Photosensitive Fiber Research Articles

Discretely tunable single‐ and dual‐wavelength fiber ring laser using a fiber Bragg gratings based Fabry‐Perot filter

AbstractA Fabry‐Perot filter is realized by inscribing two identical 1 mm uniform Bragg gratings separated by 7.7 mm space in the hydrogen‐loaded photosensitive optical fiber. It is then incorporated with a tunable chirped fiber Bragg grating into a fiber ring cavity. Fourteen single‐wavelength lasings from 1552.186 nm to 1553.453 nm with average 97.5 pm spacing, and 13 pairs of dual‐wavelength lasings within the above wavelength range are achieved stably. This kind of laser will be very useful in applications requiring close‐spacing and tunable single‐ or dual‐wavelength components and systems. © 2008 Wiley Periodicals, Inc. Microwave Opt Technol Lett 50: 2170–2173, 2008; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.23620

High-resolution refractive index anisotropy measurement in optical fibers through phase retardation modulation

We present an improved, high-resolution method for the measurement of phase retardation induced by the material birefringence of optical fibers. Such a method can be used to retrieve information about the spatial distribution of refractive index anisotropy in the fiber by comparing the accumulated phase of a polarization component oriented along the fiber transmission axis and another located in the transverse plane. The method is based on the nonlinear regression of a phase modulated signal of known modulation amplitude altered by the sample. Experimental results obtained with our method for a standard telecommunications fiber (the Corning SMF-28) as well as photosensitive fibers (Fibercore PS1250 and PS1500) are presented with a noise-limited phase resolution below 10(-4) radians and a spatial resolution below 1 microm. An analysis of the limitation of such measurement methods is also presented including diffraction by the fibers.

Limitations of Superimposed Fiber Bragg Gratings

The authors demonstrate the inscription of several Bragg gratings at the same location of a photosensitive fiber. This superimposed Bragg grating was fabricated by two-beam interference method using UV laser light. Effects due to the inscription of multiple gratings at the same location of the fiber such as changes of reflectivity and shift of wavelength are studied.

Rapid characterization of the ultraviolet induced fiber Bragg grating complex coupling coefficient as a function of irradiance and exposure time

We report the application of optical frequency domain reflectometry and a discrete-layer-peeling inverse scattering algorithm to the spatial characterization of the UV induced complex coupling coefficient during fiber Bragg grating growth. The fiber grating is rapidly characterized using this technique to give irradiance dependent growth as a function of exposure time, thereby providing the complete characterization of the coupling coefficient in the form of a "growth surface," which is related to the fiber's photosensitivity. We compare measurements of fiber Bragg grating growth in SMF-28 when exposed to continuous wave 244 nm irradiation from 0 to 90 W cm(-2) for exposure times up to 3230 s with a selection of other fibers including high germanium concentration fiber and erbium doped fiber.

Type IIA photosensitivity and formation of pores in optical fibers under intense ultraviolet irradiation

Formation of the type IIA Bragg gratings in germanosilicate optical fibers is studied. We report the observation of such a type of gratings in the standard single-mode fiber (Corning SMF-28) under different experimental conditions. A mechanism for the type IIA photosensitivity in optical fibers is proposed which is based on nucleation and evolution of pores from vacancy-type defects in fiber areas where a high level of mechanical stress is induced under intense ultraviolet (UV) light. Evolution of fiber core temperature under influence of a single 20 ns light pulse from a KrF excimer laser was measured and compared with theoretical calculations. It was shown that transient thermoinduced stress in the fiber core can achieve a level sufficient for effective nucleation of pores. A theory describing formation of pores in optical fibers has been developed and was used to estimate the pore nucleation rate, concentration, and other parameters of pore evolution for different levels of UV fluence and fiber core stress.

Three-hole microstructured optical fiber for efficient fiber Bragg grating refractometer

We present a photosensitive three-hole microstructured optical fiber specifically designed to improve the refractive index sensitivity of a standard fiber Bragg grating (FBG) sensor photowritten in the suspended Ge-doped silica core. We describe the specific photowriting procedure used to realize gratings in such a fiber. We then determine their spectral sensitivity to the refractive index changes of material filling the holes surrounding the core. The sensitivity is compared with that of standard FBGs photowritten in a six-hole fiber with a larger core diameter. We demonstrate an improvement in the sensitivity by two orders of magnitude and reach a resolution of 3 x 10(-5) and 6 x 10(-6) around mean refractive index values of 1.33 and 1.40, respectively.

