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Novel Nonlinear and Quantum Devices Based on Multimode Optical Fibers

Hamed Pourbeyram

Published Jan 1, 2018• 0 citations
OpenAlex

Abstract

In this dissertation, multimode optical fibers (MMFs) have been studied as a robust and practical platform for nonlinear optics and quantum optics applications, where by experimental investigations and theoretical calculations we present the quantum and nonlinear virtue of MMFs as a versatile tool for future applications. Our investigations can be summarized in three parts: At first, we have investigated stimulated Raman scattering (SRS) in a long MMF, where more than 20 orders of cascaded Raman peaks have been generated. Observation of beam clean-up despite the highly multimode nature of the fiber and such strong nonlinear interaction despite the large fiber core size are distinguishable results of our work. Next, we have studied four-wave mixing (FWM) via intermodal phase-matching in both step-index and graded-index MMFs (GIMFs). A rigorous and general approach in the theoretical study is presented where some of the commonly used, yet inaccurate approximations are avoided. Experimentally, FWM side-bands with minimal contamination from SRS, and residual pump photons have been generated from quantum vacuum and amplified more than 11 orders of magnitude. Finally, in the last part of this dissertation, we have done a theoretical study of photon-pair generation in a multimode optical fiber (both step-index and GIMF) via intermodal-FWM as a source of state-engineered photon-pairs.

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Four-wave mixing in optical fibers has been proven to have many applications within processing of classical optical signals. In addition, recent developments in multimode fibers have made it possible to achieve the necessary phase-matching for efficient four-wave mixing over a very wide bandwidth. Thus, the combination of multimode fiber optics and four-wave mixing is very attractive for various applications. This is especially the case for applications in quantum communication, for example in photon-pair generation. This is the subject of this work, where we discuss the impact of fluctuations in core radius on the quality of the heralded single-photon states and demonstrate experimental results of intermodal spontaneous four-wave mixing for photon-pair generation.

