Frequency Conversion of Quantum States via Intermodal Bragg Scattering

The nonlinear processes inside propagating media allow frequency conversion of light. That is useful for wavelength down-conversion of infrared light for cameras and up-conversion for quantum sources to telecommunication part of spectra. The two different approaches are based on usage of χ⁽²⁾ or χ⁽³⁾ based processes. The first one is widely used for the conversion of single photons but lacks tunability. Only χ⁽³⁾ based nonlinear interaction known as Bragg-scattering (BS) four-wave mixing (FWM) allows tunable frequency conversion in addition to no pump noise transfer to converted signal.
In this work efficient BS FWM frequency conversion is demonstrated among different spatial modes in a few-mode fiber (FMF). In the beginning, the model based on the calculation phase-matching conditions by measured group delay (GD) for every propagating mode of FMF is tested by modulation interaction (MI) FWM. Verified dispersion properties of FMF simplify conducting experiments of BS FWM, as the wavelength accuracy for each interacting component is crucial for satisfying phase-matching conditions. The efficient BS FWM is experimentally demonstrated with a wavelength span above 600nm. Two pumps excite the LP11 mode, while the signal excites the LP01 mode, and the BS idler is generated in the LP01 mode. For these processes, we experimentally characterize their conversion efficiency and bandwidth. Conversion efficiency (CE) up to -1.5dB is demonstrated between the telecommunication part of the spectrum and wavelength region of nowadays the most efficient quantum devices. Dependencies of conversion efficiency from peak pump power and separation between BS components were measured. Finally treatment of another undesirable nonlinear effect - stimulated Brillouin scattering is demonstrated in order to avoid pump power limitation for BS FWM. This is done by frequency broadening of the pump and counter-phasing them to avoid excessive broadening of the idler.