Patent Publication Number: US-2021172582-A1

Title: Rgb illuminator system having curved surfaces

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application is a continuation of International Patent Application No. PCT2019/037757, filed Jun. 18, 2019, entitled “RGB ILLUMINATOR SYSTEM HAVING CURVED SURFACES,” which claims the benefit of and priority to U.S. Provisional Patent Application No. 62/686,474, filed Jun. 18, 2018, entitled “RGB ILLUMINATOR SYSTEM HAVING CURVED SURFACES,” the contents of which are hereby incorporated by reference in their entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     Modern computing and display technologies have facilitated the development of systems for so-called “virtual reality” or “augmented reality” experiences, where digitally produced images are presented in a wearable device to a user in a manner where they seem to be, or may be perceived as, real. A virtual reality, or “VR,” scenario typically involves presentation of digital or virtual image information without transparency to other actual real-world visual input; an augmented reality, or “AR,” scenario typically involves presentation of digital or virtual image information as an augmentation to visualization of the actual world around the user. 
     The wearable device may include augmented and/or virtual reality glasses. The image can be displayed using image frames or raster scanned images. In these display devices, optical combiners are often used to combine light beams from different light sources. For example, red, green, and blue light beams can be combined to form a collimated combined colored light beam for displaying an image. Conventional optical combiners made with glass lenses, mirrors, and prisms can be bulky and heavy. They are undesirable for wearable device in VR or AR applications. 
     Therefore, improved structures and methods for optical combiners are highly desirable. 
     SUMMARY OF THE INVENTION 
     Embodiments of the present invention provide methods and systems for RGB (Red Green Blue) illuminators that are compact and light weight compared with conventional systems. In some embodiments, a single curved reflective element is used in conjunction with a cylindrical lens to provide collimated illumination having a plurality of wavelengths. As compared with a system that can utilize a plurality of right angle prisms, embodiments of the invention have a reflective element that is not only a single reflective surface for all channels of a laser package that provide light beams of different wavelengths or colors, but further has a curved surface as opposed to a linear surface or a surface to reflect each channel at a common angle (e.g., 90 degrees). Such a configuration enables smaller beam diameters per channel as compared to a linear reflector, thereby further enabling a single cylinder lens per channel to deliver the light, as opposed to alternative optical lenses necessary to support wider beam diameters that in turn may produce aberrations requiring additional optical components, resulting in large and complex optical systems. 
     According to some embodiments of the invention, an optical device includes two or more light sources configured for providing input light beams having different wavelengths, a curved reflective element configured to receive the input light beams to provide two or more reflected light beams, and a cylindrical lens configured to transmit a respective one of the reflected light beams to provide a corresponding collimated output light beam. 
     In some embodiments, each of the input light beams is configured to impinge on the curved reflective element in different regions. 
     In some embodiments, the curved reflective element is configured to have discrete prescriptions in incidence locations on the curved reflective element for each of the input light beams. 
     In some embodiments, the optical device can include a first light source configured to emit a first input light beam, a second light source configured to emit a second input light beam, and a third light source configured to emit a third input light beam. The curved reflective element is configured to receive the first input light beam and provide a first reflected light beam, receive the second input light beam and provide a second reflected light beam, and receive the third input light beam and provide a third reflected light beam. A first cylindrical lens is disposed in a first light path of the first reflected light beam to receive the first reflected light beam and provide a first collimated output light beam. A second cylindrical lens is disposed in a second light path of the second reflected light beam to receive the second reflected light beam and provide a second collimated output light beam. A third cylindrical lens is disposed in a third light path of the third reflected light beam to receive the third reflected light beam and provide a third collimated output light beam. 
     In some embodiments, the first light source is a red laser light source, the second light source is a green laser light source, and the third light source is a blue laser light source. 
     In some embodiments, the first input light beam, the second input light beam, and the third input light beam are configured to impinge on or impact the curved reflective element in different regions. 
     In some embodiments, the curved reflective element has a single prescription in different regions. 
     In some embodiments, the curved reflective element is configured to have discrete prescriptions in incidence locations on the curved reflective element for the first input light beam, the second input light beam, and the third input light beam, respectively. 
     In some embodiments, the curved reflective element has a parabolic surface contour. 
     In some embodiments, the curved reflective element has an elliptical surface contour. 
     In some embodiments, the curved reflective element has a hyperbolic surface contour. 
