Polarization axis fluctuations in a single mode VCSEL laser at 780nm

Using transfer matrix method and TFcalc thin film design software,the reflectance spectrum of distributed Bragg reflector (DBR) and vertical cavity surface emitting laser (VCSEL) are simulated.The reflectance spectra from the cavity and surface are compared with each other,thus providing the basis for white light source (WLS) optical reflectance spectrum of the VCSEL epitaxial wafer.When using WLS to characterize VCSEL wafer,it is necessary to combine the simulation results and the shape of optical reflectance spectrum to speculate the reflectance seen from the cavity.The influences of different cap layers on the reflectance of DBRs are discussed theoretically and experimentally.With a 1/4 GaAs cap layer,the reflectance reaches up to 97.8% seen from the cavity.This design can make the wavelength of the VCSEL etalon picked easily because of avoiding the influence of test noise. The active region has higher heat accumulation due to the small area and poor thermal conductivity.The characteristics of the gain spectrum of InGaAs/AlGaAs strained quantum well (QW) under different temperatures and the temperature distribution in VCSEL are simulated by Crosslight software.The gain-to-cavity wavelength detuning is used to improve the slope efficiency and the temperature stability.The temperature in active region ranges from 360 K to 370 K.The gain peak wavelength and the Fabry-Perot cavity wavelength are designed in the ranges of 829-832 nm and 845-847 nm,respectively.Epitaxial wafer with top-emitting VCSEL structure grown by metal-organic chemical vapor deposition is characterized.The room temperature photoluminescence peak is at 827.5 nm and the etalon cavity wavelength measured by optical reflectance is 847.7 nm,which are consistent with designed values. The oxide restricted VCSELs with 7.5 m oxide aperture are fabricated.The image of the infrared light source CCD shows that the oxide aperture is circular.A passivation layer of 120 nm SiO2 is finally deposited to insulate water vapor.The threshold current is 0.8 mA,and the maximum output power reaches up to 9 mW at 13.5 mA.The optical spectrum at 6.0 mA reveals multiple transverse modes.The center wavelength is 852.3 nm and the root mean square (RMS) spectrum width is 0.6 nm,meeting the high-speed Datacom standards.Shannon theory indicates that the maximum data rate is not only proportional to bandwidth but also related to signal-to-noise ratio (SNR).It is effective to reduce relative intensity noise and enhance the SNR by increasing output power.From the eye diagram of 25 Gbit/s on-off key VCSEL,it is demonstrated that fall time is 38.66 ps,rise time is 41.54 ps,SNR is 5.6,and jitter RMS is 1.57 ps.Clear eye opening is observed from eye diagram of 25 GBaud/s PAM-4 VCSEL,which indicates the qualified 50 Gbit/s high speed performance.

Remarkable progress made in performance characteristics and reliability of high-speed (> 10 GHz) 850-nm multi-mode (MM) oxide-confined vertical cavity surface emitting lasers (VCSELs) during the last decade has led them to find applications in space satellite systems. The main advantage of deploying high-speed VCSELs in space satellites over directly modulated 850-nm edge emitting lasers is the absence of COMD (catastrophic optical mirror damage). In recent years, leading VCSEL manufacturers introduced higher speed (~ 20 GHz or 25 Gbps) VCSELs with encouraging characteristics. However, little has been reported on reliability and failure modes of these state-of-the-art VCSELs although it is crucial to understand failure modes and degradation mechanisms in these VCSELs through physics of failure investigation and subsequently develop VCSELs that exceed lifetime requirements for space satellite systems. For the present study, we performed short-term and long-term accelerated life-tests on 25 Gbps oxide-confined MM VCSELs to study reliability of these devices. Our goal is to extract credible reliability model parameters (thermal activation energy and current exponent factor) from these life-tests to determine suitability of these lasers for future space satellite systems. We also performed failure mode analysis on VCSELs at different stages of degradation using various techniques. We employed nondestructive techniques including optical beam induced current (OBIC) and electron beam induced current (EBIC) techniques as well as destructive techniques including focused ion beam (FIB) and high-resolution TEM techniques. Our detailed reliability and failure mode analysis results are reported along with our understanding on the physical origin of degradation in high-speed VCSELs with strained InGaAs quantum wells.

