JP2006259132A - Stair-shaped diffraction element and optical head apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a stair-shaped diffraction element and an optical head apparatus by which desired characteristic can be easily obtained. <P>SOLUTION: The stair-shaped diffraction element 100 is equipped with a plurality of stair members 120 having stair-shaped steps of even multiple of an integer (m) of ≥2 and diffracts light of predetermined wavelength λ. The stair members 120 is constituted of a polymer liquid crystal with double refractivity. When refractive index difference between a medium 130 being in contact with the respective stair members 120 and the respective stair members 120 is Δn, level difference (d) between respective stair-shaped steps is equal to λ/(2mΔn) and respective stair-shaped steps are arranged so that width has periodicity of a half period of pitches of the stair-shaped diffraction element 100. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a staircase diffractive element and an optical head device that diffracts and emits incident light, and more particularly to a staircase diffractive element and an optical head device that can suppress the intensity of secondary diffracted light.

  Conventionally, in an optical head device that records optical information on an optical recording medium such as a CD and a DVD and reproduces the optical information recorded on the optical recording medium, in order to perform optical information reading, focusing servo, and tracking servo In addition, a diffraction element having a plurality of diffraction gratings having different pitches has been used (for example, see Patent Document 1). FIG. 9 is an explanatory diagram showing the arrangement of the diffraction element on which the return light from the optical recording medium is incident and the light receiving surface on the light receiver that receives the ± 1st order diffracted light from the diffraction element.

  In FIG. 9, the return light 90 is incident on the diffraction element 70, and ± first-order diffracted light becomes the diffracted light of the main component. Here, the + 1st order diffracted lights 91 to 94 from the diffractive element 70 enter the light receiving surfaces 71 to 74, and the −1st order diffracted lights 95 to 98 from the diffractive element 70 enter the light receiving surfaces 75 to 78. Is incident. Generation of an RF signal for reading optical information recorded on the optical recording medium and a servo signal for performing focusing servo and tracking servo is detected by the light receiving surfaces 71 to 74 and the light receiving surfaces 75 to 78. This is done by adding or subtracting (linearly combining) the received signals.

Here, when a blazed diffraction grating is used, for example, as shown in FIG. 9, the second-order diffracted light 99 becomes stray light, and one or more light receiving surfaces (in FIG. 9, light receiving surfaces 75 and 76). May cause a malfunction of the optical head device. As a diffractive element for setting the ratio of the diffraction efficiency of the + 1st order diffracted light and the −1st order diffracted light to a predetermined value without such malfunction, for example, a four-step pseudo-blazed diffraction grating described in Patent Document 1 There is a diffractive element having This diffractive element is composed of a diffraction grating composed of four steps having the same width of every step.
JP 2004-139728 A

  However, such a conventional stepped diffraction element has a problem that it is difficult to form a step having a desired shape because an inorganic material such as lithium niobate is used as the material of the step member. The diffraction efficiency changes greatly depending on the accuracy of the step width, the shape of the step, etc., and it is difficult to produce a stepped diffraction element having desired characteristics. In addition, the conventional stepped diffraction element having the pseudo-blazed diffraction grating having the number of steps exceeding 4 steps has a uniform step width, so that, for example, only the + 1st order light is generated and the −1st order light is not generated. There was another problem that it was necessary to split the beam using a Wollaston prism.

  The present invention has been made to solve such a problem, and provides a stepped diffraction element and an optical head device capable of easily obtaining desired characteristics.

  In view of the above points, the invention according to claim 1 is a stepped structure comprising a transparent substrate made of an isotropic material, and stepped steps provided on the transparent substrate and an even multiple of an integer m of 2 or more. A step-like diffraction grating in which a plurality of step members having a cross-sectional shape are arranged in a diffraction grating shape, and an optically isotropic material and an optically anisotropic material arranged so as to fill the stepped portion of the step-like diffraction grating And a stair-like diffractive element that diffracts light of a predetermined wavelength λ, wherein the stair member is composed of a polymer liquid crystal having birefringence, and the stair member When the refractive index difference between the stepped diffraction grating and the optical medium for linearly polarized light polarized in the longitudinal direction is Δn, the step d between the stepped steps is equal to λ / (2mΔn), and the step member M steps from the lowest step The width of the corresponding stepped steps are of equal construction to each other and subsequent m steps thereto.

