Laser homogenizer based on light pipe integration and micro optics

Abstract

The invention relates to laser beam shaping, intended to achieve a controlled intensity distribution, typically a flat-topped distribution, by means of a plurality of laser beam sources having a non-uniform beam intensity distribution (incident Gaussian laser beam in the visible, infrared or ultraviolet spectrum or other higher-order multi-transverse mode). The laser beam homogenization-expander uses a primary beam shaper element or light pipe (light integrator), and a second beam shaper unit or microlens array (MLA), i.e., a second light integrator defining a flat-topped intensity distribution output, to provide a second homogenization level. The invention also includes several optical devices, such as a light diffuser, a mirror, a dichroic filter, a beam shaping lens, a light pipe, and a microlens, to achieve good homogenization at the desired output angle. The invention is designed as a laser source that fully integrates / mixes one or more wavelengths, such as red, green and blue, but can also integrate other wavelengths in the visible / invisible spectrum. If multiple wavelengths are used as the initial laser source, the light engine output can provide good homogenization and color mixing with little noticeable speckles, near flat-topped intensity distribution, and also no visible hot spots or interference patterns. The invention can be used in many applications, such as microscopes, material processing, fluorescence, lighting devices, holography, fiber coupling, and the like, which require the spectral characteristics (narrowband, monochromaticity) of the laser, but at the same time require a uniform distribution of the intensity.

Description

TECHNICAL FIELD

[0001] The present invention is in the field of laser beam shaping and relates to a laser emitting source suitable for spectroscopy, material processing, fluorescence, heat treatment, communication, entertainment and many other applications. BACKGROUND

[0002] Most laser types in use today have output beam profiles with circular or elliptical cross sections with Gaussian or near-Gaussian intensity profile distribution. This is acceptable and often advantageous for many applications where the laser beam is focused to a small spot. However, there are also many different applications where a uniform, homogenized intensity distribution (“top hat”) is required. For example, in material processing applications, a uniform intensity distribution ensures that the entire laser spot is processed uniformly. This distribution is also valuable in cases where the laser is used primarily for illumination, as uniform illumination results in identical features regardless of their position in the illuminated field, which simplifies image processing tasks, improves contrast and resolution.

[0003] There are several techniques and methods to achieve a uniform distribution pattern. Low-cost laser-based devices often use a Gaussian beam source that can be physically cut by an aperture to form a pseudo-flat top distribution.

[0004] This approach is inefficient and wastes a significant amount of energy in the outer regions of the Gaussian distribution, but this engineering approach helps to minimize the complexity and overall cost of the device.

[0005] High-performance applications that require higher efficiency often use different types of refractive beam shapers.

[0006] These systems often use field-mapping phase elements such as aspherical lenses and diffractive elements to simultaneously redistribute the irradiance and phase distribution.

[0007] The amplitude and phase of the incident beam are changed after passing through refractive optical elements such as micro-optical or surface-engineered diffusers.

[0008] The output beam has high efficiency and is wavelength-independent over a range of optical setups.

[0009] Refractive beam shapers provide a uniform irradiance distribution and a flat phase wavefront.

[0010] A laser beam integrator or homogenizer is an optical element that can be formed from a light pipe (also known as a homogenizer / integrator rod, light guide, or waveguide). Through a total internal reflection (TIR) process, the light pipe can convert the output of a highly non-uniform light source into a highly uniform or homogenized illumination.

[0011] The laser beam integrator or homogenizer can be used for coherent laser sources as well as for other non-coherent light sources.

[0012] Most laser beam integrators are used to generate a uniform flat-top distribution profile from an incident Gaussian laser beam or other higher order multi-transverse mode. SUMMARY

[0013] The present invention is based on a light pipe as the primary integrator, however, due to a second order integrator based on refraction (Kohler ) integrator, as well as a custom engineered surface diffuser, and the overall optical setup and optical mount, it has a unique optical setup.

[0014] The primary integrator system consists of a light pipe, but an engineered surface diffuser can also be considered as an alternative solution, which is equally capable of delivering good results.

