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Ultrafast laser inscription of asymmetric integrated waveguide 3 dB couplers for astronomical K-band interferometry at the CHARA array
We present the fabrication and characterization of 3 dB asymmetric directional couplers for the astronomical K-band at wavelengths between 2.0 and 2.4 µm. The couplers were fabricated in commercial Infrasil silica glass using an ultrafast laser operating at 1030 nm. After optimizing the fabrication parameters, the insertion losses of straight single-mode waveguides were measured to be ∼1.2±0.5dB across the full K-band. We investigate the development of asymmetric 3 dB directional couplers by varying the coupler interaction lengths and by varying the width of one of the waveguide cores to detune the propagation constants of the coupled modes. In this manner, we demonstrate that ultrafast laser inscription is capable of fabricating asymmetric 3 dB directional couplers for future applications in K-band stellar interferometry. Finally, we demonstrate that our couplers exhibit an interferometric fringe contrast of >90%. This technology paves the path for the development of a two-telescope K-band integrated optic beam combiner for interferometry to replace the existing beam combiner (MONA) in Jouvence of the Fiber Linked Unit for Recombination (JouFLU) at the Center for High Angular Resolution Astronomy (CHARA) telescope array.
Raman-Kerr Comb Generation Based on Parametric Wave Mixing in Strongly Driven Raman Molecular Gas Medium
We report on experimental and theoretical demonstrations of an optical comb spectrum based on a combination of cascaded stimulated Raman scattering and four-wave mixing mediated by Raman-induced nonresonant Kerr-type nonlinearity. This combination enabled us to transform a conventional quasiperiodic Raman comb into a comb with a single and smaller frequency spacing. This phenomenon is achieved using a hollow-core photonic crystal fiber filled with 40 bars of deuterium and pumped with a high-power picosecond laser. The resultant comb shows more than 100 spectral lines spanning over 220 THz from 800 nm to 1710 nm, with a total output power of 7.1 W. In contrast to a pure Raman comb, a 120 THz wide portion of the spectrum exhibits denser and equally spaced spectral lines with a frequency spacing of around 1.75 THz, which is much smaller than the lowest frequency of the three excited deuterium Raman resonances. A numerical solution of the generalized nonlinear Schrödinger equation in the slowly varying envelope approximation provides very good agreement with the experimental data. The additional sidebands are explained by cascaded four-wave mixing between preexisting spectral lines, mediated by the large Raman-induced optical nonlinearity. The use of such a technique for coherent comb generation is discussed. The results show a route to the generation of optical frequency combs that combine large bandwidth and high power controllable frequency spacing.