Vertical-cavity Surface-emitting Laser Arrays Research Articles

Establishing equivalent circuits of mounted, high-power VCSEL arrays for iToF cameras

Solid-state indirect time-of-flight (iToF) cameras are crucial to numerous short-to-medium-range applications, owing to their advantages in terms of system integrability and long-term reliability. However, due to the low light intensity, the sensing range of iToF cameras is generally limited to a few meters, which hinders their wide applications. Further increasing the sensing range requires not only higher-power laser diodes but also well-designed driver circuits, which are based on prior knowledge of the laser diodes’ equivalent circuits (ECs). However, experimental studies on ECs of a mounted, high-power vertical-cavity surface-emitting laser (VCSEL) array that comprehensively incorporates all parasitic components, especially parasitic stemming from printed circuit boards (PCBs), remain absent. In this Letter, an 850 nm VCSEL array with a 15.3 W peak power and a 581 MHz bandwidth is fabricated, and more importantly, its EC is experimentally established. Leveraging the accurate EC, a compact iToF camera with a sensing range up to 11.50 m is designed. In addition, a modified precision model is proposed to better evaluate the iToF camera’s performance.

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.

Metasurface- and PCSEL-Based Structured Light for Monocular Depth Perception and Facial Recognition.

The novel depth-sensing system presented here revolutionizes structured light (SL) technology by employing metasurfaces and photonic crystal surface-emitting lasers (PCSELs) for efficient facial recognition in monocular depth-sensing. Unlike conventional dot projectors relying on diffractive optical elements (DOEs) and collimators, our system projects approximately 45,700 infrared dots from a compact 297-μm-dimention metasurface, drastically more spots (1.43 times) and smaller (233 times) than the DOE-based dot projector in an iPhone. With a measured field-of-view (FOV) of 158° and a 0.611° dot sampling angle, the system is lens-free and lightweight and boasts lower power consumption than vertical-cavity surface-emitting laser (VCSEL) arrays, resulting in a 5-10 times reduction in power. Utilizing a GaAs-based metasurface and a simplified optical architecture, this innovation not only addresses the drawbacks of traditional SL depth-sensing but also opens avenues for compact integration into wearable devices, offering remarkable advantages in size, power efficiency, and potential for widespread adoption.

Thermal characteristics of high-power vertical cavity surface emitting laser array

Due to the excellent characteristics such as good beam quality, dynamic single-longitudinal mode, low power consumption, and good wavelength stability, especially easy-to-integrate high-density 2D area arrays, vertical-cavity surface-emitting laser (VCSEL) is widely used in optical identification, optical interconnection systems, optical data storage and other fields. In recent years, with the improvement of materials and process technology, VCSEL has played an increasingly important role in the fields of smartphone face recognition, drone obstacle avoidance, virtual reality/augmented reality (VR/AR), sweeping robots, home cameras, etc., and with the rapid development and application of 5G communication, VCSEL will become an indispensable main component. However, due to the introduction of distributed Bragg mirrors, the thermal effect of VCSEL is very serious, especially when VCSEL is integrated into an array device, current-induced self-heating of each individual array cell and the thermal coupling among array cells have become the major factors contributing to thermal rollover and hence restraining the optical output performance of the VCSEL arrays. Therefor it is of great significance to study the thermal characteristics of VCSELs, in order to solve the problem of thermal crosstalk between single-tube devices, and increase the life of the device. This paper analyzes the influence of cell spacing and arrangement on the thermal crosstalk phenomenon and thermal diffusion performance of VCSEL array device based on the finite element model. The simulation results show that the maximum temperature of the device decays exponentially with the increase of cell spacing, the thermal crosstalk phenomenon and thermal diffusion performance of VCSEL array devices are significantly improved. When the cell spacing is 120 μm, the influence between the cells is small, the thermal crosstalk phenomenon is significantly improved, and the heat dissipation effect is better. On this basis, four non-square VCSEL arrays with 16 cells are designed, and it can be seen that compared with the square arrangement, the isosceles triangle, pentagonal and hexagonal configurations have improved the thermal crosstalk phenomenon and thermal diffusion performance, and the overall temperature rise of the VCSEL array is significantly reduced. The thermal crosstalk phenomenon of the pentagonal arrangement is significantly improved, and the device temperature is 37.32 ℃, which is the best effect among several arrangement methods. According to the results of theoretical simulation, the VCSEL array devices with different arrangements are prepared and characterized on the same epitaxial wafer by the same process. From the <i>P</i>-<i>I</i>-<i>V</i> characteristic curves, it can be seen that the threshold currents of isosceles triangles, pentagons and hexagons are lower than those of the square arrangement, and the maximum output power is higher than that of the square arrangement, especially the maximum output power value of the pentagonal is 150 mW. The results show that the new arrangement can effectively improve the thermal crosstalk phenomenon between the cells, increase the output power of the device, and make the VCSEL array device have good optoelectronic and thermal characteristics.