     In some embodiments, the curved reflective element has a prescription described by: 
         z=cy   2 /1+√{square root over (1−(1 +k ) c   2   y   2 )}+α 1   y   2 +α 2   y   4 ++α 3   y   6 +α 4   y   8 +α 5   y   10 +α 6   y   12  
 
     wherein c is a curvature of a base radius, k is a conic constant (for parabola=−1), and coefficients α are polynomial aspheric coefficients. 
     In some embodiments, the curved reflective element has a uniform surface finish. 
     In some embodiments, the curved reflective element has a discrete surface finishing in each region of the curved reflective element. 
     According to some embodiments, a method for generating illumination having a plurality of wavelengths is provided. The method includes providing a curved reflective element, providing two or more cylindrical lenses, receiving input light beams having different wavelengths from two or more light sources at the curved reflective element to provide two or more reflected light beams, and transmitting each of the two or more reflected light beams through a respective cylindrical lens to provide a corresponding collimated output light beam. 
     In some embodiments, each of the input light beams is configured to impinge on the curved reflective element in different regions. 
     In some embodiments, the method includes directing each of the input light beams to impinge on the curved reflective element in different regions. 
     In some embodiments, the method includes receiving a first input light beam from a first light source, using the curved reflective element, and providing a first reflected light beam. The method also includes receiving a second input light beam from a second light source using the curved reflective element, and providing a second reflected light beam. The method also includes receiving a third input light beam, a third light source using the curved reflective element, and providing a third reflected light beam. The method includes transmitting the first reflected light beam through a first cylindrical lens to provide a first collimated output light beam, transmitting the second reflected light beam through a second cylindrical lens to provide a second collimated output light beam, and transmitting the third reflected light beam through a third cylindrical lens to provide a third collimated output light beam. 
     In some embodiments, the method includes emitting a red light beam from the first light source, emitting a green light beam from the second light source, and emitting a blue light beam from the third light source. 
     In some embodiments, the method also includes directing the first input light beam, the second input light beam, and the third input light beam to impinge on or impact the curved reflective element in different regions. 
     In some embodiments, the method also includes configuring the curved reflective element to have a single prescription in different regions. 
     In some embodiments, the method also includes configuring the curved reflective element to have discrete prescriptions in incidence locations on the curved reflective element for the first input light beam, the second input light beam, and the third input light beam, respectively. 
     In some embodiments, the curved reflective element has an elliptical surface contour. 
     In some embodiments, the curved reflective element has a parabolic surface contour. 
     In some embodiments, the curved reflective element has a prescription described by: 
         z=cy   2 /1+√{square root over (1−(1 +k ) c   2   y   2 )}+α 1   y   2 +α 2   y   4 ++α 3   y   6 +α 4   y   8 +α 5   y   10 +α 6   y   12  
 
     wherein c is a curvature of a base radius, k is a conic constant (for parabola=−1), and coefficients α are polynomial aspheric coefficients. 
     In some embodiments, the method also includes applying a uniform surface finish to the curved reflective element. 
     In some embodiments, the method also includes applying a discrete surface finishing in each region of the curved reflective element. 
     Additional features, benefits, and embodiments are described below in the detailed description, figures, and claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a simplified block diagram illustrating an optical system according to some embodiments of the present invention; 
         FIG. 2  is a simplified schematic diagram illustrating a planar waveguide according to some embodiments of the present invention; 
         FIG. 3A  is a simplified diagram illustrating a perspective view of an optical device according to some embodiments of the present invention; 
         FIG. 3B  is a simplified diagram illustrating a side view of the optical device of  FIG. 3A  according to some embodiments of the present invention; 
         FIG. 4A  is a simplified diagram illustrating a timing diagram illustrating timing cycles of different light sources providing light beams of different wavelengths according to some embodiments of the present invention; 
         FIG. 4B  is a simplified diagram illustrating a timing diagram illustrating timing cycles of different light sources providing light beams of different wavelengths according to some alternative embodiments of the present invention; 
         FIG. 5  is a simplified diagram illustrating an example of power distribution of different light sources providing light beams of different wavelengths according to embodiments of the present invention; 
         FIG. 6  is a simplified schematic diagram illustrating a top cross-sectional view of optical device  300  of  FIG. 3  according to some embodiments of the present invention; 
         FIG. 7A  is a simplified diagram illustrating an exploded view of an optical device according to some embodiments of the present invention; 
         FIG. 7B  is a simplified diagram illustrating a perspective view of a laser package according to some embodiments of the present invention; 
         FIG. 8A  is a simplified diagram illustrating an exploded view of an optical device according to some alternative embodiments of the present invention; 
         FIG. 8B  is a simplified diagram illustrating a perspective view of a laser package according to some embodiments of the present invention; 
         FIG. 8C  is a simplified diagram illustrating a cross-sectional view of laser package  802  in  FIG. 8B  along Section A-A′ according to some embodiments of the present invention; 
         FIG. 8D  is a simplified diagram illustrating a cross-sectional view of laser package  802  in  FIG. 8B  along Section B-B′ according to some embodiments of the present invention; and 
         FIG. 9  is a flowchart illustrating a method for generating illumination having a plurality of wavelengths according to some embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS 
     Embodiments of the present invention are directed to optical illuminators for providing light beams from multiple light sources for a wearable device. 
