Yb-doped Optical Fibers Research Articles

Rapid photo-bleaching gamma irradiated Yb-doped optical fibers by high-energy ns pulsed laser

A rapid and efficient photo-bleaching process was demonstrated with a high-energy nanosecond pulse to recover existing and/or revealed color centers on 10 kGy Gamma-irradiated Yb-doped optical fiber. Multi-mJ pulsed laser based on an optical parametric amplifier system operating at wavelengths of 532 nm, 680 nm and 793 nm was used. The photo-bleaching performance is investigated as a function of the wavelength and energy of the pulsed light source. It was observed that the photo-bleaching level of the Yb-doped optical fiber increased when the exposure time of the pulsed laser light and the photon energy was increased. The maximum PB occurred in the pulsed laser of 532 nm wavelength in the optical fiber. Also, a drastically increase in the PB was observed due to the increasing laser energy at the wavelength of 532 nm and 680 nm pulsed laser. The results show that the recovery levels of color centers in the Yb-doped optical fibers could be reached up to 96 % in a shorter time (h) by using the pulsed laser compared to that of the studies using continuous laser.

Observation of two-wave mixing in a single-frequency fiber amplifier induced by frequency modulation.

We report on the observation of unstable two-wave mixing in a Yb-doped optical fiber amplifier induced by frequency modulation of a single-frequency laser. What is believed to be a reflection of the main signal experiences a gain much higher than that provided by the optical pumping and potentially limits power scaling under frequency modulation. We propose an explanation for the effect based on the dynamic population and refractive index gratings formed by the interference between the main signal and its slightly frequency-detuned reflection.

Numerical Study of Modal Properties of Large-Mode-Area Yb-Doped Optical Fiber in High-Power Laser Amplification

Numerical Study of Modal Properties of Large-Mode-Area Yb-Doped Optical Fiber in High-Power Laser Amplification

Stimulated Brillouin scattering mitigation using optimized phase modulation waveforms in high power narrow linewidth Yb-doped fiber amplifiers.

We demonstrate the mitigation of stimulated Brillouin scattering (SBS) in a double-clad single mode Yb-doped optical fiber amplifier through external phase modulation of narrow linewidth laser radiation using optimized periodic waveforms from an arbitrary waveform generator. Such optimized phase modulation waveforms are obtained through a multi-objective Pareto optimization based on a comprehensive model for SBS in high power narrow linewidth fiber amplifiers using Brillouin parameters determined from controlled measurements. The ability of our approach to mitigate SBS is tested experimentally as a function of RMS linewidth of the modulated optical radiation, and we measure an enhancement in SBS threshold with respect to optical linewidth of ∼ 10 GHz-1. Furthermore, we discuss the dependence of the SBS threshold enhancement on key parameters such as the amplifier length and the period of the optimized waveforms. Through simulations we find that waveforms of sufficiently long periods and optimized for a relatively long fiber (10 m) are effective for SBS suppression for shorter fibers as well. We also investigate the effect of increase in the bandwidth and amplitude of the modulation waveform on the SBS threshold enhancement observed at higher optical linewidth.

Effects of neutron irradiation on optical characteristics of Yb-doped fiber materials

Yb-doped fiber laser is a very appealing technology to implement space communication, laser radar and nuclear facilities due to its reduced weight, size, high efficiency, high peak power combined with good beam quality. However, Yb-doped fiber materials will suffer a harsh ionizing radiation (such as neutron, proton and electron) under the condition of irradiation. The radiation-induced darkening effect can lead the fiber loss to increase obviously, the laser slope efficiency to decrease drastically, and even no laser output to be observed in severe cases. Therefore, it is necessary to conduct in-depth research on the performance changes of Yb-doped fiber materials subjected to the irradiation. In this paper, a series of Yb-doped optical fibers and optical fibers is prepared by the improved chemical vapor deposition method combined with rare-earth chelate-doped. And the optical properties of the optical fiber before and after being irradiated and annealed are tested. We mainly investigate the absorption spectrum of Yb-doped fiber material. The results show that the concentration of Al-related defects in the Yb-doped fiber material increases after neutron irradiation, and the absorption loss in the visible region increases. And the color center defects produced by the irradiation will significantly reduce the Yb ion fluorescence lifetime. The doping of Ce ions can reduce the Al-oxygen hole center (Al-OHC) color center defects, and can suppresse the radiation-induced darkening effect of Yb-doped fiber to a certain extent. Thermal annealing can reduce the absorption of fiber material by reducing the concentration of neutron radiation-induced color center defects, and thus eliminating the darkening effect to a certain extent. Finally, with our previous research, we find that neutron irradiation and ray irradiation have similar effects on the optical properties of Yb-doped fiber materials. The main reason is that the electron ionization effects occur due to both ray irradiation and neutron irradiation to generate free electron and hole pairs, which then combine with the original defects in the material to turn into color center defects. However, the color center defects caused by neutron irradiation are more stable and require thermal annealing to be eliminated. And the results obtained in this study provide theoretical basis and solution for developing the Yb-doped fibers with high laser performance and high radiation resistance.

