VCSEL Research Articles

Improved optical performance in circular-grating distributed feedback nanoplasmonic lasers

Optical modes control has been driven from vertical cavity surface emitting lasers (VCSELs) applications with multi-mode operation such as optical fiber communication or optical sensing. Optical gain materials coupling with optical resonators provides an effective way to tuning the optical mode of VCSELs. In this paper, a kind of novel circular-grating distributed feedback (CGDFB) nanoplasmonic laser is designed by using a special semiconductor nanorods array VCSEL coupled optically with a second-order distributed feedback (DFB) circular grating. In the device, the mode field is cooperatively controlled by the confinement of surface plasmon around the nanorod cavity and the spatial tuning with the DFB circular grating. By optimizing the design on the device comprehensively, the CGDFB nanoplasmonic laser exhibits the optimum lasing performance with a single-longitudinal mode peak at 511.2 nm with a side-mode suppression ratio of 45 dB. The 3 dB line-width is compressed to 1.98 p.m. at a driven current of four times of the threshold current of 124 mA. A six-fold rotational symmetric far-field mode corresponding to the third-order Laguerre Gaussian azimuth mode is produced by the DFB circular grating. This work paves a fundamental way for the mode engineering design of novel nanolaser in the technical applications of optical communication, sensing and integrated photonics.

Synergistic effect of proton irradiation and electrical stress on high-speed 850 nm vertical-cavity surface-emitting lasers

The synergistic effect significantly impact VCSEL's reliability for potential applications in spatial data communication. The synergistic effect of 3 MeV proton irradiation and electrical stress on 850 nm high-speed VCSELs is investigated. The degradation of VCSEL slow down under the influence of bias current. However, device degradation is more pronounced during irradiation under high bias current density. Low frequency noise (LFN) analysis results indicate that defects induced by proton irradiation are the primary cause of the optical-electrical parameter degradation in the device. The bias current density significantly influences the final defect yield, leading to variations in the degree of degradation after irradiation at different bias currents. At a bias current density of 6.03 × 105 A/m2, the degradation of the modulation bandwidth is reduced in comparison to the unbiased state. A mechanism investigation into the synergistic effect of proton irradiation and electrical stress on VCSELs lends support for accurately evaluating the radiation hardness of optical communication system composed of VCSELs in harsh radiation environments.

Secure multi-channel information encryption based on integrated optical device

In allusion to the privacy and security problems, a secure multi-channel information protection scheme based on an integrated optical device is proposed in this paper. The information to be encrypted and shared is converted into a QR code. For supporting the multi-channel encryption, a 1-nw encryption algorithm is employed, which possesses the capability to map the original image information into nw seemingly random cipher images. In terms of hardware, an ultra-compact optical encryption device is designed, which consists of two rows of ring resonators coupled to the waveguide. This novel encryption device can output ten cipher images, each transmitted on a separate channel. Meanwhile, the vertical-cavity surface-emitting laser subject to optical injection and optical feedback is researched to generate optical chaos sequences, which are introduced for key generation and QR code encryption. Ultimately, combining the 1-nw encryption algorithm and optical chaos, information protection is implemented based on integrated optics. The simulation results demonstrate the practical feasibility and remarkable effectiveness of the proposed scheme. Furthermore, various security and robustness analyses are performed, and the results reveal that the proposed information protection scheme has a high level of security and robustness. This work is of great significance for the development of optical encryption chips and provides potential applications for secure data sharing.

