Showing papers on "Fiber Bragg grating published in 2005"

Highly sensitive fiber Bragg grating refractive index sensors

Abstract: We combine fiber Bragg grating (FBG) technology with a wet chemical etch-erosion procedure and demonstrate two types of refractive index sensors using single-mode optical fibers. The first index sensor device is an etch-eroded single FBG with a radius of 3 μm, which is used to measure the indices of four different liquids. The second index sensor device is an etch-eroded fiber Fabry-Perot interferometer (FFPI) with a radius of ~1.5 μm and is used to measure the refractive indices of isopropyl alcohol solutions of different concentrations. Due to its narrower resonance spectral feature, the FFPI sensor has a higher sensitivity than the FBG sensor and can detect an index variation of 1.4 X 10(-5). Since we can measure the reflection signal, these two types of sensors can be fabricated at the end of a fiber and used as point sensors.

High resolution optical frequency domain reflectometry for characterization of components and assemblies.

Abstract: We describe a technique for polarization sensitive optical frequency domain reflectometry (OFDR) that achieves 22 micrometer two-point spatial resolution over 35 meters of optical length with -97 dB sensitivity in a single measurement taking only seconds. We demonstrate OFDR’s versatility in both time- and frequency-domain metrology by analyzing a fiber Bragg grating (FBG) in both the spectral and impulse response domains. We also demonstrate how a polarization diversity receiver can be used in an OFDR system to track changes in the polarization state of light propagating through a birefringent component.

Optimization of sensitivity in Long Period Fiber Gratings with overlay deposition.

Abstract: The deposition of an overlay of higher refractive index than the cladding in a Long Period Fiber Grating (LPFG) permits to improve the sensitivity to ambient refractive index changes in a great manner. When the overlay is thick enough, one of the cladding modes is guided by the overlay. This causes important shifts in the effective index values of the cladding modes, and henceforward fast shifts of the resonance wavelength of the attenuations bands in the transmission spectrum. This could be applied for improving the sensitivity of LPFG sensors. The problem is analysed with a numerical method based on LP mode approximation and coupled mode theory, which agrees with so far published experimental results.

Generation and distribution of a wide-band continuously tunable millimeter-wave signal with an optical external modulation technique

Abstract: A new technique to generate and distribute a wide-band continuously tunable millimeter-wave signal using an optical external modulator and a wavelength-fixed optical notch filter is proposed. The optical intensity modulator is biased to suppress the odd-order optical sidebands. The wavelength-fixed optical notch filter is then used to filter out the optical carrier. Two second-order optical sidebands are obtained at the output of the notch filter. A millimeter-wave signal that has four times the frequency of the microwave drive signal is generated by beating the two second-order optical sidebands at a photodetector. Since no tunable optical filter is used, the system is easy to implement. A system using an LiNbO/sub 3/ intensity modulator and a fiber Bragg grating filter is built. A stable and high spectral purity millimeter-wave signal tunable from 32 to 50 GHz is obtained by tuning the microwave drive signal from 8 to 12.5 GHz. The integrity of the generated millimeter-wave signal is maintained after transmission over a 25-km standard single-mode fiber. Theoretical analysis on the harmonic suppression with different modulation depths and filter attenuations is also discussed.

High-birefringence fiber loop mirrors and their applications as sensors.

Abstract: The reflection and transmission characteristics of a high-birefringence fiber loop mirror (HiBi-FLM), which is composed of a standard fiber coupler and one-section or multisection high-birefringence fibers (HBFs), are analyzed and discussed in detail. Theoretical reflectivity and transmissivity expressions for HiBi-FLMs with one-, two-, and three-section HBFs were obtained. The procedure for calculating reflectivity and transmissivity for HiBi-FLMs with n-section HBFs is given. Experimental results have verified the theoretical model. The basic characteristics of the one-section HiBi-FLM when strain and high temperature are applied to HBFs were analyzed and investigated theoretically and experimentally. The experimental results are in good agreement with the theoretical analysis. Furthermore, a strain– temperature sensor that makes use of those characteristics, which is new for applications of HiBi-FLMs, has been proposed and demonstrated.

Stable and uniform dual-wavelength erbium-doped fiber laser based on fiber Bragg gratings and photonic crystal fiber.

