Highly energetic ultrafast pulses are necessary for many applications including, but not limited to, nonlinear microscopy, pump-probe spectroscopy, mid-IR generation, micromachining and even optical data storage. A preferred source for such applications would be a highly energetic, fiber-based, color-tunable source capable of providing multi-color time-synchronized pulses with minimal relative timing jitter, on demand. Conventional free-space sources based on bulk parametric nonlinearities are capable of providing highly energetic ultrafast pulses of tunable wavelengths, however they lack the robustness and remote delivery that fiber-based sources readily provide, and may also suffer from lack of temporal synchronicity and spatial collinearity of multi-color outputs. Notable fiber-based solutions that attempt to alleviate some of these shortcomings include soliton self-frequency shifting (SSFS), parametric oscillation and self-phase modulation (SPM) enabled spectral selection. Such techniques can yield dual-color sources with some success, however they are not capable of providing temporally synchronized dual-color or multi-color pulses on their own; because group velocity dispersion prevents time-synchronization between the pulses of different colors. Even if temporal and spatial overlap is achieved through external means; which defeats the purpose of using fibers, such sources may suffer from relative timing jitters on the order of the pulse durations themselves due to effects such as temperature, stress, pump power fluctuations culminating upon pulse propagation over long transmission lengths in fibers.
We study a recently discovered process called soliton self-mode conversion (SSMC), providing a power scalable platform for the generation of ultrafast pulses at userdefined colors. SSMC is an intermodal ultrafast Raman process which relies on the dispersion - and hence group index - diversity of higher order modes in multimode fibers. During the mode conversion process, the two participating modes at their respective wavelengths share the same group index, thus circumventing the temporal walk-off of the two pulses at distinct wavelengths. It has been shown that one can obtain pulses up to 80 nJ pulse energy (74 fs pulse width) with the SSMC process, as well as creating temporally locked dual-color pulses when the process is stopped mid power transfer.
In this thesis, we aim to understand the SSMC process by studying its fundamental properties and characteristics, in order to be able to exploit them to obtain ultrafast color-tunable sources with multi-color temporally locked output pulses. We first investigate the pulse-to-pulse coherence properties of SSMC-created pulses through interferometric techniques, which reveal it to be a spontaneous process. Crucially we find that SSMC can be the dominant ultrafast process in a fiber even in the presence of other nonlinear pathways (of which, plenty exist in multimode fibers) despite the fact that it is strictly quantum noise initiated. Hence, SSMC is a robust nonlinearity that may be easily controllable by dispersion engineering of fibers and their modes.
Another aspect of SSMC that we probe is its manifestation in the face of chirping of the input pulse. We show that we can use chirp control as a method to tune the output wavelength over a range of ∼250 nm, with ∼30 nJ pulse energies. Although we can access broad wavelength tunability with the SSMC process, it results in an inherent gap in the wavelength tuning range, owing to the fact that during the mode conversion the wavelength separation between the two modes is about one Stokes shift. We show that by taking advantage of the distinct outcomes the two signs of chirp provides, we can decrease this inaccessible region by half, leaving only∼ 30 nm (about half a Stokes shift) of gap over the tuning range.
Lastly, we leverage the group index matching condition for SSMC to induce threecolor and four-color SSMC pulses, increasing the largest available frequency separation between pulse pairs to ∼25 THz with the three-color and to ∼35 THz with the four-color SSMC schemes. Nonlinear mixing experiments conducted on each pair (of the multicolor set of pulses) reveal that the multi-color SSMC process yields timelocked pulses as well. The three color pulses were each measured to have ∼6.5 nJ pulse energy and ∼95 fs pulse width, while the four color pulses were ∼5 nJ and ∼100 fs. We demonstrate that through selection of fiber size, length and launch mode, we can generate multi-color, time synchronized pulses through SSMC, directly out of a single aperture on demand. Hence, we show that multimode fiber-based SSMC yields a versatile platform to generate tunable single-color as well as multi-color jitter free ∼ 100 fs pulses at high energies (∼10s of nJ) directly out of optical fibers.
| Identifer | oai:union.ndltd.org:bu.edu/oai:open.bu.edu:2144/45473 |
| Date | 17 January 2023 |
| Creators | Kabagöz, Havva Begüm |
| Contributors | Ramachandran, Siddharth |
| Source Sets | Boston University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis/Dissertation |
| Rights | Attribution 4.0 International, http://creativecommons.org/licenses/by/4.0/ |
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