Vertical-cavity Surface-emitting Laser Arrays Research Articles

Long-wavelength (λ=1.55 µm) monolithic VCSEL array with >3 W CW output power

Monolithically integrated long-wavelength vertical-cavity surface-emitting high-power laser arrays emit continuous wave (CW) optical output power in excess of 3 W at 1.55 µm. Regardless of the wavelength, this is the highest CW output power reported from a monolithic VCSEL structure to date.

Parallel self-mixing imaging system based on an array of vertical-cavity surface-emitting lasers

In this paper we investigate the feasibility of a massively parallel self-mixing imaging system based on an array of vertical-cavity surface-emitting lasers (VCSELs) to measure surface profiles of displacement, distance, velocity, and liquid flow rate. The concept of the system is demonstrated using a prototype to measure the velocity at different radial points on a rotating disk, and the velocity profile of diluted milk in a custom built diverging-converging planar flow channel. It is envisaged that a scaled up version of the parallel self-mixing imaging system will enable real-time surface profiling, vibrometry, and flowmetry.

Arrayed Multimode Fiber to VCSEL Coupling for Short Reach Communications Using Hybrid Polymer-Fiber Lens

We report the novel method that can give high coupling efficiency between a vertical-cavity surface-emitting laser (VCSEL) array and a multimode optical fiber ribbon by using a hybrid-polymer-fiber lens technique. The coupling efficiency as high as 91% was achieved by optimizing the polymer lens curvature for the single source to the single fiber case. The longitudinal and the transverse coupling tolerances were enhanced with the introduction of a coreless silica fiber (CSF) segment between the fiber and the lens tip. The tradeoff between the high coupling efficiency and the large tolerance was experimentally observed. The technique was further applied to the coupling between a four-channel VCSEL array and a four-core fiber ribbon for short reach applications. The procedure of fabrication and the analysis of their optical characteristics are reported.

Small-Pitch Flip-Chip-Bonded VCSEL Arrays Enabling Transmitter Redundancy and Monitoring in 2-D 10-Gbit/s Space-Parallel Fiber Transmission

We demonstrate novel pixel architectures in 2-D vertical-cavity surface-emitting laser (VCSEL) arrays offering additional functionality without sacrificing efficient fabrication, compactness, and low module cost. Very high flip chip VCSEL packing densities enable both a built-in 3-per-channel VCSEL redundancy as well as simple intracell VCSEL monitoring. Each pixel has three individually addressable oxide-confined and substrate- removed 850-nm-wavelength VCSELs directly flip bonded to the mesas that are extremely close-spaced to permit equal butt coupling to 50-mum-core-diameter multimode fibers (MMFs) without the need for external coupling optics. Built-in radial VCSEL-to- fiber launch offsets as small as 7.8 mum were reached, leading to offset coupling penalties of only 0.4 dB. Quasi-error-free transmission of 10 Gbit/s signals is achieved over 500-m-long MMFs even under offset launch conditions. New opportunities for a range of further applications offered by high-density VCSEL arrays are discussed.

Photonic Crystal Heterostructures Based on Vertical-Cavity Surface-Emitting Laser Arrays

The design and analysis of phase-coupled arrays of vertical-cavity surface-emitting lasers (VCSELs) can greatly profit from concepts related to photonic crystals (PhCs). VCSEL-arrays can be modeled as PhCs in which the refractive index varies periodically in the plane normal to the beam propagation direction. The relatively simple implementation of these structures via lithography techniques permits the exploration of complex PhC configurations and the realization of novel spatial-mode-controlled VCSEL array structures. We elaborate here the concept of VCSEL-based PhC heterostructures that permit the control of photonic envelope functions in novel and useful ways. In particular, we discuss envelope function confinement, coupling and switching. Several such heterostructures, implemented using VCSEL arrays employing Bragg mirror patterning, are demonstrated and investigated experimentally.

