Vertical cavity surface emitting lasers utilizing native oxide mirrors and buried tunnel contact junctions

ABSTRACTAn electrically pumped 1.24um GaxIn1-xAsyP1-y Vertical Cavity Surface Emitting Laser (VCSEL) has been designed and simulated. Threshold gain of 153cm-1 and threshold current of 1.00mA was obtained. The external efficiency predicted is 0.31. Also the optimized design of VCSEL structure is presented in this article. GaInAsP and AlInAs constitute the Distributed Bragg Reflectors (DBRs) multilayer stack. Reflectivity of the top DBRs is 0.97 and reflectivity of the bottom DBRs is 0.9978 and is shown to provide a good resonant cavity and sufficient lasing intensity. Compared with other reported structures, the present VCSEL has a lower threshold current and higher threshold gain.

Laser action with low threshold average pump power density (∼50 W · cm−2) at room temperature is reported for a crack-free planar vertical cavity surface emitting laser (VCSEL) structure based on a bottom lattice-matched AlInN/GaN distributed Bragg reflector (DBR) and a top dielectric DBR. The cavity region, formed by n- and p-type GaN layers surrounding only three InGaN/GaN quantum wells, corresponds to a typical active region suitable for an electrically driven VCSEL. In addition to low threshold, a spontaneous emission coupling factor β∼2×10−3 is derived for this ready-to-be-processed laser structure.

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.

Reduced current thresholds in GaAs/AlGaAs vertical cavity surface emitting lasers using 4° off-oriented (001) GaAs substrates

In the present work, we investigate threshold characteristics of a single mode 1.55 μm InGaAsP vertical cavity surface emitting laser (VCSEL) with two different optical confinement structures. The device employs InGaAsP active region, which is sandwiched between GaAs/AlGaAs and GaAs/AlAs distributed Bragg reflectors (DBRs). The optical confinement introduced by the oxide aperture or a single defect photonic crystal design with holes etched throughout the whole structure, is compared with previous work. Photonic crystal VCSEL shows 30.86% and 57.02% lower threshold current than that of the similar oxide confined VCSEL and previous results, respectively. This paper provides key results of the threshold characteristics, including the threshold current and the threshold power. Results suggest that, the 1.55 μm InGaAsP photonic crystal VCSEL seems to be the most optimal one for light sources in high performance optical communication systems.

Vertical cavity surface emitting lasers (VCSELs) have long been predicted as low-cost enabling laser sources for many applications including optical communications, sensing and imaging. Traditional VCSELs use distributed Bragg reflectors (DBRs) as mirrors, which consist of many tens layers of epitaxy layers with alternating refractive indecies. Snice 2004, we develeoped a new class of planar optics has emerged using near-wavelength dielectric structures, known as high contrast metastructures (HCM). Many extraordinary properties can be designed top-down based for integrated optics on a silicon or GaAs substrate. The one-dimensional version, a single layer high index contrast near-wavelength gratings (HCG), has been used to replace the hundred-layered DBR in a VCSEL structure. In this talk, I will review recent results using HCG/HCM in device applications of VCSELs, beam-steering optical phased array, biosensor, 4-wave generation, and spatial light modulator. I will discuss inventions and advances in VCSELs that have led to recent global deployment of commercial applications including 3D sensing, LIDAR and optical coherent tomography applications. I will also discuss future prospects for advanced applications.

