This PDF file contains the front matter associated with SPIE Proceedings Volume 7615, including the Title Page, Copyright information, Table of Contents, and the Conference Committee listing.

No systematic studies on 1060nm high speed VCSELs have been reported in terms with reliability so far to our best knowledge. In this work, a systematic and intensive study on reliability has been performed for our 1060nm VCSELs consist of double intra-cavity and oxide confined structure with >70ps eye opening margin in Ib=3mA. Estimated power dissipation per bit rate of >5Gbps/mW at Ib=2mA was obtained from low series resistance and low drive voltage characteristics due to effective current injection in our double intra-cavity structure. Aging tests for 3,467pcs discrete non-hermetic VCSELs were performed under 6mA, 70°C to 120°C and up to 5,736 hours, which is equivalent to over 10million device hours in normal operating condition of 40°C and Ib=5mA. We found one degraded device due to the disconnection of the metal interconnecting layer, resulting in 81Fits (C.L.=90%) under Ea=0.35eV and no current accelerated factor. Also, their degradation of threshold current after 1,000 hours operation was less than 0.1mA under high stress condition of >40kA/cm² and 120°C, which corresponds to more than hundreds year operation. No eye diagram degradation was observed as far as no large threshold current increase under the high stress condition up to 40kA/cm². It is experimentally proven that inherent potentiality of the VCSELs with 1060nm InGaAs-QW and double intra-cavity structure would be applicable to the future large green data traffic system.

Frequently quoted advantages of VCSELs over other optical sources include wafer scale fabrication and testing, low cost, ease of fabricating arrays and ease of fiber coupling. To benefit from these advantages a robust manufacturing process and product demand are needed. Avago Technologies produces a range of single channel and parallel optical link products incorporating 850nm band VCSEL sources operating at up to 10Gb/s per channel. This paper will explore some important factors which need to be controlled for manufacturability of VCSEL devices.

Extensive VCSEL reliability enhancements have been carried out at Emcore in the past year with significant results. In this talk, we will present the failure mechanisms, the method and effectiveness of wafer and die screening, and the approaches to eliminate these failure mechanisms. Results of improved reliability will also be discussed.

The design of an oxide confined 850 nm VCSEL has been engineered for high speed operation at low current density. Strained InGaAs/AlGaAs QWs, with a careful choice of In and Al concentrations based on rigorous band structure and gain calculations, were used to increase differential gain and reduce threshold carrier density. Various measures, including multiple oxide layers and a binary compound in the lower distributed Bragg reflector, were implemented for reducing capacitance and thermal impedance. Modulation bandwidths > 20 GHz at 25°C and > 15 GHz at 85°C were obtained. At room temperature, the bandwidth was found to be limited primarily by the still relatively large oxide capacitance, while at 85°C the bandwidth was also limited by the thermal saturation of the resonance frequency. Transmission up to 32 Gb/s (on-off keying) over multimode fiber was successfully demonstrated with the VCSEL biased at a current density of only 11 kA/cm². In addition, using a more spectrally efficient modulation format (16 QAM subcarrier multiplexing), transmission at 40 Gb/s over 200 m multimode fiber was demonstrated.

In this paper we will discuss recent results on high speed VCSELs targeted for the emerging 16GFC (Fibre Channel) standard as well as the now forming 25Gbps PCI express standard. Significant challenges in designing for reliability and speed have been overcome to demonstrate VCSELs with bandwidth in excess of 20Gbps.

Data centers and supercomputers are driving the demand for short reach aggregate bandwidth. E.g. active CXP active optical cables (AOC) with an aggregate bandwidth of 120 Gbps [1] are being installed since about one year in some of the biggest server farms in the world. As these applications require parallel optics, obviously this is a natural playground for VCSEL technology. The 10G VCSEL platform of Philips ULM Photonics is enabling operation of such AOC at less than 3 W total power by low bias currents for the individual VCSEL as low as 3.4 mA at room temperature and 5.5 mA at 85°C ambient. In combination with ideally matched driver electronics, the launch power of the VCSELs can be stabilized within 0.15dB variation across this operating temperature range [2] and thus allow for open loop power control. With more than 10⁸ hours of operation in the field and no field return reported, the FIT rate for the 1x12 VCSEL array can be calculated to be less than 10 FIT.

