David Vyhlídal

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.

Subharmonic synchronously intracavity pumped picosecond optical parametric oscillator for intracavity phase interferometry

The laser system suitable for precise intracavity phase interferometry is presented. The system is based on an intracavity pumped PPLN linear optical parametrical oscillator (OPO). For synchronous pumping of OPO a SESAM-mode-locked, picosecond, diode-pumped Nd:YVO4 linear oscillator, operating at 1.06 µm was used. The OPO cavity was set to be twice as long as the pumping Nd:YVO4 laser cavity. The pumping laser was set in such a manner that the parametric gain inside the PPLN overcame the OPO threshold only for one direction of pumping pulse propagation. This leads to the generation of two independent trains of pulses at the 1.5 µm spectral range. To verify the system performance, a LiNbO3 electro-optic phase modulator was placed inside the OPO. The RF-signal derived from the pumping pulse train, detected by a fast photodiode and divided by two, was applied on the modulator. A stable beat-note signal between the two OPO trains was successfully measured for the first time from such a compact, all-diode-pumped laser system. For RF-signal amplitude from 100 up to 700 mV beat-note frequency varied from 232 up to 1847 Hz which corresponded to detected phase-shift 36–250 µrad. The bandwidth of beat-note was less than 1 Hz (FWHM) resulting in phase-shift measurement error 1.5 × 10−7 rad.

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.

2.6 W diode-pumped actively mode-locked Tm:YLF laser

We have experimentally demonstrated an actively mode-locked laser with a Tm3+-doped yttrium lithium fluoride crystal (YLF). A continuous mode-locked regime was achieved using an acousto-optic modulator and a stable train of pulses with 149.3 MHz repetition rate, 170 ps pulse width and 2.6 W average output power at 1.91 µm in a nearly diffraction-limited beam was obtained. To the best of our knowledge, this is the first report on a diode-pumped actively mode-locked bulk thulium laser with a stable output.

Gain-switched Fe:ZnMgSe laser oscillation under cryogenic temperature

The spectroscopic and laser properties of bulk Bridgman-grown Zn1-xMgxSe single crystals with the various concentrations of Mg (x=0.19; x=0.27; x=0.38) were investigated in the wide temperature range. The pumping was provided by a 2.94 μm Q-switched Er:YAG laser with a maximal energy of 15 mJ in 120 ns pulse, 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:ZnMgSe crystal placed inside the LN cooled dewar. The 55 mm long plane-concave cavity was formed by a dichroic pumping mirror (T = 92 % @ 2.94 μm and R = 100 % @ 4 - 5 μm) and a output coupler (R = 95 % @ 4.5 μm, r = 200 mm). The strong dependence of output pulse energy on temperature was observed for all samples. The maximum output Fe,Cr:Zn1- xMgxSe laser energy was 230 μJ and 180 μJ (for Mg concentration x=0.19 and x=0.38, respectively) for gain switched operation at 88 K. The central emission wavelength of ~ 4600 nm, ~ 4700 nm and ~ 4800 nm for Mg concentration x=0.19; x=0.27, and x=0.37, respectively at 88 K was obtained. The emission wavelength was found to increase up to ~ 4700 nm and ~ 4900 nm at 250 K for Mg concentration x=0.19 and x=0.38, respectively. This results show the possibility to obtain sufficiently longer oscillation wavelengths compared to previously studied Fe:ZnSe active medium especially at liquid nitrogen temperatures when pumping by free-running Er:YAG laser becomes possible. Fluorescence spectra and lifetimes of Fe2+ ions in different Zn1-xMgxSe crystals in the range from 250 K down to 80 K were also measured.

