Photosensitive Fiber Research Articles

Form birefringence in UV-exposed photosensitive fibers computed using a higher order finite element method

The effective index change and form birefringence are calculated in UV-exposed fibers using a high-order vectorial finite element method. The birefringence is compared in optical fibers with and without photosensitive inner cladding.

Wavelength-flattened fibre couplers produced by UV-trimming of fused hybrid fibres

A novel method is described for fabricating wavelength-flattened fibre couplers with the combination of one intrinsically photosensitive fibre and one standard singlemode fibre. The spectrum monitored in real time can be shifted and flattened to the desired wavelength band by exposing the entire coupler to intense ultraviolet light.

Bragg grating performance in Er–Sn-doped germanosilicate fiber for simultaneous measurement of wide range temperature (to 500 °C) and strain

A fiber-optic sensing scheme for the simultaneous measurement of strain and a wide range of temperatures has been investigated by combining the properties of the fiber Bragg grating (FBG) and the fluorescence from a rare-earth-doped photosensitive fiber. The temperature-dependent fluorescence peak power ratio of the two peaks occurring around 1535 nm and 1552 nm from the amplified spontaneous emission due to the I13/24↔4I15/2 transitions in Er3+-doped tin–germanosilicate fiber, with 980 nm pumping, and the dual functionality of the FBG were exploited in this scheme. The sensor is based on a single FBG written in a small length (∼10 cm) of the above fiber, which can be used for the simultaneous measurement of strain and temperature over ranges of 0–1150 με and 22–500 °C, with root-mean-square errors of 36 με and 6 °C, respectively.

A Novel Method for Measuring the Photo-Induced Birefringence ofPhotosensitive Fibres Based on a Fibre Loop

We report a novel method for measuring photo-inducedbirefringence of photosensitive fibres based on a fibre loop. Themethod is simple and is only slightly affected by the surroundings. Theexperimental results show that the photo-inducing normalizedbirefringence of the photosensitive fibre ismulti-decaying-exponentially proportional to the exposure and it canattain 10−5.

Photosensitization of standard fibers with deep uv laser radiation

A strong and permanent ultraviolet photosensitivity enhancement was locked into standard optical fiber pre-treated with above-bandgap 157-nm F <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> -laser radiation. Below-bandgap 248-nm irradiation of the pre-treated fiber yielded fiber Bragg gratings (FBGs) with an index contrast of 6.5 × 10/sup -4/, a value four-fold larger than that available in 16% Ge-doped photosensitive fiber. The F <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> -laser photosensitivity locking was 300 times more effective than with KrF-laser pre-treatment in terms of the accumulated fluence required to lock in a similar refractive index change. Accelerated FBG aging tests equivalent to 25 years at 70°C revealed a less than 2% reflectivity decay in both 157- and 248-nm pre-treated fibers, which is sufficient to eliminate the postannealing step typically required in hydrogen-loaded fibers. The large photosensitization and the superior thermal stability of both 248- and 157-nm pre-treated fibers are associated with a new Si-H bond absorption feature at 1387 nm, which appears only during the FBG formation step.

Study of the behaviour of the fibre Bragg grating fabricated with cladding mode suppression fibre

One effective method for suppression of coupling from guided optical modes into cladding modes in an optical fibre Bragg grating is to use cladding mode suppression fibre. In this specially engineered fibre, the grating is written into the core and the inner cladding, both of which are photosensitive. A theory is presented in this paper to analyse the spectral characteristics of this kind of gratings. A fibre Bragg grating was experimentally fabricated with this kind of photosensitive fibre (PS-RMS-50 from StockerYale). It is shown both theoretically and experimentally that such gratings exhibit strong suppression of the coupling between core mode and cladding mode. The experimental result is in good agreement with the theoretical analysis.

Erasable holographic recording in photosensitive polymer optical fibers.

We report what is to our best knowledge the first holographic recording in photosensitive polymer optical fibers by guided beams. The fibers are made of poly(methyl methacrylate) doped with Disperse Red 1 dye. We also demonstrate what we believe is the first recording of Fourier-transform images in these fibers.

