Generation and characterization of cylindrical vector beams in few-mode fiber

For the past many decades, the Gaussian laser beam has driven major scientific discoveries that revolutionized the world of optics and photonics. In recent years, there is a burgeoning transformation where significant research has been dedicated in discovering the complex properties of cylindrical vector beams (CVBs). Increasingly, a beam of light with its intensity profile taking the shape of a single doughnut ring has attracted attention of several researchers the world over. Particularly, the so-called CVBs exhibit unique properties when focused owing to their radial and azimuthal distribution of polarization. In comparison to conventional (Gaussian-like) beams inheriting homogeneous polarization, CVBs provide unique light-matter interactions. For example, a radially polarized beam can enhance the imaging resolution of the system significantly with their spatial inhomogeneous polarization by imparting a symmetric and high numerical aperture focus. Moreover, CVBs with their phase and intensity singularities have found broad applications in quantum optics, optical micro/nano-manipulation, surface plasmon polariton, super-resolution imaging, and high-capacity fiber-optic communication. The studies of most widely used CVBs have been explored both in free space optics as well as in guided fiber optics. Further developments will require reliable techniques to generate these CVBs with strong coupling efficiency, high mode purity and high-power handling. For the past few years, the design, fabrication and study of optical fibers that supports CVBs, vortex and orbital angular momentum (OAM) beams have come to the forefront of research in this area. This is true in a sense that mode division multiplexing (MDM) is considered as a preeminent solution to the data capacity limitations faced by the standard single-mode fiber. In addition, vector beams in optical fibers constitute the fundamental basis set of linearly polarized (LP) modes (within the scalar approximation) as well as modes carrying OAM which represent another potential approach for implementing MDM based communications. Therefore, fundamental information and control over the vector beams is key to unravel future fiber communication links and CVB based fiber-optic sensors. For this purpose, it is essential to develop efficient methods to generate these CVBs. Some of the current methods reported for the generation of CVBs employ spiral phase plate, spatial light modulator (SLM), and offset fiber coupling. This thesis elucidates the generation as well as the optical characterization of such propagating cylindrical vector beams in a few-mode fiber. The ultimate purpose would be to develop simple, flexible and cost-effective photonic devices that will allow the efficient generation and stable propagation of the CVB while reducing the overall losses incurred by the system. Most of the methods reported earlier were limited to the measurements of the scalar LP mode groups of a FMF, thus neglecting the underlying vector beams that require delicate spectral and spatial control in order to be detected. In this thesis, three different techniques have been utilized for the generation of CVBs and OAM beams with high output purity. Initially, a tunable mechanical mode converter has been fabricated to demonstrate the generation of cylindrical vector beams supported by FMF in the telecom spectral range. This photonic device is utilized to demonstrate the non-destructive nonlinear characterization of CVB by utilizing the phenomenon of stimulated Brillouin scattering for the first time. We showed how the Brillouin gain spectra of the vector beams in some specialty fibers can be independently identified, measured, and subsequently exploited to probe the corresponding effective refractive indices of the vector beam retrieved from the data. This new characterization method of individual vector beam will have an impact in both light-wave and FMF-based optical sensing applications, which at present, mostly rely on the scalar LP modes. Further, a simple and low-cost technique to generate CVBs via long period fiber grating (LPFG) with very small grating pitch is reported. This work demonstrates that the cost-effective electric arc writing method for the fabrication of LPFGs is open to specialty few-mode fiber that often calls for very small pitch values. Finally, the generation of perfect cylindrical vector beams (PCVB) is demonstrated whose beam profile (i.e. transverse intensity profile) can be easily and precisely controlled. The latter novel method was used in-order to increase the free space coupling efficiency demanded by some specialty FMFs. The tailoring of the beam width and radius is performed via an iris and a diffractive phase mask implemented on a programmable SLM. The technique proposed towards the generation of PCVBs is highly adaptable for its robust nature to generate any arbitrary PCBs as well as perfect vortex beams with any topological order, using the same experimental setup. This experimental analysis is supported and validated via a rigorous theoretical framework that is in concordance with the results obtained

Transmission of optical communication signals through ring core fiber using perfect vortex beams

Orbital angular momentum can be used to implement high capacity data transmission systems that can be applied for classical and quantum communications. Here we experimentally study the generation and transmission properties of the so-called perfect vortex beams and the Laguerre-Gaussian beams in ring-core optical fibers. Our results show that when using a single preparation stage, the perfect vortex beams present less ring-radius variation that allows coupling of higher optical power into a ring core fiber. These results lead to lower power requirements to establish fiber-based communications links using orbital angular momentum and set the stage for future implementations of high-dimensional quantum communication over space division multiplexing fibers.Comment: 13 pages, 6 figure

Orbital angular momentum source generation via parametric nonlinear interactions

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

Space Division Multiplexing in Optical Fibres

Optical communications technology has made enormous and steady progress for several decades, providing the key resource in our increasingly information-driven society and economy. Much of this progress has been in finding innovative ways to increase the data carrying capacity of a single optical fibre. In this search, researchers have explored (and close to maximally exploited) every available degree of freedom, and even commercial systems now utilize multiplexing in time, wavelength, polarization, and phase to speed more information through the fibre infrastructure. Conspicuously, one potentially enormous source of improvement has however been left untapped in these systems: fibres can easily support hundreds of spatial modes, but today's commercial systems (single-mode or multi-mode) make no attempt to use these as parallel channels for independent signals.Comment: to appear in Nature Photonic