1.55-μm non-anti-reflection-coated fiber grating laser for single-longitudinal mode operation

A 1.55-m fiber grating laser (FGL) was fabricated by optically packaging a non-anti-reflection (AR) coated Fabry–Perot (FP) laser to a lensed fiber grating. The FGL demonstrates a single-longitudinal mode operation with a side-mode suppression ratio (SMSR) of up to 40 dB. The SMSR oscillates and diminishes to <30 dB as the increase of injection current above 38 mA, and the tilt of the fiber approaches 5° away from the facet normal of the FP laser. We have performed numerical simulations on the single-longitudinal mode operation for the FGL. The SMSR for the FGL increases over 40 dB as the increase of the fiber grating reflectivity (R _g) above 0.7 with non-AR-coated FP laser facet. Our calculations also show that the strong current-dependent SMSR oscillation is from the mode selection by the fiber grating external cavity and the heating effect in the FP laser.     

Simulation and optimization of 2.6–2.8 μm GaSb-based VCSELs

We present the simulation results of threshold operation of mid-infrared GaSb-based vertical-cavity surface-emitting lasers (VCSELs) obtained with the use of comprehensive fully self-consistent optical-electrical-thermal-recombination numerical model. The results show that by a proper design of VCSEL structure and composition of the active region it is theoretically possible to achieve room-temperature (RT) threshold operation for wavelength of 2.8 μm which is about 0.2 μm longer than those reported so far in the literature for III-V VCSELs with type-I quantum wells. Calculated values of the RT threshold current were equal to 2.5 and 4.0 mA for tunnel-junction diameters of 2 and 4 μm, respectively.     

High output power operation of 1.3-μm strained MQW lasers with low threshold currents at high temperature

We have investigated the temperature dependencies of the slope efficiency and the threshold current for strained multiquantum well (MQW) lasers as a parameter of the well number. Smaller well numbers mean larger temperature dependencies of the slope efficiency and the threshold current, while larger well numbers mean larger internal loss and broadening of the photoluminescence linewidth of the MQW structure. Furthermore, the change in the slope efficiency with temperature change is related to the change in internal loss. In this work, the 1.3-m strained MQW laser with a compressive strain of 1.0% and 7 wells shows the highest output power of 6.8 mW for an injection current of 50 mA and the lowest threshold current of 5.5 mA at 85°C, and the lowest variation in output power of 2.0 dB from 25–85°C at injection current of 50 mA.     

Modeling of InGaAsP–InP 1.55 μm lasers with integrated mode expanders using fiber-matched leaky waveguides

A new concept for InGaAsP–InP 1.55 μm lasers integrated with spot size converters using leaky waveguides is presented. The large fundamental mode size and the high discrimination of the higher order modes make ARROWs (Antiresonant Reflecting Optical Waveguides) and antiguided waveguides useful for fiber coupling functions. Three-dimensional (3-D) beam propagation method (BPM) results show that the devices have transformation losses lower than 0.22 dB. Fiber-coupling efficiencies of 60% are possible with standard cleaved single-mode fibers (SMF). The horizontal and vertical FWHM can be efficiently reduced to 9.70° (horizontal) and 17.80° (vertical). The fabrication of such devices avoids the growth of thick layers of quaternary material with a low Ga and As fraction, and simplifies the fabrication to one planar epitaxial growth step and one non-critical conventional etch. Received: 16 May 2001 / Published online: 30 October 2001     

High power 2-μm room-temperature continuous-wave operation of GaSb-based strained quantum-well lasers

A high power GaSb-based laser diode with lasing wavelength at 2 μm was fabricated and optimized. With the optimized epitaxial laser structure, the internal loss and the threshold current density decreased and the internal quantum efficiency increased. For uncoated broad-area lasers, the threshold current density was as low as 144 A/cm2 (72 A/cm2 per quantum well), and the slope efficiency was 0.2 W/A. The internal loss was 11 cm-1 and the internal quantum efficiency was 27.1%. The maximum output power of 357 mW under continuous-wave operation at room temperature was achieved. The electrical and optical properties of the laser diode were improved.     

