Built for the long haul

The CoreStream system Products tested in the components lab For More Information Read other articles from this issue: Table of contents, May 2005 FEATURES Built for the long haul Test fast clocks on the cheap Concurrent test Spin, look, and measure

LINTHICUM, MD

Ciena takes an unusual approach to product development, as some of the people who define the technical details of new products and upgrades also perform design-validation tests. Greg Bartolomei, director of systems validation, and James Allard, senior director of systems and verification, are two key people at the beginning and ending of a product's development cycle. Allard works closely with Ciena's customers and with marketing to obtain feedback to define a new product or feature upgrade. After marketing develops a functional feature specification, Allard's systems-engineering team transforms it into an engineering requirements and specifications document.

When Ciena begins work on a new design, Allard assigns a systems engineer to follow the product throughout its development. Allard says the engineer lives in the engineering labs and is the "one neck to grab" when other engineers need a resource or if something goes wrong. "The process not only ensures quality," said Allard, "but it ensures that we developed the product we set out to design."

James Allard, senior director of systems and verification, leads the group that writes engineering specs; his group also has the final say on when a design is ready for production.

Photonics director Jean-Luc Archambault manages eight engineers in the components lab who evaluate optical products for new designs. Upon receiving engineering specs, engineers including senior principal engineer Dr. Doyle Nichols test optical receivers, transmitters, amplifiers, and passive components such as splitters. (For a complete list, see "Products tested in Ciena's components lab" at the bottom of this page.)

Jean-Luc Archambault manages the components lab, where engineers evaluate active and passive optical components.

The components lab has four test benches for active components, two each for evaluations at 10 Gbps and two for slower speeds (1 Gbps and 2.5 Gbps). On the day of my visit, Nichols tested an optical receiver at 10 Gbps. He often tests components at the slower speeds when they will connect to a tributary line.

To test a component, Nichols uses a test mounting board roughly 2 ft x 3 ft in size, on which he mounts the device under test (DUT), fiber-optic cables, attenuators, splitters, and connectors. Nichols knows the characteristics of all components other than the DUT, which ensures that no unforeseen variables will alter his measurements.

Nichols routes the receiver's electrical output to a clock-recovery circuit, which extracts embedded clocks from data streams. He runs the receiver's recovered clock and data stream into a bit-error-rate tester (BERT), an optical spectrum analyzer (OSA), and a digital communications analyzer (DCA). With these instruments, Nichols performs tests such as BER versus optical power and BER versus optical signal-to-noise ratio (OSNR). A receiver evaluation usually takes one or two days in the optics lab. (For details on how Nichols performs BER versus OSNR tests, see "Bits battle noise," Ref. 1.)

Optical signals must pass through dozens of amplifiers as they travel the long distances between cities. Ciena engineers typically perform more than 100 measurements when they qualify an amplifier. "Testing is an Nth-dimensional problem," said Archambault. "You can spend as much time testing as you want." While Ciena purchases most of the amplifiers it uses, Archambault said the company gets heavily involved in an amplifier's design. "Sometimes, we come up with our own preliminary designs and show them to our suppliers," he noted.

Figure 1.  An optical switch routes a light pulse around a loop to simulate the effects of the pulse going through a chain of amplifiers.

The recirculating loop is but one homegrown tester in the components lab. Ciena engineers also built an automated test system that measures how well an amplifier boosts optical 1's and 0's. Other tests the system performs include gain level, gain flatness, and gain accuracy. The engineers use a comb laser source to generate more than 2000 optical signals of different wavelengths across a 38-nm wavelength band. Using an OSA, they can take measurements at each wavelength.

Archambault's team also test dispersion-compensating fibers (DCFs), which compensate for dispersion of light as it travels through a fiber-optic cable. The engineers measure loss and dispersion as a function of wavelength, and they perform multipath interference measurements. Backscattering will occur in a DCF that isn't properly designed.

Besides performing bench-level performance tests, the engineers also test components for reliability by placing them in thermal chambers. Archambault pointed out that a component will undergo a thermal stress at 75°C, 90% relative humidity for about three weeks. Components also undergo temperature cycling from –40°C to 85°C.

Operational tests take place at temperatures from 0°C to 65°C. During these tests, an engineer drives a component with a pattern generator and measures BER. The engineer also looks at a component's eye diagram with a sampling oscilloscope to measure how a device's output changes with temperature. An optical switch, controlled by a PC, lets engineers run thermal tests on up to 18 devices at once.

Senior director Cecil Smith manages 25 electrical, software, and automation engineers who develop products and test software in the electronics lab.

Hardware and software development takes place simultaneously. Hardware engineers manually test the first iteration of a product such as a line card by testing the card's physical-layer functions. Figure 2 shows the test equipment on a typical engineer's bench. Engineers use the equipment to measure parameters such as signal power, jitter, and BER.

To perform these tests, engineers built a test bed that includes a CoreStream backplane. It also provides access to optical and electrical data streams that the card needs. Engineers start by looking at waveforms from the backplane and from the line card's electrical interfaces to its optical components. The test bed connects to a PC that exercises and calibrates the card. (Calibration involves setting the card's optical power levels and wavelengths.)

