Optical scanning element and image display device using the same

Provided is an optical scanning element that includes mirror unit 1, and movable unit 2 including mounting unit 7 on which mirror unit is mounted and which is configured to be rotatable. Mirror unit 1 includes: a dielectric multilayer film that reflects a part of incident light while transmitting the remainder of the light; and a first substrate on one surface of which the dielectric multilayer film is formed, and which transmits the remainder of the light passed through the dielectric multilayer film. Mounting unit 7 includes a through-hole at a portion facing the dielectric multilayer film.

This application is a National Stage Entry of PCT/JP2011/061417 filed May 18, 2011, which claims priority from Japanese Patent Application 2010-126732 filed Jun. 2, 2010, the contents of all of which are incorporated herein by reference, in their entirety.

TECHNICAL FIELD

The present invention relates to an optical scanning element that scans a screen with an optical beam, and more particularly to an optical scanning element that includes a rotatable mirror.

BACKGROUND ART

Patent Literature 1 describes a light deflector that includes an oscillating body including a mirror and a torsion spring for rotatably supporting the mirror, a fixing unit to which the oscillating body is fixed, and a driving unit configured to reciprocally vibrate the mirror by deforming the torsion spring with its resonance frequency.

The mirror includes a base made of silicon. A refection film made of metal U (Al or Au) is formed on the base.

In the aforementioned light deflector, the temperature of the reflection film rises due to absorption of a part of incident light on the reflection film. As a result, distortion occurs in the mirror (especially, in the base) because of thermal expansion. When the distortion occurs in the mirror, accurate scanning with the optical beam is difficult.

The temperature increase causes a change in rigidity of the torsion spring, thereby changing the resonance frequency of the torsion spring. The change of the resonance frequency of the torsion spring is accompanied by the change of a deflection angle. As a result, accurate scanning with the optical beam is difficult.

Thus, Patent literature 1 describes a light deflector that has a heat releasing structure. This light deflector includes, in addition to the oscillating body, the fixing unit, and the driving unit described above, a solid heat transfer body that covers the oscillating body. The solid heat transfer body is made of metal such as Al. Heat generated by light absorption is released from the reflection film through the solid heat transfer body to the outside.

Further, Patent Literature 2 describes an image display device that includes a light source, a MEMS mirror configured to scan a screen with an optical beam from the light source, an auxiliary light source configured to irradiate the MEMS mirror with an optical beam, and control means for controlling power of the auxiliary light source to keep the temperature of the MEMS mirror constant.

The MEMS mirror includes a mirror unit including a dielectric multilayer film, a first substrate on which the mirror unit is mounted, two beams for rotatably supporting the first substrate, and a second substrate to which the beams are fixed. The beams and the second substrate correspond to the aforementioned torsion spring.

In the MEMS mirror, a part of incident light from the light source is reflected on the dielectric multilayer film, and the remainder of the light is absorbed by the dielectric multilayer film or the first substrate. Reflectance of the dielectric multilayer film in the wavelength region of the light from the light source is 98% or higher.

The light from the auxiliary light source is also applied to the mirror unit of the MEMS mirror. The wavelength region of the auxiliary light source is different from that of the light source. Reflectance of the dielectric multilayer film in the wavelength region of the auxiliary light source is several %. Accordingly, most of the light from the auxiliary light source is absorbed by the dielectric multilayer film or the first substrate.

In the image display device, the control means controls the amount of light emitted from the auxiliary light source so that the sum total of the energy amount of the light emitted from the light source and absorbed by the mirror unit and the energy amount of the light emitted from the auxiliary light source and absorbed by the mirror unit can be constant.

CITATION LIST

Patent Literature

DISCLOSURE OF THE INVENTION

However, in the light deflector having the heat releasing structure described in Patent Literature 1, because of the structure where the mirror comes into contact with the solid heat transfer body, the mirror must be reciprocally vibrated together with the solid heat transfer body.

A resonance frequency in this case is lower than that when the mirror is reciprocally vibrated independently. The reduced resonance frequency causes reduction of the scanning speed and a change of the deflection angle. As a result, high-speed and accurate scanning with the optical beam is difficult.

