VCSEL Research Articles

Vertical-cavity surface-emitting laser with subwavelength photonic double heterostructures

Control of transverse modes in the oxide-confined vertical-cavity surface-emitting laser (VCSEL) is a key issue. We demonstrate a VCSEL with a double heterostructure high-contrast grating (HCG) as the top mirror to control the transverse mode characteristics. Compared with the common HCG–VCSEL with a uniform HCG, theoretically the proposed double heterostructure HCG–VCSEL can suppress the higher-order transverse modes. Experimentally, the double heterostructure HCG–VCSEL has a reduced spectral width and red-shifted fundamental mode compared with a common HCG–VCSEL with a uniform HCG, which is consistent well with the theoretical results. This work provides a new, to the best of our knowledge, approach to control the transverse modes of the VCSEL.

Anomalous noise spikes beating from a fine-splitting mode of surface-emitting laser with imperfect true-roundness aspect

High-frequency discrete and distinct mode-beating noise (MBN) spectra are an anomalous phenomenon observed in the related intensity noise spectrum from the vertical-cavity surface-emitting laser (VCSEL), which reveals strong correlation with fine-splitting transverse modes caused by non-circular emission aspect of the VCSEL with imperfect true-roundness. Such a phenomenon is demonstrated for the first time, which seriously degrades the transmission performance of high-speed and broad-band encoding data carried by the multi-mode (MM) VCSEL biased under a large DC bias. In this work, the 4-ary quadrature-amplitude modulation (4-QAM) orthogonal frequency-division multiplexing (OFDM) and the non-return-to-zero on–off keying (NRZ-OOK) transmissions with the direct modulation of different 850-nm MM VCSELs, as suffered from the MBN-induced notches of the signal-to-noise-ratio (SNR) spectra, are characterized under their high-level lasing conditions with multi-mode numbers. The significant SNR degradation on the receiving and decoding performances caused by the cross-correlation between MBN and data spectra is also examined by tuning the lensed fiber along the transverse (X/Y)-axis of the VCSEL (perpendicular to the surface normal) when collimating the output beam, which finds the disorderly shifted MBN spectra, as attributed to the collection of light emitted from the overlapped gain region of different transverse modes. In contrast, the MBN reveals invariant spectral features while the lensed fiber is either back-and-forth tuning along the vertical (Z)-axis (parallel with the surface normal) or tilting its angles from the surface normal under the same horizontal position and distance concerning the MM VCSEL. Such MM VCSEL exhibits several MBN spikes to decay the SNRs of NRZ-OOK and QAM OFDM data encoded around 21 and 34 GHz. For other MM VCSELs with reducing numbers of their transverse lasing modes and deforming roundness of their circular emitting aspects, the corresponding MBN spectral peaks can also be respectively observed at 14/17/24 GHz and 4.5/10.5/13 GHz to degrade the SNRs of the encoded data at such frequencies. Even though pre-emphasized encoding technology is employed, such an MBN effect is strongly correlated with the asymmetric circular aspect of the VCSEL with a degraded roundness ratio, which still affects the data modulation performance of the MM VCSEL for data center communication.

Meter-Scale Long Connectorized Paper-like Polymer Waveguide Film for 100 Gbps Board-Level Optical Interconnects Application

We design and fabricate meter-scale long connectorized paper-like flexible multimode polymer waveguide film with a large bandwidth-length product (BLP) for board-level optical interconnects application. The measured BLP of the multimode waveguide is greater than 57.3 GHz·m at a wavelength of 850 nm under the strictest overfilled launch condition with a maximum length of 2.1 m and 10-dB insertion loss. The fabricated waveguide films are as flexible as regular printing paper and can be conveniently interfaced with standard mechanically transferable (MT) fiber connectors with low loss. The average insertion loss of the connectorized waveguide is about 0.042 dB/cm with inter-channel crosstalk as low as −46.4 dB, and the bending loss is less than 1 dB at a bending radius of 1 mm under the overfilled launch condition. We also demonstrate a vertical-cavity surface-emitting laser (VCSEL)-based single-lane 100 Gbps PAM4 transmission. Our results show that the meter-scale long paper-like polymer waveguide film has both excellent optical properties and large bandwidth and is ideal for high-speed board-level optical interconnects application with a single-lane data rate of 100 Gbps and beyond, especially those that have a strict requirement on the length of connection and compactness.

