High-frequency Bragg gratings in a photothermorefractive glass

Holographic UV mirrors were recorded in a volume of photothermorefractive glass. This photosensitive silicate glass doped with silver, cerium, and fluorine is transparent in the near-UV, visible, and near-IR spectral regions, and its induced refractive-index change reaches 10(-3). UV radiation of a He-Cd laser at 325 nm was used for Bragg grating recording with a spatial frequency of ~9200 mm(-1). The absolute diffraction efficiency of the recorded mirror reached 12.6% at 325 nm and did not deteriorate under the long-time effects of optical irradiation and heating up 400 degrees C.     

Compact chip-scale filter based on curved waveguide Bragg gratings

We propose a method for miniaturization of filters based on curved waveguide Bragg gratings, so that long structures can be packed into a small area on a chip. This eliminates the stitching errors introduced in the fabrication process, which compromise the performance of long Bragg gratings. Our approach relies on cascading curved waveguide Bragg gratings with the same radius of curvature. An analytical model for the analysis of these devices was developed, and a filter based on this model was designed and fabricated in a silicon on insulator platform. The filter had a total length of 920μm, occupied an area of 190μm×114μm, and exhibited a stop band of 1.7nm at 1.55μm and an extinction ratio larger than 23dB.     

Bragg gratings in ternary SiO(2):SnO(2):Na(2)O optical glass fibers

Bragg gratings have been written in an optical fiber with a core made from ternary SiO(2): SnO(2): Na(2)O glass and a SiO(2) cladding. The presence of Na(2)O allows for higher concentrations of SnO>(2) , which are believed to be responsible for the photorefractive response of this composition. In these preliminary experiments significant refractive-index modulations, up to 6.2x10(-4) , have been achieved with a 248-nm excimer laser and a phase mask to write gratings for reflectivity at ~1.5microm . The induced refractive-index changes show enhanced temperature stability, and there is no sign of any erasure up to temperatures exceeding 600 degrees C.     

光纤布拉格光栅温度灵敏性的实验研究

在-150～550℃的温度范围内测量了光纤布拉格光栅中心反射波长的温度灵敏性。光栅波长的温度响应是非线性的。通过对试验数据的三次多项式进行拟合分析,结果表明:波长为1500nm左右的光栅具有相同的温度敏感性;波长对温度变化的响应(ΔλB/ΔT)主要是由折射率的变化(dn/dT)引起的,dn/dT随着温度的降低而减小;得到了温度灵敏系数关于温度的二次表达式,并给出了在-150～550℃温度范围内波长变化的快速估算。     

Picosecond linear optical pulse shapers based on integrated waveguide Bragg gratings

We demonstrate a general linear pulse-shaping technique based on integrated III-V Bragg gratings (BGs). Such a technique allows for the synthesizing of complex waveforms with picosecond resolution using a compact single-waveguide design. This approach is experimentally demonstrated by fabricating and testing a series of integrated ultrafast optical pulse shapers based on BG geometries acting as time-domain code generators operating at 500 Gbits/s.     

Interrogating fiber Bragg grating sensors by thermally scanning a demultiplexer based on arrayed waveguide gratings

We evaluate a wavelength interrogation technique based on an arrayed waveguide grating (AWG). Initial results show that the Bragg wavelength of fiber Bragg grating (FBG) sensors can be precisely interrogated by thermally scanning an AWG-based demultiplexer. The technique potentially offers a low-cost, compact, and high-performance solution for the interrogation of FBG distributed sensors and multisensor arrays.     

Index structure of fiber Bragg gratings in Ge-SiO(2) fibers

The behavior of the concentration of photoinduced color centers in Ge-SiO(2) optical fibers was compared with that of the index modulation associated with fiber Bragg gratings (FBG's) written in the same fibers. We find that the fluence dependence of the photoinduced Ge E? center, its thermal annealing behavior, and its reaction with H(2) are similar to that of the index modulation generated in both H(2)-loaded and unloaded Ge-SiO(2) fibers. The much higher photosensitivity of H(2)-loaded Ge-SiO(2) fibers is attributed to the much higher formation efficiency of Ge E? centers, with an additional contribution from GeH. A diamagnetic structure, possibly densification, is also found to contribute to the index modulation of FBG's.     