Abstract: Based on fiber Bragg gratings (FBGs) and high nonlinear photonic crystal fiber (HN-PCF), a novel dual-wavelength erbium-doped fiber (EDF) laser is proposed and demonstrated. Experimental results show that, owing to the contributions of two degenerate four-wave mixings in the HN-PCF, the proposed fiber laser is quite stable and two output signals are uniform at room temperature. With adjustment of the attenuator, our fiber laser can selectively realize one wavelength lasing.

Slow-light, band-edge waveguides for tunable time delays.

Abstract: We propose the use of slow-light, band-edge waveguides for compact, integrated, tunable optical time delays. We show that slow group velocities at the photonic band edge give rise to large changes in time delay for small changes in refractive index, thereby shrinking device size. Figures of merit are introduced to quantify the sensitivity, as well as the accompanying signal degradation due to dispersion. It is shown that exact calculations of the figures of merit for a realistic, three-dimensional grating structure are well predicted by a simple quadratic-band model, simplifying device design. We present adiabatic taper designs that attain <0.1% reflection in short lengths of 10 to 20 times the grating period. We show further that cascading two gratings compensates for signal dispersion and gives rise to a constant tunable time delay across bandwidths greater than 100 GHz. Given typical loss values for silicon-on-insulator waveguides, we estimate that gratings can be designed to exhibit tunable delays in the picosecond range using current fabrication technology.

Characterisation of a polymer-coated fibre Bragg grating sensor for relative humidity sensing

Abstract: This paper presents a detailed study of the characteristics of a polymer-coated fibre Bragg grating (FBG) sensor for relative humidity (RH) detection. The sensing scheme used in this work builds upon previous research and extends the application of FBGs in chemical sensing by employing a moisture sensitive polymer coating to induce a mechanical strain on the device through volume expansion. The swelling of the polymer coating as a result of the moisture absorption changes the Bragg wavelength of the FBG, thus giving a direct indication of the humidity level. Sensors with different coating thicknesses were evaluated through a series of experiments carried out over a range of values of RH and temperature to investigate various sensing characteristics which include the RH and temperature sensitivity, the time response and the hysteresis effect. All the sensors tested show a linear and reproducible response with a small degree of hysteresis.

Nonreciprocal waveguide Bragg gratings.

Abstract: The use of a complex short-period (Bragg) grating which combines matched periodic modulations of refractive index and loss/gain allows asymmetrical mode coupling within a contra-directional waveguide coupler. Such a complex Bragg grating exhibits a different behavior (e.g. in terms of the reflection and transmission spectra) when probed from opposite ends. More specifically, the grating has a single reflection peak when used from one end, but it is transparent (zero reflection) when used from the opposite end. In this paper, we conduct a systematic analytical and numerical analysis of this new class of Bragg gratings. The spectral performance of these, so-called nonreciprocal gratings, is first investigated in detail and the influence of device parameters on the transmission spectra of these devices is also analyzed. Our studies reveal that in addition to the nonreciprocal behavior, a nonreciprocal Bragg grating exhibits a strong amplification at the resonance wavelength (even with zero net-gain level in the waveguide) while simultaneously providing higher wavelength selectivity than the equivalent index Bragg grating. However, it is also shown that in order to achieve nonreciprocity in the device, a very careful adjustment of the parameters corresponding to the index and gain/loss gratings is required.

High sensitivity evanescent field fiber Bragg grating sensor

Abstract: Increased sensitivity to change in surrounding index is shown by etching the core of a fiber Bragg grating. A maximum sensitivity of 1394 nm/riu is achieved as the surrounding index approaches the core index. Assuming a detectable spectral resolution of 0.01 nm realized in the experiment, the sensor achieves a minimum detectable index resolution of 7.2/spl times/10/sup -6/ when the index of the surrounding medium is 1.44. The sensor can be used as a chemical and biosensor and multiple sensors can be multiplexed and interrogated along a single fiber.

Continuous-wave ultraviolet light induced fiber Bragg gratings in few- and single-mode microstructured polymer optical fibers

Abstract: We report observations and measurements of the inscription of fiber Bragg gratings (FBGs) in two different types of microstructured polymer optical fiber: few-mode and an endlessly single mode. Contrary to the FBG inscription in silica microstructured fiber, where high-energy laser pulses are a prerequisite, we have successfully used a low-power cw laser source operating at 325 nm to produce 1 cm long gratings with a reflection peak at 1570 nm. Peak reflectivities of more than 10% have been observed.

