Short Fiber Section Research Articles

Opto-mechanical fiber sensing with optical and acoustic cladding modes

Optical fibers are an excellent sensor platform. However, the detection and analysis of media outside the cladding and coating of standard fibers represent a long-standing challenge: light that is guided in the single optical core mode does not reach these media. Cladding modes help work around this difficulty, as their transverse profiles span the entire cross-section of the fiber cladding and reach its outer boundary. In this tutorial, we introduce and discuss in detail two recent advances in optical fiber sensors that make use of cladding modes. Both concepts share optomechanics as a common underlying theme. First, we describe a spatially continuous distributed analysis using the optical cladding modes of the fiber. Light is coupled to these modes using Brillouin dynamic gratings, which are index perturbations associated with acoustic waves in the core that are stimulated by light. Unlike permanent gratings, which are routinely used to couple light with cladding modes, Brillouin dynamic gratings may be switched on and off at will and can be confined to short fiber sections at arbitrary locations in a random-access manner. Second, we present the extension of the cladding mode sensor concept to include acoustic rather than optical modes. The acoustic cladding modes may be stimulated and monitored by guided light in the single core mode, and their linewidths are modified by the elastic properties of surrounding media. The principles and analyses of both concepts are provided in detail, alongside examples of experimental setups and results.

Cascaded Fiber-Optic Interferometers for Multi-Perimeter-Zone Intrusion Detection With a Single Fiber Used for Each Defended Zone

A new fiber-optic perimeter intrusion detection system employing only one single-mode fiber as a disturbance sensor for each perimeter zone is presented. A disturbance signal generated through interference between lights from two resonators, i.e., a linear laser cavity and an extended resonator, was used for determining intrusion for each defended zone. A single-mode fiber was used not only for sensing the intrusion-induced disturbance but also for transmission of pump and signal lights. Although many papers have reported multi-perimeter-zone intrusion detection systems, the study presents, for the first time, a system with a single-mode fiber used all along the perimeter that is in a line pattern and is sectioned into multiple defended zones. An outdoor test for a four perimeter-zone intrusion detection system was carried out with a short section of fiber attached on a netted fence in each defended zone. Experimental results demonstrated that there was no cross interference between any two defended-zone light circuits, and that a high alarm-upon-intrusion rate and a zero false alarm rate could be reached by using the presented detection algorithm.

Development of Ammonia Gas Sensor Using Optimized Organometallic Reagent

Reliable, continuous, and spatially distributed monitoring of dangerous or irritating chemical substances belongs to standard functions of contemporary industrial and public security systems. Fiber-optic-based detection provides feasible platform to fulfill such aims. This paper deals with characterization of ammonia sensing elements based on multimode polysiloxane-clad silica-core optical fibers sensitized with 5-(4′-dioctylamino phenylimino) quinoline-8-1 cobalt bromide complex reagent immobilized into the cross-linked polymer matrix from a proper mixture of organic solvents and a radical scavenger contributing to the desired long-term stability of optical properties. The applied sensing mechanism combines optical detection principle with chemical reaction of the reagent and ammonia resulting in changes in the visible near-infrared optical absorption spectrum of the cladding layer, influencing via evanescent optical field interactions the spectral distribution of the guided light intensity. Reaction kinetics of short fiber sections exposed to ammonia/nitrogen mixture of various ammonia concentrations is tested and evaluated. The obtained sensitivity, limit of detection, and forward response time of the prepared sensors amount to1.52⁎10-5 ppm−1, 31 ppm, and 25 s, respectively. The obtained results are promising for fabrication of distributed fiber-optic sensors applicable to detection and location of ammonia gas leaks in industrial as well as general public premises.

Selectively coupling core pairs in multicore photonic crystal fibers: optical couplers, filters and polarization splitters for space-division-multiplexed transmission systems

Selective coupling a single pair of cores in a photonic crystal fiber with multiple, initially decoupled, cores is demonstrated through the use of a technique to locally post-process the fiber cross section. Coupling occurs when the hole between the selected core pair is collapsed over a short fiber section, which is accomplished by heating the section while the hole is submitted to an air pressure that is lower than that applied to all other holes in the microstructure. The demonstrated couplers present an estimated insertion loss of ~1 dB and exhibit spectral modulations with a depth of up to 18 dB and a high polarization sensitivity that can be exploited for polarization splitting or filtering in space-division-multiplexed optical interconnection and telecommunication links.

A semiclassical model for noise propagation in depleted-pump optical amplifiers

Erbium-doped and parametric fiber-optic amplifiers can be modeled as a cascade of amplifying beamsplitters, each representing a short fiber section in which the pump is approximately constant. Each beamsplitter obeys simple noise combination relations and its gain is calculated from the standard rate equations. The proposed model allows for the management of pump depletion in a very simple way by numerical analysis, avoiding direct quantum mechanical calculations. Sample noise propagation diagrams are reported for technically significant values of the parameters. For the four-wave-mixing amplifier, it has been found that the 3-dB noise limit can be reached even for a strong gain compression.

Modal interference in a short fiber section: Fiber length, splice loss, cutoff, and wavelength dependences

When a short fiber section is inserted within a fiber link, it is known that modal interference can create periodic oscillations in the wavelength dependence of the insertion loss and, accordingly, the insertion loss can exceed the sum of the interconnection losses (in decibels). This modal interference phenomenon is a potential cause of noise in telecommunications transmission systems. Experimental and theoretical investigations were performed in order to better understand its dependences on relevant fiber parameters and to generate engineering guidelines to minimize it in a telecommunication link. >

Modal noise in short fiber sections

The authors present the results of a theoretical and experimental investigation of modal noise in single-mode fiber lengths of less than 1 m. Simulations show that modal noise effects improve as the fiber lengths get very short (several centimeters or less). Experiments illustrating the applicability of the model are carried out for two cases-the interaction of two fiber discontinuities and the interaction of laser-fiber launch with a single fiber discontinuity. The latter case is applicable to laser and transmitter package design. The results of these measurements indicate that low-loss connectors are of paramount importance in high-quality transmitter packaging. >

Wavelength dependence of birefringence in single-mode fiber

Real single-mode fibers generally exhibit elliptical birefringence caused by deviations of the core shape from circularity, by transverse internal stress, and by residual twist. These three contributions can be individually determined by analyzing on a Poincaré sphere the wavelength dependence of the output state of polarization of a short section of fiber with fixed input polarization. Various types of polarization eigenstates of a twisted birefringent fiber are discussed, and the resulting group delay difference is derived.