This PDF file contains the front matter associated with SPIE Proceedings Volume 12020, including the Title Page, Copyright information, Table of Contents and Conference Committee list.

Lumentum’s short pulse single-junction VCSEL arrays are currently being deployed in Time of Flight (ToF) sensors for short-range Light Detection and Ranging (LIDAR) applications in consumer electronics. However, single-junction VCSEL devices have a slope efficiency (SE) of only around 1.1 W/A making them unsuitable for longer range, higher power applications. With the rise of autonomous vehicle market, there is a need for ultra-short pulse, high peak power VCSELs for medium to long-range LIDAR systems. Multi-junction VCSELs are ideal candidates for this segment. The multi-junction VCSEL consists of stacked gain regions connected by highly doped tunnel junctions. An electron which radiatively recombines with a hole in one gain region, generating a photon, can then tunnel back into the conduction band via the highly doped region and is again available to generate another photon in the subsequent gain region, and so on. The total photons or output power scales with the number of junctions in the device. The optical power density and chip size are critical parameters of a LIDAR module. In this paper, we report the results of compact multi-junction arrays capable of delivering very high peak power.

Flip-chip VCSELs with backside emission revolutionize 3D sensing systems. Etching lenses directly into the GaAs substrate is the most compact way to integrate optics with the VCSEL. Various lens structures can be implemented enabling collimation as well as the uniform illumination of a defined field of view. Superior to separate optical elements the integrated optics avoids the need for individual alignment of laser die and optics and makes them an irremovable part of the chip, which is beneficial for long-term laser safety. The realization of both contacts on the epitaxy side enables flip-chip assembly without the need for wire bonds. This facilitates thermal management compared to top-emitting VCSELS and reduces system impedance. Individually addressable zones can be easily implemented, contacting is done via fine-pitch copper pillars. Backside emitting VCSELs can be designed for a large emission area thus enabling a very high active area of the array. This is especially useful for operating schemes with low duty-cycle enabling ultra-high pulse power output per chip area. VCSEL chips with integrated flood illumination optics for time-of-flight applications with narrow or broad field of view will be presented. Addressability of multiple zones on a chip can be implemented for enhanced illumination and sensing schemes. The ViBO technology platform allows for a wide variation of VCSEL chip properties and a significant miniaturization of illumination systems.

In this paper, a new method for short pulse generation below 100 ps is reported by controlling the scale of transverse mode distribution by free carrier plasma effect in an oxide-confined VCSEL. A thin tapered oxide layer is the significant structure for controlling transverse mode. That structure provides small built-in transverse optical confinement, which enables changing the transverse mode distribution even with a slight change in the refractive index caused by carrier plasma effect. When a current is injected into the VCSEL, the refractive index around the oxide aperture decreases due to increase in the carrier density, and the transverse mode distribution spreads from center of the current injection region. This reduces the stimulated emission rate and suppresses lasing, and carriers accumulate in QWs. When the current pulse falls, the transverse mode distribution is narrowed as the refractive index increasing, and large stimulated emission rate achieved by accumulated carriers. Thus, Q-switch like single pulse lasing without tailing can be obtained with a conventional VCSEL structure. In a 940 nm top-emitting VCSEL having a thin oxide layer with the oxide aperture area of 29 μm2 , the pulse width of 53 ps and the peak power of 50 mW were observed in synchronization with falling of several nanosecond current pulse at room temperature.

High power density VCSELs are attracting attentions in the field of LiDAR applications. Multi-junction VCSEL is the key technology to obtain high power density. High slope efficiency, high filling factor and small divergence angle are the straight-forward research directions to realize this high performance multi-junction VCSEL array. We optimize the epitaxial design and fabrication process, such as tunnel junction, oxidation layer and array layout. The progress on high performance multi-junction VCSEL array emitting around 940 nm is reported. Selectively oxidized, top-emitting VCSEL emitter array with 59.7% power conversion efficiency and slope efficiency of 8.3 W/A are developed as the basic laser source targeting at the LiDAR applications. The fabricated VCSEL array devices with emitting area of 234*250 um² exhibit power density higher than 1800 W/mm² and divergence angle lower than 21 degree (1/e²) with 15A peak current, 10kHz 10 nano-second pulsed (FWHM) driver.