  With this configuration, since the staircase member is composed of a birefringent polymer liquid crystal, a stepped diffractive element that can easily form a step with a desired shape and easily obtain a desired characteristic is realized. it can. The polymer liquid crystal has good workability compared to inorganic materials, and does not require a proton exchange region as described in Patent Document 1, and can easily perform second-order diffraction simply by adjusting the refractive index. Light can be suppressed. Further, since the thickness of the portion having birefringence is thin, even if the polarization direction is slightly deviated from the desired direction, it is difficult for second-order diffraction to occur. Furthermore, characteristics depending on the polarization direction can be imparted.

  According to a second aspect of the present invention, in the first aspect, the optical medium is composed of a liquid crystal.

  With this configuration, in addition to the effect of the first aspect, the refractive index of the liquid crystal can be switched by switching the voltage applied to the liquid crystal, and a stepped diffraction element that functions for light of a plurality of wavelengths can be realized.

  The invention according to claim 3 is the structure according to claim 1 or 2, wherein the refractive indices of the stepped diffraction grating and the optical medium are equal to each other for linearly polarized light polarized in a direction orthogonal to the longitudinal direction of the stepped member. have.

  According to this configuration, in addition to the effect of claim 1 or 2, it is arranged in an optical system configured to pass linearly polarized light whose polarization direction is orthogonal in the reciprocating optical path, and only one of the reciprocating light is diffracted. Can be realized.

  According to a fourth aspect of the present invention, in any one of the first to third aspects, the integer m is any one of 2, 3, and 4.

  With this configuration, in addition to the effect of any one of claims 1 to 3, since the number of stepped steps is as small as 4, 6, and 8, a stepped diffraction element that can be easily formed can be realized. .

  The invention according to claim 5 has a configuration in which, in any one of claims 1 to 4, the integer m is 2, and the width ratio of adjacent stepped steps is 3: 7. is doing.

  With this configuration, since the number of stepped steps is 4 and the ratio of the width of adjacent stepped steps is 3: 7, the intensity of the + 1st order diffracted light and the −1st order diffracted light is 9: A stepped diffraction element that can be reduced to about 1 can be realized.

  Further, the invention according to claim 6 is provided on the transparent substrate, and a plurality of step members having a step-like cross-sectional shape including stepped steps of an even number multiple of an integer m of 3 or more are arranged in a diffraction grating shape. A stepped diffraction element that diffracts light of a predetermined wavelength λ, and when the refractive index difference between the medium in contact with the stepped step and the stepped diffraction grating is Δn, The step d between the stepped steps is equal to λ / (2mΔn), and the widths of the corresponding stepped steps from the lower step of the stepped member to the step m are equal to each other. is doing.

  With this configuration, the number of stepped steps is 6 or more, the step d between the stepped steps is equal to λ / (2mΔn), and the width of each stepped step is a period of half the pitch of the stepped diffraction element. Therefore, it is possible to realize a stepped diffraction element that has a degree of freedom in design and can suppress ± second-order diffracted light. Here, the pitch of the stepped diffraction element refers to a distance from a stepped step to an adjacent stepped step having the same height in a direction orthogonal to the longitudinal direction of the stepped diffraction grating. Further, the maximum diffraction efficiency is 81.1% at m = 2, whereas the maximum diffraction efficiency can be 91.2% and 95% at m = 3 and m = 4, respectively. Can be high.

  The invention according to claim 7 has a configuration comprising liquid crystal arranged in claim 6 so as to fill a stepped portion of the stepped diffraction grating.

  With this configuration, in addition to the effect of the sixth aspect, the refractive index of the liquid crystal can be switched by switching the voltage applied to the liquid crystal, and a stepped diffraction element that functions for light of a plurality of wavelengths can be realized.

  The invention according to claim 8 has a configuration in which the integer m is 3 or 4 in claim 6 or 7.

  With this configuration, in addition to the effect of the sixth or seventh aspect, the number of stepped steps is as small as 6 or 8, and therefore a stepped diffraction element that can be easily formed can be realized. Further, the maximum diffraction efficiency is 81.1% at m = 2, whereas the maximum diffraction efficiency can be 91.2% and 95% at m = 3 and m = 4, respectively. Can be high.

  According to a ninth aspect of the invention, there is provided a light source, an objective lens that condenses the light emitted from the light source onto an optical recording medium, and return light that is collected by the objective lens and reflected by the optical recording medium. An optical head device comprising a photodetector that performs the step-like diffraction element according to any one of claims 1 to 8, wherein the stepped diffraction element is disposed in an optical path between the photodetector and the light source. It has a configuration.