Coherence of Photonic Crystal Vertical-Cavity Surface-Emitting Laser Arrays

We measure and compare the coherence properties of 2 times 1 arrays of photonic crystal vertical-cavity surface-emitting lasers. Antenna array theory applied to the measured far field intensity patterns is used to determine the phase of the complex degree of coherence, which is found to vary with current injection. The amplitude of the complex degree of coherence is determined by calculating the visibility from the far field patterns and making near field measurements of the relative intensities between lasing defects. We find that the amplitude and phase of the complex degree of coherence are correlated, such that coherence is maximized near in-phase and out-of-phase coupling conditions, and controllable by independent current injection to each array element

High-Speed 985 nm Bottom-Emitting VCSEL Arrays for Chip-to-Chip Parallel Optical Interconnects

For chip-scale interconnection, 4 times 12 vertical-cavity surface-emitting laser (VCSEL) arrays have been optimized. Each flip-chip bondable bottom-emitting oxide-confined 985 nm VCSEL have integrated backside lenses, and is capable of modulation at >20 Gb/s with a low current density of only 9.9 kA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . An aggregate data rate of 960 Gb/s was obtained from a chip area of only 1.4 mm times 3.75 mm, or 18.3 Tb/(sldrcm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ).

High-Speed Subcarrier Based on High-Resistivity Silicon for Long-Wavelength VCSEL Array

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> We propose a chip-to-chip optical interconnection using a silicon (Si) subcarrier integrated with a long-wavelength vertical-cavity surface-emitting laser (LW-VCSEL) array. As the basic study of that, we examined the microwave and optical characteristics of the Si subcarrier. Finally, a demonstrator that mounted the four channels and 1550-nm LW-VCSEL array on the subcarrier has been shown with no kink L-I curves and with clear 2.5-Gb/s eye diagrams. </para>

Thermoelectrical model for vertical cavity surface emitting lasers and arrays

An analytical model of thermal and electrical characteristics of vertical cavity surface emitting lasers (VCSELs) and VCSEL arrays is presented. The model is based on analytical expressions for the thermal resistance and for the I-V curve of the devices and incorporates only a few macroscopic parameters. It accounts for three-dimensional current and heat flows and provides thermo-electrical self-consistency. Good agreement between the model’s predictions and the measured characteristics of InGaAs∕AlGaAs VCSEL arrays is obtained.

Stochastic Characterization and Models to Predict Performance Uncertainty in Photonic Packages

There is considerable uncertainty in the prediction of performance of a system, due mainly to idealizations in geometry, material behavior, and loading history. Uncertainties in geometry can be predicted and controlled using tighter tolerances. However, the models currently used to describe material behavior are mostly deterministic. To predict the coupling efficiency of a photonic system to a greater degree of confidence, stochastic analysis procedures are necessary. As part of such an analysis, the behavior of materials must be stochastically characterized. In this paper, we present extensive experimental data on thermally and UV-cured epoxies, typically used in photonic packages to enable stochastic analysis. We perform dynamic mechanical analysis over a wide frequency and temperature range to determine the viscoelastic behavior of the epoxies. We next derive an analytical description of the time-dependent behavior of a vertical cavity surface emitting laser (VCSEL) array bonded to a substrate. We further characterize the variation in the displacement of the VCSEL array due to the stochastic, viscoelastic behavior of the bond epoxy. We carry out Monte Carlo simulation to predict the uncertainty in the coupling efficiency of a generic photonic package. We finally relate the size of the VCSEL laser array to its ability to achieve the required coupling efficiency.

