Flame-fused type-II silica glass and synthetic type III silica glass were exposed to gamma radiation and then thermally poled. Second-harmonic generation (SHG) is reduced by several orders of magnitude in Suprasil glass for doses higher than 100 kGy and in Herasil glass for doses higher than a few MGy. In Herasil glass, however, SHG is unchanged for doses lower than about one MGy. In the latter case, Maker fringe measurements show that, like in the poled pristine glass, the second-order nonlinearity is located in a thin layer beneath the anodic surface. On the other hand, in the former case, Maker fringe measurements reveal the existence of weak bulk nonlinearity. This unexpected result suggests that dipole orientation takes place in the glass modified by gamma radiation.
We demonstrate numerically a secure communication scheme based on the synchronization of two chaotic laser diodes that are respectively subjected to incoherent optical feedback and incoherent optical injection. In this scheme, the optical fields emitted by the two lasers and the fields that are fed back and injected into these two lasers have orthogonal polarizations. Consequently, the external fields do not coherently interact with the lasing fields but only act on the population inversions. Synchronization of both lasers does not require fine tuning of their optical frequencies neither accurate control of the external cavity lengths contrary to the cryptographic systems based on conventional optical feedback. The message encoding/decoding is achieved by chaos shift keying.
Bragg gratings were written in H2-loaded polarization- maintaining fiber (PM-FBGs) and inserted in an actively mode-locked Er-doped fiber laser. The use of PM-FBG in sigma laser cavity allows to effectively build all polarization- maintaining fiber laser. Long term stabilization of the laser was ensured by a feedback loop that controlled the cavity length. Peak wavelengths, reflection bandwidths and reflectivity values of the grating were equal to 1545.5 nm and 1540.5 nm, 1.6-nm and 0.8-nm, of 99% and 90%, respectively. At a 3-GHz repetition rate, pulses of 9.4-ps and 27.9-ps duration were generated with first and second gratings, respectively. By inserting additional long piece of dispersion-shifted fiber in the cavity, nonlinear pulse shortening was observed due to formation of average soliton inside the cavity. In this case, pulses of 7.8-ps and 22.7- ps duration were generated with first and second gratings, respectively. With the first grating, generated pulses were close to transform limit for hyperbolic secant pulse shape. Longer pulse duration obtained with second grating is attributed to narrower bandwidth and residual chirp of the grating.
We observed and investigated self-starting quasi-periodic pulsation in Er-doped fiber laser at 30 - 100 mW pump power. Pulse with duration of 10 - 50 ns and peak power of 50 - 200 W are generated at a quite stable repetition rate in the range of 300 - 500 microsecond(s) . In contrast with previous experiments the pump level in our experiment is significantly lower. At this low pump power we found no nonlinear effect except SBS influencing on the laser dynamics. The experimental results were explained by a theoretical model based on cooperative dynamics of Rayleigh backscattering and Stimulated Brillouin Scattering (SBS). Using digital oscilloscope, we traced in details different stages of Q-switching pulse formation process: growth of the spontaneous radiation, lasing due to Rayleigh backscattering, appearance and growth of the first order SBS Stokes radiation and the second order Stokes radiation, lasing suppression due to saturation of the population inversion in Er-doped fiber by the SBS Stokes radiation. Good agreement between theory and experiment have been demonstrated.
We demonstrate for the first time that pulse-to-pulse amplitude fluctuations occurring in the rational-harmonic repetition-rate-doubling regime of actively mode-locked fiber laser are eliminated when modulation frequency is properly tuned. Irregularity of the pulse position in the train is found to be the only drawback of this technique. The irregularity can be reduced to value acceptable for applications by a proper laser design.
