High power,widely tunable femtosecond MgO:PPLN optical parametric oscillator

We demonstrate a high power,widely tunable femtosecond MgO-doped periodically poled lithium niobate(MgO:PPLN)optical parametric oscillator(OPO)at 151 MHz,pumped by a Kerr-lens mode-locked Yb:KGW laser.With a maximum pump power of 7 W,the OPO is capable of delivering as high as 2.2 W of the signal centered around 1500 nm with tunable signal spectrum ranges of 1377 nm-1730 nm at an extraction efficiency of 31.4%,which exhibits a long-term passive power stability better than 0.71%rms over 4 h.The maximum idler bandwidths of 185 nm at 3613 nm are obtained across the idler tuning ranges of 2539 nm-4191 nm.By compensating intracavity dispersion,the signal has the shortest pulse duration of 170 fs at 1428 nm.

All-fiber laser seeded femtosecond Yb:KGW solid state regenerative amplifier

A high efficiency compact Yb:KGW regenerative amplifier using an all-fiber laser seed source was comprehensively studied.With thermal lensing effect compensated by the cavity design,the compressed pulses with energy of 1 mJ at 1 kHz and 0.4 mJ at 10 kHz in sub-400-fs pulse duration using chirped fiber Bragg grating(CFBG)stretcher were demonstrated.A modified Frantz-Nodvik equation was developed to emulate the dynamic behavior of the regenerative amplifier.The simulation results were in good agreement with the experiment.Numerical simulations and experimental results show that the scheme can be scalable to higher energy of multi-mJ,sub-300 fs pulses.

Heat transfer model of two-phase flow across tube bundle in submerged combustion vaporizer

In order to optimize the design of the submerged combustion vaporizer(SCV), an experimental apparatus was set up to investigate the heat transfer character outside the tube bundle in SCV. Several experiments were conducted using water and CO_2 as the heat transfer media in the tubes, respectively. The results indicated that hot air flux, the initial liquid level height and the tube pitch ratio had great influence on the heat transfer coefficient outside the tube bundle(ho). Finally, the air flux associated factor β and height associated factor γ were introduced to propose a new hocorrelation. After verified by experiments using cold water, high pressure CO_2 and liquid N_2 as heat transfer media, respectively, it was found that the biggest deviation between the predicted and the experimental values was less than 25%.

A 515-nm laser-pumped idler-resonant femtosecond BiB_(3)O_(6) optical parametric oscillator

We report on an idler-resonant femtosecond optical parametrical oscillator(OPO)based on BiB_(3)O_(6)(BiBO)crystal,synchronously pumped by a frequency-doubled,mode-locked Yb:KGW laser at 515 nm.The idler wavelengths of OPO can be tuned from 1100 nm to 1540 nm.At a repetition rate of 75.5 MHz,the OPO generates as much as 400 mW of idler power with 3.1 W of pump power,the corresponding pulse duration is 80 fs,which is 1.04 times of Fourier transform-limited(FTL)pulse duration at 1305 nm.In addition,the OPO exhibits excellent beam quality with M^(2)<1.8 at 1150 nm.To the best of our knowledge,this is the first idler-resonant femtosecond OPO pumped by 515 nm.

Achieving robust nitritation in a modified continuous-flow reactor: From micro-granule cultivation to nitrite-oxidizing bacteria elimination

In this study,a modified continuous-flow nitrifying reactor was successfully operated for rapid cultivation of micro-granules and achieving robust nitritation.Results showed that sludge granulation with mean size of ca.100μm was achieved within three weeks by gradually increasing settling velocity-based selection pressure from 0.48 to 0.9 m/hr.Though Nitrospira like nitrite-oxidizing bacteria(NOB)were enriched in the micro-granules with a ratio between ammonia-oxidizing bacteria(AOB)and NOB of 5.7%/6.5% on day 21,fast nitritation was achieved within one-week by gradually increasing of influent ammonium concentration(from 50 to 200 mg/L).Maintaining ammonium in-excess was the key for repressing NOB in the micro-granules.Interestingly,when the influent ammonium concentration switched back to 50 mg/L still with the residual ammonium of 15–25 mg/L,the nitrite accumulation efficiency increased from 90%to 98%.Experimental results suggested that the NOB repression was intensified by both oxygen and nitrite unavailability in the inner layers of micro-granules.Unexpectedly,continuous operation with ammonium in excess resulted in overproduction of extracellular polysaccharides and overgrowth of some bacteria(e.g.,Nitrosomonas,Arenimonas,and Flavobacterium),which deteriorated the micro-granule stability and drove the micro-granules aggregation into larger ones with irregular morphology.However,efficient nitritation was stably maintained with extremely high ammonium oxidation potential(>50 mg/g VSS/hr)and nearly complete washout of NOB was obtained.This suggested that smooth and spherical granule was not a prerequisite for achieving NOB wash-out and maintaining effective nitritation in the granular reactor.Overall,the microgranules exhibited a great practical potential for high-rate nitritation.