[0015] According to the total internal reflection (TIR) principle, light from a non-uniform laser source is collected in the focused coupling region and provided with uniform irradiance in the Fourier plane of the Fourier lens.

[0016] The outer illumination area is the superposition of these individual and overall internal multiple reflections, providing a sharp cutoff of the flat-top or flat beam profile known in the art. When a diffuser is added in front of the light pipe entrance face, the uniformity is improved.

[0017] As a second stage of the present invention, a second integrator based on a microlens array is added downstream to further improve the uniformity.

[0018] To eliminate potential hot spots and achieve a more uniform distribution profile, the beam can be diffused with a holographic refractive diffuser with a custom engineered surface that provides the required exit angle with a transmittance greater than 97%.

[0019] The optical behavior and mechanical characteristics of the light pipe, microlens, and engineered diffuser are also custom designed and carefully tuned to achieve superior beam homogenization performance while minimizing light loss.

[0020] The purpose of the present invention is to achieve a controlled, typically flat-top intensity distribution based on a non-uniform intensity laser beam source (supporting an incident Gaussian laser beam or other higher order multi-transverse mode in the visible, infrared, or ultraviolet spectrum).

[0021] The laser beam homogenization-beam expander uses a primary beam shaping element or light pipe (light integrator) and a second beam shaping element or microlens array (MLA), i.e., a second light integrator, which defines a flat-top intensity distribution output, thus providing a second homogenization stage.

[0022] The present invention also includes various light diffusers, mirrors, filters, and output lenses.

[0023] The present invention provides very precise beam shaping performance with minimal light loss. Specifically, it achieves good homogenization at the desired output angle by fully integrating / mixing one or more laser source wavelengths (such as red, green, blue and other wavelengths in the visible / non-visible spectrum).

[0024] When using multiple wavelengths as the initial laser source, the output of the light engine provides good homogenization and color mixing with little to no visible speckle, near top-hat intensity distribution, and no visible hot spots or interference patterns. BRIEF DESCRIPTION OF DRAWINGS

[0025] Figure 1 is a perspective view of a light engine according to the present invention.

[0026] Figure 2 is a side view of a light engine according to the present invention.

[0027] Figure 3 is a schematic diagram showing all beam path settings of all optical elements and laser sources.

[0028] Figure 4 is a schematic diagram of another beam path setting, including another example of all optical elements and RGB laser sources. DETAILED DESCRIPTION

[0029] The present invention relates to a laser homogenizing-beam expander for generating a uniform laser output beam from one or more incident laser sources.

[0030] The laser homogenizing-beam expander includes various optical components arranged along a beam path for shaping and homogenizing an incident laser beam to achieve an output laser beam with a top-hat intensity distribution suitable for applications in spectroscopy, material processing, fluorescence, heat treatment, communication, and entertainment.

[0031] Figure 1 and Figure 2 An overview of the complete lighting device (light engine) according to the present invention and the cooling system (18, 19) required to maintain all laser sources at the desired operating temperature is provided.

[0032] The cooling system is based on a capillary heat pipe cooler consisting of an actively forced air-cooled heat sink and a copper heat pipe embedded in the heat sink.

[0033] The capillary heat pipe allows for efficient internal liquid phase change in any direction, which is desirable depending on the application requirements.

[0034] The output window (17) contains an optical diffuser and a wide-angle aspheric condenser lens. The output window (17) is centered along the optical axis, this arrangement allows to improve the convenience according to the application, especially in cases where more output optics are required.

[0035] Figure 3 is a schematic structural diagram showing the beam path arrangement including all optical elements and laser light sources.

[0036] The light emitter is composed of four laser light sources (1a, 1b, 2, 3), each of which is composed of several laser diode arrays.

[0037] Each light emitter (1a, 1b, 2, 3) comprises a multi-chip package (MDP) composed of a packaged laser die emitter and a corresponding short focal collimator lens array.

[0038] The red laser color MDPs (1a, 1b) have a central emission wavelength of about 638 nm, each MDP is composed of 28 laser emitters arranged in a 7 by 4 matrix.