Fiber-coupling efficiency for free-space optical communication through atmospheric turbulence.

Abstract: High-speed free-space optical communication systems have recently used fiber-optic components. The received laser beam in such a system must be coupled into a single-mode fiber at the input of the receiver module. However, propagation through atmospheric turbulence degrades the spatial coherence of a laser beam and limits the fiber-coupling efficiency. We numerically evaluate the fiber-coupling efficiency for laser light distorted by atmospheric turbulence. We also investigate the use of a coherent fiber array as a receiver structure and find that a coherent fiber array that consists of seven subapertures would significantly increase the fiber-coupling efficiency.

Thinned fiber Bragg gratings as refractive index sensors

Abstract: In this work, highly sensitive refractive index measurements have been experimentally demonstrated by using thinned fiber Bragg grating (FBG) sensors. When the cladding diameter is reduced, significant changes in the effective refractive index occur due to surrounding medium refractive index modifications, leading to Bragg wavelength shifts. Uniformly thinned FBGs have been obtained by using wet chemical etching in hydrofluoric acid solutions. In order to prove sensor sensitivity, experimental tests have been carried out by using glycerine solutions with well-known refractive indices. Obtained results agree well with the numerical analysis carried out by using the three-layer fiber model. If the cladding layer is completely removed, resolutions of /spl ap/10/sup -5/ and /spl ap/10/sup -4/ for the outer refractive index around 1.450 and 1.333, respectively, are possible. Finally, a novel approach based on the selective etching along the grating region has been analyzed, leading to high-sensitivity refractive index sensors based on intensity measurements.

Watts-level frequency doubling of a narrow line linearly polarized Raman fiber laser to 589nm.

Abstract: A single-mode, linearly polarized, 1118 nm ytterbium fiber laser was applied to pumping of a short fiber length, polarization-maintaining Raman cavity, resulting in a 0.4 nm linewidth, 23 W CW source at 1179 nm. Efficient, single-pass frequency doubling of the Raman source in MgO doped PPLN to 589 nm was demonstrated with CW power levels in excess of 3 W. No beam quality degradation was observed due to photorefraction at pump power densities up to 2 MW/cm2. The proposed approach can be readily extended to Watt-level generation of any desired wavelength in the 560 to 770 nm range.

Switchable and tunable multiwavelength erbium-doped fiber laser with fiber Bragg gratings and photonic crystal fiber

Abstract: We propose and report on a novel multiwavelength-switchable-tunable erbium-doped fiber laser with excellent stability and uniformity based on four-wave mixing (FWM) in a highly nonlinear photonic crystal fiber. By adjusting the attenuators, the single, dual, or triple wavelengths can be lasing simultaneously. Under the influence of the FWM, the spectrum is stabilized and the uniformity is less than 0.6 dB.

High extinction ratio in-fiber polarizers based on 45° tilted fiber Bragg gratings

Abstract: We report a near-ideal in-fiber polarizer implemented by use of 45° tilted fiber Bragg grating structures that are UV inscribed in hydrogenated Ge-doped fiber. We demonstrate a polarization-extinction ratio of 33?dB over a 100-nm operation range near 1550?nm. We further show an achievement of 99.5% degree of polarization for unpolarized light with these gratings. We also theoretically investigate tilted grating structures based on the Green's function calculation, therein revealing the unique polarization characteristics, which are in excellent agreement with experimental data.

230-kW peak power femtosecond pulses from a high power tunable source based on amplification in Tm-doped fiber

Abstract: We report for the first time an all-fiber laser system that generates tunable Watt-level femtosecond pulses at around 2 μm without an external pulse compressor. The system is based on amplification of a Raman shifted Er-doped fiber laser in a Tm-doped 25-μm-core fiber. We obtain 108-fs pulses at 1980 nm with an average power of 3.1 W and a pulse energy of 31 nJ. The peak power at the output of the amplifier is estimated as ~230 kW, which to the best of our knowledge is the highest peak power obtained from a femtosecond or a few-picosecond amplifier based on any doped fiber. The amplified output is frequency-doubled to produce 78-fs pulses at 990 nm with an average power of 1.5 W and a pulse energy of 15 nJ. We demonstrate broad wavelength tunability around 2 μm as well as around 1 μm.

