JP2010008545A - Laser projection device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a laser projection device for reducing speckles of a projection image on an optional screen without significantly affecting the resolution of an image. <P>SOLUTION: The laser projection device includes: a laser beam source unit including laser elements LB, LR and LG; a scanning device MS for two-dimensionally scanning with the laser beam from the laser beam source unit; and an imaging optical system L4 or the like. An optical element LS including a large transparent medium which is divided into a plurality of areas having different optical path lengths and has an optical path length difference between the areas larger than a coherence distance of a laser beam is provided between the laser beam source unit and the scanning device MS. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention is applicable to, for example, an image projection apparatus such as a pocket projector, a data projector, a rear projection television, and the like, and relates to a laser projection apparatus that projects an image on a screen using a laser beam. The present invention relates to a technique for reducing speckles in a projected image generated by light.

  In Patent Document 1, in a rear projection type laser TV, speckles and scintillation are reduced by rotating the screen in parallel with the screen. In Patent Document 2, the optical system, not the screen, is driven perpendicularly to the optical axis for reduction. In Patent Document 3, a birefringent plate is disposed between a scanning unit of a scanning laser projector and a screen, and the optical axis and the polarization direction of the laser are inclined.

JP 2006-343663 A JP 2003-21806 A JP 2006-284749 A JP-T-2004-529375 JP 2004-45684 A

  Although Patent Document 1 is effective to some extent in reducing speckles, it is necessary to move a large screen, which inevitably increases the drive configuration. Furthermore, since usable screens are limited, for example, it is not suitable for portable projectors targeting unspecified screens.

  In Patent Document 2, since the projection lens is minutely vibrated in the direction perpendicular to the optical axis, the pixel size is substantially increased in the vibration direction, resulting in resolution degradation.

  Patent Document 3 aims to reduce speckle by superimposing polarized light, but since polarization has only two directions orthogonal to each other, the reduction effect obtained by superposition is only 50%.

  Patent Document 4 aims to reduce speckles by arranging a wavefront modulation element that generates phase modulation in an optical path. However, this is a reduction effect due to the superposition of speckles in different phase states, and the speckle reduction is performed by scanning the row image. Therefore, in the case of point image scanning, due to the influence of diffraction by the wavefront modulation element, the scanning means The amount of light incident on the screen becomes small, and as a result, the image on the screen becomes dark.

  In Patent Document 5, in order to reduce the coherency of laser light, the one-dimensional spatial modulation element is illuminated with a plurality of light sources having optical path lengths different from each other by a coherent distance or more. However, although the optical path length is greatly changed one-dimensionally, it is not a two-dimensional speckle reduction. Therefore, when applied to a two-dimensional scanning projector such as the present invention, the resolving power in the one-dimensional direction is large. descend.

  An object of the present invention is to provide a laser projection apparatus that can reduce speckle of a projected image on an arbitrary screen without greatly affecting the resolution of the image.

In order to achieve the above object, a laser projection apparatus according to the present invention comprises:
A laser light source;
A scanning device for two-dimensionally scanning laser light from a laser light source,
An optical element provided between a laser light source and a scanning device, divided into a plurality of regions having different optical path lengths, and including a transparent medium in which the optical path length difference between the regions is larger than the coherence distance of the laser light It is characterized by.

In the present invention, the optical path length difference T between the regions is defined as λ as the wavelength of the laser beam and Δλ as the spread width of the wavelength.
T> λ ² / Δλ
Is preferably satisfied.

  In the present invention, each divided region of the transparent medium is preferably arranged in a concentric ring shape with the optical axis as the center.

In the present invention, the object distance incident on the transparent medium is L, the outer diameter of the transparent medium is D,
| L / D |> 50
Is preferably satisfied.

In the present invention, the laser light source includes a blue laser element, a red laser element, and a green laser element,
A color synthesizing element for synthesizing laser light from each laser element;
The optical element is preferably disposed behind the color synthesizing element.

