Recent technical advances in fiber optic sensor technology have brought fiber sensors into the mainstream. Using a wide
variety of sensing elements, and interrogation techniques, these devices are finding applications in fields from power line
management to homeland security. A variety of fiber sensor technologies, applications, and markets are discussed.
Over the last few years, Motorola has been applying a different kind of semiconductor laser technology to a family of datalink and discrete products. The laser technology is commonly referred to as Vertical Cavity Surface Emitting Lasers (VCSELs). This technology is now emerging from advanced development and research laboratories into the market place, and there is a number of introductory texts, inaddition to journal articles that describe the technology . Motorola, has chosen the VCSEL technology to be the back-bone of it's optical program and has developed products such as the OPTOBUSTM datalink [1,2,3], which interconnects arrays of VCSELs inside a small, compact module via parallel fiber ribbon to receiver modules. As an extension to this technology, Motorola has designed new and novel ways to package the VCSELs. This paper will detail, inaddition to the packaging used in OPTOBUSTM, two approaches to discrete VCSEL packaging that are commercially competitive; the flip-chip and the angled angle TO-can. The essence of both these package designs is what is usually termed as 'auto-power control' (APC). This allows a feedback mechanism to feedback a signal to the laser, to control or change its output power level with respect to system conditions. Usually, this is accomplished by back facet monitor photodetectors in conventional edge emitting laser systems. As the VCSEL does not have facets, alternative solutions have to developed; the flip-chip and angled TO-can are shown to be good candidate.
High performance, low cost, and highly reliable vertical cavity surface emitting lasers (VCSELs) have been developed and are currently being used in both parallel and serial optical interconnect applications. For example, Motorola's OPTOBUSTM parallel optical interconnect relies heavily on the unique characteristics of arrays of GaAs based VCSELs emitting at 850 nm to achieve its stringent performance goals at low cost. Representative parametric results of discrete VCSELs and VCSEL arrays will be compared, including `optical power output-current' and `current-voltage' curves, optical wall plug efficiencies, and modulation characteristics. The use of statistical parameter analysis across a wafer and subsequent parametric wafer maps has proven to be a valuable tool for maintaining control of the fabrication process. The consistency of VCSEL parameters across individual VCSEL arrays will be discussed. VCSELs are very robust devices. Life times at room ambient in excess of 3E6 hours have been reported by several groups. Degradation behavior of selected device parameters will be discussed. Failure analysis demonstrating the effect of proton implant depth on reliability will be presented. ESD damage at forward bias is shown to be process related, while ESD damage at reverse bias is shown to be material related. These VCSELs are ESD Class 1 devices.
The OPTOBUSTM family of products provides for high performance parallel interconnection utilizing optical links in a 10-bit wide bi-directional configuration. The link is architected to be 'transparent' in that it is totally asynchronous and dc coupled so that it can be treated as a perfect cable with extremely low skew and no losses. An optical link consists of two identical transceiver modules and a pair of connectorized 62.5 micrometer multi mode fiber ribbon cables. The OPTOBUSTM I link provides bi- directional functionality at 4 Gbps (400 Mbps per channel), while the OPTOBUSTM II link will offer the same capability at 8 Gbps (800 Mbps per channel). The transparent structure of the OPTOBUSTM links allow for an arbitrary data stream regardless of its structure. Both the OPTOBUSTM I and OPTOBUSTM II transceiver modules are packaged as partially populated 14 by 14 pin grid arrays (PGA) with optical receptacles on one side of the module. The modules themselves are composed of several elements; including passives, integrated circuits optoelectronic devices and optical interface units (OIUs) (which consist of polymer waveguides and a specially designed lead frame). The initial offering of the modules electrical interface utilizes differential CML. The CML line driver sinks 5 mA of current into one of two pins. When terminated with 50 ohm pull-up resistors tied to a voltage between VCC and VCC-2, the result is a differential swing of plus or minus 250 mV, capable of driving standard PECL I/Os. Future offerings of the OPTOBUSTM links will incorporate LVDS and PECL interfaces as well as CML. The integrated circuits are silicon based. For OPTOBUSTM I links, a 1.5 micrometer drawn emitter NPN bipolar process is used for the receiver and an enhanced 0.8 micrometer CMOS process for the laser driver. For OPTOBUSTM II links, a 0.8 micrometer drawn emitter NPN bipolar process is used for the receiver and the driver IC utilizes 0.8 micrometer BiCMOS technology. The OPTOBUSTM architecture uses AlGaAs vertical cavity surface emitting lasers (VCSELs) at 850 nm in conjunction with unique opto-electronic packaging concepts. Most laser based transmitter subsystems are incapable of carrying an arbitrary NRZ data stream at high data rates. The receiver subsystem utilizes a conventional GaAs PIN photo-detector. In parallel interconnect systems. The design must take into account the simultaneous switching noise from the neighboring systems. If not well controlled, the high density of the multiple interconnects can limit the sensitivity and therefore the performance of the system. The packaging approach of the VCSEL and PIN arrays allow for high bandwidths and provide the coupling mechanisms necessary to interface to the 62.5 micrometer multi mode fiber. To allow for extremely high electrical signals the OPTOBUSTM package utilizes a multilayer tape automated bonded (TAB) lead frame. The lead frame contains separate signal and ground layers. The ground layer successfully provides for a pseudo-coaxial environment (low inductance and effective signal coupling to the ground plane).
The use of vertical cavity surface emitting lasers (VCSELs) in a parallel optical interconnect for Motorola's OPTOBUSTM interconnect was made public over 1 year ago. This was the first time VCSELs were introduced into a product which took advantage of the excellent qualities of VCSELs over edge-emitting lasers. Motorola's OPTOBUSTM interconnect is a ten channel parallel bi-directional data link based on two 10 channel multimode fiber ribbons. One of the key differences in this type of interconnect compared with previous data link designs is the use of the VCSELs as the optical source for the link's fiber optic transmitter. A single 1 X 10 VCSEL array from a GaAs wafer is die attached to a 10 channel GUIDECASTTM optical interface unit which couples the emission from each laser device to its corresponding fiber ribbon channel and thus negates the use of expensive manufacturing techniques such as active alignment and pig-tailing. The OPTOBUSTM interconnect achieves its performance goals (which include low cost) via the unique characteristics of the GaAs VCSELs arrays. For example, the 850 nm devices produce a circular symmetric beam with a half angle of about 10 degrees allowing the coupling loss into the waveguide to be less than 3 dB. In addition, to maintain low manufacturing costs, each VCSEL array is individually and automatically probe tested (just as in the silicon industry) to verify that each VCSEL achieves the OPTOBUSTM interconnect's stringent electrical, optical, thermal and mechanical specifications. Typical computer generated wafer maps from automated production tooling and statistical parametric results are discussed. The combination of low threshold currents with superior thermal and optical performance allow the devices to be modulated under fixed bias conditions. Typical drive currents of 3X threshold are used to obtain nominal FDA Class 1 safety optical power levels from the GUIDECASTTM optical interface unit.