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

High-power and high-quality Gaussian beam in VCSEL via anisotropic modes control

Gaussian beam in the vertical cavity surface emitting laser (VCSEL) hasimportantapplicationin 3D sensing and optical communications, due to its concentrate energy at center and long propagation distance. However, its beam quality under high power tends to be serious degradation and become annular hollow distribution, severely limiting the use of VCSEL. Here, the anisotropic elliptical oxide aperture VCSEL (AEOA-VCSEL) was designed to achieve high-power Gaussian beam with high-quality via anisotropic modes control. The saturation output power reached up to 8.2 mW under low injection current, to our best knowledge, which was the highest power generated in single VCSEL device. Moreover, the AEOA-VCSEL can control the output beam profile only by adjusting injection current, and could also homogenize the mode spacing among adjacent high-order modes. The facile preparation process and low cost makes the device present great potential in augmented and virtual reality (AR/VR), 3D sensing and optical communications.

High-power operations of single-mode surface grating long oxide aperture VCSELs

A high-power single-mode vertical cavity surface emitting laser (VCSEL) based on a slow light mode (SLM) concept is demonstrated. Shallow surface gratings were implemented to select a single slow light mode in mm long oxide aperture VCSELs. The vertical structure and well-established fabrication processes are unchanged from those of conventional 850 nm VCSELs. An electron-beam lithography is used to form a 1st-order shallow grating on the surface. Thanks to the low effective index and the intensity penetration of the SLM to the surface, the grating fabrication process is much easier than conventional guided mode distributed feedback lasers. A stable single mode with a high side suppression ration of more than 30 dB is obtained for the entire current range under CW operations. A narrow divergence of 0.038° in far field patterns is achieved for a 6 mm long device. A record single-mode power of more than 5 W is realized before saturation under 50 ns pulse operations. Unlike long cavity in-plane edge emitting lasers, a SLM-based VCSEL can provide higher output powers without limitation of catastrophic optical mirror damage. The high power, superior mode stability, and good beam quality of our unique design paves a way for VCSELs to be used for wide ranges of high-power applications.

Improved thermal property of strained InGaAlAs/AlGaAs quantum wells for 808-nm vertical cavity surface emitting lasers

The 808-nm vertical cavity surface emitting laser (VCSEL) with strained In0.13Ga0.75Al0.12As/Al0.3Ga0.7As quantum wells is designed and fabricated. Compared with the VCSELs with Al0.05Ga0.95As/Al0.3Ga0.7As quantum wells, the VCSEL with strained In0.13Ga0.75Al0.12As/Al0.3Ga0.7As quantum wells is demonstrated to possess higher power conversion efficiency (PCE) and better temperature stability. The maximum PCE of 43.8% for 10-μm VCSEL is achieved at an ambient temperature of 30 °C. The size-dependent thermal characteristics are also analyzed by characterizing the spectral power and output power. It demonstrates that small oxide-aperture VCSELs are advantageous for temperature-stable performance.

The Vertical-Cavity Surface-Emitting Laser as a Sensing Device

We show that a change to the quality ( Q ) factor of a laser diode cavity affects the electrical properties of the device. The mechanism is demonstrated experimentally and numerically via a 980-nm InGaAs/GaAs oxide-aperture vertical-cavity surface-emitting laser (VCSEL) wherewith thin, sub-half-lambda-thick Si x N y layers are deposited on top of the VCSEL's top-coupling distributed Bragg reflector (DBR) mirror to modify the DBR's power reflectance. The presence of such a mechanism enables the detection of a substance that rests on or is in close proximity to the exposed DBR when the substance modifies the Q factor of the laser diode. As an example, we numerically analyze the influence of the change of the refractive index and absorption of a substance in the proximity of a 1651-nm AlInGaAs/InP VCSEL with a top mirror in the form of a monolithic subwavelength high-index contrast grating (MHCG). Such a design enables the detection of methane near the MHCG by changing the properties of the resonating light in the proximity of the grating. We show that this sensing mechanism enables the construction of a new compact optical sensing system without the need for a photodetector.