  In the present invention, each divided region of the transparent medium preferably has a phase modulation structure in which the optical path length is changed in the wavelength order of the laser beam.

  In the present invention, it is preferable that a birefringent material or a birefringent structure is provided in different thicknesses and / or different directions on the light incident surface and / or the light emitting surface of each divided region of the transparent medium.

  In the present invention, the laser light source preferably includes a semiconductor laser element.

  In the present invention, it is preferable to further include an imaging optical system for condensing the scanned laser light on the fixed screen.

  According to the present invention, speckles of a projected image on an arbitrary screen can be reduced without greatly affecting the resolution of the image.

  The laser projection device according to the present invention is provided between a laser light source, a scanning device for two-dimensionally scanning laser light from the laser light source, and the laser light source and the scanning device, and a plurality of optical path lengths different from each other. And an optical element including a transparent medium in which the optical path length difference between the regions is larger than the coherence distance of the laser light.

  In recent years, a projector using a laser light source has been proposed, and since a color reproduction range can be increased as compared with a projector using a conventional discharge lamp light source, it is attracting attention as a next-generation high-quality video display device. However, when a coherent light source such as a laser is used, the projected image appears to float due to the generation of speckles, so the video quality is degraded. However, since speckle generation is caused by using the laser itself, it is theoretically difficult to completely remove it. Therefore, there is a demand for a method of reducing speckles to an inconspicuous level rather than removing speckles.

  There are two main methods for reducing speckle. The first method is to reduce the generated speckle by temporally changing and overlapping each other. As a method for reducing speckle while maintaining the uniformity of polarization and wavelength, temporal integration is performed so that it cannot be perceived by human eyes. Patent document 1 also uses the principle. However, in the method of Patent Document 1, it is necessary to vibrate and rotate the screen and the optical element in order to generate a change with time. As a result, a drive device is necessary, and there are problems such as power of the drive device, noise caused by the drive, and reliability of the drive device, resulting in high costs and side effects.

  The second method is to reduce the apparent coherence of the light source. Specifically, the light source side is devised such as superimposing a plurality of polarization states, a superposition of a plurality of wavelengths, a superposition of a plurality of phase states, and further dividing the light source into a plurality of superpositions. How to add.

  Since the polarization state can be decomposed into two directions orthogonal to each other, the polarization state can only be superimposed, so that the reduction effect is only 50% at the maximum.

  The superposition of a plurality of wavelengths needs to change the state of the light source itself, and is synonymous with the use of a low-coherence light source itself, so it cannot be used in a method using a general laser light source.

  The method of superimposing a plurality of phase states as in Patent Document 4 aims at speckle reduction by arranging a wavefront modulation element that generates phase modulation in the optical path, but reduction by superimposing speckles of different phase states. Since speckle reduction is performed by scanning the row image, it is preferable in the case of point image scanning because the amount of light incident on the scanning means becomes small due to the influence of diffraction by the wavefront modulation element, resulting in darkness. Absent.

  A method generally used as a method of overlapping a plurality of light sources includes a reduction method in which a large number of minute light sources are created in a pseudo manner using a diffusion plate, and the respective light sources do not interfere with each other. However, if the laser light passes through the diffusion plate, it diffuses, and the original straightness of the laser light is lost, so that the light amount of the entire system is lost.

  Therefore, the present invention divides the transparent medium into a plurality of regions, and makes the light path length (= thickness × refractive index) of each divided region equal to or greater than the coherence distance, so that the emitted light from each divided region is independent. We propose a reduction method by treating it as a light source.

  In recent years, there is a laser scanning projector as an ultra-compact projector attracting attention. As a specific configuration, there is a laser projection apparatus that has a laser light source and scanning means for two-dimensionally scanning the laser light, and projects the image by scanning the modulated laser light with the scanning means. Compared with a method of illuminating a two-dimensional spatial modulation element such as a liquid crystal panel or DMD (Digital Micromirror Device) and projecting a modulated image with a projection lens, a projection lens is not required, so the size can be reduced. By direct modulation of the laser light source, for example, when the screen is black, the laser does not emit light and low power consumption is achieved.

