This PDF file contains the front matter associated with SPIE Proceedings Volume 11704, including the Title Page, Copyright information and Table of Contents

Wafer bow/warp in high performance 940nm VCSEL epitaxial wafers has been eliminated through the use of 150 mm Ge substrates, replacing conventional GaAs substrates. Ge is a drop-in replacement for GaAs for this application and has additional benefits in that it is zero EPD and mechanically more robust. High performance 940nm VCSELs have been fabricated on Ge and compared directly with those grown on GaAs with the same structure, with no discernible difference in device performance between the two approaches. Use of Ge also provides an immediate route to 200 mm VCSEL growths as Ge is readily available at that diameter.

Precise lasing wavelength control of VCSELs is attractive for several applications: 3D sensing, atomic clock, laser pumping, and the like. The wavelength of commercially available VCSELs is typically varied for each chip. This is because the thickness of epitaxial layers inevitably varies on a wafer due to the distribution of temperature and gas flow. VCSEL users on the module side have tolerated the problem. Here, we propose the novel strategy for precise wavelength control of VCSELs with simple fabrication by applying multi-wavelength (MW) VCSEL. We proposed two ways to achieve that. One is uniform wavelength method on wafer, and another is wavelength selection method. In both concepts, MW-VCSELs with wavelength-tuning-layer (WTL) inside DBR are suitable. Uniform wavelength method is followed by three steps: 1) A lower DBR, a cavity consisting of active layers, a first top DBR, and a WTL are grown. 2) The dip wavelength of the Fabry-Perot cavity is measured over the wafer. The WTL thickness is processed as to cancel out the wavelength variation in the two-dimensional data, which is performed by photolithography and etching techniques. 3) A remaining second top DBR is formed by regrowth technique to achieve more uniform wavelength than epitaxial growth alone. In our experiment, the reduction of wavelength variation from 6.6 nm (epitaxy only) to 2.0 nm was demonstrated. The concept can provide large scale array with uniform wavelength. Therefore, it is advantageous on high power applications as well as applications in which VCSELs with precise lasing wavelength are required. In wavelength selection method, precise wavelength is obtained by selecting emitters that met wavelength specification from emitters that are formed with different wavelengths by applying MW-VCSEL technology.

High-volume low-cost production of vertical cavity surface emitting lasers (VCSELs) will allow their exploitation in new commodity markets. We report the successful scaling up from research level fabrication to produce oxide confined VCSELs across a whole 150mm wafer. On-wafer light-current-voltage (L-I-V) and spectral measurements are analyzed to determine the cross-wafer variations in threshold current, threshold current densities and emission wavelength, which is compared with reflectivity measurements taken immediately after growth. We examine the dependence of VCSEL performance on fabrication parameters over a range of device dimensions to assess whether variation arises from non-uniformity of the epitaxial material or wafer processing.

Vertical cavity surface emitting laser (VCSEL) have recently emerged as highly promising electro optic device in 3D sensing and Lidar due to excellent properties such as high reliability, attractive high power performance, design flexibility and low manufacturing costs. To become a dominant player in serving the consumer electronics and driverless cars markets, we develop 6-inch VCSEL production line include device design, epitaxial growth and device-fabrication. By optimizing the device structure and manufacturing process, high power and high efficiency VCSEL devices are developed. We demonstrate that the maximum power conversion efficiency of the triple active regions VCSEL with 61.6%. In this paper, we will present you the evolution of VCSEL manufacturing technology and device characterization.

Development of a quick fabrication (QF) method for commercial wafer characterisation based on rapid feedback of VCSEL performance. We report on the design of the fabrication process including the systematic removal of time-consuming steps of planarization, oxidation and substrate lapping, and the associated impact on device performance and yield. We show comparable performance of the oxide-confined QF etched trench VCSELs and full process devices and we show that unoxidised devices behave as large aperture oxidised devices. Further, we demonstrate similar performance of substrate-lapped and -unlapped VCSELs between 1.0-1.2 Ith with a difference in current tuning typically 0.064nm/mA.

While time-of-flight applications have led to VCSEL arrays operating at currents measured in the amperes and producing very high aggregate powers, the current through each individual VCSEL aperture is not substantially higher than in many other applications. Driving a single VCSEL emitter of moderate size to extremely high currents requires specialized circuits and operation in a regime where thermal effects will not destroy it, meaning low duty cycles and pulse on-times measured in single-digit nanoseconds. In that regime traditional VCSEL performance and geometry scaling rules no longer apply and surprising behaviors emerge. We describe results for small area single-emitter 850-nm VCSELs designed for high power extraction operating at peak currents of several amperes. The electrooptical behaviors observed afford opportunities for VCSELs in nontraditional areas, but they may also indicate some previously unsuspected limitations.