  With this configuration, the optical head device having the effect of any one of claims 1 to 8 and having high light utilization efficiency can be realized.

  In the present invention, since the staircase member is composed of a polymer liquid crystal having birefringence, a stepped diffraction having an effect that a step having a desired shape can be easily formed and a desired characteristic can be easily obtained. An element and an optical head device can be provided. Further, the number of stepped steps is 4 or more, the step d between the stepped steps is equal to λ / (2mΔn), and the width of each stepped step has a periodicity that is half the pitch of the stepped diffraction element. Therefore, it has a degree of freedom in design and can suppress ± second-order diffracted light, and can easily obtain desired characteristics. In addition, installing a polarization-type stepped diffraction element between the polarizing beam splitter and the objective lens makes it easy to secure space in the optical head device, so that the axis can be easily adjusted, and its characteristics are sufficient. It can be demonstrated.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
(Embodiment)
FIG. 1 is a cross-sectional view showing a schematic configuration of a stepped diffraction element according to an embodiment of the present invention. In FIG. 1A, a stepped diffraction element 100 includes a pair of transparent substrates 111 and 112 arranged in parallel and four or more even numbers (4 in FIG. 1A) provided on the transparent substrate 111. A plurality of step members 120 having stepped steps), an optical medium 130 having an isotropic refractive index filled between the transparent substrate 111 and the step member 120 and the transparent substrate 112, and the optical medium 130 sealed. And a polarization type diffraction element.

  In FIG. 1A, as the transparent substrates 111 and 112, glass substrates are preferable from the viewpoint of durability, reliability, and the like. However, since it is lightweight and inexpensive, an organic resin such as an acrylic resin, an epoxy resin, a vinyl chloride resin, or polycarbonate may be used.

  The staircase member 120 has birefringence and is periodically formed on the transparent substrate 111 so that its longitudinal direction is in a direction perpendicular to the cross section shown in FIG. 1A and forms a diffraction grating. Here, the staircase member 120 is configured such that the direction of the extraordinary light refractive index is, for example, the above-described longitudinal direction. Moreover, as shown to Fig.2 (a), as for each step-like step 121-124 of the staircase member 120, the level | step difference (henceforth a step difference between steps) between each step-like step is d.

Here, when the number of stepped steps constituting the staircase member 120 is 2 m, the m stepped steps starting from the thinnest stepped step are referred to as a first stepped step group, and the rest The thick m stepped steps are referred to as a second stepped step group. Further, the ratio of the widths of the stepped steps constituting the first stepped step group is set to r ₁₁ : r ₁₂ :...: R _{1 m} from the smallest thickness to form the second stepped step group. The ratio of the width of each stepped step is set to r ₂₁ : r ₂₂ :...: R _2m from the smallest thickness, and the first stepped step group 125 and the second stepped step group 126 are the same. It is assumed that it is the width of. Here, the width of the step refers to the width when the cross-sectional shape is a staircase.

It said stair member 120 is an example of m = 2, each stepped steps 121 to 124 _{r 11: r 12 = r 21} : has a _{r 22.} In other words, the first stepped step group 125 and the second stepped step group 126 have a periodicity of a period p / 2 in which the width of the stepped steps 121 to 124 is half the pitch p of the diffraction grating. . Specifically, the widths of the first stepped step 121 and the third stepped step 123 are both w, and the widths of the second stepped step 122 and the fourth stepped step 124 are both (p / 2). ) -W.

  For the staircase member 120, for example, an acrylic polymer liquid crystal can be used. A liquid crystal compound (1), (2), (3) and (4) shown in [Chemical Formula 1] is mixed at a molar ratio of 14: 14: 36: 36 to prepare a polymer liquid crystal. To do.

  In this case, a liquid crystal compound is applied on the transparent substrate 111, irradiated with UV light or the like to be solidified to obtain a polymer liquid crystal, and the above stepped shape is formed by using a photolithography technique and an etching technique. Since the polymer liquid crystal is a material that can easily obtain a desired shape by using a photolithography technique and an etching technique, the stepped cross-sectional shape of the staircase member can be easily formed using the polymer liquid crystal.

In addition, the alignment direction of the polymer liquid crystal can be set by providing an alignment film (not shown) and adjusting the rubbing direction of the alignment film, or by obliquely depositing SiO ₂ or the like, irradiating an ion beam, etc. Can also be set. Hereinafter, it is assumed that a polymer liquid crystal is used for the staircase member 120 and a rubbing alignment film is formed on the interface between the transparent substrate 111 and the staircase member 120.

