Beam expander and method of operating the same

Abstract

The beam expander includes first and second optical elements spaced apart from each other, and a light diffuser having an angular aperture through which incident light is diffused, wherein the first optical element in-couples the diffused light such that light exiting the first optical element has a first cross-sectional shape, and light having a second cross-sectional shape different from the first cross-sectional shape is incident on the second optical element, and the second optical element out-couples the light incident from the first optical element.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to the field of optical systems, and more particularly, to beam expanders for illumination in three-dimensional (3D) images (e.g., 3D displays, holographic displays, etc.). BACKGROUND

[0002] In optical systems, particularly in holographic imaging and display systems, beam expanders can be used, which are optical devices that generally receive collimated light and expand its size.

[0003] Among the requirements for the holographic display devices currently being developed, as well as for the configuration systems for illumination and beam expansion, the main requirements are to minimize the size of the device, increase the efficiency of homogenization, provide a beam with a specific cross-sectional shape (aperture), and remove speckle.

[0004] Here, "speckle" is a random interference pattern formed by the mutual interference of coherent waves with a random phase shift and / or a random intensity set.

[0005] The requirements for holographic display devices, as well as for the configuration systems for illumination, beam expansion, and collimation, are beam shaping with the required cross-sectional shape and size (aperture) and homogenization, as well as effective suppression of laser beam speckle, while maintaining the coherence of the output light.

[0006] However, optical systems with beam expanders generally have the following problems.

[0007] 1) Beam homogeneity

[0008] A beam expanded by a conventional high-efficiency beam expander provides limited possibilities in maintaining the coherence of the laser beam and ensuring high homogeneity.

[0009] 2) Beam cross-sectional (aperture) shaping

[0010] Assuming that at the input of the beam expander the beam has circular or elliptical symmetry, it is not possible to ensure beam homogeneity and beam cross-sectional shaping (shaping the beam as needed) while maintaining high efficiency of the system (e.g., without significant energy loss).

[0011] 3) Reduction of speckle

[0012] The use of coherent laser light can produce speckle and speckle patterns, which are produced due to random interference. SUMMARY

[0013] Example embodiments have been made taking into account the disadvantages of the above prior art solutions, and can aim to eliminate or at least reduce the disadvantages of the prior art.

[0014] Example embodiments provide coherence beam uniformity, and provide a desired cross-sectional shaping and reduction or suppression of speckle, while maintaining coherence of the output beam.

[0015] Additional aspects will be set forth in part in the description which follows, and, in part, will be apparent from the description, or can be learned by practice of the presentation of embodiments of the disclosure.

[0016] According to an aspect of the disclosure, a beam expander includes a first optical element, a second optical element spaced apart from the first optical element, and a light diffuser having an angular aperture, the light diffuser diffusing incident light through the angular aperture, wherein the first optical element in-couples the diffused light such that light exiting from the first optical element has a first cross-sectional shape, and light having a second cross-sectional shape different from the first cross-sectional shape is incident on the second optical element, and wherein the second optical element out-couples the light incident from the first optical element.

[0017] The angular aperture of the light diffuser can be greater than about 0° and equal to or less than 5°.

[0018] The intensity of the light diffused by the light diffuser can have greater uniformity on a light cross-section than the intensity of the incident light.

[0019] The light diffuser can output the incident light as a plurality of sub-lights spatially separated from each other.

[0020] The light diffuser can modulate a phase of the incident light and output the incident light as a plurality of sub-lights.

[0021] The light diffuser can perform spatially non-uniform phase modulation on the incident light and output a plurality of sub-lights.

[0022] The light diffuser can include at least one of a microlens grating, a phase mask, a matte diffuser, a diffractive optical element, and a holographic optical element.

[0023] The second cross-sectional shape can be a polygonal shape.

[0024] The first cross-sectional shape can be a circular or elliptical shape.

[0025] The size of the second cross-sectional shape can be smaller than the size of the first cross-sectional shape.

[0026] The incident light can have an optical cross-sectional profile of at least one of a Gaussian beam, a multimode beam, a cylindrical beam, a super-Gaussian beam, and a Laguerre-Gaussian beam.

[0027] At least one of the first optical element and the second optical element can include at least one of a diffractive optical element and a holographic optical element.

[0028] The beam expander can further include a waveguide disposed on the first optical element and the second optical element, which gradually converts light having the first cross-sectional shape from the first optical element into light having a second cross-sectional shape by total internal reflection and guides the converted light to the second optical element.

[0029] The waveguide can gradually reduce a size of the first cross-sectional shape by total internal reflection to convert the light having the first cross-sectional shape into the light having the second cross-sectional shape.

[0030] The light diffuser and the first optical element can be arranged to overlap each other in a direction perpendicular to a length direction of the waveguide.

