Yonggang Zou

Ultraviolet-enhanced electroluminescence from individual ZnO microwire/p-Si light-emitting diode by reverse tunneling effect

In this study, heterojunction light-emitting diodes (LEDs) based on a ZnO single microwire (SMW) were fabricated on a p-Si substrate. Electroluminescence (EL) characteristics of this device operated at different biases were investigated. Ultraviolet (UV) and visible emission bands of the n-ZnO SMW/p-Si heterojunction device were observed under forward and reverse biases, respectively. When the reverse bias voltage reached 55 V, a sharp UV emission peak at ∼400 nm with a full width at half maximum of 20 nm dominated the EL spectra, and a satellite peak was located at ∼490 nm. The ratio of UV and defect emission intensity under the 55 V reverse bias was almost 28 times the ratio obtained under the 55 V forward bias. Our results indicated that the carrier transport process was affected by the tunneling effect, and this process could effectively suppress the defect emission and cause high UV emission efficiency in ZnO/Si-based ultraviolet LEDs. The emission mechanism of the heterojunction LED was discussed in terms of interface defect and energy band theory.

The Surface and Optical Properties of Passivated GaSb with Different Passivating Agents

In this paper, (NH4)2S and Na2S were used as passivating agent for the sulphuration treatment of GaSb. Although the oxide layer can be removed by both (NH4)2S and Na2S, the etching rate of Na2S was faster than (NH4)2S, which caused the high RMS. After passivation treatment, PL instensity of GaSb increased, however, if the passivation time was too long, PL instensity would decrease. At low temperature PL spectrum, the emission located at 777meV and 795meV which can be associated with the transition from the conduction band to the native acceptor level VGaGaSb and the other peak corresponds to bound-edge-related transitions can be observed.

Beam shaping for kilowatt fiber-coupled diode lasers by using one-step beam cutting-rotating of prisms

The beam quality mismatch of laser diode stacks in both axes limits many direct applications for fiber or solid laser pumping and material processing. In this paper, a one-step cutting-rotating beam shaping system has been designed to homogenize the beam quality of two polarization-multiplexing laser diode stacks. Coupling laser diode stacks consisting of eight bars into a standard fiber with a core diameter of 600 μm and an NA of 0.22 is achieved. The simulative result shows that the system will have an output power over 1056 W. By using the technique, the production of compact and high brightness fiber-coupling diode lasers can be directly used for laser cladding and laser surface hardening processes.

Design of high-brightness 976nm fiber-coupled laser diodes based on ZEMAX

Fiber-coupled laser diodes have become essential sources for fiber laser pumping and direct energy applications. To obtain high power, high brightness semiconductor laser beam output, a 976nm wavelength fiber coupling module with 12 single-emitter laser diodes has been designed using ZEMAX optical design software, and single-emitter has an output power of 10 W with a 105μm wide emission aperture. The core diameter of output fiber is set as 105μm with a numerical aperture (NA) of 0.15. Finally, the simulated result indicates that the module will have an output power over 100W with the brightness of 16.63MW·cm-2·str-1, and the coupling efficiency achieved 85%.

Research on narrow linewidth picosecond pulsed fiber lasers based on graphene saturable absorber

The graphene films have been proved to be potentially useful as optical elements in fiber lasers. In this paper, the graphene films are prepared by both pulsed laser deposition (PLD) method and solution deposition method as saturable absorbers in mode-locked fiber laser. The pulse evolution process in the laser cavity is simulated by RP Fiber Power software. The influence of graphene saturable absorber with different modulation depth on pulse duration and linewidth of laser is analyzed. Finally, a ring-cavity passively mode-locked picosecond pulse Erbium-doped fiber laser based on graphene saturable absorber is constructed experimentally. At the pump power of 30mW, the laser with each saturable absorber can work stably without damage in the mode-locked state.

Morphology and electrical characteristics of p-type ZnO microwires with zigzag rough surfaces induced by Sb doping

Sb-doped p-type ZnO microwires with zigzag rough surfaces were synthesized by two zone chemical vapor deposition. The zigzag morphology characteristics analyzed by high resolution scanning electron microscopy and transmission electron microscopy show the existence of surface defects caused by Sb doping. The incorporation of Sb into a ZnO lattice induces lattice imperfection, which is the origin of the zigzag rough surface. Photoluminescence and electrical properties of the obtained Sb-doped ZnO microwires were determined. The crossed structure microwire-based p–n homojunction device was fabricated by applying as-synthesized Sb-doped p-type ZnO microwires and undoped n-type ZnO microwires. The doped microwires demonstrate reproducible p-type conduction and enhanced rectifying behavior with increasing Sb doping concentration. The results demonstrated that the optimizable optical and electrical characteristics, controlled by increasing the doping concentration, are reflected in the surface morphology changes which would be helpful for characterizing the doping effects in micro/nanoscale materials.

Design of 20 W fiber-coupled green laser diode by Zemax*

We represent a design of a 20 W, fiber-coupled diode laser module based on 26 single emitters at 520 nm. The module can produce more than 20 W output power from a standard fiber with core diameter of 400 μm and numerical aperture (NA) of 0.22. To achieve a 20 W laser beam, the spatial beam combination and polarization beam combination by polarization beam splitter are used to combine output of 26 single emitters into a single beam, and then an aspheric lens is used to couple the combined beam into an optical fiber. The simulation shows that the total coupling efficiency is more than 95%.

Single higher-order mode ring-shaped VCSEL with surface relief

A novel vertical-cavity surface-emitting laser (VCSEL) with single mode, high-power, low divergence, and temperature stability is presented. The most prominent structural feature of the device is that the high optical loss region is formed by an anti-phase surface relief above the top Distributed Bragg Reflectors (DBRs) and the light-emitting aperture is ringshaped with larger region. The simulation results show the device with 15μm oxide aperture and 5μm width ring light emitting region achieves stable single-higher-order transverse mode emission with a side mode suppression ratio (SMSR) of more than 80dB. The maximum continuous-wave (CW) single mode power is up to 15.2 mW and far-field divergence angle (FWHM) is lower than 4.5°. Moreover, the VCSEL maintains CW single mode emission up to a record high temperature of 450K.

Design of 150W, 105-μm, 0.22NA, fiber coupled laser diode module by ZEMAX

We represent a design of a high brightness, fiber coupled diode laser module based on 16 single emitters at 915nm. The module can produce more than 150 Watts output power from a standard fiber with core diameter of 105μm and numerical aperture (NA) of 0.22. To achieve a high power and high brightness laser beam, the spatial beam combination and polarization beam combination are used to combine output of 16 single emitters into a single beam, and then an aspheric lens is used to couple the combined beam into an optical fiber. The simulation show that the total coupling efficiency is more than 95% and the highest brightness is estimated to be 11MW/ (cm2*sr).

Improving efficiency of spectral beam combining by optimizing reflectivity of output coupler for laser diode array

This paper reports a study on the relationship between the combining efficiency and reflectivity of output coupler of diode array in spectral beam combining. The combining efficiency is analyzed theoretically by using principle of the resonator. The simulation shows that high reflectivity will lead to low combining efficiency, and low reflectivity may cause the failure of wavelength locking. With increasing of the reflectivity of the OC, the combining efficiency changes like a downward parabola which has a maximum value of ~10%. The experiments demonstrate that the highest efficiency is obtained at a reflectivity of 10%, and the experimental results agree well with the theoretical analysis.