Petr Koranda

Oscillation properties of dysprosium-doped lead thiogallate crystal

The room-temperature laser-oscillation properties of lead thiogallate crystals doped with dysprosium ions PbGa(2)S(4):Dy(3+) without additional codoping and codoped with potassium ion for excessive charge compensation were investigated. A slope efficiency of 1% was obtained for the codoped crystal. YAP:Er -1.66 microm laser resonant pumping of PbGa(2)S(4):Dy(3+) laser was compared with YAG:Nd -1.32 microm nonresonant pumping. The maximum energy obtained was 57 microJ with the negligible influence of the atmosphere.

Fe:ZnSe laser - comparison of active materials grown by two different methods

The aim of the presented project was comparison of two Fe:ZnSe lasers based on Fe:ZnSe bulk active crystals grown by two different methods - Bridgman and floating zone. For pumping the Q-switched Er:YAG laser generating 15 mJ and 300 ns giant pulses was used. The highest Fe:ZnSe laser generated output energy was 1.2 - 1.3 mJ for both investigated crystals, the pulse duration was 150 - 200 ns. The Fe:ZnSe laser threshold was reached at absorbed pumping energy of ~ 1 mJ. Tuning properties using intracavity CaF2 prism were also investigated and tuning range ~ 4 - 5 μm was observed for both crystals.

Room-temperature lasing, gain-switched bulk, tunable Fe:ZnSe laser

The goal of this work was to design and investigate a gain switched, at room temperature lasing Fe:ZnSe laser. The active medium was a bulk, by Bridgman-technique grown Fe:ZnSe sample with the thickness 3.4 mm. The pumping was provided by electro-optically Q-switched Er:YAG laser with the oscillation wavelength 2.937 μm matching the local maximum of the Fe:ZnSe absorption. The Er:YAG Q-switched operation was obtained by the Brewster angle cut LiNbO3 Pockels cell placed between the rear mirror and the laser active medium. No additional intracavity polarizers were used. The maximum pumping pulse energy and length was 15 mJ, and ~300 ns, respectively. This pulse-length is close to room-temperature measured lifetime of Fe2+ ions in Fe:ZnSe crystal. The pump radiation was directed into the Fe:ZnSe crystal which was placed inside the cavity formed by dichroic pumping mirror (THR=92% at 2.94 μm and RHR~100% for 3.5-5.2 μm) and optimal output coupler with the reflectance ROC=90% at 4.5 μm, radius of curvature r = -200 mm. The maximum obtained output Fe:ZnSe laser energy was 1.2 mJ, the generated output pulse duration on the wavelength 4.5 μm was 65 ns (FWHM). The output pulse profile was approximately Gaussian. The crystal showed rather high uniformity of oscillation properties throughout its volume. For the case of tuning the CaF2 prism was implemented into the resonator. The tuning curve of generated Fe:ZnSe laser radiation covered the spectral range 3.9 - 4.7 μm.

Diode pumped Tm:YAP laser for eye microsurgery

Pulsed tunable diode-pumped Tm:YAP laser was characterized and used for preliminary investigation in eye microsurgery. By means of Lyot filter, the laser emission was tuned between a high (120 cm-1 @ 1940 nm) and two times lower (50 cm-1 @ 2040 nm) value of radiation absorption in water. In the interaction experiment, the eye tissue (in vitro) was irradiated with tunable Tm:YAP laser radiation, and the effects of cutting and coagulation depth in wavelength range from 1940 nm up to 2040 nm were investigated. The results were documented by optical microscope.

