Decreasing pores in a laser cladding layer with pulsed current

A pulsed current is introduced into the traditional coaxial laser cladding process to decrease the porosity of the cladding layer. The magneto contraction force caused by pulsed current exerted on the molten pool squeezes the gas out and compensates the shrinkage during molten pool solidification. As a result the porosity of the cladding layer is decreased to an extremely low degree. Simultaneously, the grain of the cladding layer is finer with the added supercooling degree with pulsed current. The microhardness of an equiaxed zone in the cross section of a cladding layer also increases.     

Optical Switching of a Quantum Cascade Laser in Continuous Wave Operation

We demonstrate an optical switching in a middle infrared continuous-wave quantum cascade laser operated in single mode by illuminating its front facet with a near infrared laser. A decrease in the laser net gain is observed in the amplified spontaneous emission spectrum. This is attributed to an increase of the carrier concentration caused by the near infrared excitation. The net gain reduction allows the quantum cascade laser to be completely switched off from single mode lasing. This optical switching can be used to convert near infrared signals into middle infrared signals for free space communication.     

Terahertz Quantum Cascade Laser Operating at 2.94THz

The development of quantum cascade laser at 2.94 THz is reported. The laser structure is based on a bound-to-continuum active region and a semi-insulating surface-plasmon waveguide. Lasing is observed up to a heat-sink temperature of 70 K in pulsed mode with light power of 4.75 mW at 10 K and 1 mW at 70 K. A threshold current density of 296.5 A/cm2 and an internal quantum efficiency of 1.57 × 10-2 per cascade period are also observed at 10 K. The characteristic temperature of this laser is extracted to be T0 = 57.5 K.     

Continuous Wave Performance and Tunability of MBE Grown 2.1μm InGaAsSb/AlGaAsSb MQW lasers

InCaAsSb/AlGaAsSb multi quantum well ridge waveguide lasers at 2.1 μm wavelength are fabricated by using molecular beam epitaxy. Continuous wave performance and tunability of the lasers are evaluated in a wide temperature range extend to 80℃. Output power of the laser at 30℃ exceeds 30 m W/facet at driving current of 0.5 A, the characteristic temperature To is 89K in 0-50℃ range. No fast degradation is observed in accelerated aging test at 90℃ for those lasers with lower Al content in cladding layers. Temperature tunability of the lasers is 1.36 nm/K. Single-mode output with side mode suppression ratios greater than 20 dB is achieved in a certain driving current region; current tunability is 8 × 10^-3 nm/mA regardless of mode hopping.     

Quantum dot quantum cascade photodetector using a laser structure

We report on a quantum dot quantum cascade detector(QD-QCD), whose structure is derived from a QD cascade laser. In this structure, more ordered In As QD layers formed in the Stranski–Krastanow growth mode on a thin Ga As buffer layer are incorporated into the active region. This QD-QCD can operate up to room temperature with a peak detection wavelength of 5.8 μm. A responsivity of 3.1 mA/W at 160 K and a detectivity of 3.6 × 10~8 Jones at 77 K are obtained. The initial performance of the detector is promising, and, by further optimizing the growth of InA s QDs, integrated QD-quantum cascade laser/QCD applications are expected.     

10-W cladding-pumped fiber laser with single transverse mode output

A Yb-doped double-clad fiber laser is demonstrated with a measured power output of 10.6 W and a fundamental spatial mode. The optical-to-optical conversion efficiency is 44% and the slope efficiency is 86% closed to quantum efficiency of optical conversion. In our laser system, a D-shape (340 μm/400 μm) inner cladding Yb-doped fiber is used as the gain material within the Fabry-Perot cavity. Multimode diode pump radiation is injected into the cladding through an end facet of the composite fiber.     

Strain-Engineered Low-Density InAs Bilayer Quantum Dots for Single Photon Emission

We investigate the growth of strain-engineered low-density 1hAs bilayer quantum dots （BQDs） on GaAs by molecular beam epitaxy. Owing to increasing dot size and In composition of the upper QDs, low-density BQDs in a GaAs matrix with an emission wavelength up to 1.4 μm at room temperature are achieved. Such a wavelength is larger than that of conventional QDs in a GaAs matrix （generally of about 1.3μm）. The optical properties of the BQDs are sensitive to annealing temperature used after spacer layer growth. Significant decrease of integrated PL intensity is observed as the annealing temperature increases. At 10 K, single photon emission from the BQDs with wavelength around 1.3μm is observed.     

Laser multi-layer cladding on ZM6 magnesium base alloy

A pulsed Nd: YAG laser is used in multi-layer cladding on ZM6 Mg base alloys. The microstructure is studied with an optical microscope and a scanning electron microscope (SEM). The composition within the layer was determined by electron probe microanalysis (EPMA). X-ray diffraction (XRD) was also used to investigate the phase of constitutes of the cladding zone. The results show that microstructure in solidified cladding layer changes much when treated by high energy laser beam. The microstructure of the ZM6 alloy consists of a-Mg and Mg9Nd, while the L-ZM6 of a-Mg, MggNd and a-Zr. The depth of the cladding is over 1 mm. Many fine particles were found to be distributed homogeneously throughout the matrix and the columnar grain grows along substrate.     

