Changchun Institute of Optics, Fine Mechanics and Physics researchers created binary phase-engraved (BiPE) superpixel CFM to overcome the efficiency limits of superpixel CFM applied using DMDs while simultaneously taking advantage of the exciting features given by the new PLM device technology. The findings were published in Light: Advanced Manufacturing.
Principle of complex field modulation (CFM) using binary phase-engraved (BiPE) superpixels. A system schematic. BS1 and BS2, beamsplitters; L1 and L2, lenses; M1–M3, mirrors; PLM, phase light modulator. b Side-profile illustration of a one-dimensional (1D) BiPE superpixel of size m = 4 . c Exploded view of the structure of each PLM micromirror. d Illustration of the phase pre-factor created at the PLM surface, with boundary phase values and the footprints of superpixels and individual micromirrors highlighted. e Normalized micromirror displacement response is illustrated as a function of electrode state (left) and as a distribution of displacement levels (right), adapted from data reported in Proc. SPIE 11698, 116980O (2021). Image Credit: Patrick Kilcullen, Jingdan Liu et al.
Complex modulation of optical fields (controlling the amplitude and phase of light) is critical for laser applications in various domains, including optical communication, three-dimensional display, biomedical imaging, and astronomy. Despite the impressive progress made by complex field modulation (CFM) applications, advancements in technical capabilities such as accuracy, speed, and versatility are still in high demand.
Spatial light modulators (SLMs) are critical to the overall performance of CFM technologies. These advanced devices enable two-dimensional arrays of electronically programmable pixels whose optical properties can be individually manipulated to alter a small area of an incident light field.
However, despite continual improvements to SLM technology, CFM's requirements for a huge pixel count (> 1 megapixel) and a fast refresh rate (> 1,000 frames per second) limit the range of optical impacts that SLM pixels can experience.
The influence of its pixels is specifically limited to the basic “ON” or “OFF” control of light amplitude, which is accomplished by reflection from microscopic mirrors that are electronically tilted in one of two directions. This is due to the high-resolution and high-speed modulation offered by digital micromirror devices (DMDs).
Notwithstanding these drawbacks, combining the optical effect of individual SLM pixels through multi-pixel clusters known as "superpixels" allows for precise control of both amplitude and phase. Binary amplitude-based superpixel CFM offers practical benefits in speed, resolution, device calibration, and effective data storage when used with DMDs.
It enables the encoding of complicated optical fields by a single binary hologram. However, the functioning of DMD-based superpixel CFM results in the majority of incident light being lost because of the constraints of binary modulation, severely restricting the overall efficiency to less than 10%.
Fortunately, advances in SLM technology now provide the foundation for overcoming these restrictions. In contrast to DMD's “tip-tilt” micromirror actuation, the phase light modulator (PLM), which Texas Instruments are developing, actuates micromirrors via “piston-mode” displacements, with each reflective pixel able to influence light phase by selecting one of sixteen positional states.
Importantly, the PLM's structure is very similar to that of DMDs, allowing it to retain the benefits of high speed and pixel counts while also being immune to pixel crosstalk, polarization independency, and operating at high stability.
Unfortunately, current PLMs produce a non-uniform distribution of phase levels due to fabrication restrictions, limiting the information imparted by pixel control to less than 4 bits in practice. More crucially, this phase-distribution characteristic may imply that device-specific calibration and adjustment are required for sensitive CFM employing PLMs—a condition unlike the pixels of DMDs, which do not have intermediate states between “ON” and “OFF.”
The approach uses a unique superpixel design to enhance the PLM’s phase-only modulation capabilities. BiPE superpixels can exploit all incident light components because PLM micromirrors do not lose light through tilting, allowing the approach to achieve an optical efficiency that is intrinsically better than amplitude-based SLMs.
Furthermore, the operation of BiPE superpixels completely avoids problems with PLM nonlinearity by requiring only the use of binary phase states corresponding to the selective introduction of half-wavelength phase shifts, whereas the 4-bit operation of PLM micromirrors faces a non-uniform phase response.
The researchers demonstrate that the combinatorics of binary phase superpixel modulation enriches BiPE superpixels with a 10-fold increase in the number of accessible complex field states compared to binary amplitude-based superpixels, despite the surprising 1-bit operation of PLM pixels.
BiPE superpixels have demonstrated high-accuracy spatial amplitude and phase modulation at up to 1.44 kHz when used with existing PLM prototypes. Additionally, they have shown off BiPE-superpixel-based CFM for several uses, including augmented reality (AR) display, improved image projection, and structured beam shaping.
In the future, the researcher plans to use multi-core computing gear, such as graphics processing units, to develop pattern-generating algorithms that could expedite the computation of BiPE superpixel holograms. This capability, according to researchers, will strengthen the BiPE-superpixel method for CFM applications that require adaptable displays and/or little processing overhead.
Journal Reference:
Kilcullen, P., et al. (2025) High-speed complex field modulation using binary phase-engraved superpixels. Light: Advanced Manufacturing. doi.org/10.37188/lam.2025.017.