Recent VCSEL drivers for high data rate parallel transceivers are designed to DC couple to the VCSELs. These drivers normally include no back termination to save power. Due to mechanical restrictions, the wire bond between the driver's output and the laser is usually quite long. Such laser drivers in a transceiver can cause excessive optical eye distortions (overshoot, pattern dependent jitter, etc.) to a VCSEL which performs well when driven by a 50Ohm source. Therefore more careful design optimizations (of the VCSEL's intrinsic laser behavior and its parasitic elements) are needed for such applications. In most cases, this is the only way to achieve good transceiver performance for a given VCSEL driver IC. In this talk, we present Emcore's recent effort to optimize the 850nm 10G VCSEL array for the real world laser drivers used in parallel transceivers and active cables.

Faraday Rotation Spectroscopy (FRS) is a polarization based spectroscopic technique which can provide higher sensitivity concentration measurements of paramagnetic gases and free radicals than direct absorption spectroscopic techniques. We have developed sensor systems which require only 0.2W to perform TDLAS (tunable diode laser absorption spectroscopy), and can additionally be quickly duty cycled, enabling operation in wireless sensor networks of laser-based trace gas sensors We adapted our integrated TDLAS electronics to perform FRS in a compact and more sensitive system for quantification of molecular oxygen (O₂) using a 762.3nm VCSEL in the A band. Using an AC magnetic field, we demonstrate detector noise dominated performance, achieving 2.1×10^-6/Hz^1/2 equivalent detectable fractional absorption and a minimum detection limit of 462 ppmv O₂ in 1 second in a 15cm path. At longer paths and integration times, such a sensor will enable oxygen measurements at biotic respiration levels (<1ppmv) to measure CO₂ - O₂ exchange for mapping natural exchange of greenhouse gases. Potential improvement of detection limits by increasing various system performance parameters is described.

We report a new method for monitoring vapor concentration of volatile organic compounds using a vertical-cavity surface-emitting laser (VCSEL). The VCSEL is coated with a polymer thin film on the top distributed Bragg reflector (DBR). The analyte absorption is transduced to the electrical domain through modulation of the VCSEL output power as the polymer swell. We have investigated the responsivity of this technique experimentally using a plasma polymerized polystyrene coating and explain the results theoretically as a reflectance modulation of the top DBR.

A demonstration of non-mechanical beam steering of high speed VCSEL arrays is reported. 980 nm oxide-confined VCSELs were grown on a GaAs substrate with AlGaAs distributed Bragg reflectors (DBRs) and an InGaAs active region. Up to 24 separate VCSEL arrays can be independently switched to sweep the beam across the illumination area. Three array designs with varying total areas were fabricated with 97, 137, and 278 elements. The corresponding mesa diameters were 24, 14 and 10 μm with pitches of 52, 40 and 26 μm. The relative output power, speed, and thermal performance of an array design is reported.

Compact, non-contact, and low operating power sensors are desirable for position sensing applications. Vertical-cavity surface-emitting lasers integrated monolithically with PIN photodetectors have been designed and fabricated for optical position sensing. This compound semiconductor component has in turn been integrated onto a Si-platform to form a microsystem. Using a metallic grating as a position gauge, the sensor microsystem can measure differences in reflected power from the grating as it travels parallel to the sensors. This measurement technique allows for a high spatial resolution. Calculations indicate that such a device can detect spatial changes on the order of the wavelength of light emitted from the laser. Measurements from the work described here show the potential to use VCSEL/PIN chips to determine position with an accuracy of sub-micron resolution.