Enhanced photonic reservoir computing using an optically injected VCSEL with random polarized optical feedback.

We propose and numerically demonstrate a photonic time-delay reservoir computing (TDRC) system exhibiting enhanced parallel task processing performance, where an optically injected vertical-cavity surface-emitting laser (VCSEL) under random distributed optical feedback acts as the reservoir computer. To assess its effectiveness, we perform two benchmark tasks including chaotic time-series prediction and waveform recognition task, where the TDRC is associated with two different random feedback structures, i.e., orthogonally polarized optical feedback (OPOF) and parallelly polarized optical feedback (PPOF). Benefiting from the enhanced nonlinearity offered by the random distributed optical feedback, the proposed TDRC excels at parallel task processing with the PPOF structure, whereas the performance of the OPOF structure may be deteriorated. Additionally, we reveal the effect of the injection strength, feedback strength, pump current, and number of virtual nodes on the proposed TDRC. Our work paves the way for the performance enhancement of parallel task processing based on polarization multiplexing in a VCSEL-based TDRC.

Multi-sensors image fusion method for non-destructive inspection in vertical-cavity surface-emitting lasers

Multi-sensors image fusion method for non-destructive inspection in vertical-cavity surface-emitting lasers

Design of a calibratable optical antenna system based on ring array light source

Abstract This study presents a multi-wavelength, multi-channel, and high-capacity optical antenna system. This approach is intended to allow parallel transmission with many channels and wavelengths by employing a ring array of vertical-cavity surface-emitting lasers, with reception accomplished via a ring detector array. The system employs spatially multiplexed techniques to send the target with numerous independent signals at the same time, effectively increasing bandwidth and reducing signal interference. The method used in this design provides benefits such as effective decreasing losses caused by blocking and removing aberrations. To overcome the spatial alignment difficulty caused by the array light source’s tiny beam size during long-distance transmission, this study introduces a high-precision acquisition, pointing, and tracking system as a reverse communication channel. The system’s wide beam can be used to align the installation of optical antenna equipment and facilitate information transmission.

Single-mode chirped high-contrast metastructure VCSEL for 106 Gbps PAM4 transmission

Short-reach optical interconnects, employing an 850 nm wavelength multimode vertical cavity surface emitting laser (VCSEL) and multimode fiber (MMF), are confronted with transmission distance limitation as the transmission speed increases. Achieving higher speeds over the same transmission distance necessitates a significant enhancement in the fiber’s effective bandwidth, resulting in substantial costs. Employing single-mode VCSEL with single-mode fiber transmission presents as a more economically viable solution. Here we report, to our knowledge, the first demonstration of an 850 nm single-mode VCSEL using a chirped high-contrast metastructure (HCM) as the top mirror with spatially graded reflectivity to suppress higher-order modes. The chirped HCM top reflector is designed to favor lasing of the fundamental mode. We show stable single-mode lasing with a >40dB side mode suppression ratio (SMSR) up to 15 times threshold current and an open eye through a short single-mode fiber SM800 transmission at 106 Gbps PAM4 modulation. This study highlights the potential for scalable single-mode VCSEL in advanced optical interconnects, providing a cost-effective pathway for high-speed applications.

Versatile VCSEL source of thermal and super-thermal light.