Multiple active regions connected in series with low-resistance tunnel junctions enable a new class of high-brightness VCSEL arrays that enhance the capability for 3DS sensing applications. Multiple photons can be generated by each injected electron which proportionally increases the power and brightness of the VCSEL with additional benefit of reduced inductance penalty at the same output power. Two and three junction VCSEL arrays have been demonstrated for mobile Time-of-Flight applications with +30% module efficiency. Five junction VCSEL arrays reach 100W at 25A and 400W at 100A for automotive LIDAR applications. Preliminary reliability data appears promising.

Recently, there has been some interest in multi-junction vertical-cavity surface-emitting lasers (VCSELs) due to their scaling properties. In particular the power density and power conversion efficiency (PCE) can be significantly increased. A PCE in excess of 63% has been demonstrated for multi-junction VCSELs, as well as a peak output power of 1kW from chips as small as 1mm^2. Multi-junction VCSELs thus present many interesting opportunities and we will review our development efforts of this technology across the near-infrared wavelength range of 800-1100nm, and covering several applications including the industrial, automotive (LIDAR in particular), and consumer fields.

Due to their superior electro-optical characteristics, including single-frequency symmetrically circular emission, wavelength tunability, and GHz modulation bandwidth, single-mode VCSELs are well suited for many applications, e.g., spectroscopy, encoders, datacom transceivers, and heat assisted magnetic recording. Conventional small aperture oxide confined single-mode VCSELs show excellent electro-optical performance, but there are also limitations that make them less attractive for some applications: Rather low output power, high series resistance, broad beam divergence, and due to their high current densities in combination with high thermal resistance, significant stress at operating conditions that may harm reliability and cause unwanted parameter drifts over time. Implementing a shallow surface relief into the cap layer of a VCSEL allows to increase the aperture size of a singlemode VCSEL, while staying in the single-mode emission regime. As a result, the series and thermal resistance is significantly decreased, as well as the current density at operating conditions. More narrow beam divergence, better MTTF values, and finally better stability of output characteristics, in particular output power and emission wavelength, is achieved. Such stable emission over time is most beneficial for TDLAS systems, but is also advantageous for other applications.

We experimentally demonstrate and elucidate by numerical simulations that breaking circular symmetry of large apertures of vertical-cavity surface-emitting lasers (VCSELs) significantly enhances their emission properties by increasing the optical density of states. Specifically, deformed shapes of circular oxide apertures of VCSELs enhance stimulated emission and suppress undesired non-radiative recombination contributing to an increase in output optical output power of more than 60% and in quantum efficiency of more than 10%. Our example deformed VCSEL structures demonstrate that the optical density of states appears to be of high importance for conventional optoelectronic devices in accordance to the predictions of quantum electrodynamics theory.

We report on the latest developments of very broadband gain MECSELs operating in the 9XX to 10XX nm spectral range. Preliminary results show room temperature operation with barrier pumping of the gain structure.

There is significant interest in developing laser wavelengths between 700 and 800 nm that may then be frequency doubled to the UV for applications in spectroscopy and atomic physics. We present our most recent results on both a 739 nm AlGaAs/AlGaInP VECSEL, where we demonstrate 150 mW of CW power suitable for frequency doubling to the Yb+ cooling transition at 369.5 nm, and a 780nm AlGaAs/AlGaInP VECSEL which was utilised in a novel demonstration of second harmonic generation in a Zinc-indiffused MgO:PPLN waveguide. In the latter we have generated 1 mW of power at 390 nm.

DBR-free VECSELs overcome long-standing limitations in standard devices and offer improved heat management, reduced growth complexity, larger material choice, and broader gain tunability. We present the latest advances in DBR-free VECSELs emitting at 1178 nm targeting sodium guide star applications. We compare barrier-pumping and in-well pumping schemes employing 808 nm and 1070 nm pump lasers, respectively. A maximum output power of >20 W is attained with the barrier-pumping configuration, while a ~52 % slope efficiency with single pass is obtained with the in-well pumping scheme. Linewidth narrowing of < 0.25 nm and frequency conversion to 589 nm is also presented.