ViP stands for a VCSEL with integrated photodiode. It features a photodiode embedded in the VCSEL resonator. A single intra-cavity contact serves as VCSEL cathode as well as photodiode anode. The VCSEL is single-mode and a sub wavelength grating on the output facet is used to stabilize the polarization of the emitted light. The ultra-compact chip has two separately addressable mesas. In addition, good production capability and reliability make the device ideal for mass products. Exploiting the principle of self-mixing interference (SMI), the ViP can be used in systems precisely measuring e.g., velocity, distance, quantitative particle concentration as a measure for air quality or fast eye-movements. The interferometric precision of velocity measurements enables demanding industrial applications like a new contactless encoder with high accuracy. ViP and the SMI principle make the detection almost insensitive for environmental background light. The functionality of the sensor has been demonstrated in bright sunlight and measuring speed over ground up to 250km/h in automotive applications. Low latency is ideal for the detection of fast eye movements. The miniature sensor fits into the frame of an AR/VR goggle and can detect eye-gestures. An ultra-compact air quality detector measuring PM2.5 as well as ultra-fine particles is described as well. The system does not require any enforced air flow nor openings in the body thus could be embedded in wearable consumer devices. With a size of a match head, it enables a precise, real-time, and personalized measurement.

In designing LiDAR systems for a mass market, it is desirable to scan a scene without moving parts and with optics and packaging that are robust and simple to manufacture. VCSEL arrays have already emerged in consumer LiDAR systems that reach several meters. Extending those distances requires packing higher peak optical power into narrower angular slices or blocks and addressing those electronically. Detector arrays are commonly read out line-by-line; accordingly, sources for these arrays must ideally emit power into narrow angular slices. An array arranged into narrow addressable stripes shown through a lens will do just that. We present 940nm arrays of 100 or more narrow (<0.16mm) stripes that each emit <50W peak power when driven for a few ns, <0.1% duty-cycle. The arrays have multiple anode contacts and a single backside cathode contact. Multiple light-emitting junctions enable high slope-efficiency (<4W/A) that reduces the current per channel required. Such stripe arrays may still illuminate more than one line on the detector or may require stripes so narrow as to cause high electrical resistance. For detector arrays that may be read-out in blocks with aspect ratio closer to one, we present 905nm and 940nm matrix-addressable VCSEL arrays that emit over 50W peak-power from regions < 0.06mm2 when driven with short (few ns) pulses at low (<0.1%) duty-cycle over 80oC. Such arrays have a linear array of cathode traces perpendicular to a linear array of anode traces. This arrangement permits MxN regions to be addressed with only M+N electrical contacts.

VCSEL arrays can play an important role in the increasing the data throughput of VCSEL-based optical interconnects both due to the need to increase the channel density and due to new emerging technologies like optical wireless. In this work we show the progress in the development of high-speed VCSEL arrays suitable for multicore fiber transmission leading to an increase of the total throughput through single fiber to 600 Gbps. We also discuss a novel type of compact VCSEL mini-arrays capable of high-speed modulation and coherent emission at the same time. Photon-photon resonance and coherent effects can help increase the resonant frequency and the bandwidth of the VCSELs and enable devices capable of 100 GHz operation.

This paper reviews the VCSEL technology used to enable 100 Gb/s multi-mode optical links. Link performance, device characterization over temperature and wear-out lifetime will be presented. The manufacturability of these high performance and reliable VCSELs will be discussed.

We study the supermode dynamics of optically coupled but electrically isolated dual-element photonic crystal vertical cavity surface emitting laser (VCSEL) arrays. The optical coupling is characterized by experimentally extracting a complex coupling coefficient, where the real component is related to the frequency splitting between the two array nonHermitian supermodes, whereas the imaginary coupling coefficient represents the gain difference between these two supermodes. The imaginary coefficient is derived from the coupling-induced excess output power. We compare three photonic crystal periods and find the largest value arising from the closest separation between two cavities. For the array with the highest imaginary component, the real part is determined which is related to the photon-photon modulation resonance, which has the potential to increase the VCSE digital modulation rate.