As a key part of vertical cavity surface emitting laser (VCSEL), active region will seriously affect the threshold and efficiency of the device. To obtain the appropriate laser wavelength and material gain, the design of In<sub>0.18</sub>Ga<sub>0.82</sub>As strain compensated quantum well is optimized. The relationship between the lasing wavelength of multiple quantum wells (MQWs) and the thickness is calculated. Considering the influence between the active region temperature and the lasing wavelength, the thickness of the quantum well is chosen as 6 nm, and the quantum barrier thickness is chosen as 8 nm, corresponding to the lasing wavelength of 929 nm. The material gain characteristics of the MQWs at different temperatures are simulated by Rsoft. The material gain exceeds 3300/cm at 300 K, and the temperature drift coefficient of the peak wavelength is 0.3 nm/K. In this work, Al<sub>0.09</sub>Ga<sub>0.91</sub>As and A<sub>l0.89</sub>Ga<sub>0.11</sub>As are chosen as the high- and the low-refractive index material of distributed Bragg reflector (DBR), and 20 nm graded layer is inserted between two types of materials. The influence of the graded layer thickness of DBR on the valence band barrier and reflection spectrum are calculated and analyzed. The increase of graded layer thickness can lead the band barrier peak and the reflection spectrum bandwidth to decrease. The reflection spectrum and phase spectrum of P-DBR and N-DBR are calculated by the transmission matrix mode (TMM): the reflectance of DBR is over 99% and the phase shift is zero at 940 nm. The optical field distribution of the whole VCSEL structure is simulated, in which the standing wave peak overlaps with the active region, and the maximum gain can be obtained. Using the finite element method (FEM), the effect of oxidation confined layer on the injection current is simulated. The current in the active region is effectively limited to the position corresponding to the oxidation confined hole, and its current density is stronger and more uniform. The optical field distributions in different modes of photonic crystal-vertical cavity surface emitting laser (PC-VCSEL) are simulated, and different modes have different resonant wavelengths. The values of quality factor Q in different modes of VCSEL and PC-VCSEL are calculated, Q of the fundamental mode is higher than that of higher transverse mode. It is demonstrated that the photonic crystal air hole structure can realize the output of basic transverse mode by increasing the loss of high order transverse mode. The VCSEL and PC-VCSEL with oxidation hole size of 22 μm are successfully fabricated, in which the photonic crystal period is 5 μm, the air pore diameter is 2.5 μm, and the etching depth is 2 μm. Under continuous current test, the maximum slope efficiency of VCSEL is 0.66 mW/mA, the output power is 9.3 mW at 22 mA, and the lasing wavelength is 948.64 nm at 20 mA injection current. Multiple wavelengths and large spectrum width are observed in the spectrum of VSCEL, which is an obvious multi-transverse mode. The maximum fundamental transverse mode output of PC-VCSEL reaches 2.55 mW, the side mode suppression ratio (SMSR) is more than 25 dB, and the spectrum width is less than 0.2 nm, indicating that the photonic crystal air hole has a strong control effect on the transverse mode, and the laser wavelength is 946.4 nm at 17 mA.

The rapid growth of internet and cloud computing applications drives a huge demand for bandwidth capacity in communication networks, while power consumption, cost, and space density must scale down. This growth leads to an increase in the size of data centers (longer optical links), and of the fibers’ channel data rate, rooted in Moore’s Law. Until now, multi-mode fibers (MMF) have been largely employed in datacom applications due to the large coupling tolerance. However, the data-carrying capability of MMF decreases with the transmission distance due to pulse broadening resulting from modal and chromatic dispersion. In order to overcome those limits, transceivers based on single mode fiber (SMF) are under development and the first systems are on the market. Vertical-cavity surface-emitting lasers (VCSELs) are the transmitters of choice for short-reach applications due to their low cost, energy efficiency , and small footprint. InP-based VCSELs emitting at long wavelengths (i.e. 1.3 and 1.55 µm) have gained large interest due to their intrinsic lower power consumption (lower band gap) and low losses in silicon waveguides and silica-based optical fibers, which allows longer transmission distances. While short-wavelength GaAs-based VCSELs have achieved small-signal modulation bandwidths up to 30 GHz [1], InP-based VCSELs show inferior modulation capabilities [2, 3]. Up to date, the highest small-signal bandwidth demonstrated on InP-based devices is 22 GHz [3]. The distributed Bragg reflectors (DBRs) commonly used for GaAs-based VCSELs are made of binary and ternary semiconductor compounds, which offer several advantages such as high refractive-index contrast between the layers, good electrical conductivity and low thermal resistivity. The inferiority of semiconductor DBRs lattice matched to InP challenges the modulation bandwidth enhancement of InP-based devices which suffer of poor thermal conductivity, and high lateral spreading resistance. A further challenge is the single-mode laser operation that has motivated the transition from MMF to SMF in datacom systems. In this chapter, the challenges related to InP-based VCSELs are discussed with focus on active region design, cavity engineering, and current and optical confinement. These arguments apply to all InP-based VCSELs with emission wavelength between 1.3 and 2.0 µm. Stationary and dynamic characteristics are presented for a 1.55 µm VCSEL. Finally, datacom and telecom transmission experiments are presented.

