Michal Němec

Diode-pumped Er:CaF 2 ceramic 2.7 μm tunable laser

Spectroscopic and laser properties of a newly developed high optical quality Er:CaF2 hot-formed ceramic were investigated. Under pulsed 968 nm laser diode pumping, the mid-infrared (2.7 μm) radiation was obtained with a slope efficiency of 3%. Laser tunability was reached using a birefringent filter and the laser tuning range of 118 nm, from 2687 up to 2805 nm, was demonstrated. The maximal output energy reached was 0.48 mJ at 2730 nm for the absorbed pumping energy 34 mJ.

1.6 μm Er:YAP and Er:YAG Lasers Resonantly Pumped by Er:Glass Laser

Abstract1623 nm Er:YAP and 1648 nm Er:YAG lasers resonantly pumped by a solid state Er:glass laser operating at 1535 nm were investigated. Laser generation was reached for Er:YAP and two Er:YAG crystals with different Er ion concentration. The maximal output energies were 20 and 45 mJ for Er:YAP and Er:YAG laser systems, respectively.

Q-switched Er:YAG lasers resonantly pumped by Erbium fiber laser

The study describes efficient, acousto-optic Q-switching of Er:YAG laser at the 1645 nm eye-safe wavelength. For longitudinal pumping at wavelength 1532 nm, linear-polarized 10 W Erbium fiber laser radiation was used. The investigated Er:YAG crystals were 25 and 40 mm long and their Erbium concentration was 0.20 and 0.25%, respectively. For giant pulse generation, a fused silica acousto-optic modulator was inserted inside the Er:YAG laser oscillator. For a maximum incident pump power of 7.95 W, pulse energy up to 4.1 mJ was generated with pulse duration 34 ns at 500-Hz repetition rate; the corresponding peak power was 119 kW.

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.

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.

Diode pumped tunable lasers based on Tm:CaF2 and Tm:Ho:CaF2 ceramics

The Tm:CaF2 (4% of TmF3) and Tm:Ho:CaF2 (2% of TmF3, 0.3% of HoF3) ceramics, prepared using hot pressing, and hot formation technique had been used as an active medium of diode pumped mid-infrared tunable laser. A fibre (core diameter 400 μm, NA = 0.22) coupled laser diode (LIMO, HLU30F400-790) was used to longitudinal pumping. The laser diode was operating in the pulsed regime (6 ms pulse length, 10 Hz repetition rate). The duty-cycle 6% ensures a low thermal load even under the maximum diode pumping power amplitude 25W (ceramics samples were only air-cooled). The laser diode emission wavelength was 786 nm. The 80mm long semi-hemispherical laser resonator consisted of a flat pumping mirror (HR @ 1.85 − 2.15 μm, HT @ 0.78 μm) and a curved (r = 150mm) output coupler with a reflectivity of ∼ 98% @ 1.85 − 2.0 μm for Tm:CaF2 laser or ∼ 99.5% @ 2.0 − 2.15 μm for Ho:Tm:CaF2. Tuning of the laser was accomplished by using a birefringent filter (single 1.5mm thick quartz plate) placed inside the optical resonator at the Brewster angle. Both samples offered broad and smooth tuning possibilities in mid-IR spectral range and the lasers were continuously tunable over ∼ 100 nm. The obtained Tm:CaF2 tunability ranged from 1892 to 1992nm (the maximum output energy 1.8mJ was reached at 1952nm for absorbed pumping energy 78 mJ). In case of Tm:Ho:CaF2 laser tunability from 2016 to 2111nm was reached (the maximum output energy 1.5mJ was reached at 2083nm for absorbed pumping energy 53 mJ). Both these material are good candidates for a future investigation of high energy, ultra-short, laser pulse generation.

Multiline generation capabilities of diode-pumped Nd:YAP and Nd:YAG lasers

Multiline generation capabilities of diode-pumped Nd:YAP and Nd:YAG lasers within 1.3?1.5??m wavelength range at room temperature are reported. Two optical resonators designed for 1.3 and 1.4??m laser operation have been realized. Using a single quartz plate as a tuning element, six single emission lines within the 1.3?1.5??m spectral range for both Nd:YAG and Nd:YAP lasers have been reached. Moreover, as also demonstrated, it was possible to obtain Nd:YAG/YAP dual frequency regime operation for some line combinations.