30 GHz bandwidth 850 nm VCSEL with sub‐100 fJ/bit energy dissipation at 25–50 Gbit/s

A high-speed and energy-efficient oxide-confined 850 nm vertical-cavity surface-emitting laser (VCSEL) for optical interconnects is presented. A record-high modulation bandwidth of 30 GHz is reached for a 3.5 mu m oxide aperture VCSEL, with 25 GHz bandwidth already at a bias current of 1.8 mA. The high bandwidth at low currents enables energy-efficient transmission with a dissipated heat energy in the VCSEL of <100 fJ/bit at 25, 40 and 50 Gbit/s.

Temperature-Dependent Impedance Characteristics of Temperature-Stable High-Speed 980-nm VCSELs

The temperature dependence of the impedance characteristics of high bit rate, highly temperature-stable 980-nm oxide-aperture vertical-cavity surface-emitting lasers (VCSELs) is investigated. A small-signal equivalent circuit model is fitted to measured small-signal reflection S11 and S21 scattering parameters across a large range of operating bias currents, temperatures, and oxide-aperture diameters to extract the circuit elements, −3 dB modulation bandwidth, relaxation resonance frequency, and parasitic cutoff frequency for each particular operating condition. The parasitic cutoff frequency of our small oxide-aperture diameter VCSELs is highly temperature insensitive from room temperature up to 85 °C, and does not limit our VCSELs’ maximum data transmission rate. Finally, the dependence of the impedance characteristics and modulation bandwidth of our VCSELs on the oxide-aperture diameter is analyzed at 25 °C and 85 °C. The larger capacitance is the main reason of our larger aperture VCSELs, compared with smaller aperture VCSELs, have a lower parasitic cutoff frequency.

Single-Mode Vertical Cavity Surface Emitting Laser via Oxide-Aperture-Engineering of Leakage of High-Order Transverse Modes

Properly designed oxide-confined vertical cavity surface emitting laser (VCSEL) allows leakage of the optical modes from the all-semiconductor core region to the selectively oxidized periphery if the orthogonality between the core mode and the modes on the periphery is broken by the oxidation-induced optical field redistribution. The leakage losses are stronger for high-order transverse modes, which have a higher field intensity close to the oxidized region. Single mode lasing in the fundamental mode can thus proceed up to large aperture diameters. The 850-nm GaAlAs thick oxide aperture VCSEL based on this concept is designed, modeled, and fabricated, showing single-mode lasing with the aperture diameters up to 5 μm. Side mode suppression ratio >20 dB is realized at the current density of ~10KA/Cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> in devices with the series resistance of 90 Ω.

High-speed operation of bow-tie-shaped oxide aperture VCSELs with photon–photon resonance

This paper presents experimental and modeling results for extending the 3-dB modulation bandwidth of a 980-nm quasi-single-mode (QSM) vertical-cavity surface-emitting laser (VCSEL) with a passive transverse-coupled cavity (TCC). While the bandwidth of a conventional VCSEL is 9 GHz, the 3-dB modulation bandwidth of a QSM TCC VCSEL with the same epi-wafer structure can reach 27 GHz, which is three times larger than the conventional VCSEL without optical feedback. A clear eye opening is obtained for large-signal modulations at 36 Gbps. A numerical simulation for further enhancement of the bandwidth is also conducted.

Thermal analysis of asymmetric intracavity-contacted oxide-aperture VCSELs for efficient heat dissipation