[0031] The light diffuser can be disposed on a first surface of the waveguide, and the first optical element can be disposed on a second surface of the waveguide opposite the first surface.

[0032] The light diffuser can be integrated with the first optical element.

[0033] The beam expander can further include a laser diode that provides the incident light.

[0034] The second optical element can collimate the light incident onto the second optical element with a divergence angle or a convergence angle less than 0.25°.

[0035] Uniformity of intensity of the light out-coupled by the second optical element can be greater than uniformity of intensity of the light incident onto the light diffuser.

[0036] According to an aspect of the disclosure, a method of operating a beam expander includes diffusing incident light with a light diffuser, changing the diffused light having a first cross-sectional shape into light having a second cross-sectional shape different from the first cross-sectional shape while making the diffused light propagate through a waveguide, and collimating the light having the second cross-sectional shape with an optical element.

[0037] According to an aspect of the disclosure, a beam expander includes a waveguide tube, an input optical element that receives incident light and outputs first light having a first cross-sectional shape to the waveguide, and an output optical element that receives light transmitted by the waveguide and outputs second light having a second cross-sectional shape different from the first cross-sectional shape.

[0038] Uniformity of a cross-sectional intensity distribution of the first light is greater than uniformity of a cross-sectional intensity distribution of the incident light.

[0039] The input optical element can include a diffuser that diffuses the incident light and at least one of a diffractive optical element (DOE) and a holographic optical element (HOE).

[0040] The diffuser can be integrated with at least one of the DOE and the HOE.

[0041] The first cross-sectional shape can be at least one of an ellipse and a circle, and the second cross-sectional shape can be a rectangle.

[0042] The second cross-sectional shape can be the same as a cross-sectional shape of the output optical element. BRIEF DESCRIPTION OF DRAWINGS

[0043] The above and other aspects, features, and advantages of certain embodiments of the disclosure will be more apparent from the following description taken in conjunction with the accompanying drawings, in which:

[0044] Figure 1 FIG. 1 is a diagram illustrating a beam expander according to an embodiment;

[0045] Figure 2 FIG. 2 is a flowchart illustrating a method of expanding coherent light according to an embodiment;

[0046] Figure 3 FIG. 3 is a diagram illustrating a path of light in a beam expander according to an embodiment;

[0047] Figure 4 FIG. 4 is a diagram illustrating a cross-section of light in a waveguide according to an embodiment; and

[0048] Figure 5 FIG. 5 is a diagram illustrating a cross-section of light output from a beam expander according to an embodiment. DETAILED DESCRIPTION

[0049] Reference will now be made in detail embodiments, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout. In this regard, the embodiments can have different forms and should not be construed as being limited to the descriptions set forth herein. Accordingly, the example embodiments are described below, by referring to the drawings, to explain aspects. As used herein, the term "and / or" includes any and all combinations of one or more of the associated listed items. Expressions such as "at least one of," when preceding a list of two or more items, cover also each of the individual items in the list.

[0050] Hereinafter, embodiments will be described in detail with reference to the accompanying drawings. In the drawings, the same reference numerals denote the same elements, and the size of each element can be exaggerated for the sake of clarity and convenience in description. The embodiments described below are merely examples, and various modifications can be made therein.

[0051] As used herein, the term "on" or "above" an element can be understood to mean that the element can be directly on another element, or the element can be on another element without being in contact with the other element.

[0052] As used herein, singular expressions shall include plural aspects unless the context clearly dictates otherwise. It will be understood that when a component is referred to as being "comprises" another component, the component can further include other components unless otherwise specified.

[0053] The term "the" and its analogous determiners can be understood to include both singular and plural forms.

[0054] Terms such as "first" and "second" can be used to describe various elements, but the elements should not be limited by the terms. The terms are used only to distinguish one element from another.

[0055] Figure 1 FIG. 1 is a diagram illustrating an expander 10 according to an embodiment.

[0056] A light source can provide light to the expander 10. The light source can include a laser diode. In an embodiment, the laser diode as the light source can be disposed outside the expander 10, or can be embedded in the expander 10. The laser diode can provide coherent light having a specific intensity profile.

[0057] The light source is not limited to the laser diode. In addition to the laser light source, any source having a narrow spectral range, such as a light-emitting diode (LED) or an organic light-emitting diode (OLED), can also be used as the light source. The light source that can be used in the expander 10 according to an embodiment is not limited to a specific light source. In some embodiments, the light source can include a gas-discharge lamp capable of generating several narrow spectral lines of light.

[0058] Example embodiments are described using a laser beam as an external source, which is a light source, but are not limited thereto. In addition to the laser beam, other types of coherent light or even incoherent light from an external source or a built-in source can also be used as the light source.