Tunable eye-safe Er:YAG laser

Er:YAG crystal was investigated as the gain medium in a diode (1452 nm) pumped tunable laser. The tunability was reached in an eye-safe region by an intracavity birefringent filter. The four tuning bands were obtained peaking at wavelengths 1616, 1632, 1645, and 1656 nm. The broadest continuous tunability was 6 nm wide peaking at 1616 nm. The laser was operating in a pulsed regime (10 ms pulse length, 10 Hz repetition rate). The maximum mean output power was 26.5 mW at 1645 nm. The constructed system demonstrated the tunability of a resonantly diode-pumped Er:YAG laser which could be useful in the development of compact diode-pumped lasers for spectroscopic applications.

Fe:ZnSe laser oscillation under cryogenic and room temperature

The goal of this work was to design and investigate a Fe:ZnSe laser operating at room and cryogenic (down to liquid nitrogen) temperature. Pumping was provided by a Q-switched Er:YAG laser at the wavelength of 2.94 μm, the output energy 15 mJ, pulse duration 120 ns, and the repetition rate 1 Hz. Q-switched operation was achieved by the Brewster angle cut LiNbO3 Pockels cell placed between the rear mirror and the Er:YAG laser active medium. The pump radiation was directed into the Fe:ZnSe crystal placed in the vacuum chamber cooled by liquid nitrogen. The resonator was formed by a dichroic pumping mirror (T = 78 % at 2.94 μm and R = 100 % at 4.5 μm), and a concave output coupler (R = 95 % at 4.5 μm, r = 500 mm). Fluorescence spectra and lifetime of the bulk Bridgman-grown Fe:ZnSe crystal in the range from room temperature down to liquid nitrogen temperature were measured as well as the output characteristics of the Fe:ZnSe laser. The shift of the generated spectral line maximum of ~ 400 nm towards the shorter wavelengths was found for the change of temperature from room to the liquid nitrogen. Also the increase of lifetime was measured from 300 ns at the room temperature up to 100 μs at the temperature of 130 K. Maximum of generated output radiation at 130 K was 150 μJ with the central emission wavelength of 4.1 μm. At the room temperature the central emission wavelength of 4.45 μm was measured with the spectral line-width of ~100 nm. The generated output energy was 1.3 mJ. The comparison of results obtained for Fe:ZnSe active material with the new bulk Fe,Cr:ZnMgSe crystal was also made. The results obtained for Fe:ZnSe active material were compared with the investigation of new bulk crystal Fe,Cr:ZnMgSe.

Resonantly pumped Er:YAG and Er:YAP lasers

Resonant pumping by a solid state Er:glass laser was successfully examined for Er:YAG and for the first time also for Er:YAP laser. The maximal incident pumping energy on the wavelength 1535 nm was 640 mJ with a repetition rate of 0.5 Hz; the corresponding pulse length was 1.9 ms (FWHM). The Er:glass laser radiation was focused into the active crystal by a CaF2 lens with 70 mm focal length. The measured beam diameter in focal plane was ~ 400 μm. The Er:YAG and Er:YAP rods had 10 mm in length and 5 mm in diameter. Various concentrations of Er3+ ions were used: 0.5 at.% for YAG and 1 at.% for YAP crystal. The resonator consisted of pumping and output dielectric mirrors. For both cases, the pumping dielectric mirror with high transmittance at pumping wavelength (T > 95 % @ 1532 nm) and maximal reflectance at the oscillating wavelength (around 1640 nm) was used. The output coupler reflectance was 85 % and 90 % for 1532 nm and 1640 nm, respectively. The advantage of resonantly pumped lasers is low thermal load corresponding to low quantum defect, and, therefore, it was not necessary to cool the active crystals. The output generated energy for the Er:YAG laser medium was 45 mJ at 1648 nm for 465 mJ incident pumping energy. For Er:YAP crystal the energy reached was 20 mJ at the lasing wavelength 1623 nm. The incident pumping was 640 mJ. For both resonantly pumped laser systems other characteristics i.e., spatial beam structure, divergence, and efficiency were investigated.