Laser action in bulk Cr:ZnSe crystals

Cr:ZnSe laser active material is one of the favourite possibility how to generate broadly tunable mid-infrared laser radiation at room-temperature. The aim of this study was to demonstrate and analyze pulsed as well as continuous-wave laser action in bulk Cr:ZnSe crystals grown by the floating-zone method or by the Bridgman method. The absorption spectra of Cr:ZnSe were measured to be from 1500 to 2000 nm, therefore various lasers were utilized for coherent longitudinal pumping of Cr:ZnSe laser, namely flashlamp-pumped Er:YAP laser (generated wavelength 1658 nm), diode-pumped Tm:YLF laser (generated wavelength 1912 nm), and diodepumped Tm:YAP laser (generated wavelength 1980 nm). In the first case, the Cr:ZnSe crystal grown by the Bridgman method was investigated. In the second case, the Cr:ZnSe crystal grown by the floating zone method was studied. In both cases, the homogeneity of the active Cr:ZnSe crystals was found reasonable good. The emission spectrum was from 2000 up to 2800 nm. The Cr:ZnSe laser generated radiation was broadly continuously tunable in the range from 2050 nm up to 2750 nm. The generated radiation beam spatial structure was close to TEM00.

Mid-infrared tunable laser based on the Cr:ZnSe active crystal

Broadly tunable mid-infrared laser sources operated at room-temperature are desired in many technological and medical applications. The aim of the project was to design and construct broadly tunable powerful Cr:ZnSe laser. The investigated Cr:ZnSe various shaped bulk crystals were grown by the Bridgman method or by the floating zone method. The absorption spectrum was measured to be from 1500 to 2000 nm and the emission spectrum was from 2100 to 2800 nm. Three different lasers were utilized for coherent longitudinal pumping of Cr:ZnSe laser, namely flashlamp-pumped Er:YAP laser (generated wavelength 1660 nm), diode-pumped Tm:YLF laser (generated wavelength 1912 nm) and diode-pumped Tm:YAP laser (generated wavelength 1980 nm). The constructed Cr:ZnSe laser operated in pulsed as well as in continuous-wave regime. In the first case the Cr:ZnSe crystal grown by the floating zone method was studied. The maximal output power in continuous-wave regime was 310 mW with the slope-efficiency 73% for the Tm:YAP laser pumping. In the second case the Cr:ZnSe prism grown by the Bridgman method which served simultaneously as laser active medium and intracavity dispersive element was investigated. For the Er:YAP laser pumping the maximal output energy was 20 mJ with the slope-efficiency 36%. The output radiation was tunable in the range from 2050 nm up to 2750 nm. For the Tm:YAP laser pumping the maximal output power in continuous-wave regime was 175 mW with the slope-efficiency 24%. The output radiation was tunable in the interval from 2220 nm up to 2680 nm. The generated radiation beam spatial structure was close to TEM00.

Bulk Fe:ZnSe Laser Gain-switched by the Q-switched Er:YAG Laser

Fe:ZnSe is one of the most promising materials capable of generating broadly tunable laser radiation in the wavelength range from 3.5 to 5 μm. The aim of the work was to test laser properties of the Bridgman-method-grown Fe2+:ZnSe crystal activated through the synthesis process as an active medium coherently pumped with the Q-switched Er:YAG laser whose oscillation wavelength (2937 nm) corresponds to the maximum of the Fe2+:ZnSe absorption spectrum. The Er:YAG laser generated giant pulses with the duration 160 - 200 ns and energy 20 - 30 mJ. The repetition-rate was set to be 1 Hz. The oscillation properties, such as the pulse length, energy, and generated beam spatial structure, of the Bridgman-method-grown Fe2+:ZnSe crystal used as an active medium of Fe2+:ZnSe laser operated at room temperature were investigated. The maximal obtained output energy of room temperature Fe2+:ZnSe laser was 580 μJ for the absorbed energy of 5.3 mJ which corresponds to slope efficiency of 38%. The generated pulse waveform was found to follow that of the pump one.

Laser brackets debonding - Tm:YAP, Nd:YAG, and GaAs diode lasers evaluation

The study demonstrates the possibility of using laser radiation for the ceramic bracket removing. Three laser radiations were examined for this effect and the removing possibility and velocity together with enamel and root damage were investigated. A diode pumped Tm:YAP microchip laser generating a wavelength 1.9 μm, diode pumped Nd:YAG laser with 1.44 μm wavelength, GaAs diode with 0.808 μm were used for the debonding purpose. The measurement of transmission and absorption of the basic element - bracket, adhesive resin, and enamel was also made with the goal to explain the source of the heat and bracket debonding. The explanation of the debonding effect is also presented. From the results it is possible to conclude that continuously running diode pumped microchip Tm:YAP laser having output power 1W can be a good candidate for ceramic bracket debonding procedure.

