It weighs in at a scant one ounce per kilometer. Yet just one fiber-optic strand, smaller than a human hair, can carry all the telephone traffic in the U.S. at the peak busy period of the year. With such credentials, it's understandable why fiber is well on the way to becoming the predominant transmission medium of the 21st century. Fiber also brings extraordinarily wide bandwidths that will become commonplace 20 years from now (see "Fiber Optics' Ascendance In Digital Transport Networks," p. 62).
As is well known, fiber trunks tie the nation together, though much of it is idle. But connecting the trunks to the end users through the "last mile" has been limited by the sluggish economy and lack of financial incentives for local operating companies to bring wideband into every home and enterprise, mixed together with a variety of technical issues.
But never mind that these obstacles have yet to be swept aside. Optical product development is flourishing these days, driven in part by a zeal to bring component costs down. Optical devices are being combined with electrical devices and, in some cases, moving onto a common substrate, shrinking size and cost. These innovations pressure optical test manufacturers to keep pace by introducing innovative instrumentation.
Consider dense wavelength-division multiplexing (DWDM) fiber-optic communication systems. For their sales to accelerate, production costs must diminish so component prices can begin to spiral downward. One manufacturer predicts that prices need to drop by as much as 40%. This is the situation facing designers of fiber-optic products, such as laser-diode modules (LDMs)—a critical element of DWDM communication systems. Testing is costly in LDM production due to the high value added during manufacturing.
Instruments of primary interest to those designing products fall into two categories: device and module testing.
Device Testing Issues: "It is the need to merge optical and electrical engineering that makes optical device testing such a challenge," says John McLin, chief technology officer at Cottonwood Technology Group, Scottsdale, Ariz. Cottonwood specializes in testing devices such as vertical-cavity surface-emitting lasers (VCSELs), laser diodes on wafers, packaged detectors, and transmitter/receiver modules.
VCSELs were developed in the mid-1990s. From a testing standpoint, the VCSEL has the advantage that its vertical-cavity construction can be tested as a laser right on the wafer. VCSELs are becoming the predominant laser for short-haul applications, like last-mile. The long haul primarily falls to edge-emitter lasers. But the latter can't really be fully tested at the wafer level because it's the cleaving process that turns them into lasers. So one must devise special technologies to test, but not until the bars are cleaved.
McLin adds that the major hurdle is the "at speed" test of optical electronic devices, which means testing at full data rates. At the wafer level there are a few standards, device topologies, and test methodologies. McLin points out that his customers require several kinds of testing. There are the dc parametrics that characterize basic electro-optical characteristics of devices—whether they are emitters or detectors. These tests are often called "structural," distinguishing them from performance parameters.
"What our customers are looking primarily at," says McLin, "is the electro-optical performance of laser diodes—light emission versus voltage and current, and slope efficiency." This is the efficiency at which electrical energy is converted into optical energy (plus parasitic issues, like unwanted capacitance, that may be present on the device in the wafer form) prior to singulation.
The basic characteristic customers test is electrical-to-light conversion. This is achieved by sweeping a current while measuring both the forward voltage and the optical output. Usually, users evaluate the "slope efficiency," which is the optical output as a function of that current. Because such testing requires the integration of diverse technologies, McLin and his design group create the test systems to fit the bill.
Pulsed Power For Nondestructive Test: Weeding out defective laser diodes early in the production process is another cost cutter. Typically, a laser diode is coupled to a fiber-optic pigtail during the final stages of manufacturing, prior to its integration into a complete, temperature-controlled laser-diode module. At this point in the manufacturing process, the module contains temperature measurement and control components, as well as the laser diode.