PhD defence of Md Hosne Mobarok Shamim – Mid-infrared soliton self-frequency shift and supercontinuum generation in cascaded nonlinear fibers

Abstract

The spectral window of the mid-infrared (MIR, 2-20 µm) is important for spectroscopy, sensing, and optical coherence tomography applications. Such emerging applications have intensified research in developing fiber-based MIR light sources using nonlinear wavelength conversions such as soliton self-frequency shift (SSFS) and supercontinuum generation (SCG). This thesis explores SSFS and SCG in cascaded nonlinear fibers (NLFs). In the first half of the thesis, wavelength converters based on SSFS are presented. Chapter 3 presents a soliton order preservation mechanism leading to broad tunability with high energy conversion efficiency (ECE) in a passive fiber-based SSFS. Here, a pulse compressing fiber, placed before a Raman shifting NLF, acts as a soliton order preserver. Soliton order at the input of the NLF is preserved within 2.0<N<2.3 despite a 6.7-fold energy increase. The resulting pulse leads to an SSFS that is spectrally tunable over 1.96–2.14 µm while keeping an ECE of 80–85%. This is the first time an ECE>80% is maintained over a tunable range as large as 180 nm. In Chapter 4, SSFS in a cascade involving three types of NLFs made of three different glasses, that is, silica, fluoride, and ChG, is demonstrated for the first time. Due to high nonlinearity, incorporating ChG NLF in a cascade significantly reduces the pump pulse energy required to achieve a target SSFS, simplifying the wavelength converter. The dispersion and nonlinearity of the NLFs convert a fundamental soliton in one NLF to become a high-order soliton in the subsequent NLF, allowing soliton fission followed by SSFS at each cascade stage. A pump pulse with an energy of 10.6 nJ at a wavelength of 1.94 µm is converted to a tunable soliton in the range of 2.09-2.14 µm in the silica NLF, 2.30-2.52 µm in the ZBLAN NLF, and 3.05-3.28 µm in the ChG NLF, resulting in a cumulative wavelength offset of 1340 nm (63.2 THz).

In the second half of the thesis, wavelength converters based on SCG are presented. Chapter 5 presents the first all-fiber MIR coherent supercontinuum spanning a spectral range of 1.7-5.0 µm from a cascade of silica, ZBLAN, and ChG NLFs. Coherence is maintained across the three NLFs by employing femtosecond pumping and deterministic spectral broadening at each stage of the cascade, as opposed to the commonly used nanosecond-picosecond pumping and non-deterministic spectral broadening via noise-seeded modulation instability. Once in the NLF, the pump pulse is converted into a soliton of order maintained at N<6 in the silica and the ZBLAN NLFs, ensuring soliton fission followed by SSFS. Finally, in the ChG NLF, where dispersion is normal, spectral broadening is facilitated through self-phase modulation, Raman-induced frequency shift, and dispersive wave generation. The deterministic nature of these nonlinear phenomena results in generating a coherent supercontinuum. Chapter 6 demonstrates the widest SCG in the wavelength range of 1.1-3.9 µm from a hybrid microtaper made of an As2S3 core and polycarbonate cladding. The microtaper is the last stage of a three-stage NLF cascade. In the first stage, made of silica fiber, a pump pulse at a wavelength of 1.94 µm is converted to a fundamental soliton at 2.14 µm via SSFS. In the second stage, made of ZBLAN fiber, this soliton experiences further SSFS up to 2.86 µm to become a new pump for the hybrid microtaper. The new pump can access the anomalous dispersion in the microtaper with a relatively large core diameter of 4.4 µm compared to the 2.0 µm core diameter required for a 1.94 µm pump. The large core diameter increases modal confinement and reduces transmission loss associated with C-H bonds in the polymer cladding. Additionally, a short microtaper length of 2.5 cm further mitigates the effect of C-H induced loss, and a cladding diameter of 270 µm provides mechanical strength to the microtaper.