Improvement of current crowding effect in VCSEL arrays with non-uniform oxidation aperture design

A compact electro-opto-thermal model of 2-D vertical cavity surface emitting laser (VCSEL) arrays considering the current crowding effect in each array cell is established to study the impact of oxidation aperture on the device performance. Simulated results shows that increasing oxidation aperture of array cell is helpful to improve the uniformity of current density distribution. With careful design of non-uniform oxidation aperture layout, both the uniformity of the temperature distribution and the current distribution is improved by 36.52% and 42.08%, respectively. Furthermore, 3×3 VCSEL arrays with uniform oxidation aperture (array-1) and non-uniform oxidation aperture (array-2) are fabricated and the L-I-V curves of two types of VCSEL arrays at different biases are also measured. The peak output optical power of array-2 is enhanced to 1.83mW with an improvement of 8.91% when compared with that of array-1. Moreover, the total optical output power of array-2 is always superior to that of array-1 over a wide bias current range.

Polarized Vertical-Cavity Surface-Emitting Laser Arrays With Asymmetrical Structures for Three-Dimensional Sensing With Polarization Imaging

Polarized Vertical-Cavity Surface-Emitting Laser Arrays With Asymmetrical Structures for Three-Dimensional Sensing With Polarization Imaging

Zone-addressable 20 × 20 940 nm VCSEL array with a 5-bit binary number pattern.

This article presents a monolithically zone-addressable 20 × 20 940 nm vertical-cavity surface-emitting laser (VCSEL) array with a binary number pattern design for sensing applications. The emitters in this VCSEL array have a uniquely designed binary pattern design, with each row representing a 5-bit pattern designed to aid pattern-matching algorithms to deduce the shape and depth information efficiently. Approximately 200 VCSELs are arranged in four individually addressable light-emitting zones, with ∼50 emitters in each zone. Each zone generates laser pulses up to 7.2 W in peak power.

Pixelated vertical-cavity surface-emitting laser arrays from colloidal quantum dot films

Pixelated arrays of vertical-cavity surface-emitting lasers from colloidal quantum dot thin films have been demonstrated. CdSe/ZnCdS core/shell structured colloidal quantum dots have been used as laser gain medium. 10 pairs of alternating layers between GaN and nanoporous GaN consisted highly reflective distributed Bragg reflectors, exhibiting more than 99% of reflectivity in the spectral range of 600 nm–650 nm. Close-packed colloidal quantum dot solid thin films were sandwiched between the two GaN/nanoporous GaN distributed Bragg reflectors to achieve well-defined lasing outputs under pulsed optical excitation with the lasing threshold as low as 11.0 μJ/cm2. The resulting CQD-VCSEL devices exhibited a lasing output power of 1.5 μW and a conversion efficiency of 11.4%. Finally, a two-axis scanning micro-mirror was used to complete the 25 (5 by 5) pixelated laser arrays system.

