Ge-doped Fibers Research Articles

Mode-Locked Thulium-Doped Fiber Lasers Based on Normal Dispersion Active Fiber

We report and compare three configurations of the semiconductor saturable absorber mirror mode-locked, linear cavity femtosecond all-fiber master oscillator power amplifier based on a highly Ge-doped thulium-doped normal dispersion fibers. We have studied the performance of the system and have obtained stable mode-locking in a wide cavity dispersion range $\sim 1.88~\mu \text{m}$ . In this letter, we focus our attention on three mode-locking regimes: laser operating in the anomalous, nearly zero and normal cavity dispersion regimes without the use of the additional dispersion compensating elements. For the nearly zero and normal cavity dispersion regimes the femtosecond pulses with several nanojoule energy could be obtained. The pulses were further compressed down to 630 fs using a simple fiber compressor consisting of a piece of the conventional telecommunication fiber, making the laser design particularly simple and cost effective.

Silica-based fiber Raman laser at > 2.4 μm.

Raman laser generation at the long wavelength transmission edge of silica fiber is explored. Numerical simulation reveals that high efficiency 2nd Stokes Raman laser operation around 2.5μm is feasible with highly Ge-doped silica fiber as the gain medium and pulsed 2-μm fiber laser as the pump source. Based on the spontaneous cascaded Raman amplification process, 0.30-W laser at 2.43μm is obtained with an optical efficiency of 16.5% pumped at 2008nm. And 0.15-W laser at 2.48μm is achieved with an optical efficiency of 7.9% pumped at 2040nm. To the best of our knowledge, the results represent the longest wavelength operation of silica-based fiber laser.

Comparison of thermoluminescence response of different sized Ge-doped flat fibers as a dosimeter

Abstract Prime dosimetric properties, including dose–response, linearity with dose, energy response, fading and threshold doses were investigated for three different dimension Ge-doped flat fibers. The results of measurement were also compared with two of the more commonly used standard TLD media, TLD-100 (LiF:Mg,Ti−7.5% 6 LiF) and TLD−700 ( 7 LiF:Mg,Ti−99.9% 7 LiF) chips. The flat cross-section samples (60×180) µm 2 , (100×350) µm 2 and (200×750) µm 2 were fabricated using the Modified Chemical Vapor Deposition (MCVD) process and pulled from the same “preform.” In the study, all flat fiber samples provided good linear dose–response for the photon and electron beams generated using a medical linear accelerator (LINAC), for doses in the range 0.5–8 Gy. Among the samples, the smallest dimension flat fiber provided the best response, with a sensitivity of some 61% and 54%, respectively of that of the TLD-100 and TLD-700 chips. The energy responses of the samples were studied for various photon (6 MV, 10 MV) and electron (6 MeV, 9 MeV) beam energies. TL fading of around 20% was observed over a period of thirty (30) days. These favorable TL characteristics point towards promising development of Ge-doped flat fibers for use in radiotherapy dosimetry.

Heavily Germanium-Doped Silica Fiber With a Flat Normal Dispersion Profile

A heavily germanium-doped (Ge-doped) silica fiber with a four-layer refractive index profile is proposed to obtain all normal flat dispersion property. The waveguide dispersion in the fiber can be modified by adjusting the fiber parameters, including the refractive indices and the core radiuses. As a result, the flat normal dispersion in the fiber can be obtained in the wavelength range of 1540–2600 nm, where the values of the dispersion slope are between $-$ 0.0058 and 0.03 $\hbox{ps/nm}^{2}/\hbox{km}$ . Furthermore, the numerical results show that the flat-top supercontinuum spectrum ranging from 1000 to 2600 nm can be generated by launching pump pulses at the wavelength of 1550 nm in the heavily Ge-doped fiber with a four-layer refractive index profile.

