This PDF file contains the front matter associated with SPIE Proceedings Volume 10938, including the Title Page, Copyright information, Table of Contents, Introduction, Author and Conference Committee lists

Higher speed VCSELs at different wavelengths and adapted to different modulation formats are needed to meet demands for higher optical interconnect capacity and bandwidth density. We discuss VCSEL dynamics and speed limitations, recent progress on high-speed VCSELs from 850 to 1060 nm, and less conventional VCSEL designs for potentially more significant speed improvements. We also discuss the interconnect capacity enabled by equalization, pulse shaping, higher order modulation formats, digital signal processing, and wavelength division multiplexing.

We review our recent experimental results that demonstrate a path to 1Tbps VCSEL links using wavelength division multiplexing with emphasis on the limitations of bandwidth and RIN. The impact of modal dynamics on 50Gbaud PAM- 2 and PAM-4 modulations will be explored including the influence of mode correlations and aperture size. Our recent demonstrations of 100Gbps error-free serial links at multiple wavelengths from 850nm to 1060nm will be presented.

Via single 980 nm VCSELs with oxide aperture diameters (ϕ) of about 3 to 16 μm and via electrically-parallel triple and septuple two-dimensional arrays of optically-uncoupled 980 nm VCSELs with ϕ of about 3.5 μm we achieve small signal modulation bandwidths of 20-30 GHz and operating optical output powers of about 5 to 50 mW.

This abstract is submitted by the Chair who will review the abstracts.

Fibre-Channel and Gigabit Ethernet (GbE) standardization opened the first significant VCSEL commercial market, accelerating development, and sealing 850nm as the standard VCSEL wavelength for a couple decades. In succession, 10-Gigabit Ethernet (10GbE) accelerated oxide VCSEL development. Standards continue to shape VCSEL technological developments, e.g. to today’s 4-level 50 Gigabit/sec devices and 2-4-wavelength WDM.

Monolithically grown, electrically-injected VCSELs of a generic design - a short cavity, sandwiched between two distributed Bragg reflectors (DBRs) - can only be realized easily in a gallium arsenide (GaAs) material system which restricts the emission wavelength to ~600 - 1100 nm range. The smartphones market and emerging applications such as LIDAR (light detection and ranging), free space communication and face recognition create a demand for VCSELs emitting outside of this range. We demonstrate electrically-injected VCSELs incorporating a monolithic high contrast grating (MHCG) - a special case of a subwavelength high contrast grating mirror (HCG). MHCG can be made of most of the common materials used in optoelectronics and provides reflectivity close to 100% at a wavelength of interest in range from ultraviolet to infrared. In contrast to the HCG, the MHCG doesn't have to be surrounded by a low refractive index material and hence, can be monolithically integrated with the rest of the VCSEL cavity. In our design the greater part of the top DBR is substituted by an MHCG mirror which reduces the amount of required material and growth time by about 20%. We show continuous wave emission around 980 nm up to 75 °C ambient temperature. Our devices are quasi-single- and double-mode from threshold to rollover for 13.5 μm and 16.5 μm oxide aperture diameters respectively. Our MHCG VCSEL concept can be produced using material systems where lattice-matched and high reflectivity DBRs are not available to create devices emitting at wavelengths from ultraviolet to infrared.

We demonstrate the characteristics of 2 mm to 1 cm long vertical-cavity surface-emitting laser (VCSEL) amplifiers that emitted continuous wave (CW).

VCSELs have various applications in different fields, including datacom, optical interconnects, printers, and sensors. However, VCSEL technology has not been applied in laser material processing that needs a light source with high power density. This is because single-mode (SM) VCSELs have fundamental difficulty in producing beams having an output power of more than several milliwatts. On the contrary, the application range of VCSEL amplifiers is expected to be extended into fields that simultaneously require lasers with high power and high beam quality because they have the potential to attain both characteristics.

