JP2014116875A - Imaging optical system and imaging device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an imaging optical system and an imaging device that are capable of accurately variably controlling a cutoff frequency of an optical low-pass filter with a simple structure and control system.SOLUTION: There is provide an imaging optical system comprising: an optical low-pass filter that separates and non-parallelly emits incident light; an image sensor; and distance adjustment means that varies a relative distance between the optical low-pass filter and the image sensor, or an imaging device having the imaging optical system.

Description

  The present invention relates to an imaging optical system having an optical low-pass filter, a two-dimensional imaging device such as a CCD (Charge Coupled Device) and a CMOS (Complementary Metal-Oxide Semiconductor), and an imaging apparatus using the imaging optical system.

Imaging devices such as digital still cameras and digital video cameras use image sensors such as CCDs and CMOSs, convert the amount of light and darkness of light that is input as an external signal for each pixel of the image sensor into electrical signals, and Generate.
In the process of generating an image, moiré and false colors are generated when an imaging target having a spatial frequency higher than the Nyquist frequency determined by the pixel pitch of the imaging element is captured. In order to prevent such moire and false color, an optical low-pass filter is used.

  The ideal optical low-pass filter is that the Nyquist frequency of the image sensor is the cut-off frequency, the contrast value of the image to be photographed having a spatial frequency lower than the cut-off frequency is 1, and the image target has a spatial frequency higher than the cut-off frequency. In other words, the contrast value is 0. However, an optical low-pass filter having such characteristics does not actually exist, and it is impossible for an actual low-pass filter to reduce the decrease in contrast to zero even in a region lower than the cutoff frequency. Contrast reduction is inevitable. Thus, except for the effect of preventing moiré and false color, the optical low-pass filter is also a cause of image degradation. Therefore, in recent years, there has also been proposed a digital still camera that attempts to obtain a high-definition image by setting the cutoff frequency of the optical low-pass filter on the high frequency side at the expense of the effect of preventing moire and false colors.

In an imaging device that can capture still images and moving images, it is preferable to set an appropriate cutoff frequency for both still images and moving images. In particular, for a digital still camera that can shoot not only still images but also moving images, high-precision image quality is required for both still images and moving images. For example, a digital single-lens reflex camera uses 10 million pixels or more in a still image, but thins out pixels used up to about 2 million pixels in a moving image, so that the Nyquist frequency is different. Therefore, it is preferable that different characteristics of the optical low-pass filter can be set in accordance with each photographing mode.
Some digital still cameras can select the screen resolution of a photographed photo in several stages. Also in this case, since pixels to be used are thinned out, it is preferable if different characteristics of the optical low-pass filter can be set in accordance with the photographing mode.

Against this background, a technique for making the cutoff frequency of the optical low-pass filter variable has been proposed. If the cutoff frequency of the optical low-pass filter can be made variable, the effect of the optical low-pass filter can be set according to the photographer's intention, and the effect of the low-pass filter can be set according to the shooting mode. .
In the prior art document, the cut-off frequency can be varied by applying stress to the photoelastic material. However, in order to control the setting of the cut-off frequency, the light projecting element and the test light that emit the test light are used. A feedback means that requires a light receiving element for receiving light and a characteristic calculation unit for calculating the birefringence characteristic of the photoelastic member based on the intensity of transmitted inspection light is required, which complicates the structure and system.

JP 2003-167219 A

  An object of the present invention is to provide an imaging optical system and an imaging apparatus that can variably control the cutoff frequency of an optical low-pass filter with high accuracy with a simple structure and control system.

The present inventors solve the above-mentioned problems by using an optical low-pass filter that separates incident light and emits it non-parallel, and provided with a distance adjusting means that varies the relative distance between the optical low-pass filter and the image sensor. As a result, the present invention has been completed.
Specifically, the present invention has the following configuration.

(Configuration 1)
An imaging optical system comprising: an optical low-pass filter that separates incident light and emits the light in parallel; an imaging element; and a distance adjusting unit that varies a relative distance between the optical low-pass filter and the imaging element.
(Configuration 2)
The imaging optical system according to Configuration 1, wherein the optical low-pass filter is a diffractive optical low-pass filter.
(Configuration 3)
The distance adjusting means is 0.1 ≦ fc / when the Nyquist frequency of the image sensor is fn, and the cutoff frequency evaluated by the action of the optical low-pass filter and the distance between the low-pass filter and the image sensor light receiving unit is fc. The imaging optical system according to Configuration 1 or 2, wherein a relative distance between the optical low-pass filter and the imaging element is controlled so that fn ≦ 10.
(Configuration 4)
In the imaging optical system, the center wavelength of the used wavelength band is λ [μm], the diffraction grating period of the diffractive optical low-pass filter is d [μm], and the pixel pitch of the imaging element is W [μm]. When fc / fn is α ≦ fc / fn ≦ β, the grating period d is
4. The imaging optical system according to any one of configurations 1 to 3, wherein d ≧ λ / sin θ.
However, θ satisfies the following formula.

