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Fiber Bragg Grating Sensors: Recent Advancements, Industrial Applications and Market Exploitation, 2011, 53-77 53 Andrea Cusano, Antonello Cutolo and Jacques Albert (Eds) All rights reserved - © 2011 Bentham Science Publishers Ltd. CHAPTER 4 Photonic Bandgap Engineering in FBGs by Post Processing Fabrication Techniques Andrea Cusano 1,* , Domenico Paladino 1 , Antonello Cutolo 1 , Agostino Iadicicco 2 and Stefania Campopiano 2 1 Optoelectronic Division – Engineering Department, University of Sannio, Corso Garibaldi 107, 82100 Benevento, Italy and 2 Department of Technology, University of Naples “Parthenope”, Centro Direzionale di Napoli Isola C4, 80143 Napoli, Italy Abstract: The incredible growth of Fiber Bragg Gratings (FBGs) from both the research and the commercial points of view, has forced the development of several methods able to tailor their spectral characteristics for specific applications. Basically, two different approaches can be adopted to specialize the grating spectra: one is based on complex grating profiles induced directly at the fabrication stage. Another, more interesting, approach relies on post processing methodologies that introduce localized defects along the grating, breaking its periodic structure. The presence of the defects leads to the formation of allowed bands or defect states within the grating bandgap. The introduction of finer scale spectral features results in new interesting perspectives in both telecommunications and sensing fields. In addition, the possibility to accurately control the defect states spectral features – depth, bandwidth, and spectral position – could allow the complete engineering of the FBG bandgap, opening the way to the realization of several new advanced and attractive photonic devices. This chapter is focused to review the main advancements in FBGs post processing to easily control the spectral features of the final device for specific applications. INTRODUCTION Thanks to their peculiar spectral characteristics, Fiber Bragg Gratings (FBGs) are widely popular devices within and out the photonic community [1-2]. Basically – by means of a UV induced periodic modulation of the core refractive index along the hosting optical fiber – FBGs are able to selectively reflect only a narrow wavelength range (typically 0.1-1 nm), giving rise to an effective Photonic BandGap (PBG). Consequently, FBGs – as they stand – represent an useful tool to manipulate the propagating features of the light traveling along an optical fiber [3], and can be usefully exploited to develop wavelength encoded sensing elements in numerous application fields [4-5]. As a consequence of the technological assessment in FBGs fabrication, in the mid 1990’s many research groups have been engaged in the study and realization of new fiber grating devices through more complex refractive index modulation profiles [6- 12]. The common idea was to complicate the core refractive index modulation allowing further manipulation of the light propagating within the fiber grating structures. In this way, it is possible to realize fiber grating with peculiar spectral characteristics either in reflection and transmission. Since FBGs can be considered one-dimensional PBG structures realized within optical fibers by UV writing, among the different complex grating structures, a particular and interesting approach relies on the inclusion of local defects breaking the FBG periodicity along the grating axis. One of the most important modern research field, in fact, is related to the novel and unique way to control many features of the electromagnetic radiation by opportune PBG structures. These artificial structures are characterized by one-, two-, or three-dimensional periodic arrangements of dielectric materials and are commonly called Photonic Crystals (PhCs) [13]. PhCs are the optical analogue to electronic semiconductors, i.e., PhCs can be considered semiconductors for photons: in the electronic case, the wave functions of electrons interact with the periodic potential of the atomic lattice and, for a certain range of energies (similar to the frequencies of photons), electronic states cease to exist, thus giving rise to an electronic bandgap. For PhCs, the analogous of the electronic potential in an atomic crystal is the spatial distribution of the dielectric constant of the PhC structure. Moreover, due to a periodic interaction, certain PBGs appear, forbidding the propagation of certain frequency ranges of light. In such a structure, a line or point defect, which *Address correspondence to this author Professor Andrea Cusano: Optoelectronic Division – Engineering Department, University of Sannio, Corso Garibaldi 107, 82100 Benevento, Italy; Tel: +39 0824305835; Fax: +39 0824305846; Email: a.cusano@unisannio.it
