Ge-doped Fibers Research Articles

Hydrogen-induced O–H peak growth in Ge-doped optical fibers: Verification of empirical and theoretical models

Migration of hydrogen in optical fibers and its chemical reactions with the glass fiber core create added optical loss, that may deteriorate the light transmission through the fiber. Although this subject has been extensively studied and a few theoretical and empirical models linking the attenuation to hydrogen pressure, temperature and time were proposed, none of the models was verified in a broad range of experimental conditions. In this work we investigate a single mode germanium doped fiber that was exposed to 25–––100 psi H2 pressures at temperatures in the range 150 – 250 °C and the exposure times up to 28 days. Different aging protocols were applied to the fiber, and the focus was given to O–H peak development. An empirical approach and a theoretical model, that assumes Gaussian distribution of the activation energies were applied to fit the experimental results. From the theoretical model, it was found that the concentration of precursor available for reaction with hydrogen is orders of magnitude higher than that of non-bridging oxygen hole centers, and that at the applied temperatures the reacted sites belong to the lower-energy wing of the Gaussian distribution. It was also found that parameters of the theoretical model cannot be accurately determined via fitting even a large array of experimental data. In contrast, parameters of the empirical model are easily obtainable from the experiment which makes this approach more practical in hydrogen-related lifetime predictions.

Dopant, coating, and grating effects in silica optical fibers under extreme neutron irradiation

Radiation effects on fiber Bragg gratings (FBGs) have been studied but data gaps related to the effects of displacement damage resulting from high fast neutron fluence remain. In this work, Type-I and Type-II FBGs inscribed in optical fibers with various core dopants (Ge and F) and fiber coatings (acrylate, polyimide) were monitored in situ during 75 days of neutron irradiation to a peak fast (>0.1 MeV) neutron fluence of 3×1021 nfast/cm2. The reflected intensity of the Type-I FBGs inscribed in a Ge-doped core fiber decreased by >30 dB within 7 h of irradiation (1019 nfast/cm2), whereas Type-II FBGs inscribed in a pure silica core fiber eventually approached >40 dB attenuation after accumulating a fast neutron fluence on the order of 1020 nfast/cm2. Type-II FBGs inscribed in F-doped core fiber improved stability: the attenuation approached an equilibrium value in the range of 10 to 20 dB.

Phenomena observed in electron EBRT using a pulse-by-pulse radioluminescence dosimetry system with cloud-based analytics

The pulse-by-pulse characteristics of electron beams from a clinical linear accelerator (linac) at high dose rates are crucial for ensuring precise dose delivery in radiation therapy. In this research, we employ a radioluminescence scintillator made from Ge-doped silica optical fiber to analyse electron beams at standard therapy dose rates 600 cGy/min with energy of 15 MeV. The scintillator is combined with a photomultiplier tube (PMT) and a photon-counting setup to achieve microsecond (μs) gating times, providing high temporal resolution. For sub-microsecond (sub-μs) temporal resolution, we utilize a multi-pixel photon counter and a rapid digital oscilloscope. Our study examines the peak time and the time it takes for pulses to return to baseline, uncovering the effects of afterglow at elevated dose rates and its potential implications for dosimetry precision. Our results indicate that monitor chambers, which have lower temporal resolution than the radioluminescence system outlined in this study, may struggle to accurately track high-rate dose delivery, emphasizing the necessity for advanced monitoring methods in FLASH radiotherapy. The dosimetric data was further analysed for deeper analytics on a cloud-based system. The dosimetric data underwent advanced analysis on a cloud-based platform, employing a prototype analytics software system comprised of Dosi-Client and Dosi-Cloud Backend components for real-time data acquisition and processing. This innovative approach revealed significant insights into radiation dosimetry, including the detection of anomalies through a reconstruction convolutional autoencoder, enhancing the precision and reliability of dosimetric measurements.

