VCSEL Research Articles

Misestimate of the performance in VCSEL-based reservoir computing systems with optical information injection by high surface reflectivity.

In reservoir computing (RC) systems based on semiconductor lasers (SLs), the information that must be processed usually enters the reservoir through optical injection. Part of the injection information directly reflected by the front facet of the SLs is inevitably hybridized into the output of the SLs and contributes to the state of virtual nodes. For an RC system based on vertical-cavity surface-emitting lasers (VCSELs), the proportion of the reflected information coupled to the laser output is relatively huge due to the high surface reflectivity. Thus the influence of the directly reflected information will be much more obvious. Using a Santa Fe chaotic time series prediction task and waveform recognition task, we theoretically investigate the influence of high front facet reflectivity on the evaluation of the performance of a VCSEL-based RC system with optical information injection. The simulation results demonstrate that, after taking the directly reflected information into account, a lower error rate is obtained for each benchmark task. The physical mechanism to misestimate the RC performance has been studied through memory correlation and a statistical histogram of virtual node states.

850 nm VCSEL with sub quantum well and p-type [formula omitted]-doping in the active layers for improved high-speed and high-temperature performance

Thermal rollover is one of the fundamental limitations that affect the output power and modulation bandwidth of high-speed laser in monolithic laser integrated circuits (ICs). Herein, an 850 nm vertical cavity surface emitting laser (VCSEL) with p-doping and sub quantum well (sub QW) centers active layer is proposed and investigated. The p-doping and sub QW insert in the offset position of each barrier, which are designed for reducing the thermal rollover and increasing the maximum intrinsic modulation bandwidth of the laser. Numerical simulations show that adding the p-doping and sub QW in active region could reduce the threshold currents (Ith), improve the slope efficiency (SE) and delay the thermal rollover in VCSEL. Comparing with previous VCSEL, our proposed VCSEL has excellent high temperature output performance, and the maximum simulation 3-dB bandwidth is 43.9 GHz at 25 °C and 33.1 GHz at 85 °C, respectively. Furthermore, temperature-dependent time-resolved photoluminescence (PL) and PL excitation reveal this novel QW has strong radiative recombination rate, which is suitable for designing active region of high-speed and high power lasers.

Security mesh-based optical network exploiting the double masking scheme.

In order to tackle the security and privacy problems in optical networks, a novel mesh-based optical security network exploiting double masking (DM) scheme for multipoint confidential communication is proposed and studied theoretically. For each node in the network, a pair of mutually asynchronous vertical-cavity surface-emitting lasers (VCSELs) are required as transceivers, and the delay fiber (DF) is used to set different time delays as network node markers. In this security network, the encryption of the message is implemented on the transmitter of the source node by using the DM scheme, and the encrypted message is transmitted to the receiver of the destination node through the optical network for decryption. Each network node can output its individual chaotic signals separately with different time delay markers. By regulating different internal parameter mismatches, the synchronization characteristics of transceivers in a security network are numerically analyzed by using the cross correlation coefficient. Simulation results show that the chaos synchronization between transceivers enjoys fantastic robustness to mismatched parameters. Meanwhile, the tolerance of the DM scheme to the inherent parameter mismatch is excellent, so it is suitable for constructing secure networks in optical networks. Besides, based on the high quality synchronization with a correlation coefficient of 0.983, the communication performances of the longest path channel are investigated for a given metropolitan area network scale. Two pieces of 10 Gb/s messages can be effectively concealed in the chaos and decoded gratifyingly behind 100 km transmission, and the system has reliable security to resist illegal attacks. Finally, the network performance simulation is conducted for diverse configurations of the mesh-based optical networks. All the results confirmed the chaotic encryption scheme provides a novel way for any two legitimate nodes to establish security keys in optical networks.