On the Use of Brillouin Scattering to Evaluate Quantum Conversion Efficiency in Yb-doped Optical Fibers

A method, based on Brillouin scattering, to evaluate the quantum conversion efficiency (QCE) in Yb-doped optical fibers is designed and tested. An adapted version of the heterodyne approach to measuring the Brillouin spectrum is first used to precisely quantify the temperature change of the fiber upon pumping. Next, a theoretical model based on the thermal conduction equation is constructed in order to simulate the temperature change assuming a QCE of one. Lastly, the QCE value of the fiber can be determined by comparing the experimental and simulation results. Importantly, the analysis can provide insight into the origin of thermal energy in a fiber. Serving in the proof-of-concept measurement, a commercial, lightly Yb-doped fiber is determined to have a QCE of 99.07%. Finally, this result is compared with slope efficiency measurements from a linear cavity laser using the same fiber as the active medium. Excellent agreement between the two measurements demonstrates the feasibility of the approach, with greater accuracy afforded by the Brillouin-based measurement. The proposed method is particularly useful for optical fiber optimization in high-power fiber laser and amplifier applications. It can also be valuable as a sensitive temperature probe in the study of anti-Stokes fluorescence cooling in optical fibers.

Study on X-ray enhancement in Laser-Compton scattering for auger therapy

Purpose Monochromatic hard X-rays with high brightness are desired for medical applications including Auger therapy. One can generate such X-rays through laser-Compton scattering (LCS) by allowing photons from a compact laser system to interact with electrons accelerated by a compact electron accelerator. In this paper, after a brief description of laser-Compton X-ray sources, a scheme called crab crossing to enhance the X-ray intensity is proposed. The effect of crab crossing is evaluated, and we report our dedicated laser system for the crab crossing LCS research. Materials and methods The luminosity enhancement factor by crab crossing is evaluated. For the electron beam, a rf deflector will be used to generate a tilted electron beam. For the laser system, chirped pulsed amplification is adopted. Yb-doped optical fibers and a Yb:YAG thin-disk is used for the laser gain media. Results The luminosity enhancement factor by crab crossing is expected to be 3.8 when the crossing angle is 45 degrees. 10mJ pulse energy was achieved by thin-disk regenerative amplifier. The pulse duration after the pulse compressor was about 1.5 ps. Conclusion We are going to demonstrate the LCS X-ray enhancement by crab crossing of electron beam and laser pulse. The expected enhancement factor is 3.8. We have successfully finished the laser development and the proof-of-principle experiment will be conducted soon.

Paving way for fabrication of silica-based single -frequency seed laser: Ultrahighly Yb-doped optical fibers via sol-gel method combined with silica tube inner wall coating and fusion-tapering technique

A method based on sol-gel preparation combined with silica tube inner wall coating and fusion-tapering technique for fabrication of ultrahighly rare-earth-doped fibers without fluorescent quenching is provided. Ultrahighly Yb-doped high SiO2 content glass fiber is developed and is easily spliced to commercial fiber-components. A ~ 1.03 μm laser with 70 dB-SNR and 0.2 nm-linewidth is achieved. The problem of low dissolution rate to rare-earth ions in silica fibers and the splicing problem between soft glass fibers and commercial fiber components have been overcome, indicating the method has potential in fabrication of short-cavity high-gain silica fiber which is applicable to single-frequency seed source. The method is worth exploring in that the core can extend to any heavily rare-earth-doped system as long as it can be realized in the sol form with good dispersion, therefore paving way for fabrication of silica-based single-frequency seed laser.

