Germanium-doped Optical Fiber Research Articles

Performance of a Ge-doped optical fibre dosimeter for CT real-time measurements

This study investigates the dosimetric characteristics of a real-time radioluminescence (RL) Ge-doped optical fiber sensor, model LS-1000, providing measurement of absorbed dose in terms of counts in use of a computed tomography (CT) system. A CT dose index head phantom has been used as the reference tool. The response of the LS-1000 sensor has been compared against that of two other forms of dosimeter - a RaySafe X2 CT sensor (an ionization chamber system) and a Black Piranha CT dose profiler (a point dose solid-state detector). The three dosimeters have been irradiated in each of five available holes within the CT dose index head phantom. The results show RL response at the position of the peripheral holes of around 250 counts/gate for a gate time of 10 ms, while at the position of the central hole the response was 80 counts/gate. The maximum dose rate for the RaySafe X2 CT sensor has been found to be 5 mGy/s while that observed for the CT dose profiler was 6 mGy/s. In respect of CT applications, this trial of the luminescence device demonstrates that the germanium-doped optical fiber device may offer a promising alternative to conventional dosimeters, also allowing measurements at high accuracy and precision.

Real-time germanium-doped optical fibers for clinical computed tomography dosimetry

In studies focusing on the practice of clinical computed tomography (CT) a particular concern has been that of delivering unnecessary dose to patients and with it the added risk for radiation-induced cancers. For a CT system, present work has investigated the dosimetric characteristics of a Ge-doped optical fiber real-time dosimetry system (model LS-2000, Lumisyns), measuring beam quality, exposure linearity and exposure duration accuracy. For comparison, the study has used a RaySafe X2 test device (Unfors RaySafe) and a Black Piranha (RTI) QA meter. Irradiations were made using a Somatom Definition Flash dual-source 128-slice CT scanner (Siemens Healthcare). The LS-2000, RaySafe X2, and Black Piranha sensors were exposed to beams energized at accelerating potentials ranging from 80 to 140 kVp, with tube current-time products from 50 to 300 mA, and exposure durations from 750 to 2000 ms. Constant signal responses from the LS-2000 have been obtained, measured in respect of beam quality, exposure linearity, and time accuracy. In respect of accuracy of beam quality, the RaySafe X2 and Black Piranha maximum deviations were 1% and 5%, respectively; the coefficients of linearity of RaySafe, Black Piranha and LS-2000 are 1%, 0.4% and 1.4%, respectively; the maximum deviation for the time accuracy test for the three sensors are 0.5%, 2% and 1.3%, respectively. Evaluation of the LS-2000 real-time dosimetry system with a Ge-doped optical fiber scintillator points to potential for its use in clinical CT dosimetry.

Temperature Dependence of Low-Dose Radiation-Induced Attenuation of Germanium-Doped Optical Fiber at Infrared Wavelengths

Ge-doped optical fibers (OFs) are considered radiation-tolerant waveguides. Even if their transmission in the infrared (IR) domain is degraded under irradiation, for most of the applications, their radiation-induced attenuation (RIA) remains acceptable. Space harsh environment is characterized by low dose and dose-rate constraints meaning that germanosilicate fibers are often employed for data links. However, the temperature can largely vary in space and is known to impact the RIA levels and kinetics. We studied here systematically the combined temperature and radiation effects induced on the transmission, at the telecom wavelengths, of a Ge-doped fiber, between −80 °C and 80 °C up to a total ionizing dose (TID) of 10 kGy(SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> )–1 Mrad. Our measurements highlight larger RIA levels at low temperatures than at room temperature (RT). At our highest TID, it increases by a factor of ~40 and ~20, respectively, at 1310 and 1550 nm, when the irradiation is performed at −80 °C instead of RT. A model is reported to study the activation energy of the radiation-induced point defects responsible for the IR-RIA. This simple model could help in predicting the fiber vulnerability for various mission profiles.

