Analysis on the saturation of refractive index modulation in fiber Bragg gratings (FBGs) written by partially coherent UV beams

Mahakud, R.; Prakash, Om; Nakhe, S. V.; Dixit, S. K.

doi:10.1364/ao.51.001828

Cited by 6 publications

(5 citation statements)

References 22 publications

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“…We suggest that, similarly to silica-based fibers, longer exposure to laser radiation during the FBG inscription leads to nonlinear changes and saturation in the refractive index modification. 40,41 This means that the interplay of irradiation conditions and photosensitivity of fiber material is important for the scattering performance of the grating. Unfortunately, the photosensitivity and refractive index modification in InF 3based optical fibers remain unexplored.…”

Section: ■ Discussionmentioning

confidence: 99%

Controllable Light Scattering on Fiber Bragg Gratings in Multimode Fibers: Tailoring Angular Emission for Advanced Fiber-Based Light Sources

et al. 2023

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We study lateral light scattering on fiber Bragg gratings (FBG) with the goal of creating an optical fiber-based linear light source with controllable emission angles. The scattering from two FBGs was measured for polar angles (measured from the fiber axis) in the range θ = 27.1–152.9° and for all azimuth angles around the fiber. The observed light emission is strongly concentrated in one or more scattering cones around the fiber axis, showing four intensity peaks on opposite sides. These scattering phenomena are described and explained using the volume current method. This method shows a novel and simple way to understand the side scattering on FBGs as a combined effect of the grating’s longitudinal period, the grating harmonics, and its transversal shape. Further, this work contributes to a better understanding of the azimuth FBG-scattering caused by an interplay of the transversal grating shape and the fiber modes. The presented method can generate tailored side emissions for fiber light source applications, create light power sensors, or suppress unwanted scattering on FBGs for low optical loss applications.

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Section: ■ Discussionmentioning

confidence: 99%

Controllable Light Scattering on Fiber Bragg Gratings in Multimode Fibers: Tailoring Angular Emission for Advanced Fiber-Based Light Sources

et al. 2023

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“…The transition of an electron from state D to D 1 depends on the available energy bands with band gap energy corresponding to incidence UV wavelength. The defect center density N d (t) is given as [33] N…”

Section: Discussionmentioning

confidence: 99%

“…The trapped states (D 1 ) population density, which is equal to the depleted defect cen ters density, can be given as The UV excitation induced RI change in the fiber core increases due to the colorcenter [34]. Taking the change in RI (δn), linearly proportional to depleted defect center population per unit volume, the induced RI change is described as [33]…”

Section: Discussionmentioning

confidence: 99%

“…where δn and δn (1) represents the period averaged and amplitude of the first harmonic component of induced RI, δn(z) respectively. By using the exponential series expan sion, carrying out necessary integrations and ignoring higher order terms, the Fourier coefficients δn and δn (1) of δn(z) are expressed as [33] δn ≈ ∆n max 1 − (1 + 0.25 γ 2 β 2 F 2 ) exp(−βF) (10) δn defect = δn (1) ≈ γβF exp (−βF) ∆n max 1 + 0.125 γ 2 β 2 F 2 .…”

Section: Discussionmentioning

confidence: 99%

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Role of UV fluence on the thermal regeneration characteristics of fiber Bragg gratings: experiment and analysis

Kumar

Prakash

Choudhary

et al. 2020

Meas. Sci. Technol.

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In this paper, the effect of UV exposure time on the thermal regeneration characteristics of fiber Bragg gratings (FBGs) inscribed using 255 nm laser radiation is presented for the first time. This effect is studied in terms of reflectivity, refractive index (RI) modulation and thermal stability of the gratings. TypeI FBGs of RI modulation varying from 2.03 × 10 −4 to 3.38 × 10 −4 were inscribed in hydrogenated SMF28 fiber by controlling UV exposure fluence from 0.9 kJ cm −2 to 28.8 kJ cm −2 . Thermally regenerated gratings were fabricated from typeI FBGs by applying a stepannealing schedule up to 900 °C. The reflectivity of the regenerated FBGs increases with UV fluence and after reaching a maximum value of 46.05%, it decreases. The different behavior of the thermal regeneration characteristics of FBGs is linked with the stress developed inside the fiber during the writing of FBGs and their relaxation during thermal annealing. The theoretical calculations using the defect center and densification model provided an indirect novel approach to estimate the contribution of stress relaxation in thermal regeneration. The work presented in this paper is significant for optimizing the thermal regeneration of FBGs with the final aim of developing a highly reflective regenerated grating for high temperature sensing applications.

