Delivery of focused short pulses through a multimode fiber

Abstract

Light propagation through multimode fibers suffers from spatial distortions that lead to a scrambled intensity profile. In previous work, the correction of such distortions using various wavefront control methods has been demonstrated in the continuous wave case. However, in the ultra-fast pulse regime, modal dispersion temporally broadens a pulse after propagation. Here, we present a method that compensates for spatial distortions and mitigates temporal broadening due to modal dispersion by a selective phase conjugation process in which only modes of similar group velocities are excited. The selectively excited modes are forced to follow certain paths through the multimode fiber and interfere constructively at the distal tip to form a focused spot with minimal temporal broadening. We demonstrate the delivery of focused 500 fs pulses through a 30 cm long step-index multimode fiber. The achieved pulse duration corresponds to approximately 1/30th of the duration obtained if modal dispersion was not controlled. Moreover, we measured a detailed two-dimensional map of the pulse duration at the output of the fiber and confirmed that the focused spot produces a twophoton absorption effect. This work opens new possibilities for ultra-thin multiphoton imaging through multimode fibers. References and links

Ultra-thin rigid endoscope: two-photon imaging through a graded-index multi-mode fiber

“…With 4 cm of MMF the output mode has a duration FWHM of 280 fs compared to 240 fs before the MMF, or a difference in second-order spectral phase of 2000 fs 2 (transform-limited FWHM 124 fs). A similar observation was also made in [11], by Morales-Delgado et al which demonstrated that if one employs a subset of MMF eigenmodes one can to a large extent retain the short pulse duration. The difference in our present work is that we can retain the short pulse duration while employing all MMF eigenmodes simultaneously, i.e.…”

“…The so-called lensless endoscopes which have recently been demonstrated [5,6,7,8,9] might have the potential to overcome this challenge, since they are able to utilize standard multi-mode fiber (MMF) as probes which consequently have diameters down to 100 µm, and which might in principle be used as insertable needle-like imaging probes. However, achieving two-photon contrast in a lensless endoscope is a challenge due to dispersion in optical fibers, and only sparse reports of a two-photon lensless endoscopes exist [10,11]. Here, we draw upon concepts from light control in complex media [12,13,14,15,16] in order to propose an amalgam of the rigid endoscope and the lensless endoscope compatible with two-photon imaging which is, in effect, an ultra-thin rigid endoscope consisting of a short piece of graded-index MMF.…”

Minimizing Group Index Variations in a Multicore Endoscope Fiber

“…We note that this optimization is not applicable to the approach utilizing a single multimode optical fiber [e.g. 9, 10], for which the spread in group indices for the different modes makes the transmission and focusing of ultrashort optical pulses more challengingalthough we note that a small subset of modes with similar optical path lengths can be selected for short pulse transmission [13]. …”

Digital confocal microscopy through a multimode fiber

“…Optimization techniques [13][14][15] iteratively find the output pattern associated with each image from a predetermined set of inputs. In digital optical phase conjugation [16][17][18] such output patterns are recorded with a holographic acquisition. The transmission matrix method [19][20][21][22][23] captures the propagation characteristics of the fiber in a matrix linking the input field with the output field.…”

“…The scanning rate is currently in the kilohertz range in fastest implementations [19,21,24], yielding an effective frame rate of about 1Hz for highresolution scans. Most implementations to date were based on narrowband lasers, but extension to broadband pulsed lasers is underway [15,18].…”