Thermal analysis and efficient high power continuous-wave and?mode-locked thin disk laser operation of Yb-doped sesquioxides

We report on thermal conductivity analysis and thin disk laser power scaling of Yb:Lu₂O₃, Yb:Sc₂O₃, and Yb:LuScO₃. Using a volume Bragg-grating stabilized pump diode we have obtained cw output powers up to 301?W with optical-to-optical efficiencies of up to 73%. In mode-locked operation of an Yb:Lu₂O₃ thin disk laser 141?W of average output power with 738 fs pulses and an optical-to-optical efficiency of 40% were achieved.     

Geometry and material optimization of the extraordinary magnetoresistance in the semiconductor–metal hybrid structure

The effects of geometry and material parameters on the extraordinary magnetoresistance (EMR) of the rectangular semiconductor–metal hybrid structure have been studied systematically using the finite-element method (FEM). We find that the EMR depends sensitively on the placement of the voltage probes and explain the origin of the EMR enhancement in the asymmetric voltage probe configuration. The width of the metal is important for the EMR effect as well as the width of the semiconductor. The low-field EMR shows an approximate quadratic with the mobility of semiconductor, while the high-field EMR gradually saturates with the increase of mobility due to the little change of hall angle. To obtain significant EMR effect, the ration of conductivity of metal and semiconductor should be larger than 10⁴${10}^4$.     

Dispersion flattened photonic crystal fiber with high nonlinearity for supercontinuum generation at 1.55 μm

A robust design for a photonic crystal fiber (PCF) based on pure silica with small normal dispersion and high nonlinear coefficient for its dual concentric core structure is presented.This design is suitable for flat broadband supercontinuum (SC) generation in the 1.55-μm region.The numerical results show that the nonlinear coefficient of the proposed eight-ring PCF is 33.8 W -1 ·km -1 at 1550 nm.Ultraflat dispersion with a value between -1.65 and -0.335 ps/(nm·km) is obtained ranging from 1375 to 1625 nm.The 3-dB bandwidth of the SC is 125 nm (1496–1621 nm),with a fiber length of 80 m and a corresponding input peak power of 43.8 W.The amplitude noise is considered to be related to SC generation.For practical fabrication,the influence of the random imperfections of airhole diameters on dispersion and nonlinearity is discussed to verify the robustness of our design.     

Fabrication of Ge Nano-Dot Heterojunction Phototransistors for Improved Light Detection at 1.55μm

Heterojunction phototransistors （HPTs） with several Ge/Si nano-dot layers as the absorption region are fabricated to obtain improved light detectivity at 1.55μm. The HPT detectors are of n-p-n type with ten layers of Ge（8ML ） /Si（45nm） incorporated in the base-collector junction and are grown by an ultrahigh-vacuum chemicalvapor-deposition system. The detectors are operated with normal incidence. Because of the good quality of the grown material and fabrication process, the dark current is only 0.71pA/μm^2 under 5 V bias and the breakdown voltage is over 20 V. Compared to the positive-intrinsic-negative （PIN） reference detector with the same absorption layer, the responsivity is improved over 17 times for normal incidence at 1.55μm.     

Low-bias,high-temperature operation of an InAs–InGaAs quantum-dot infrared photodetector with peak-detection wavelength of 11.7 μm

We report on a low-bias InAs–InGaAs quantum-dot (QD) infrared photodetector (QDIP) with operating temperature of 150 K. Longwave-infrared (LWIR) detection at the peak wavelength of 11.7 μm was achieved. Peak specific photodetectivity D¹ of 1.7 × 10⁹ and 9.0 × 10⁷ cm Hz^1/2/W were obtained at the operating temperature T of 78 K and 150 K, respectively. A large photoresponsivity of 8.3 A/W and high photoconductive gain of 1100 were demonstrated at a low-bias voltage of V = 0.5 V at T = 150 K. The low-bias and high-temperature performance demonstration based on InAs–GaAs material systems indicates that the QDIP technology is promising for LWIR sensing and imaging.     