Influence of Hydrogen on the Radiation-Induced Attenuation of Ge-doped Optical Fibers

Influence of Hydrogen on the Radiation-Induced Attenuation of Ge-doped Optical Fibers

Combined Radiation and Temperature Effects on Brillouin-Based Optical Fiber Sensors

The combined effects of temperature (from −80 °C to +80 °C) and 100 kV X-ray exposure (up to 108 kGy(SiO2)) on the physical properties of Brillouin scattering and losses in three differently doped silica-based optical fibers, with varying dopant type and concentration (4 wt%(Ge), 10 wt%(Ge) and 1 wt%(F)), are experimentally studied in this work. The dependencies of Brillouin Frequency Shifts (BFS), Radiation-Induced Attenuation (RIA) levels, Brillouin gain attenuation, Brillouin frequency temperature (CT) and strain (Cε) sensitivity coefficients are studied under X-rays in a wide temperature range [−80 °C; +80 °C]. Brillouin sensing capabilities are investigated using a Brillouin Optical Time Domain Analyzer (BOTDA), and several properties are reported: (i) similar behavior of the Brillouin gain amplitude decrease with the increase in the RIA; (ii) the F-doped and heavily Ge-doped fibers do not exhibit a temperature dependence under radiation for their responses in Brillouin gain losses. Increasing Ge dopant concentration also reduces the irradiation temperature effect on RIA. In addition, Radiation-Induced Brillouin Frequency Shift (RI-BFS) manifests a slightly different behavior for lower temperatures than RIA, presenting an opportunity for a comprehensive understanding of RI-BFS origins. Related temperature and strain sensors are designed for harsh environments over an extended irradiation temperature range, which is useful for a wide range of applications.

Watt-level 1.6 μm dissipative soliton ultrafast laser and external generation of ∼70 fs noise-like pulses for supercontinuum generation

Here, we propose a high power dissipative soliton (DS) laser with a central wavelength of ∼1612 nm. The whole laser system includes the mode-locking oscillator and the corresponding master oscillator power amplifier (MOPA). The mode-locking oscillator is a nonlinear amplifier loop mirror (NALM) cavity, which has a net normal dispersion enabling the generation of nJ-level DS pulses. To further enhance the output power of the 1.6 μm DS pulses, the specially-designed MOPA system consisting of multiple-stage single-pass amplifier is constructed, which can amplify the output power to watt level and meanwhile keeps a high suppression ratio to C-band amplified spontaneous emission (ASE). This high power 1.6 μm DS laser will have further applications in biomedical research and material processing. In the amplifier, we observe the automatic evolution of the DS pulses into the noise-like (NL) pulses under high pumping strength. At the maximum pump power, the MOPA system delivers the NL pulses with an output power of over 2 W. The narrowest spike duration is ∼70 fs (fs). This 1.6 μm fs-scale NL laser is a good pump source for generating the mid-infrared supercontinuum without short wavelength region loss. To demonstrate the superiority of this NL laser, a broad supercontinuum from ∼0.87 μm to ∼3.75 μm waveband is achieved with a piece of highly germanium-doped (Ge-doped) fiber.

Combined Photobleaching and Temperature Effects on 1550 nm Radiation-Induced Attenuation of Germanosilicate Optical Fiber

Combined temperature and photobleaching (PB) effects on the 1550 nm radiation-induced attenuation (RIA) levels and kinetics of a commercial Ge-doped single-mode optical fiber have been investigated. The main goal of this study was to evaluate if synergetic effects could impact the performances of free space optical communication links in space. For this, we performed accelerated (0.5 Gy(SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> )/s) irradiation runs with 100 kV X rays up to the total ionizing dose of 150 kGy(SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ), varying the irradiation temperature between -80 °C and 80 °C. First, we evaluate the irradiation temperature effects: infrared RIA largely increases at negative temperatures. Second, the effects of injecting different optical powers at the different temperatures have been characterized: if at temperatures exceeding room temperature (RT), PB effects are almost negligible, a PB effect is clearly observable at lower temperatures where the germanosilicate optical fiber present large RIA levels due to the contribution of metastable defects absorbing at 1550 nm. Increasing the injected powers allows reducing the concentration of these radiation-induced unstable defects, but not as much as thermal treatment consecutive to the irradiation. These results highlight the complex physics of radiation induced defects absorbing at 1550 nm in Ge-doped optical fiber, but also the vulnerability of this class of fiber for space telecommunication links. Regarding the unstable nature of point defects causing the RIA at lower temperature, it seems possible to mitigate the loss excess by thermal cycles at RT or higher temperatures. We also could expect that our accelerated tests represent a worst-case scenario in terms of RIA levels at the maximum dose.

