily selected as long as it is other than the branching optical path for Stokes parameter measurement.

  According to the ellipsometer of the present invention, since the light is branched using the diffraction grating, the deviation of the optical axis is not a problem. Furthermore, by using a diffraction grating, the problem of deviation of the polarization component between the branched optical paths hardly occurs, so that highly accurate measurement can be performed.

Embodiments of the present invention will be described below with reference to the drawings. In addition, in each figure, the same code | symbol is attached | subjected to the mutually same or equivalent member, and the overlapping description is abbreviate | omitted. Further, as the 3 _1, is that there is a subscript in numerals, there are a plurality of the like represented by reference numeral 3 indicates that one of them. In the present specification, the “n-th branch optical path” has no relation to the subscript number or the like and “n”. In addition, “′” in the reference sign indicates that the function is the same, but includes a part that differs in configuration.

Prior to the description of the embodiments, the Stokes parameter measurement principle will be briefly described.
FIG. 12A is a diagram showing a measurement principle by four branches. The light beam 1 to be measured is branched into four branch light paths (# 1 to # 4) by the branching means 2, and the light flux in the branch light path # 1 changes the phase difference between two polarization components orthogonal to each other by 1 °. / 4 wavelength plate 12, after passing through only a 45 ° polarizer _{11 45B} passing polarized component inclined 45 ° with respect to the reference polarization, is detected as the power _{I q45} in photoelectric conversion unit _61. Light beam branching optical paths of # 2 passes only polarized component inclined 45 ° with respect to the reference angle of polarization, after passing through a 45 ° polarizer 11 _45A, the optical electric converting means 6 ₂ detects a power I _45. # 3 of the light beam branching optical paths reference polarization (reference angle: 0 °) was passed through the 0 ° polarizer 11 ₀ to pass only components, from photoelectric conversion unit 6 ₂ light energy (light intensity) into electric energy Convert and detect as power I ₀ . Light beam branching optical paths of # 4 directly enters the photoelectric conversion unit 6 ₄ is detected as the power I _t. Using the obtained four power detection results, the Stokes parameter is obtained by the following equation using the calculation means 9.
S ₀ = I _t (1-1)
S ₁ = 2 · I ₀ −I _t (1-2)
S ₂ = 2 · I ₄₅ −I _t (1-3)
S ₃ = 2 · I _q45 −I _t (1-4)

FIG. 12 (b) shows the principle of measurement by another method. (A) differs from the point for detecting the branched light path is 90 ° polarizer 11 ₉₀ is passed through only 90 ° polarization component intensity of the branched light path # 4, the operation content is different from the operation unit 9 . Since the configurations of the branched optical paths of # 1, # 2, and # 3 are the same as those of # 1, # 2, and # 3 of (a), the outputs of the photoelectric conversion means 6 ₁ , 6 ₂ , and 6 ₃ are respectively I _q45 , I ₄₅ , and I ₀ . The output of the photoelectric conversion unit 6 _4, when I _90,, determine the Stokes parameters by the following equation using an arithmetic unit 9 '.
S ₀ = I ₀ + I ₉₀ (2-1)
S ₁ = I ₀ −I ₉₀ (2-2)
S ₂ = 2 · I _45- (I ₀ + I ₉₀ ) (2-3)
S ₃ = 2 · I _q45 − (I ₀ + I ₉₀ ) (2-4)

  As described above, the Stokes parameters can be obtained by the method described in FIG. If Stokes parameters are used, it is possible to uniformly evaluate the polarization state including complete polarization such as linearly polarized light, circularly polarized light, elliptically polarized light, and partial polarized light. In this measurement method, it is important that the measurement system does not disturb the polarization state of the light to be measured as much as possible in order to obtain highly accurate measurement data. In particular, the branching unit 2 is susceptible to fluctuations in the polarization state.