Photosensitive polymer optical fibres and gratings

This paper reports the development of photosensitive polymer optical fibres for gratings. These fibres, processed by the preform drawing technique, are composed of polymethylmethacrylate cladding and methyl methacrylate copolymer cores. Three core systems with enhanced UV sensitivity are proposed, including benzyl methacrylate (with strong UV absorption) co-monomer contained, benzophenone (photo-initiator)-doped, and trans-4-stilbenemethanol (a molecule which undergoes photo-isomerization)-doped cores, respectively. These cores display significant absorption at wavelength longer than ∼250 nm in which the cladding PMMA is basically transparent. Irradiation tests disclosed the occurrence of photo-induced change in absorption upon exposure to UV at wavelength longer than 250 nm for these cores, ie, at 260 and 280 nm for benzyl methacrylate contained, at around 320 nm for both benzophenone-doped and trans-4-stilbenemethanol-doped. The photo-induced change in refractive index was detected on trans-4-stilbenemethanol-doped core polymer by the Prism Coupler. The photosensitivity of this doped system was demonstrated by Bragg gratings in the core of the fibre fabricated by use of a pulse laser at 325 nm with a low photo-energy and comparatively short exposure time.

Characterization of Single-Mode Fiber With Fiber Bragg Gratings for the Design of Long-Period Gratings

We propose a simple method to determine the dispersion characteristics of the core mode and the cladding modes of a single-mode fiber for the design of long-period gratings (LPGs). The method is based on measuring the effective indexes of the core mode and the cladding modes from the transmission spectra of a number of slightly tilted fiber Bragg gratings (FBGs) that are composed of properly separated Bragg wavelengths. By using four FBGs written in a photosensitive fiber, we obtained 19 phase-matching curves for the design of LPGs over the (S + C + L)-band, which correspond, respectively, to the couplings between the core mode and the first 19 symmetric cladding modes of the fiber. The accuracy of the phase-matching curves is verified with a number of LPGs fabricated in the same fiber using various grating pitches.

A novel measurement method for the photosensitivity of fibers

An online measurement method for the photosensitivity of fibers is proposed. This method is simple and useful. Theoretical and experimental studies have been carried out. This method is based on the Mach–Zehnder interferometer (MZI). One arm of the MZI is externally exposed to UV light at 248 nm originating from a KrF excimer laser. Then the output optical spectrum is measured, by some relevant analysis, and the photosensitivity of the fiber can be obtained. The relationship between the refractive index increment of fibers and the exposure time is in accordance with the absorbing model that is brought forward by H. Patrick, which shows that this method has high precision.

Methods of achieving linear index-change response for narrow-band fiber Bragg grating sequential writing

This investigation elucidates the refractive index-change response of the photosensitive optical fiber to exposed UV flux. The nonlinear UV photosensitivity deforms the index profile of FBGs on the tail edges where the writing UV flux is low. Pre-UV treatment eliminates the nonlinear index-change response and ensures the linearity of the response of subsequent grating writing. A new method that involves interference between two unequal beams in a single writing scan is proposed and demonstrated to be able to improve the index profile and the spectral response of fabricated FBGs. The procedure is stable for writing weak fiber Bragg gratings with pre-designed index profiles.

Modeling, design, fabrication, and testing of a fiber Bragg grating strain sensor array

The modeling, design, simulation, fabrication, calibration, and testing of a three-element, 15.3 cm fiber Bragg grating strain sensor array with the coherent optical frequency domain reflectometry (C-OFDR) interrogation technique are demonstrated. The fiber Bragg grating array (FBGA) is initially simulated using in-house software that incorporates transfer matrices. Compared to the previous techniques used, the transfer matrix method allows a systemwide approach to modeling the FBGA-C-OFDR system. Once designed and simulated, the FBGA system design is then imprinted into the core of a boron-germanium codoped photosensitive fiber using the phase mask technique. A fiber optic Fabry-Perot interferometric (FPI) strain gauge calibrator is then used to determine the strain gauge factor of a single fiber Bragg grating (FBG), and the results are used on the FBGA. The FPI strain gauge calibrator offers nondestructive testing of the FBG. To test the system, the FBGA is then attached to a 75 cm cantilever beam and interrogated using an incremental tunable laser. Electric strain gauges (ESGs) are then used to independently verify the strain measurements with the FBGA at various displacements of the cantilever beam. The results show that the peak strain error is 18% with respect to ESG results. In addition, good agreement is shown between the simulation and the experimental results.

Thermal decay characteristics of strong fiber Bragg gratings showing high-temperature sustainability

Strong fiber Bragg gratings (FBGs) with high-temperature sustainability were fabricated by writing the gratings into several specially developed photosensitive fibers. The thermal decay characteristics of these gratings were investigated over a temperature range from room temperature to 950°C. A cation-hopping model is presented to account for the obvious differences between the FBGs in terms of their thermal properties. A related cation-oriented trap distribution model is also used to simulate the decay properties of the gratings during high-temperature annealing and is found to yield a good fit to the experimental data. An accurate operating lifetime of these specially fabricated gratings can be predicted by using the cation-oriented trap distribution simulation.