Over 20% wall plug efficiency of on-wafer GaN-based vertical-cavity surface-emitting laser

We demonstrated an over 20% wall plug efficiency of an on-wafer GaN-based vertical-cavity surface-emitting laser (VCSEL) with a 5 μm aperture. The cavity length of the VCSEL was 4λ optical length, containing a 3.7λ part of GaN-based layers controlled with an in situ reflectivity spectra measurement and a 0.3λ part of an ITO electrode and a Nb2O5 spacer layer adjusted with an ex situ measurement. In order to calibrate the thickness of ITO and N2O5, we directly evaluated resonance wavelength shifts of a 4λ GaN cavity test structure with additional ITO or N2O5 depositions on the cavity. We then fabricated GaN-based VCSELs with various aperture sizes from 5 to 20 μm by implementing the tuned ITO electrode and the tuned N2O5 spacer layer into the 4λ cavity of the VCSELs. The GaN-based VCSEL with an 8 μm aperture showed a light output power of 13.1 mW and an emission wavelength of 417.7 nm, which was only a 0.3 nm away from a designed wavelength.

In Situ Center Wavelength Control of AlInN/GaN Distributed Bragg Reflectors with In Situ Reflectivity Spectra Measurements

An in situ control of a center wavelength in a 40‐pair AlInN/GaN distributed Bragg reflector (DBR) for GaN‐based vertical‐cavity surface‐emitting lasers (VCSELs) with an in situ reflectivity spectra measurement is proposed and developed. A procedure of the in situ control consists of the following three steps: 1) to grow an initial 5‐pair DBR; 2) to obtain the center wavelength value just after the initial DBR growth by the in situ reflectivity spectra measurement; and 3) to adjust growth times as a result of the obtained center wavelength value and continue the remaining 35‐pair DBR. It is experimentally demonstrated that the in situ control of the center wavelength in the 40‐pair DBRs with the ±2% thickness deviations in the initial 5‐pair DBR results in reductions of the deviation down to ±0.3%. The in situ DBR center wavelength control is a key technique for highly reproducible GaN‐based VCSELs.

Ultrathin Parylene C-based sensitivity-gain nanoplasmonic sensor integrated on VCSEL for Aβ42 detection

As Alzheimer's disease prevalence continues to rise, there is an increasing demand for efficient on-chip biosensors capable of early biomarker detection. This study presents a novel biosensor chip leveraging vertical cavity surface emitting laser (VCSEL) technology, with Parylene C serving as the antibody coupling layer and utilizing a streamlined one-step antibody modification method. Integration of Parylene C enhances chip sensitivity from 34.28 μW/RIU to 40.32 μW/RIU. Moreover, post-testing removal of Parylene C enables chip reusability without significant alteration of results. The sensor demonstrates effective detection of Aβ42, an Alzheimer's biomarker, exhibiting a linear range of 1–200 ng/mL and a detection limit of 0.26 ng/mL. These findings underscore the reusability and reliability of the ultrathin Parylene C-based VCSEL biosensor chip, highlighting its potential for point-of-care Alzheimer's disease diagnosis.

Operation of a Ramsey-CPT microcell atomic clock with driving current-based power modulation of a VCSEL

We report on the operation of a coherent population trapping (CPT) microcell atomic clock using a pulsed Ramsey-like interrogation. The Ramsey-CPT sequence, defined by two-step optical pulses separated by a free-evolution dark time, is produced by switching on and off the output power of a low-power vertical-cavity surface-emitting laser, through direct modulation of its driving current. High-contrast and narrow Ramsey-CPT fringes are detected without the use of any external optical modulator stage. We demonstrate closed-loop operation of the clock based on high-speed digital signal processing implemented in a field programmable gate array board. The clock's short-term fractional frequency stability is 1.3 × 10−10τ−1/2 until 2000 s. A power light-shift coefficient of 8 × 10−11/μW, in relative value, is obtained for a dark time of 150 μs. This value is about ten times lower than in the continuous regime. These results show the feasibility of fully integrated atomic clocks based on Ramsey spectroscopy, which could provide enhanced long-term stability.

High-performance multi-junction cascade 1.3 µm quantum dot vertical cavity surface-emitting laser.

