JP2012037460A - Optical sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To ensure and keep the good sensitivity of a sensor, by a one-quater wavelength plate which certainly converts linear polarized light into a circular polarized, even if the use wavelength of light source light in an optical system of an optical sensor is varied.SOLUTION: A vibration detecting optical sensor 1 comprises a light source 2, a polarized light beam splitter 3, an objective lens 4, a wavelength plate unit 5, and one light receiving element 6. The wavelength plate unit has an integrated structure of a one-quater wavelength plate 7 using structural birefringent and a vibration plate 8 having a reflecting surface. The light receiving element is disposed so as to make its optical axis x2 in parallel with and slightly shift from an optical axis x1 of reflection light, and an incident beam spot shape is projected at a position where its center c is slightly shifted from a center O of a light receiving surface 6a, so that detected light amount is increased and decreased according to the displacement of the vibration plate.

Description

  The present invention relates to an optical sensor for detecting, for example, fluid pressure and sound pressure, displacement of a measurement surface, vibration, and the like.

  Conventionally, in an optical pickup for reproducing or recording information from an optical disk such as a CD, a knife edge method, an astigmatism method, a critical angle method, Optical methods such as Foucault method are adopted. Various methods and apparatuses for optically measuring the position, displacement, and surface roughness of an object to be measured using these techniques have been proposed.

  For example, in order to measure a minute displacement or surface roughness of an object using the knife edge method, a laser beam is projected onto a measurement object so as to be focused on the surface, and the reflected light is received by a two-divided diode. A measuring instrument is known (see, for example, Patent Document 1). In this optical system, a knife edge is arranged in the middle of the optical path, and the cross section of the reflected beam incident on the two-divided diode becomes a semicircle. When the surface of the object to be measured is shifted backward from the focal position, the reflected beam is shifted downward from the two-divided diode, and when it is shifted forward, it is shifted upward. The displacement amount of the object surface can be obtained.

  An optical displacement sensor that measures the surface shape and surface roughness of the measurement surface using the astigmatism method projects laser light onto the measurement surface as a minute spot, and passes the reflected light through a cylindrical lens to produce astigmatism. And the center of the beam spot is made to coincide with the center of the four-divided light receiving element (see, for example, Patent Documents 2 and 3). When the measurement surface is at the focal position of the laser beam, the output signal of each light receiving element is equal, but when the measurement surface is shifted backward or forward, the output signal of the light receiving element at one diagonal position is the other diagonal. It becomes larger than the output signal of the light receiving element at the position. Therefore, by calculating the displacement signal of the measurement surface from the output signal of each light receiving element, the displacement amount from the focal position of the measurement surface can be obtained.

  As a microphone device that detects a change in sound pressure using an astigmatism method, a vibration detection device using light such as laser light is known (see, for example, Patent Document 4). In this vibration detection apparatus, linearly polarized laser light is converted into circularly polarized light by a quarter wavelength plate and reflected by the vibration plate, and reflected light is a straight line in which circularly polarized light is orthogonal to outgoing light by the quarter wavelength plate. It is converted into polarized light and detected by a quadrant photodetector. The diaphragm is placed at a position where the laser beam is focused by the focusing lens, and when the diaphragm vibrates, the deviation from the focal position is converted into an electrical signal as a change in the intensity distribution of the laser beam striking the photodetector. By doing so, sound waves are detected.

  Furthermore, an optical pressure sensor that detects the displacement of a pressure receiving element that responds to pressure in the form of an electrical signal is known (see, for example, Patent Document 5). This optical pressure sensor uses an optical system used for an optical head. Laser light is condensed and irradiated onto a diaphragm surface through a beam splitter, a quarter wavelength plate, and an objective lens, and the reflected light is reflected by a photodiode. When it is detected and deviates from the position where the diaphragm surface is completely condensed in response to the pressure, the pressure is detected from the fluctuation of the output voltage of the photodiode.

  In general, a quarter-wave plate includes a resin film made of an organic material such as polycarbonate that has been birefringent by a stretching process, a retardation plate made of a liquid crystal cell having a polymer liquid crystal layer sandwiched between transparent substrates, and a crystal. A crystal plate made of an inorganic crystal material having birefringence such as the above is used. Furthermore, a wave plate using structural birefringence due to a periodic structure with a sub-wavelength smaller than the wavelength of light to be used is known (for example, see Non-Patent Documents 1 and 2).

  This structural birefringent wave plate is characterized in that the size of the birefringence, that is, the phase difference amount and the broadband property can be controlled by selecting the dimensions of the fine periodic structure. However, if the ratio of the height to the pitch (structure period) of the convex portions forming the fine periodic structure, that is, the aspect ratio, is difficult to manufacture with high accuracy, there is a possibility that high transmittance cannot be maintained. is there. Therefore, a quarter of a combination of two wave plates in which the height of the fine periodic structure is approximately half the height required for the desired wavelength or wavelength range, with the convex portions of each periodic structure facing each other. Wave plates have been developed (for example, see Non-Patent Documents 1 and 2).

