Wavelength Vertical-cavity Surface-emitting Laser Research Articles

Structural control in atomic layer deposited hafnium oxide thin films through vertical cavity surface-emitting laser (VCSEL)-Based ultra-rapid heating

HfO2 thin films were formed by plasma-enhanced atomic layer deposition (PEALD) using TDMAHf precursor as a Hf source and oxygen plasma as an oxygen source. Rapid annealing assisted by a 980 nm wavelength vertical cavity surface-emitting laser (VCSEL) was employed to assess the dielectric properties of the HfO2 thin films, compared to furnace annealing. Operational power and time of VCSEL was precisely controlled by computational simulation. In addition to decreased thickness of the interfacial layer, rapid annealing enabled reduced leakage current density as annealing temperature increased due to suppression of crystallization, resulting from the extremely short time duration of 0.98 s–8.14 s. Cross-sectional microstructures exhibited incomplete grain boundary formation of the rapidly annealed HfO2 thin film. VCSEL-based rapid annealing was verified to be an efficient process to enhance the dielectric characteristics of high-k dielectric oxide films.

New Packaging Technology for 2-dimensional VCSEL Arrays and Their Electro-Optical Performance and Applications

At IMAPS2021, we reported on 1-dimensional VCSEL arrays and the packaging technology developed for these arrays, but this time we present the manufacturing workflow and look at the challenges presented by the packaging of special long-wavelength 2-dimensional vertical-cavity surface-emitting laser (VCSEL) arrays in various dimensions. We will show that available packaging technologies for epoxy application by silk screening, die- and wire bonding can be adapted for the unique properties of the 2D VCSEL arrays, delivering high accuracy for production with great overall yield. Proof of the excellent reliability and lifetime will be given through mechanical and electro-optical testing results. The technologies developed for 2D laser arrays will enable new devices for various new applications like high channel count interconnects for data communication and high power VCSEL solutions for 3D sensing. The perfected wire bonding process secured a homogeneous monometallic structure of the wire bonds, a reduced, well defined bond loop height, with sufficient clearance of the bonding wire above the VCSEL surface. Properly set-up manufacturing procedures for the packaging of the VCSEL arrays are necessary to achieve high performance and reliability of the products presented. Processing of large 2D-VCSEL arrays will be part of the paper. 2D arrays have enabled VCSEL technologies to address a wide range of new laser-based sensing applications. VCSEL products vary from laser chips with only a few emitters for proximity sensors up to several thousands of emitters for LiDAR systems. Applications include systems for the automotive, industrial, medical and consumer markets. There is a strong and growing market opportunity for long wavelengths VCSELs, e.g. based on InP material enabling single mode and multi-mode 2D-VCSEL arrays from 1.3 to beyond 2 µm. For 3D sensing systems and free-space optical communication, long wavelength 2D-VCSEL arrays offer improved eye safety and further benefits. The long wavelength VCSELs discussed in this paper can be deployed in a wide range of sensing and optical communication applications.

Investigation of the Static and Dynamic Characteristics for a Wafer-Fused C-band VCSEL in the Mode of the Optical-Electric Converter

The results of an experimental study for a long wavelength vertical cavity surface-emitting laser of a wafer-fused construction as an effective resonant cavity enhanced photodetector of analog optical signals are described. The device is of interest for a number of promising microwave photonics applications and for creation of a low-cost photoreceiver in a high-speed fiber optics telecommunication system with dense wavelength division multiplexing. The schematic of the testbed, the original technique allowing to calculate the passband of the built-in optical cavity, and the results of measuring dark current, current responsivity, amplitude- and phase-frequency characteristics during the process of photo-detection are demonstrated.

Исследование статических и динамических характеристик сплавного VCSEL C-диапазона в режиме оптико-электрического преобразователя

AbstractThe results of an experimental study for a long wavelength vertical cavity surface-emitting laser of a wafer-fused construction as an effective resonant cavity enhanced photodetector of analog optical signals are described. The device is of interest for a number of promising microwave photonics applications and for creation of a low-cost photoreceiver in a high-speed fiber optics telecommunication system with dense wavelength division multiplexing. The schematic of the testbed, the original technique allowing to calculate the passband of the built-in optical cavity, and the results of measuring dark current, current responsivity, amplitude- and phase-frequency characteristics during the process of photo-detection are demonstrated.