FIG. 1 shows a first embodiment of the invention according to the present application. The light beam 1 to be measured is branched into four by the branching means 2 to be branched optical paths 3 ₁ , 3 ₂ , 3 ₃ , 3 ₄ . The branching means 2 may be composed of one branching means element (diffraction grating 2 _g ) as shown in FIG. 4, or two or more branching means elements (see FIGS. 1B and 1C). _2g , _2h , _2h ', etc.) may be combined. Moreover, it is preferable that two or more branching means elements (2 _g , 2 _h , 2 _h ′) are diffraction gratings. Each of the light processing means 4 includes a function of allowing only a specific polarization component to pass therethrough and shifting the phase between the polarizations. For example, the light processing means 4 includes the polarizer 11 and the quarter wavelength plate 12 described with reference to FIG. Is. Specific contents of the light processing means 4 will be described later. The photoelectric conversion element array 5, for example, a photoelectric converter ₆₁ through _4, such as a photodiode obtained by arranging a plurality of light processing means 4 of the output light flux of just the light beam spot 7 is a photoelectric conversion element array on 5 It overlaps with the light receiving part of the electrical conversion means 6. In this way, the intensity of the branched light flux that has undergone the predetermined light processing is converted into the detected electrical signals 8 ₁ to 8 ₄ , and the calculation means uses the equations (1) and (2) to calculate S _{0 to} S ₃ . The value is calculated and output as measurement output 10. In addition, although what was integrated as the photoelectric conversion element array 5 was shown in this figure, you may comprise separately. The arithmetic means 9 can be integrated with the photoelectric conversion element array 5.

The feature of this embodiment is that the branching means 2 is configured to include a diffraction grating. As is well known, when a parallel light beam is incident on the diffraction grating, the light beam is separated at various diffraction angles. This is shown in FIG. Phase difference angle as (optical path difference phase angle corresponding to the distance h in FIG. 2) is an integral (m) times the wavelength (diffraction angle of the diffraction grating 2 _g collimated light beam 1 incident on a like adjacent slits 17 A bright line is formed in the direction of θ), and the luminous flux can be separated and extracted. Here, the m value is referred to as the order. The branching angle (diffraction angle θ) can be controlled by selecting the pitch Λ of the diffraction grating. Deviation of the polarization component of the diffraction grating 2 _g is in the range (about 15 °) angle of diffraction is not large, it is known that not a serious problem, in the diffraction grating 2 _g, branched light path at a diffraction angle within this range If 3 is obtained, highly accurate measurement can be performed. Various With orders of branch light path can be simultaneously separating a plurality of branched lights little deviation of the polarization components at one time by the diffraction grating 2 _g.

As shown in FIG. 1C, a diffraction grating can also be used for the branching element 2 _h ′. Between the two branching element elements (2 _g , 2 _h ) shown in (b), either one may be a diffraction grating or both may be a diffraction grating.

FIG. 3 is for explaining the second embodiment, and shows an example in which the branching means 2 is constituted by two branching means elements, that is, a diffraction grating 2 _g and a branching element 2 _h . Further, FIG. 3 specifically shows the light processing means 4 inserted in each branch optical path (3 _{1 to} 3 ₄ ) of FIG. 1 as 4 _A , 4 _B and 4 _{C in the} drawing. The branched light path _{3 1,} 1/4-wave plate 12 and 45 ° polarizer _{11 45B,} is inserted. The branched light path _{3 2,} 45 ° polarizer _{11 45A} is inserted. The branched light path _{3 3,} 0 ° polarizer 11 ₀ is inserted. In branched light path 3 _4, branched light beam is directly irradiated to the light converting means 6 _4. This configuration corresponds to the configuration of FIG. The collimator lens 15 converts the optical signal from the optical fiber 14 into the parallel light beam 1. Note that the optical processing method having the configuration described here is the same as that in the case of FIG. 12A, and the calculation of the calculation means 9 is performed using the equation (1). Further, as another embodiment, further, there is also a form of inserting a 90 ° polarizer branched light path 3 _4. In this case, the calculation of the calculation means 9 is performed using equation (2).

FIG. 4 illustrates another configuration of the second embodiment, in which a single diffraction grating 2 _g is used as the branching means 2. If constituted in this way, since all the branched optical paths (3) can be obtained with one diffraction grating _2g , the apparatus can be further miniaturized. Moreover, since the diffraction grating is used, a highly accurate measurement result can be obtained with little change in the polarization component at the branching means.

FIG. 5A illustrates the third embodiment. The branching unit 2 includes a diffraction grating 2 _g , a branching unit 2 _h , and an orthogonal polarization separating unit 2 _s . The diffraction grating 2 _{g divides} the parallel light beam 1 into the branched optical paths 3 ₅ and 3 ₆ , and then the branched optical path 3 ₅ is further branched by the branching means element 2 _h , and the branched optical path 3 ₁ and the branched optical path 3 ₂ obtain. Branched light path 3 ₆ is connected to the orthogonal polarization separator 2 _s, is separated into 0 ° polarized light component and 90 ° polarization component, respectively are output as a branch path 3 _3, 3 _4. The light intensity of the light beams of the above four branched optical paths (3 ₁ , 3 ₂ , 3 ₃ , 3 ₄ ) is measured by the photoelectric conversion means (6 ₁ , 6 ₂ , 6 ₃ , 6 ₄ ).

