Orbital angular momentum (OAM) modes have attracted immense attention for their fundamental properties such as helical phase fronts, zero intensity at the beam centers as well as the phase singularities. Due to these novel characteristics, they have broad application prospects in the fields of super-resolution imaging, laser machining, particle manipulation, classical and quantum communications. These application spaces span an extensive range of wavelengths from the visible range for stimulated emission depletion microscopy to ~1550 nm for telecommunications, for instance. They also span a large range of power levels, from kilowatts (kW) peak powers for laser machining to single photons for secure quantum communications. However, access to this vast space is challenging because of the limitations in available laser sources at wavelengths of interest. More importantly, since the conventional way of creating OAM light involves discrete mode conversion of the Gaussian light that is emitted by a typical laser system, mode converters that can work at all the desired wavelengths and potentially can handle high powers are critically needed. Furthermore, in certain applications where simultaneous creation of multiple OAM modes of equal weights are necessary, such as in the case of higher-dimensional entanglement, an additional requirement of distinct OAM mode excitations with similar efficiencies is of interest.
Here, we borrow from the extensive progress made in the field of single-mode fiber nonlinear optics to develop nonlinear signal generation and conditioning schemes in fibers where light propagates in desired OAM states. Single-mode nonlinear fiber optics has shown that by frequency-converting existing commercial laser sources via nonlinear interactions such as four-wave mixing (FWM), Raman scattering, etc., novel colors of power-levels ranging from kW to single photons can be created. Therefore, it motivates us to develop a similar platform for the OAM modes as well, which is only now possible due to recent developments that show that a large ensemble of OAM modes can be stably guided through optical fibers. As of this writing, fibers supporting over ~50 OAM modes even over km-length scales, with mode areas ranging from 150 to 600 μm^2 are now available, making this platform readily amenable for nonlinear investigations.
This thesis has two primary aims: (1) to study nonlinear optical phenomena of OAM modes in fibers, especially FWM and Raman scattering processes, to investigate whether they behave the same as any other modes in multi-mode fibers (MMFs) or whether the fact that they carry OAM alters the efficiencies and selection rules of nonlinear processes; and (2) to exploit them for two distinct applications spanning both a large wavelength range as well as power levels.
Our studies indicate FWM interactions among OAM modes not only share the attributes with other multimode systems in terms of the variety of phase matching possibilities offered by the expanded modal space, but also show extra advantages of being more diverse and efficient due to the similar intensity profiles of a larger ensemble of guided modes. In addition, the helical phase terms that are unique to OAM modes induce an extra OAM conservation rule for the FWM processes, which provides a high degree of selectivity one would desire when creating specific sources at desired OAM values and wavelengths. We also study Raman scattering in these modes and find some rather counterintuitive behaviors. While Raman scattering is conventionally considered as a phase-insensitive process, its dynamics for a linearly polarized OAM mode are instead governed by a special phase matching equation. Specifically, the phase dependency arises from the optical activity that a linearly polarized OAM mode experiences due to the circular birefringence that is induced by the spin-orbit interaction in the OAM fiber, which manifests in a rotating linear polarization state along the propagation axis, with the rotation rate determined by the modal dispersion characteristic. Since the Raman gain maximizes for co-polarized light, the differences in polarization evolutions for the pump and Stokes light lead to the special phase matching conditions, which can be used to spectrally-reshape and modulate the strength of Raman scattering signals.
Next, we exploit the aforementioned unique and beneficial attributes for specific applications. We first demonstrate a high-power FWM-based OAM source at both ~888 nm and ~1326 nm, with peak powers of ~3 kW and ~2 kW, respectively. We also show extra-cavity second harmonic generation, to access the blue-green wavelengths ranges at which compact, kW peak-power level source generation is both highly desirable for many applications, and also hard to achieve today. The results indicate that FWM not only provides a convenient way to create high power OAM light, but also allows creation of new colors. This is because the multi-mode system can circumvent the near-zero dispersion constraints that are required for phase matching in single-mode systems.
Secondly, we demonstrate OAM-FWM-based photon-pair generation at the single-photon level and reveal the two benefits offered by OAM modes: (1) the ability to engineer the spectral correlations of the photon pairs by switching the angular momentum content of the pump; and (2) simultaneous creation of photon pairs at ~1550 nm and ~780 nm through different FWM paths, hence linking the transmission of flying qubits in the telecom wavelength range to the stationary quantum memory systems that operate in the near-infrared. For all the different FWM processes we probe, we measure the coincidence-to-accidental ratio to be higher than ~400, the second-order correlations to be less than ~0.1, which indicate the high signal to noise ratio and low multi-photon pair generation probability single-photon sources enabled by our OAM-FWM platform.
In summary, FWM and Raman scattering among OAM modes in fibers provide new, interesting nonlinear coupling pathways that allow high power generation as well as control of bi-photon spectra for quantum applications. The benefits of OAM modes compared to either the fundamental mode in single-mode system or traditional modes in MMFs mainly lie in the versatile phase matching possibilities enabled by the large modal space that the OAM-supported fiber offers, as well as the large gains for all FWM pathways ensured by the large and similar mode effective areas for all OAM modes. These two fundamental properties may lead to future development of high-power laser sources at other desired wavelengths, hybrid- and higher-dimensional entanglement sources in the quantum regime and other applications where OAM sources at user-defined wavelengths are desired.
| Identifer | oai:union.ndltd.org:bu.edu/oai:open.bu.edu:2144/45462 |
| Date | 17 January 2023 |
| Creators | Liu, Xiao |
| Contributors | Ramachandran, Siddharth |
| Source Sets | Boston University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis/Dissertation |
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