Beam combining device having a diffractive grating surface

Abstract

The present application discloses a beam combining device, which includes a reflective diffractive grating surface configured to combine a first, a second and a third incident light beam having different colors to a single diffracted mixed-color light beam when impinging on the reflective diffractive grating surface, wherein a profile of the grating surface is configured according to an optimization criterion with respect to a diffraction efficiency.

Claims

1. A beam combining device, comprising: a reflective diffractive grating surface configured to combine a first incident light beam, a second incident light beam and a third incident light beam into a single diffracted mixed-color light beam when impinging on the reflective diffractive grating surface, wherein the first incident light beam, the second incident light beam, and the third light beam are different colors, wherein a profile of the grating surface is configured according to an optimization criterion with respect to a diffraction efficiency, and wherein the optimization criterion is based on at least one of: angles of the first incident light beam, the second incident light beam, and the third incident light beam to a normal; an angle of the diffracted light beam to normal; wavelengths of the first incident light beam, the second incident light beam, and the third incident light beam; a diffractive order number; and a grating period of the grating surface.

2. The beam combining device of claim 1, wherein the reflective diffractive grating surface comprises a blazed grating.

3. The beam combining device of claim 2, wherein the blazed grating comprises grooves having an asymmetric triangular form.

4. The beam combining device of claim 1, wherein the reflective diffractive grating surface comprises a sub-wavelength grating.

5. The beam combining device of claim 4, wherein the sub-wavelength grating comprises grooves arranged on the grating surface with a period of less than a wavelength of one of the first incident light beam, the second incident light beam, or the third incident light beam.

6. The beam combining device of claim 5, wherein the grooves of the sub-wavelength grating have a symmetric triangular form.

7. The beam combining device of claim 5, wherein the grooves of the sub-wavelength grating have a sinusoidal form.

8. The beam combining device of claim 1, wherein the grating surface comprises a reflective metal layer covering the grating surface.

9. The beam combining device of claim 1, wherein the profile of the grating surface is concave.

10. The beam combining device of claim 1, wherein the optimization criterion is based on the relation: $\sin {\theta }_i+\sin {\theta }_w=\frac{m{\lambda }_i}{T},$ wherein θ.sub.i denotes an angle of an i-th incident light beam to normal, wherein i is a positive integer, wherein θ.sub.w denotes an angle of a diffracted light beam to a normal, wherein λ.sub.i denotes the wavelength of the i-th incident light beam, wherein m denotes the diffractive order number; and wherein T denotes the grating period of the grating surface.

11. The beam combining device of claim 10, wherein the diffractive order number (m) is equal to negative one.

12. The beam combining device of claim 1, wherein the grating period of the grating surface is approximately 2500 lines per millimeter.

13. The beam combining device of claim 1, wherein the wavelengths of the first incident light beam, the second incident light beam, and the third incident light beam correspond to wavelengths of a red laser light, a green laser light and blue laser light.

14. A laser scan pico-projector, comprising: a first laser configured to generate a first incident light beam; a second laser configured to generate a second incident light beam; and a third laser configured to generate a third incident light beam, wherein the first incident light beam, the second incident light beam, and the third incident light beam are different colors, wherein the first laser, the second laser, and the third laser are arranged such that the first incident light beam, the second incident light beam, and the third incident light beam are directed to a reflective diffractive grating surface of a beam combining device, wherein the reflective diffractive grating surface comprises a sub-wavelength grating, and wherein the sub-wavelength grating comprises grooves arranged on the grating surface with a period of less than a wavelength of one of the first incident light beam, the second incident light beam, or the third incident light beam.

15. The laser scan pico-projector of claim 14, wherein the grooves of the sub-wavelength grating have a symmetric triangular form.

16. The laser scan pico-projector of claim 14, wherein the grooves of the sub-wavelength grating have a sinusoidal form.

17. The laser scan pico-projector of claim 14, wherein the grating surface comprises a reflective metal layer covering the grating surface.

18. The laser scan pico-projector of claim 14, wherein the profile of the grating surface is concave.

19. A method for generating a mixed-color light beam based on a combination of a first incident light beam, a second incident light beam and a third incident light beam, the method comprising: arranging the first incident light beam, the second incident light beam, and the third incident light beam with respect to a reflective diffractive grating surface of a beam combining device, according to the relation: ${\theta }_i=\arcsin \left(\frac{m{\lambda }_i}{T}-\sin {\theta }_w\right),$ wherein θ.sub.i denotes an angle of an i-th incident light beam to normal, wherein i is a positive integer, wherein θ.sub.w denotes an angle of a mixed-color light beam to a normal, wherein λ.sub.i denotes a wavelength of the i-th incident light beam, wherein m denotes a diffractive order number, and wherein T denotes a grating period of the grating surface.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 shows a schematic diagram illustrating combining of red, green and blue laser beams into the one RGB color controlled light beam.