  In the laser scanning projector, the light flux from the light source to the scanning means is very small, and has a certain divergence angle behind the scanning means. Therefore, an optical element having an effect of reducing speckles between the light source and the scanning means, specifically, an optical element made of a transparent medium having different optical path lengths by dividing the laser beam into a plurality of regions may be disposed. preferable.

  Specifically, the different optical path lengths need to have optical path lengths different from each other by a coherent distance of the laser light. Thereby, since the coherence between different divided areas is lost, it is possible to regard the emitted light from each divided area as an independent light source. The presence of light sources that differ by the number of divided regions causes speckles to be superimposed, resulting in a reduction to an inconspicuous level.

Specifically, the optical path length difference T between the divided regions preferably satisfies T> λ ² / Δλ, where λ is the wavelength of the laser light and Δλ is the wavelength spread. This expression is a conditional expression for defining that the light emitted from each divided region does not interfere.

  However, the coherent distance is a distance at which the influence of the interference becomes a certain amount or less, and it is preferable that the optical path length difference is set to be more than the extent that the interference is sufficiently reduced from the degree of the influence of the actual speckle.

  Regarding the dimensions of the transparent medium, it is preferable that | L / D |> 50 is satisfied, where L is the object distance incident on the transparent medium and D is the outer diameter of the transparent medium. This expression defines the diffraction angle (Sinθ≈θ (= D / L)) of the first-order diffracted light generated by the divided regions of the transparent medium.

  Further, the number of the divided areas of the transparent medium is preferably 3 or more. In the case of the method of eliminating the coherence using the polarization element which is the prior art, since the polarized light is decomposed into two orthogonal components, the speckle reduction effect is at most 50%. On the other hand, the advantage of the method of the present invention is that the laser beam can be divided by the number of divided regions, and the speckle reduction effect is improved accordingly. In order to obtain an effect over the conventional polarization method, it is desirable that at least three divided regions exist. More desirably, when the number is four or more, the speckle amount is reduced to 25% (half of the polarization method), and thus the effect becomes more remarkable.

  Each divided area of the transparent medium has a configuration in which cylinders with different lengths are arranged in a concentric ring shape with the optical axis as the center (first embodiment), or rectangular columns with different lengths in a plane perpendicular to the optical axis. The form (2nd Embodiment) etc. which are arrange | positioned by the matrix form can be considered.

  The method of the present invention uses each divided region as an individual light source in order to reduce the coherence of the laser light. When a plurality of square pillars are arranged in a matrix, the individual light source has a size corresponding to the end face of the square pillar. This means that the effective diameter of the original optical system is reduced by dividing. As a result, the light from any of the divided regions becomes as large on the projection plane as the light, so that the light spot on the screen becomes large and the resolution may be lowered. Therefore, consideration must be given to the design of the laser scanning optical system.

  On the other hand, when a plurality of cylinders are arranged in a concentric ring shape, the individual light source has a size corresponding to the end face of the cylinder. As described above, the innermost cylinder has a smaller effective diameter and a larger light spot on the screen. However, the outer diameter cylinder has a larger effective diameter and therefore a smaller light spot on the screen. Therefore, when individual light sources composed of a plurality of cylinders are combined, as shown in the graph of FIG. 3, the light spot becomes an image in which the core portion at the center remains clearly and the skirt spreads. This means that there is almost no resolution degradation and speckle reduction is possible. When the bottom of the light spot is widened, the contrast at a low spatial frequency is lowered, but since the reduction in resolution can be suppressed by the central core portion, this is a method suitable for a two-dimensional scanning projector.

  It is preferable that substantially parallel light rays are incident on the transparent medium. If a light beam in a condensed state or a divergent state is incident on a transparent medium, vignetting occurs and the efficiency of the optical system is reduced. In order to make the amount of vignetting not practically problematic, the object distance incident on the transparent medium is L, the outer diameter of the transparent medium (diameter in the case of a cylinder, the length of one side in the case of a prism) is D, It is preferable that | L / D |> 50 is satisfied. This formula defines the maximum incident angle at the periphery and is about 1 degree (exactly 1.145 degrees).