We report on the progress of a volume manufacturable oxide-confined 850-nm vertical-cavity surface-emitting laser (VCSEL) operating at 25 (NRZ) to 56 (28 GBd PAM-4) Gbit/s data rate from room temperature (RT, 25 °C) to 105 °C. Our devices offer clear eye openings over a wide-temperature range without applying equalization or forward error correction (FEC) at either transmitter or receiver sides. Even at extreme operating temperatures, the meticulously optimized devices offer ameliorated dynamics characteristics without the need for these additional power-hungry signalprocessing techniques. The equalization-free characteristic reduces the reliance on signal compensating circuits in the transceiver modules with the flexibility of operating over a wide temperature range for short-teach high-speed optical interconnects and automotive applications.

We report on development and characterization of 850 nm vertical-cavity surface-emitting lasers (VCSELs) having a -3dB modulation bandwidth above 24 GHz with a flat frequency response at temperatures up to 85°C. Aperture size is optimized for a high relaxation oscillation frequency with a narrow spectral width and low relative intensity noise. Two types of VCSELs (Gen 1 and Gen 2) with different epitaxial designs are fabricated with an optimized aperture size. Large-signal modulation at 53 GBd PAM-4 (106 Gb/s) is performed for eye diagram and TDECQ measurements. The Gen 1 VCSEL is capable of 53 GBd PAM-4 modulations at temperatures up to 70°C, but performance is insufficient at 85°C. The Gen 2 VCSEL with a stronger optical confinement achieves higher modulation bandwidth with an extremely suppressed resonance peak in frequency response, leading to reduction in TDECQ compared to the Gen 1 VCSEL. TDECQ below 4.5 dB are verified at temperatures up to 85°C without any pre-emphasis in the transmitter. Also, we use a pre-emphasis with 3-tap feed forward equalizer to improve the TDECQ by 2 dB. Furthermore, after the transmission over 100 m multimode fiber (OM5), the TDECQ keeps below 3.0 dB even at 85°C. These results demonstrate the capability of 850 nm VCSELs for 100 Gb/s per optical lane short-reach interconnects operating over a wide temperature range

We report a novel concept of tuning the VCSEL wavelength by varying the optical thickness of the laser cavity. This is achieved by implementing an intra-cavity grating pattern with a variable fill factor. The optical thickness of the cavity and the corresponding resonance wavelength will depend on the fill factor of the grating, its height and the refractive index contrast between the overgrowth and the grating layers. We developed a fabrication process based on two steps growth epitaxy and a lithographic intra-cavity grating to define the VCSEL wavelength at the emitter level. A 20 nm wide tuning range around 940nm has been demonstrated using this technique.

Precise wavelength control of light sources is necessary for applications such as 3D sensing, OCT, and spectroscopy. Tunable MEMS-VCSELs are attractive candidates because of high-speed modulation and low manufacturing cost. However, there are problems such as non-linearity of swept-wavelength and decrease in activation range over MHz-speed for conventional electrothermally and electrostatic-driven MEMS-VCSELs, which leads to performance degradation. In this paper, high linearity of wavelength sweep has been demonstrated by using MEMS actuators which consists of piezoelectric PZT layers and meander structures. The 940 nm MEMS-VCSEL consists of MEMS and bottom-emitting half-VCSEL. As for MEMS, two kinds of actuator are employed, a large meander structure and a small membrane structure. The meander structures are utilized to move dielectric DBR mirrors deposited on MEMS linearly with an applied voltage, and perpendicularly to the chip surface. Half-VCSEL has n-DBR, active layers inserted in spacer layers and several pairs of p-DBR that is located between an air gap and spacer layers on n-GaAs substrate. MEMS and Half-VCSEL are integrated by thermal bonding via bonding layers. The Coefficient of determination between an applied voltage and swept wavelength is over 0.99 with the large MEMS actuator. Also, modulation frequency over 1.3 MHz with 2.98nm wavelength sweep is obtained with the small actuator. These actuators overcome the trade-off between high-speed wavelength modulation and precise adjustment of wavelength compared to conventional MEMS-VCSEL. There are remaining challenges for further improvement. Modifying the stiffness of actuators and reduction in mass enables faster resonance frequency more than a ten MHz order.