High temporal contrast 1053 nm laser source based on optical parametric amplification and second-harmonic generation

Temporal contrast directly affects the interaction between ultraintense and ultrashort pulse lasers with matter.Seed laser sources with broad bandwidth and high temporal contrast are significant for overall temporal contrast enhancement.The technique of cascaded nonlinear processes with optical parametric amplification and second-harmonic generation is demonstrated for high temporal contrast seed source generation.Within 40 ps before the main pulse,the temporal contrast reaches over 10^(11).The pulse energy and duration of the high-contrast pulse are 112μJ and 70 fs,respectively.Considering its high beam quality and stability,this laser source can serve as a high-quality seed for Nd:glass-based ultraintense and ultrashort pulse laser facilities.

Diode-pumped 10 W femtosecond Yb:CALGO laser with high beam quality

We demonstrate a diode-pumped femtosecond Yb:CaGdAlO_(4)(Yb:CALGO)laser with a semiconductor saturable absorber mirror(SESAM)for stable mode-locking operation.A perfect beam profile is measured under 10 W output power with M_(x)^(2)=1.017 and M_(y)^(2)=1.016 in the horizontal and vertical directions,respectively.At the repetition rate of 71.66 MHz,the optical pulse duration is 247 fs and the pulse energy is 140 nJ at the central wavelength of 1041 nm,corresponding to a peak power of 0.56 MW.In addition,we also generate continuous wave(CW)power of more than 15 W with TEM00 mode,corresponding to an optical-to-optical efficiency of 44.1%.

Harmonically pump a femtosecond optical parametric oscillator to 1.13 GHz by a femtosecond 515 nm laser

We demonstrate a harmonically pumped femtosecond optical parametric oscillator(OPO)laser using a frequency-doubled mode-locked Yb:KGW laser at a repetition rate of 75.5 MHz as the pump laser.Based on a bismuth borate nonlinear crystal,repetition rates up to 1.13 GHz are realized,which is 15 times that of the pump laser.The signal wavelength is tunable from 700 nm to 887 nm.The maximum power of the signal is 207 m W at the central wavelength of 750 nm and the shortest pulse duration is 117 fs at 780 nm.The beam quality(M^2 factor)in the horizontal and vertical directions of the output beam are 1.077 and 1.141,respectively.

Diode-pumped 13 W Yb:KGW femtosecond laser

We report on a high-power diode-pumped Yb∶KG(WO_(4))_(2)(Yb:KGW) mode-locked laser with a semiconductor saturable absorber mirror(SESAM). For 32.7 W of incident pump power, we generate 261 fs pulses with the maximum average output power of up to 13.0 W and spectrum centered around 1039 nm at 68.4 MHz, corresponding to 190 n J of single pulse energy and0.72 MW of peak power. The optical-to-optical conversion efficiency is 39.8%, and the slope efficiency is 64.4%. The Yb:KGW laser exhibits a power stability better than 0.543% of the root-mean-square in 2 h.

Temporal contrast enhancement by nonlinear elliptical polarization rotation in a multi-pass cell

We demonstrate the simultaneous temporal contrast improvement and pulse compression of a Yb-doped femtosecond laser via nonlinear elliptical polarization rotation in a solid state multi-pass cell.The temporal contrast is improved to 109,while the pulse is shortened from 181 to 36 fs,corresponding to a compression factor of 5.The output beam features excellent beam quality with M^(2) values of 1.18×1.16.The total efficiency of the contrast enhancement system exceeds 50%.This technique will have wide applications in high temporal contrast ultra-intense femtosecond lasers.

Generation of 601 fs pulse from an 8 kHz Nd:YVO_(4) picosecond laser by multi-pass-cell spectral broadening

We demonstrate nonlinear pulse compression of an 8 k Hz Nd:YVO4 picosecond laser using the multi-pass-cell(MPC) technique with fused silica as the nonlinear medium. The pulse duration is compressed from 12.5 ps to 601 fs, corresponding to a pulse shortening factor of 20.8. The output pulse energy is 154 μJ with an efficiency of 74.5%. To the best of our knowledge,this is the highest pulse compression ratio achieved in a single-stage MPC with bulk material as the nonlinear medium. The laser power stability and the beam quality(M2) before and after the MPC are also experimentally studied. Both the laser power stability and the beam quality are barely deteriorated by the MPC device.