JP2006268009A - Wide-band wavelength plate and adjusting method for wide-band wavelength plate - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a wide-band wavelength plate which is so structured as to have high transmissivity and also easily adjustable to a desired phase difference even when the phase difference deviates from the desired value and an adjusting method for the wide-band wavelength plate. <P>SOLUTION: This wide-band wavelength plate 10 is constituted by arranging two wavelength plates 11 and 12, which have periodic structures 15 and 16 generating phase differences by microstructures having cycles of ≥1/n<SB>min</SB>of the wavelength of the light with the shortest wavelength among lights in use, opposite each other so that their main axes are not in parallel to each other. Here, n<SB>min</SB>represents the refractive index of the material of the wavelength plates to the light with the shortest wavelength. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a broadband wavelength plate using a plurality of wavelength plate elements and a method for adjusting a broadband wavelength plate.

  A wave plate using structural birefringence requires a high aspect ratio to obtain a generally required phase difference (λ / 4, λ / 2) in addition to the structure being in the wavelength order. Therefore, there is a problem that it is difficult to manufacture and it is difficult to obtain reproducibility. Furthermore, in the case of a structure having a broadband property, the transmittance varies depending on the height of the structure. Therefore, a high transmission efficiency is not always obtained at the height of the structure for obtaining an optimum phase difference.

  Further, as shown in FIG. 5, in the conventional configuration in which two wave plates A and B are combined to match the direction of the two wave plates A and B (main axes a and b), the wave plates A and B are combined. However, if a manufacturing error occurs, it will deviate from the desired phase difference.

  The following Patent Document 1 describes a polarization axis rotating laminated phase difference in which at least two retardation plates having a retardation value of 160 to 300 nm are laminated so that their slow axes are not parallel or orthogonal to each other. Disclose the board. This phase difference plate is intended to have a broadband property with respect to the wavelength by shifting the slow axis of each of the two wave plates.

Patent Document 2 below discloses a holographic optical element in which two relief gratings are juxtaposed on an optical axis at a predetermined relative grating groove angle, and the relative angle of the grating grooves of the two relief gratings is adjusted. Thus, the phase difference is adjusted, but no consideration is given to the transmittance of the optical element.
JP-A-10-90521 JP 63-155107 A

  In view of the above-described problems of the prior art, the present invention has a structure capable of obtaining a high transmittance, and a broadband wave plate and a broadband that can be easily adjusted to a desired phase difference even when the phase difference deviates from a desired value. An object of the present invention is to provide a method for adjusting a wave plate.

In order to achieve the above object, the broadband wave plate according to the present invention has a periodic structure in which a phase difference is generated by a fine structure having a period of 1 / n _min or more of the wavelength of the shortest light in the light to be used. It is characterized in that at least two wave plates are arranged to face each other so that their main axes are non-parallel. Where n _{min is} the refractive index of the wave plate material for the shortest wavelength light.

According to this broadband wave plate, in the wave plate having a periodic structure in which a phase difference is generated by a fine structure having a period of 1 / n _min or more of the wavelength of the shortest wavelength, the height of the structure capable of obtaining a high transmittance. Even if the phase difference deviates from the desired value, it can be adjusted to the desired phase difference by making at least two wave plates face each other so that their main axes are not parallel. A broadband wave plate having transmittance and a desired phase difference can be realized with a simple configuration.

  In the broadband wave plate, when the phase difference of the at least two wave plates is δ1 and δ2, respectively, 0 ° <δ1 <135 °, 0 ° <δ2 <135 °, 90 ° <δ1 + δ2 <270 °. It is preferable.

  Further, since at least one of the wave plates has a structural dimension that satisfies the following expression (1), high transmittance and a desired phase difference (λ / 4) can be obtained.

