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A tunable bandwidth add-drop filter in ideal quasi-critical coupling (QCC) state was analyzed in theory in detail and demonstrated in experiment, with two vector modes HE $^{\rm X}_{11}$ and HE $^{\rm Y}_{11}$ coupled single whispering gallery mode (WGM) as its basic configuration. In the experiment, the bandwidth tuning range at the Drop port was from 54.0 MHz to 775.8 MHz with the conversion efficiency maintained above 95%, and bandwidth tuning range at the Through port was from 6.6 MHz to 720.6 MHz with the insertion loss fluctuation less than 5%. The demonstrated filter gains advantages of stable efficiency, narrow band, and broad bandwidth tuning range, which will be helpful for its application in microwave photonics, narrow linewidth lasers and nonlinear optics. [ABSTRACT FROM AUTHOR]

A compact and robust in-fiber acoustically-induced Mach-Zehnder interferometer (AI-MZI) was demonstrated based on the phase mismatch in an acoustic wave driving fiber Bragg grating (FBG). The fiber structure of the AI-MZI was easy to fabricate and of a uniform diameter with larger refractive index difference in its arm part, which helped to reduce the length of the arm. In the experiment, the total length of the AI-MZI was less than 7 cm and the response time was about 120  μs. The bandpass isolation and resonant center wavelength could be optimized by changing the power and frequency of the radio frequency source, respectively. As its application, a tunable and individually switchable dual-wavelength fiber ring laser was realized and characterized. The proposed structure gains advantages of robustness, easy fabrication, compact size and fast tuning speed accordingly. At the same time, it provides another practical way to use an FBG to introduce phase mismatch into an acousto-optic fiber grating. [ABSTRACT FROM AUTHOR]

We experimentally obtain a lower frequency shift based on mode conversion by using a mode-selective coupler (MSC) and an acoustically induced fiber grating (AIFG) for optical heterodyne micro-vibration measurement. Through efficient conversion between LP11 and LP01 core modes in a few-mode fiber, a mode conversion frequency shifter (MCFS) can be achieved and the magnitude of frequency shift is several hundred kHz, which is generally challenging for traditional bulk acousto-optic frequency shifter. Moreover, the signal-to-signal beating interference (SSBI) can be eliminated by using the MCFS without requirement of guard band. The scheme of optical heterodyne micro-vibration measurement based on the MCFS has a high-quality signal-to-noise ratio (SNR) over 85 dB. An all-fiber Bragg grating (FBG) scheme is also designed to improve the measurement sensitivity of heterodyne detection, which has a measurable vibration frequency range of 1 Hz to 300 kHz and minimal detectable amplitude of 0.019 nm. The all-fiber MCFS has the advantages of compactness, easy integration and can be applied directly to satellite ground station and laser feedback tomography. [ABSTRACT FROM AUTHOR]

Rayleigh scattering in graded-index polymer optical fibers (GI-POF) is studied under the assumption that spherical scatterers are uniformly distributed within the layers of concentric ring structure. This approximation allows to describe typical imperfections occurring during the fabrication of GI-POFs. For the purpose of experimental characterization by side illumination, an analytical model of the light conversion into waveguide modes has been developed. This model describes the mode excitation by a multiplicative contribution of three different physical mechanisms. The corresponding experimental analysis of the light scattering has been performed and compared with numerical simulation results for the fibers with single-ring and multi-ring structures. Besides the application to GI-POFs, this article can be also applied to multi-step index fibers. [ABSTRACT FROM AUTHOR]

We demonstrate the fabrication of long-period fiber gratings (LPFGs) in an elliptical core few-mode fiber (e-FMF) by CO2 laser. The mode conversion from fundamental mode to high-order modes (LP11a, LP11b, and LP21a mode) can be realized by the LPFGs with high efficiency of more than 95%. The effect of bending, torsion, temperature, and strain on the resonance wavelength of the e-FMF based LPFG mode converters are experimentally investigated. The temperature and strain sensitivities of the LPFGs are characterized to be −36 pm/°C and −4.9 pm/μϵ, and the LPFGs are found to be insensitive to the bending and torsion. Due to the existence of trench in the e-FMF, the higher-order LP02 and LP31a mode can also be stimulated by the e-FMF based LPFGs with a grating period of 515 μm and 400 μm, respectively. The proposed e-FMF based LPFGs could have a promising prospect in the application of fiber sensing and mode division multiplexing optical communication system. [ABSTRACT FROM AUTHOR]

