Rare-earth Doped Optical Fibers Research Articles

Two-Photon Excited Fluorescence in Rare-Earth Doped Optical Fiber for Applications in Distributed Sensing of Temperature

Distributed temperature sensing based on time-correlated two-photon excited fluorescence (TPF) in doped optical fiber is described. Counter-propagating laser pulses generate a TPF flash at the position of their overlap which is scanned along the fiber by a variable relative time delay. The flash is transmitted to one end where it is detected and analyzed to yield the temperature from its thermal dependence. To identify suitable dopants, the two-photon excitation spectra of glass doped with various rare-earths were recorded. Preliminary results on TPF in praseodymium doped single-mode fiber are presented.

Thermal stability and transport studies of (100−2x)TeO2–xAg2O–xWO3 (7.5≤x≤30) glass system

Typical modulated DSC (NRHF and C p ) results during a heating scan in the 85TeO 2 –7.5Ag 2 O–7.5WO 3 glass sample. ▶ The addition of equal mol% of WO 3 and Ag 2 O to TeO 2 increases the thermal stability. ▶ The 55TeO 2 –22.5WO 3 –22.5Ag 2 O shows the highest thermal stability (Δ T = 237 °C). ▶ These glasses are more useful for drawing of optical fibers. ▶ The present glass system shows higher conductivity . Differential scanning calorimetry (DSC), infrared (IR) and direct current (DC) conductivity studies have been carried out on (100 − 2 x )TeO 2 – x Ag 2 O– x WO 3 (7.5 ≤ x ≤ 30) glass system. The IR studies show that the structure of glass network consists of [TeO 4 ], [TeO 3 ]/[TeO 3+1 ], [WO 4 ] units. Thermal properties such as the glass transition ( T g ), onset crystallization ( T o ), thermal stability (Δ T ), glass transition width (Δ T g ), heat capacities in the glassy and liquid state ( C pg and C pl ), heat capacity change (Δ C p ) and ratios C pl / C pg of the glass systems were calculated. The highest thermal stability (237 °C) obtained in 55TeO 2 –22.5Ag 2 O–22.5WO 3 glass suggests that this new glass may be a potentially useful candidate material host for rare earth doped optical fibers. The DC conductivity of glasses was measured in temperature region 27–260 °C, the activation energy ( E act ) values varied from 1.393 to 0.272 eV and for the temperature interval 170–260 °C, the values of conductivity ( σ ) of glasses varied from 8.79 × 10 −9 to 1.47 × 10 −6 S cm −1 .

Fabrication of high aluminium containing rare-earth doped fiber without core–clad interface defects

The unwanted core–clad interface defect generation and related scattering loss in rare-earth doped optical fibers containing Al-oxide is up to now an unsolved problem as it cannot be avoided without modifying fiber design or composition. Here a simple technique is proposed to eliminate the problem without affecting fiber parameters. The method comprises addition of fumed silica in appropriate amount under dispersed condition to the soaking solution containing Al and rare-earth salts during the solution impregnation stage. This facilitates the networking between Al 2O 3 and SiO 2 during sintering. The method leads to an improvement in optical properties of the fibers and enables doping of high concentration of Al-oxide even in presence of GeO 2 in the core.

Tunable doped-fibre vertical cavity surface emitting laser

A simple way to improve the output power of singlemode tunable VCSELs is demonstrated. It is based on a master oscillator power amplifier (MOPA)-like embodiment of a fibre-VCSEL. The reported device, operating at 1.55 µm, uses a single pump source to photo-excite both a semiconductor structure and a doped fibre without compromise in compactness. Initial results show continuous tunability over 11 nm and fibre-coupled output power of 3.1 mW. It is noted that this approach can be extended to any part of the spectrum for which rare-earth doped optical fibres are available.

Radiation effects on Yb- and Er/Yb-doped optical fibers: A micro-luminescence study

The integration of rare-earth doped optical fibers as part of fiber-based systems in space implies the development of waveguides tolerant to the radiation levels associated with the space missions. Erbium (Er)- or Ytterbium/Erbium (Yb/Er)-doped fibers have been shown to be very sensitive to ionizing radiations. Radiations lead to a strong increase of the fiber attenuation around the pump and amplified signal wavelengths. In this paper, we investigate by confocal luminescence microscopy the radiation-induced spectroscopic changes on prototype Yb- or Yb/Er-doped optical fibers. The set of tested fibers allows us to provide new insights into the relative influence of the P, Al doping on the radiation responses of their silica-based host matrix and on the transitions between the energy states of rare-earth ions.