Parallel Optical Transmitter Module Using Angled Fibers and a V-Grooved Silicon Optical Bench for VCSEL Array

We propose an advanced structure of optical subassembly (OSA) for packaging of the vertical-cavity surface-emitting laser (VCSEL) array, using (111) facet mirror of the V-groove ends formed in a silicon optical bench (SiOB) and angled fiber apertures. The feature of our OSA can provide a low optical crosstalk between neighboring channels, a low feedback reflection, and a large misalignment tolerance along the V-groove. We describe the optimized design of fiber angle, VCSEL position, and fiber position. The fabricated OSA structure consists of 12 channels of angled fiber array, 54.7deg V-grooves, Au-coated mirrors on (111) end facet of the V-grooves, and flip-chip-bonded VCSEL array on a SiOB. In this structure, the beam emitted from the VCSEL is deflected at the 54.7deg mirror of (111) end facet and propagated into the angled fiber. The angled fiber array was polished by 57deg. Fabricated OSAs showed a coupling efficiency of 30%-50% that is 25 times larger than that obtained from an OSA with a vertically flat fiber array. Our OSA showed large misalignment tolerance of about 90 mum along the longitudinal direction in the V-groove. We fabricated a parallel optical transmitter module using the OSA and demonstrated 12 channels times2.5 Gb/s data transmission with a clear eye diagram

Optical Micro Cell System: Smart Optical Wireless Access Data-Communication for Moving-User Terminals

Multiple high-speed optical wireless access terminals using vertical-cavity surface-emitting laser (VCSEL) and receiver arrays are proposed on the basis of an optical micro cell (OMC) system. Based on this concept, data multibeam directions can be controlled fastly using a single lens, a VCSEL array, electronic matrix switches, a hub-node phase locked loop (PLL)-large scale integration (LSI), and an end-node laser diode (LD)/photodiode (PD)-receiver array.

Optoelectronic and Microwave Transmission Characteristics of Indium Solder Bumps for Low-Temperature Flip-Chip Applications

This paper describes low-temperature flip-chip bonding for both optical interconnect and microwave applications. Vertical-cavity surface-emitting laser (VCSEL) arrays were flip-chip bonded onto a fused silica substrate to investigate the optoelectronic characteristics. To achieve low-temperature flip-chip bonding, indium solder bumps were used, which had a low melting temperature of 156.7degC. The current-voltage (I-V) and light-current (L-I) characteristics of the flip-chip bonded VCSEL arrays were improved by Ag coating on the indium bump. The I-V and L-I curves indicate that optical and electrical performances of Ag-coated indium bumps are superior to those of uncoated indium solder bumps. The microwave characteristics of the solder bumps were investigated by using a flip-chip-bonded coplanar waveguide (CPW) structure and by measuring the scattering parameter with an on-wafer probe station for the frequency range up to 40 GHz. The indium solder bumps, either with or without the Ag coating, provided good microwave characteristics and retained the original characteristic of the CPW signal lines without degradation of the insertion and return losses by the solder bumps

Trade-offs between lens complexity and real estate utilization in a free-space multichip global interconnection module

The FAST-Net (Free-space Accelerator for Switching Terabit Networks) concept uses an array of wide-field-of-view imaging lenses to realize a high-density shuffle interconnect pattern across an array of smart-pixel integrated circuits. To simplify the optics we evaluated the efficiency gained in replacing spherical surfaces with aspherical surfaces by exploiting the large disparity between narrow vertical cavity surface emitting laser (VCSEL) beams and the wide field of view of the imaging optics. We then analyzed trade-offs between lens complexity and chip real estate utilization and determined that there exists an optimal numerical aperture for VCSELs that maximizes their area density. The results provide a general framework for the design of wide-field-of-view free-space interconnection systems that incorporate high-density VCSEL arrays.

