Oxide-confined Vertical-cavity Surface-emitting Lasers Research Articles

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.

25 Gb/s NRZ transmission at 85°C using a high-speed 940 nm AlGaAs oxide-confined VCSEL grown on a Ge substrate.

In this Letter, we present a comprehensive analysis of the high-speed performance of 940 nm oxide-confined AlGaAs vertical-cavity surface-emitting lasers (VCSELs) grown on Ge substrates. Our demonstration reveals a pronounced superiority of Ge-based VCSELs in terms of thermal stability. The presented Ge-VCSEL has a maximum modulation bandwidth of 16.1 GHz and successfully realizes a 25 Gb/s NRZ transmission at 85 ∘C. The experimental results underscore the significance and potential of Ge-VCSELs for applications requiring robust performance in high-temperature environments, laying the cornerstone for the future development of VCSEL devices.

Polarization-stable single-mode multi-junction VCSELs with integrated anti-phase mode filter

In this paper, polarization-stable and spectrally single-mode operation of oxide-confined multi-junction vertical-cavity surface-emitting lasers (VCSELs) are achieved by using a non-circular integrated anti-phase transverse mode filter. A record-high single-mode output power is obtained by radially reducing effective mirror reflectivity at the surface areas where the higher order modes are expected to lase, leading to an increase in the threshold gain for these transverse modes, all the while minimal losses are introduced to the fundamental mode. The polarization control of such devices is achieved by using a non-circular mode filter in order to perturb the cylindrical symmetry of the cavity. An elliptically etched surface relief is shown to be effective in realizing a single-mode single-polarization device over a wide range of operating bias currents and temperatures. The continuous-wave light–current–voltage characteristics of 8-junction 940 nm VCSELs show single-mode output power >13 mW at room temperature, spectrally measured with a side-mode suppression ratio of >30 dB. This single-mode performance can be achieved depending on the ratio between the relief opening and the oxide aperture size. Additionally, the single-polarization behavior of such devices (with an extinction ratio of >15 dB) is shown to be dependent on the orientation of the elliptical mode filter, where the light is mainly polarized along the minor axis of the elliptical feature. Overall, combined with a multi-junction active region, the elliptical model filter is able to achieve highly efficient single-mode linearly polarized VCSELs.

Electrostatic discharge induced degradation of optical-electrical properties and defect evolution of GaAs-based oxide-confined VCSELs.

GaAs-based oxide-confined vertical-cavity surface-emitting lasers (VCSELs) exhibit relatively low resistance against reliability-related damage. In order to gain a deeper understanding of the degradation and failure mechanism in oxide-confined VCSELs caused by electrostatic discharge (ESD)-induced defect proliferation, we investigated the effects of ESD stress on the degradation of optical-electrical characteristics and the evolution of defects in VCSELs under human body model test condition. The degradation threshold values for forward and reverse ESD pulse amplitudes were estimated to be 200 V and -50 V, respectively. Notably, VCSELs demonstrated greater sensitivity to reverse bias ESD compared to forward bias ESD. Analysis of optical emission and microstructure provided evidence that the device failure is attributed to an increase in ESD current density, leading to the multiplication of dark line defects (DLDs) originating from the edge of the device's oxide aperture. The formation of defects occurred suddenly in discrete events within small regions, rather than progressing gradually and uniformly. These defects propagated and led to damage across the entire active region. We believe that our results would be meaningful for improving the reliability of VCSEL in the future.

Oxide-confined GaAs-based vertical-cavity surface-emitting laser: Measurement and modeling of the strain field

The strain field of the emitters of one unstressed and two electrically stressed oxide-confined GaAs-based vertical-cavity surface-emitting lasers has been analyzed. The components of the strain tensor in the optical aperture, in the quantum wells, and at the oxide edge of the lamellas have been measured by nano-beam precession electron diffraction and geometrical phase analysis. The measurements have been used to validate the simulations based on the finite element method (FEM) of the mechanical behavior of the emitters. The FEM model is used to quantify the strain caused by the lattice mismatch of the GaAs-based epitaxial layers and the volume variation of the Al 98Ga 2As layer due to selective oxidation. The first is calculated as a function of the misfit strain and corresponds very well to the measured values. The latter is calibrated assuming that the thickness of the oxide is expanded by 4 % in both the unstressed sample and the stressed sample with dislocations. Instead, it is contracted by 8 % for the sample where the stress test has caused further oxidation.