Mode-locked vertical external-cavity surface emitting lasers are promising compact sources for high-power, ultrafast pulses with excellent beam quality and the flexibility offered by an external cavity. Typical models of these lasers use macroscopic or quasistatic approaches based on rate or delay differential equations. Although these approaches have shown widespread success, they often require numerous experimentally tuned parameters and cannot capture the ultrafast nonequilibrium dynamics present as the field interacts with the quantum well. The Maxwell Semiconductor Bloch Equations has reduced parametrization and captures the carrier dynamics by coupling together a numerical wave propagator to a first principles of quantum mechanical description of the induced microscopic polarization within the active semiconductor quantum well. We expand on this model utilizing a reference frame transform to model modelocking within VECSEL cavities with non-normally incident semiconductor heterostructures. Specifically, we demonstrate the effect of increased pumping on the fundamental and harmonic modelocking behaviors of V-cavity VECSELs as well as transverse kinetic hole burning during colliding pulse operation as seen in modelocked ring cavities.

We have fabricated 3D printed micro-optics to feedback light into an 850-nm VCSEL with reduced top-mirror reflectivity and control its transverse modes. Our goal is to create a single-frequency VCSEL with output power on the order of 10 mW for use in atomic and quantum physics. Feedback of 50% can reduce threshold current 5-fold and preferentially select the fundamental transverse mode. We will compare theory and experiment for micro-optic length scales near 100 microns, yielding Gaussian mode diameters near 10 microns.

A photonic MOSFET includes an MOS field effect transistor, a VCSEL in the drain region, and a photon sensor or avalanche photo diode (APD) in the channel / well regions of the MOSFET. The MOSFET, VCSEL, and APD are fabricated as one integral transistor. When a voltage is applied to the drain, and a voltage is applied to the gate, both MOSFET and VCSEL are on (VCSEL is forward biased). Light from the VCSEL is absorbed by the APD (which is reversed biased) that triggered avalanche breakdown. A large breakdown current flows into the drain. When the MOSFET is switched off, VCSEL and APD are also turned off. Nonlinear optical films are fabricated in the substrate and isolation regions. In the paper we will discuss how to achieve better thermal stability with the embedded field effective devices (MOSFET and avalanche breakdown device), output power efficiency, integration of multiple wavelength arrays for optical signal processing, VCSEL-based nonlinear optical operations, and Ultra Large Scale Integration with VCSEL for optical computing.

Coherent optical coupling in VCSEL arrays introduces novel and desirable behaviors that can manifest themselves in many ways. We explore some of these behaviors using common characterization techniques but using large datasets, and show how we use computational data analysis methods to analyze datasets in an automated and scalable manner.

Vertical-emitting laser arrays operating in a coherently coupled regime offer the potential for high brightness and lowpower. Previous work focused on quantifying the beam propagation factor of on-wafer single-emitter photonic crystal vertical cavity surface emitting laser (PCSEL) devices for comparison with typical measured values of the spectral linewidth and side-mode suppression ratio. Expanding on this work, here we report on a novel method of characterizing the beam propagation factor for 2x1 multiple-defect coherently coupled PCSEL arrays. First, the on-wafer 2x1 PCSEL arrays were characterized to determine the range of injection currents that produced a coherently or incoherently coupled output using a 3-D power map. Both operating regions are explored here. After measuring the spectrum, the beam profiles were captured using a vertically mounted beam profiling system. Each individual laser in the 2x1 array was first operated and characterized independently. The device was then characterized operating in an incoherent and coherent coupled mode, respectively. The beam propagation factor, or M2, was calculated for each set of data using a weighted least-squares curve fit and in accordance with the ISO Standard 11146. As expected, the individual lasers making up the 2x1 array produced near-Gaussian beam profile with M² close to 1. With both laser elements operating, regardless of the state of coherence, the output beam adopted an asymmetry and the M2 value increased predominately on the lateral axis. In this effort, a parametric study of the beam propagation factor of devices emitting near 850nm is presented.

Optical speckle patterns relate to the topological characteristics of the scattering surface through commonly used parameters such as contrast and various correlation functions. We use a probe with a single-mode laser source, lenses, and associated electronics for fast acquisition of large sets of images of distinct material types. We use the principal component analysis (PCA) technique for generating dictionaries of images for speckle image datasets of known materials. We subsequently acquire speckle data for an unknown material and represent it via its orthogonal basis vectors and use least square errors for accurate classification of the unknown material.