(Configuration 5)
The optical low-pass filter has a refractive index change region having a refractive index difference different from that of the base material inside the glass, and the refractive index difference is modulated at a predetermined period, thereby functioning as a transmissive diffraction grating 1 5. The imaging optical system according to any one of items 1 to 4.
(Configuration 6)
An imaging apparatus having the imaging optical system according to any one of configurations 1 to 5.

  According to the present invention, there is provided an imaging optical system and an imaging apparatus capable of variably controlling the cutoff frequency of an optical low-pass filter with high accuracy with a simple structure and control system.

It is an example of the beam separation of the diffraction type optical low-pass filter used for the Example in this invention. It is a pattern of light separation by a checkered diffraction grating, the horizontal axis and depth axis are diffraction orders, and the vertical axis is diffraction efficiency. It is a MTF characteristic in Table 2 of Example 1. FIG. It is an MTF characteristic in Table 4 in Example 2.

The imaging optical system and imaging apparatus of the present invention will be described below.

  The imaging optical system of the present invention includes at least an optical low-pass filter that separates incident light and emits the light in a non-parallel manner, and an imaging element. By changing the relative distance between the optical low-pass filter and the image sensor, the cut-off frequency is changed using the fact that the separation width of the light beam in the attending part of the image sensor changes. If the beam separation angle of the optical low-pass filter at a specific wavelength is known, the cutoff frequency of the imaging optical system including the optical low-pass filter can be controlled simply by accurately controlling the relative distance between the optical low-pass filter and the image sensor using the distance adjusting means. Can be controlled with high accuracy.

As an optical low-pass filter that separates incident light and emits it non-parallel, an optical low-pass filter having an external cone refraction effect, a diffractive optical low-pass filter, or the like can be used.
In the diffractive optical low-pass filter, it is preferable that the refractive index change region in the glass substrate is modulated with a predetermined period. With this configuration, it is possible to impart a light diffracting action to the glass substrate, so that the light rays incident on the optical low-pass filter are separated, and the separated light rays are emitted non-parallel. Crystals conventionally used in optical low-pass filters have a light separation effect, but the separated and emitted light beams are parallel, so use them alone in an imaging optical system that only has the light separation effect. I can't.

  In order to form a plurality of refractive index changing regions on a glass substrate, for example, by irradiating the glass substrate with a pulse laser and applying energy to the inside of the glass substrate, it can be partially changed into a heterogeneous layer. It is done.

Assuming that the diffraction grating period of the diffractive optical low-pass filter is d, the angle 2θ formed by the ± first-order diffracted light, which is a separated light beam of wavelength λ perpendicularly incident on the diffractive optical low-pass filter, is 2θ = 2sin ⁻¹ (λ / d). Assuming that the relative distance between the exit surface of the separated light and the light receiving portion of the image sensor in the optical low-pass filter is L, the light beam separation width w in the attending portion of the image sensor is w = 2L tan θ. Therefore, if L is controlled by the distance adjusting means, the cutoff frequency of the imaging optical system can be controlled.

  The distance adjusting unit that varies the relative distance between the optical low-pass filter and the image sensor includes a moving unit that moves the optical low-pass filter or the image sensor along the optical axis. The distance adjusting means preferably includes an optical low-pass filter or detection means for detecting the position of the image sensor. Furthermore, it is preferable that the distance adjusting means includes a feedback control means for controlling a relative distance between the optical low-pass filter or the image sensor by feeding back a detection result obtained by the detecting means. Even in this case, since it is only necessary to perform feedback control using the position detection result, the structure and system are not complicated.

  In the imaging optical system, from the viewpoint of facilitating control for focusing an image on the imaging device, the distance adjusting unit is a unit that fixes the imaging device to an imaging device or the like and moves a low-pass filter. Is preferred.

  As the movement adjusting means for moving the optical low-pass filter or the image pickup device, known means such as a mechanism for driving the rack and pinion with a motor, an electromagnetic drive mechanism, and a piezoelectric actuator can be used.