Integrated optical semiconductor sensors are a promising technology for both lab-on-a-chip and molecular imaging applications due to their low cost, small size, high sensitivity, and flexible designs. We present the design and fabrication of a GaAs-based monolithically integrated fluorescence sensor incorporating 670nm VCSELs and PIN photodetectors. This is the first integrated, VCSEL-based fluorescence sensor with excitation at a far-red wavelength and is specifically designed for in vivo sensing applications. In addition, we discuss considerations to simultaneously achieve high power VCSELs and low dark current PIN photodetectors required for sensitive fluorescence detection. These fabricated sensors incorporate 670nm VCSELs emitting 2.0mW at room temperature (RT) with adjacent detectors exhibiting RT dark less than 2pA/mm² (100mV reverse bias). Fluorescence emission filters suitable for transmitting Cy5.5 fluorescent dye emission were integrated with the photodetectors. The sensor detects Cy5.5 molecules in vitro at 5nM concentration with linear response for concentrations up to 25μM. These miniature sensors are suitable for portable diagnostic assays and in vivo rodent studies.

Recent advances in Vertical-cavity Surface-emitting Laser (VCSEL) efficiency and packaging have opened up alternative applications for VCSELs that leverage their inherent advantages over light emitting diodes and edge-emitting lasers (EELs), such as low-divergence symmetric emission, wavelength stability, and inherent 2-D array fabrication. Improvements in reproducible highly efficient VCSELs have allowed VCSELs to be considered for high power and high brightness applications. In this talk, Aerius will discuss recent advances with Aerius' VCSELs and application of these VCSELs to miniature optical sensors such as rangefinders and illuminators.

Many applications require laser pump sources with high output power (tens to hundreds of Watts) in the smallest spot, with the smallest divergence. Such high-brightness pump sources typically use edge-emitting semiconductor lasers. However, it is also possible to use high-power two-dimensional vertical-cavity surfaceemitting laser (VCSEL) arrays for this purpose. Using a single 976nm 2D VCSEL array chip in an external cavity configuration, combined with a matching micro-lens array, we have demonstrated more than 30W output power from a 50μm/0.22NA fiber, corresponding to a brightness of 10MW/cm².sr. This represents a substantial reduction in module complexity compared to edge-emitter based modules with similar brightness. These novel high-brightness pump sources exhibit some well-known intrinsic VCSEL performance features such as wavelength stability and narrow spectrum. Power and brightness can be scaled up using polarization and spectral combining.

The performance of high power VCSELs in a specific application depends on the geometrical and thermal design as well as on the quality of the epitaxially grown material. Due to the relatively high heat load in densely packed high power arrays the temperature in the active zone and the DBR mirrors changes significantly with the applied current and the traditional characterization methods become less meaningful than for low power devices. This paper presents a method to measure temperature independent power curves with the help of short pulse techniques and data mapping at different heat sink temperatures. In addition the internal quantum efficiency, the transparency current and the gain coefficient are measured by a novel method which operates the VCSEL material as an edge emitter and applies a cut-back technique. The optical losses in the DBR mirrors are determined using external feedback. In summary all relevant parameters which determine the quality of an epitaxial design are measured independently and can be directly compared with modeling and help to optimize the high power VCSEL performance.

The properties of high-power and low-noise seed lasers are key for high performance master oscillator-power amplifier (MOPA) fiber-lasers. We have successfully demonstrated high-power and low-noise seed lasers using our VCSEL technology. We used an external-cavity configuration with optimum cavity design for single-mode control, and the mode-beating problem can be fully avoided compared to the edge-emitter seed lasers. The external-cavity VCSEL achieved high-power single-mode pulsed operation with good mode quality that allowed it to be efficiently coupled into a single-mode PM or non-PM fiber. Using high-speed driving electronics, optical pulse widths of 12ns and shorter were obtained with repetition rates of up to 1 MHz. The optical output peak power obtained is over 10 W. We have also demonstrated a CW version of this high-power VCSEL seed laser achieving single transverse and longitudinal mode with an output power of greater than 0.5 W. The high-power external cavity VCSELs were operated in single longitudinal mode demonstrating narrow spectral line-width of 200kHz, and having very low RIN of -155 dBc/Hz at 1MHz, which was even lower at higher frequencies.