     In some embodiments, the light patterns generated by the illuminator described herein are coupled into one or more planar waveguides having a structure similar to that proposed for use in augmented reality. A description of a device for creating an augmented reality using such waveguides can be found in U.S. patent application Ser. No. 15/146,296, having a priority date of May 4, 2015 and U.S. patent application Ser. No. 14/331,218, having a priority date of Jul. 12, 2013, the disclosures of each of which are hereby incorporated by reference in their entirety. 
       FIG. 1  a simplified block diagram illustrating an optical system according to some embodiments of the present invention. As shown in  FIG. 1 , an optical system  100  can include a waveguide apparatus  102  that includes a planar waveguide  1 . The planar waveguide is provided with one or more diffractive optical elements (DOEs)  2  for controlling the total internal reflection of the light within the planar waveguide. The optical system further includes an optical coupler subsystem  104  and a control subsystem  106 , as described further below. 
       FIG. 2  is a simplified schematic diagram illustrating a planar waveguide according to some embodiments of the present invention. As illustrated in  FIG. 2 , the planar waveguide  1  has a first end  108   a  and a second end  108   b,  the second end  108   b  opposed to the first end  108   a  along a length  110  of the planar waveguide  1 . The planar waveguide  1  has a first face  112   a  and a second face  112   b,  at least the first and the second faces  112   a,    112   b  (collectively,  112 ) forming a partially internally reflective optical path (illustrated by arrow  114   a  and broken line arrow  114   b,  collectively,  114 ) along at least a portion of the length  110  of the planar waveguide  1 . 
     The planar waveguide  1  may take a variety of forms that provide for substantially total internal reflection (TIR) for light striking the first face  112   a  and the second face  112   b  at less than a defined critical angle. The planar waveguide  1  may, for example, take the form of a pane or plane of glass, fused silica, acrylic, or polycarbonate. 
     The DOE  2  (illustrated in  FIGS. 1 and 2  by dash-dot double line) may take a large variety of forms which interrupt the TIR optical path  114 , providing a plurality of optical paths (illustrated by arrows  116   a  and broken line arrows  116   b,  collectively,  116 ) between an interior  118  and an exterior  120  of the planar waveguide  1  extending along at least a portion of the length  110  of the planar waveguide  1 . The DOE  2  may advantageously combine the phase functions of a linear diffraction grating with that of a circular or radial symmetric zone plate, allowing positioning of apparent objects and a focus plane for apparent objects. The DOE may be formed on the surface of the waveguide or in the interior thereof. 
     With reference to  FIG. 1 , the optical coupler subsystem  104  optically couples light to the waveguide apparatus  102 . Alternatively, the light may be coupled directly into the second end  108   b  of the waveguide if the coupler is not used. As illustrated in  FIG. 1 , the optical coupler subsystem may include an optical element  5 , for instance a reflective surface, mirror, dichroic mirror, or prism to optically couple light into an edge  122  of the planar waveguide  1 . The light can also be coupled into the waveguide apparatus through either the front or back faces. The optical coupler subsystem  104  may additionally or alternatively include a collimation element  6  that collimates light. 
     The control subsystem  106  includes one or more light sources and drive electronics that generate image data which may be encoded in the form of light that is spatially and/or temporally varying. As noted above, a collimation element  6  may collimate the light, and the collimated light is optically coupled into one or more planar waveguides  1  (only one waveguide is illustrated in  FIGS. 1 and 2 ). 