  The beam from each laser is collimated. Accordingly, since the object distance after collimated transmission is ∞, the object distance L incident on the element is L = ∞. Therefore, the value of the conditional expression, | L / D |, is infinite and satisfies the conditional expression. Since it is collimated, it passes through the element to form an image on a finite screen, and then (in the embodiment) a lens with a focal length of 400 mm is used to form an image on a screen 400 mm ahead. The collimating lens may not be completely collimated, but may be slightly convergent and L may be finite rather than infinite. For example, the imaging lens can be omitted by setting the object distance L = 400 mm. For that purpose, it is important to satisfy the conditional expression. In the case of the first embodiment, in order to satisfy the conditional expression, since D = 1.49, L is preferably 75 or more, and setting L = 400 is preferable from the viewpoint of efficiency.

  Further, the light source of the scanning projector is usually a multicolor (for example, blue, red, green) light source in many cases. There are two speckle-reducing optical element arrangements: a method for arranging each light source and a method for arranging the optical elements after passing through the color synthesizing elements. It is preferable to arrange it behind.

  The speckle reduction method according to the present invention is to divide into regions having optical path lengths different from each other by a coherence distance of light, and can be combined with other speckle reduction methods. Specifically, it can be used in combination with a speckle reduction method using polarized light (for example, Patent Document 3) and a speckle reduction method using phase superposition (for example, Patent Document 4). It is possible to increase the speckle reduction effect by increasing the number of speckle states to be superimposed.

  As another speckle reduction method, each divided region of the transparent medium preferably has a phase modulation structure in which the optical path length is changed in the wavelength order of the laser beam. Due to the presence of the phase modulation structure in the region, it is possible to superimpose different phase states, and speckle can be further reduced.

  As another speckle reduction method, a birefringent material or a birefringent structure is attached in different thicknesses and / or different directions on the light incident surface and / or the light emitting surface of each divided region of the transparent medium. Preferably it is. Also in this case, by partially changing the polarization state of the region, it is possible to superimpose different phase states, and speckle can be further reduced.

  A semiconductor laser element is preferably used as the light source. The semiconductor laser element is known as a small and efficient light source, and is more preferable as a light source for a small projector. In addition, although the quality of laser light is very good, gas lasers and the like have a long resonator length and a very large configuration, and a coherence distance becomes too long (for example, 1 m or more, etc.) so that they do not interfere with each other. For this reason, the difference in thickness of the optical element must be very large, which is not realistic.

Therefore, the optical path length difference T between the regions preferably satisfies T> λ ² / Δλ, where λ is the wavelength of the laser beam and Δλ is the wavelength spread. Specifically, the coherence distance of the laser light is preferably 5 mm or less. For example, when λ = 532 nm, T = 5 mm and Δλ is preferably 0.05 nm or more. Many semiconductor laser elements have a wavelength width of 0.05 nm or more, and thus are preferable light sources for the speckle reduction method according to the present invention. Specifically, many semiconductor laser elements having Δλ = 0.5 nm and Δλ = 2 nm are commercially available, and the use of such semiconductor devices enables downsizing of the optical element.

  Next, a specific configuration of the present invention will be described.

(First embodiment)
FIG. 1 is a configuration diagram showing a first embodiment of the present invention. The laser projection device includes a laser light source unit and a projection optical system for projecting laser light from the laser light source unit onto a fixed screen SC.

  The laser light source unit includes a laser element LB that generates B (blue) light, a laser element LR that generates R (red) light, and a laser element LG that generates G (green) light. The laser element LB is composed of, for example, a semiconductor laser that generates light having a wavelength of 445 nm. The laser element LR is constituted by, for example, a semiconductor laser that generates light having a wavelength of 630 nm. The laser element LG is composed of, for example, a semiconductor-pumped solid-state laser that generates light having a wavelength of 532 nm using second harmonic generation by a PPLN waveguide.

