Progress in MHCG VCSELs (Conference Presentation)

In this paper, the design and preparation of polymer based monolithic high-contrast grating (MHCG) structures and their Talbot self-imaging effect are presented. For the preparation of MHCG structures to IP-Dip negative photoresist material, 3D laser lithography was used. The shape and morphological properties of the prepared MHCG structure were investigated by scanning electron microscope (SEM). Finally, the highly resolved near-field scanning optical microscope (NSOM) for the vertical optical field distribution over the grating structure was used. The demonstration of the Talbot effect for different wavelengths in a diffraction and a subwavelength regime is presented theoretically and experimentally from NSOM measurements.

The key component of a quantum cascade vertical cavity surface emitting laser (QC VCSEL) is a monolithic high-contrast grating (MHCG) that replaces one of the distributed Bragg mirrors (DBR). The grating induces the polarization component necessary to stimulated emission in quantum cascade active regions embedded in MHCG. The complex electrical and optical phenomena defining the performance of the structure depends on the grating parameters (stripes dimensions, position, thickness and doping concentrations). This work presents optimization of QC VCSELs that is aimed to achieve minimal thresholds currents in the pulse operation regime.

We present results of computer simulations of vertical cavity surface emitting lasers (VCSELs) using novel, highreflectivity monolithic high refractive-index contrast grating (MHCG) mirrors and their more advanced version, partially covered by a thin metal layer - metallic MHCG (mMHCG) mirrors. The first experimental realization of this new class of mirrors is presented and discussed. We show that the metal layer does not deteriorate the high reflectivity of an mMHCG mirror, but in contrary, is a crucial element which allows high reflectivity and additionally opens a way for a more efficient electrical pumping of a VCSEL. Comparison of results of thermal-electrical-carrier-gain self-consistent simulations of both MHCG- and mMHCG-based VCSELs is presented and discussed. It is shown that using mHCG mirror as a top mirror of a VCSEL improves electrical characteristics and greatly decreases the differential resistance of the device.

This paper presents results of computer simulation of 1D monolithic high refractive index contrast grating (MHCG) reflector also called surface grating reflector (SGR). We analyzed optical properties of the GaAs reflector designed for 980 nm wavelength with respect to the grating parameters variation. We also determined the electric field patterns after reflection from the structure in several cases of parameters variation. We show that thanks to the scalability and design simplicity, proposed design is a promising candidate for simple, next generation vertical cavity surface emitting lasers emitting from ultra-violet to infrared.

Diode laser parameters such as emission wavelength, threshold current and optical output power, sensitively depend on the junction temperature of the laser diode. The temperature in the active layer results from the loss processes inside the laser structure as well as from the thermal characteristics of the semiconductor material and the heat sink configuration. In this paper, we present numerical model calculations employing a two dimensional finite element method in order to study the transient temperature behavior of high power laser arrays in a time range between 10 ns and cw-operation. Furthermore, we investigated the influence of different heat sinks on the thermal performance of whole device. Numerical and experimental results are compared.

We present the firstexperimental demonstration of a planar focusingmonolithic subwavelength grating mirror. The grating is formed onthe surface of GaAs and focuses 980 nm light in one dimension on thehigh-refractive-index side of the mirror. According to our measurements,the focal length is 475 μm (300 μm of which is GaAs) andthe numerical aperture is 0.52. The intensity of the light at thefocal point is 23 times larger than that of the incident light. Tothe best of our knowledge, this is the highest value reported fora grating mirror. Moreover, the full width at half-maximum (FWHM)at the focal point is only 3.9 μm, which is the smallest reportedvalue for a grating mirror. All of the measured parameters are closeto or very close to the theoretically predicted values. Our realizationof a sophisticated design of a focusing monolithic subwavelength gratingopens a new avenue to technologically simple fabrication of the gratingsfor use in diverse optoelectronic materials and applications.