We report on vertical cavity surface emitting lasers (VCSELs) having a -3dB modulation bandwidth above 30 GHz and a narrow spectrum down to single mode (SM) operation. The 850 nm and 910 nm SM VCSELs in combination with the IN5612 VCSEL driver from Inphi Corporation allowed to reach 106 Gb/s PAM4 with the TDECQ values of only 1.5 dB. For the multimode VCSELs, TDECQ of ~2.6 dB were achieved in combination with the same driver chip. VCSELs with the reduced spectral width allow to cover transmission distance over multimode fiber reaching 1.0-2.5 km at 50 Gbaud. Furthermore, reduction of the aperture size to a certain limit allows to reach ultimate modulation bandwidths at the same current density as applied in the large aperture VCSELs but at lower total currents and thus much lower current-induced overheating. The latter enables a significant improvement in the reliability of the devices and stimulates further research in novel types of VCSEL-based devices.

An electrically parallel, optically uncoupled, 19-element 980 nm top-surface-emitting vertical-cavity surface-emitting laser (VCSEL) array with inter-VCSEL metal and top mesa ridge connectors, processed using a two-dimensional honeycomb lattice basis with oxide aperture diameters of ϕ ~10.5 m per VCSEL exhibits a record combination of continuous wave optical output power (150 mW), -3 dB small-signal modulation bandwidth (17 GHz), and wall plug efficiency (30%) at a conservative operating bias current density of 10 kA/cm². To demonstrate the potential of the VCSEL array as an optical source for future fifth generation free-space optical communication systems we perform a data transmission test at 25 Gb/s across OM1 multiple-mode optical fiber using simple 2-level pulse amplitude modulation and pseudorandom binary patterns of word length 2⁷-1. We achieve a bit rate of 25 Gb/s with a bit error ratio below 1 x 10^-12.

As global IP traffic demands are exponentially growing, the increase in sub-system and system level requirements for higher speeds and longer transmission distances from multi-mode fiber (MMF) based vertical-cavity surface-emitting laser (VCSEL) optical links are fundamentally inhibited by bandwidth limits due to inherent fiber dispersions as well as transmitter noise impairments. Noise variances such as spectral linewidth (Δ𝜆rms), relative intensity noise (RIN), mode noise (MN), mode partition noise (MPN), chromatic dispersion (CD), frequency chirp and reflection feedback etc., all have serious ramifications on signal-to-noise ratio (SNR) in degrading receiver sensitivity. In order to drive the systems to higher bandwidth levels, it is important to identify the source of noise impairments, and try to minimize them with optimum component designs that facilitate solutions to achieve high SNR at the receivers. In this contest the author propose a multipurpose effective refractive index (Δneff) model of VCSEL cavity to push data transmission of non-return zero (NRZ) based 100G SR4 to beyond eSR4 limits that can potentially offer stable industrial temperature narrow Δ𝜆rms and low RIN at chip level, low MPN in optical fiber, and high SNR at receiver, respectively. This simple and effective concept with further bandwidth, power and thermal budget improvements likely to have high potential for 850nm VCSEL use in PAM-4 based top of the rack (TOR)-leaf and leaf-spine 400G (SR8, SR4.2)/800G (SR16) Datacenter optical interconnects (DCI). The Δneff occurred from multilayer distributed Bragg reflector (DBR) stacks and quantum well active regions due to oxide layer is critical in fixing Δ𝜆rms of VCSELs as the data transmission distance is inversely proportional to Δ𝜆rms. The author discusses the impact of Δneff in creating narrow Δ𝜆rms and its benefit on low MPN in basic light transmitter units of NRZ 25.78125Gb/s and 28.05Gb/s transmissions of VCSELs in 100G eSR4 optical links with performance margin up to 400m in OM4 MMF at 85C with inexpensive power penalties at the receiver without equalization, pre-emphasis and error correction techniques.

Vertical-cavity surface-emitting lasers (VCSELs) are widely used in optical data communication mainly in data centers for short-haul transmissions. However, their intensity modulation resonance frequency does not exceed 40 GHz which also limits the achievable modulation bandwidth and data rate. In contrast, spin-VCSELs can overcome these bandwidth limitations by modulating spin and polarization instead of current and intensity. In spin-VCSELs, the birefringence determines the resonance frequency of the polarization dynamics as well as the modulation bandwidth. We control the birefringence and thus the polarization dynamics via the elasto-optic effect by mechanically or thermally induced strain providing polarization oscillation frequencies up to more than 200 GHz. Detailed analysis shows that spin-VCSELs offer polarization dynamics with good signal strength even when operating close to threshold and at high temperatures. Here, we analyze devices with integrated surface gratings. VCSELs with different grating periods as well as mesa diameters and resulting different oxide apertures were investigated.