Patent Publication Number: US-2023152593-A1

Title: Wearable heads-up display with optical path fault detection

Description:
TECHNICAL FIELD 
     This disclosure relates to wearable heads-up displays (WHUDs) that use laser light to form a display and to minimization of risk of laser accidents while using the WHUD. 
     BACKGROUND 
     WHUDs are wearable electronic devices that use optical combiners to combine real world and virtual images. The combiner may be integrated with one or more lenses to provide a display lens that is fitted into a support frame of a WHUD. The display lens would provide an eyebox that is viewable by a user when the WHUD is worn on the head of the user. One class of combiners uses a lightguide (or waveguide) to transfer light. In general, light from a projector of the WHUD enters the lightguide of the combiner through an in-coupler, propagates along the lightguide by total internal reflection (TIR), and exits the lightguide through an out-coupler. If the pupil of the eye is aligned with one or more exit pupils provided by the out-coupler, at least a portion of the light exiting through the out-coupler will enter the pupil of the eye, thereby enabling the user to see a virtual image in a display space associated with the WHUD. If the combiner is transparent or semitransparent, the user will also be able to see the real world. 
     Some WHUDS use laser projectors to project the light that is used to form a display. Since WHUDS are worn near the eyes of the user, use of laser light to form a display raises safety concerns. In general, lasers are classified based on their potential to cause injury to human eyes and skin. There are four main classes for visible beam lasers, with Class 1 lasers being the safest and Class 4 lasers being the least safe. Class 1 lasers are safe under all conditions of use—the lasers are safe either because of a low power output or due to an enclosure that prevents user access to the laser beam during normal operation. Class 2 lasers are relatively safe to the extent that the user does not stare at the beam without blinking for an extended period of time. Class 3 lasers are generally considered to be dangerous to the eye, especially when used in combination with optical devices that change the beam diameter and/or power density. Class 4 lasers put out high optical powers that are considered to be dangerous with or without optical devices that change beam diameter and/or power density. The safest WHUD will be one whose maximum accessible exposure is managed such that the device is effectively a Class 1 laser product. 
     When a lightguide-based combiner is used in a WHUD, there is a risk that the lightguide may fracture, e.g., if the user accidentally drops the WHUD on a hard surface such that the lightguide is subjected to loads that exceed the fracture strength of the lightguide. If a fractured lightguide is used to transfer laser light, uncontrolled laser light leakage is likely to occur at the fracture(s) in the lightguide. If the pupil of the eye happens to be aligned with the fracture(s), the leaking laser light may enter the pupil of the eye and impinge on the retina. In some cases, a much higher laser power than would ultimately be delivered to the eye may be coupled into the lightguide. As an example, the much higher laser power may have been selected to accommodate inefficiencies in the system such that when the laser light is coupled out of the lightguide by the out-coupler, the appropriate level of laser power will be delivered to the eye to achieve a display with the appropriate brightness within laser safety limits. In cases where laser light with a relatively high level of laser power is coupled into a fractured lightguide, there is a risk of exposing the eye to unsafe laser power levels. Safety concerns may be not only to the WHUD user but also to an observer that happens to be standing in front of the WHUD user and in the field of view of the combiner. 
     Being able to make a WHUD that looks like normal eyeglasses has been one of the most interesting and complex problems in the art. The support frame of such a WHUD would have a frame front and temples that are attached to the frame front by hinges. The hinges would allow the temples to be foldable in the same manner as normal eyeglasses. The laser projector of the WHUD could be hidden within the temple, and the frame front could carry the display lens. This means that there needs to be a way of getting the laser light from the laser projector to the combiner that is carried by the display lens while maintaining the normal eyeglasses appearance. One approach has been to transmit the laser light from the temple through an opening at the front end of the temple and an opening at the back end of the frame front to the combiner. In this case, if the temple is folded, the front opening of the temple through which the laser light exits will be exposed to the environment. If for some reason the laser projector is outputting laser light while the temple is folded, laser light that was meant to be delivered to the combiner would not reach the combiner and would instead spill into the environment, possibly creating a laser hazard. 
     SUMMARY 
     An improved WHUD may be summarized as including a power source, a plurality of laser sources, a lightguide having at least one optical path defined therein, at least one photodetector, and a laser safety circuit. The at least one photodetector is positioned to detect an intensity of a portion of a test light that reaches a surface portion of the lightguide from the at least one optical path. The laser safety circuit is communicatively coupled to the power source and the at least one photodetector. The laser safety circuit provides a control to reduce or shut off a supply of electrical power from the power source to the plurality of laser sources in response to an output signal from the at least one photodetector indicating that the detected intensity is below a threshold. 
     Another improved WHUD may be summarized as including a first laser source, a second laser source, a power source, a lightguide having a first optical path and a second optical path defined therein, a first in-coupler positioned to receive a first laser light outputted by the first laser source and couple the first laser light into the first optical path, a second in-coupler positioned to receive a second laser light outputted by the second laser source and couple the second laser light into the second optical path, a photodetector, and a laser safety circuit. The photodetector detects an intensity of a portion of the first laser light that reaches a surface portion of the lightguide from the first optical path. The laser safety circuit is communicatively coupled to the power source and the photodetector. The laser safety circuit provides a control to reduce or shut off a supply of electrical power from the power source to the second laser source in response to an output signal from the photodetector indicating that the detected intensity is below a threshold. 
     Another improved WHUD may be summarized as including a first laser source; a second laser source; a power source; a lightguide having a first optical path, a second optical path, and a third optical path defined therein; a first in-coupler positioned to receive a first laser light outputted by the first laser source and couple the first laser light into the first optical path and the second optical path; a second in-coupler positioned to receive a second laser light outputted by the second laser source and couple the second laser light into the third optical path; a first photodetector; a second photodetector; and a laser safety circuit. The first photodetector detects a first intensity of a portion of the first laser light that reaches a first portion of a surface of the lightguide from the first optical path. The second photodetector detects a second intensity of a portion of the second laser light that reaches a second portion of the surface of the lightguide from the second optical path. The laser safety circuit is communicatively coupled to the first photodetector, the second photodetector, and the power source. The laser safety circuit provides a control to reduce or shut off a supply of electrical power from the power source to the second laser source in response to an output signal from the first photodetector indicating that the detected first intensity is below a first threshold and/or an output signal from the second photodetector indicating that the detected second intensity is below a second threshold. 
     Another improved WHUD may be summarized as including a power source, a laser module including at least one laser source, a lightguide having at least one optical path defined therein, an in-coupler positioned to receive a display light outputted by the laser module and couple the display light into the at least one optical path, at least one light source to output a test light that is directed into the lightguide through a first portion of a surface of the lightguide, a photodetector, and a laser safety circuit. The photodetector is positioned to detect an intensity of a portion of the test light that reaches a second portion of the surface of the lightguide from within the lightguide. The laser safety circuit is communicatively coupled to the power source and the photodetector. The laser safety circuit provides a control to reduce or shut off a supply of electrical power from the power source to the laser module in response to an output signal from the photodetector indicating that the detected intensity is below a threshold. 
     A method of safely operating a WHUD having a field a view, a lightguide positioned within the field of view, a plurality of laser sources, a power source, and a support frame may be summarized as including testing the WHUD for optical path faults at select times during use of the WHUD. The testing may include generating a test light, directing the test light along a projection optical path, and detecting an intensity of a portion of the test light that reaches a portion of a surface of the lightguide from an optical path within the lightguide. In response to detecting an intensity of the portion of the test light that is below a threshold, reducing or shutting off a supply of electrical power from the power source to the plurality of laser sources. 
     The foregoing general description and the following detailed description are exemplary of the invention and are intended to provide an overview or framework for understanding the nature of the invention as it is claimed. The accompanying drawings are included to provide further understanding of the invention and are incorporated in and constitute part of this specification. The drawings illustrate various embodiments of the invention and together with the description serve to explain the principles and operation of the invention. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       In the drawings, identical reference numbers identify similar elements or acts. The sizes and relative positions of elements in the drawings are not necessarily drawn to scale. For example, the shapes of various elements and angles are not necessarily drawn to scale, and some of these elements are arbitrarily enlarged and positioned to improve drawing legibility. Further, the particular shapes of the elements as drawn are not necessarily intended to convey any information regarding the actual shape of the particular elements and have been solely selected for ease of recognition in the drawing. 