Miniaturized Vertical‐Cavity Surface‐Emitting Laser Array with a Novel Electrode Design for High‐Speed, Low‐Noise, and High‐Brightness Performance

Herein, it is shown how the novel layout and arrangement of electrodes of a vertical‐cavity surface‐emitting laser (VCSEL) array can simultaneously improve its high‐speed data transmission performance and the brightness of the output beam. In contrast to the layout of the traditional VCSEL array with its isolated mesas and a single electrode to electrical parallel arrangement of all active elements, the new inverse design can effectively reduce the pitch size between neighboring light emission apertures thereby allowing significant downscaling of the whole active area of the array and high brightness output. Moreover, there are two separate electrodes in demonstrated compact 7 × 7 VCSEL array, one for pure dc current injection and the other for large ac signal modulation. Compared with the single electrode reference device, the demonstrated array shows heavier dampening of the electrical–optical (E–O) frequency response, a wider maximum 3‐dB E–O bandwidth (17 vs 13 GHz), and a Gaussian‐like optical far‐field pattern with a higher brightness output (65.95 vs 40.8 kW cm−2 sr−1), under the same high output power (≈145 mW). The advantages of this novel VCSEL array lead to a much better quality of 32 Gbps eye‐opening with a higher brightness output.

Compact structured light generation based on meta-hologram PCSEL integration

Metasurfaces, a catalog of optical components, offer numerous novel functions on demand. They have been integrated with vertical cavity surface-emitting lasers (VCSELs) in previous studies. However, the performance has been limited by the features of the VCSELs such as low output power and large divergence angle. Although the solution of the module of VCSEL array could solve these issues, the practical application is limited by extra lens and large size. In this study, we experimentally demonstrate reconstruction of a holographic images using a compact integration of a photonic crystal surface-emitting laser and metasurface holograms designed for structured light generation. This research showcases the flexible design capabilities of metasurfaces, high output power (on the order of milliwatts), and the ability to produce well-uniformed images with a wide field of view without the need for a collection lens, making it suitable for 3D imaging and sensing.

High Slope Efficiency Double and Triple Junction 808 nm Vertical Cavity Surface Emitting Lasers

High-performance 808 nm double and triple-junction vertical cavity surface emitting lasers (VCSELs) are designed and investigated. The light-current-voltage (L-I-V) characteristics of 808-nm single, double, and triple-junction VCSELs are compared. In the paper, the double-junction VCSELs with two oxide apertures exhibit a very high slope efficiency (SE) of 2.32 W/A at 25 °C, and the peak SE of 2.51 W/A is achieved at 10 °C. The power conversion efficiency (PCE) of 53.14% is achieved for double-junction individual VCSEL emitter. The peak SE of the triple-junction VCSELs reach 3.40 W/A with the PCE of 53.02% for individual VCSEL emitter. Under narrow pulse test conditions (400 Hz, 60 ns pulse width), the peak output powers of the designed double-junction and triple-junction VCSEL arrays reach 216.11 W and 325.77 W under the injection current of 80 A, respectively. Such high-performance multi-junction 808-nm VCSELs are very important for pumping solid state lasers and medical aesthetics.

Design of a compact optical antenna system with high transmission efficiency utilizing a ring-like VCSEL array.

Vertical cavity surface-emitting lasers (VCSELs) with gigahertz bandwidth and good beam quality possess great potential for multi-wavelength free-space optical communication. In this Letter, a compact optical antenna system utilizing a ring-like VCSEL array that can realize the parallel transmission of multi-channel and multi-wavelength collimated laser beams and has the advantages of aberration elimination and high transmission efficiency is proposed. Ten different signals can be transmitted simultaneously, greatly increasing the channel capacity. Based on the vector theory of reflection, ray tracing and the performance of the proposed optical antenna system are demonstrated. This design method has a certain reference value for designing complex optical communication systems with high transmission efficiency.

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.