  In addition, in a quarter wavelength plate, when the reflectance of the incident surface is high or the incident angle dependency of the transmittance is large, the sensitivity decreases when used in an optical sensor, for example, due to a decrease in transmitted light intensity. There is a possibility that the recording / reproducing function is lowered in the optical pickup. Therefore, a structural birefringent wave plate in which an antireflection structure is formed on a light incident surface by forming a convex portion of a fine periodic structure in a tapered shape has been proposed (see, for example, Patent Documents 6 and 7). . Furthermore, in an optical element such as a lens, in order to suppress reflection without degrading optical performance, an antireflection film in which a large number of fine conical irregularities, for example, are formed on the incident surface at a pitch below the wavelength of light is provided. It is known (see, for example, Patent Documents 8 and 9).

Japanese Patent Laid-Open No. 7-4914 JP-A-5-231848 JP-A-8-29664 JP 58-145299 A JP-A-1-253626 Japanese Patent No. 3913765 JP-A-2005-44429 JP 2008-197216 A JP 2009-31610 A

Makiko Imae and three others, “Optimum design of broadband quarter-wave plates using structural birefringence”, KONICA MINOLTA TECHNOLOGYREPORT, Konica Minolta Technology Center, Inc., 2006, VOL. 3, p. 62-67 Osamu Masuda, 4 others, “Creation of broadband waveplate using nanoimprint technology”, KONICA MINOLTA TECHNOLOGYREPORT, Konica Minolta Technology Center, Inc., 2008, VOL. 5, p. 101-106

  However, in the optical system of the above-described optical sensor or optical pickup, there is a possibility that the optical component is exposed to a considerably high temperature by shortening the wavelength of the semiconductor laser used as the light source or setting the operating temperature range. Therefore, an optical element used in these optical devices may be required to have sufficient heat resistance and / or light resistance.

  In addition, when a semiconductor laser is used as a light source, it may generate high heat and cause a wavelength drift of the oscillation laser. In particular, a quarter-wave plate that converts linearly polarized light into circularly polarized light causes a phase shift in the emitted light due to the wavelength drift of the laser light, and elliptically polarized light is emitted. For this reason, when used in an optical sensor, the light intensity received by the photodetector from the reflecting surface of the object to be measured is reduced, which may reduce the sensor sensitivity. Therefore, it is preferable that the quarter wavelength plate has a wider usable wavelength range so that an optical device such as an optical sensor on which the quarter wavelength plate is mounted can stably exhibit good performance.

  Furthermore, the conventional optical sensors described above have a problem that all of them have a large number of optical components, are large, and have a complicated structure and are expensive.

  Therefore, the present invention has been made in view of the above-described conventional problems, and an object of the present invention is when the wavelength of light source light used in the optical system of the optical sensor varies due to, for example, a wavelength drift of a semiconductor laser. However, the ¼ wavelength plate can surely convert linearly polarized light into circularly polarized light so that good sensor sensitivity can be secured and maintained. It is a further object of the present invention to provide an optical sensor that can be reduced in size and cost by reducing the number of optical components and simplifying the structure.

In order to achieve the above object, an optical sensor according to the present invention collects a light source, a photodetector, a measurement object including a diaphragm, and light emitted from the light source so as to be focused on a reflection surface of the diaphragm. A first optical system that emits light, and a second optical system that condenses the reflected light from the reflecting surface on the light receiving surface of the photodetector,
The light receiving surface of the photodetector is arranged with its optical axis parallel to the optical axis of the reflected light and shifted from the optical axis of the reflected light, and the amount of deviation is always only part of the beam spot shape of the reflected light. Determined to be incident on the light receiving surface,
A first optical system that transmits or reflects outgoing light from a light source; an objective lens that focuses the outgoing light transmitted or reflected by the polarizing beam splitter onto a reflecting surface of the diaphragm; and It consists of a quarter wave plate that is arranged on the opposite side of the polarizing beam splitter and through which the emitted light is transmitted,
The quarter-wave plate comprises a structural birefringent wave plate having a fine periodic structure in which a plurality of convex portions are arranged at a constant pitch smaller than the used wavelength of the emitted light,
The quarter-wave plate and the diaphragm are integrated.