Tensile-strained 1.3 μm InGaAs/InGaAlAs quantum well structure of high temperature characteristics

A new tensile strained InGaAs/InGaAlAs quantum well structure in the 1.3 μm wavelength region is proposed for high temperature characteristics via quantum well band structure and optical gain calculations. To obtain such features, a tensile-strained InGaAs/InGaAlAs quantum well structure, which emits light dominated by TM polarization, is considered. This proposed structure has very high temperature characteristics (T ₀ > 130 K) due to its high density of state at the first transition edge. This results clearly show the potential of tensile strained quantum well structure usage for the high temperature operation of quantum well semiconductor lasers.     

Spectroscopy and cw operation of a 1.85 μm Tm:KY3F10 laser

Room-temperature cw laser operation on the ³ F ₄?³ H ₆ transition at 1.85 μm of Tm³⁺ ions in a KY₃F₁₀ single crystal is reported here for the first time. Using a cw Ti:sapphire laser as a pump source, a threshold absorbed pump power of 120 mW and a laser slope efficiency of 42.5% were achieved by using a 45% transmissive output coupler. Optimization of the activator concentration and crystal length is discussed taking into account self-quenching and pump-absorption efficiencies as well as parasitic and intrinsic reabsorption losses. The emission cross-section at the laser wavelength is determined using different methods, showing that the result of the J–O approach is, in this case, very uncertain. Received: 2 January 2001 / Revised version: 7 February 2001 / Published online: 27 April 2001     

Passive Q-switching of 1.44 μm and 1.34 μm diode-pumped Nd:YAG lasers with a V:YAG saturable absorber

Spectroscopic data of a V³⁺:YAG passive Q-switch crystal were measured. The absorption recovery time was determined to be of 37±7 ns and the ground state absorption cross section was estimated to be 0.7×10^-18 cm² at 1.44 μm and 3.5×10^-18 cm² at 1.34 μm. Passively Q-switched operation of diode pumped 1.44 μm and 1.3 μm Nd:YAG lasers was demonstrated using this crystal as saturable absorber. Average output powers of 1.42 W (1.44 μm) and 1.56 W (1.34 μm) and pulse energies of 24 μJ (1.44 μm) and 25 μJ (1.34 μm) were observed, respectively. Received: 19 August 2002 / Published online: 12 February 2003 RID="*" ID="*"Corresponding author. Fax: +49-40/42838-6281, E-mail: kretschmann@physnet.uni-hamburg.de     

High efficiency lasing of a dye-doped polymer laser with 1.06 μm pumping

For the first time, tunable lasing from a dye laser with an active polymer medium has been obtained using 1.06 μm pumping. The conversion efficiency of 43% and the tunable range of Δλ=63 nm have been reached with the use of polymethine dye in a polyurethane matrix. Received: 20 April 2001 / Revised version: 16 July 2001 / Published online: 19 September 2001     

Impacts of Ti on electrical properties of Ge metal–oxide–semiconductor capacitors with ultrathin high-k LaTiON gate dielectric

Ge Metal–Oxide–Semiconductor (MOS) capacitors with LaON gate dielectric incorporating different Ti contents are fabricated and their electrical properties are measured and compared. It is found that Ti incorporation can increase the dielectric permittivity, and the higher the Ti content, the larger is the permittivity. However, the interfacial and gate-leakage properties become poorer as the Ti content increases. Therefore, optimization of Ti content is important in order to obtain a good trade-off among the electrical properties of the device. For the studied range of the Ti/La₂O₃ ratio, a suitable Ti/La₂O₃ ratio of 14.7% results in a high relative permittivity of 24.6, low interface-state density of 3.1×10¹¹ eV⁻¹ cm⁻², and relatively low gate-leakage current density of 2.0×10⁻³ A cm⁻² at a gate voltage of 1 V.     

