We study the dynamics of a multimode VCSEL with an elliptical oxide aperture for datacom applications. We simulate the laser dynamics through a set of coupled rate equations for the modal components of the electric field and the carrier density, accounting for coherent mode mixing and spatial hole burning. Our simulations show what are the relevant frequency detuning configurations to control in order to improve noise performance. Simulations with NRZ PRBS performed in order to explore the applications of these devices in short-reach data transmission show potentially reachable transmission speeds of 65 Gbit/s.
We report high frequency (20-100 GHz range) optical field intensity oscillations in laterally-coupled-cavity verticalcavity surface-emitting lasers with several different techniques. The oscillation frequency is defined by the photon energy splitting of the coupled states. The resonance effect is stable in an extended current range and can enable modulation frequency resonances at higher frequencies as compared to the conventional relaxation oscillation frequency of the laser. This paves a way towards high-speed data transmission solutions at data rates beyond ~200 Gb/s with the advantage of better laser stability, as the resonance observed can reach high frequencies even at low current densities. A ~75 GHz intensity modulation between optical modes of a coupled-cavity VCSEL array was first reported by the authors in a two-aperture configuration in 2023 applying optical excitation [1]. Studies of 4- and 10-element coupled VCSEL arrays give further insight into the effects observed. New 3D numerical simulations and electrical modulation techniques have been applied to address the specific nature of the photon-photon resonance studies.
In this work, two-photon polymerization three-dimensional laser writing is used to integrate a microlens on the surface of a single mode polarization-stable vertical-cavity surface-emitting laser (VCSEL) to be used as a current-driven tunable source in a compact optical guided-wave gas sensor. The writing conditions are optimized to enable on-demand room temperature and single-step fabrication at a post-mounting stage. We show that a writing time of 5 min is sufficient to fabricate a microlens that efficiently reduces the VCSEL beam divergence, without significant change on its emitted power or polarization stability. The lens addition reduces the spectral available range at high injection currents. A two-dimensional optical modeling of the gain characteristics is used to explain this effect and a new transverse design is proposed to avoid this issue.
Vertical-cavity surface-emitting lasers (VCSELs) are widely used in optical data communication mainly in data centers for short-haul transmissions. However, their intensity modulation resonance frequency does not exceed 40 GHz which also limits the achievable modulation bandwidth and data rate. In contrast, spin-VCSELs can overcome these bandwidth limitations by modulating spin and polarization instead of current and intensity. In spin-VCSELs, the birefringence determines the resonance frequency of the polarization dynamics as well as the modulation bandwidth. We control the birefringence and thus the polarization dynamics via the elasto-optic effect by mechanically or thermally induced strain providing polarization oscillation frequencies up to more than 200 GHz. Detailed analysis shows that spin-VCSELs offer polarization dynamics with good signal strength even when operating close to threshold and at high temperatures. Here, we analyze devices with integrated surface gratings. VCSELs with different grating periods as well as mesa diameters and resulting different oxide apertures were investigated.
In single-mode vertical-cavity surface-emitting lasers (VCSELs) the frequency difference between the two orthogonal modes, which is defined by the birefringence present in the cavity, is the key factor to enable ultrafast polarization dynamics in spin-lasers. This could be a promising alternative to overcome the bandwidth limitations in short-haul data transmission. Therefore, controlling the birefringence is indispensable to utilize the full potential of the polarization dynamics. Splittings of around 100GHz were realized with an on-chip approach by integration of a surface grating in an oxide-confined AlGaAs-based VCSEL. In this paper we present further details of the parameter search process using a three-dimensional vectorial optical VCSEL electro-magnetics (VELM) model. We also show the geometrical properties of the processed grating structure.
The operation of vertical-cavity surface-emitting lasers (VCSELs) results from the interplay among different physical mechanisms. For this reason, even a basic VCSEL model must address the coupling of electrical injection, stimulated/spontaneous emission and optical resonances, without disregarding the strong thermal effects affecting each of these models, leading to the need of an entangled multiphysical description. With the aim to fill the present gap of advanced comprehensive VCSEL models, in this work we present our VCSEL electro-opto-thermal numerical simulator (VENUS). The paper describes the VENUS constitutive blocks and their coupling strategy. The model is then validated by comparing the most significant lasing features with experimental results.
