Fairy Lights

Figure 2. System setup. (Image: Ochiai et al.)

Figure 3. Rendering voxels (Image: Ochiai et al.)

Figure 4. Example applications of laser-based graphics technology. (Image: Ochiai et al.)

Fairy Lights refers to the images created by a method of rendering aerial and volumetric graphics using a femtosecond laser.

The aerial plasma, induced by laser, emits light without interaction with physical matter. The practical result of the ionization of air molecules is crackling, photon-emitting pockets of plasma (voxels) that can be arranged to create moving and interactive images.

The technology was developed by researchers at the University of Tsukuba, Utsunomiya University, the Nagoya Institute of Technology and the University of Tokyo and first published in a scientific paper called Fairy Lights in Femtoseconds: Aerial and Volumetric Graphics Rendered by Focused Femtosecond Laser Combined with Computational Holographic Fields. According to Ochiai et al., the main contribution of the paper was “the production of an in-air spatial light modulator-based laser-plasma graphics that enables physical contact and interaction by ultra-short pulse duration laser.”

There are two main methods to render graphics in air using a femtosecond laser. The first is by spatial light modulation technology and the second is the scanning of a laser beam by a galvano mirror. Currently, the display size and workspace for these systems is 1cm³ and 5 cm³, respectively. Although these sizes are still too small for the practical applications proposed in the research paper, the study will allow for further development and design of laser-based aerial volumetric displays that could be scalable according to the optical gadgets and setup used.

With volumetric displays, users can see the images produced from any angle. They can be divided into two categories according to the characteristics of the voxels. The first is voxels that emit light, and the second voxels that reflect light. Those in the first category can be LEDs, end points of optical fibers, or laser-induced plasma. The voxels in the second category that reflect projected light can take the form of a fog, water drops, or floating small particles.

In the study conducted by Ochiai and colleagues, they used laser-induced plasma which has some advantages: 1) “it does not require physical matter arranged and suspended in air to emit light,” 2) “it does not require wires and structures that possibly obstruct the line-of-sight because power is transmitted wirelessly”, and 3) “the laser can be precisely controlled owing to the progress in optical technologies.” By using a femtosecond-laser display system, the technology is also safer than using a nanosecond laser.

Previous studies pioneered the development of laser-plasma 3D displays using a nanosecond and femtosecond laser. Nevertheless, the studies were incomplete and Ochiai and colleagues intended to provide a complete discussion on laser-plasma graphics technology.

Fairy lights is that they are interactive (Figure 1). A person can touch them, feel them, and control them. The haptic feedback is a result of the shockwave created when plasma interacts with material. The current displays are very small, the spatiotemporal resolution is relatively high (up to 200,000 voxels), and the image framerate varies according to the voxel density needed.

Creating Fairy Lights

The basic setup to generate a simultaneous-multi-point volumetric display consists of a femtosecond laser source, an XYZ scanner (galvano scanner + varifocal lens), and a liquid crystal on silicon SLM (LCOS-SLM) displaying a CGH for simultaneously addressed voxels (Figure 2).

To generate fairy lights, there is a need for a medium that can reflect or emit light at precise locations throughout a given volume. The researchers used air, turning it into plasma. Focusing a laser very precisely, it is possible to produce a small, isolated volume of plasma, thereby creating a voxel. These can be seen as the 3D equivalent of pixels and stand for “volume element” (Figure 3).

Previous successful attempts at creating aerial plasma used nanosecond laser pulses. These type of lasers had the effect of burning tissue since the energy associated the lit voxels was high. Ochiai and colleagues used femtosecond (a quadrillionth of a second) laser pulses to generate short lived, light-emitting plasma voxels that would not burn tissue when touched, although they can still burn softer tissue like eyeballs. Since plasma emits ultraviolet and infrared radiation, the researchers still advise the use of glasses with infrared filters as a precaution.

Each voxel is generated by a laser with pulse durations of 30-100 fs and 269 fs, resulting in a safer technology that doesn’t produce any relevant skin damage “unless the laser is firing at that same spot at one shot per millisecond for a duration of 2,000 milliseconds.”

According to Doss (2015), “the location of the voxels is determined by using a computer-generated hologram, which is a method of digitally creating interference patterns that are printed on a mask or film. The mask or film is also called a spatial light modulator and its role is to alter the phase and location of the coherent light source illuminating it.“ He concludes that using the femtosecond laser, it was possible “to create multiple plasma voxels with one pulse and create voxels at a faster rate, leading to a well defined, bright 3D holographic image safe enough to touch, and feel.”

Limitations

The first main limitation of this technology that needs to be addressed in future research in the system’s scale up, which is dependent on the optical gadgets used and the setup. The fairy lights produced are only 1cm³ in volume, with 5 cm³ to move around in. A Japanese firm - Aerial Burton - has a technology that projects large images through ionization but they are too dangerous to touch and loud.

According to Ochiai et al., three factors need to be developed in order to scale up their system: 1) increasing power of laser source; 2) shortening pulse width to increase peak power; 3) and increasing scanning speed. The development of these makes possible to “have an amount of simultaneously addressed and scanned voxels within a single frame, keeping visible and touchable features.”

Another limitation of the system is the safety issue. Plasma has high energy and can be harmful to humans. While the use of a femtosecond laser was proven to reduce the damage to tissue, the plasma can still damage the retina.

Currently, it seems that there is a trade-off between large and dangerous displays and small, interactive ones. Furthermore, the color of the fairy lights cannot be changed, and plasma can be a little noisy.

Applications

The viable implementation of laser-induced plasma aerial images (i.e. Fairy lights) has many useful applications such as spatial aerial augmented reality (AR), aerial user interfaces, and aerial volumetric images (Figure 4). These would be effective means to display 3D information, and could even be applied to wearable materials and spatial user interactions. In the AR field, this technology offers an advantage since the content is generated in a 3D space instead of a 2D computer display.