Recently, to achieve high luminance of a displayed image, there is a tendency to increase the output of the light source for emitting a beam. For example, light sources of more than a dozen W to several tens of W may be used. When such a light source of a high output is used, it is difficult to prevent a temperature increase in the mirror by only the heat releasing effect of the solid heat transfer body. Especially, since the base and the reflection film are made of incident light absorbing materials, the light deflector described in Patent Literature 1 is easily affected by the temperature increase caused by as increase in the output of the light source.

In the image display device described in Patent Literature 2, because of the use of the auxiliary light source, the cost and size of the device have increased.

In the MEMS mirror, 98% of the entire incident light is reflected on the dielectric multilayer film, while remaining 2% is absorbed by the dielectric multilayer film or the first substrate. When the aforementioned light source of the high output is used, even in the case of 2% of the light, its absorption increases the temperature of the dielectric multilayer film or the first substrate, consequently causing distortion of the mirror or a change in rigidity of the torsion spring.

It is therefore an object of the present invention to provide an optical scanning element that can solve the aforementioned problems and prevent the temperature increase in a mirror unit, and in an image display device using the same.

Solution to Problem

To achieve the object, an optical scanning element according to the present invention includes: a dielectric multilayer film that reflects a part of incident light while transmitting the remainder of the light; a first substrate on one surface of which the dielectric multilayer film is formed, and which transmits the remainder of the light passed through the dielectric multilayer film; and a movable unit which includes a mounting unit on which the first substrate is mounted, the movable unit being configured so that the mounting unit can be rotated. The mounting unit includes a through-hole at a portion facing the dielectric multilayer film.

An image display device according to the present invention includes: a light source; a first optical scanning element that scans a screen with an optical beam from the light source in a first direction; a second optical scanning element that scans the screen with an optical beam from the first optical scanning element in a second direction intersecting the first direction; and a control unit that controls, based on an input video signal, the first and second optical scanning elements and the irradiation timing of the laser light source. Each of the first and second optical scanning elements includes the aforementioned optical scanning element according to the present invention.

DESCRIPTION OF REFERENCE NUMERALS

DESCRIPTION OF EMBODIMENTS

Hereinafter, the exemplary embodiments of the present invention will be described with reference to the drawings.

FIG. 1is a perspective view schematically showing the configuration of an optical scanning element according to the first exemplary embodiment of the present invention.FIG. 2is an exploded perspective view showing the main components of the optical scanning element.

Referring toFIGS. 1 and 2, the optical scanning element, which is used for a projection display device for scanning a screen with a laser beam to form an image, includes mirror unit1, movable unit2, base3, and absorption member4.

FIG. 3is a schematic view showing an example of mirror unit1. As shown inFIG. 3, mirror unit1includes dielectric multilayer film5that reflects a part of incident light while transmitting the remainder of the light, and mirror substrate6on one surface of which dielectric multilayer film5is formed and which transmits the remainder of the light passed through dielectric multilayer film5.

Dielectric multilayer film5has high reflectance in the wavelength region of a used laser beam, and the light absorbance of the wavelength region is nearly 0%.

FIG. 4is a schematic view showing an example of dielectric multilayer film5. In this example, dielectric multilayer film5has a structure where ZnS—SiO2layers5aand SiO2layers5bare alternately stacked. The film thickness tz of ZnS—SiO2layer5ais, for example, 47.1 nm. The film thickness is of SiO2layer5bis, for example, 69.1 nm.

According to dielectric multilayer film5having the laminate structure, in the wavelength region of 405 nm, reflectance of about 99% can be achieved, and light absorbance in the wavelength region can be nearly 0%.

Generally, the dielectric multilayer film has incident angle dependence where reflectance changes depend on a light incident angle. However, the influence of the incident angle dependence can be prevented by limiting wavelength regions (wavelength regions of incident light) that are used.

