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.
VCSEL arrays can play an important role in the increasing the data throughput of VCSEL-based optical interconnects both due to the need to increase the channel density and due to new emerging technologies like optical wireless. In this work we show the progress in the development of high-speed VCSEL arrays suitable for multicore fiber transmission leading to an increase of the total throughput through single fiber to 600 Gbps. We also discuss a novel type of compact VCSEL mini-arrays capable of high-speed modulation and coherent emission at the same time. Photon-photon resonance and coherent effects can help increase the resonant frequency and the bandwidth of the VCSELs and enable devices capable of 100 GHz operation.
Strain-induced birefringence in GaAs-based oxide-confined VCSELs (Vertical-Cavity Surface-Emitting Laser) can split the optical modes into orthogonally polarized components. A polarization switching at very high frequencies can occur between these components, which is of great interest for optical communication systems of the future. In this study, we focus our investigation on the frequency characteristics of the polarization switching between the optical modes, which is caused by polarization self-modulation (PSM) in fiber-coupled systems. Moreover, we analyze the PSM that is originating in different optical modes of the VCSEL and compare multi-mode and single-mode VCSELs.
We report on vertical cavity surface emitting lasers (VCSELs) having a -3dB modulation bandwidth above 30 GHz and a narrow spectrum down to single mode (SM) operation. The 850 nm and 910 nm SM VCSELs in combination with the IN5612 VCSEL driver from Inphi Corporation allowed to reach 106 Gb/s PAM4 with the TDECQ values of only 1.5 dB. For the multimode VCSELs, TDECQ of ~2.6 dB were achieved in combination with the same driver chip. VCSELs with the reduced spectral width allow to cover transmission distance over multimode fiber reaching 1.0-2.5 km at 50 Gbaud. Furthermore, reduction of the aperture size to a certain limit allows to reach ultimate modulation bandwidths at the same current density as applied in the large aperture VCSELs but at lower total currents and thus much lower current-induced overheating. The latter enables a significant improvement in the reliability of the devices and stimulates further research in novel types of VCSEL-based devices.
In this paper, we perform a comparison of three modulation formats: NRZ, duo binary and DMT in combination with the state of the art 850 and 910nm VCSELs for their application in short reach high speed optical links. The system for NRZ and DB utilizes feasible for deployment equalization including a 9-tap finite impulse response filter and raised cosine filtering in the transmitter and a 7-tap UI-spaced feed forward equalizer in the receiver. The 100 Gbit/s net link with DB modulation can be realized without applying receiver equalization. For DMT the highest gross data rate of 224 Gbit/s/lambda is achieved.
KEYWORDS: Vertical cavity surface emitting lasers, Polarization, Birefringence, Near field optics, Switching, Near field, Multimode fibers, Data transmission, Receivers, Modulation
We report high-frequency polarization self‐modulation (PSM) in high speed vertical-cavity surface emitting lasers (VCSELs) connected to the stress-induced birefringence in oxide-confined aperture VCSELs. Polarization oscillations up to 45 GHz were captured. We analyze the far and the near field of the device and show how the fiber-coupling conditions induce optical feedback, affect emission properties of the device and influence the polarization switching phenomenon. In conditions where the PSM was suppressed, we demonstrate NRZ high-speed multi-mode fiber data transmission up to 90 Gbit/s.
New applications in sensing, automotive and on-board applications require vertical-cavity surface-emitting lasers (VCSELs) operating at high data rates up to very high ambient temperatures. We study temperature stability of the 850 nm Quantum-Dot (QD) VCSELs and benchmark them to Quantum-Well (QW) VCSELs of similar design.
QD VCSELs enable extension of the temperature stability and demonstrate threshold currents below 1 mA for operation range from 30°C to 200°C. The role of gain to cavity detuning is discussed in details. 25 Gbit/s NRZ multi-mode fiber transmission with QD VCSELs is realized at temperatures up to 180°C. Pulsed operation of QD VCSELs with 8 μm oxide aperture diameter is studied at temperatures from 30°C to 125°C and 1 W peak power is realized on 100 ns pulses at room temperature.