Collapsed-Hole Ge-Doped Photonic Crystal Fiber as a Diagnostic Radiation Dosimeter

A new type of microstructured optical fiber, fabricated by collapsing down the entire array of holes formed by a Photonic Crystal Fiber (referred to herein as PCF-collapsed), is used in this study as an ionizing radiation dosimeter. The performance of the optical fiber is evaluated for diagnostic applications, covering doses from a fraction of 1 mGy up to 10 Gy. The effect of different x-ray beam accelerating potentials from 40 up to 125 kVp photon beam, 6 MeV electrons, and 6 MV photons are demonstrated. The performance of the PCF-collapsed is compared with the uncollapsed PCF (PCF-structured) and a conventional 20-μm-core-diameter optical fiber, all three types of fiber being fabricated from the same Ge-doped-preforms. It is shown that while a PCF-structured has relatively low sensitivity, by collapsing all of the holes in the PCF, the TL response of the fiber is increased by more than 16 times, being also more than four times greater than that of the conventional form optical fiber. The PCF-collapsed can detect a minimum dose as low as 0.6 mGy measured and 5.75 ± 0.11 mGy calculated. The TL analysis suggests that a collapsed-hole-PCF enhances structural defects in an uncollapsed-hole-PCF, highly desirable for dosimeter applications.

Ionizing radiation detectors based on Ge-doped optical fibers inserted in resonant cavities.

The measurement of ionizing radiation (IR) is a crucial issue in different areas of interest, from environmental safety and industrial monitoring to aerospace and medicine. Optical fiber sensors have recently proven good candidates as radiation dosimeters. Here we investigate the effect of IR on germanosilicate optical fibers. A piece of Ge-doped fiber enclosed between two fiber Bragg gratings (FBGs) is irradiated with gamma radiation generated by a 6 MV medical linear accelerator. With respect to other FBG-based IR dosimeters, here the sensor is only the bare fiber without any special internal structure. A near infrared laser is frequency locked to the cavity modes for high resolution measurement of radiation induced effects on the fiber optical parameters. In particular, we observe a variation of the fiber thermo-optic response with the radiation dose delivered, as expected from the interaction with Ge defect centers, and demonstrate a detection limit of 360 mGy. This method can have an impact in those contexts where low radiation doses have to be measured both in small volumes or over large areas, such as radiation therapy and radiation protection, while bare optical fibers are cheap and disposable.

Characterization of amorphous thermoluminescence dosimeters for patient dose measurement in X-ray diagnostic procedures

We investigate the use of novel Ge-doped amorphous silica flat fibers as thermoluminescence dosimeters (TLDs) in verifying patient entrance surface-dose (ESD) in diagnostic examinations. Selected fibers with established dosimetric characteristics (including energy dependence, linearity, reproducibility, and fading) were loaded into plastic capsules in groups of six. The fibers have been calibrated against a parallel plate ionization chamber, use being made of x-rays generated at 70 kVp, accessing a Secondary Standards Dosimetry Laboratory (SSDL) facility. The fiber characterization measurements were made using a Toshiba X-ray machine operating within the nominal energies range 40 kVp to 150 kVp, for doses in the range 0.02 mGy up to 3 mGy. For doses from 2 mGy up to 150 mGy, the flat fibers exhibit linearity between TL yield and dose, reproducible to better than 3% standard deviation following repeat measurements (n=3). A marked energy-dependent response is observed for photons generated at potentials from 40 kVp to 150 kVp. From present results, it is concluded that Ge-doped fibers represent a viable system for use in diagnostic dosimetry, corrections being made for the various factors influencing TL yield.

Effect of steady-state and transient γ radiation on Ge-doped quartz optical fiber

Effect of steady-state and transient γ radiation on Ge-doped quartz optical fiber

The Thermoluminescence Performance of Ge- Doped Optical Fibres To 6 Mv Photon Irradiations

The study focus on thermoluminescence (TL) response of five different diameters ~120, 241, 362, 483 and 604 μm of 8 mol% Ge-doped optical fibres. The irradiation was performed using Linear Accelerator (LINAC) Model Primus MLC Siemens provided by the Hospital Sultan Ismail, Johor Bahru at 6 MV photon irradiation with delivered doses ranging from 0.5 – 4.0 Gy. The results show the linear dose response against TL signals up to 4 Gy. The diameter of 483 μm optical fibre shows the highest in TL dose response and sensitivity compared to other fibres. The sensitivity of 483 µm optical fibre are almost 1.6 times more than 604 µm. The linear dose response at low dose makes this optical fibre suitable to be used in radiation dosimetry application.

