We demonstrate wafer-scale integration of a saturable absorber in a surface emitting semiconductor laser. Vertical
external cavity surface-emitting lasers (VECSELs) have high quality circular output beams, 2D-array scalability, and
high average power. To date, ultrafast VECSELs required a folded cavity with a separate saturable absorber device for
passive modelocking. In the result presented here, we integrate the saturable absorber into the same semiconductor
wafer, optimize its performance for integration with quantum dots and demonstrate stable passive modelocking in a
simple straight external cavity which allows for a fully monolithically wafer-integrated structure to reduce cost and
improve ease of mass production. We refer to this class of devices as the modelocked integrated external-cavity surface emitting laser (MIXSEL). Such devices would be ideally suited for many applications where the current ultrafast laser technology is considered to be too bulky and expensive.
We numerically investigate how polarization properties of vertical cavity surface emitting lasers (VCSELs) are affected by optical feedback from an extremely short external cavity. In order to do it we use a two modes rate equation model that accounts for infinite round trips in the external cavity. With it we perform maps of bistability finding out a modulation of the polarization switching currents when increasing the external cavity length with a period equal to half the wavelength of operation of the device. When the external mirror reflectivity is high enough there is a region within each period of modulation where the VCSEL polarization is stable at any injected current within the operation range. Moreover, using polarized feedback the map of bistability is channelled and the bistable region is almost suppressed. A preliminar study of the effects of temperature variations on the map of bistability is also carried out and presents polarized feedback as a more robust technic to obtain polarization stabilization. An experimentally obtained map of bistability supports the model showing very good agreement with it.
Vertical-cavity surface-emitting lasers (VCSELs) have now entered
the stage of implementation in optical devices and networks.
Besides the number of properties superior to EEL, VCSELs have the
drawback of less stabilized polarization. As any semiconductor
laser, VCSELs are sensitive to Optical Feedback (OF) due to
parasitic reflections inevitable in practical applications. Here
we experimentally and numerically investigate the spectral and
polarization properties of VCSELs subject to polarization
insensitive optical feedback from an extremely short external
cavity -ESEC (few microns). We observe that the total output power
and the wavelength of laser emission are sinusoidally, in-phase
modulated with the external cavity (EC) length and with a period
equal to half the wavelength of the VCSEL. Moreover the currents
at which the switches between the two linearly polarized (LP)
modes happen and the hysteresis width are also sinusoidally
modulated with the same period. When biasing the VCSEL at the
switching current polarization switching between two orthogonal
linearly polarized states happens at different EC lengths. This
polarization switching happens through a hysteresis when
decreasing and increasing the EC length. We have also develop a
two modes rate equation model that proves and explains the changes
of all VCSEL characteristics observed in the experimental part,
showing very good agreement with the experimental results.
We report on in-depth experimental and theoretical studies of the polarization behavior and mode structure of Vertical-Cavity Surface-Emitting Lasers with an elliptical surface relief. The aim of the relief is twofold, first to enhance the single mode operation by introducing spatially distributed losses, and second, to enhance polarization stability by making these losses anisotropic, i.e. polarization dependent. We first identify the transverse mode structure for different dimensions of the relief ellipse as a function of the injection current. Then we proceed with a systematic measurement of the change in orientation of the two linearly polarized (LP) fundamental modes emitted by VCSELs and their frequency splitting as a function of an externally applied strain, anisotropic in the plane of the quantum wells. We investigate two orientations of the surface relief ellipse: longer sides along [110] and [-100] directions, respectively. These studies show that whereas the polarization direction is governed by the orientation of the index ellipsoid, the selection of the lasing LP mode is mainly determined by the anisotropy in the mirror losses introduced by the elliptical surface relief. We confirm these experimental studies by theoretically estimating the effect of the relief and the anisotropic strain on both the refractive index and the losses.
We report on recent progress in the design of short-wavelength vertical-cavity surface-emitting lasers (VCSELs) for 10 Gbit/s datacom applications. Topics of interest include differential mode delay characterizations of high-performance multimode fibers and their interplay with transverse single- and multimode VCSELs, flip-chip integrated two-dimensional arrays at 850 nm wavelength, as well
as experiments toward the realization of optical backplanes. In
the latter case, reliable 10 Gbit/s data transmission has been
achieved over low-loss integrated polymer waveguides with up to 1
meter length. Moreover we present VCSELs with output powers in the 10 mW range that are employed in multi-beam transmitters for free-space optical data transmission with Gbit/s speed over distances of up to about 2 km.