H = a ₁ × f + b ₁ + c ₁ (1)
However, −d ₁ ≦ c ₁ ≦ + d ₁
a ₁ = −10 × P + 4.6
b ₁ = −18.560 × P ² + 27.684 × P−6.8299
d ₁ = −27.273 × P ² + 18.994 × P-3.15
P: Structural period (μm)
H: Structure height (μm)
f: Filling factor (= L / P, where L: structure width (μm))

  Further, since at least one of the wave plates has a structural dimension that satisfies the following formula (2), high transmittance and a desired phase difference (λ / 4) can be obtained.

H = a ₂ × f + b ₂ + c ₂ (2)
However, −d ₂ ≦ c ₂ ≦ + d ₂
a ₂ = −10 × P + 4.6
b ₂ = −76.515 × P ² + 69.335 × P-13.825
d ₂ = −54.663 × P ² + 36.782 × P−6.02
P: Structural period (μm)
H: Structure height (μm)
f: Filling factor (= L / P where L: structure width (μm))

  In the method for adjusting a broadband wave plate according to the present invention, at least two wave plates having a structure causing a phase difference due to a fine periodic structure are arranged to face each other, and the angle formed by the principal axes of the wave plates is set to each phase difference. And a desired phase difference and polarization state are obtained by adjusting according to the polarization dependence of the transmittance.

  According to this wideband wave plate adjusting method, if a manufacturing error occurs in each wave plate, a single plate configuration or a configuration in which two plates match the direction of the structure (main axis direction) will deviate from a desired phase difference. , At least two wave plates are placed facing each other, and the angle between the main axes of each wave plate is adjusted according to the polarization dependence of each phase difference and transmittance to adjust to the desired phase difference Can be adjusted to a desired polarization state in consideration of the polarization dependence of transmittance, for example, a λ / 4 wavelength plate can convert linearly polarized light into complete circularly polarized light, With a λ / 2 wavelength plate, linearly polarized light can be converted into linearly polarized light that is completely orthogonal.

  In a birefringent element such as a wave plate, the direction in which light travels fast (the phase advances) is called the fast axis of the element, and the slow direction (the phase is delayed) is called the slow axis. The fast axis and slow axis are collectively referred to as the main axis.

  According to the broadband wavelength plate of the present invention, a high transmittance and a desired phase difference can be realized with a simple configuration. In addition, according to the method for adjusting a broadband wave plate of the present invention, a structure with a high transmittance can be obtained, and even when the phase difference deviates from a desired value, it can be easily adjusted to the desired phase difference.

  The best mode for carrying out the present invention will be described below with reference to the drawings. FIG. 1 is a perspective view schematically showing a broadband wave plate according to the present embodiment. FIG. 2 is a graph showing an example of the relationship between the structural height, transmittance, and phase difference (wavelength λ = 405 nm) of one wave plate element. FIG. 3 is a view for explaining the deviation angle (shift direction) of the principal axis of each wave plate element in the broadband wave plate of FIG. FIG. 4 is a graph illustrating an arrangement for obtaining a quarter wavelength plate with the broadband wavelength plate of FIG. FIG. 6 is a diagram showing the relationship between the filling factor and the phase difference normalized by the phase difference when the wavelength is 405 nm in the wave plate element.

  As shown in FIG. 1, the broadband wave plate 10 includes a first wave plate element 11 having a fine uneven periodic structure 15 on a back plate 13 and a second wave plate having a fine uneven periodic structure 16 on the back plate 14. Wavelength plate element 12. The first wave plate element 11 and the second wave plate element 12 are arranged to face each other so that the planes are parallel to each other, but the main shaft 11a and the main shaft 12a of each wave plate element 11, 12 are arranged. They are arranged so as to be shifted and non-parallel.

  The uneven periodic structure 15 of the first wave plate element 11 and the uneven periodic structure 16 of the second wave plate element 12 have, for example, the same design and the same characteristics.

