Q-switched Mode Research Articles

Comparative analysis of damageability produced by powerful pulsed ion/plasma streams and laser radiation on the plasma-facing W samples

Parallel use of directed plasma/fast ion streams from Dense Plasma Focus and of laser radiation (in a free running and Q-switched modes) for irradiations of targets looks perspective for tests of materials designed to withstand extreme thermal loads in the mainstream fusion reactors with inertial and magnetic plasma confinement. It is so because of a wide range of power flux densities and pulse durations generated by them. Tungsten is counted as the most appropriate material for the plasma-facing components. Main features of degradation of the surface layer of double forged tungsten were determined at different conditions of its irradiation by these streams. Chief similarities and differences in damageability obtained in the regimes are fixed – blisters, craters, porosity, microcracks, and discontinuity flaws etc. – that are peculiar characteristics for each mode of irradiation. The observed resemblances and dissimilarities that are specific for each regime of irradiation are explained with the help of numerical modelling of the treatment processes. It is shown that a so-called Integral Damage Factor may be used only in restricted ranges of parameters of an irradiation. It was found that in the regime of irradiation with the well-developed gas dynamic motion of secondary plasma the overall energy of radiation will be spent preferentially either on mass removal from the material's surface or on heating of a small amount of matter to high temperature (and consequently into its fast movement) depending on power flux density of radiation and its pulse structure.

Investigation of regime switching from mode locking to Q-switching in a 2 µm InGaSb/AlGaAsSb quantum well laser.

A two-section InGaSb/AlGaAsSb single quantum well (SQW) laser emitting at 2 μm is presented. By varying the absorber bias voltage with a fixed gain current at 130 mA, passive mode locking at ~18.40 GHz, Q-switched mode locking, and passive Q-switching are observed in this laser. In the Q-switched mode locking regimes, the Q-switched RF signal and mode locked RF signal coexist, and the Q-switched lasing and mode-locked lasing happen at different wavelengths. This is the first observation of these three pulsed working regimes in a GaSb-based diode laser. An analysis of the regime switching mechanism is given based on the interplay between the gain saturation and the saturable absorption.

Validity Analysis of Holmium-Doped Fiber as Saturable Absorber for Passively Q-Switched Thulium-Doped Fiber Laser

We propose and demonstrate a method for measuring nonlinear transmission (NLT) of holmium-doped fiber (HDF) as saturable absorber (SA) by using two calibrated photodiodes. The HDF with a length of 10 cm is under test as reference, and the NLT measurement is carried out from 1930 to 1970 nm by using a tunable continuous-wave (CW) thulium-doped fiber laser (TDFL). The result finds that modulation depth of HDF is 56% at 1950 nm and decreases down to 37% at both 1930 and 1970 nm. Saturation power is about 20 mW for all measured wavelengths. The validity of the HDF SA for a passively Q-switched TDFL is also studied by taking the measured parameters into account. The laser operates in Q-switched mode from 1944 to 1960 nm, whereas in CW mode beyond the wavelength range due to insufficient modulation depth. The pulse temporal characteristics are also discussed.

Superluminescent high-efficient parametric generation in PPLN crystal with pumping by a Q-switched mode locked Nd:YAG laser

We present results on parametric superluminescence in a periodically poled lithium niobate crystal pumped by a train of 45 ps pulses using a Q-switched mode locked Nd:YAG laser. The conversion efficiency (with respect to the absorbed power) was ~83%. To the best of our knowledge, this is the highest efficiency obtained with powerful superluminescent parametric sources. At the average pumping power of the laser of ~0.5 W and repetition rates of 1 and 1.7 kHz, the peak total output powers were as high as 210 and 200 kW, and the powers of the idler wavelength (3.82 µm) were 55 and 50 kW. New lines in the visible and UV spectrum were observed and are explained. The experiments demonstrated that the spectral and angular characteristics of superluminescence are determined by the pumping laser. In particular, the line width of the signal wave was close to that of the pumping line at ~200 GHz, and the divergence of the signal and idler waves depended only on the convergence (divergence) angle of the pumping radiation (30 mrad) and was independent of the wavelength.

Sum frequency generation under conversion of Q-switched cryogenic slab RF discharge CO laser radiation in ZnGeP2

For the first time broadband sum frequency generation in ZnGeP2 crystal has been accomplished with a compact cryogenic slab RF discharge CO laser. The spectrum of the CO laser operating in a repetitively pulsed multiline Q-switching mode consisted of ~90 emission lines in the wavelength range of 4.95–6.60 µm. Internal sum frequency generation efficiency was reached up to 8.2%. The resulted spectrum contained ~200 spectral lines in the wavelength range of 2.5–3.2 µm.