  The laser elements LR and LB may directly modulate the current injected into the laser chip, or an optical modulator such as an AO (acousto-optic) element may be provided separately. The laser element LG may directly modulate the current injected into the excitation laser chip, or an optical modulator such as an AO element may be separately provided.

  The projection optical system includes an incident optical system, a scanning device MS, and an imaging optical system.

  The incident optical system has a function of transmitting the laser light from the laser light source unit to the scanning device MS. For example, in FIG. 1, the incident optical system has lenses L1, L2, and L3 for condensing laser beams from the laser elements LB, LR, and LG, and coaxially the laser beams from the lenses L1, L2, and L3. Are composed of color combining mirrors M1, M2, M3 and the like.

  The scanning device MS has a function of scanning a laser beam two-dimensionally. The scanning device MS may have a configuration in which a main scanning mirror and a sub scanning mirror are separately provided, or may have a configuration in which a main scanning drive mechanism and a sub scanning drive mechanism are provided in a single mirror. For example, the scanning device MS may be configured by a MEMS mirror mechanism including a scanning mirror that vibrates in a horizontal direction and a vertical direction by a piezoelectric element.

  The imaging optical system has a function of condensing the scanned laser light on a fixed screen. For example, in FIG. 1, as an imaging optical system, a lens L4 (for example, f = 400 mm) is disposed on the exit side of the optical element LS.

  Thus, in accordance with the image signal supplied from the outside, the laser light from each of the laser elements LB, LR, LG is intensity-modulated, and the scanning device MS performs vertical scanning at a predetermined frame frequency, and at a predetermined horizontal scanning frequency. By performing horizontal scanning, a two-dimensional image can be projected onto the screen SC.

  When the screen SC is a transmissive type, it can be configured as a rear projection display. On the other hand, when the screen SC is a reflection type, it can be configured as a front projection type display.

  In the present embodiment, an optical element LS for reducing speckles is provided between the laser light source unit and the scanning device MS. As described above, the optical element LS includes a transparent medium that is divided into a plurality of regions having different optical path lengths (= thickness × refractive index) and in which the optical path length difference between the regions is larger than the coherence distance of the laser light.

  FIG. 2 shows an example of the optical element LS, FIG. 2 (a) is a plan view, and FIG. 2 (b) is a side view. Each divided area of the transparent medium is arranged in a concentric ring shape around the optical axis. The transparent medium is formed of, for example, a glass material such as BK-7 or a resin material such as acrylic resin, and as a manufacturing method, individual cylinders may be bonded, or a plastic injection mold or glass mold using a mold. It is also possible to integrally mold by the method.

  As specific dimension examples, the width of each ring zone is 3 mm, and the diameter is 0.6 mm, 0.849 mm, 1.039 mm, 1.200 mm, 1.342 mm, 1.470 mm as shown in the following (Table 1). The shape is such that a total of 6 cylindrical bodies are joined.

  Therefore, each divided area of the transparent medium is 1) a cylindrical portion having a diameter of 0.6 mm and a thickness of 18 mm, 2) a cylindrical portion having a diameter of 0.6 mm to 0.849 mm and a thickness of 15 mm, and 3) a diameter of 0.849 mm to 1.039 mm. A cylindrical portion having a thickness of 12 mm, 4) a cylindrical portion having a diameter of 1.039 mm to 1.200 mm, and a thickness of 9 mm, 5) a cylindrical portion having a diameter of 1.200 to 1.342 mm, and a thickness of 6 mm, 6) a diameter of 1.342 mm to 1. A cylindrical portion having a thickness of 470 mm and a thickness of 3 mm is obtained.

  Here, if the diameter of each divided region is changed at equal intervals, the area of the light emitting end surface increases as the outer divided region becomes larger, so the diameter of each divided region is set so that the areas of the light emitting end surfaces are equal to each other. It is preferable to do.

