Fiber-based high energy ultrafast sources for nonlinear microscopy
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Nonlinear microscopy (NM) techniques are important for biologists. For example, two-/three-photon excited fluorescence (2PEF/3PEF) microscopy helps deep visualization of the brain; and coherent anti-Stokes Raman scattering (CARS) and stimulated Raman scattering (SRS) allow the acquisition of chemical information in biological tissues. However, in contrast to conventional, linear microscopy, the minuscule cross-sections of nonlinear interactions require the use of specialized high energy ultrafast sources. Furthermore, in order to reduce scattering and heating related problems, ~1300 nm is the preferred imaging wavelength window, given maximal tissue transparency. Currently, these requirements are met by commercial solid-state optical parametric oscillators (OPOs) and optical parametric amplifiers (OPAs), which are bulky, costly, require significant user-expertise for operation, and often yield non-perfect (i.e. non-Gaussian-shaped) beam profiles. Fiber-based sources are alternative options. They are alignment-free, lightweight, compact, cost-effective, and have high output beam quality (perfect Gaussian or a pure higher order mode (HOM)). Moreover, the flexibility of a fiber can be exploited to implement endoscopy, which enables imaging studies in freely moving animals. In these schematics, a commercial, alignment-free fiber laser can be combined with another specialty fiber that utilizes its own nonlinear interactions to produce pulses, on-demand, at user-desired wavelengths. One such effect is soliton self-frequency shift (SSFS)—an intrapulse Raman scattering process in optical fibers—using which one can continuously transfer energy to longer wavelengths, such as from ~1050 nm (Yb fiber laser wavelength) to a desired target wavelength (e.g. ~1300 nm). SSFS using HOMs in step-index multimode fibers (MMFs) has been demonstrated to generate pulses with pulse energies of ~30 nJ and peak powers of ~0.5 MW at ~1300 nm. More recently, using both SSFS and an intermodal interpulse Raman scattering process called soliton self-mode conversion (SSMC), strong pulses with pulse energies of ~80 nJ and peak powers of ~1.1 MW have been generated at ~1300 nm. The efficacy of this fiber-based source generation technique has been validated by the fact that it has been successfully utilized for acquiring 3PEF images from mouse brain samples. In this thesis, we conduct an in-depth study of this newly discovered SSMC phenomenon. We show that this effect can be cascaded to produce multiple temporally aligned, but spectrally separated pulses, as may be needed from schematics that use more than one excitation or readout source, and that it can be extended across wide bandwidths, satisfying the needs of multiple microscopy applications. Salient results we have obtained include the following: (1) both higher input powers and longer fiber lengths keep lowering the mode order, and at the same time red-shifting the soliton wavelength, without loss of spatial coherence; (2) group refractive index matching plays a key role, and hence this process can be controlled by simple fiber design; (3) pump-to-soliton energy conversion efficiencies are ≥ ~30%, and inter-soliton conversion always results in the transfer of ~100% of the photons; and (4) these attributes can be combined to yield wavelength translations as much as ~62% of the carrier wavelength (1045-1696 nm), with further translation currently limited by the pump power available in the lab, and ultimately by the self-action/dielectric breakdown effect in the fiber. The second focus of the thesis is using the SSFS process to develop a compact fiber-based dual color source for CARS/SRS microscopy. In the Stokes arm, the HOM excitation and HOM-to-Gaussian conversion are both implemented using commercially available physical axicons. The pump output is at 1045nm, with a pulse energy of ~400 nJ and peak power of ~4 MW. The Stokes output is tunable from 1339 nm to 1376 nm, with a pulse energy of ~14.5 nJ and peak power of ~0.2 MW. Finally, we conduct theoretical simulations, based on initial conditions obtained from experimental values, on the prospects of using SSFS and SSMC in MMFs for further power-scalability and wider wavelength tunability. We find that the power scaling by way of dispersion-area product scaling is ultimately limited by the fiber numerical aperture (NA) and self-action/dielectric breakdown. However, the soliton pulse energy/peak power can still potentially approach the level of commercial solid-state OPOs/OPAs. Furthermore, the SSFS/SSMC mechanisms can also be applied in fluoride and chalcogenide glass fibers to generate strong pulses in the mid infrared (MIR) range. Our study indicates SSFS and SSMC in step-index MMFs are effective ways to generate high energy, ultrafast pulses covering both near infrared (NIR) and MIR windows. This source can be useful for nonlinear microscopy, endoscopy, spectroscopy, chemical sensing, and potentially offer an attractive alternative to bulk, alignment-sensitive OPOs and OPAs that are currently the only known options for high-power source development at wavelengths where lasing gain media do not exist.