High power VCSELs can be realized by scaling up the active area of bottom-emitting devices. This results in a large Fresnel number of the laser cavity. The laser beam cannot be described with Gauss modes in a simple way anymore, but is best described in terms of tilted plane waves, called Fourier modes. The beam quality and mode spectra depending on the applied current and the temperature of the VCSEL are investigated. Two-dimensional measurements of the near and the far field are combined with power and spectral measurement to characterize the VCSEL. Polarization and Fourier filtering are used to examine the spatially-dependent emission in detail. A rich dynamic in the angular emission profile for large-area VCSELs is observed and can be explained by considering the residual reflections from the AR-coated substrate-air interface and thermal effects. The presented theoretical model simulates the dynamics of the angular emission. The calculated angular and spectral profiles match the experimental observations very well over the whole parameter range. The influence of the active area is studied for diameters of the oxide aperture from 20 up to 300 μm. For smaller diameters diffraction effects become more dominant, the Fresnel number is reduced and the emission spectrum gets closer to the Gauss mode description.

In this article, we report our results on 980nm high-index-contrast subwavelength grating (HCG) VCSELs for optical interconnection applications. In our structure, a thin undoped HCG layer replaces a thick p-type Bragg mirror. The HCG mirror can feasibly achieve polarization-selective reflectivities close to 100%. The investigated structure consists of a HCG mirror with an underneath λ/4-thick oxide gap, four p-type GaAlAs/GaAs pairs for current spreading, three InGaAs/GaAs quantum wells, and an n-type GaAlAs/GaAs Bragg mirror. The HCG structure was defined by e-beam lithography and dry etching. The current oxide aperture and the oxide gap underneath the HCG were simultaneously formed by the selective wet oxidation process. Compared to air-gap high contrast grating mirrors demonstrated elsewhere, our grating mirrors are particular since they are supported by thinner λ/4 aluminium oxide layer, and thus are mechanically robust and thinner than usual designs. Sub-milliamp threshold currents and single-transverse-mode operation was obtained. A hero device exhibited maximum singlemode output power of more than 4 mW at room temperature and 1 mw at 70°C, which are the highest values ever reported from the HCG structures. These results build a bridge between a standard VCSEL and a hybrid laser on silicon, making them of potential use for the realization of silicon photonics.

A widely-tunable single-mode long wavelength vertical-cavity surface-emitting laser structure employing a MEMStunable high-index-contrast subwavelength grating (HCG) is suggested and numerically investigated. A very large 80- nm linear tuning range was obtained as the HCG was actuated by -220 to 250 nm. The large tuning range results from making the air gap part of the optical cavity, which was achieved by inserting an antireflection layer below the air gap and by the absence of partial top DBR for current spreading. The single mode operation was maintained throughout the tuning range, thanks to the selective pumping of the fundamental mode and the moderate mode selection by the HCG itself. Analytic expressions for tuning range and tuning sensitivity were derived, using the penetration depth of the HCG for the first time.

The first investigation of the effect of mesa plated sidewall heatsinks on the performance of red oxide-confined verticalcavity surface-emitting lasers (VCSELs) is reported. Single mode (SMSR>20dB) visible (670 nm) VCSELs were fabricated with an AlGaInP active region and AlGaAs distributed Bragg reflectors (DBRs). Copper is electroplated on and around VCSELs with different mesa diameters with a variety of overlap sizes. A trend of reduction in thermal impedance for some devices has also been observed with increasing the plating overlap size.

We have fabricated proton-implanted photonic crystal (PhC) vertical-cavity surface-emitting lasers (VCSELs) emitting in the visible spectrum. The active region of the VCSELs is composed of multiple InGaAlP quantum wells resulting in an emission wavelength of 674 nm. A threshold current of 1.3 mA and single mode output power higher than 1 mW at room temperature have been achieved. The maximum continuous wave (CW) lasing temperature was found to be 55° C. The PhC VCSELs operate in a single fundamental mode with a side-mode suppression ratio (SMSR) larger than 30 dB and show a constant beam divergence of 8 degree (full angle) for all levels of injection current and various ambient temperatures. We compare ion-implanted and photonic crystal VCSELs and demonstrate that the controllable refractive index guidance effect of the PhC results in a stable beam output which makes these red VCSELs interesting for imaging applications.