Enhancing the radiation dose detection sensitivity of optical fibres

A method for improving the thermoluminescence (TL) yield of silica-based optical fibres is demonstrated. Using silica obtained from a single manufacturer, three forms of pure (undoped) fibre (capillary-, flat-, and photonic crystal fibre (PCF)) and two forms of Ge-doped fibre (capillary- and flat-fibre) were fabricated. The pure fibre samples were exposed to 6 and 21MeV electrons, the doped fibres to 6MV photons. The consistent observation of large TL yield enhancement is strongly suggestive of surface-strain defects generation. For 6MeV irradiations of flat-fibre and PCF, respective TL yields per unit mass of about 12.0 and 17.5 times that of the undoped capillary-fibre have been observed. Similarly, by making a Ge-doped capillary-fibre into flat-fibre, the TL response is found to increase by some 6.0 times. Thus, in addition to TL from the presence of a dopant, the increase in fused surface areas of flat-fibres and PCF is seen to be a further important source of TL. The glow-curves of the undoped fibres have been analysed by computational deconvolution. Trap centre energies have been estimated and compared for the various fibre samples. Two trap centre types observed in capillary-fibre are also observed in flat-fibre and PCF. An additional trap centre in flat-fibre and one further trap centre in PCF are observed when compared to capillary fibre. These elevated-energy trap centres are linked with strain-generated defects in the collapsed regions of the flat fibre and PCF.

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.

Assessment of Ge-doped optical fibres subjected to x-ray irradiation

We have reported the thermoluminescence (TL) response of five different diameters ~120, 241, 362, 483, and 604 μm of 6 mol percent Ge-doped optical fibres. The perfomance of the Ge-doped optical fibre are compared with commercially available TLD-100 chips (LiF:Mg,Ti) in terms of their sensitivity and minimum detectable dose (MDD). The irradiation was performed using X-ray machine (Model ISO 'Narrow Spectrum Series') provided by the Malaysian Nuclear Agency (MNA) at 60 kV X-ray irradiation in low doses ranging from 1-10 mGy. The results show the linear TL dose response from the fibres up to 10 mGy. The smallest diameter of 120 pm optical fibre shows the highest TL dose response compared to above mentioned fibres. The minimum detectable dose (MDD) is 0.82, 0.20, 0.14, 0.08, and 0.13 mGy for Ge-doped with diameters of 120, 241, 362, 483 and 604 μm. All TL materials show the MDD value within the delivered dose 0.01-1.00 mGy subjected to x-ray irradiation. The Ge-doped fibre with diameter of 483 pm was matched the MDD value of TLD-100 chips that equivalent to 0.08 mGy at the same irradiation. We have observed that among the five different diameters of optical fibre, 120 μm shows the best results and its better response than TLD-100 chips (by a factor of 5). The linear response at low dose levels makes this optical fibre most suitable for medical application.

Thermoluminescence characteristics of Ge-doped optical fibers with different dimensions for radiation dosimetry