We present a high-speed swept-source optical coherence tomography (SS-OCT) imaging system using an electrically pumped, micro-electromechanical-system (MEMS) tunable HCG-VCSEL operating at the 1060 nm wavelength regime. Comparing to existing MEMS VCSEL light sources for SS-OCT, a movable high-contrast grating (HCG) is used as the top mirror of the laser cavity, replacing the conventional distributed Bragg reflector mirror design. By applying a reverse bias voltage, the HCG mirror actuates downward toward the VCSEL cavity, changing the effective cavity length and resulting in wavelength tuning responses. The developed SS-OCT system allows an A-scan rate of 250 kHz, a detection sensitivity of 98 dB, and an axial imaging resolution of 22 µm (full-width at half-maximum (FWHM), in air). The A-scan rate can be further improved to 500 kHz if both the backward (long to short wavelength) and forward laser sweep are used. In the experimental setup, a dual-channel acquisition scheme was utilized to provide calibration of the OCT signal with a separate calibration interferometer. Volumetric imaging of the human fingernail/nail fold junction in vivo shows the feasibility of providing high-speed imaging of the tissue architectures. The MEMS tunable HCG-VCSEL light source can provide high-speed OCT imaging with a more compact light source footprint and potentially a lower cost

Spectroscopy applications, e.g., TDLAS for gas sensing, require very stable single-mode laser sources to avoid or reduce in-situ or undesirably frequent wavelength calibration. Such stability of laser characteristics can be achieved, in general, by the quality of epitaxial growth and quality of processing technology but must be complemented by a robust epitaxial and device design. In addition to outstanding electro-optical performance, we present exceptionally stable laser characteristics, like threshold current, slope efficiency, output power, and lasing wavelength over observation periods exceeding 4000 hours at accelerating operation conditions correspondent to 8 years of continuous operation.

We report on a novel sensor concept based on a coupled resonator configuration and the employment of vertical cavity surface-emitting laser (VCSEL) sources. The back reflection of a sample surface next to the emission window of the laser source affects the internal resonator conditions of the VCSEL resulting in a change of the emitted wavelength and operating current, respectively, if the operating voltage is kept constant. The behavior of the VCSEL in this scenario was investigated for both the near and the far field which offers the potential for different types of measurement applications. First experimental results show a measurable and reproducible change of the operating current when moving the sample by as little as a few nm in vertical direction. This behavior was also verified with a simulation based on ANSYS Lumerical by creating distributed Bragg reflection (DBR) stacks with different layers and quantifying the influence of the movable third resonator surface on the emission wavelength. In the next steps, the new sensor system will be integrated into an inline production chain for additive optics manufacturing to supervise the manufacturing accuracy and realize a feedback loop for the correction of process imperfections.

Vertical-cavity surface-emitting lasers (VCSELs) are of utmost importance as key components for high-speed datacom, sensor and free-space applications. Therefore, for a successful further optimization of their performance, understanding their aging behavior is of crucial importance. Photocurrent spectroscopy (PCS) is a powerful, nondestructive technique which can be used to analyze semiconductor materials. Applying it on VCSELs makes it a powerful tool to investigate these tiny devices. In this work, we present room temperature high-resolution PCS analyses of fresh vs. aged 850 nm VCSELs. These VCSELs are characterized before and after aging by means of PCS, which measures essentially the convolution of the top mirror and intrinsic region absorption spectra. Heavy hole and light hole quantum well transitions are revealed and the related quantum-confined Stark effect is studied. The VCSELs used in this study are mounted on a standard V-connector and were intentionally aged at extreme conditions to accelerate their degradation till reaching optical damage. It was found that in these VCSELs, a reduced PCS current is observed, which is possibly caused by nonradiative recombination centers generated by the aging-related processes. Moreover, we observe that aging of the devices at very high current densities results in the evolution of defect related states, which modify the IV-curve under reverse bias. Degraded devices also show a systematic shift in breakdown voltage towards lower values, indicating a possible shrinkage of the undoped region by impurity electromigration and diffusion. Interestingly, these changes are minimal in stable devices that were aged under normal conditions.