  Here, in the concavo-convex periodic structure of one wave plate element, the relationship between the height H (see FIG. 1) of the concavo-convex periodic structure and the transmittance (TE and TM) and the relationship between the structural height H and the phase difference are as follows. In each case as shown in FIG. 2, when the structure height H is 2 μm, a desired phase difference (λ / 4) can be obtained as shown by a broken line in the figure, but the transmittance is low. As shown in FIG. 2, in the case of a structure in which a single wave plate element has a wide bandwidth, the transmittance varies depending on the height of the structure. Therefore, the structure height for obtaining a desired phase difference (λ / 4) is not necessarily used. High transmission efficiency is not always obtained.

In FIG. 6, the pitch (period) of the concavo-convex periodic structure is P, the structure width is L, and the filling factor is f (= L / P). Compare the 700 nm phase difference. Assuming that the broadband property is within ± 20 degrees as a general value, it can be seen from FIG. 6 that the design value that can exist is λ _min / n <P (n: refractive index). In addition, although the material was calculated as polyolefin resin, there is no big difference in a general optical resin material.

  In order to obtain the phase difference λ / 4 with a wave plate made of a general optical resin material, the structure height H of the concavo-convex periodic structure is required to be at least 2 μm, and the manufacture is not easy. Manufacture becomes easy by combining the wave plate elements. Further, in this region, the transmittance at a short wavelength greatly varies as shown in FIG. 2, and therefore, when a high transmittance is desired, a desired phase difference (for example, an integral fraction of λ / 4 or λ / 2) is obtained. ) Is not obtained, and it is necessary to adjust the orientation (angle) of the main axis in order to obtain a high transmittance and a desired phase difference.

  Therefore, when each of the wave plate elements 11 and 12 in FIG. 1 has the concave and convex periodic structure in FIG. 2, for example, the structure height H is designed to have a high transmittance (H is about 1.3 μm) as shown by a solid line. Then, the desired phase difference (λ / 4) is obtained by combining the first and second wave plate elements 11 and 12 as shown in FIG. 1 and adjusting their main axes 11a and 12a so as to be non-parallel. Can do.

  Here, as shown in FIG. 3, the main axis a before adjustment of the first wave plate element A and the main axis b before adjustment of the second wave plate element B are, for example, the polarization directions of incident light on the x axis. However, the main axis a before adjustment of the first wave plate element A is shifted by θa to be the main axis a ′, and the main axis b before adjustment of the second wave plate element B is θb. The main axis a ′ after adjustment of the first wave plate element A and the main axis b ′ after adjustment of the second wave plate element B are shifted so as to be non-parallel by using the shifted main axis b ′. At this time, a desired phase difference (λ / 4) is obtained by adjusting the deviation angle θa and the deviation angle θb.

For example, when combining the first wave plate element 11 and the second wave plate element 12 having the same design and the same characteristic with a fine irregular periodic structure as shown in FIG. 1, the first wave plate as shown in FIG. By determining the deviation angle θa of the principal axis of the element 11 and the deviation angle θb of the principal axis of the second wave plate element 12, a quarter wavelength plate can be obtained. That is, assuming that the phase difference between the wave plate elements 11 and 12 is δ 1 and δ 2, the phase difference δ 1 between the wave plate elements 11 and 12 is within a range that satisfies the following conditional expressions (3), (4), and (5). , Δ2 is, for example, 100 °, the broadband wavelength plate 10 shown in FIG. 1 is adjusted by adjusting the main shafts 11a and 12a so that θa is about −5 ° and θb is about 40 °. Can function as a quarter-wave plate.
0 ° <δ1 <135 ° (3)
0 ° <δ2 <135 ° (4)
90 ° <δ1 + δ2 <270 ° (5)

  Conventionally, in a wave plate using structural birefringence, the structure is fine in the wavelength order, and a high aspect ratio is required to obtain the required phase difference (λ / 4, λ / 2). It is difficult to manufacture and it is difficult to obtain reproducibility. Furthermore, in the case of a wideband structure, high transmission efficiency is not always obtained with a structure height that obtains an optimum phase difference. According to the broadband wave plate of the embodiment, each wave plate element is designed to have a structure height capable of obtaining a high transmittance, and even if the phase difference deviates from a desired value, the two wave plate elements are combined and desired By adjusting to the phase difference, a desired phase difference can be obtained with a simple configuration like the broadband wave plate 10 of FIG.