Irradiation effect on strain sensitivity coefficient of strain sensing fiber Bragg gratings

The effect of irradiation on the strain sensitivity coefficient of strain sensing fiber Bragg gratings(FBGs) has been investigated through experiments. FBGs were fabricated in single mode fibers with 3 mol% Ge-concentration in the core and with a H2-loading treatment. In experiments, the FBGs were subjected to γ-radiation exposures using a Co60 source at a dose-rate of 25 Gy/min up to a total dose of 10.5 kGy. The GeO defect in fiber absorbs photons to form a GeE’ defect; the interaction with H2 is a probable reason for the γ-radiation sensitivity of gratings written in hydrogen loaded fibres. The effect mechanism of radiation on the strain sensitivity coefficient is similar to that of radiation on the temperature sensitivity coefficient. Radiation affects the effective index neff, which results in the change of the thermo-optic coefficient and the strain-optic coefficient. Irradiation can change the strain sensitivity coefficient of FBGs by 1.48%–2.71%, as well as changing the Bragg wavelength shift(BWS) by 22 pm–25 pm under a total dose of 10.5 kGy. Our research demonstrates that the effect of irradiation on the strain sensitivity coefficient of FBG is small and that strain sensing FBGs can work well in the radiation environment.     

Experimental investigation of pre-irradiation effect on radiation sensitivity of temperature sensing fiber Bragg gratings

The effect of pre-irradiation on radiation sensitivity of fiber Bragg gratings(FBGs) is verified experimentally.FBGs are fabricated in photosensitive optical fibers and single mode fibers with Ge-concentration in a range from 3 mol% to 23.37 mol% in the core.In experiments,the FBGs are subjected to twice γ-radiation exposures to a Co60 source at a dose-rate of 0.1 Gy/s up to a total dose of 50 kGy.Pre-irradiation treatment can reduce the temperature sensitivity variation of FBGs by 18.12%-35.91%,as well as Bragg wavelength shift(BWS) by 8%-27.08 %.Our research demonstrates that pre-irradiation treatment is a feasible method to improve the radiation tolerance of FBGs.     

Multiband processing of multimode light: combining 3D photonic lanterns with waveguide Bragg gratings

The first demonstration of narrowband spectral filtering of multimode light on a 3D integrated photonic chip using photonic lanterns and waveguide Bragg gratings is reported. The photonic lanterns with multi‐notch waveguide Bragg gratings were fabricated using the femtosecond direct‐write technique in boro‐aluminosilicate glass (Corning, Eagle 2000). Transmission dips of up to 5 dB were measured in both photonic lanterns and reference single‐mode waveguides with 10.4‐mm‐long gratings. The result demonstrates efficient and symmetrical performance of each of the gratings in the photonic lantern. Such devices will be beneficial to space‐division multiplexed communication systems as well as for units for astronomical instrumentation for suppression of the atmospheric telluric emission from OH lines.     

Industrial and medical applications of fiber Bragg gratings(Invited Paper)

The uses of optical fibers are numerous, and over the past few decades, they have extended from optical fiber communications to a wide variety of sensing applications. In particular, fiber Bragg grating(FBG) sensors inscribed in single-mode optical fibers offer significant advantages over more conventional electrical sensors and have been successfully deployed in many different industries. In this Review, we review the applications of intrinsic FBG pressure and flow sensors in oil and gas and the deployment of FBG sensing networks in railways.The promising prospect of using polymer FBGs in wearable medical devices is also described.     

Efficient frequency doubling of a pulsed laser diode by use of a periodically poled KTP waveguide crystal with Bragg gratings

Blue light with an average power of as much as 7.5 mW in picosecond pulses has been generated at 486, 488, and 491 nm from a frequency-doubled, nonresonant injection seeded, gain-switched InGaAs/GaAs diode laser by use of a periodically poled KTP waveguide crystal that incorporates a Bragg grating section.     

