What is the optical path of a 1280x720 AR waveguide?

The optical path of a 1280x720 AR waveguide is not a single fixed route but a carefully engineered sequence of light interactions that take a micro-display image and deliver it to your eye as a see-through overlay. In a typical birdbath or freeform prism design, the light goes straight from the display to a combiner, but waveguides use total internal reflection (TIR) to bounce the image along a thin glass or plastic slab before coupling it out into your pupil. For a 1280x720 resolution, which is HD-ready, the waveguide must handle a pixel pitch usually around 3.5 to 5 microns on the micro-OLED or LCoS panel, and the optical path must preserve that detail across a field of view (FOV) that typically ranges from 25 to 40 degrees diagonal. The path starts at the micro-display, which for 1280x720 is often a 0.39-inch or 0.5-inch OLED from Sony or an LCoS from Himax. The display emits unpolarized or polarized light, depending on the architecture, and that light hits a collimating lens system. This lens system, usually a set of 2 to 4 plastic or glass aspheres, turns the diverging rays from each pixel into a nearly parallel beam. The collimated beam then enters the waveguide through an input grating or a prism coupler. For a 1280x720 waveguide, the input coupler is typically a surface-relief grating (SRG) with a period around 300 to 450 nanometers, or a volume Bragg grating (VBG) with a thickness of 10 to 30 microns. The grating diffracts the light into the waveguide at a specific angle that satisfies TIR, meaning the angle of incidence inside the slab must be greater than the critical angle, which for glass with a refractive index of 1.5 to 1.7 is around 40 to 45 degrees.

Once inside the waveguide, the light bounces between the two parallel surfaces. The number of bounces depends on the waveguide thickness and the FOV. For a 1.5mm thick glass waveguide with a 30-degree FOV, the light might bounce 5 to 10 times along a 30mm path. Each bounce must maintain the polarization state and the angular content of the image, because any scattering or absorption at the TIR surfaces will reduce contrast and resolution. The waveguide material is usually high-index glass like Schott N-SF11 or N-LAF2, with an Abbe number above 50 to minimize chromatic aberration. For a 1280x720 color image, the waveguide must handle red (630nm), green (530nm), and blue (460nm) wavelengths simultaneously. This is where the optical path gets tricky. Single-layer gratings have strong chromatic dispersion, so the red and blue light will diffract at different angles. To fix this, engineers use either a multi-layer grating stack or a two-dimensional grating that splits the path into separate color channels. For example, a common approach is to use a diffractive waveguide with a 2D grating that has a period of 400nm for the in-coupling and a 1D grating for the out-coupling. The in-coupling grating diffracts the light into the waveguide, and the light propagates by TIR. The out-coupling grating then extracts the light toward the eye. The extraction efficiency of the out-coupling grating is typically 10% to 20% per bounce, so the light gradually leaks out over multiple bounces to create a uniform exit pupil. For a 1280x720 waveguide, the exit pupil size is usually 10mm to 15mm in diameter, which is the eye box. The eye box must be large enough to accommodate different eye positions, and the waveguide must maintain a uniform brightness across that area. The uniformity is measured in terms of luminance variation, which should be less than 20% across the eye box for a good user experience. The optical path also includes a pupil expander in some designs. In a 1D waveguide, the light only expands in one direction, so the eye box is a slit. To get a 2D eye box, you need a two-dimensional grating or a folded path. For example, a popular design from Lumus uses a series of partial reflectors embedded in the waveguide, called a light-guide optical element (LOE). The reflectors are coated with dielectric layers that have a reflectivity of 10% to 30% for the specific wavelength range. The light enters the LOE, bounces, and hits the reflectors, which split the beam into multiple copies that exit the waveguide. The number of reflectors can be 10 to 20, each with a slightly different angle to create a uniform exit pupil. The optical path in this case is more complex because the reflectors must be precisely aligned to avoid ghost images and double vision.

For a 1280x720 waveguide, the resolution is limited by the waveguide's ability to preserve the modulation transfer function (MTF). The MTF at 30 cycles per degree (cpd) should be above 0.3 for a sharp image. The waveguide itself introduces a low-pass filter effect because the grating diffracts light into multiple orders, and the TIR bounces cause a slight blur due to the finite thickness of the waveguide. The blur is typically 1 to 2 arcminutes, which is acceptable for a 1280x720 display where the pixel pitch is about 2.5 arcminutes per pixel at a 20-degree FOV. The contrast ratio is another critical parameter. The waveguide must have a contrast ratio of at least 100:1 in the dark areas of the image. The main source of contrast loss is stray light from the TIR bounces and from the grating's zero-order diffraction. The zero-order is the undiffracted light that passes straight through the waveguide, which can cause a veiling glare. To reduce this, the waveguide is often coated with an anti-reflective coating on the front surface, and the back surface is coated with a black absorbing layer to capture any stray light. The polarization management is also crucial. Many micro-displays emit polarized light, and the waveguide must maintain that polarization to avoid ghosting. For a 1280x720 waveguide, the typical polarization extinction ratio (PER) is 100:1 or better. The waveguide's birefringence must be less than 10 nanometers per centimeter to avoid depolarization. The temperature stability of the waveguide is another factor. The refractive index of glass changes with temperature, which shifts the TIR angle. For a 1280x720 waveguide, the temperature range is usually -20°C to 60°C, and the optical path must be designed to keep the image stable within 0.5 arcminutes over that range. This is achieved by using athermalized materials or by compensating with the display's position.