We propose and demonstrate efficient \textLP01-\text{ LP}11 selective mode conversion over a wide spectral range based on tilted binary phase plate. Due to the extended optical path length of the transverse wave through the tilted phase plate, the matched wavelength for efficient mode conversion can be tuned toward longer wavelength by controlling the tilt angle. In the mode profile characterization, the mode purity of the output \textLP11 beam profile has been theoretically analyzed based on the difference in peak intensities between the two lobes of the \textLP11 mode. The presented model is used for determining the \textLP01:\textLP11 ratio of an impure \textLP11 mode in the experiment. The proposed instrument is a simple yet powerful tool for efficient mode conversion in SDM communication network. [ABSTRACT FROM PUBLISHER]

A design method for fully adiabatic waveguide branches is developed for the first time. Mode conversion between local normal modes is entirely suppressed through the explicit decoupling of the modes, where novel parametric and graphical representations of the self-consistent orthogonal coupled-mode theory are utilized to describe the strongly coupled part of the waveguide branches. A closed-form expression for an adiabatic condition is derived, and the significance of degenerated waveguide patterns with nonuniform waveguide widths is clarified. Validity of the method is tested by numerical simulations, and dependence of the converted power to a circuit length is found. These results raise a fundamental question about the meaning of the adiabaticity in the waveguide branches. [ABSTRACT FROM PUBLISHER]

The propagation characteristics of near-Z-axis lithium-niobate waveguides are investigated, considering the finite crystallographic-alignment tolerances. We expand on the previous models that have considered only the departures from the Z-axis in the plane of the substrate ("yaw") to include the effects of "pitch" and "roll." We find that a combination of yaw and pitch couples the major electric-field components of the fundamental quasi-TE and quasi-TM modes through off-diagonal permittivity components. When these two rotations are both larger than ~0.1deg, the imaginary coupling coefficient associated with the overlap of the major field components becomes larger than the real coefficient, associated with the coupling of the longitudinal TM component to the transverse TE component, that results from yaw alone. We show numerically, as well as experimentally, that unintentional crystallographic-alignment errors can result in substantial mode conversion, affecting the extinction ratio and linearity of Z-propagating modulators. The results also indicate that waveguides with intentional yaw will have mode-conversion characteristics that are highly sensitive to any unintentional pitch resulting from the boule slicing/polishing process. We highlight the importance of crystallographic alignment as well as the role of modal birefringence in suppressing undesired mode conversion [ABSTRACT FROM PUBLISHER]

This paper presents a simple and effective design of taper-based higher order mode (HOM) filters that can effectively remove HOMs from the few-mode fibers. For this purpose, a taper manufacturing technique that allows the production of long and highly uniform tapers was developed. A filter performance was demonstrated on a standard single-mode telecommunication fiber operating at 850 nm. The HOM suppression was better than -39 dB, and the insertion loss of the fundamental mode was less than 0.15 dB [ABSTRACT FROM PUBLISHER]

In-fiber devices enable a vast array of critical photonic functions ranging from signal conditioning (amplification, dispersion control) to network management (add/drop multiplexers, optical monitoring). These devices have become mainstays of fiber-optic communication systems because they provide the advantages of low loss, polarization insensitivity, high reliability, and compatibility with the transmission line. The majority of fiber devices reported to date are obtained by doping, designing, or writing gratings in the core of a single-mode fiber (SMF). Thus, these devices use the fiber only as a platform for propagating light-the device effect itself is due to some extraneously introduced material or structure (dopants for amplification, gratings for phase matching, etc.) There exists another, relatively less explored degree of freedom afforded by fibers-the ability to copropagate more than one mode. Each mode may have a uniquely defined modal dispersion and propagation characteristic. In this paper, we will describe the variety of fiber devices enabled by few-mode fibers-fibers that typically support two to four modes with suitably tailored dispersive properties. We will show that the unique dispersive properties of various modes, in conjunction with the ability to couple between them with gratings, leads to devices that offer novel solutions for dispersion compensation, spectral shaping, and polarization control, to name a few. [ABSTRACT FROM PUBLISHER]

Novel photonic devices based on a new type of waveguide, hollow optical fibers (HOF), are described. Utilizing unique three layered structure of HOF, the central air hole, germanosilicate ring core, and silica cladding along with its adiabatic mode transformation capability we demonstrated versatile applications in short-haul, long-haul optical communications, and tunable wavelength selective devices. Detailed design parameters, fabrication arts of the fibers, and operation principles of the devices are discussed. [ABSTRACT FROM PUBLISHER]

An analysis of multimode waveguides where several modes are coupled via quasiperiodic perturbations is presented. The supermodes (or eigenmodes) of the structure are derived and orthonormality considerations are discussed. In addition, a new type of mode converter between copropagating modes is proposed, where mode conversion is mediated by a backward propagating mode. Adiabatic and nonadiabatic coupling coefficients are considered and the supermode formalism is used to conveniently describe the mode of operation of the device [ABSTRACT FROM PUBLISHER]