Tapered rib adiabatic following fiber couplers in etched GaAsmaterials for monolithic spot-size transformation

Design details and demonstration data are presented for an (Al,Ga)As monolithic tapered rib waveguide achieving modal spot-size transformation. The tapered rib adiabatic following fiber coupler structure (TRAFFIC) achieves two-dimensional (2-D) expansion of the output optical mode of single-transverse-mode semiconductor waveguide modulators and lasers using a one-dimensional (1-D) taper between noncritical initial and final taper widths which are compatible with optical lithographic techniques. Measurements are presented of total mode expansion losses between ~1.5-2.0 dB and semiconductor to single-mode-fiber waveguide coupling losses of ~0.5-1.0 dB for doped pin optical-modulator-type waveguides using the TRAFFIC waveguide. A semiconductor laser with a TRAFFIC tapered-rib mode-expansion section and measured coupling loss between the laser output and single-mode fiber of only 0.9 dB is described. Finally, a TRAFFIC Spot-size transformer for undoped waveguide modulators with total mode expansion losses of 1.84 dB and excellent modal behavior at 1.32-μm wavelength is presented. The TRAFFiC structure is particularly well suited for integration with both active and passive etched rib waveguide devices. Fabrication is relatively simple, requiring only patterning and etching of the tapered waveguide and uniform-width outer mesa waveguide without any epitaxial regrowth     

锂离子电池正极材料稀土掺杂研究进展

作为一种新型的高效绿色电池,稀土掺杂材料在锂离子电池中得到了广泛的应用,从而成为稀土应用的重要应用领域之一。阐述了锂离子电池技术发展的重要性,综述了稀土掺杂对锂离子电池正极材料结构和电化学性能的影响。重点介绍了稀土掺杂尖品石锰酸锂正极材料研究进展,展望了稀土掺杂在锂离子电池正极材料中的应用发展前景,并认为随着稀土掺杂研究的深入,采用稀土掺杂以进一步推动高性能锂离子电池的发展,是今后高比容量电池发展的一个重要领域。     

Design and characteristics of high-power (>0.5-W CW)diode-pumped vertical-external-cavity surface-emitting semiconductorlasers with circular TEM00 beams

We describe the design, fabrication, and measured characteristics of the high-power optically pumped-semiconductor (OPS) vertical-external-cavity surface-emitting lasers (VCSELs). Using diode laser pumping, we have recently demonstrated operation of such lasers, which for the first time generate high (watt-level) power and a circular Gaussian beam directly from a semiconductor laser. These OPS-VECSELs have a strain-compensated multi-quantum-well InGaAs-GaAsP-GaAs structure and operate CW near λ~1004 nm with output power of 0.69 W in TEM ₁₁ mode, 0.52 W in TEM₀₀ mode and 0.37 W coupled to a single-mode fiber. With multiple pump and gain elements, OPS-VCSEL technology is scalable to the multiwatt power levels. Such lasers will prove useful in a variety of applications requiring compact and efficient sources with high-power output in a single-mode fiber or with diffraction-limited beam quality     

Synchronization of high-power broad-area semiconductor lasers

Semiconductor lasers offer significant operational advantages due to their compactness and high electrical-optical conversion efficiency. The major drawback in considering semiconductor lasers for many applications is the relatively small emission power that can be obtained from a single semiconductor laser. Synchronization of laser arrays provides a unique solution to this limitation. In this paper, we describe our recent research on the synchronization of high-power broad-area semiconductor lasers and laser arrays. We demonstrate experimental results on 1) simultaneous injection locking of multiple broad-area lasers to achieve single longitudinal/transverse mode beams and 2) synchronization and coherent beam combination of an integrated 19 broad-area laser array based on a scalable external cavity. A number of issues in the synchronization of broad-area lasers have been addressed in this paper. These include the effects of laser coupling on the array synchronization performance and the gigahertz complementary intensity oscillations occurring at different transverse modes of broad-area lasers subject to the optical injection.     