Short cavity single frequency fiber laser for in-situ sensing applications over a wide temperature range

A novel Er-doped silica fiber, with heavy Er doping, was specially developed for application to a single frequency fiber laser. Two high temperature-sustainable fiber Bragg gratings, written into Bi-Ge codoped photosensitive fiber, were chosen for the application and spliced to the specialist Er doped silica fiber to form a compact, linear cavity, fiber laser. The fiber laser retained single mode oscillation over a wide temperature range, from room temperature to 400 degrees C. The wavelength of the laser output could be tuned smoothly, without mode hopping being observed, when the temperature was changed. A narrow linewidth of less than 1 kHz was measured at the output of fiber laser and this indicates the potential of the fibre laser sensing system with extremely high sensitivity and resolution over this wide range.

Fabrics that "See": Photosensitive Fiber Constructs

We can now construct conformal detectors that allow us to have optically functional clothing.

Optimisation of superimposed chirped fibre Bragg gratings for the generation of ultra-high speed optical pulse bursts

Compact superstructure fibre Bragg gratings for the generation of ultra-high speed optical pulse bursts are reported. The gratings are equivalent in response to multi-pass superimposed chirped fibre Bragg gratings (SI-CFBGs), but an arbitrary sequence of consecutive gratings can be replicated in a single fabrication step using a simple trigonometric amplitude-phase function. SI-CFBGs are fabricated in photosensitive optical fibre and the time-resolved reflection of a 10GHz picosecond pulse train is measured via frequency-resolved optical gating and optical sub-harmonic sampling detection techniques. The experimental results are compared to the theoretically simulated grating response and good agreement is found.

Temperature monitoring of vehicle engine exhaust gases under vibration condition using optical fibre temperature sensor systems

Two optical approaches, comprising and contracting both the fluorescence decay lifetime and the fibre Bragg grating (FBG) methods, were developed and evaluated for temperature monitoring of exhaust gases for use on a vehicle engine. The FBGs used in the system were written into specially designed Bi-Ge co-doped photosensitive fibres, to enable them to sustain high temperatures to over 800°C, which is far beyond that of FBGs written into most commercial photosensitive fibres. The sensors were subjected to a range of vibration tests, as a part of an optical exhaust monitoring network under development, and results from the test carried out are reported.

Phase separation in highly photosensitive tin-codoped silica optical fibers and fiber preforms exposed to UV radiation

Sn-doped silicate fibers exhibit extremely high photosensitivity when exposed to 248nm KrF excimer laser. Electron micrograph probing has shown that nanoscale phase separation occurs in fibers and fiber preforms after UV exposure. This phenomenon has been observed only in SnO2-doped germanosilicate and phosphosilicate fibers and fiber preforms when the SnO2 concentration is high. At low SnO2 concentrations no phase separation has been observed. SnO2 codoped glasses are the first optical fibers to exhibit phase separation.

Birefringence relaxation in photosensitive polymer optical fiber preform

The birefringence relaxation of photosensitive polymer optical fiber preform with low azo concentration is analyzed. Instead of bi-exponential function, tri-exponential one is introduced to describe the relaxation process. Three types of relaxation are given considering the movements of chromophores, side chains, and main chains. The conserved birefringence, which depends on the orientation of main chains, is also studied through comparison of three kinds of materials with different interaction between side chain and main chain.

Medium-range ordering in glass structures as a background of photosensitivity in silica fibers

Using picosecond stimulated Raman scattering (SRS) in silica-based fibers as a spectroscopic tool, some correlations between medium-range ordering and photosensitivity in silica glasses were experimentally established. Apart from the fundamental Raman-active vibrations attributed to the silica tetrahedra and dopant groups and their overtones, the intense Raman combination bands were observed. Based on the overtone band sequences, the vibrational anharmonicity constants were calculated and analyzed, including the ones corresponding to the 570cm−1 band. This band is forbidden in Raman scattering in silica crude selection rules are used. However, the transition is activated owing to the tetrahedron distortions by outer shells of the first and second peripheral atoms in a silica network. It is the intensity of the 570cm−1 Stokes band that grows enormously in the SRS spectra of germanium-doped silica fibers as a result of photoinduced changes in the glass structure resulting in second harmonic generation (SHG). The experimental data are interpreted using a modern glass theory. Practical applications of picosecond SRS spectroscopy for glass microstructural studies are discussed.

Optical fibre for fabricating long-period gratings with low temperature sensitivity within selected temperature range

Reported is a photosensitive fibre for making long-period gratings with temperature sensitivity of the resonance wavelength as low as 0.25 nm over 0–70°C. Furthermore, a convenient method for selecting the temperature of minimum sensitivity, by controlling the boron concentration in the core and monitoring it with attenuation at 1550 nm, is presented.