At the recent Frontiers in Optics and Laser Science conference, a paper by Peter Smith from the University of Southampton described how IR wave-mixing can create visible AR transparent display solutions. His concept is to fabricate a transparent non-linear material on the lenses of a pair of glasses. An IR laser beam is then divided into two paths. One path illuminates an LCOS or DLP imager whose light is coupled into the eyeglasses and transported to the non-linear material via total internal reflection. The second path can condition the beam and is likewise directed from the opposite direction into the glasses and toward the non-linear material. The non-linear material converts these beams via second-harmonic generation to visible light (515 nm for the 1030 nm IR laser).
Basic Set-up for Wave-Mixing in AR Glasses
What is particularly interesting about this concept is that the DLP or LCOS chip can be used as a spatial light modulator – i.e. writing a computer-generated hologram to it. On the reference beam side, the shape of the beam can be used to change the distance of the virtual object. A plane wave places the object at infinity while a curved wave can move virtual objects closer.
One way to power this solution is using Sony’s new DPSSEL IR lasers (see Recent Advancements in Laser-based AR Engines article). The 1030 nm device creates green light while the 1313 nm device and the 946 nm devices create red (656 nm) and blue light (473 nm) when converted within the non-linear material. Smith also notes that he is developing a new organic non-linear material for this application that is only 10 microns thick and can support images up to 120 pixels per degree (20/10 vision).
These devices are still lab prototypes so much work needs to be done to optimize and potentially commercialize a solution. However, his team is making good progress as noted in the chart below.
Specs for Wave-Mixing Display
Interestingly, a paper from Universidad de Murcia noted that non-linear conversion can also occur naturally in the photoreceptors of the eye. Some anecdotal evidence suggests that light in the 1.11 to 1.18 nm wavelength has been perceived as yellow-green or orange light by some observers.
The team then set out to verify this and measure the visual acuity of 2-photon vision. They also wondered if it might be possible to make direct-view wave-mixing displays using 3 different IR lasers to create white light and hence, multiple colors. They created an experimental set up to do just that and had test subjects look at patterns to assess visual acuity. They found it is similar to visible light acuity at best focus but shifted accordingly for chromatic aberration. The reduced chromatic aberration of an IR-RGB display could offer advantages in creating display solutions compared to using RGB illumination. It might also enable people with cataracts or opaque corneas to “see” in the future.