As an example, a case where the image sensor is fixed and the optical low-pass filter is moved by an electromagnetic drive mechanism will be described.
The optical low-pass filter is fixed to the support frame, and the support plate frame is fixed to the guide rail so as to restrict movement in the direction other than the front-rear direction (direction perpendicular to the image sensor surface) with respect to the main body of the imaging device. As the support frame moves in the front-rear direction, the relative distance between the optical low-pass filter and the image sensor changes. The optical low-pass filter is fixed to the support frame so that the light incident surface and the light exit surface are parallel to the imaging element surface. The guide rail is cylindrical and made of stainless steel, and a plurality of guide rails (for example, one each on the left and right sides) are fixed along the front-rear direction. Guide holes are formed on the left and right sides of the support frame, and the guide rails are inserted through the guide holes. The portion of the support frame that forms the guide hole is thickened in the front-rear direction, and the friction between the left and right rails is made uniform, so that the low-pass filter is prevented from being tilted in the vertical and horizontal directions. Thereby, the support frame of the optical low-pass filter moves in the front-rear direction along the guide rail. A main coil is fixed to the left and right side surfaces of the support frame in the main body of the imaging apparatus. Permanent magnets are fixed as main magnets on the left and right side surfaces of the support frame, and are opposed to the main coils with a predetermined gap therebetween. The main magnet and the main coil constitute an electromagnetic drive mechanism, and a driving force having a predetermined force constant is generated by supplying electric power to the main coil. The mechanism can move the optical low-pass filter fixed to the support frame in the front-rear direction along the guide bar by controlling the direction of the current to the main coil.
In addition, a hall element for detecting the position of the optical low-pass filter is installed in the main body of the imaging apparatus, and a sensor magnet is fixed to the support frame in accordance with the position of the hall element. Accordingly, the position of the optical low-pass filter can be detected on the order of several μm from the change in magnetic flux density in the Hall element. When the moving distance is 1 mm or more, a plurality of Hall elements may be used.
The position of the optical low-pass filter can be controlled with high accuracy by providing feedback means for controlling the position of the optical low-pass filter using the signal of the Hall element in the main body of the image sensor.
The distance adjusting means having such a configuration makes it possible to control the cutoff frequency of the imaging optical system including the optical low-pass filter with high accuracy.

The distance adjusting means is 0.1 ≦ fc / when the Nyquist frequency of the image sensor is fn, and the cutoff frequency evaluated by the action of the optical low-pass filter and the distance between the low-pass filter and the image sensor light receiving unit is fc. It is preferable to control the relative distance between the optical low-pass filter and the image sensor so that fn ≦ 10.
If the relative distance between the optical low-pass filter and the image sensor can be controlled within the range of 0.1 ≦ fc / fn ≦ 10, the effect of the optical low-pass filter is increased or decreased within a sufficient range according to the photographer's intention. It becomes possible.
The closer the relative distance between the optical low-pass filter and the image sensor, the higher the value of fc / fn, and the smaller the effect of the optical low-pass filter. In order to obtain a high-definition image at the expense of the effect of preventing moire and false colors, fc / fn may be set higher. Conversely, as the relative distance increases, fc / fn decreases and the effect of the optical low-pass filter increases. Pixel thinning is performed in the moving image mode of the digital still camera and the screen resolution setting of the photographed photo. In this case, it is possible to cope with this by setting fc / fn lower than 1.
In the case of an optical low-pass filter made of crystal, the cutoff frequency can be evaluated as a characteristic unique to the optical low-pass filter. However, in the case of an optical low-pass filter that emits non-parallel separated light according to the present invention, the cutoff frequency can be reduced only by the low-pass filter. Regardless, the cut-off frequency is evaluated by the distance between the optical low-pass filter and the image pickup device attending part.

In the imaging optical system of the present invention, the center wavelength of the wavelength band to be used is λ [μm], the diffraction grating period of the diffractive optical low-pass filter is d [μm], and the pixel pitch of the imaging element is W [μm]. When fc / fn is α ≦ fc / fn ≦ β, the grating period d is preferably d ≧ λ / sin θ.
However, θ satisfies the following formula.

If the grating period d is in the range satisfying d ≧ λ / sin θ, the positioning accuracy of the distance adjusting means for changing the relative distance between the optical low-pass filter and the imaging device is required in the market if the positioning accuracy is, for example, at least ± 10 μm in repeated positioning accuracy. In view of the performance of the imaging apparatus to be performed, it is possible to control the cutoff frequency with sufficiently high accuracy.
It is more preferable that θ satisfies the following expression because the cutoff frequency can be controlled more precisely and with high accuracy.