In-core temperature measurment is a critical issue for the safe operation of nuclear reactors. Classical thermocouples require shielded connections and are known to drift under high neutron fluence. As an alternative, we propose to take advantage of the multiplexing capabilities of FThre Bragg Grating (FBG) temperature sensors to perform the in-core temperature measurements. We first report on our irradiation experiments on multiplexed FBGs, written in different fibres, employed to measure the in-pile temperature of an air-cooled graphite-moderated nuclear reactor. For some FBGs the difference between the measurements and the readings of calibrated backup thermocouples was within the measurement uncertainty. In the worst case, the difference saturated after 30 hours of reactor operation at about 5°C. Afterwards, we irradiated multiplexed FBG sensors in our material testing nuclear reactor, evidencing the possibility to use FBG sensing technology for in-core monitoring of nuclear reactors with specific care and in well-specified conditions.
Optical fiber technology is considered now for communication and sensing applications in various radiation environments, like space and nuclear industry. We report on results from an on-going experimental program, which aims at using multiplexed Fiber Bragg gratings (FBGs), essential photonic components, for in-pile temperature monitoring in a nuclear reactor. To the best of our knowledge, it is the first time that multiplexed FBG-sensors are used in such conditions.
We demonstrate numerically and experimentally that low- frequency fluctuations (LFF) in a laser diode subject to delayed optical feedback can be suppressed or stabilized by a second optical feedback with a short delay. The second feedback suppresses LFF by shifting antimodes far away from the external cavity modes in phase space, or by making them disappear, with the consequence that the crises that induce the power dropouts are no longer possible. Moreover, as the second feedback strength increases, the laser undergoes a bifurcation cascade with successive regions where it exhibits chaos or LFF and regions where it locks to a newly-born stable maximum gain mode. This all-optical stabilization technique is easier to implement from an experimental point of view than many existing methods since it does not require modification of any laser parameters or of the first optical feedback.
Optical fiber sensors (OFSs) offer numerous advantages, which include immunity to electromagnetic interference, intrinsic safety, small size, a possibly high sensitivity, multiplexing capabilities, and the possibility of remote interrogation. However, OFSs have a relatively low penetration in the commercial market, which is still dominated by standard electromechanical sensors. Nuclear environments are an example where particular OFSs might have a distinct superiority in the competition, but the feasibility of using OFSs in radiation environments still needs to be assessed. In the present paper we report on irradiation experiments performed to provide a sound basis for the design of a fiber Bragg grating based sensor capable to operate even under high total dose exposure.
Generation of picosecond pulses at two distinct wavelengths is interesting for wavelength-division-multiplexing, fiber communication and sensing. For this purpose, we achieved harmonic active mode locking simultaneously at two wavelengths separated by about 15 m in an Erbium-doped fiber laser. Dual- wavelength lasing was obtained with two wide-bandwidth (greater than 1 nm) nonchirped high-reflectivity fiber Bragg gratings inserted in the laser cavity. The fiber Bragg gratings were written with 275-nm light from an Ar laser in hydrogen-loaded fibers. Optical path lengths and losses were carefully adjusted at each wavelength to obtain perfect mode locking at both wavelengths. Total cavity dispersion was set in the anomalous dispersion regime and optimized at each wavelength independently to generate solitons. Pulses at 3-GHz repetition rate were obtained at two wavelengths simultaneously with pulse widths of 16 ps and 13 ps, at 1547 nm and 1562 nm respectively. Time-bandwidth products of 0.37 and 0.34 respectively confirmed that the pulses were nearly transform-limited at each wavelength.