[0059] In an embodiment, the light source can provide light having various electric field and light cross-sectional profiles. For example, the light can have a cross-sectional profile of at least one of a Gaussian beam, a multi-mode beam, a cylindrical beam, a super-Gaussian beam (a beam having a ring-shaped cross-section), and a Laguerre-Gaussian beam.

[0060] In the coherent light according to an embodiment, a plurality of components can be arranged in a specific distribution in a cross-section. For example, the coherent light can include blue (a wavelength of about 460 nm), red (a wavelength of about 640 nm), and green (a wavelength of about 515 nm). The distribution of the three components of the light in the cross-section of the incident light described above can vary for each wavelength. However, the present disclosure is not limited thereto.

[0061] The beam expander 10 according to an embodiment does not need to adjust any device to use a specific light source. The above-described light source is a light source that makes light incident on the beam expander 10 from the outside of the beam expander 10, and can also be referred to as a main light source. However, the present disclosure is not limited thereto. The beam expander 10 itself can include a light source, and the light source can be controlled by the beam expander 10 itself and the entire system. In some cases, the light source can be an essential element of the beam expander 10.

[0062] Examples of implementation of an embodiment using an external light source can provide the light source to be used as a separate device for a laboratory stand, use in metrology, use in the same type of device (monitors and glasses for augmented or virtual reality) for various manufacturers' end users, etc. Examples of implementation of an embodiment with a built-in light source can provide the use of the light source in a single manufacturer's user device when the element is integrated.

[0063] The beam expander 10 according to an embodiment can include a first optical element 110 (e.g., an input optical element) and a second optical element 120 (e.g., an output optical element), each of which can be implemented as a holographic optical element (HOE) or a diffractive optical element (DOE). The first optical element 110 can be a HOE, and the second optical element 120 can be a DOE, or vice versa.

[0064] DOEs and HOEs are basically based on the same physical principle, and both of these optical elements can essentially be referred to as DOEs. The difference between these two optical elements is that a DOE is an optical element in which diffraction occurs on a surface relief, and a HOE includes a Bragg body grating in which diffraction occurs in the material due to a local change in optical properties.

[0065] The purpose of the first optical element 110 is to guide light onto the plane of the second optical element 120 so that it has a specific cross-sectional shape and uniformity when the light is incident on the second optical element 120. The purpose of the second optical element 120 is to collimate light having a cross section formed by the first optical element 110 while maintaining the coherence of the light at the output end of the second optical element 120.

[0066] The expander 10 according to an embodiment can further include a waveguide 130 connecting the first optical element 110 to the second optical element 120. The waveguide 130 can be a substrate or a substrate coated with a thin film. Alternatively, the waveguide 130 can include a substrate and a multi-layer film coated on one or both surfaces of the substrate. The substrate can be made of any optically transparent material used in the art. The thin film coating can be single or multi-layered, and can be reflective (specular). In particular, the thin film coating can be metallized or can be an oxide film. The thin film coating can be implemented as an anti-reflection coating selectively and partially transmitting a spectrum of light. The thin film coating can be formed by, for example, spraying or chemical vapor deposition.

[0067] The expander 10 according to an embodiment can further include a light diffuser 140 (i.e., a diffuser) having a specific angular aperture. The light diffuser 140 can be manufactured as a separate element or an element attached to a surface of the waveguide 130. Alternatively, as shown, each of the light diffuser 140 having a specific angular aperture and the first optical element 110 can be arranged on different sides of the waveguide 130, and the light diffuser 140 and the first optical element 110 can at least partially overlap with each other in a direction perpendicular to a length direction of the waveguide 130. Other arrangement relationships between the light diffuser 140 having a specific angular aperture and the first optical element 110 (input grating) can also be easily designed by those skilled in the art. Figure 1

[0068] The light diffuser 140 having a specific angular aperture according to an embodiment can diffuse light incident from a light source and irradiate the first optical element 110. In the context of an embodiment, the term "specific" angular aperture means that the angular aperture of the light diffuser 140 is specially calculated or matched with specific parameters of an optical system and coordinated with the remaining elements (e.g., the first optical element 110 and the second optical element 120).

[0069] In an embodiment, the light diffuser 140 having a specific angular aperture can be implemented in the form of a so-called calculated "phase mask" produced by etching, cutting, casting, or photolithography. The "phase mask" can shape light as needed to diffuse light. The phase mask can achieve higher efficiency because the coverage of the second optical element 120 with respect to the diffused light is high. The light diffuser 140 having a specific angular aperture in the form of a "phase mask" can be made of any optically transparent material such as plastic, glass, resin, polymer, or photopolymer, but is not limited thereto.

[0070] ​In an embodiment, the light diffuser 140 with a certain angular aperture can be implemented as a micro-lens array with a periodic structure or a random structure. Because the micro-lens array has a wide angular range, the micro-lens array can be slightly less efficient than the light diffuser that is a "phase mask". Therefore, the shape of the beam cross-section in the plane of the second optical element 120 can be larger than the size of the second optical element 120 itself.

