VCSEL Research Articles

On-demand polarization by a vertical-cavity surface-emitting laser with two tilted sub-wavelength gratings.

We show that each polarization state on the Poincaré sphere (PS) can be accessed on-demand (Poincaré sphere tailoring) by a semiconductor-based vertical-cavity surface-emitting laser (VCSEL) with two tilted sub-wavelength gratings (SWGs). We develop a vectorial Barkhausen criterion that answers the question: what conditions must the cavity fulfill to support a given desired polarization state? Addressing this inquiry leads to a completely different strategy based on the entangled interplay between two tilted SWGs, resulting in an overall chiral cavity, whose features depend on the gratings and their mutual rotation. This leads to the emission of a well-controllable polarization state based on standard technologies used in polarization-stable VCSELs, which paves the way for inspiring several new potential applications.

Detector-integrated vertical-cavity surface-emitting laser with a movable high-contrast grating mirror

In this paper, we present a detector-integrated vertical-cavity surface-emitting laser (VCSEL) with a movable high-contrast grating (HCG) mirror in an n-i-p-i-n manner. The detector-integrated VCSEL with a movable HCG can achieve three functions, including wavelength tuning, power monitoring, and resonant-cavity-enhanced (RCE) photon detection. Currently, the device can achieve a wavelength tuning range of 27 nm at room temperature when the suspended HCG is driven by the reverse-bias voltage. The n-i-p structure located at the upper part of the device can serve as an intra-cavity photodiode to monitor the output power due to the defect absorption. The RCE photon detection function of the detector-integrated VCSEL with a movable HCG is measured, and it has a peak responsivity at about 926 nm. This detector-integrated VCSEL with a movable HCG will be useful for sensing and imaging.

Study on Bottom Distributed Bragg Reflector Radius and Electric Aperture Radius on Performance Characteristics of GaN-Based Vertical-Cavity Surface-Emitting Laser.

This article presents the results of a numerical analysis of a nitride-based vertical-cavity surface-emitting laser (VCSEL). The analyzed laser features an upper mirror composed of a monolithic high-contrast grating (MHCG) and a dielectric bottom mirror made of SiO2 and Ta2O5 materials. The emitter was designed for light emission at a wavelength of 403 nm. We analyze the influence of the size of the dielectric bottom mirrors on the operation of the laser, including its power-current-voltage (LIV) characteristics. We also study the effect of changing the electrical aperture radius (active area dimensions). We demonstrate that the appropriate selection of these two parameters enables the temperature inside the laser to be reduced, lowering the laser threshold current and increasing its optical power output significantly.

Comparison of TJ and ITO GaN VCSELs in terms of their frequency characteristics

The work focuses on vertical cavity surface emitting lasers (VCSELs) made of nitride materials that emit a wavelength of 445 nm. Two structures were examined: a laser with a tunnel junction and implantation (TJ VCSEL) and an ITO contact (ITO VCSEL). The analysis delves into capacitance phenomena influencing the modulation speed of these lasers. The results highlight differences in active currents between two structures, i.e., currents which contribute to the modulation of the laser emission. According to the authors’ simulations, the TJ VCSEL is more effective in modulating the number of carriers in the active region than the ITO VCSEL, assuming the same modulation amplitude of driving current.

Modeling full PCSELs and VCSELs using modified rigorous coupled-wave analysis.

An integrated rigorous coupled-wave analysis (RCWA) algorithm is presented in this paper, which can simulate full vertical-cavity surface-emitting laser (VCSEL) and photonic crystal surface-emitting laser (PCSEL) structures. A classic RCWA can only analyze a structure when the light source is incident from the top, bottom, or both sides of the device. However, for VCSEL applications, the light source is generated in the middle and propagates in both directions. A bidirectional scattering matrix method and doubling algorithm are implemented in RCWA. The resonant wavelength and Q factor of a VCSEL can then be found in the output spectrum. The accuracy and execution speed are compared with those of the Lumerical finite-difference time-domain (FDTD) method for several VCSEL and PCSEL designs. The results show that the maximum discrepancy between RCWA and FDTD is less than 3 nm, and the difference in the far-field divergence angle is less than 0.5°. The speed of RCWA also outperforms FDTD simulation significantly.

