Monolithic integration of vertical-cavity surface-emitting laserand 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 4×4 array of focused spots 10 mm from the substrate with spot sizes down to less than 400 μm, almost at the diffraction limit. Diffraction efficiencies of 29% and uniformity errors of 14% were measured     

Optical broadcast and clock distribution for multi-chip-modules

In this letter, we report a new architecture for clock and broadcast distribution using optical interconnect components, such as vertical cavity surface emitting lasers (VCSEL) and pin photodiodes with benefits of diffractive optical elements (DOE) fan-out. A two-bit-large bus for broadcast or clock distribution demonstration is presented using collective wiring technologies and MCM hybridization process in a standard BGA package. Diffractive optical elements allow one to four distribution scene through an optical plate. Specific laser drivers for VCSELs and photodiode receiver are realized in complete CMOS 0.6 μm transmitter and receiver chips.     

Integration of microdiffractive lens with continuous relief withvertical-cavity surface-emitting lasers using focused ion beam directmilling

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 μm, 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° before integration of the VCSEL/diffractive optical elements (DOEs) to 0.6° after FIB integration, allowing for an interconnect length of ~4 mm. It was proven by device testing that there is little influence upon the VCSEL performance after FIB processing     

Densely packed pie shaped vertical-cavity surface-emitting laserarray incorporating a tapered one-dimensional wet oxidation

Wavelength-division-multiplexing (WDM) photonic integrated emitter (PIE) vertical-cavity surface-emitting laser (VCSEL) arrays are fabricated using a post growth wet oxidation technique. High-density integration of WDM VCSEL arrays is possible by combining the technique of one-dimensional oxidation and large-scale tapered oxidation. Eight channels are integrated into a circle of 60 μm in diameter. Seven channels are found to operate as lasers. The lasing wavelengths range from 823 to 836 nm corresponding with the distance between the VCSEL mesa and the tuning trench. The successful demonstration of incorporating wet oxidation into the wavelength control of the PIE VCSEL array opens a new way of fabricating mask-defined densely packed WDM VCSEL arrays     

Power minimization and technology comparisons for digitalfree-space optoelectronic interconnections

This paper investigates the design optimization of digital free-space optoelectronic interconnections with a specific goal of minimizing the power dissipation of the overall link, and maximizing the interconnect density. To this end, we discuss a method of minimizing the total power dissipation of an interconnect link at a given bit rate. We examine the impact on the link performance of two competing transmitter technologies, vertical cavity surface emitting lasers (VCSELs) and multiple quantum-well (MQW) modulators and their associated driver-receiver circuits including complementary metal-oxide-semiconductor (CMOS) and bipolar transmitter driver circuits, and p-n junction photodetectors with multistage transimpedance receiver circuits. We use the operating bit-rate and on-chip power dissipation as the main performance measures. Presently, at high bit rates (>800 Mb/s), optimized links based on VCSELs and MQW modulators are comparable in terms of power dissipation. At low bit rates, the VCSEL threshold power dominates. In systems with high bit rates and/or high fan-out, a high slope efficiency is more important for a VCSEL than a low threshold current. The transmitter driver circuit is an important component in a link design, and it dissipates about the same amount of power as that of the transmitter itself. Scaling the CMOS technology from 0.5 μm down to 0.1 μm brings a 50% improvement in the maximum operating bit rate, which is around 4 Gb/s with 0.1 μm CMOS driver and receiver circuits. Transmitter driver circuits implemented with bipolar technology support a much higher operating bandwidth than CMOS technology; they dissipate, however, about twice the electrical power. An aggregate bandwidth in excess of 1 Tb/s-cm² can be achieved in an optimized free-space optical interconnect system using either VCSELs or MQW modulators as its transmitters     

Fast optical thresholding with an array of optoelectronictransceiver elements

In this letter, we present an optoelectronic system for fast gray-level image decomposition into binary slices. To perform thresholding, we are making use of the differential nature of optical thyristors and compare the light level of the image pixels to that of a reference intensity. As input a gray-level image was generated using a nematic liquid crystal spatial light modulator (SLM) and as a reference intensity a VCSEL light source combined with a diffractive fan-out element was used. We also introduced a compact, large field of view telecentric optical system based on gradient refractive index lenses to image between the SLM and the array. A frame rate decomposition of a six gray-level image has been obtained at 1.8 kHz. Future prospects for system improvements are discussed     

Single-mode power dependence on surface relief size formode-stabilized oxide-confined vertical-cavity surface-emitting lasers

We have studied the mode behavior of oxide-confined vertical-cavity surface-emitting lasers (VCSELs) with a surface relief for fundamental mode selection. The dependence of single-mode power on the surface relief diameter was measured and compared with numerically calculated values. VCSELs with diameters of 9 and 12 μm were equipped with surface reliefs with diameters in the range 4-10 μm. The results show that there exists an optimum relief diameter for each VCSEL size. A maximum single-mode power of 2.2 mW was achieved for a 9-μm-diameter VCSEL with a 4-μm-diameter surface relief     

