Optical scanning device

An optical scanning device of the invention includes: a substrate; torsion bar portion which is connected to the substrate; a mirror portion which is supported by the torsion bar portion; a drive source which causes the substrate to oscillate; and a light source which projects light onto the mirror portion, where the mirror portion resonates and vibrates in accordance with a vibration imparted to the substrate by the drive source, and the direction of reflection light from the light projected onto the mirror portion from the light source changes in accordance with the vibration of the mirror portion, and a spring constant in a longitudinal direction of the torsion bar portion supporting the mirror portion is distributed along the longitudinal direction of the torsion bar portion.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a 35 U.S.C.§371 National Phase conversion of PCT/JP2007/068636, filed Sep. 26, 2007, which claims benefit of Japanese Application No. 2006-261604, filed Sep. 27, 2006, the disclosure of which is incorporated herein by reference. The PCT International Application was published in the Japanese language.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical scanner which performs scans by the scanning of an optical beam, and, in particular, to an optical scanning device having a structure in which a micro mirror which is supported by torsion bars is made to oscillate so as to cause the direction of an optical beam to change.

2. Background Art

In recent years, optical scanners which scan optical beams of laser light or the like have been used as optical instruments such as bar code readers, laser printers, and head mounted displays, or as the optical intake devices of input devices such as infrared cameras and the like. Optical scanners having a structure in which a micro mirror obtained via silicon micromachining technology is oscillated have been proposed for this type of optical scanner. For example, the structure described in Japanese Unexamined Patent Application, First Publication No. H07-65098 (Patent document 1) is known (referred to below as ‘Conventional technology1’). As shown inFIG. 19, this optical scanner irradiates light which is emitted from a light source100and reflected by a mirror portion101onto a detection object102, and then vibrates the mirror portion101so that the light is scanned in a predetermined direction of the detection object102, and is provided with two mutually parallel drive sources103which are formed as cantilevered beams with one end respectively thereof formed as a fixed end and which perform bending operations, a linking component104which links together the free end sides of the two drive sources103, a torsional deformation component105which extends from a center portion of the linking component104, and the mirror portion101which is provided on this torsional deformation component105. The center of gravity of the mirror portion101is made to sit on the torsion center axis of the torsional deformation component105. If the two drive sources103are driven, for example, by a bimorph structure on which a piezoelectric material has been adhered, and are vibrated in antiphase, then torsional vibration is induced in the torsional deformation component105, and the two drive sources are driven at the resonance frequency of the torsional deformation component105. As a result, it is possible to vibrate the mirror portion101over a sizable amplitude.

Moreover, as shown inFIG. 20, the optical scanner described in Japanese Unexamined Patent Application, First Publication No. H04-95917 (Patent document 2, referred to below as ‘Conventional technology2’) is a scanner in which a mirror surface111is formed by a surface of a vibrator110having two elastic deformation modes, namely, a bending deformation mode and a torsional deformation mode, and in which this vibrator110is vibrated at the respective resonance frequencies of the two modes. Optical beams irradiated towards the mirror surface111of the vibrator110are reflected by that mirror surface111so that the light is scanned in two directions. If the vibrator110is vibrated in a single mode, then this scanner becomes a one-dimensional scanning optical scanner.

Moreover, as an optical scanner in which a micro mirror obtained by means of silicon micromachining technology is oscillated, the structure described in Japanese Unexamined Patent Application, First Publication No. H10-197819 (Patent document 3) is known (referred to below as ‘Conventional technology3’).

As shown inFIG. 21, this optical scanner is provided with a plate-shaped micro mirror121which is used to reflect light, a pair of rotation supporting bodies122which are positioned on a straight line and support both sides of the micro mirror121, a frame portion123to which the pair of rotation supporting bodies122are connected and which surrounds the periphery of the mirror121, and a piezoelectric element124which applies translational motion to the frame portion123. In addition, this optical scanner is structured such that the center of gravity of the mirror121is located at a position away from the straight line connecting together the pair of rotation supporting bodies122.

When voltage is applied to the piezoelectric element124, the piezoelectric element124is made to expand and contract, so as to vibrate in the Z axial direction. This vibration is transmitted to the frame portion123. When the micro mirror121is made to move relative to the driven frame portion123and the vibration component in the Z axial direction is transmitted to the micro mirror121, because the micro mirror121has a left-right asymmetrical mass component relative to the axis formed by the X axis rotation supporting bodies122, rotational moment is generated in the micro mirror121centered on the X axis rotation supporting bodies122. In this manner, the translational motion which has been applied to the frame portion123by the piezoelectric element124is transformed into rotational motion centering on the X axis rotation supporting bodies122of the micro mirror121.