  In this optical sensor, the first linearly polarized light separated from the light emitted from the light source by the polarization beam splitter is converged by the objective lens, converted into circularly polarized light by the quarter wavelength plate, and incident on the reflecting surface of the diaphragm. Then, it is reflected as circularly polarized light in the reverse direction, converted again to second linearly polarized light by the quarter wavelength plate, and incident on the photodetector. The shape of the beam spot received by the light receiving surface of the photodetector changes depending on the position of the reflecting surface of the diaphragm on the optical axis, and the amount of light detected from the photodetector corresponds to the displacement of the reflecting surface from the in-focus position. Therefore, the amount of displacement of the reflecting surface can be obtained by detecting this change in the amount of light.

  The structural birefringent wave plate can change the phase difference amount and the broadband property depending on the dimension of the fine periodic structure. By using this for the quarter wavelength plate of the first optical system and setting its dimensions appropriately, even if the wavelength of the emitted light from the light source varies somewhat, linearly polarized light is more completely circularly corresponding to it. It can be converted to polarized light. Therefore, the optical sensor of the present invention can stably secure and maintain good sensor sensitivity.

  Furthermore, by integrating the quarter-wave plate and the diaphragm, the number of optical components is reduced as compared with the prior art, the optical path length from the diaphragm to the photodetector is shortened, and 1 / The alignment between the four-wavelength plate and the diaphragm can be omitted, and the alignment between them and the other optical system can be completed only once. Therefore, the entire apparatus can be miniaturized, the structure and assembly can be simplified, and a more accurate and reliable optical sensor can be obtained at low cost.

  In one embodiment, the quarter-wave plate comprises a pair of structural birefringent wave plates each having a fine periodic structure formed on one main surface of the substrate, and the pair of structural birefringent wave plates. Are arranged with the convex portions of the fine periodic structure facing each other. As a result, each structural birefringent wave plate makes the height of the convex part of the fine periodic structure about half of the height required for the desired use wavelength or use wavelength range, and reduces the aspect ratio with respect to the structure period. Because it can, it can be manufactured more easily and with high accuracy. As a result, the sensor sensitivity of the optical sensor can be stabilized and improved using a low-cost and high-performance quarter-wave plate.

  In another embodiment, the quarter-wave plate has an antireflection film having a large number of fine concavo-convex structures provided on the side opposite to the diaphragm, thereby reducing the incident angle dependency and preventing oblique incidence. However, since the high transmittance can be secured, the sensor sensitivity of the optical sensor can be further stabilized.

(A) is a block diagram of the first embodiment of the photosensor according to the present invention, and (B) is a diagram showing the beam shape on the light receiving element. (A) The figure is sectional drawing which shows 1st Example of a wavelength plate unit, (B) The figure is a perspective view of a quarter wavelength plate. (A) The figure is sectional drawing which shows the modification of the wavelength plate unit of 1st Example, (B) The figure is the elements on larger scale which show the uneven surface of an antireflection film. Sectional drawing which shows 2nd Example of a wavelength plate unit. Sectional drawing which shows the modification of the wave plate unit of 2nd Example. Sectional drawing which shows 3rd Example of a wavelength plate unit. Sectional drawing which shows the modification of the wave plate unit of 3rd Example. The block diagram of 2nd Example of the optical sensor by this invention. The block diagram of 3rd Example of the optical sensor by this invention. FIGS. 9A to 9C are diagrams showing the shape of a beam spot incident on a four-divided light receiving element when the diaphragm is in a different position in the optical sensor of the third embodiment, and FIG. The diagram which shows a displacement signal.

  Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. In the accompanying drawings, the same or similar components are denoted by the same or similar reference numerals.

  FIG. 1A shows the configuration of the first embodiment of the optical sensor according to the present invention. The optical sensor 1 of the present embodiment is for detecting vibrations, and includes a light source 2 composed of a laser diode, a flat polarizing beam splitter 3, an objective lens 4, a wave plate unit 5, and light detection. One light receiving element 6 is provided as a part. The wave plate unit 5 is a unitary combination of a quarter wave plate 7 and a diaphragm 8 as a measurement object.

FIG. 2 (A) shows a first embodiment of the wave plate unit of the present invention. Waveplate unit 5 ₁ of this embodiment comprises a quarter-wave plate 7 made of a structural birefringent waveplate, a diaphragm 8 made of flat metal, joined together across the spacer 9 at their periphery Has been. As shown in FIG. 2B, the quarter-wave plate 7 is a fine substrate composed of a flat substrate 10 having a predetermined thickness and a large number of minute protrusions 11 formed on one main surface of the substrate. And a periodic structure 12. The convex portions 11 are arranged at a constant pitch smaller than the wavelength of the laser light output from the light source 2 or the minimum wavelength in the usable wavelength range.