  In addition, when a manufacturing error occurs in each wave plate element, in a configuration in which one plate is configured or two plates are aligned in the direction of the structure (main axis direction) as illustrated in FIG. Even in such a case, the desired phase difference can be easily adjusted by shifting the main axis as shown in FIG. As described above, according to the adjustment method of the broadband wave plate of the present embodiment, even if there is a manufacturing error in each wave plate element, it is possible to adjust the desired phase difference of the design by adjusting the shift of the main axis.

  The retardation plate of the above-mentioned Patent Document 1 is intended to give a broadband property to the wavelength by shifting the slow axis of each of the two wavelength plates, whereas the broadband wavelength plate of the present invention is In addition, the structure is provided with a wide band and the phase difference of the design is adjusted by shifting the main axis.

  Next, in the present embodiment, two wavelength plate elements are combined and a desired phase difference is obtained by adjusting a deviation angle of each main axis, and angle adjustment in consideration of polarization dependency of transmittance is illustrated. This will be described with reference to FIG.

  The basic action of the quarter wave plate when the main axis is arranged at 45 ° with respect to the polarization direction of incident light having linear polarization is to convert the linear polarization into circular polarization or vice versa. When incident light is used, it is converted into linearly polarized light. However, even if the phase difference is λ / 4, if the transmittance due to the polarization component is different, even if the main axis is arranged at 45 ° with respect to the polarization direction of the incident light, complete circular polarization cannot be obtained and the elliptical It becomes polarized light. As a result, it is not a satisfactory wave plate for general optical equipment.

  In particular, a wave plate using structural birefringence has a polarization direction orthogonal to incident light (TE wave) having a polarization direction parallel to the structure due to the shape parameters (height, pitch, filling factor) of the microstructure. Because the transmittance deviation differs depending on the incident light (TM wave), the transmittance of the TE wave and the TM wave does not always match even with a design value that is judged to be good from the broadband property and the average transmittance of TE / TM. Absent. Therefore, when two wave plate elements are arranged to face each other, it is necessary to adjust the angle in consideration of the polarization dependency of the transmittance as well as the phase difference.

  Hereinafter, an angle calculation method when two wave plate elements are arranged to face each other will be described. When considering the polarization-dependent component of the wave plate and the component causing the phase difference, the Stokes parameter conversion matrix is used to represent the partial polarizer matrix (PO) and the phase shifter matrix (C), respectively. (Refer to "Applied Optics I and II" by Tatsuta Tatsuo, Bafukan).

If the partial polarizer matrix (PO) and the retarder matrix (C) of the wave plate element A and the wave plate element B in FIG. 3 are PO _A , C _A , PO _B , and C _B , respectively, PO _A , C _A , PO _B and C _B can be expressed as in formulas (6), (7), (8), and (9) in the following equation (1). When the Stokes parameter of the incident light is S and the Stokes parameter of the outgoing light is S ′, the relationship can be expressed by the following equation (11). If the polarization dependence is not taken into consideration, it can be expressed by equation (10).

  Here, the Stokes parameter S representing the linearly polarized light with an angle of 0 degrees is expressed by the following formula 2.

  Further, the Stokes parameters S ′ representing the clockwise circularly polarized light and the counterclockwise circularly polarized light are respectively expressed by the following Equation 3.

  Therefore, δ1 and δ2 satisfying the formulas (12) and (12 ′) in the following formula 4 mean appropriate angles between the wave plate element A and the wave plate element B.