A high-performance 5-junction cascade quantum dot (QD) vertical cavity surface-emitting laser (VCSEL) with 1.3 µm wavelength was designed. The characteristics of the QD as active regions and tunnel junctions are combined to effectively increase output power. The photoelectric characteristics of single-junction, 3-junction cascade, and 5-junction cascade QD VCSELs are compared at continuous-wave conditions. Results indicate that the threshold current gradually decreases, and the output power and slope efficiency exponential increase with the increase of the number of active regions. The peak power conversion efficiency of 58.4% is achieved for the 5-junction cascade individual QD VCSEL emitter with 10 µm oxide aperture. The maximum slope efficiency of the device is 6.27 W/A, which is approximately six times than that of the single-junction QD VCSEL. The output power of the 5-junction cascade QD VCSEL reaches 188.13 mW at injection current 30 mA. High-performance multi-junction cascade 1.3-µm QD VCSEL provides data and theoretical support for the preparation of epitaxial materials.

VCSEL-based photonic spiking neural networks for ultrafast detection and tracking

Inspired by efficient biological spike-based neural networks, we demonstrate for the first time the detection and tracking of target patterns in image and video inputs at high-speed rates with networks of multiple artificial spiking optical neurons. Using photonic systems of in-parallel spiking vertical cavity surface emitting lasers (VCSELs), we demonstrate the implementation of multiple convolutional kernel operators which, in combination with optical spike signalling, enable the detection and tracking of target features in images/video feeds at an ultrafast photonic operation speed of 1 ns per pixel. Alongside a single layer optical spiking neural network (SNN) demonstration, a multi-layer network of photonic (GHz-rate) spike-firing neurons is reported where the photonic system successfully tracks a large complex feature (Handwritten Digit 3). The consecutive photonic layers perform spike-enabled image reduction and convolution operations, and interact with a software-implemented SNN, that learns the feature patterns that best identify the target to provide a high detection efficiency even in the presence of a distractor feature. This work therefore highlights the effectiveness of combining neuromorphic photonic hardware and software SNNs, for efficient learning and ultrafast operation, thanks to the use of spiking light signals, towards tackling complex AI and computer vision problems.

Temperature Effects on Power Characteristics of Vertical-Cavity Surface-Emitting Lasers

This paper presents a numerical study based on temperature dependency on Auger process, examining the impact of temperature change and laser cavity active region parameters on the output power characteristics (Pout) of vertical-cavity surface-emitting lasers (VCSELs). The active region parameters include active region diameter, effective length, active region thickness, injection current, and mirror reflectivity. With increasing temperature, Pout reduction rate rises in proportion to active region diameter growth as opposed to effective length or active region thickness growth. It was determined that Pout thermal effects change minimally with injection current variation but can be reduced or completely mitigated by increasing the mirror reflectivity. This study provides notable insights in the design of VCSEL with consideration of temperature dependency on the Auger process.