Increased wavelength VCSELs reach out

Work from Sweden and the US is extending the reach of low-cost laser and fibre technology to meet the growing needs of optical interconnects in the massive data centres that underpin our Internet-enabled world. Team member Ewa Simpanen holding up a set of VCSEL chips in the cleanroom at Chalmers University of Technology. VCSELs (Vertical-cavity surface-emitting lasers) are power efficient semiconductor lasers that can be produced in high volumes at low cost. They can also be directly modulated at high speed when biased at low currents. All of this makes them the preferred light source for the short-reach optical interconnects used in data centres and high-performance computing systems. In these applications, VCSELs are used with high modal bandwidth multimode fibre (MMF) to relax the laser-to-fibre alignment tolerance, which also reduces costs associated with assembly and packaging. With the development of cloud-based applications and Internet-delivered services, data centres operated by service providers like Facebook and Google are developing into very large facilities. These ‘mega’ data centres need optical interconnects that can reach beyond 1 km. This cannot be supported by current VCSEL-MMF technology owing to the large chromatic dispersion and attenuation in MMF at the standard wavelength – 850 nm. At longer wavelengths, dispersion and attenuation are much lower, allowing greater reach. This is the need that the team, from Chalmers University of Technology in Sweden, and Hewlett Packard Enterprise (HPE) in the US, are working to meet. Team member Ewa Simpanen told us, “The purpose of this work is to develop high-speed, longer wavelength those while still using the GaAs-based material systems as it enables the fabrication of VCSELs which are superior to those based on other material systems in terms of speed, efficiency, manufacturability, and cost.” Currently, large-scale data centres use single-mode fibres and long-wavelength silicon photonic transceivers to cover the needed distances. With longer wavelength GaAs-based VCSELs, it would be possible to build transmitters for long-range interconnects that are as power and cost efficient as those used for shorter reach interconnects. In this issue of Electronics Letters, the Chalmers and HPE team present a VCSEL design operating at 1060 nm. This high-speed, single-mode, low-resistance VCSEL can support bit-rates up to 50 Gbit/s while dissipating no more than 100 fJ of energy per bit; making it very energy efficient – on par with the state-of-the-art at 850 nm. “We have limited the wavelength extension to 1060 nm since the long-term reliability of GaAs-based VCSELs may be compromised if extending the wavelength beyond ∼1100 nm,” explained Simpanen. At 1060 nm, the fibre chromatic dispersion is reduced by 70% and attenuation by 50% compared to 850 nm. Using a single-mode VCSEL the narrow spectral width of the laser further reduces the impact of chromatic dispersion. With the single-mode VCSEL properly aligned to the MMF, only the fundamental mode of the fibre, or the lowest order mode group, is excited, which also reduces the impact of modal dispersion. “In these conditions, the fibre ideally acts as a pure attenuator without degrading the signal quality. All this together should enable data transmission distances compatible with what is required in mega data centres,” said Simpanen. The results reported in this issue deal with the modulation capacity of the VCSEL itself. Since then, the team have started investigating the data transmission capacity of optical interconnects using the VCSEL together with MMF designed for high modal bandwidth at 1060 nm and so far the results are very promising, according to Simpanen. “This prototype fibre is fabricated at and provided by Corning. In a first preliminary experiment, we have transmitted data at 25 Gbit/s over 1 km with some margin. We believe that higher data rates and/or longer transmission distances are possible with further optimisations. We will also investigate the alignment tolerances under mode selective launch as they are of utmost importance for practical implementations.” A microscope image of an array of VCSEL devices on chip; newly fabricated and ready for characterisation. Looking further ahead, the team think that continued engineering of conventional VCSEL designs and the implementation of more exotic VCSEL concepts for further significant speed improvements will play a big part in developing much higher capacity interconnects. Combined with electronic compensation techniques, such as equalisation and digital signal processing, higher order modulation formats, and more advanced forms of spatial and wavelength division multiplexing, they believe such lasers could eventually achieve aggregate interconnect capacities approaching 100 Tbit/s.

Controlled inhibition of spiking dynamics in VCSELs for neuromorphic photonics: theory and experiments.