In this article, a record-high single-mode output power from a multi-junction vertical-cavity surface emitting laser (VCSEL) emitting at 905 nm is achieved. Single spectral mode emission of the VCSEL is realized by using an anti-phase mode filter, placed at the top surface of the device, that acts as a relief layer to suppress the higher order transverse modes. Placement of such anti-phase layer at the surface areas in which the higher order modes are expected to emit, leads to reduction in the effective mirror reflectivity at those areas, increased optical losses, and consequently, increased threshold gain for the higher order modes compared to the fundamental mode. The room-temperature light-current-voltage characteristics of a 6-junction 905 nm VCSEL with an oxide aperture size of ∼8 μm and mode filter size of 5 μm shows a single-mode optical power of 12.55 mW and maximum power conversion efficiency of 56.48%. Moreover, the emission spectrum of this device shows a stable side-mode suppression ration of >30 dB in the single-mode operation regime. The VCSEL remains reliably single-mode at higher temperatures, generating a maximum optical output power of 9.48 mW at 60 °C. Such remarkable single-mode performance can be mainly attributed to the multi-junction epitaxial structure of the device, in which the output optical power is expected to proportionally increase according to the number of bipolar cascade active regions.

This study investigates the thermal design of 2-D vertical-cavity surface-emitting laser (VCSEL) arrays for optical output power improvement. Considering the temperature dependencies of bias current, internal quantum efficiency, internal optical loss, and thermal resistance of each array cell, a compact electro-opto-thermal model of 2-D VCSEL arrays with linear power dissipation and quadratic power dissipation in each array cell is established to study the effects of array cell spacing and layout on the device performance. At the same time, the power dissipation mechanisms contributing to thermal rollover are also studied. It is found that the array cell spacing has a strong influence on the linear power dissipation, and the layout (such as hexagonal design) influences quadratic power dissipation obviously. With the careful design of array cell spacing and layout, both the temperature profile and the thermal resistance matrix of VCSEL arrays are improved, which is helpful to delay the onset of thermal rollover and hence enhance the optical output power. The simulation results are consistent with experimental results.

P-type AlGaN/GaN/InGaN superlattices were incorporated in a InGaN based blue light emitting diode as electron blocking layer to minimize the temperature dependence on optical output power. For the characteristic temperatures in range of 10 to 100 °C and at operation current of 350 mA, the external quantum efficiency varied by less than 0.5%. For the presented device, the negative characteristic temperature was shown to occur below temperature of 50 °C. The improved temperature stability in optical output power is thought to be attributed to (1) the efficiency of hole carrier transport in AlGaN/GaN/InGaN superlattices and (2) the enhanced blocking of electron overflow between multiple quantum wells and AlGaN/GaN/InGaN superlattices.

A high power ultraviolet (UV) light emitting diode (LED) multi-chip module package based on aluminum nitride (AlN) and alumina (Al2O3) is presented. The AlN substrate with a high thermal conductivity of up to 180 W/(m· K) and LED chips based on a copper alloy provide superb thermal management and heat extraction. Efficient cooling is an important prerequisite for the increase of extractable optical power and decrease of thermally induced wavelength shift. A design of a stackable module featuring arrays of 7×2 indium-gallium-aluminum- nitride UV LED chips at 395 nm is developed. This configuration of sub-modules allows for the scalable assembly of line sources with different lengths. Applications using UV LEDs cover market segments such as curing of adhesives, inks and coatings, sterilization of medical equipment and treatment of potable water, as well as various uses in chemical detection, biochemical analytics and spectroscopy. Thermal and thermo-mechanical modelling of the sub-mount is conducted using finite elements analysis. Die attach using eutectic gold-tin solder, lower melting tin-lead solder and silver-filled adhesive are compared with respect to optical output power and wavelength drift. Mechanical strength and structure of the resulting joints are investigated using shear force measurements, cross-sectioning and micro-tomography. An optical output power of 7.7 W is achieved using a cluster of 14 LED chips at 1050 mA resulting in a peak irradiance of 30.8 W/cm2 at the LED surface with respect to the footprint and pitch of the attached chips.

Effects of the number of wells on the characteristics of a 1.3-µ m edge-emitting light-emitting diode (LED) are investigated. The edge-emitting LED with 10 quantum wells had the highest optical output power at 100 mA and the lowest temperature coefficient of the optical output power in the temperature range from 10 to 75°C. The experimental result shows that the multiple quantum well structure improves the thermal stability of the optical output power.