Gamma irradiation of Ge-doped and radiation-hard silica fibers at cryogenic temperatures: Mitigating the radiation-induced attenuation with 1550 and 970 nm photobleaching

We investigated the effects of gamma irradiation on radiation-induced attenuation (RIA) in photosensitive (Ge-doped) and radiation-hard (F-doped) fibers at cryogenic temperatures (77 K) under different photobleaching conditions. We show that increasing the probe power (1550 nm) and injecting lower wavelength light (970 nm) both resulted in a significant reduction in RIA in both fiber types, where radiation-hard fibers were intrinsically more resistant to the RIA. Deconvolution of RIA growth curves revealed that the RIA was composed of transient and long-term growth components that were correlated with distinct radiation-induced defects specific to each fiber composition. The 1550 nm light more effectively suppressed the transient RIA, while 970 nm more effectively suppressed the long-term RIA. Ultimately, we show that cryogenic RIA may be effectively managed in fiber optic sensing systems using radiation-hard fibers and dual-wavelength photobleaching strategies.

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.

Enhanced backscatter and unsaturated blue wavelength shifts in F-doped fused silica optical fibers exposed to extreme neutron radiation damage

Amorphous fused silica (a-SiO2) optical fibers, with and without inscribed Bragg gratings, were interrogated using infrared (∼1550 nm) optical backscatter reflectometry during ∼25 days of intense neutron irradiation to a fast (energy > 0.1 MeV) neutron fluence of ∼1021 n/cm2, or ∼1.5 atomic displacements per atom. The reflected light amplitudes in Ge-doped core telecommunications fiber dropped below detection limits (>15 dB attenuation) within 3 days of irradiation (∼1020 fast n/cm2). Amplitudes from a pure silica core, F-doped silica cladding fiber reached equilibrium levels ∼1.5–2 dB higher than pre-irradiation values, whereas Bragg gratings inscribed in the same fiber using a femtosecond laser (point-by-point method) suffered > 45 dB attenuation. Blue wavelength shifts were initially consistent with previous radiation-induced compaction models but increased linearly with increasing neutron fluence (no evidence of saturation) beyond 1020 n/cm2 and exceeded 0.6%, which corresponds to >1,000 °C drift if used to measure temperature changes.

Inverse Design of Broadband Dispersion Compensation Fiber Based on Deep Learning and Differential Evolution Algorithm

A Ge-doped dual-core dispersion compensation photonic crystal fiber (DC-DCPCF) is proposed. The small diameters of two layers’ air holes make DC-DCPCF form a dual-core structure, which is conducive to broadband dispersion compensation. Low Ge-doped silica as the only background material reduces the preparation difficulty and cost. It is inversely designed by using artificial neural network (ANN) combined with differential evolution algorithm (DE) to obtain target dispersion compensation. ANN replaces the finite element method to accomplish fast forward prediction of DC-DCPCF properties. DE solves the single solution problem of single or cascade network that makes it flexible and reproducible. The results demonstrate that the designed DC-DCPCF can not only compensate 45 and 25 times its length of Corning single-mode fiber 28 (SMF28) in S+C+L+U bands and E+S+C+L+U bands respectively, but also accurately compensate the residual dispersion with effective dispersion compensation being only +0.005∼+0.842ps/(nm·km) and −0.03∼+1.31ps/(nm·km), respectively. In addition, the kappa values of DCP-PCF are well matched with SMF28 in the broadband wavelength range. It takes only about 10 seconds to complete the inverse design of the target DC-DCPCF. It provides a design method for custom DC-DCPCF and an efficient inverse design solution for photonic automation in fiber optical communication systems.