     As illustrated in  FIG. 2 , the light propagates along the planar waveguide with at least some reflections or “bounces” resulting from the TIR propagation. It is noted that some implementations may employ one or more reflectors in the internal optical path, for instance thin-films, dielectric coatings, metalized coatings, etc., which may facilitate reflection. Light that propagates along the length  110  of the waveguide  1  intersects with the DOE  2  at various positions along the length  110 . The DOE  2  may be incorporated within the planar waveguide  1  or abutting or adjacent to one or more of the faces  112  of the planar waveguide  1 . The DOE  2  accomplishes at least two functions. First, the DOE  2  shifts an angle of the light, causing a portion of the light to escape TIR, and emerge from the interior  118  to the exterior  120  via one or more faces  112  of the planar waveguide  1 . Second, the DOE  2  can also be configured to direct the out-coupled light rays to control the virtual location of an object at the desired apparent viewing distance. Thus, someone looking through a face  112   a  of the planar waveguide  1  can see the virtual light source as if from a specific viewing distance. As will be discussed below, in various embodiments of the invention, the illustrator can be configured to operate within or complement the DOE and waveguide technology discussed above. 
       FIG. 3A  is a simplified diagram illustrating a perspective view of an optical device according to some embodiments of the present invention, and  FIG. 3B  is a simplified diagram illustrating a side view of the optical device of  FIG. 3A  according to some embodiments of the present invention. As described below, optical device  300  is configured to generate illumination having a plurality of wavelengths, or a plurality of colors. As shown in  FIGS. 3A and 3B , optical device  300  includes a laser package  302 , a collimation optics block  304 , a flex connector  306 , and a stiffener/heat sink  308 . Laser package  302  can include diode-laser based light sources with red, green, and blue channels. In some embodiments, as illustrated in  FIGS. 3A and 3B , laser package  302  is coupled to a collimation optics block  304 , which may be similar to collimation element  6  described above in reference to  FIGS. 1 and 2 . In some embodiments, laser package  302  and collimation optics block  304  are coupled to flex connector  306  and stiffener/heat sink  308 . 
     In some embodiments, laser package  302  is configured to output a red channel with a red light beam  311  having a wavelength of 635 nm with a tolerance of 5 nm, a green channel with a green light beam  312  having a wavelength of 520 nm with a tolerance of 5 nm, and a blue channel with a blue light beam  313  having a wavelength of 455 with a tolerance of 5 nm. In other embodiments, additional channels characterized by other wavelengths and tolerances are utilized. As a result, embodiments of the present invention are not limited to three colors. One of ordinary skill in the art would recognize many variations, modifications, and alternatives. 
       FIG. 4A  is a simplified diagram illustrating a timing diagram illustrating timing cycles of different light sources providing light beams of different wavelengths according to some embodiments of the present invention, and  FIG. 4B  is a simplified diagram illustrating a timing diagram illustrating timing cycles of different light sources providing light beams of different wavelengths according to some alternative embodiments of the present invention. In some embodiments, laser package  302  outputs light beams from each light source per a duty factor of approximately 25% to 50% of any given channel (wherein the sum of all channels equals a duty cycle of 100%).  FIGS. 4A and 4B  depict exemplary timing cycles pursuant to such duty cycle parameters for various frequencies. For example, in  FIG. 4A , in a period of 8.4 msec, a red light beam  411  is turned on for 2.1 to 4.2 msec, a green beam  412  is turned on for 2.1 to 4.2 msec, and a blue light beam  413  is turned on for 2.1 to 4.2 msec. In  FIG. 4B , in a period of 4.2 msec, a red light beam  421  is turned on for 1.1 to 2.1 msec, a green beam  422  is turned on for 1.1 to 2.1 msec, and a blue light beam  423  is turned on for 1.1 to 2.1 msec. 
       FIG. 5  is a simplified diagram illustrating an example of power distribution of different light sources providing light beams of different wavelengths according to embodiments of the present invention. In some embodiments, power outputs per wavelengths of given duty cycles are configured relative to one another, such that there is variable power input to a channel based on its duty factor within the cycle.  FIG. 5  illustrates an exemplary power distribution for a given duty cycle of 50% red, 25% green, and 25% blue. For example, the red light beam has a peak optical output of 130 mW, an average optical output of 65 mW, and an average electrical power of 330 mW. The green light beam has a peak optical output of 185 mW, an average optical output of 46 mW, and an average electrical power of 600 mW. The blue light beam has a peak optical output of 115 mW, an average optical output of 29 mW, and an average electrical power of 141 mW. 
