Tunable visible comb using Raman self-frequency shift, intermodal phase matching and cascading of nonlinearities in an all-fiber platform

Abstract

Up-conversion of IR frequency combs from compact erbium fiber mode-locked lasers (MLLs) around 1550 nm is enabling the development of small-footprint visible combs for ultralow-power spectroscopy and quantum optics. Limited tunability of the IR MLLs, due to the small gain bandwidth (∼30 nm) of erbium fiber amplifiers, and difficulty in achieving broadband phase matching for up-conversion in a nonlinear crystal or waveguide restricts the visible comb wavelengths to second- and third-harmonic wavelengths. Here, we harness the soliton self-frequency shift (SSFS) of sub-nJ energy mode-locked IR pulses in a standard single-mode silica fiber and combine it with the up-conversion, through cascading of optical nonlinearities, in a dispersion-engineered silica nanowire to achieve wideband tuning of the second-, third-, fourth-, and sixth-harmonic generated (SHG, THG, FHG, SiHG) signals. By varying the fiber length and the IR pump power, we Raman shift the pump wavelength from 1560 to ∼1750 nm using the SSFS and use it to tune the wavelengths of the THG and SHG signals in a 10-mm-long silica nanowire from 520 to 578 nm and 780 to ∼850 nm, respectively. Four-wave mixing between the THG and SHG signals creates a tunable signal close to the fourth-harmonic (390 nm), and SHG of THG creates a tunable SiHG in the deep UV (260 nm). The generation and tuning of deep UV (∼260 nm) to near-IR combs (∼780 nm) using redshifted IR solitons demonstrate that the cascading of optical nonlinearities in a silica nanowire enables spectral translation between wavelength regimes that are more than five octaves apart.