Control of Q-switched laser radiation characteristics via changing the gain distribution

The advantages of simultaneous side and end diode pumping of Nd:YAG laser to enhance the spatio-temporal characteristics in the Q-switched mode of operation has been demonstrated. It has been shown that using a hybrid pump geometry in a short linear resonator provides a superior combination of output beam optical quality, energy, and pulse duration in contrast to the solitary use of an end-pump or a side-pump scheme at similar levels of average output power. We have demonstrated a compact single active rod Nd:YAG laser design in Q-switching mode with a pulse duration of 18 ns, pulse energy up to 3mJ, a repetition rate of 8 kHz, and М2 <2.

Exploring the self-mode locking of the 2 μm Tm:YAG laser with suppression of the self-pulsing dynamic.

A continuous-wave (cw) self-mode-locked Tm:YAG laser at 2015nm is successfully demonstrated by suppressing the self-pulsing behavior. By using rate equations to simulate the laser temporal dynamic, the theoretical analysis indicates that the reabsorption-induced self-pulsing for the Tm:YAG laser can be suppressed with sufficient pump power. It is experimentally confirmed that at absorbed powers higher than 6W, the self-pulsing can be eliminated and a cw self-mode-locked pulse is generated instead of the Q-switched mode locking. At an absorbed power of 6.8W, the output power of the self-mode-locked Tm:YAG laser reaches 1.22W with corresponding pulse duration of 3ps and a repetition rate of 3.376GHz.

Spectral characteristics of Q-switched CO laser

The spectrum of low-pressure cryogenic CO laser operated in Q-switched mode was measured at frequencies of 70 Hz and 120 Hz. CO laser spectrum consisted of 115 spectral lines in the wavelength range from 4.89 μm to 6.54 μm at 70 Hz, and 102 lines in the wavelength range from 4.90 μm to 6.44 μm at 120 Hz. For the first time the dynamics of generation on each spectral line for full CO laser spectrum were measured. We identified rotational-vibrational transitions and analyzed the processes, which affect the formation of the CO laser spectrum. This data were taken into account to determine the peak powers of all the spectral lines correctly.

Collective acceleration of laser plasma in a nonstationary and nonuniform magnetic field

This paper presents the new experimental results concerning acceleration of deuterium ions extracted from laser plasma in the rapid-growing nonuniform magnetic field in order to initiate the nuclear reactions D(d, n)3He and T(d, n)4He. For obtaining of laser plasma a Nd: YAG laser (λ = 1,06 μm) that generates in Q-switched mode the radiation pulses with the energy W ≤ 0,85 J and duration of τ ≈ 10 ns was used. Rapid-growing magnetic field was created with the discharge of Arkadyev-Marx pulsed-voltage generator to conical coil with the inductance of 0,65 μΗ. At characteristic discharge time of 30 ns, the rate of magnetic field growth achieved 2·107 T/s. Ion velocity was determined with the time-of-flight technique. During the experiment on deuterium plasma an ion flux velocity of ∼3 · 108 cm/s was obtained, which corresponds to the deuteron energy of ∼100 keV. Herewith, for target power density of ∼5·1011 W/cm2 obtaining of up to 1015 of accelerated deuterons and up to 108 of neutrons per a pulse is expected.

Systematic analysis on linear and nonlinear optical traits of citrulline doped NH4H2PO4 (ADP) crystal

The current investigation is aimed to explore the optical properties of citrulline doped ammonium dihydrogen phosphate (NH4H2PO4,ADP) crystal by employing crystal growth, structural, UV–vis, Kurtz-Perry test and Z-scan characterization techniques. The growth of citrulline doped ADP single crystal has been achieved by slow solvent evaporation technique at ambient temperature. The pure and citrulline doped ADP crystals have been characterized by single crystal X-ray diffraction technique for structural analysis. The functional groups of grown crystal have been qualitatively identified by means of Fourier transform infrared analysis. The influence of citrulline on optical transmittance of ADP crystal has been evaluated within 200–900nm by means of UV–vis spectral analysis. The enhanced second harmonic generation (SHG) efficiency of citrulline doped ADP crystal has been determined using the Q-switched mode Nd:YAG laser and it is found to be 2.18 times higher than standard KDP crystal. The third order nonlinear optical (TONLO) behavior of citrulline doped ADP crystal has been investigated using He-Ne laser operating at 632.8nm. The close and open aperture Z-scan configuration has been used to evaluate the nature of TONLO refraction and absorption. The magnitude of TONLO parameters of citrulline doped ADP crystal has been determined using the Z-scan transmittance data. The potential liability of citrulline doped ADP crystal for optical applications has been discussed.

