Picosecond Diode-Pumped Laser System with 9.3-W Average Power and 2.3-mJ Pulse Energy

We describe a diode-pumped Nd:YAG laser that is suitable for micromachining applications and is capable of generating 2.3-mJ pulses at a 4-kHz pulse repetition frequency. The output pulse duration is 20.5 ps. The system is based on a Nd:YAG regenerative amplifier with a novel four-pass postamplifier. The postamplifier incorporates birefringent depolarization compensation and simultaneously prevents parasitic laser oscillation by use of a nonreciprocal beam path. These output pulse energies are achieved without the use of chirped pulse amplification.     

Dependence of gold nanoparticle production on pulse duration by laser ablation in liquid media

The dependence on laser fluence and laser pulse duration of size, size distribution and concentration of gold nanoparticles synthesized by laser ablation in liquid media was investigated. It was demonstrated that increasing laser energy from 1 to 5 mJ/pulse enhances the ablation rate by a factor of 100. The behavior of the ablation rate, hence of the nanoparticle concentration, as a function of pulse duration (varied from 40 fs to 200 ps) was found to strongly differ from that in air, which can be explained by photoionization and important losses of laser energy in the femtosecond regime. The optimal pulse duration for maximum ablation rate in liquid media was found to be equal to 2 ps.     

Periodic pulse oscillation in an intracavity-doubled Nd:YVO(4) laser

We report periodic pulse oscillation in an intracavity-doubled Nd:YVO(4) laser. The pulse oscillation is caused by alternate oscillation between two modes. We investigated improvement of green-light power with the pulse oscillation in a Nd:YVO(4) laser. The pulse oscillation gave a factor of approximately 2 higher average green-light power than that for cw oscillation at the same pumping power.     

Intracavity pumped nanosecond optical parametric oscillator emitting in the eye-safe range

We report on a 1.57-mum wavelength singly resonant optical parametric oscillator (OPO) using a noncritically phase-matched KTP crystal intracavity pumped by an electro-optically Q-switched Nd:YAG laser. A simple model for numerical calculation of the temporal profile of the laser and signal pulses developed inside the intracavity pumped OPO (IPOPO) resonator is described. Working conditions for single-pulse and multipulse parametric oscillation are established. A single signal pulse of 8.1-mJ output energy was obtained when the laser was pumped three times above the pulsed IPOPO threshold. A pulse duration of 5.9 ns was measured at 1.5 times above threshold for the IPOPO.     

Stable room-temperature LiF:F2 (+*) tunable color-center laser for the 830-1060-nm spectral range pumped by second-harmonic radiation from a neodymium laser

Simultaneous photostability and thermostability of a room-temperature LiF:F2(+*) tunable color-center laser, with an operating range over 830-1060 nm, pumped by second-harmonic radiation of a YAG:Nd(3+) laser with a 532-nm wavelength has been achieved. The main lasing characteristics of the obtained LiF:F2 (+*) laser have been measured. Twenty-five percent real efficiency in a nonselective resonator cavity and 15% real efficiency in a selective resonator cavity have been obtained. The stable LiF:F2 (+*) laser operates at a 1-100-Hz pulse-repetition rate with a 15-ns pulse duration, a 1-1.5-cm(-1) narrow-band oscillation bandwidth, and divergency of better than 6 × 10(-4). Doubling the fundamental frequencies of F2(+*) oscillation made it possible to obtain stable blue-green tunable radiation over the 415-530-nm range.     

Multiline, superbroadband and sun-color oscillation of a LiF:F(2)(-) color-center laser

We report experimental and theoretical studies of the temporal, spectral, and spatial features of a superbroadband laser. The results obtained show that the superbroadband room-temperature operable LiF:F(2)(-) color-center laser can provide low-coherence, high-intensity laser radiation with a spectral width of 1400 A centered at 1.14 mum and 700 A in the visible range from green to red and exhibit good spatial collimation with a divergence of between 5 and 6 mrad. Oscillation of all the lines of a superbroadband spectrum is completely synchronous and occurs almost simultaneously with the pump pulse, exhibiting 4-9-ns pulse delay at 20-ns pump pulse duration. Second-harmonic generation of superbroadband oscillation spectrum was realized with an overall efficiency of 10%.     