For example, as described in Non-Patent Document 1, the sub-wavelength order fine periodic structure has effective refractive indexes nTE and nTM that are different in the periodic direction and the direction orthogonal thereto in the effective medium theory. This difference in the effective refractive index causes a difference in the propagation speed of light in each polarization direction, and as if it is a birefringent material, the passing light produces a phase difference and expresses a function as a wave plate. It has been known. The line width of the convex portion 11 of the fine periodic structure 12 is L, the height is H, the pitch is P, the refractive index of the material forming the fine periodic structure 12 is n ₁ , and the material filling the grooves between the convex portions 11 ( When in this embodiment the refractive index of air) and n _2, the effective refractive index of the fine periodic structure 12 nTE, nTM is represented by the following formula (1) (2).

nTE = {f · n ₁ exp2 + (1−f) · n ₂ exp2} exp (1/2) (1)
n TM = {f · n ₁ exp (−2) + (1−f) · n ₂ exp (−2)} exp (−1/2) (2)
However, f = L / P.

  When the wavelength of the laser beam to be used is λ, the phase difference δ of the fine periodic structure 12 with respect to the incident wavelength λ is expressed by the following equation (3).

δ = (nTE−nTM) · H (3)
Therefore, a desired quarter-wave plate can be obtained by selecting the height H so as to satisfy δ = λ / 4 in the above equation (3).

  The quarter wavelength plate 7 having the sub-wavelength structure described above can be integrally formed by using a known material such as a nanoimprint method or precision molding, for example, using a resin material such as polyolefin, acrylic, or polycarbonate. The fine periodic structure 12 of the quarter wave plate 7 can be processed by etching an inorganic material such as a glass material.

  Moreover, the quarter wavelength plate 7 can also form the board | substrate 10 and the convex part 11 separately. For example, the fine periodic structure 12 can be formed by attaching another material on the main surface of the substrate 10 and processing the other material. Alternatively, the fine periodic structure 12 can be formed as an independent separate part and integrally coupled to the main surface of the substrate 10.

  The polarizing beam splitter 3, the objective lens 4 and the wave plate unit 5 constituting the first optical system for condensing the laser beam emitted from the light source 2 on the vibration plate 8 of the measurement object include the quarter wave plate 7 as the objective. On the lens side, they are arranged on the same optical axis x1 orthogonal to the optical axis of the light source 2. The light receiving element 6 is made of, for example, a photodiode, and its optical axis x2 is arranged in parallel with the optical axis x1 of the polarization beam splitter 3 or the like and shifted by a slight distance d therefrom.

  The light source 2 is connected to a drive circuit 19 which is a power source and controls the output of the emitted laser light. The light receiving element 6 is connected to a signal processing circuit 20 that processes the output signal and outputs a displacement signal of the diaphragm 8. The signal processing circuit 20 includes, for example, a photoelectric converter, a differential amplifier, and the like.

  In the optical sensor 1, the laser light L emitted from the light source 2 as divergent light is reflected by the polarization beam splitter 3 with the linearly polarized component of S-polarized light, converged by the objective lens 4 and incident on the wave plate unit 5. The laser light L incident on the wave plate unit is converted from linearly polarized light to circularly polarized light when passing through the quarter wave plate 7 and projected on the surface of the vibration plate 8 to be reflected as circularly polarized light in the reverse direction. Is done. The reflected light from the surface of the diaphragm 8 passes through the quarter-wave plate 7 again and is converted into P-polarized linearly polarized light, converged by the objective lens 4, transmitted through the polarizing beam splitter 3, and diffused after being imaged. Then, it enters the light receiving element 6.

  FIG. 1B shows the shape of a beam spot incident on the light receiving surface 6 a of the light receiving element 6. The optical axis x2 of the light receiving element 6 is arranged so as to be shifted from the optical axis x1 of the polarization beam splitter 3 as described above. Therefore, the beam spot shape m0 is projected at a position where the optical axis x1 passes through the center c, that is, at a position away from the center O of the light receiving surface 6a by a distance d.

  As shown in FIG. 1A, the diaphragm 8 is arranged so that the laser light L is focused on the reflecting surface 8a when in the non-displaced state. At this time, the reflected light R0 from the reflecting surface 8a follows the same optical path as that at the time of incidence, reaches the polarization beam splitter 3 while maintaining the same beam cross section as at the time of incidence, passes through this, and enters the light receiving element 6. .

  When the diaphragm 8 is displaced from the non-displacement position S0 toward the optical axis direction, that is, to the position S1 on the ¼ wavelength plate 7 side, the laser light L is reflected on the reflection surface 8a before the focal position S0. Therefore, the reflected light R1 follows the inner optical path from the time of incidence, reaches the polarization beam splitter 3 with a beam cross section somewhat narrower than that at the time of incidence, passes through this, and enters the light receiving element 6. Since the reflected light R1 forms an image before the reflected light R0, the beam spot shape m1 on the light receiving surface 6a appears concentrically and larger than the original beam spot shape m0.

