Light control in a multicore fiber using the memory effect

Abstract

Endoscopic imaging through a multicore fiber (MCF) is widely used but is affected by pixelated images, which limits its resolution to a few micrometers. This is due to the spacing between the cores in the MCF, which is large enough to avoid core-to-core light coupling. Wavefront shaping techniques have been shown to focus light to a resolution finer than the inter core spacing, however a long calibration procedure is needed. Moreover the calibration depends on the optical fiber conformation. Here, we show a calibration method using only one digital hologram. The method is based on digital phase conjugation and the memory effect of the MCF to focus and scan a spot. In addition, we show how simple patterns can be projected using the same multicore fiber.

Ultrathin endoscopes based on multicore fibers and adaptive optics: a status review and perspectives

“…45,47,[58][59][60][61] The SLM plane can be placed in a plane that is conjugated with the MCF proximal end face, which was done in the early reports. 44,59 However, it is advantageous to offset the SLM from the conjugate plane, as in Fig.…”

“…45, a faster version of this method was used, where a spatial Fourier transform replaces the need to scan over ϕ proximal i . Also, an even faster method is based on digital phase conjugation, 61,62 which additionally eliminates the need for sequential, pairwise illumination of cores, but requires an external reference beam.…”

“…More recently, the work of Stasio et al 61 has also highlighted the use of the memory effect for point-scanning in a lensless endoscope. The imaging fiber bundle employed by the authors had a significant amount of cross talk between cores, which led to a decreased memory effect.…”

Lensless two-photon imaging through a multicore fiber with coherence-gated digital phase conjugation

“…Combining a widefield imaging modality with TPF imaging would provide a powerful tool for endoscopic use. Also, in a previous paper, 27 we showed that the LP 01 mode exhibits a large scan range when a linear phase gradient is applied to the DPC hologram. This "memory effect" ability could be utilized to increase the imaging speed with high speed galvo-mirror scanning within a single calibration point, or with high speed digital mirror SLMs.…”

“…However, to create the focus, we use coherence-gated digital phase conjugation (DPC), as we have done previously with a GRIN and step-indexed MMFs. 24,25 DPC compensates for core-to-core coupling 27 and allows for use of commercially available MCFs, which contain a large number (several thousand to 100,000) of densely packed cores arranged in a quasiperiodic array. The closely spaced cores spread the diffraction orders in the focal plane away from the desired focus, which allows for a larger field of view than possible with highly spaced core MCFs 18 and also reduces the power distributed to the diffraction orders.…”