Impact of thermal oxidation uniformity on 150 mm GaAs- and Ge-substrate VCSELs

Vertical cavity surface emitting laser (VCSEL) devices and arrays are increasingly important in meeting the demands of today’s wireless communication and sensing systems. Understanding the origin of non-uniform wet thermal oxidation across large-area VCSEL wafers is a crucial issue to ensure highly reliable, volume-manufactured oxide-confined VCSEL devices. As VCSEL wafer diameters approach 200 mm, germanium (Ge) is emerging as an alternative substrate solution. To this end, we investigate the uniformity of 940 nm-emitting VCSEL performance across 150 mm diameter GaAs- and Ge-substrates, comparing the oxidation method in each case. Nominally identical epitaxial structures are used to evaluate the strain induced wafer bow for each substrate type with Ge exhibiting a reduction of over 100 μm in the peak-to-valley distortion when compared with GaAs. This wafer bow is found to be the principal cause of centre-to-edge oxidation non-uniformity when utilising a conduction-heated chuck furnace, in comparison to a convection-heated tube furnace. Using on-wafer testing of threshold current, differential resistance, and emission wavelength, device performance is demonstrated for the first time across a 150 mm Ge wafer, and is shown to be comparable to performance on GaAs substrates, when the effects of oxidation uniformity are removed. These results provide evidence that there is a realistic path to manufacturing high yield VCSELs, over wafer diameters approaching those used in Si-photonics, via Ge substrates.

(Invited) Achieving High-Power Single-Mode Operation in Vertical-Cavity Surface-Emitting Lasers Via Scalable, Higher-Order Mode Suppression Techniques

Achieving high-power single-mode operation in Vertical-Cavity Surface-Emitting Lasers (VCSELs) has received renewed interest because of applications that span facial recognition and 3D imaging in mobile telephones, Light Detection and Ranging (LiDAR) for autonomous vehicles, and Pulse Amplitude Modulation (PAM-4) for high-speed (>100 Gbps) data transmission within data centers. A key challenge with oxide-confined VCSELs is that the strong refractive index contrast between the oxidized ~AlAs aperture and surrounding AlGaAs layers combined with the oxide aperture size create a photonic cavity that fundamentally supports multiple transverse modes. When the size of the aperture is reduced to create a single-mode cavity, the output power is limited by the electrically pumped gain region within the aperture. There is thus a fundamental trade-off between size and power which manifests itself in a limited capability to simultaneously achieve high power and single mode operation. Various methods have been employed by groups working on VCSELs to overcome this limitation. Neglecting external cavity methods which are fundamentally not scalable, the most promising results have been achieved by groups using etched surface relief where a pattern is etched in the top layer or layers to change the standing wave pattern in the cavity and reduce modal gain by lowering the magnitude of the electric field in regions overlapping with the quantum wells, which provide laser gain. A downside of this approach is that material is removed, impacting the uniformity of current injection across the area of the device. A key question of interest is therefore how to achieve mode suppression in large area devices in a way that does not impact current injection. In this presentation, novel methods for mode control that rely on spatial variation of modal gain across the area of the VCSEL aperture will be discussed, and results on high-power single-mode operations of VCSELs will be presented. Two distinct mode control methods will be reviewed. The first method of mode control uses deposited optical coatings on the top surface of the VCSEL to spatially modify the electric field standing wave in the quantum well heterostructure region. Through patterning the coating in either a pillar or anulus, specific modes can be suppressed by selectively changing the gain available for those modes. These filters have been made using both SiO2/TiO2 stacks and alternatively a single layer of silicon. The capability to pattern these layers in a wafer-scale process makes this method attractive for volume manufacturing. The second method of mode control utilizes impurity-induced layer disordering via patterned diffusion of Zn to selectively intermix Distributed Bragg Reflector (DBR) layers, thereby creating a top laser mirror whose reflectivity varies spatially across the aperture. Designing the diffused region to maximize overlap with high-order modes while minimizing overlap with the fundamental mode provides mode suppression and enhanced single-mode power. An added benefit of this approach is a reduction in resistance through the device top DBR layers brought about by the higher p-doping level in the Zn-diffused regions, minimizing Ohmic losses and heat generation. Through the use of strain control in the diffusion mask, diffusion front shape is also shown to be controllable in a way that can be used to minimize the impact of the diffused region on fundamental mode reflectivity. Tensile and compressive strain of diffusion masks are shown to have differing effects on vertical (into the device) versus lateral (across the area of the device under the mask) diffusion, allowing more precise spatial control of reflectivity. Using these methods, single-mode output powers in excess of 10 mW have been demonstrated, with side-mode suppression ratios in excess of 30 dB. These results will be discussed, as well as further work to enhance VCSEL performance. Figure 1