Important thermoluminescence (TL) properties of five (5) different core sizes Ge-doped optical fibers have been studied to develop new TL material with better response. These are drawn from same preform applying different speed and tension during drawing phase to produce Ge-doped optical fibers with five (5) different core sizes. The results of the investigations are also compared with most commonly used standard TLD-100 chips (LiF:Mg,Ti) and commercial multimode Ge-doped optical fiber (Yangtze Optical Fiber, China). Scanning Electron Microscope (SEM) and EDX analysis of the fibers are also performed to map Ge distribution across the deposited region. Standard Gamma radiation source in Secondary Standard Dosimetry Lab (SSDL) was used for irradiation covering dose range from 1Gy to 10Gy. The essential dosimetric parameters that have been studied are TL linearity, reproducibility and fading. Prior to irradiation all samples ∼0.5cm length are annealed at temperature of 400°C for 1h period to standardize their sensitivities and background. Standard TLD-100 chips are also annealed for 1h at 400°C and subsequently 2h at 100°C to yield the highest sensitivity. TL responses of these fibers show linearity over a wide gamma radiation dose that is an important property for radiation dosimetry. Among all fibers used in this study, 100μm core diameter fiber provides highest response that is 2.6 times than that of smallest core (20μm core) optical fiber. These fiber-samples demonstrate better response than commercial multi-mode optical fiber and also provide low degree of fading about 20% over a period of fifteen days for gamma radiation. Effective atomic number (Zeff) is found in the range (13.25–13.69) which is higher than soft tissue (7.5) however within the range of human-bone (11.6–13.8). All the fibers can also be re-used several times as a detector after annealing. TL properties of the Ge-doped optical fibers indicate promising applications in ionizing radiation dosimetry.

Dosimetric characteristics of fabricated silica fibre for postal radiotherapy dose audits

Present investigation aims to establish the dosimetric characteristics of a novel fabricated flat fibre TLD system for postal radiotherapy dose audits. Various thermoluminescence (TL) properties have been investigated for five sizes of 6 mol% Ge-doped optical fibres. Key dosimetric characteristics including reproducibility, linearity, fading and energy dependence have been established. Irradiations were carried out using a linear accelerator (linac) and a Cobalt-60 machine. For doses from 0.5 Gy up to 10 Gy, Ge-doped flat fibres exhibit linearity between TL yield and dose, reproducible to better than 8% standard deviation (SD) following repeat measurements (n = 3). For photons generated at potentials from 1.25 MeV to 10 MV an energy-dependent response is noted, with a coefficient of variation (CV) of less than 40% over the range of energies investigated. For 6.0 mm length flat fibres 100 μm thick × 350 pm wide, the TL fading loss following 30 days of storage at room temperature was < 8%. The Ge-doped flat fibre system represents a viable basis for use in postal radiotherapy dose audits, corrections being made for the various factors influencing the TL yield.

Characterisation of the thermoluminescence (TL) properties of tailor-made Ge-doped silica glass fibre for applications in medical radiation therapy dosimetry

We have investigated the characterisation of new fabricated material Ge doped silica glass thermoluminescence TL dosimeter (Photonic Research Centre, University of Malaya) for medical radiation dosimetry at therapy energy. Previously, the dosimeter has been studied to provide ideal dosimetry system, suitable to ensure an accurate delivery of radiation doses to tumour tissue while minimising the amount of radiation administrated to healthy tissue. Both energies of photon and electron were used in this experiment for a dose range of 1 to 5 Gy. The various sizes of core diameter Ge doped silica glass (120, 241, 362, 483 and 604 μm) were exposed by using linear accelerator at Pantai Medical Centre. For both energies, the optical fibres were found to produce a flat response to a fixed photon and electron doses to within 4% (S.D) of the mean of the TL distribution. In terms of dose response, the fibres provide linear response over the range investigated, from a fraction of 1-5 Gy. The finding shows 120 μm fibres have 1.82 greater dose response than 604 pm fibres irradiated at 6 MV photon with a fixed dose of 3 Gy. While for electron energy 12 MeV, the response shows 120 μm fibres have 1.58 greater dose response compared to 604 μm fibres. The good responses are suitable to make these tailor-made doped silica fibres a promising TL material for use as a dosimetric system in medical radiation therapy.