Technology development of a high-density 32-channel 16-Gb/s opticaldata link for optical interconnection applications for theoptoelectronic technology consortium (OETC)

A parallel, 32-channel, high density (140 μm pitch), 500 Mb/s NRZ, point-to-point, optical data link has been fabricated using existing GaAs IC, silicon optical bench (SiOB), and multichip module (MCM-D) technologies. The main components of the transmitter and the receiver modules are a GaAs-based vertical cavity surface emitting laser (VCSEL) array at 850 mn with its IC driver array chip and an integrated metal-semiconductor-metal (MSM) receiver (photodetector and signal processing circuits) array at 850 nm. The package module uses a modified 164 I/O JEDEC premolded plastic quad flat pack (PQFP) in combination with a polymer film integrated circuit (POLYFIC) chip carrier. The electrical input and output are 500 Mb/s NRZ binary signals. The optical I/O in both modules consists of a directly-connectorized (nonpigtail) fiber array block that plugs into the 32×1 optical fiber ribbon directly on one side and accepts 32 optical signals from the SEL array or delivers them to the MSM receiver array via a gold-coated 45° polished fiber array mirror. The MACII-32 ribbon cable is an enhanced version of the standard MACII connector ribbon cable. This paper characterizes key components of the optical data link, describes its package design, and discusses preliminary component and optical data link test results     

Wafer-bonded 1.55 μm vertical cavity laser arrays for wavelengthdivision multiplexing

The first electrically pumped 1.55 μm multiple wavelength VCSEL array is demonstrated. The wafer bonded array consists of four channels operating between 1509 and 1524 nm. Multiple wavelengths were defined using an etched intracavity superlattice prior to bonding. Threshold currents of 0.9 mA and peak output powers of 0.45 mW were measured     

Grating-coupled surface-emitting laser with a hyperbolic unstableresonator producing a stable focused output beam

A grating-coupled surface-emitting semiconductor laser has been integrated with a focusing diffractive beamforming element. A hyperbolic unstable resonator is used to introduce mode discrimination and suppress filamentation in a broad geometry, resulting in a 200-μm-wide coherent output. The light is focused 500 μm above the laser surface to a spot size of 5×7 μm. A remarkable beam stability is observed with very small spot size variation and beam-steering up to three times the threshold current under continuous operation     

Independently addressable VCSEL arrays on 3-μm pitch

We present an exceedingly dense linear vertical-cavity surface-emitting laser (VCSEL) array with independently addressable elements on a staggered 3-μm pitch. Our devices utilize an all-epitaxial structure and operate at a wavelength of 813 mm with threshold currents below 400 μA. The high-packing density is enabled by combining transparent contact technology with a planar laterally oxidized device architecture. The array exhibits low interelement thermal crosstalk and has electrical resistances of 3 MΩ between adjacent array elements     

High-performance BiCMOS 100 K-gate array

A BiCMOS gate array in 0.8-μm technology with CMOS intrinsic gate delays of 100 ps plus 60 ps/fan-out and BiCMOS intrinsic delays of 200 ps with a 17-ps/fan-out drive factor is discussed. A compact base cell (750 μm²/gate) has been designed with full bipolar drive capability for the efficient layout of both primitive gates and large-arrayed macros, such as register files and multipliers. A 106 K-gate array has been built on a 1.14-cm² chip with ECL I/O capability. The place and route in three levels of metal provide array utilization greater than 90%. The gate array was used to implement a 74 K-gate filter design with testability features such as JTAG and two-phase scan     

Electroless remetallization of aluminum bond pads on CMOS driverchip for flip-chip attachment to vertical cavity surface emitting lasers(VCSEL's)

A method for remetallizing the bond pads of electronic chips, which are initially metallized with aluminum or aluminum alloy is presented. Application of electroless plating process for the remetallization of aluminum to a solderable gold surface can reduce the cost and complication of the widely accepted flip-chip interconnection technology. We have developed a step by step nickel/gold wafer bumping technique (remetallized bump height is 5.0 μm) for the appropriate solder (15.0 μm of In:Pb). Variation of roughness of the remetallized surface has been studied carefully. We have completed prototype research studies on test devices and successfully packaged the flip-chip bonded hybrid pair of a CMOS driver chip and a dummy structure of vertical cavity surface emitting laser (VCSEL) array. Cross section of the flip-chip solder joint is studied. Also, adhesion strength of the metal deposit is investigated     

Experimental evaluation of the 3-D optical shuffle interconnectionmodule of the sliding Banyan architecture

A free-space optical interconnection module for the sliding Banyan (SB) multistage interconnection network is experimentally evaluated. This three-dimensional (3-D) optical shuffle topology employs a macro-lens array in a reflective architecture. Interconnections for multiple stages are interleaved across a single two-dimensional (2-D) multichip array of “smart pixels”. The experimental module implements five interleaved stages of shuffle interconnections with approximately 10 μm resolution and 10 μm registration accuracy across a 10×10 cm, 256 node, simulated optoelectronic (OE) backplane. The experiments demonstrate the use of conventional refractive optical elements to implement the required shuffle interconnection pattern in a SB network. The results suggest that this reflective 3-D shuffle interconnected SB approach may lead to ATM switching fabrics with aggregate throughputs scaleable to >1 Tb/s. Such a system could be implemented with vertical cavity surface emitting laser (VCSEL) based smart pixel OE technology     