We report experimentally and theoretically on the controllable inhibition of spiking regimes in a 1300 nm wavelength vertical-cavity surface-emitting laser. Reproducible suppression of spiking dynamics is demonstrated at fast operation speeds (up to sub-ns rates) and with total control on the temporal duration of the spiking inhibition windows. This Letter opens new paths toward a photonic inhibitory neuronal model system for use in future neuromorphic photonic information processing modules and which are able to operate at speeds up to 8 orders of magnitude faster than biological neurons.

Polarization switching with low power consumption induced by optical feedback in long-wavelength vertical-cavity surface-emitting lasers

The polarization switching (PS) characteristics of vertical cavity surface emitting lasers(VCSELs) have received sustained attention for the past years. With the development of manufacturing technology, the performances of 1550 nm VCSELs have been improved, however the researches on the PS of 1550 nm VCSELs are relatively inadequate for the PS characteristics in the long-wavelength VCSELs may have wide application prospects in optical information processing and optical communications. In this paper, based on the extended spin-flip model (SFM), we theoretically investigate the PS with low power consumption induced by optical feedback in long-wavelength VCSELs. Results show that the PS, which is failed to realize in free-running long-wavelength VCSELs, can be achieved by introducing a moderate-strength polarization-rotation optical feedback. By comparing two different linear dispersion effects, some interesting phenomena have been found. For weak linear dispersion, the PS is relatively easy to realize for a low injection current level, and the range of feedback strength used to control the PS is wide. However, for strong dispersion effect, the PS cannot be obtained all the time since two mode-coexisting zones will appear, and the value of injection current where the PS happens is relatively high. Meanwhile, as observed in short-wavelength VCSELs, the polarization mode hopping and multiple PS have also been found in long-wavelength VCSELs, indicating that the physics nature thet induces the PS is similar for both long and short wavelength VCSELs. In addition, because the PS in long-wavelength VCSEL is more difficult to realize as compared with that in short-wavelength VCSELs, reasonable analyses and explanations may be as follows: since the linear dispersion effect in 1550 nm-VCSEL is much stronger than that of short wavelength VCSEL, the frequency difference between the two linear polarization modes is up to 60 GHz (or 0.48 nm), thus leading to the decrease of the correlation between two linear polarization modes. As a result, it is relatively difficult to obtain the PS phenomenon at low injection current level in long-wavelength VCSEL; while using suitable polarization-rotated optical feedback can partially compensate the deficiency of this correlation. We believe that the results obtained in this work will be helpful in investigation of low power consumption for all optical buffers by using long-wavelength VCSELs.

大功率VCSEL三角列阵的空间相干性

The spatial coherence properties of high power vertical cavity surface emitting lasers( VCSELs) triangle-arrays were studied based on the theorem of Hermitian-Gaussian beams and spatial coherence. The far-field divergence angle and spatial coherence degree were gauged using the second-moment method and spatial coherence degree integral average value method for the devices of980 nm wavelength VCSEL arrays. The influence of array-units distance on far-field characteristics and coherent characteristics was also discussed. With the increasing of array element spacing from200 μm to 300 μm,the far-field divergence angle of VCSEL arrays device increases from 9. 5° to12. 5°,and the spatial coherence degree decreases from 0. 719 to 0. 526,while the unit aperture of triangle array is 150 μm. Under the same injection current condition,the optical powers of three VCSEL array devices are almost equal.

Single-mode temperature and polarisation-stable high-speed 850nm vertical cavity surface emitting lasers

A new intracavity-contacted design to realize temperature and polarization-stable high-speed single-mode 850 nm vertical cavity surface emitting lasers (VCSELs) grown by molecular-beam epitaxy is proposed. Temperature dependences of static and dynamic characteristics of the 4.5 pm oxide aperture InGaAlAs VCSEL were investigated in detail. Due to optimal gain-cavity detuning and enhanced carrier localization in the active region the threshold current remains below 0.75 mA for the temperature range within 20-90°C, while the output power exceeds 1 mW up to 90°C. Single-mode operation with side-mode suppression ratio higher than 30 dB and orthogonal polarization suppression ratio more than 18 dB was obtained in the whole current and temperature operation range. Device demonstrates serial resistance less than 250 Ohm, which is rather low for any type of single-mode short- wavelength VCSELs. VCSEL demonstrates temperature robust high-speed operation with modulation bandwidth higher than 13 GHz in the entire temperature range of 20-90°C. Despite high resonance frequency the high-speed performance of developed VCSELs was limited by the cut-off frequency of the parasitic low pass filter created by device resistances and capacitances. The proposed design is promising for single-mode high-speed VCSEL applications in a wide spectral range.