         FIG.  1    is a front elevational view of a WHUD. 
         FIG.  2    is a schematic diagram of a portion of a WHUD positioned relative to an eye. 
         FIG.  3    is a plan view of a display combiner. 
         FIG.  4    is a side view of the display combiner shown in  FIG.  3   . 
         FIG.  5    is a plan view of a display combiner showing alternative positioning of an exit pupil expander (EPE) on a lightguide. 
         FIG.  6    is a plan view of a display combiner with two EPEs. 
         FIG.  7    is a plan view of a lightguide showing example fracture sites that intersect optical paths. 
         FIG.  8    is a schematic portion of a WHUD including a single EPE combiner and an optical path fault (OPF) detection system. 
         FIG.  9    is a schematic portion of a WHUD including a dual EPE combiner and an OPF detection system. 
         FIG.  10    is a plan view of a display combiner showing an optical path for OPF testing that is separate from optical paths for transferring display images. 
         FIG.  11    is a plan view of a display combiner showing two optical paths for OPF testing that are separate from optical paths for transferring display images. 
         FIG.  12    is a schematic portion of a WHUD including a display combiner with a dedicated optical path for OPF testing. 
         FIG.  13    is a schematic portion of a WHUD showing a light engine that provides light to dedicated optical paths for OPF testing and dedicated optical paths for transferring display images. 
         FIG.  14    is a schematic portion of a WHUD showing light sources positioned to direct light into a lightguide for OPF testing. 
         FIG.  15    is a top plan view of a WHUD with the temples in an open position. 
         FIG.  16    is a top plan view of a WHUD with one of the temples in an open position and the other in a folded position. 
         FIG.  17    is a flow diagram showing an example method of operating a WHUD safely. 
         FIG.  18    is a schematic portion of a WHUD with a photodetector behind an in-coupler for optical fault detection in a support frame optical path. 
         FIG.  19    is a schematic portion of a WHUD with a mirror behind an in-coupler for optical fault detection in a support frame optical path. 
         FIG.  20    is a schematic portion of a WHUD with a photodetector positioned to detect light emitted proximate an input region of a lightguide for optical fault detection in a support frame optical path. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description, certain specific details are set forth in order to provide a thorough understanding of various disclosed embodiments. However, one skilled in the relevant art will recognize that embodiments may be practiced without one or more of these specific details, or with other methods, components, materials, etc. In other instances, well-known structures associated with portable electronic devices and head-worn devices have not been shown or described in detail to avoid unnecessarily obscuring descriptions of the embodiments. For the sake of continuity, and in the interest of conciseness, same or similar reference characters may be used for same or similar objects in multiple figures. For the sake of brevity, the term “corresponding to” may be used to describe correspondence between features of different figures. When a feature in a first figure is described as corresponding to a feature in a second figure, the feature in the first figure is deemed to have the characteristics of the feature in the second figure, and vice versa, unless stated otherwise. 
     In this disclosure, unless the context requires otherwise, throughout the specification and claims which follow, the word “comprise” and variations thereof, such as, “comprises” and “comprising” are to be construed in an open, inclusive sense, that is as “including, but not limited to.” 
     In this disclosure, reference to “one implementation” or “an implementation” or to “one embodiment” or “an embodiment” means that a particular feature, structures, or characteristics may be combined in any suitable manner in one or more implementations or one or more embodiments. 
     In this disclosure, the singular forms “a,” “an,” and “the” include plural referents unless the content clearly dictates otherwise. It should also be noted that the term “or” is generally employed in its broadest sense, that is, as meaning “and/or” unless the content clearly dictates otherwise. 
     The headings and Abstract of the disclosure provided herein are for convenience only and do not interpret the scope or meaning of the embodiments. 
       FIG.  1    shows a WHUD  100  according to one illustrative implementation. WHUD  100  may be a retinal display that projects light carrying an image to an eye of a user. The user will have a perception of seeing an image on a display screen floating in space some distance in front of the eye. However, in reality the image is on the retina of the eye and not on a display screen. WHUD  100  includes a support frame  102  that can be worn on the head of the user. Support frame  102  carries the devices, electronics, and software that enable WHUD  100  to display content. In one example, support frame  102  includes a frame front  106  and temples (or arms)  110 ,  111  attached to opposite sides of frame front  106 . Many of the components of WHUD  100 , such as the components that form a laser projector, are carried by or within temples  110 ,  111 . Other components, such as lenses  112 ,  113 , are carried by frame front  106 . In the example shown in  FIG.  1   , lens  112  is a display lens in that it has an integrated display combiner. In general, either of lenses  112 ,  113  may be a display lens, or both lenses  112 ,  113  may be display lenses to enable a binocular display. 
       FIG.  2    is a schematic diagram showing a portion of WHUD  100  positioned relative to an eye  104  according to one illustrative implementation. In  FIG.  2   , a display combiner  200  is integrated with lens  112 . In one example, lens  112  has two lens parts  112   a,    112   b,  and display combiner  200  is stacked with lens parts  112   a,    112   b.  Each of lens parts  112   a,    112   b  is itself a lens, e.g., lens part  112   a  may be a planoconvex lens or a meniscus lens, and lens part  112   b  may be a planoconcave lens, a meniscus lens, or a biconcave lens. Display combiner  200  includes a lightguide  204 , an in-coupler  208  to couple light into lightguide  204 , and an out-coupler  212  to couple light out of lightguide  204 . In one example, lightguide  204  may be an optical substrate having a higher refractive index compared to a surrounding medium (e.g., air). In another example, lightguide  204  may be a dielectric waveguide including a core layer between two cladding layers, where the core layer has a higher refractive index compared to the cladding layers. Light that enters lightguide  204  at an angle equal to or greater than an associated critical angle will propagate within lightguide  204  by TIR. When eye  104  is aligned with out-coupler  212 , eye  104  can receive at least a portion of the light coupled out of lightguide  204  by out-coupler  212 . 
     In one implementation, WHUD  100  includes a light engine  116  that outputs laser light. Light engine  116  includes a laser module  120 , which includes a number of laser sources to generate laser light. For illustrative purposes, laser module  120  is shown as including three laser sources  120   a,    120   b,    120   c.  In one example, laser sources  120   a,    120   b,    120   c  are visible laser sources—visible laser light generated by these laser sources may be used to form a display image that is seen by eye  104 . As an example, laser source  120   a  may be a red laser diode that outputs red light in the visible range, laser source  120   b  may be a green laser diode that outputs green light in the visible range, and laser source  120   c  may be a blue laser diode that outputs blue light in the visible range. The adjectives used before the term “laser diode” refer to a characteristic of the output of the laser diode, e.g., the wavelength(s) or band of wavelengths of light outputted by the laser diode. Also, other types of laser sources besides laser diodes may be used as the visible laser sources. Also, laser sources other than visible laser sources may be included in laser module  120 . Light engine  116  may include a beam combiner  124  with optical elements  124   a,    124   b,    124   c,  such as dichroic filters, arranged to combine light from laser sources  120   a,    120   b,    120   c  into an aggregated beam  128 . 
     In one implementation, WHUD  100  includes a display modulator  132  that receives laser light from light engine  116  and modulates the laser light to form a virtual image that is seen by eye  104 . In one example, display modulator  132  includes scan mirrors  136 ,  140 , each of which is a mono-axis mirror that oscillates or rotates about its respective axis. Alternatively, display modulator  132  may include a 2D scan mirror that is rotatable about two different axes instead of two mono-axis mirrors. The scan mirror(s) may be microelectromechanical systems (MEMS) mirror(s), piezoelectric mirror(s), or the like. The scan mirror(s) may be controlled to form a predetermined display pattern, such as a raster pattern or a Lissajous pattern. In the illustrated example, for each scan orientation (i.e., relative positions of the scan mirrors  136 ,  140 ), scan mirror  136  receives beam  128  from beam combiner  124  and reflects beam  128  to scan mirror  140 , which redirects beam  128  towards in-coupler  208 . Although not shown in  FIG.  2   , there may be an arrangement of relay optics between display modulator  132  and in-coupler  208  to guide beam  128  from display modulator  132  to in-coupler  208 . 