High-peak-power large-angular-momentum beams generated from passively Q-switched geometric modes with astigmatic transformation

The criterion of achieving efficient passive Q-switching is analyzed to design an off-axis pumped Nd:YVO4/Cr4+:YAG laser with a degenerate cavity. Experimental results reveal that pure high-order HG0,m or HGm,0 eigenmodes with the order m between 0 and 14 can be generated, depending on the off-axis displacement along the y axis or the x axis. On the other hand, lasing modes naturally turn into planar geometric modes when the off-axis displacement is larger than the value for exciting the HG0,m or HGm,0 eigenmodes with m>14. The overall peak powers for high-order eigenmodes or geometric modes can exceed 140 W. Furthermore, the high-order eigenmodes and geometric modes are employed to generate vortex beams with large orbital angular momentum by using an external cylindrical mode converter. Theoretical analyses are performed to confirm experimental results and to manifest the phase structures of the generated vortex beams.

Voltage-on-Type RTP Pockels Cell for Q-switch of an Er:YAG Laser at 1,617 nm

We report a resonantly diode-pumped electro-optic Q-switched Er:YAG laser operating at 1,617 nm using a voltage-on-type rubidium titanyl phosphate (RTP) Pockels cell as the modulator. The Er:YAG laser operates at a very stable Q-switching mode with a per pulse energy yield of 1.5 mJ and a pulse duration of 114 ns at 1 kHz PRF under an incident pump power of 21.6 W.

A diode-pumped Tm:CaYAlO4 laser at 1851 nm

Laser emission at ~1850 nm is of great interest for neural stimulation applications. In this letter, we report on the diode-pumped continuous-wave (CW) and Q-switched (QS) laser operation of Tm:CaYAlO4 at 1851 nm, for the first time to our knowledge. In the CW regime, a maximum output power up to 0.62 W is obtained with a laser slope efficiency of about 18.0%. Using a Cr:ZnSe saturable absorber, QS laser operation is achieved with a maximum average output power of 0.25 W, the narrowest pulse width of 107 ns and the highest repetition rate of 5.85 kHz. The corresponding pulse peak power and pulse energy are about 388 W and 42.8 µJ, respectively. In this Q-switched mode, wavelength tuning is also realized over about 3 nm by slightly tilting the saturable absorber.

Passively Q-Switched Tm:CaYAlO4 Laser Using a MoS2 Saturable Absorber

We report here for the first time on the diode-pumped continuous-wave laser operation of an a-cut Tm:CaYAlO4 (CYAO) crystal in a two-mirror laser configuration at about 1850 nm, which is the shortest laser wavelength ever achieved with this material, with a maximum output power of 0.88 W. Using a MoS2 saturable absorber deposited directly onto the surface of the output coupler, the Tm:CYAO laser is then operated in the Q-switched mode with a maximum output power of 0.49 W and the narrowest pulse width of $0.48~\mu \text{s}$ .

Multi-operation laser oscillator: an example of multi-operation laser

A multi-operation laser oscillator is developed and built with multiple operation modes (OMs): injectionseeding mode, cavity-dumping mode and Q-switching mode.With the same electrical energy pumping, the multioperation laser oscillator provides different output energies and pulse durations for different OMs. In the Q-switching mode, the output coupling is optimized for different electrical energy pumping. The laser oscillator operation can be switched between different modes conveniently. The multi-operation laser sources could be operated in multiple OMs for various research and application requirements.

Compact and Flexible Dual-Wavelength Laser Generation in Coaxial Diode-End-Pumped Configuration

A novel coaxial diode-end-pumping configuration with combined two laser crystals was proposed for simultaneous, compact, and flexible dual-wavelength laser generation. Theoretical simulations showed that by balancing the gain in both laser crystals by varying the pump focusing depth or pump wavelength, the power ratio for each wavelength could be tuned for either continuous wave or Q-switched operation, and the time interval between two pulses at different wavelengths in Q-switched mode was also tunable. Experimental verifications were performed, demonstrating coincident conclusions. As there was no gain competition between two wavelengths, the output characteristics were much more stable than dual-wavelength generation in a single-laser crystal. It is believed that this is a feasible and promising method for generating dual-wavelength laser for applications of spectroscopy, precision measurement, nonlinear optical frequency conversion to terahertz wave, and so on.