       FIG. 6  is a simplified schematic diagram illustrating a top cross-sectional view of optical device  300  of  FIG. 3  according to some embodiments of the present invention.  FIG. 6  is a cross-sectional view of optical device  300  in  FIG. 3  taken on along a plane between the collimation optics block  304  and laser package  302 , or at the top surface of laser package  302 .  FIG. 6  shows the flex connector  306 , the laser package  302 , and light beams  311 ,  312 , and  313  in  FIG. 3 . In some embodiments, laser package  302  is configured to substantially collimate the output of each light source at a “beam waist” where laser package  302  meets collimation optics block  304 . In  FIG. 6 , the beam diameters of light beams  311 ,  312 , and  313  as based on the 1/e 2  width of each light beam. As is understood in optics, the 1/e 2  width of a light beam is equal to the distance between the two points of the light beam on the marginal distribution that are 1/e 2 =0.135 times the maximum value of the light beam. It can be seen from  FIG. 6  that the beam diameters is a factor in determining the dimensions of the optical device. Embodiments of the present invention provide a method and structure that can reduce the dimensions of the optical device. 
       FIG. 7A  is a simplified diagram illustrating an exploded view of an optical device  700  according to some embodiments of the present invention. It can be seen from  FIG. 7A , optical device  700  includes a laser package  702 , a collimation optics block  704 , a flex connector  706 , and a stiffener/heat sink  708 , similar to optical device  300  in  FIG. 3 . 
       FIG. 7B  is a simplified diagram illustrating a perspective view of the laser package  702  in  FIG. 7A  according to some embodiments of the present invention. Laser package  702  includes a first light source  711  for emitting a first input light beam  721 , a second light source  712  for emitting a second input light beam  722 , and a third light source  713  for emitting a third input light beam  723 . In this example, the first light source  711  is a first laser diode, the second light source  712  is a second laser diode, and a third light source  713  is a third laser diode. Laser package  702  also includes a first right-angle prism  731  configured to receive the first input light beam  721  and provide a first reflected light beam  741 , a second right-angle prism  732  configured to receive the second input light beam  722  and provide a second reflected light beam  742 , a third right-angle prism  733  configured to receive the third input light beam  723  and provide a third reflected light beam  743 . The first reflected light beam  741 , the second reflected light beam  742 , and the third reflected light beam  743  are collimated in the collimation optics block  704  as shown in  FIG. 7A . 
     In the example of  FIG. 7B , the first light source  711  is a red laser light source, the second light source  712  is a green laser light source, and the third light source  713  is a blue laser light source. 
     Laser package  702  can also include Zener diodes for protection of the laser diodes. For example, the first light source  711  is coupled to a first Zener diode  751 , the second light source  712  is coupled to the second Zener diode  752 , and the third light source  713  is coupled to a third Zener diode  753 . 
     In some embodiments, photodiodes are used to monitor the laser diodes. For example, in  FIG. 7B , a first photodiode  761  is coupled to the optical path of the first light source  711 , a second photodiode  762  is coupled to the optical path of the second light source  712 , and a third photodiode  763  is coupled to the optical path of the third light source  713 . 
     In some embodiments, laser package  702  can also include a thermistor  765  to monitor temperature fluctuations or other temperature parameters in the laser package. In some embodiments, the first light source  711 , the second light source  712 , and the third light source  713 , the first Zener diode  751 , the second Zener diode  752 , the third Zener diode  753 , the first photodiode  761 , the second photodiode  762 , and the third photodiode  763 , and the thermistor  765  can all be coupled to a submount  767 . 
       FIG. 8A  is a simplified diagram illustrating an exploded view of an optical device according to some alternative embodiments of the present invention, and  FIG. 8B  is a simplified diagram illustrating a perspective view of a laser package in the optical device of  FIG. 8A  according to some embodiments of the present invention. Optical device  800  in  FIGS. 8A and 8B  is similar to optical device  700  in  FIGS. 7A and 7B . One difference is that the three right-angle prisms in  FIG. 7B  are replaced by a single curved reflective element in  FIG. 8B , as described below. 
     As illustrated in  FIG. 8A , optical device  800  includes a laser package  802 , a collimation block  804 , a flex connector  806 , and a stiffener/heat sink  808 , similar to optical device  300  in  FIG. 3  and optical devices  700  in  FIGS. 7A and 7B . 
     As illustrated in  FIG. 8B , laser package  802  includes a first light source  811  for emitting a first input light beam  821 , a second light source  812  for emitting a second input light beam  822 , and a third light source  813  for emitting a third input light beam  823 . Laser package  802  also includes a curved reflective element  830  configured to receive the first input light beam  821  and provide a first reflected light beam  841 , to receive the second input light beam  822  and provide a second reflected light beam  842 , to receive the third input light beam  823  and provide a third reflected light beam  843 . 