The field of view for a 1280x720 waveguide is directly related to the waveguide's thickness and the grating period. For a given thickness, the FOV is limited by the angular bandwidth of the grating. A typical 1D grating can support a FOV of about 30 degrees diagonal. To get a larger FOV, say 40 degrees, you need a thicker waveguide or a higher refractive index. For example, a waveguide made of N-LASF44 with a refractive index of 1.8 can support a 40-degree FOV with a 1.5mm thickness. The FOV also determines the eye relief, which is the distance from the waveguide to the eye. For a 1280x720 waveguide, the eye relief is usually 15mm to 20mm. The eye relief must be long enough to accommodate glasses, but it also affects the size of the eye box. The optical path includes a combiner that merges the waveguide image with the real world. The combiner is the waveguide itself, which is transparent to ambient light. The transparency is typically 80% to 90% for a diffractive waveguide, but it can be lower for a reflective waveguide. The ambient light passes through the waveguide and into the eye, while the display light is diffracted into the eye. The brightness of the display must be high enough to overcome the ambient light. For a 1280x720 waveguide, the display luminance is usually 500 to 1000 nits, but the waveguide efficiency is only 10% to 20%, so the perceived brightness is 50 to 200 nits. This is sufficient for indoor use, but for outdoor use, you need a display with 2000 to 5000 nits and a waveguide with higher efficiency. The efficiency is measured as the ratio of the output light to the input light. For a 1280x720 waveguide, the efficiency is typically 5% to 15% for a diffractive design and 20% to 30% for a reflective design. The efficiency is affected by the grating's diffraction efficiency, which is wavelength-dependent. For a single-layer grating, the diffraction efficiency at the center wavelength can be 80%, but it drops to 50% at the edges of the spectrum. To get uniform color, the waveguide must have a balanced efficiency across the red, green, and blue channels. This is often achieved by using a multi-layer grating or a chirped grating that varies the period across the surface.

The eye box for a 1280x720 waveguide is typically 10mm x 10mm to 15mm x 15mm. The eye box is the area where the eye can see the full image without vignetting. The optical path must deliver a uniform image across the entire eye box. The uniformity is measured by the variation in brightness and color. For a 1280x720 waveguide, the brightness variation should be less than 20%, and the color variation should be less than 0.01 in CIE 1931 chromaticity coordinates. The eye box is created by the pupil expander, which replicates the exit pupil. In a 1D waveguide, the pupil expander is a grating that diffracts the light into multiple angles, creating a 1D array of exit pupils. In a 2D waveguide, the pupil expander is a 2D grating that creates a 2D array of exit pupils. The number of exit pupils is typically 10 to 20 in each direction, so the total number of exit pupils is 100 to 400. Each exit pupil must have the same image content, but the intensity can vary. The variation is caused by the interference between the different copies of the light. The waveguide must be designed to minimize these interference fringes, which are called rainbow artifacts. The rainbow artifacts are more visible in diffractive waveguides because the gratings are sensitive to the wavelength. To reduce them, the waveguide is often coated with a thin film that reduces the reflection at the TIR surfaces. The film is usually a quarter-wave stack of SiO2 and TiO2, which has a reflectivity of 99.9% at the TIR angle. The film also reduces the scattering loss, which is typically 0.1% to 1% per bounce. The total loss in the waveguide is the sum of the absorption loss, the scattering loss, and the extraction loss. For a 1280x720 waveguide, the total loss is usually 50% to 70%, so the output light is 30% to 50% of the input light. The display must be bright enough to compensate for this loss. The micro-display for a 1280x720 waveguide is typically a 0.5-inch OLED with a resolution of 1280x720, which gives a pixel density of 3000 PPI. The OLED has a contrast ratio of 10000:1 and a color gamut of 100% sRGB. The display is driven by a driver IC that supports 60Hz to 120Hz refresh rates. The display's brightness is 500 to 1000 nits, but the waveguide reduces it to 50 to 200 nits. To get a brighter image, you can use a laser-based display, which can produce 10000 nits, but the waveguide must be designed to handle the coherence of the laser light, which can cause speckle. The speckle is reduced by using a diffuser or by vibrating the waveguide.

