Martin Grabherr

High-performance oxide-confined GaAs VCSELs

We present GaAs based selectively oxidized VCSELs with record high 57% wallplug efficiencies emitting in the 820-860-nm wavelength regime. Solid source molecular beam epitaxy with carbon as p-type dopant is used for crystal growth. Multimode devices show continuous-wave (CW) output powers up to 42 mW and stable operation from -80/spl deg/C up to +185/spl deg/C. Efficient single-mode output power of some milliwatts is maximized by controlling optical waveguiding that depends on the position of the 30-nm thin oxide aperture. Elliptically shaped current apertures are applied to stabilize output polarization.

High-power VCSEL arrays for emission in the watt regime at room temperature

Selectively oxidized InGaAs vertical-cavity surface emitting lasers (VCSELs) at an emission wavelength of /spl lambda/=980 mm are investigated for high-power applications. Densely packed arrays consisting of 19 single devices with an active diameter of 50 /spl mu/m emit 1.08 W of continuous-wave (CW) optical output power at room temperature. At 10/spl deg/C, heat sink temperature the output power increases to 1.4 W, which corresponds to a chip size averaged power density of 1 kW/spl middot/cm/sup 2/. Low divergence angle of less than 16/spl deg/ full-width at half-maximum (FWHM) and the circularly symmetric far-field pattern allow for simple focusing of the beam with power densities above 10 kW/spl middot/cm/sup 2/.

High volume production of single-mode VCSELs

Up to now applications for singlemode VCSELs were in low volume and high prized applications like tunable diode laser absorption spectroscopy (TDLAS, [1,2]) or optical interferometers. Typical volumes for these applications are in the range of thousands of pcs per year, with pricing levels of several 100 USD/pcs. New applications for singlemode VCSELs in consumer markets require manufacturing in very high volumes and at very low cost. Examples are laser-based optical mouse sensors, optical encoders, and rubidium atomic clocks for GPS systems [3,4]. U-L-M photonics presents manufacturing aspects, device performance and reliability data for these devices. The first part of the paper is dealing with high volume manufacturing of 850 nm singlemode VCSEL chips with very high efficiency and low operation current. Special processing technologies have been developed to achieve yields on 3 inch wafers of more than 90%. Wafer qualification procedures are discussed as well. The second part of the paper covers high volume packaging in TO and SMT type packages where very high packaging yields must be achieved. In the last part of the paper reliability issues are discussed, focused on the very high susceptibility of these devices to electrostatic discharge.

Volume production of polarization controlled single-mode VCSELs

Over the past 3 years laser based tracking systems for optical PC mice have outnumbered the traditional VCSEL market datacom by far. Whereas VCSEL for datacom in the 850 nm regime emit in multipe transverse modes, all laser based tracking systems demand for single-mode operation which require advanced manufacturing technology. Next generation tracking systems even require single-polarization characteristics in order to avoid unwanted movement of the pointer due to polarization flips. High volume manufacturing and optimized production methods are crucial for achieving the addressed technical and commercial targets of this consumer market. The resulting ideal laser source which emits single-mode and single-polarization at low cost is also a promising platform for further applications like tuneable diode laser absorption spectroscopy (TDLAS) or miniature atomic clocks when adapted to the according wavelengths.

Commercial VCSELs reach 0.1-W cw output power

Following the success in fiber based DataCom, VCSELs start to conquer additional market shares in a variety of other applications like free space optics (FSO), lighting, printing, and sensing. U-L-M photonics presents a new family of commercial high power VCSELs emitting powers of up to 50 mW cw at RT based on top-emitting technology. The devices are available at 850 nm emission wavelength. All devices can be operated passively cooled and provide modulation bandwidths of up to 1 GHz. Wallplug efficiencies are in excess of 25 %. Even higher output power of 250 mW cw from a 80 μm active diameter bottom-emitting VCSEL operating at 980 nm has already been obtained although just beeing passively cooled. Further power up scaling is achieved by arrangement of multiple VCSELs in 2D arrays. For the first time we demonstrate cw output power of 10 Watt cw at RT from compact monolithic VCSEL module of 14 mm2 chip area. Transfer of the technology to other wavelengths, e.g. 808 nm and 945 nm, is presented, too, and shows perspectives towards homogeneous optical pumping of solid state lasers. Almost identical device performance levels can be presented for the entire wavelength span. All discussed results are based on highest quality epitaxy optimized for maximum intrinsic efficiency and differential slope efficiency. Oxide confinement is used for current constriction that provides most efficient electrical pumping of the active area. In combination with advanced mounting techniques all mentioned aspects sum up to allow for cost effective VCSEL products in the medium and high power laser regime. The circular output beam in addition to simple heat sinking offers attractive solutions for advanced system integration.

