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

In-fibre Bragg grating (FBG) sensors are one of the most exciting developments in the field of optical fibre sensors in recent years. Compared with conventional fibre-optic sensors, FBG sensors have a number of distinguishing advantages. Significant progress has been made in applications to strain and temperature measurements. FBG sensors prove to be one of the most promising candidates for fibre-optic smart structures. This article presents a comprehensive and systematic overview of FBG sensor technology regarding many aspects including sensing principles, properties, fabrication, interrogation and multiplexing of FBG sensors. It is anticipated that FBG sensor systems will be commercialized and widely applied in practice in the near future due to the maturity of economical production of FBGs and the availability of cost effective interrogation and multiplexing techniques.

https://iopscience.iop.org/article/10.1088/0957-0233/8/4/002/pdf

In-fibre Bragg grating sensors

Since the discovery of photosensitivity in optical fibers there has been great interest in the fabrication of Bragg gratings within the core of a fiber. The ability to inscribe intracore Bragg gratings in these photosensitive fibers has revolutionized the field of telecommunications and optical fiber based sensor technology. Over the last few years, the number of researchers investigating fundamental, as well as application aspects of these gratings has increased dramatically. This article reviews the technology of Bragg gratings in optical fibers. It introduces the phenomenon of photosensitivity in optical fibers, examines the properties of Bragg gratings, and presents some of the important developments in devices and applications. The most common fabrication techniques (interferometric, phase mask, and point by point) are examined in detail with reference to the advantages and the disadvantages in utilizing them for inscribing Bragg gratings. Reflectivity, bandwidth, temperature, and strain sensitivity of the Bragg reflectors are examined and novel and special Bragg grating structures such as chirped gratings, blazed gratings, phase-shifted gratings, and superimposed multiple gratings are discussed. A formalism for calculating the spectral response of Bragg grating structures is described. Finally, devices and applications for telecommunication and fiber-optic sensors are described, and the impact of this technology on the future of the above areas is discussed.

Fiber Bragg gratings

Recent developments in rare-earth doped materials for optoelectronics

A linearly chirped in-fiber Bragg grating is reported that can compensate at 1549 nm for the dispersion [ approximately -19 ps/(nmkm)] of standard telecommunications optical fiber optimized for 1300-nm operation.

Chirped in-fiber Bragg gratings for compensation of optical-fiber dispersion.

A wavelength multiplexing/demultiplexing device is fabricated and used to drop/insert a single wavelength channel from/into a multiple wavelength transmission link with 100 GHz channel-spacing at 1550 nm. The device consists of an all-fiber Mach-Zehnder interferometer with photoinduced Bragg gratings. The following performances were measured: extraction/coupling efficiency =99.4%, excess loss 20 dB, and return loss >23 dB. >

/pdf/an-all-fiber-dense-wavelength-division-multiplexer-48bzu67076.pdf

An all-fiber dense wavelength-division multiplexer/demultiplexer using photoimprinted Bragg gratings

An apodised in-fibre Bragg grating reflector is fabricated using the phase mask photoimprinting technique. The reflector has a centre wavelength of 1550 nm, a bandwidth of 0.22 nm and a peak reflectivity of 90%. At 0.4 nm (50 GHz) from the centre wavelength the reflectivity is 40 dB lower than the peak reflectivity; this is an improvement of more than 20 dB over an unapodised Bragg grating reflector with similar bandwidth and peak reflectivity.

/pdf/apodised-in-fibre-bragg-grating-reflectors-photoimprinted-20skttvpnj.pdf

Apodised in-fibre Bragg grating reflectors photoimprinted using a phase mask

Bragg gratings with sidelobe levels 26 dB lower than the peak reflectivity have been fabricated in standard telecommunication optical fibres by exposure to ultraviolet light through a phase mask with a locally varying diffraction efficiency This represents a reduction of 14 dB in the sidelobe levels compared to uniform gratings with the same bandwidth and reflectivity

Apodisation of the spectral response of fibre Bragg gratings using a phase mask with variable diffraction efficiency

The growth rate of fiber Bragg gratings written using 193 nm light from an ArF excimer laser is linearly proportional to the laser pulse energy density for fibers with high germanium doping but proportional to the square of the pulse energy density for standard telecommunications fibers with low germanium concentration. The two‐photon process in standard fibers yields refractive index increases that saturate around 10−3, an order of magnitude improvement over previous results in this type of fiber without sensitization treatment. The two types of photoinduced refractive index gratings have comparable thermal stability and preserve about 50% of their initial magnitude after 30 min at 600 °C.

Comparison of one-photon and two-photon effects in the photosensitivity of germanium-doped silica optical fibers exposed to intense ArF excimer laser pulses

S. Theriault

Papers

Chirped in-fiber Bragg gratings for compensation of optical-fiber dispersion.

An all-fiber dense wavelength-division multiplexer/demultiplexer using photoimprinted Bragg gratings

Apodised in-fibre Bragg grating reflectors photoimprinted using a phase mask

Apodisation of the spectral response of fibre Bragg gratings using a phase mask with variable diffraction efficiency

Comparison of one-photon and two-photon effects in the photosensitivity of germanium-doped silica optical fibers exposed to intense ArF excimer laser pulses