Chalcogenide Glass-Fiber-Based Mid-IR Sources and Applications

The Naval Research Laboratory (NRL) is developing chalcogenide glass fibers for applications in the mid- and long-wave IR wavelength regions from 2 to 12 $mu$m. The chalcogen glasses (i.e., glasses based on the elements S, Se, and Te) are transparent in the IR, possess low phonon energies, are chemically durable, and can be drawn into fiber. Both conventional solid core/clad and microstructured fibers have been developed. Chalcogenide glass compositions have been developed that allow rare earth doping to enable rare-earth-doped fiber lasers in the IR. Also, highly nonlinear compositions have been developed with nonlinearities ${sim}hbox{1000} times$ silica that enables nonlinear wavelength conversion from the near IR to the mid- and long-wave IR. In this paper, we review rare-earth-doped chalcogenide fiber for mid- and long-wave IR lasers, and highly nonlinear chalcogenide fiber and photonic crystal fiber for wavelength conversion in the mid- and long-wave IR.      

Materials for Photonic Applications From Sol-Gel*

A review of sol-gel materials developed in our laboratory for photonic applications is presented. These materials include planar and strip waveguides for integrated optics (IO) passive devices, Er doped waveguides for IO amplifiers, films doped with semiconductor quantum dots for optical switching and fullerene doped materials for optical limiting.     

Materials for Photonic Applications From Sol-Gel*

A review of sol-gel materials developed in our laboratory for photonic applications is presented. These materials include planar and strip waveguides for integrated optics (IO) passive devices, Er doped waveguides for IO amplifiers, films doped with semiconductor quantum dots for optical switching and fullerene doped materials for optical limiting.     

Glass-fiber lasers in the ultraviolet and visible

The recent fabrication of rare earth-doped fluoro-zirconate (ZBLAN) glass fiber has spurred the development of a family of visible and ultraviolet fiber lasers pumped by upconversion. The performance of CW room temperature devices demonstrated to date is reviewed with emphasis on the recently reported Nd-doped ZBLAN fiber laser operating in the ultraviolet at 381 nm     

Progress in GaN-based quantum dots for optoelectronics applications

Our recent progress in GaN-based quantum dots (QDs) for optoelectronics application is discussed. First, we discussed an impact of the use of GaN-based QDs on semiconductor lasers, showing theoretically that reduction of threshold current by using the QDs in GaN-based lasers is much more effective compared to those in GaAs-based or InP-based lasers. Then discussed are our growth technology including self-assembling growth of InGaN QDs on sapphire substrates by atmospheric-pressure metalorganic chemical vapor deposition. Using the self-assembling growth technique, we have succeeded in obtaining lasing action in an edge-emitting laser structure with the InGaN QDs embedded in the active layer under optical excitation with the emission wavelength of 410 nm. Toward UV light wavelength emission, we have recently established self-assembled GaN QDs of high quality and high density under very low V-III ratio. We clearly observed two photoluminescence peaks from both the QDs and the wetting layer at room temperature, which clearly shows the nanostructures are formed with the Stranski-Krastanow growth mode.     

Unidirectionally coupled synchronization of optically injected semiconductor lasers

Three different synchronization scenarios, namely identical chaos synchronization, chaotic driven oscillation, and chaotic optical modulation, are experimentally observed for the chaotic optical fields generated by optically injected semiconductor lasers that are unidirectionally coupled. In this fully optical system, the channel signal is different from the output field of the transmitter laser by an additional injection optical field delivered by a master laser. In the case of identical chaos synchronization, the output field of the receiver is synchronized, and frequency locked, to that of the transmitter but is not synchronized to the channel signal. In chaotic driven oscillation, the output field of the receiver is synchronized, and frequency locked, to the channel signal but is not synchronized to the output field of the transmitter. In chaotic optical modulation, the output field of the receiver is synchronized, but not frequency locked, only to the channel signal. These three different synchronization scenarios are identified in the same optical-injection system under different operating conditions.     

Monolithically integrated multiwavelength sampled grating DBRlasers for dense WDM applications

For accurate control of the channel spacing in fabricating multiwavelength laser arrays or discrete multicolor lasers, we proposed a novel approach that exploits sampled grating distributed Bragg reflector (DBR) mirrors to vary the laser wave length across the wafer. This approach can realize a set of lasers with a wavelength spacing that meets the ITU recommendations for dense wavelength-division multiplexing systems and a wavelength range that can cover up to 40 nm or more. The wavelength variation across an array is achieved by changing the sampling periods of the DBR mirrors from laser to laser. The accuracy on the channel spacing of sampled grating DBR laser arrays was shown to be the same as that of conventional distributed feedback or DBR laser arrays, but their wavelengths can be better controlled for the gratings are fabricated with single holographic exposure. Arrays of 21 lasers have been successfully fabricated and have around 0.8-nm wavelength spacing with a simple tuning mechanism     