54 Fiber Bragg Grating Sensors Cusano et al. breaks the lattice periodicity, can be introduced wherein a mode is localized in virtue of being suppressed by the lattice. In practice, defects break the PhC structure symmetry and may give rise to a defect state or an allowed frequency within the PBG region. Several research groups proposed and experimentally demonstrated optical trapping and dropping functions by exploiting resonant tunneling [14], cavity wave-guiding coupling phenomena in one-dimensional [15-16], two-dimensional [17], and three-dimensional [18] structures. In the case of FBGs, the introduction of local defects introducing a phase shift along the grating can be substantially obtained by adopting two different methods: - by acting at the fabrication stage through the modification of the photo-induced core refractive index modulation generating punctual or distributed phase shift; - by post processing approaches aimed to locally modify the FBG physical and/or geometrical features generating distributed phase shift. The common spectral effect is to induce narrow transmitted windows or defect states within the standard FBG stopband. Clearly, narrower spectral features would further increase the FBG potentialities in both telecommunications and sensing. Even if this chapter is focused on the second class of FBG based devices, here it is important to briefly summarize also the main milestones characterizing the first category. In 1986, it was originally demonstrated by Alferness et al. [19] that the insertion of a single quarter-wave phase shift at the center of a Bragg grating waveguide opens a band-pass peak within the stop-band. The same principle of operation was adopted in 1990 by Reid et al. to realized phase-shifted Moiré grating resonators [20]. The Moiré approach to fiber grating transmission filter fabrication was reported for application to side-etched, surface-relief structures. Later, the same technique was adopted using the UV direct-write grating fabrication method and uniform FBGs [21]. The Moiré grating resonators were fabricated using the same technique as for standard FBGs, but adopting the holographic exposure. Moirè gratings were formed by a double exposure to two interference patterns of slightly different periods. In 1994, Agrawal and Radic modeled the response of phase-shifted FBGs and showed that the transmitted peak shifts with the amount of the phase shift in the middle of the grating [22]. FBGs fabricated with single or multiple phase shifts at various precise locations along the grating have been proposed for creating narrow spectral resonances for both filtering and laser applications [23-24]. Such structures can be produced at the fabrication stage by using specially designed phase masks incorporating the required phase shifts with restrictions on the magnitude, position, and number [24-26]. However, the cost of this procedure, the dependence on the operating wavelength and the need of a large variety of phase masks to produce structures with different and optimized spectral properties for specific applications are the major limitations of this approach. Phase shifts were also realized by detuning the relative position between the phase mask and the optical fiber hosting the grating at properly selected longitudinal positions [27-28]. In this case a piezoelectric ceramic translation stage was used to change the relative position during the fabrication stage, but, in principle, it would be extremely useful to separate the FBG fabrication stage from the successive introduction of defects along their structure. In this sense, during the last decades several mechanisms have been adopted to introduce phase shifts along FBGs, allowing also a more accurate and adjustable control of the induced phase shift amount. In particular, many research studies focused their attention on the development of wavelength independent post processing techniques able to induce distributed phase shifts along the grating structure enabling the tailoring of the bandgap features. In this chapter, the main technological steps are carefully reviewed. POST PROCESSING APPROACHES: A VIEW BACK The first attempt to introduce a defect along a FBG by post processing methods was dated 1994 by Canning and Sceats [29]. Starting from the demonstration of grating writing by point-by-point technique [30] – having the significant advantage of allowing arbitrary phase gratings to be written – here the basic idea relied on the demonstration that a
Page 2 and 3: FIBER BRAGG GRATING SENSORS: RECENT
Page 4 and 5: CONTENTS Foreword i Preface ii Cont
Page 6 and 7: ii PREFACE The field of fiber optic
Page 8 and 9: iv CONTRIBUTORS Jacques Albert Depa
Page 10 and 11: vi Kate Sugden Photonics Research G
Page 12 and 13: 2 Fiber Bragg Grating Sensors Jacqu
Page 14 and 15: Fiber Bragg Grating Sensors: Recent
Page 16 and 17: Fiber Bragg Gratings: Advances in F
Page 18 and 19: 36 Fiber Bragg Grating Sensors Joha
Page 22 and 23: Photonic Bandgap Engineering in FBG
Page 24 and 25: Fiber Bragg Grating Interrogation S
Page 28 and 29: Multiplexing Techniques for FBG Sen
Page 30 and 31: Polarization Properties of Fiber Br
Page 34 and 35: Fiber Bragg Grating Sensors in Civi
Page 36 and 37: 172 Fiber Bragg Grating Sensors Tak
Page 40 and 41: Fiber Bragg Grating Sensors in Ener
Page 42 and 43: 198 Fiber Bragg Grating Sensors Tam
Page 44 and 45: 218 Fiber Bragg Grating Sensors: Re
Page 46 and 47: 220 Fiber Bragg Grating Sensors Ber
Page 48 and 49: Fiber Bragg Grating Evanescent Wave
Page 50 and 51: 270 Fiber Bragg Grating Sensors: Re
Page 52 and 53: 272 Fiber Bragg Grating Sensors Tom
Page 54 and 55: Polymer Fiber Bragg Gratings Fiber
Page 58 and 59: Fiber Bragg Grating Sensors: Market
Page 60: 322 Fiber Bragg Grating Sensors Cus

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