Efficient green-light generation by frequency doubling of a picosecond all-fiber ytterbium-doped fiber amplifier in PPKTP waveguide inscribed by femtosecond laser direct writing.

We have demonstrated an ultrashort, compact green light radiation by frequency doubling of an all-fiber ytterbium-doped fiber laser source in a PPKTP waveguide fabricated by femtosecond laser pulses. Using the fabricated PPKTP waveguide crystal containing a 10 mm single grating with a period of 9.0 µm, we generate 310 mW of picosecond radiation at 532 nm for a fundamental power of 1.6W, corresponding to a conversion efficiency of 19.3%. The temperature tuning range of 8°C is achieved for a fixed fundamental wavelength of 1064 nm, the FWHM of the wavelength tuning curve is 4.2 nm at room temperature. The generated ultrashort pulses at 532 nm are of great importance and have comprehensive applications in photobiology research and high-resolution spectroscopy.

[Effect of different levels of pressure support on physiological variables and patient-ventilator synchronicity during noninvasive pressure support ventilation in patients with respiratory failure].

OBJECTIVE: To evaluate the effects of different levels of pressure support (PS) during noninvasive pressure support ventilation (NPPV) on physiological variables, patient-ventilator synchronicity in patients with acute exacerbation chronic obstructive pulmonary disease (AECOPD) and hypercapnic respiratory failure. METHODS: Fifteen hypercapnic respiratory failure AECOPD patients requiring NPPV in respiratory intensive care unit (RICU) were enrolled. Three different PS levels of 5, 10, 15 cm H2O (1 cm H2O=0.098 kPa) were applied randomly in all the 15 patients. At each PS level the patient was ventilated for 30 minutes, whereas physiological variables, ineffective effort (IE) expressed as IE index were recorded within the last 2 minutes. RESULTS: The IE index, tidal volume (VT), minute ventilation (VE), coefficient of variation of VT, ventilator inspiratory time (TI), expiratory time (TE), leak volume at 15 cm H2O PS level were significantly higher than those at 5 cm H2O level [IE index: (33.8+/-9.1)% vs. (8.0+/-6.0)%, VT (ml): 626+/-203 vs. 339+/-115, VE (L/min): 11.1+/-4.7 vs. 7.7+/-2.7, coefficient of variation of VT: (32.6+/-15.5)% vs. (11.3+/-6.9)%, TI (s): 1.14+/-0.31 vs. 0.76+/-0.15, TE (s): 2.49+/-0.44 vs. 1.87+/-0.28, leak volume (L/min): 8.28+/-4.86 vs. 2.22+/-1.58, all P<0.05], while the ventilator respiratory rate [(17+/-3) breaths/min] was lower at 15 cm H2O PS level than that at 5 cm H2O level [(23+/-3) breaths/min, P<0.05]. At 5 cm H2O level, the IE index was positively correlated with TI (r=0.62, P<0.05). Furthermore, the increase in IE index followed by PS level increase was correlated negatively with the change of VT (Delta VT) and positively with that of the TI (Delta TI, R2=0.88, P=0.000). CONCLUSIONS: (1) The IE at low assistance is associated with prolonged TI. (2) High PS level can significantly increase VE and VT, and reduce the ventilator respiratory rate. However, high PS level also lead to higher IE than low level. The factors favoring a higher increase in IE index as PS level increased include a less increase in VT and a higher increase in TI. However, an increase in leak volume is not associated with an elevated incidence of IE. (3) IE is high at high PS level during NPPV, even the Shape-signal is used as a trigger mechanism.

Femtosecond second-harmonic generation in periodically poled lithium niobate waveguides written by femtosecond laser pulses.

We present in this Letter the second-harmonic generation of femtosecond pulses in double-line-written waveguides fabricated in periodically poled lithium niobate (PPLN) with femtosecond laser pulses. In a 10-mm-long sample, a normalized conversion efficiency of 12.6% W(-1) cm(-2) has been achieved for 40 fs pump pulses with the wavelengths centered at 1550 nm. Simulation results show that in PPLN waveguides the FWHM of wavelength tuning curve for 40 fs pump pulses is 42 nm, which is 15 times of that for 40 ps pump pulses.

