In the context of optical diffraction tomography 27, Lee et.al demonstrated a time-multiplexing structured illumination control scheme using DMDs 28. In display application, the DMD is used to modulate the time integrated light intensity through sequential pulse width modulation 23, 24, 25, 26. They used phase retrieval methods to design a binary computer generated hologram and demonstrated speed enhancement by approximately 2 orders of magnitude as compared to a liquid crystal SLM with a low playback latency of ~ 5 ms. In recent years 22, Wang et.al implemented a binary-based digital optical phase conjugation (DOPC) system based on the use of DMD instead of SLMs. A Bayesian phase retrieval algorithm 21 was used to precisely estimate the transmission matrix as well as for beam focusing through a white paint layer which acted as the highly scattering medium. In 20, Drémeau et.al used the DMD as a binary input in a reference-less optical system for beam focusing. An improved optical performance was achieved by projecting Lee holograms on the DMD along with spatial filtering and misaligned optical lenses for phase modulation using “Super-pixel” technique where each super-pixel is composed of multiple pixels of the DMD 19. demonstrated the use of Lee computer generated holograms 18 to achieve beam focusing in scattering media with signal to background ratio up to ~ 160 in ~ 34 ms. Despite its binary amplitude-modulation, it was shown that DMDs can outperform SLMs in beam-shaping applications 16. However, as an “on–off” device, DMD allows only binary amplitude-modulation as compared to the grayscale and phase modulation that is possible with SLMs. DMD devices have a maximum refresh rate of 32 kHz 15. As opposed to SLMs, DMDs are characterized by their much faster refresh rates.
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This has opened the door for another technology, DMDs, to be used for similar applications 9, 10, 11, 12, 13, 14. This limitation can make it impossible to achieve high performance in applications that require high speed such as beam focusing in live samples.
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A main limitation of liquid crystal SLMs is their low refresh rates, typically less than 120 Hz. SLMs have been widely used in optical applications including wave-front shaping and light focusing 1, 2, 3, 4, 5, 6, 7, 8. For faster modulation speed, an electro-optic modulator was used in synchronization with the DMD in an amplitude modulation mode to create grayscale patterns with frame rate ~ 833 Hz with display time of only 1.2 ms instead of 38.4 ms for time multiplexing gaining a speed up by a factor of 32. We performed phase conjugation by compensating the distortions incurred due to propagation through free-space and a scattering medium.
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With this method, we demonstrated 8-bit complex field modulation with a frame time of 38.4 ms (around 0.15 s for the entire complex-field). We used the built-in time domain dynamic range representation of the DMD to project 8-bit complex-fields. DMDs however can only display binary, unipolar patterns and utilize temporal modulation to represent with excellent accuracy multiple gray-levels in display applications. As compared to spatial light modulators (SLMs), they are characterized by their much faster refresh rates (full-frame refresh rates up to 32 kHz for binary patterns) compared to 120 Hz for most liquid crystal SLMs.
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Digital micro-mirror devices (DMDs) have been deployed in many optical applications.