With a thickness of only 2.5 millimeters, NVIDIA developed a holographic optical waveguide VR solution

With the development of technology, vr's visual effects and sense of experience are getting better and better, but there are still a variety of technical limitations of this technology, the most obvious of which is vr's bulky shape. In fact, since the advent of the Rift headset in 2016, the volume of the C-end VR headset has indeed gradually shrunk, but the existing VR headset products are still generally bulky. This is mainly because VR optics usually have a certain thickness, which has not improved much after years of development. To this end, some VR manufacturers have begun to explore Pancake-based ultra-short throw optics, but there is no VR all-in-one product that combines 6DoF Inside-Out positioning and short-throw solutions.

Pancake-based ultra-short throw VR is already lightweight enough, but in NVIDIA's eyes, VR can be smaller, especially the thickness of the VR optical module, and the distance between the lens and the display can be shortened. Therefore, NVIDIA has partnered with Stanford University to develop a holographic VR display scheme based on SLM elements and optical waveguides, and recently announced this result.

According to Qingting Network, NVIDIA's holographic solution can shorten the thickness of VR glasses to 2.5 mm, and can not only display 2D images, but also support 3D display, and the visual effect is more three-dimensional. Its biggest breakthrough is the use of precision-designed optical structures, which greatly reduce the thickness of optical modules. It is worth noting that the scheme can be combined with an eye-tracking module and uses a Pupil-HOGD algorithm to calibrate the user's dynamic pupil size (error less than 0.5 mm) to improve VR image quality. Cleverly, the user's pupil can also be used as a natural Fourier filter.

Holographic VR optical principles

We know that the essence of a VR headset is to place a display very close to your eyes. The human eye is too close to the screen to see things clearly, so in order to make you see the VR screen clearly at close range, you need to add a lens structure to enlarge the image. For a long time, VR has often used Fresnel lens structure, and the disadvantage of this lens scheme is that the optical path is longer, so the thickness of the optical module is large. In contrast, Pancake lenses use a folding light path that significantly reduces the overall thickness.

Still, Pancake's optical solution still has a certain thickness and can only display 2D images, so NVIDIA has developed a holographic VR solution to solve the pain points of Pancake optics.

To put it simply, this holographic VR solution consists of three core parts: 1, a holographic display in virtual mode; 2, a geometric phase lens; and 3, a pupil-bionic optical waveguide.

Among them, the holographic display contains elements such as eyepieces, beam splitters, lasers, and phase-only SLM. Compared with the real mode, the main difference between the holographic display in the virtual mode is the different position of the imaging plane, and the advantage is that the optical path can be shortened and the hardware thickness can be reduced. In fact, pure phase SLM can display holographic images in front of and behind the module, and when displayed in the front, the light is reflected to the side of the eyepiece through the beam splitter, and when displayed in the rear, the distance between the eyepiece and the SLM optical path can be further shortened, thereby reducing the overall thickness.

To further reduce the size of the hardware, NVIDIA replaced the beam splitter with a light waveguide structure that mimics the pupil, changing the holographic position to the human eye. In addition, lighter geometric phase lenses (GPs) are used instead of eyepieces. GPs only support a few specific beam polarizations, and since most SLMs only support linearly polarized light, NVIDIA adds a quarter waveform (QWP) between the SLM and GP lenses.

VR prototyping and optimization

To verify the effect, NVIDIA designed three prototype schemes of desktop, monocular and binocular, in which the biocular eyewear prototype is only 2.5 mm thick, weighs 60 g, the diagonal FOV is 22.8°, the fixed eye track range is 2.3 mm, and the dynamic eye track range can reach 8 mm. Of course, due to the scalability of the solution, existing prototype parameters can be further optimized. NVIDIA said: The display characteristics of the holographic display scheme are mainly determined by SLM and eyepieces, such as the larger the volume of SLM, the larger the FOV, the smaller the SLM pixels, the greater the eye track range, and the shorter the focal length of the GP lens, the shorter the eye distance.

So while the prototype VR glasses have a FOV of only 22.8°, NVIDIA says it can boost monocular FOV to 120° if it uses a 2-inch SLM and a 15mm focal length GP lens. In addition, stacking two identical GP lenses and a circular polarizer can also halve the focal length without significantly increasing the thickness. In addition, the prototype of the VR glasses designed by NVIDIA has an eye track range of only 2.3 mm, but by combining it with the eye tracking module, the eye track range can be extended to 8 mm. At the same time, the size of the eye track range is also directly related to the pixel pitch of SLM.

In order to optimize pixel detail and image quality, NVIDIA also uses THE HOGD (High Gradient Descent) algorithm to replace the SGD (Random Gradient Descent) algorithm to improve contrast. In the future, the use of optical waveguides designed for holographic VR glasses can further improve image quality.

In short, NVIDIA's holographic VR optical solution mainly solves the following problems: to create an ultra-thin optical solution that supports 3D display, to further shorten the distance between the eyepiece/lens and the display panel, and then to reduce the thickness of the VR headset to achieve a lightweight all-weather design. It is a scheme based on SLM and optical waveguide, not a traditional combination of lens and display panel structure.

Compared with traditional VR optics, the holographic display scheme has two different points: 1) higher diffraction level; 2) with dynamic eye tracking range, SLM light direction can be controlled by the angle of the input beam, after combining the eye tracking module, you can follow the fixation point and the angle of view to synchronously change the angle of the input beam, so as to achieve dynamic zoom and solve the problem of visual spoke adjustment conflict.

In fact, NVIDIA's holographic VR solution is also reminiscent of a HOE-based VR glasses solution that Meta has shown before. In contrast, Meta's scheme uses a 2D holographic + liquid crystal combination and cannot display 3D stereo vision, while NVIDIA uses a holographic + optical waveguide scheme, based on a laser light source. However, the thickness of the NVIDIA holographic VR optical module is only about 2.5 mm, the FOV of the prototype is 22.8 °, and the weight without SLM is 60g, as a comparison of the prototype thickness of the Meta holographic VR glasses is about 9 mm, and the field of view is as high as 90 ° range, and the two technical paths are different, resulting in obvious differences.