The extraordinary reflectivity and resonance features of high-contrast gratings are consequences of the interference of two grating modes. From this observation, an extension of the Fabry-P´erot interferometer to the bimodal case is proposed, in view of describing rigorously the physical mechanisms occurring in the grating. The closedform expressions of the interferometer reflection and transmission, obtained starting from a novel parametrization of the unitary symmetric 3 × 3 scattering matrix characterizing the bar-air interface, allow a complete exploration of the device parameter space, explaining and predicting phenomena such as ultra-broadband quasi-100% reflectivity.
We discuss some of the key issues to be addressed along the way to complement, and possibly to replace, the standard semiclassical Boltzmann picture with genuine quantum approaches for the simulation of carrier transport and recombination in GaN-based LEDs, with the goal of gradually removing the fitting parameters presently required by semiempirical "quantum corrections" and to better understand the processes responsible for the efficiency droop. As examples of augmented semiclassical models, we present a three-step description of trap-assisted tunneling, especially relevant below the optical turn-on, and a carrier-density-dependent estimate of the phonon-assisted capture rate from bulk states to quantum wells (QWs). Moving to genuine quantum models, we solve the semiconductor Bloch equations to calculate the gain/absorption spectra of AlGaN/GaN QWs, and we discuss our first simulations of spatially and energetically resolved currents across the active region of a single-QW LED based on the nonequilibrium Green’s function approach.
High Contrast Gratings (HCG) have become a hot research topic, because of their new functionalities at very small
volumes. However no efficient 3D VCSEL model capable to account for HCG has been reported so far. HCG design is
therefore mainly based on 1D simulations. For realistic structures usually FDTD is the most popular approach, with its
well known cumbersome computation drawbacks. VELM code,1 the well established VCSEL electromagnetic solver developed in the last ten years in the Torino group, has now been upgraded to rigorously handle HCG layers. The efficiency of the tool is preserved, and an entire set of HCG VCSEL modes can be computed in minutes on an ordinary desktop. A full set of design tools and guidelines, starting from 1D HCG properties up to 3D simulations which include HCG in the VCSEL design, will be presented and applied to the design of a structure that is in fabrication.
We report the investigation of the state of polarization (SOP) of a tunable vertical-cavity surface-emitting laser
(VCSEL) operating near 850 nm with a mode-hop free single-mode tuning range of about 12 nm and an amplitude
modulation bandwidth of about 5 GHz. In addition, the effect of a sub-wavelength grating on the device and
its influence on the polarization stability and polarization switching has been investigated. The VCSEL with an
integrated sub-wavelength grating shows a stable SOP with a polarization mode suppression ratio (PMSR) more
than 35 dB during the tuning.
We present surface micro-machined tunable vertical-cavity surface-emitting lasers (VCSELs) operating around
1550nm with tuning ranges up to 100nm and side mode suppression ratios beyond 40 dB. The output power
reaches 3.5mW at 1555 nm. The electro-thermal and the electro-statical actuation of a micro electro-mechanical
system (MEMS) movable distributed Bragg reflector (DBR) membrane increases/decreases the cavity length
which shifts the resonant wavelength of the cavity to higher/lower values. The wavelength is modulated with
200 Hz/120 kHz. Both tuning mechanisms can be used simultaneously within the same device. The newly
developed surface micro-machining technology uses competitive dielectric materials for the MEMS, deposited
with low temperature plasma enhanced chemical vapor deposition (PECVD), which is cost effective and capable
for on wafer mass production.
A simple and low-cost technology for tunable vertical-cavity surface-emitting lasers (VCSELs) with curved movable
micromirror is presented. The micro-electro-mechanical system (MEMS) is integrated with the active optical
component (so-called half-VCSEL) by means of surface-micromachining using a reflown photoresist droplet as
sacrificial layer. The technology is demonstrated for electrically pumped, short-wavelength (850 nm) tunable
VCSELs. Fabricated devices with 10 μm oxide aperture are singlemode with sidemode suppression >35 dB,
tunable over 24 nm with output power up to 0.5mW, and have a beam divergence angle <6 °. An improved
high-speed design with reduced parasitic capacitance enables direct modulation with 3dB-bandwidths up to
6GHz and error-free data transmission at 5Gbit/s. The modulation response of the MEMS under electrothermal
actuation has a bandwidth of 400 Hz corresponding to switching times of about 10ms. The thermal
crosstalk between MEMS and half-VCSEL is negligible and not degrading the device performance. With these
characteristics the integrated MEMS-tunable VCSELs are basically suitable for use in reconfigurable optical
interconnects and ready for test in a prototype system. Schemes for improving output power, tuning speed, and
modulation bandwidth are briefly discussed.