  The optical medium 130 is made of a material such as polyester or acrylic, and can be used as a so-called filler. Here, for example, the ordinary light refractive index of the staircase member 120 and the refractive index of the optical medium 130 are substantially matched. The step difference d between the steps of the staircase member 120 is determined in accordance with the refractive index difference Δn between the staircase member 120 and the optical medium 130 (> 0, hereinafter referred to as a staircase member refractive index difference).

  Specifically, when the wavelength of the incident light is λ, the step difference d between steps of the staircase member 120 is λ / (4Δn). In the general case, that is, when the number of stepped steps is 2 m, the step difference d between steps is λ / (2mΔn). Due to this step, a λ / (2 m) optical path length difference is generated.

  The seal 140 is formed outside the optically effective area of the transparent substrate 112 using a printing method, and the distance between the transparent substrates 111 and 112 is kept constant by bonding the transparent substrate 111 and the transparent substrate 112 together. It is provided as follows. As the seal 140, a thermosetting polymer such as an epoxy resin, an ultraviolet curable resin, or the like can be used, and a spacer such as a glass fiber may be mixed into the seal 140 in order to obtain a desired cell interval.

FIG. 2C shows the sum of the intensities of the + 1st order diffracted light and the −1st order diffracted light, and the + 1st order diffracted light with respect to the ratio (hereinafter referred to as width ratio) r ₁₁ of the first stepped step 121. 4 is a table showing the intensity ratio of -1st order diffracted light, the intensity ratio of + 2nd order diffracted light and + 1st order diffracted light, and the intensity ratio of -2nd order diffracted light and -1st order diffracted light. The sum of the intensities of the + 1st order diffracted light and the −1st order diffracted light is expressed as a ratio to the incident light. FIG. 2D is a diagram showing the relationship between the intensity ratio of the + 1st order diffracted light and the −1st order diffracted light with respect to the width ratio r ₁₁ shown in FIG.

Here, as shown in FIG. 2 (a), each wavelength lambda 0.405 .mu.m, the level difference d between steps about 2.154Myuemu, the ordinary refractive index _{n o} and extraordinary index _{n e} of the polymer liquid crystal 1. 541 and 1.588, and the refractive index of the optical medium 130 is 1.541. The pitch p is 26.1 μm. From FIGS. 2 (c) and 2 (d), + 1-order intensity ratio of the diffracted light and -1-order diffracted light, the width ratio _{r 11} is 0.15 or near 0.35 neighborhood, be a 10 I understand.

3 (c) is a view corresponding to FIG. 2 (c) in the case of m = _{3, r 12} is fixed to 0.1. FIG. 3D is a diagram showing the relationship between the intensity ratio of the + 1st order diffracted light and the −1st order diffracted light with respect to the width ratio r ₁₁ shown in FIG. Here, as shown in FIG. 3 (a), each wavelength lambda 0.405 .mu.m, the level difference d between steps about 1.436Myuemu, the ordinary refractive index _{n o} and extraordinary index _{n e} of the polymer liquid crystal 1. 541 and 1.588, and the refractive index of the isotropic optical medium is 1.541. From FIG. 3C and FIG. 3D, the intensity ratio of the + 1st order diffracted light and the −1st order diffracted light is 10 when the width ratio r ₁₁ is about 0.08 or about 0.32. It can be seen that the change in the intensity ratio of the + 1st order diffracted light and the −1st order diffracted light is smaller than that when m = 2. Since the change in the intensity ratio can be reduced in this way, the process margin can be widened, and the stepped diffraction element can be easily manufactured.

FIG. 4C corresponds to FIG. 2C when m = 4, and both r ₁₂ and r ₁₃ are fixed to 0.05. FIG. 4 (d) is a diagram showing the relationship of FIG. 4 +1 order diffracted light to the width ratio r ₁₁ shown in (c) and -1-order diffracted light intensity ratio of. Here, as shown in FIG. 4 (a), each wavelength lambda 0.405 .mu.m, the level difference d between steps about 1.077Myuemu, the ordinary refractive index _{n o} and extraordinary index _{n e} of the polymer liquid crystal 1. 541 and 1.588, and the refractive index of the isotropic optical medium is 1.541. From FIG. 4 (c) and FIG. 4 (d), + 1-order intensity ratio of the diffracted light and -1-order diffracted light, the width ratio _{r 11} 0.11 near or 0.28 neighborhood, be a 10 It can be seen that the change in the intensity ratio of the + 1st order diffracted light and the −1st order diffracted light is smaller than that when m = 2. Since the change in the intensity ratio can be reduced in this way, the process margin can be widened, and the stepped diffraction element can be easily manufactured.

