Petr Navratil

Fluxgate sensor for magnetopneumometry

Abstract A race-track single-core fluxgate magnetic gradient sensor has achieved a noise level of 30 pT cm −1 r.m.s. (50 mHz–10 Hz) with a gradiometric base of only 3.75 cm. Such a sensor has sensitivity high enough to replace SQUIDs in the detection of ferromagnetic particles deposited in human lungs. Measurements on tissue samples, dust from respiration filters and experiments with phantoms have proved that the total amount and spatial distribution of dust in the lungs may be measured in the case of welders, grinders and other metal workers. Measurements on coal miners and asbestos workers are less specific. The construction of the magnetization device for in vivo experiments is in the progress.

Self-induced laser line sweeping and self-pulsing in double-clad fiber lasers in Fabry-Perot and unidirectional ring cavities

Rare-earth doped fiber lasers are subject to instabilities and various self-pulsed regimes that can lead to catastrophic damage of their components. An interesting self-pulsing regime accompanied with laser wavelength drift with time is the so called self-induced laser line sweeping (SLLS). Despite the early observations of the SLLS in solid-state ruby lasers, in fiber lasers it was first time mentioned in literature only in 2009 where such a laser wavelength drift with time was observed in a relatively broad range of about 1076 -1084 nm in ring ytterbium-doped fiber laser (YDFL). The main characteristic of the SLLS is the scanning of the laser wavelength from shorter to longer wavelength, spanning over large interval of several nanometers, and instantaneous bounce backward. The period of this sweeping is usually quite long, of the order of seconds. This spectacular effect was attributed to spatial-hole burning caused by standing-wave in the laser cavity. In this paper we present experimental investigation of the SLLS in YDFLs in Fabry-Perot cavity and ring cavities. The SLLS was observed also in erbium-doped fiber laser around 1560 nm. We present for the first time observation of the laser wavelength sweep in reverse direction, i.e., from longer towards shorter wavelengths. It was observed in YDFL around 1080 nm.

Cryogenic Yb:YAG Laser Pumped by VBG-Stabilized Narrowband Laser Diode at 969 nm

We present the laser performance of Yb:YAG at different cryogenic temperatures pumped by a volume Bragg grating (VBG)-stabilized narrowband fiber-coupled diode laser emitting at 968.7 nm for the first time to the best of our knowledge. A maximum output power of 6.54 W with a slope efficiency as high as 64.6% is achieved at 140 K. The optimum temperature for 968.7 nm and 0.35 nm bandwidth VBG-stabilized pump source is found to be 140 K for Yb:YAG.

Effect of pump wavelength on self-induced laser line sweeping in Yb-doped fiber laser

We have characterized ytterbium–doped fiber laser and described in detail the effect of pump wavelength on self–induced laser line sweeping (SLLS). SLLS is a transient laser regime manifested by a relatively slow laser line shifting and usually observed in the near of pump laser threshold. The fiber laser under consideration is cladding–pumped by a temperature stabilized multimode laser diode (LD) at about 976 nm. The output wavelength of the diode is tunable by changing the diode temperature and current through the diode. The cavity of the laser is formed by perpendicularly cleaved fiber ends. Using this laser layout we made detailed study of sweeping dependences on pump wavelength by adjustment of the LD current and LD case temperature. The laser manifested laser line sweeping within the range of 5–7 nm on a wide span of pump laser diode power and temperature: 15–45°C LD temperature scale and pump range reaching to more than twice the amount of excess over threshold. We performed the measurement of the laser line sweeping span, period and rate as the dependence of pump wavelength.

Diode pumped compact cryogenic Yb:YAG/Cr:YAG pulsed laser

In this work, we have shown the possibility of performing pulsed laser operation at cryogenic temperature with innovative modular compact laser cavity. Using Yb:YAG/Cr:YAG configuration, at cryogenic temperature, in the preliminary laser experiments, we obtained a maximum average output power of 2.7 W with a repetition rate of around 50 kHz. The minimum pulse width, maximum pulse energy and peak power obtained were 2.5 ns, 60 μJ. and 25 kW respectively. To the best of our knowledge, an experimental approach on compact pulsed Q-switched lasers based on Yb:YAG/Cr:YAG at cryogenic temperature is tried for the first time.

