Bottom-emitting Vertical-cavity Surface-emitting Lasers Research Articles

Enabling VCSEL-on-silicon nitride photonic integrated circuits with micro-transfer-printing

New wavelength domains have become accessible for photonic integrated circuits (PICs) with the development of silicon nitride PICs. In particular, the visible and near-infrared wavelength range is of interest for a range of sensing and communication applications. The integration of energy-efficient III-V lasers, such as vertical-cavity surface-emitting lasers (VCSELs), is important for expanding the application portfolio of such PICs. However, most of the demonstrated integration approaches are not easily scalable towards low-cost and large-volume production. In this work, we demonstrate the micro-transfer-printing of bottom-emitting VCSELs on silicon nitride PICs as a path to achieve this. The demonstrated 850 nm lasers show waveguide-coupled powers exceeding 100 µW, with sub-mA lasing thresholds and mW-level power consumption. A single-mode laser with a side-mode suppression ratio over 45 dB and a tuning range of 5 nm is demonstrated. Combining micro-transfer-printing integration with the extended-cavity VCSEL design developed in this work provides the silicon nitride PIC industry with a great tool to integrate energy-efficient VCSELs onto silicon nitride PICs.

Fabrication and performance of bottom-emitting flip-chip bonded 980 nm vertical-cavity lasers with copper- and indium-plated heat-sinks

Bottom-emitting, flip-chip bonded, oxide-confined, 980 nm vertical-cavity surface-emitting lasers (VCSELs) with improved performance were fabricated and characterized. Flip-chip bonding, bottom-emitting VCSELs with 20 µm active area diameter on copper- and indium-plated heat spreaders reduced the devices’ thermal resistance by 35% and increased the bias current density at which the output optical power saturates by 19%, which increased the maximum output optical power achieved by 26%. Bonded VCSELs with active area diameters of 20 µm demonstrated an ∼30% decrease in the active region temperature at the maximum output optical power. A 10% improvement in the devices’ modulation bandwidth is calculated due to reduced temperature and increased power. Devices with different active area diameters exhibited a threshold current density and differential quantum efficiency of (0.88 ± 10%) kA cm−2 and 0.57 ± 8%, respectively, implying good active area scaling. A simple rate-equation-based thermal VCSEL model was used to extract different VCSELs’ parameters, which enabled the estimation of internal temperature for different device sizes at different bias currents. A genetic algorithm was utilized in model parameter extraction and optimization. Good agreement between measured and simulated characteristics for different device sizes was obtained. Detailed protocols for critical fabrication steps are presented.

HIGH POWER AND HIGN BEAM QUALITY 980nm BOTTOM-EMITTING VERTICAL-CAVITY SURFACE-EMITTING LASER

A high-power and high beam quality 980nm bottom-emitting vertical-cavity surface-emitting laser (VCSEL) with optimized p-contact aperture was reported. A numerical simulation of current density in a large aperture bottom-emitting VCSEL with oxidation between the active region and the top p-type mirror was conducted. It is found that the simulated current density profiles of VCSEL are dependent on the oxide aperture diameter and the p-contact diameter. For a fixed oxide aperture diameter, the homogeneous current density of the VCSEL could he realized by optimizing the p-contact diameter. Thus, the edge mode in far-field patterns was suppressed. The far-field divergence angle from a 600 pm diameter VCSEL was suppressed from more than 30 to 15 and no strong sidelobe was observed in far-field pattern when the p-contact diameter decreased from 650 mu m to 580 mu m. There is a slight rise both in threshold current and optical output power due to the p-contact optimization. The VCSEL device produces the maximum optical output power of 2.01 W with lasing wavelength of 982.6nm by improving the device packaging method.

High-Pulsed-Power (49W) Vertical-Cavity Surface-Emitting Laser with Five Quantum Wells by Uniform Current Injection into Large Emitting Area

We report on the high-power lasing characteristics of a large area bottom-emitting vertical-cavity surface-emitting laser (VCSEL). There have been difficulties in uniform current injection for large area VCSELs, which are caused by the band discontinuity at the interface between AlAs and GaAs of the n-type Distributed Bragg Reflector (DBR). We have reported the n-type DBR using a graded composition interface of 20nm thick suppresses the crowding of current to the edge of emitting area and improves external efficiency [1]. The highest peak pulsed power of over 49W was achieved by a five-quantum-well VCSEL with a current aperture diameter of 200µm.