P‐44: A Smart Eye Tracking System Based on Linear Array Sensors

Eye tracking as an important human‐machine interface plays avital role in immersive displays such as augmented reality (AR) and virtual reality (VR). Different from near infrared (NIR) camerabased methods, the proposed corneal reflection method based on principle of eyeball co‐vision takes advantage of using eye‐safe 940‐ nm vertical cavity surface emitting laser (VCSEL) and dual‐column linear array sensor (DCLAS) to obtain coordinates of the eyeball's moving positions through detecting its reflected light. The DCLAS not only delivers high sensitivity at eye‐safe NIR wavelength, but also has high spatial resolution for position accuracy and wide dynamic range to suppress ambient light influence. Together with low power consumption, good wearability, low data redundancy and simple computing algorism, a great promise can be made to enable smart eye tracking for AR/VR.

Assessing the quality of random number generators through neural networks

In this paper we address the use of Neural Networks (NNs) for the assessment of the quality and hence safety of several Random Number Generators (RNGs), focusing both on the vulnerability of classical Pseudo Random Number Generators (PRNGs), such as Linear Congruential Generators (LCGs) and the RC4 algorithm, and extending our analysis to non-conventional data sources, such as Quantum Random Number Generators (QRNGs) based on Vertical-Cavity Surface-Emitting Laser (VCSEL). Among the results found, we have classified the generators based on the capability of the NN to distinguish between the RNG and a Golden Standard RNG (GSRNG). We show that sequences from simple PRNGs like LCGs and RC4 can be distinguished from the GSRNG. We also show that sequences from LCG on elliptic curves and VCSEL-based QRNG can not be distinguished from the GSRNG even with the biggest long-short term memory or convolutional neural networks (CNNs) that we have considered. We underline the fundamental role of design decisions in enhancing the safety of RNGs. The influence of network architecture design and associated hyper-parameters variations was also explored. We show that longer sequence lengths and CNNs are more effective for discriminating RNGs against the GSRNG. Moreover, in the prediction domain, the proposed model is able to deftly distinguish between the raw data of our QRNG and data from the GSRNG exhibiting a cross-entropy error of 0.52 on the test data-set used. All these findings reveal the potential of NNs to enhance the security of RNGs, while highlighting the robustness of certain QRNGs, in particular the VCSEL-based variants, for high-quality random number generation applications.

Structured-Light 3D Imaging Based on Vector Iterative Fourier Transform Algorithm.

Quasi-continuous-phase metasurfaces overcome the side effects imposed by high-order diffraction on imaging and can impart optical parameters such as amplitude, phase, polarization, and frequency to incident light at sub-wavelength scales with high efficiency. Structured-light three-dimensional (3D) imaging is a hot topic in the field of 3D imaging because of its advantages of low computation cost, high imaging accuracy, fast imaging speed, and cost-effectiveness. Structured-light 3D imaging requires uniform diffractive optical elements (DOEs), which could be realized by quasi-continuous-phase metasurfaces. In this paper, we design a quasi-continuous-phase metasurface beam splitter through a vector iterative Fourier transform algorithm and utilize this device to realize structured-light 3D imaging of a target object with subsequent target reconstruction. A structured-light 3D imaging system is then experimentally implemented by combining the fabricated quasi-continuous-phase metasurface illuminated by the vertical-cavity surface-emitting laser and a binocular recognition system, which eventually provides a new technological path for the 3D imaging field.

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.

Characterizing the aggregated encoding method utilizing bursts activated by a VCSEL-neuron with a feedback structure

The rapid advancement of photonic technologies has facilitated the development of photonic neurons that emulate neuronal functionalities akin to those observed in the human brain. Neuronal bursts frequently occur in behaviors where information is encoded and transmitted. Here, we present the demonstration of the bursting response activated by an artificial photonic neuron. This neuron utilizes a single vertical-cavity surface-emitting laser (VCSEL) and encodes multiple stimuli effectively by varying the spike count during a burst based on the polarization competition in the VCSEL. By virtue of the modulated optical injection in the VCSEL employed to trigger the spiking response, we activate bursts output in the VCSEL with a feedback structure in this scheme. The bursting response activated by the VCSEL-neuron exhibits neural signal characteristics, promising an excitation threshold and the refractory period. Significantly, this marks the inaugural implementation of a controllable integrated encoding scheme predicated on bursts within photonic neurons. There are two remarkable merits; on the one hand, the interspike interval of bursts is distinctly diminished, amounting to merely one twenty-fourth compared to that observed in optoelectronic oscillators. Moreover, the interspike period of bursts is about 70.8% shorter than the period of spikes activated by a VCSEL neuron without optical feedback. Our results may shed light on the analogy between optical and biological neurons and open the door to fast burst encoding-based optical systems with a speed several orders of magnitude faster than their biological counterparts.