  As described above, by adjusting the wave plate element A and the wave plate element B in FIG. 3 to the appropriate angle, a desired phase difference can be obtained, and even when the transmittance due to the polarization component is different, for example, The light having linearly polarized light enters and exits the wave plate element A and the wave plate element B, so that it does not become elliptically polarized light but can be emitted from the wave plate as completely circularly polarized light. A satisfactory broadband wave plate can be realized for the equipment.

  Hereinafter, the present invention will be described more specifically with reference to Examples 1 and 2. However, the present invention is not limited to these Examples.

  <Example 1>

In the first embodiment, when two wave plate elements are combined, the pitch P, the structural height H, and the filling factor are expressed by the above formulas (1) and (2) as shown in the designs 1 to 6 in Table 1 below. The structural dimension of f (= L / P, where L: structural width) was determined, and the transmittance and phase difference were evaluated. The results are also shown in Table 1. In the simulation, vector analysis, Rigorous Coupled Wave Analysis, was used. Further, a polyolefin resin is used as the material, and the refractive index n is as follows.
n = 1.551088 (λ = 405 nm)
n = 1.533454 (λ = 650 nm)
n = 1.530011 (λ = 780 nm)

In Table 1, the comprehensive evaluation * 3 was performed as follows.
(1) More than 75% transmittance,
(2) Within Φ = ₆₅₀ ± 0.25 ± 0.02 [λ], the phase difference at λ = 650 nm when adjusted to the phase difference φ ₄₀₅ = 0.25 [λ] at λ = 405 nm,
(3) Phase difference at λ = 405 nm φ ₄₀₅ = 0.25 [λ] When adjusted to λ = 780 nm, phase difference at ₇₈₀ = 0.25 ± 0.04 [λ],
Those that satisfy each of the above are marked as ○. The overall evaluation shall be ○ if all of the above (1), (2) and (3) are satisfied.

  From Table 1, high transmittance and a desired phase difference of λ / 4 can be obtained by satisfying the formula (1) or the formula (2) and arranging the two wave plate elements at the optimum principal axis angle. It can be seen that a broadband wave plate can be obtained.

  <Example 2>

  In the second embodiment, the angle adjustment is performed in consideration of the polarization dependence of the transmittance when combining two wave plate elements. 7A is a diagram illustrating the polarization state at the point n in FIG. 3 and FIG. 7B is a diagram illustrating the polarization state at the point m in FIG. 3 when the angle is adjusted without considering the polarization dependence. is there. 8A is a diagram illustrating the polarization state at the point n in FIG. 3 and FIG. 8B is a diagram illustrating the polarization state at the point m in FIG. 3 when the angle is adjusted in consideration of the polarization dependence. .

In this embodiment, a case where the following two wave plate elements are combined is considered.
Wave plate element A: phase difference 65 deg, TE transmittance 100% TM transmittance 50%
Wave plate element B: phase difference 60 deg, TE transmittance 100% TM transmittance 50%

  As a result of adjusting the angle by the above equation (10) that does not consider the polarization dependence, δ1 and δ2 are obtained as 16.7 deg and 53.9 deg, respectively, and the polarization states thereof are shown in FIG. Shown in (b). On the other hand, as a result of adjusting the angle by the above equation (11) in consideration of the polarization dependence, 11.2 deg and 37.9 deg are obtained as one of the solutions for δ1 and δ2, respectively, and the polarization state is shown in FIG. Shown in (a), (b).

  When the angle is adjusted without considering the polarization dependence, as shown in FIG. 7B, the perfect circular polarization cannot be obtained and becomes an elliptical polarization, whereas the angle is adjusted in consideration of the polarization dependence. In this case, it can be seen that complete circularly polarized light can be obtained as shown in FIG.

  As described above, the best modes and examples for carrying out the present invention have been described. However, the present invention is not limited to these, and various modifications are possible within the scope of the technical idea of the present invention. is there. For example, in FIG. 1, the wave plate elements 11 and 12 are arranged so that the concavo-convex periodic structures face each other, but may be arranged so that the concavo-convex periodic structure and the back plate 13 or 14 face each other. Moreover, you may arrange | position so that the backplates 13 and 14 may oppose. The light may be incident from the concave-convex periodic structure side or from the back surface. Furthermore, the incident light may be from the first wave plate element 11 side or from the second wave plate element 12 side.