  In the above description, the example in which the optical medium 130 is made of an isotropic material such as a filler has been described. However, the optical medium 130 may be made of a predetermined liquid crystal. Here, the liquid crystal used as the optical medium 130 has a difference between the ordinary light refractive index and the extraordinary light refractive index larger than the difference between the ordinary light refractive index and the extraordinary light refractive index of the step member 120. Further, it is assumed that the ordinary light refractive indexes of the liquid crystal used as the optical medium 130 and the staircase member 120 are substantially equal.

FIG. 1B shows a stepped diffraction element 200 having a configuration equivalent to such a configuration. In this case, as shown in FIG. 1B, a transparent for applying a voltage to the liquid crystal between the optical medium 131 made of liquid crystal and the transparent substrate 111, and between the staircase member 132 and the transparent substrate 112. An electrode 141 and a transparent electrode 142 may be provided. It is assumed that the liquid crystal constituting the optical medium 131 is aligned in the longitudinal direction of the diffraction grating. FIGS. 5 (a1) to 5 (a3) show the characteristics shown in FIGS. 2 (c) and 2 (d) for light of wavelengths λ0.405 μm and 0.66 μm when m = 2. It is a figure which shows the structure for obtaining the same characteristic. Here, Δn _ee shown in FIG. _5A3 is obtained by subtracting the extraordinary refractive index of the polymer liquid crystal composing the step member 120 from the extraordinary refractive index of the liquid crystal composing the optical medium 131. The step member refractive index difference Δn shown in FIG. 5A1 is obtained by adjusting the voltage applied between the transparent electrodes 141 and 142. That is, when the wavelength λ is 405 nm, a voltage is applied to the liquid crystal to set the refractive index n of the liquid crystal to 1.664, thereby setting the step member refractive index difference Δn to 0.086, and when the wavelength λ is 660 nm, No voltage is applied to the liquid crystal, and the step member refractive index difference Δn is set to 0.139.

In the above description, the configuration in which the optical medium 130 is filled has been described. However, as illustrated in FIG. 1C, the configuration without the optical medium 130, that is, the stepped diffraction element 300 is configured by the transparent substrate 111 and the stepped member 133. It may be. Further, when m is 3 or more, as indicated above, changes to width ratio r ₁₁ of the intensity ratio of + 1-order diffracted light and -1-order diffracted light becomes smaller than that in the case of m = 2 Therefore, even if the step difference d between steps is set to λ / (2mΔn) and a solid optical member is used as the staircase member 133, the intensity ratio of desired + 1st order diffracted light and −1st order diffracted light is made higher than before. In addition to being easily obtainable, the intensity ratio of + 2nd order diffracted light and + 1st order diffracted light and the intensity ratio of −2nd order diffracted light and −1st order diffracted light can be made substantially zero.

  5 (b1) and 5 (b2) use a solid optical member as the staircase member 133. When m = 3, FIG. 3 (c) and FIG. It is a figure which shows the structure for obtaining the characteristic similar to the characteristic shown to d). Similarly, FIGS. 5 (c1) and 5 (c2) use a solid optical member as the step member 133, and when m = 4, FIG. 4 (c) and FIG. It is a figure which shows the structure for obtaining the characteristic similar to the characteristic shown in FIG.4 (d). In the above description, the polarization type stepped diffraction element has been described. However, the step member 133 may be formed using a material having an isotropic refractive index to form a non-polarization type stepwise diffraction element.

Hereinafter, the operation of the stepped diffraction element 100 according to the embodiment of the present invention will be described. Hereinafter, the width ratio _{r 11} is assumed to be set to 0.15. About 80% of the light incident on the stepped diffraction element 100 is divided into + 1st order diffracted light and −1st order diffracted light, and is emitted from the stepped diffraction element 100. On the other hand, the ± second-order diffracted light is suppressed to almost 0 because each step between the stepped steps 121 to 124 is λ / (4Δn).

  FIG. 6 is a diagram showing a block configuration of the optical head device according to the embodiment of the present invention. In FIG. 6, the optical head device 1 includes a light source 11, a collimator 12 that converts the divergent light from the light source 11 into a parallel beam, a polarization beam splitter 13, a stepped diffraction element 600, a quarter-wave plate 14, The objective lens 15, the condenser lens 16, and the light receiver 60 are configured. Here, the stepped diffraction element 600 has a polarization type diffraction grating.

