Chun-Lin Chang

Strain effect on the thermoelectric power of YBa2−xSrxCu3O7

Abstract The temperature dependent thermoelectric power (TEP) for a series of superconducting Y(Ba 2− x Sr x )Cu 3 O 6.95 with x =0 to 0.8 has been investigated. We have kept the oxygen content at the optimal value (6.95±0.02) and have changed the lattice parameters by Sr-doping to study the strain dependent TEP. We have observed that a hump occurs at 240 K for all samples except that with x =0. With increasing the Sr-doping level, S shifts to a higher value and changes its sign from negative to positive. This study demonstrates that the sign as well as the absolute value of TEP has been affected by the lattice strains.

Two-Dimensional Simulations on Heat Transfer and Fluid Flow for Yttrium Aluminium Garnet Single-Crystal Fiber in Laser-Heated Pedestal Growth System

Heat transfer and fluid flow in a laser-heated pedestal growth (LHPG) system are analyzed near the deformed interfaces. The global thermal distributions of the crystal fiber, the melt, and the source rod are described by their temperature and axial gradient over a length of ~10 mm. Compared with the growth of bulk crystal of several centimeters in diameter, natural convection is 6 orders of magnitude smaller owing to the smaller melt volume; therefore, conduction rather than convection determines the temperature distribution in the molten zone. Moreover, thermocapillary convection rather than mass-transfer convection becomes dominant. The symmetry and mass flow rate of the double eddy pattern are significantly influenced by the molten-zone shape owing to the diameter reduction and the surface-tension-temperature coefficient when it is more than 10-4–10-3 dyn cm-1 K-1.

Observation of the thermal hysteresis of thermoelectric power in Pr0.5(Sr, Ca)0.5MnO3

The strong thermal hysteresis of thermoelectric power (TEP) in Pr0.5(Sr, Ca)0.5MnO3 is observed. A simple estimation shows that the electrostatic energy between Mn3+/Mn4+ alone cannot account for the large shift of Tco with changing A size. Based on our experimental evidence, we attribute the distinct features exhibited in the thermal hysteresis to the manifestation of the underlying competition between the delocalization and localization effects assisted respectively by the double exchange interaction and the charge ordering. The observed features include the following: (1) the A-size effect can depress Tco by as much as 70 K for Pr0.5Sr0.5-xCaxMnO3; (2) large thermal hysteresis of resistivity and TEP are exhibited in Pr0.5Sr0.5-xCaxMnO3; (3) for x = 0.3 and 0.4, the thermal hysteresis is larger than those of x = 0.1 and 0.5 and (4) magnetic field induces a large shift in Tco.

Side-Coupling Scheme for a High-Power Laser Diode Array With Grating Couplers: Thermal and Geometrical Issues

Grating couplers for side coupling of the light from a high-power laser diode array (LDA) into the inner cladding of a double-clad fiber were studied theoretically and implemented experimentally. In the experiments, two types of grating couplers were designed and fabricated: a gold-coated surface relief grating coupler and a gold-embedded silica grating coupler. A suitable design for heat dissipation for each grating coupler was employed to minimize thermal expansion due to the heat accumulated from the light absorption by the metal part of the grating coupler. The experimental results show that the gold-embedded silica grating coupler is superior to the surface relief gold grating coupler, because of its higher resistance to thermal expansion and better heat removal capability. In addition, the grating pitch and groove width were optimized for the highest overall coupling efficiency by taking account the backward diffraction loss and the groove wall nonverticality due to fabrication distortion. The experimental results show that the gold-embedded silica grating coupler is capable of coupling light power up to 21 W from a 976-nm continuous-wave LDA into the inner cladding of a 400- m-diameter double-clad fiber with an overall coupling efficiency of 50%.

Demonstration of side coupling between high power laser diode array and double-clad fiber using sub-wavelength grating

A sub-wavelength gold-embedded silica binary grating for side coupling light emission from a 976-nm high-power laser diode array into the 400-μm-diameter inner cladding of a double-clad fiber was demonstrated with near 50% overall coupling efficiency.

Influences of amplified spontaneous emission on fiber laser amplifier chain

The time-dependent coupled rate equations based on the multi-channel treatment has been applied to study the influences of amplified spontaneous emission on fiber laser amplifier with the experimental verification and practical solutions for suppressing ASE.

A high-peak power nanosecond all-fiber MOPA system at high-repetition rate

By limiting the core diameter of 15 μm at maximum with NA=0.07±0.01 for the near-diffraction-limited output (V <3.6), we successfully generate the pulse with the peak power of 36 kW and the duration of 4.6 ns in FWHM at the repetition rate of 20 kHz. To the best of our knowledge, the signal pulse energy corresponding to 264 μJ is the highest to date in the diode-seeded 15-μm all-fiber MOPA system with the efficiency of 35%. The success in the energy/power scaling is attributed to the further raise of input energy for more extracted energy, the tradeoff between the Raman-limited signal energy and the amplifier slope efficiency for more signal energy ratio, and the proper adjustment of both pump wavelength and power for avoiding coat damage without forced cooling.

Investigation of parasitic stimulated emission in a nanosecond diode-seeded high gain fiber pre-amplifier

Parasitic stimulated emission produces a front spike in a pulsed fiber MOPA system at a gain threshold of∼11.5 dB. Mitigation of the parasitic stimulated emission can enable high gain for diode-seeded fiber pre-amplifier.

High-Intensity Nanosecond All-Fiber-Coiled Laser and Extreme Ultraviolet Generation

High-efficiency fiber-based extreme ultraviolet driver in the alignment-free configuration has been experimentally achieved with a maximum intensity of 6.4×1010 W/cm2 on target at a repetition rate of 20 kHz. The output EUV signal within 10~20 nm in wavelength was confirmed with a Si/Zr-coated x-ray photodiode by varying numbers of Be and Al foil filters. The measured spectral range is consistent with that obtained by the weighted oscillator strengths of Sn8+ to Sn+13 ions using an one-dimensional hydrodynamic code coupled with the ionization model of collisional-radiative equilibrium. The driver is based on a 1064-nm nanosecond coiled ytterbium all-fiber laser system in diode-seeded master oscillator power amplification. With an overall optical efficiency up to 56%, it can deliver a 1.16-mJ, 117-kW, 6.1-ns laser pulses with a FWHM linewidth of 10 nm and beam propagation factor of M2~1.55. The full advantages of using fiber laser for a movable LPP EUV metrology source are revealed.

Intense supercontinuum generation in a nanosecond nonlinear all-PM- fiber power amplifier

Intense nanosecond emission with spectral broadening from 980 to 1600 nm was generated with peak power up to 117 kW, close to the damage threshold of fiber fuse. Both laser amplification and nonlinear conversion were simultaneously employed in a fiber power amplifier giving power scaling free from significant depletion. In a diode-seeded all-PM-fiber master oscillation power amplifier system under all normal dispersion, a core-pumped preamplifier using double-pass scheme can significantly improve the energy extraction. This produced the pulse energy of 1.2 mJ and duration of 6 ns with a conversion efficiency of 66% at the moderate repetition of 20 kHz, which is consistent with the coupled laser rate equations including the stimulated Raman scattering. For the comparable nonlinear strength in each stage from single to few modes, the onset and interplay of four kinds of fiber nonlinearities can be addressed.