     In general, light engine  116  and display modulator  132  with scan mirror(s) constitute a scanning laser projector. In alternative implementations, WHUD  100  may use other types of display light sources besides a scanning laser projector, such as organic light-emitting diode (OLED) micro-display, liquid crystal on silicon (LCOS) micro-display, and digital light processing (DLP) display. 
     WHUD  100  includes an application processor (AP)  144 . In one example, AP  144  is a microprocessor that runs the operating system and applications software of the WHUD. AP  144  may include, for example, a processor, graphics processing unit (GPU), and memory (not shown separately). WHUD  100  includes a display engine  148  that is communicatively coupled to AP  144 . In one example, display engine  148  includes a display controller  152 , a laser diode driver (or laser source driver in general)  156 , and a scan mirror driver  160 . In one example, display controller  152  receives image data from AP  144  and processes the image data for display. For example, the display controller  152  may pre-warp and apply various corrections to the image data to enhance the quality of the eventual display. Display controller  152  provides laser controls to laser diode driver  156  and sync controls to scan mirror driver  160  based on the image data. Laser diode driver  156  modulates the outputs of the laser sources in laser module  120  according to the laser controls that it receives from display controller  152 . For example, an image to be displayed typically has pixels, each of which is made of a combination of red, green, and blue colors. Laser diode driver  156  would control the laser sources in laser module  120  to generate a combination of red, green, and blue light that represents each pixel in the image. Scan mirror driver  160  applies driving voltages to the scan mirror(s) of display modulator  132  so that beam  128  outputted by light engine  116  lands on the correct spot in the display space associated with the WHUD. 
     Light that is outputted from light engine  116  travels to display modulator  132  and then to in-coupler  208  along a “support frame optical path”, enters lightguide  208 , propagates along one or more optical paths within lightguide  204  by TIR, and comes out of lightguide  204  at out-coupler  212 . “Support frame optical path” is an optical path that is defined within the support frame ( 102  in  FIG.  1   ) of the WHUD. In an implementation of the WHUD where light engine  116  and display modulator  132  are carried within temple  110  (in  FIG.  1   ), support frame optical path would extend along temple  110 , through an opening in frame front  106  (in  FIG.  1   ), to display combiner  200  carried by lens  112 . The support frame optical path and lightguide optical path(s) make up the optical path network of the WHUD. Any faults in the optical path network of the WHUD may be a potential source of laser light leakage from the WHUD. For laser safety reasons, it is generally desirable to detect faults in the optical path network as soon as possible so that steps can be taken to minimize the risk of a laser accident. 
     WHUD  100  includes a power subsystem  164  that supplies electrical power to laser power source  168 , display engine  148 , AP  144 , and other components of the WHUD not specifically mentioned. Power subsystem  146  may include a power manager and an energy storage, and energy may be stored in the energy storage via charging of the WHUD from an external source. Laser power source  168  provides electrical power to laser sources in laser module  120  according to the laser controls provided by laser diode driver  156 . WHUD  100  includes a laser safety circuit  172 , which is in communication with laser power source  168  and display engine  148 . Accordingly, if there is a fault in the optical path network of the WHUD, laser safety circuit  172  can take actions to minimize the risk of a laser accident. 
     In one implementation, faults in the optical path network of the WHUD are detected by monitoring light emitted at a perimeter of lightguide  204  from known optical path(s) within lightguide  204 . In one implementation, WHUD  100  includes one or more photodetectors positioned to detect light emitted at selected positions on the perimeter of lightguide  204 —these selected positions are aligned with known optical paths within lightguide  204 . For illustration purposes, two photodetectors  180 ,  184  are shown in  FIG.  2   . However, the number of photodetectors will generally depend on the number of optical paths within the lightguide that are to be monitored. In one implementation, laser safety circuit  172  receives the outputs of photodetectors  180 ,  184 . Under certain conditions, the output(s) of the photodetector(s) may cause laser safety circuit  172  to trigger an optical path fault (OPF) event. 
     In one implementation, upon triggering an OPF event, laser safety circuit  172  may send a laser power shutoff control to laser power source  168 , which would prompt laser power source  168  to stop supplying electrical power to laser module  120 , which would stop light engine  116  from outputting laser light. Laser safety circuit  172  may also send a notification of the OPF event to display engine  148 , which may prompt display engine  148  to stop providing display controls to laser diode driver  156  and scan mirror driver  160 . 
     An alternative to shutting off the light engine  116  in response to an OPF event may be to allow the light engine  116  to have a low power mode and a normal power mode. When an OPF event is triggered and the triggering condition has not been resolved, the light engine  116  may operate in the low power mode. When the WHUD does not have an active OPF event, light engine  116  may operate in a normal power mode. In one implementation, in the low power mode, laser light that is outputted by light engine  116  will have an optical power density that does not exceed the maximum permissible exposure for a Class 1 laser—this would generally result in a dim display given the inefficiencies of the display combiner. In the low power mode, any laser light that may escape from faults in the optical path network should generally be safe for the eye. In the normal power mode, laser light that is outputted by light engine  116  may be outside of Class 1 laser safety limits, with the expectation that the laser light that will be ultimately delivered to the eye will generally be within the Class 1 laser safety limits. In this case, when an OPF event is triggered, laser safety circuit  172  may send a control to laser power source  168  to change the power mode of light engine  116  to low power mode, which would limit the power provided to the laser sources in laser module  120  to comply with the requirements of the low power mode. Laser safety control  172  may also notify display engine  148  of the OPF event. Display engine  148  may use such information to limit the amount of processing applied to image data before generating controls for laser source driver  156  and scan mirror driver  160 . For example, if the display will be dim, certain processing to improve the quality of the display may be omitted. 
       FIGS.  3  and  4    show display combiner  200  with lightguide  204 , in-coupler  208 , and out-coupler  212  according to one illustrative implementation. Lightguide  204  has major surfaces  216 ,  220  (in  FIG.  4   ) and a perimeter surface  224 . Major surfaces  216 ,  220  may be referred to as front and back surfaces, respectively, for convenience. Front and back surfaces  216 ,  220  are opposed surfaces that are separated by an axial thickness T (in  FIG.  4   ) of lightguide  204 —axial thickness T may be constant or may vary across the lightguide. Perimeter surface  224  wraps around a perimeter of lightguide  204  and may include portions of front and back surfaces  216 ,  220  proximate the perimeter of lightguide  204 . A light absorbing material or structure may be selectively disposed on perimeter surface  224  to manage stray light from the perimeter of lightguide  204 . In general, portion(s) of perimeter surface  224  where light emission is to be detected for the purpose of OPF detection would not have the light absorbing material or structure. Lightguide  204  may be planar, i.e., front and back surfaces  216 ,  220  are planar surfaces. Alternatively, lightguide  204  may be non-planar, e.g., one or both of front and back surfaces  216 ,  220  are non-planar. For example, one or both of front and back surfaces  216 ,  220  may be curved surfaces (i.e., a surface that does not lie flat in a plane) or complex surfaces (e.g., a surface having flat portion(s) and curved portion(s)). 
     Lightguide  204  has an input region  204   a  (in  FIG.  4   ) to receive an input light. In-coupler  208  is positioned at input region  204   a  to couple the input light into input region  204   a.  The input light is directed or steered to strike in-coupler  208  at an appropriate incident angle to encourage propagation of the light by TIR once coupled into lightguide  204 . In one example, in-coupler  208  may be an optical grating, which may be designed, for example, with surface relief grating(s), volume hologram grating(s), or metasurface(s). In-coupler  208  may be a transmission grating or a reflection grating. A transmission grating transmits light and applies desired optical function(s) to the light during transmission, whereas a reflection grating reflects light and applies desired optical function(s) to the light during reflection. In-coupler  208  may be physically coupled (e.g., adhered, embedded, or otherwise attached) to lightguide  204  at input region  204   a.  In-coupler  208  may be positioned at or proximate a portion of front surface  216  at input region  204   a  (as shown in  FIGS.  3  and  4   ) or may be positioned at or proximate a portion of back surface  220  at input region  204   a  (not illustrated), depending on whether in-coupler  208  is a transmission grating or a reflection grating and the incoming direction of the input light from the source. In some cases, in-coupler  208  may be a prism. 