Characterization of Bivoj/DiPOLE 100: HiLASE 100-J/10-Hz diode pumped solid state laser

The HiLASE “Bivoj” laser system developed at CLF Rutherford Appleton Laboratory in collaboration with HiLASE team as DiPOLE100 was relocated to Dolni Brezany near Prague, Czechia at the end of 2015 and fully re-commissioned at the end of 2016. In 2016, the system demonstrated average output power of 1kW generating pulses of 105 J at 10 Hz repetition rate for the first time in the world. Since then the system has been subjected to several testing campaigns in order to determine some of its key characteristics. Beam quality, wavefront quality, pointing stability, energy stability and experience with long term operation of 1 kW laser are presented. In addition, depolarization effects have been detected inside the main amplifier. Details on these results along with numerical simulations are presented.

Self-sweeping of laser wavelength and associated mode instabilities in fiber lasers

Fiber lasers may exhibit instabilities and self-pulsed regimes that can have catastrophic consequences on their components. One of the self-pulsing regimes is the recently observed spontaneous laser line sweeping (SLLS). The SLLS is characterized by periodic wavelength drift over broad spectral interval of several nanometers followed by quick bounce backward. The sweeping rate is relatively slow, of the order of nm per second. The SLLS can be explained by spatial-hole burning in the active fiber and it can be considered as a special case of instability of longitudinal modes of the laser cavity. Thanks to narrow linewidth and simple construction, the SLLS fiber lasers are attractive sources for testing of photonics components, interrogation of optical fiber sensor arrays and for laser spectroscopy. We review recent advances in investigation of the SLLS in fiber lasers including its effect on the triggering of the self-Q-switched regime and the generation of giant laser pulses.

Diode-pumped cryogenic Yb:KLu(WO 4 ) 2 laser

In recent years, the development of compact all solid-state lasers has attracted the scientific community because of their vast scientific and technological applications. In particular, ytterbium (Yb3+) laser sources have been developed with a great number of hosts, due to the promise of the Yb3+ ion related to the very simple energy level structure and absence of higher energy levels, as well as a small quantum defect. However, at room temperature (RT) these materials behave as quasi three-level lasers due to a finite population of the ground state leading to reabsorption limited power scaling [1]. To overcome this issue, the material has to be cooled to cryogenic temperatures, where four-level operation is accomplished. The monoclinic potassium lutetium double tungstate crystal, KLu(WO4)2, considered here exhibits favourable spectroscopic properties when doped with Yb3+, characterized by large absorption and stimulated-emission cross sections and attractive thermo-optic properties for certain “athermal” crystal cuts (e.g., along the Ng-axis). It is very suitable for moderate power levels as demonstrated with Yb3+ and Tm3+ doping at RT [2]. In this work, we studied the continuous-wave laser characteristics of Yb:KLu(WO4)2 (Yb:KLuW) at cryogenic temperatures pumped by a VBG-stabilized fiber coupled diode laser at 981 nm.

Continuous-wave and passively Q-switched cryogenic Yb:KLu(WO_4)_2 laser

We study cryogenic laser operation of an Yb-doped KLu(WO4)2 crystal pumped with a volume Bragg grating (VBG) stabilized diode laser at 981 nm. In the continuous wave laser regime, a maximum output power of 4.31 W is achieved at 80 K with a slope efficiency of 44.0% with respect to the incident pump power. Using a 85% initial transmission Cr:YAG crystal for passive Q-switching, an average output power of 2.11 W is achieved at 100 K for a repetition rate of 19 kHz. The pulse energy, pulse duration and peak power amount to 111 µJ, 231 ns and 0.48 kW, respectively.

Cryogenic Yb:YGAG ceramic laser pumped at 940 nm and zero-phonon-line: a comparative study

We present a comparative study on a continuous-wave laser operation of Yb-doped Y3Ga2A13O12 (Yb:YGAG) ceramic at cryogenic temperatures with conventional pumping (940 nm) and zero phonon line pumping (969 nm) under identical experimental conditions. In CW laser operation, at 80 K with ZPL pumping, a maximum output power of 6.53 W with a slope efficiency of 52.0% is achieved with respect to incident power. When compared between two pump sources at cryogenic temperatures, ZPL pumping performs better due to the difference of quantum defect between the two pump sources that results in different heat load in the sample. In passive Q-switching experiment, at 100 K, with 85% initial transmission of Cr:YAG, an average output power of 3.37 W with a repetition rate of 17.6 kHz was achieved. The pulse energy, pulse width and peak power obtained in this case were 0.19 mJ, 164 ns and 1.16 W respectively.