Distributed optical fiber sensing using forward propagation of multiple modes

Fiber optic distributed sensors typically demand bidirectional propagation in optical fibers to localize each point of measurement. This imposes barriers to their deployment in long-haul, amplified telecommunication links and prevents massive range enhancement through in-line amplification, due to the typical presence of non-reciprocal elements such as isolators. In this work, we propose and experimentally demonstrate a new method for distributed sensing that localizes the measurand information in a single unidirectional transmission measurement in fibers carrying at least two weakly coupled non-degenerate co-propagating modes, using swept wavelength interferometry for interrogation. We experimentally demonstrate our proof-of-principle, achieving quantitative strain and temperature measurements with measurand resolutions of 1.2 $\mu\varepsilon$ and 0.135 K, respectively, and spatial resolutions in the tens of meter range, using SMF-28 as a three-mode fiber at 1064 nm center wavelength. These results can be extended to other space-division-multiplexing-ready fibers operating in telecom wavelength bands. Our work paves the way for ultra-long range distributed sensing that is compatible with currently evolving amplified space-division multiplexed telecommunication links.Comment: 9 pages, 5 figure

Novel Specialty Optical Fibers and Applications

Novel Specialty Optical Fibers and Applications focuses on the latest developments in specialty fiber technology and its applications. The aim of this reprint is to provide an overview of specialty optical fibers in terms of their technological developments and applications. Contributions include:1. Specialty fibers composed of special materials for new functionalities and applications in new spectral windows.2. Hollow-core fiber-based applications.3. Functionalized fibers.4. Structurally engineered fibers.5. Specialty fibers for distributed fiber sensors.6. Specialty fibers for communications

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

Time-resolved imaging of guided wave phenomena

In the past decade, increasing demand and rapid developments in classical and quantum sciences resulted in advanced novel multipixel single photon detector arrays engineered on a single electronic chip. Silicon single photon avalanche detector (Si-SPAD) is one of the mainstream solution for low level light detection in visible and near-infrared wavelength region due to the dependable amplification of light signal. This thesis mainly focusses on three key experiments to showcase the potential applications of a single photon detector (Megaframe 32) consists of 32×32 square array Si-SPADs with picosecond timing circuits. With ≈ 50 ps timing resolution, each SPAD can perform time-correlated single photon counting independently. First, the concept of multiplexed single-mode wavelength-to-time mapping (WTM) of multimode light was investigated. The spacetime imaging capability of the Megaframe was then demonstrated by imaging the spatial modes emerging from a few-mode fibre enabling WTM of spatial modes. Finally, timeresolved discrete imaging in laser inscribed photonic lattices was demonstrated. By placing a photonic lattice in a linear cavity and re-injecting the output mode profile back to the lattice, the propagation of light was measured in quasi-real time manner. The experimental demonstrations using Megaframe will find applications in Raman spectroscopy, soliton imaging, quantum optics, and discrete waveguide optics

Mode Evolution in Fiber Based Devices for Optical Communication Systems

Space division multiplexing (SDM) is the most promising way of increasing the capacity of a single fiber. To enable the few mode fiber (FMF) or multi-mode fiber (MMF) transmission system, several major challenges have to be overcome. One is the urgent need of ideal mode multiplexer, the second is the perfect amplification for all spatial modes, another one is the modal delay spread (MDS) due to group velocity difference of spatial modes. The main subject of this dissertation is to model, fabricate and characterize the mode multiplexer for FMF transmission. First, we designed a novel resonant mode coupler (structured directional coupler pair). After that, we studied the adiabatic mode multiplexer (photonic lantern). 6-mode photonic lantern using graded-index (GI) MMFs is proposed and demonstrated, which alleviates the adiabatic require-ment and improves mode selectivity. Then, 10-mode photonic lantern is demonstrated using novel double cladding micro-structured drilling-hole preform, which alleviates the adiabatic requirement and demonstrate a feasible way to scale up the lantern modes. Also, multi-mode photonic lantern is studied for high order input modes. In addition, for the perfect amplification of the modes, cladding pump method is demonstrated. The mode selective lantern designed and fabricated can be used for the characterization of few mode amplifier with swept wavelength interferometer (SWI). Also, we demonstrated the application of the use of the few mode amplifier for the turbulence-resisted preamplified receiver. Besides, for the reduction of MDS, the long period grating for introducing strong mode mixing is demonstrated