For example, when the wavelength λ of the laser light source is 532 nm and the wavelength width Δλ is 0.2 nm, λ ² /Δλ=1.4 mm is calculated. On the other hand, assuming that the refractive index of the transparent medium is 1.5 and the refractive index of air is 1, the optical path length difference T between the divided regions is calculated as 1.5 × 3 mm−1 × 3 mm = 1.5 mm. Therefore, the optical element LS in FIG. 2 satisfies the above-described formula T> λ ² / Δλ, and this reduces the coherence of the laser light, thereby reducing speckle.

  Here, the case where the laser beam is incident on the left side of the optical element LS and is emitted from the right side is illustrated, but the laser beam is configured to be incident on the right side of the optical element LS and emitted from the left side due to the reversibility of the light. May be.

(Second Embodiment)
FIG. 4 shows another example of the optical element LS. FIG. 4 (a) is a plan view and FIG. 4 (b) is a side view. The overall configuration of the laser projection apparatus according to the present embodiment is the same as that shown in FIG. 1, but as the optical element LS, rectangular columns having different lengths are arranged in a matrix in a plane perpendicular to the optical axis. is doing. The transparent medium is formed of, for example, a glass material such as BK-7 or a resin material such as acrylic resin, and as a manufacturing method, individual square pillars may be bonded, or a plastic injection mold or glass using a mold. It is also possible to integrally form by a molding method.

  As a specific dimension example, it is a shape having a square cross section of 0.4 mm × 0.4 mm, and rectangular columns having lengths of 3 mm, 6 mm, 9 mm, and 12 mm are randomly joined in a 4 × 4 matrix arrangement. Therefore, each divided area of the transparent medium is 1) a quadrangular column of 0.4 mm × 0.4 mm × 3 mm, 2) a quadrangular column of 0.4 mm × 0.4 mm × 6 mm, 3) 0.4 mm × 0.4 mm × 9 mm square column, 4) 0.4 mm × 0.4 mm × 12 mm square column.

In this case, for example, when the wavelength λ of the laser light source is 532 nm and the wavelength width Δλ is 0.2 nm, λ ² /Δλ=1.4 mm is calculated. On the other hand, assuming that the refractive index of the transparent medium is 1.5 and the refractive index of air is 1, the optical path length difference T between the divided regions is calculated as 1.5 × 3 mm−1 × 3 mm = 1.5 mm. Therefore, the optical element LS of FIG. 4 satisfies the above-described formula T> λ ² / Δλ, and this reduces the coherence of the laser light, thereby reducing speckle.

  Here, the case where the laser beam is incident on the left side of the optical element LS and is emitted from the right side is illustrated, but the laser beam is configured to be incident on the right side of the optical element LS and emitted from the left side due to the reversibility of the light. May be.

(Third embodiment)
FIG. 5 shows still another example of the optical element LS. FIG. 5A is a cross-sectional view, and FIG. 5B is a side view. The overall configuration of the laser projection apparatus according to this embodiment is the same as that shown in FIG. 1, but the optical element LS has a shape in which the transparent medium and air are reversed in the shape shown in FIG. 2. The transparent medium is formed of, for example, a glass material such as BK-7 or a resin material such as acrylic resin, and as a manufacturing method, individual square pillars may be bonded, or a plastic injection mold or glass using a mold. It is also possible to integrally form by a molding method.

  Each divided region of the transparent medium has 1) a cylindrical portion having a diameter of 0.6 mm to 0.849 mm and a thickness of 5 mm, 2) a cylindrical portion having a diameter of 0.849 mm to 1.039 mm and a thickness of 10 mm, and 3) a diameter of 1.039 mm to 1 Cylindrical portion of 200 mm and thickness of 15 mm, 4) Cylindrical portion of diameter of 1.200 to 1.342 mm, thickness of 20 mm, 5) Cylindrical portion of diameter of 1.342 mm to 1.470 mm, thickness of 3 mm, and central cylindrical portion (diameter 0.6 mm, thickness 5 mm) is air.