Large-Signal Equivalent Circuit for Datacom VCSELs – Including Intensity Noise

Optical interconnects (OIs) continue to require more and more sophisticated driver and receiver electronics as higher baud rates are pushed in datacom, mainly due to the bandwidth stagnation of the optoelectronic components such as the vertical-cavity surface-emitting laser (VCSEL) in the transmitter as well as the photodetector (PD) in the receiver. Another important focus is maintaining high energy efficiency for the OIs. For both cases, a reliable equivalent circuit model for high-speed VCSELs is required for link optimization. This work is an extension of our previously presented large-signal VCSEL equivalent circuit model, where noise is added to the physical processes that the VCSEL model is based on. Thus, a new step is taken in the strive of developing an even more accurate physics-based large-signal VCSEL equivalent circuit model for datacom applications. Following a detailed description of the VCSEL noise modelling, a presentation is given on simulated results of VCSEL relative intensity noise (RIN) spectrum and eye diagrams under 28 Gbaud on-off keying (OOK) and pulse-amplitude 4 (PAM4) modulation, and that are compared with corresponding measurement results. Good agreement is found over a wide range of VCSEL driving conditions and temperatures. A 28-GHz-bandwidth VCSEL is applied for the demonstration, and the presented noise extended VCSEL equivalent circuit model was implemented in Verilog-A, and the simulations were performed using Cadence Spectre.

Nondestructive Evaluation of Detuned Wavelength for As-Grown VCSEL Epi-Wafer

We propose a nondestructive method to estimate the peak wavelength of the active quantum well (QW) region of an as-grown vertical-cavity surface-emitting laser (VCSEL) epi-wafer based on room temperature electroluminescence (EL) measurements. Contacts for the electrical excitation are established using flexible conductive materials on the epilayers without the fabrication of metal electrodes. The angle dependence of the EL properties is investigated to analyze the detuned wavelength of the active QW region for the VCSEL structure. The wavelength offset caused by detuning obtained from the as-grown epi-wafer correlates with the laser threshold properties of the VCSEL chips fabricated from the epi-wafer. The results show the possibility of a nondestructive evaluation method without metal electrodes for as-grown VCSEL epi-wafers, particularly the active QW region, which has been challenging to optically analyze from completed VCSEL epi-structures.

Orbital Angular Momentum Wave Generation and Multiplexing: Experiments and Analysis Using Classical and Quantum Optics

Optical fiber communication is known as the backbone of communication today for voice, video, and data transmission. Recent advancements in optical technologies are leading us to many new forms of generation of orbital angular momentum (OAM) waves and its transmission technologies. Among them are hypergeometric waveform (HG), Bessel’s waveforms, Laguerre Gaussian waveforms, and other forms of OAMs. This mode of communication is by the twisted form of electromagnetic wave which can accommodate a larger number of channels making communication simpler for wired and wireless communication. In this paper, various methods of OAM waveform generation are presented and are compared in details discussing the various needs in communication. OAM multiplexing is done by spatial division multiplexing (SDM) and wavelength division multiplexing (WDM). Methods of OAM communication, generation techniques, power analysis, and bit error analysis prove efficient transmission capability of OAM waves using low power consumption and low bit error rate in optical communication networks. OAM from fiber can be directly communicated to atmosphere which is found very useful for short distance wireless communication. Experimental validation is discussed for HG, Bessel’s waveforms, and LG waveforms. Pure Optical Vertices (OV) are generated by VCSELs (vertical cavity surface emitting lasers) and transmitted by (MMF) multimode fibers. Various atmospheric effects and challenges are also elaborated in this paper. RSoft OptSim software is used for simulation.

VCSEL quick fabrication of 894.6 nm wavelength epi‐material for miniature atomic clock applications

A vertical cavity surface emitting laser (VCSEL) quick fabrication (VQF) process is applied to epitaxial materials designed for miniature atomic clock applications (MACs). The process is used to assess material quality and uniformity of a full 100 mm (4-inch) wafer against the stringent target specification of VCSELs for MACs. Target specifications in optical power (>0.6 mW) and differential efficiencies (<0.5 W/A) are achieved over large portions of a wafer; however, the variation in the oxide aperture diameter is shown to limit the yield. The emission of the fundamental mode at 894.6 nm at 70 ℃ $^{\circ}\mathrm{C}$ is met over a significant area of the wafer for ∼4 μ $\mu $ m aperture multi-mode devices. The consideration of the on-wafer variation reveals that further optimisation is required to increase the device yield to levels required for volume manufacture.