Photosensitivity of germanosilicate fibres and preforms doped with nitrogen inhomogeneously over the cross section

The effect of the inhomogeneous distribution of nitrogen impurity on the photosensitivity of germanosilicate fibres is studied. For this purpose, single-mode fibres with different alternations of the core layers sintered in nitrogen and oxygen atmospheres were specially prepared by the MCVD technique. Photoinduced variations in the quadratic nonlinear susceptibility and refractive index of these fibres were recorded. It is shown that the recording efficiency depends on the radial distribution of germanium oxygen-deficient centres, which can be controlled at the stage of sintering through a nonuniform doping by nitrogen over the cross-section.

A novel method for fabricating apodized fiber Bragg gratings

In this paper, a new implementation of the double exposure method for fabricating apodized fiber Bragg gratings by controlling the parameters of rotation stage and the number of UV-shots is presented. As a result, the average refractive index is kept nearly constant along the Bragg grating, and various apodized fiber Bragg grating with high reflectivity and high side lobe suppression ratio have been fabricated in single-mode hydrogen-loaded photosensitive fiber . The achieved side-lobe suppression ratio is larger than 25 dB .

Ultraviolet photon-induced absorption bands and paramagnetic centers in Ge and Sn co-doped SiO 2 glass

Germanium and Sn co-doped SiO 2 glass, a material for photosensitive optical fiber core that can be used for fabrication of optical fiber gratings, was exposed to photons from a KrF excimer laser (5.0 eV) or a XeCl lamp (4.0 eV). The photo-induced paramagnetic centers and optical properties of the glass have been examined with electron spin resonance and absorption measurements. Precursors of the paramagnetic centers are also discussed.

An All-Fibre Dynamic Gain Equalizer Based on a NovelLong-Period Fibre Grating Written by High-Frequency CO2 Laser Pulses

An all-fibre dynamic gain equalizer based on a novel long-period fibregrating (LPFG) written on a photosensitive fibre by high-frequencyCO2 laser pulses is demonstrated for the dynamic gain flattening ofEr-doped fibre amplifiers. The resonant peak amplitude of such adynamic gain equalizer can be adjusted linearly by bending the LPFG.The gain profile of the Er-doped fibre amplifier constructed has beenflattened to within ±0.7 dB over a 32 nm bandwidth using such adevice.

Cavity-enhanced spectroscopy in optical fibers

Cavity-enhanced methods have been extended to fiber optics by use of fiber Bragg gratings (FBGs) as reflectors. High-finesse fiber cavities were fabricated from FBGs made in both germanium/boron-co-doped photosensitive fiber and hydrogen-loaded Corning SMF-28 fiber. Optical losses in these cavities were determined from the measured Fabry-Perot transmission spectra and cavity ring-down spectroscopy. For a 10-m-long single-mode fiber cavity, ring-down times in excess of 2 ms were observed at 1563.6 nm, and individual laser pulses were resolved. An evanescent-wave access block was produced within a fiber cavity, and an enhanced sensitivity to optical loss was observed as the external medium's refractive index was altered.

Growth Modeling of Fiber Gratings: A Numerical Investigation

We have developed a simple method to analyze the growth process of fiber gratings (FGs) in the core of photorefractive optical fibers. It is based on the exponential saturation model and it takes into account the type of photosensitive fiber used, the exposure times, and the incident optical power. The photorefractive fiber has been characterized by two parameters: the saturation photoinduced refractive index change and the saturation energy, which can be obtained experimentally. We carry out a numerical investigation for the growth process of a uniform FG. The experimentally observed phenomena of shifting the Bragg optical frequency with exposure times and the appearance of higher-order Bragg resonances are predicted and discussed.

Dual temperature and strain measurement with the combined fluorescence lifetime and Bragg wavelength shift approach in doped optical fiber.

We have constructed fiber-optic sensors to measure temperature and strain by combining the properties of fiber Bragg gratings with the fluorescent lifetimes of various doped fibers. Sensors have been made with the fiber Bragg grating written directly into the doped fiber to ensure the collocation of the strain and temperature measurement points. Results are compared with those obtained previously from a Bragg grating written into standard photosensitive fiber spliced to doped fiber. Standard deviation errors of 7 microepsilon and 0.8 degrees C have been obtained for strain and temperature ranges of up to 1860 microepsilon and 120 degrees C, respectively.