  When the diaphragm 8 is displaced from the non-displacement position S0 to the rear side in the optical axis direction, that is, to the position S2 on the opposite side of the quarter wavelength plate 7, the laser light L is reflected to the reflecting surface 8a behind the focal position S0. Accordingly, the reflected light R2 follows the optical path outside the incident time, reaches the polarizing beam splitter 3 with a beam section somewhat thicker than the incident light, passes through this, and enters the light receiving element 6. Since the reflected light R2 forms an image behind the reflected light R0, the beam spot shape m2 on the light receiving surface 6a appears concentrically and smaller than the original beam spot shape m0.

  Therefore, the distance d, which is the deviation between the center c of the light receiving surface 6a and the center O of the beam spot shape, always has only a part of the beam spot shape on the light receiving surface 6a regardless of the position on the optical axis of the diaphragm 8. Set to enter. As a result, the beam spot shape on the light receiving surface 6a changes depending on the position of the diaphragm 8 as described above. Accordingly, the amount of light detected from the light receiving element 6 increases or decreases in accordance with the displacement of the diaphragm 8, and the amount of displacement of the diaphragm 8 can be obtained by detecting the change in the amount of light.

  In the optical sensor 1 of this embodiment, the number of optical components is significantly smaller than that of the conventional one. In addition, by using the wave plate unit 5 in which the quarter wave plate 7 and the vibration plate 8 are integrated, the optical path length from the vibration plate 8 to the light receiving element 6 can be significantly shortened. Further, when the optical sensor 1 is assembled, it is not necessary to align the quarter-wave plate 7 and the diaphragm 8, and only one alignment between them and the objective lens 4 is required. As a result, the entire apparatus can be downsized, the assembly of the optical sensor 1 can be simplified, and a highly accurate and reliable apparatus can be obtained at a lower cost.

FIG. 3 (A) shows a modification of the wave plate unit 5 ₁ of FIG. Waveplate unit 5 ₂ of the present embodiment, the fine periodic structure 12 of the entrance surface or the quarter-wave plate 7 in that a reflection preventing film 13 on the opposite side, different from the wavelength plate unit of Figure 2 . As shown in FIG. 3B, the antireflection film 13 has a large number of fine projections 13a having a regular quadrangular pyramid shape formed on the surface thereof. Projections 13a are arranged with a small pitch than the wavelength of the incident light, thereby eliminating the abrupt change in refractive index at the interface between the antireflection film 13 with air, the effective reflection of light entering the waveplate unit 5 ₂ Is suppressed. The antireflection film 13 is manufactured, for example, by etching a transparent inorganic material substrate such as synthetic quartz glass or by molding a resin material, and is bonded to the incident surface of the quarter-wave plate 7. .

FIG. 4 shows a second embodiment of the wave plate unit of the present invention. Waveplate unit ₃ of this embodiment, 1/4 board 10 ₁ wavelength plate ₇₁ is also serves as a diaphragm of the measuring object, thereby being smaller and thinner. On the main surface of the substrate 10 ₁ on the incident side, similarly to the waveplate unit of the above embodiments, a large number of fine periodic structure 12 ₁ consisting of minute projections 11 ₁ are formed. The said fine periodic structure of the substrate 10 ₁ on the outer surface of the opposite side, the reflecting surface 8a is subjected to metal film 14 ₁ is formed. Linearly polarized incident wave plate unit ₃ is converted through the fine periodic structure 12 ₁ into circularly polarized light, is reflected further a circularly polarized light in the opposite direction at the reflection surface 8a passes through the substrate 10 ₁ .

Figure 5 shows a modification of the wave plate unit ₃ of FIG. Waveplate unit ₄ of this embodiment, similarly Although the substrate 10 _{and second} quarter-wave plate 7 ₂ also serves as the diaphragm of the measuring object, the fine periodic comprised of many minute projections 11 ₂ the structure 12 ₂ is constructed as a separate part. Substrate 10 _2, the reflection surface 8a is subjected to metal film 14 ₂ is formed on the main surface of the incident side. Fine periodic structure 12 ₂ is formed as a separate part independent, they are coupled together on the metal film 14 _2. Linearly polarized incident wave plate unit 5 ₄ is converted into circularly polarized light passes through the fine periodic structure 12 _1, is reflected as circularly polarized light opposite in reflecting surface 8a. Substrate 10 ₂ of the present embodiment can select various materials without any consideration of the optical characteristics as the material. Further, when the substrate 10 ₂ has a high reflectance becomes and the surface of a metal plate, it is possible to omit the metal film 14 _2.