Comparison of effective atomic number, output factor, and glow curve of fabricated Ge-doped optical fibers (FGDOFs) for electron beams dosimetry

The present study is extended from a previous study on advanced characteristics for Fabricated Ge-Doped Optical Fibers (FGDOFs) in electron beam dosimetry with comparison in terms of Effective Atomic Number (Zeff), Output Factor (OF), and Glow Curve Analysis. Other than that, the different shapes and germanium dopant concentrations of FGDOFs were studied. Regarding Zeff, Flat Fiber (FF) showed lower Zeff than Cylindrical Fiber (CF) at a difference of ±7%. No significant difference at p > 0.05 for the of 6, -9, and 12 MeV electron energies. WinGCF software was used to deconvolute the glow curve, with the best fitting for FF and CF. Results from Glow Curve Analysis showed that the Maximum Temperature (Tmax) was energy independent for all CFs and FFs. Peak 3 (P3) was for the significantly deep holes for FF (2.3% mol; 643 μm × 356 μm), CF (2.3% mol; 481 μm), and CF (6.0% mol; 486 μm) while peak 4 (P4) for FF (6.0% mol; 620 μm × 165 μm) and CF (6.0% mol; 616 μm), respectively. Both FF (2.3% mol and 6.0% mol) have the greater sensitivity with the highest thermoluminescence (TL) signal due to the highest activation energy compared to CFs.

Dose mapping in Cesium-137 blood irradiator using novel Ge-doped silica optical fibres in comparison with gafchromic EBT-XD film: A preliminary study

The purpose of dose-mapping in blood irradiation is to ensure radiation dose delivered to the blood bags is within the specified limits of minimum 15 Gy and maximum 50 Gy, thus preventing any blood transfusion-related diseases. In the present study, we investigated the feasibility of novel fabricated germanium-doped (Ge-doped) silica-based optical fibre as a potential passive dosimeter for dose-mapping of a Cesium-137 (Cs-137) gamma blood irradiator. The calibration curve of optical fibres and Gafchromic EBT-XD film was established by irradiating the samples in the dose range from 5 to 35 Gy at the central of a water-equivalent phantom in a dedicated blood irradiator machine. For dose mapping, the fabricated optical fibres and EBT-XD film were irradiated for a gamma exposure of 8 min and 56 s to deliver a central dose of 25 Gy. Thermoluminescence measurements were performed using a Harshaw 3500 TLD reader at a maximum acquisition temperature of 400 °C. The results of optical fibres were compared with EBT-XD film. The absorbed dose obtained from optical fibres shows deviation of within 0.1%–10.4% compared to that of dose of the film. The fabricated optical fibre studied in this work have a good potential as a new passive dosimeter for characterisation and dose mapping of gamma blood irradiator due to its high sensitivity and reproducible characteristics.

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.

Ge-Doped Optical Fibers for Passive and Active Radiation Detection Modes

Silica-based optical fibers made by Modified Chemical Vapor Deposition (MCVD) technique have been doped with different concentrations of germanium (Ge). After characterization of the thermoluminescence (TL) process in each of these fibers, their real-time responses were studied by radioluminescence (RL) under X-rays. First, the same counterintuitive trend was observed for the intensity of the two phenomena as a function of the Ge content, namely the decrease in TL and RL signals with increasing Ge content. This behavior was explained by the existence of thermally disconnected traps. Then, the RL response as a function of the dose-rate of the fiber doped with 1% Ge was judiciously chosen and investigated in detail. The results show that up to Total Ionizing Dose (TID) of 1.9 kGy(SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ), this response is not only insensitive to the accumulated dose effect but also exhibits an afterglow decay at the end of the radiation much faster than that of a Ce-doped fiber, a very useful feature where irradiations are repetitive as in the medical field of flash-therapy. In addition to the medical field where the Ge-doped fiber seems to be quite suitable, slightly larger core diameters would also allow this same fiber to meet the challenge of high sensitivity in the environmental radiation monitoring around nuclear power plants or radioactive waste storage areas.

Influence of Ambient Light on the Radiation-Induced Attenuation of Germanosilicate Optical Fibers in the Visible and Near-Infrared Domains

We investigated the influence of the ambient light on the Radiation-Induced Attenuation (RIA) of Ge-doped optical fibers measured at the telecommunication wavelengths when exposed to a <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">60</sup> Co source, and in the visible and Near-InfraRed (NIR) domains when exposed to X-rays. From our results, it is possible to observe that this class of optical fibers are sensitive to the ambient light. Indeed, in all cases, their RIA value changes depending on the illumination conditions. In particular, the γ-ray irradiations revealed jumps in the measured RIA when changing the lamp of the irradiation chamber, whereas the X-ray irradiations reported that the relative photobleaching effect caused by the ambient light increases with the wavelength, up to ~1300 nm. The spectral study revealed no direct correlation between the absorption bands of the known Ge-related point defects and the external photobleaching phenomenon.