Fe:ZnMnSe laser active material properties at room and cryogenic temperature

Fe:Zn(1-x)Mn(x)Se solid solution spectroscopic and laser properties were investigated in the temperature range 80- 290 K. Two novel samples with different zinc - manganese (Zn–Mn) ratio described by the Mn content x (0.1 or 0.2) were used and the results were compared to the known Fe:ZnSe crystal. The samples had a broad absorption spectra with the maximum around 3 μm and therefore an Er:YAG laser (2.94 μm, 10 mJ, 120 ns) was used as a pump radiation source. The Fe:ZnMnSe fluorescence spectra are generally broad in the range 3.5 – 5.5 μm. In the case of Fe:ZnMnSe x = 0.1, the fluorescence spectrum at 290 K is ranging from 3.5 to 5.5 μm. Lowering the temperature down to 80 K lead to the spectral narrowing mainly in the mid-IR part, but the fluorescence is still up to 5 μm at 80 K. In the case of Fe:ZnMnSe x = 0.2 the fluorescence is shifted towards mid-IR up to 5.2 μm even at 80 K. The fluorescence lifetime decreases from tens of us at 80 K down to 1 us at 240 K. The laser oscillations were successfully achieved with both novel Fe:ZnMnSe crystals in the temperature range 80- 290 K. In the case of x = 0.1, the central wavelength was ~4.2 μm at 80 K and the temperature increase up to 290 K led to almost linear increase of the wavelength up to ~4.75 μm. The tendency was similar in the case of Fe:ZnMnSe x = 0.2: the output wavelength increased from ~4.3 μm up to ~4.8 μm with the temperature increase from 80 to 290 K. The laser spectral linewidth was about 300 nm. In comparison with the Fe:ZnSe crystal, the laser output wavelength shift toward mid-IR region without any spectrally tunable element in the laser cavity can be clearly observed.

Time-to-Digital Converter With 2.1-ps RMS Single-Shot Precision and Subpicosecond Long-Term and Temperature Stability

This paper presents the design and performance measurement results of a prototype time-to-digital converter (TDC) implemented using commercially available components. The TDC is based on an interpolation principle. In this principle, the time interval is first roughly digitized by a coarse counter driven by a high-stability reference clock, and the fractions between the clock periods are measured by two high-resolution interpolators based on the time-to-amplitude conversion method. This principle ensures theoretically unlimited measurement range with fine time resolution. The TDC achieves 2.1-ps rms time interval measurement precision with a resolution of 0.17 ps and the maximum measurement rate of 200 Hz. The estimated single-channel precision is 1.5-ps rms. A unique circuit structure was developed to allow individual channel calibration after each event. During the calibration, a value corresponding to one reference clock period is obtained. This gives the TDC an outstanding long-term stability of 110 fs peak-peak and a long-term drift of 0.2 fs/h over a 24-h interval. The calibration also helps to obtain a high temperature stability of 0.2 ps/°C. The power consumption is 10 W. The circuit structure allows for programmable customization of the TDC to satisfy many potential applications in time-of-flight systems, such as laser range finding or event counting.

Optimization of passively mode-locked Nd:GdVO4 laser with the selectable pulse duration 15-70 ps

In this paper the optimization of a continuously diode-pumped Nd:GdVO4 laser oscillator in bounce geometry passively mode-locked using semiconductor saturable absorber mirror is presented. In the previous results the Nd:GdVO4 laser system generating 30 ps pulses with the average output power of 6.9 W at the repetition rate of 200 MHz at the wavelength of 1063 nm was reported. Now we are demonstrating up to three times increase of peak power due to the optimization of mode-matching in the laser resonator. Depending on the oscillator configuration we obtained the stable continuously mode-locked operation with pulses having selectable duration from 15 ps to 70 ps with the average output power of 7 W and the repetition rate of 150 MHz.

Femtosecond diode-pumped mode-locked neodymium lasers

Fluoride-type crystals (CaF2, SrF2) doped with neodymium Nd3+ and codoped with buffer ions for breaking clusters of active ions and increasing fluorescence efficiency, present interesting alternative as laser active media for the diode-pumped mode-locked lasers. In comparison with widely used materials as Nd:YAG or Nd:YVO4, they have broad emission spectra as well as longer fluorescence lifetime, in comparison with Nd:glass, SrF2 and CaF2 have better thermal conductivity. In spite of the fact, that this thermal conductivity decreases with Nd3+ doping concentration, these crystals are alternative for the Nd:glass in subpicosecond mode-locked laser systems. In this paper we review the basic results reported recently on these active materials and in the second part we present our results achieved in low power diode pumped passively mode locked lasers with Nd,La:CaF2 and Nd,Y:SrF2 crystals. The pulses as short as 258 fs at wavelength of 1057 nm were obtained in the first case, while 5 ps long pulses at 1065 nm were generated from the second laser system.