We present recent results on the integration of polymer microlenses on single mode Vertical-Cavity
Surface-Emitting Lasers (VCSELs) to achieve output beam control. We describe in particular low
cost and collective fabrication methods developed to allow for a self-alignment of the lens with the
laser source. These approaches are based either on surface tension effects or on a self-writing
process using novel Near Infra-Red (NIR) photopolymers. Results on beam collimation at 850nm are
presented and compared to a fully vectorial and three-dimensional optical model that takes into
account the complete geometry of laser resonator is used. Results on short distance focusing using
self-aligned microtips are presented. Considerations to achieve an active beam control by means of
polymer-based MEMS (Micro-electro-mechanical System) are also discussed. Potential applications
may concern the improvement of VCSEL insertion in optical interconnects or sensing systems, as
well as the fabrication of optical micro-probes for near-field microscopy.
Widely tunable vertical cavity surface emitting lasers (VCSEL) are of high interest for optical communications,
gas spectroscopy and fiber-Bragg-grating measurements. In this paper we present tunable VCSEL operating at
wavelength around 850 nm and 1550 nm with tuning ranges up to 20 nm and 76 nm respectively. The first versions
of VCSEL operating at 1550 nm with 76 nm tuning range and an output power of 1.3mW were not designed for
high speed modulation, but for applications where only stable continious tuning is essential (e.g. gas sensing).
The next step was the design of non tunable VCSEL showing high speed modulation frequencies of 10 GHz with
side mode supression ratios beyond 50 dB. The latest version of these devices show record output powers of
6.7mW at 20 °C and 3mW at 80 °C. The emphasis of our present and future work lies on the combination of
both technologies. The tunable VCSEL operating in the 850 nm-region reaches a modulation
bandwidth of 5.5GHz with an output power of 0.8mW.
A model to simulate Vertical-Cavity Surface-Emitting Laser (VCSEL) operation above threshold is presented. The power
- injected current (PI) curves are computed accounting for mode competition arising from spatial hole-burning and temperature
profiles. The latter affect many laser parameters, such as the gain spectra and the optical modes, which change their
shapes and wavelengths during operation. This comprehensive model aims to describe the details of VCSEL operation
above threshold, in a non-circularly symmetric geometries, preserving at the same time computational efficiency. The optical
treatment is vectorial, using the in-house developed VELM code. The model is based on a solution of the dynamical
equations for field-carrier interactions. Similarly to the more mature vectorial optical treatment, the numerical efficiency
is achieved by expanding in proper basis of simple and analytical functions all the involved 3D variables: carrier densities,
temperature and optical fields.
VCSELs have several advantages with respect to edge emitting semiconductor lasers; one of their major drawbacks
is the lack of polarization control due to the almost absence of any selecting mechanism in the resonator and
active medium. In this paper we will review a technique based on the integration of a shallow grating into the
VCSEL output facet, which gives very good results, provided that a careful design is performed.
Polarization stability of vertical-cavity surface-emitting lasers (VCSELs) is of crucial importance in particular for single-mode devices used in optical communications and optical sensing. We
will consider major approaches to polarization control and then focus on surface gratings, which provide a monolithically integrated type of polarization-dependent feedback. Single-mode as well as multimode grating VCSELs have been fabricated in large
quantity and have been shown to be polarization-stable with high orthogonal polarization suppression ratio not only for static operation but also under digital and analog modulation, temperature variation, optical feedback, as well as externally
applied stress. After reviewing published work on grating VCSELs,
we will discuss grating design variations by means of extensive
vectorial simulations. The reliability of polarization control is then investigated by studying the characteristics of several thousand VCSELs on a given sample. Finally, as an application example in optical communications, we demonstrate free-space transmission of an aggregate data rate of 16 Gbit/s using polarization division multiplexing with two multimode grating VCSELs.