(2) FIG. 2 shows a schematic diagram illustrating the general structure of a laser projection system with a dispersive light beam combiner element in form of a prism.

(3) FIG. 3 shows a schematic diagram illustrating the principle of transmittance diffractive grating working with angles calculations.

(4) FIG. 4 shows a schematic diagram illustrating a beam combining device including a reflective diffractive grating surface for combining red, green and blue light beams into a single white light beam according to an implementation form.

(5) FIG. 5 shows a schematic diagram illustrating a reflective diffractive grating surface with a curved profile according to an implementation form.

(6) FIG. 6 shows a schematic diagram illustrating a laser scan pico-projector illumination part according to an implementation form.

(7) FIG. 7 shows a schematic diagram illustrating a method for generating a mixed-color light beam according to an implementation form.

DESCRIPTION OF EMBODIMENTS

(8) In the following detailed description, reference is made to the accompanying drawings, which form a part thereof, and in which is shown by way of illustration specific aspects in which the disclosure may be practiced. It is understood that other aspects may be utilized and structural or logical changes may be made without departing from the scope of the present disclosure. The following detailed description, therefore, is not to be taken in a limiting sense, and the scope of the present disclosure is defined by the appended claims.

(9) The devices and methods described herein may be based on diffractive grating, such as reflective diffractive grating. It is understood that comments made in connection with a described method may also hold true for a corresponding device configured to perform the method and vice versa. For example, if a specific method step is described, a corresponding device may include a unit to perform the described method step, even if such unit is not explicitly described or illustrated in the figures. Further, it is understood that the features of the various exemplary aspects described herein may be combined with each other, unless noted otherwise.

(10) FIG. 4 shows a schematic diagram illustrating a beam combining device 400 including a reflective diffractive grating surface for combining red, green and blue light beams into a single white light beam according to an implementation form.

(11) The beam combining device 400 may include a reflective diffractive grating surface 401 to combine a first incident light beam 1, a second incident light beam 2 and a third incident light beam 3 having different colors, e.g. blue, green and red into a single diffracted mixed-color light beam 7 when impinging on the reflective diffractive grating surface 401. A profile 403 of the grating surface 401 is designed according to an optimization criterion with respect to diffraction efficiency. The first incident light beam 1, a second incident light beam 2 and a third incident light beam 3 may be reflected from the reflective diffractive grating surface 401. The grating surface 401 may be highly reflective, similar to a mirror. No energy or only a small energy portion of the incident light beams is lost due to transmittance through the grating surface 401. Thus, an energy efficiency of the beam combining is high. In one example, the grating surface 401 is covered with a metal layer or some other layer having mirroring properties to improve the reflective property of the grating surface 401. In one example, the metal layer or the layer having mirroring properties is embedded in the grating surface.

(12) By optimally designing the profile 403 of the grating surface 401, diffractive gratings can be designed to=o provide amplitude diffraction efficiencies higher than 15% and phase diffraction efficiencies higher than 41% for first order binary profiles, higher than 31% for first order sinusoidal profiles, higher than 60% for first order blazed profiles and higher than 60% for first order subwavelength profiles.

(13) In an implementation form, the reflective diffractive grating surface 401 may include a blazed grating. In an implementation form, the blazed grating may have grooves having an asymmetric form, such as an asymmetric triangular form. In an implementation form, the reflective diffractive grating surface 401 may include a sub-wavelength grating. In an implementation form, the sub-wavelength grating may have grooves arranged on the grating surface 401 with a period of less than a wavelength of one of the three incident light beams 1, 2, 3. In an implementation form, the grooves of the sub-wavelength grating may have a symmetric triangular or a sinusoidal form.

(14) The efficiency of diffractive grating may depend on the quality of the profile 403, on the wavelength and on the depth of the profile 403. Efficiency of blazed or subwavelength gratings may be higher than 90%. Blazed gratings may define gratings with grooves having asymmetric triangular form. Subwavelength gratings may define gratings with the period of grooves being less than a wavelength. The grooves may have symmetric triangular or sinusoidal form.

(15) The highest efficiency for red, green and blue light sources simultaneously can be achieved when optimizing the profile 403 of the grating 401. Subwavelength gratings may be preferable because their diffraction angle increases when decreasing the grating period as can be seen from the general diffractive grating equation (1) described below. An increasing of the diffraction angle may be needed for placement of laser sources.