FIG. 5is a characteristic diagram showing an example of the spectral reflection characteristics of the dielectric multilayer film. A vertical axis indicates reflectance (%), and a horizontal axis indicates a wavelength (nm). Curve A indicates spectral reflection characteristics when light obliquely enters a film surface (e.g. at incident angle of 25°), and curve B indicates spectral reflection characteristics when the light vertically enters the film surface.

As shown inFIG. 5, when dielectric multilayer film5is irradiated with a laser beam having a wavelength of about 405 nm, reflectance of about 99% can be achieved in both cases of oblique incidence and vertical incidence. Absorbance of the laser beam is roughly 0% in both cases of oblique incidence and vertical incidence, and light of about 1% is transmitted through dielectric multilayer film5.

Referring again toFIGS. 1 to 3, mirror substrate6is made of a material (specifically, material having a transmittance of roughly 100% in the wavelength region of laser beam) having spectral transmittance characteristics, which is plate-like and transparent to the laser beam applied to dielectric multilayer film5. For example, as materials having spectral transmission characteristics transparent to the light having the wavelength region of 405 nm, there are SiO2, CaF2, and MgF2.

Movable unit2includes mounting unit7on which mirror substrate6is mounted. Movable unit2is configured so that mounting unit7can be rotated. Mounting unit7includes through-hole7a. Among laser beams applied to mirror unit1, a beam transmitted through dielectric multilayer film5and mirror substrate6passes through through-hole7a. The position and the size of through-hole7aare designed such that the transmitted beam is not blocked by mounting unit7.

FIG. 6is a top view of movable unit2. As shown inFIG. 6, movable unit2includes two torsion beams11aand11bwhich rotatably support mounting unit7, and frame-shaped substrate10to which torsion beams11aand11bare fixed. Mounting unit7, torsion beams11aand11b,and substrate10can be made of, for example, Si, and integrally formed. The length and the width of the torsion beam11aare nearly equal to those of torsion beam11b.

When seen from a direction vertical to the surface of substrate10, mounting unit7is located almost on the center of the opening (within frame) of substrate10. Frame-shaped mounting unit7includes first and second opposing sides71aand71b. The center (specifically, position of center of gravity) of first side71ais fixed to substrate10via torsion beam11a, and the center (specifically, position of center of gravity) of second side71bis fixed to substrate10via torsion beam11b.

Substrate10includes first and second opposing arms. One end of torsion beam11ais fixed to the center of the first arm, and one end of torsion beam11bis fixed to the center of the second arm. The length and the width of the first arm are nearly equal to those of the second arm.

The first arm includes arm portion10aextending toward one side from the center, and arm portion10bextending toward the other side. Piezoelectric element12ais arranged on arm portion10a, and piezoelectric element12bis arranged on arm portion10b.

The second arm includes arm portion10cextending toward one side from the center, and arm portion10dextending toward the other side. Piezoelectric element12cis arranged on arm portion10c, and piezoelectric element12dis arranged on arm portion10d.

Referring again toFIGS. 1 to 3, substrate10of movable unit2is fixed to base3of U-shaped cross section. Base3includes first and second opposing convex portions. Substrate10includes first and second fixed portions extending in a direction that intersects the first and second arms. The first fixed portion faces the second fixed portion. The first fixed portion is fixed to the first convex portion, and the second fixed portion is fixed to the second convex portion.

Absorption member4, which is made of a material for absorbing light of a specific wavelength region including that of the laser beam applied to mirror unit1, absorbs, from among the laser beams applied to mirror unit1, the beam that is transmitted through dielectric multilayer film5and mirror substrate6. Absorption member4is disposed in the entire area of the concave portion of base3opposite dielectric multilayer film5and irradiated with the transmitted beam.

Base3can be made of a material or formed into a structure capable of releasing heat generated by light absorption at absorption member4.

Next, the operation of the optical scanning element according to this embodiment will be described.

First, the operation of reciprocally vibrating mirror unit1will be described.

Mirror unit1is reciprocally vibrated by supplying a voltage based on a first driving signal to piezoelectric elements12aand12cand by supplying a voltage based on a second driving signal to piezoelectric elements12band12d. In this case, the first driving signal is reverse in phase to the second driving signal.