FIGS. 5B and 5C are views of the branched optical path 3 ₆ , the orthogonal polarization separation means 2 _s , and the branched optical paths 3 ₃ , 3 ₄ in FIG. (B) uses a birefringent element 2 _sA as the orthogonal polarization separation means 2 _s . As is well known, a birefringent element emits a light beam to the outside with a different optical path for each polarized light component with respect to a light beam containing orthogonal polarized light components, for example, a calcite crystal. (C) uses a combination of the polarization beam splitter 2 _sB1 and the right-angle prism 2 _sB2 as the orthogonal polarization separation means 2 _s . Each orthogonal polarization components is obtained is separated from the transmission optical path 3 ₄ by the polarization beam splitter 2 _SB1 as reflected light path 3 _3. Right-angle prisms 2 _SB2 provided in contact with the polarizing beam splitter 2 _SB1 is to bend the direction of the branch light path 3 ₃ perpendicularly oriented in the direction of the optical path to the photoelectric conversion unit 6 _3. Note that even if the polarization component fluctuates due to reflection by the right-angle prism _2sB2 , it does not affect the light intensity, so that no measurement error occurs. The photoelectric conversion element array 5 ′ is equivalent to the photoelectric conversion element array 5 described with reference to FIG. 3, but on the photoelectric conversion element array 5 of the photoelectric conversion means 6 according to the spatial arrangement of the branch optical path 3. The arrangement of is different.

With this configuration, a plurality of branched optical paths can be obtained with one diffraction grating _2g , and a 0 ° polarizer is not necessary, thus simplifying the apparatus. Further, since the diffraction grating _2g is used, a highly accurate measurement result can be obtained with little fluctuation of the polarization component at the branching means.

FIG. 6 explains another configuration of the third embodiment, and shows a more specific configuration of FIG. 5. A single diffraction grating 2 _g as the branching means 2 and orthogonal polarization are shown. The separation means _2s is used. If comprised in this way, a measuring apparatus can further be simplified.

FIG. 7A shows a specific structure of the diffraction grating _2g . (A) is a so-called slit-type diffraction grating, in which thin non-transparent parallel lines are arranged on a thin transparent substrate 19, and the transmissive portion 20 _B (corresponding to 17 in FIG. 2) and the non-transmissive portion 20 _A are pitched. These are provided at intervals of Λ. As described with reference to FIG. 2, the diffraction angle θ is determined by the wavelength λ of the light beam to be measured and the pitch Λ, and the diffraction angle (branching angle θ) decreases as the value of the pitch Λ increases. As described above, the diffraction grating has the characteristic that the deviation of the polarization component is small, but if an extremely large diffraction angle is taken, there may be a problem of fluctuation of the polarization component. According to the experiment of the inventors of the present invention, it is desirable to select the value of Λ so as to satisfy the expression (3).
Λ> 10λ (3)
The diffraction angle θ is also determined by dimensional conditions determined by the distance from the diffraction grating 2 _g to the photoelectric conversion element arrays 5, 5 ′, the distance between the photoelectric conversion means 6, and the branched light beam diameter. If the value of Λ is reduced within a range satisfying the expression (3), the distance between the branching means 2 and the photoelectric conversion element arrays 5 and 5 ′ can be reduced, and the ellipsometer can be miniaturized. On the other hand, when the interval between the branching unit 2 and the photoelectric conversion element arrays 5 and 5 ′ is given, the interval between the photoelectric conversion units 6 _{1 to} 6 ₄ can be reduced by increasing the value of Λ.