Fabrication and performance of tuneable single-mode VCSELs emitting in the 750- to 1000-nm range

The growing demand on low cost high spectral purity laser sources at specific wavelengths for applications like tuneable diode laser absorption spectroscopy (TDLAS) and optical pumping of atomic clocks can be met by sophisticated single-mode VCSELs in the 760 to 980 nm wavelength range. Equipped with micro thermo electrical cooler (TEC) and thermistor inside a small standard TO46 package, the resulting wavelength tuning range is larger than +/- 2.5 nm. U-L-M photonics presents manufacturing aspects, device performance and reliability data on tuneable single-mode VCSELs at 760, 780, 794, 852, and 948 nm lately introduced to the market. According applications are O2 sensing, Rb pumping, Cs pumping, and moisture sensing, respectively. The first part of the paper dealing with manufacturing aspects focuses on control of resonance wavelength during epitaxial growth and process control during selective oxidation for current confinement. Acceptable resonance wavelength tolerance is as small as +/- 1nm and typical aperture size of oxide confined single-mode VCSELs is 3 μm with only few hundred nm tolerance. Both of these major production steps significantly contribute to yield on wafer values. Key performance data for the presented single-mode VCSELs are: >0.5 mW of optical output power, >30 dB side mode suppression ratio, and extrapolated 10E7 h MTTF at room temperature based on several millions of real test hours. Finally, appropriate fiber coupling solutions will be presented and discussed.

25 Gbps and beyond: VCSEL development at Philips

In comparison to widely used InGaAs Quantum Wells (QW) in high speed VCSELs operating at 25 Gbps and beyond, we present an investigation on the use of GaAs QWs, which have proven their ability to serve reliably in 10 Gbps and 14 Gbps VCSEL products and allow for an evolutionary extension of data rates based on mature technology. As data centers continuously increase in size, the demand for longer reach optical links within these data centers is addressed by the proposal of using small spectral width single-mode VCSELs that offer the potential of significantly reduced chromatic dispersion along optical fibers of several 100 m length. Performance and modeling parameters of single-mode VCSELs are being compared to those of typical multi-mode VCSELs built from identical epitaxy and process technology.

New applications boost VCSEL quantities: recent developments at Philips

Besides the mature and steadily growing datacom market for which VCSELs are key components in Transceivers, Active Optical Cables (AOC), Mid Board Optical Modules (MBOM) or Embedded Optical Modules (EOM), VCSELs have proven to be key components also for other volume applications. Laser mice emerged 2004, just after the burst of the dotcom bubble and the related downturn in the Datacom industry, and dominated the shipped quantities for some years, accompanied by various smaller applications like atomic clock, oxygen sensing, encoders, and many more. Over the past years, two other major applications came into focus: optical interconnects in high performance computers or datacenters and smart sensors for mobile devices. In addition, VCSELs are penetrating into more and more power applications, primarily for illumination or IR heating. We present recent developments in technology, products, and addressed market segments that will have a major impact on the VCSEL industry.

Integrated photodiodes complement the VCSEL platform

Many VCSEL based applications require optical feedback of the emitted light. E.g. light output monitor functions in transceivers are used to compensate for thermally induced power variation, power degradation, or even breakdown of pixels if logic for redundancy is available. In this case integrated photodiodes offer less complex assembly compared to widely used hybrid solutions, e.g. known in LC-TOSA assemblies. Especially for chip-on-board (COB) assembly and array configurations, integrated monitor diodes offer a simple and compact power monitoring possibility. For 850 nm VCSELs the integrated photodiodes can be placed between substrate and bottom-DBR, on top of the top-DBR, or inbetween the layer sequence of one DBR. Integrated intra-cavity photodiodes offer superior characteristics in terms of reduced sensitivity for spontaneously emitted light [1] and thus are very well suited for power monitoring or even endof- life (EOL) detection. We present an advanced device design for an intra-cavity photodiode and according performance data in comparison with competing approaches.

Speed it up to 10 Gb/s and flip chip it: VCSEL today

Accumulation of all advantageous properties VCSELs are famous for, like low power consumption, circular low divergent beam profile, high modulation bandwidth, and scalability of monolithic arrangements, results in two-dimensional (2D) VCSEL arrays that appear as key components to reach highest aggregate bandwidths of tomorrows parallel optical transceivers. We report on 2D VCSEL arrays, substrate emitting although operating at 850 nm and prepared for flip-chip bonding, that are well suited for the customers needs in terms of speed, power consumption, reliability and compact integration. Based on advanced technology, our arrays target the requirements of transceivers in the OC-192 VSR and 10 Gigabit Ethernet arena. In this paper we present the basic technology, static and dynamic device characteristics as well as reliability data for a 4x12 850 nm bottom-emitting VCSEL array. A13