Mode-Hopping-Free Single-Longitudinal-Mode Actively Q-Switched Ring Cavity Fiber Laser With an Injection Seeding Technique

With a fiber pigtailed electro-optic modulator as the active Q-switch, mode-hopping-free single-longitudinal-mode pulses are generated by a ring cavity fiber laser based on the injection seeding technique. The experimental results show that the single-longitudinal-mode oscillating properties of the laser may be determined by the power and the center frequency of the injection seed rather than the Q-switch waveform and modulation frequency. With the injected seed power of 200 μW, single-longitudinal-mode Q-switched pulses with the repetition rate and the pulse width from 1 to 100 kHz, and 50 to 400 ns are achieved with the laser. The output pulse linewidth can be narrowed effectively by reducing the coupling ratio of the output coupler of the laser. A stable single-longitudinal-mode pulse stream with a linewidth of 1.5 MHz is obtained with our laser, which may be the narrowest linewidth for a Q-switched fiber laser so far to our best knowledge.

Passively Q-switched mode-locked Tm, Ho:LLF laser with a WS2 saturable absorber

Using few-layer tungsten disulfide (WS2) doped polyvinyl alcohol as a saturable absorber for the initiation of the pulse generation, we experimentally demonstrate stable passively Q-switched mode-locked operations of Tm, Ho:LiLuF4 laser at 1895 nm for the first time. The laser is designed with an X-type four-mirror cavity and pumped by a Ti:sapphire laser operated at 785 nm, and its continuous operation is initiated when the absorbed pump power is 143 mW. When the absorbed pump power reaches 2.645 W, we obtain a maximum output power of 985 mW and a crystal slope efficiency of 39.8% by linear fitting. When the saturable absorber WS2 is inserted in the cavity, the threshold of the absorbed pump power is increased to 234 mW. With the increase of the pump power, Q-switch pulse sequence is first observed. When the absorbed pump power reaches 1.39 W, the stable operation of the Q-switched mode locked pulse is realized. A maximum average output power of 156 mW is achieved at an absorbed pump power of 2.6 W, which corresponds to a 25 kHz Q-switched repetition rate and a 300 μs-long pulse envelope. In this case, the modulation depth in Q-switching envelopes is close to 100%. After the passively Q-switched mode-locked is obtained stably, the mode-locked pulses inside the Q-switched pulse envelope have a repetition rate of 131.6 MHz, corresponding to a mode locked pulse energy of 1.19 nJ and a cavity length of 1.14 m. According to the definition of the rise time and considering the symmetric shape of the mode locked pulse, we can assume that the duration of the pulse is approximately 1.25 times more than the rise time of the pulse. Then the width of the mode locked pulse is estimated to be about 878 ps. These experimental results show that WS2 is a promising broadband saturable absorption material for generating a 2 μm-wavelength mid-infrared solid-state laser pulse. By increasing the pump power and reducing the loss of WS2 material, it is possible to realize a continuous mode locking operation which has a narrower pulse duration. The mode-locked mid-infrared pulses are very stable and have a lot of potential applications such as ultrafast molecule spectroscopy, mid-IR pulse generation, laser radar, atmospheric environment monitoring, etc.

The performance of a novel Ho:LuAG ceramic laser Q-switched by a polycrystalline Cr2+:ZnS saturable absorber

A novel Ho:LuAG ceramic laser Q-switched by a polycrystalline Cr2+:ZnS saturable absorber was reported for the first time in this paper. We took a diode-pumped Tm:YLF laser emitting at 1.9075 μm as the pump source. The laser operated in both continuous wave mode and passively Q-switching (PQS) mode. The maximum PQS output power of 2.67 W was obtained with a slope efficiency of 26.4%. When the absorbed pump power increased from 4.78 to 10.8 W, with three output couplers of T = 2%, T = 10% and T = 25%, the pulse widths decreased as the pump power increased, from 102.9 to 89.2 ns, from 147.1 to 127.6 ns, and from 173 to 150 ns, respectively, and the repetition frequency varied from 10.2 to 20.1 kHz, from 9.3 to 18.3 kHz, and from 8.45 to 16.66 kHz as well. The central wavelength remained constant 2100.64 nm with the change of output couplers and operation modes. Furthermore, the output laser had a beam quality factor M 2 of 1.1.

Integration of helicity-control and pulse-modulation for vortex laser based on a black phosphorus plate.

Using a home-made black phosphorus plate (BPP) as handedness controller and Q-switch modulator synchronously, a ~1.6 µm pulsed vortex laser with well-determined handedness is demonstrated in this letter. Stable vortex pulses of LG0, + 1, LG0,-1, LG0, + 2 and LG0,-2 modes were respectively achieved from compact resonant cavities in this experiment. Such pulsed vortex laser should have promising applications in various fields based on its simple structure, controllable handedness, and carried orbital angular momentum.