Output Characteristics of an InP/InGaAsP Triangle Microcavity Laser

Mode competitions between modes with different output coupling efficiencies can result in optical bistability under certain asymmetric nonlinear gain. For a GaInAsP/InP equilateral triangle microlaser with the side length of 10μm, the drop of the output power with the increase of the injection current is observed corresponding to transverse mode transitions. Furthermore, the measured laser spectra up to 270K show that lasing modes coexist with the wavelength interval of 39nm at 240K. The emission at 5.2THz can be expected by the mode frequency beating with the 39nm interval.     

Effect of Intense Laser Irradiation on the Lattice Stability of Al_2Au

The structural, electronic and lattice-dynamical properties of the intermetallic Al 2 Au at different electronic temperatures have been investigated via density functional calculations. The results of electronic density of state indicate that, although its value changes considerably, Al2Au is still of metal with the increasing of electronic temperature. The acoustic mode of Al2Au gets negative which leads to lattice dynamical instability when the electronic temperature is beyond 1.44 eV. Moreover, with the increasing of the electronic temperature, the vibrational frequencies of the T1u optical mode (triply degenerate) of Al2Au at Γ point decrease first and increase then, the turning point is at Te = 1.40 eV. T2g optical mode at Γ point has a similar situation, but the turning point is at Te = 1.80 eV. The predicted melting temperatures of Al2Au undergo a sharp decrease from 1333K at normal temperature to 1172 K at Te = 1.8 eV after intense laser irradiation.     

All-optical modulator based on a ferrofluid core metal cladding waveguide chip

We propose a novel optical intensity modulator based on the combination of a symmetrical metal cladding optical waveguide （SMCW） and ferrofluid, where the ferrofluid is sealed in the waveguide to act as a guiding layer. The light matter interaction in the ferrofluid film leads to the formation of a regular nanoparticle pattern, which changes the phase match condition of the ultrahigh order modes in return. When two lasers are incident on the same spot of the waveguide chip, experiments illustrate all-optical modulation of one laser beam by adjusting the intensity of the other laser. A possible theoretical explanation may be due to the optical trapping and Soret effect since the phenomenon is considerable only when the control laser is effectively coupled into the waveguide.     

Photoluminescence of Self－Assembled InAs／GaAs Quantum Dots Covered by InAlAs and InGaAs Combination Strain－Reducing Layer

Self-assembled lnAs/GaAs quantum dots covered by the 1-nm InxAI(1-x)As (x = 0.2, 0.3) and 3-nm In0.2Ga0.8As combination strain-reducing layer are fabricated, whose height can take up to 30-46 nm. The luminescence emission at a long-wavelength of 1.33μm and the energy separation between the ground and the first-excited state of 86meV are observed at room temperature. Furthermore, comparative study proves that the energy separation can increase to 91 meV by multiple stacking.     

A Novel Mach–Zehnder Interferometer Based on Hybrid Liquid Crystal–Photonic Crystal Fiber

We propose a novel all fiber Mach–Zehnder interferometer(MZI) based on photonic crystal fiber(PCF) filled with liquid crystal(LC). The interference between the core mode and the cladding modes of a PCF is utilized.To excite the cladding modes, a region is formed using fiber fusion splicer. Due to the fact that varying effective index difference between the core region and the LC-filled cladding region can cause different transmission spectra,we mainly study the MZIs with different LC-filled structures and different lengths of LC filling. The measured results demonstrate that quite clear interference spectra can be obtained. Through analysis spatial frequency spectrum and temperature spectrum of two MZIs with different LC-filled structures, we can obtain that the MZI with adjacent two LC-filled holes has clearer interference spectrum and higher temperature sensitivity. Thus we choose this MZI to measure the temperature sensitivity with different lengths of LC filling. When the length of LC filling is 2 cm, the temperature sensitivities can be enlarged to 1.59 nm/C. The interferometer shows a good temperature tunability and sensitivity, which can be a good candidate for a highly tunable optical filtering and temperature sensing applications.     

1.82-μm distributed feedback lasers with InGaAs/InGaAsP multiple-quantum wells for a H2 O sensing system

High-strained InGaAs/InGaAsP multiple quantum wells （MQWs） distributed feedback （DFB） lasers, fabricated using metal organic chemical vapor deposition, are presented at 1.82 μm with a high side-mode-suppression ratio of 49.53 dB. The current- and temperature-tuning rates of the DFB mode wavelength are 0.01 nm/mA and 0.13 nm/℃, respectively. A characteristic temperature of 51 K is also confirmed. The DFB laser demonstrates good performance and can be applied to H₂ O concentration sensing.     