FIG. 7Ais a schematic view showing the deformed state of arm portions10aand10bof substrate10when voltage based on the first driving signal is supplied to piezoelectric element12aand when voltage based on the second driving signal is supplied to piezoelectric element12b.FIG. 7Bis a schematic view showing the deformed state of arm portions10cand10dof substrate10when voltage based on the first driving signal is supplied to piezoelectric element12cand when voltage based on the second driving signal is supplied to piezoelectric element12d.

At the half cycles of the first and second driving signals, arm portions10aand10bare set in the deformed state shown inFIG. 7A, and arm portions10cand10dare set in the deformed state shown inFIG. 7B. In these states, the portion (hereinafter, first end side) of mounting unit7on the side of arm portions10aand10bis higher than the surface of substrate10, and the portion (hereinafter, second end side) of mounting unit7on the side of arm portions10cand10dis lower than the surface of substrate10.

At the remaining half cycles of the first and second driving signals, the state of arm portions10aand10band the state of arm portions10cand10dare reverse to each other. That is, arm portions10aand10bare set in the deformed state shown inFIG. 7B, and arm portions10cand10dare set in the deformed state shown inFIG. 7A. In these states, the first end side of mounting unit7is lower than the surface of substrate10, and the second end side of mounting unit7is higher than the surface of substrate10.

By setting frequencies of the first and second driving signals nearly equal to the resonance frequencies of torsion beams11aand11b, torsion beams11aand11bcan be deformed based on these resonance frequencies. Accordingly, mirror unit1mounted on mounting unit7is reciprocally vibrated. This reciprocal vibration enables spatial scanning with the incident laser beam.

Next, a principle for preventing temperature increases at mirror unit1and mounting unit7during light irradiation will be described.

FIG. 8is an explanatory schematic view showing the principle for suppressing a temperature increase. As shown inFIG. 8, when dielectric multilayer film5is irradiated with irradiation beam (laser beam)100, a part of irradiation beam (laser beam)100is reflected on dielectric multilayer film5, while the remainder of the beam is transmitted through dielectric multilayer film5.

In the wavelength region of irradiation beam100, the light absorbance of dielectric multilayer film5is nearly 0, and thus the light absorbance causes no increase in the temperature of dielectric multilayer film5. Irradiation beam100is divided into reflection beam101and transmission beam102by dielectric multilayer film5.

Transmission beam102is transmitted through mirror substrate6. In the wavelength region of irradiation beam100, the light absorbance of mirror substrate6is nearly0, and thus the light absorbance causes no increase in the temperature of mirror substrate6.

Transmission beam102transmitted through mirror substrate6passes through through-hole7aof mounting unit7, and then reaches absorption member4. Transmission beam102is absorbed by absorption member4. In absorption member4, light energy is converted into thermal energy, and the temperature of absorption member4increases. The thermal energy is transferred from absorption member4to base3, and then is released from the outer surface of base3to the outside (heat releasing).

In the reciprocally vibrated state of mirror unit1, the position and the size of through-hole7aare designed such that transmission beam102is not blocked by a part of mounting unit7. Thus, transmission beam102always passes through through-hole7anot to be absorbed by mounting unit7.

The optical scanning element according to this embodiment is an example of the present invention, and its configuration can appropriately be changed without departing from the gist of the invention.

For example, an engagement structure, in which mirror substrate6is fixed to mounting unit7, can be employed.FIG. 9is a schematic view showing an example of such an engagement structure.

Referring toFIG. 9, engagement units6aand6bhaving hook-shaped cross sections are provided to both ends of mirror substrate6which are fixed to mounting unit7. The cross sectional shapes of first and second sides71aand71bof mounting unit7are square. The shape of engagement unit6amatches that of the corner of first side71a, and the shape of engagement unit6bmatches that of the corner of second side71b.

According to such an engagement structure, by fitting engagement units6aand6bof mirror substrate6to the corners of first and second sides71aand71bof mounting unit7, mirror substrate6and mounting unit7can be easily and accurately positioned.

