All-Optical Self-Switching of 1560nm Femtosecond Pulses in Highly Nonlinear Soft Glass Multicore Fiber

The concept of all-optical switching, based on a nonlinear dual-core fiber coupler, was introduced theoretically in the early 1980s. After decades of imperfect demonstrations, our group has achieved high-contrast, low-energy, ultrafast switching using a specially designed dual-core fiber (DCF). Sub-100 fs C-band pulses were switched in both self-switching and cross-switching regimes at sub-nJ energies [1]. Following these promising approaches, a new fiber sample comprising five dual-core units with smaller intercore distances and various dual-core asymmetries was self-fabricated (Fig. 1a). By independently analyzing the nonlinear response of all five units, we could differentiate between them and identify the best one, as the switching performance is highly sensitive to dual-core asymmetry, which varies among central and side units. A Menlo C Fiber fs laser was used as the source, generating 4.5 nJ pulses at 1560 nm with a duration of 75 fs at a repetition rate of 100 MHz. By launching the laser beam separately into one fiber core via a microobjective, the nonlinear response of the respective unit was studied as a function of pulse energy. The pulse energy was controlled by a half-wave plate and polarizer system, ranging from 10 pJ to 4.4 nJ. The basis of the experimental study was fiber length optimization, accomplished by repeating the entire nonlinear propagation analysis for various lengths. The cores of the DCF were monitored by an IR camera and optical spectrum analyzer. Cutting back the fiber, all ten cores were subsequently excited, and the input energy dependence of the dual-core extinction ratio <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$(\text{ER}=10 \log (E_{\text {exc }} / E_{\text {non-exc }})$</tex> in dB, where <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$E_{\mathrm{x}}$</tex> is the output energy in the corresponding core) was measured.