Enhanced Rayleigh backscattering optical fibers, interrogated by an optical frequency domain reflectometer, are used to perform remote real-time measurements of X-ray irradiation profiles, with possible application as dosimeters in radiotherapy treatments. The enhanced Rayleigh backscattering is obtained by proper engineering of the composition of fiber core, either by introduction of Aluminum or Magnesium silicate nanoparticles as radiation-sensitive dopants. A detectable radiation-induced refractive index change can be spatially resolved through the measurement of the frequency shift of the Rayleigh backscattering along the fiber. It is experimentally demonstrated that two mechanisms of radiation-induced refractive index change take place. At doses nearly compatible with those delivered in radiotherapy, a negative refractive index is induced, whereas at high doses the change is positive. This behavior is also confirmed by the shift of Bragg wavelength of a fiber Bragg grating inscribed in the nanoparticles-doped fiber and used as a reference.
The theoretical model of the Faraday rotation in the low birefringence optical fiber is proposed to serve as a convenient tool for the determination of the birefringence upper limit allowed to retain current sensor sensitivity. The measurement technique offers a fast and efficient determination of the ultra-low linear birefringence when other techniques are not sensitive enough or they are difficult to implement. A temperature dependence of the Faraday rotation and its causes are investigated.
We investigate theoretically and experimentally the possibility of electrostatic actuation of nanomechanical optical fibers with integrated electrodes. The fiber has two optically guiding cores suspended in air by thin flexible membranes. This fiber structure allows for control of the optical properties via nanometer-range mechanical core movements. The electrostatic actuation of the fiber is generated by electrically charged electrodes embedded in the fiber cladding. Fiber designs with one to four electrodes are analyzed and, in particular, a quadrupole geometry is shown to allow for all-fiber optical switching in a 10cm fiber with an operating voltage of 25 - 30V. A multi-material fiber draw technique is demonstrated to fabricate a fiber with well-defined dual core structure in the middle and four continuous metal electrodes in the cladding. The fabricated fiber is analyzed and compared with the modeled requirements for electrostatic actuation.
Passive Q-switching of a diode-pumped (Tm,Yb):KLu(WO4)2 laser, operating between 1910 and 1950 nm, has been
achieved using polycrystalline Cr2+:ZnS as a saturable absorber in a hemispherical L-shaped cavity. The dependence on
the doping levels of Tm and Yb and the low-signal absorption of the saturable absorber has been studied. The highest
average output power reached 272 mW at a repetition rate of 2.04 kHz corresponding to a pulse energy of 133 μJ. This
was achieved with (8.8 at. % Tm, 2.3 at. % Yb):KLu(WO4)2 active element and a saturable absorber with 85% lowsignal
transmission. The highest pulse energy, 148 μJ, was achieved with the same combination at lower (0.94 kHz)
repetition rate. The obtained pulse energy with the different samples is compared with theoretical analysis and the pulse
duration and peak power are estimated from the calculations.
Monoclinic crystals of Tm-doped KLu(WO4)2 were used to demonstrate pulsed laser operation near 2 μm. Passive Qswitching
and passive mode-locking were the techniques employed to produce such laser pulses. For passive Qswitching
we used an AlGaAs -based diode laser to pump the active elements and Cr:ZnSe and Cr:ZnS crystals as
saturable absorbers. For passive mode-locking we used a Ti:sapphire laser as pump source and single-walled carbon
nanotubes as saturable absorbers. In the former case, maximum pulse energies of 200 μJ for a pulse duration of 70 ns
were achieved at a repetition rate of 3 kHz with Cr:ZnS saturable absorber, while in the latter case, ultrashort pulse
durations of ~10 ps were measured with a maximum average power of 240 mW. In both laser regimes the oscillation
wavelength was ~1945 nm.
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