The asymmetric intracavity-contacted oxide-aperture vertical-cavity surface-emitting lasers (VCSELs), operating at λ ∼ 980 nm, with different oxide aperture diameters were fabricated and their thermal analysis was theoretically performed using a three-dimensional cylindrical heat dissipation model. The heat flux, temperature profile, and thermal resistance ( R th) of the devices were investigated by incorporating heat source values, obtained from experimentally measured results, into the thermal simulation. For the fabricated VCSELs with benzocyclobutene passivation layer, the R th decreased from 4612 K/W to 1130 K/W as the oxide aperture diameter ( D a) increased from 8 μm to 16 μm and it increased significantly below 8 μm. The use of the thin substrate and the passivation layer with a high conductivity enhances the heat dissipation, allowing for a low R th. Furthermore, thick Au layers on contact pads and top DBR in intracavity-contacted VCSEL structures help increase heat removal from the active region. For D a = 8 μm and 16 μm, the VCSELs with SiN x passivation layer, 5 μm thick extra Au layer, and 100 μm thick substrate indicate R th = 3050 K/W and 778 K/W, respectively, leading to an improvement by >30% compared to the fabricated devices.

Low thermal resistance, high-speed 980 nm asymmetric intracavity-contacted oxide-aperture VCSELs

We demonstrated high-speed characteristics of an oxide-aperture vertical-cavity surface-emitting laser (VCSEL) with intracavity structures for both p- and n-contacts, based on InGaAs/GaAs multiple quantum wells operating at λ ∼ 980 nm, indicating a low thermal resistance (Rth). The asymmetric current injection scheme is employed for reducing current crowding around the rim of the oxide aperture. A high aluminium content undoped Al0.88Ga0.12As and GaAs distributed Bragg reflector (DBR) mirror is used for efficient heat dissipation. The VCSEL with a 7 μm oxide aperture exhibited an output power of 2.5 mW and a threshold current of 0.8 mA with a slope efficiency of 0.39 mW/mA at 20 °C under continuous-wave operation and it still worked with 1.3 mW at 90 °C. The temperature tuning coefficient of 0.081 nm/°C and dissipated electrical power tuning coefficient of 0.104 nm/mW were observed, leading to a low Rth of 1.28 °C/mW. A high modulation bandwidth up to 13 GHz with a modulation current efficiency factor of 6.1 GHz/mA1/2 was achieved. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Structure-induced effects on the selective wet thermal oxidation of digital AlxGa1–xAs alloys

A thorough study of the selective wet oxidation in digital AlxGa1–xAs alloys is presented. We report experimental results and physical interpretation on the oxidation kinetics within those ranges of the AlGaAs composition (x = 0.95 to 1) and layer thickness (20 to 50 nm) of interest for oxide-aperture vertical-cavity surface-emitting laser (VCSEL) application. We demonstrate the high controllability of the oxidation reaction between different Al compositions; made different thanks to the use of digital alloys. Unlike standard alloys, we measured an invariability of the oxidation rates in the studied thickness range (20–50 nm), implying a better control of the fabrication process. The dependence of the reaction rate with the temperature is expressed as an Arrhenius law. Two activation energies (1.2 and 0.55 eV) have been derived for composition ranges of x = 0.95–0.98 and x = 0.99–1, respectively, revealing that two different mechanisms are involved depending on the Al content and the superlattice structure of the digitally-grown AlGaAs.

Full Recess Integration of Small Diameter Low Threshold VCSELs within Si-CMOS ICs

Oxide-aperture vertical-cavity surface-emitting lasers (VCSELs) have been integrated as individual device pills within the dielectric stacks of commercially produced silicon integrated circuits and monolithically connected electrically with the underlying circuitry using technology compatible with wafer-scale processing. The 55 microm diameter, 8 microm tall device pills were bonded in recesses etched to reveal buried contact/bond pads included in the IC layout; the surface was replanarized, contact vias formed, and interconnect metal deposited and patterned. The typical CW threshold current, 1 to 2.5 mA, was the same before and after integration, and integrated devices had thermal impedances similar to devices on their native GaAs substrates.