     At least a portion of the input light coupled into input region  204   a  by in-coupler  208  may propagate to an output region  204   b  (in  FIG.  4   ) of lightguide  204 . The light may propagate by TIR along one or more optical paths defined within lightguide  204 . Out-coupler  212  is positioned at output region  204   b  to couple light out of output region  204   b  and direct the output light to a target, e.g., eye  104  in  FIG.  2   . In one example, out-coupler  212  may be an optical grating, which may be designed, for example, with surface relief grating(s), volume hologram grating(s), or metasurface(s). Out-coupler  212  may be a transmission grating or a reflection grating. Out-coupler  212  may be physically coupled (e.g., adhered, embedded, or otherwise attached) to lightguide  204  at output region  204   b.  Out-coupler  212  may be positioned at or proximate a portion of front surface  216  at output region  204   b  (in  FIGS.  3  and  4   ) or may be positioned at or proximate a portion of back surface  220  at output region  204   b  (not illustrated), depending on whether out-coupler  212  is a transmission grating or a reflection grating and the outgoing direction of the output light to the target. 
     Display combiner  200  may include light redirecting elements between in-coupler  208  and out-coupler  212 . For example, display combiner  200  may include an exit pupil expander (EPE)  228  between in-coupler  208  and out-coupler  212 . EPE  228  is an optical structure that provides beam expansion and changes in beam propagation direction. EPE  228  is positioned to receive light that is propagating from input region  204   a,  expand the light, and redirect the light towards output region  204   b.  A region  204   c  (in  FIG.  4   ) of lightguide  204  that is in registration with EPE  228  may be referred to as a fold region in that it is neither an input region nor an output region. EPE  228  may be a fold grating, which may be designed, for example, with surface relief grating(s) or volume hologram grating(s) or metasurface(s). In  FIG.  3   , EPE  228  is located to the right of out-coupler  212  and below in-coupler  208 , which means that light will go down from input region  204   a  (in  FIG.  4   ) and then turn left towards out-coupler  212 .  FIG.  5    shows optical combiner  200 ′ with an alternative location for EPE  228 ′ that is above out-coupler  212 ′ and to the left of in-coupler  208 ′, where light will go to the left from input region  204   a  (in  FIG.  3   ) and then turn down towards out-coupler  212 ′. Where to locate an EPE on lightguide  204  is a design choice and may be constrained by the geometry of lightguide  204 , which may be constrained by the geometry of the lens into which display combiner  200  is integrated. 
     Referring to  FIGS.  3 ,  4 , and  5   , EPE  228  ( 228 ′) enables optical combiner  200  ( 200 ′) to support a larger field of view (FOV) than would be possible without an EPE and for the same lightguide geometry. In general, in-coupler  208  receives an input light from a source, e.g., display modulator  132  in  FIG.  2   , couples the input light into input region  204   a  and directs the input light to a primary optical path  232  ( 232 ′) within lightguide  204 . Primary optical path  232  passes through fold region  204   c.  EPE  228  ( 228 ′) is positioned along (or aligned with) primary optical path  232  ( 232 ′) such that the light propagating along primary optical path  232  ( 232 ′) bounces along EPE  228  ( 228 ′). EPE  228  ( 228 ′) will expand at least a portion of the light that it receives from primary optical path  232  ( 232 ′) and redirect the expanded light to a branch optical path  236  ( 236 ′) within lightguide  204 . Branch optical path  236  ( 236 ′) passes through output region  204   b.  Out-coupler  212  is positioned at output region  204   b  such that the light propagating along branch optical path  236  ( 236 ′) bounces along out-coupler  212 . Out-coupler  212  will couple light that it receives from branch optical path  236  ( 236 ′) out of the lightguide. 
     Two EPEs may be used to support a larger FOV than possible with a single EPE.  FIG.  6    shows optical combiner  200 ″ with EPEs  228   a,    228   b  positioned at intermediate stages between in-coupler  208 ″ and out-coupler  212 ″. EPEs  228   a,    228   b  can have the same characteristics described above for EPE  228  (in  FIG.  3   ). In this alternative implementation, in-coupler  208 ″ directs a first portion of an input light into a first primary optical path  232   a  within lightguide  204  and a second portion of the input light into a second primary optical path  232   b  within lightguide  204 . EPE  228   a  is positioned along first primary optical path  232   a  to receive light from first primary optical path  232   a,  expand at least a portion of the light, and redirect the expanded light to a first branch optical path  236   a  within lightguide  204 . EPE  228   b  is positioned along second primary optical path  232   b  to receive light from second primary optical path  232   b,  expand at least a portion of the light, and redirect the expanded light to a second branch optical path  236   b  within lightguide  204 . Out-coupler  212 ″ is positioned to receive light from both first branch optical path  236   a  and second branch optical path  236   b  and to couple at least a portion of the received light out of lightguide  204 . In-coupler  208 ″ and out-coupler  212 ″ may be optical gratings, which may be designed with, for example, surface relief grating(s), volume hologram grating(s), or metasurface(s). Examples of suitable optical gratings for use with dual EPEs are described, for example, in U.S. Provisional Application No. 62/846,979 (“Single RGB Combiner with Large Field of View”). 
     Returning to  FIG.  3   , some of the light traveling along primary optical path  232  will be redirected to branch optical path  236  by EPE  228 . The remaining portion of the light that is not redirected to branch optical path  236  by EPE  228  will continue to travel along primary optical path  232  until reaching a portion  225  of perimeter surface  224  that is aligned with primary optical path  232 . Also, some of the light redirected to branch optical path  236  will be coupled out of lightguide  204  by out-coupler  212 . The remaining portion of the light that is not coupled out by out-coupler  212  will continue to travel along branch optical path  236  until reaching a portion  227  of perimeter surface  224  that is aligned with branch optical path  236 . Thus, for every input light that in-coupler  208  receives and couples into lightguide  204  at an angle that encourages TIR, some of the input light will reach perimeter surface portion  225  from primary optical path  232  and some of the input light will reach perimeter surface portion  227  from the branch optical path  236 , provided that there are no breaks in optical paths  232 ,  236 . If there are breaks in optical paths  232 ,  236 , light may not reach perimeter surface portions  225 ,  227 , or the amount of light that reaches perimeter surface portions  225 ,  227  may be diminished compared to when there are no breaks in the optical paths. Breaks in optical paths  232 ,  236  may occur if fractures develop in lightguide  204  and the fractures cross optical paths  232 ,  236 . 
     For illustrative purposes,  FIG.  7    shows example sites  240   a,    240   b,    240   c  where fractures may cross primary optical path  232  if lightguide  204  has fractures. Site  240   a  is located somewhere on the portion of primary optical path  232  between input region  204   a  and fold region  204   c.  Site  240   b  is located somewhere on the portion of primary optical path  232  in fold region  204   c.  Site  240   c  is located somewhere on the portion of primary optical path  232  after fold region  204   c.  A fracture that crosses any of sites  240   a,    240   b,    240   c  will interrupt propagation of light along primary optical path  232 , diminishing the amount of light that reaches perimeter surface portion  225  from primary optical path  232 . A fracture that crosses site  240   a  will interrupt diversion of light from primary optical path  232  to branch optical path  236 , diminishing the amount of light that reaches perimeter surface portion  227  from branch optical path  236 . 
       FIG.  7    also shows example sites  244   a,    244   b,    244   c  where fractures may cross branch optical path  236  if lightguide  204  has fractures. Site  244   a  is located somewhere in a portion of branch optical path  236  between fold region  204   c  and output region  204   b.  Site  244   b  is located somewhere in a portion of branch optical path  236  in output region  204   b.  Site  244   c  is located after output region  204   b.  A fracture that crosses any of sites  244   a,    244   b,    244   c  will interrupt propagation of light along branch optical path  236 , diminishing the amount of light that reaches perimeter surface portion  227  from branch optical path  236 . 