Picosecond SESAM-based ytterbium mode-locked fiber lasers

Using semiconductor saturable absorber mirrors and a grating-pair dispersion compensator, we obtain reliable self-starting mode locking of a ytterbium (Yb) fiber laser tunable over 125 nm. The 980-1105-nm tuning range is achieved by optimization of nonlinear reflection and bandgap characteristics of the multiple-quantum-well saturable absorber and by proper engineering of the laser cavity. A short-length Yb-doped double-clad amplifier seeded with mode-locked Yb-fiber laser produces picosecond pulses with energy of 30 nJ (700 mW of average power). A compact version of the fiber laser was built using a Gires-Tournois compensator and short length (1-cm long) of highly doped Yb fiber. Using a novel semiconductor saturable abserver mirror based on GaInNAs structure, self-started 1.5-ps pulse mode-locked operation was obtained at 1023 nm with a repetition rate of 95 MHz. A mode-locked Yb-doped fiber laser was also developed without using any dispersion compensation technique. Overall group-velocity dispersion was minimized by using highly doped Yb fiber in a compact amplifying loop cavity. Self-started mode-locked operation was obtained in 980-1030-nm wavelength range with a fundamental repetition rate of 140 MHz. Without using dispersion compensation, the lasers produced pulses in a range from 15 to 26 ps.     

Frequency plan and wavelength switching limits for widely tunablesemiconductor transmitters

As wavelength-division-multiplexing (WDM) channel spacing continues to decrease in size, and with the application of tunable lasers in dense wavelength-division-multiplexing (DWDM) systems, we demonstrate the ability of tunable semiconductor lasers to cope with demanding channel spacing and inevitable low frequency setting error. By finding the stable operating points of a single tunable laser at the desired frequencies, using advanced software a lookup table to drive the laser was generated. Once the drive currents to access 2000 channels with the laser are found, their frequency setting error and side mode suppression ratio (SMSR) were found. These results open up new possibilities for DWDM access networks as well as providing a limit of achievement for channel density in the network. Meanwhile, since the sampled-grating distributed Bragg reflector (SG-DBR) laser is among the most attractive sources for DWDM, it is important to investigate its wavelength switching characteristics. This behavior will affect wavelength routing and the capability limits for channel reallocation in future networks. We present new detailed experimental studies on a high-speed SG-DBR laser by using a Fabry-Perot interferometer technique adapted for the noncontinuous wave case. Measurements of fast intramodal (i.e., cavity mode) and intermodal (i.e., supermode) wavelength switching and insights into the device's dynamic behavior are obtained. Implications are given for transmitter design in dynamic wavelength routing and channel reallocation     

Silicon-on-silicon rib waveguides with a high-confiningion-implanted lower cladding

The realization of single-mode rib waveguides in standard epitaxial silicon layer on lightly doped silicon substrate, using ion implantation to form the lower cladding, is reported. The implanted buffer layer enhances' the vertical confinement and improves the propagation characteristics. Respect to similar standard all-silicon waveguides a propagation loss reduction of about 7 dB/cm, in the single-mode regime, has been measured. A numerical analysis has been performed to evaluate the theoretical attenuation and the transverse optical field profiles. As a result of the presence of the ion implanted buffer layer, an increase of the fundamental mode confinement factor from 0.3 to 0.85 has been calculated. This results in a great enhancement of the coupling efficiency with standard single-mode optical fibers. Moreover, the proposed technique is low cost, fully compatible with standard VLSI processes, and allows a great flexibility in the integration of guided-wave devices and electronic circuits. Finally, the very high thermal conductivity characterizing these waveguides makes them attractive host-structures for electrically and thermally controlled active optical devices     

Nonlinear properties of tapered laser cavities

The nonlinear phenomena accompanying the process of light generation in high-power tapered semiconductor lasers are studied using a combination of simulation and experiment. Optical pumping, electrical overpumping, filamentation, and spatial hole burning are shown to be the key nonlinear phenomena influencing the operation of tapered lasers at high output powers. In the particular tapered laser studied, the optical pumping effect is found to have the largest impact on the output beam quality. The simulation model used in this study employs the wide-angle finite-difference beam propagation method for the analysis of the optical propagation within the cavity. Quasi-three-dimensional (3-D) thermal and electrical models are used for the calculation of the 3-D distributions of the temperature, electrons, holes, and electrical potential. The simulation results reproduce key features and the experimental trends.     