Mode characteristics of VCSELs with different shape and size oxidation apertures

Vertical cavity surface emitting laser (VCSELs) as the ideal light source for rubidium (Rb) and cesium (Cs) atomic clocks is analyzed for its mode and polarization control. We fabricated three kinds of shapes: triangular, elliptic, and circular oxidation apertures which also have different sizes. We formed three different shape oxide apertures by wet-oxidation with 36 μm–39 μm circular mesa. Our results show that triangular oxidized-VCSEL has the advantages of mode and polarization selection over elliptic and circular oxide apertures. When triangular oxide-confined VCSELs emit in single mode, the measured side mode suppression ratio (SMSR) is larger than 20 dB and orthogonal polarization suppression ratio achieves 10 dB. Resonant blueshift of VCSELs with triangular and elliptic apertures is observed with the decrease of aperture size.

Analysis of optical and thermal properties of 940-nm vertical-cavity surface-emitting lasers

We achieve 13.5 mW optical output power, 48% power conversion efficiency, 1.17 W/A slope efficiency and 17 kW/cm2 laser power density with top-surface-emitting 940 nm oxide-confined vertical-cavity surface-emitting laser (VCSEL). The physical mechanism of minimum threshold current generation in oxide-confined VCSEL has been thoroughly studied theoretically and experimentally. Further, we also succeeded in 90.8 mW optical output power, 40% power conversion efficiency with 2 × 4 VCSEL arrays. We find an increase in output power and PCE of 2 × 2 VCSEL arrays as we increase the array spacing which we attribute primarily to increased heat dissipation and reduced thermal crosstalk between the emitters. Thermal properties in oxide-confined 2 × 2 VCSEL arrays were analyzed numerically and experimentally. The simulated results are in good agreement with the measurement. It is proved that theoretical simulation is very useful for the future device optimization.

Parasitic Parameters Extraction of High-Speed Vertical-Cavity Surface-Emitting Lasers

Parasitic parameters, including electrical capacity and inductance, are the key limiting factors for bandwidth improvement of high-speed vertical-cavity surface-emitting lasers (VCSELs). The traditional parasitic extraction method, which uses a first-order low-pass filter transfer function, is oversimplified, and there are large deviations between the obtained data and the actual measured data. In this paper, we proposed a modified parasitic extraction method that described the extrinsic behaviour of the high-speed oxide-confined VCSELs well and easily extracted the values of all parasitic parameters. This method can also precisely fit microwave reflection coefficient S11 data even at high frequencies and provide design guidance for high-speed VCSELs. Using this method, a high-speed 850 nm VCSEL featuring a six-layer oxide aperture with −3 dB bandwidth up to 23.3 GHz was analysed. The electrical parasitics have been systematically extracted from VCSELs with different oxide apertures. The enhanced bandwidth based on the improvement of parasitic parameters was discussed. It was found that the critical parasitic factors that affect the −3 dB bandwidth of VCSELs are pad capacitance and inductance.

High Speed VCSEL Technology and Applications

Historically optical links up to 100–300m distances are served by light emitting devices in the 850 nm spectral range in combination with multimode glass fibers (MMF). As the silicon scaling continues, a single channel data rate is to double each 24 months. Light-emitting diodes had to be replaced by vertical cavity surface emitting lasers (VCSELs) and the data rate increased from 100 Mb/s to 10 Gb/s. At higher data rates problems with further scaling evolved. To avoid the collapse an anti-waveguiding VCSEL cavity design was invented, applied, and presently serves data links operating up to 50–100 Gb/s per channel. Another requirement in data communication is the bandwidth density scaling, with the number of channels per link increasing approximately 5-fold each 10 years, while keeping a similar space for connectors. A coarse short-wavelength division multiplexing allowing 850 nm, 880 nm, 910 nm, and 940 nm wavelengths in a single MMF is introduced. The bandwidth density increase is also possible by using multicore fiber (MCF) coupled to on-chip VCSEL arrays. The data rates up to 224 Gb/s are already reached by 850nm VCSELs. At such data rates significant transmission distance over MMF is only possible by applying ultra-narrow spectrum VCSELs minimizing the chromatic dispersion effects. On-chip mini-arrays of oxide-confined VCSELs allow a high coupling efficiency to MMFs, a narrow spectrum, a high power, a significant transmission distance at high data rates. Coherent lasing in such arrays allow photon-photon resonance engineering aimed at modulation bandwidths ∼50–100 GHz.

