We report on our recent developments at II-VI Laser Enterprise of laser diode sources for the 79x nm range. High power conversion efficiency in excess of 62% was demonstrated. For high power applications like Thulium fiber laser pumping we have achieved an output power of more than 12.5W in CW operation for 94 μm wide broad-area single-emitters. We added the functionality of wavelength stabilization to the laser diodes by using a distributed feedback grating (DFB). Locking has been obtained over the full current range between 1A and 4A tested so far with some margin for temperature variation. For efficient fiber laser pumping the laser diodes were integrated in a multi-emitter platform, achieving 38 W out of a 105 μm fiber within 0.15 NA.
Multi-mode VCSEL arrays are candidates for compact illumination modules with applications as flash light illumination
for the detailed imaging of fast moving objects (> 100 m/s) or for time-of-flight cameras with modulation frequency in
the > 10 MHz domain. Rise and fall times have to be as short as a few nanoseconds, while the optical output power has
to be in the order of one Watt. In this paper we investigate, for multi-mode VCSEL arrays, the dependency of the far-field
pattern on the drive current and on the distance to a reflecting surface. We demonstrate that, with the help of
diffusing elements, the current dependency of the far-field pattern can be reduced. We have realized an illumination unit
with modulation frequencies of up to 80 MHz and an optical output of 1 Watt.
VCSELs (Vertical-Cavity Surface-Emitting Lasers) emit circularly symmetric beams vertical to the substrate; the small footprint of the active area (around 400 um2) enables the simultaneous fabrication of several thousand devices on a single wafer. Micro-optical components can modify the free-space optical properties of VCSELs for applications such as fiber-coupling in transceiver modules, illumination purposes, or beam profiling in sensing applications. However, the alignment of a laser towards a lens, for example, is expensive when performed separately for each device. Here we demonstrate a wafer-scale replication process to realise microlenses directly on top of the undiced VCSEL wafers. The process combines uv-casting and lithography to achieve material-free bonding pads and dicing lines. Several examples of lenses and gratings are given. An organically modified sol-gel material (ORMOCER) has been used as lens material. The micro-optical components on the wafer show good stability while sawing and bonding, where temperatures up to 220°C may occur. We have compared refractive lenses on top of the VCSELs with lenses on glass substrates. The lenses on the glass wafers were illuminated from the back-side by a planar wave. Spot diameters around 1.2 um and focal lengths of 30 um to 100 um were measured depending on the radii of curvature. On the VCSELs the lenses showed a strong influence on the transversal mode behaviour.
High data-rate communication links are placing increasing demands on the performance and cost of semiconductor-laser diodes. Vertical-cavity surface-emitting lasers (VCSELs) are ideal light sources for 10 Gbit/s applications. At Avalon Photonics Ltd., high-performance multimode VCSELs and VCSEL arrays are developed and fabricated for applications in low-cost fiber-optic communication links. An overview of static and dynamic characteristics of oxide-confined 850 nm VCSELs with data rates of 10 Gbit/s is presented. These 10 Gbit/s VCSELs are developed for the next generation 10 Gigabit Ethernet standard. Results show low threshold, high temperature operation, high modulation efficiency, short rise and fall times, and well-open eye-diagrams at different temperatures. Transmission over 600 m high-bandwidth multimode fiber at 10 Gbit/s is demonstrated. Mainly due to their low noise level and high linearity, these state-of-the-art devices are also well suited for transparent fiber-optic links using subcarrier multiplexed modulation schemes. Spurious-free dynamic ranges greater than 100 dBHz2/3 are reported.
At Avalon Photonics Ltd. high-performance multimode VCSELs and VCSEL arrays are developed and fabricated for applications in low-cost fiber-optic communication links. We report on state-of-the-art oxide-confined 850 nm VCSELs for current-generation parallel optical link modules with data-rates up to 3.125 Gbit/s per channel. The high performance and high reliability of these devices is reviewed. Moreover, 10 Gbit/s VCSELs are developed for the next-generation 10-Gigabit Ethernet standard (10-GbE). Transmission over 600 m high-bandwidth multimode fiber at 10 Gbit/s is demonstrated. Mainly due to their low noise level and high linearity, these high-performance devices are also well suited for transparent fiber-optic links using subcarrier multiplexed modulation schemes in the low GHz range. Spurious-free dynamic ranges larger than 100 dBHz2/3 were measured, which is sufficient for important applications like cable television distribution and remote antenna addressing in mobile phone systems.
