Junying Xu

Passively Q-switched diode-pumped Yb:YAG laser using Cr4+-doped garnets

Abstract We investigate the repetitive modulation in the kHz frequency domain of a passively Q-switched, diode-pumped Yb:YAG laser, by Cr 4+ :YAG, Cr 4+ :LuAG, and Cr 4+ :GSGG saturable absorbers. The results presented here are focused towards the design of a passively Q-switched Yb:YAG microlaser. The free-running performance of both rod and a disk Yb:YAG is characterized and experimental parameters such as gain and loss are evaluated. These values, together with the value of the stimulated emission cross-section, e.g. σ em =3.3×10 −20 cm 2 were found to fit between our experimental results and an existing numerical model which relates the experimental and physical parameters to the minimal threshold pumping power. Q-switched pulses with maximum peak power of ≈10.4 kW, with energy of ≈0.5 mJ/pulse, were extracted with 30% extraction efficiency.

Theoretical investigation on quantum well lasers with extremely low vertical beam divergence and low threshold current

A specially designed quantum well laser for achieving extremely low vertical beam divergence was reported and theoretically investigated. The laser structure was characterized by two low index layers inserted between the waveguide layers and the cladding layers. The additional layers were intended to achieve wide optical spread in the cladding layers and strong confinement in the active region. This enabled significant reduction of beam divergence with no sacrifice in threshold current density. The numerical results showed that lasers with extremely low vertical beam divergence from 20° down to 11° and threshold current density of less than 131 A/cm2 can be easily achieved by optimization of the structure parameters. Influences of individual key structure parameters on beam divergence and threshold current density are analyzed. Attention is also paid to the minimum cladding layer thicknesses needed to maintain low threshold current densities and low internal loss. The near and far field patterns are given...

Design and fabrication of 980-nm InGaAs/A1GaAs quantum well lasers with low beam divergence

We report on the design and fabrication of 980nm InGaAs/AlGaAs double quantum well lasers with low vertical beam divergence. The specially designed structure, which characterized by two low refractive index layers inserted between the cladding and waveguide layers, was theoretically studied using the shooting method and systematic calculation. Results for the beam divergence and far-field distribution are given, and the physical concepts are discussed. Experimental investigation by molecular beam epitaxy was performed. For the fabricated as-cleaved 3micrometers - wide ridge waveguide structure lasers having a cavity length of 800micrometers , a threshold current of 30mA and an external quantum efficiency of 0.8mW/mA were achieved. The measured far field pattern has a vertical divergence of 21 degrees and a horizontal divergence of 8 degrees.

Intrinsic Strength Asymmetry Between Tension and Compression of Perfect Face-Centered-Cubic Crystals

The strength asymmetry between tension and compression is a typical case of mechanical response of materials. Here we achieve the intrinsic strength asymmetry of six face-centered-cubic perfect crystals (Cu, Au, Ni, Pt, Al and Ir) through calculating the ideal tensile and compressive strength with considering the normal stress effect and the competition between different crystallographic planes. The results show that both the intrinsic factors (the ideal shear strength and cleavage strength of low-index planes) and the orientation could affect the strength asymmetry, which may provide insights into understanding the strength of ultra-strong materials.

Anisotropy of tensile strength and fracture mode of perfect face-centered-cubic crystals

This study presents an effective method to calculate the ideal tensile strength of six face-centered-cubic (fcc) crystals (Cu, Au, Ni, Pt, Al, and Ir) along an arbitrary tensile direction by considering the coupling effect of normal stress and shear stress on a given crystallographic plane. Meanwhile, the fracture modes of the six crystals can also be derived from the competition between shear and cleavage fracture along different crystallographic planes. The results show that both the intrinsic factors (the ideal shear strength and cleavage strength of low-index planes) and the orientation may affect the tensile strength and fracture modes of ideal fcc crystals, which may give the reliable strength limit of fcc metals and well interpret the observed high strength in nano-scale mechanical experiments.