Coherent polarization stabilization in large-aperture rectangular post bottom-emitting vertical-cavity surface-emitting lasers

The output characteristics of large-aperture rectangular post bottom-emitting vertical-cavity surface-emitting lasers (VCSELs) were investigated. It was shown that the output power of the rectangle VCSELs can be up to 660mW at a current of 5A. Both H-polarization (horizontal) and V-polarization (vertical) demonstrated a coherent stabilization over the entire range of operation current, and coherent spectrum blue-shift of H-polarization light occurred with respect to V-polarization light at three different injected currents. The polarization states of output light were stabilized in the two orthogonal directions and H-polarization was the most principal polarization which was parallel to the longer side of the rectangular aperture. From the relationship between polarization ratio and aspect ratio of the oxidation confinement aperture (OCA), it was found that the highest polarization ratio (about 2:1) took place when the appropriate aspect ratio was 5:3, which meant better polarization stabilization in large-aperture VCSELs.

High power and good beam quality of two-dimensional VCSEL array with integrated GaAs microlens array

High power and good beam quality of two-dimensional bottom-emitting vertical-cavity surface-emitting laser array with GaAs microlens on the substrate is achieved. Uniform and matched convex microlens is directly fabricated by one-step diffusion-limited wet-etching techniques on the emitting windows. The maximum output power is above 1 W at continuous-wave operation at room temperature, and the far-field beam divergence is below 6.6° at a current of 4 A. These properties between microlens-integrated and conventional device at different operating current are demonstrated.

High-power vertical-cavity surface-emitting laser with an optimized p-contact diameter

A 980 nm bottom-emitting vertical-cavity surface-emitting laser (VCSEL) with a p-contact diameter is reported to achieve high power and good beam quality. A numerical simulation is conducted on the current spreading in a VCSEL with oxidation between the active region and the p-type distributed Bragg reflector. It is found that, for a particular oxide aperture diameter, somewhat homogeneous current distribution can be achieved for a VCSEL with an optimized p-contact diameter. The far-field divergence angle from a 600 microm diameter VCSEL is suppressed from 30 degrees to 15 degrees, and no strong sidelobe is observed in the far-field pattern by using the optimized p-contact diameter. There is a slight rise in threshold and optical output power that is due to the p-contact optimization. By improving the device packaging method, the maximum optical output power of the device is 2.01 W.

High power nonuniform linear vertical-cavity surface-emitting laser array with a Gaussian far-field distribution

A 980 nm bottom-emitting vertical-cavity surface-emitting laser (VCSEL) array with a nonuniform linear arrangement is reported to realize high power with a Gaussian far-field distribution. This array is composed of five symmetrically-arranged elements of 200 μm, 150 μm, and 100 μm diameters, with the center spacing of 300 μm and 250 μm respectively. This structure makes it possible to discriminate against the higher order array supermodes. The theoretical simulation of the far-field distribution is in good agreement with the experimental result. An output power of 880 mW with a power density of 1 KW/cm 2 is obtained. The divergence angle is below 20° in the range of operating current from 0 A to 4 A. The comparison between this nonuniform linear array and the conventional two-dimensional array is carried out to demonstrate the good performance of the linear array. A peak power of over 20 W is achieved under a short pulsed operation with a repetition frequency of 1 kHz.

A linear array of 980 nm VCSEL and its high temperature operation characteristics

A 980 nm bottom-emitting vertical-cavity surface-emitting laser linear array with high power density and a good beam property of Gaussian far-field distribution is reported. This array is composed of five linearly arranged elements with a 200 μm diameter one at the center, the other two 150 μm and 100 μm diameter ones at both sides of the center with center to center spacing of 300 μm and 250 μm, respectively. A power of 880 mW at a current of 4 A and a corresponding power density of up to 1 kW/cm2 is obtained. The temperature dependent characteristics of the linear array are investigated. The thermal interaction between the individual elements of the VCSEL linear array is smaller due to its optimized element size and device spacing, which make it more suitable for high power applications. A peak power of over 20 W has been achieved in pulsed operation with a 60 ns pulse length and a repetition frequency of 1 kHz.