  In FIG. 1, two wave plate elements having the same design and the same configuration are combined. However, the wave plate of the present invention is not limited to this, and three or more wave plate elements may be combined. You may combine the wave plate element of an uneven | corrugated periodic structure.

It is a perspective view which shows schematically the broadband wavelength plate by this Embodiment. It is a graph which shows the example of relationship between the structural height of one wave plate element, the transmittance | permeability, and phase difference (wavelength (lambda) = 405 nm). It is a figure for demonstrating the shift | offset | difference angle (shift direction) of the principal axis of each wavelength plate element in the broadband wavelength plate of FIG. It is a graph explaining arrangement | positioning for obtaining the quarter wavelength plate with the broadband wavelength plate of FIG. It is a perspective view which shows roughly the wavelength plate which combined the conventional 2 sheets. It is a figure which shows the relationship between the filling factor and the phase difference normalized by the phase difference at the wavelength of 405 nm in the wave plate element. 4A is a diagram illustrating a polarization state at a point n in FIG. 3 and FIG. 3B is a diagram illustrating a polarization state at a point m in FIG. 3 when the angle is adjusted without considering polarization dependency. FIG. FIG. 4A is a diagram illustrating a polarization state at an n point in FIG. 3 when an angle is adjusted in consideration of polarization dependence, and FIG. 5B is a diagram illustrating a polarization state at an m point in FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Broadband wave plate 11, 12 Wave plate element 15, 16 Fine uneven | corrugated periodic structure a, b Main axis H Structure height P Structure period (pitch)
f Filling factor θa, θb Spindle deviation angle

Claims (5)

At least two wave plates having a periodic structure that causes a phase difference due to a fine structure having a period of 1 / n _min or more of the wavelength of the shortest wavelength of the light used so that the principal axes thereof are not parallel to each other. A broadband wave plate characterized by being disposed opposite to the substrate.
Where n _{min is} the refractive index of the wave plate material for the shortest wavelength light. When the phase difference between the at least two wave plates is δ1 and δ2, respectively.
The broadband wave plate according to claim 1, wherein 0 ° <δ1 <135 °, 0 ° <δ2 <135 °, 90 ° <δ1 + δ2 <270 °. The broadband wave plate according to claim 1 or 2, wherein at least one of the wave plates has a structural dimension that satisfies the following formula (1).
H = a ₁ × f + b ₁ + c ₁ (1)
However, −d ₁ ≦ c ₁ ≦ + d ₁
a ₁ = −10 × P + 4.6
b ₁ = −18.560 × P ² + 27.684 × P−6.8299
d ₁ = −27.273 × P ² + 18.994 × P-3.15
P: Structural period (μm)
H: Structure height (μm)
f: Filling factor (= L / P, where L: structure width (μm)) The broadband wave plate according to claim 1 or 2, wherein at least one of the wave plates has a structural dimension that satisfies the following formula (2).
H = a ₂ × f + b ₂ + c ₂ (2)
However, −d ₂ ≦ c ₂ ≦ + d ₂
a ₂ = −10 × P + 4.6
b ₂ = −76.515 × P ² + 69.335 × P-13.825
d ₂ = −54.663 × P ² + 36.782 × P−6.02
P: Structural period (μm)
H: Structure height (μm)
f: Filling factor (= L / P where L: structure width (μm)) At least two wave plates having a structure that generates a phase difference due to a fine periodic structure are arranged to face each other, and the angle formed by the main axes of each wave plate depends on the polarization dependency of each phase difference and transmittance. A method for adjusting a broadband wave plate, wherein a desired phase difference and polarization state are obtained by adjustment.

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