In this case, for example, when the wavelength λ of the laser light source is 532 nm and the wavelength width Δλ is 0.2 nm, λ ² /Δλ=1.4 mm is calculated. On the other hand, assuming that the refractive index of the transparent medium is 1.5 and the refractive index of air is 1, the optical path length difference T between the divided regions is calculated as 1.5 × 5 mm−1 × 5 mm = 2.5 mm. Therefore, the optical element LS of FIG. 5 satisfies the above-described formula T> λ ² / Δλ, and this reduces the coherence of the laser light, thereby reducing speckle.

  Here, the case where the laser beam is incident on the left side of the optical element LS and is emitted from the right side is illustrated, but the laser beam is configured to be incident on the right side of the optical element LS and emitted from the left side due to the reversibility of the light. May be.

(Fourth embodiment)
6 shows still another example of the optical element LS. FIG. 6A is a side view and FIG. 6B is a partially enlarged view thereof. The overall configuration of the laser projection apparatus according to the present embodiment is the same as that shown in FIG. 1, but the optical element LS has a birefringent material or birefringence on the light incident surface and / or light emitting surface of each divided region of the transparent medium. The structure is affixed in different thicknesses and / or different directions.

  The transparent medium has a shape as shown in FIG. 2, and the width of each annular zone is 5 mm. Therefore, each divided region has 1) a cylindrical portion having a diameter of 0.6 mm and a thickness of 30 mm, 2) a cylindrical portion having a diameter of 0.6 mm to 0.849 mm and a thickness of 25 mm, and 3) a diameter of 0.849 mm to 1.039 mm and a thickness of 20 mm. 4) Cylindrical portion having a diameter of 1.039 mm to 1.200 mm and a thickness of 15 mm 5) Cylindrical portion having a diameter of 1.200 to 1.342 mm and a thickness of 10 mm, 6) A diameter of 1.342 mm to 1.470 mm, a thickness 5 mm cylindrical part.

In this case, for example, when the wavelength λ of the laser light source is 532 nm and the wavelength width Δλ is 0.2 nm, λ ² /Δλ=1.4 mm is calculated. On the other hand, assuming that the refractive index of the transparent medium is 1.5 and the refractive index of air is 1, the optical path length difference T between the divided regions is calculated as 1.5 × 5 mm−1 × 5 mm = 2.5 mm. Therefore, the optical element LS of FIG. 6 satisfies the above-described formula T> λ ² / Δλ, and this reduces the coherence of the laser beam, thereby reducing speckle.

  Further, as shown in FIG. 6B, at least two polarizing regions P1 and P2 are provided by installing polarizing elements such as a polarizer, a wave plate, and a birefringent material on the end face of each divided region. ing. By changing the thickness and / or direction of the polarizing element in each of the polarizing regions P1 and P2, the transmission axis of the first polarizing region P1 and the transmission axis of the second polarizing region P2 are made to be orthogonal to each other. . Therefore, when the laser light is incident on the same divided region, the light passing through the polarization region P1 and the light passing through the polarization region P2 are perpendicular to each other in polarization direction, so that the coherence of the laser light further decreases. As a result, more effective speckle reduction is achieved.

(Fifth embodiment)
FIG. 7 shows still another example of the optical element LS. FIG. 7A is a side view and FIG. 7B is a partially enlarged view thereof. The overall configuration of the laser projection apparatus according to the present embodiment is the same as that shown in FIG. 1, but as the optical element LS, each divided region of the transparent medium has a phase modulation structure in which the optical path length is changed in the wavelength order of the laser light. I am using something.

