Instruments Race To Stay Abreast Of Advances In Fiber Optics

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.

Keithley Instrument's Model 2520INT Integrating Sphere and Model 2520 Pulsed Laser-Diode Test System compose a duo that can accurately measure laser-diode optical power at the wafer, bar, or chip level (Fig. 1). There's no risk of thermal damage that might otherwise occur when testing without cooling components. This instrument pair can be used for light-current-voltage (LIV) production testing of 980- and 1480-nm EDFA pump lasers, Raman amplifiers, telecommunication laser diodes, and high-power telecommunication VCSELs.

The Model 2520INT consists of a 1-in. sphere with a germanium detector to provide a wide operating range of telecom wavelengths. It promotes ease of setup and integration and achieves very low-level power measurement. It also shortens testing time by providing an SMA fiber tap to enable measured light to be sent to another instrument simultaneously for additional optical measurements.

Test Systems For Do-It-Yourselfers: Over 90% of the fiber-optic component test systems used in R&D and manufacturing are assembled in-house. Test engineers usually choose instruments or instrument modules, one by one, and then add a computer. So it's understandable that modularizing is a popular approach to fulfilling test needs.

Modularization is attractive because it enables test engineers to economize. They can configure and later reconfigure test systems, and swap and interconnect modules as testing requirements change. A number of customizable test-configuration entries have arrived in the marketplace this past year.

Anritsu's version, the ME7894A optical component test system, evaluates optical components slated for wideband transmission (WDM) systems operating in the C- and L-bands (1530 to 1570 nm and 1570 to 1610 nm, respectively). This test system comprises a tunable laser source, an optical power meter, a high-sensitivity sensor, and control software. It exhibits a wide dynamic range, enabling it to conduct accurate analysis of optical filters, couplers, isolators, and splitters during R&D and manufacturing.

Another entry is Agilent's E2156A customizable optical amplifier (OA) test system. This open, extensible platform can be customized to meet user-specific needs. Test engineers can configure it with just the right amount of test capabilities for budget-sensitive applications and still protect their investment, upgrading as necessary.

FlexTest, until recently known as Instrumentation Engineering, has introduced two test platforms (see the table, again). They are aimed at implementing test solutions for various protocols, including Gigabit Ethernet, 10-Gbit Ethernet, InfiniBand, and Fibre Channel. These test stations can be supplied with single or multiple device-under-test (DUT) sites. Both single- and parallel-channel configurations are available.

JDS Uniphase has six new cassettes that are actually modules for its Multiple Application Platform (MAP). These additions, which enable the testing of fiber-optic systems and components, broaden the MAP portfolio to 13 high-performance cassettes.

The modules host a tunable source cassette with 110-nm of tunable range over C- and L-bands and a broadband source cassette that provides an amplified spontaneous emission output. This output exhibits a flattened, high-power density across the C-band. A utility cassette simplifies the mechanical integration of passive optical components for test sets. Finally, an electrical clock and data recovery cassette supports five key 10-Gbit/s data rates.

Optical Switches To The Rescue: A critical question for test engineers is if light signals can switch between multiple input and output channels. If they can, costs during the design and manufacturing test phases can be slashed.

Typically, optical tests require expensive stimulus sources, like optical transmitters and acquisition tools (signal analyzers and bit-error-rate testers, or BERTs). But by automating an optical test station, utilization of optical test instrumentation can be increased. This boosts productivity and reduces costs.

Such is the path pursued by Mink Hollow Systems Inc. Its designers configured a National Instruments PXI chassis and controller with an optical switch in a compact solution that can be easily customized and reproduced. National Instruments' LabView was used to write an automated test that was quick to develop and increase equipment usage, thereby reducing costs. With this optical switching configuration, users can connect a unit under test (UUT) to four optical test instruments that can run specified tests (Fig. 2). Conversely, four UUTs can be connected to a cluster of instruments.

First, the software identifies and displays the characteristics of any optical switches in the PXI chassis. By selecting a specific switch, a control appears on the screen of the companion PC to show the number of available connections. Clicking on a specific switch location then toggles the connection.

Another interesting optical switch development comes via Tektronix. Its OSW8000 optical switching modules use microelectromechanical systems (MEMS) mirror technology. These switches let optical equipment designers and manufacturing test engineers switch instruments and UUTs without compromising quality or productivity of the tests. The switches display excellent repeatability, low insertion loss, and low polarization dependent loss. Modules include LabView and LabWindows/CVI drivers to provide an easy-to-use programming environment that's compatible with other Tektronix optical test system instruments.

Moving Up To 40 Gbits/s: Although 40-Gbit/s deployment is still in its infancy, a rush of products with this technology is being developed. To meet instrumentation needs, a number of new module test systems have debuted. Agilent Technologies' OmniBER optical transport network (OTN), 40-Gbit/s communications performance analyzer (CPA) is a multirate Sonet/synchronous digital hierarchy (SDH) and OTN test set that can assist engineers in developing and qualifying 40- and 43-Gbit/s line cards, modules, and subsystems. Equipped with all line rates from 52 Mbits/s to 40 Gbits/s, the CPA makes it possible to validate device operation and conformance to design criteria and Telcordia/ITU-T standards.

The analyzer provides an intrusive "thru" mode for both Sonet/SDH and OTN at 40 and 43 Gbits/s. At 43 Gbits/s, the OmniBER OTN analyzer offers ITU-T G.709-compliant OTU-3 testing with forward-error-correction (FEC) analysis plus error-add capability. This lets engineers simulate realistic network conditions. High-accuracy jitter testing to 10 Gbits/s for Sonet/SDH and the ITU-T G.709 optical channel is another feature. Moreover, the analyzer can map defects when encapsulating Ethernet payloads into Sonet/SDH, up to 2.5 Gbits/s.