In this paper, we demonstrate the CW operation of high-power SM 2 mm to 1 cm long VCSEL amplifiers that had an output power of several watts at 20 °C. By optimizing the top-mirror reflectivity and seed-laser wavelength, the output power was linearly proportional to the amplifier length when the growth of amplified spontaneous emission (ASE) was suppressed. The I–L characteristics for different amplifier lengths were measured. All devices emitted narrow beams with divergence angles of approximately 0.1°. The maximum SM output power of the devices with lengths of 2 mm, 6 mm, and 1 cm were 0.62, 2.13, and 3.25 W, respectively. This result shows the possibility to achieve high power and high beam quality for direct semiconductor laser processing.

Optically pumped VCSELs with a 1λ thick optical cavity lasing at 375 nm have been demonstrated using a pulsed 248 nm KrF excimer laser source. To realize a high-reflectivity mirror on the bottom of the cavity, five-period airgap/ Al0.05Ga0.95N DBRs with a large refractive index contrast have been employed while the top mirror was formed by dielectric DBRs consisting of twelve pairs HfO2/SiO2. The lowest threshold incident power density measured at room temperature was estimated to be ~270 kW/cm2. The achieved optically pumped VCSEL demonstrates the possibility that the airgap/AlxGa1-xN DBRs can be used as a mirror for injection laser devices.

We demonstrated all-monolithically-integrated self-scanning vertical-cavity surface-emitting laser (VCSEL) arrays whose emitters emit light individually. Each array needs only three to five bonding pads even when the number of VCSELs is over hundreds. Recently, an array with hundreds of VCSELs that lase individually is needed for some applications that do not require mechanical devices, such as MEMS and polygon mirrors. However, conventional VCSEL arrays need the same number of bonding pads, gold wires, substrate wirings, and IC driver terminals as emitters, thus taking up space and increasing application costs. We applied the self-scanning light emitting device (SLED) technology to the VCSEL array to reduce the number of bonding pads and other components mentioned above. In this paper, we discuss the temperature characteristics of the all-monolithically-integrated self-scanning VCSEL array. All layers in the epilayer structure of the array were formed at a time by MOCVD. The array consists of AlGaAs based thyristors and conventional 850 nm oxide-confined VCSELs on a p-type GaAs substrate. The array includes a transmission region, an emission region with 16 emitters, five bonding pads on the top side, and one anode electrode on the bottom of the substrate. Switching of the thyristor and lasing of the VCSEL were achieved up to 90°C. Self-scanning of the array was also possible at a temperature as high as 90°C. This is a key technology to dramatically reduce the size and cost of the chip itself and its applicable applications that require many emitters.

The design and optimization of two-dimensional VCSEL arrays operating with duty-cycles from 2 to 5% (quasicontinuous- wave or QCW) operating at 940 nm are presented. Designs for nominal 8 W and 100 W peak power using TriLumina’s flip-chip-bondable, back-side-emitting VCSELs are reviewed. Performance as a function of duty cycle, including peak power output, spectral width and beam divergence are presented. Performance from -40°C to 125°C, corresponding to automotive grade 1 requirements, is reviewed. Optimization of the VCSEL arrays as a function of the number of emitters per chip is analyzed for trends in wall-plug efficiency, slope efficiency and operating conditions.

VCSEL arrays are the ideal light source for 3D imaging applications. The narrow emission spectrum and the ability for short pulses make them superior to LEDs. Combined with fast photodiodes or special camera chips spatial information can be obtained which is needed in diverse applications like camera autofocus, indoor navigation, 3D-object recognition, augmented reality or autonomously driving vehicles. Pulse operation at the ns scale and at low duty cycle can work with significantly higher current than traditionally used for VCSELs in continuous wave operation. With reduced thermal limitations at low average heat dissipation very high currents become feasible and tens of Watts output power could be realized with small VCSEL chips. Good reliability is demonstrated up to current densities above 100kA/cm². At higher current density and temperature gradual degradation can be observed. With this limitation in mind VCSEL arrays can be operated at 15 times higher current than in most continuous operation cases. The good VCSEL properties like robustness, stability over temperature and the potential for integrated solutions open a huge potential for VCSELs in new mass applications in the consumer and automotive markets.