A wafer-scale CMOS-compatible process for heterogeneous integration of III-V epitaxial material onto silicon for photonic device fabrication is presented. Transfer of AlGaAs-GaAs Vertical-Cavity Surface-Emitting Laser (VCSEL) epitaxial material onto silicon using a carrier wafer process and metallic bonding is used to form III-V islands which are subsequently processed into VCSELs. The transfer process begins with the bonding of III-V wafer pieces epitaxy-down on a carrier wafer using a temporary bonding material. Following substrate removal, precisely-located islands of material are formed using photolithography and dry etching. These islands are bonded onto a silicon host wafer using a thin-film non-gold metal bonding process and the transfer wafer is removed. Following the bonding of the epitaxial islands onto the silicon wafer, standard processing methods are used to form VCSELs with non-gold contacts. The removal of the GaAs substrate prior to bonding provides an improved thermal pathway which leads to a reduction in wavelength shift with output power under continuous-wave (CW) excitation. Unlike prior work in which fullyfabricated VCSELs are flip-chip bonded to silicon, all photonic device processing takes place after the epitaxial transfer process. The electrical and optical performance of heterogeneously integrated 850nm GaAs VCSELs on silicon is compared to their as-grown counterparts. The demonstrated method creates the potential for the integration of III-V photonic devices with silicon CMOS, including CMOS imaging arrays. Such devices could have use in applications ranging from 3D imaging to LiDAR.

The paper presents the results of the research and development of 1300-nm vertical-cavity surface-emitting lasers, fabricated by wafer fusion technique for hybrid integration of an InAlGaAs/InP optical cavity with two AlGaAs/GaAs distributed Bragg reflectors using molecular-beam epitaxy. The active region is based on InGaAs/InAlGaAs superlattice, while current and optical confinement is provided by n⁺⁺-InGaAs/p⁺⁺-InGaAs/p++-InAlGaAs buried tunnel junction (BTJ). The proposed device design results in low internal loss (about 3.2 cm^-1 at 20 °C). The devices with BTJ diameter of 5 μm demonstrate a stable single-mode lasing with threshold current less than 1.3 mA and output optical power more than 6 mW and operation in a wide temperature range. The measured -3 dB bandwidth is more than 8 GHz at 20 °C and about 5.5 GHz at 85 °C, the eye diagrams are open with a bit rate up to 20 Gbps using nonreturn-to-zero (NRZ) modulation standard. Using 5-tap feedforward equalizer, the NRZ transmission at 25 Gbps has been demonstrated up to 5km single-mode fiber.

Although VCSELs are intrinsically single-longitudinal mode devices, they usually show complex polarization characteristics. Most VCSEL are not designed to emit in a single polarization state. There is typically no control about the polarization angle. The light emitted by the VCSEL is typically linearly polarized along one of two orthogonal directions and abrupt polarization switching can be observed when temperature or bias current is changed. In this work we show a method of measuring spatially resolved polarization characteristics of VCSELs. This is achieved using a combination of polarization filtered microscope optics and a CMOS camera.

The paper contains charts of the micro-channel structures´ simulation data from a benchmark done using air as a cooling medium. It is showing Rth and other performance data in comparison to needed air volume in m³/h. Therefore, several Rogers’ standard micro-channel-structures, such as Qua 50, 100, Hex 50 and off the shelf standard lamellar heat sinks were benchmarked in thermal simulations using Solid Works. Comparing the results at a given flow rate of 1 m³/h all Rogers’ cooling structures show a lower chip temperature then the reference coolers. This is due to a higher heat transfer coefficient of these cooling structures resulting in up to 43 % lower thermal resistances versus the benchmark. Summing up the results show a clear picture, for certain use cases Rogers’ micro-channel coolers can close a gap for customers using air cooling but reach the limits of current solutions and future needs

We investigate the performance VCSEL based 400G transmission over OM3 multimode fibers (MMF) that are TIA- 492AAAC compliant using 4-level Pulse Amplitude Modulation (PAM-4) standardized in IEEE 802.3cm. We analyze the relationship between fiber differential mode delay (DMD) tilt, fiber effective modal bandwidth (EMB), and bit error ratio (BER). We demonstrate that fiber tilt is not a reliable indication that fiber performance will be optimized based on directional barring. It is affirmed that EMB and BER are more favorable considerations for fiber communication performance.