Thermal properties of fibre Bragg gratings inscribed point-by-point by infrared femtosecond laser

Abstract: Direct, point-by-point inscription of fibre Bragg gratings by an infrared femtosecond laser has been reported recently. Response of these gratings to annealing at temperatures in the range 500 to 1050°C is studied for the first time. Gratings inscribed by infrared femtosecond lasers were thermally stable at temperatures up to 900°C, representing a significant improvement in comparison with the 'common', UV-inscribed, gratings. Annealing at temperatures up to 700°C increased grating reflectivity. © IEE 2005.

Transmission improvement in fiber wireless links using fiber Bragg gratings

Abstract: We demonstrate a simple, passive technique for significant improvement of transmission performance in fiber wireless links through the application of a narrow-band fiber Bragg grating. The grating is used to optimize the optical modulation depth in the transmitted signal by reducing the power of the optical carrier. We present experimental measurements that show improvements in receiver sensitivity (at bit-error rate =10/sup -9/) of up to 7 dB for both double- and single-sideband modulation schemes. The scheme is applicable to a wide range of radio frequencies and modulation depths.

Compensation of nonlinear phase shifts with third-order dispersion in short-pulse fiber amplifiers

Abstract: We show that nonlinear phase shifts and third-order dispersion can compensate each other in short-pulse fiber amplifiers. This compen-sation can be exploited in any implementation of chirped-pulse amplification, with stretching and compression accomplished with diffraction gratings, single-mode fiber, microstructure fiber, fiber Bragg gratings, etc. In particular, we consider chirped-pulse fiber amplifiers at wavelengths for which the fiber dispersion is normal. The nonlinear phase shift accumulated in the amplifier can be compensated by the third-order dispersion of the combination of a fiber stretcher and grating compressor. A numerical model is used to predict the compensation, and experimental results that exhibit the main features of the calculations are presented. In the presence of third-order dispersion, an optimal nonlinear phase shift reduces the pulse duration, and enhances the peak power and pulse contrast compared to the pulse produced in linear propagation. Contrary to common belief, fiber stretchers can perform as well or better than grating stretchers in fiber amplifiers, while offering the major practical advantages of a waveguide medium.

A novel long period fiber grating sensor measuring curvature and determining bend-direction simultaneously

Abstract: A novel long period fiber-grating (LPFG) sensor that can not only measure curvature directly, but also determine every bend-direction within the circular range of 0/spl deg/-360/spl deg/, is proposed and demonstrated in this paper. Such a bend-sensor consists of one LPFG induced by a UV laser and two LPFGs induced by high-frequency CO/sub 2/ laser pulses. The curvature is measured by the UV laser-induced LPFG whose bend-sensitivity is independent of the bend-directions, and the bend-direction is determined by the CO/sub 2/ laser-induced LPFGs whose bend-sensitivities depend strongly on the curved directions. In addition, the unique bend-characteristics of LPFGs induced by high-frequency CO/sub 2/ laser pulses are demonstrated.

Response of fiber Bragg gratings to longitudinal ultrasonic waves

Abstract: In the last years, fiber optic sensors have been widely exploited for several sensing applications, including static and dynamic strain measurements up to acoustic detection. Among these, fiber Bragg grating sensors have been indicated as the ideal candidate for practical structural health monitoring in the light of their unique advantages over conventional sensing devices. Although this class of sensors has been successfully tested for static and low-frequency measurements, the identification of sensor performances for high-frequency detection, including acoustic emission and ultrasonic investigations, is required. To this aim, the analysis of feasibility on the use of fiber Bragg grating sensors as ultrasonic detectors has been carried out. In particular, the response of fiber Bragg gratings subjected to the longitudinal ultrasonic (US) field has been theoretically and numerically investigated. Ultrasonic field interaction has been modeled, taking into account the direct deformation of the grating pitch combined with changes in local refractive index due to the elasto-optic effect. Numerical results, obtained for both uniform and Gaussian-apodized fiber Bragg gratings, show that the grating spectrum is strongly influenced by the US field in terms of shape and central wavelength. In particular, a key parameter affecting the grating response is the ratio between the US wavelength and the grating length. Normal operation characterized by changes in the wavelength of undistorted Bragg peak is possible only for US wavelengths longer than the grating length. For US wavelengths approaching the grating length, the wavelength change is accompanied by subpeaks formation and main peak amplitude modulation. This effect can be attributed to the nonuniformity of the US perturbation along the grating length. At very high US frequencies, the grating is not sensitive any longer. The results of this analysis provide useful tools for the design of grating-based ultrasound sensors for meeting specific requirements in terms of field intensity and frequencies.