  The light source 11 is made of a semiconductor laser, for example, and emits divergent light having a predetermined wavelength. For example, as shown in FIG. 7, the stepped diffraction element 600 may have a configuration in which pseudo-blazed diffraction gratings 101 to 104 having different pitches are arranged with their longitudinal directions aligned. Here, the pseudo blazed diffraction gratings 101 to 104 have the same configuration as that of the stepped diffraction element 100 except that there is no seal at the portions in contact with each other.

  Here, the light receiver 60 has light receiving surfaces 61 to 68 that receive the + 1st order diffracted light and the −1st order diffracted light. Specifically, the light receiving surfaces 61 to 64 receive + 1st order diffracted light, and the light receiving surfaces 65 to 68 receive −1st order diffracted light. With this configuration, the ± first-order diffracted light from each diffraction grating can be used as a light beam for a servo signal for focusing and tracking the objective lens 15.

  The operation of the optical head device 1 according to the embodiment of the present invention will be described below. Light of linearly polarized light (hereinafter referred to as P-polarized light) emitted from the light source 11 passes through the collimator 12, the polarizing beam splitter 13, and the stepped diffraction element 600 and enters the quarter-wave plate 14. The light incident on the quarter wavelength plate 14 becomes circularly polarized light, exits the quarter wavelength plate 14 toward the objective lens 15, and is condensed on the information recording surface of the optical recording medium 10. The light incident on the optical recording medium 10 is reflected by the optical recording medium 10, becomes return light, passes through the objective lens 15, and enters the quarter-wave plate 14. The return light incident on the quarter-wave plate 14 is converted from circularly-polarized light to S-polarized return light that is a polarization direction orthogonal to the P-polarized light, and exits the quarter-wave plate 14 toward the stepped diffraction element 600. To do. The S-polarized return light incident on the stepped diffraction element 600 becomes ± first-order diffracted light having a diffraction angle corresponding to the pitch of the pseudo blazed diffraction gratings 101 to 104 by the stepped diffraction element 600, and enters the polarization beam splitter 13. The stair-like diffraction element 600 is emitted. The ± first-order diffracted light of the return light incident on the polarization beam splitter 13 is reflected and transmitted through the condensing lens 16 and collected on predetermined light receiving points 61 to 68 of the light receiver 60.

  FIG. 8 is a diagram showing a block configuration of an optical head device equipped with a non-polarization type stepped diffraction element. In the configuration of the optical head device 2 shown in FIG. 8, a non-polarization type stepped diffraction element 800 is provided between the condenser lens 16 and the light receiver 80. In the configuration of the optical head device 2 shown in FIG. 8, the return light incident on the quarter-wave plate 14 becomes S-polarized return light, is reflected by the polarization beam splitter 13, passes through the condenser lens 16, and is stepped. The light is diffracted by the diffraction element 800 and collected on predetermined light receiving points 81 to 88 of the light receiver 80.

  In the above (FIG. 6), the configuration in which the stepped diffraction element 600 is provided between the polarizing beam splitter 13 and the quarter wavelength plate 14 has been described. It may be provided on another predetermined optical path between 13 and the condenser lens 16. In the above description, the stepped diffraction element 800 is described as being provided between the condenser lens 16 and the light receiver 80. However, the stepped diffraction element 800 includes the polarization beam splitter 13, the condenser lens 16, and the like. May be provided on another predetermined optical path.

  As described above, in the staircase diffraction element according to the embodiment of the present invention, since the staircase member is composed of a polymer liquid crystal having birefringence, a step having a desired shape can be easily formed and desired. These characteristics can be easily obtained.

  Further, since the number of stepped steps made of polymer liquid crystal is as small as 4, 6 and 8, it can be easily formed.

  Further, since the number of stepped steps made of polymer liquid crystal is 4 and the width ratio of adjacent stepped steps is 3: 7, the intensity of the + 1st order diffracted light and the −1st order diffracted light Can be about 9: 1.

  Further, even when the liquid crystal is not used for the staircase member, the number of stepped steps is 6 or more, the step d between the stepped steps is equal to λ / (2mΔn), and the width of each stepped step. Has a periodicity with a period that is half the pitch of the step-like diffractive element, so that it has a degree of freedom in design and can suppress ± second-order diffracted light.

  Moreover, even if it is the structure which does not use a polymer liquid crystal for a staircase member, since the number of stepped steps is as few as 6 or 8, it can be created easily.