Vertical cavity surface emitting lasers (VCSELs) are low cost and reliable light sources for high-speed local area and storage area network (LAN/SAN) optical fiber data communication systems and short-reach computer interconnects. The continuing rapid increase of serial transmission data rates driven by multi-core microprocessor's bandwidth upgrades cannot be sustained via conventional copper-based links as bit rates move beyond 10 Gbit/s and distances greater than 1 m. The intrinsic limitation of copper at high single-channel data rates facilitates the need to transition to optical fiberbased links at ever shorter distances. For LAN/SAN applications the 850 nm wavelength is standard. This same wavelength is also the standard for several other evolving short-reach application areas including Fibre Channel, CEI, USB, InfiniBand, and HDMI optical link systems. Herein we present our recent results on 850 nm oxide-confined VCSELs operating at data bit rates up to 40 Gbit/s. The low operational current density in the range of ~10 kA/cm2 ensures viable device reliability and long-term stability based on well-known industry certification specifications. Key VCSEL device parameters including the relaxation resonance frequency, damping, and parasitic cut-off frequency are determined for VCSELs with oxide-confined apertures of various diameters. We find that a parasitic cut-off frequency of 24-28 GHz limits the VCSEL's high speed operation at the highest optical modulation rates. We believe that with some effort the device parasitics can be further reduced such that current modulated VCSELs can be realized with larger than 30 GHz optical modulation bandwidth and reliable and practical operation beyond 40 Gbit/s.

High reliability, high transmission speed and low power consumption laser diodes are required for optical interconnect. We have developed 1060 nm Vertical Cavity Surface Emitting Lasers (VCSELs) with InGaAs/GaAs strained quantum wells, oxide-confined and double intracavity structures for the requirement. In this paper, an extremely long term aging test close to the actual operating temperature was performed in condition of 90°C and 6 mA. No degradation of optical power and threshold current has been observed up to 26,500 hours (3 years). The aging duration is equivalent to 80 years at 40°C and 10 years at 70°C using Ea = 0.7 eV. It indicates that our 1060 nm VCSELs can operate for the long duration.

The vertical cavity surface emitting laser (VCSEL) is recognized as one important device in applications such as optical interconnects, optical communications and so on. Most of VCSELs have been fabricated on (100)-oriented substrates, and exhibit linear polarization. But usually it is unstable due to the small polarization selectivity [1]. The polarization control is, therefore, an important subject for polarization-sensitive applications. One of the control methods is to use the effect of optical anisotropy of strained quantum wells (QWs) grown on (n11)-oriented substrates [2]. We present here the first result on the stable polarization characteristics of a (311)A-oriented InGaAs/GaAs VCSEL operated at room temperature

The electro-optical efficiency of semiconductor vertical-cavity surface-emitting lasers (VCSELs) strongly depends on the efficient carrier injection into the quantum wells (QWs) in the laser active region. However, carrier injection degrades with increasing temperature which limits the VCSEL performance particularly in high power applications where self heating imposes high temperatures in operation. By simulation we investigate the transport of charge carriers in 808 nm AlGaAs multi-quantum-well active layers with special attention to the temperature dependence of carrier injection into the QWs. Experimental reference data was extracted from oxide-confined, top-emitting VCSELs. The transport simulations follow a drift-diffusion-model complemented by a customized, energy-resolved, semi-classical carrier capture theory. QW gain was calculated in the screened Hartree-Fock approximation with band structures from 8x8 k.p-theory. Using the gain data and by setting losses and the optical confinement factor according to experimental reference results, the appropriate threshold condition and threshold carrier densities in the QWs for a VCSEL are established in simulation for all transport considerations. With the combination of gain and transport model, we can explain experimental reference data for the injection efficiency and threshold current density. Our simulations show that the decreasing injection efficiency with temperature is not solely due to increased thermionic escape of carriers from the QWs. Carrier injection is also hampered by state filling in the QWs initiated from higher threshold carrier densities with temperature. Consequently, VCSEL properties not directly related to the active layer design like optical out-coupling or internal losses link the temperature dependent carrier injection to VCSEL mirror design.