Infrared-to-Visible Upconversion Emissions in Er<sup>3+</sup> Doped TeO<sub>2</sub>-Based Oxyhalide Glasses

The fluorescence and up-conversion spectral properties of Er3+-doped TeO2-ZnO and TeO2-ZnO-PbCl2 glasses suitable for developing optical fiber amplifier and laser have been fabricate and characterized. Strong green (around 527-550 nm) and red (around 661 nm) up-conversion emissions under 977 nm laser diode excitation were investigated, corresponding to 2H11/2, 4S3/2, ® 4I15/2 and 4F9/2 ® 4I15/2 transitions of Er3+ ions respectively, have beenobserved and the involved mechanisms have been explained. The dependence of up-converted fluorescence intensity versus laser power confirm that two-photons contribute to up-conversion of the green-red emissions. The novelty of this kind of optical material has been its ability in resisting devitrification, and its promising optical properties strongly encourage for their further development as the rare-earth doped optical fiber amplifiers and upconversion fiber laser systems.

Rare Earth Doped Optical Fibers for Temperature Sensing Utilizing Ratio-Based Technology

A new and practical fluorescence temperature detecting system based on fluorescence intensity ratio was proposed. The background theory of fluorescence intensity-ratio method was presented simply. And the characters of rare earth doped samples were detailed. The erbium-doped fiber was chosen as the sensing element. The energy levels of 2H 11/2 and 4S 3/2 are responsible for the emission of radiation at approximately 530 and 555 nm. The erbium-doped (960 ppm) fiber of length 20 cm and core diameter 3.2 μm was used as the sensing part. A silica photodiode transfers the fluorescence signal to electric signal, then the ratio of the average of the two different signals was calculated by the computer and the temperature was obtained. The ratio R of the intensity resulting from the transition between the two levels varies proportionly with temperature interval from 293 K to 373 K. The sensitivity of the sensor is approximately 0.05 K −1.

Spectral control of optical gain in a rare earth-doped optical fiber using novel triple layered structures

Novel techniques to control the effective emission cross section of a rare earth-doped optical fiber with a functional cladding structure are reviewed for applications in active fiber devices. We discuss evanescent wave filtering (EWF) technique based on the overlap in the radiative transition cross sections between the emitting ion in the core and the absorbing ion in the inner cladding. Experimental applications of EWF concept are reviewed for a composite gain medium with the erbium ions in the core and samarium ions in the inner cladding for self-gain flattening in C-band. W-type three layered fiber structure is also analyzed in terms of modal guidance and location of the LP 01 mode cut-off. Its application in neodymium and thulium-doped silica fiber laser is discussed to facilitate the radiative transition near 940 and 1600 nm region, respectively. The design concepts, fabrication process, and device performances are discussed for each application.

Induced attenuation in Ce 3+ and Nd 3+ doped fibers irradiated with electron beams under low dose regime

In this paper, we study the feasibility of rare earth doped optical fibers as dosimeters based on the attenuation induced by electron beams under standard conditions in radiotherapy. Neodymium and cerium doped fibers have been irradiated with 10 MeV electrons, the radiation-induced loss and the dynamic response have been measured in the wavelength range 1200–1600 nm. The sensitivity of the fibers is of the order of 0.1 (dB/m)/Gy and has a linear dependence on the dose.

Programmable control system for wavelength tuning and stabilization of optical fiber lasers

We study the performance of a simple computer-based control system for tuning and wavelength stabilization of an erbium-doped single-mode fiber laser. Tuning is achieved by stretching a fiber grating used as one of the mirrors for the laser cavity, while wavelength is monitored through an optical spectrum analyzer. The desired wavelength of emission is fed to the computer through a graphic computer interface, and an actuator stretches the grating to tune the fiber laser. Once the specified wavelength is achieved, the control systems stabilize the fiber laser by monitoring wavelength fluctuations due to parameters such as temperature variations. Accuracy for wavelength stabilization is limited only by the resolution of the optical spectrum analyzer (0.06 nm). Being a general purpose system, any other tuning methods or wavelength tracking devices such as etalons can be used. Thus, by using the proper devices, the computer-based system could be easily adapted for the tuning and stabilization of any other rare earth-doped optical fiber laser.

Frequency upconversion properties of Yb3+-Er3+co-doped oxyfluoride germanate glass