FIG. 7 (b) shows another specific structure of the diffraction grating _2g . (B) (c) is what is called a phase grating, and (b) is a diffraction grating in which a bank-like portion 21 having a height d is formed on a transparent substrate with a pitch Λ.
The characteristics of the phase grating are evaluated using the following Q values. In other words, the Q value gives a rough indication of the thickness of the grating. As this value increases, the effect of Bragg diffraction becomes stronger, and the difference in diffraction efficiency due to the vibration direction of the light wave becomes significant. According to an experimental study by the inventors of the present invention, it is desirable that the following condition is satisfied as the Q value.
Q = (2πλd / (nΛ ² )) <1/10 (4)

As described above, by setting the specifications of the diffraction grating 2 _g to a value within the range calculated by the equations (3) and (4), the fluctuation of the polarization component which becomes a problem when the branch angle is large, and the branch optical path The difference in diffraction efficiency can be suppressed to a certain value or less, and a highly accurate measurement result can be obtained. As described above, there is a lower limit on the value of Λ from the viewpoint of measurement accuracy, but the distance between the diffraction grating 2 _g and the photoelectric conversion element array 5 is reduced so that the size of the analyzer is as small as possible. In addition, there is an upper limit in the value of Λ in order to separate the light beams of the branch optical path 3 on the photoelectric conversion element array.
As a diffraction grating 2 _g, in the description of the present application embodiment has been described of a transmissive type, it is of course possible using a reflection type and stairs lattice type.

FIG. 8 is a diagram for explaining the configuration of the fourth embodiment, which is a modification of the configuration of FIG. The functions of the 45 ° polarizers 11 _45A and 11 _45B in FIG. 4 are shared by one polarizer 11 ₄₅ . As shown in FIG. 8, it is arranged two single 45 ° polarizer 11 ₄₅ of the two branch light path 3 ₁ and 3 ₂ to cover. With such a configuration, it is possible to reduce the number of the polarizer 11 _45. Note that this embodiment is not limited to the application to the configuration of FIG. 4 and can be applied to all cases where a plurality of 45 ° polarizers are used.

FIG. 9 is a diagram for explaining the fifth embodiment. The configuration for improving the measurement accuracy by compensating for the imbalance in the branching efficiency of the branching unit will be described as an example applied to the configuration of FIG. FIG. It is needless to say that the present invention is not limited to the configuration of FIG. 4 and can be applied to all other configurations of the present invention. When the branching unit 2 is used, the light levels of the branched branching optical paths 3 may not be uniform due to unbalanced branching efficiency. This arrangement is such to insert the light level adjusting means 23 ₁ to 23 ₄ to provide a predetermined attenuation with respect to light of each branch light path 3 ₁ to 3 ₄ if, on the basis of the amount of attenuation to the difference in branch efficiency Set. Further, each loss amount of each branched light path 3 from the entrance end of the branch unit 2 _g to the entrance end of the optical electric converting means 6 pre-measured, a light level adjusting means 23 ₁ to 23 ₄ in consideration of the loss by adjusting the attenuation, splitting means not only unbalanced branching efficiency of 2 _g, the propagation loss of the polarizer 11,1 / 4 loss wavelength plate 12 and the branch optical path 3 ₁ to 3 _4, photoelectric converting means 6 can be compensated including imbalances such as conversion efficiency, and the measurement accuracy of the ellipsometer can be increased. The level compensation means 24 is performed after the branching efficiency imbalance including the branching efficiency is converted into an electrical signal, and the same effect as the compensation of the optical stage using the light level adjusting means 23 can be obtained. . Further, it can be compensated by the calculation of the calculation means 9.

Figure 10 shows a sixth embodiment, the configuration of the branched light path 3 ₀ other than the branch path to be used for ellipsometric branched from the diffraction grating 2 _g, enters the optical fiber 14 'as a signal light It is. In the figure, it passes through holes 18 in the photoelectric conversion element array 5 for passing a signal branched light path 3 ₀ is provided. By adopting such a configuration, the polarization state can be measured while the signal light is transmitted, so that it can be used as a polarization monitor for polarization mode dispersion compensation in an optical fiber communication system. In this figure, the 0th-order optical path is taken as an example as the signal branching optical path. However, the order is not limited to this, and other orders may be selected.

Figure 11 shows a seventh embodiment, the branch in the optical path ₃ 1 to 3 ₄ to insert the deflecting prism ₂₅ to 253 _4, the optical axis of the branch path 3 branched by the diffraction grating _{2 g} Is made perpendicular to the photoelectric conversion element array 5 so that all branched light beams are incident on the photoelectric conversion means 6 in the vertical direction. By adopting such a configuration, the measurement light beam can be incident on all the photoelectric conversion means at a constant angle, the unbalance of the photoelectric conversion efficiency due to the branched light paths 3 _{1 to} 3 ₄ can be reduced, and the photoelectric conversion efficiency can be reduced. Can be maximized. Here, the case where the incident angle is vertical has been described. However, as long as the countermeasure is only for imbalance, if the incident angle is the same, the incident angle is not limited to being vertical. Further, the detection value compensation of the light intensity by the unbalance of the incident angle can be performed even with the configuration shown in FIG.

