Miroslav Cech

Quasi-continuously pumped passively mode-locked operation of a Nd:GdVO4 and Nd:YVO4 laser in a bounce geometry

The comparative study of pulsed diode pumped Nd:GdVO4 and Nd:YVO4 slab lasers in a bounce geometry in a free running and a passively mode-locked regime was performed. The better efficiency of Nd:GdVO4 in both regimes was obtained. In the free running regime the efficiency of 39% was achieved from Nd:GdVO4 comparing with 28% from Nd:YVO4. Passive mode-locking of both lasers using a semiconductor saturable absorber in the transmission mode was demonstrated. Trains containing 6 pulses were generated in both cases but the pump energy for Nd:GdVO4 was 50% lower. The single pulse extraction using cavity dumping was demonstrated with contrast better than 10−3.

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.

Spectroscopic and laser properties of bulk iron doped zinc magnesium selenide Fe:ZnMgSe generating at 4.5 - 5.1 µm.

The Fe:Zn(1-x)Mg(x)Se (x = 0.19, 0.27, and 0.38) solid solutions spectroscopic properties were investigated and laser oscillations were achieved for the first time. The increase of the magnesium concentration in the Fe:ZnMgSe crystal was shown to result in an almost similar long wavelength shift of both absorption and fluorescence spectra of about 60 nm per each 10% of magnesium. With the Fe:ZnMgSe crystal temperature decrease, the fluorescence spectrum maximum shifts towards shorter wavelength resulting mainly from strong narrowing of the longest wavelength fluorescence line. Laser radiation wavelength dependence on the magnesium concentration as well as on temperature was observed. The Fe:ZnMgSe x = 0.38 laser oscillation wavelength increased from 4780 nm at 80 K to 4920 nm at 240 K using the optical resonator without any intracavity spectrally-selective element. In comparison with the Fe:ZnSe laser operating in similar conditions, these wavelengths at both temperatures were shifted by about 500 nm towards mid-IR region.

Passively Q-switched quasi-continuously pumped 2.4% Nd:YAG laser in a bounce geometry

We report on quasi-continuously pumped laser based on highly 2.4 at.% doped crystalline Nd:YAG in a bounce geometry passively Q-switched by a Cr:YAG and V:YAG saturable absorber respectively. The optimization of laser parameters and successive pulse amplification in the second Nd:YAG crystal led to efficient linearly polarized operation. At 1.06 μm the oscillator 5 ns long output pulse with energy of 1.3 mJ was further amplified to 3.5 mJ. At 1.3 μm the 13 ns long output pulse with energy of 600 μJ was amplified to 800 μJ in single pass.

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.

Mode locked Nd:YVO4 laser with intracavity synchronously pumped optical parametric oscillator

The motivation of this work is the development of laser sensor and gyroscope based on short pulse solid state ring laser. In comparison with regular ring laser containing the gain medium and saturable absorber, where counterpropagating pulses overlap, a ring synchronously pumped optical parametric oscillator, in which the pulse crossing point is controlled externally by the time of arrival of the pump pulses, is the ideal source for short pulse laser sensor. The optimum configuration is a synchronously pumped parametric oscillator inserted inside the optical resonator of the diode pumped mode-locked solid state laser. We are developing a such system, as a first step we have demonstrated operation of a diode pumped Nd:YVO4 passively mode-locked laser using semiconductor saturable absorber with synchronously pumped intracavity optical parametric oscillator in linear configuration. The repetition rate of the pump laser was 132 MHz and the pulse duration of 15 ps. Parametric oscillator was based on 20 mm long Brewster cut single grating (with poling periode of 30.3 μm) periodically poled magnesium doped lithium niobate (MgO:PPLN) crystal. The temperature tuning of parametric luminescence from the crystal with peak wavelength at 1537 nm - 1550 nm for temperature variation from 30 °C to 57 °C was observed.

Er:YAG laser giant-pulse generation

Three possibilities of mid-infrared Er:YAG lasers Q-switching were investigated: mechanical and two electro-optical ones. The mechanical method of Q-switching was proved by using of a rotating mirror placed inside the resonator; Pockels or Kerr cells were used for the electro-optical Q-switching. The Pockels cell was constructed on the basis of the Brewster angle cut LiNb03 crystal; the Kerr cell used a ceramic PLZT material. In all the three cases the Er:YAG laser resonator was plan parallel and it consisted of the rear copper mirror and output coupler with 50-70% reflectivity. The Er:YAG crystal was pumped by one xenon flashlamp in a single elliptical silver coated cavity. The generated giant pulse output parameters were the following: the length was measured in the range of 200-400 ns; the output energy was from 10 mJ up to 30 rnJ. With the rotating mirror the output was stable in a wide range ofthe switching interval. All the three cases can be used in applications. The particular Q-switch can be chosen from the point of view of output parameters and construction requirements.

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.

Cavity dumping of single 19 ps pulses from passively mode-locked quasi-continuously pumped highly doped Nd:YAG laser

The common approach for a picosecond laser source with output energy in millijoule level is a continuously pumped mode locked oscillator with single pulse selector followed by a regenerative amplifier. Systems can operate with repetition rates up to 100 kHz with single pulse duration around 10 ps. For many applications, as for example laser ranging, lasers operating at repetition rates below 1 kHz with longer pulse durations are more suitable. Such pulses can be obtained from passively mode-locked lasers with pulsed pumping, where the microjoule pulses are generated directly from oscillators and simple single or double pass amplifiers are sufficient for additional pulse amplification. Recently we reported about development of a 0.8 mJ laser with output pulse duration of 113 ps, where for passive mode-locking special semiconductor saturable absorbers with 100 quantum wells were used under Brewster angle in resonator in transmission mode [1].

Quasi-continuously pumped operation of 2.4% doped crystalline Nd:YAG in a bounce geometry

We report on efficient operation of highly doped 2.4 at. % crystalline Czochralski grown Nd:YAG at 1.06μm, 1.3μm and 1.4μm in a diode pumped bounce amplifier configuration under quasi-continuous pumping. At wavelength of 1064nm the linearly polarized pulses with energy of 16.8 mJ in free running regime with repetition rate of 10 Hz (optical to optical efficiency of 44.6 % and slope efficiency of 50%) and 1 mJ in passively Q-switched regime with pulse duration of 6.4 ns were generated.The passively Q switched operation at 1.3μm was also demonstrated.