The history, fabrication, theory, numerical modeling, optical properties, guidance mechanisms, and applications of photonic-crystal fibers are reviewed

Photonic-Crystal Fibers

Aircraft operators are faced with increasing requirements to extend the service life of air platforms beyond their designed life cycles, resulting in heavy maintenance and inspection burdens as well as economic pressure. Structural health monitoring (SHM) based on advanced sensor technology is potentially a cost-effective approach to meet operational requirements, and to reduce maintenance costs. Fiber optic sensor technology is being developed to provide existing and future aircrafts with SHM capability due to its unique superior characteristics. This review paper covers the aerospace SHM requirements and an overview of the fiber optic sensor technologies. In particular, fiber Bragg grating (FBG) sensor technology is evaluated as the most promising tool for load monitoring and damage detection, the two critical SHM aspects of air platforms. At last, recommendations on the implementation and integration of FBG sensors into an SHM system are provided.

Fiber optic sensors for structural health monitoring of air platforms.

This paper describes a method of multiplexing several optical signals onto a single spatial channel (e.g., a single-mode fiber) using a short coherence length continuous wave light source. Several system configurations which utilize this technique are proposed, and some design considerations are discussed. Experimental results for a single sensor and receiver are presented and compared with theoretical predictions.

Coherence Multiplexing of Fiber-Optic Interferometric Sensors

A wavelength-swept single-longitudinal-mode erbium-doped fiber ring laser capable of operating at sweeping frequency in the order of a few kHz is designed and demonstrated by using a fiber Fabry-Perot tunable filter and a Sagnac loop incorporated with a 3.5-meter unpumped erbium-doped fiber. The laser operates in continuous-wave (CW) mode and can sweep approximately 45 nm over the entire C-band (1520nm-1570nm) window with linewidth less than 0.7 kHz. The optimum wavelength sweeping frequency in order to achieve the best output power stability was found to be approximately20Hz with sweeping-induced power fluctuation of only 0.1%.

C-band wavelength-swept single-longitudinalmode erbium-doped fiber ring laser.

A particular photonic crystal fiber (PCF) designed with all circle air holes is proposed. Its characteristics are studied by full-vector finite element method (FEM) with anisotropic perfectly matched layer (PML). The simulation results indicated that the proposed PCF can realize high birefringence (up to 10(-2)), high nonlinearity (50W(-1)·km(-1) and 68W(-1)·km(-1) in X and Y polarizations respectively) and low confinement loss (less than 10(-3)dB/km at 1.55um wavelength).

High birefringence photonic crystal fiber with high nonlinearity and low confinement loss.

A photonic crystal fiber (PCF) with circular air holes in the fiber cladding and elliptical air holes in the fiber core is proposed. According to calculation, both ultrahigh birefringence (larger than 0.01) and ultralow confinement loss (less than 0.001dB/km) can be achieved simultaneously over a large wavelength range for a PCF with only four rings of circular air holes in the fiber cladding. The confinement loss in this PCF can be effectively reduced while the birefringence almost remains the same. The proposed design of the PCF is a solution to the tradeoff between the birefringence and the confinement loss for the originally reported highly birefringent elliptical-hole PCF. Moreover, an approach to modify the effective index of fiber core is also suggested in this letter

Ultrahigh Birefringent Photonic Crystal Fiber With Ultralow Confinement Loss

A multi-wavelength fiber ring laser of tunable channel spacing is proposed by employing an optical variable delay line (OVDL) in a Mach-Zehnder interferometer. Stable lasing is achieved at room temperature with the hybrid gains of a single mode fiber (as the Raman gain medium) and an Erbium-doped fiber (EDF) in a ring structure. The channel spacing of the present multi-wavelength fiber ring laser can be continuously tuned by adjusting the computer-controlled OVDL.

Channel-spacing-tunable multi-wavelength fiber ring laser with hybrid Raman and Erbium-doped fiber gains

A novel elliptical-hole photonic crystal fiber (PCF) with double defect is theoretically investigated, and its high birefringence [including both the phase-index birefringence (PIB) and the group-index birefringence (GIB)] is demonstrated, which can reach an order of 10-2 Such a PCF can especially have a very high PIB, with a negligible GIB in a small band of normalized frequencies. The PIB of this PCF is much higher than that of the circular-hole PCF with double defect, under comparable parameters. Compared with the elliptical-hole PCF with single defect, the present PCF possesses the advantages of both a higher PIB and a larger modal area for modest ellipticities; thus, it is more suitable in linear applications, as well as in fabrication. Moreover, a novel single-polarization single-mode (SPSM) PCF with a broad wavelength band is proposed and demonstrated. In addition, a new approach in designing broadband SPSM PCFs is suggested.

Highly Birefringent Elliptical-Hole Photonic Crystal Fibers With Double Defect

A novel fiber Bragg grating (FBG) sensing system based on a spectrum-limited Fourier domain mode-locking (SL-FDML) fiber laser is proposed. Multiple FBGs cascaded in a long fiber are utilized as both the sensors in the system and the wavelength-selected components in the SL-FDML fiber laser. Both wavelength-division multiplexing and spatial-division multiplexing techniques are demonstrated for interrogation of multiple FBGs by mapping the wavelength measurement to the time measurement and by adjusting the driving frequency of the SL-FDML fiber laser. The proposed FBG sensing system, employing techniques of the wavelength- and spatial-domain interrogation of multiple FBGs, can be used in remote and quasi-distributed multipoint sensing.

Multiple fiber Bragg grating interrogation based on a spectrum-limited Fourier domain mode-locking fiber laser

A novel approach for generating high-frequency microwave signals is proposed and experimentally demonstrated. With a pair of wavelength matched fiber Bragg gratings written directly in a polarization-maintaining erbium-doped fiber, a stable short cavity dual-wavelength single-longitudinal-mode (DW-SLM) distributed-Bragg-reflector fiber laser with orthogonal oscillation modes is realized at room temperature. The wavelength spacing between the two lasing modes is 0.374 nm. By heterodyning the two wavelengths of the DW-SLM fiber laser at a photodetector, microwave signal at over 46 GHz is achieved.

Dual-Wavelength Single-Longitudinal-Mode Polarization-Maintaining Fiber Laser and Its Application in Microwave Generation

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