We have built highly controllable sources of thermal and super-thermal states on the basis of a single-mode (SM) and a multi-mode (MM) vertical cavity surface emitting laser (VCSEL) with noised driving current operating above the threshold. Varying the average driving current and the amplitude and bandwidth of noise, one can robustly obtain light with the temporal second-order intensity correlation function reaching 2.5 and correlation times from 1 µs to 10 ns. For the SM VCSEL, generated thermal lights retain its single-mode quality negating a need for a spectral filtering. For the MM VCSEL, one can still find a near-threshold regime when the generated thermal radiation is close to a single-mode one.

Electrical liquid crystal integration with VCSEL arrays for tunable orthogonal polarization laser.

Vertical-cavity surface-emitting laser (VCSEL) arrays that can emit orthogonally polarized light have shown broad application potential and market demand in the fields of optical communication, optical sensors, and biomedicine. Two kinds of polarization-switching VCSEL arrays based on anti-parallel (AP) oriented liquid crystal (LC) external cavities and twisted nematic (TN) oriented LC external cavities are proposed in this paper. The light is independently guided by the LC electrically, achieving uniform lasing of linearly polarized light with mutually orthogonal polarization modes and equal power. The uniformity and polarization tunable ability of the lasing orthogonally polarized light power is guaranteed by the integration of the LC external cavity and the VCSEL array.

Enhanced performance of GaN based VCSELs through graded electron-blocking layer design

Electron leakage from the active region to the p-type region restricts the performance of GaN-based Vertical-Cavity Surface-Emitting Lasers (VCSELs). AlGaN EBL can decrease the leakage current, but also raises the hole injection barrier and reduce the hole injection efficiency. Then, it is important to design EBL structures that can enhance both electron blocking and hole injection. In this study, VCSEL devices with three different EBL, basic structure Al0.6Ga0.4N EBL labeled as Device A, newly proposed 16 nm thick Al0.75–0.80Ga0.25–0.20N EBL labeled as Device B, and 18 nm thick Al0.75–0.80Ga0.25–0.20N EBL labeled as Device C, respectively, are designed and analyzed using PICS 3D simulation. Device A represents a basic VCSEL structure, while Device C incorporates a graded electron-blocking layer (EBL) with adjusted thickness. Through simulation results, it is observed that the introduction of a graded EBL in Device C leads to significant performance enhancements compared to the basic structure of Device A. Specifically, the graded EBL effectively reduces band bending and increases the electron barrier height, thereby improving carrier confinement and reducing electron leakage. Additionally, the utilize of a Graded structure in Device C aids in strain relief at the layout between the QB and EBL, resulting in improved electron-blocking capability and potentially enhanced hole transport characteristics. These findings underscore the importance of EBL grading in optimizing the performance of VCSELs, highlighting its potential for advancing the efficiency and functionality of these semiconductor devices. Power of the device C is being improved upto 4.16%, similarly the conduction band barrier height is improved upto 26.6% which is beneficial for better VCSEL performance as it enhances electron confinement in the active region, leading to increased efficiency and reduced carrier leakage and valance band barrier height decreases upto 6.52% and threshold current is decreased upto 4.8% so if valence band barrier height decrease the hole injection efficiency increases and if threshold current decreases the emitting power of the device increase.

Photonic reservoir computing for parallel task processing based on a feedback-free spin-polarized VCSEL

Time delayed reservoir computing (RC) is a novel artificial neural network that is easy to implement in hardware due to its extremely simple structure. Because of its time-division multiplexed information processing, laser-based photonic time-delayed RCs usually realize parallel processing with polarization/wavelength multiplexing. However, the performance of two different tasks is difficult to regulate separately and simultaneously in the time delayed RC system, especially for the chip-scale configuration. Here, we propose a feedback-free RC system based on a spin-polarized vertical-cavity surface-emitting semiconductor laser (VCSEL), which simplifies the whole system structure and can process time series prediction and waveform recognition tasks in parallel, with employing the input and output coding to provide the effect from past states. By separately setting the number of past states introduced by the coding for the two tasks, the performance of the two tasks can be adjusted respectively. Furthermore, by appropriately tuning the pump polarization ellipticity which is the unique feature for the spin-polarized VCSEL, the computational ability of the proposed RC can be focused on one of the two parallel tasks. Therefore, the proposed RC system is capable of dealing with different tasks with high performance, and also expected to provide a viable solution for integrated neuromorphic computing systems due to its compact, feedback-free structure.

