Elham Nazemosadat

Phase matching for spontaneous frequency conversion via four-wave mixing in graded-index multimode optical fibers

A detailed analysis of four-wave mixing (FWM) and its required phase-matching conditions in graded-index multimode optical fibers is presented. The analysis is relevant mainly for spontaneous frequency conversion and photon pair generation via FWM, when the fiber length is of the order of a meter or shorter, and the signal and idler are generated at large frequency separations from the pump. An analytic expression for calculating the phase mismatch occurring as a result of the waveguide dispersion among the pump, signal, and idler beams is derived, and it agrees well with independent numerical analysis. This expression simplifies calculations considerably, as it allows the prediction of the wavelengths of the signal and idler by using only the mode group number of the spatial modes involved in the FWM process and the refractive index profile of the fiber. The scaling of the signal–pump–idler frequency separations with the mode group numbers is explored. The abundance of the configuration degeneracies of the FWM process in graded-index multimode fibers indicates that nearly identical signal and idler frequencies can be obtained for many different pump, signal, and idler spatial mode profile combinations.

Nonlinear multimodal interference and saturable absorption using a short graded-index multimode optical fiber

A detailed investigation of the nonlinear multimodal interference in a short graded-index multimode optical fiber is presented. The analysis is performed for a specific device geometry, where the light is coupled in and out of the multimode fiber via single-mode fibers. The same device geometry was recently used to obtain ultra-low-loss coupling between two single-mode optical fibers with very different mode-field diameters. Our results indicate the potential application of this simple geometry for nonlinear devices, such as in nonlinear switching, optical signal processing, or as saturable absorbers in mode-locked fiber lasers. Saturable absorption in this all-fiber configuration is discussed and it is shown that it provides attractive properties that can potentially be used in high pulse energy mode-locked fiber lasers.

Nonlinear switching in multicore versus multimode waveguide junctions for mode-locked laser applications

The main differences in nonlinear switching behavior between multicore versus multimode waveguide couplers are highlighted. By gradually decreasing the separation between the two cores of a dual-core waveguide and interpolating from a multicore to a multimode scenario, the role of the linear coupling, self-phase modulation, cross-phase modulation, and four-wave mixing terms are explored, and the key reasons are identified behind higher switching power requirements and lower switching quality in multimode nonlinear couplers.

Detailed investigation of intermodal four-wave mixing in SMF-28: blue-red generation from green.

A short piece of commercial-grade SMF-28 optical fiber is pumped with a 680 ps high-peak power green laser. Red Stokes and blue anti-Stokes beams are generated spontaneously from vacuum noise in different modes in the fiber via intermodal four-wave mixing. Detailed experimental and theoretical analyses are performed and are in reasonable agreement. The large spectral shifts from the pump protect the Stokes and anti-Stokes from contamination by spontaneous Raman scattering noise. This work highlights the predictive power and limitations of a theoretical model to explain the experimental results for a process that relies on the amplification of quantum vacuum energy over more than 11 orders of magnitude.

Saturable absorption in multicore fiber couplers

The saturable absorption characteristics of two-, three-, and five-core one-dimensional fiber coupler arrays and the seven-core hexagonal fiber coupler array are investigated. It is shown that the performance of all these saturable absorbers is comparable and not much is gained, if anything, by going from a two-core nonlinear coupler geometry to a higher number of cores. This observation is supported by the similarity of the saturable absorption curves, as well as comparable pulse characteristics obtained from the simulation of a generic mode-locked fiber laser cavity.

Nonlinear switching in a two-concentric-core chalcogenide glass optical fiber for passively mode-locking a fiber laser

We propose an all-fiber mode-locking device, which operates based on nonlinear switching in a novel two-concentric-core fiber structure. The design is particularly attractive given the ease of fabrication and coupling to other components in a mode-locked fiber laser cavity. The nonlinear switching in this coupler is studied, and the relative power transmission is obtained. The analysis shows that this nonlinear switch is practical for mode-locking fiber lasers and is forgiving to fabrication errors.

Design considerations for multicore optical fibers in nonlinear switching and mode-locking applications

We explore the practical challenges that should be addressed when designing a multicore fiber coupler for nonlinear switching or mode-locking applications. The inevitable geometric imperfections formed in these fiber couplers during the fabrication process affect the performance characteristics of the nonlinear switching device. Fabrication uncertainties are tolerable as long as the changes they impose on the propagation constant of the modes are smaller than the linear coupling between the cores. It is possible to reduce the effect of the propagation constant variations by bringing the cores closer to each other, thereby increasing the coupling. However, higher coupling translates into a higher switching power, which may not be desirable in some practical situations. Therefore, fabrication errors limit the minimum achievable switching power in nonlinear couplers.

Enhancement of nonlinearity and phase-matching in Anderson localized 1D optical waveguides

We show that the presence of disorder in transverse Anderson-localized optical waveguides enhances the nonlinear coefficient of the modes and creates an abundance of multimodal phase-matching opportunities, resulting in rich nonlinear behavior.

An analytical approach for calculating the FWM idler and signal wavelengths in a graded index multimode optical fiber

We present a simple yet powerful analytical approach for calculating the four-wave mixing signal and idler wavelengths in a graded-index multimode optical fiber using the core radius, core-cladding index step, and mode group numbers.

Detailed analysis of amplified spontaneous four-wave mixing in a multimode fiber

Detailed theoretical and experimental analyses of the amplified spontaneous FWM in a multimode optical fiber hint at strong non-universality of the third order Kerr nonlinear tensor as a function of its frequency components.