Detailed study of external quantum efficiency (eta) QE is reported for AlGaInP-based Microcavity Light-Emitting Diodes (MCLEDs). Unlike conventional LED's the extraction efficiency (gamma) ext and far field profile depend on the linewidth of the intrinsic spontaneous emission and wavelength detuning between cavity mode and peak electroluminescence. This dependence makes it difficult to estimate the intrinsic spectrum, hence the performances of MCLED's. By using a non- destructive deconvolution technique, the intrinsic spectra of a MCLED and a reference LED (with the same active regions) could be determined at different current densities. This allowed precise calculation of (gamma) ext for both devices (values close to 11% were found for the MCLED), hence of their apparent internal quantum efficiencies (eta) int. At 55 A/cm2, values of 90% and 40% were determined for the LED and MCLED respectively. In order to explain this difference, we measured (eta) QE for devices with different sizes. From a fitting procedure based on a simple model taking into account the device size, we found out that the radiative efficiencies of LEDs and MCLEDs were close to 90%. We concluded that the low (eta) int of MCLED was due to a bad current injection, and especially to electron leakage current, as confirmed by numerical simulations.
VCSEL arrays are attractive low-cost high-speed sources for free space and fiber coupled links. An overview of existing device types and technologies as well as trends in device technology, optical interconnect and parallel optical datacom link applications is given. We discuss applications and performance limits of 1D and 2D VCSEL arrays. At Avalon Photonics Ltd. a broad variety of devices (up to 16x16) is developed and fabricated. Top and bottom emitting arrays offer new perspectives in optical and electrical packaging and are therefore of high interest for short haul parallel optical links, board-to-board and on-board ultra high-speed optical interconnects. Bandwidth requirements are increasing rapidly. The limits of VCSEL arrays in terms of modulation speed are analyzed and RF device optimization is discussed. Device and package limitations have to be considered. Important system level implications are also pointed out. Results of selectively oxidized 850nm multi transverse mode VCSEL arrays demonstrating low threshold current, high modulation efficiency and excellent multi-gigabit performance are presented. The paper ends with a discussion of future prospects in the field.
Coupled arrays of vertical-cavity surface-emitting lasers were realized by patterning the reflectivity of the top-distributed Bragg reflector using a phase-matching layer and a metal grid. For improved current injection and better heat dissipation the devices were selectively oxidized. Continuous-wave room-temperature operation of these anays has been achieved at 960 urn. Polarization resolved measurements revealed a stable behavior without any polarization flips in the fundamental lasingmode regime. Spectrally resolved measurements of the near- and far-field showed an abrupt transition from the fundamental super-mode with a TEM00-shaped near-field in every individual pixel with a beam divergence of 5.5°in both arraydirections to a TEM01-like super-mode with 5.5° beam divergence in one direction and 1 1° in the other direction. Measurements with high angular resolution showed full-width at half-maximum of the far-field intensity lobes as small as 0.89° for a lOxlO array.
In this contribution, we bring forward and compare the polarization switching (PS) dynamics and the polarization modulation characteristics of gain- and index-guided VCSELs. We then discuss the steady-state and dynamic characteristics of both types of VCSELs. Finally we focus on the polarization modulation limit and the average mode hopping frequency, which both scale over 8 orders of magnitude when the switching current is varied from just above threshold up to 2 times the threshold current.
The use of VCSELs in spectrally-demanding applications is considered. Vertical cavity lasers with emission in the (lambda) equals 763 nm, 780 nm and 852 nm wavelength ranges have been developed and optimized for spectrally-pure emission. Measurements of output spectra, linewidth, noise, polarization and spectral aging properties are considered in detail. Typical linewidth values below 5 MHz are seen, which makes these lasers very attractive for a variety of spectroscopic applications. Two of these optically-demanding applications are considered, namely oxygen sensing and pump sources for atomic clocks; performance results for the former are presented.
Femtosecond and picosecond pulses can find many applications if they can be produced with laser sources that are not only powerful and efficient but also compact and reliable. In continuous wave operation, diode pumping of solid-state lasers has allowed for a rapid progress towards powerful, compact and reliable sources, while the often used technique of Kerr lens modelocking for pulsed operation tends to be in conflict with requirements for diode-pumpable high power designs. Passive modelocking with semiconductor saturable absorber mirrors solves this problem as it relaxes the restrictions on the cavity design. We report on our recent achievements in this field. In particular we present a novel semiconductor device for dispersion compensation and various improved diode-pumped passively modelocked lasers. Also we show which laser parameters determine the stability of a passively modelocked lasers against Q-switching instabilities.