FIG. 6 shows a third embodiment of the wave plate unit of the present invention. Waveplate unit _{5 5} of the present embodiment has 1/4-wavelength plate _{7 3} a pair of structural birefringence wave plate 15a, a 15b. Each of the structural birefringent wave plates 15a and 15b is configured in the same manner as the quarter wave plate 7 in FIG. 2, and has flat substrates 10 ₃ and 10 ₄ each having a predetermined thickness, and one of the main substrates. It has fine periodic structures 12 ₃ and 12 ₄ made up of a large number of fine convex portions 11 ₃ and 11 ₄ formed on the surface. Structural birefringence wave plate 15a, 15b is made to face the fine periodic structure sides, they are joined together to sandwich the spacer 9 ₁ at their periphery. At this time, the structural birefringent wave plates 15a and 15b arrange their optical axes non-parallel so that the sum of their phase differences is 90 °.

Diaphragm ₈₁ is made of a thin plate, that on one of the incident side reflecting surface 8a is formed by applying a metallic film 14 _3, the peripheral portion is thicker in a short rectangular tubular for reinforcement. Diaphragm ₈₁ is integrally joined to the outer surface of the side opposite to the 1/4 incident side of the wavelength plate 7 ₃ in the rectangular tubular portion thereof. Linearly polarized incident wave plate unit 5 ₅ is converted through the 1/4-wavelength plate 7 ₃ into circularly polarized light, it is reflected as circularly polarized light in the opposite direction by the reflection surface 8a. In the case where the diaphragm ₈₁ has a high reflectance becomes and the surface of a metallic material, it is possible to omit the metal film 14 _3.

Figure 7 shows a modification of the wave plate unit 5 ₅ of FIG. Waveplate unit 5 ₆ of this embodiment, in that the same anti-reflection film 13 and the wavelength plate unit 5 ₂ of Figure 3 is provided on the incident surface of the quarter-wave plate 7 _3, waveplate unit of FIG. 6 And different. On the surface of the antireflection film 13, a large number of fine projections 13a having a regular quadrangular pyramid shape are formed at a pitch smaller than the wavelength of the incident light, and the reflection of the incident light is effectively suppressed.

  FIG. 8 shows the configuration of a second embodiment of the vibration detecting photosensor according to the present invention. The light sensor 21 of the present embodiment includes a light source 2 composed of a laser diode, a flat polarizing beam splitter 3, an objective lens 4, a wave plate unit 5, and one light detection unit arranged in the same manner as in the first embodiment. In addition to the light receiving element 6, a collimator lens 22 is further provided between the light source 2 and the polarization beam splitter 3 on the same optical axis. A driving circuit 19 is connected to the light source 2, and a signal processing circuit 20 is connected to the light receiving element 6. The wave plate unit 5 has the structure shown in FIG. 2, but can be used by replacing the wave plate unit shown in FIGS.

  In the optical sensor 21, the laser light L emitted from the light source 2 is converted into parallel light by the collimating lens 22, the S-polarized linearly polarized component is reflected by the polarization beam splitter 3, and converged by the objective lens 4 to be converged by the wave plate unit. 5 is incident. The laser light L incident on the wave plate unit is converted from linearly polarized light to circularly polarized light when passing through the quarter wave plate 7 and projected on the surface of the vibration plate 8 to be reflected as circularly polarized light in the reverse direction. Is done. Reflected light from the surface of the diaphragm 8 passes through the quarter-wave plate 7 again and is converted into P-polarized linearly polarized light, converted into parallel light by the objective lens 4, and transmitted through the polarizing beam splitter 3 to receive the light receiving element 6. Is incident on.

  The diaphragm 8 is arranged so that the laser light L is focused on the reflecting surface 8a when the diaphragm 8 is in a non-displaced state. At this time, the reflected light from the reflecting surface 8 a follows the same optical path as that at the time of incidence, maintains the same beam cross section as at the time of incidence, passes through the polarization beam splitter 3, and enters the light receiving element 6. The beam spot shape incident on the light receiving surface of the light receiving element 6 is projected at a position where the optical axis x1 passes through the center, that is, at a position away from the center of the light receiving surface by a distance d.

  When the diaphragm 8 is displaced from the non-displacement position S0 toward the optical axis direction, that is, to the position S1 on the ¼ wavelength plate 7 side, the laser light L is reflected on the reflection surface 8a before the focal position S0. The reflected light R follows the inner optical path from the time of incidence, passes through the polarization beam splitter 3 with a beam cross section somewhat narrower than that at the time of incidence, and enters the light receiving element 6. In this case, the beam spot shape on the light receiving surface of the light receiving element 6 is smaller than the original beam spot shape.

  When the diaphragm 8 is displaced from the non-displacement position S0 to the rear side in the optical axis direction, that is, to the position S2 on the opposite side of the quarter wavelength plate 7, the laser light L is reflected to the reflecting surface 8a behind the focal position S0. The reflected light R follows the optical path outside the incident time, passes through the polarization beam splitter 3 with a beam cross section somewhat thicker than the incident time, and enters the light receiving element 6. In this case, the beam spot shape on the light receiving surface of the light receiving element 6 is larger than the original beam spot shape.

