Veterans Affairs

Combination amplitude modulation SLM with a phase modulating SLM in the same optical illumination system – benefits to the fields of fluorescent microscopy, 3D printing, and direct laser writing

Device provides a dynamic means to create complex illumination patterns with high light utilization efficiency and void of unwanted diffraction patterns

Photonics

While the applications for this technology are broad, it holds promise for advancing the state of the art in bioprinted skin tissue. Credit: Lucia Liu, Molly Boutin and M.J. Song, NCATS

Scientists at the U.S. Department of Veterans Affairs (VA) have recently combined an amplitude modulating spatial light modulator with a phase modulating spatial light modulator in a single optical illumination system. The patented technology is available via patent license agreement to companies that would make, use, or sell it commercially.

One conventional means to create spatial light patterns is to modulate the amplitude of light as a function of position within a 2D plane. Examples devices include traditional film projectors, LCD projectors, and digital micromirror device (DMD) based projectors. DMDs are reflective devices that can project very high intensities of light at high switching speeds. A limitation of DMDs is that they are extremely inefficient at utilizing the available light energy when only a small portion of the target area (sample) needs to be illuminated. This is particularly problematic if small localized regions of a sample require very high light intensities, such as during photolysis of a caged compound and stimulation for optogenetics. In addition, DMD is also a 2D technology and cannot simultaneously control light in 3 dimensions.

An alternate means to create spatial light patterns is to modulate the phase of the light – as done with holography. But limitations of digital holographic systems include the production of zero and higher-order diffraction patterns as well as ghost images that must be blocked. Typically, a physical barrier is used to block the zero-order diffraction pattern but there are no practical solutions to block the higher-order diffraction patterns. In addition, most phase-only SLMs are unable to rapidly switch between different patterns. Moreover, the diffraction efficiency and thus the distribution of light in the field of view of the objective lens depends on the lateral position. In most applications for patterned illumination, it is best to have a flat field in terms of intensity as a function of space.

To address the above limitations, VA researchers have now combined an optical illumination system with both an amplitude modulating SLM and a phase modulating SLM. The combination of the amplitude modulation SLM and the phase modulation SLM allows the optical illumination system to compensate for the limitations of each mode.

Certain applications of the system include multi-site photostimulation, two-photon photostimulation, structured illumination for microscopy, and light-controlled 3D fabrication, just to name a few. Specific to 3D fabrication, the system improves the performance by masking the unwanted diffraction pattern and by allowing the DMD to independently control the light energy that can be delivered to specific locations. Advantageously, although the axial resolution might be limited in such an embodiment, this can be compensated for by moving the position of the DMD at critical locations. This fabrication method is particularly useful for initial substrates that may be in a fluid state and which are not highly scattering (e.g., clear plastics, tissue scaffolds and the like).

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