Monolithically integrated surface gratings have proven to control the polarization of vertical-cavity surface-emitting lasers (VCSELs) very reliably and effectively. To overcome the drawbacks of these devices with respect to threshold current or differential quantum efficiency we will present in this paper monolithically integrated surface gratings with a modified longitudinal position relative to the standing wave inside the laser cavity. It turns out that an optimum is reached for an additional quarter-wave layer on top of the upper Bragg mirror. In that way the diffraction from the grating and therefore its influence on the other laser properties like threshold, differential quantum efficiency or maximum output power can be reduced significantly, while the polarization control is still maintained. At the same time the requirements on the fabrication accuracy are very relaxed. 118 out of 120 fabricated devices with grating periods between 0.5 and 1.2 μm and grating depths between 35 and 105 nm exhibit a stable polarization orthogonal to the grating grooves. If one limits the lateral extension of the grating to a diameter smaller than the oxide aperture of the laser, the single-mode output power can be increased simultaneously.
EBEX is a balloon-borne polarimeter designed to measure the intensity and polarization of the cosmic microwave background radiation. The measurements would probe the inflationary epoch that took place shortly after the big bang and would significantly improve constraints on the values of several cosmological parameters.
EBEX is unique in its broad frequency coverage and in its ability to provide critical information about the level of polarized Galactic foregrounds which will be necessary for all future CMB polarization experiments.
EBEX consists of a 1.5 m Dragone-type telescope that provides a resolution of less than 8 arcminutes over four focal planes each of 4 degree diffraction limited field of view at frequencies up to 450 GHz. The experiment is designed to accommodate 330 transition edge bolometric detectors per focal plane, for a total of up to 1320 detectors. EBEX will operate with frequency bands centered at 150, 250, 350, and 450 GHz. Polarimetry is achieved with a rotating
achromatic half-wave plate. EBEX is currently in the design and construction phase, and first light is scheduled for 2008.
We report on advances in the fabrication and performance of monolithic 850 nm, linearly polarized vertical-cavity surface-emitting lasers (VCSELs) incorporating a semiconductor surface grating at the outcoupling facet. Depending on the grating parameters, the light is polarized either parallel or perpendicular to the grating grooves. Deep-etched gratings enable complete polarization pinning even in directions that are 45 degrees off the preferred crystal axes. On the other hand, such devices can show strong side-lobes in the far-field which may limit the available output power for some applications. Shallow-etched VCSELs with almost undistorted far-fields deliver output powers as high as 29 mW with about 12 dB orthogonal polarization suppression ratio. A combination
of surface relief and grating is used to increase the transverse single-mode output power while maintaining polarization stability.
Based on design guidelines from a three-dimensional, fully
vectorial model, we have fabricated vertical-cavity
surface-emitting lasers (VCSELs) with a monolithically integrated
dielectric surface grating for polarization control. For VCSELs
with emission wavelengths of 850 and 980 nm we have achieved
orthogonal polarization suppression ratios (OPSRs) above 15 dB for
all modes up to thermal rollover, which very well agrees with
theory. It is shown both theoretically and experimentally that the
grating has no influence on the emission far-field. The surface
grating has also been combined with a surface relief to stabilize
the polarization and to increase the fundamental mode output power
at the same time.
We present a detailed study of oxide-confined, vertical-cavity surface-emitting lasers (VCSELs), where the reflectivity of the top mirror has been patterned by means of a metal grid, which at the same time acts also as an electrode. Owing to their features, these kind of devices are commonly referred to as phase-coupled VCSEL arrays. The anlaysis is based on a joint experimental and theoretical effort: the former is devoted to a complete characterization of the emission properites, while the latter is based on a comprehensive fully vectorial model for the structure eigenmodes with the details of their complex structure. The detected characteristics make them quite attractive for various applications and the comparison of their modal properties with the model is proven to be essential for a deep understanding of these lasers. In particular we explain for the first time, a characteristic behavior of the lasing array, which displays spatially inhomogeneous polarization characteristics with symmetry properties with respect to the array diagonals. The good matching between theory and experiment opens new perspectives for optimized devices.