Design and characterization of a nonuniform linear vertical-cavity surface-emitting laser array with a Gaussian far-field distribution

A 980 nm bottom-emitting vertical-cavity surface-emitting laser array with a nonuniform linear arrangement is reported to realize emission with a Gaussian far-field distribution. This array is composed of five symmetrically arranged elements of 200 microm, 150 microm, and 100 microm diameter, with center spacing of 300 microm and 250 microm, respectively. An output power of 880 mW with a high power density of 1 kW/cm2 is obtained. The divergence angle is below 20 degrees in the range of operating current from 0 A to 6 A. The theoretical simulation of the near-field and the far-field distribution is in good agreement with the experimental result. The comparison between this nonuniform linear array, the single device, and the conventional two-dimensional array is carried out to demonstrate the good performance of the linear array.

High-power InGaAs VCSEL's single devices and 2-D arrays

Single devices and 2-D arrays of bottom-emitting vertical-cavity surface-emitting lasers operating in the 980 nm wavelength regime, have been fabricated for high continuous-wave optical output power. Single devices with active diameters of 500 μm show high output powers of 1.95 W at room temperature. Its threshold current is 510 mA, and the maximum spatially averaged optical power density is 0.93 kW/cm 2. Arrays consisting of 16 elements of 200 μm active diameters arranged in a square structure achieve output powers of 1.21 W corresponding to a power density of 1 kW/cm 2 spatially averaged over the effective array chip size. The device threshold current is 1.1 A.

Large aperture VCSELs with a continuous-wave output power of 1.95 W

In this study, 980 nm high-power bottom-emitting vertical-cavity surface-emitting lasers (VCSELs) with large emission aperture are described. The devices have been fabricated by using oxidation confinement technology. Al 2O 3 film, instead of SiN x film, is used as the passivation layer to enhance heat dissipation. A distinguished device performance is achieved. The maximum continuous-wave (CW) output power of the large emission aperture devices with active diameters up to 500 μm is as high as 1.95 W at room temperature.

Integration of microdiffractive lens with continuous relief with vertical-cavity surface-emitting lasers using focused ion beam direct milling

A novel one-step integration of bottom-emitting vertical-cavity surface-emitting laser (VCSEL) (operation wavelength of 980 nm) with microdiffractive lens by means of focused ion beam (FIB) technology is described. A diffractive lens with continuous relief, diameter of 140 /spl mu/m, and six annulus was designed and fabricated using FIB direct milling on the backside of VCSEL with GaAs substrate for beam collimation. The divergence angle (half angle) of the VCSEL was reduced from 12/spl deg/ before integration of the VCSEL/diffractive optical elements (DOEs) to 0.6/spl deg/ after FIB integration, allowing for an interconnect length of /spl sim/4 mm. It was proven by device testing that there is little influence upon the VCSEL performance after FIB processing.

High performance selectively oxidized VCSELs and arrays for parallel high-speed optical interconnects

High-bandwidth single-mode selectively oxidized vertical-cavity surface-emitting laser (VCSEL) arrays operate at 980 nm or 850 nm emission wavelength for substrate or epitaxial side emission. Coplanar feeding lines and polyimide passivation are used to reduce electrical parasitics in top-emitting GaAs and bottom-emitting InGaAs VCSELs. To enhance fundamental single-mode emission for larger devices of reduced series resistance a surface relief transverse mode filter is employed. Fabricated VCSELs are applied in various interconnect schemes. InGaAs quantum-well based VCSELs at 935 nm emission wavelength are investigated for use in perfluorinated graded-index plastic-optical fiber (GI-POF) links. We obtain a 7 Gb/s pseudo random bit sequence (PRBS) nonreturn-to-zero (NRZ) data transmission over 80 m long 155 /spl mu/m diameter GI-POF. We investigate data transmission over standard 1300 nm, 9 /spl mu/m core diameter single-mode fiber with selectively oxidized single-mode GaAs and InGaAs VCSELs. We achieve biased 3 Gb/s and bias-free 1 Gb/s pseudo-random data transmission over 4.3 km at 830 nm emission wavelength where a simple fiber mode filter is used to suppress intermodal dispersion caused by the second order fiber mode. For the first time, we demonstrate 12.5 Gb/s data rate transmission of PRBS signals over 100 m graded-index multimode fiber or 1 km single-mode fiber using high performance single-mode GaAs VCSELs of 12.3 GHz modulation bandwidth emitting at /spl lambda/=850 nm.