In addition, it is preferable to use the median value of the shortest wavelength and the longest wavelength of the wavelength region of light used in the application in which the imaging optical system or the imaging apparatus using the imaging optical system is used as the center wavelength of the used wavelength band. In the case of a digital still camera or a digital video camera generally used in the market, the center wavelength of the used wavelength band is set to 587.56 nm.

The optical low-pass filter of the present invention has a refractive index change region having a refractive index difference different from that of the base material inside the glass, and functions as a transmission diffraction grating by modulating the refractive index difference at a predetermined period. It is preferable.
At this time, the glass is preferably a thin glass substrate.
A case will be described in which a plurality of refractive index changing regions are arranged at a predetermined period on a glass substrate in order to impart a light diffraction effect. The refractive index changing region is formed, for example, by irradiating a glass substrate with a pulse laser beam and making the irradiated portion a heterogeneous layer. The refractive index changing region has a wavelength of 587 with respect to a region that includes at least a part of both main surfaces of the glass substrate and is continuous at the same refractive index (that is, a portion where no heterogeneous layer is formed). The absolute value (| Δnd |) of the refractive index difference of light of .56 nm is preferably 0.001 or more. By setting | Δnd | to be 0.001 or more, a sufficient optical path length difference can be obtained with a short light path length, and the degree of freedom in designing the optical low-pass filter is increased. In order to obtain this effect more easily, | Δnd | is more preferably 0.002 or more, and most preferably 0.005 or more. The upper limit of | Δnd | is not particularly limited and is preferably as large as possible. However, the upper limit of | Δnd | actually obtained is about 0.5.
The main surface refers to a surface on which light reaching from the subject enters or exits.

The glass substrate constituting the optical low-pass filter of the present invention is made of glass mainly composed of any one of SiO ₂ , B ₂ O ₃ , P ₂ O ₅ , Bi ₂ O ₃ , and TeO _2. When irradiated, it is preferable because it is easy to change to a heterogeneous layer inside the glass.
A main component means a component with the largest content rate compared with the other component which comprises glass.

  In this specification, “heterogeneous” means that a part of the glass causes structural changes in the material, changes in the bonding state of elements, vacancy generation, composition distribution, etc. due to pulse laser irradiation. The “heterogeneous phase” means a portion having a refractive index different from the refractive index of the glass base material, which is generated by making a part of the glass heterogeneous. The refractive index change of the heterogeneous phase is caused by density change, composition change, cavity formation, crystallization, ion valence change, and the like. They are also not accompanied by cracks that cause scattering. In the optical low-pass filter according to the present invention, the heterogeneous phase preferably has a minimum width of 3 nm or more. When the width of the heterogeneous phase is smaller than 1/100 of the wavelength used by the optical low-pass filter, even if it exists inside the glass, the transmitted light is not affected, and a change in refractive index and a phase difference can be realized. Because it becomes difficult.

  If the minimum width of the heterogeneous phase, that is, the refractive index change region is smaller than the wavelength used, the polarization characteristics may be adversely affected. In order to suppress the influence on the polarization in the visible light region, it is preferably larger than the wavelength size of the visible light region, for example, larger than 830 nm. On the other hand, the upper limit of the width of the heterogeneous phase is smaller than the refractive index change region described later, and can be appropriately changed depending on the width of the refractive index change region.

  In the present specification, “visible light” means an electromagnetic wave having a wavelength in the range of 360 nm to 830 nm.

  As the shape of the refractive index changing region, a linear shape, a spherical shape, an elliptical shape, a cube shape, a polygonal column shape, a polygonal pyramid shape, a polyhedron shape, or the like can be used, or a plurality of these shapes can be used in combination. The minimum width of the refractive index change region is defined by the diameter of the cross section of the line if it is a linear structure, and it is a sphere, ellipse, cube, polygonal column, polygonal pyramid, polyhedron, etc. or an irregularly shaped structure Is defined by the equivalent sphere diameter of the same sphere.

The glass composition suitable for the glass substrate used for this invention is demonstrated.
The glass substrate is made of glass mainly composed of any one of SiO ₂ , B ₂ O ₃ , P ₂ O ₅ , Bi ₂ O ₃ , and TeO ₂ , and it is easy to generate a heterogeneous layer by pulse laser irradiation. In addition, | Δnd | can be increased.