Carrier diffusion and depletion effects on multiwave mixing insemiconductor lasers

A detailed theoretical treatment has been undertaken of multiwave mixing in semiconductor lasers, taking into account the effects of pump/probe depletion, carrier diffusion, usual gain saturation, nonlinear gain compression, total power dependence of the coupling coefficients as well as the longitudinal dependence of the nonlinear interaction. It is shown that the effect of carrier diffusion can considerably enhance the probe and conjugate reflectivity for detuning frequency near the relaxation oscillation frequency of the pump laser. It is demonstrated, in particular that, for relatively high input probe power, the probe and conjugate reflectivity can be enhanced significantly near the relaxation oscillation frequency of the pump laser, compared to that for low input probe power. Furthermore, both the probe reflectivity and the conjugate reflectivity show asymmetric characteristics with respect to the zero pump/probe frequency detunings. The pump/probe depletion effect plays an important role in determining the optical output power when the input probe power is larger than ~0.1 μW     

Two-dimensional photonic crystal semiconductor lasers: computational design, fabrication, and characterization

We have designed, fabricated, and tested two-dimensional (2-D) slab photonic crystal semiconductor lasers at communication wavelengths. Wavelength-size microresonators defined on the 2-D slab photonic crystal have been effective in photon confinement and functioned well as ultra-small lasers by optical pumping. The photonic crystal laser structures that we have tested have shown large quality factors and low thresholds.     

The promise of cryogenic solid-state lasers

Cryogenic cooling of solid-state lasers has a number of important benefits, including the near vanishing of optical distortion in high average power lasers, as well as enhanced spectroscopic and lasing properties. These benefits are just beginning to be exploited to produce compact high average power lasers whose output is scalable, near diffraction limited, and whose efficiencies will exceed those of modern bulk solid-state lasers. In this paper, we review the history of cryogenically cooled solid-state lasers and the benefits of cryogenic cooling, including optical and laser properties and thermal and thermooptic properties; examine cryogenic amplifiers and cooling methods, including a straight-through propagation thin-disk configuration that does not perform well at room temperature, and summarize the experimental performance demonstrated to date. As a specific example, we examine the spectroscopic and lasing properties of Yb:YAG and show that compact high efficiency and high average power, near diffraction limited lasers (>100 kW) can be realized in the near future using presently available technology.     

Cleaved and etched facet nitride laser diodes

Room-temperature (RT) pulsed operation of blue (420 nm) nitride-based multiquantum-well laser diodes grown on a-plane and c-plane sapphire substrates has been demonstrated. Structures investigated include etched and cleaved facets as well as doped and undoped quantum wells. A combination of atmospheric and low-pressure metal organic chemical vapor deposition using a modified two-flow horizontal reactor was employed. Threshold current densities as low as 12.6 kA/cm² were observed for 10×1200 μm lasers with uncoated reactive ion etched facets on c-plane sapphire. Cleaved facet lasers were also demonstrated with similar performance on a-plane sapphire. Laser diodes tested under pulsed conditions operated up to 6 h at RT. Lasing was achieved up to 95°C and up to a 150-ns pulselength (RT). Threshold current increased with temperature with a characteristic temperature T₀ of 114 K     

Fundamental characteristics of semiconductor ring laser gyroscopes

Semiconductor ring lasers have many capabilities of realizing new functional devices. In this paper, we propose a novel optical inertial rotation sensor using a semiconductor ring laser. If a semiconductor ring laser operates as an optical inertial rotation sensor, a very small and simple optical gyroscope can be realized. To our knowledge this is the first demonstration of a semiconductor ring laser gyroscope (S‐RLG). Experimental results are as follows. (1) The Sagnac frequency shift can be detected as a beat note by the terminal voltage change of the semiconductor ring laser without branching the circulating optical power. Therefore, the S‐RLG system can be constructed very simply as compared with already proposed optical gyroscopes. (2) The detected beat frequency between two counterpropagating lasers in the S‐RLG is directly proportional to the applied rotation rate. (3) Furthermore, we present data demonstrating the injection locking phenomenon around low rotation rate. These results verify that the proposed S‐RLG operates as an optical inertial rotation sensor based on the Sagnac effect. © 2000 Scripta Technica, Electr Eng Jpn, 132(4): 73–78, 2000