We present the time dependence, steady state behavior and spectra of a dual fiber-laser compound cavity. This particular cavity is formed with two Er-doped fiber amplifiers, each terminated with a fiber Bragg grating, and coupled through a 50/50 coupler to a common feedback and output coupling element. The experiment and theory show that a low Q, high gain symmetric compound cavity extracts nearly all the incoherent power in a coherent mode when the two fiber polarizations are aligned. This extraction is maintained even when there is significant difference in the optical pathlengths of the two component elements.
Far-infrared p-Ge laser operation in an active crystal prepared by transmutation doping is demonstrated for the first time. Though saturated current density in the prepared active crystal is twice lower than optimal, the laser performance is comparable to that of good lasers made from commercially produced melt grown p-Ge. The current saturation behavior of this material confirms the expected higher doping uniformity over melt grown laser rods.
Photonic-lattice structures with modulated gain, that is active photonic lattices (APLs), of large index steps and gain preferentially enhanced on the low-index lattice sites have been used, as early as 1988, for effective lateral-mode control range in large-aperture (100-200 microns) high-power coherent devices. Photonic-bandpass (PBP) structures relying on long-range resonant leaky-wave coupling, so called ROW arrays, have allowed stable, near-diffraction-limited beam operation to powers as high as 1.6W CW and 10W peak pulsed. Photonic-bandgap (PBG) structures with a built-in lattice defect, so called ARROW lasers, have provided up to 0.5W peak-pulsed stable, single-mode power and hold the potential for 1W CW reliable single-mode operation from apertures 8-10 microns wide. The solution for high-efficiency surface emission, from 2nd-order DFB/DBR lasers, in an orthonormal, single-lobe beam pattern was found in 2000. Recently, single-lobe and single-mode operation in a diffraction-limited beam orthonormal to the chip surface was demonstrated from 1.5mm-long DFB/DBR ridge-guide lasers. That opens the way for the realization of 2-D surface-emitting,2nd-order APLs for the stable generation of watts of CW single-lobe, single-mode power from large 2-D apertures, as well as scalability of such devices at the wafer level.
Tapered unstable resonator lasers incorporating laterally finite mirrors are reported. By reducing the lateral extent of mirrors, cavity losses may be shifted from internal losses (contributing to scattering and absorption) to mirror losses (contributing to output power). Design of these cavities will be discussed and evaluated. Experimental data substantiates theoretical analysis, showing an increase in slope efficiency of 40% over conventional infinite aperture tapered lasers.
The susceptiblity of curved-grating, surface-emitting, distributed-feedback lasers to filamentation and multi-mode instability was studied by numerical modeling and analysis. It was found that reflections from the ends, edges and output window can intiate instability, leading to filmantation. These effects can be suppressed by using absorbers between the end of the stripe and teh cleaved ends of the chip, wide unpumped-grating edges, and introducing wedge into the substrate to alleviate the effect of window reflections. It was also found that increasing the optical-confinement factor improves stability. A specific design is presented that is stable up to values of antiguiding that is larger than that measured in InGaAs quantum well lasers.
High efficiency, high power and excellent beam quality has been
achieved in optically-pumped semiconductor disc lasers (OPS-disc
laser) emitting at 1000nm. Minimizing the thermal resistance
between active region and heat-sink, more than 5.5W of continuous
wave (cw) output has been obtained at room-temperature. Even more
remarkable, the laser characteristics corresponding to this power
display differential efficiencies of better than 50% and
optical conversion efficiencies of better than 40%. This
combination of high power and high efficiency represents the best
reported values so far. As such, a highly efficient beam converter
has been realized, transforming low-brightness optical pump power
into high-brightness laser emission.
We describe design and performance of novel, electrically pumped, vertical compound cavity semiconductor lasers emitting at 980 nm. The laser combines a vertical cavity semiconductor laser with a partially reflecting output coupler and an external cavity for mode control. The concept is scalable and has been demonstrated in monolithic low power (few miliwatts) devices all the way to high power extended cavity devices which generate over 950 mW CW multimode power and 0.5 W CW power in a TEM00 mode, the latter with 90% coupling efficiency into a single mode telecommunication fiber. The concept has been applied to the development of uncooled lasers, mounted in TO-56 cans, capable of producing 50 to 100 mW of fiber-coupled power. We have also demonstrated the extended cavity lasers at wavelengths of 920 nm and 1064 nm. We present reliability data for the chips used in the extended cavity lasers.