  Also in this embodiment, since the shape of the beam spot incident on the light receiving surface of the light receiving element 6 changes depending on the position of the diaphragm 8 as described above, the amount of light detected from the light receiving element 6 depends on the displacement of the diaphragm 8. Increase or decrease correspondingly. Therefore, the displacement amount of the diaphragm 8 can be obtained by detecting the change in the light amount.

  Also in the present embodiment, the number of optical components is significantly reduced as compared with the prior art. Furthermore, the wavelength plate unit 5 in which the quarter wavelength plate 7 and the diaphragm 8 are integrated can significantly shorten the optical path length from the diaphragm 8 to the light receiving element 6. The alignment between the plate 7 and the diaphragm 8 is not necessary, and the alignment between the plate 7 and the objective lens 4 can be performed only once. Therefore, the entire apparatus can be miniaturized, the structure and the assembly can be simplified, and a more accurate and reliable optical sensor can be obtained at a lower cost.

  FIG. 9 shows the configuration of a third embodiment of the vibration detection photosensor according to the present invention. The optical sensor 31 of this embodiment includes a light source 2 composed of a laser diode, a collimating lens 22, a flat polarizing beam splitter 3, an objective lens 4, and a wave plate unit 5 arranged in the same manner as in the second embodiment. A quadrant light receiving element 32 as a detection unit is used, and a cylindrical lens 33 is disposed between the quadrant light receiving element and the polarization beam splitter 3. The four-divided light receiving element 32 and the cylindrical lens 33 are disposed on the same optical axis as that of the polarization beam splitter 3. For example, a quadrant photodiode is used as the quadrant light receiving element 32. The wave plate unit 5 has the structure shown in FIG. 2, but can be used by replacing the wave plate unit shown in FIGS.

  In the optical sensor 21, the laser light L emitted from the light source 2 is converted into parallel light by the collimating lens 22, the S-polarized linearly polarized component is reflected by the polarization beam splitter 3, and converged by the objective lens 4 to be converged by the wave plate unit. 5 is incident. The laser light L incident on the wave plate unit is converted from linearly polarized light to circularly polarized light when passing through the quarter wave plate 7 and projected on the surface of the vibration plate 8 to be reflected as circularly polarized light in the reverse direction. Is done. Reflected light from the surface of the diaphragm 8 passes through the quarter-wave plate 7 again and is converted into P-polarized linearly polarized light, converted into parallel light by the objective lens 4, transmitted through the polarizing beam splitter 3, and the cylindrical lens 33. , And astigmatism is given, and enters the four-divided light receiving element 32.

  The diaphragm 8 is arranged so that the laser light L is focused on the reflecting surface 8a when the diaphragm 8 is in a non-displaced state. At this time, the reflected light R from the reflecting surface 8a follows the same optical path as that at the time of incidence, passes through the polarization beam splitter 3 while maintaining the same beam cross section as that at the time of incidence, and converges on the cylindrical lens 33 to form an image. Thereafter, the light enters the four-divided light receiving element 32. FIG. 10A shows the beam spot shape M0 incident on the light receiving surface 32a of the four-divided light receiving element at this time. The beam spot shape M0 is a circle with its center placed at the center of the four-divided light receiving element 32.

  When the diaphragm 8 is displaced from the non-displacement position S0 toward the optical axis direction, that is, to the position S1 on the ¼ wavelength plate 7 side, the laser light L is reflected on the reflection surface 8a before the focal position S0. The reflected light R follows an inner optical path from the time of incidence, passes through the polarization beam splitter 3 with a beam cross section somewhat narrower than that at the time of incidence, converges on the cylindrical lens 33, and forms an image. 32 is incident. FIG. 10B shows a beam spot shape M1 incident on the light receiving surface 32a of the four-divided light receiving element at this time. The beam spot shape M1 is a horizontally long ellipse whose center is located at the center of the four-divided light receiving element 32.

  When the diaphragm 8 is displaced from the non-displacement position S0 to the rear side in the optical axis direction, that is, to the position S2 on the opposite side of the quarter wavelength plate 7, the laser light L is reflected to the reflecting surface 8a behind the focal position S0. The reflected light R follows an optical path outside the incident light, passes through the polarization beam splitter 3 with a beam cross section somewhat thicker than the incident light, converges on the cylindrical lens 33, and forms an image. 32 is incident. FIG. 10C shows a beam spot shape M2 incident on the light receiving surface 32a of the four-divided light receiving element at this time. The beam spot shape M2 is a vertically long ellipse whose center is placed at the center of the quadrant light receiving element 32.