Moreover, as shown inFIG. 22, an optical scanning device is also described in Japanese Unexamined Patent Application, First Publication No. H10-104543 (Patent document 4, referred to below as ‘Conventional technology4’). In this optical scanning device, beam portions133and133extend in mutually opposite directions from both sides of a movable portion132in a vibrator131, and are connected to two arm portions134and134of a fixed portion136. Piezoelectric thin films135and135are provided respectively on the arm portions134and134of the fixed portion136, and these piezoelectric thin films135and135are driven by the same signal which includes higher order vibration frequencies.

In the optical scanning devices of the above-described prior technologies, in order to achieve a small-size, portable laser projector or the like, it is necessary to position the above-described optical scanning device in a compact arrangement together with a laser light source and other optical systems, so that it is essential that such apparatuses are designed to be as small as possible. In order to achieve this, it is possible to miniaturize the optical scanning device using Si micro machining and the like, however, in contrast, in the case of a laser projector system which performs a single mirror scan, because the optical aperture width is determined by the size of the mirror, if this optical aperture width is too small, then it is not possible to achieve a sufficiently small spot size on the projection surface. As a result, there is a considerable deterioration in image resolution. Because of this, it is necessary for the size of the mirror to be greater than or equal to at least 1 mmφ, and depending on the application, a surface whose area is greater than or equal to 5 mm square is considered necessary. In this case, because the length of the hinge which supports the mirror portion is added to the mirror size, the size of those structural portions of the optical scanning device which generate torsional resonance ends up being greater than or equal to at least 5 mm square, and in some cases, greater than or equal to 1 cm which hinders attempts to make the size of the device any smaller. This is a serious problem as it makes it difficult for the scan angle of the optical scanning device to be made greater than or equal to 30°, and in the case of a low-scanning speed optical scanning device having a resonance frequency that is less than or equal to 100 Hz which is used for the vertical scanning of a two-dimensional scan, makes it difficult for the design to be made more compact (referred to below as Problem 1).

Moreover, in the design of an optical scanning device having a resonance frequency that is greater than or equal to 10 kHz in which the length of the torsion hinge is comparatively short, when this is made to resonate by a large driving force and the mirror portion is scanned using a large torsion angle that is greater than or equal to 20°, because the torsion angle of the torsion bars per unit length increases considerably, if the torsion bars are made from a metal material or the like, the problem has occurred that because of metal fatigue there is an abrupt deterioration in performance. Moreover, when the torsion bars are made from a brittle material such as silicon monocrystals, in order to achieve a large scan angle, there is a limit in the torsion angle per unit length. Because of this, it is necessary to design the hinge length to be comparatively long, and reducing the size of the mirror resonance structural portions and of the overall optical scanning device continues to remain a difficult design problem (referred to below as Problem 2).

If, for example, the spring constant of the elastic deformation portion (i.e., the torsion bar portions) in Conventional technology1is taken as k, and the moment around the rotation axis (i.e., the Y axis or the Z axis) is taken as I, then the resonance frequency f in a vibrator1can be expressed by the following formula.

The spring constant in the bending deformation mode (in a θB direction) of the elastic deformation portion is taken as kB, while the spring constant in the torsion deformation mode (in a θT direction) is taken as kT. If the spring constant k in Formula (1) is replaced by these spring constants kB and kT, then Formula (1) shows the resonance frequency fB in the bending deformation mode, and shows the resonance frequency fT in the torsion deformation mode, and the spring constant kB in the bending deformation mode is expressed by the following formula.

Here, E is Young's modulus, w is the width (i.e., the Length in the Y (direction) or the elastic deformation potion, t is the thickness (i.e., the length in the X direction) of the elastic deformation portion, and L is the length (i.e., the length in the Z direction) of the elastic deformation potion.

The spring constant kT in the torsion deformation mode is expressed by the following formula.

Here, G is the modulus of transverse elasticity, and β is a coefficient relating to the shape of the cross section. In Formula (3), more typically, w represents the length of a long side of the cross section of the elastic deformation portion, and t represents the length of a short side of the same cross section.

It is understood from Formula (1) that, as a result of the spring constant k changing, the resonance frequency of the vibrator is changed.

Moreover, in actual devices in which the above-described optical scanning devices are used such as laser projectors and barcode readers, because of the necessity for two-dimensional scans or in order to obtain a reduction in size, it is necessary not only to reduce the size of the optical scanning device itself, but to additionally create designs in which various combinations of reflection mirrors and the like are used in order to modify the optical path. However, each time light is reflected onto one of the respective minor portions, the overall light amount is decreased by the absorptivity of that mirror portion, and there is a deterioration in the projected image and in the luminance of the optical beams. In particular, when the optical scanning device is used in a portable device, increasing the amount of light from the light source, and consequently, securing sufficient voltage source capacity have proven to be sizable problems (referred to below as Problem 3).