Large-aperture single-mode 795 nm VCSEL for chip-scale nuclear magnetic resonance gyroscope with an output power of 4.1 mW at 80°C.

Transverse optical confinement in oxide-confined vertical-cavity surface-emitting lasers (VCSELs) crucially depends on thickness of oxide layer and its position relative to a standing wave. Modifying the structure reduces the overlap between the oxide layer and the standing wave as well as effective refractive index difference between core and cladding of the VCSEL that subsequently decreases of the number of transverse modes and increases the mode extension beyond oxide aperture. A 795 nm VCSEL is designed and fabricated with this concept. The proposed device achieves high single-mode operation of 4.1 mW at 80 °C, SMSR of 41.68 dB, and OPSR of 27.4 dB. VCSEL is applied in a nuclear magnetic resonance gyroscope (NMRG) system as pump source due to its excellent device performance and satisfactory test results are obtained.

795-nm high-temperature and high-power operating vertical-cavity surface-emitting laser and application in atomic gyroscope

Single-transverse mode vertical-cavity surface-emitting lasers (VCSELs) are preferable optical sources for small low-power atomic sensors, including chip-scale atomic clocks, magnetometers, and gyroscopes.When VCSEL is used as the pump source of nuclear magnetic resonance gyroscope, it is required to have high single-mode output power. Oxide aperture diameter must be sufficiently small (< 4 µm) in a conventional oxide-confined VCSEL to support the fundamental mode alone. However, high series resistance (typically > 200 Ω for GaAs-based VCSEL) from the small aperture limits its output power and reliability due to excessive current-induced self-heating and high current density. It is a very attractive idea to achieve high power operation of an intrinsic single mode VCSEL based on a large oxide aperture by means of epitaxial structure design without introducing additional process steps. Transverse optical confinement in oxide-confined VCSELs crucially depends on the thickness of oxide layer and its position relative to standing wave. Modifying the structure reduces the overlap between the oxide layer and the standing wave as well as the difference in effective refractive index between core and cladding of the VCSEL, thereby reducing the number of transverse modes andincreasing the mode extension beyond oxide aperture. A 795-nm VCSEL is designed and fabricated based on this concept. A cavity structure of VCSEL with gain-cavity detuning of ~10.8 nm at room temperature is adopted in this paper. The effective refractive index and the standing wave distribution of the VCSEL are calculated, and the position of the oxide layer in the epitaxial structure of the VCSEL is optimized according to the standing wave distribution. Finally, the structure with low effective refractive index difference is obtained. The proposed device achieves high single-mode operation of 4.1 mW at 80 ℃, SMSR of 41.68 dB, and OPSR of 27.4 dB. The VCSEL is applied to a nuclear magnetic resonance gyroscope (NMRG) system as pump source due to its excellent device performance, and satisfactory test results are obtained. This paper presents a new method of designing single-mode high power VCSEL and its feasibility is also demonstrated through experimental results.

Fabrication and Characterization of Low-Threshold Single Fundamental Mode VCSELs With Dielectric DBR Mirror

The SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /SiN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> dielectric film stacks deposited by inductively coupled plasma chemical vapor deposition (ICP-CVD) are employed on the top of the oxide-confined vertical-cavity surface-emitting lasers (VCSELs). The reflecting mirror characteristics of the dielectric films are measured with varying numbers of pairs from 4 to 12. The prepared devices have low threshold current of 0.3 mA, single-mode peak power of 1.4 mW at room temperature. The thermal resistance of the device depending on ambient temperature is estimated according to the relationship of wavelength shift and dissipated power, allowing us to extract the actual temperature in active region and the temperature dependence of the output characteristics of the VCSEL. The beam property of the device is characterized by the far field pattern (FFP). Depending on the drive current, the average divergence angles of the output beam estimated from the full-width-half-maximum (FWHM) of the FFP varies between 20.4° and 21.2°. Furthermore, polarization-controlled single mode emission is achieved by introducing a built-in grating into the VCSEL with orthogonal polarization suppression ratio (OPSR) around 19 dB.

Mechanical strain mapping of GaAs based VCSELs

We report an investigation of the strain field in mesa structures for oxide-confined vertical-cavity surface-emitting lasers (VCSELs) using the polarization-resolved micro-photoluminescence (PL) measurement of the degree of polarization (DOP) at room temperature. The DOP of the PL is correlated with the spatial distribution of the embedded anisotropic strain in a VCSEL structure. Measurements normal to (100) surfaces of the samples and from (110) cross-sectional planes were performed. The effect of two processes required in the fabrication of GaAs-based VCSELs was studied: the plasma etching of the P-doped distributed Bragg reflector and the wet oxidation process used to control current flow and lateral optical confinement. The DOP method allows very sensitive measurements of the mechanical strain (on the order of 10−5) accumulated in VCSEL devices even during different steps of the fabrication process.

