Baining Liu

Quartz fibre phase conjugators with high fidelity and reflectivity

Abstract Phase conjugators based on stimulated Brillouin scattering in undoped multimode quartz fibres can be operated with high fidelity and reflectivity. The threshold, reflectivity and fidelity of fibres with a core diameter of 200 μm and different lengths were measured with 30 ns pulses at 1.06 μm wavelength with different coherence lengths of the input beams. The depolarization of SBS pulses could be reduced to less than 0.5% by proper configuration of the fibre. Such fibre phase conjugators operate with more than 50% reflectivity, 93% fidelity, and less than 17 kW threshold.

Effects of hydrogen/deuterium exchange on photosynthetic water cleavage in PS II core complexes from spinach

H/D isotope exchange effects on P680+· reduction by Yz and electron abstraction from the water oxidizing complex (WOC) in redox state S3 by YZ OX were analyzed in PS II core complexes from spinach by measurements of laser flash induced absorption changes at 820 nm and 355 nm. The results obtained reveal: (1) the rate of YZ oxidation by P680+· is almost independent of the substitution of exchangeable protons by deuterons; and (2) the reaction between YZ OX and the WOC in S3 exhibits a kinetic H/D isotope exchange effect of similar magnitude as that recently observed in PS II membrane fragments [Renger, G., Bittner, T. and Messinger, J. (1994) Biochem. Soc. Trans. 22, 318–322]. Based on these results it is inferred that photosynthetic dioxygen formation comprises the cleavage of at least one hydrogen bond.

Studies on the recombination kinetics of the radical pair P680+Pheo− in isolated PS II core complexes from spinach

Yield and decay kinetics of the laser flash-induced primary radical pair P680+Pheo− were analyzed by measuring flash-induced absorption changes at 820 nm (ΔA820) and fluorescence decay kinetics in PS II core complexes under different redox conditions for the primary plastoquinone acceptor, QA. If QA is chemically reduced in the dark by Na2S2O4, the yield of long-lived (τ > 1 ns) P680+Pheo− states is 30% with lifetimes of 4 ns (about 23 of the total decay) and 30 ns (about 13 of the total decay). The fluorescence data suggest that the 30 ns radical pair and probably the 4 ns radical pair as well decay predominately by recombination to P680∗Pheo. Irradiation of PS II core complexes in the presence of Na2S2O4 for only 10 s with intense visible light increases the yield of the 30 ns primary radical pair by a factor of 3. This result can be explained by a double reduction of QA and subsequent protonation leading to QAH2. At short irradiation times this effect is reversible because subsequent addition of K3[Fe(CN)6] fully restores the capability for a stable charge separation giving rise to P680+QA formation. However, longer irradiation of Na2S2O4-treated PS II core complexes leads to an irreversible impairment of the primary charge separation.

Er:YAG-laser at 2.94 μm Q-switched by a FTIR-shutter with silicon output coupler and polarizer

Abstract A FTIR-shutter based on the principle of frustrated total internal reflection was studied as an active Q-switch for 3 μm lasers. The FTIR-shutter used consists of two truncated YAG crystal prisms with a small air gap between them. The gap width is modulated by two piezoelectrical cells attached on the truncated facets of the prisms. The switching time and the spatial transmission profile of the shutter were measured. An Er:YAG-laser was Q-switched with this shutter. Furthermore, we demonstrate that conventional semiconductors like silicon and germanium wafers are effective output couplers and Brewster polarizers for 3 μm lasers. Linearly polarized and stable laser pulses with pulse energy up to 50 mJ and pulse duration of less than 60 ns are achieved.

Diode-pumped self-Q-switched single-frequency 946-nm Nd 3+ ,Cr 4+ :YAG microchip laser

A stable low-threshold self-Q-switched diode-pumped 946-nm Nd(3+), Cr(4+):YAG microchip laser operating in single frequency and fundamental transverse mode is reported. The output characteristics of the microchip laser were investigated under cw and pulsed pumping. By combination of the infrared microchip and an external frequency doubler, 473-nm pulses with a conversion efficiency of 18% were achieved.

Theoretical and experimental studies of noise suppression for intracavity frequency doubled lasers with phase matching type I or II

A general physical model consisting of two arbitrary birefringent elements is presented and studied based on Jones matrix calculation. The resonant frequencies and polarization directions of the two kinds of fundamental wave eigenmodes are given in general case. The stability of the intracavity frequency doubled cw laser emission is analyzed under multi-longitudinal modes operation. The conditions for noise suppression of the second harmonic output are derived for both types of phase matching. Experiment results are in good agreement with the theoretical analyses.

Efficient blue cw Nd:YAG microchip laser with two intracavity frequency doublers

A microchip cavity including a half-monolithic Nd:YAG- KNbO(3) chip and a second KNbO(3) crystal is presented. An output power of up to 30 mW at 473 nm pumped by a single stripe 1-W at 808-nm diode is achieved. The laser can be operated in either single-longitudinal or multilongitudinal mode by rotation of the second frequency-doubling crystal to 0 degrees or 90 degrees from the first one. The cw output noise is less than 2% for both cases.

Efficient acousto-optic Q switching of Er:YSGG lasers at 2.79-µm wavelength

A flash-lamp-pumped Er:Cr:YSGG laser at 2.79-microm wavelength has been acousto-optically Q switched. The Q-switched pulse energy and duration depend on pump pulse level and relative Q-switching time. Limits of single-pulse operation with the given acousto-optic diffraction efficiency have been determined. Resonator length, position of the Q switch, and output mirror reflectivity have been varied to obtain high pulse energy and the shortest pulse duration in the TEM00 transverse laser mode. A maximum single-pulse energy of 27 mJ and a minimum pulse duration of 120 ns were obtained with an output mirror reflectivity of approximately 25%. The highest Q-switched single-pulse energy amounted to 52% of the free-running, fundamental mode output pulse energy.

Fiber phase conjugators at 1064-nm, 532-nm, and 355-nm wavelengths.

Undoped multimode quartz fibers can be used as effective phase conjugators with high fidelity and reflectivity for improving the beam quality of lasers with high average power. Such phase conjugators based on stimulated Brillouin scattering (SBS) in undoped multimode quartz fibers with a core diameter of 100 microm and a length of more than 2 m were characterized at wavelengths of 1064, 532, and 355 nm. The power thresholds at these three wavelengths are 6.4, 3.3, and 3 kW, respectively, and the maximum SBS reflectivity is more than 50% in all cases, without a significant depolarization owing to the fiber. Direct measurement of the far-field fidelity, i.e., the phase-conjugated part of the reflected light in the far field at 1064 nm, results in a fidelity of more than 90%.

Low mode-locking saturation intensity of co-doped Nd3+,Cr4+:YAG crystal as saturable absorber

We report the characteristics of low saturation intensity of Nd,Cr:YAG as saturable absorber both in a diode pumped Nd:YAG laser and a self-mode-locking Nd,Cr:YAG laser. The difference of saturation intensity between Q-switched and mode-locked operation in co-doped Nd,Cr:YAG was only one to two orders of magnitude while the reported difference in Cr:YAG was five orders of magnitude. More than 80% passive mode-locking modulation depth was achieved at an intracavity intensity of 6 x 10(4) W/cm(2). Self-mode-locking phenomenon was observed in a fiber-coupled laser diode pumped Nd,Cr:YAG laser at an intracavity intensity of 1.0 MW/cm(2). The modulation depth is approximately 40%