Moreover, the optical scanning device of the above-described Conventional technology4has the drawback that a large torsion angle cannot be formed in the movable portion132.

Namely, if a piezoelectric film is formed in the two narrow-width cantilever beam portions which support the two torsion bars protruding from the frame portion, then the rigidity of this portion increases and vibration which is induced in the piezoelectric film is not transmitted efficiently to the torsion bars. As a result, the torsional vibration of the mirror becomes smaller. Moreover, unless the vibration characteristics of the vibration source portion formed by the two cantilever beam portions and the piezoelectric film which is formed thereon are matched precisely, then the vibration amplitude of the torsional vibration of the mirror becomes suppressed and, at the same time as this, a vibration mode other than torsional vibration is superimposed thereon so that accurate laser beam scanning cannot be achieved. Furthermore, in order to increase the drive force for the mirror by increasing the surface area of the piezoelectric film portion, it is necessary to increase the width of the cantilever beam portions. Because of this, an unnecessary two-dimensional vibration mode is generated in the same cantilever beam portion, so that at the same time as the vibration amplitude of the torsional vibration of the mirror is restricted, a vibration mode other than the torsional vibration is superimposed thereon. As a result, the problem arises that it is not possible to achieve accurate laser beam scanning. Moreover, because the width of the cantilever beams is restricted to a narrow width, the formation of the top portion electrodes which are used to drive the piezoelectric film formed on this portion is made more difficult because of the narrow width, so that problems arise such as the yield during production being greatly affected (referred to below as Problem 4).

FIG. 23shows the same case as that of Conventional technology4, and shows a structure in which a piezoelectric film is formed on two narrow-width cantilever beam portions which support two torsion bars which protrude from a frame portion. The drive efficiency of the mirror portion scan angle was checked by a simulation calculation. The surface where Y=0was taken as a plane of symmetry, and half of this was used as a model.

FIG. 24shows the torsion angle of a mirror having a structure in which a piezoelectric film is formed on two narrow-width cantilever beam portions which support two torsion bars which protrude from the frame portion shown inFIG. 23. The drive voltage was set at 1 V, while the characteristics of a PZT-5A which are typical parameters were used for the electrical characteristics of the piezoelectric body, while SUS 304 characteristics were used for the material of the scanner frame main body. The torsion angle of the mirror portion was small at only 0.63°.

SUMMARY OF THE INVENTION

It is an object of the invention to provide an optical scanning device in which it is possible to reduce the overall size of an optical scanning device while securing sufficient mirror size.

It is a further object of the invention to provide an optical scanning device in which torsional vibration can be generated efficiently in a mirror portion.

A description will now be given of the basic elements of the optical scanning device of the invention which is intended to achieve the above-described objects with reference made to the drawings.

Firstly, the basic structure of the optical scanning device which is the subject of the invention is shown inFIG. 1.

InFIG. 1, for example, by etching or by press working a plate material, a substrate10is cut out so as to leave a mirror portion11and torsion bar portions12remaining. The mirror portion11is supported from both sides by the torsion bar portions12and12which are joined to the substrate10. Outer side ends of the torsion bar portions12and12are supported respectively by cantilever beam portions14.

In addition, one end of the substrate10is supported in a cantilever form, for example, by a supporting component13.

In the present specification, the term ‘substrate10’ refers to a frame structural portion of a device which excludes the mirror portion11and the torsion bar portions12, but includes the cantilever beam portions14(hereinafter, the substrate10may be referred to as a frame structural portion). The portion of the substrate10which excludes the cantilever beam portions14is referred to as the substrate main body20.

A piezoelectric film15which is used to drive an optical scan is formed on a portion of the substrate10away from the linking portion between the substrate10and torsion bar portions12. This piezoelectric film15is formed using a thin-film formation method such as an aerosol deposition method (may be referred to below on occasion as an ‘AD method’), a sputtering method, or a sol-gel method, or may be formed by adhering a bulk piezoelectric thin plate material thereon. When voltage is applied from a power source16to a top portion electrode17on the optical scan driving piezoelectric film15and to the substrate10which serves as a bottom portion electrode, the optical scan driving piezoelectric film15vibrates in piezoelectric oscillation, and a plate wave or vibration is induced in the substrate10. Using this, torsional vibration is generated in the mirror portion11. As a result, it is possible to generate torsional vibration efficiently in the mirror portion11using a simple structure.

In this case, the optical scan driving piezoelectric film15forms a drive source to cause the substrate10to vibrate.