SiO ₂ is a very useful component in the glass component of the present invention, but if only this is used, the refractive index difference is likely to change in the positive direction, and it is difficult to provide a large refractive index difference. That is, SiO ₂ is a component that forms a glass network structure in glass, and in order to break the bond, it is necessary to apply a large power during laser irradiation. However, SiO ₂ is effective in increasing the refractive index difference of the heterogeneous phase formed by laser irradiation when other components coexist. That is, it is considered that a component having a low contribution to glass network formation and having a lower binding force moves in the glass and causes a change in refractive index.

When the SiO ₂ component is included, the lower limit of SiO ₂ is preferably 20%, more preferably 30%, and most preferably 35%. If it is 20% or more, it can be vitrified stably. The glass itself is stable even if it contains other modified oxides that affect the refractive index of the glass itself and the refractive index difference of the heterogeneous phase formed by laser irradiation. The upper limit of SiO ₂ is preferably 75%, more preferably 70%, and most preferably 65%. If it is 90% or less, the glass can be melted at a relatively low temperature, and a homogeneous glass with few striae and bubbles is easily obtained, which is preferable.

B ₂ O ₃ can lower the melting temperature and obtain a stable glass. When the B ₂ O ₃ component is included, the upper limit of B ₂ O ₃ is preferably 17% or less, more preferably 10% or less, and most preferably 5% or less. If it is 17% or less, it is difficult to cause phase separation of the glass, a stable glass is easily obtained, and a relatively large amount of TiO ₂ is easily contained, which is preferable.

P ₂ O ₅ is a glass forming oxide that acts as a network forming oxide of the glass composition. The content of P ₂ O ₅ varies depending on the coexisting component and the amount thereof. However, when the P ₂ O ₅ component is contained, if the amount is too small, the stability of the glass deteriorates, so the content is 10% or more. Preferably, 15% or more is more preferable, and 20% or more is most preferable. Moreover, even if there is too much content, there exists a problem that it is hard to vitrify and glass becomes unstable, and the upper limit is preferably 90%, more preferably 80%, and 70%. Most preferred.
When the P ₂ O ₅ component is contained, the SiO ₂ component is preferably 10% or less.

Bi ₂ O ₃ is a glass-forming oxide that acts as a network-forming oxide of the glass composition. Moreover, since it can contribute to a higher refractive index, it may be added as a subcomponent for the purpose of increasing the refractive index. When the Bi ₂ O ₃ component is contained, in order to obtain glass stably, the content is preferably 0 to 40%, more preferably 0 to 20%, and most preferably 0% or more and less than 10%.

TeO ₂ is a glass-forming oxide that acts as a network-forming oxide of the glass composition. Moreover, since it can contribute to a higher refractive index, it may be added as a subcomponent for the purpose of increasing the refractive index. When the TeO ₂ component is contained, in order to obtain glass stably, the content is preferably 0% or more and less than 50%, more preferably 0% or more and less than 30%, and most preferably 0% or more and less than 20%.

Alkali metal oxides such as Li ₂ O, Na ₂ O, K ₂ O have the effect of lowering the melting temperature and stabilizing the glass. If the content of these alkali metal oxides is too low, the melting temperature increases, and if the content is too high, the chemical durability decreases, and in any case, the stability of the glass decreases.
The content of Li ₂ O is preferably 0 to 15%, more preferably 0 to 12%, and most preferably 0 to 10%. The content of Na ₂ O is preferably 0 to 30%, more preferably 0 to 25%, and most preferably 0 to 20%. The content of K ₂ O is preferably 0 to 30%, more preferably 0 to 25%, and most preferably 0 to 20%.
The alkali metal oxide is not essential, but it is preferably contained at 1% or more, more preferably 3% or more. The upper limit of the content is preferably 30%, more preferably 25%, and most preferably 20%.

Bivalent metal oxides such as MgO, SrO, CaO, BaO, and ZnO have a function of cutting bonds in the glass network structure, increase the solubility during glass production, and make the glass more stable. Although the contribution of these own glass changes to the heterogeneous layer is considered to be small, it is thought to work to enhance the effect of other components that are largely affected by the change to the heterogeneous layer.
The content of MgO is preferably 0 to 10%, more preferably 0 to 7%, and most preferably 0 to 5%.
The content of SrO is preferably 0 to 10%, more preferably 0 to 7%, and most preferably 0 to 5%.
The content of CaO is preferably 0 to 30%, more preferably 0 to 25%, and most preferably 0 to 20%.
The content of BaO is preferably 0 to 30%, more preferably 0 to 25%, and most preferably 0 to 20%.
The content of ZnO is preferably 0 to 40%, more preferably 0 to 30%, and most preferably 0 to 20%.
The divalent metal oxide is not essential, but is preferably contained in an amount of 1% or more, more preferably 3% or more. The upper limit of the content is preferably 40%, more preferably 30%, and most preferably 20%.