大功率VCSEL器件的空间相干性的研究

The spatial coherence properties of high power vertical cavity surface emitting lasers (VCSELs) are studied based on the theorem of partially coherent light proposed by Van Cittert-Zernike. The interference stripe patterns are gauged using the Young's double-slit experiment for the devices of 980 nm wavelength VCSEL single emitters. Then interference patterns are switched to grayscale by intensity distribution patterns for data collection. In this paper, the integral average value method is proposed to calculate the degree of spatial coherence. The proposed integral average value method and traditional average method are used respectively to calculate the light intensity of the pattern. The results are compared with those calculated by the theoretical value of Van Cittert-Zernike theorem. The VCSEL optical apertures' influence on coherent characteristics are also discussed. The experimental results show that, a relative error within 2.5%~9.4% is reached by our integral average value method. On the contrary, a relative error of 7.5%~67.4% are got from the traditional average method. Apparently, traditional average value method causes the error generally 1.5~27 times worse than our integral average value method. Moreover, for single emitter VCSELs with optical aperture from 200 m to 500 m, the smaller the optical aperture are, the larger the coherent degrees (between 0.731~0.426) are. The conclusion can be wildly used on theoretical and experimental design for VCSEL coherent arrays.

Optical Injection Locking of Polarization Modes in VCSELs Emitting at 1.3 $\mu{\rm m}$ Wavelength

Injection locking of polarization modes of 1.3 μm wavelength vertical-cavity surface-emitting lasers (VCSELs) fabricated via wafer fusion is investigated. Maps of the injected modes in the frequency-power plane reveal the detailed response of the slave VCSEL to the optical injection, in agreement with numerical simulations of the system.

Voltage Spectroscopy and the Operating State of an Optically Injected Long Wavelength VCSEL

A study on the correspondence between the voltage and optical properties of long wavelength vertical cavity surface-emitting lasers (VCSELs) during tuned optical injection is presented. A direct link between the voltage response and the induced optical dynamics for parallel and orthogonal injection is highlighted. The induced dynamics include unlocked stable polarization switching at large frequency offsets from the free-running VCSEL lasing wavelength for both injection orientations. The carrier density change during stable locking is estimated using the cavity resonance shift and voltage change. This estimate is confirmed using a full numerical simulation of the injection locked rate equations.

VCSEL Technology for Green Optical Interconnects

State-of-the-art vertical-cavity surface-emitting laser (VCSEL) technology will be reviewed in terms of power consumption and reliability to realize the energy-efficient high-performance computing (HPC) and data centers. Intermediate wavelength (between short and long wavelength) VCSELs emitting at 1060 nm are promising candidates for the upcoming higher performance computing with energy saving.

Experimental and numerical analysis on temporal dynamics of polarization switching in an injection-locked 155-μm wavelength VCSEL

We present experimental and numerical study of temporal characteristics of injection-locked polarization switching of a conventional-type 1.55-μm wavelength single-mode vertical cavity surface-emitting laser (VCSEL). Delayed recovery response of the main-mode of the VCSEL was observed when short and strong optical injection pulses of an orthogonal polarization to the VCSEL's main polarization-mode were applied. Numerical analysis based on a spin-flip model describes that the relatively long upper level lifetimes compared to a short injection-pulse width and long cavity photon lifetimes cause delayed recovery response of the main-mode of the VCSEL. An optimum bias current of the VCSEL was also observed for the shortest recovery time of its free-running polarization mode after the orthogonal polarization beam pulse injection.

Wafer-fused heterostructures: application to vertical cavity surface-emitting lasers emitting in the 1310 nm band

Currently, wafer-fusion technology represents an effective technique for enhancing the performance of long-wavelength vertical cavity surface-emitting lasers (VCSELs) based on classical double heterostructures with multi-quantum well active regions. Using the example of 1310 nm wavelength VCSELs, we demonstrate the status of this technology for wafer-level scale, wavelength-controlled devices with high performance, capable of operation in a wide temperature range up to 90 °C with single-mode output power levels in excess of 1 mW and a side mode suppression ratio (SMSR) in excess of 40 dB. No degradation was observed in a qualification lot that operated at 10 mA and 90 °C for 2000 h.