Modal characteristics of coupled vertical cavity surface emitting laser diode arrays

The modal characteristics of dual-element coupled vertical cavity surface emitting laser (VCSEL) arrays are analyzed numerically and experimentally. A photonic crystal pattern etched into the top mirror optically defines the two elements of the array that are independently electrically biased. Using a two-dimensional complex waveguide analysis, we incorporate the effects of varying temperature and electron plasma-induced index suppression arising from asymmetric injection. The simulations are compared to experimental characterization of output power, lasing spectra, and far-field beam profile as a function of the two independent injection currents. Three distinct operating regimes are identified for the arrays: single independent local mode; a region of two modes that are primarily localized into a specific cavity; and a region of two supermodes whose fields extend across both elements. This analysis provides a physical intuition for the behavior of the dual-element coupled VCSEL array across its full operating range for emerging applications.

An 8-A Sub-1ns Pulsed VCSEL Driver IC With Built-In Pulse Monitor and Automatic Peak Current Control for Direct Time-of-Flight Applications

An integrated laser diode driver for multi-junction vertical-cavity surface-emitting laser (VCSEL) arrays is presented in this brief. The driver chip, fabricated in 180nm bipolar-CMOS-DMOS (BCD) process, working under 12V supply, can pump more than 8A peak current into a triple-junction VCSEL array, producing up to 20.3W optical pulses. Measured optical pulses have tunable widths from 276ps to 2352ps with 120ps/160ps rise/fall time. When emitting 10MHz-frequency 960ps-width optical pulses with peak power ranging from 3.6W to 17.5W, the driver-VCSEL system has over 20% electrical-to-optical power conversion efficiency. The driver can record its output current waveform with a built-in pulse monitor. The peak data from the recorded waveform is used for automatic peak current control.

Static Beam Steering by Applying Metasurfaces on Photonic-Crystal Surface-Emitting Lasers

Photonic-crystal surface-emitting laser (PCSEL) holds promise for laser beam with high efficacy while retain the small divergence angle, being now all the rage as laser source in modern opto-electronic technologies to replace edge emitting lasers and vertical-cavity surface-emitting lasers (VCSELs). Arbitrary beam shaping and deflection of VCSELs with ultra-thin flat optics, aka metasurfaces, has recently demonstrated, making possibility to realize a compact system in many technologies. However, the large divergence angle and limited output power at single fundamental mode of the VCSEL limits its application. Here, we report the static steering of laser beam with high efficiency and near 1° divergence angle by shining the laser beam of the PCSEL through the metasurface array placed on top. The small divergence angle is contributed from PCSEL. On the other hand, the metasurface array offers the higher efficiency beam deflection, without considering additional collimating or focusing. This provides highly flexible platform and is capable to manipulate the laser emission through the design of the metasurfaces and the PCSELs. Moreover, the beam proprieties can be further modified for specific application through the metasurfaces. The integration of metasurface with PCSEL provides the compact and collimated laser source, promising more accurate generation of versatile laser sources on demand for automotive ultra-compact light detection and ranging (LiDAR), depth sensing, optical communication, etc.

1.55-µm range optical transmitter based on a vertical-cavity surface-emitting laser

1.55-µm range optical transmitter based on a vertical-cavity surface-emitting laser

Lead-free Cs3Cu2I5 perovskite vertical cavity surface emitting lasers with low threshold

Due to their non-toxicity, excellent thermal stability and optical properties, ternary copper halide perovskites, such as Cs3Cu2I5 and CsCu2I3, have been widely used in light-emitting devices (LEDs), UV and X-ray scintillation detectors. However, laser devices based on these materials have not been fully investigated. In this work, we demonstrated thermally stable Cs3Cu2I5 thin films, deposited by dual-source co-evaporation, possessing a photo-luminescence quantum yield (PLQY) up to 50.62%. Using the Cs3Cu2I5 as gain media, optically pumped perovskite vertical-cavity surface-emitting lasers (Pe-VCSELs) have been fabricated, integrating with a pair of distributed reflectors (DBRs), and shown lasing at 440 nm with a threshold of ∼1.12 μJ/cm2 pumped by a 288 nm fs laser. Furthermore, a monolithic dual-wavelength laser, with lasing wavelengths of 458.8 nm and 505.6 nm, has been achieved by depositing an additional gain layer and DBR mirror. Those results indicate that the lead-free ternary copper halide perovskites could be suitable as laser gain materials.