Further, in the optical scanning element according to this embodiment, to reduce the weight of mirror unit1, a concave portion can be formed on a surface opposite the surface of mirror substrate6where dielectric multilayer film5is formed.

FIG. 10shows an example of the weight reduction structure of mirror substrate6. In this example, four concave portions60aare formed on a surface (rear surface) opposite the surface of mirror substrate6where dielectric multilayer film5is formed. Concave portions60aare equal in size, and rectangular in shape when seen from a direction vertical to the rear surface. Each concave portion60ahas a rectangular cross sectional shape. Because of the inclusion of concave portions60a, the weight of mirror substrate6is lower than that shown inFIG. 3. Reducing the weight of mirror substrate6enables an increase in the scanning speed.

FIG. 11shows another example of the weight reduction structure of mirror substrate6. In this example, a plurality of regularly arranged concave portions61aare formed on the surface (rear surface) opposite the surface of mirror substrate6where dielectric multilayer film5is formed. Concave portions61aare equal in size, and circular in shape when seen from the direction vertical to the rear surface. Each concave portion61ahas a rectangular cross sectional shape. Because of the inclusion of concave portions61a, the weight of mirror substrate6is lower than that shown inFIG. 3.

The periodic arrangement (2 rows and 5 columns) of concave portions61aenables a part where no concave portion61ais formed to secure rigidity of mirror substrate6.

In the optical scanning element according to this embodiment, mounting unit7is not limited to the square shape. Mounting unit7can be formed into other shapes such as a circular shape. However, in view of processability, mounting unit7is desirably formed into a square shape.

In this embodiment, mounting unit7, torsion beams11aand11b, and substrate10are made of, for example, Si. However, these components can be made of sheet metals such as stainless steel. In such a case, there is an advantage of using cheaper elements than Si.

FIG. 12is a perspective view schematically showing the configuration of an optical scanning element according to the second exemplary embodiment of the present invention.

Referring toFIG. 12, the optical scanning element, which is used in a projection display device that forms an image by scanning a screen with a laser beam, mainly includes mirror unit1and movable unit20. Mirror unit1is similar to that of the first exemplary embodiment (including modified example).

Mounting unit21includes through-hole21a, through which among laser beams that are applied to mirror unit1, a laser beam that is transmitted through dielectric multilayer film5and mirror substrate6pass. The position and the size of through-hole21aare designed such that the transmitted beam is not blocked by mounting unit21.

One end of shaft22is rotatably fixed to a bearing unit (not shown). The other end of shaft22is connected to the output shaft of a motor via power transmission means such as a gear (not shown). Shaft22is parallel to the film surface of dielectric multilayer film5, and a line passing through the center of the section of shaft22passes through the center of gravity of the film surface of dielectric multilayer film5when seen from a direction vertical to the film surface of dielectric multilayer film5.

In the optical scanning element according to this embodiment, by driving the motor, mirror unit1is rotated around shaft22. This enables spatial scanning with an incident laser beam.

In the optical scanning element according to this embodiment, by the same principle as that of the first exemplary embodiment for preventing the temperature increase (FIG. 8), the temperature increase of mirror unit1and mounting unit21is prevented.

In the wavelength region of irradiation beam100, the light absorbance of dielectric multilayer film Sand mirror substrate6is nearly 0, and thus the light absorbance causes no increase in the temperature of dielectric multilayer film5and mirror substrate6.

Transmission beam102transmitted through mirror substrate6passes through through-hole21aof mounting unit21. This prevents any increase in the temperature of mounting unit21.

In this embodiment, as in the case of the first exemplary embodiment, an absorption member can be disposed in a position facing dielectric multilayer film5to absorb transmission beam102that passed through through-hole21aof mounting unit21.

FIG. 13is a perspective view schematically showing the configuration of an optical scanning element according to the third exemplary embodiment of the present invention.

The optical scanning element according to this embodiment is similar to that of the first exemplary embodiment except for a mounting unit. InFIG. 3, components similar to those of the first exemplary embodiment are denoted by similar reference numerals. To avoid repeated description, detailed description of the similar components will be omitted.