Fabrication of 7×6 multimode optical fiber gratingdemultiplexer-star coupler using a single GRIN-rod lens

A multimode optical fiber grating demultiplexer-star coupler having seven demultiplexing channels and six fan-out channels is demonstrated. This device consists of an input-output fiber array, in which 42 output fibers are aligned radially around an input fiber, a single gradient-index (GRIN)-rod lens and a multifacet blazed reflection grating. In this device, the incident light beam is split into six beams, each of which is diffracted at the multifacet grating and couples to the output fibers. The device has a working band from 0.62 to 0.88 μm, channel separation from 36 to 45 nm and 3 dB bandwidth from 13 to 24 nm. We also evaluate the unevenness in the center wavelength and the minimum excess loss among the fan-out channels and discuss the relationship between the unevenness and the fabrication accuracy of the optical components in detail     

Collimating diode laser beams from a large-area VCSEL-array usingmicrolens array

In this letter, we demonstrate the fabrication and bonding of a 1 cm×1 cm monolithic two dimensional (2D) vertical-cavity surface-emitting laser (VCSEL) array. We coupled the array to a matched microlens array to individually collimate light from each laser. We found the beam divergence of the collimated array to be 1.6° (1/e2) for the entire array. Using a 1-cm diameter F2 lens, we were able to focus the collimated beams to a spot of 400 μm in diameter and to couple more than 75% of the array power into a 1-mm core fiber. Our results show that it is possible to uniformly bond large area VCSEL arrays to heat sinks, and to collimate light from each element into parallel beams using a single 2-D microlens array. Our results also show that the brightness of the focused beam can be further increased with a lens to near 10⁵-W/cm² Steradian, a level that is useful for many high-power applications     

Two-dimensional 8×8 photoreceiver array and VCSEL drivers forhigh-throughput optical data links

Two custom GaAs integrated circuits (ICs) have been developed for enabling vertical cavity surface emitting laser (VCSEL) arrays to be used for high throughput spatial division multiplexed (SDM) optical data links. A 16-channel driver IC was developed to drive the VCSEL array and an 8×8 monolithic photoreceiver, which spatially matches the VCSEL array, was developed for receive. Both of these circuits were fabricated in a standard commercial GaAs MESFET process with parasitic photodetectors used for the photoreceivers. Power dissipation and circuit size were primary design challenges for both circuits. The present 8×8 array size along with an estimated usable channel speed of 1 Gb/s allows for an aggregate throughput of 64 Gb/s     

High-efficiency dual-beam vertical cavity surface emitting lasers for inteferometric applications

Reported are the characteristics of 850 nm vertical cavity surface emitting lasers (VCSELs) with integrated linear diffractive gratings emitting a dual-beam output with negligible increase in threshold and excellent zero-order cancellation. Power emitted into the first orders is greater than 80%. The use of a dual-beam VCSEL for measurement of surface topography using interferometry is demonstrated     

Uniform wafer-bonded oxide-confined bottom-emitting 850-nm VCSELarrays on sapphire substrates

Uniform bottom-emitting 850-nm vertical-cavity surface-emitting laser (VCSEL) arrays on sapphire substrates have been demonstrated using wafer bonding technology to transfer the epitaxially-grown VCSEL structures from GaAs substrates onto sapphire substrates. The uniformity of the VCSEL arrays were improved by placing thin oxide aperture at the standing wave node to reduce scattering loss for small aperture devices. The averaged threshold current of a 5×5 VCSEL array is as low as 346 μA, while the averaged external quantum efficiency approaches 57%. The maximum wall-plug efficiency is 25% and the single-mode output power is more than 2 mW under continuous-wave current excitation at room temperature. We have also demonstrated a large (10×20) VCSEL array with variations of threshold current and external quantum efficiency less than 4% and 2%, respectively     

Multi-oxide layer structure for single-mode operation invertical-cavity surface-emitting lasers

We propose a novel vertical-cavity surface emitting laser (VCSEL) with Al(Ga)As multi-oxide layer (MOX) structure for the purpose of enlarging window aperture maintaining single transverse mode operation. We have fabricated an InGaAs-GaAs VCSEL with the proposed MOX structure formed on GaAs (311)B substrate. We have performed a numerical simulation to investigate single-mode behavior of the proposed structure and showed a possibility of single-mode VCSEL's with a large active area. We have fabricated an 11-μm current aperture 960-nm wavelength VCSEL with this MOX structure. The threshold current and voltage were 1.0 mA and 2.0 V, respectively, which are comparable to those of conventional oxide VCSELs. In 8-μm aperture, single-mode operation was maintained with a driving current up to four times the threshold