[0071] In an embodiment, the light diffuser 140 with a certain angular aperture can be implemented as a matte diffuser with a surface roughness.

[0072] In an embodiment, the light diffuser 140 with a certain angular aperture can be implemented as a DOE or a HOE. When the light diffuser 140 is implemented as a DOE or a HOE, the light diffuser 140 can be a separate element or can be implemented integrally with the first optical element 110. For example, in an embodiment where the first optical element 110 is implemented as a grating structure, the light diffuser 140 with a certain angular aperture can be "recorded" in the grating structure. Specifically, the light diffuser 140 with a certain angular aperture can be integrated into the first optical element 110 in a "recording" step, which allows the light diffuser 140 with a certain angular aperture to be referred to as a "virtual" diffuser in the context of some embodiments.

[0073] The "recording" process of the light diffuser 140 with a certain angular aperture in the grating structure implementing the first optical element 110 can be performed by an analog type or by a digital type.

[0074] Analog recording is essentially recording the interference result (interference pattern) between two light beams (reference beam and object beam on photographic material). When the resulting structure is irradiated by one of the light beams involved in the recording or a light associated therewith, as a result of diffraction, light that is exactly the same as the light involved in the recording in all parameters can be recovered. In other words, a conventional diffraction grating can essentially be an interference pattern between two collimated light beams. When the light diffuser 140 is placed in one of the light beams involved in the recording, the light diffuser 140 can be integrated in the first optical element 110. Therefore, when light is irradiated on the first optical element 110, the light can pass through the first optical element 110 integrated with the light diffuser 140, and light having an energy distribution corresponding to the pattern of the light diffuser 140 can be output.

[0075] After the diffraction structure of the first optical element 110 is calculated / simulated, digital recording is formed by a photolithography or a mechanical unit.

[0076] The method of setting the angular aperture can be different according to the design type of the light diffuser 140. When the light diffuser 140 is a matte diffuser, the angular aperture can be "set" as the surface roughness, that is, the size of irregularities present on the surface of the light diffuser 140. When the light diffuser 140 is a microlens array (microlens grating), the angular aperture can be "set" by the focal length, size, and period of the microlenses. When the light diffuser 140 is a HOE or a DOE, or is integrated in the first optical element 110, the angular aperture can be "set" by the diffractive structure included in the optical element.

[0077] The light diffuser 140 having a specific angular aperture can be implemented in the form of a separate element arranged at regular intervals in front of the first optical element 110, and can also be made in the form of an element that is glued to or otherwise adhered to the waveguide 130 or the first optical element 110. Furthermore, the light diffuser 140 having a specific angular aperture can be implemented on a portion of the surface of the waveguide 130 by mechanical means, chemical means (by etching), or casting.

[0078] The above is merely an example of the implementation of the light diffuser 140 having a specific angular aperture used in the device according to an embodiment. However, the present disclosure is not limited thereto. The selection of the embodiment of the light diffuser 140 having a specific angular aperture can be determined by the specific features of the implementation of the device according to an embodiment and / or specific requirements for the device.

[0079] The light incident on the light diffuser 140 can be converted into a combination of secondary light sources. The light incident on the light diffuser 140 can be light having a Gaussian distribution, and the combination of secondary light sources can include a plurality of sub-lights spatially separated (i.e., a plurality of sub-lights spatially separated from each other). The secondary light source can have higher light uniformity on a light cross-section than the incident light. In other words, the intensity distribution of the cross-section of the secondary light source can be more uniform than the intensity distribution of the light incident on the light diffuser 140.

[0080] The mixing quality and uniformization of the light output from the expander 10 can be ensured by the density and light pattern (e.g., the cross-sectional shape of the light) of the light emitted from the combination of secondary light sources and the optical path length of the waveguide 130. Furthermore, the efficiency of the entire optical system of the proposed device can vary according to the light pattern of the combination of secondary light sources and the optical path length of the waveguide 130.

[0081] The light diffuser 140 can output a plurality of sub-lights by performing phase modulation on the incident light. For example, the light diffuser 140 can perform phase modulation of the light that is spatially non-uniform. Thereby, speckle noise of the light diffused by the light diffuser 140 can be reduced.

[0082] The light diffuser 140 with a specific angular aperture can multiply the incident coherent light and output the result of the multiplication as secondary light sources, i.e., multiple sub-lights. Each sub-light can form its own speckle pattern while forming a different speckle pattern. Because the speckle patterns of the multiple sub-lights acquired by the light diffuser 140 with a specific angular aperture overlap with each other when passing through the waveguide, the contrast of the combined speckle pattern formed on the second optical element 120 can be significantly reduced. The total contrast of the speckle pattern on the second optical element 120 can be reduced by a factor of where n is the number of sub-lights output by the light diffuser 140 with a specific angular aperture. In the expander 10 according to an embodiment, the implementation of the optical scheme of the light diffuser 140 with a specific angular aperture can reduce the formation of speckles output from the second optical element 120 by up to about 80%.