Surface–trapped electromagnetic waves can be localized at a boundary between a semiconductor distributed Bragg reflector (DBR) and a homogeneous dielectric medium or air. These waves enable a novel class of both in–plane–emitting and vertically–emitting optical devices including edge–emitting lasers, disk microlasers, near–field fiber–coupled lasers as well as vertical cavity surface emitting lasers (VCSELs). We show that the surface–trapped modes can be controlled by tuning the thickness of a single DBR layer. Diagrams in variables “wavelength – thickness of the control layer” are constructed for both TM and TE optical modes revealing the parameter domains, in which surface–trapped modes exist. The domains contain cusps, in the vicinity of which surface–trapped modes are allowed only in a narrow spectral region, enabling wavelength–stabilized operation of a laser. For an edge–emitting structure designed for lasing at ~1 μm, the lasing wavelength shifts upon temperature at a rate ~0.07 nm/K. The fraction of the optical power of the surface–trapped mode accumulated in the air can reach ~60%. In oxide–confined VCSEL structures the surface–trapped mode can be used for engineering of the interaction with the VCSEL lasing modes. Deposition of an effective (3λ/4)–thick additional layer on top of the top DBR of the VCSEL allows surface–trapped modes to reach the wavelength of the VCSEL lasing modes. Interaction of these two types of generally non–orthogonal optical modes results in the lateral leakage of the VCSEL emission. Mapping of the VCSEL wafers in areas with the variable aperture diameters D shows non–monotonous behavior of side mode suppression ratio (SMRS) versus D oscillating in the range from 7 dB to ~30 dB with three clearly revealed maxima in the SMSR at particular aperture diameters varied in the range from ~3 μm to ~5 μm. Similar oscillatory behavior was previously predicted for a different type of leaky VCSELs. VCSELs with SMSR above 20 dB are tested for data transmission over a multimode fiber (MMF). 40Gb/s open eye data transmission over 1.4 km OM5 MMF without pre–emphasis or equalization is demonstrated is such device.
Optical VCSEL-based links operating in on-off keying (OOK) modulation represent a robust energy-efficient solution for short-reach optical interconnects in datacenters. We report on the optical and electronic elements of such link and their integration into the transmitter (TR) and receiver (RX) assemblies. A single channel transceiver link capable of 40-56 Gbit/s OOK transmission over multimode fiber at record energy-efficiency of ~4.5 pJ/bit is demonstrated. VCSEL driver and receiver transimpedance amplifier (TIA) circuits capable of generating 80-100 Gbit/s error-free signals are characterized on a special test-board assembly. Real-time 56 Gbit/s transmission experiments of the complete link are done, resulting in bit-error ratios (BER) below standard Forward Error Correction (FEC) levels without equalization or signal processing.
We consider comprehensive description of vertical cavity surface emitting laser (VCSEL) structures in practical cylindrically symmetric geometry. A complete drift-diffusion model for carrier transport in a multilayer semiconductor laser heterostructure including p-n junction is developed. We evaluate the impact of interface grading in distributed Bragg reflectors (DBRs), modulation doping of the DBRs and surrounding layers of the quantum well (QW) as well as drastically material–dependent carrier mobility and recombination constants. Solution of the drift–diffusion model yields spatial profiles of the nonequilibrium carrier concentrations and current. The model is applied as an example to an oxideconfined GaAs/AlGaAs VCSEL. We address both depletion and diffusion capacitance of the device and show that both contributions to the capacitance as well as the differential series resistance critically depend on the injection current and the VCSEL chip design such that, in general, VCSEL cannot be properly modeled by an equivalent circuit approximation. We consider current injection into the aperture region and illustrate the current crowding effects, which result in substantial enhancement of the current density at the tips of the oxide aperture. The effective RC-product can be kept low at a small size of the oxide-confined aperture but only in case where the diameter of the VCSEL mesa is also small.
The development of advanced OM5 wideband multimode fiber (WBMMF) allowing high modal bandwidth in the spectral range 840-950 nm motivates research in vertical-cavity-surface-emitting-lasers (VCSELs) at wavelengths beyond the previously accepted for short reach communications. Thus, short wavelength division multiplexing (SWDM) solutions can be implemented as a strategy to satisfy the increasing demand of data rate in datacenter environments. As an alternative solution to 850 nm parallel links, four wavelengths with 30 nm separation between 850 nm and 940 nm can be multiplexed on a single OM5-MMF, so the number of fibers deployed is reduced by a factor of four. In this paper high speed transmission is studied for VCSELs in the 850 nm – 950 nm range. The devices had a modulating bandwidth of ~26-28 GHz. 50 Gb/s non-return-to-zero (NRZ) operation is demonstrated at each wavelength without preemphasis and equalization, with bit-error-rate (BER) below 7% forward error correction (FEC) threshold. Furthermore, the use of single-mode VCSELs (SM-VCSELs) as a way to mitigate the effects of chromatic dispersions in order to extend the maximum transmission distance over OM5 is explored. Analysis of loss as a function of wavelength in OM5 fiber is also performed. Significant decrease is observed, from 2.2 dB/km to less than 1.7 dB/km at 910 nm wavelength of the VCSEL.