In this paper we present a device concept for long-wavelength vertical-cavity surface-emitting lasers (VCSELs) in the InGaAlAs/InP material system incorporating a buried tunnel junction (BTJ). A major issue of long-wavelength VCSELs is the dissipation of heat because of the low thermal conductivity of ternary and quaternary alloys. With the BTJ-VCSEL, a significant reduction of the thermal resistance is achieved by the use of a hybrid backside mirror made of a stack of amorphous dielectrics with Au-coating and the monolithic integration of a heat sink. These provide improved heat sinking capability compared to a conventional epitaxial semiconductor DBR. In addition, the tunnel junction facilitates a substitution of most of the p-doped layers by n-doped material, reducing heat generation due to ohmic losses. These features significantly improve the VCSEL characteristics. At 1.55 μm wavelength, we demonstrated single-mode cw-output powers of 1.7mW at room temperature [1], multi-mode cw-output powers of 7mW [2], laser operation up to heat sink temperatures of 110 °C [2], and optical data transmission with 10 Gbit/s and low bit error rates [3]. These are record values to the best knowledge of the authors. Using strained quantum wells, the emission wavelength can be tailored to any value in the range between 1.3 μm and 2.0 μm [4], sample results are presented for the telecommunication wavelengths 1.3 μm and 1.55 μm, 1.8 μm, and the currently upper limit of 2.0 μm. The slight wavelength tuning with driving current is brought about by the tiny volume of the devices and makes VCSELs ideal components for tunable diode laser absorption spectroscopy (TDLAS) [5, 6]. The maximum detuning typically reaches 4 nm (≙500 GHz). (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Theoretical investigations of the generation of light with sub-shot-noise intensity fluctuations by vertical cavity surface-emitting lasers are performed with a semiclassical model. From these results we conclude that, because of their dimensions and structure, vertical cavity surface-emitting lasers are promising candidates for generation of amplitude squeezed states. Finally, the influence of internal loss, distributed reflection coefficients, and gain suppression is emphasized and discussed.

InAsP/InGaAsP periodic gain structure for 1.5 μm vertical cavity surface emitting laser applications

Strong commercial interest in inexpensive 1.3-1.55 /spl mu/m sources for short to intermediate-reach optical networks has recently yielded demonstrations of 1.3 /spl mu/m vertical cavity surface emitting lasers (VCSELs) grown on GaAs substrates employing InGaAsN quantum wells. InGaAsN appears particularly attractive for this application on the basis of fundamental material properties such as favorable band offsets, as well as the ability to grow this material both by molecular beam epitaxy and metalorganic chemical vapor deposition. However, initial devices fabricated using these materials have demonstrated relatively poor performance compared to shorter-wavelength devices, both as a result of the lower quality of InGaAsN compared to GaAs or InGaAs, and the relatively simple device structures employed. In the present work, we report significantly improved properties for these devices as a result of enhancements to the InGaAsN quality and VCSEL structures.

A comprehensive beam propagation method (BPM) for the modeling of oxide-confined visible emitting (665nm) vertical-cavity surface-emitting lasers (VCSELs) aimed at polymer optical fibre (POF) communications is presented. In this model all the major physical processes, including the current density distribution, the self-heating effect of the devices, the carrier lateral diffusion in quantum wells, and the optical field modes are considered self-consistently. Using the model, the current flow, carrier diffusion in the active quantum wells, and temperature distribution are calculated. The model indicates severe current crowding around the edge of the oxide aperture within the VCSEL. Using a simple function to describe the variation of optical gain with temperature and wavelength, the threshold properties, transverse modes and the optical output current-light characteristics are calculated. The simulation results are compare favorably with measurements.

