Delivery of ultrashort spatially focused pulses through a multimode fiber

Two-photon imaging through a multimode fiber

“…The experiments were conducted using a Ti:sapphire oscillator (Coherent Mira 900; central wavelength λ 𝑐 =800 nm; spectral width σ λ =10 nm; pulse width=100 fs). The capability of our method to form a sharp focused pulse through the fiber depends on the number of modes than can be controlled simultaneously [32]. In step-index fibers, the pulse spreading in time is given by: 𝜎 𝜏 ≈ 𝐿∆/(2𝑐 1 ) while in graded-index fibers: 𝜎 𝜏 ≈ 𝐿∆ 2 /(4𝑐 1 ) [36].…”

“…The field emerging from the fiber on the proximal side is imaged by a 20x objective (OBJ1) and a 150 mm lens (L1) onto the CCD, where it is interfered by the pulsed reference beam. The pulsed reference can be delayed and is set at the time in which the visibility of the fringes is maximum [32].…”

“…As demonstrated in [32], the counter-propagation of a time-sampled set of modes excites only fiber modes that travel at approximately the same speed (group velocity). These modes generate a focused pulse which does not suffer from modal dispersion.…”

“…Andersen et al have demonstrated two-photon imaging through a custom MCF with 169 cores [31]. We have previously shown scanning and focusing of pulses in a step index MMF while maintaining a pulse duration of 500 fs through 30 cm fiber propagation [32]. In graded index fibers, spatial focusing of pulses has been shown [33].…”

“…In this paper, we demonstrate, for the first time, high resolution two-photon excitation imaging through a graded-index multimode optical fiber. We use time-gated digital phase conjugation to focus and scan high intensity femtosecond pulses through the fiber [32]. Sectioned images of fluorescent beads suspended in a PDMS polymer volume are imaged.…”