Detecting hybridization of DNA by highly sensitive evanescent field etched core fiber Bragg grating sensors

Abstract: Highly sensitive fiber Bragg grating sensors were developed by etching away the cladding and part of the core of the fiber and detecting the change of Bragg wavelength due to the change of index of the surrounding medium. A sensitivity of 1394 nm/riu was achieved when the diameter of the grating core was 3.4 /spl mu/m and the index of the surrounding medium was close to the index of the core of the fiber. Assuming a detectable spectral resolution of 0.01 nm realized in the experiment, the sensor achieves a minimum detectable index resolution of 7.2/spl times/10/sup -6/. Higher sensitivity at lower surrounding index was achieved by using higher order modes excited in the Bragg grating region. The use of the fiber Bragg grating sensor was further investigated to detect hybridization of DNA. Single stranded DNA oligonucleotide probes of 20 bases were immobilized on the surface of the fiber grating using relatively common glutarahyldehyde chemistry. Hybridization of complimentary target single strand DNA oligonucleotide was monitored in situ and successfully detected. The demonstrated fiber Bragg grating sensors provide an elegant method to monitor biological changes in an in situ manner, and provide temporal information in a single experiment.

Delamination monitoring of laminated composites subjected to low-velocity impact using small-diameter FBG sensors

Abstract: Small-diameter fiber Bragg grating (FBG) sensors were applied for the monitoring of delamination induced by low-velocity impact. The FBG sensors were embedded into carbon fiber reinforced plastic (CFRP) laminates [04/904/04]. Using a drop-weight impact tester, an impact loading was applied to the laminates at four impact energy levels. After the impact tests, the internal damages including delaminations were observed by ultrasonic C-scan, and the reflection spectra from the embedded FBG sensors were measured. The form of the spectrum changed sensitively depending on the delamination size. Furthermore, the spectra were calculated theoretically for confirmation of the measured spectra. Since the change in the measured spectrum was consistent with that in the calculated spectrum, the relationship between the delamination size and the form of the spectrum could be clarified. From the results, the present method using small-diameter FBG sensors was found to be effective for the monitoring of the delamination.

Polymer based distributive waveguide sensor for pressure and shear measurement

Abstract: According to embodiments of the present invention, a distributed pressure and shear stress sensor includes a flexible substrate, such as PDMS, with a waveguide formed thereon. Along the waveguide path are several Bragg gratings. Each Bragg grating has a characteristic Bragg wavelength that shifts in response to an applied load due to elongation/compression of the grating. The wavelength shifts are monitored using a single input and a single output for the waveguide to determine the amount of applied pressure on the gratings. To measure shear stress, two flexible substrates with the waveguide and Bragg gratings are placed on top of each other such that the waveguides and gratings are perpendicular to each other. To fabricate the distributive pressure and shear sensor, a unique micro-molding technique is used wherein gratings are stamped into PDMS, for example.

Ultrasonic hydrophone based on distributed Bragg reflector fiber laser

Abstract: We demonstrate a novel fiber-optic hydrophone that uses a dual polarization distributed Bragg reflector (DBR) fiber laser as sensing element. The operation principle is based on the modulation of the birefringence of the fiber laser by high-frequency ultrasound. By measuring the amplitude and frequency of the sidebands as well as the polarization beat frequency of the output of the fiber laser using a photodetector and a radio-frequency spectrum analyzer, the amplitude and frequency of the acoustic pressure, and temperature can be determined simultaneously. The DBR fiber laser hydrophone has a linear response to acoustic pressure and can detect acoustic frequency up to at least 40 MHz.