Analysis of a deep waveguide Bragg grating

Spectral properties of a very deep Bragg grating operating in a first diffraction order on a single-mode planar waveguide have been studied theoretically. It is shown that the scattering loss can be low (a few percent), the reflectivity very high (over 90%), the reflection band is shifted against the Bragg wavelength toward the shorter wavelengths, and its spectral shape is very different from that of a shallow grating. Inside a reflection band, a part of the input optical power penetrates through the grating even if it is infinitely long. These properties are predicted by modelling using two independent computer codes based on different modelling methods, namely the bi-directional mode expansion and propagation method (BEP), and a method of lines (MoL). The first method is discussed in some detail here. The work has been performed within the framework of European Action COST 240.     

Modeling of waveguide UV-induced Bragg gratings with arbitrary apodization function: Tools for designing high-performance related devices

We present a semi-analytical modeling suitable for the investigation of the spectral characteristics of UV-waveguide Bragg gratings (UV-WBGs) having arbitrary apodization function and extending both in the film layer and in the overlayer. The modeling is based on a proper combination of coupled mode theory and transfer matrix formalism and provides a powerful tool for designing and simulation of complex UV-grating based structures. The results of a detailed investigation of UV-WBGs in doped silica based channel optical waveguides are presented and critical issues for upgrading the performances of related device are discussed. Validation of the modeling is provided by means of an extensive comparison between theoretical and experimental results relevant to UV-WBGs realized in phosphorous (P)- and boron (B)-doped silica based waveguides.     

F2-laser patterning of indium tin oxide (ITO) thin film on glass substrate

This paper reports the controlled micromachining of 100 nm thick indium tin oxide (ITO) thin films on glass substrates with a vacuum-ultraviolet 157 nm F₂ laser. Partial to complete film removal was observed over a wide fluence window from 0.49 J/cm² to an optimized single pulse fluence of 4.5 J/cm² for complete film removal. Optical microscopy, atomic force microscopy, and energy dispersive X-ray analysis show little substrate or collateral damage by the laser pulse which conserved the stoichiometry, optical transparency and electrical conductivity of ITO coating adjacent to the trenches. At higher fluence, a parallel micron sized channel can be etched in the glass substrate. The high photon energy and top-hat beam homogenized optical system of the F₂ laser opens new means for direct structuring of electrodes and microchannels in biological microfluidic systems or in optoelectronics. PACS 79.20.Ds; 42.62.Cf; 42.55.Lc     

High-reflectivity Bragg gratings fabricated by 248-nm excimer laser holographic ablation in thin Ta2O5 films overlaid on glass waveguides

We demonstrate strong Bragg grating reflection in Ta₂O₅ (tantalum pentoxide) thin films overlaid on potassium ion-exchanged channel waveguides in BK-7 glass, inscribed using 248-nm excimer laser holographic ablation. The experimental data presented are divided into two sections: the first section refers to the study of the grating ablation process of thin Ta₂O₅ films with respect to the exposure conditions, while the second focuses on the implementation of these relief gratings in functional waveguide devices. Firstly, experimental data on grating morphology versus exposure conditions, accomplished with scanning electron microscopy microscans, are presented. In the second section diffraction spectra for waveguide gratings are presented and analysed. Spectral notches in transmission of depth -18 dB for the TM polarisation were obtained from 16-mm-long gratings patterned in waveguides overlaid with a 105-nm-thick Ta₂O₅ film, using 50 pulses of 60-mJ/cm² energy density. PACS 42.82.Cr; 42.79.Dj; 81.65.Cf     

Synthesis of 1D Bragg gratings by a layer-aggregation method

We present what we believe to be a novel method for the synthesis of complex 1D (fiber and waveguide) Bragg gratings, which is based on an impedance reconstruction layer aggregation technique. The main advantage brought by the method is the possibility of synthesizing structures containing defects or discontinuities of the size of the local period, a feature that is not possible with prior reported methods. In addition, this enhanced spatial resolution allows the synthesis of very strong fiber Bragg grating devices providing convergent solutions. The method directly renders the refractive index profile n(z) as it does not rely on the coupled-mode theory.