Figure 2.  Engineers’ benches in the electronics lab contain optical and electrical test equipment.

For time-domain measurements, Smith's engineers use DCAs to observe eye diagrams of transport signals in electrical form on the card and in optical form as the signals enter and exit a card's line interfaces. They look for reflections and check for correct waveform shapes, and they use logic analyzers to investigate data patterns. They then adjust drive levels, extinction ratios, or impedances to get the best jitter performance.

The engineers also measure a card's optical output power and make BER measurements on both transmitted and received data streams. To make the optical BER measurements, they use SONET test sets that transport payloads of pseudorandom bit sequences encapsulated in SONET frames. The frames start as OC-3 (155 Mbps) and OC-12 (622 Mbps) streams multiplexed up to OC-192 (10 Gbps) streams. They also add noise to the signals so they can measure a card's ability to accurately transport data under less than ideal conditions.

If a card requires design changes, its second iteration usually goes to the automation group, where engineers run the card through a series of automated tests. "We have an extensive set of I/O libraries that we use to bring up new cards," said Smith. Automation engineers use those libraries to write test code in LabView, and they often reuse the code in production test systems.

As software engineers complete software modules, testing moves from checking the hardware to checking that the hardware and software work together. For example, a line card must report errors if SONET frames fail to arrive at their destination. The software must also fill a payload frame when errors occur.

When engineers are convinced that a card and its embedded software meet specifications, they perform environmental tests. "Some countries don't have good environmental controls," said Smith. "Transport systems could reside in central offices that, although heated, could receive a blast of cold air if someone opens the door in the winter, so we test our cards for thermal shock." A typical test subjects a card to a temperature change of 40°C in one minute.

When a card passes its tests in the electronics lab, it's ready to run in a system and communicate over a simulated network. In the lightwave systems (LWS) lab, Dr. B. Sridhar tests cards and systems under simulated real-world conditions. He uses racks of test and network equipment, 30,000 km of fiber, and dozens of optical amplifiers to emulate networks in the field. "People characterize communications systems in terms of distance and capacity," said Sridhar. "We test line cards and systems to see how they perform under different conditions." Sridhar also uses the lab's network to test purchased optical amplifiers by looking at the signals they produce and by running BER tests.

From his network measurements, Sridhar can evaluate new products and can recommend to customers how they should configure new networks for optimal performance. Ciena has developed a network-design software tool that customers use to configure their networks with amplifiers and fibers. The tool is based on Sridhar's work at characterizing networks in the LWS lab.

Figure 3.  In the LWS lab, engineers test the effects of amplifiers in an optical network.

Sridhar tests each transport system or line card with different vintages of fiber because each produces different amounts of dispersion in the optical signal. He then adds dispersion-compensation fiber to compensate for the network's dispersion, as Ciena's customers will do. "Once we design a transport system," said Sridhar, "we test it with different fiber types to ensure that it will work in the real world."

To perform his tests, Sridhar uses a tunable filter that selects each wavelength. He looks at each channel's optical power with an OSA. Although an OSA indicates a wavelength's power, it provides no information about the signal's integrity. Therefore, Sridhar uses a DCA to view a signal's shape and jitter in the time domain.

Because communications service providers demand that their networks operate at 10^–12 BER or lower, Sridhar designs networks to run at 10^–16 BER, which lets him attain a good margin of error. "We're trying to figure out how far we can go between amplifiers," noted Sridhar. "We look for the optimum configuration for each customer. More channels results in shorter distances because channels close in wavelength will more likely interfere with each other as distance increases."

Director of systems validation Greg Bartolomei and his team perform system-level tests on Ciena's new products to verify that they meet engineering specifications.

Allard and his engineering team test new products under conditions that simulate a customer's existing network, bringing hardware, its embedded software, and system software together for the first time. In this lab, engineers verify that a product or product upgrade will work with the network-management software that Ciena's customers use. Ciena provides network operators with network-management software, but so do third-party developers. Some operators develop their own management software, and Ciena uses that software in the systems-validation lab to verify that products are ready for production.

Product upgrades must also pass tests in the systems-validation lab. "Customers don't need a new box to get the latest features," said Allard, "They can add the latest features to existing equipment in the field." Bartolomei and Allard verify all product upgrades because they have at least one of every Ciena product built over the last 10 years. Thus, they can emulate any customer's network in the lab.

As part of systems validation, Bartolomei and Allard configure optical switches controlled through a LabView program that simulates real-world hazards such as broken fiber-optic cables. When a simulated break occurs, they check that the system properly recovers from the break. They also check a system's physical layer to make sure it's compatible with the customer's network. "It's a luxury to be able to simulate so many customer networks," said Bartolomei, to which Allard added "we can quickly mimic a customer's network from the management software layer to the physical layer."

When Allard's group approves a product or upgrade, it goes to production. The systems that test production units were developed by automation engineers in the electronics lab, which completes the circle of light.