The optical path length from the display to the eye is typically 30mm to 50mm for a waveguide. The path length determines the size of the waveguide and the weight of the AR glasses. For a 1280x720 waveguide, the waveguide itself is usually 30mm to 40mm long, 20mm to 30mm wide, and 1mm to 2mm thick. The weight of the waveguide is 5 to 10 grams. The total weight of the AR module, including the display and the optics, is 10 to 20 grams. The module must be mounted in a frame that holds it in front of the eye. The alignment of the waveguide to the eye is critical. The waveguide must be tilted at an angle of 5 to 10 degrees relative to the line of sight to avoid reflections from the front surface. The tilt also affects the eye relief and the FOV. The optical path also includes a fold mirror in some designs. The fold mirror is used to reduce the size of the module. The mirror is placed between the display and the waveguide, and it reflects the light into the waveguide. The mirror is usually a flat mirror with a reflectivity of 95% to 99%. The mirror must be coated with a dielectric coating that is durable and scratch-resistant. The alignment of the mirror is critical because any tilt will shift the image. The tolerance for the mirror tilt is typically 0.1 degrees. The display's alignment is also critical. The display must be positioned so that the image is centered on the waveguide's input coupler. The tolerance for the display position is 0.1mm in the x and y directions and 0.05mm in the z direction. The display's rotation must be within 0.1 degrees. The alignment is done during assembly using a camera and a laser. The waveguide itself must be manufactured with high precision. The grating period must be accurate to within 1 nanometer, and the grating depth must be accurate to within 10 nanometers. The waveguide's thickness must be uniform to within 0.1 microns. The surface roughness must be less than 1 nanometer RMS to avoid scattering. The manufacturing process for a 1280x720 waveguide is typically nanoimprint lithography or holographic exposure. The nanoimprint process uses a stamp to create the grating pattern on a polymer layer. The polymer is then cured with UV light. The holographic process uses two laser beams to create an interference pattern in a photosensitive material. The material is then developed to create the grating. The cost of a 1280x720 waveguide is typically $10 to $50 per unit in volume, but the tooling cost is $100,000 to $500,000. The yield is 50% to 80% for a mature process.

The color uniformity of a 1280x720 waveguide is a major challenge. The grating's diffraction efficiency is wavelength-dependent, so the red, green, and blue light are diffracted at different angles. This causes the image to have a color shift across the FOV. The color shift is measured as the difference in the chromaticity coordinates between the center and the edge of the FOV. For a 1280x720 waveguide, the color shift should be less than 0.02 in the CIE 1931 diagram. To achieve this, the waveguide is often designed with a chirped grating that varies the period across the surface. The chirp compensates for the chromatic dispersion by diffracting the red light at a slightly different angle than the blue light. The chirp rate is typically 0.1 to 1 nanometer per millimeter. The grating's depth is also varied to balance the efficiency across the spectrum. The depth is typically 100 to 200 nanometers for a single-layer grating. The color uniformity is also affected by the display's color gamut. The display must have a wide color gamut to produce accurate colors. The typical color gamut for a 1280x720 waveguide is 100% sRGB or 70% DCI-P3. The display's color temperature is usually 6500K. The waveguide's color temperature can be adjusted by using a color filter or by adjusting the display's white balance. The brightness uniformity is another issue. The light from the display is not uniform across the FOV because the waveguide's extraction efficiency varies with the angle. The extraction efficiency is highest at the center of the FOV and lowest at the edges. The variation is typically 20% to 30%. To improve the uniformity, the waveguide is designed with a graded extraction grating that has a higher efficiency at the edges. The grating's efficiency is graded by varying the depth or the duty cycle. The duty cycle is the ratio of the grating's ridge width to the period. The duty cycle is typically 0.5 for a uniform grating, but it can be varied from 0.3 to 0.7 to achieve a graded efficiency. The graded extraction grating is often combined with a pupil expander that has a graded efficiency as well. The pupil expander's efficiency is graded by varying the number of bounces or the reflectivity of the partial reflectors. The number of bounces is typically 5 to 10 for a 1D expander and 10 to 20 for a 2D expander. The reflectivity of the partial reflectors is graded from 10% to 30% across the waveguide. The grading is done by using a variable coating thickness or by using a mask during the coating process. The brightness uniformity is also affected by the display's luminance uniformity. The display must have a luminance uniformity of 90% or better across the active area. The display's pixels are driven by a current source that can be adjusted to compensate for non-uniformity. The compensation is done by using a calibration table that stores the correction values for each pixel. The calibration is done during the manufacturing process using a camera and a photometer.

The ghost images in a 1280x720 waveguide are caused by multiple reflections between the waveguide's surfaces. The ghost images appear as faint copies of the main image, shifted by a few pixels. The ghost images are more visible in high-contrast scenes. The main source of ghost images is the reflection from the front surface of the waveguide. The front surface reflects about 4% of the light, which can bounce back into the waveguide and create a ghost. To reduce the ghost images, the front surface is coated with an anti-reflective coating that reduces the reflection to 0.1% to 0.5%. The back surface is also coated with an anti-reflective coating to reduce the reflection from the back. The ghost images can also be caused by the grating's higher-order diffraction. The grating diffracts the light into multiple orders, and