Development of Digital Holographic Microscopy on an All-Optical Phase Shifter

We propose an all-optical phase-shifting method using Yb-doped optical fibers and demonstrate its potential in on-axis digital holography microscopy (DHM) system. The Yb-doped optical fiber induces a phase change under optical pumping conditions. The induced phase shift shows a good linear response to applied pumping currents, and N-step phase-shifting up to 2p can be properly performed. Four-, six-, and eight-step phase-shifting methods are performed, and the phase retrievals from their phase-shifted interferograms are achieved using an interframe correlation matrix method. An 8-step phase shifting-based reconstruction, which is effective for background noise reduction, is used for 3D imaging of an indium tin oxide (ITO) thin film and an organic light-emitting diode (OLED) thin film. The reconstructed surface profile was in agreement with the step structure of the samples. The proposed all-optical phase-shifting method is expected to be applied to various optical imaging systems using fiber-based phase-shifting interferometers.

3 kW narrow-linewidth all-fiber amplifier with near diffraction limited beam quality

A theoretical model is established to simulate the stimulated Brillouin scattering (SBS) effect in fiber amplifiers using white noise signal (WNS) phase-modulated seed. Based on the theoretical model, the influences of amplifier structure and cut-off frequency of WNS on SBS threshold are discussed in detail. Through theoretical analysis, optimized amplifier structure and phase modulation signal are achieved to suppress SBS, and a fiber laser based on a main oscillator power amplifier with counter-pumped scheme is established. The oscillator, with 3 dB spectral linewidth of 48 GHz, is produced by a WNS phase-modulated single-frequency seed. The amplifier uses a 915 nm diode laser pump and 25/400 Yb-doped optical fiber. The output power reaches 3.02 kW at 1064 nm, with 3 dB root-mean-square linewidth of 48 GHz. Amplified quality is obtained with M, M at maximum power.

Suppression of photo-darkening effect in Yb-doped silica glass fiber by co-doping of group 2 element

We found that co-doping of group 2 element effectively suppresses the photo-darkening (PD) effect in Yb-doped silica glass fiber and Mg co-doped Yb-doped silica glass fiber (YbMgSGF) is the most effective. This phenomenon is well understood on the basis of bond valence concept in the glass. In our model, Mg2+ ion with Al3+ stabilizes Yb3+ valence state with charge compensation in the silica glass and Mg2+ could smoothly match silica glass network because the ionic radius is the closest to the radii of Al3+ and Si4+. YbMgSGF is not only a promising Yb-doped optical fiber for high power fiber laser, but also for ultimate short length fiber lasers. This technique could also be applied to reduce PD effect in the other rare-earth ion doped fibers, such as Tm, Tb, Eu, and Sm.

Thermoluminescence response of ytterbium-doped silicon dioxide fiber

In this study, thermoluminescence properties of Yb-doped of SiO2 optical fiber such as linearity, sensitivity and dose response have been investigated. Samples were placed in the gelatin capsules to irradiate with photons from Cobalt-60 of the dose range between 10–100 Gy. The beams were provided by Gammacell 220, available at Universiti Kebangsaan Malaysia, Bangi, Malaysia. The glow curves were analyzed to determine various characteristics of the thermoluminescence. The TL response as a function of dose is linear up to 40 Gy. The sensitivity of Yb-doped optical fiber is 0.136 nC mg−1 Gy−1.

Thermoluminescence responses of the Yb- and Yb–Tb-doped SiO2 optical fibers to 6-MV photons

Characteristics of the thermoluminescence (TL) responses of Yb- and Yb–Tb-doped optical fibers irradiated with 6MV photons are reported. The concentration of Yb in the Yb-doped optical fiber was 0.26mol%; the concentrations of Yb and Tb in the Yb–Tb-doped optical fiber were 0.62 and 0.2mol%, respectively. The TL dose responses are linear in the dose range 0.5–4Gy. The radiation sensitivity of the Yb–Tb material is almost two orders of magnitude higher than the sensitivity of the material doped with Yb alone.