GaInP is a very important material for opto-electronic components operating in the visible spectra range ((lambda) equals 630...690 nm). Depending on MOVPE growth conditions, spontaneous ordering occurs which is established in form of a monoatomic superlattice arrangement of the group III components in {111}B planes. The reduced symmetry in ordered crystals leads to a polarization anisotropy of the absorption coefficient, which was investigated with transmission experiments. The polarization anisotropy was utilized to realize polarization threshold switches and polarization detectors. The polarization threshold switches consist of a n-i-p photodiode with an intrinsically integrated junction field-effect-transistor amplifier. The polarization anisotropy of the ordered GaInP absorption layer results in a polarization dependent source- gate photocurrent. This finally determines the working point of the device and yields in a very sensitive control of the source-drain conductivity GSD by the polarization of the incident light. First measurements reveal a switching contrast of 50 dB between light linearly polarized along the [011] and [01-1] crystal direction, respectively.
We present two monolithically integrated optical sensor systems based on semiconductor photonic integrated circuits. These compact, robust and highly functional transducers perform all necessary optical and electro-optical functions on-chip; extension to multi-sensor arrays is easily envisaged. A monolithic Michelson interferometer for high-resolution displacement measurement and a monolithic Mach-Zehnder interferometer for refractometry are discussed.
System miniaturization is a key issue in further development of integrated optical sensors. We present an integrated optical GaAs/AlGaAs-based Mach-Zehnder interferometer with a 2 mm long TiO2 on SiO2 embedded waveguide sensor pad and its performance as a bioaffinity sensor. Preliminary experiments to evaluate the performance of our Mach-Zehnder interferometer device were done by studying the binding of immunoglobulin (IgG) to protein A. The current resolution limit of IgG surface coverage is 3 pg/mm2, corresponding to less than 1/1600 of a saturated IgG monolayer. Since the design of the basic GaAs/AlGaAs waveguide structure is fully compatible with monolithic integration of lasers, modulators and detectors, improved performance can be expected from integration of these optoelectronic components on the same chip.
`Real-world' applications of femtosecond pulses require laser sources that are reliable, compact, and easy-to-use. Diode-pumped lasers are one key step in this direction, and as a next key step we have developed and demonstrated a simple technique using semiconductor saturable absorbers to passively start and stabilize mode-locked lasers. Because the saturable absorber stabilizes soliton modelocking, we achieve self-starting modelocking over a wide cavity stability range, in contrast to KLM, which tends to require critical cavity alignment and is usually not self-starting. We discuss different saturable absorber designs. An A-FPSA is used in a diode-pumped Nd:glass laser (130 fs, 100 mW avg output power) and a Cr:LiSAF laser (45 fs, 80 mW output). A thin saturable absorber design provides self-starting mode- locking over a wavelength range of 30 nm, and a low-loss design supports a record mode- locked output power of 120 mW from a Cr:LiSAF laser. A dispersive saturable absorber mirror design combines both negative dispersion compensation and saturable absorption within one semiconductor device and produces a compact mode-locked Cr:LiSAF laser (160 fs, 25 mW) without the need of prisms for dispersion compensation. Finally, we present an optimized diode-pumped cavity layout for Cr:LiSAF which allows for higher output powers (> 1 W cw).
Vertical cavity surface emitting lasers (VCSELs) are the light source of choice for a broad range of applications in sensing due to their potential for low cost manufacturability and unique optical properties. Tunable low cost, single mode lasers with an emission wavelength around 762 nm are important for applications in oxygen sensing. We present results on top- emitting AlAs/AlGaAs VCSELs with an emission wavelength of 765 nm. Minimum threshold currents of 0.6 mA and threshold voltages of 1.9 V are achieved. The dependence of these characteristics on laser size is determined. VCSELs with an emission window radius of 8 micrometer exhibit threshold currents of 1.5 mA, wallplug efficiencies of 11.2% and a slope efficiency of 0.46 W/A. The maximum output powers exceed 5 mW for large area lasers. These characteristics represent a significant improvement compared to previously published data for VCSELs operating in this wavelength range. The emission wavelength can be tuned by the operating current over a range of 2.7 nm without the appearance of higher order lateral modes. The current tuning coefficients range from 0.38 nm/mA for small area to 0.05 nm/mA for large area devices.
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