If optical beams are irradiated from a light source18onto the mirror portion11while voltage is being applied to the optical scan driving piezoelectric film15which serves as a drive source, then because the mirror portion11vibrates, light which is reflected by the mirror portion11vibrates at a uniform torsion angle.

In the basic structure of the optical scanning device which is the subject of the invention shown inFIG. 1, firstly, in order to solve the above-described Problem 1, as shown in Part (b) ofFIG. 2, according to the invention, either the mirror portion11is extended so as to surround the torsion bar portions12, or notches which are parallel with the axial direction of the torsion bar portions12are cut into the inner sides of the mirror portion11so that the torsion bar portions12are extended as far as the inner sides of the mirror portion11. As a result, essentially, without changing the overall length of the mirror portion11and torsion bar portions12, the mirror size is enlarged. At this time, there is a loss in the amount of reflection light corresponding to the surface area of the notches which separate the mirror portion11from the torsion bar portions12, however, the width of these notches only needs to be enough for them to not be connected mechanically, and it is easy to reduce the surface area of these portions compared with the entire mirror surface area, and the reduced light amount does not pose any problems. For example, if this surface area is set less than or equal to 10% of the diameter of the light spot projected onto the mirror surface, then the loss in the amount of light can be essentially disregarded. PART (a) ofFIG. 2shows a conventional device.

Moreover, in order to solve Problem 2, as shown inFIG. 3, in the invention, by distributing the spring constant in the axial direction of the torsion bar portions12which support the mirror portion11, namely, by changing the spring constant in the longitudinal direction, compared with when there is no distribution in the spring constant of the torsion bar portions12, it is possible for the length of the torsion bar portions12to be designed shorter in both the resonance frequency and the scan angle.

In the present specification, any change in the spring constant which is due to the shape of the torsion bar portions being changed is included as being a change in the spring constant of the torsion bar portions.

Moreover, in order to solve Problem 3, as shown inFIG. 4, in the invention, the torsion bar portions12which support the mirror portion11are formed from a metal material such as stainless steel or from a plastically deformable material such as a resin material. By causing these torsion bar portions12to become plastically deformed so that the mirror portion11is fixed on an angle relative to the substrate10supporting the torsion bar portions12, then an optical beam is incident onto the mirror portion11without using reflective mirrors, it is possible to impart a deflection angle to the scanning optical beam. As a result, structurally, the above-described optical scanning device can be designed with a smaller size.

Moreover, in order to solve Problem 4, by forming a single piezoelectric film (i.e., body) as a vibration source in the frame portion, the rigidity of the two cantilever beam portions is lowered and mirror torsional vibration is efficiently induced. At the same time as this, by only providing a single vibration source to drive the mirror, the invention solves the above-described problem of the creation of an unnecessary vibration mode as well as the problem of a consequent reduction in amplitude which are caused by a lack of uniformity in the vibration source. Moreover, by separating the piezoelectric film formation portion which forms the vibration source from the mirror torsional vibration portions which are formed by the mirror portion and the torsion bar portions which support the mirror portion by means of the two cantilever beam portions, the surface area of the piezoelectric film forming the drive source can be set freely irrespective of the width of the cantilever beam portions, and it becomes possible to apply a large drive force more efficiently to the mirror torsional vibration portions. Furthermore, it is also easy to form the electrodes for driving the piezoelectric film, so that it becomes possible to improve the yield in industrial production.

FIG. 5is a plan view showing an optical scanning apparatus according to the invention which is structured such that one piezoelectric film15forming the drive source is provided on the substrate10, wherein the surface where Y=0 is taken as a plane of symmetry, and only half of this is used as a model.

The dimensions of the mirror portion11and the dimensions of the torsion bar portions12, the mounting position where the torsion bar portions12are mounted on the mirror portion11(i.e., the position of the center of gravity of the mirror portion11), the shape of the substrate10as well as the method which is used to support it, and also the thickness and the value of the total surface area of the piezoelectric film15which all provide the basic structure of the optical scanning apparatus are made the same. This optical scanning apparatus only differs in the position where the piezoelectric film15is formed.

FIG. 6shows the torsion angle of the mirror portion11of the device shown inFIG. 5. The drive voltage was set at 1 V, while the characteristics of a PZT-5A which are typical parameters were used for the electrical characteristics of the piezoelectric body, while SUS 304 characteristics were used for the material of the scanner frame main body. Basically, the resonance frequency of the invention shown inFIG. 5is substantially the same as in Conventional technology4shown inFIG. 16, however, while the torsion angle of the mirror portion11was only 0.63° in Conventional technology4, in the structure of the invention shown inFIG. 5, it was confirmed to have an approximately 4.3 times greater torsion, namely, the torsion angle was 2.69° (80.7° at a conversion of 30V).