  In addition, the optical head device according to the embodiment of the present invention has the above effects and can improve the light utilization efficiency.

Specific examples based on the above-described embodiments of the present invention will be described below.
(Example)
Fig.1 (a) is sectional drawing which shows the typical structure of the step-shaped diffraction element based on the Example of this invention. First, glass substrates are used as the transparent substrates 111 and 112. Use the stairs member 120, ordinary extraordinary refractive index using the polymer liquid crystal of _{n e} is 1.588 in the refractive index _{n o} is 1.541, the refractive index in the optical medium 130 is a filler 1.541 It was. Below, the preparation method of a stair-like diffraction element is demonstrated.

  First, an alignment film is formed on one substrate surface of a predetermined substrate (hereinafter referred to as a replacement substrate), and an alignment process is performed by a rubbing method. The rubbing direction is the longitudinal direction of the diffraction grating. Next, an epoxy resin adhesive is used as the material for the seal 140, and the epoxy resin adhesive is printed on the outer periphery of the optically effective area of the substrate surface on which the alignment film of the replacement substrate is formed. Similarly, an alignment film is formed on one substrate surface of the transparent substrate 111, and alignment processing is performed by a rubbing method. Next, the replacement substrate and the transparent substrate 111 are opposed to each other so that the surfaces on which the alignment films are formed are opposed to each other, and the seal 140 is formed, whereby a cell is manufactured.

Next, the above polymer liquid crystal is injected into this cell by a vacuum injection method, UV-exposed, and solidified to obtain a polymer liquid crystal. Next, the replacement substrate is removed, and the polymer liquid crystal is etched using photolithography technology and etching technology, m is 2, pitch p is 26.1 μm, 31.8 μm, 40.7 μm and 56.5 μm, width ratio. A stepped shape with r ₁₁ of 0.15 is assumed. Finally, a filler is filled on the polymer liquid crystal, and the transparent substrate 112 is attached to form a staircase diffraction element.

  When linearly polarized (S-polarized) laser light having a wavelength of 405 nm and a polarization direction parallel to the longitudinal direction of the diffraction grating is incident on the stepped diffraction element 100 configured as described above, the + 1st order and −1st order are incident. Diffracted light with a diffraction efficiency ratio of approximately 9: 1 can be obtained. On the other hand, when P-polarized laser light is incident, it is transmitted without being diffracted. The intensity ratio between the + 2nd order diffracted light and the + 1st order diffracted light is substantially 0, and the intensity ratio between the −2nd order diffracted light and the −1st order diffracted light is also substantially 0.

  FIG. 6 is a diagram showing a block configuration of the optical head device according to the embodiment of the present invention. First, a semiconductor laser is used as the light source 11. As shown in FIG. 7, the stepped diffraction element 600 is composed of pseudo-blazed diffraction gratings having four different pitches, except that no seal is provided on the boundary surface where the pseudo-blazed diffraction gratings 101 to 104 are in contact with each other. It is fabricated in the same manner as the stepped diffraction element 100 shown in the embodiment. Here, the pitches of the pseudo-blazed diffraction gratings constituting the stair-like diffraction element 100 are 21.6 μm, 31.8 μm, 40.7 μm, and 56.5 μm for the pseudo-blazed diffraction gratings 101 to 104, respectively.

  In the optical head device 1 configured as described above, linearly polarized (P-polarized) laser light having a wavelength of 405 nm emitted from the light source 11 and a polarization direction perpendicular to the longitudinal direction of the diffraction grating is collimator 12 and polarization beam splitter 13. Then, the light passes through the stepped diffraction element 600 without being diffracted and enters the quarter-wave plate 14. The light incident on the quarter-wave plate 14 becomes circularly polarized light, passes through the objective lens 15 and is reflected by the optical recording medium 10, and becomes return light. The return light from the optical recording medium 10 is converted to S-polarized light by the quarter-wave plate 14, and is diffracted by the step-like diffraction element 600 at a diffraction angle corresponding to the pitch of each diffraction grating and output to the polarization beam splitter 13. To do. The light incident on the polarization beam splitter 13 is reflected by the polarization beam splitter 13 and is incident on the light receiving surfaces 61 to 68 on the light receiver 60 by the condenser lens 16.

  The step-like diffractive element according to the present invention can easily suppress the second-order diffracted light and can easily set desired characteristics such as a light amount ratio between the + 1st-order diffracted light and the −1st-order diffracted light. The present invention can also be applied to uses such as a stair-like diffractive element having a useful effect and an optical head device with less noise.