  As shown in the figure, the output signals of the light receiving elements of the four-divided light receiving element 32 are Va, Vb, Vc, and Vd from their positions. At this time, the displacement signal V0 of the diaphragm 8 is expressed by V0 = (Va + Vd) − (Vb + Vc). FIG. 10D shows the displacement amount of the diaphragm 8 with the zero point on the horizontal axis as the focal position of the laser beam L, and the displacement signal V0 on the vertical axis. The displacement signal V0 is an S-order curve in which a certain range changes substantially linearly on both sides of +/− with respect to the zero point. Therefore, the displacement amount of the diaphragm 8 can be obtained from the displacement signal V0 within this linear range.

  Also in this embodiment, the wave plate unit 5 in which the quarter wave plate 7 and the vibration plate 8 are integrated reduces the number of optical components compared to the conventional one, and the optical path length from the vibration plate 8 to the light receiving element 6 is greatly increased. Therefore, when the optical sensor 1 is assembled, the alignment of the quarter-wave plate 7 and the diaphragm 8 is unnecessary, and the alignment between the objective lens 4 and the quarter-wave plate 7 can be completed only once. Therefore, the entire apparatus can be miniaturized, the structure and the assembly can be simplified, and a more accurate and reliable optical sensor can be obtained at a lower cost.

  Each of the above embodiments is an optical sensor for vibration detection, and is provided with a diaphragm for this purpose. However, the present invention can be used to measure the position and displacement of the measurement object separate from the optical sensor, the vibration of the surface to be measured, and the like. In that case, if the wave plate unit is replaced with a structure composed of a quarter wave plate and a phase correcting element, the vibration plate is omitted, and the laser beam is set to be focused on the surface to be measured of the object to be measured. Good.

  Various configurations other than the above-described embodiment are conceivable as an optical system for condensing and projecting laser light on the diaphragm and an optical system for causing reflected light from the diaphragm to enter the light receiving element. For example, the linearly polarized component of the laser light emitted from the light source and transmitted through the polarizing beam splitter can be projected onto the diaphragm, and the reflected light can be reflected by the polarizing beam splitter and incident on the photodetector. .

  The present invention is not limited to the above embodiments, and can be implemented with various modifications or changes within the technical scope thereof. For example, in the first and second embodiments, as long as the displacement of the diaphragm can be detected as a change in the amount of light, the light detection unit can be configured with a plurality of light receiving elements. The polarizing beam splitter can be a prism type. Various known light sources other than the laser diode can be used as the light source.

DESCRIPTION OF SYMBOLS 1, 21, 31 ... Optical sensor, 2 ... Light source, 3 ... Polarizing beam splitter, 4 ... Objective lens, 5 ... Wave plate unit, 6 ... Light receiving element, 6a, 32a ... Light receiving surface, 7 ... 1/4 wavelength plate, DESCRIPTION OF SYMBOLS 8 ... Diaphragm, 8a ... Reflective surface, 9 ... Spacer, 10 ... Substrate, 11 ... Convex part, 12 ... Fine periodic structure, 13 ... Antireflection film, 13a ... Projection, 14 ... Reflective film, 15a, 15b ... Structural property Birefringent wave plate, 19... Driving circuit, 20... Signal processing circuit, 22... Collimating lens, 32.

Claims (3)

A measurement object including a light source, a photodetector, a diaphragm, a first optical system for condensing the emitted light from the light source so as to be focused on the reflection surface of the diaphragm, and A second optical system for collecting the reflected light on the light receiving surface of the photodetector;
The light receiving surface of the photodetector is arranged with its optical axis parallel to the optical axis of the reflected light and shifted from the optical axis of the reflected light, and the amount of shift is always the beam spot shape of the reflected light. Only a part is determined to be incident on the light receiving surface,
A polarizing beam splitter for transmitting or reflecting light emitted from the light source; and an objective lens for collecting the emitted light transmitted or reflected by the polarizing beam splitter on the reflecting surface of the diaphragm. And a quarter-wave plate disposed on the opposite side of the objective lens from the polarizing beam splitter and transmitting the emitted light,
The quarter-wave plate comprises a structural birefringent wave plate having a fine periodic structure in which a plurality of convex portions are arranged at a constant pitch smaller than the use wavelength of the emitted light,
The optical sensor, wherein the quarter-wave plate and the diaphragm are integrated.   The quarter wave plate is composed of a pair of structural birefringent wave plates each having the fine periodic structure formed on one main surface of the substrate, and the pair of structural birefringent wave plates are mutually connected to the fine wave plate. The optical sensor according to claim 1, wherein the convex portions of the periodic structure are arranged to face each other.   3. The optical sensor according to claim 1, wherein the quarter-wave plate has an antireflection film having a large number of fine concavo-convex structures provided on a side opposite to the vibration plate.

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