     In some embodiments, laser package  802  can include a first cylindrical lens  881  disposed in a first light path of the first reflected light beam  841  to receive the first reflected light beam  841  and provide a first collimated output light beam  891 . Laser package  802  can also include a second cylindrical lens  882  disposed in a second light path of the second reflected light beam  842  to provide a second collimated output light beam  892 . Laser package  802  can also include a third cylindrical lens  883  disposed in a third light path of the third reflected light beam  843  to provide a third collimated output light beam  893 . 
     In the example of  FIG. 8B , the first light source  811  is a red laser light source, for example, a red laser diode. The second light source  812  is a green laser light source, for example, a green laser diode. The third light source  813  is a blue laser light source, for example, a blue laser diode. 
     Laser package  802  can also include Zener diodes for protection of the laser diodes. For example, the first light source  811  is coupled to a first Zener diode  851 , the second light source  812  is coupled to a second Zener diode  852 , and the third light source  813  is coupled to a third Zener diode  853 . 
     In some embodiments, laser package  802  can also include photodiodes for monitoring the light sources, for example, laser diodes. For example, a first photodiode  861  is coupled to the optical path of the first light source  811 , a second photodiode  862  is coupled to the optical path of the second light source  812 , and a third photodiode  863  is coupled to the optical path of the third light source  813 . 
     In some embodiments, laser package  802  can also include a thermistor  865  to monitor temperature fluctuations or other temperature parameters in the laser package. In some embodiments, the first light source  811 , the second light source  812 , and the third light source  813 , the first Zener diode  851 , the second Zener diode  852 , the third Zener diode  853 , the first photodiode  861 , the second photodiode  862 , and the third photodiode  863 , and the thermistor  865  can all be coupled to a submount  867 . 
     The light beams output by edge-emitting laser diodes can have different parallel and perpendicular divergence angles, also referred to as fast divergence and slow divergence angles. These asymmetric divergence angles result in an elliptical beam spot, instead of a circularly symmetric beam spot. In some embodiments, the curved reflective element  830  and the cylindrical lenses  881 ,  882 , and  883  can provide collimation and circularization of the light beams. In contrast, conventional collimation and circularization are achieved by a combination of refractive lens (for instance, toroidal) and mirrors (rotational symmetric/off-axis parabolic or freeform), which can be more complicated and bulkier. 
     A cylindrical lens is a lens that focuses light into a line instead of a point as a spherical lens would. The curved face of a cylindrical lens is a section of a cylinder and can focus the image passing through it into a line parallel to the intersection of the surface of the lens and a plane tangent to it. A cylindrical lens can also be used to collimate or otherwise shape a light beam along the line of focus. 
     The curved reflective element  830  has a length in the longitudinal direction and a curved cross section. In the example of  FIG. 8B , the fast axis of the elliptical laser light beam is perpendicular to the longitudinal direction of the curved reflective element  830 . The reflected light off the curved reflective element  830  is collimated or focused in the direction perpendicular to the longitudinal direction of the curved reflective element  830 , but divergent along the longitudinal direction of curved reflective element  830 . The longitudinal axes of the cylindrical lenses are disposed perpendicular to the longitudinal direction of the curved reflective element  830 , to collimate the reflected light in the direction parallel to the longitudinal direction of the curved reflective element  830  to produce a collimated output light that is collimated in both directions. This arrangement can result in smaller beam sizes by limiting divergence of the light beams along the light pathway. 
       FIG. 8C  is a simplified diagram illustrating a cross-sectional view along Section A-A′ of laser package  802  in  FIG. 8B  according to some embodiments of the present invention. As shown in  FIG. 8B , the Section A-A′ is perpendicular to the longitudinal direction of the curved reflective element  830  and intersects a cross section of the curved reflective element  830 .  FIG. 8C  shows the first light source  811  (in this example, a laser diode), curved reflective element  830 , and cylindrical lens  881 .  FIG. 8C  also shows the first input light beam  821 , the first reflected light beam  841 , and the first output light beam  891 . It can be seen that the first input light beam  821  emitted from the first laser diode  811  is divergent as it impinges on or impacts the curved reflective element  830 , and the reflected light beam  841  reflected off the curved reflective element  830  is collimated in the upward direction in  FIG. 8C . Although not shown in  FIG. 8C , the first cylindrical lens  881  further collimates in a perpendicular direction. In  FIG. 8C , the second laser diode  812  and the third laser diode  813  are disposed behind the first laser diode  811  in the direction perpendicular to the drawing and are not visible. However, a top view of output light beams  891 ,  892 , and  893  is shown in an upper portion of  FIG. 8C . 