Generation of 6.6-microm optical parametric oscillation with periodically poled LiNbO3

Greater than 6-mum-oscillation was demonstrated by means of optical parametric oscillation with periodically poled LiNbO(3) (PPLN). The interaction length and thickness were 40 mm and 500 mum. The pump source used was a Q-switched Nd:YAG laser with a pulse duration of 120 ns and a repetition rate of 1 kHz. The tuning ranges of the idler waves were 6.576.56, 6.226.12, and 6.065.94 mum for PPLN wafers of 20-, 21.3-, and 22-mum periods, respectively.     

Bandwidth dependence of pulse duration in intra-cavity frequency-doubled lasers: theory and experiment

A model is presented of pulse evolution in broadband intra-cavity frequency-doubled lasers. The model utilizes normalized coupled rate equations for each mode, including terms that represent the loss due to nonlinear mixing between longitudinal modes. The pulse energy, shape, peak power and duration are calculated by numerical solution of these equations. The model shows that the pulse duration depends not only on the initial population inversion, photon lifetime and the effective nonlinear coupling coefficient, as is the case for narrowband lasers, but also on the fundamental bandwidth. A gain-switched Ti:sapphire laser, pumped by a Q-switched Nd-doped yttrium aluminium garnet laser at 532?nm, was frequency doubled using an intra-cavity β-barium borate crystal. The bandwidth was reduced from about 25?nm to about 1.5?nm in two steps using a series of prisms, and the resulting changes in experimental pulse durations and energies agree well with the model.     

Low-repetition-rate, high-energy, twin-pulse, passively mode locked Yb3+-doped fiber laser

We report an all-normal-dispersion, low-repetition-rate, high-energy, twin-pulse, passively mode locked ytterbium-doped fiber laser. The mode-locking mechanism of the laser is based on nonlinear polarization evolution and strong pulse shaping with a cascade long-period fiber grating bandpass filtering in a highly chirped pulse. The laser generates a highly stable twin-pulse group with 248?ps and 296?ps duration simultaneously and maximum pulse energy of 26.8?nJ-each pulse at a 2.5445?MHz repetition rate. Energy quantization is observed, which demonstrates the nonparabolic nature of these pulses. The laser can also work in third-harmonic mode locking with 17.8?nJ energy (at a repetition rate of 7.65?MHz and pulse width of 780?ps).     

Influence of the laser pulse duration on spectrochemical analysis of solids by laser-induced plasma spectroscopy

Quantitative analysis of aluminum and copper alloys by means of laser-induced plasma spectroscopy (LIPS) has been investigated for three representative laser pulse durations (80 fs, 2 ps, and 270 ps). The experiments were carried out in air at atmospheric pressure with a constant energy density of 20 J/cm2. Because the decay rate of the spectral emission depends on the laser pulse duration, the optimum detection requires an optimization of the temporal gating acquisition parameters. LIPS calibration (sensitivity and nonlinearity) and the limit of detection (LOD) are discussed in detail. While the LOD of minor elements embedded in alloy samples obtained by sub-picosecond or sub-nanosecond laser pulses are both time and element dependent, provided an appropriate temporal window is chosen, the optimum LODs (several parts per million (ppm)) prove to be independent of the laser pulse duration. Finally, it is found that for elements such as those detected here, gated LIPS spectra using picosecond or sub-picosecond laser pulses provide much better LOD values than non-gated spectra.     

Passively Q-switched Yb3+:YCa4O(BO3)3 laser with InGaAs quantum wells as saturable absorbers

A diode-pumped Yb:YCOB laser at 1086 nm is passively Q switched by using InGaAs quantum wells as saturable absorbers and utilizing the Bragg mirror structure as an output coupler. With an absorbed pump power of 9.2 W the laser produces pulses of 100 ms duration with average pulse energy of as much as 165 microJ at a pulse repetition rate of 7 kHz.     