Degradation Characteristics and Mechanism of High Speed 850 nm Vertical-Cavity Surface-Emitting Laser during Accelerated Aging

The degradation process of Vertical-cavity Surface-emitting lasers with high speed and a central wavelength at 850 nm is investigated via constant-current accelerated aging experiments. Degradation of the photoelectric performances under different operating conditions are characterized by optical output power, and forward and reverse bias current–voltage. The 1/f noise characteristics and formation mechanism are discussed in terms of the experimental results of low frequency noise below threshold current. The main composition of low frequency noise before and after aging, the bias current dependence and the origin of the noise are analyzed emphatically. The correlation between the degradation suggests that the loss of photoelectric performance and the fluctuation of low frequency noise characteristic can be attributed to the contagion of defects towards the active region of Vertical-cavity Surface-emitting lasers. Furthermore, the results of failure analysis confirmed the conclusion that the contagion of defects occurred towards the active region of the samples after aging.

Synaptic delay plasticity based on frequency-switched VCSELs for optical delay-weight spiking neural networks.

In this Letter, we propose an optical delay-weight spiking neural network (SNN) architecture constructed by cascaded frequency and intensity-switched vertical-cavity surface emitting lasers (VCSELs). The synaptic delay plasticity of frequency-switched VCSELs is deeply studied by numerical analysis and simulations. The principal factors related to the delay manipulation are investigated with the tunable spiking delay up to 60 ns. Moreover, a two-layer spiking neural network based on the delay-weight supervised learning algorithm is applied to a spiking sequence pattern training task and then a classification task of the Iris dataset. The proposed optical SNN provides a compact and cost-efficient solution for delay weighted computing architecture without considerations of extra programmable optical delay lines.

Long-Distance High-Power Wireless Optical Energy Transmission Based on VECSELs

Wireless charging systems are critical for safely and efficiently recharging mobile electronic devices. Current wireless charging technologies involving inductive coupling, magnetic resonance coupling, and microwave transmission are bulky, require complicated systems, expose users to harmful radiation, and have very short energy transmission distances. Herein, we report on a long-distance optical power transmission system by optimizing the external cavity structure of semiconductor lasers for laser charging applications. An ultra-long stable oscillating laser cavity with a transmission distance of 10 m is designed. The optimal laser cavity design is determined by simulating the structural parameters for stable operation, and an improved laser cavity that produces an output of 2.589 W at a transmission distance of 150 cm is fabricated. The peak power attenuation when the transmission distance increases from 50 to 150 cm is only approximately 6.4%, which proves that this wireless power transfer scheme based on a vertical external cavity surface-emitting laser can be used to realize ultra-long-distance power transmission. The proposed wireless energy transmission scheme based on a VECSEL laser is the first of its kind to report a 1.5 m transmission distance output power that exceeds 2.5 W. Compared with other wireless energy transmission technologies, this simple, compact, and safe long-distance wireless laser energy transmission system is more suitable for indoor charging applications.

Impact of Diffusion Profile on the Modulation Response of Single-Mode Disorder-Defined VCSELs

The impact of diffusion profile shaping through the use of tensilely and compressively strained diffusion masks on the modulation response of single-mode vertical-cavity surface-emitting lasers (VCSELs) using disorder-defined apertures is investigated. VCSELs designed for high-power single-fundamental-mode emission through the use apertures defined via impurity-induced disordering (IID) in conjunction with a standard oxide-confinement process are characterized to extract high-frequency optical cavity parameters across oxide aperture and diffusion mask strain variations. The 3-dB small-signal bandwidth is maintained for a 7.68 mW single-mode 850 nm VCSEL with an oxide aperture of 13 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula> using a tensilely strained diffusion mask relative to a non-disordered multimode device of the same oxide aperture. A large K-factor reduction is also observed, from 0.248 ns to 0.045 ns, indicating that damping and photon lifetimes within the cavity of VCSELs employing disorder-defined apertures are substantially reduced. Performance implications to data communication applications are discussed.