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“…To incorporate spatial variation of fringe visibility, as in case of FBG writing by a biprism interferometer, the modified form of Eq. (12) may be expressed as [22] where Δn (x, z) is the distribution of RI change, V(x,z) is fringe visibility, F 0 (=NI 0 τ p , N is the number of incident UV pulses, I 0 (x,z) is fringes mean intensity and τ p is pulse duration) is mean fluence and χ is a constant. Equation (13) is based on single-quantum excitation to the UV absorption band (usually, 5 eV) of germanium-oxygen-deficient centers in the fiber core, and the RI change is proportional to depletion of germanium-oxygen-deficient centers (GODCs) depleted on UV absorption.…”

Section: Refractive Index (Ri) Modulationmentioning

confidence: 99%

FBG inscription by a biprism interferometer: analysis and experiment

et al. 2015

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Bragg grating is made by inducing refractive index (RI) modulation, by spatially patterned UV radiation, in the core of a photosensitive optical fiber [4][5][6][7][8]. Phase mask, interferometric and point-by-point methods are used for FBG fabrication. Phase mask technique is most efficient, but wavelength specific. Distributed sensing requires multiplexing of many FBGs, each of different Bragg wavelengths, in the fiber network. Both phase mask-Talbot interferometer and point-by-point technique are suitable for writing FBGs at different Bragg wavelengths [9][10][11][12]. The long arm of Talbot or mirror-based interferometers necessitates a stable UV beam for multi-pulse FBG inscription. The stability of fringe system in a mirror-based interferometer, involving large number of optical and mechanical components, is prone to environmental disturbances. Submicron focal spot and positional accuracy of translation stages are required for fabrication of submicron period FBG by the point-by-point method. FBG fabrication by focused femtosecond laser by point-by-point method relies on refractive index changes by multi-photon absorption [12]. Such FBGs reveal large birefringence and high degree of polarization. The prisms interferometers, simple and cost effective, are used as a method of wavefront division to holographically write first-order Bragg gratings [13][14][15][16]. A biprism interferometer offers many advantages such as high damage threshold in the absence of surface coatings, inherent fringe stability due to a single optical element, ease of fabrication and handling and suitable for Bragg wavelength tuning. FBGs at different Bragg wavelengths can be inscribed by the same biprism using a geometrically diverging UV beam incident on the biprism [17]. The tuning is accomplished by translation of the fiber in the biprism fringe depth, which avoids the necessity to rotate the biprism or changing the angle of incidence of the UV beam. The tuning can also be accomplished by changing the geometrical divergence Abstract In this paper, the effect of spatial variation of fringe visibility on the length, reflectivity and wavelength tuning of fiber Bragg gratings (FBGs), written by a biprism interferometer, is analyzed. The variation of fringe visibility and usable fringe area are the cumulative effect of spectral and spatial frequency spectrum of the laser source, beam profile and fringe stability. It is analytically shown that with the increase in the distance of FBG writing plane from the biprism, the saturation of UV fringes-induced refractive index modulation decreases, whereas the reflectivity of FBGs inscribed will pass through a maximum. Fiber Bragg gratings are written by a UV beam (255 nm, 5.6 kHz, 30 ns) at different distances from a 24 o refraction angle biprism. The trends on experimental results matched the analysis. The maximum reflectivities observed are of 93.6 % (12 dB) for FBGs in Ge-B co-doped fiber and 99.8 % (29 dB) for FBGs in hydrogen-loaded SMF-28 fiber, attributed to the consideration of spatial vari...

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Analysis on the saturation of refractive index modulation in fiber Bragg gratings (FBGs) written by partially coherent UV beams

Cited by 6 publications

References 22 publications

Controllable Light Scattering on Fiber Bragg Gratings in Multimode Fibers: Tailoring Angular Emission for Advanced Fiber-Based Light Sources

Controllable Light Scattering on Fiber Bragg Gratings in Multimode Fibers: Tailoring Angular Emission for Advanced Fiber-Based Light Sources

Role of UV fluence on the thermal regeneration characteristics of fiber Bragg gratings: experiment and analysis

FBG inscription by a biprism interferometer: analysis and experiment

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