(16) In an implementation form, the grating surface 401 may include a reflective metalized grating, such as a metal layer covering the grating surface 401. A reflective metalized grating may be preferable because its utilizing allows avoiding light energy loss on transparent surfaces.

(17) The general diffractive grating equation:

(18) $\begin{array}{cc}\sin {\theta }_i+\sin {\theta }_w=\frac{m{\lambda }_i}{T}& \left(1\right)\end{array}$
describes a relation between the angles θ.sub.i, i=1, 2, 3, . . . normal 402 of the incident light beams 1, 2, 3 to normal 402, an angle θ.sub.w of the diffracted light beam 7 to normal 402, wavelengths λ.sub.i of the incident light beams 1, 2, 3, a diffractive order number m and a grating period T of the grating surface 401.

(19) θ.sub.i may be denoted as the angle to normal 402 of red, green and blue incident light beams, where i may be an index for r, g, b, i.e. for a red incident light beam 1, green incident light beam 2 and blue incident light beam 3 respectively. θ.sub.w may be denoted as the angle to normal direction 402 of the output combined (“white”) light beam 7. λ.sub.i may be denoted as the wavelength for the red incident light beam 1, green incident light beam 2 and blue incident light beam 3 light beams, respectively. The diffractive order number m may obtain the values m=±1, ±2, ±3, ±4, . . . .

(20) In one example, positive diffractive orders (m=+1, +2, . . . ) may be in the opposite to incident beams 1, 2, 3 half-space relative to the normal 402. Negative orders (m=−1, −2, . . . ) may be in the same half-space with incident beams 1, 2, 3 relative to the normal 402. θ.sub.w may be determined as a constant value and λ.sub.r, λ.sub.g, λ.sub.b may also be constants because they are determined by the light sources. Therefore the angle of incidence for each RGB light beam 1, 2, 3 may be written as:

(21) $\begin{array}{cc}{\theta }_i=\arcsin \left(\frac{m{\lambda }_i}{T}-\sin {\theta }_w\right)& \left(2\right)\end{array}$

(22) A reflective diffractive grating with optimized profile may be used for combination of red, green, and blue laser beams into a single beam. In an implementation form, a diffractive order of m=−1 is used in order to avoid mixing of designed “white” light beam 7 with zero orders of initial RGB laser beams 1, 2, 3. Replacing m=−1 in equation (2) results in:

(23) $\begin{array}{cc}{\theta }_i=\arcsin \left(-\frac{{\lambda }_i}{T}-\sin {\theta }_w\right)& \left(3\right)\end{array}$

(24) In one example, zero orders may be used for feedback control of light sources.

(25) The diffraction efficiency for diffractive gratings may depend on depth and quality of the phase profile of the diffractive grating on the one hand. In one embodiment, the diffractive efficiency may depend on the wavelength and polarization of light. When using deeply optimized diffractive grating profiles 403, the maximal efficiency of diffracted light with three wavelengths into the designed order can be achieved.

(26) Deeply optimized diffractive grating profiles 403 may be provided by optimizing the grating profile according to equations (2) or (3), i.e. the optimization criterion may be based on equation (2) or equation (3).

(27) In an implementation form, the profile 403 of the grating surface 401 may be curved, such as concave as described below with respect to FIG. 5.

(28) In an implementation form, the grating period T of the grating surface may lie in a range of approximately 2500 lines per millimeter (mm).

(29) In an implementation form, the wavelengths λ.sub.i of the incident light beams 1, 2, 3 may correspond to wavelengths of red, green and blue laser light, e.g. 450 nm (nanometer) for blue laser light, 532 nm (nanometer) for green laser light and 640 nm (nanometer) for red laser light. Applying equation (2) may result in exemplary incident angles for the three light beams of approximately θ.sub.3≈9.7° for the blue light beam 3, approximately λ.sub.2≈22° for the green light beam 2, approximately θ.sub.1≈40° for the red light beam 1 and approximately θ.sub.w≈72° for the white emitting light beam 7.

(30) Although FIG. 4 depicts three incident light beams, the beam combining device is not limited to three light beams such as RGB light beams. In examples, any other number of light sources can also be combined by the beam combining device 400, e.g. 2 light beams, 4, 5, 6, 7, etc. light beams. The colors are not limited to red, green and blue. In examples, different colors are used to create a mixed color light beam. The mixed color light beam does not necessarily have to be a white color light beam. In one example, the light beams 1, 2, 3 may be generated by lasers. In one example, the light beams 1, 2, 3 may be generated by light emitting diodes (LEDs).