This paper is going to review the state-of-the-art of the high-speed 850/940-nm vertical cavity surface emitting laser (VCSEL), discussing the structural design, mode control and the related data transmission performance. InGaAs/AlGaAs multiple quantum well (MQW) was used to increase the differential gain and photon density in VCSEL. The multiple oxide layers and oxide-confined aperture were well designed in VCSEL to decrease the parasitic capacitance and generate single mode (SM) VCSEL. The maximal modulation bandwidth of 30 GHz was achieved with well-designed VCSEL structure. At the end of the paper, other applications of the near-infrared VCSELs are discussed.

We report the first visible-light (657 nm) operation of a vertical-cavity surface-emitting laser in a photopumped intrinsic structure. Vertical-cavity surface-emitting lasers (VCSELs) have recently attracted a great deal of interest because of their surface-normal operation, potential for extremely low-threshold currents, ease of fabrication of closely-spaced 1- and 2-dimensional arrays and integration with other devices such as transistors for photonic switching applications. To date operation of vertical-cavity surface-emitting laser has been limited to the range between 770 nm and 980 nm. We have extended this range into the visible where numerous applications exist including: ultrafast holographic memory, visible laser projection displays, and local area networks based on inexpensive, rugged plastic fibers. The structure was grown using low-pressure metalorganic vapor phase epitaxy in a single growth sequence on 6° misorientated (100) GaAs substrates. The active region consists In0.54Ga0.46P strained quantum wells and In0.48(AlyGal-y)0.52P (y=0.7-1.0) graded barriers whereas the distributed Bragg reflectors are AlAs/Al0.5Ga0.5As. Room temperature optical-pump lasing was achieved with a very low threshold power (~ 1 kW/cm2 average power, ~ 4 ps pulses, 82 MHz), comparable to our best AlGaAs/GaAs VCSELs operating at 850 nm. This work effectively demonstrates the viability of visible-VCSEL technology, and opens the door for a broad range of new materials and device physics research and visible-light optoelectronic applications. The SNL research is supported by DOE contract No. DE-ACO4-76DP00789.

Blue nitride semiconductor laser diodes and light emitting diodes have recently been developed and are having a tremendous impact on industry as well as on fundamental research. Blue lasers with Indium gallium nitride (InGaN) multiple quantum wells as the active layer have been demonstrated under continuous-wave operation. This chapter describes blue vertical cavity surface emitting lasers (VCSELs) and blue InGaN quantum dot lasers. It explains the fabrication of these two types of blue lasers using metal organic chemical vapor deposition and the observation of lasing oscillation in these devices at room temperature. To produce practical blue VCSELs, the operation temperature must be increased from the cryogenic temperature to room temperature. A laser with quantum dots in the active layer is expected to have a lower threshold current and other superior performance characteristics when compared to quantum well lasers. The key issue for commercial blue VCSELs is still the growth of highly reflective nitride Bragg reflectors.

Electrostatic Discharge (ESD) events can cause irreversible damage during production, packaging and application of Vertical-Cavity Surface Emitting Lasers (VCSELs). Experimental investigation of those damage patterns inside a real device is a complex and expensive task. Simulation tools can provide insight into the physics during an actual discharge event. This paper aims to analyze ESD events in VCSELs with a microscopic simulation. With the help of a state-of-the art Technology Computer Aided Design (TCAD) virtual ESD tests are performed on oxide-confined VCSELs. The 2-D simulation model takes into account high-field effects and self-heating in a hydrodynamic framework that allows time-dependent spatially resolved monitoring of critical quantities (such as electric field across the oxide, temperature profile, current densities) during the ESD events. Human Body Model (HBM), Machine Model (MM) and Charged Device Model (CDM) show typical local heating and current crowding effects which may lead to irreversible damaging of the device. For slow ESD events the temperature peak is found near the center of the device. Faster pulses show maximum temperature at the interface between oxide and aperture. Physics-based explanations in terms of local electric field, heat generation and heat transport are given. Oxide aperture, thickness and its position relative to the intrinsic region strongly influence self-heating, electric fields, current density profiles and the dielectric breakdown conditions. The impact of those factors on ESD robustness are analyzed and guidelines for robust ESD design in VCSELs are presented.