3.2 kW, 0.22 nm narrow-linewidth MOPA configuration fiber laser with a homemade polarization-maintaining Yb-doped fiber

In this work, a narrow-linewidth linearly polarized fiber amplifier with a record output power of 3.2 kW was achieved based on a homemade polarization-maintaining Yb-doped fiber corresponding to a slope efficiency of 79% and a 3 dB linewidth of 0.2227 nm. By examining various numerical aperture (NA) PMYDFs, the experimental investigation on expanding mode instability (MI) threshold in PM fiber amplifiers was put on display. And the results reveal that the MI threshold is enhanced by more than 370 W for every 0.004 decrease in core numerical aperture. Increasing the seed linewidth from 0.0454 nm to 0.0976 nm by adding 200 m polarization maintaining Ge-doped fiber the stimulated Brillouin scattering threshold increased from 805 W to above 3.2 kW. By applying the MI suppression method, a double-eight-shaped aluminum plate was adopted to coil the gain fiber, and the MI threshold increased by more than 1100 W.

Irradiation of Multimode Ge-doped and F-doped Optical Fibers at Cryogenic Temperatures Down to 10 K

The combined effect of radiation and cryogenic temperatures on the response of two different multimode fibers (MMF), one conventional and one radiation resistant, has been investigated. This article presents the outcome of the measurements of the radiation induced attenuation (RIA) in those fibers at 830 nm and 1310 nm under gamma irradiation and at temperatures of 300 K, 218 K, 143 K and 10. The results reveal a strong increase of the RIA when decreasing the temperature down to cryogenic levels, with substantial different growth dynamics between the two fibers. However, both cases show a non-linear dependence of the RIA on the temperature. It is also observed that after a long recovery time, including the heating of the fiber from cryogenic temperature to room temperature, a large amount of the defects created are annealed, bringing the radiation resistant fiber close to its initial performance.

Nanoscale morphology and thermal properties of low insertion loss fiber Bragg gratings produced using the phase mask technique and a single femtosecond laser pulse.

Fiber Bragg gratings with a very low insertion loss are inscribed using the phase mask technique and a single infrared (800 nm) femtosecond laser pulse. The morphology of the resultant light-induced structural changes in the Ge-doped silica fiber (SMF-28) is analyzed using scanning electron microscopy. The electron microscopy images reveal that each Bragg grating period incorporates an elongated micropore embedded in a region of homogeneous material modification. The Bragg wavelength drift and reflectivity of fiber Bragg gratings produced with single pulses having the same energy but different duration (80 fs and 350 fs) are monitored for 1000 hours in the course of isothermal annealing at 1000°C. The annealing data demonstrate that both the isothermal Bragg wavelength drift and the decrease in the reflectivity of the fiber Bragg gratings under test are statistically slower for the 350 fs inscription pulses.

1.7 µm - 1.73 µm tunable ultrafast Raman fiber laser pumped by 1.6 µm dissipative soliton pulses.

Here, we report an all-fiber tunable ultrafast Raman laser synchronously pumped by a home-made 1.6 µm dissipative soliton (DS) picosecond (ps) laser, which produces Stokes light beyond 1.7 µm. The Raman gain medium is a segment of highly germanium-doped (Ge-doped) fiber offering a high Raman gain coefficient at the target wavelength. Once the Raman conversion cavity is synchronized with the pump light, a stable 1.7 µm Raman laser (the first Stokes light) can be obtained at a low pump threshold. The maximum output power of the 1.7 µm Raman laser can reach ∼ 22.62 mW. The wavelength tuning operation is independent of tunable pump source and intra-cavity filter. By adjusting the intra-cavity delay line simply, the different spectral component within the broad Raman gain bandwidth can be selectively synchronized with the pump light so that the Raman laser wavelength can be tuned continuously from 1702.6 nm ∼ 1728.84 nm. This tunable 1.7 µm waveband ultrafast laser will have potential applications in multiphoton microscopy for e.g. deep bio-imaging.