Abstract
We examine the impact of fiber bends on ultrashort pulse propagation in a 169-core multicore fiber (MCF) by numerical simulations and experimental measurements. We show that an L-shaped bend (where only one end of the MCF is fixed) induces significant changes in group delays that are a function of core position but linear along the bending axis with a slope directly proportional to the bending angle. For U- and S-shaped bends (where both ends of the MCF are fixed) the induced refractive index and group delay changes are much smaller than the residual, intrinsic inter-core group delay differences of the unbent MCF. We further show that when used for point-scanning lensless endoscopy with ultrashort pulse excitation, bend-induced group delays in the MCF degrade the point-spread function due to spatiotemporal coupling. Our results show that bend-induced effects in MCFs can be parametrized with only two parameters: the angle of the bend axis and the amplitude of the bend. This remains valid for bend amplitudes up to at least 200 degrees.
Highlights
In recent years challenges were tackled on the way towards robust fiber-based minimally invasive lensless endoscopes that would operate in a clinical setting
In the scope of designing and building flexible lensless 2-photon endoscopes, this paper investigates numerically and experimentally the deformation-induced inter-core group delays resulting from bending multicore fiber (MCF) with large angular bends and assesses the impact on imaging performances
The first important conclusion that can be drawn from the data presented in Figs. 2, 3 is that only the L-type bending geometry has a considerable impact on the inter-core group delays in the studied MCF
Summary
In recent years challenges were tackled on the way towards robust fiber-based minimally invasive lensless endoscopes that would operate in a clinical setting. MCF, in strong contrast with MMF, are made with a multitude of single mode fiber cores which show weak or zero coupling, they maintain and translate, to some extent, their output diffraction patterns when a simple phase tip and/or tilt is applied on the input wavefront This so-called "memory effect" [11] has been extensively used for imaging using scanning [12,13,14] or wide field modalities [15,16,17]. When dealing with ultrashort pulses, the bending may affect the group delays between pulses traveling in the different fiber cores such that they don’t overlap and interfere anymore at the distal tip [19, 20], precluding any focusing and imaging This deformation-induced inter-core group delays are important in the context of 2-photon flexible lensless endoscope [14, 21] that might require active phase [18] and group delay controls [20]
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