[0039] Both red MDPs exhibit vertical polarization (S) and are combined by a polarizing beam splitter (PBS) (5) and a mirror (4) on the optical axis, the mirror reflects the light beams from the first MDP (1a) to the PBS (5) to combine the P-polarized light and the S-polarized light.

[0040] The red MDPs located behind the PBS (5) include a half-wave plate in front of the collimator array for aligning the S-polarized component with the P-polarized component to make the PBS work properly.

[0041] The green MDP (2) has a central emission wavelength of about 525 nm and is also composed of 28 laser emitters arranged in a 7 by 4 matrix.

[0042] The blue MDP (3) has a central emission wavelength of about 455 nm and is composed of 14 laser emitters arranged in a 7 by 2 matrix.

[0043] The mirror (4a) reflects the 28 green light beams at a right angle to a dichroic filter (6) that reflects the blue light while allowing the green and red light to pass (B / GR).

[0044] The light beam array from the green MDP passes through the dichroic filter (6) and is reflected to the mirror (4b) located in front of the dichroic filter (6).

[0045] The light beam array from the blue MDP (3) is reflected by the dichroic filter (6) at an angle of 90 degrees, thus also being directed to the mirror (4b).

[0046] At mirror (4b), the array of overlapping beams from green MDP (2) and blue MDP (3) presents a color that can be perceived as cyan. Non-coherent beams on the spectrum are effectively combined together.

[0047] Another dichroic filter (7) reflects green and blue light while allowing red light to pass (GB / R). Red beams output from PBS (5) pass through this dichroic filter (7) to reach PCX focusing lens (8).

[0048] Green and blue beams resulting from the combination of each MDP (2, 3) are reflected perpendicularly by another mirror (4b) downstream of dichroic filter (7), which can selectively reflect the beams perpendicularly to focusing lens (8).

[0049] At focusing lens (8), the spectrally non-coherent RGB beams resulting from all MDPs (1a, 1b, 2, 3) are finally combined.

[0050] In this example, the laser sources are based on MDPs. It is worth noting that alternative embodiments can apply different types of packaged laser array sources, single TO-CAN laser diodes, optically pumped or other semiconductor lasers, solid-state pumped lasers, gas lasers, groups of lasers, or even non-coherent light such as laser-pumped phosphor variants or light-emitting diodes. All light sources should exhibit a high degree of collimation in order to function properly within the system.

[0051] Downstream of focusing lens (8), the combined RGB beams are reflected upwards to another mirror (10) and pass through beam-shaping lens (9).

[0052] The aspherical short focal length lens focuses the RGB beams from mirror (10) into light pipe (12) through a holographic diffuser (16a), the purpose of which is to "break" the spatial coherence inherent to laser radiation to some extent, thus creating multiple light rays (small beams). This improves the homogenization effect within light pipe (12) by increasing the number of reflections caused by the total internal reflection phenomenon (TIR) within the light pipe.

[0053] Light pipe (12) has a hexagonal cross-section and is composed of fused silica glass, both faces of which are coated with a broadband anti-reflective (BBAR) coating to minimize reflection losses upon beam incidence.

[0054] The angles and numerical apertures (NA) of focusing lenses (8, 9, 11a) and light pipe (12) are calculated according to Snell's law to achieve complete total internal reflection (TIR) within light pipe (12).

[0055] The initial RGB beam homogenization is effectively obtained after the light pipe (first integrator) and the output of the light pipe is collimated by an aspheric lens (11b) acting as a condenser lens.

[0056] Downstream of the condenser lens (11b), an achromatic PCX lens (13) acts as a relay beam shaping lens. The telescope formed by the two long focal length PCX lenses, namely the Fourier lens (14a) and the imaging lens (14b), transmits the light to a second optical or "Kohler" integrator formed by a dual microlens array (15).

[0057] The imaging lens (14b) directs the output of the light pipe (12) to two micro lenses (15) through zigzag mirrors (4d, 4e), and further directs it upward through a reflecting mirror (4f).