Comparison of tunable lasers based on diode pumped Tm-doped crystals

We report on continuously tunable operation of a diode pumped lasers based on Tm-doped materials, emitting in the 1.8 - 2.μ1 m spectral band. In our study we compare results obtained with three various single crystals doped by Tm3+ ions: Yttrium Aluminum perovskite YAP (YAlO3), Gadolinium orthovanadate GdVO4, and Yttrium Lithium Fluoride YLF (YLiF4). Following samples were available: the 3mm long a-cut crystal rod of Tm:YAP with 4% at. Tm/Y (diameter 3 mm); the 8mm long b-cut crystal rod of Tm:YLF with 3.5% at. Tm/Y (diameter 3 mm); the 2.7mm long a-cut crystal block of Tm:GdVO4 with 2% at. Tm/Gd (crystal face 5×3 mm). For active medium pumping, the laser diode radiation was used. Because the tested samples differs significantly in absorption spectra, two fibre-coupled (core diameter 400 µm) temperature-tuned laser diodes were used: first operating at wavelength 793nm was used for Tm:YAP and Tm:YLF; the second operating at wavelength 802nm was used for Tm:GdVO4. In both cases, the continuous power up to 20W was available for pumping. The diode radiation was focused into the active crystal by two achromatic doublet lenses with the focal length f = 75 mm. The measured radius of pumping beam focus inside the crystal was 260 µm. The longitudinally diode pumped crystals were tested in linear, 80mm long, hemispherical laser cavity. The curved (radius 150mm) output coupler reflectivity was ~ 97 % in range from 1.8 up to 2.1 μm. The pumping flat mirror had maximal reflectivity in this range and it had high transmission around 0.8 μm. A 1.5mm thick birefringent plate made from quartz (Lyot filter) inserted under a Brewsters angle was used as a tuning element. This plate was placed inside the resonator between the crystal and the output coupler. Using Tm:YAP crystal, the maximal output power of 2.8W in this set-up was obtained. The laser could be tuned from 1865nm up to 2036nm with a maximum at 1985 nm. Laser based on Tm:YLF crystal was tunable from 1835nm up to 2010nm with a maximum at 1928 nm (3.0W was reached). Using the Tm:GdVO4 tunable operation with greater that 1W output at 1920nm and 130nm tuning range (1842-1972 nm) was demonstrated. The overall reached tuning range of over 200nm covers many important atmospheric absorption lines and contains also the local absorption peak of liquid water, making them attractive for applications such as high resolution spectroscopy, atmospheric remote sensing, laser radar, and laser microsurgery.

Er:YAG laser radiation applications in different medical branches

For the purpose of biophotonics, free running and Q-switched Er:YAG lasers were constructed. As Q-switches the rotating mirror and Pockels cell were used. In the case of rotating mirror placed inside the resonator the maximum of generated laser energy was 210 mJ in a free-running mode regime when pulses up to 110 μs long (FWHM) were generated. The resulted parameters of the giant pulses were 30 mJ energy, and 250 ns pulse length. For the Pockels cell Q-switching, the laser was generating 325 mJ of energy in a 250 μs pulse, and 60 mJ of energy in a 60 ns pulse in the case of free running and Q-switched regime, respectively. This output properties together with the generating wavelength (2.94 μm), coinciding exactly with the absorption peak of water, giving us the possibility of using this radiation to the efficient interaction with biological tissue. The transport of the radiation to the interaction place was solved by the special cyclic olefin polymer coated silver hollow glass waveguides with the inner diameter of 700 μm and the length of 10 - 50 cm. For the contact treatment the sealed caps were used for preventing delivery system damage. The aim of this work was except of special laser systems development, the investigation ofthe effect differences between long (free running) and short (nanosecond) laser pulses on ophthalmic (cornea, sclera), urologic (ureter wall), or dental (enamel, dentine) tissue.