Vertical cavity surface emitting lasers (VCSELs) are low cost and reliable light sources for high-speed local area and storage area network (LAN/SAN) optical fiber data communication systems and short-reach computer interconnects. The continuing rapid increase of serial transmission data rates driven by multi-core microprocessor's bandwidth upgrades cannot be sustained via conventional copper-based links as bit rates move beyond 10 Gbit/s and distances greater than 1 m. The intrinsic limitation of copper at high single-channel data rates facilitates the need to transition to optical fiberbased links at ever shorter distances. For LAN/SAN applications the 850 nm wavelength is standard. This same wavelength is also the standard for several other evolving short-reach application areas including Fibre Channel, CEI, USB, InfiniBand, and HDMI optical link systems. Herein we present our recent results on 850 nm oxide-confined VCSELs operating at data bit rates up to 40 Gbit/s. The low operational current density in the range of ~10 kA/cm2 ensures viable device reliability and long-term stability based on well-known industry certification specifications. Key VCSEL device parameters including the relaxation resonance frequency, damping, and parasitic cut-off frequency are determined for VCSELs with oxide-confined apertures of various diameters. We find that a parasitic cut-off frequency of 24-28 GHz limits the VCSEL's high speed operation at the highest optical modulation rates. We believe that with some effort the device parasitics can be further reduced such that current modulated VCSELs can be realized with larger than 30 GHz optical modulation bandwidth and reliable and practical operation beyond 40 Gbit/s.

We report developments at Emcore on serial 850 nm vertical-cavity surface-emitting lasers (VCSELs) operated up to 25 Gb/s. They have been designed to provide a solution not only to meet stringent 10 Gb/s IEEE and Fiber Channel specifications but also for emerging demands of 17 Gb/s Fiber Channel serial and 100 Gb/s (4x25 Gb/s or 5x20 Gb/s) parallel applications in local and storage area networks. This paper covers 10 Gb/s GenX production distributions and improved GenX VCSEL device design to meet low-power requirements at 20 Gb/s. We have successfully demonstrated low threshold current of 0.65 mA at 25°C using nominal 7.3 μm oxide-aperture GenX VCSELs. They can be directly modulated up to 25 Gb/s with open eyes at 6 mA bias. With the same design, open eyes of 20 Gb/s is achieved at bias current as low as 4 mA (9 KA/cm²) at 25°C and 8 mA (18 KA/cm²) at 70°C. These operation conditions are comparable to current 10 Gb/s GenX VCSELs in production which have been shown a great field history.

Single fundamental mode, oxide-confined, polyimide planarized 980nm vertical cavity surface emitting lasers (VCSELs) with a multi-oxide layer (MOL) structure to increase oxide aperture diameter and maintain single-mode operation are fabricated and characterized. VCSELs with an 8μm active diameter and 16 mode suppression layers maintained single transverse mode operation under continuous wave (CW) condition with a side-mode suppression ratio (SMSR) of more than 32 dB at current densities up to 20 times threshold, which is five times higher than the previously reported value for similar devices with only 3 mode suppression layers, and exhibited a maximum 3-dB modulation frequency bandwidth of 13GHz. The threshold current and voltage were as low as 260μA and 1.45V, respectively, with a maximum optical power and slope efficiency of 1mW and 0.31W/A, respectively.

An investigation into the carrier and spectral dynamics of a 1.55 μm Buried Tunnel Junction (BTJ) VCSEL was carried out by examining the emission spectra under high resolution and the voltage across the junction as polarisation resolved light from a tunable laser source was injected into the cavity. The VCSEL combines an epitaxial InGaAlAs distributed Bragg reflector with a Si/ZnS dielectric reflector and an oval shaped BTJ leading to a predominantly single transverse polarisation mode and laser linewidths as low as 20 MHz. Around lasing threshold and injecting into the primary mode, the voltage required to maintain the current drops due to stimulated emission and a consequent reduction in the carrier density. Locking behaviour associated with this characteristic is measured with increased input power. Voltage drops as large as 6 mV are measured. Above threshold, injection locking is measured in addition to features associated with the relaxation oscillations of the carriers.