[0083] The light diffuser 140 with a specific angular aperture according to an embodiment can be integrated into the structure of the first optical element 110. This can be implemented in the manufacture of the first optical element 110 and can allow reducing the number of components of the expander 10 while providing similar efficiency to that in the previous embodiments described above, because the light diffused by the first optical element 110 has a high coverage of the second optical element 120.

[0084] In various embodiments, the light diffuser 140 with a specific angular aperture can be made of any material that is optically transparent in the spectral range of the incident light. For example, the light diffuser 140 can include at least one of plastic, glass, resin, polymer, and photopolymer, and can be one or more of the following forms, but is not limited thereto.

[0085] — a separate element;

[0086] — an element attached on the surface of the waveguide 130 or formed as part of the surface of the waveguide 130;

[0087] — an element combined with the first optical element 110 (input grating);

[0088] — an element with a specific surface relief;

[0089] — a reflective or transmissive DOE (relief optical element or volume Bragg grating)

[0090] In various embodiments, the first optical element 110 (input grating) can be made of any material that is optically transparent in the spectral range of the incident light. For example, the first optical element 110 can include at least one of plastic, glass, resin, polymer, and photopolymer, and can be one or more of the following forms, but is not limited thereto.

[0091] — a separate element;

[0092] - an element attached on the surface of the waveguide 130 or formed as part of the surface of the waveguide 130;

[0093] - an element combined with a light diffuser 140 having a specific angular aperture;

[0094] - a reflective or transmissive DOE (a relief optical element or a volume Bragg grating)

[0095] The second optical element 120 (an output grating) can also be made of any material that is optically transparent in the spectral range of the incident light. The second optical element 120 can include at least one of plastic, glass, resin, polymer, and photopolymer, and can be implemented in one or more of the following forms, but is not limited thereto.

[0096] - a separate element;

[0097] - an element attached on the surface of the waveguide 130 or formed as part of the surface of the waveguide 130;

[0098] - a reflective or transmissive DOE (a relief optical element or a volume Bragg grating)

[0099] The light diffuser 140 having a specific angular aperture can diffuse the incident light and output a plurality of sub-lights. The light diffuser 140 having a specific angular aperture can increase the uniformity of light intensity by converting a Gaussian light into a light having the same intensity distribution regardless of the coordinates in the light cross-section, and can provide a reduction in speckle.

[0100] The first optical element 110 can in-couple the diffused light (that is, a plurality of sub-lights) to the waveguide 130. The first optical element 110 can be designed so that a light spot having a specific cross-section is formed on the plane of the second optical element 120.

[0101] The light diffused through the waveguide 130 can be transmitted from the first optical element 110 to the second optical element 120. Due to the fact that the diffused light can be mixed while propagating through the waveguide 130, the waveguide 130 (which converts a Gaussian light into a light having a constant intensity distribution regardless of the coordinates in the light beam cross-section) can ensure the uniformization of light and contribute to speckle suppression.

[0102] The light traveling through the waveguide 130 by total internal reflection can be incident on the second optical element 120, and the second optical element 120 can collimate each sub-light formed by the light diffuser 140 having a specific angular aperture and combine all the plurality of sub-lights into one light spot having a desired shape.

[0103] The light traveling through the waveguide 130 can be diffracted by the second optical element 120 and then out-coupled from the waveguide 130. The second optical element 120 can be designed to collimate the light reaching through the waveguide 130 and give the collimated light a desired cross-sectional shape. Thus, the out-coupled light can have a desired degree of collimation and a desired cross-sectional shape.

[0104] The cross-sectional shape of the light incident on the second optical element 120 can be based on the design of at least one of the light diffuser 140 having a certain angular aperture, the first optical element 110, the waveguide 130, and the second optical element 120. At the output end of the second optical element 120, the light can have a high level of collimation and uniformity and can have a certain cross-sectional shape with a low speckle contrast.

[0105] Preferably, the light on the plane of the second optical element 120 can have a shape and cross-sectional size each of which substantially corresponds to the shape and size of the second optical element 120. That is, the light can substantially completely cover the plane of the second optical element 120 without exceeding the limits of the second optical element 120. This can ensure a high illumination efficiency by the collimated output light.