Mounting unit30includes a lattice-shaped frame, and a plurality of through-holes31are formed by the frame. The sizes of through-holes31are nearly equal to one another. Mounting unit30, torsion beams11and11b, and substrate10can be made of, for example, Si, and integrally formed.

In the optical scanning element according to this embodiment, mirror unit1includes dielectric multilayer film5and mirror substrate6shown inFIG. 3. A beam transmitted through mirror unit1passes through each through-hole31of mounting unit30. Accordingly, the temperature increase of dielectric multilayer film5, mirror substrate6, and mounting unit30can be prevented.

The transmitted beam that passed through each through-hole31of mounting unit30is absorbed by absorption member4. In absorption member4, light energy is converted into thermal energy, and the temperature of absorption member4increases. The thermal energy is transferred from absorption member4to base3, and then released from the outer surface of base3to the outside (heat releasing).

In the optical scanning element according to this embodiment, a part of the transmitted beam is absorbed by the lattice-shaped portion of mounting unit30. As compared with a mounting unit having no through-hole, the following effects can be provided.

In the mounting unit having no through-hole, most of the transmitted beam is absorbed by the mounting unit. This light absorbance increases the temperature of the mounting unit, thus causing distortion of the mirror unit or a change in rigidity of the beam.

On the other hand, in the optical scanning element according to this embodiment, a part of the transmitted beam is absorbed by the lattice-shaped portion of mounting unit30, and the remainder of the transmitted beam passes through each through-hole31of mounting unit30. Accordingly, the light absorbance amount of mounting unit30is smaller than that of the mounting unit having no through-hole. Thus, a light source whose output is higher than that of the mounting unit having no through-hole can be used.

Since a part of the transmitted beam is absorbed by the lattice-shaped portion to increase the temperature of mounting unit30, the increase in the output of the light source is limited within a range that will not cause any distortion to the mirror unit or change the rigidity of the beam.

In the optical scanning element according to this embodiment, because of the lattice shape of mounting unit30, the rigidity of mounting unit30is higher than that of mounting units7of the optical scanning elements of the first and second exemplary embodiments.

The lattice-shaped frame of mounting unit30can be applied to mounting unit21of the optical scanning element of the second exemplary embodiment. This can increase the rigidity of mounting unit21of the optical scanning element of the second exemplary embodiment.

The aforementioned optical scanning element according to each of the embodiments can be used for a projection display device that forms an image by scanning a screen with a laser beam.

FIG. 14is a schematic view showing the configuration of a projection display device that includes the optical scanning element according to the present invention.

Referring toFIG. 14, the projection display device includes laser light source100, horizontal optical scanning element101, vertical optical scanning element102, screen103, and control unit104. It is presumed that the display surface of screen103is a two-dimensional plane of X and Z axes and a horizontal direction and a vertical direction on the display screen are respectively an X axis direction and a Z axis direction.

Horizontal optical scanning element101and vertical optical scanning element102include any of the optical scanning elements of the first to third exemplary embodiments.

Horizontal optical scanning element101, which is located in the traveling direction of a laser beam emitted from laser light source100, reflects the laser beam from laser light source100toward vertical optical scanning element102.

Vertical optical scanning element102, which is located in the traveling direction of the laser beam reflected by horizontal optical scanning element101, reflects the laser beam from horizontal optical scanning element101toward screen103.

Control unit104controls, based on an input video signal from the outside, the light emission timing of laser light source100and the rotation of the mirror units of horizontal optical scanning element101and vertical optical scanning element102. When horizontal optical scanning element101executes scanning in the horizontal direction, vertical optical scanning element102simultaneously executes scanning in the vertical direction, thereby forming a two-dimensional image on screen103.

The present invention has been described referring to the embodiments. However, the present invention is not limited to the embodiments. Various changes understandable to those skilled in the art can be made to the configuration and the specifics of the present invention.

This application claims priority from Japanese Patent Application No. 2010-126732 filed Jun. 2, 2010, which is hereby incorporated by reference herein in its entirety.