Dreaded "chirp" becomes a huge bandwidth-limiting factor as telecommunication networks soar from 10-Gbit/s DWDM systems to 40-Gbit/s systems. Chirp is a carrier wavelength shift that occurs when a laser rapidly generates optical pulses. Measures must be taken to ensure that chirp doesn't impair signal quality.

Advantest's Model Q7607 Chirp Test Set (CTS) measures chirp at data rates up to 50 Gbits/s. It can be used to develop active optical components, like modulators, systems, and subsystems, as well as components composing such products. Measurements can be performed in less than 30 seconds.

The CTS exhibits a frequency resolution of 30 MHz and includes an interferometer with a free spectral range of 300 GHz. An optical amplifier, available as an option, covers optical frequencies in the C- and L-bands.

Turn Noisy Lasers Into Sharp Sources: Enhanced channel isolation is essential in situations that require a single channel to be analyzed for preproduction testing of new DWDM designs. Needless to say, transforming inexpensive, noisy lasers into sharp laser sources for a fraction of the cost of high-end lasers becomes invaluable.

Digital Lightwave's Automatic Channel Locking Filter (ACLF) suppresses noise on laser sources and isolates and tracks transmission channels (Fig. 3). It can isolate and select a single WDM channel while attenuating all other channels. In a single, 6.5- by 3.9- by 0.9-in. package that weighs 6.6 pounds, the ACLF combines the high performance of the fiber Fabry-Perot Tunable Filter (FFP-TF) with automatic scan and lock circuitry.

After light enters the ACLF through the input port, it's passed via an isolator and the FFP-TF. A small fraction of the optical power is split through a 1% optical tap and sent to the detector. The electrical signal from the detector is then fed to the ACLF scan and lock circuitry, where a phase-locked loop (PLL) ensures that the peak of the tunable filter passband locks to the frequency of peak lasing intensity. The remaining 99% of the optical signal is fed from the ACLF to the output port.

Bit-Error-Rate Testing: Another essential qualifier of data-handling equipment is bit-error-rate testing. Agilent recently enhanced its modular test platform, the 81250 ParBERT 10-Gbit Ethernet, and its 40-Gbit/s testing capabilities. It can test a wide range of bit rates and devices without the inconvenience and extra cost of mastering a totally new test instrument. In fact, measuring is rather easy. The instrument can test electrical, optical, and combined integrated product configurations at 45 Gbits/s, matching the range of OC-768 DUTs.

Enhancements include new modules that support 45-Gbit/s electrical and optical measurements and expand ParBERT 81250's scalable, SFI-5 capabilities. The platform now offers an integrated solution for users who need to test devices like SFI-5 40-Gbit/s transponders, or optical components and optical transport semiconductors used in such transponders. Other enhancements are a 10-Gbit Ethernet frame editor and post-processing software that enable parametric, OSI layer 1 tests of XAUI and 10-Gbit Ethernet interfaces.

Stressed Eye Testing For 802.3ae: An economical solution for stressed eye testing, as specified in IEEE 10-Gbit Ethernet Standard 802.3ae, is becoming a must. This measurement involves controlling the extinction ratio (ER), the electrical bandwidth for ISI, AM interference, and timing jitter for receiver conformance testing. To meet these test requirements, JDS Uniphase has introduced a stressed eye tester, optical pattern generator, and optical error detector.

The company's versatile OPTX10A stressed eye generator, a 1310- or 1550-nm reference transmitter, supports data rates of 155 Mbits/s to 10.71 Gbits/s. It features an adjustable ER from 10 to 3 dB, selectable electrical signal paths (internal pass through and standard four-pole Bessel Thomson, and external filter) and a sinusoidal, amplitude interference input.

The OPG10A optical pattern generator and OED10A optical error detector compose a BERT with high-level sensitivity and overload performance. The OPG10A includes an option for input frequency offset, which targets sinusoidal timing jitter generation and serial electrical output.

Jitter Analysis: With rapidly rising clock speeds and data rates resulting from the exponential growth in data transmission, designers are coming to grips with a new reality—control of jitter has reached critical status. To that end, the Wavecrest SIA-3000 family of instruments enables high-speed design, debug, characterization, and production testing of optical signals in Gigabit Ethernet, Fibre Channel, Very Short Reach (VSR), Sonet, and InfiniBand applications.

SIA-3000 family members can perform a 1000-sample period measurement in less than 9 ms. These instruments can separate jitter into its random and deterministic components and analyze jitter in a wide range of applications. They also can measure data rates up to 4.5 Gbits/s with 200-fs resolution and high accuracy.

Looking Ahead: Much of the fiber-optic component industry is now where the semiconductor industry stood 30 to 40 years ago—fabricating discrete components and then assembling them manually. Integration and automation in both manufacturing and testing are just getting started. No silicon lasers exist, so materials of primary interest are gallium arsenide and indium phosphide.

DWDM is slated to comprise 160 wavelengths of 10 Gbits/s (1.6 Tbits/s) spaced 25 GHz apart. However, some vendors propose narrowing the spacing to 12.5 GHz, which translates to 320 wavelengths of 10 Gbits/s (3.2 Tbits/s). That's why the challenges to those who supply the components and modules, and those who develop the instrumentation to meet test requirements, are formidable indeed.