It is also possible to position a plurality of vibration sources on a substrate in order to increase the mirror scanning amplitude, however, in this case, because of irregularities in the mounting state due to the characteristics of the vibration sources or the mounting positions, or to the adhesion or film formation, it becomes easy for two-dimensional vibration which is asymmetrical relative to the axis of symmetry in a perpendicular direction relative to the torsion bars supporting the mirror portion to be induced in the substrate, which results in a deterioration in the scanning accuracy of the optical beams due to the torsional vibration of the mirror portion. In contrast, in the invention, torsional vibration is induced efficiently in the minor portion even though there is only one vibration source, and it is possible to largely reduce scan jitter in the optical beams, and suppress product irregularities.

As shown inFIG. 1of the invention, in order to transmit vibration energy generated at a position separated from the mirror portion11as energy which efficiently becomes torsional vibration in the minor portion11, it is necessary to considerably set the resonance frequency (fm) of the mirror portion11which is mainly determined by the mass of the mirror portion11and by the spring constant of the torsion bars12away from the resonance frequency (fb) which includes the division oscillation mode of the frame portion itself. When the piezoelectric film15of the optical scanning device is driven so as to match the resonance frequency (fm) of the torsional vibration of the mirror portion11, then if a resonance mode is also induced in the substrate10, the vibration energy generated by the vibration source becomes distributed between torsional vibration of the mirror portion11and two-dimensional division vibration of the substrate10due to the law of conservation of energy. Accordingly, the amplitude (i.e., the torsion angle) of the torsional vibration of the mirror portion11becomes smaller by the amount of vibration energy from the drive source which is consumed by the two-dimensional division vibration of the substrate10, so that it becomes impossible to efficiently drive the optical scanning device. Moreover, if unnecessary two-dimensional division vibration is induced in the substrate10, then there are also cases in which a vibration mode other than pure torsional vibration which has the torsional bar portions12as its axis of rotation becomes superimposed on the mirror portion11that positions at the distal end of the substrate10, so that it becomes impossible to achieve an optical scan having a high level of accuracy in the rectilinear scan performance thereof. In contrast, as shown inFIG. 7, the invention is designed such that the torsional resonance frequency a (fm (n): n=0, 1, 2, . . . ) which includes elements up to the higher orders induced in the mirror portion does not overlap with the torsional resonance frequency b (fb (n): n=0, 1, 2, . . . ) which includes elements up to the higher orders induced in the substrate10.

The optical scanning device of the invention has a basic structure in which the thin plate-shaped substrate10shown inFIG. 1is cantilever supported by a supporting component13on the opposite side from the mirror portion11. Because of this, if a vertical disturbance vibration is applied to the entire optical scanning device, then the entire optical scanning device vibrates, and optical beams which are reflected and scanned by the mirror portion11are affected by this vibration and do not vibrate stably, so that the problem arises that it is not possible to guarantee an accurate optical scan. Accordingly, assuming that the optical scanning device will be used in practical applications such as in portable devices and the like, it is necessary to improve this instability which is caused by this entire optical scanning device being supported by a cantilever structure.

Therefore, as shown inFIG. 8, in the invention, the optical scanning device is fixed by means of narrow-width substrate connecting bars23to an extremely rigid substrate fixing frame22which is positioned so as to surround the entire cantilever supported optical scanning device at positions separated from the supported portion supported by the supporting component13.

At this time, the resonance state of the optical scanning device itself changes depending on the fixing positions of the substrate connecting bars23, and is affected by the scan angle and resonance frequency of the mirror portion11.

FIGS. 9 and 10show the results when this state was examined. As shown in PART (a) of FIG.9., if the optical scanning device is fixed by the substrate connecting bars23at the base of cantilever beam portions14whose vibration amplitude close to the center of the vibration is large when the mirror portion11is in torsional resonance state, then the scanning amplitude of the mirror portion11is considerably reduced, namely, by approximately 17° compared to the scanning amplitude of approximately 55° which is obtained when the optical scanning device is not fixed. This is because if a large portion of the vibration amplitude is fixed at the outer peripheral portion of the optical scanning device so that this vibration is suppressed, then the vibration mode of the entire optical scanning device substrate10is changed, which results in it becoming impossible to efficiently transmit energy to the torsional vibration of the mirror portion11.