[0058] To avoid overfilling of the lens aperture and the consequent loss of brightness, the diameter of the individual beamlets at the downstream microlens array (15) is lower than its lens pitch. Otherwise, such overfilling of the lens array could lead to undesirable double images in the Fourier plane.

[0059] Downstream of the optical integrator, another field lens in the form of a condenser lens (11c) is used to shape the output beam into an inverted cone with a divergence angle θ, the desired angle typically depending on the envisaged application or integration with any downstream optical devices. The focal length of the integrator does not have a decisive influence other than achieving the stated divergence angle θ.

[0060] A second holographic diffuser (16b) with a custom engineered surface reduces any hot spots, speckles or interference that may remain in the beam when passing through the "second integrator" (15), further improving the mixing of all RGB wavelengths and the distribution profile of the final output beam.

[0061] exist Figure 4 In the variation shown, the optical elements again include dichroic mirrors (24, 25) for combining red (21), green (23) and blue (22) laser light from corresponding laser chip arrays; a dielectric filter (26); a plano-convex lens (27) for focusing the combined laser light into a hexagonal light pipe homogenizer rod that provides a first stage of homogenization by total internal reflection; a plano-convex achromatic relay lens (30) for collimating the homogenized light exiting the light pipe; a Keplerian 1:1 telescope (31); a microlens array pair (32) for providing a second stage of homogenization; an aspheric condenser lens (33) for focusing the further homogenized light; and a holographic diffuser (34) for diffusing the focused light to achieve a flat-top intensity distribution output beam.

[0062] A dichroic mirror (24, 25) combines the individual red, green and blue laser wavelengths into a single multi-wavelength beam. A dielectric filter (26) ensures high transmission of the combined beam. A plano-convex lens (27) focuses the beam into the entrance face of a light pipe (12), where the focused light undergoes multiple total internal reflections to achieve the first stage of homogenization. The homogenized light exiting the light pipe (12) is collimated by a plano-convex achromatic relay lens (30). A Keplerian telescope (31) provides unit magnification of the collimated beam. A pair of microlens arrays (32) act on the collimated beam to provide the second stage of homogenization. An aspheric condenser lens (33) focuses the further homogenized collimated beam and a holographic diffuser (34) diffuses the focused light to achieve a flat-top intensity distribution output laser beam.

[0063] Additional components can be included, such as a holographic diffuser (28) between the plano-convex lens (27) and the light pipe (12) to improve homogeneity. All optical elements are carefully designed, arranged and adjusted to maximize transmission efficiency while minimizing light loss, speckle, interference patterns and non-uniformity. The optical elements can be mounted in a housing with opto-mechanical components to achieve stable alignment.

[0064] According to Figure 4 An exemplary laser homogenization-beam expander has a hexagonal cross-section light pipe homogenizer rod (12) made of fused silica. Three separate arrays of semiconductor laser chips (1, 2, 3) emit red (638 nm), green (525 nm) and blue (455 nm) laser light, respectively.

[0065] The red and green beams are combined together using a long-pass dichroic mirror (24) that reflects the red light at 638 nm at 45° and transmits the green light at 525 nm at 0°. The blue beam is then added using a short-pass dichroic mirror (25) that reflects the blue light at 455 nm at 45° and transmits the combined red and green beams at 0°. A dielectric filter (26) ensures high transmission of the combined RGB beam.

[0066] The combined beam is focused by a plano-convex lens (27) through a holographic diffuser (28) into the entrance face of the light pipe (12). Total internal reflections in the light pipe (12) highly homogenize the beam. The homogenized output beam from the light pipe (12) is collimated by a plano-convex achromatic lens (30). A Keplerian 1:1 telescope (31) provides unit magnification of the collimated beam.

[0067] A pair of 1 mm spaced microlens arrays (32) further improve homogeneity. An aspheric condenser lens (33) focuses the beam over a length of 50 mm and an engineered holographic diffuser (34) provides the final diffusion to achieve a flat-top distributed output beam of high uniformity suitable for entertainment laser projection applications.

