Focusing light through multimode fibres using a digital micromirror device: a comparison study of non-holographic approaches

Abstract

Focusing light through a multimode fibre (MMF) has attracted significant research interest, mainly driven by the need for miniature endoscopes in biomedicine. In recent years, digital micromirror devices (DMD) have become increasingly popular as a high-speed alternative to liquid-crystal spatial light modulators for light focusing via wavefront shaping based on binary amplitude modulations. To exploit the potentials and limitations of the state-of-the-art DMD-based wavefront shaping methods, in this study, for the first time, we compared four representative, non-holographic and DMD-based methods that are reported so far in literature with the same experimental and simulation conditions, including a real-valued intensity transmission matrix (RVITM)-based algorithm, a complex-valued transmission matrix (TM)-based algorithm, a conditional probability algorithm and a genetic algorithm. We investigated the maximum achievable peak-to-background ratio (PBR) in comparison to theoretical expectations, and further improved the performance of the RVITM-based method. With both numerical simulations and experiments, we found that the genetic algorithm offered the highest PBR but suffered from the lowest focusing speed, while the RVITM-based algorithm provided a comparable PBR to that of the genetic algorithm, and the highest focusing speed.

All-optical multimode fibre photoacoustic endomicroscopy with scalable spatial resolution and field-of-view

“…MMF characterisation was performed using a high-speed algorithm developed by the author's group, namely realvalued intensity transmission matrix [7]. It took ~ 3 min to characterise the MMF for raster-scanning at 7850 spatial positions.…”

“…Wavefront shaping algorithm Since mode dispersion scrambles coherent light transport through MMFs, the MMF was calibrated for shaping the transmitted light into a tightly focused beam prior to imaging implementation. The calibration method was detailed in our previous works [7]. Here briefly, a series of binary masks constructed using Hadamard patterns were applied to the micromirrors of a digital micromirror device (DMD) to modulate the wavefront of the excitation laser and projected onto the proximal fibre tip while the corresponding output speckles were recorded by a camera.…”

Video-rate dual-modal photoacoustic and fluorescence imaging through a multimode fibre towards forward-viewing endomicroscopy

“…The speed of high-quality PA imaging of RBCs is mainly limited by the decrease of laser pulse energy when the pulse repetition rate of the laser is larger than 10 kHz. Although the RVITM method had a higher usage ratio of laser energy compared to holographic approaches [49] , the laser pulse energy transmitted through the optical fibre was 100 nJ at a pulse repetition rate of 10 kHz and declined to 62 nJ at 22.7 kHz and 30 nJ at 47 kHz. Further, the focusing enhancement obtained in this work was lower than that reported with holographic approaches with a larger number of input modes [29] and thus resulted in a lower power in light foci.…”

Video-rate dual-modal forward-viewing photoacoustic and fluorescence endo-microscopy through a multimode fibre

“…Although the RVITM method had a higher usage ratio of laser energy compared to holographic approaches, 39 the laser pulse energy transmitted through the optical fibre declined to ∼100 nJ at a pulse repetition rate of 10 kHz. Further, the focusing enhancement obtained in this work was lower than that reported with holographic approaches with a larger number of input modes 21 and thus resulted in lower power in light foci.…”