In contrast, in the state shown inFIG. 10in which the substrate connecting bars23are not connected, when the mirror portion11is in torsional resonance state, if, as shown in PART (d) ofFIG. 9, the optical scanning device is connected and fixed by the substrate connecting bars23in a portion in the vicinity of a bottom point25where a vibration amplitude in the Z axial direction of the edge portion of the optical scanning device substrate10(i.e., a portion indicated by the symbol24inFIG. 10) is at the minimum, then the scanning amplitude of the mirror portion11becomes a slightly larger scanning amplitude, namely, approximately 55°, than when it is not fixed to the substrate fixing frame22. In this case, because the vibration mode of the entire optical scanning device substrate10is not changed, it is possible to maintain a substantially equivalent resonance state compared with when the optical scanning device is not fixed, and any effects on the scanning amplitude of the mirror portion11of the optical scanning device substrate10from the substrate connecting bars23are kept to a minimum.

Accordingly, if the optical scanning device is fixed via the outer edge portion of the optical scanning device by means of the substrate connecting bars23at the bottom point of the vibration when the mirror is resonating, or at the point where the vibration amplitude is the smallest and which is also furthest away from the optical scanning apparatus supporting component13, then it is possible to stably support the optical scanning device against any external disturbance vibration without attenuating the scanning amplitude of the mirror portion11.

When the scan jitter and optical face tangle error (i.e., the stability of the beam scanning speed) of the optical beams in the above-described optical scanning device of the invention were evaluated by a MEMS scanner measurement system (ALT-9A44) manufactured by ALT Ltd., it was found that while scan jitter of a conventional silicon MEMS optical scanner (manufactured by Nippon Signal) was Jp-p: 0.2 to 0.3%, irrespective of the fact that the optical scanning device of the invention was formed from a metal material, the scan jitter at scan resonance frequencies of 6 kHz, 16 kHz, and 24 kHz was smaller by a factor of 10, namely, Jp-p: 0.06%. It was thus possible to achieve a high-accuracy optical beam scan which equated to conventional polygon mirror technology. Moreover, in conventional polygon mirror technology, the optical face tangle error is approximately Wp-p: 30 to 40 seconds, and it is necessary to apply correction using an f-Θ lens or the like and lower the value by a factor of 10. However, in the optical scanning device of the invention, the optical face tangle error is approximately Wp-p: 5 seconds which is a lower value by a factor of 10, so that it is possible to achieve a highly stable beam scanning speed without a correction lens system, so that reductions in both size and cost can be achieved easily. From the above-described measurement results, in the optical scanning device of the invention, it is evident that it is possible to obtain an excellent optical beam scanning accuracy which can be used in a laser printer and the like.

The invention provides the excellent effects described below.(1) By distributing the spring constant over the longitudinal direction of the torsion bar portions that supports the mirror portion, it is possible to design a shorter length for the torsion bar portions in both the resonance frequency and scan angle compared with when there is no distribution of the spring constant of the torsion bar portions.(2) By making the substantial length of the torsion bar portions supporting the mirror portion longer than the space between the mirror portion end surfaces at the two ends of the torsion bar portions and the substrate, it is possible to obtain a larger mirror size without changing the overall length of the mirror portion and torsion bar portions.(3) By setting the mirror portion in a desired position by pivoting the mirror portion from its initial set position around the supporting axis of the torsion bar portions, and imparting plastic deformation to the torsion bar portions, then the mirror portion is fixed in position on an angle, an optical beam is incident onto the mirror portion without using reflective mirrors, it is possible to impart a deflection angle to the scanning optical beam. As a result, structurally, the above-described optical scanning device can be designed with a smaller size.(4) By sizably setting the resonance frequency of the mirror portion which is determined by the mass of the mirror portion and by the spring constant of the torsion bars away from the resonance frequency of the substrate, it is possible to transmit vibration energy generated at a position separated from the mirror portion efficiently as energy which becomes torsional vibration in the mirror portion.(5) By positioning a substrate fixing frame so as to surround the substrate main body and cantilever beam portions and then fixing it on the fixing end side of the substrate main body, and by connecting together the substrate main body and the substrate fixing frame by means of substrate connecting bars at a position away from the supporting component and also in the vicinity of the minimum amplitude of the substrate vibration, it is possible to stably support the optical scanning device against external disturbance vibration without attenuating the scanning amplitude of the mirror portion.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The best embodiments for implementing the optical scanning device according to the invention will now be described based on examples with reference made to the drawings.

FIG. 2is a plan view illustrating Example 1 according to the invention. PART (a) ofFIG.2shows a conventional optical scanning device, while PART (b) ofFIG. 2shows the optical scanning device of Example 1. InFIG. 2, a supporting component which supports a substrate10and a power supply which supplies voltage to the optical scan driving piezoelectric film15have been omitted from the drawing.

In PART (b) ofFIG. 2, by either extending the mirror portion11so as to surround the torsion bar portions12, or by inserting notches which are parallel with the axial direction of the torsion bar portions12into the inner sides of the mirror portion11, the torsion bar portions12are made to extend as far as the inner sides of the mirror portion11, so that, essentially, without changing the size and resonance frequency of the mirror portion11from those shown in PART (a) ofFIG. 2, the space between the mirror portion11and the substrate10can be shortened compared to that shown in PART (a) ofFIG. 2. As a result, it is possible to reduce the overall size of the optical scanning device.