Surface relief VCSELs at 670 nm with integrated polymer microlens for highly collimated fundamental-mode emission.

We demonstrate the integration of a wet-chemically etched surface relief on a vertical-cavity surface-emitting laser (VCSEL) emitting in the red spectral range for higher-order mode suppression. With this relief, fundamental-mode emission is achieved over the entire power range from threshold beyond thermal rollover. For collimation of the emitted beam, we implement polymer microlenses fabricated on-chip by a thermal reflow technique. We reduce the angle of divergence for all injected currents to a maximum of 2∘. By measuring high-resolution spectra, we show that Gaussian beam profiles correspond to pure fundamental-mode emission which is preserved after implementation of the polymer microlens onto the etched relief, proving the compatibility of the two processes.

Stably polarized 795 nm vertical-cavity surface-emitting lasers with anti-phase SiNx surface gratings

795 nm vertical-cavity surface-emitting lasers (VCSELs) with dielectric surface gratings to control the output polarization are designed and fabricated. The calculated results demonstrate that a well-designed SiNx surface grating positioned on the surface of an anti-phase VCSEL structure enhances the reflectivity difference between the two polarization modes compared to a conventional GaAs surface grating, consequently resulting in a larger gain anisotropy in VCSELs and a high orthogonal polarization suppression ratio (OPSR). Characterization shows that a peak-to-peak OPSR of 30.3 dB is achieved at 85°C for 795 nm VCSELs with a SiNx surface grating of 5 µm in diameter and an oxide aperture of ∼4µm, demonstrating the effectiveness of the SiNx surface grating in polarization control for 795 nm VCSELs.

High Performance Electrically‐Injected InGaN Microdisk Lasers through Simultaneous Enhancement of Optical Confinement and Overlap Factor

AbstractDespite the considerable research interest in InGaN‐based microdisk lasers, owing to their unique circular geometry distinct from vertical cavity surface‐emitting lasers (VCSEL) or edge‐emitting lasers, commercial products remain scant due to deficient performance under electrical injection. This study proposes an innovative method integrating a thin‐film configuration with a metallic undercut, significantly augmenting optical confinement, overlap factor, and thus lasing performance. This approach results in a diminished lasing threshold from 1500 to 670 Wcm−2 and a record high quality (Q) factor of 10100 under electrical injection. Strain relaxation in the metallic undercut, identified via scanning near‐field optical microscopy (SNOM), beneficially mitigates the Quantum Confined Stark Effect (QCSE), boosting the internal quantum efficiency of the laser. Furthermore, a beneficial blue‐shift in the emission spectrum aligns with the lasing peak, strengthening the Q factor. A tactfully implemented recessed top contact enables electrical injection lasing without optical performance compromise. The study provides invaluable insights for future microdisk laser technology advancements, potentially leading to specialized functionalities, including quantum optics, thereby opening new avenues for research and applications in this field.

Temperature effect on a weighted vortex spin-torque nano-oscillator for neuromorphic computing

In this work, we present fabricated magnetic tunnel junctions (MTJs) that can serve as magnetic memories (MMs) or vortex spin-torque nano-oscillators (STNOs) depending on the device geometry. We explore the heating effect on the devices to study how the performance of a neuromorphic computing system (NCS) consisting of MMs and STNOs can be enhanced by temperature. We further applied a neural network for waveform classification applications. The resistance of MMs represents the synaptic weights of the NCS, while temperature acts as an extra degree of freedom in changing the weights and TMR, as their anti-parallel resistance is temperature sensitive, and parallel resistance is temperature independent. Given the advantage of using heat for such a network, we envision using a vertical-cavity surface-emitting laser (VCSEL) to selectively heat MMs and/or STNO when needed. We found that when heating MMs only, STNO only, or both MMs and STNO, from 25 to 75 °C, the output power of the STNO increases by 24.7%, 72%, and 92.3%, respectively. Our study shows that temperature can be used to improve the output power of neural networks, and we intend to pave the way for future implementation of a low-area and high-speed VCSEL-assisted spintronic NCS.