TiO _{2 is} also an optional component useful in the glass component of the present invention, and is highly effective as a component that increases the difference in refractive index, although it depends on the combination with other modified oxides and the mixing ratio. In particular, it is effective for increasing the negative refractive index difference Δn.
The upper limit of TiO ₂ is preferably 50%, more preferably 40%, and most preferably 30%. If it is 50% or less, the effect of devitrification of the glass containing SiO ₂ is reduced, and the glass having high transparency in the visible light region is suppressed by suppressing coloring due to the valence change of Ti ions. Since it is easy to do, it is preferable. The lower limit is 0%.

Al ₂ O ₃ can obtain a stable glass by coexisting with SiO ₂ . The upper limit of Al ₂ O ₃ is preferably 4%. Nb ₂ O ₅ can increase the refractive index of the glass itself. The upper limit of Nb ₂ O ₅ is preferably 20%. Further, ZrO ₂ can improve the fire resistance by adding it during glass production, and has a function of suppressing devitrification. The upper limit of ZrO ₂ is preferably 6%. Sb ₂ O ₃ is not an essential component, but can be used as a defoaming agent during glass production. The upper limit of Sb ₂ O ₃ is preferably 0.4%. Although fluorine is not an essential component, it has the effect of reducing the refractive index and dispersion of the glass itself, and the effect of reducing the difference in refractive index caused by laser irradiation. Therefore, it is preferable to contain 5% or less for the purpose of adjusting the refractive index and dispersion of the glass itself. In addition, at least one component consisting of Y ₂ O ₃ and SnO may be contained. These upper limits are preferably 15%.

Note that oxides containing at least one component consisting of the group of WO ₃ , Ta ₂ O ₅ , La ₂ O ₃ , Gd ₂ O ₃ or elements contained in the oxides are reported as having an environmental impact or biotoxicity. This is not preferable.

  In addition, each transition metal component such as V, Cr, Mn, Fe, Co, Ni, Cu, Ag, and Mo is colored in the visible region even when a small amount of each is contained alone or in combination. In order to cause absorption at a specific wavelength, it is preferable that the optical low-pass filter of the present invention is not substantially included in the visible wavelength region. In addition, since each rare earth component may be colored alone or in combination, it tends to cause absorption at a specific wavelength in the visible range. When using a filter, it is preferable not to contain substantially.

  Furthermore, components such as Be, Pb, Th, Cd, Tl, As, Os, S, Se, Br, Cl, and I tend to refrain from being used as harmful chemicals in recent years. However, since measures for environmental measures are required until the processing steps and disposal after commercialization, it is preferable that they are not substantially included when importance is placed on the environmental impact.

  The glass substrate used in the present invention is obtained by processing and polishing a glass surface flatly. In order to give light diffraction to the glass substrate, it is necessary to prevent the laser light from being irregularly reflected on the surface because the pulse laser is irradiated from the surface of the glass substrate to the inside. It is preferable to be as flat as possible. With the recent increase in the number of pixels in the image sensor, the pixel pitch is narrowed. Specifically, from the viewpoint of reducing the deviation of the incident position of the diffracted light, the flatness is preferably 10 μm or less, and preferably 2 μm or less. It is more preferable that The surface roughness is preferably 150 mm or less in terms of the point average roughness Rz. The flatness is a distance from the highest peak portion to the lowest valley portion in a region where light enters as an optical low-pass filter.

  The optical low-pass filter used in the present invention is preferably coated with a light reflection preventing film on at least one main surface of the glass substrate in order to increase light transmittance.

  The refractive index changing region according to the present invention can be formed by irradiating a part of the glass substrate with a pulsed laser. Therefore, the refractive index change region can control the diffraction action of light by any one of the volume, the shape, the number density, the refractive index change amount, or the combination of the refractive index change regions arranged.

Further, in forming the refractive index changing region, the glass base material is obtained by performing the pulse laser irradiation repeatedly at the same place, adjusting the pulse laser irradiation output, and / or adjusting the scan speed of the pulse laser. The refractive index difference with respect to can be controlled.