     Thus, for display combiner  200  shown in  FIG.  3   , if a fracture crosses primary optical path  232  anywhere, the amount of light that reaches perimeter surface portion  225  from primary optical path  232  will be diminished compared to if there are no fractures that cross primary optical path  232 . Also, if a fracture crosses branch optical path  236  anywhere, the amount of light that reaches perimeter surface portion  227  from branch optical path  236  will be diminished compared to if there are no fractures that cross branch optical path  236 . Thus, fractures in lightguide  204 , if they occur, can modulate the amount of light that reaches perimeter surface portions  225 ,  227  from optical paths  232 ,  236 , respectively. 
     For display combiner  200 ′ variation shown in  FIG.  5   , if a fracture crosses primary optical path  232 ′ anywhere, the amount of light that reaches perimeter surface portion  225 ′ from primary optical path  232 ′ will be diminished compared to if there are no fractures that cross primary optical path  232 ′. Also, if a fracture crosses branch optical path  236 ′ anywhere, the amount of light that reaches perimeter surface portion  227 ′ from branch optical path  236 ′ will be diminished compared to if there are no fractures that cross branch optical path  236 ′. Thus, fractures in lightguide  204 , if they occur, can modulate the amount of light that reaches perimeter surface portions  225 ′,  227 ′ from optical paths  232 ′,  236 ′. 
     With respect to detecting fractures, the main difference between display combiner  200  in  FIG.  3    and display combiner  200 ′ variation in  FIG.  5    is the location of the perimeter surface portions on lightguide  204 . In  FIG.  5   , perimeter surface portion  225 ′ that is aligned with primary optical path  232 ′ is located at the bottom of lightguide  204 , and perimeter surface portion  227 ′ that is aligned with branch optical path  236 ′ is located at the side of lightguide  204 —these locations are transposed for corresponding perimeter surface portions  225 ,  227  in  FIG.  3   . 
     For the dual EPE combiner  200 ″ variation shown in  FIG.  6   , if a fracture crosses first primary optical path  232   a  anywhere, the amount of light that reaches perimeter surface portion  225   a  from first primary optical path  232   a  will be diminished compared to if there are no fractures that cross primary optical path  232   a.  If a fracture crosses first branch optical path  236   a  anywhere, the amount of light that reaches perimeter surface portion  227   a  from first branch optical path  236   a  will be diminished compared to if there are no fractures that cross first branch optical path  236   a.  Similarly, if a fracture crosses second primary optical path  232   b  anywhere, the amount of light that reaches perimeter surface portion  225   b  from second primary optical path  232   b  will be diminished compared to if there are no fractures that cross second primary optical path  232   b.  Also, if a fracture crosses second branch optical path  236   b  anywhere, the amount of light that reaches perimeter surface portion  227   b  from second branch optical path  236   b  will be diminished compared to if there are no fractures that cross second branch optical path  236   b.  Thus, fractures in lightguide  204 , if they occur, can modulate the amount of light that reaches perimeter surface portions  225   a,    225   b,    227   a,    227   b  from primary optical paths  232   a,    232   b  and branch optical paths  236   a,    236   b,  respectively. 
     From the examples above, fractures in lightguide  204 , if they occur, can modulate the amount of light that reaches perimeter surface portions (e.g.,  225 ,  227  in  FIG.  3   ;  225 ′,  227 ′ in  FIG.  5   ;  225   a ,  227   a ,  225   b ,  227   b  in  FIG.  6   ) that are aligned with optical paths (e.g.,  232 ,  236  in  FIG.  3   ;  232 ′,  236 ′ in  FIG.  5   ;  232   a ,  236   a ,  232   b ,  236   b  in  FIG.  6   ) within the lightguide. By monitoring the changes in the amount of light that reach perimeter surface portions of lightguide  204  from known optical paths within lightguide  204 , it is possible to determine whether lightguide  204  has fractured or not. This monitoring will detect fractures within the lightguide that cross the known optical paths but may not detect other fractures within the lightguide that do not cross the known optical paths. However, the optical paths monitored can be the same as the optical paths carrying display light so that any faults detected can be a reliable indication of laser light leakage that requires mitigating actions to be taken. 
       FIG.  8    shows a portion of WHUD  100  with an optical path fault detection (OPFD) system based on the principles described above. In  FIG.  8   , photodetector  180  is positioned to detect light that reaches perimeter surface portion  225  from primary optical path  232 , and photodetector  184  is positioned to detect light that reaches perimeter surface portion  227  from branch optical path  236 . In this implementation, photodetectors  180 ,  184  are responsive to laser light emitted by at least one laser source in laser module  120 . In one example, AP  144  may run an OPF test routine that includes providing a test image to display engine  148 —the test image may be a point, a geometric shape, or a pattern, and pixels of the test image may be set to white, although this is not strictly necessary. Upon receiving the image data, display engine  148  would control light engine  116  to generate test light according to the test image data and would control display modulator  132  to redirect the test light to in-coupler according to the test image data. At in-coupler  208 , the test light would be coupled into lightguide  204 . Photodetectors  180 ,  184  would output signals that are representative of the amount (or intensity) of the light that reaches edge surface portions  225 ,  227  from corresponding optical paths  232 ,  236 . The output signals of photodetectors  180 ,  184  may be interpreted to determine if there are breaks in optical paths  232 ,  236  that could indicate the presence of fractures in lightguide  204 . 
     In one implementation, laser safety circuit  172  receives the outputs of photodetectors  180   184 , compares each of the photodetector outputs to a threshold, and generates an output signal that is representative of this comparison. In one implementation, an optical path within the lightguide is considered to have a fault if an intensity detected by the photodetector associated with the optical path is below an associated threshold—that is, little to no light is detected from the optical path. In one example, laser safety circuit  172  includes a first comparator  189  having a negative input connected to receive an output signal PD 1  from photodetector  180  and a positive input connected to a reference voltage signal VR 1 . PD 1  indicates the intensity of light detected by photodetector  180 . VR 1  indicates the threshold against which the output of photodetector  180  is to be compared. Comparator  189  generates an output signal CS 1  that is representative of a comparison between output signal PD 1  and reference voltage VR 1 . Under normal conditions where there are no breaks in optical path  232 , PD 1  will be higher than VR 1 , and CS 1  will be LOW. PD 1  will drop in response to breaks in optical path  232  that disrupt propagation of light along optical path  232 . If PD 1  drops below VR 1 , CS 1  will go HIGH. 
     Laser safety circuit  172  may further include a second comparator  190  having a negative input connected to receive an output signal PD 2  from photodetector  184  and a positive input connected to a reference voltage signal VR 2 . PD 2  indicates the intensity of light detected by photodetector  184 . VR 2  indicates the threshold against which the output of photodetector  184  is to be compared. Comparator  190  generates an output signal CS 2  that is representative of a comparison between output signal PD 2  and reference voltage VR 2 . Under normal conditions where there are no breaks in optical path  236 , PD 2  will be higher than VR 2 , and CS 2  will be LOW. PD 2  will drop in response to breaks in optical path  236  that disrupt propagation of light along optical path  236 . If PD 2  drops below VR 2 , CS 2  will go HIGH. 
     The threshold indicated by reference voltage VR 2  may be the same or may be different from the threshold indicated by reference voltage VR 1 . The thresholds against which PD 1  and PD 2  are compared should take into account environmental light that may artificially inflate the intensity of the light detected. In some cases, using a narrow bandwidth test light and tuning the photodetectors to respond to the narrow bandwidth may improve the accuracy of the detection by the photodetectors. The thresholds (or reference voltages) should be selected such that when the photodetector outputs are below the respective thresholds, this can be taken as an indication that there are faults in the optical paths that the photodetectors are monitoring. The thresholds may be determined experimentally, e.g., by measuring light detected with and without faults in the optical paths, or by modeling. 