Fabricated germanium-doped optical fibres for computed tomography dosimetry: Glow curve characteristics

Fabricated germanium (Ge)-doped optical fibre glow curve characteristics are investigated with respect to computed tomography (CT) dosimetry. 2.3 mol% and 6 mol% Ge-dopant concentration preforms have been used to produce flat and cylindrical fibres (FF and CF) of various size and diameter. The fibres are irradiated to doses of 20, 30 and 40 mGy for each of the beam qualities RQT 8 (100 kV), RQT 9 (120 kV) and RQT 10 (150 kV). The thermoluminescence (TL) kinematic parameters studied are maximum temperature (Tmax), activation energy (Ea) and peak integral (PI). The glow curve formations are reconstructed from the Windows®–based radiation evaluation and management system (WinREMS), deconvoluted using glow curve deconvolution (CGCD) analysis software. The structures of the glow curves are broad single or double-peaked, occurring at relatively high glow peak temperatures, TL response increases with radiation dose and peak height decreases with increasing energy, showing clear photoelectric dependence. The deconvoluted glow curves for all fibres are seen to consist of five individual glow peaks, P1 to P5, P1 being dominant in all cases other than for 6 mol% Ge-FF for which P3 is dominant due to the formation of a double-peaked glow curve. Tmax increases from P1 to P5 for all fibres, throughout the energy range used. P1 and P3 (6 mol% Ge-FF) have the lowest Ea, while P4 shows the greatest Ea for all fibres. The results indicate that electrons in P1 and P3 (6 mol% Ge-FF) are occupied at low energy traps while for P4, the electrons are trapped at a deeper energy level. The lowest PI value, indicative of the least number of electrons, is shown to be that of the deeper trap P4 for all energies investigated. This study provides support for the use of 6 mol% and 2.3 mol% preform fibres for CT dosimetry, each with similar kinetic parameters.

Development of optical fibers for food irradiation dosimeter

Different technologies and methods for enhancing food quality have been developed and applied in the last few decades. One of the latest technologies is by using ionizing radiation. Food irradiation is the technology that improves the safety and extends the shelf life of food by reducing or eliminating harmful microorganisms and insects. In order to provide for food safety, proper control or radiation detector of irradiated food seems very critical to facilitate international trade of irradiated foods and to enhance consumer confidence. In present studies, germanium-doped (Ge-doped) optical fibers of various form and dimensions were used as radiation detector. The fibers were irradiated using electron beam (EPS 3000), with doses from 1 kGy up to 10 kGy, exceeding the dose range of all commercial high dose dosimeters used in food irradiation industry. A study has been made of linearity, reproducibility, and fading. The fibers show a linear dose response over the studied range doses with mean of reproducibility less than 5 % variation between 1st exposure and 2nd exposure. TL fading of Ge-doped flat fibers has been found to be &lt; 8.7%.

Germanium-doped optical fiber for real-time radiation dosimetry

Over the past three decades growing demand for individualized in vivo dosimetry and subsequent dose verification has led to the pursuit of newer, novel and economically feasible materials for dosimeters. These materials are to facilitate features such as real-time sensing and fast readouts. In this paper, purposely composed SiO2:Ge optical fiber is presented as a suitable candidate for dosimetry. The optical fiber is meant to take advantage of the RL/OSL technique, providing both online remote monitoring of dose rate, and fast readouts for absorbed dose. A laboratory-assembled OSL reader has been used to acquire the RL/OSL response to LINAC irradiations (6 MV photons). The notable RL characteristics observed include constant level of luminescence for the same dose rate (providing better consistency compared to TLD-500), and linearity of response in the radiotherapy range (1 Gy/min to 6 Gy/min). The OSL curve was found to conform to an exponential decay characteristic (illumination with low LED source). The Ge doping resulted in an effective atomic number, Zeff, of 13.5 (within the bone equivalent range). The SiO2:Ge optical fiber sensor, with efficient coupling, can be a viable solution for in vivo dosimetry, besides a broad range of applications.

The Opposite Effects of the Heating Rate on the TSL Sensitivity of Ge-doped Fiber and TLD500 Dosimeters

One of the main criteria for choosing a thermoluminescent dosimeter (TLD) is its sensitivity to the radiations under investigation. Increasing the heating rate during readout often appears necessary to reduce the time between the measurements and the dose evaluation, especially in routine dosimetry. However, doing this degrades the radiation sensitivity of common dosimeters, as illustrated in this work for the TLD500 dosimeter. It is shown that the germanium-doped optical fiber (GDF) is not only more sensitive to radiation than these COTS dosimeters but, unlike them, its sensitivity is enhanced when the heating rate increases. The physical origin of this rare effect of sensitivity enhancement is probably due to the temperature dependence of the recombination rate by which the detrapped electrons upon stimulation are transferred to the luminescent centers. The effect of light exposure on the dose information stored in both GDF and a commercial TLD is also reported.

