Chirped Fiber Bragg Gratings (CFBG) for high-speed fiber optic communications systems

A chirp is a linear variation in the grating period, that can be added to the refractive index profile of the grating. The reflected wavelength fluctuates with the grating period, broadening the reflected spectrum. A grating possessing a chirp has the ability to add dispersion—especially, different wavelengths reflected from the grating will be subject to different delays.

A non-uniform resonance wavelength along the length of the grating in a CFBG can be accomplished by varying the period or by varying the average effective refractive index. The average refractive index can be changed using different methods, for example, changing the amplitude of the reflective index modulation profile or variation the fiber in the region of the grating length. The chirped FBG was manufactured with the usage of a chirped phase mask to generate a variation in the period of the refractive index.

Chirped fiber Bragg gratings have been widely used for dispersion compensations in high-speed fiber optic communications systems because they are able to retard pulsed light depending on its wavelength. Experience has proven that ideas in one field find applications in another. Actually, this type of optical device has been attracting significant attention in the fiber optic sensing community, in high sensitivity sensors or wavelength discriminators in interrogation systems.

There are two prevailing fields of application of chirped FBG: measurement of curvature based on chirped fiber Bragg gratings and new interrogation system, written in an Erbium-doped fiber. The increasing demand for measurement of curvatures has stimulated the appearance of few sensing systems that depend on the intrinsic characteristics of fiber Bragg gratings. A curvature measurement technique using a smart composite consists of two chirped fiber Bragg gratings. The two gratings are embedded on the opposite sides of the composite laminate and serve as curvature sensors and as wavelength discriminators enabling a temperature-independent intensity-based scheme for the measurement of the radius of curvature.

FBG interrogation relies on the usage of the edge filtering concept applied to a chirped fiber Bragg grating written in an erbium-doped fiber as the processing element. Through the combination of the photon amplification of the erbium-doped fiber and of the distributed wavelength reflection characteristics of the chirped FBG, it becomes possible to reach different reading sensitivities and amplification of the remote sensing signal. The ability of chirped FBG has also been employed successfully in the development of interrogation techniques. One of these techniques uses the group-delay in a Sagnac loop interferometer and another the spectra response of broadband chirped gratings.

Tilted FBG – optical Fiber Bragg Gratings (TFBG)

Tilted FBGsThe tilted FBG is a new kind of sensor that includes all the advantages of established usage Bragg grating technology in addition to being able to excite cladding modes resonantly. This device opens possibilities for single-point sensing in hard-to-reach places.

Tilted fiber Bragg gratings are also known as slanted gratings or blazed gratings. This is a special type of short-term optical fiber gratings. TFBGs are fabricated using the same tools and techniques as standard FBGs, i. e. from a permanent refractive index change induced in doped glasses by an interference pattern between two intense ultraviolet laser beams. The boundary surface of the varied index is not vertical with respect to the fiber axis but has a certain angle. One feature of tilted FBG is that they can couple guided modes with copropagating modes or counterpropagating modes in specific wavelengths.

As we have known that tilted FBG can unite light from guided modes into radiation modes or cladding modes. The cladding modes are investigated theoretically by studying a three-layer model of optical fibers, whereas the core mode is investigated by studying a two-layer model of optical fibers. The analysis reveals that to increase the coupling of the energy transferred from the core mode to cladding modes, the cladding radius needs to be decreased. Such behavior is illustrated by studying the change in the electric field distribution and is used to enhance the sensitivity of the sensing refractive index of the surrounding medium.

Connection efficiency with the help of tilted FBGis sensitive to light polarization, various sensors, and devices based on these characteristics have been proposed or developed for a wide range of applications. TFBG sensors are using for mechanical and biochemical applications, including one-dimensional TFBG vibroscopes, accelerometers, and micro-displacement sensors; two-dimensional TFBG vector vibroscopes and vector rotation sensors; reflective TFBG refractometers with in-fiber and fiber-to-fiber configurations; polarimetric and plasmonic tilted FBG biochemical sensors for in-situ detection of a cell, protein, and glucose.

Apodized Fiber Bragg Gratings (FBG)

Apodized FBGsFiber Bragg Gratings is one of the most meaningful developments in the areas of optical fiber technology, due to their flexibility and unique filtering performance. FBG is defined as the key component in dense wavelength division multiplexing on the basis of their low insertion loss, high wavelength selectivity, low polarization dependent loss, and low polarization modal dispersion.

When light propagates through the FBG in the narrow range of wavelength, the total internal reflection appears at Bragg wavelength and other wavelengths don`t have influence by the Bragg grating except some side lobes existing in the reflection spectrum. These side lobes are sometimes interfering, e.g. in some applications of fiber Bragg gratings as optical filters. They can be largely brought out with the technique of apodized FBG: the strength of the index modulation is smoothly ramped up and down along the grating.

 

The term apodization is concerned with the grading of the refractive index to approach zero at the end of the grating. Apodized gratings introduce the essential improvement in side-lobe suppression while maintaining reflectivity and narrow bandwidth. Gaussian and raised cosine methods are typically used to apodize an FBG. Each method has some special features and different methods of fabrication. The fabrication of apodized Gaussian Bragg gratings is using the two UV-pulse interfere with variable time delay, which strongly reduces the reflectivity at sidelobes and this method makes it possible to write off truly apodized gratings by the single exposure of a uniform phase mask.

The fabrication of Apodized Fiber Bragg Gratings has raised much interest because of its reduced reflectivity at sidelobes. It, therefore, increases the quality of optical filters and improves the dispersion compensation by simultaneously reducing the group delay ripples. Apodized FBG can be used to optimize the parameters of the introduced optical switch and may also prove to be useful as the optical sensing element in a range of other fiber sensor configurations, especially for grating-based chemical sensors, pressure sensors, and accelerometers.