Photonic crystal fibers are a new class of optical fibers. Their artificial crystal-like microstructure results in a number of unusual properties. They can guide light not only through a well-known total internal reflection mechanism but using also photonic bandgap effect. In this paper different properties possible to obtain in photonic crystal fibers are reviewed. Fabrication and modeling methods are also discussed.

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Photonic Crystal Fibers

Although mainstream grating writing, more often than not using single photon excitation of germanosilicate based defects with CW 244 nm light, remains the key technology for complex devices it is now being complemented by a whole host of processes which can enhance and tailor the properties of both conventional and not-so-conventional fibre Bragg gratings. Further, processes for writing of gratings in non-germanosilicate fibres have also continued to develop and include multi-photon excitation directly into the band edge of the glass. It is now possible to custom tailor a gratings property based on the application and the nature of production as well as custom tailor the grating writing process to suit the type of fibre and application. Examples and suggestions where these can benefit sensors and lasers are outlined.

Fibre gratings and devices for sensors and lasers

The development of highly-sensitive miniaturized sensors that allow real-time quantification of analytes is highly desirable in medical diagnostics, veterinary testing, food safety, and environmental monitoring. Photonic Crystal Fiber Surface Plasmon Resonance (PCF SPR) has emerged as a highly-sensitive portable sensing technology for testing chemical and biological analytes. PCF SPR sensing combines the advantages of PCF technology and plasmonics to accurately control the evanescent field and light propagation properties in single or multimode configurations. This review discusses fundamentals and fabrication of fiber optic technologies incorporating plasmonic coatings to rationally design, optimize and construct PCF SPR sensors as compared to conventional SPR sensing. PCF SPR sensors with selective metal coatings of fibers, silver nanowires, slotted patterns, and D-shaped structures for internal and external microfluidic flows are reviewed. This review also includes potential applications of PCF SPR sensors, identifies perceived limitations, challenges to scaling up, and provides future directions for their commercial realization.

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Photonic crystal fiber based plasmonic sensors

Based on fiber Bragg gratings (FBGs) and high nonlinear photonic crystal fiber (HN-PCF), a novel dual-wavelength erbium-doped fiber (EDF) laser is proposed and demonstrated. Experimental results show that, owing to the contributions of two degenerate four-wave mixings in the HN-PCF, the proposed fiber laser is quite stable and two output signals are uniform at room temperature. With adjustment of the attenuator, our fiber laser can selectively realize one wavelength lasing.

Stable and uniform dual-wavelength erbium-doped fiber laser based on fiber Bragg gratings and photonic crystal fiber.

In this paper, we describe the properties of Fabry-Perot fiber cavity formed by two fiber Bragg gratings in terms of the grating effective length. We show that the grating effective length is determined by the group delay of the grating, which depends on its diffraction efficiency and physical length. We present a simple analytical formula for calculation of the effective length of the uniform fiber Bragg grating and the frequency separation between consecutive resonances of a Fabry-Perot cavity. Experimental results on the cavity transmission spectra for different values of the gratings' reflectivity support the presented theory.

Effective length of short Fabry-Perot cavity formed by uniform fiber Bragg gratings.

We describe an Er3+-doped aluminosilicate core photonic crystal fibre laser incorporating distributed Bragg reflectors written by two-photon 193nm irradiation through an optical phase mask as the feedback elements. The laser is diode pumped at 980nm and evidence of dual linewidth laser operation close to threshold is observed. However, at higher pumping levels gain competition preferentially selects one laser line.

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All-fibre photonic crystal distributed Bragg reflector (PC-DBR) fibre laser

Propagation of light at 1.5 µm with peak intensity in an air hole is achieved within an air–silica-structured Fresnel waveguide. Thus a simpler fiber design alternative to photonic crystal fibers is possible for high-peak-power propagation with reduced nonlinear interactions. Multiple foci are observed in the far field.

Propagation in air by field superposition of scattered light within a Fresnel fiber.

An air-silica structured fibre is designed with air holes placed in circular rings marking the effective Fresnel zones of the waveguide. By educing the holes to six in each ring and pulling the fibre to appropriate dimensions, light at 633nm leaks out between the holes and is unable to propagate along the fibre. On the other hand confinement and propagation is achieved at 1550nm despite a large hole separation.

Wavelength Dependent Leakage in a Fresnel-based Air-silica

Wavelength dependent leakage in a Fresnel-based air–silica structured optical fibre

In this paper we review recent results describing the generation of optical modes within waveguides based on coherent scattering from artificially structured interfaces. The generation of optical waveguide propagation similar to free space propagation enables possible solutions to controlling and shaping optical field generation in free space using coherent scattering of multiple sources. It is shown that the controlled fabrication of such sources can be done simply with air-material structured waveguides such as air-silica structured fibres. Further, the technique of coherent superposition is well known in Fresnel optics, exploiting zone plates to correct the necessary phase adjustments for a desired lens performance. Similarly, in waveguide form this allows fine control of the interference process resulting in the desired mode field and its properties within the waveguide, at the end of the waveguide in the near field regime and well beyond the waveguide into the far field. A factor that can contribute significantly to the coherent scattering within the Fresnel waveguide is resonant-like scattering inside the low index regions since the critical angle of propagation can be very small, increasing Fresnel reflections between interfaces. The results presented here open up a range of hitherto unexplored possibilities in controlling and shaping at first glance disparate phenomena, including free space diffraction.

E. Buckley

Papers

All-fibre photonic crystal distributed Bragg reflector (PC-DBR) fibre laser

Propagation in air by field superposition of scattered light within a Fresnel fiber.

Wavelength Dependent Leakage in a Fresnel-based Air-silica

Wavelength dependent leakage in a Fresnel-based air–silica structured optical fibre

Multiple source generation using air-structured optical waveguides for optical field shaping and transformation within and beyond the waveguide