We report on high resolution analysis of vertical cavity surface emitting lasers (VCSELs) to detect and assess defects in sub-surface layers. We employ a focussed ion beam scanning electron microscope (FIB-SEM) to sputter and image successive cross sections (slice and view technique) in order to produce a 3D reconstruction of the oxide aperture region. High resolution images and measurements of the multilayers and oxide apertures of VCSEL devices were obtained. The process took ∼2.5 h and produced over 270 slice SEM images for a device volume of approximately 13.2 × 16.0 × 13.8 μm3, with a voxel size of 50 nm. On-wafer, single mode VCSEL devices with high and low output powers were analysed to compare their oxide apertures and distributed Bragg reflector (DBR) layer structures. It was found that the low output power VCSEL had DBR layer defects and a 41.8% reduction of effective oxide aperture area, explaining the lower power obtained. The results provide evidence that oxide aperture area and structural defects are major factors that affect the optical output power of VCSEL devices. Outcomes in this work show FIB-SEM slice and view is a valuable method for 3D reconstruction of VCSEL devices, which enables top view, cross-sectional view and angled view of the whole device region as well as designated structures such as oxide aperture or structural defects in various layers. This work demonstrates a promising technique with high resolution (50 nm) 3D imaging for analysis of complex semiconductor devices.

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

Random source is important for the security of key distribution. In this paper, a novel secure key distribution scheme based on unidirectional injection of vertical cavity surface emitting laser (VCSEL) system is proposed. In the proposed scheme, a chaotic signal without time delay signature is generated by a VCSEL subject to unidirectional optical injection, which is regarded as a master laser. The chaotic signal generated by the master VCSEL is further injected into two slave VCSELs to obtain synchronized bandwidth-enhanced chaotic signals. After that, by sampling, quantizing and XOR operation on the two synchronized chaotic signals, two key streams can be obtained. Based on the well-known spin-flip model, the time delay signature of chaotic signals generated by master VCSEL and the synchronization performance between the master VCSELs and two slave VCSELs are numerically investigated in detail. It is shown that by the unidirectional injection, the chaotic outputs can be achieved in the master VCSEL in a wide range of frequency detuning and coupling strength. More importantly, no time delay signature can be observed in the auto correlation function of the chaotic intensity time series generated by the master VCSEL. Besides, we find that high quality synchronization is achieved between the bandwidth-enhanced chaotic signals generated by two slave VCSELs under the common driving of master VCSEL. The synchronization coefficient between two slave VCSELs increases up to 0.99, and the synchronization coefficient between master VCSEL and salve VCSEL is only 0.74. Note that such a high quality synchronization between two slave VCSELs while relatively low quality synchronization between the master and slave VCSEL is conducible to ensuring the security of key distribution. In addition, the effects of tunable parameters on key bit error rate are considered, and two quantization methods are employed for comparison. Numerical simulation results show that the key bit error rate between two legitimate users is as low as 1%, and the key bit error rate between legitimate user and eavesdropper is higher than 10% in the single-threshold case; the bit error rate can even be as low as 10-6 in the double-threshold case. The influence of parameter mismatch on key bit error rate is also discussed, and it is suggested that two salve VCSELs should be finely matched to ensure low bit error rate. Finally, NIST randomness test is performed for the generated key streams. Hence, the proposed scheme enhances the security of key distribution, which is valuable for further developing the chaos communication systems.