Improved Nonlinear Model Implementation for VCSEL Behavioral Modeling in Radio-Over-Fiber Links

This paper proposes an improved model for the nonlinear behavior of the components of Radio-over-Fiber (RoF) links focusing here on the laser source. In a previous work, it was shown with a flexible new opto-microwave design approach a first behavioral model of RoF links that exploited the hyperbolic tangent function (the so-called <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">tanh</i> model) to model the high-power nonlinearities. Since this was not sufficient to simulate adequately all the nonlinearities, in this work, for the first time in the authors’ knowledge, the improved nonlinear model based on Cann's model is developed, on an electro optic transducer. Then the amplitude-to-amplitude conversion of the link is better taken into account with the introducing of nonlinear properties of the link at high input power in the model. The new transfer equation includes several parameters that can be adjusted to adapt the model to any of the components, especially the photo-emitters under study, which are the main cause of this type of nonlinear behavior of the system. In particular, this improved model is applied on the state-of-the-art 850 nm GaAs Vertical Cavity Surface Emitting Laser (VCSEL), simulating several Figures of Merit such as the Error-Vector Magnitude (EVM) of a Long Term Evolution (LTE) signal with a Quadrature Phase Shift Keying (QPSK)and the output amplitude-modulated power versus input amplitude-modulated power, also called AM/AM curve. This new proposed nonlinear model outperforms other existing behavioral models and its non linear simulation better fits with the measurements. Finally, we conclude that the performance improvement highly depends on the nonlinearity sharpness parameter, depending on the frequency and the bias current.

Impact of an AlGaN thin insertion layer in the p-GaN region on InGaN/GaN vertical-cavity surface-emitting laser diodes

In this work, we propose a p-GaN/p-AlxGa1-xN/p-GaN structure in p-type hole injection layer for InGaN/GaN vertical-cavity surface-emitting laser diodes (VCSEL), and this design can efficiently improve the hole injection into the multiple quantum wells (MQWs). In the p-GaN/p-AlxGa1-xN/p-GaN structure, the polarization induced charges generated at the heterojunction interface can produce the polarization induced electric field, which can accelerate holes. Meanwhile, the p-AlxGa1-xN layer also favors the current transport into the cavity center that can better enhance the device performance, i.e., the threshold current can be reduced and the lasing power can be enhanced. Moreover, we also find that the device performance is sensitive to the structural designs for the p-GaN/p-AlxGa1-xN/p-GaN junctions. Thus, the parametric investigations have also been conducted in this work.

Design of High-Power Red VCSEL on a Removable Substrate

In this work, the architecture of a high-power InAlGaP/InGaP vertical-cavity surface-emitting laser (VCSEL) with an emission wavelength of 680 nm was studied. The design of quantum well, including the well thickness, indium composition, and barrier aluminum composition targeting the emission wavelength, was elaborately optimized. Moreover, the influences of leakage current, temperature dependence of optical gain, and resonance mode gain to threshold current under different barrier aluminum compositions were investigated. Lastly, the temperature characteristics of InAlGaP/InGaP VCSEL with substrate removal have also been calculated with 24% and 40.6% improvement in thermal resistance and operating current range, respectively. It holds great promise for high-power red VCSEL application.

A Simple Method to Build High Power PCSEL Array with Isolation Pattern Design

Photonic crystal surface-emitting lasers (PCSELs) hold promising properties of both edge emitting lasers (EELs) and vertical-cavity surface-emitting lasers (VCSELs). They possess high output power while radiating light vertically, being thought of as the next generation laser source. One of the main advantages of PCSELs is their scalability of size and power, which makes them applicable to high power applications or long-distance detection. However, due to problems such as current injection and mode competition, there are limits on their dimensions. To further increase the power, the capability of two-dimensional array integration paves the way. In this work, we demonstrate a new method to fabricate PCSEL arrays by defining an isolation pattern. We also investigate the influence of aperture size and array arrangement on lasing performance.