Multimode light generation in an injection semiconductor laser based on a chiral AlAs/(Al, Ga)As/GaAs microcavity

Experimental investigations of chiral injection AlAs/(Al, Ga)As/GaAs vertical-cavity surface-emitting lasers in the multimode generation regime are performed. A high circular polarization degree 70% of different generation modes measured with a high spectral resolution, is demonstrated. Detailed maps of spatial and angular distribution of laser radiation intensity were constructed.

On the relationship between electrical and electro-optical characteristics in vertical cavity surface emitting lasers for chip scale Rb atomic clocks

We report on a comprehensive study of the electrical and electro-optical properties of 795 nm vertical cavity surface emitting lasers (VCSELs) designed for chip scale Rb atomic clocks. We highlight several key findings including the observation that the current flow at moderate bias levels comprises several parallel paths which are identified by an analysis of the I−V characteristic also confirmed by a numerical simulation. Resistance is a key parameter in any VCSEL. We analyze it in detail at all bias levels and find that above transparency, when the VCSEL enters the high injection regime, the current flow mechanism is modified significantly from an exponential to a power law characteristic. Consequently, resistance attains a nonlinear contribution which is quadratic in the spontaneous emission regime and quasi-linear above the threshold. This nonlinear contribution is not considered in common models. The optoelectronic properties are strongly correlated with the electrical characteristics what allow to explain several peculiarities of the VCSEL performance. We designed and fabricated the VCSELs according to the requirements of miniature Rb atomic clocks, including optimal operation at high temperatures. Their minimum threshold occurs at 363 K where they emit at 794.7 nm. The modal and polarization discrimination in the bias range where these VCSELs operate in practical miniature atomic clocks is well above 30 dB.

Learning Gradient-Based Feed-Forward Equalizer for VCSELs

Vertical cavity surface-emitting laser (VCSEL)-based optical interconnects (OI) are crucial for high-speed data transmission in data centers, supercomputers, and vehicles, yet their performance is challenged by harsh and fluctuating thermal conditions. This paper addresses these challenges by integrating an ordinary differential equation (ODE) solver within the VCSEL communication chain, leveraging the adjoint method to enable effective gradient-based optimization of pre-equalizer weights. We propose a machine learning (ML) approach to optimize feed-forward equalizer (FFE) weights for VCSEL transceivers, which significantly enhances signal integrity by managing inter-symbol interference (ISI) and reducing the symbol error rate (SER).

Stable Single-Mode 795 nm Vertical-Cavity Surface-Emitting Laser for Quantum Sensing.

Vertical-cavity surface-emitting lasers (VCSELs) are essential for exhibiting single-transverse-mode output characteristics, which are critical for applications in quantum sensing, optical interconnection, and laser printing. In this study, we achieved stable single-transverse-mode lasing using extended-2λ-cavity with an oxide aperture diameter of 7.08 μm. The device demonstrated a high output power of 6.8 mW and a narrow linewidth of 49.8 MHz at room temperature. Additionally, it maintained stable single-mode emission at 794.8 nm and achieved a side-mode suppression ratio (SMSR) exceeding 40 dB within the temperature range of 25 °C~85 °C, thereby meeting the requirements of 87Rb atom quantum sensors. The fabricated device obtained high-power and narrow linewidth single-transverse-mode operation by a monolithic extended cavity without introducing additional processing procedures, which is expected to promote the commercial viability of VCSELs in quantum sensing.