All-Monolithically Integrated Self-Scanning Addressable VCSEL Array for 3D Sensing

We propose an all-monolithically integrated self-scanning addressable vertical-cavity surface-emitting laser array for time-of-flight measurement. Some advantages of addressable VCSELs for ToF measurement applications include large reductions in power consumption, heat generation, multi-pass noise, and flare noise. In this paper, we discuss the characteristics of the proposed self-scanning addressable VCSEL array. All layers in the epilayer structure of the proposed VCSEL array were formed at once by metal-organic chemical vapor deposition. The device consists of an (Al)GaAs-based thyristor and a conventional top-emitting 940-nm oxide-confined VCSEL on an n-type GaAs substrate. The array contained 12 blocks (4 × 3) that have more than 40 emitters each. The device required only four signals from a field-programmable gate array to select the emitting blocks and one emission signal from a conventional VCSEL driver, even for arrays containing hundreds of blocks. The proposed module is capable of one-block sequential emission, parallel emission from several blocks, and emission from all blocks. The rise and fall times of the fabricated VCSEL array were observed to be 200 and 400 ps, respectively, for each emission mode. The influence of flare noise from an obstacle in front of the camera was dramatically reduced by avoiding emission to the obstacle.

Solid-State dToF LiDAR System Using an Eight-Channel Addressable, 20-W/Ch Transmitter, and a 128 × 128 SPAD Receiver With SNR-Based Pixel Binning and Resolution Upscaling

An entire solid-state direct time-of-flight (dToF) light detection and ranging (LiDAR) system that incorporates innovations for both the transmitter (TX) and the receiver (RX) is presented in this work. For the illumination TX, we demonstrate solid-state channel addressability, which significantly reduces the transmit power and improves the ranging distance by dividing the field of view (FoV) into separately illuminated sub-regions. In the RX, we introduce single-photon avalanche diodes (SPADs) pixel binning, which enables reconfigurability of the sensor’s spatial resolution. Finally, we introduce a machine learning (ML) technique that enables this pixel-binned depth sensor to upscale its spatial resolution after training/inference fusion with the intensity image. The laser diode driver (LDD) chip is implemented in the 180-nm bipolar-CMOS-DMOS (BCD) process and is capable of pumping more than 8-A peak current into a multi-junction vertical-cavity surface-emitting laser (VCSEL) array, producing up to 20.3-W optical pulses under 12.5-V supply voltage. The sensor chip is also implemented in the 180-nm BCD process with a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$128 \times 128$ </tex-math></inline-formula> SPAD array and reconfigurable pixel binning. Hardware and software co-optimization under low signal-to-noise ratio (SNR) conditions with ML-based spatial resolution upscaling is demonstrated.

Injection locking and coupling the emitters of large VCSEL arrays via diffraction in an external cavity.

Networks of semiconductor lasers are the foundation of numerous applications and fundamental investigations in nonlinear dynamics, material processing, lighting, and information processing. However, making the usually narrowband semiconductor lasers within the network interact requires both high spectral homogeneity and a fitting coupling concept. Here, we report how we use diffractive optics in an external cavity to experimentally couple vertical-cavity surface-emitting lasers (VCSELs) in a 5×5 array. Out of the 25 lasers, we succeed to spectrally align 22, all of which we lock simultaneously to an external drive laser. Furthermore, we show the considerable coupling interactions between the lasers of the array. This way, we present the largest network of optically coupled semiconductor lasers reported so far and the first detailed characterization of such a diffractively coupled system. Due to the high homogeneity of the lasers, the strong interaction between them, and the scalability of the coupling approach, our VCSEL network is a promising platform for experimental investigations of complex systems, and it has direct applications as a photonic neural network.

Study on Supermode Control of External Cavity VCSEL Array with Parallel-Coupled Model