Long Wavelength VCSEL-by-VCSEL Optical Injection Locking

Vertical-cavity surface-emitting laser (VCSEL)-by-VCSEL optical injection locking to obtain high cutoff frequencies of 1.3-mum VCSELs is demonstrated. A detailed physical explanation of the underlying mechanism is presented. VCSELs from the same wafer have been used in a master-follower configuration. Two probe stations are used in this experiment to power-up two VCSELs simultaneously. Polarization insensibility of the injection locking is demonstrated and a novel architecture is proposed to achieve cutoff frequency doubling. For the first time, a high cutoff frequency is achieved through optically injection locking the satellite mode of a long wavelength VCSEL. Injection-locking spectra with variable injection powers and variable detuning values have been obtained and methods have been proposed to obtain high cutoff and/or resonance frequencies. A rate-equation-based model is presented. Simulations have been carried out using this model. Finally, a linear increases in the follower VCSEL cutoff frequency with increasing injected power is demonstrated by using a semiconductor optical amplifier.

Experimental Study of the Lasing Modes of 1.3-<formula formulatype="inline"><tex Notation="TeX">$\mu$</tex></formula>m Highly Strained InGaAs–GaAs Quantum-Well Oxide-Confined VCSELs

We present an experimental study of the main modes involved in the emission properties of InGaAs-GaAs quantum-well oxide-confined long wavelength vertical-cavity surface-emitting lasers. Lasing properties are dominated by the so-called "oxide modes" and by aperture modes, respectively, for small and large driving currents. We present complementary investigations of the laser emission including far-field angular distribution and spectroscopic near-field optical microscopy to a better understanding of the nature of the "oxide modes".

In(Al)GaAs–AlGaAs Wafer Fused VCSELs Emitting at 2-$\mu$m Wavelength

We demonstrate 2-mum wavelength, wafer-fused In(Al)GaAs-InP-AlGaAs-GaAs vertical-cavity surface-emitting lasers (VCSELs) emitting single-mode power of 0.5 mW at room temperature with a threshold current of 4 mA and sidemode suppression ratio of over 30 dB. These devices can be continuously tuned with current by 5 nm without mode hopping, with a tuning rate of 0.31 nm/mA. These features demonstrate the potential of these long wavelength VCSELs for gas sensing and other optical spectroscopy applications.

2.5-Gb/s Hybrid Single-Mode and Multimode Fiber Transmission of 1.5-$\mu$m Wavelength VCSEL

We report, for the first time, a successful 2.5-Gb/s transmission performance of a 1.5-mum emission monolithic vertical-cavity surface-emitting laser (VCSEL) over hybrid links composed of 25-km-long conventional single-mode fibers (SMFs) and a 2.2-km-long 50-mum core multimode fibers (MMFs). This result suggests that 1.5-mum wavelength VCSELs can be effectively used for multigigabit-per-second transmission over hybrid links interconnecting SMF-based long-distance and (or) subscriber network lines with MMF-based local-area network lines in the future ubiquitous network era

Investigation and design of widely tunable vertical-cavity surface emitting lasers

The tuning characteristics of widely tunable wavelength vertical-cavity surface-emitting lasers (VCSELs) have been investigated based on transfer matrix model and finite-element structure-electric coupled-field analysis model and experiment, in which the electrostatic tuning of wavelength and microelectronic mechanical system (MEMS) cantilever of the tunable VCSELs with central wavelength 980nm were designed. The analysis shows that the characteristics of wavelength tuning of VCSELs will be affected by the maximal displacement of cantilever and the structure of wavelength resonator. The tuning range can be improved based on the optimization of sacrificial layer for a given geometry of cantilever and active region of the device. In this paper, the structure of tunable VCSEL is designed. Further numerical simulations show that a continuous tuning range up to 32nm is obtainable, tuning efficiency is 0.12, and the whole tuning range of wavelength lies within the high gain region of the InGaAs quantum wells with GaAs barrier.