     In one example, comparator signals CS 1 , CS 2  are fed to an OR gate  191 . Either or both of CS 1  and CS 2  going HIGH will cause an output signal LS 1  of OR gate  191  to go HIGH. Output signal LS 1  of OR gate  191  may be fed to a port of a safety microcontroller (MCU)  188 , which is part of laser safety circuit  172 . If output signal LS 1  is HIGH, this may trigger an OPF event at safety MCU  188 . In response, safety MCU  188  may send a laser power shutoff control to laser power source  168 . For example, laser power source  168  may be coupled to laser module  120  by a switch, and the laser power shutoff control may open the switch so that laser power source  164  no longer provides current to the laser sources in laser module  120 . Also, safety MCU  188  may notify display engine  148  of the OPF event so that display engine  148  can stop sending display controls to display modulator  132  and light engine  116 . Alternatively, as previously mentioned, light engine  116  may have a low power mode and a normal mode, and a response to the OPF event may be to send a control to laser power source  168  that changes the power mode of light engine  116  from a normal power mode to a low power mode. In the low power mode, display engine  148  may drive display modulator  132  and light engine  114  with low resolution image data. 
     In  FIG.  8   , two photodetectors  180 ,  184  are used to monitor light that reaches perimeter surface portions of lightguide  204  from optical paths within lightguide  204 . In general, one or more photodetectors may be used to monitor light that reaches perimeter surface portion(s) of lightguide  204  from one or more known optical paths within lightguide. For example, photodetector  180  may be omitted in  FIG.  8   , and photodetector  184  alone may be used to detect light that reaches perimeter surface  227  from branch optical path  236 . This would allow detection of fractures that cross branch optical path  236  and fractures that cross a portion of primary optical path  232  between in-coupler  208  and EPE  228 . Alternatively, if the display combiner does not have an EPE, this simplifies to one optical path that runs from the input region to the output region to a portion of the perimeter surface of the lightguide, requiring one photodetector for OPF testing. 
     Display combiner  200 ″ variation in  FIG.  6    can be monitored for OPF events in much the same manner described for display combiner  200  above. Referring to  FIG.  9   , photodetectors  180   a,    184   a,    180   b    184   b  are positioned to detect light that reaches perimeter surface portions  225   a,    227   a,    225   b,    227   b  from optical paths  232   a,    236   a,    232   b,    236   b,  respectively. Photodetectors  180   a,    184   a,    180   b,    184   b  are responsive to light emitted by at least one laser source in laser module  120 . To detect if there are fractures in lightguide  204 , a test light may be generated by at least one of the laser sources in laser module  120  and directed to display modulator  132 , which would redirect the test light to in-coupler  208 . Photodetectors  180   a,    184   a,    180   b,    184   b  generate output signals PD 3 , PD 4 , PD 5 , PD 6 , which are representative of the amount (or intensity) of light that reaches perimeter surface portions  225   a,    227   a,    225   b,    227   b  from optical paths  232   a,    236   a,    232   b,    236   b,  respectively. In one implementation, laser safety circuit  172 ″ receives the photodetector outputs PD 3 , PD 4 , PD 5 , PD 6 , compares each of the photodetector outputs to a respective threshold, and generates an output signal that is representative of this comparison. 
     In one example, laser safety circuit  172 ″ includes a first comparator  193  having a negative input connected to receive output signal PD 3  from photodetector  180   a  and a positive input connected to a reference voltage VR 3 . PD 3  indicates the intensity of light detected by photodetector  180   a.  VR 3  corresponds to the threshold against which the output of photodetector  180   a  is to be compared. Comparator  193  generates an output signal CS 3  that is representative of a comparison between output signal PD 3  and reference voltage VR 3 . Under normal conditions where there are no breaks in first primary optical path  232   a,  PD 3  will be higher than VR 3 , and CS 3  will be LOW. PD 3  will drop in response to breaks in optical path  232   a.  If PD 3  drops below VR 3 , CS 3  will go HIGH. 
     Laser safety circuit  172 ″ may further include a second comparator  194  that takes output signal PD 4  from photodetector  184   a  and reference voltage VR 4  as inputs and generates an output signal CS 4  that is representative of a comparison between output signal PD 4  and reference voltage VR 4 . Under normal conditions where there are no breaks in first branch optical path  236   a,  PD 4  will be higher than VR 4 , and CS 4  will be LOW. 
     Laser safety circuit  172 ″ may further include a third comparator  195  that takes output signal PD 4  from photodetector  180   b  and reference voltage VR 5  as inputs and generates an output signal CS 5  that is representative of a comparison between output signal PD 5  and reference voltage VR 5 . Under normal conditions where there are no breaks in second primary optical path  232   b,  PD 5  will be higher than VR 5 , and CS 5  will be LOW. 
     Laser safety circuit  172 ″ may further include a fourth comparator  196  that takes output signal PD 6  from photodetector  184   b  and reference voltage VR 6  as inputs and generates an output signal CS 6  that is representative of a comparison between output signal PD 6  and reference voltage VR 6 . Under normal conditions where there are no breaks in second branch optical path  236   b,  PD 6  will be higher than VR 6 , and CS 6  will be LOW. 
     In one example, comparator signals CS 3 , CS 4 , CS 5 , CS 6  are fed to an OR gate  197 . Any of CS 1  and CS 2  going HIGH will cause an output signal LS 2  of OR gate  197  to go HIGH. Output signal LS 2  of OR gate  197  may be fed to safety MCU  188 . Safety MCU  188  upon receiving a HIGH signal from OR gate  197  may trigger an OPF event. Safety MCU  188  may act on the OPF event as previously explained with reference to  FIG.  8   . 
     In  FIG.  9   , four photodetectors  180   a,    184   a,    180   b,    184   b  are used to monitor light that reaches four perimeter surface portions from four different optical paths within lightguide  204 . In alternative implementations, less than four photodetectors may be used to monitor light from less than four perimeter surface portions of lightguide  204 . For instance, it may suffice to use only two photodetectors to monitor light that reaches two perimeter surface portions from two optical paths within the lightguide. For example, photodetectors  180   a,    180   b  alone may be used to monitor light that reaches perimeter surface portions  225   a,    225   b.  Alternatively, photodetectors  180   a,    184   a  alone may be used to monitor light that reaches perimeter surface portions  225   a,    227   a.  Alternatively, photodetectors  180   b,    184   b  alone may be used to monitor light that reaches perimeter surface portions  225   b,    227   b.  Where two photodetectors are used, laser safety circuit  172  in  FIG.  8    may be used instead of laser safety circuit  172 ″ in  FIG.  9   . There is also the possibility of using only one photodetector to monitor the light that reaches one of perimeter surface portions  225   a,    227   a,    225   b,    227   b  and basing assessment of an OPF event on the output of the single photodetector. In general, the greater the number of optical paths monitored, the more robust may be the lightguide fracture detection as it would allow more coverage of the lightguide. 
     In the WHUD of  FIGS.  8  and  9   , the same optical paths are used for OPF testing and normal transfer of display images. This generally means that when OPF testing is being carried out, display images are not being sent, and vice versa. In one implementation, the WHUD may be operated by carrying out OPF testing prior to each instance of sending a display image to the eye through the display combiner. If the OPF testing does not indicate an OPF event, then the display image may be sent immediately after the testing. In another implementation, the WHUD may be operated by carrying out an OPF testing upon starting or waking up the WHUD and carrying out subsequent OPF testing at intervals during use of the WHUD—the intervals may be preset or based on triggering activities. In the case where OPF testing shares the same optical paths used for normal transfer of display images, low power visible light may be used as the test light so as to minimize visibility of the test light to the user. There is the possibility of using infrared as the test light, which would be invisible to the eye. However, an optical path that is tuned for visible light may not work for infrared light because of the much longer wavelength of the infrared light compared to the visible light. 
     In another approach, one or more separate optical paths may be provided for OPF testing so that OPF testing may be carried out simultaneously with transferring display images to the eye. In this alternative approach, the separate optical path(s) for OPF testing may be tuned for infrared light, which would enable invisible OPF testing. For illustrative purposes,  FIG.  10    shows a display combiner  200 ′″ having an optical path for OPF testing that is separate from optical paths for transferring display images. In addition to the features of display combiner  200  (in  FIG.  3   ) previously described, display combiner  200 ′″ includes an in-coupler  248  that is positioned to couple a test light into lightguide  204 . For discussion purposes, in-coupler  248  may be referred to as “auxiliary in-coupler”, and previously described in-coupler  208  may be referred to as “primary in-coupler”. In one example, auxiliary in-coupler  248  receives a test light and couples the test light into an auxiliary optical path  252  within lightguide  204 . Auxiliary optical path  252  is separate from primary and branch optical paths  232 ,  236  used to transfer display images. To detect an OPF event, light that reaches portion  255  of perimeter surface  224  of lightguide  204  from auxiliary optical path  232  may be monitored. 