Thermoluminescence Response of Germanium-Doped Optical Fibers to X-Ray Irradiation

We present the characteristics of the thermoluminescence (TL) response of Ge-doped optical fibers with various energies and exposures of photon irradiation. To investigate the Ge-doped SiO2 as an efficient TL material, the TL responses are compared with commercially available standard TLD100 media. The Ge-doped optical fiber and TLD100 are placed in gelatin capsules and irradiated with x-ray using a Toshiba model KXO-15R x-ray generator. The Ge-doped fiber and TLD-100 show linear response as a function of current and time using x-ray photon of energy 60, 80 and 100 kV. When irradiated with 60, 80 and 100 kV x-ray energy at various currents (mA), tube distance (cm) and exposure time (second) ranges, TLD100 media provide a TL yield up to two times that of Ge-doped fibers. The energy response of the Ge-doped fibers is linear and similar over the 60–100 kV energy range, and its sensitivity is 0.39±0.05 of the TLD100 media. The glow curves of TLD 100 and doped optical fiber are also compared.

Phase-matched third-harmonic generation in highly germanium-doped fiber

Phase-matched third-harmonic generation is demonstrated in a germanium-doped optical fiber. Green light at 514.4 nm is generated in an LP(03) mode when a pump field at ~1543.3 nm is launched into the fiber in the fundamental LP(01) mode. The phase matching is achieved for a particular combination of the germanium doping concentration and the fiber core diameter.

Thermoluminescence Energy Response of a Germanium-doped Optical Fiber Obtained Using a Monte Carlo N-particle Code Simulation

This paper reports on the energy response of a Ge-doped optical fiber subjected to photon irradiation. The thermoluminescence (TL) responses of the Ge-doped optical fiber for various photon energies ranging from 20 keV to 6 MeV were investigated as energy absorbed in the TL material by using the Monte Carlo N Particle transport code version 5 (MCNP5). The results obtained are compared against results for the thermoluminescence dosimeter 100 (TLD-100). The input parameters included in this study were the geometry specification, the source information, the material information and tallies. Tally F6 is an important parameter in data card which was used in this simulation as a energy-dependant heating function instead of flux. Similar patterns of response were found for each dosimeter. The simulation shows that the optical fiber has a greater response than TLD 100 in the lower energy range, but the responses overlap in the higher energy ranges.

Influence of the drawing process on the defect generation in multistep-index germanium-doped optical fibers

Variation of germanium lone pair center (GLPC) concentration in germanosilicate multistep-index optical fibers and preforms was studied using confocal microscopy luminescence technique. The experimental results provide evidence that in the central core region ([Ge] approximately 11 wt.%) of our specific canonical samples the ratio [GLPC]/[Ge] is five times larger in fiber than in preforms. The relative influence of the glass composition and of the drawing process on the generation efficiency of the GLPC defects that drive the glass photosensitivity is discussed. The radial distribution of these defects suggests a possible enhancement of the defect creation related to the internal stress of the fiber core.

Permanent refractive-index modification in germanium-doped optical fibers by use of red light

We observed photoinduced birefringence in elliptical-core optical fibers by using a continuous-wave krypton-ion laser. We induced the birefringence by injecting 20 mW of 647-nm or 50 mW of 676-nm light into the fiber at 45 degrees to the slow axis. The rate of change of the refractive index was found to be proportional to the square of the average power. Polarization mode couplers written into the fibers have been stable for more than 2 years and can be erased by use of light polarized perpendicularly to the original writing beam.