[0106] The second optical element 120 can collimate the incident light in one direction and output the collimated light. However, the present disclosure is not limited thereto. The second optical element 120 can output the light having a desired cross-sectional shape with a divergence angle or a convergence angle of less than 0.25°. That is, the light output from the second optical element 120 can not be strictly collimated but can be slightly divergent or convergent, and the size of the light at a distance considerably far from the second optical element 120 can have a cross-sectional size different from that of the light at the output end of the second optical element 120. That is, at the output end and at a distance considerably far from the expander 10, the light can not be cylindrical but can have a conical shape with a size of about 0.25° or about 15 angular minutes.

[0107] The expander 10 according to an embodiment can provide light having a high uniformity to an optical system following the expander 10. Thus, the expander 10 can provide uniform light even when light sources having different intensity distributions are used, and various types of light sources can be used.

[0108] The expander 10 according to an embodiment can output uniform light, have a certain cross-sectional shape, and maintain spatial coherence even if light from various sources is incident on the input end of the expander 10. In addition, as described above, the expander 10 can provide a speckle suppression effect for light. Furthermore, the expander 10 can provide an improved color gamut and improved polarization characteristics of a formed image to form a very realistic 3D image.

[0109] Figure 2is a flowchart illustrating a method of expanding coherent light according to an embodiment. According to an embodiment, the method can be implemented by the expander 10 as a coherent expander.

[0110] In operation S1, the incident coherent light can be diffused by the light diffuser 140 having a specific angular aperture, and then can be incident on the first optical element 110. The cross-section of the incident light can be spatially non-uniform. For example, the light can have a light cross-sectional profile of at least one of a Gaussian beam, a multi-mode beam, a cylindrical beam, a super-Gaussian beam, and a Laguerre-Gaussian beam.

[0111] Figure 3 is a diagram illustrating a path of light in the expander 10 according to an embodiment. As Figure 3 indicated, the light diffuser 140 having a specific angular aperture can multiply the incident coherent light Li and output a plurality of sub-lights L1, L2, and L3 spatially separated. The light diffuser 140 can modulate a phase of the incident coherent light Li and output the plurality of sub-lights L1, L2, and L3. For example, the light diffuser 140 can perform spatially non-uniform phase modulation on the incident coherent light Li and the plurality of sub-lights L1, L2, and L3 output.

[0112] The light diffuser 140 can include at least one of a microlens grating, a phase mask, a matte diffuser, a DOE, and a HOE. The light diffuser 140 can have an angular aperture greater than about 0° and equal to or less than 5°, and a method of setting the angular aperture can vary according to the type of the light diffuser 140.

[0113] In operation S2, the diffused light can be in-coupled by the first optical element 110 and travel to the waveguide 130. The first optical element 110 can function as a coupling element that in-couples the incident light to the waveguide 130, and as a lens that focuses the light on a plane of the second optical element 120.

[0114] The first optical element 110 can in-couple the light diffused by the light diffuser 140, that is, the plurality of sub-lights L1, L2, and L3, and thus the diffused light can be incident on the second optical element 120 as light having a specific cross-section.

[0115] The first optical element 110 can include at least one of a DOE and a HOE. The structure of the surface relief or volume Bragg grating of the first optical element 110 can be designed such that the cross-section of the light output from the first optical element 110 and the cross-section of the light incident on the second optical element 120 are different from each other. For example, the cross-section of the light (that is, the combination of the plurality of sub-lights L1, L2, and L3) output from the first optical element 110 can be circular or elliptical. In addition, the cross-section of the light L0 incident on the second optical element 120 can have a polygonal shape, such as a quadrilateral.

[0116] As shown in operations S1 and S2, Figure 2 The light incident on the expander 10 is diffused and then in-coupled. However, the disclosure is not limited thereto. When the light diffuser 140 and the first optical element 110 are implemented as a single optical element, the light incident on the expander 10 can be simultaneously diffused and in-coupled.

[0117] In operation S3, the light in-coupled by the first optical element 110, that is, the plurality of sub-lights L1, L2, and L3, can be guided to the second optical element 120 through total internal reflection in the waveguide 130. The plurality of sub-lights L1, L2, and L3 have different speckle patterns. However, because the plurality of speckle patterns overlap during the total internal reflection in the waveguide 130, the contrast of the entire speckle pattern is reduced. The reduction in the contrast of the speckle pattern can be proportional to the number of the sub-lights L1, L2, and L3 separated by the light diffuser 140, and can be proportional to the length of the waveguide 130.

[0118] Figure 4 is a diagram illustrating a cross-section of light in the waveguide 130 according to an embodiment. As shown in Figure 4 The cross-section of the in-coupled light can be elliptical. However, as the in-coupled light travels through the total internal reflection in the waveguide 130, the cross-section of the in-coupled light can gradually change to a rectangular cross-section. As described above, the change in the cross-section of the light is based on the structure of the surface relief of the first optical element 110 or the design of the volume Bragg grating of the first optical element 110, and the change in the cross-section of the light can change the speckle pattern corresponding to each sub-light and reduce the contrast of the entire speckle pattern.