(31) FIG. 5 shows a schematic diagram illustrating a reflective diffractive grating surface 500 with a curved profile according to an implementation form.

(32) For correcting the divergence of incident light 501 on a grating surface from laser diodes or LEDs, a curvature may be applied to the grating surface. The curved grating surface 500 may reduce this divergence. The curvature may be shaped concave. The curved grating surface 500 may focus the emergent light beam 502.

(33) FIG. 6 shows a schematic diagram illustrating a laser scan pico-projector illumination part 600 according to an implementation form. The laser scan pico-projector illumination part 600 may include a first laser 601, a second laser 603 and a third laser 605 generating a first incident light beam 602, a second incident light beam 604 and a third incident light beam 606, respectively. The incident light beams may have different colors, e.g. such as red, green and blue as exemplary illustrated in FIG. 6. The laser scan pico-projector illumination part 600 may include a beam combining device 400 as described above with respect to FIG. 4, such as a beam combining device 500 having a curved surface as depicted in FIG. 5. The first laser 601, second laser 603 and third laser 605 may be arranged such that the first incident light beam 602, second incident light beam 604 and third incident light beam 606 are directed to the reflective diffractive grating surface of the beam combining device as depicted in FIG. 6. The arrangement may be designed according to equation (2) or (3) described above.

(34) The combining of the red, green and blue light beams into one combined light beam 608 has been modeled by a ZEMAX simulation. An exemplary parameter set may be the following: grating period 2500 lines per mm, the angles from normal may be for blue laser light (450 nm)≈9.7°, for green laser light (532 nm)≈22°, for red laser light (640 nm)≈40°, for the resulting (“white”) laser beam≈72°.

(35) The results of the simulation demonstrate good performance of combining of the three light beams with different colors into one for providing a light beam with designed color (RGB) for image projection techniques.

(36) The devices presented in this disclosure may be employed into laser scan pico-projector systems creating light beams with designed colour and intensity and may thus replace conventional image projection techniques which are using two or more dichroic mirrors.

(37) FIG. 7 shows a schematic diagram illustrating a method 700 for generating a mixed-color light beam according to an implementation form.

(38) The method 700 may include arranging 701 a first, second and third incident light beam with respect to a reflective diffractive grating surface according to the relation:

(39) ${\theta }_i=\arcsin \left(\frac{m{\lambda }_i}{T}-\sin {\theta }_w\right),$
i.e., according to equation (2), where θ.sub.i denotes an angle of the i-th incident light beam to normal, i=1, 2, 3, θ.sub.w denotes an angle of the mixed-color light beam to normal, λ.sub.i denotes a wavelength of the i-th incident light beam, i=1, 2, 3, m denotes a diffractive order number, and T denotes a grating period of the grating surface.

(40) The reflective diffractive grating surface may correspond to a reflective diffractive grating surface as described above with respect to FIG. 4, FIG. 5 and FIG. 6. The reflective diffractive grating surface may form a beam combining device 400 as described above with respect to FIG. 4.

(41) The method 700 may be used to operate a laser scan pico-projector illumination part as described above with respect to FIG. 6. The reflective diffractive grating surface may be formed as described above with respect to FIG. 4 and FIG. 5.

(42) While a particular feature or aspect of the disclosure may have been disclosed with respect to only one of several implementations, such feature or aspect may be combined with one or more other features or aspects of the other implementations as may be desired and advantageous for any given or particular application. Furthermore, to the extent that the terms “include”, “have”, “with”, or other variants thereof are used in either the detailed description or the claims, such terms are intended to be inclusive in a manner similar to the term “comprise”. Also, the terms “exemplary”, “for example” and “e.g.” are merely meant as an example, rather than the best or optimal.

(43) Although specific aspects have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that a variety of alternate and/or equivalent implementations may be substituted for the specific aspects shown and described without departing from the scope of the present disclosure. This application is intended to cover any adaptations or variations of the specific aspects discussed herein.

(44) Although the elements in the following claims are recited in a particular sequence with corresponding labeling, unless the claim recitations otherwise imply a particular sequence for implementing some or all of those elements, those elements are not necessarily intended to be limited to being implemented in that particular sequence.

(45) Many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the above teachings. Of course, those skilled in the art readily recognize that there are numerous applications of the embodiment of the application beyond those described herein. While the present embodiment of the application has been described with reference to one or more particular embodiments, those skilled in the art recognize that many changes may be made thereto without departing from the scope of the present embodiment of the application. It is therefore to be understood that within the scope of the appended claims and their equivalents, the embodiment of the application may be practiced otherwise than as described herein.