SILICON PHOTODIODE FOR TELECOMMUNICATIONS (FSO) WITH INCREASED SPEED AND SENSITIVITY AT 980 nm

One of the basic elements of Free Space Optics is a photodiode, which must have a high level of sensitivity to radiation in the range of 0.8-1 μm, and be fast-acting. In our case, the chosen wavelength is 980 nm, which corresponds to the wavelength of a vertical-cavity surface-emitting laser (VCSEL). The optimization of the photodiode design is carried out taking into account the uniform generation of the photocurrent with a uniform volume charge density inside the crystal when receiving radiation with a working wavelength. The current of the photo signal in the external circuit, in this case, depends on the modulation frequency, the width of the space charge region (SCR), the mobility of minor charge carriers and the amount of bias at the p-n junction. This allows you to significantly manipulate the rise time of the transient characteristic of the photodiode, and the use of double passage of the radiation beam throughout the SCR, you can obtain a significant increase in sensitivity. The subject is the construction of a photodiode, which at a wavelength of 980 nm has small values of the rise time and high values of current monochromatic sensitivity. The goal is to substantiate and create a design of a photodiode with increased current monochromatic sensitivity and reduced rise time at 980 nm wavelengths for operation in FSO. Task: To analyze the factors affecting the sensitivity and rise time of the photodiode when working at a wavelength of 980 nm, as well as to develop the design of the photodiode and investigate it. Methods: mathematical analysis of photoelectric parameters of a photodiode taking into account the mechanism of generation of minor charge carriers. Modeling of the photodiode design. A planar technology for silicon was used to manufacture the photodiode. The technical solution is based on the optimization of the modulation frequency, the width of the SCR, the mobility of minor charge carriers and the amount of displacement at the p-n junction, as well as ensuring the double passage of radiation through the photodiode crystal. The result was also achieved by etching the mesastructure from the reverse side of the photodiode, the area of which is the projection of the photosensitive element of the photodiode, and the width of the silicon plate above the mesastructure should be about 50 μm. Results: Factors affecting the current monochromatic sensitivity of the photodiode and its rise time were determined and taken into account. A new photodiode design with increased sensitivity and reduced rise time has been created. Comparative studies of the manufactured batch of created photodiodes and FD-255A photodiodes were conducted, which showed that the proposed photodiode really has a current monochromatic sensitivity at a wavelength of 980 nm of at least 0.55 A/W. At the same time, its rise time was reduced from 50 ns to 20 ns. compared to the serial photodiode FD-255A.

Multi-junction cascaded vertical-cavity surface-emitting laser with a high power conversion efficiency of 74%

High electro-optical conversion efficiency is one of the most distinctive features of semiconductor lasers as compared to other types of lasers. Its further increase remains a significant objective. Further enhancing the efficiency of edge-emitting lasers (EEL), which represent the highest efficiency among semiconductor lasers at present, is challenging. The efficiency of vertical cavity surface emitting lasers (VCSELs) has always been relatively low compared to EEL. This paper, combining modeling with experiments, demonstrates the potential of multi-junction cascaded VCSELs to achieve high efficiency beyond that of EELs, our simulations show, that a 20-junction VCSEL can achieve an efficiency of more than 88% at room temperature. We fabricated VCSEL devices with different numbers of junctions and compared their energy efficiency. 15-junction VCSELs achieved a maximum efficiency of 74% at room temperature under nanosecond driving current, the corresponding differential quantum efficiency exceeds 1100%, being the largest electro-optical conversion efficiency and differential quantum efficiency reported until now for VCSELs.

An InGaAs Vertical-Cavity Surface-Emitting Laser Emitting at 1130 nm for Silicon Photonics Application

A highly strained InGaAs quantum well (QW) vertical-cavity surface-emitting laser (VCSEL) with low threshold current density, high efficiency and output power emissions around 1130 nm was grown by MOCVD. Its static characteristics at room temperature and high operation temperature were studied in detail. The 7 μm oxide aperture device exhibits a threshold current of 0.68 mA, corresponding to a threshold current density of 1.7 kA/cm2. The slope efficiency is 0.43 W/A and the maximum output power is 3.3 mW. Continuous-wave (CW) operation in the 10–80 °C temperature range is observed. The slope efficiency is almost constant at 10–80 °C. The threshold current becomes lower at high temperatures thanks to the alignment between gain peak and cavity mode. The 3 μm oxide aperture device’s lasing in single mode with the RMS spectral width of 0.163 nm and orthogonal polarization suppression ratio (OPSR) is ~15 dB at 25 °C. The small-signal response analysis indicates that reducing the parasitics of the device and refining the fabrication process will improve the dynamics response characteristics. These results indicate that the 1130 nm GaAs-based VCSEL with highly strained InGaAs QWs is expected to be used as source for silicon photonics.

High-beam-quality low-resistance vertical-cavity surface-emitting laser array with graphene electrode.