       FIG. 8D  is a simplified diagram illustrating a cross-sectional view along the Section B-B′ of laser package  802  in  FIG. 8B  according to some embodiments of the present invention. The Section B-B′ extends along the longitudinal direction of the curved reflective element  830 .  FIG. 8D  shows curved reflective element  830  and cylindrical lenses  881 ,  882 , and  883 .  FIG. 8C  also shows reflected light beams  841 ,  842 , and  843  as they are reflected off the curved reflective element  830 , and the reflected light beams are divergent in the longitudinal direction of the curved reflective element  830 .  FIG. 8D  also shows output light beams  891 ,  892 , and  893  being collimated by cylindrical lenses  881 ,  882 , and  883 . 
     In some embodiments, as shown in  FIG. 8D , the curved reflective element  830  in laser package  802  is configured to receive the first input light beam  821  in a first region  831  of the curved reflective element  830  and provide a first reflected light beam  841 , to receive the second input light beam  822  in a second region  832  and provide a second reflected light beam  842 , and to receive the third input light beam  823  in a third region  833  and provide a third reflected light beam  843 . 
       FIGS. 8A-8D  illustrate an embodiment having a single curved reflective element  830  and three cylindrical lenses  881 ,  882 , and  883 . As compared with  FIG. 7 , which has a plurality of right angle prisms, the system of  FIGS. 8A-8D  illustrates a reflective element  830  that is not only a single reflective surface for all channels of laser package  802 , but further has a curved surface as opposed to a linear surface or surface to reflect each channel at a common angle (e.g. 90 degrees) generally. As shown in connection to  FIGS. 8A-8D , the longitudinal dimensions of the cylindrical lens are disposed perpendicular to the longitudinal direction of the curved reflective surface. This arrangement collimates the output light beams in both directions. Such a configuration enables smaller beam diameters per channel as compared to a linear reflector, such as the right angle prisms, thereby further enabling a single cylinder lens to deliver the light for each channel, as opposed to alternative optical lens necessary to support wider beam diameters that in turn may produce aberrations requiring additional optical components. Referring to  FIG. 6 , the dimension of the output beam diameters can determine the size of the optical device. Therefore, the optical device described in  FIGS. 8A-8D  can lead to a smaller device size, which is highly desirable in augmented reality applications. 
     In some embodiments, light beams from different light channels may impact the curved reflective element at different locations. 
     In some embodiments, the curved reflective element  830  has a single prescription for all channels. 
     In some embodiments, the curved reflective element  830  has discrete prescriptions for each individual channel&#39;s incidence location on curved reflective element  830 . 
     In some embodiments, the curved reflective element  830  has an elliptical surface contour, such as an elliptical mirror. 
     In some embodiments, the curved reflective element  830  has a parabolic surface contour, such as a parabolic mirror. 
     In some embodiments, the curved reflective element  830  has a hyperbolic surface contour, such as a hyperbolic mirror. 
     In some embodiments, the curved reflective element  830  follows a prescription corresponding to the following: 
         z=cy   2 /1+√{square root over (1−(1 +k ) c   2   y   2 )}+α 1   y   2 +α 2   y   4 ++α 3   y   6 +α 4   y   8 +α 5   y   10 +α 6   y   12  
 
      where c is a curvature of the base radius, k is the conic constant (for parabola=−1) and coefficients alpha are polynomial aspheric coefficients. Output facet of laser may be offset in z and y to position laser at a desired focus. In some embodiments, the curvature is defined in a z-y plane and is generally constant in the x-dimension (with x-dimension being the one that spans outputs from one or more lasers). In some embodiments, x is swept through an arc. In some embodiments, the curved reflective element can have aspheric coefficient values of zero; in some embodiments, at least one aspheric coefficient is non-zero. 
     In some embodiments, a uniform surface finish is applied to the curved reflective element. In some embodiments, discrete surface finishing per channel incidence upon the curved reflective element is applied. 
       FIG. 9  is a flowchart illustrating a method for generating illumination having a plurality of wavelengths according to some embodiments of the present invention. As shown in  FIG. 9 , the method  900  can be summarized as follows:
         At  910 , provide light sources with a plurality of wavelengths;   At  920 , provide a curved reflective element;   At  930 , provide cylindrical lenses;   At  940 , receive input light beams having different wavelengths from two or more light sources at the curved reflective element to provide two or more reflected light beams; and   At  950 , transmit each of the two or more reflected light beams through a respective cylindrical lens to provide a corresponding collimated output light beam.