[Measuring and analyzing infrared spectrum of periodically polarized lithium niobate].

The present paper reports theoretical and experimental research on the tunable output characteristics of periodically polarized lithium niobate. We made six equal distance crystal grating sections, with each distance being 0.5 mm and the polarization period range of 29.0-31.5 mm. Especially, at room temperature, the signal-wavelength tunable output in the range of 1 449.7-1 665.0 nm and idle-wavelength tunable output in the range of 3 989.2-2 946.0 nm were achieved by LD end-pumped Nd : YVO4 laser with Q-switch in sound-wave. The lowest excitation threshold was 108.0 mW, while the highest excitation threshold was 149.2 mW. When the pump power was 649 mW, the highest gained signal-wave output was 118.5 mW and its conversion efficiency was 18.26%. Meanwhile, the idle-wave output was 46.6 mW and its conversion efficiency was 7.18%. These parameters approached the practicality level.

Numerical study on self-similar pulses in mode-locking fiber laser by coupled Ginzburg-Landau equation model.

A theoretical model is established to study the self-similar pulses in nonlinear polarization evolution (NPE) mode-locked fiber lasers. The propagation of pulse in single mode fibers and gain fibers are described by coupled Ginzburg- Landau equation (GLE). Two wave plates and a polarizer are considered to realize the NPE mechanism in simulation. This model describes the laser completely and provides some useful pulses' information. In our simulation the laser generates high quality self-similar pulses output. The region of steady self-similar pulses operation is found. The polarization states of different parts across the pulse are simulated along the laser cavity. It is found that polarization states across the pulse are modulated from elliptical to almost circular before the pulse passing through the polarizer.

Second harmonic generation of periodically poled potassium titanyl phosphate waveguide using femtosecond laser pulses.

The authors have presented in this paper the fabrication and characterization of double line written type waveguides in c-cut periodically poled potassium titanyl phosphate crystals. The waveguides were fabricated by using a femtosecond laser, and were utilized for second harmonic generation at 1064 nm. Our experiments have shown that single mode propagation was observed at optimal waveguide width of 14.5 microm. And a conversion efficiency of 39.6% can be achieved.

[Physiologic response to external positive end-expiratory pressure in mechanically ventilated chronic obstructive pulmonary disease patients].

OBJECTIVE: To evaluate the effects of external positive end-expiratory pressure (PEEPe) on respiratory system mechanics in passively ventilated chronic obstructive pulmonary disease (COPD) patients. METHODS: Fifteen patients with acute exacerbation of COPD were randomly enrolled into the study. The patients were admitted into respiratory intensive care unit (RICU) from Jan, 2005 to Jun, 2006 for controlled mechanical ventilation. PEEPe was applied stepwise from zero to 120% of intrinsic PEEP (PEEPi) in all the 15 patients. At each PEEPe level, airway resistance, compliance (Crs), plateau pressure (Pplat), and total PEEP (PEEPtot) were measured. According to the changes of Pplat after PEEPe application, the patients were divided into 2 sub-groups, abnormal response group (n=11) and a perplexing response group (n=4). RESULTS: In all of the 15 patients, airway resistance at PEEPe equal to 80% and 100% of PEEPi (18.5+/-2.0) cm H2O.L(-1).s(-1) (1 cm H2O=0.098 kPa), (18.0+/-2.2) cm H2O.L(-1).s(-1) were significantly lower than that at zero PEEPe [(23.0+/-2.9) cm H2O.L(-1).s(-1), t=5.36, 6.27 respectively, all P<0.01]. At PEEPe levels equal to 120% of PEEPi, airway resistance (17.3+/-2.1) cm H2O.L(-1).s(-1) and PEEPtot (12.7+/-2.2) cm H2O were significantly different compared with those at zero PEEPe (23.0+/-2.9) cm H2O. L(-1).s(-1), (10.0+/-1.1) cm H2O (t=6.79, -3.90 respectively, all P<0.01). There were no significant differences in baseline PEEPi levels (10.0+/-1.0) cm H2O, (10.0+/-1.4) cm H2O, resistance (22.8+/- 1.9) cm H2O.L(-1).s(-1), (23.1+/-4.1) cm H2O.L(-1).s(-1), Crs (39+/-6) ml/cm H2O, (42+/-9) ml/cm H2O and Pplat (20+/-4) cm H2O, (21+/-3) cm H2O between the normal response and the perplexing response groups (t=0.03, 0.10, 0.60, 0.15 respectively, all P>0.05). However, the corresponding values of Pplat changes at different PEEPe levels in the normal response group [DeltaPplat40 (-0.020+/-0.970) cm H2O, DeltaPplat80 (1.6+/-1.0) cm H2O, DeltaPplat100 (4.0+/-2.9) cm H2O, DeltaPplat120 (6.4+/-3.3) cm H2O] were found to be significantly different compared with those in the perplexing response group [DeltaPplat40 (-7.500+/-0.920) cm H2O, DeltaPplat80 (-4.4+/-1.4) cm H2O, DeltaPplat100 (-3.8+/-1.9) cm H2O, DeltaPplat120 (-1.6+/-1.2) cm H2O] (t=-9.64, -5.90, -3.80, -3.92 respectively, all P<0.01). CONCLUSION: At least in some passively ventilated COPD patients, the applied PEEPe may offer benefit by decreasing the Pplat.