Thermal crosstalk and current crowding effects are pressing issues that significantly impact the beam quality and efficiency of vertical-cavity surface-emitting laser (VCSEL) arrays. In this paper, by taking advantage of the excellent current transmission characteristics of graphene, what we believe to be a novel VCSEL array based on graphene electrode is designed to realize vertical current injections. The series resistance and self-heating of arrays are reduced by controlling the transport direction of the current, effectively suppressing the thermal crosstalk effect. Furthermore, high array beam quality is obtained by optimizing the current density distribution in active regions. Ultimately, the high-power quasi-single mode emission of VCSEL arrays is achieved by introducing graphene electrodes (Gr-VCSEL array) designs. Compared to traditional VCSEL arrays, the 10 × 10 Gr-VCSEL array demonstrates a 41% reduction in series resistance, a side mode suppression ratio of 32 dB, and a divergence angle around 12 °. This structure simultaneously achieves quasi-single mode emission and effectively suppresses the thermal crosstalk effect, providing a new method for the development of high-beam quality VCSEL arrays.

Study of the free carrier characteristics and surface morphology of AlGaAs/GaAs thin films deposited using MOCVD

Ultra-low loss optical thin films find broad applications in fields such as vertical-cavity surface-emitting lasers and optical atomic clocks. The main optical losses in AlGaAs/GaAs distributed Bragg reflectors (DBRs) prepared using metal-organic chemical vapor deposition (MOCVD) arise from absorption loss caused by free carriers within the layers and scattering loss caused by surface roughness. In this study, we fabricated AlGaAs and GaAs single-layer thin films with varying Al compositions on substrates of three crystal orientations and under different V/III ratios. The dependence of carrier concentration and surface morphology on different substrates and growth conditions was investigated. Thin films grown on substrates with three different crystal orientations exhibited three distinct growth modes (step-flow mode, SK mode, and FM mode). The impact of the V/III ratio on the growth mode was found to be complex. Higher V/III ratios resulted in poorer morphology for films grown on (100) substrates, while better morphology was observed on (211) B substrates. Furthermore, the surface morphology of films grown on (100) 15° off substrates showed less sensitivity to changes in the V/III ratio. With increasing Al composition, the carrier concentration of the films significantly increased. Elevating the V/III ratio proved effective in suppressing the incorporation of carbon, thereby reducing the carrier concentration of AlGaAs films. GaAs films exhibited a low carrier concentration at an appropriate V/III ratio. Additionally, the distinct abilities of different substrates to adsorb impurities exerted a significant impact on the carrier concentration of the films. This study demonstrates that, under optimal conditions, it is feasible to fabricate AlGaAs/GaAs Bragg mirrors with low carrier concentration and relatively small roughness on (100) 15° off substrates.

Antireflective vertical-cavity surface-emitting laser for LiDAR

Multijunction vertical-cavity surface-emitting lasers (VCSELs) have gained popularity in automotive LiDARs, yet achieving a divergence of less than 16° (D86) is difficult for conventional extended cavity designs due to multiple-longitudinal-mode lasing. Our innovation, the antireflective vertical-cavity surface-emitting laser (AR-VCSEL), addresses this challenge by introducing an antireflective light reservoir, where the electric field intensity is substantially higher than the gain region. This reduces the required cavity length for minimal divergence, preserving the single-longitudinal-mode lasing. A 6-junction AR-VCSEL array showcases a halved divergence and tripled brightness compared to its conventional counterpart. Various multijunction AR-VCSEL array designs achieve a divergence range of 8° to 16° (D86). Notably, a 7 μm AR-VCSEL emitter achieves 28.4 mW in single transverse mode lasing. AR-VCSEL stands out among semiconductor lasers, offering a well-balanced power density and brightness, making it a cost-effective solution for long-distance LiDARs. The antireflective cavity concept may inspire diverse applications in photonic devices beyond LiDARs.