Sectional drawing which shows the typical structure of the step-shaped diffraction element which concerns on embodiment of this invention The figure for demonstrating the 1st structure and characteristic of the step-shaped diffraction element which concerns on embodiment of this invention The figure for demonstrating the 2nd structure and characteristic of the step-shaped diffraction element which concerns on embodiment of this invention The figure for demonstrating the 3rd structure and characteristic of the step-shaped diffraction element which concerns on embodiment of this invention Configuration and characteristics (a) when the staircase member of the staircase diffraction element according to the embodiment of the present invention is configured using liquid crystal and polymer liquid crystal, and configuration and characteristics (b) when configured with a six-step transparent member ), A diagram for explaining the configuration and characteristics (c) when configured with an 8-step transparent member The figure which shows the 1st block configuration of the optical head apparatus which concerns on embodiment of this invention. The figure for demonstrating the planar structure of the stair-like diffraction element 600 shown in FIG. The figure which shows the 2nd block configuration of the optical head apparatus which concerns on embodiment of this invention. Explanatory drawing which shows arrangement | positioning of the light-receiving surface on the light receiving device which receives the diffraction element into which the return light from an optical recording medium injects, and the ± 1st order diffracted light from a diffraction element

Explanation of symbols

DESCRIPTION OF SYMBOLS 1, 2 Optical head apparatus 10 Optical recording medium 11 Light source 12 Collimator 13 Polarizing beam splitter 14 1/4 wavelength plate 15 Objective lens 16 Condensing lens 60, 80 Light receiver 61-68, 71-78, 81-88 Light-receiving surface 70 , 100, 200, 300, 600, 800 Stepped diffraction element 90 Return light 91-99 Diffracted light 111, 112 Transparent substrate 120, 132 Step member 121-124 Stepped step 131 Optical medium consisting of liquid crystal 133 Step member or isotropic Refractive index member

Claims (9)

A transparent substrate made of an isotropic material;
A step-like diffraction grating provided on the transparent substrate, wherein a plurality of step members having a step-like cross-sectional shape composed of even-numbered steps of an integer m of 2 or more are arranged in a diffraction grating shape;
An optical medium made of one of an optically isotropic material and an optically anisotropic material, and arranged to fill the stepped portion of the stepped diffraction grating, and diffracts light of a predetermined wavelength λ A stepped diffraction element,
When the stepped member is made of a polymer liquid crystal having birefringence, and the refractive index difference between the stepped diffraction grating and the optical medium with respect to linearly polarized light polarized in the longitudinal direction of the stepped member is Δn, The step d between the stepped steps is equal to λ / (2mΔn), and the widths of the corresponding stepped steps from the lower step of the stepped member to the step m are equal to each other. Stepwise diffraction element.   The step-like diffraction element according to claim 1, wherein the optical medium is composed of liquid crystal.   3. The stepped diffraction element according to claim 1, wherein refractive indexes of the stepped diffraction grating and the optical medium with respect to linearly polarized light polarized in a direction orthogonal to a longitudinal direction of the stepped member are equal to each other.   The step-like diffractive element according to any one of claims 1 to 3, wherein the integer m is any one of 2, 3, and 4.   The stepped diffraction element according to any one of claims 1 to 4, wherein the integer m is 2 and a ratio of widths of adjacent stepped steps is 3: 7. A transparent substrate made of an isotropic material;
A stepped diffraction grating provided on the transparent substrate and having a stepped cross-sectional shape composed of stepped steps of an even number multiple of an integer m of 3 or more, and a plurality of stepped diffraction gratings arranged in a diffraction grating shape; A stepped diffraction element for diffracting light of wavelength λ,
When the difference in refractive index between the medium in contact with the stepped step and the stepped diffraction grating is Δn, the step d between the stepped steps is equal to λ / (2mΔn), and the lower step of the stepped member A stepped diffraction element characterized in that widths of stepped steps corresponding to m steps and m steps subsequent thereto are equal to each other.   The staircase-like diffraction element according to claim 6, further comprising a liquid crystal arranged to fill a staircase portion of the staircase-like diffraction grating.   The step-like diffraction element according to claim 6 or 7, wherein the integer m is 3 or 4.   An optical head device comprising: a light source; an objective lens that condenses light emitted from the light source onto an optical recording medium; and a photodetector that detects return light that is collected by the objective lens and reflected by the optical recording medium. An optical head device according to claim 1, wherein the stepped diffraction element according to any one of claims 1 to 8 is disposed in an optical path between the photodetector and the light source.

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