The High Frequency Instrument of the Planck satellite is dedicated to the measurement of the anisotropy of the Cosmic Microwave Background (CMB). Its main goal is to map the CMB with a sensitivity of ΔT/T=2.10-6 and an angular resolution of 5 arcmin in order to constrain cosmological parameters. Planck is a project of the European Space Agency based on a wide international collaboration, including United States and Canadian laboratories. The architecture of the satellite is driven by the thermal requirements resulting from the search for low photon noise. Especially, the passively cooled telescope should be at less than 50K, while a cascade of cryo-coolers will ensure the cooling of the HFI bolometers down to 0.1K. This last temperature will be produced by a gravity insensitive 3He/4He dilution cooler. This will be achieved at the L2 Lagrangian point of the Sun-Earth system. The whole sky will be observed two times in the 14 months mission with a scanning strategy based on a 1RPM rotation of the satellite. In addition to the cosmological parameters that can be derived from the CMB maps, Planck will deliver nine high sensitivity submillimeter maps of the whole sky that will constitute unique data available to the whole astronomical community.
BaR-SPOrt (Balloon-borne Radiometers for Sky Polarisation
Observations) is an experiment to measure the linearly polarized
emission of sky patches at 32 and 90 GHz with sub-degree angular
resolution. It is equipped with high sensitivity correlation
polarimeters for simultaneous detection of both the U and Q stokes
parameters of the incident radiation. On-axis telescope is used to
observe angular scales where the expected polarization of the
Cosmic Microwave Background (CMBP) peaks. This project shares most
of the know-how and sophisticated technology developed for the
SPOrt experiment onboard the International Space Station. The
payload is designed to flight onboard long duration stratospheric
balloons both in the Northern and Southern hemispheres where low
foreground emission sky patches are accessible. Due to the
weakness of the expected CMBP signal (in the range of microK),
much care has been spent to optimize the instrument design with
respect to the systematics generation, observing time efficiency
and long term stability. In this contribution we present the
instrument design, and first tests on some components of the 32
GHz radiometer.
We present a model to compute the linewidth in vertical-cavity surface-emitting
lasers, accounting for the 3D-structure of real devices. To this aim
we include the noise source in the field equations and treat both the noise
and the structural characteristics by means of coupled mode theory.
In this way we obtain an expression for the linewidth that is given as the
standard
relation, modified by two correction factors that account for spatial effects
and modal dispersion of the resonator.
In the numerical results we give some guidelines towards devices aimed to
narrow-line emission and, as a significant example, we study the transition
from gain to index-guided devices in oxide-confined VCSEL's.
An electromagnetic model of Vertical Cavity Surface Emitting Lasers based on semiconductor compounds is developed to study the threshold conditions, the mode field distributions and the noise properties, in particular the linewidth. The model relies on the solution of Maxwell equations by using an expansion in plane waves. The main purpose is the description of the optical field confinement due to the gain mechanism induced by carrier injection. This strongly influences the noise properties and gives rise, in the case of a waveguide device, to the Petermann factor. The solution of the problem is based on an integral equation of the Fredholm type whose eigenvalues are related to the threshold operating conditions and whose eigenvectors give the field distributions. The noise characteristics are analyzed by introducing a Langevin source term in the integral equation.
Frequency conversion in Multiple Quantum Well travelling wave amplifiers, when a strong pump is injected in the device and under highly saturated regime, can show longitudinal power fluctuations up to hundreds gigahertz beat frequencies. A model which explains such phenomena is presented.
A density matrix approach has been used to describe fast nonlinear phenomena, such as carrier heating and spectral hole burning, in semiconductor laser devices. The aim of this paper is to investigate highly non degenerate frequency mixing in semiconductor lasers amplifiers and oscillators. Coupled mode approach is used to analyze intermodulation in travelling wave laser amplifiers.
A model is presented for the dynamical evolution of optical bistable devices based on Multiple Quantum Well structures, in which the
electric field is near resonance with an excitonic line, and the
parameters which appear in the equations are discussed. For room ternperature
operation, examples of steady state response curves are presented.
In conditions of adiabatic elimination of the exciton density
and within the assumption of small carrier density, it is shown that
the set of equations is equivalent to the well known model for optical
bistability in two level systems.
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