The demand for high-power vertical-cavity surface-emitting laser (VCSEL) arrays is increasing continuously due to the growing market for 3D sensing solutions. In these applications (e.g. face recognition or drive-assistance systems), the distances of the objects of interest vary by orders of magnitude. For this reason, a flexible tailoring of the beam divergence is desired. In this work, we discuss methods to tune the emitted beam profile by only optimizing the epitaxial structure of a VCSEL. We show results of VCSEL arrays with beam divergences ranging between ~10° and 45°. This technique is also power scalable and multi-watt VCSEL arrays can be realized.

Vixar presents a novel monolithic tunable vertical cavity surface emitting laser (VCSEL) design with a demonstrated tunable wavelength range of 41 nm in the near-infrared. This design combines a microelectromechanical system (MEMS) top mirror over a gallium arsenide based VCSEL cavity, and a monolithic construction which side steps any complex external cavity structures needed for tuning. We will present results which show the electro-thermally tunable mirror physically and reproducibly moving up to 400 nm, which corresponds to potential tuning range of over 50 nm in output wavelengths. These results also illustrate the use of this single mode, continuous wave, tunable VCSEL as a light source for biological tomographic imaging. Data taken in collaboration with Notre Dame presents the use of such tunable light sources for diffuse optical spectroscopic imaging (DOSI) of breast tumors and other anomalous tissue inclusions. Previous work confirms the viability of this design as an optical source for DOSI instruments and the work presented here shows improved results with even greater tuning capabilities and higher optical power. In conjunction with past work centered at 775 nm, current work at 940 nm, and future proposed VCSEL examples at 905 nm, Vixar expects to show that this design is capable of supporting tomographic imaging spanning 765 -782 nm, and 885 – 955 nm in total. Above and beyond these immediate imaging goals, these results continue to enable future visions of many possible applications for which a monolithic tunable light source might be ideal.

Shallow semiconductor grating has been developed as an important add-on feature of Finisar’s VCSEL product family for low cost and yet highly effective polarization control. Our device simulation based on RCWA is capable of surface grating design for a variety of power levels and wavelength range. Data are presented to demonstrate stable polarization locking with side mode suppression ratio (SMSR) up to 30dB.

Optical VCSEL-based links operating in on-off keying (OOK) modulation represent a robust energy-efficient solution for short-reach optical interconnects in datacenters. We report on the optical and electronic elements of such link and their integration into the transmitter (TR) and receiver (RX) assemblies. A single channel transceiver link capable of 40-56 Gbit/s OOK transmission over multimode fiber at record energy-efficiency of ~4.5 pJ/bit is demonstrated. VCSEL driver and receiver transimpedance amplifier (TIA) circuits capable of generating 80-100 Gbit/s error-free signals are characterized on a special test-board assembly. Real-time 56 Gbit/s transmission experiments of the complete link are done, resulting in bit-error ratios (BER) below standard Forward Error Correction (FEC) levels without equalization or signal processing.

The growing demand for cost effective LIDAR sensors in automobile and military applications in LIDAR and free space optics could be met by compact and solid state, high power 2D VCSEL arrays. In this type of an array hundreds or more high power VCSELs can be monolithically integrated. This greatly reduces the cost over separately packaged lasers. In addition, beam quality and spectral quality make high power VCSELs important laser sources for free space data links. In this talk we introduce a different type of high power VCSEL that uses an oxide-free, lithographic aperture. The aperture enables a very low effective index change to help prevent trapping of internal modes that can occur with oxide VCSELs. The zero index aperture was designed to remove the internal ring modes that oxide VCSELs have, resulting in a lasing pattern with low optical loss. The novel zero index aperture is placed near the cavity spacer to improve its effect on confinement and electrical injection, and yet minimizing internal guiding of the optical mode. The VCSELs using the new aperture have been tested for their pulsed power, pulse response, device efficiency and beam quality. Dense arrays show high beam quality and record narrow spectral emission. Measured results of multi and single transverse mode VCSELs arrays with dense packing are also reported.