  The transparent medium has a shape as shown in FIG. 2, and the width of each annular zone is 3 mm. Accordingly, each divided region has 1) a cylindrical portion having a diameter of 0.6 mm and a thickness of 18 mm, 2) a cylindrical portion having a diameter of 0.6 mm to 0.849 mm and a thickness of 15 mm, and 3) a diameter of 0.849 mm to 1.039 mm and a thickness of 12 mm. 4) Cylindrical portion having a diameter of 1.039 mm to 1.200 mm and a thickness of 9 mm, 5) Cylindrical portion having a diameter of 1.200 to 1.342 mm, a thickness of 6 mm, and 6) A diameter of 1.342 mm to 1.470 mm, a thickness. 3mm cylindrical part.

In this case, for example, when the wavelength λ of the laser light source is 532 nm and the wavelength width Δλ is 0.2 nm, λ ² /Δλ=1.4 mm is calculated. On the other hand, assuming that the refractive index of the transparent medium is 1.5 and the refractive index of air is 1, the optical path length difference T between the divided regions is calculated as 1.5 × 3 mm−1 × 3 mm = 1.5 mm. Accordingly, the optical element LS in FIG. 7 satisfies the above-described formula T> λ ² / Δλ, and this reduces the coherence of the laser beam, thereby reducing speckle.

  Further, as shown in FIG. 7B, the end face of each divided region is divided into a plurality of phase regions, for example, a phase region having a step for one wavelength of laser light, and a phase having a step for two wavelengths. Regions, phase regions having steps for three wavelengths, and the like are randomly arranged. Accordingly, when the laser light is incident on the same divided region, the phase changes to a different phase for each passing phase region, so that the coherence of the laser light further decreases. As a result, more effective speckle reduction is achieved.

  INDUSTRIAL APPLICABILITY The present invention is extremely useful industrially in that speckle of a projected image on a screen can be reduced.

It is a block diagram which shows 1st Embodiment of this invention. An example of the optical element LS is shown, FIG. 2A is a plan view, and FIG. 2B is a side view. It is a graph which shows light intensity distribution of the light spot on a screen. Other examples of optical element LS are shown, Drawing 4 (a) is a top view and Drawing 4 (b) is a side view. Still another example of the optical element LS is shown, FIG. 5A is a cross-sectional view, and FIG. 5B is a side view. FIG. 6A is a side view and FIG. 6B is a partially enlarged view of still another example of the optical element LS. FIG. 7A is a side view and FIG. 7B is a partially enlarged view of still another example of the optical element LS.

Explanation of symbols

LB, LR, LG Laser element L1, L2, L3, L4 Lens LS Optical element M1, M2, M3 Color composition mirror MS Scanning device SC screen

Claims (9)

A laser light source;
A laser projection device including a scanning device for two-dimensionally scanning laser light from a laser light source,
An optical element provided between a laser light source and a scanning device, divided into a plurality of regions having different optical path lengths, and including a transparent medium in which the optical path length difference between the regions is larger than the coherence distance of the laser light A laser projection device characterized by the above. The optical path length difference T between the regions is defined as follows.
T> λ ² / Δλ
The laser projection apparatus according to claim 1, wherein:   The laser projection device according to claim 1, wherein the divided areas of the transparent medium are arranged in a concentric ring shape with the optical axis as a center. The object distance incident on the transparent medium is L, and the outer diameter of the transparent medium is D.
| L / D |> 50
The laser projection apparatus according to claim 1, wherein: The laser light source includes a blue laser element, a red laser element, and a green laser element,
A color synthesizing element for synthesizing laser light from each laser element;
The laser projection apparatus according to claim 1, wherein the optical element is disposed behind the color synthesis element.   2. The laser projection device according to claim 1, wherein each divided region of the transparent medium has a phase modulation structure in which an optical path length is changed in a wavelength order of the laser beam.   2. The laser projection according to claim 1, wherein birefringent materials or birefringent structures are provided in different thicknesses and / or different directions on the light incident surface and / or the light exit surface of each divided region of the transparent medium. apparatus.   The laser projection apparatus according to claim 1, wherein the laser light source includes a semiconductor laser element.   The laser projection apparatus according to claim 1, further comprising an imaging optical system for condensing the scanned laser light on a fixed screen.