High-contrast 40 Gb/s operation of a 500 μm long silicon carrier-depletion slow wave modulator

In this Letter, we demonstrate a highly efficient, compact, high-contrast and low-loss silicon slow wave modulator based on a traveling-wave Mach-Zehnder interferometer with two 500?μm long slow wave phase shifters. 40 Gb/s operation with 6.6?dB extinction ratio at quadrature and with an on-chip insertion loss of only 6?dB is shown. These results confirm the benefits of slow light as a means to enhance the performance of silicon modulators based on the plasma dispersion effect.     

'On-wafer' surface implanted high power, picosecond pulse InGaAsP/InP (λ = 1.53–1.55 μm) laser diodes

A cost-effective, on-wafer surface implantation technique was applied for the fabrication of short pulse InGaAsP/InP laser diodes. Based on thick electroplated Au masks, local ion implantation could be performed, allowing for a versatile design of the absorber region. Picosecond pulses with typical FWHM of 20 ps and optical power exceeding 1.5 W were obtained. The measured emission spectrum in the pulsed regime demonstrates the potential of such devices to pump commercially available EDFA's.     

Free-running and Q-switched operation of a diode pumped Er:YSGG laser at the 3 μm transition

A study is made of a diode pumped Er³⁺:YSGG laser crystal operating at 2.797 μm. Lasers were constructed in the bounce geometry, using a transversely cooled 50 at.% Er:YSGG slab and a face-cooled 38 at.% Er:YSGG slab. Results from these are compared with those from a 50 at.% Er³⁺:YAG laser, also in the bounce geometry. With quasi-continuous wave diode pumping, free-running pulse energies of up to ~55 mJ and a slope efficiency of 20.5% are obtained from 50 at.% Er:YSGG. Better thermal performance is obtained from the face-cooled 38 at.% Er:YSGG slab, allowing average power of ~2 W to be obtained at a repetition rate and pump pulse duration of 140 Hz and 500 μs, respectively. Both Er:YSGG systems perform better than Er:YAG. Numerical modelling of the free-running 50 at.% Er:YSGG and Er:YAG systems is undertaken with good qualitative agreement with experimental results. Electro-optic Q-switching of the 50 at.% Er:YSGG laser using a LiNbO₃ crystal yields ~0.5 mJ pulses with ~77 ns duration.     

Laser diode spectroscopy of H2O at 2.63 μm for atmospheric applications

The 2.7 μm spectral range is highly suitable for the in situ monitoring of atmospheric H₂O using compact balloonborne laser diode spectrometers. Water vapour spectroscopic parameters of the 2₀₂   1₀₁ and the 4₁₃   4₁₄ transitions of the ν₃ band are revisited in this spectral region using a new distributed-feedback InGaAsSb laser diode emitting at 2.63 μm. Accurate line strengths are provided which are well adapted for the in situ probing of the middle atmosphere. Our measurements are thoroughly compared to an existing molecular database, laboratory measurements and ab-initio calculations. A laser hygrometer was developed for operation from small stratospheric balloons using this new laser diode technology, with emission at 2.6 μm. The realized sensor is described and results from a recent test-flight are reported. PACS 07.57.Ty; 92.60 Jq     

Estimation of required parameters for detection of small smoke plumes by lidar at 1.54 μm

Parameters of eyesafe lidar at 1.54 μm for detection of small plumes of smoke from burning wood or oil have been evaluated. It was assumed that a diode-pumped solid-state Er:glass laser at 1.54 μm or a Nd:YAG laser with a Raman cell or optical-parametric oscillator is used as a light source and that detection of backscattered light is performed with an avalanche photodiode. Ash and soot particle size distributions were taken from experiments. A backscattering coefficient at 1.54 μm for various source of smoke was estimated. In computing the laser energy, range between lidar and smoke, receiver optics diameter, fuel mass burned in unit time, fire source radius, laser pulse duration and visibility were varied. Results of the computations enabled estimation of the required laser energy, which ranges from 0.05 to 1400 mJ depending on the parameters. Received: 5 January 2000 / Revised version: 3 March 2000 / Published online: 11 May 2000