Growth and fabrication of high performance 980nm strained InGaAs quantum well lasers using novel hybrid material system of InGaAsP and AlGaAs

In this paper, we report on the design, growth and fabrication of 980 nm strained InGaAs quantum well lasers employing novel material system of Al-free active region and AlGaAs cladding layers. The use of AlGaAs cladding instead of InGaP provides potential advantages in laser structure design, improvement of surface morphology and laser performance. We demonstrate an optimized broad-waveguide structure for obtaining high power 980 nm quantum well lasers with low vertical beam divergence. The laser structure was grown by low-pressure metalorganic chemical vapor deposition, which exhibit a high internal quantum efficiency of approximately 90% and a low internal loss of 1.5 - 2.5 cm-1. The broad-area and ridge-waveguide laser devices are both fabricated. For 100 micrometers wide stripe lasers with cavity length of 800 micrometers , a low threshold current of 170 mA, a high slope efficiency of 1.0 W/A and high output power of more than 3.5 W are achieved. The temperature dependences of the threshold current and the emitting spectra demonstrate a very high characteristic temperature coefficient (T0) of 200 - 250 K and a wavelength shift coefficient of 0.34 nm/ degree(s)C. For 4 micrometers - width ridge waveguide structure laser devices, a maximum output power of 340 mW with COD-free thermal roll-over characteristics is obtained.

High power 980 nm Al-free strained quantum-well lasers for erbium-doped fiber amplifiers

In this paper, we reported on the fabrication of 980 nm InGaAs/InGaAsP strained quantum-well (QW) lasers with broad waveguide. The laser structure was grown by low-pressure metalorganic chemical vapor deposition on a n(+)- GaAs substrate. For 3 mu m stripe ridge waveguide lasers, the threshold current is 30 mA and the maximum output power and the output power operating in fundamental mode are 350 mW and 200 mW, respectively. The output power from the single mode fiber is up to 100 mW, the coupling efficiency is 50%. We also fabricated 100 mu m broad stripe coated lasers with cavity length of 800 mu m, a threshold current density of 170 A/cm(2), a high slope efficiency of 1.03 W/A and a far-field pattern of 40 x 6 degrees are obtained. The maximum output power of 3.5 W is also obtained for 100 mu m wide coated lasers

High-power 0.98-μm broad-waveguide lasers using novel material system of InGaAs/InGaAsP/AlGaAs

We describe the theoretical design and experimental fabrication of high power 0.98 micrometers strained quantum well lasers employing broad waveguide structure and novel hybrid material system of Al-free InGaAs/InGaAsP active region and AlGaAs cladding layers. The use of AlGaAs cladding, instead of InGaP, provides advantages in flexibility of laser structure design, simple epitaxial growth, improvement of surface morphology and laser performance. In addition to the theoretical study of the new structure, we successfully demonstrate the design by obtaining high performance laser devices. The as-grown InGaAs/InGaAsP/AlGaAs laser material exhibits very high quality with low threshold current density of 200 A/cm2, high internal quantum efficiency of approximately 93 percent, and low internal loss of 1.2 cm-1. For 100 micrometers -wide stripe lasers with cavity length of 800 micrometers , a high slope efficiency of 1.03 W/A, low vertical beam divergence of 36 degrees, high output power of 3.65 W, and very high characteristic temperature coefficient of 250 K are achieved.

Influence of graded index waveguides on the gain difference between TEO and TMO modes of semiconductor optical amplifiers

Polarization independent semiconductor optical amplifiers are highly desirable for optical communication. In this paper, the influence of graded index waveguides on the gain difference between TE0 and TM0 modes of semiconductor optical amplifiers is studied in detail. A new numerical procedure called complex shooting method was proposed at first to solve the eigenvalues of waveguides with graded complex refractive index. The results show that a graded index waveguide will increase the polarization sensitivity of the modal gain while an unplanar waveguide can decrease it. A polarization independent semiconductor optical amplifier can be obtained by adopting an unplanar waveguide.

High-power 980-nm InGaAs/GaAs/AlGaAs window structure lasers fabricated by impurity-free vacancy diffusion

For the first time, we report on the 980 nm InGaAs/GaAs/AlGaAs strained quantum well lasers with window structure in regions near the laser facet fabricated by impurity-free vacancy diffusion for quantum well intermixing. In the fabricating procedure, SrF2 was used as a film suppressing the quantum well intermixing. 3 micrometers - stripe ridge waveguide lasers with window region of 25 micrometers at each end exhibit a high slope efficiency of 0.8 W/A and a threshold current of 20 mA. The characteristics are comparable with that of conventional lasers. Catastrophic optical mirror damage is not observed at thermal limited powers as high as 360 mW. Accelerated lifetesting at 100 mW and 50 degree(s)C shows devices to have good reliability.