In experiments, substantially without altering the scan angle (100° and the torsion resonance frequency (500 to 600 Hz) of the mirror portion11, as shown inFIG. 2, it is possible to shorten the length of the connection between the outer ends of the two torsion bar portions12and12which support the mirror portion11from both sides thereof from 16 mm to 12 mm, namely, by approximately 25%, which is effective in allowing the optical scanning device to be designed with a smaller size.

FIG. 11is a plan view illustrating Example 2 according to the invention. InFIG. 11, a supporting component which supports a substrate10, an optical scan driving piezoelectric film, and a power supply and the like have been omitted from the drawing.

FIG. 11shows an example of a method of distributing the above-described spring constant in which, as shown in the top right portion of the drawing, a portion of the torsion bar portions12is formed as a resilient zig-zag structure, so that the length of the connection between the outer ends of the two torsion bar portions12and12can be designed shorter compared to the case shown in the top center of the drawing in which there is no distribution of the spring constant of the torsion bar portions12in both the resonance frequency and the scan angle.

In this case, if the length of a straight line connecting together the two ends of each torsion bar portion12is considered as being fixed, then as a result of the actual length of the torsion bar portions12themselves being extended, it is possible to lower the resonance frequency and, at the same time, increase the scan angle. Moreover, as shown in the graph in the bottom ofFIG. 11, if the same resonance frequency is considered, then in this example the length of the straight line connecting together the two ends of each torsion bar portion12can be shortened by approximately ⅓, namely, from 3 mm to 1 mm (see the symbols ▪ in the drawing), and additionally the scan angle can be increased by approximately 20% (see the symbol ● in the drawing). Thus, the overall size of the optical scanning device can be reduced.

As shown inFIG. 12, if the length of a straight line connecting together the two ends of each torsion bar portion12is considered as being fixed at 1 mm, then as a result of the extended length provided by the zig-zag structure, the same tendency can be shown as when the length of the bars is extended. For example, it is possible to lower the resonance frequency and, at the same time, increase the scan angle. In this example, if the length of the straight line connecting together the two ends of each torsion bar portion is considered as being fixed at 1 mm, then the substantial length of the torsion bars in this zig-zag structure is extended from 1 mm (see A inFIG. 12) to 3 mm (see B inFIG. 12) or 4.6 mm (see C inFIG. 12). As a result, the scan angles can be increased by 33% and 51% respectively. Thus, the overall size of the optical scanning device can be reduced.

Moreover, it is also possible while maintaining a high scan angle that is greater than or equal to 40° to precisely adjust the resonance frequency.

FIG. 13shows an example in which, by moving the position of the zig-zag structure, the resonance frequency is precisely adjusted up or down.

In this case, with the length of a straight line connecting together the two ends of each torsion bar portion12and the substantial length of each torsion bar portion fixed, by moving the position of the zig-zag structure in a center portion of each torsion bar portion away from this center portion towards the cantilever beam portion side, it is possible to lower the resonance frequency and, at the same time, decrease the scan angle. Moreover, by moving the position of the zig-zag structure in the center portion of each torsion bar portion away from this center portion towards the mirror portion11side, it is possible to raise the resonance frequency and, at the same time, increase the scan angle. By employing this technique, it is possible while maintaining a high scan angle greater than or equal to 50° to precisely adjust the resonance frequency.

Moreover, at this time, the torsion angle of each portion of the torsion bar portions12per unit length of the length of a straight line connecting together the two ends of each torsion bar portion12is smaller than when a simple straight rod-shaped torsion bar structure is used, and when the torsion bar portions are made from a metal material, this enables the fatigue characteristics to be improved. Moreover, when a brittle material such as silicon monocrystals is used, it is possible to cause the mirror portion to resonate at a larger scan angle that is greater than or equal to the brittle fracture limit.

FIG. 14is a plan view illustrating Example 3 according to the invention. InFIG. 14, a supporting component which supports a substrate10, an optical scan driving piezoelectric film, and a power supply and the like have been omitted from the drawing.

FIG. 14shows an example of a method of distributing the above-described spring constant in which, by distributing the spring constant between a portion of the width of the torsion bar portions12, it is possible to partially increase the mechanical rigidity and, in spite of the overall length of the torsion bar portions12remaining fixed, the torsion resonance frequency of the mirror portion11is increased.

