There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

We review the recent developments in the area of optical fiber grating sensors, including quasi-distributed strain sensing using Bragg gratings, systems based on chirped gratings, intragrating sensing concepts, long period-based grating sensors, fiber grating laser-based systems, and interferometric sensor systems based on grating reflectors.

Fiber grating sensors

In this paper, we describe the spectral characteristics that can be achieved in fiber reflection (Bragg) and transmission gratings. Both principles for understanding and tools for designing fiber gratings are emphasized. Examples are given to illustrate the wide variety of optical properties that are possible in fiber gratings. The types of gratings considered include uniform, apodized, chirped, discrete phase-shifted, and superstructure gratings; short-period and long-period gratings; symmetric and tilted gratings; and cladding-mode and radiation-mode coupling gratings.

Fiber grating spectra

The historical beginnings of photosensitivity and fiber Bragg grating (FBG) technology are recounted. The basic techniques for fiber grating fabrication, their characteristics, and the fundamental properties of fiber gratings are described. The many applications of fiber grating technology are tabulated, and some selected applications are briefly described.

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Fiber Bragg grating technology fundamentals and overview

Femtosecond laser micromachining can be used either to remove materials or to change a material's properties, and can be applied to both absorptive and transparent substances. Over the past decade, this technique has been used in a broad range of applications, from waveguide fabrication to cell ablation. This review describes the physical mechanisms and the main experimental parameters involved in the femtosecond laser micromachining of transparent materials, and important emerging applications of the technology. Interactions between laser and matter are fascinating and have found a wide range of applications. This article gives an overview of the fundamental physical mechanisms in the processing of transparent materials using ultrafast lasers, as well as important emerging applications of the technology.

Femtosecond laser micromachining in transparent materials

We present a detailed investigation into the sensitivity of long-period fiber gratings (LPFGs) as a function of temperature, strain, and surrounding refractive index, with particular attention to the higher order cladding modes and the possibilities for ultrasensitive sensors. From a general theoretical analysis, we identify a general sensitivity factor which offers new physical insight into LPFG behavior and represents a useful design aid in conjunction with a set of measurand-specific sensitivity factors. Our analysis reveals the existence of turning points in the mode dispersion characteristics at which ultrasensitive operation may be obtained. In an extensive set of coordinated experiments, we verify the theoretical predictions with close agreement and provide demonstrations of the device behavior close to the turning points. Alternative sensor schemes for temperature, strain, and refractive index based, respectively, on measurement of the dual resonance characteristic of the modes and on the transmission characteristics close to the turning points, utilizing higher order modes of the LPFG, are presented. For two variables at least, we record the highest LPFG sensitivities yet reported.

Sensitivity characteristics of long-period fiber gratings

Direct, point-by-point inscription of fibre Bragg gratings by infrared femtosecond laser is reported for the first time. Gratings of first to third order have been produced in non-photosensitised, standard telecommunication fibre and dispersion shifted fibre.

Direct writing of fibre Bragg gratings by femtosecond laser

The authors examine the formation of stable soliton-like pulses in optical fibres with a periodic dispersion map. It is found that increased energy is required to launch a pulse of given width compared to the equivalent uniform fibre with equal path-average dispersion. The stable pulse shapes in dispersion managed systems are significantly different to the hyperbolic secant solitons found in uniform fibres.

Enhanced power solitons in optical fibres with periodic dispersion management

Simultaneous continuous wave multiwavelength operation of an erbium-doped fiber laser has been demonstrated using two types of recently developed in-fiber grating comb filters. The lasing wavelengths and linewidths were determined by the comb filter, which was inserted in a ring cavity with a single inhomogeneously broadened gain medium.

Multiwavelength generation in an erbium-doped fiber laser using in-fiber comb filters

We describe what is to our knowledge the first use of fiber Bragg gratings written into three separate cores of a multicore fiber for two-axis curvature measurement. The gratings act as independent, but isothermal, fiber strain gauges for which local curvature determines the difference in strain between cores, permitting temperature-independent bend measurement.

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Ian Bennion

Papers

Sensitivity characteristics of long-period fiber gratings

Direct writing of fibre Bragg gratings by femtosecond laser

Enhanced power solitons in optical fibres with periodic dispersion management

Multiwavelength generation in an erbium-doped fiber laser using in-fiber comb filters

Two-axis bend measurement with Bragg gratings in multicore optical fiber