       FIG.  11    shows a display combiner  200 ″″ variation where auxiliary in-coupler  248 ′ couples a test light into two auxiliary optical paths  252 ,  256  within lightguide  204 . Auxiliary optical paths  252 ,  256  are separate from primary and branch optical paths  232 ,  236 . To detect an OPF event, light that reaches perimeter surface portions  255 ,  257  from auxiliary optical paths  252 ,  256  may be monitored. In  FIG.  11   , even though EPE  228  appears to be aligned with auxiliary optical path  256 , EPE  228  will typically be designed to be unresponsive to the light that auxiliary optical path  256  is carrying. For example, auxiliary optical path  256  may carry infrared light, while EPE  228  is designed to be responsive to visible light. In general, it is not expected that EPE  228  will redirect the light in auxiliary optical path  256  towards out-coupler  212 .  FIG.  11    also shows that it is possible to have auxiliary optical paths  252 ,  256  proximate primary and branch optical paths  232 ,  236 , respectively, such that fractures that cross primary and branch optical paths  232 ,  236  will most likely also cross auxiliary optical paths  252 ,  256 . 
       FIG.  10    shows auxiliary in-coupler  248  laterally offset from primary in-coupler  208  as an example.  FIG.  11    shows a different example where auxiliary in-coupler  248 ′ is stacked with primary in-coupler  208 . Both auxiliary in-coupler  248  ( 248 ′) and primary in-coupler  208  may be located on or proximate the same major surface (i.e., front or back surface) of lightguide  204 . Alternatively, auxiliary in-coupler  248  ( 248 ′)and primary in-coupler  208  may be located on or proximate different major surfaces of lightguide  204 , e.g., auxiliary in-coupler on the front surface and primary in-coupler  208  on the back surface, or vice versa. 
     Auxiliary in-coupler  248  ( 248 ′) may be made with optical grating as previously described for in-coupler  208 . In general, auxiliary in-coupler  248  ( 248 ′) may be designed to be responsive to light in a first wavelength range and unresponsive to light in a second wavelength range, and primary in-coupler  208  may be designed to be responsive to light in the second wavelength range and unresponsive to the light in the first wavelength range. “Responsive” herein means that the coupler redirects the light and may modify the light while redirecting the light. “Unresponsive” herein means that the coupler transmits or reflects the light generally without modifying the light. The first wavelength range to which auxiliary in-coupler  248  ( 248 ′) is responsive may be an infrared wavelength range, while the second wavelength range to which primary in-coupler  208  is responsive is a visible wavelength range. Alternatively, the first wavelength range to which auxiliary in-coupler  248  ( 248 ′) is responsive may be a subset of the visible wavelength range, and the second wavelength to which primary in-coupler  208  is responsive may be another subset of the visible wavelength range. 
       FIG.  12    shows a photodetector  180   a  positioned to detect light that reaches perimeter surface portion  255  of lightguide  204  from auxiliary optical path  252  within lightguide  204 . In this case, laser safety circuit  172 ′″ may include a comparator  192  that compares the output PD 7  of photodetector  180   a  to a threshold (reference voltage VR 7 ) and generates a signal CS 7  that indicates whether or not there are breaks in auxiliary optical path  252 . CS 7  is received by safety MCU  188 , which may generate an OPF event if the conditions for an OPF event are met. OPF testing may be carried out simultaneously with providing display light to primary in-coupler  208 . If the output of laser safety circuit  172 ′″ indicates an OPF event, safety MCU  188  may stop light engine  116  from outputting light or may put light engine  116  in a low power mode as previously described. 
       FIG.  12    shows light source  120   d  outside of laser module  120 , which means that test light is not aggregated with the output of light engine  116  and may take a different sub-path of support frame optical path compared to light outputted by light engine  116 . In some cases, it may be easier to package light source  120   d  with laser module  120  so that the test light and display light share the same optical path from the light engine to the display combiner. For illustrative purposes, in  FIG.  13   , laser module  120 ′ includes laser source  120   d′  for generating test light. Beam combiner  124 ′ includes optical elements  124   a,    124   b,    124   c,    124   d′  to combine the test light generated by laser source  120   d′  with the display light generated by laser sources  120   a,    120   b,    120   c.  The aggregated beam  128 ′ outputted by beam combiner  124 ′ is directed to display modulator  132 , which redirects the light to the in-couplers  208 ,  248 ′. Auxiliary in-coupler  248 ′ will couple the test light portion of the incident light into auxiliary optical paths  252 ,  256 . Primary in-coupler  208  will couple the display light portion of the incident light into primary optical path  232 . In one example, laser source  120   d′  that generates the test light is an infrared laser source, and laser sources  120   a,    120   b,    120   c  that generate the display light may be visible laser sources. In another example, all of the laser sources may be visible laser sources.  FIG.  13    is illustrated using display combiner  200 ″″ that has two auxiliary optical paths  252 ,  256 . Photodetectors  180 ,  184  and laser safety circuit  172  previously described with respect to  FIG.  8    may be used to monitor the auxiliary optical paths for faults. 
     Testing light may be coupled into auxiliary optical paths without use of an in-coupler.  FIG.  14    shows light sources  300 ,  304  positioned to direct light into auxiliary optical paths  252 ,  256  through portions  308 ,  312  of perimeter surface  224  of lightguide  204 . The test light directed into auxiliary optical paths  252 ,  256  travels to perimeter surface portions  255 ,  257 , respectively. Light that reaches perimeter surface portions  255 ,  257  can be detected by photodetectors  180 ,  184 , respectively. Laser safety circuit  172  can receive the outputs of photodetectors  180 ,  184  and determine if there is an OPF event that requires further action as previously explained. Light sources  300 ,  304  may be laser sources, such as laser diodes, or other type of light sources, such as light emitting diodes. Light sources  300 ,  304  may be visible light sources or infrared light sources. The WHUD shown in  FIG.  14    may be alternatively implemented with one light source and one auxiliary optical path or with more than two light sources and more than two auxiliary optical paths. 
     For each WHUD variation described in  FIGS.  8 ,  9 ,  12 ,  13 , and  14   , all the components shown other than lens  112  and photodetectors ( 180 ,  184  in  FIGS.  8 ,  13 , and  14   ;  180   a  in  FIGS.  9  and  12   ;  180   b ,  184   a ,  184   b  in  FIG.  9   ) may be carried by one or both of the temples ( 110 ,  114  in  FIG.  1   ) of the WHUD. Typically, light engine  116  and display modulator  132  will be carried by the same temple. Lens  112  will be carried by the frame front ( 106  in  FIG.  1   ) of the WHUD, as shown in  FIG.  1   . For each WHUD variation, the respective photodetectors may be embedded in or carried by a portion of the frame front circumscribing the display lens, as shown at  180 ,  184  in  FIG.  1   . In general, the positioning of photodetectors in the frame front should be such that the photodetectors can monitor light that reaches the perimeter of lightguide  204  from known optical paths within the lightguide. For the WHUD variation shown in  FIG.  14   , light sources  300 ,  304  can be embedded in or carried by the portion of the front frame circumscribing lens  112  (similar to what is shown in  FIG.  1    for photodetectors  180 ,  184 ). The portion of the frame front where light sources  300 ,  304  are carried will be close to the input region of lightguide  204  (i.e., close to where in-coupler  208  is located). 