Heiko Unold, Sawat Mahmoud, Roland Jaeger, Matthias Golling, Max Kicherer, Felix Mederer, Michael Riedl, T. Knoedl, Michael Miller, Rainer Michalzik, Karl Ebeling
We compare various approaches aiming at large-area high-power single-mode oxidized VCSELs. Stable and reproducible single-mode emission with SMSR (side-mode suppression ratio) greater than 30dB and output powers well above 5mW are reported for the long monolithic cavity and self-aligned shallow surface etching approaches, both of which are suitable for commercial production. Additionally, Photonic Crystal Surface-Emitting Lasers (PCSELs) are introduced, which enable advanced mode control by novel transverse optical guiding techniques.
We present the investigation of several methods to increase the active diameter of single transverse mode oxide confined VCSELs in both the 850 and 980 nm wavelength regimes. Among the concepts considered are mode intensity specific shallow surface etched reliefs, monolithically increased cavity lengths, current confinement matching the fundamental mode intensity distribution and saturable absorbers. All approaches are introduced in theoretical considerations and corresponding measurement results are presented. Additionally, numerical simulations are performed to gain an increased understanding of some of the mode selection mechanisms. The considered concepts are evaluated in terms of decrease of the series resistance (for impedance matching/driving reasons) and device lifetime (as derived from maximum current densities). The results obtained are also compared to other approaches found in literature (e.g. metal apertures, photocurrent feedback, Fabry-Perot etalon, half-symmetric cavity). Conventional devices with optimized thin oxide aperture location have shown single-mode output powers above 4 mW with an active diameter of 3.5 micrometer. A record high single-transverse mode output power of 5 mW at a series resistance of 98 (Omega) is obtained for a 7 micrometer aperture device by increasing the cavity length monolithically by 4 micrometer.
Vertical-cavity surface-emitting lasers (VCSELs) with emission wavelengths in the range from 850 nm to 950 nm are highly attractive light sources for low-cost, high-speed data transmission over several hundred meters of perfluorinated graded-index plastic-optical fibers (GI- POFs). Multi-Gigabit/s data transmission over common PMA- based step-index POFs is generally limited to several meters predominantly due to dispersion. Here, we demonstrate 3 Gbit/s and 7 Gbit/s pseudo-random-bit-sequence non-return- to-zero data transmission over 80 m perfluorinated GI-POF made of CYTOP using a single mode butt-coupled selectively oxidized InGaAs VCSEL source emitting at 935 nm emission wavelength. For 3 Gbit/s data rather the received optical power for a bit error rate (BER) of 10-11 is -22.5 dBm for back-to-back (BTB). A power penalty of 1 dB is found for transmission over 100 m graded-index multimode glass fiber and 2 dB for 80 m GI-POF for 80 m GI- POF transmission. Reduced power penalty observed at 7 Gbit/s is attributed to the restricted modulation bandwidth of the VCSEL used in the experiment.
We have designed and fabricated a 64 channel optical module using a self-alignment flip-chip packaging technique for 2D GaAs epitaxial-side emitting vertical-cavity surface- emitting laser (VCSEL) array mounting without substrate removal on Si subcarrier. Light emission is obtained through a wet-chemically etched window in the Si subcarrier. The 2D independently addressable selectively oxidized GaAs laser array is arranged in an 8 X 8 matrix with a device pitch of 250 micrometers and each laser is supplied with two individual top contacts. This metallization scheme allows flip-chip mounting junction-side down on Si subcarrier. The VCSEL array chip is placed above the window in the Si subcarrier and is assembled using a self-aligned bonding technique with PbSn solder bumps. Arrays with 4 micrometers active diameter investigated before and after packaging show quite homogeneous optical and electrical continuous wave output characteristics exhibiting threshold currents of less than 1.1 mA and single-mode output powers of 2 mW. Driving characteristics of the lasers in the array are fully compatible to advanced 3.3 V CMOS technology. The modules are used to demonstrate free-space directional transmission applying beam steering.
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