  In the embodiment of the present application, the optical processing for measuring the Stokes parameter has been described with respect to the configuration in which the phase compensation processing, the polarization filter processing, or the orthogonal polarization separation processing as shown in FIGS. 3 and 5 is combined. It is obvious that the technical scope of the present application extends to a configuration in which non-measurement light is branched into a plurality of branch optical paths and predetermined optical processing is performed.

  The present invention can be used for a measuring instrument that measures the polarization state of an optical fiber and a polarization monitor of a polarization mode dispersion compensator of a communication system using the optical fiber. Further, it can be widely used for detection of various stresses by utilizing the fact that the stress on the fiber affects the polarization state of the transmitted light.

It is a figure explaining 1st Embodiment which concerns on this invention. It is a figure explaining a mode that a parallel light beam is isolate | separated into a several branch optical path with a diffraction grating. It is a figure explaining the structure of 2nd Embodiment. It is a figure explaining another structure of 2nd Embodiment. It is a figure explaining the structure of 3rd Embodiment. It is a figure explaining another structure of 3rd Embodiment. It is a figure explaining the structure of a diffraction grating. It is a figure explaining the structure of 4th Embodiment. It is a figure explaining the structure of 5th Embodiment. It is a figure explaining the structure of 6th Embodiment. It is a figure explaining the structure of 7th Embodiment. It is a figure explaining the measurement principle of a Stokes parameter.

Explanation of symbols

1 Light beam 2 Branching means 2 _g diffraction grating (branching means element)
2 _h , 2 _h ′ Branch means element 2 Diffraction grating 2 _gB using _gA slit, 2 _gC phase grating 2 _s orthogonal polarization separation means 2 _sA birefringence element 2 _sB1 polarization beam splitter 2 _sB2 right angle prism 3 branch optical path 4 optical processing Means 5, 5 ′ Photoelectric conversion element array 6, 6 ′ Photoelectric conversion means 7 Optical spot 8 Detected electrical signal 9 Calculation means 10 Measurement output 11 Polarizer 11 ₀ 0 ° polarizer 11 ₉₀ 90 ° polarizer 11 _45A , 11 _45B 45 ° polarizer 12 ¼ wavelength plate 14, 14 ′ optical fiber 15, 15 ′ collimator lens 16 orthogonal polarization separation means 16 _A birefringence element 16 _B polarization beam splitter 17 slit 18 passage hole 19 substrate 20 _A non-transmission portion 20 _B transmission portion 21 bank-shaped portions 23 the light level adjusting means 24 level compensation means 25 deflecting prism 100 ellipsometer 101,1 1 'polarization state detecting portion

Claims (8)

Branching means for branching the light beam into at least four branching optical paths;
Means for performing predetermined light processing after generating the four branched light paths, means for measuring the light intensity from each of the branched light paths, and means for calculating a Stokes parameter indicating a polarization state from the light intensity;
The branching unit includes a diffraction grating, and uses an optical path generated corresponding to a different diffraction angle as the branching optical path;
Ellipsometer. Of the branched light paths, the first branched light path includes a quarter wavelength plate and a 45 ° polarizer;
Among the branched light paths, a second branched light path has a 45 ° polarizer;
A third branching light path of the branching light paths comprises a 0 ° polarizer;
The ellipsometer according to claim 1. The branching means comprises a single diffraction grating;
The ellipsometer according to claim 2. Of the branched light paths, the first branched light path includes a quarter wave plate and a 45 ° polarizer;
A second branched light path of the branched light paths includes a 45 ° polarizer;
The branching unit separates light from one branching optical path other than the first branching optical path and the second branching optical path into a 0 ° polarization component and a 90 ° polarization component, and provides two branching optical paths. Having orthogonal polarization separation means to generate;
The ellipsometer according to claim 1. The branching means comprises a single diffraction grating and the orthogonal polarization separating means;
The ellipsometer according to claim 4. The 45 ° polarizer provided in the first branch optical path and the second branch optical path are integrally configured;
The ellipsometer according to any one of claims 2 to 5. Adjusting the light intensity with the efficiency of branching into the branching optical path;
The ellipsometer according to any one of claims 1 to 6. Using the light obtained in the branch optical path as signal light;
The ellipsometer according to any one of claims 1 to 7.

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