In the present invention, a pulse laser having a pulse width of picosecond (10 ⁻¹² s) or less can be suitably used. A femtosecond pulse laser having a center wavelength of about 800 nm, a [pulse width of 100 to 400 fs (10 ⁻¹⁵ s)], a repetition frequency of 10 Hz to 100 MHz, and a pulse energy of about 0.1 to 4.0 μJ is preferable. Such a pulse laser beam is passed through an objective lens having a magnification of 20 times (aperture ratio of 0.45), for example, under conditions of a scanning depth of 10 to 5000 μm and a scanning speed of about 0.005 to 100 mm / second. By condensing and irradiating the inside of the glass substrate, a heterogeneous phase having a different refractive index difference can be generated due to a difference in molecular structure and composition distribution from the mother glass.

  The laser irradiation conditions can be changed depending on the desired heterogeneous phase shape, refractive index change amount, and the like. For example, in the case of processing a spot-like heterogeneous phase formed at the focal position by combining a laser beam with a lens and using a stage scanning or the like, the pulse energy is 10 nJ or more, preferably 1 μJ or more. There is no particular upper limit if pulse energy that can cause movement is ensured, but if the upper limit is 100 MHz or less, the repetition frequency can suppress the occurrence of thermal distortion and undesirable cracks due to heat accumulation during processing, and the speed of the stage. This is preferable because it is easy to control and processing with higher accuracy is possible. The lower limit of the repetition frequency is preferably 10 Hz or more, and most preferably 1 kHz or more in consideration of the influence of machining between pulses and the machining time.

  On the other hand, the pulse width must be short enough to generate a pressure wave (shock wave) by laser irradiation, preferably 1 ps or less, more preferably 500 fs or less, and most preferably 300 fs or less. However, if the pulse width is too short, there is a problem that the pulse width is easily widened in the process of transmitting or reflecting the dispersion material of the lens and various optical components, so that the handling becomes difficult. Preferably, 100 fs or more is most preferable.

  The wavelength of the laser to be irradiated is preferably a transparent wavelength region that does not absorb the glass substrate. Although depending on the transmittance of the glass to be irradiated and the presence or absence of absorption at a specific wavelength, it is preferably about 200 nm to 2100 nm, more preferably 400 to 1100 nm, and most preferably 500 to 900 nm. Within this range, it is preferable because multiphoton absorption is caused only at a portion with high light intensity near the condensing position and precise processing can be performed.

Such a pulse laser can form a heterogeneous phase having a refractive index different from the refractive index of the glass base material locally in the glass substrate by condensing and irradiating laser light. Further, since the pulse width in this embodiment is short, there is little thermal influence on the transparent material. For this reason, generation | occurrence | production of the thermal distortion and crack by the accumulation | storage of the heat | fever at the time of a process can also be suppressed, and the heterogeneous phase of a smooth shape can also be formed.

Further, in the pulse laser irradiation according to the present embodiment, it is preferable to have a step of dividing the pulse laser beam to be irradiated into a plurality of parts because of high manufacturing efficiency. The step of dividing the pulsed laser beam into multiple parts is, for example, by placing a hologram for forming a heterogeneous phase of a predetermined shape between the pulsed laser irradiation device and the glass substrate, and irradiating the glass substrate with the pulsed laser beam It is a process to do. In the embodiment in which the pulse laser beam is divided into a plurality of parts using a hologram, it is possible to form a heterogeneous phase pattern at a time, so the processing time is shortened and a highly accurate heterogeneous phase pattern is formed. be able to.

  By using the imaging optical system of the present invention as an imaging optical system for a known digital still camera, digital video camera, etc., the cut-off frequency of the optical low-pass filter can be variably controlled with a simple structure and control system with high accuracy. A possible imaging device can be obtained.

Example 1
Setting example in which the control range of the cut-off frequency fc / fn at the wavelength λ = 587.56 nm is set to 1.0 ≦ fc / fn ≦ 1.5 for an image pickup device having a pixel pitch W of 6 μm and is changed in six steps. Indicates. The relative distance between the low-pass filter and the light receiving unit of the image sensor is determined by fixing the image sensor and moving the low-pass filter.
A heterogeneous layer was formed by irradiating a P ₂ O ₅ —Nb ₂ O ₅ —TiO ₂ glass substrate with a pulse laser, and a diffraction low-pass filter having a light separation pattern as shown in FIG. 1 was used. The grating period of the diffractive low-pass filter was set to 430 μm. ± 1 at a relative distance L (μm) between the low-pass filter and the light-receiving part of the image sensor when the effect of the low-pass filter is uniformly controlled in six steps within the above fc / fn range, wavelength λ = 587.56 nm Table 1 shows the calculated value of the separation width w (μm) on the light receiving portion of the imaging device for the next light beam.