Single-fundamental-mode cryogenic (3.6 K) 850-nm oxide-confined VCSEL

Cryogenic oxide-confined vertical-cavity surface-emitting laser (VCSEL) has promising application in cryogenic optical interconnect for cryogenic computing. In this paper, we demonstrate a cryogenic 850-nm oxide-confined VCSEL at around 4 K. The cryogenic VCSEL with an optical oxide aperture of 6.5 μm in diameter can operate in single fundamental mode with a side-mode suppression-ratio of 36 dB at 3.6 K, and the fiber-coupled output power reaches 1 mW at 5 mA. The small signal modulation measurements at 298 and 292 K show the fabricated VCSEL has the potential to achieve a high modulation bandwidth at cryogenic temperature.

Gain characteristics of In0.60Ga0.40As/In0.53Al0.20Ga0.27As superlattice active regions for vertical-cavity surface-emitting lasers

Gain characteristics of In0.60Ga0.40As/In0.53Al0.20Ga0.27As superlattice active regions for vertical-cavity surface-emitting lasers

Progress in Research on Co-Packaged Optics

In the 5G era, the demand for high-bandwidth computing, transmission, and storage has led to the development of optoelectronic interconnect technology. This technology has evolved from traditional board-edge optical modules to smaller and more integrated solutions. Co-packaged optics (CPO) has evolved as a solution to meet the growing demand for data. Compared to typical optoelectronic connectivity technology, CPO presents distinct benefits in terms of bandwidth, size, weight, and power consumption. This study presents an overview of CPO, highlighting its fundamental principles, advantages, and distinctive features. Additionally, it examines the current research progress of two distinct approaches utilizing Vertical-Cavity Surface-Emitting Laser (VCSEL) and silicon photonics integration technology. Additionally, it provides a concise overview of the many application situations of CPO. Expanding on this, the analysis focuses on the CPO using 2D, 2.5D, and 3D packaging techniques. Lastly, taking into account the present technological environment, the scientific obstacles encountered by CPO are analyzed, and its future progress is predicted.

A Modified Current-Mode VCSEL Driver for Short-Range LiDAR Sensor Applications in 180 nm CMOS

This paper presents a modified current-mode vertical-cavity surface-emitting laser (VCSEL) driver as a transmitter for short-range light detection and ranging (LiDAR) sensors, where a stable bias generator is suggested with a regulated cascode current mirror circuit to provide the bias current of 1 mA with a trivial deviation of 5.4%, even at the worst-case process–voltage–temperature (PVT) variations. Also, a modified current-steering logic circuit is exploited with N-type MOSFET (NMOS) switches to deliver the modulation currents of 0.1~10 mApp to the VCSEL diode simultaneously, with no overshoot distortions. Post-layout simulations of the modified current-mode VCSEL driver (m-CMVD), using 180 nm CMOS technology, demonstrate very large and clean output pulses with significantly reduced signal distortions. Hereby, the VCSEL diode is transformed into an equivalent circuit with a 1.6 V DC voltage and a 50 Ω resistor for circuit simulations. The proposed m-CMVD consumes a maximum of 11 mW from a 3.3 V supply voltage and the chip core occupies an area of 0.196 mm2.

Magnetic resonance linewidths independent with the laser linewidth in coherent population trapping (CPT) magnetometers

Coherent population trapping (CPT) magnetometers are the magnetic measuring instrument that utilize the interference effect between light and atoms. It has a broad range of applications in weak magnetic measurement. However, current CPT magnetometers have low sensitivity, which does not satisfy the ultra-high sensitivity requirements of weak magnetic measurement. This paper is aimed to improve this problem by studying the relationship between the sensitivity of CPT magnetometer and the laser linewidth. It has been concluded that the magnetic resonance linewidth of the CPT system remains independent of the laser linewidth according to the Liouville equation, firstly. Furthermore, the correctness of this theory has been verified through design experiments. Additionally, we have identified certain issues that require attention in the miniaturization and chip-scale CPT magnetometer on vertical cavity surface emitting laser (VCSEL).