[0068] The optical elements are arranged in a housing on an opto-mechanical mount for stabilizing the alignment of the beam path. The resulting output is a homogenized multi-wavelength laser beam without significant speckle and with intensity variations across the multi-wavelength laser beam individually less than 5%.

[0069] Industrial applicability

[0070] The present invention is applicable to the entire entertainment industry. Its benefits further apply to a wide range of other applications, which can be broadly categorized as "lighting", including machine vision, detection, entertainment special effects (FX), medical uses, etc. Specifically, the present invention can be used in applications such as microscopy, material processing, fluorescence, lighting devices, holography, fiber coupling, etc., which require the spectral properties of a laser (narrowband, monochromaticity), but at the same time require a uniform intensity distribution.

[0071] List of reference signs

[0072] Reference signs Description 1a, 1b, 2, 3 MDP laser source 4, 4a, 4b, 4c, 4d, 4e, 4f, 10 Dielectric >95% (400nm-700nm) mirror 6、7 Dichroic filter 5 Polarizing beam splitter 15 Microlens array 12 Light pipe 8, 9, 11a, 11b, 13, 14a, 14b, 11c Lens 16a, 16b Engineered surface diffuser Theta Output divergence angle 21 Multi-chip package red laser chip array 22 Multi-chip package blue laser chip array 23 Multi-chip package green laser chip array 24 Dichroic R / GB 25 Dichroic B / GR 26 Dielectric full spectrum mirror 27 Plano-convex lens (light pipe coupling) 28 First holographic diffuser 30 Plano-convex achromatic relay lens 31 Kepler 1 : 1 telescope 32 Kohler integrator 33 Aspheric condenser lens 34 Second holographic diffuser

Claims

1. A laser beam homogenizer-beam expander device, comprising: a light pipe (12) for receiving and homogenizing light from a laser source; a microlens array (32) for further homogenizing the light output from the light pipe (12); as well as A holographic diffuser (8, 14) for diffusing light output from the microlens array (32), wherein the light pipe (12) and the microlens array (32) are configured to create a flat-top intensity distribution of homogenized light.

2. The apparatus according to claim 1, wherein the light pipe (12) is a hexagonal light pipe homogenizing rod.

3. The device according to claim 1 or 2, further comprising a plano-convex lens (27) for coupling light into the light pipe (12).

4. The device according to any of the preceding claims, wherein the microlens array (32) forms a Kohler integrator.

5. A device according to any one of the preceding claims, wherein the holographic diffuser (8, 14) has a surface formed to provide a given exit angle and preferably has a given transmission higher than 97%.

6. The apparatus of any preceding claim, further comprising a plano-convex achromatic relay lens (30) positioned to receive light output from the light pipe (12).

7. Apparatus according to any preceding claim, further comprising a Keplerian 1:1 telescope (31) positioned to relay light output from the microlens array (32).

8. Apparatus according to any preceding claim, further comprising an aspherical condenser lens (33) positioned to focus light output from the holographic diffuser (34).

9. The device according to any of the preceding claims, wherein the laser light source comprises a red laser chip array (21), a blue laser chip array (22) and a green laser chip array (23).

10. The apparatus according to claim 9, further comprising: a dichroic mirror (24) configured to combine light from the red laser chip array (21) and the blue and green laser chip arrays (2, 3); and A dielectric full spectrum mirror (26) is positioned to reflect the light combined by the dichroic mirror (24).

11. The apparatus of claim 10, further comprising a second dichroic mirror (25) configured to combine light from the blue laser chip array (22) with light from the green laser chip array (23).

12. The apparatus of any one of claims 9 to 11, wherein the laser source is configured to emit light in the visible, infrared or ultraviolet spectrum.

13. A device according to any preceding claim, designed for use in an application selected from the group consisting of: spectroscopy, materials processing, fluorescence, heat management, communications and entertainment.

14. Apparatus according to any one of the preceding claims, wherein the light pipe (12) consists of an alternative engineered surface for achieving homogenisation.

15. The apparatus of any preceding claim, wherein the laser source is configured to mix multiple wavelengths to provide color mixing within the output of the light engine.

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