In the experiment data shown inFIG. 14, the resonance frequency and scan angle data are shown for when the length of straight torsion bar portions12having a fixed width was changed in order to provide a comparison. When both were compared, it was confirmed that when the supporting portions of the substrate10which support the torsion bar portions12were formed in a triangular shape so that the spring constant was distributed over the width of the torsion bar portions12, while the scan angle was kept substantially constant, the resonance frequency was increased by substantially 30% from, 8.6 kHz to 12 kHz (as shown by the arrow inFIG. 14). In this case, it was found that a high resonance frequency which was impossible to achieve even when the length of the torsion bar portions12was shortened to the minimum possible limit was able to be achieved by forming the supporting portions of the substrate10which support the torsion bar portions12in a triangular shape so that the spring constant was distributed over the width of the bars.

Due to the effect of the triangle shape in the supporting portion of the cantilever beam portions14which support the torsion bar portions12, the transmission efficiency of the vibration energy being transmitted from the substrate10side is strengthened. As a result, in order for it to be possible to increase the resonance frequency while the scan angle is kept in a substantially unchanged state, it is desirable for the height of the triangle shapes to be less than or equal to half the overall length of the torsion bar portions12.

FIG. 17is a perspective view illustrating Example 4 according to the invention.FIG. 17shows an example of the distribution of the spring constant over the longitudinal direction of the torsion bar portions12. In this example, by changing the material of the torsion bar portions12in the longitudinal direction supporting the mirror portion11, the resonance frequency or scan angle are changed.

By raising the mechanical rigidity in the longitudinal direction by a factor of 0.7 relative to the material forming the substrate10of portions of the material forming the torsion bar portions12, it is possible to increase the resonance frequency thereof while keeping the scan angle substantially constant.

Moreover, by lowering the mechanical rigidity in the longitudinal direction by a factor of 0.8 relative to the material forming the substrate10of portions of the material forming the torsion bar portions12, it is possible to increase the scan angle thereof while keeping the resonance frequency substantially constant.

FIG. 18is a perspective view illustrating Example 4 according to the invention.FIG. 18shows an example of the distribution of the spring constant over the longitudinal direction of the torsion bar portions12. In this example, by providing a different material from the material forming the torsion bar portions12integrally with a portion of the torsion bar portions12supporting the mirror portion11, the resonance frequency or scan angle are changed.

By providing as the material which is different from the material forming the torsion bar portions12a material having a higher mechanical rigidity (e.g., TiN, W, Al203) than the material forming the torsion bar portions12integrally with a portion of the torsion bar portions12supporting the mirror portion11, it is possible to increase the scan angle thereof while keeping the resonance frequency substantially constant. In particular, by adjusting the thickness of the material having a high mechanical rigidity, it is possible to precisely adjust any desired increase in the resonance frequency. Moreover, by adjusting the position on the portion of the torsion bar portions12where the material which is different from the material forming the torsion bar portions12is located as well as by adjusting the size thereof, it is possible to precisely adjust the resonance frequency.

The material which is different from the material forming the torsion bar portions12which is formed on the torsion bar portions12is desirably a thick film formed by an AD method.

Shot peening is a cold working technique in which small, hard spheres having a particle diameter of approximately 20 μm to 1.3 mm which are known as shot material are jetted at an accelerating speed by a projection apparatus so as to collide at high speed with a processing component. Torsion bar portions12which have been subjected to the shot peening have a certain amount of roughness formed on the surface thereof, however, the surface layer portion thereof becomes work-hardened, and has a high level of compressive residual stress imparted thereto. As a result, the mechanical rigidity is increased in portions of the torsion bar portions12so that it is possible to increase the resonance frequency thereof while keeping the scan angle substantially constant. Moreover, by adjusting the material used for the shot peening material as well as the position and size of the area where the shot peening is performed, it is possible to precisely adjust the resonance frequency.

FIG. 4is perspective view illustrating Example 6 according to the invention. In this example, the torsion bar portions12which support the mirror portion11are formed from a metal material such as stainless steel or from a plastically deformable material such as a resin material. By causing these torsion bar portions12to become plastically deformed so that the mirror portion11is fixed on an angle relative to the substrate10supporting the torsion bar portions12, then an optical beam is incident onto the mirror portion11without using reflective mirrors, it is possible to impart a deflection angle to the scanning optical beam. In experiments it was confirmed that, as shown inFIG. 4, as a result of the plastic deformation of metal torsion bar portions, even if the deflection angle of the mirror portion11relative to the substrate10is changed from 0° to 90°, a state of resonance could still be achieved.

Moreover, as shown inFIGS. 15 and 16, the increase in the deflection angle caused by twisting of the torsion bar portions12has substantially no effect on the torsion resonance frequency of the mirror portion11, however, the scan angle decreases in conjunction with the increase in the deflection angle.