Inverted n–p junction 940‐nm vertical‐cavity surface‐emitting laser arrays consisting of 875 devices on p‐GaAs substrate

AbstractThe authors demonstrate a large‐area vertical‐cavity surface‐emitting laser (VCSEL) 25 × 35 array on a p‐type GaAs substrate with an emission wavelength of ∼940 nm and optical output power > 1 W, using an inverted junction (n‐up) epitaxial structure for emerging sensing applications. The electrical characteristic (series resistance) of the inverted n–p VCSELs is better than that of standard p–n VCSELs, but optical characteristics (the threshold current density and slope efficiency) are worse possibly due to crystalline defects originating from the p‐type substrate whose effect could be avoided in small‐area VCSELs. Other inverted junction structures on an n‐type substrate are needed for further development.

Photonic implementation of the input and reservoir layers for a reservoir computing system based on a single VCSEL with two Mach-Zehnder modulators

Hardware implementation of reservoir computing (RC), which could reduce the power consumption of machine learning and significantly enhance data processing speed, holds the potential to develop the next generation of machine learning hardware devices and chips. Due to the existing solution only implementing reservoir layers, the information processing speed of photonics RC system are limited. In this paper, a photonic implementation of a VMM-RC system based on single Vertical Cavity Surface Emitting Laser (VCSEL) with two Mach Zehnder modulators (MZMs) has been proposed. Unlike previous work, both the input and reservoir layers are realized in the optical domain. Additionally, the impact of various mask signals, such as Two-level mask, Six-level mask, and chaos mask signal, employed in system, has been investigated. The system's performance improves with the use of more complex mask(t). The minimum Normalized mean square error (NMSE) can reach 0.0020 (0.0456) for Santa-Fe chaotic time series prediction in simulation (experiment), while the minimum Word Error Rate (WER) can 0.0677 for handwritten digits recognition numerically. The VMM-RC proposed is instrumental in advancing the development of photonic RC by overcoming the long-standing limitations of photonic RC systems in reservoir implementation. Linear matrix computing units (the input layer) and nonlinear computing units (the reservoir layer) are simultaneously implemented in the optical domain, significantly enhancing the information processing speed of photonic RC systems.

Experimental reservoir computing with diffractively coupled VCSELs.

We present experiments on reservoir computing (RC) using a network of vertical-cavity surface-emitting lasers (VCSELs) that we diffractively couple via an external cavity. Our optical reservoir computer consists of 24 physical VCSEL nodes. We evaluate the system's memory and solve the 2-bit XOR task and the 3-bit header recognition (HR) task with bit error ratios (BERs) below 1% and the 2-bit digital-to-analog conversion (DAC) task with a root mean square error (RMSE) of 0.067.

Performance improvement of GaN-based vertical cavity surface emitting laser by employing a hole storage layer and enhancing the carrier transport behavior

In this paper, the transport behavior of carriers between multiple quantum wells (vertical) and inside a single quantum well (radial) in a GaN-based Vertical Cavity Surface Emitting Laser (VCSEL) is investigated, and a new device structure with a hole storage layer is proposed to enhance the laser performance. The theoretical results indicate that the vertical transport capability of holes is the key factor for total stimulated recombination rate and hole leakage. The VCSEL with a double-QW structure exhibits better output performance compared to the devices with three or more QWs owing to the lower light absorption loss and the stronger optical field in the active region. Furthermore, a thicker undoped GaN layer is proposed to be added as the hole storage layer to mitigate hole leakage for the VCSEL with a double-QW structure. This new structure can increase the tilt of energy band in the region of the hole storage layer and effectively improve the uniformity of radial distribution of holes in QWs. Therefore, it is beneficial to reduce the threshold current and enhance the laser output power.

26.5625 Gbaud PAM-4 short-reach transmission at 75°C using 850 nm VCSELs with strategic oxide guiding layer positioning.

This article presents an all-epitaxy approach to reduce the root mean square spectral width (ΔλR M S) of 850 nm oxide-confined vertical cavity surface-emitting lasers (VCSELs) with a large aperture of 7 µm through strategic optimization of the oxide guiding layer within the epitaxy structure. At 75°C, the VCSEL demonstrates a ΔλR M S of ∼0.3 nm at a bias current of 7.5 mA. Furthermore, the VCSEL achieves successful transmission of 26.5625 Gbaud PAM-4 modulation over a short-reach (SR) OM4 fiber link while maintaining a TDECQ budget below the 4.5 dB specified by 50G IEEE Ethernet standards.

Vertical-Cavity Surface-Emitting Laser Linewidth Narrowing Enabled by Internal-Cavity Engineering

Vertical-Cavity Surface-Emitting Laser Linewidth Narrowing Enabled by Internal-Cavity Engineering