The influence of hydrogen loading and the fabrication process on the mechanical strength of optical fibre Bragg gratings

Abstract This paper investigates the influence of hydrogen loading and other stages of the fabrication process on the mechanical strength of fibre Bragg gratings. Following UV irradiation, tensile tests were carried out on Ge–B codoped photosensitive fibres with and without hydrogen loading. Fibre Bragg gratings (FBGs) were written using a range of UV wavelengths, namely 246, 255 and 266 nm. The tensile strength of the optical fibres was determined in their as-received status and following the various stages of FBG fabrication. The mechanical strength was assessed using Weibull statistics. The results indicate that the strength of FBGs is influenced by the UV irradiation parameters and by the hydrogen-loading process. FBGs fabricated using shorter UV wavelengths and low pulse power intensity exhibit a high mechanical strength. The FBGs written in hydrogen loaded fibres have less than 50% of the strength of FBGs that have not been hydrogen loaded. Fibre fracture morphology observed by scanning electron microscope reveals fracture mechanisms of FBGs, which are correlated with the structural change of the silica fibres induced during the FBG inscription process. Combined with surface information gathered by atomic force microscopy, fracture mechanics is applied to understand the mechanisms of strength degradation caused by the FBG inscription process.

Laser assisted jacket removal and writing of fiber Bragg gratings using a single laser source

The 289 and 255nm outputs of a frequency doubled copper laser are used to remove the polymer jacket of optic fiber and write fiber Bragg gratings respectively. Our studies show that 289nm laser light is ideal for stripping the polymer jacket in a well defined manner without interacting significantly with the core of photosensitive fibers.

Increasing fiber photosensitivity to near-UV radiation by rare earth doping

The writing of Bragg gratings with near-UV light is desirable because there exist fiber coatings transparent to this wavelength. This will enable gratings to be written without first removing the protective coating. However, the writing time is typically long when near-UV wavelengths are used. We demonstrate that the writing time can be substantially reduced by using fibers doped with holmium and thulium rare earth ions. These ions are absorbing at these wavelengths but transparent at telecommunication wavelengths. We used the 355-nm output from an efficient all solid-state laser to write the gratings and observed a 30% reduction in writing time.

Ultraviolet photosensitive response in an antimony-doped optical fiber

A silica optical fiber doped with Sb is fabricated with a refractive-index profile that is comparable with standard single-mode fiber. In D(2)-loaded samples, we observe UV photosensitivity with an initial refractive-index-modulation growth rate six times higher than that of the equivalent Ge-doped standard fibers. Enhanced temperature stability of the Bragg grating strength up to 200 degrees C is also observed. Grating growth kinetics in the Sb-doped fiber is compared with those of other Ge-doped photosensitive fibers.

Strong 157 nm F 2 -laser photosensitivity-locking of hydrogen-loaded telecommunication fibre for 248 nm fabrication of long-period gratings

Strong (>20 dB) long-period fibre gratings were fabricated by combining F/sub 2/-laser photosensitivity-locking of hydrogen-soaked telecommunication fibre and KrF-laser grating formation. The 157 nm pretreatment permanently enhances the fibre photosensitivity response to the low-energy 248 nm photons. Thermal annealing at 120/spl deg/C for 24 h improves the grating stability without diminishing the enhancement.

Effect of gamma–neutron nuclear reactor radiation on the properties of Bragg gratings written in photosensitive Ge-doped optical fiber

We have experimentally studied the effect of mixed gamma–neutron nuclear reactor radiation on the effective refractive index of Bragg gratings fabricated in Ge-doped photosensitive optical fiber. We compared the effect of radiation on gratings written with and without hydrogen sensitization and show that the use of hydrogen loading results in an increase of the sensitivity to reactor radiation. Neither grating annealing at 500 °C nor pre-irradiation of the fiber results in a significant increase of the radiation hardness of the Bragg gratings. The best radiation stability is observed for gratings fabricated without hydrogen loading.