Nanocavity laser diodes and especially nano-VCSELs will fill a need for future laser diodes used in data communications, sensing, and materials processing that can be more efficient and higher speed. In this talk we will contrast the key physical requirements of the future laser diodes that have nano-scale cavity volumes than now possible with today's commercial laser diodes, along with the needed laser diode physics for the new technology. Key parameters include scaled values from today's commercial laser diodes but with higher speed, higher efficiency, operating temperature and reliability, and a host of other improved parameters. The analysis shows that much of the benefits will come from being able to drive the nano-cavity laser diodes to higher current density. Parameters such as laser diode reliability can be improved. The ability to drive the nano-cavity laser diode to much higher current density and stimulated emission rate than now possible could lead to new physical regimes of laser diode operation. The high drive levels will create the operating conditions needed to observe Rabi oscillation, but in an operating laser cavity. Increased reliability can also be possible due to improved heat flow and reduced mechanical stress in the nano-cavity lasers.

We consider comprehensive description of vertical cavity surface emitting laser (VCSEL) structures in practical cylindrically symmetric geometry. A complete drift-diffusion model for carrier transport in a multilayer semiconductor laser heterostructure including p-n junction is developed. We evaluate the impact of interface grading in distributed Bragg reflectors (DBRs), modulation doping of the DBRs and surrounding layers of the quantum well (QW) as well as drastically material–dependent carrier mobility and recombination constants. Solution of the drift–diffusion model yields spatial profiles of the nonequilibrium carrier concentrations and current. The model is applied as an example to an oxideconfined GaAs/AlGaAs VCSEL. We address both depletion and diffusion capacitance of the device and show that both contributions to the capacitance as well as the differential series resistance critically depend on the injection current and the VCSEL chip design such that, in general, VCSEL cannot be properly modeled by an equivalent circuit approximation. We consider current injection into the aperture region and illustrate the current crowding effects, which result in substantial enhancement of the current density at the tips of the oxide aperture. The effective RC-product can be kept low at a small size of the oxide-confined aperture but only in case where the diameter of the VCSEL mesa is also small.

Quantum-cascade vertical-cavity surface-emitting lasers (QC VCSELs) [1] combine features of VCSELs in respect of low threshold current, high quality of output beam, possible high speed modulation and fabrication of two dimensional phase-coupled arrays and quantum cascade lasers (QCLs) due to their emission in a broad range of infrared radiation up to about 100 m. In those structures vertical resonance and stimulated emission of photons is possible due to embedding QCs in the stripes of a monolithic high-refractive-index contrast grating (MHCG). Unipolar QCs provide flexibility in the number of the active regions used in the structure, leading to designs with distributed active regions enabling efficient stimulated emission. The expected high performance of QC VCSELs relies on sophisticated designing of MHCG and active regions which takes into account distributions of the QC VCSEL modes. Spatial distributions of modes are highly unintuitive and anticipation of them requires the use of numerical methods solving fully vectorial Maxwell eigenvalue problem. In this article, we present the principles of QC VCSELs designing illustrated by examples of optimization of a structure emitting at the wavelength of 9 µm. Particularly, we demonstrate optimization of the MHCGs, the resonant cavities and the numbers of active regions in QC VCSELs. In this contribution, optimal designs with respect to minimal threshold current and maximal output power are presented. [1] T. Czyszanowski: Quantum Cascade Vertical Cavity Surface Emitting Laser, IEEE Photon. Technol. Lett. vol.29, pp. 351-354, 2018