Glass beads and Ge-doped optical fibres as thermoluminescence dosimeters for small field photon dosimetry

An investigation has been made of glass beads and optical fibres as novel dosimeters for small-field photon radiation therapy dosimetry. Commercially available glass beads of largest dimension 1.5 mm and GeO2-doped SiO2 optical fibres of 5 mm length and 120 µm diameter were characterized as thermoluminescence dosimeters. Results were compared against Monte-Carlo simulations with BEAMnrc/DOSXYZnrc, EBT3 Gafchromic film, and a high-resolution 2D-array of liquid-filled ionization chambers. Measurements included relative output factors and dose profiles for square-field sizes of 1, 2, 3, 4, and 10 cm. A customized Solid-Water® phantom was employed, and the beads and fibres were placed at defined positions along the longitudinal axis to allow accurate beam profile measurement. Output factors and the beam profile parameters were compared against those calculated by BEAMnrc/DOSXYZnrc. The output factors and field width measurements were found to be in agreement with reference measurements to within better than 3.5% for all field sizes down to 2 cm2 for both dosimetric systems, with the beads showing a discrepancy of no more than 2.8% for all field sizes. The results confirm the potential of the beads and fibres as thermoluminescent dosimeters for use in small photon radiation field sizes.

Radiation Vulnerability of Fiber Bragg Gratings in Harsh Environments

The difficulties encountered in the implementation of a temperature or strain sensor based on fiber Bragg grating (FBG) in a harsh radiative environment are introduced. We present the choices made to select both a radiation-resistant fiber in terms of transmission and also the grating inscription conditions necessary to write radiation tolerant FBGs in such fibers with a femtosecond laser. The radiation response of these gratings was also studied under radiation at dose up to 1 MGy. The comparison between Ge-free and Ge-doped fibers was highlighted.

광민감 광섬유로 제작한 광섬유 브래그 격자 센서의 방사선 민감도에 대한 연구

In this study, we studied the effect of Co 60 gamma-radiation on the FBGs written in photo-sensitive and commercial Ge-doped single-mode optical fibers. The FBGs were exposed to gamma-radiation up to a dose of 17.8 kGy at the dose rate of 300 Gy/min. According to the experimental data and analysis results, the lowest Bragg wavelength shift (18 pm) was obtained by a grating written in photosensitive fiber without H2-loading. Also, we confirmed that the H2 loading process has considerably more influence on the Bragg wavelength shift change under gamma radiation than GeO2 contents in the fiber core.

Photoluminescence mapping of oxygen-defect emission for nanoscale spatial characterization of fiber Bragg gratings

Confocal photoluminescence (PL) microscopy is used to gain insight into the inner structure of Ge-doped Fiber Bragg Gratings (FBGs). These measurements pinpoint room temperature PL emission from oxygen-related defects in the visible range, whose spatial distribution exhibits a periodicity associated with the spatial modulation of the refractive index printed inside the fiber core of the FBG. The period measured by PL mapping performed at submicrometric resolution matches the period of the refractive index variation determined from the optical transmission wavelength using the Bragg condition. Since the PL emission of oxygen-related defects can be used to probe local chemical changes inside fused silica, this novel and non-destructive experimental approach can be implemented for the direct characterization of FBGs, to study the effects of gas conditioning, ageing, and degradation under various environments.

X-ray irradiation effects on fluorine-doped germanosilicate optical fibers

We report an experimental investigation on the effects of fluorine codoping on the radiation response of Ge-doped Optical Fibers (OFs) obtained by three different drawing conditions. The OFs were irradiated with 10 keV X-rays up to 300 Mrad and studied by online Radiation-Induced-Attenuation (RIA) measurements. Confocal Micro- Luminescence (CML) and Electron Paramagnetic Resonance (EPR) were also employed to investigate the permanent radiation-induced-defects. The variation of the Germanium-Lone-Pair-Center (GLPC) and Non-Bridging- Oxygen-Hole-Centers (NBOHC) concentration with the radiation dose is investigated by CML, whereas the ones of the induced Ge(1), Ge(2) and Eʹ centers by EPR. No relevant differences are found in the RIA of the three fibers, as well as in the induced concentrations of Ge(1) and Ge(2) and in the decrease of the GLPC, showing minor relevance of changing the drawing conditions. We found that fluorine codoping does not affect the RIA and that, unexpectedly, the fluorine co-doped zones of the OFs show an enhanced photoluminescence of the radiation induced NBOHC enabling to suggest the presence of both Si and Ge variants. Moreover, an overall increase of the radiation induced Eʹ(Ge) centers is registered in relation to the presence of fluorine showing that this codopant has relevant effects.