Coupled-mode structure in oxide aperture vertical-cavity surface-emitting lasers and its effects on lasing threshold

Mode coupling caused by the presence of an oxide dielectric aperture is addressed. For rays passing through the dielectric, we apply the complex reflection coefficient that corresponds to a Fabry–Perot subcavity formed by a slab dielectric and the nearest distributed Bragg reflector (DBR) mirror. The DBR reflectivity applies to rays passing through the aperture opening. The diffraction effects for curved wave-front incidence are also included. The expansion coefficients of the cavity eigenmodes into pure Gauss–Laguerre (GL) modes are obtained for the lowest eigenmodes in the optimum waist representation. The effects of the aperture location in the cavity standing wave are addressed. Over the entire range of aperture diameters, higher cavity losses and higher threshold currents result for antinode placement, owing to wide-angle scattering. This agrees with experimental results and earlier scattering analysis by use of pure GL (uncoupled) eigenmodes. Mixing with higher modes increases round-trip losses at small apertures, compared with uncoupled-mode results. In the limit of a large aperture diameter, the cavity eigenmodes decouple to pure GL modes.

Quasi-Continuous Operation of 1.55- μm Vertical-Cavity Surface-Emitting Lasers by Wafer Fusion

Room temperature quasi-continuous operation is achieved near 1556 nm with threshold current as low as 2.2 mA from a 5.6- ${\mu}{\textrm}{m}$ oxide-aperture vertical-cavity surface-emitting laser. Wafer fusion techniques are employed to combine the GaAs/AlGaAs mirror and the InP-based InGaAs/InGaAsP active layer. In this structure, an $Al_x/O_y$ /GaAs distributed bragg reflector and intra-cavity contacts are used to reduce free carrier absorption.

Design of low-loss single-mode vertical-cavity surface-emitting lasers

An in depth study of aperture placement relative to the electric field standing wave of oxide aperture vertical-cavity surface-emitting lasers (VCSELs) is presented. VCSELs with oxide apertures placed at a node and at an antinode are studied for their dependence of internal loss, far field, threshold current, and efficiency on the position of a thin AlO/sub x/ current aperture relative to the longitudinal standing wave in the cavity.

Lasing characteristics of InGaAs-GaAs polarization controlled vertical-cavity surface-emitting laser grown on GaAs [311] B substrate

We have demonstrated an oxide confinement polarization controlled vertical-cavity surface-emitting laser (VCSEL) grown on a GaAs [311]B substrate. The polarization state was well controlled along the [2~33] crystal direction due to an anisotropic gain in the [311]B plane. We fabricated a small oxide aperture VCSEL with a threshold of 260 /spl mu/A and realized single-transverse mode and single-polarization operation for the first time. The sidemode suppression ratio (SMSR) was 35 dB and the orthogonal polarization suppression ratio (OPSR) was 25 dB. In addition, we have measured polarization and transverse mode characteristics of multi- and single-transverse mode devices under high-speed modulation. In the multimode device of 12 /spl mu/m/spl times/12 pm oxide aperture, we have achieved stable polarization operation of over 25-dB OPSR up to 10 Gb/s and have observed no power penalty due to polarization instability under 2.5-Gb/s pseudorandom modulation. The single-mode device showed stable single-transverse mode and polarization under the modulation conduction up to 5 GHz of sinusoidal modulation. SMSR and OPSR were over 30 and 10 dB, respectively.

Polarisation anisotropy in asymmetric oxide aperture VCSELs

Strong polarisation discrimination is observed from asymmetric oxide-aperture vertical cavity surface emitting lasers (VCSELs). The polarisation state perpendicular to the longer direction of the slit-shaped oxide aperture dominates when the aperture width is &lt; 4 µm. Generally, the mode size for TM polarisation is slightly less than that for TE polarisation, indicating polarisation-dependent scattering losses. A 1.4 µm oxide aperture VCSEL gives a polarisation extinction ratio (PER) of up to 184 .

Very small oxide-confined vertical-cavity surface-emitting lasers with a bulk active region

Data are presented characterizing very small oxide aperture vertical-cavity surface-emitting lasers with a 0.5-μm-thick bulk GaAs active region. The transverse far field is independent of the aperture size for diameters ⩽3 μm, although threshold reduction occurs with reducing aperture size down to ∼0.5 μm diam. Threshold reduction is attributed to improved overlap between the optical mode and gain profile. The large signal temporal response is characterized using optical gain switching.