Recently a new mechanism for passive Q-switching in fiber lasers based on cooperative dynamics of linear Rayleigh backscattering (RS) and Stimulated Brillouin Scattering (SBS) has been reported in Yb- and Er-doped fiber lasers with high pump powers (greater than 2 W). At such high pump levels, the intensity of the light generated inside the fiber laser cavity exceeds considerably the SBS threshold, so that the conditions for passive Q-switching due to nonlinear backscattering are easily achieved and, for this reason, no special optimization of the laser configuration is required. Here, we report on experiments in Er-doped fiber lasers with low pump power levels. The Q-switching operational mode of Er-doped fiber lasers was observed at pump power levels in the range of 30 - 100 mW. Throughout the experiments, we traced in details different stages of Q-switching pulse formation process: growth of the spontaneous radiation, lasing due to Rayleigh backscattering, appearance and growth of the first order SBS Stokes radiation and the second order Stokes radiation, lasing suppression due to saturation of the population inversion in Er-doped fiber by the SBS Stokes radiation. In general, the process was slower in comparison with previous experiments at high pump power level. The output pulse duration was in the range of 10 - 20 ns and the peak power of the pulses was less than approximately 100 - 200 W. For this reason, all nonlinear processes except SBS did not influence the pulse formation process. The experimental results are well explained by a theoretical model based on RS-SBS dynamics.
Nuclear industry shows an increasing interest in the possibilities offered by optical fiber technology. The radiation hardness of many fiber-optic devices still needs to be assessed if optical fiber telecommunication and sensing systems have to be deployed in nuclear facilities. In this paper, we report on recent gamma irradiation experiments on modern fiber optic devices such as special optical fibers, fiber temperature sensors, Bragg gratings and VCSELs. The results indicate that these components can be applied in ionizing radiation environments, even at very high levels of exposure.
We report on our irradiation experiments on different types of fiber-optic sensors, including three types of commercially available temperature sensors, a multimode extrinsic Fabry-Perot cavity strain sensor and fiber Bragg- gratings. For the temperature sensors, results show that gamma radiation does not interfere with the basic sensing mechanism and that the most critical component turns out to be the optical fiber itself. Semiconductor absorption temperature sensor showed no degradation up to total doses of 250 kGy, whereas the specifications of Fabry-Perot type sensor and fluorescence temperature sensors were already dramatically influenced below the kGy-level. Replacing the optical fiber by a more radiation resistant version allowed to increase the radiation hardness of the fluorescence sensors system by orders of magnitude. The use of fiber- optic sensors in the presence of neutron radiation remains compromised. Similar conclusions are valid for the Fabry- Perot type fiber-optic strain sensors. We finally show that the Bragg-grating resonance wavelength can shift with radiation dose, but that the temperature sensitivity remains unaltered.
Thin F-doped cladding, pure silica core optical fibers might be excellent candidates for space applications due to their good resistance
to high ionising radiation doses. However, besides radiation, thermally-induced microbending losses might also degrade the
transmission of such polyimide-coated fibers at telecommunication wavelengths. In the infrared, this attenuation can reach levels
comparable to radiation-induced loss in the visible. To evidence this effect, a worst-case experiment was conducted in which fiber
samples (10 m) were wrapped around 54mm diameter aluminum mandrels and heated up to 60°C during irradiation. We compared
the results with those obtained by in situ spectral measurements of transmission loss on similar but unirradiated samples. Here, the
temperature was cycled between room temperature and 120 °C. Infrared transmission loss increased with temperature but disappeared
after cooling back to room temperature. At a wavelength of 1 tm, the temperature-induced loss (reference loss at room temperature)
in a polyimide-coated fiber reached 0.4, 0.9 and 2.0 dB/m at 50 °C, 73 °C and 114 °C respectively. Whereas the behavior of
polyimide-coated radiation-resistant fibers is strongly influenced by temperature-induced microbending losses, acrylate-coated fibers,
however, showed practically no sensitivity of transmission loss to temperature. An aluminium-coated radiation-resistant fiber showed
an intermediate sensitivity.
It is shown that fibers doped with (Ge + P) or with (GE + B) can be used with good accuracy for fluence determinations of fast or slow neutrons, respectively, when they are calibrated at sources of known flux. Calibration at easily available gamma sources, as well as fast neutron dose determination, are complicated by the facts that energetic recoil protons generated by fast neutrons in H-containing fiber coatings contribute to the dose in the fiber core, and that the highly ionizing secondary particles released by fast and slow neutrons will cause significantly lower light absorption than beta or gamma rays of the same dose.
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