The introduction of high power diode laser systems in industry has boosted the interest in these devices for a wide range of applications. Besides printing and soldering, cutting and deep penetration welding are becoming more important. An overview about the developments, an update on today's high power laser activities and an outlook will be given, what characteristics laser bars will have to fulfil in the near future.
For higher brightness, laser bars with lower fill factors, monolithic integrated laser junctions and tapered laser designs were investigated. High power diode laser (HPDL) bars with 25% - 50% fill factor were operated between 40 W and 80 W and lifetimes up to 100 000 hours could be extrapolated. Tapered laser bars with 50W output power and high wall plug efficiencies were developed.
Wavelength multiplexing and polarisation coupling were used in order to reach multi-kilo-Watt diode laser emission. Examples for applications will be given.
The long-term reliability of high-power, single-mode, 980 nm, InGaAs/GaAlAs/GaAs, laser diodes is reported. We have performed constant-current aging at at 85°C for three operating currents, 450 mA (~300 mW), 550 mA (~350 mW) and 700 mA (~420 mW). The data for 450 mA aging indicate a total failure rate of less than 250 FITs at a confidence level of 60%. For 550 mA and 700 mA operating currents, no degradation in laser performance within the 5% measurement accuracy of our test equipment have been observed during the first thousand hours of testing.
This study examines catastrophic optical damage in failed, single-mode, 980 nm, InGaAs/GaAlAs/GaAs, ridge wave-guide laser diodes. Analysis techniques were selected for their simplicity to provide quick evaluation of material and device quality. The analysis techniques are chemical etching, optical microscopy, infrared microscopy, and scanning electron microscopy.
Facet overheating is considered a potential source for device degradation of diode lasers. We test two different concepts for the reduction of facet temperatures of high-power diode lasers by measuring the facet temperatures by means of Raman spectroscopy. For conventional high-power broad area lasers we demonstrate the reduction of the facet overheating by the introduction of current blocking layers by a factor of 3-4. For another set of devices among them quantum well and quantum-dot lasers with almost the same device design we find a reduction of the overheating by 40 to 60 percent for the dot devices. Thus we qualify two very different but promising technological approaches for increasing device reliability.
A general scheme for the determination of vital operating characteristics of semiconductor lasers from low intensity photo-luminescence spectra is outlined and demonstrated. A fully microscopic model for the optical properties is coupled to a drift-diffusion model for the mesoscopic charge and field distributions to calculate luminescence and gain spectra in barrier-doped laser material. Analyzing experiments on an optically pumped multi quantum-well structure it is shown that the electric fields arising from the charges of ionized dopants lead to strongly excitation dependent optical properties like significant differences between luminescence and gain wavelengths.
We present a comparison of experimental and microscopically based model results for optically pumped vertical external cavity surface emitting semiconductor lasers. The quantum well gain model is based on a quantitative ab-initio approach that allows calculation of a complex material susceptibility dependence on the wavelength, carrier density and lattice temperature. The gain model is coupled to the macroscopic thermal transport, spatially resolved in both the radial and longitudinal directions, with temperature and carrier density dependent pump absorption. The radial distribution of the refractive index and gain due to temperature variation are computed. Thermal managment issues, highlighted by the experimental data, are discussed. Experimental results indicate a critical dependence of the input power, at which thermal roll-over occurs, on the thermal resistance of the device. This requires minimization of the substrate thickness and optimization of the design and placement of the heatsink. Dependence of the model results on the radiative and non-radiative carrier recombination lifetimes and cavity losses are evaluated.
The problem of delivering a multimode pump to a single mode core is considered. Original designs of double-clad fiber amplifiers are analyzed. A slab pump geometry of a fiber amplifier is suggested. The partially coherent pump should be coupled to the narrow slab which supports propagation of the pump and absorbs highest modes of the core, then the core itself has no need to be single-mode. The slab plays the role of cladding in conventional double-clad fibers. Such a cladding has no need to surround the core and its refractive index can be higher than that of the core. Then the highest modes of the slab are absorbed at the very beginning of the amplifier. This allows for tapering of the slab, providing almost constant pump power in the core. The end result is a the high efficiency of a compact device with scaling to high pump power. We provide the key design formulars for such a device.
Modeling of high-power diodes poses several numerical problems. They require algorithms capable of capturing accurately the fast temporal and spatial dynamics in a broad spectral range. Another problem is how to reconcile vastly different time scales of various physical processes involved. We present an outline of a semiconductor laser simulation engine that incorporates both the first-principles many body gain calculations, and the carrier and heat transport simulation into an interactive computer laser model.