Mid-infrared (IR) wavelength range (2–12 μm) is tremendously important for various spectroscopy applications. In comparison to widely used silica and fluoride fibers, chalcogenide fibers have much wider transmission windows (up to 15 μm), and high intrinsic nonlinearity enables emission virtually at any wavelength based on nonlinear gain. In this thesis, I have investigated nonlinear fiber oscillators based on chalcogenide microwires in the wavelength band of 2 μm. I have also developed highly sensitive frequency-resolved optical gating (FROG) pulse characterization techniques based on extremely high nonlinearity in chalcogenide microwires.
This thesis is composed of two main parts. In the first part, two approaches to mid-IR pulse generation are demonstrated. First approach is a 2 μm Raman fiber laser using a multimaterial chalcogenide microwire. Raman laser oscillation in a ring cavity is achieved at a low threshold peak power, with 8 nm of wavelength tunability range. Numerical simulations are performed by solving the generalized nonlinear Schrödinger equation using the split-step Fourier method, and good agreement is found between numerical and experimental results. The second approach is a 2 μm fiber optical parametric oscillator using a chalcogenide microwire cladded with Cyclo Olefin polymer (COP). Parametric oscillation in a ring cavity is achieved at a low threshold peak power, with 55 nm of signal wavelength tunability range, and total emission wavelength covering 290 nm in total with the help of cascaded parametric generation. Output wavelength of these optical sources can be further extended into the mid-IR with optimization of the cavity configuration and microwire structure.
In the second part of this thesis, two approaches to mid-IR FROG pulse characterization are demonstrated in the 2 μm wavelength band. First approach is a modified FROG pulse characterization technique based on highly efficient four-wave mixing in a COP-cladded chalcogenide microwire fabricated with anomalous chromatic dispersion. The FROG traces are identical to the ones obtained from conventional FROGs based on second-harmonic generation. The amplitude and phase of chirped and unchirped picosecond pulses are accurately characterized with a high sensitivity of 0.16 mW2, which is several orders of magnitude better than the sensitivity of conventional FROGs. Second approach is another modified FROG pulse characterization technique, but this time based on cross-phase modulation in a COP-cladded chalcogenide microwire fabricated with zero chromatic dispersion. Without the direction-of-time ambiguity and the need for a continuous wave probe laser, the amplitude and phase of pulses as short as 390 fs with femtojoule energy are accurately characterized with a high sensitivity of 18 mW2. These all-fiber FROG architectures enable alignment-free and highly sensitive pulse characterization for the mid-IR.