Specialty Fiber Lasers and Novel Fiber Devices

At the Dawn of the 21st century, the field of specialty optical fibers experienced a scientific revolution with the introduction of the stack-and-draw technique, a multi-steps and advanced fiber fabrication method, which enabled the creation of well-controlled micro-structured designs. Since then, an extremely wide variety of finely tuned fiber structures have been demonstrated including novel materials and novel designs. As the complexity of the fiber design increased, highly-controlled fabrication processes became critical. To determine the ability of a novel fiber design to deliver light with properties tailored according to a specific application, several mode analysis techniques were reported, addressing the recurring needs for in-depth fiber characterization. The first part of this dissertation details a novel experiment that was demonstrated to achieve modal decomposition with extended capabilities, reaching beyond the limits set by the existing mode analysis techniques. As a result, individual transverse modes carrying between ~0.01% and ~30% of the total light were resolved with unmatched accuracy. Furthermore, this approach was employed to decompose the light guided in Large-Mode Area (LMA) fiber, Photonic Crystal Fiber (PCF) and Leakage Channel Fiber (LCF). The single-mode performances were evaluated and compared. As a result, the suitability of each specialty fiber design to be implemented for power-scaling applications of fiber laser systems was experimentally determined. The second part of this dissertation is dedicated to novel specialty fiber laser systems. First, challenges related to the monolithic integration of novel and complex specialty fiber designs in all-fiber systems were addressed. The poor design and size compatibility between specialty fibers and conventional fiber-based components limits their monolithic integration due to high coupling loss and unstable performances. Here, novel all-fiber Mode-Field Adapter (MFA) devices made of selected segments of Graded Index Multimode Fiber (GIMF) were implemented to mitigate the coupling losses between a LMA PCF and a conventional Single-Mode Fiber (SMF), presenting an initial 18-fold mode-field area mismatch. It was experimentally demonstrated that the overall transmission in the mode-matched fiber chain was increased by more than 11 dB (the MFA was a 250 ?m piece of 50 ?m core diameter GIMF). This approach was further employed to assemble monolithic fiber laser cavities combining an active LMA PCF and fiber Bragg gratings (FBG) in conventional SMF. It was demonstrated that intra-cavity mode-matching results in an efficient (60%) and narrow-linewidth (200 pm) laser emission at the FBG wavelength. In the last section of this dissertation, monolithic Multi-Core Fiber (MCF) laser cavities were reported for the first time. Compared to existing MCF lasers, renown for high-brightness beam delivery after selection of the in-phase supermode, the present new generation of 7-coupled-cores Yb-doped fiber laser uses the gain from several supermodes simultaneously. In order to uncover mode competition mechanisms during amplification and the complex dynamics of multi-supermode lasing, novel diagnostic approaches were demonstrated. After characterizing the laser behavior, the first observations of self-mode-locking in linear MCF laser cavities were discovered

Processamento ótico e digital de sinal em sistemas de transmissão com multiplexagem por divisão espacial