Effect of High Current Density Pulses on Performance Enhancement of Optoelectronic Devices

Thermal annealing is commonly used in fabrication processing and/or performance enhancement of electronic and opto-electronic devices. In this study, we investigate an alternative approach, where high current density pulses are used instead of high temperature. The basic premise is that the electron wind force, resulting from the momentum loss of high-energy electrons at defect sites, is capable of mobilizing internal defects. The proposed technique is demonstrated on commercially available optoelectronic devices with two different initial conditions. The first study involved a thermally degraded edge-emitting laser diode. About 90% of the resulting increase in forward current was mitigated by the proposed annealing technique where very low duty cycle was used to suppress any temperature rise. The second study was more challenging, where a pristine vertical-cavity surface-emitting laser (VCSEL) was subjected to similar processing to see if the technique can enhance performance. Encouragingly, this treatment yielded a notable improvement of over 20% in the forward current. These findings underscore the potential of electropulsing as an efficient in-operando technique for damage recovery and performance enhancement in optoelectronic devices.

25 Gb/s NRZ transmission at 85°C using a high-speed 940 nm AlGaAs oxide-confined VCSEL grown on a Ge substrate.

In this Letter, we present a comprehensive analysis of the high-speed performance of 940 nm oxide-confined AlGaAs vertical-cavity surface-emitting lasers (VCSELs) grown on Ge substrates. Our demonstration reveals a pronounced superiority of Ge-based VCSELs in terms of thermal stability. The presented Ge-VCSEL has a maximum modulation bandwidth of 16.1 GHz and successfully realizes a 25 Gb/s NRZ transmission at 85 ∘C. The experimental results underscore the significance and potential of Ge-VCSELs for applications requiring robust performance in high-temperature environments, laying the cornerstone for the future development of VCSEL devices.

Photonic engineering of InP towards homoepitaxial short-wavelength infrared VCSELs

Many emerging opportunities, such as three-dimensional (3D) sensing, biophotonics, and optical data links, call for vertical cavity surface-emitting lasers (VCSELs) that operate in the short-wavelength infrared (SWIR) range. In this paper, we report the use of InP distributed Bragg reflector (DBR) mirrors to overcome an impasse in wafer-level mass production of SWIR VCSELs. The DBRs were based on homoepitaxial InP structures and selectively converted through electrochemistry into quarter-wavelength stack structures of alternating nanoporous (NP) and nonporous InP layers with a record index contrast (Δn∼1.0) and near-unity reflectivity. We demonstrated VCSEL operation at both 1380 and 1550 nm from two separate structures prepared on InP substrates using NP–InP DBRs as the bottom mirror and dielectric DBRs as the top mirror. Room temperature continuous-wave (CW) operation of SWIR VCSELs was successfully achieved at both wavelengths with a threshold current density below 2kA/cm2, greater than milliwatt optical output, and a peak power conversion efficiency of 17%. Our work provides strong evidence that the decades-old challenge, in preparing an InP-compatible, high-performance DBR to support the SWIR-emitting vertical cavity, has been addressed and is poised to enable new applications.

Sub-bandgap optical absorption processes in 300-nm-thick Al1−xInxN alloys grown on a c-plane GaN/sapphire template

We investigate the sub-bandgap optical absorption (SOA) in 300-nm-thick Al1−xInxN alloys used in cladding layers of edge-emitting laser diodes and distributed Bragg reflectors of vertical-cavity surface-emitting lasers. Al1−xInxN alloys, with indium content x ranging from 0.114 to 0.185, were grown by metal-organic chemical vapor deposition on a c-plane GaN/sapphire template. SOAs on 300-nm-thick thin films were characterized using photothermal deflection spectroscopy (PDS). Thermal emission, such as nonradiative recombination with phonon emission, is the dominant energy relaxation process occurring after SOA in Al1−xInxN alloys. The absorption coefficient of the SOA was estimated to be 0.6–7.0 × 103 cm−1 in these samples by combining PDS and spectroscopic ellipsometry. The drastic increase in the SOA, when x exceeded the lattice-matched composition of the GaN/sapphire template, indicates that impurities, vacancy-type defects, and their complexes with increasing x are possible candidates that result in SOA in Al1−xInxN alloys.