Short wavelength output compact solid state lasers have become one of the most active areas of lasers research and development today [1]. Compared with other techniques, upconversion is of interest because it does not need either stringent constraint of phase matching or of high excitation wavelength stability and the output wavelength is not restricted to a given harmonic. In the past decades, the host materials widely studied for upconversion were fluoride crystals and glasses because fluorides have low nonradiative relaxation rates [2]. However, the mechanical strength, chemical durability and thermal stability of fluoride glasses are generally less than those of oxide glasses. These properties are important for developing rare-earth doped optical fibers. Recently, heavy metal oxide (HMO) glasses, which contain high content of PbO or Bi2O3, have considerable potential as lasing materials such as upconversion and planar optical waveguides [3, 4]. The HMO glasses mainly includes tellurite based glasses, bismuth based materials and germanate based glasses, which are desirable hosts for optically active ions. Of these HMO materials, lead germanate glasses are of growing interest, because they combine higher mechanical strength, chemical durability and better thermal stability even though the maximum phonon energy (810 cm−1) is slightly larger than that of tellurite glasses (750–780 cm−1) [5–8]. In addition, the recent study of glasses containing more than one anion has revealed the possibility of the creation of glasses where the properties are varied by replacing the traditional oxygen by another anion, particularly by halides [9]. The substitution of a fluoride for an oxygen can improve the glass forming behavior of melts and the thermal stability of glasses [10, 11]. However, only a few reports have concentrated on the oxyfluoride germanate glasses and, few investigations have been carried out on the spectroscopic properties of rare-earth doped oxyfluoride germanate glasses. The present paper investigated the upconversion spectroscopic properties of Yb3+Er3+ co-doped 5.5NaF2-8Na2O-11.3PbO-14.5BaF23.6Al2O3-4.1AlF3-51.2GeO2 (mol%; GPAB, hereafter) glass. The doping concentrations of Yb3+ and Er3+ ions were 7.8 × 1020 and 0.78 × 1020 ions/cm3, respectively. The glass sample was synthesized by a conventional melting and quenching method [12]. Reagent grade

Rare-earth doped optical fiber approach to an alarm system for fire and heat detection

A fluorescence based optical fiber fire alarm system has been further refined in this work to determine the presence of a localized temperature excursion (as low as 50–100 °C) against a varying background at temperatures up to 500–600 °C.

Does photosensitivity pave the way towards the fabrication of miniature coherent light sources in inorganic glass waveguides?

Photosensitivity designates the ability to permanently change the refractive index of a glass by optical processing. The phenomenon allows the fabrication of numerous phase structures, the simplest of which is the Bragg grating obtained by photoimprinting a periodic index modulation within the material. Bragg gratings have changed the way in which optical fibre (or planar waveguide) lasers are now used. Distributed Bragg Reflector (DBR) or Distributed Feed Back (DFB) lasers, when intracore Bragg gratings are used for linear cavity feedback, are commonly fabricated in rare earth doped optical silica-based fibres. On the other hand, photosensitivity can also result in effects which can prove to be detrimental to the fabrication of miniature coherent light sources. The paper will cover some of the advances that have been made in improving the photosensitivity of inorganic glasses, in searching new photosensitive materials and in characterizing Bragg grating properties relevant to laser applications.

Rare earth doped fibre for structural damage assessment

A technique that uses laser excited fluorescence in rare earth doped optical fibres to determine the location of structural damage is demonstrated. It has been shown experimentally that the position of a 'fracture' in a sensing erbium doped optical fiber can be evaluated by measuring the backward fluorescence power (BFP) from this erbium doped optical fiber (EDOF). This sensor can locate the position of fracture with a spatial accuracy of 3 mm within a gauge length of 0.43 m. A review of the theoretical basis for this sensing technique is provided and a discussion on optimization is presented.

Rare-earth doped optical fibers for temperature sensing

The feasibility of using a variety of rare-earth doped optical fibers for measuring spatially averaged temperatures from approximately 0 to approximately 100 degrees C over distances of 10 to 20 m is discussed. Such distributed temperature sensors would be particularly well-suited for building climate control systems and industrial processing applications. The temperature-dependent absorption spectra of 6 MCVD processed fibers containing different concentrations of Nd/sup 3+/, Pr/sup 3+/, and Yb/sup 3+/ rare-earth ions were characterized and used to determine thermally active dopant species, optimal dopant concentrations, and most sensitive operating wavelengths for use as dual wavelength distributed temperature sensors. >

Ground-state and excited-state absorption in rare-earth doped optical fibres

We present a modelling of absorption in rare-earth doped optical fibres. The transmission through the fibre is derived from ground-state absorption (GSA) and excited-state absorption (ESA) parameters. Fitting to experimental data obtained with thulium at 676.4 nm (GSA only) and 647.1 nm (GSA + ESA) yields experimental values for the absorption parameters. At 647.1 nm, ESA gives rise to transmission decreasing with power, which modifies the usual behaviour of absorption saturation.

Fabrication of high-concentration rare-earth doped optical fibers using chelates

A process which uses rare-earth complexes of 2,2,6,6,-tetramethyl-3,5-heptanedione for uniform incorporation of rare earths in the cores of optical fibers is demonstrated. It is a vapor-phase transport process capable of producing fibers having a rare-earth content of at least 11 wt.% and easily lends itself to incorporation of multiple rare-earth dopants. The process gives high rare-earth concentration with low loss and is readily adaptable to multiple rare-earth doping. High-concentration fibers, 1.0 wt.% Nd/sub 2/O/sub 3/, necessary for the double-clad fiber laser configuration with low loss, less than 10 dB/km, have been fabricated and shown to have efficiencies approaching quantum limits when pumping well above threshold. >