Polarization control and mode optimization of 850\xa0nm multi-mode VCSELs using surface grating

To control the polarization of 850 nm multi-mode oxide-confined vertical-cavity surface-emitting lasers (VCSELs), monolithically integrated surface gratings are investigated. VCSELs with different grating parameters are simulated by the rigorous coupled-wave analysis (RCWA) method. Optical field intensity of two polarization orientations with different grating periods and duty cycles is obtained. The mode characteristics of VCSELs with different grating parameters are achieved by FDTD method. 850 nm surface grating VCSELs are fabricated and characterized, a maximum orthogonal polarization suppression ratio (OPSR) of 20.7 dB is observed. Meanwhile, VCSEL’s output mode characteristics are improved by integrating the surface grating. Application of integrated surface grating technique to achieve dynamically polarization-stable multi-mode VCSELs which enable polarization division multiplexed data transmission, the data throughput over a given link can be doubled.

Transverse-mode control in VCSELs by electrically tunable liquid crystal mode filters

A distinctive method is used to achieve transverse-mode control by building and integrating a tunable liquid crystal (LC) mode filter on top of traditional vertical-cavity surface-emitting lasers (VCSELs). An LC cell is fabricated by injecting a type of nematic LC material (E7) in an annular groove patterned by SiO2. By electrically tuning the refractive index of the LC, a spatially dependent reflectivity profile can be achieved and optimized, which directly influences the threshold modal gain of each transverse mode provided by oxide-confined VCSELs. The flexible and accurate control performance of the LC mode filter structure have been demonstrated by the simulation results and our analysis. This design is a completely novel method to realize VCSELs with an accurate and real-time controllable transverse-mode, and it will probably play a significant role in the future.

Analysis of Thermal Properties of 940-nm Vertical Cavity Surface Emitting Laser Arrays

The output characteristics of vertical cavity surface emitting lasers (VCSELs) arrays are largely influenced by temperature. We systematically studied thermal properties in 20-element oxide-confined VCSEL arrays via both theory and experiment. The output power, conversion efficiency, and threshold current dependent on ambient temperatures were measured. According to the relationship of wavelength shift and dissipated power, the device thermal resistance dependent on ambient temperature was obtained. The actual temperature in active region was successfully extracted. A 3-D heat diffusion model was established to simulate temperature distribution accurately.

Investigation of the current influence on near-field and far-field beam patterns for an oxide-confined vertical-cavity surface-emitting laser.

This experiment presents dynamic behaviors between the operating current and the optical beam images of vertical-cavity surface-emitting lasers (VCSELs) with two different aperture diameters of 3 µm (single-mode) and 5 µm (multi-mode). These VCSELs exhibit complex optical phenomena under current injection such as thermal effects, modal competition, carrier distribution, and laser coherence which make the light field distribution difficult to predict. In this report, the DC properties, optical spectrum, and optical images were measured together at different operating currents to accurately evaluate the characteristics of the lasers. Unlike previous works, the variations of the far-field angle were precisely evaluated by the side-mode-suppression ratio (SMSR) of the optical spectrum. In addition to commonly used transform functions such as the Gaussian beam formula, the SMSR provides another tool for the judgment of far-field divergence which could prevent inaccurate analysis. Moreover, the impact of thermal lensing was calculated by the DC measurement and demonstrated by the far-field measurement at high injection current. Through this experiment, the interaction between the injection carrier, thermal lens effect, and current spreading was described as fully as possible.

19-Element 2D Top-Emitting VCSEL Arrays

We achieve 200 mW optical output power, 18 GHz bandwidth, and 35% wall plug efficiency with 19-element (novemdecuple) top-surface-emitting 980 nm oxide-confined vertical-cavity surface-emitting laser (VCSEL) arrays - a record combination of these three attributes. We grow wafers by metal-organic vapor phase epitaxy and process arrays into a unique two-dimensional honeycomb geometry with top mesa inter-VCSEL ridge connectors and p-metal anode interconnections. We study the impact on array performance of inter-VCSEL spacing and bottom n-doped mesa area. The top emitting VCSELs are optically uncoupled but electrically connected in parallel via anode ring-to-ring p-metal along the ridges and a common horseshoe ohmic cathode n-metal. We demonstrate the potential of our arrays as sources for optical wireless communication via 25 Gb/s data transmission tests.