Crystalline and amorphous phases of GeO 2 in VAD silica–germania soot preform

In germanium doped optical fiber, produced by vapor-phase axial deposition (VAD) technique, the refractive index profile is controlled by the radial distribution profile of germanium. At the porous soot state, GeO 2 may be deposited as amorphous and/or crystalline phases in a SiO 2–GeO 2 system. To correlate the quantitative deposition between amorphous and crystalline phases of GeO 2, several silica–germania soot preforms were prepared under different deposition conditions and their X-ray diffraction (XRD) patterns and X-ray fluorescence spectra were analyzed. The presence of GeO 2 crystalline (hexagonal) phase was detected in all soot samples, and its concentration was affected by deposition conditions, mainly the soot surface temperature and H 2/O 2 ratio in the flame. The concentration of the crystalline phase is minimum at the center of the silica–germania soot preform and increases along the radial direction. The greater concentrations at the outer diameter of the silica–germania soot preform are due to the lower deposition temperature (<700°C) in comparison to the one at the center (>850°C). The GeO 2 is eliminated at the center of preform in reductive atmosphere (H 2/O 2>2) and higher temperature ( T ∼ 1000°C), independently of SiCl 4/GeCl 4 ratio. These results are important to understand the main parameters of the doping process, to produce high quality materials with a given germanium concentration profile.

Bragg gratings in defect-free germanium-doped optical fibers

Bragg gratings have been written in germanium-doped optical fibers that have been treated to remove the UV absorption bands associated with oxygen-deficient defects. When one is using high-intensity 193-nm light from an ArF excimer laser to fabricate the gratings, the refractive index increases and the grating transmission spectra are similar to those obtained in standard (untreated) fiber.

Growth of Bragg gratings produced by continuous-wave ultraviolet light in optical fiber

We have written Bragg gratings of as much as 94% reflectance in germanium-doped optical fiber by two-beam interference of 244-nm continuous-wave UV light. We measured grating reflectance as a function of exposure time for UV light intensities ranging from 1.5 to 47 W/cm2. The observed dependence of index modulation on time and intensity does not agree with the predictions of a model based on depletion of a defect population by one-photon absorption.

Growth rate of second-harmonic generation in glass

We study the growth rate of the light-induced second-order nonlinearity in germanium-doped optical fiber preforms. We seed the glass with both infrared light and green light (Nd:YAG and doubled Nd:YAG) to create the second-order nonlinearity and measure its formation rate while varying the intensity of either the fundamental or the second-harmonic seeding beams. We find that the formation rate varies as a power law of the intensities, but with an exponent larger than predicted by recent models.

Observation of direct correlation between induced absorption and second harmonic generation in germanium-doped glass optical fibres

A direct correlation is reported between second harmonic generation (SHG) and the appearance of an induced absorption band in germanium-doped silica optical fibres. The correlation is drawn from two experiments comparing the SHG efficiency and the magnitude of the induced absorption.

Simultaneous observation of photobleaching and photorefractive effects in germanium-doped optical fibers

Photobleaching and photorefractive effects caused by intense pulsed laser irradiation at 532 nm have been observed simultaneously in a germanium-doped monomode fiber. A decrease of up to 4.1*10/sup -4/ in the refractive index of the core at 633 nm was inferred from the change in the radiation pattern of a probe beam and also from the change in the cutoff wavelength of the LP/sub 11/ mode. The effect is attributed to the bleaching of preexisting Ge(1) color centers in the fiber. >

Ultraviolet radiation- and γ radiation-induced color centers in germanium-doped silica glass and fibers

Abstract Induced optical losses and paramagnetic Ge(n) centers were investigated in germanium-doped silica glass and optical fibers after γ and ultraviolet (UV) irradiation. It was found that both types of irradiation created similar effects. By means of selective UV irradiation, Ge (1 and 3) centers were identified in optical absorption spectra, presumably as induced bands centered at 4.4 eV and 6.2 eV, respectively. Moreover, photobleaching of Ge(1) centers took place under 266-nm wavelength excitation. In optical fibers no difference was observed between γ- and UV-induced loss spectra in the wavelength range from 480 nm to 1,900 nm. Partial reversibility of the photocoloration was observed. For comparison, the coloration effects were studied in glass prepared by means of modified chemical vapor deposition (MCVD) vapor-phase axial deposition (VAD), and plasma-activated chemical vapor deposition (PCVD) techniques.

Nitrogen-related absorption bands in optical fibres

Two absorption bands are reported in germanium-doped optical fibres containing low levels of nitrogen. The bands are not present in the fibre as drawn, but can be activated by exposure to elevated temperatures. Germanium, along with nitrogen, plays a role in forming the defects responsible for the absorption.