 
The method is described in more detail below.
       

     At  910 , the method provides light sources with a plurality of wavelengths. Examples of light sources providing a plurality of wavelengths are described above in connection with  FIGS. 3A-8D . The light sources can be laser diodes providing light beams in the red, green, and blue wavelength ranges. In this embodiment, the diode-laser based light sources can have red, green, and blue channels. 
     At  920 , the method provides a curved reflective element. An example of curved reflective element is described above in connection to  FIGS. 8B-8D . Curved reflective element  830  has a longitudinal direction along its length and a curved cross section, which can have different prescriptions. For example, the curved reflective element  830  can have a single prescription for all channels. Alternatively, the curved reflective element can have discrete prescriptions in incidence locations on the curved reflective element for the first input light beam, the second input light beam, and the third input light beam. In some embodiments, the curved reflective element can have a parabolic surface contour. In some embodiments, the curved reflective element can have an elliptical surface contour. In some embodiments, the curved reflective element can have a hyperbolic surface contour. In some embodiments, the curved reflective element can have a hyperbolic contour, etc. The curved reflective element can also have a surface contour described by a mathematical equation with adjustable parameters. In some embodiments, the curved reflective element is used to collimate light beams in a cross-sectional plane perpendicular to its longitudinal direction. 
     At  930 , the method provides cylindrical lenses. An example of cylindrical lenses is described above in connection to  FIGS. 8B-8D . A cylindrical lens can focus or collimate light beams in its cross-sectional direction, while leaving the longitudinal direction unchanged. As described above in connection to  FIGS. 8B-8D , cylindrical lenses  881 ,  882 , and  883 , are disposed perpendicular to the longitudinal direction of the curved reflective element  830  in such a way to collimate input light beams in two perpendicular directions. Such a configuration enables smaller beam diameters per channel as compared to a linear reflector, e.g., a right angle prism. The smaller beam diameters allow reduction of the optical device. 
     At  940 , the method includes receiving input light beams having different wavelengths from two or more light sources at the curved reflective element to provide two or more reflected light beams. As described above in connection with  FIGS. 8A-8D , the method includes receiving a first input light beam, e.g., a red light beam, from a first light source, using the curved reflective element, and providing a first, red, reflected light beam. The method also includes receiving a second input light beam, e.g., a green light beam, from a second light source using the curved reflective element, and providing a second, green, reflected light beam. The method further includes receiving a third input light beam, e.g., a blue light beam, from a third light source using the curved reflective element, and providing a third, blue, reflected light beam. 
     At  950 , the method includes transmitting each of the two or more reflected light beams through a respective cylindrical lens to provide a corresponding collimated output light beam. With reference to  FIGS. 8A-8D , the method includes transmitting the first reflected light beam through the first cylindrical lens to provide a first, e.g., red, collimated output light beam. The method also includes transmitting the second reflected light beam through the second cylindrical lens to provide a second, e.g., green, collimated output light beam. The method can also include transmitting the third reflected light beam through the third cylindrical lens to provide a third, e.g., blue, collimated output light beam. 
     In some embodiments, the method can also include directing the first input light beam, the second input light beam, and the third input light beam to impact the curved reflective element in different regions. 
     In some embodiments, the method can also include configuring the curved reflective element to have a single prescription in different regions. 
     In some embodiments, the method can also include configuring the curved reflective element to have discrete prescriptions in incidence locations on the curved reflective element for the first input light beam, the second input light beam, and the third input light beam. 
     In some embodiments, the curved reflective element has a prescription described by: 
         z=cy   2 /1+√{square root over (1−(1 +k ) c   2   y   2 )}+α 1   y   2 +α 2   y   4 ++α 3   y   6 +α 4   y   8 +α 5   y   10 +α 6   y   12  
 
     wherein c is a curvature of a base radius, k is a conic constant (for parabola=−1), and coefficients α are polynomial aspheric coefficients. 
     In some embodiments, the method can include applying a uniform surface finish to the curved reflective element, or applying a discrete surface finishing in each region of the curved reflective element. 
     While the preferred embodiments of the invention have been illustrated and described, it will be clear that the invention is not limited to these embodiments only. Numerous changes, modifications, variations, substitutions and equivalents will be apparent to those skilled in the art without departing from the spirit and scope of the invention as described in the claims.