  From the calculated values in Table 1, the relative distance L for each stage was set with the moving pitch of the low-pass filter as 100 μm units. Table 2 shows the separation width w (μm) of the ± primary rays on the light receiving portion of the imaging element and the value of the cutoff frequency fc / fn at the wavelength λ = 587.56 nm. Further, FIG. 2 shows the MTF characteristics for the settings in Table 2.

Assuming that the repetitive positioning accuracy of the moving means of the low-pass filter is ± 10 μm, the relative distance variation in the step 6 is in the range of 1490 μm to 1510 μm with the relative distance in the step 5 as a reference, and the corresponding variation in the separation width. Is in the range of 4.07 μm to 4.13 μm. The variation in the value of fc / fn at this time is in the range of 1.45 to 1.46, and this difference is extremely small, about 1/90 of the difference in fc / fn at each stage.
As described above, the imaging optical system of the present invention can control the effect of the low-pass filter with high accuracy even if the positioning accuracy of the distance adjusting means is not high.

(Example 2)
Setting example in which the control range of the cut-off frequency fc / fn at the wavelength λ = 587.56 nm is set to 0.5 ≦ fc / fn ≦ 4.0 and is changed in 8 steps with respect to an image sensor having a pixel pitch W of 3 μm. Indicates. The relative distance between the low-pass filter and the light receiving unit of the image sensor is determined by fixing the image sensor and moving the low-pass filter.
A heterogeneous layer was formed by irradiating a P ₂ O ₅ —Nb ₂ O ₅ —TiO ₂ glass substrate with a pulse laser, and a diffraction low-pass filter having a light separation pattern as shown in FIG. 1 was used. The grating period of the diffractive low-pass filter was set to 500 μm. ± 1 at a relative distance L (μm) between the low-pass filter and the light-receiving portion of the image sensor when the effect of the low-pass filter is uniformly controlled in eight steps within the above fc / fn range, wavelength λ = 587.56 nm Table 3 shows the calculated value of the separation width w (μm) of the next light beam on the light receiving portion of the image sensor.

  From the calculated values in Table 3, the relative distance L for each stage was set with the moving pitch of the low-pass filter as 20 μm units. Table 4 shows the separation width w (μm) of the ± primary rays on the light receiving portion of the imaging element and the value of the cutoff frequency fc / fn at the wavelength λ = 587.56 nm. Further, FIG. 3 shows the MTF characteristics for the settings in Table 4.

Assuming that the repeat positioning accuracy of the moving means of the low-pass filter is ± 2 μm, the relative distance variation in the step 6 is in the range of 418 μm to 422 μm with the relative distance in the step 5 as a reference, and the corresponding variation in the separation width. Is in the range of 0.98 μm to 0.99 μm. The variation in the value of fc / fn at this time is in the range of 3.03 to 3.05, and this difference is extremely small, about 1/30 of the difference in fc / fn at each stage.
As described above, the imaging optical system of the present invention can control the effect of the low-pass filter with high accuracy even if the positioning accuracy of the distance adjusting means is not high.

Claims (6)

  An imaging optical system comprising: an optical low-pass filter that separates incident light and emits the light in parallel; an imaging element; and a distance adjusting unit that varies a relative distance between the optical low-pass filter and the imaging element.   The imaging optical system according to claim 1, wherein the optical low-pass filter is a diffractive optical low-pass filter.   The distance adjusting means is 0.1 ≦ fc / when the Nyquist frequency of the image sensor is fn, and the cutoff frequency evaluated by the action of the optical low-pass filter and the distance between the low-pass filter and the image sensor light receiving unit is fc. The imaging optical system according to claim 1, wherein a relative distance between the optical low-pass filter and the imaging element is controlled so that fn ≦ 10. In the imaging optical system, the center wavelength of the used wavelength band is λ [μm], the diffraction grating period of the diffractive optical low-pass filter is d [μm], and the pixel pitch of the imaging element is W [μm]. When fc / fn is α ≦ fc / fn ≦ β, the grating period d is
The imaging optical system according to claim 1, wherein d ≧ λ / sin θ.
However, θ satisfies the following formula.
  The optical low-pass filter has a refractive index change region having a refractive index difference different from that of a base material inside the glass, and functions as a transmissive diffraction grating when the refractive index difference is modulated at a predetermined period. The imaging optical system according to any one of 1 to 4.   An imaging apparatus having the imaging optical system according to claim 1.