An approach of open-path gas sensor based on tunable diode laser absorption spectroscopy

Tunable diode laser absorption spectroscopy(TDLAS)is a new method to detect trace-gas qualitatively or quantificationally based on the scan characteristic of the diode laser to obtain the absorption spectroscopy in the characteristic absorption region.A time-sharing scanning open-path TDLAS system using two near infrared distributed feedback(DFB)tunable diode lasers is designed to detect CH_4 and H_2S in leakage of natural gas.A low-cost Fresnel lens is used in this system as receiving optics which receives the laser beam reflected by a solid corner cube reflector with a distance of up to about 60 m.High sensitivity is achieved by means of wavelength-modulation spectroscopy with second-harmonic detection.The minimum detection limits of 1.1 ppm·m for CH_4 and 15 ppm·m for H_2S are demonstrated with a total optical path of 120 m.The simulation monitoring experiment of nature gas leakage was carried out with this system. According to the receiving light efficiency of optical system and detectable minimum light intensity of detection,the detectable optical path of the system can achieve 1-2 km.The sensor is suitable for natural gas leakage monitoring application.     

Comparison of two kinds of active layers for high-power narrow-stripe distributed feedback laser diodes

Usually GaAs/AlGaAs is utilized as an active layer material in laser diodes operating in the spectral range of 800--850 nm. In this work, in addition to a traditional unstrained GaAs/AlGaAs distributed feedback (DFB) laser diode, a compressively strained InGaAlAs/AlGaAs DFB laser diode is numerically investigated in characteristic. The simulation results show that the compressively strained DFB laser diode has a lower transparency carrier density, higher gain, lower Auger recombination rate, and higher stimulated recombination rate, which lead to better a device performance, than the traditional unstrained GaAs/AlGaAs DFB laser diode.     

A Single Mode Hybrid Ⅲ-Ⅴ/Silicon On-Chip Laser Based on Flip-Chip Bonding Technology for Optical Interconnection

A single mode hybrid Ⅲ-Ⅴ/silicon on-chip laser based on the flip-chip bonding technology for on-chip optical interconnection is demonstrated.A single mode Fabry-Perot laser structure with micro-structures on an InP ridge waveguide is designed and fabricated on an InP/AlGaInAs multiple quantum well epitaxial layer structure wafer by using i-line lithography.Then,a silicon waveguide platform including a laser mounting stage is designed and fabricated on a silicon-on-insulator substrate.The single mode laser is flip-chip bonded on the laser mounting stage.The lasing light is butt-coupling to the silicon waveguide.The laser power output from a silicon waveguide is 1.3mW,and the threshold is 37 mA at room temperature and continuous wave operation.     

High-Temperature Operation of 8.5 μm Distributed Feedback Quantum Cascade Lasers

We present a distributed feedback quantum cascade laser （DFB-QCL） emitting at a wavelength of 8.5μm and operating up to 420K （147℃） with a low-threshold current density in pulsed mode. The DFB-QCLs studied are based on a four-well active design; the central portion of the waveguide consists of 60 periods of lattice matched InP-based InGaAs/AlInAs. In the design of the device, an active structure with lower doping and a deep-top grating process are utilized to achieve high temperature operation with a lower-threshold current density. At 420K, a low-threshold current density of 3.28 kA/cm^2 and a single mode peak power of 15mW are achieved on an epilayer-up mounting device with ridge width of 26μm and cavity length of 3.0mm. A side mode suppression ratio of 25 dB at 420 K is obtained.     

Time delay in InGaN multiple quantum well laser diodes at room temperature

This paper reports that a long delay between the beginning of pumping current pulse and the onset of optical pulse is observed in InGaN laser diodes.The delay time decreases as the pumping current increases,and the speed of the delay time reduction becomes slower as the current amplitude increases further.Such delay phenomena are remarkably less serious in laser diodes grown on GaN substrate than those on sapphire.It attributes the delay to the traps which cause a large optical loss by saturable absorption and retard the laser action.The traps can be bleached by capturing injected carriers.The effect of GaAs laser irradiation on InGaN laser action demonstrates that the traps responsible for the delay are deep centres which can be filled by the photo-assisted processes.     

Low Timing Jitter and Tunable Dual-Wavelength Picosecond Pulse Generation from a Fabry--Pérot Laser Diode with External Injection

A novel scheme to generate tunable dual-wavelength optical pulses with low timing jitter at arbitrary repetition rates is proposed and demonstrated experimentally. The pulses are generated from a gain-switched Fabry-Pérot laser diode with two external cw beams for injection seeding simultaneously. The cw light is generated by two independent distributed feedback laser diodes, and their wavelengths can be tuned independently by two temperature controllers. The dual-wavelength pulses with the pulse width of 57 ps, the timing jitter of 340 fs, are obtained. The sidemode-suppression ratio of the output pulses is better than 23dB over a 10-nm wavelength tuning range.