Experimental investigation of high-energy wave-breaking-free-pulse generation in bidirectional-pumping all-fiber laser

We report a passively mode-locked all-fiber laser with bidirectional pumping, emitting high-energy wave-breaking-free pulses. Experimental investigations show that forward and backward pump powers mainly contribute on the chirp and the nonlinear phase shift of pulses, respectively. Nonlinear chirp pulses with 50 nJ pulse energy, 340 ps duration at a 8.2 MHz repetition rate are directly emitted from the all-fiber laser pumped by two 550 mW, 977 nm laser diodes. The pulses cannot be compressed to near the transform limit, showing that the chirp of pulses is nonlinear.     

A long cavity passive mode-locking fibre laser with the reflective non-linear optical loop mirror

In this paper, we report a long cavity passively mode-locked fibre laser. The proposed mode locker is a reflective long cavity non-linear optical loop mirror (NOLM) which consists of a 50:50 coupler and 2-km single-mode fibres. The laser achieves stable mode locking at a fundamental repetition rate of 100 kHz. The rectangular pulses operating in dissipative soliton resonance region is generated in the laser. The relationship between the pulse duration and the pump power is investigated in detail. When the pump power is 200 mW, the laser generates rectangular pulses at 1565.57 nm (central wavelength) with pulse duration of 81.5 ns. The single pulse energy as high as 33.34 nJ is obtained. The results show that the reflective NOLM is an efficient mode locker and useful for the generation of high energy pulse.     

Optimisation of GaAs nanocrystals synthesis by laser ablation in water

Laser ablation of a gallium arsenide (GaAs) wafer immersed in distilled water was carried out using the fundamental wavelength of a high frequency Nd:YAG laser with 240?ns pulse duration. Rate of nanoparticles generation through laser ablation for various amounts of laser pulse energies (0.4–0.94?mJ) and repetition rates (400–2000?Hz) were studied and a maximum ablation rate of 19.6?µgr/s was obtained. Formation of the pure GaAs nanocrystals (NCs) is confirmed using TEM micrograph and X-ray diffraction analysis. Band-gap energy of generated GaAs NCs is calculated by Tauc method to be between 2.48 and 2.60?eV which is larger than the band-gap energy of bulk GaAs. The band-gap energy of NCs is increased by increasing the energy of laser pulses and is decreased by increasing the pulse repetition rate.     

Statistics of intensity of backscattered semiconductor laser radiation in single-mode optic fiber

We have studied the statistics of the intensity of backscattered radiation of a semiconductor laser in single-mode optic fiber as a function of the probing laser pulse duration and coherence time. It is shown that, at a given radiation source coherence time, the intensity distribution function varies, depending on the pulse duration, from nearly exponential to nearly Gaussian. The exponential statistics provides a better sensitivity for a coherent reflectometer with direct detection. Using the calculated distribution function, it is possible to qualitatively determine the degree of deterioration of the reflectometer sensitivity with respect to external factors during increasing probing pulse duration or decreasing laser coherence time. These data provide criteria for the optimum choice of a radiation source for the coherent fiber-optic reflectometer.     

Efficient 550-600-nm tunable laser based on noncritically phase-matched frequency doubling of room-temperature LiF:F2- in lithium triborate

We report on efficient conversion of a 1064-nm Nd:YAG laser to tunable visible light. The conversion scheme uses noncritically phase-matched second-harmonic generation of a pulsed Nd:YAG-pumped LiF:F2- laser in lithium triborate. Optimization yields 42% LiF:F2- laser efficiency and 56% frequency-doubling efficiency, providing >20% conversion from 1064 nm to broadly tunable visible output. A dramatic sensitivity of laser efficiency to pump pulse duration is recorded, with a fundamental efficiency improvement of over 30-fold measured for an increase in pump pulse duration from 5 to 34 ns.     