       FIG.  15    shows a top plan view of WHUD  100  according to one illustrative implementation. Light engine  116 , display modulator  132 , laser power source  168  are shown within temple  110 . However, not all the components that temple  110  may carry are shown. Temple  110  is coupled to frame front  106  by a hinge  114 , which allows temple  110  to be moved between an open position (or extended position), as shown in  FIG.  15   , and a folded position, as shown in  FIG.  16   . When temple  110  is in the open position, an opening at a front end  110   a  of temple  110  is aligned with an opening at a back corner  106   a  of frame front  106 . Laser light outputted by light engine  116  is directed to display modulator  132 , which directs the laser light towards the opening at front end  110   a  of temple  110 . Frame front  106  carries display lens  112 , which carries display combiner  200  (or a variant thereof). If temple  110  is in the open position, as shown in  FIG.  15   , the light will reach front end  110   a  of temple  110  and will be able to enter the opening at back corner  106   a  of frame front  106  to reach display combiner  200 . Support frame optical path (i.e., the optical path extending from within temple  110  through the opening at the back corner  106   a  of frame front  106  to display combiner  200 ) will be broken when temple  110  is in the folded position, as shown in  FIG.  16   , and in at least some intermediate positions between the open and folded positions of the temple. 
     In some cases, display light coming out of light engine  116  may need to be at an optical power that exceeds Class 1 laser safety limits. The optical power would typically be reduced by the time the light is delivered to the eye due to inefficiencies in the display combiner. However, while traveling along the support frame optical path, the power may exceed Class 1 laser safety limits. If temple  110  is not in the open position, e.g., is in the folded position, the opening at front end  110   a  of temple  110  will be exposed to the environment, as shown in  FIG.  16   . If light engine  116  is outputting laser light while the opening at front end  110   a  of temple  110  is exposed to the environment, the light outputted by light engine  116  and modulated by display modulator  132  will be directed to the environment rather than to an optical path that is connected to display combiner  200 . This means that potentially dangerous laser light may be spilling out of temple  110  to the environment. If the front end of temple  110  is aimed at an eye in this condition, this potentially dangerous laser light could potentially cause eye damage. 
     It may be possible to install a shutter at the front end of temple  110  that slides over the opening at the front end of temple  110  when temple  110  is in the open position and slides away from the opening when temple  110  is in the folded position. This would avoid potential spilling of laser light to the environment when temple  110  is in the open position. However, it may still be desirable to know when the support frame optical path is no longer defined so that light engine  116  can be prevented from outputting light. In some cases, the same system used to detect faults in optical paths within the lightguide can be used to detect faults in the support frame optical path. If the light used in the OPF testing is carried by temple  110  (e.g., in the systems shown in  FIGS.  8 ,  12 , and  13   ), a test light generated for OPF testing will reach the lightguide of display combiner  200  only if there is a defined support frame optical path from temple  110  to display combiner  200 , i.e., the temple is not in the folded position, or if a shutter is used at the front end of the temple, the shutter is not in a closed position. This means that an OPF testing using a light source carried within temple  110  can detect faults in both lightguide and support frame optical paths. 
       FIG.  17    is a flow diagram illustrating one example of operating a WHUD safely. It is useful to contemplate the system described in any one of  FIGS.  8 ,  12 , and  13    while considering the flow diagram in  FIG.  17   . At  400 , the display is turned on—turning on the display may mean that the display engine ( 148  in  FIGS.  8 ,  12 ,  13   ) is communicatively coupled to the display modulator ( 132  in  FIGS.  8 ,  12 ,  13   ) and light engine ( 116  in  FIGS.  8 ,  12 ,  13   ). The display may be turned on by the user or may be triggered by some application that is running on the WHUD. At  404 , in response to the display being turned on, a test light is generated from within the temple carrying the light source for the test light. At  408 , the test light is directed to an in-coupler ( 208 ,  248 ,  248 ′ in  FIGS.  8 ,  12 ,  13   ) positioned to couple light into a lightguide ( 208  in  FIGS.  8 ,  12 ,  13   ) of the display combiner of the WHUD. At  412 , an output of at least one photodetector positioned to detect light emitted from a known optical path within the lightguide is received at the laser safety circuit. At  416 , the laser safety circuit ( 172 ,  172 ″″ in  FIGS.  8 ,  12 , and  13   ) determines if the output of the photodetector is below a threshold. If the output of the photodetector is below a threshold, then an OPF event is triggered. At  420 , in response to the OPF event, a control may be sent to the laser power source ( 168  in  FIGS.  8 ,  12 ,  13   ) to switch the light engine to a low power mode. Alternatively, a control may be sent to the laser power source to stop supplying electrical power to the laser module ( 120  in  FIGS.  8 ,  12 ,  13   ), essentially shutting off the light engine. At some point, OPF testing may be carried out again by returning to act  404 . If from the determination of  416  the output of the photodetector is above a threshold, then it may be concluded that there are no faults in the lightguide and support frame optical paths and that the light engine could operate in the normal power mode. While operating in the normal power mode, the WHUD may from time to time return to act  404  to verify that the lightguide and support frame optical paths are still free of faults and that the WHUD can still operate in the normal power mode. 
       FIG.  18    shows a system that may be used to test a support frame optical path for faults. In this case, a photodetector  500  is stacked behind in-coupler  208 . Light from light engine  116  is directed to in-coupler  208  along a support frame optical path. If the support frame optical path is free of faults up to in-coupler  208 , in-coupler  208  will couple the light into lightguide  204 , and photodetector  500  will detect the light that is coupled into lightguide  208 . The output of photodetector  500  may be received by laser safety circuit  172  (or a variant thereof) and compared to a threshold to determine whether the support frame optical path has faults or not as previously described for the other OPFD systems. The system shown in  FIG.  18    will not detect faults in optical paths within lightguide  204 . However, the system shown in  FIG.  18    can be combined with other systems that detect faults in lightguide optical paths but do not detect faults in support frame optical path, such as the system shown in  FIG.  14   . 
       FIG.  19    shows another system that may be used to test a support frame optical path for faults. In this case, a mirror  504  is stacked behind in-coupler  208 . Light from light engine  116  is directed to in-coupler  208  along a support frame optical path. If the support frame optical path is free of faults up to in-coupler  208 , in-coupler  208  will couple the light into lightguide  204 . At the same time, mirror  504  will reflect a portion of the light coupled into lightguide  204  back along the support frame optical path. A photodetector may be positioned to detect if there is returned light in the support frame optical path. As an example, a photodetector  508  may be provided in light engine  116  to detect the returned light. The output of photodetector  508  may be received by laser safety circuit  172  (or a variant thereof), which may process the output to determine whether or not to trigger an OPF event. 
     In the examples of  FIGS.  18  and  19   , OPF testing may be combined with transferring display images to the eye. That is, for example, an aggregated beam of display light and test light may be generated by light engine  116  and directed to in-coupler  208 . Test light may be, for example, infrared light to which in-coupler  208  does not respond, which means that the test light will simply pass through in-coupler  208  to be detected by photodetector  500  (in  FIG.  18   ) or returned by mirror  504  (in  FIG.  19   ). In  FIG.  19   , mirror  504  essentially functions as a light source near display combiner  200  that directs light into the support frame optical path. Thus, in an alternative implementation of the system of  FIG.  19   , mirror  504  may be replaced with a light source. The light source may be stacked with in-coupler  208  or may be offset from in-coupler  208  but still in a position to direct light into the support frame optical path for detection within the temple. 
     When light is coupled into lightguide  204  by in-coupler  208 , not all of the light will be directed into the primary optical path within the lightguide. Some of the light will be back-reflected to a portion of the perimeter surface of the lightguide near the input region of the lightguide. It is possible to detect this back reflection with a photodetector in order to verify that the support frame optical path is defined. For illustration purposes,  FIG.  20    shows such a system, where a photodetector  512  is positioned to detect light reflected towards a portion of the perimeter surface of the lightguide proximate the input region of the lightguide. The output of photodetector  512  can be received by laser safety circuit  172  (or a variant thereof), which may process the output to determine whether or not to trigger an OPF event. 
     The above description of illustrated embodiments, including what is described in the Abstract of the disclosure, is not intended to be exhaustive or to limit the embodiments to the precise forms disclosed. Although specific embodiments and examples are described herein for illustrative purposes, various equivalent modifications can be made without departing from the spirit and scope of the disclosure, as will be recognized by those skilled in the relevant art. The teachings provided herein of the various embodiments can be applied to other portable and/or wearable electronic devices, not necessarily the exemplary wearable electronic devices generally described above.