Optimization of VCSEL spatiotemporal operation in MMF links for 10-gb ethernet

A complete model of the spatiotemporal behavior of multimode vertical-cavity surface-emitting lasers (VCSELs), through static and dynamic response, noise, thermal effects, and its coupling to multimode fibers (MMF) has been investigated in low-cost 10-Gb Ethernet (10-Gb E) prototype optoelectronic packages using GaAs VCSEL arrays coupled to MMF ribbons for short-distance optical links. Eye diagram simulations compared to measurements allow the analysis of critical parameters, such as launching conditions, in order to maximize the MMF bandwidth as well as VCSEL static and dynamic conditions. A new bit error rate (BER) estimation method is proposed by taking into account not only the Q-factor but random and deterministic jitter and digital pattern effects. Global system optimization is discussed for a BER less than 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-12</sup> for an aggregate link speed of 120 Gb/s over 300 m of a 12-fiber MMF ribbon

1 /spl times/ 12 VCSEL array with optical monitoring via flip-chip bonding

A vertical-cavity surface-emitting laser (VCSEL) 1 times 12 array flip-chip bonded to a 1 times 12 GaAs monitor photodiode array is demonstrated, and faithful monitoring of the VCSEL threshold current, optical power, and subthreshold emission is recorded. Extremely small monitor crosstalk is measured. This is the first published report of an optically monitored VCSEL array

Parallel Translation of Microscopic Objects in Three-Dimensional Traps by Sequential Change of Emitting Vertical-Cavity Surface-Emitting Lasers

This paper deals with the parallel and independent translation of microscopic objects in three-dimensional traps by switching individual sources of a vertical-cavity surface-emitting laser (VCSEL) array on and off. Difficult calculations for dynamic manipulation are avoidable because light distributions are controlled in real time by direct modulation of the VCSELs. We constructed an experimental system in which target objects were irradiated from below with a one-dimensional VCSEL array. Experimental results demonstrated that two microscopic particles were trapped in three dimensions simultaneously, and then translated independently with a sequence of light distributions. The lower limit of intervals between times of changes of emitting VCSELs for translation was measured as 200 ms with 2 mW for each beam.

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.

Lasers—an effective artificial source of radiation for the cultivation of anoxygenic photosynthetic bacteria

The laser diode (LD) is a unique light source that can efficiently produce all radiant energy within the narrow wavelength range used most effectively by a photosynthetic microorganism. We have investigated the use of a single type of LD for the cultivation of the well-studied anoxygenic photosynthetic bacterium, Rhodobacter capsulatus (Rb. capsulatus). An array of vertical-cavity surface-emitting lasers (VCSELs) was driven with a current of 25 mA, and delivered radiation at 860 nm with 0.4 nm linewidth. The emitted light was found to be a suitable source of radiant energy for the cultivation of Rb. capsulatus. The dependence of growth rate on incident irradiance was quantified. Despite the unusual nearly monochromatic light source used in these experiments, no significant changes in the pigment composition and in the distribution of bacteriochlorophyll between LHII and LHI-RC were detected in bacterial cells transferred from incandescent light to laser light. We were also able to show that to achieve a given growth rate in a light-limited culture, the VCSEL required only 30% of the electricity needed by an incandescent bulb, which is of great significance for the potential use of laser-devices in biotechnological applications and photobioreactor construction.

A method for improved VCSEL packaging using MEMS and ink-jet technologies

Vertical-cavity surface-emitting lasers (VCSELs) have been recognized as low-cost low-power light sources for applications, such as, optoelectronic (OE) interconnects for high-speed optical data communication. However, many VCSEL applications have not been fully realized due to the lack of solutions to technical issues such as optical coupling, alignment, interconnects, and RF compatibility. In this paper, the authors propose microelectromechanical systems (MEMS) and MEMS-enabled ink-jet printing technologies to provide improved solutions to these technically challenging problems in OE-device packaging. Wafer-level microoptical elements consisting of transparent polymeric pedestals and microlenses were designed and fabricated directly on top of the VCSEL-emitting facets to improve the optical-coupling efficiency between the VCSEL and the optical fiber. Self-aligning MEMS clampers and micromold structures were designed and fabricated for precise packaging and reliable electrical connection of diced VCSEL arrays. This novel packaging process is substrate independent and relatively simple. This technique will provide a reliable assembly of OE devices in miniature optical systems on various substrates