This paper presents data on wafer-fused 1550-nm vertical-cavity surface-emitting lasers (VCSELs) based on the active region and distributed Bragg reflectors (DBRs) grown by molecular beam epitaxy. VCSELs with a tunnel junction aperture diameter of 8 μm show lasing at a threshold current density j th <; 3 kA/cm 2 , an output optical power of ~4 mW, and a -3 dB bandwidth of approximately 7 GHz at a 10-mA bias current. The devices demonstrate single-mode continuous wave operation with the transverse side-mode suppression ratio (SMSR) varying in the range of 40-45 dB up to roll-over currents. The increase in mirror losses due to the etching of the top DBR makes the output optical power increase to 6 mW and causes the wallplug efficiency value to reach 20%, but SMSR remains in the range of 40-45 dB. This also makes it possible to reduce both the photon lifetime and, as a result, the effect of damping and increase the modulation bandwidth to 9 GHz. The observed open and clear eye diagrams indicate that non-return-to-zero operation is possible at bit rates of up to 30 Gbps without equalization or forward error correction. The high-output optical power and modulation performance pave the way for the dense wavelength division multiplexing application of wafer-fused 1550-nm VCSELs.

Stimulated Brillouin scattering (SBS) is the major factor that limits the maximum optical fiber output power in narrow linewidth applications, which include important fields such as passive optical networks (PONs), high-power fiber amplifiers, and lasers. Great efforts have been dedicated to suppressing the SBS effect and increasing the maximum optical fiber output power. This paper focuses on key fiber-based techniques to suppress SBS. These techniques take advantages of the properties of optical fibers. We present how these properties (electric modes, acoustic modes, and material properties) could be utilized to suppress SBS. The fiber-based techniques are divided into transverse optical fiber design, longitudinal variant fiber design, and external perturbations (strain and temperature) on optical fibers. Transverse optical fiber design focuses on the mechanism electro-acoustic interaction. Large effective area fiber design and acoustic tailoring techniques have been discussed. Longitudinal variant fiber design considers the nonlinear SBS interaction along propagation distance, and various techniques related have been presented. External perturbations (strain and temperature) on optical fibers emphasize on how external static perturbations could modify the SBS effect.

Fiber lasers and amplifiers are capable of producing kilowatts of optical output power with diffraction-limited beam quality. We present high precision high power fiber arrays which can be used for coherent and spectral beam combining for scaling of fiber laser systems to high brightness and high power. The availability of CO2 laser-based fiber splicing systems that can control the position and size of the heating zone has opened up new possibilities in the splicing of multiple fibers (fiber arrays) to large optical elements. In our experiments we realized linear fiber arrays with PM and non-PM large mode area (LMA) fiber with no gap between the individual fiber channels (DC-PM-20/400 and DC-PM-25/250). The fibers were spliced to a large AR-coated fiber end cap. The misalignment of the PM axes of all fiber channels was less than ±1 degree and the inline tolerance of the fiber array was below ±2.5µm. Each fiber channel was tested up to an optical output power of 1kW.

Though micro‐LED (light‐emitting diode) displays are considered the emerging display technology, the micron‐scale LED chip size encounters severe efficiency degradation that may impact the power budget of the displays. This work proposes an ion implantation method to deliberately create a high‐resistivity sidewall in the InGaN/GaN green LED. Our study demonstrates that ion implantation suppresses reverse leakage current due to the mitigation of sidewall defects. For an LED mesa size of 10 × 10 μm2, optical output power density is improved by 36.2% compared to the device without implantation. Compared to a larger 100 × 100 μm2 device without implantation, we achieve only 21.3% degradation of output power density under 10 A/cm2 injection for a 10 × 10 μm2 LED with implantation. In addition, the ion implantation method can lower the wavelength shift by reducing light emission from the region near the sidewall (where the amount of Indium [In] clustering differs from the mesa region). The results show promise in addressing the efficiency challenges of micro‐LED displays by selective ion implantation.