[0119] In addition, the plurality of sub-lights L1, L2, and L3 incident on the waveguide 130 can mix with each other while propagating through the total internal reflection in the waveguide 130, and further increase light uniformity.

[0120] In addition, as the plurality of sub-lights L1, L2, and L3 travel through the waveguide 130, the cross-sectional size of the combination of the plurality of sub-lights L1, L2, and L3 traveling through the waveguide 130 can gradually decrease. As described above, the change in the size of the light can be based on the structure of the surface relief of the first optical element 110 or the volume Bragg grating of the first optical element 110, and on the design of the waveguide 130. However, the disclosure is not limited thereto. The cross-sectional size of the combination of the plurality of sub-lights L1, L2, and L3 traveling through the waveguide 130 can gradually increase as the plurality of sub-lights L1, L2, and L3 travel through the waveguide 130. The degree of the size change can be adjusted according to the structure of the surface relief of the first optical element 110 or the volume Bragg grating of the first optical element 110, and on the design of the waveguide.

[0121] In Figure 2In operation S4, light can be incident on the second optical element 120 while maintaining optical coherence, and be out-coupled from the waveguide 130 through the second optical element 120. Due to diffraction of light in the second optical element 120, collimation of light in the second optical element 120 can be achieved. The second optical element 120 can collimate the incident light in one direction and output the collimated light. However, the disclosure is not limited thereto. The second optical element 120 can output light having a desired cross-sectional shape with a divergence angle or a convergence angle of less than 0.25°. That is, the light output from the second optical element 120 can not be strictly collimated, but can be slightly divergent or convergent, and the size of the light at a distance considerably far from the expander 10 can have a size different from that of the light at the output end of the second optical element 120.

[0122] To achieve collimation of light, the second optical element 120 can be manufactured as a surface relief structure after calculation, or can be obtained by recording a volume Bragg grating structure as a result of interference between two light beams. Here, the two light beams can include a perfectly collimated reference beam sequentially recovered, and an object beam identical to the light beam traveling along each of the light diffuser 140, the first optical element 110, and the waveguide 130 having a specific angular aperture.

[0123] To check light uniformity, the first optical element 110 and the second optical element 120 are manufactured in the form of a volume Bragg grating, the waveguide 130 is a flat glass plate, and the light diffuser 140 uses a matte diffuser having a 5° angular aperture. A semiconductor laser diode emits light having a diameter of about 2 mm and a Gaussian energy distribution to the light diffuser 140.

[0124] Figure 5 is a diagram illustrating a cross-section of light output from the expander 10 according to an embodiment. As Figure 5 indicated, light incident from a semiconductor laser diode can pass through the light diffuser 140, the first optical element 110, the waveguide 130, and the second optical element 120, and be converted into a light beam having a square cross-section of about 5x5 mm 2 The light output from the expander 10 can have a uniformity of 80% or more.

[0125] In an embodiment, the first optical element 110 and the second optical element 120 can be manufactured in the form of a volume Bragg grating, the waveguide 130 can be a flat glass plate, and a microlens array having a focal length of about 500 µm, a period of about 500 µm, and a lens size of 20x20 can be used as the light diffuser 140. In this embodiment, a collimated light beam having a diameter of about 2 mm and a Gaussian energy distribution from a semiconductor laser diode can also be converted into a light beam having a square cross-section of about 5x5 mm 2a uniform beam (80% or more) with a square cross-section of about 5x5 mm

[0126] In an embodiment, the first optical element 110 and the second optical element 120 can be fabricated in the form of volume Bragg gratings, the waveguide 130 can be a flat glass plate, and the light diffuser 140 can be integrated into the first optical element 110 in a recording operation and use a matte diffuser with an angular aperture of about 5°. In this embodiment, a collimated beam from a semiconductor laser diode with a diameter of about 2 mm and a Gaussian energy distribution can be converted into a uniform beam (80% or more) with a square cross-section of about 5x5 mm 2 In an embodiment, the matte diffuser does not need to be perfectly matched to the first optical element 110. In the light output from the expander 10, noise in the form of a halo / halo light can be reduced.

[0127] The light output from the second optical element 120 can be transmitted to an external device (such as a diverter, such as a lens or a mirror), but the external device is not limited thereto. In an embodiment, the light out-coupled in the second optical element 120 can directly illuminate a screen, a liquid crystal display (LCD) panel, or an imaging matrix.