RF crosstalk in multiple-wavelength vertical-cavity surface-emitting laser arrays

We present the crosstalk measured in multiple-wavelength vertical-cavity surface-emitting laser (VCSEL) arrays under RF modulation. The array consists of eight bottom-emitting VCSEL's arranged with a "pie"-like configuration within a 60-μm-diameter circle with a 3-μm spacing for coupling into a multimode fiber. The crosstalk is investigated by measuring a cross-modulation response of two VCSEL's in the array using a monochromator in an optical spectrum analyzer as a receiver. Excess crosstalk is minimized by flip-chip mounting the array and introducing a proper bias condition. The crosstalk observed In the adjacent VCSEL's is less than -40 dB at the modulation frequency below 700 MHz and it Increases at 20 dB/decade above this frequency when both VCSEL's are biased at 5.0 mA.

Spatial mode structure of bottom-emitting broad-area vertical-cavity surface-emitting lasers

The spatial mode structure of circular broad-area bottom-emitting vertical-cavity surface-emitting lasers is investigated close to threshold. The devices display very high-order flower-like Laguerre modes as well as structures with a Cartesian symmetry in spite of the original O(2) symmetry. In some cases the latter resemble tilted waves more than Hermite-Gaussian modes. Spatial structures appear for both sides of the detuning between the gain peak and the cavity resonance. The results indicate the necessity to refine the theoretical models to include realistic boundary conditions and inhomogeneities.

High-performance wafer-bonded bottom-emitting 850-nm VCSEL's on undoped GaP and sapphire substrates

We have demonstrated high performance wafer-bonded bottom-emitting 850-nm vertical-cavity surface-emitting lasers (VCSEL's) on transparent substrates. The free-carrier absorption of the substrate was avoided by using undoped GaP or sapphire substrates. The maximum external quantum efficiency approaches 48% while the threshold current remains as low as 550 /spl mu/A for the 6/spl times/6 /spl mu/m/sup 2/ VCSEL's bonded on GaP substrates. VCSEL's with 8.6/spl times/8.6 /spl mu/m/sup 2/ aperture bonded on sapphire substrates also exhibit threshold currents of 800 /spl mu/A and external quantum efficiencies of 33.2%. The difference in efficiency between these two devices results from the change of the refractive index of the exit medium.

Vertical cavity surface emitting lasers incorporating structured mirrors patterned by electron-beam lithography

We demonstrate the operation of bottom-emitting vertical cavity surface-emitting lasers (VCSELs) with linear mode polarization which is controlled in an arbitrary orientation by the use of patterned metallic mirrors on top of the VCSEL surface. The top mirror is made in the shape of a 200 nm pitch grating, composed of alternating high reflectivity (Au) and low reflectivity (Cr) metal lines, whose orientation determines the polarization of the laser mode. The gratings were fabricated by high-resolution (<50 nm) electron-beam lithography and lift-off technique, and were aligned with the other parts of the VCSEL structure (top electrode, ion-implanted zone) fabricated by conventional photolithography. Various types of mirror shapes and sizes were fabricated, including square and circular grating envelopes, as well as circular mirrors with an average (radial) Gaussian reflectivity.

Wafer-bonded bottom-emitting 850-nm VCSEL's on GaP substrates

Bottom-emitting 850-nm vertical-cavity surface-emitting lasers were fabricated using wafer bonding technology to replace the absorbing GaAs substrates with transparent GaP substrates. Ohmic-like p-type GaAs-GaP bonded interfaces were obtained with proper bonding condition. The devices with 4/spl times/4 /spl mu/m/sup 2/ current aperture exhibit 24% external quantum efficiency, and threshold current as low as 400 /spl mu/A. The threshold voltages range from 1.71 to 1.8 V for different aperture size devices.

Monolithic integration of vertical-cavity surface-emitting laser and diffractive optical element for advanced beam shaping

We have demonstrated a vertical-cavity surface-emitting laser (VCSEL) with a monolithically integrated diffractive optical element (DOE) for advanced beam shaping. The DOE is a two-level surface relief, etched into the GaAs substrate beneath a bottom-emitting VCSEL. The combination generates a 4/spl times/4 array of focused spots 10 mm from the substrate with spot sizes down to less than 400 /spl mu/m, almost at the diffraction limit. Diffraction efficiencies of 29% and uniformity errors of 14% were measured.