Advanced attenuation and fluorescence measurement methods in the investigation of photodarkening and related properties of ytterbium-doped fibers

Considering the continuous development of ytterbium (Yb)-doped fibers for fiber lasers and amplifiers, thorough characterization of the features and quality of such fibers is necessary now more than ever. In particular, the evaluation of the very strong core attenuation that results from Yb absorption and the losses accompanying photodarkening (PD) processes in Yb-doped optical fibers is still of immense interest. Keeping that in mind, the potential of a method using pinhole fibers will be demonstrated here. Moreover, the investigation of fluorescence properties of Yb3+ is also important; it can be affected by phenomena such as reabsorption, inhomogeneous pump intensity distribution, amplified spontaneous emission, lasing, and, lastly, PD. As these phenomena occur while measuring the fluorescence properties of Yb3+ in optical fibers, they can distort the fluorescence spectra and fluorescence decay significantly. We suggest a twice-perpendicular measurement method and strict time management to avoid these interfering phenomena during the fluorescence measurements.

Investigations of Yb-doped optical fiber using selective laser excitation

The method of tunable site-selective laser excitation was used for a study of the spectral and fluorescence characteristics of Yb3+ optical centers in laser optical fiber. Decay curves and positions of fluorescence spectra maxima at different wavelengths of selective laser excitation and positions of excitation spectra maxima at different fluorescence selective registration provide new information on lifetimes and Stark energy distribution among inhomogeneously broadened Yb3+ lines. The obtained Stark splitting energy distribution diagram demonstrates the tuneability of Yb3+ laser oscillation wavelength under tunable excitation of a pump laser.

Rate Equation for the Nonlinear Phase Shift in Yb-Doped Optical Fibers Under Resonant Diode-Laser Pumping

Rate Equation for the Nonlinear Phase Shift in Yb-Doped Optical Fibers Under Resonant Diode-Laser Pumping

Dynamics of pump-induced refractive index changes in single-mode Yb-doped optical fibers

We quantify the refractive index changes (RIC) in single-mode ytterbium-doped optical fibers induced by optical pulses at 980 nm and, for the first time, report details of the effect dynamics. The RIC dynamics is shown to follow that of the population of the excited/unexcited ion states with a factor proportional to their polarizability difference (PD). The absolute PD value is evaluated in the spectral range of 1460-1620 nm for different fiber samples and is found to be independent on the fiber geometry and on the ion concentration. The PD dispersion profile indicates to a predominant far-resonance UV rather than near-resonant IR transitions contribution to the RIC.

Spectrally Resolved Transmission Loss in Gamma Irradiated Yb-Doped Optical Fibers

Yb <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> -doped silicate fibers are commonly employed in optical systems utilizing fiber lasers and amplifiers. Deployment of such materials and systems in space-based and other adverse radiation environments requires knowledge of their response to fluxes of ionizing radiation. This paper reports the results of gamma radiation exposures on a suite of passive, modern, highly Yb <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> -doped aluminosilicate fibers. Of interest are the effects of total dose and dose rate as well as the development of radiation-induced absorption across a broad spectral window (1.0-1.7 mum). Results indicate that these fibers exhibit reasonable radiation resistance to gamma exposures typical of a five-year low-Earth-orbit environment. Maximum transmittance losses of less than 10% in the 1.0-1.7-mum spectral region for total gamma exposures of 2-5 krad (Si) were observed. In addition, it was found that the dependence of transmittance on radiation dose generally followed a power law that was dependent on dose rate.

Temperature effects on the emission properties of Yb-doped optical fibers

Ytterbium-doped fiber lasers are making impressive leaps in power production. Yet in spite of fiber’s large surface area to volume ratio which is beneficial for cooling, such power inevitably leads to high core temperatures that in turn affect the laser performance. In this paper, the temperature effects on the emission and fluorescence lifetime of ytterbium-doped optical fibers are investigated. From these the temperature dependent emission and absorption cross-sections are calculated.