Passive mode locking at harmonics of the free spectral range of the intracavity filter in a fiber ring laser.

We report the passive mode-locking at harmonics of the free spectral range (FSR) of the intracavity multi-channel filter in a fiber ring laser. The laser uses a sampled fiber Bragg grating (SFBG) with a free spectral range (FSR) of 0.8 nm, or 99 GHz at 1555 nm, and a length of highly nonlinear photonic crystal fiber with low and flat dispersion. Stable picosecond soliton pulse trains with twofold to sevenfold enhancement in the repetition rate, relative to the FSR of the SFBG, have been achieved. The passive mode-locking mechanism that is at play in this laser relies on a dissipative four-wave mixing process and switching of repetition rate is realized simply by adjustment of the intracavity polarization controllers.

Stable and uniform dual-wavelength erbium-doped fiber laser based on fiber Bragg gratings and photonic crystal fiber.

Based on the fiber Bragg gratings (FBGs) and high nonlinear photonic crystal fiber (HN-PCF), a novel dual-wavelength erbium-doped fiber (EDF) laser is proposed and demonstrated. The experimental results show that, owing to the contributions of two degenerate four-wave mixings in the HN-PCF, the proposed fiber laser is great stable and two output signals are uniform at room temperature. With adjustment of the attenuator, our fiber laser can selectively realize one wavelength lasing.

Tunable high-peak-power, high-energy hybrid Q-switched double-clad fiber laser.

A cw laser-diode-pumped Yb-doped double-clad fiber laser operating in a hybrid Q-switched regime was demonstrated. The output pulses had a duration as short as 4.2 ns, a tunable wavelength range from 1080.8 to 1142.7 nm, and a linewidth of less than 0.05 nm. Maximum peak power of approximately 175 kW and single-pulse energy of 1.57 mJ were obtained.

Narrow-linewidth widely tunable hybrid Q-switched double-clad fiber laser.

A laser-diode-pumped Yb-doped double-clad fiber laser operating in a hybrid Q-switched regime has been demonstrated. With pulsed pump light and stimulated Brillouin scattering of the gain fiber as the Q-switching mechanisms, the laser generated nanosecond pulses with a stable repetition rate. A single-pulse energy of as much as 143.1 microJ with a peak power of 28.6 kW was obtained. Use of an external-cavity diffraction grating in the Littman configuration permitted tuning of the laser wavelength over a 15.7-THz range from 1080 to 1140 nm, and a linewidth of 0.04 nm over the whole tuning range was achieved.