GaN-based vertical-cavity surface-emitting lasers (VCSELs) have drawn interest in recent years for their potential applications in data storage, laser printing, solid-state lighting, optical communications, sensing, and displays. Several research groups have demonstrated electrically injected GaN-based VCSELs utilizing different growth and fabrication techniques to address the many challenges associated with III-nitride materials. One such challenge is fabrication of highquality conductive epitaxial distributed Bragg reflectors (DBRs). A relatively new approach that yields high-index-contrast lattice-matched epitaxial DBRs is to introduce subwavelength air-voids (nanopores) in alternating layers of doped/undoped GaN. These nanoporous layers can be achieved by the controlled anodic electrochemical etching of highly doped n-type GaN in acids. The selective formation of the nanopores in the doped layers effectively lowers the refractive index compared to the adjacent undoped GaN layers, resulting in a refractive index difference of ~0.83, allowing high reflectance (>99%) with only ~16 pairs. Here, we will present electrically injected nonpolar m-plane GaN-based VCSELs with lattice-matched nanoporous GaN bottom DBRs and top dielectric DBRs. Lasing under pulsed operation at room temperature was observed at 409 nm with a linewidth of ~0.6 nm and a maximum output power of ~1.5 mW. The nonpolar m-plane orientation offers low transparency, high material gain, and anisotropic gain characteristics. The VCSELs were linearly polarized with a polarization ratio of ~0.94 and polarization-pinned emission along the a-direction. The mode profiles, thermal properties, and lasing yield of the VCSELs are also discussed.

High-efficiency and high-power operation have been demonstrated for blue GaN-based vertical-cavity surface-emitting lasers (VCSELs) with AlInN/GaN distributed Bragg reflectors. The high-efficiency performance was achieved by introducing a novel SiO2-buried lateral index guide and adjusting the front mirror reflectivity. Lateral optical confinement has been shown to greatly lower the otherwise significant loss of transverse radiation exhibited by typical VCSELs based on GaN. Employing a long (10λ) cavity can also enhance the output power, by lowering the thermal resistance of the VCSEL and increasing the operating current associated with thermal rollover. This modification, in conjunction with optimized front mirror reflectivity and a buried SiO2 lateral index guide, results in a blue VCSEL (in the continuous wave mode with an 8 μm aperture at 20 °C) having a superior differential quantum efficiency value of 31% and an enhanced 15.7 mW output power. This unit also exhibits a relatively high output power of 2.7 mW at temperatures as high as 110 °C. Finally, a 5.5 μm aperture VCSEL was found to generate a narrow divergence (5.1°) single-lobe far field pattern when operating at an output power of approximately 5 mW.

This paper reports on the optimization of red vertical-cavity surface-emitting laser (VCSEL) diodes grown by metalorganic vapour phase epitaxy. The VCSEL structure has an GaInP/AlGaInP multiquantum well active zone sandwiched between AlGaAs/AlAs distributed Bragg reflectors (DBRs). We present results on the optimization of the electrical resistance of the p:DBR and the wavelength tuning. Using these optimized parameters pulsed laser operation is demonstrated for wavelengths between 639 nm and 662 nm. At 650 nm the threshold current density is 3.7 kA/cm/sup 2/. Cw laser operation is achieved at room temperature. With wet oxidized VCSELs maximum cw output powers of 160 /spl mu/W are obtained at a wavelength of 656 nm and 10/spl deg/C.

Long-wavelength vertical cavity surface emitting lasers (VCSELs) operating in the 1.3μm to 1.5μm wavelength range are ideal light sources for optical fibre communication applications. However, a number of problems have hindered progress in the development of cost-effective long wavelength VCSELs. These are (a) intrinsically high non-radiative losses, (b) difficulty in fabricating highly reflecting mirrors, lattice-matched to InP, and (c) the disparity between the predicted and observed temperature dependence of the operation of the devices. In this report we present the results of our studies concerning the pulsed operation of a bulk GaInAsP/InP VCSEL fabricated at BT laboratories. The device is tailored to emit at around 1.5 μm at room temperature. The structure has a 45 period n-doped GaInAsP/InP bottom Distributed Bragg Reflector (DBR), and a 4 period Si/Al 2 O 3 dielectric top reflector. Spectral electroluminescence (EL) from a 16 μm diameter window is measured in the pulsed injection mode. Device parameters are recorded in the temperature range between 90 K and 240K. Threshold current exhibits an approximate parabolic temperature dependence with a broad minimum of J th = 13.2 kA cm -2 , at temperatures between 170K and 195K. Temperature dependence of the threshold current is compared with the theoretical calculations which consider radiative transitions with and without k-selection. Best agreement with the experimental results is obtained when a partial k- selection model, with an energy broadening of about 4.4 meV, is used in the calculations.