[0128] The expander 10 and the method of operating the expander 10 according to an embodiment can be used as an illumination element for forming a holographic and / or another three-dimensional image in various application fields. As non-limiting examples, the expander 10 can be used in various display devices, such as monitors and televisions. The example embodiments can be used to form images in augmented or virtual reality (AR / VR) systems for various applications, head-mounted display devices, indicators on vehicle windshields, systems for projecting information on vehicle windshields, and fingerprint scanners. These application fields of the example embodiments are for illustrative purposes only and do not limit the scope of the example embodiments. Numerous other uses of the example embodiments will be apparent to those skilled in the art

[0129] The expander 10 has been described with reference to the example embodiments shown in the drawings. However, this is merely an example and those of ordinary skill in the art can understand that various modifications and other equivalent embodiments can be made.

[0130] It should be understood that the embodiments described herein should be considered in a descriptive sense only and not for purposes of limitation. Descriptions of features or aspects within each embodiment should typically be considered as available for other similar features or aspects in other embodiments. While one or more embodiments have been described with reference to the figures, it will be apparent to those with ordinary skill in the art that various changes and modifications can be made therein without departing from the spirit and scope of the appended claims and their equivalents.

[0131] This application is based on and claims priority to Russian Patent Application No. 2020124347, filed on July 22, 2020, in the Russian Patent Office, and Korean Patent Application No. 10-2021-0014402, filed on February 1, 2021, in the Korean Intellectual Property Office, the disclosures of which are incorporated herein in their entireties by reference.

Claims

1. An expander comprising: a first optical element; a second optical element spaced apart from the first optical element; a waveguide connecting the first optical element and the second optical element; and a light diffuser having an angular aperture through which the incident light is diffused, wherein the first optical element in-couples the diffused light such that the light exiting from the first optical element has a first cross-sectional shape that is circular or elliptical, and light having a second cross-sectional shape that is polygonal and different from and smaller than the first cross-sectional shape is incident on the second optical element, and wherein the second optical element out-couples the light incident from the first optical element, wherein the waveguide is configured to gradually convert the light having the first cross-sectional shape from the first optical element into light having the second cross-sectional shape by total internal reflection, and guide the converted light to the second optical element. 2.The expander of claim 1, wherein the angular aperture of the light diffuser is greater than 0° and equal to or smaller than 5°. 3.The expander of claim 1, wherein an intensity of the light diffused by the light diffuser has greater uniformity over a light cross-section than an intensity of the incident light. 4.The expander of claim 1, wherein the light diffuser outputs the incident light as a plurality of sub-lights that are spatially separated from each other. 5.The expander of claim 1, wherein the light diffuser modulates a phase of the incident light and outputs the incident light as a plurality of sub-lights. 6.The expander of claim 1, wherein the light diffuser performs spatially non-uniform phase modulation on the incident light and outputs a plurality of sub-lights. 7.The expander of claim 1, wherein the light diffuser comprises at least one of a microlens grating, a phase mask, a matte diffuser, a diffractive optical element, and a holographic optical element. 8.The expander of claim 1, wherein the incident light has an optical cross-sectional profile of at least one of a Gaussian beam, a multi-mode beam, a cylindrical beam, a super-Gaussian beam, and a Laguerre-Gaussian beam. 9.The expander of claim 1, wherein at least one of the first optical element and the second optical element comprises at least one of a diffractive optical element and a holographic optical element. 10.The expander of claim 1, wherein the light diffuser and the first optical element are arranged to overlap each other in a direction perpendicular to a length direction of the waveguide. 11.The expander of claim 1, wherein the light diffuser is arranged on a first surface of the waveguide, and wherein the first optical element is arranged on a second surface of the waveguide opposite to the first surface. 12.The expander of claim 1, wherein the light diffuser is integrated with the first optical element. 13.The expander of claim 1, further comprising a laser diode that provides the incident light. ​ 14. The beam expander of claim 1, wherein the second optical element collimates light incident on the second optical element with a divergence or convergence angle less than 0.25°.

15. The beam expander of claim 1, wherein a uniformity of intensity of light out- coupled by the second optical element is greater than a uniformity of intensity of light incident on the light diffuser.

16. A method of operating a beam expander, the method comprising: diffusing incident light with a light diffuser; changing the diffused light having a first cross-sectional shape that is circular or elliptical to light having a second cross-sectional shape that is polygonal and different from and smaller than the first cross-sectional shape while propagating the diffused light through a waveguide; and collimating the light having the second cross-sectional shape and uniformity with an optical element, wherein the waveguide is configured to gradually convert light having the first cross-sectional shape to light having the second cross-sectional shape by total internal reflection.

17. A beam expander comprising: a waveguide; an input optical element that receives incident light and outputs first light having a first cross-sectional shape that is circular or elliptical to the waveguide; and an output optical element that receives light transmitted by the waveguide and outputs second light having a second cross-sectional shape that is polygonal and different from and smaller than the first cross-sectional shape, wherein the waveguide is configured to gradually convert light having the first cross-sectional shape from the input optical element to light having the second cross-sectional shape by total internal reflection and direct the converted light to the output optical element. ​

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