New applications in industrial, automotive and datacom applications require vertical-cavity surface-emitting lasers (VCSELs) operating at very high ambient temperatures at ultrahigh speed. We discuss issues related to high temperature performance of the VCSELs including temperature response and spectral properties. The influence of the gain-to-cavity wavelength detuning on temperature performance and spectral width of the VCSELs is discussed. Performance of the oxide-confined 850 nm VCSELs with increased temperature stability capable of operating at bit rates up to 25 Gbit/s at heat sink temperature of 150°C and 35Gbit/s at 130°C. Furthermore, opposite to previous studies of VCSELs with large gain-to-cavity detuning, which demonstrated strongly increased spectral width and a strong redistribution of the mode intensities upon current increase. VCSELs demonstrated in this work show good reproducibility of a narrow spectrum in a wide range of currents and temperatures. Such performance strongly improves the transmission distance over multi-mode fiber and can reduce mode partition noise during high speed operation.
Novel lasing modes in a vertical-cavity surface-emitting laser (VCSEL)-type structure based on an antiwaveguding cavity are studied. Such a VCSEL cavity has an effective refractive index in the cavity region lower than the average index of the distributed Bragg reflectors (DBRs). Such device in a stripe geometry does not support in–plane waveguiding mode, and all modes with a high Q-factor are exclusively VCSEL-like modes with similar near field profile in the vertical direction. A GaAlAs–based VCSEL structure studied contains a resonant cavity with multiple GaInAs quantum wells as an active region. The VCSEL structure is processed as an edge-emitting laser with cleaved facets and top contact representing a non–alloyed metal grid. Rectangular-shaped ~400x400 µm pieces are cleaved with perpendicular facets. The contact grid region has a total width of ~70 μm. 7 μm–wide metal stripes serve as non–alloyed metal contact and form periodic rectangular openings having a size of 10x40 μm. Surface emission through the windows on top of the chip is measured at temperatures from 90 to 380 K. Three different types of modes are observed. The longest wavelength mode (mode A) is a VCSEL–like mode at ~854 nm emitting normal to the surface with a full width at half maximum (FWHM) of the far field ~10°. Accordingly the lasing wavelength demonstrates a thermal shift of the wavelength of 0.06 nm/K. Mode B is at shorter wavelengths of ~840 nm at room temperature, emitting light at two symmetric lobes at tilt angles ~40° with respect to the normal to the surface in the directions parallel to the stripe. The emission wavelength of this mode shifts at a rate 0.22 nm/K according to the GaAs bandgap shift. The angle of mode B with respect to the normal reduces as the wavelength approaches the vertical cavity etalon wavelength and this mode finally merges with the VCSEL mode. Mode B hops between different lateral modes of the VCSEL forming a dense spectrum due to significant longitudinal cavity length, and the thermal shift of its wavelength is governed by the shift of the gain spectrum. The most interesting observation is Mode C, which shifts at a rate 0.06 nm/K and has a spectral width of ~1 nm. Mode C matches the wavelength of the critical angle for total internal reflection for light impinging from semiconductor chip on semiconductor/air interface and propagates essentially as an in–plane mode. According to modeling data we conclude that the lasing mode represents a coupled state between the TM–polarized surface–trapped optical mode and the VCSEL cavity mode. The resulting mode has an extended near field zone and low propagation losses. The intensity of the mode drastically enhances once is appears at resonance with Mode B. A clear threshold is revealed in the L–I curves of all modes and there is a strong competition of the lasing mechanisms once the gain maximum is scanned over the related wavelength range by temperature change.
Design of the oxide–confined vertical cavity surface emitting laser (VCSEL) with enhanced engineered lateral leakage of high–order transverse optical modes is studied by three–dimensional optical modeling to evaluate the robustness of the leakage selection approach with respect to thermal effects. Both Joule heat and heat generated by the free carrier absorption of the optical mode in the doped semiconductor layers and their impact on the refractive index profile are considered. We show that for typical regimes of the VCSEL design and operation absorption–induced heat exceeds by several times the Joule heat while the shape of the generated heated domains strongly differ. Modeling shows that well defined spectral separation between the transverse optical modes persists upon increase in injection current. Further, upon increase in current the lateral extension of the fundamental mode decreases and the mode shrinks towards the center of the VCSEL structure thus reducing the lateral leakage and increasing the mode lifetime, whereas similar effect for high–order transverse modes is much weaker. Thus the preferred conditions for the lasing of the fundamental mode persist and even improve upon current increase. At high currents the fundamental mode becomes favorable at all aperture diameters, also for those where the cold cavity approximation predicts preference for the excited mode lasing.