A vertical-cavity surface-emitting laser (VCSEL) is easily made into a two-dimensional array to improve its output power. However, if the near-field complex amplitude output cannot be stabilized in the lowest supermode generated by the fundamental transverse mode, the spot cannot converge on the center normal of the far-field array geometry, and the energy will be diverged. The external cavity is prepared for the VCSEL array, and the output of the array can be stabilized in the lowest order supermode generated by the fundamental transverse mode by supermode control through diffraction coupling. In this study, the parallel-coupled model for the 2-dimensional VCSEL array based on Fresnel diffraction integration was established and used to numerically calculate and analyze the influence on the supermodes’ thresholds of different array distributions, array elements’ transverse modes and duty ratios versus the length of the external cavity. The simulation results show that the square array provides a wider range of external cavity control length (0.2d2/λ and approximately 0.9 to 1.0d2/λ). The lowest supermode of the fundamental transverse mode can be outputted. For the regular hexagonal array with the length of the external cavity of 0.15d2/λ and 0.6d2/λ. The results have important reference significance for the design of an external cavity VCSEL array, which can control the supermode and obtain VCSEL coherent array beams with high power and near diffraction limits.

Perspective on 3D vertically-integrated photonic neural networks based on VCSEL arrays

Abstract The rapid development of artificial intelligence has stimulated the interest in the novel designs of photonic neural networks. As three-dimensional (3D) neural networks, the diffractive neural networks (DNNs) relying on the diffractive phenomena of light, has demonstrated their superb performance in the direct parallel processing of two-dimensional (2D) optical data at the speed of light. Despite the outstanding achievements, DNNs utilize centimeter-scale devices to generate the input data passively, making the miniaturization and on-chip integration of DNNs a challenging task. Here, we provide our perspective on utilizing addressable vertical-cavity surface-emitting laser (VCSEL) arrays as a promising data input device and integrated platform to achieve compact, active DNNs for next-generation on-chip vertical-stacked photonic neural networks. Based on the VCSEL array, micron-scale 3D photonic chip with a modulation bandwidth at tens of GHz can be available. The possible future directions and challenges of the 3D photonic chip are analyzed.

Low-cost scanning LIDAR architecture with a scalable frame rate for autonomous vehicles.

Autonomous vehicles need accurate 3D perception with a decent frame rate and high angular resolution to detect obstacles reliably and avoid collisions. We developed a low-cost scanning multichannel light detection and ranging sensor architecture allowing scalable frame rates by adjusting the number of laser and detector pairs. Scanning is achieved by a pair of micro-electro-mechanical system (MEMS) mirrors. A control pattern for the MEMS mirrors to maximize the frame rate is presented. A built prototype based on the proposed architecture achieves a frame rate of 11.5Hz, a field of view of 70∘×30∘, and an angular resolution of 0.4°. The distance resolution is 6cm. Reliable single-shot detection for low-reflective objects up to 19m indoors and 11m under direct sunlight exposure is achieved. A performance assessment based on the presented measurement system for recently available vertical-cavity surface-emitting laser arrays with power densities up to 1k W/m m 2 shows promising improvement potential.

3D-printed facet-attached optical elements for connecting VCSEL and photodiodes to fiber arrays and multi-core fibers.

Multicore optical fibers and ribbons based on fiber arrays allow for massively parallel transmission of signals via spatially separated channels, thereby offering attractive bandwidth scaling with linearly increasing technical effort. However, low-loss coupling of light between fiber arrays or multicore fibers and standard linear arrays of vertical-cavity surface-emitting lasers (VCSEL) or photodiodes (PD) still represents a challenge. In this paper, we demonstrate that 3D-printed facet-attached microlenses (FaML) offer an attractive path for connecting multimode fiber arrays as well as individual cores of multimode multicore fibers to standard arrays of VCSEL or PD. The freeform coupling elements are printed in situ with high precision on the device and fiber facets by high-resolution multi-photon lithography. We demonstrate coupling losses down to 0.35 dB along with lateral 1 dB alignment tolerances in excess of 10 μm, allowing to leverage fast passive assembly techniques that rely on industry-standard machine vision. To the best of our knowledge, our experiments represent the first demonstration of a coupling interface that connects individual cores of a multicore fiber to VCSEL or PD arranged in a standard linear array without the need for additional fiber-based or waveguide-based fan-out structures. Using this approach, we build a 3 × 25 Gbit/s transceiver assembly which fits into a small form-factor pluggable module and which fulfills many performance metrics specified in the IEEE 802.3 standard.