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In this dissertation, multimode optical fibers (MMFs) have been studied as a robust and practical platform for nonlinear optics and quantum optics applications, where by experimental investigations and theoretical calculations we present the quantum and nonlinear virtue of MMFs as a versatile tool for future applications. Our investigations can be summarized in three parts: At first, we have investigated stimulated Raman scattering (SRS) in a long MMF, where more than 20 orders of cascaded Raman peaks have been generated. Observation of beam clean-up despite the highly multimode nature of the fiber and such strong nonlinear interaction despite the large fiber core size are distinguishable results of our work. Next, we have studied four-wave mixing (FWM) via intermodal phase-matching in both step-index and graded-index MMFs (GIMFs). A rigorous and general approach in the theoretical study is presented where some of the commonly used, yet inaccurate approximations are avoided. Experimentally, FWM side-bands with minimal contamination from SRS, and residual pump photons have been generated from quantum vacuum and amplified more than 11 orders of magnitude. Finally, in the last part of this dissertation, we have done a theoretical study of photon-pair generation in a multimode optical fiber (both step-index and GIMF) via intermodal-FWM as a source of state-engineered photon-pairs.
Four-wave mixing in optical fibers has been proven to have many applications within processing of classical optical signals. In addition, recent developments in multimode fibers have made it possible to achieve the necessary phase-matching for efficient four-wave mixing over a very wide bandwidth. Thus, the combination of multimode fiber optics and four-wave mixing is very attractive for various applications. This is especially the case for applications in quantum communication, for example in photon-pair generation. This is the subject of this work, where we discuss the impact of fluctuations in core radius on the quality of the heralded single-photon states and demonstrate experimental results of intermodal spontaneous four-wave mixing for photon-pair generation.
Summary form only given. Single-photon sources are key components in applications of photonic quantum technologies such as quantum key distribution (QKD) [1]. One way of realizing single-photon sources is generation of photon pairs (PP) using spontaneous four-wave mixing (FWM): two photons from a pump p annihilate and create two side-band photons at frequencies determined partly by the energy conservation 2ωρ= ω1+ ω2, where ωp,ω1,ω2are the frequencies of the pump and the two side-bands, respectively, and partly by the phase-matching condition. PP generated spontaneously arrive at indeterministic times but even so, they are useful for QKD because one of the photons can be heralded by detecting the other. The heralded photons are then used for transmitting the quantum key.We propose a higher order mode scheme for generating PP in silica fibers; the side-band photons are generated at center wavelength λ1 = 810 nm in the linearly polarized LP02-mode and at λ2 = 1550 nm in the LP01mode, respectively, by pumping in the LP11-mode at λ7, = 1064 nm, where laser sources are readily available. The 810 nm photon is detectable with relatively cheap and efficient silicon-based single-photon detectors, and the heralded 1550 nm photon is mode-matched with transmission fibers in the low-loss spectral window. We design a silica fiber that phase matches these modes and wavelengths while it also reduces the impacts of core radius fluctuations (CRF) caused by fabrication imperfections; the fiber design is inspired by B. P.-P. Kuo et al. [3] and a sketch of the refractive index profile is shown in Fig. 1(a). The scalar wave equation is solved using appropriate Sellmeier data for Germanium-doped silica and the calculated effective indexes for the three modes versus wavelength are shown in Fig. 1(b) together with the core and cladding refractive indexes; the inset shows the wavelength configuration of the pump and two side-bands; phase matching is achieved when the effective indexes of the three modes lie on a straight line [2]. The nonlinear overlap of the waves in this configuration is approximately 18% of the corresponding overlap where all waves are in the LP01-mode.
Being able to generate and process quantum states are essential functionalities in quantum communication. Examples of the processing include conversion of the wavelength of quantum states or modification of their temporal shape. The combined use of four-wave mixing and higher order modes in optical fibers has enabled such functionalities. The use of intermodal four-wave mixing among higher order modes, enable frequency conversion over a wavelength bandwidth, large enough to avoid spontaneous Raman scattering, which is otherwise detrimental for quantum communication. The process of intermodal four-wave mixing furthermore has the advantage that the same process may be used to generate single photon states. We review recent results on the use of intermodal four-wave mixing in optical multi-mode fibers for quantum information science and show a theoretical/numerical model that may be used to predict the performance of quantum state processing devices based on four-wave mixing.
Part I of this thesis describes the theoretical and experimental aspects of a new area in coherent optics known as phase conjugate optics. By using nonlinear optical mixing techniques, the complex conjugate of an arbitrary incident monochromatic wave can be generated. In particular, the theory of the degenerate four-wave mixing technique is presented. Experimental observation of phase conjugate wave front generation in a liquid crystal material (MBBA) is described. The ability of the conjugate wave front to correct for phase aberration is demonstrated. The quantum mechanical origin of the nonlinear third order susceptibility responsible for the four-wave mixing process is illustrated in an atomic system. Because of a finite memory time of such an atomic system, the electromagnetic waves taking part in the mixing process need not occur simultaneously. The applications of the phase conjugation process to real-time information processing and the characteristics of a phase conjugate resonator are discussed in detail. The ability of an optical fiber to confine the propagation of electromagnetic waves over a long distance and within a small cross sectional area implies that large optical intensities can be maintained over a long " interaction length even at moderate incident optical power. Part II of this thesis describes some third order nonlinear optical phenomena that have been observed under this condition. Especially, phase conjugation by degenerate four-wave mixing in a fiber has been shown to require significantly less pumping power. The theoretical and experimental studies of four-wave mixing in a liquid-filled fiber are presented. Extensive experimental efforts have been devoted over the last few years to the study of stimulated Raman scattering and the oscillation of the Stokes radiation inside fibers. A detailed theoretical treatment will be given here to describe these effects. The analysis shows that pump depletion due to the nonlinear process is very important in describing the stimulated Raman scattering in low loss fibers and is the dominant saturation effect in a fiber Raman oscillator. Finally, the implications of nonlinear optical phenomena to fiber optic communication will be mentioned.
We investigate by theory ultra broadband photon pair generation in graded index multimode optical fibers (GIMFs). It has been shown how the unique dispersive properties of GIMFs provide an opportunity to generate state engineered photon pairs. Our findings hint on interesting features such as: - GIMFs provide can be used as an ultra tunable source of state engineered photon pairs. - Quantum correlations are independent of fiber mode and group number and only depend on spectral separation from the pump. - It is possible to simultaneously generate uncorrelated and correlated photon pair in the same fiber.
We theoretically reveal the potential of the parallelism of squeezed state generation by nonlinear pulse propagation in an optical fiber. Starting from a nonlinear Schr\"odinger equation coupling with phonon modes that cause Raman noise, we develop a multimode quantum theory of nonlinear propagation in an optical fiber. Based on our proposed method, we numerically simulate fiber nonlinear propagation in two conditions: solitonlike and zero-group-delay-dispersion (zero-GVD) propagation. As a result, we find that zero-GVD propagation enables the large-scale parallel generation of squeezed states relative to solitonlike propagation owing to the broadband phase matching of the four-wave mixing process.