A CuBr laser with high efficiency in the double-pumping-pulse mode

The results of examination of characteristics of a CuBr laser operated in the double-pumping-pulse mode with a lasing pulse repetition rate of 50 Hz are presented. The potential to enhance the laser efficiency by choosing the optimum voltages of dissociation and excitation pulses and time delay values is discussed. It is confirmed that a more than twofold increase in the laser efficiency may be achieved through optimization and matched injection of the excitation-pulse energy into the active medium plasma. A laser efficiency as high as 2.6% (with respect to an excitation pulse) with an average power of 16 mW, an energy of 0.32 mJ, a halfamplitude laser pulse duration of 40 ns, and a time delay of 150 μs is demonstrated. The maximum average radiation power is 37 mW with an energy of 0.7 mJ and a laser efficiency of 0.7% at an energy density of 24 (2.7) mJ/cm³ of a dissociation (excitation) pumping pulse.     

Laser-diode-pumped passively Q-switched Nd:YVO4 green laser with periodically poled KTP and GaAs saturable absorber

A laser-diode-pumped passively Q-switched Nd:YVO₄ green laser with periodically poled KTP (PPKTP) and GaAs saturable absorber has been realized. The dependences of pulse repetition rate, pulse energy, pulse width and peak power on incident pump power for the generated-green-light pulses are measured. At the maximum incident pump power of 4.1 W, the maximum average output power of 113 mW is obtained, corresponding to an optical conversion efficiency of 2.8%. At the same pump power, stable green laser pulses of duration of 44.6 ns and energy of 0.28 µJ are generated at a repetition rate of 403.4 kHz. The coupling wave rate equations for a passively Q-switched laser are also given and the numerical solutions agree with the experimental results.     

An analytical modeling of heat transfer for laser-assisted nanoimprinting processes

Laser-assisted direct imprinting (LADI) technique has been proposed to utilize an excimer laser to irradiate and heat up the substrate surface through a highly-transparent quartz mold preloaded on this substrate for micro- to nano-scaled structure fabrications. While the melting depth and molten duration are key issues to achieve a satisfactory imprinting pattern transfer, many material property issues such as crystalline phase alteration, grain size change and induced film stress variation are strongly affected by transient thermal response. With one-dimensional simplification as a model for the LADI technique, the present paper has successfully derived an analytical solution for the arbitrary laser pulse distribution to predict the relevant imprinting parameters during the laser induced melting and solidification processes. The analytical results agree quite well with the experimental data in the literature and hence can be employed to further investigate the effects of LADI technique from laser characteristics (wavelength, fluence and pulse duration) and substrate materials (silicon and copper) on the molten duration, molten depth and temperature distributions. Three kinds of excimer laser sources, ArF (193 nm), KrF (248 nm) and XeCl (308 nm) were investigated in this study. For the silicon substrate, the melting duration and depth were significantly dictated by the wavelength of laser used, indicating that employing the XeCl excimer laser with longer pulse duration (30 ns in the present study) will achieve the longest molten duration and deepest melting depth. As for the copper substrate, the melting duration and depth are mainly affected by the laser pulse duration; however, the wavelength of laser still plays an insignificant role in LADI processing. Meanwhile, the laser fluence should properly be chosen, less than 1.4 J/cm² herein, so as to avoid the substrate temperature exceeding the softening point of the quartz mold (~1950 K) and to make sure that the mold can still maintain the original features.     

Effect of anodizing on pulsed Nd:YAG laser joining of polyethylene terephthalate (PET) and aluminium alloy (A5052)

A series of laser joining experiments between polyethylene terephthalate (PET) and aluminium alloy (A5052) were conducted to investigate the effect of anodizing on A5052 surface on dissimilar materials used in joining. In this study, PET/A5052 joints with anodized A5052 surface exhibited greater shear strength compared to PET/A5052 joints without anodizing. The shear strength of the joints was increased with increasing of heat input and pulse duration. This indicates that the anodizing process could improve shear strength of the laser joining specimens. Significant molten pools were formed in both PET/A5052 (anodized) and PET/A5052 (as-received) joints except for PET/A5052 (as-received) sample joined at the lowest heat input and pulse duration. For the test results from laser joining under different pulse duration at the constant heat input, it was shown that joining behaviour was dominantly controlled by heat input and not by pulse duration.