KEYWORDS: Waveguides, Semiconductor lasers, Photonic crystals, Active optics, Cladding, Near field, Near field optics, High power lasers, Crystals, Phase matching
The concepts, features, modeling and practical realizations of high power high brightness semiconductor diode lasers
having ultrathick and ultrabroad waveguides and emitting in the single vertical single lateral mode are analyzed.
Ultrathick vertical waveguide can be realized as a photonic band crystal with an embedded filter of high order modes. In
a second approach a tilted wave laser enables leakage of the optical wave from the active waveguide to the substrate and
additional feedback from the back substrate side. Both designs provide high power and low divergence in the fast and the
slow axis, and hence an increased brightness. Lateral photonic crystal enables coherent coupling of individual lasers and
the mode expansion over an ultrabroad lateral waveguide. Experimental results are presented. Obtained results
demonstrate a possibility for further expansion of the concept and using the single mode diodes having an ultrabroad
waveguide to construct single mode laser bars and stacks.
We have designed, fabricated and measured the performance of two types of edge emitting lasers with unconventional
waveguides and lateral arrays thereof. Both designs provide high power and low divergence in the fast and the slow axis,
and hence an increased brightness. The devices are extremely promising for new laser systems required for many
scientific and commercial applications. In the first approach we use a broad photonic crystal waveguide with an
embedded higher order mode filter, allowing us to expand the ground mode across the entire waveguide. A very narrow
vertical far field of ~ 7° is resulting. 980 nm single mode lasers show in continuous wave operation more than 2 W,
ηwp ~ 60%, M2 ~ 1.5, beam parameter product of 0.47 mm×mrad and a brightness ~ 1×108 Wsr-1cm-2 respectively. First
results on coherent coupling of several lasers are presented. In the second approach we use leaky designs with feedback.
The mode leaks from a conventional waveguide into a transparent substrate and reflects back, such that only one mode at
a selected wavelength is enhanced and builds up, others are suppressed by interference. 1060 nm range devices
demonstrate an extremely narrow vertical far field divergence of less than 1°.
A mechanism of Kerr-lens mode locking in fiber laser is proposed and analyzed. It could be realized in active photonic-crystal fibers with finite number of the air holes hexagonally arranged around the core with refractive index larger than refractive index of the glass matrix and relies at decrease of the mode confinement loss for higher intensity and mode transformation from leaky to guided when refractive index in the core increases owing to Kerr self-focusing.
We introduce a phenomenological model, based on steady state analytical solution adapted to transient regime through modification of the Brillouin spectrum with the pulse spectrum. This model can accurately de-convolve the strain profiles from measured spectra. The model includes experimental parameters such as the electro-optic modulator Extinction Ratio, the pulse width, pulse and pump powers, position and sensing fibre length. The pulse base is treated as pure steady state contribution. A systematic numerical analysis has been carried out and the results are qualitatively matched with our experimental results. The experimental results have been used to validate the model and evaluate its limitations. Within this context, the approach has been applied to experimental data obtained under well-controlled laboratory conditions. The agreement is good and reflects the Brillouin frequency and then the strain distribution along the fibre. The approach is also successful when used to deconvolve the main strain contributions of a pipe subjected to a compression stress. The strength of the model lies in its simplicity of implementation because it is quasi-analytical and is not restricted to short fibre lengths.
We report on lasers and light emitting diodes based on the longitudinal photonic bandgap crystal (PBC) concept. The PBC design allows achieving a robust and controllable extension of the fundamental mode over a thick multi-layer waveguide region to obtain a very large vertical optical mode spot size and a very narrow vertical beam divergence. An efficient suppression of high order modes can be realized either by the optical confinement factor selection of the fundamental mode, which is localized at the "optical defect" region and has a higher overlap with the gain region. All the other modes spread across the thicker PBC waveguide. In another approach leakage loss selection can be used to suppress excited modes in case of absorbing substrate or the substrate with a higher-refractive index. In this paper we concentrate on growth and performance of high power single mode visible (650 nm) GaInP/AlGaInP PBC lasers, giving a comprehensive example. The devices show narrow far field pattern (full width at half maximum of vertical beam divergence of about 7°), which is stable up to the highest output powers. Differential efficiency up to 85% is demonstrated. Total continuous wave single mode output power as high as 120 mW is achieved in 4 micrometer-wide stripes. Infrared (980 nm) InGaAs/AlGaAs PBC lasers with a beam divergence down to 4.2 degrees and a high temperature stability of the threshold current are also demonstrated.
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