The present thesis focuses on the development of optical and digital signal processing techniques for coherent optical transmission systems with spacedivision multiplexing (SDM). According to the levels of spatial crosstalk, these systems can be grouped in the ones with and the ones without spatial selectivity; drastically changing its operation principle. In systems with spatial selectivity, the mode coupling is negligible and therefore, an arbitrary spacial channel can be independently routed through the optical network and post-processed at the optical coherent receiver. In systems without spatial selectivity, mode coupling plays a key role in a way that spatial channels are jointly transmitted and post-processed at the optical coherent receiver. With this in mind, optical switching techniques for SDM transmission systems with spatial selectivity are developed, whereas digital techniques for space-demultiplexing are developed for SDM systems without spatial selectivity. With the purpose of developing switching techniques, the acoustic-optic effect is analyzed in few-mode fibers (FMF)s and in multicore fibers (MCF)s. In FMF, the signal switching between two arbitrary modes using flexural or longitudinal acoustic waves is numerically and experimentally demonstrated. While, in MCF, it is shown that a double resonant coupling, induced by flexural acoustic waves, allows for the signal switching between two arbitrary cores. Still in the context of signal switching, the signal propagation in the multimodal nonlinear regime is analyzed. The nonlinear Schrödinger equation is deduced in the presence of mode coupling, allowing the meticulous analysis of the multimodal process of four-wave mixing. Under the right conditions, it is shown that such process allows for the signal switching between distinguishable optical modes. The signal representation in higher-order Poincaré spheres is introduced and analyzed in order to develop digital signal processing techniques. In this representation, an arbitrary pair of tributary signals is represented in a Poincaré sphere, where the samples appear symmetrically distributed around a symmetry plane. Based on this property, spatial-demultiplexing and mode dependent loss compensation techniques are developed, which are independent of the modulation format, are free of training sequences and tend to be robust to frequency offsets and phase fluctuations. The aforementioned techniques are numerically validated, and its performance is assessed through the calculation of the remaining penalty in the signal-to-noise ratio of the post-processed signal. Finally, the complexity of such techniques is analytically described in terms of real multiplications per sample.A presente tese tem por objectivo o desenvolvimento de técnicas de processamento ótico e digital de sinal para sistemas coerentes de transmissão ótica com multiplexagem por diversidade espacial. De acordo com a magnitude de diafonia espacial, estes sistemas podem ser agrupados em sistemas com e sem seletividade espacial, alterando drasticamente o seu princípio de funcionamento. Em sistemas com seletividade espacial, o acoplamento modal é negligenciável e, portanto, um canal espacial arbitrário pode ser encaminhado de forma independente através da rede ótica e pós-processado no recetor ótico coerente. Em sistemas sem seletividade espacial, o acoplamento modal tem um papel fulcral pelo que os canais espaciais são transmitidos e pós-processados conjuntamente. Perante este cenário, foram desenvolvidas técnicas de comutação entre canais espaciais para sistemas com seletividade espacial, ao passo que para sistemas sem seletividade espacial, foram desenvolvidas técnicas digitais de desmultiplexagem espacial. O efeito acústico-ótico foi analisado em fibras com alguns modos (FMF) e em fibras com múltiplos núcleos (MCF) com o intuito de desenvolver técnicas de comutação de sinal no domínio ótico. Em FMF, demonstrou-se numérica e experimentalmente a comutação do sinal entre dois modos de propagação arbitrários através de ondas acústicas transversais ou longitudinais, enquanto, em MCF, a comutação entre dois núcleos arbitrários é mediada por um processo de acoplamento duplamente ressonante induzido por ondas acústicas transversais. Ainda neste contexto, analisou-se a propagação do sinal no regime multimodal não linear. Foi deduzida a equação não linear de Schrödinger na presença de acoplamento modal, posteriormente usada na análise do processo multimodal de mistura de quatro ondas. Nas condições adequadas, é demonstrado que este processo permite a comutação ótica de sinal entre dois modos de propagação distintos. A representação de sinal em esferas de Poincaré de ordem superior é introduzida e analisada com o objetivo de desenvolver técnicas de processamento digital de sinal. Nesta representação, um par arbitrário de sinais tributários é representado numa esfera de Poincaré onde as amostras surgem simetricamente distribuídas em torno de um plano de simetria. Com base nesta propriedade, foram desenvolvidas técnicas de desmultiplexagem espacial e de compensação das perdas dependentes do modo de propagação, as quais são independentes do formato de modulação, não necessitam de sequências de treino e tendem a ser robustas aos desvios de frequência e às flutuações de fase. As técnicas referidas foram validadas numericamente, e o seu desempenho é avaliado mediante a penalidade remanescente na relação sinal-ruído do sinal pós-processado. Por fim, a complexidade destas é analiticamente descrita em termos de multiplicações reais por amostra.Programa Doutoral em Engenharia Eletrotécnic