Production - Finding the suite spot with modular measurement systems
Building-block test structures allow component manufacturers to mix, match, and reuse test equipment as needed.
GUNNAR STOLZE, Agilent Technologies
As service providers face investment in their infrastructures that is higher than revenues, their key task is to cut network costs. That implies the development and use of innovative network technologies, higher equipment integration levels, new mesh architectures, and the reduction of operational costs.
On the component level, this essential cut in deployment costs recently caused steep price erosions in the passive-device market. Consequently, manufacturers were forced to improve productivity and profitability. The number one goal in component manufacturing is reduced time-to-profit. Achieving this goal requires a range of measures, but driving down manufacturing cost by optimizing production and test processes is among the most important.
To stay competitive, component manufacturers must drive down costs while pushing technological boundaries. Current trends in fiber-optic networks call for increased data rates and channel counts per optical fiber. The resulting increase in revenue per fiber has placed enormous pressure on the fiber-optic-component manufacturer to evaluate performance characteristics and failure modes more extensively. The amount of testing per device has increased, and the types of tests are becoming more complex.
In an effort to provide faster test development time and more reuse of high-performance equipment, some vendors are pursuing the concept of building block structures for passive-component test and measurement. By mixing and matching various tests into a common working environment, test solution suites allow manufacturers and end users of fiber-optic components to quickly tailor tests to meet specific demands.
The passive-component-test (PCT) system is an example of this type of building block. The PCT system measures insertion loss and polarization dependent loss (PDL) over wavelength using a tunable-laser source and broadband detector. The system also can utilize a customized data analysis block to calculate from the collected data scans, center wavelength, peak ripple and insertion loss, bandwidth, crosstalk, and other measurements. Additional building blocks can be added with minimal bridging code, such as backreflection tests, environmental thermal control, fiber alignment, and dispersion measurements. Ultimately, fully integrated assembly and test can be developed using the same platform.
Many aspects factor into the calculation of component manufacturing cost-throughput and yield are most important. The hiring and acquisition activities during last year's explosive industry growth met only short-term needs. Automation is the only alternative to volume manufacturing in the long run. It speeds up manufacturing processes, reduces labor costs, and overcomes the yield limitations set by human potential. Implementing automated production processes is a powerful way to reduce costs, especially at the labor-intensive adjustment, alignment, and packaging workstations.
Automation depends on suitable instrumentation for process control, thus immediately posting new challenges for optical-component test solutions in terms of accuracy, reliability, and measurement cycle time. Ultimately, the combination of automation and optical test is mandatory to successfully enter the next generation of optical-component manufacturing.
Testing represents as much as 40% of the optical-component cost structure. It is divided into cost of test execution, determined mainly by test cycle times; labor cost, maintenance, and service expenses; and cost of test development, of which the labor and equipment investments are a big part.
Any test sequence in production increases the production cycle time, consequently decreasing throughput. However, yield can only be guaranteed by implementing a suitable quality management tool. Balancing quality-control and throughput requirements is critical in meeting the cost-reduction goal.
Optimizing the cost of test will ultimately involve increasing the level of automation. The pass-on effect is the increased complexity of each manufacturing station, driven by higher automation levels as well as more advanced components. Manual, multistation production cycles are replaced by highly automated and fewer station manufacturing environments with integrated test and measurement capabilities.
To reasonably achieve that re quires flexible, adaptive, intelligent test solutions. The intelligence herein lies in the capability to address, with a flexible combination of test-solution engines, a great variety of components that differ in functionality, technology, connectivity, and physical size. These engines are so-called building blocks available as a measurement system or test library.
The modular concept of the measurement system, enhanced by the comprehensive collection of measurement and analysis functions of the test software, ease test-solution integration and reduce engineering time from a couple of months to just a few hours. It also provides the versatility to meet various testing requirements, ultimately lowering test costs.
To allow component manufacturers to handle emerging technologies such as 40-Gbit/sec (OC-768) transmission rates, test solutions need to offer upgrades with new functionality for parameters such as dispersion. Chromatic dispersion (CD) and polarization-mode dispersion (PMD) have attracted enormous attention among component manufacturers.
The lack of standardized definitions of component specifications is another issue. Only the most flexible analysis tools can account for the variety of parameter definitions for bandwidth, passband ripple, and crosstalk, among others.
Realization of the cost-reduction goal requires suitable tools to evaluate production processes be implemented at the three main areas of test: incoming inspection to verify the quality of supplied subcomponents, in-process test to monitor production processes, and final test to specify the packaged product. These test steps are generic for every manufacturing floor and have one goal in common: to guarantee yield and prevent potential follow-up costs through device qualification and process control (see Figure 1). Let's take a look at several test solutions based on modular measurement systems used in conjunction with test and measurement software:
- Thin-film filters are used for a variety of optical components such as multiplexers/demultiplexers or WDM filters. The quality of the final device depends on the quality of the filter alignment with the input and output fibers and on the spectral transmission properties of the filter, which is verified in the incoming inspection. High throughput is achieved by automating the positioning of the test devices. The automated alignment requires optical feedback in the form of real-time measurements of the spectral transmission and reflection properties. Fast data acquisition and analysis are mandatory for high throughput. The test and measurement software in conjunction with a tunable-laser source and optical power sensor or head provides real-time updates and analysis with the wavelength accuracy and resolution required for WDM components, thus representing a solution for automated filter alignment systems. Increased throughput, reduced development effort, and simplified integration into the automated system are the key contributors to the test cost reduction.
- Verifying the quality of fiber connectors involves testing for insertion and return loss under special testing requirements such as instantaneous and repeatable measurements. Small footprint solutions embedding return-loss modules with integrated laser sources and power sensors can offer accurate connector test capabilities. Internal power monitoring and compensation make measurement accuracy independent from the light source used. Return-loss modules enable users to extensively improve the manufacturing yield, enabled by better specifications for return-loss measurements. Performing measurements is simplified by pre-calibration of the modules, which speeds up the test.
- Fiber Bragg gratings are used for a variety of optical components such as add/drop modules, gain-flattening filters, WDM filters, and dispersion compensators. The process of writing the Bragg grating into a fiber is critical for the quality of the final component. The spectral transmission properties of the fiber grating are monitored during the writing process in a so-called in-process test. Performing spectral transmission tests during the production process optimizes throughput. That reduces the number of required test steps afterwards. The test solution provides feedback on the process quality and demands real-time spectral-loss measurements. High wavelength accuracy is guaranteed by employing a high-performance tunable laser. Using the modular concept of the measurement system allows technicians to choose the tunable-laser source according to the wavelength band of interest. Filters and pump couplers for the 1400-nm region can be tested using an E- and S-band tunable-laser source.
- Multiplexers/demultiplexers are among the most demanding components for testing, because these devices combine high port counts with high dynamic range and narrow channel spacing, all of which set special requirements on test solutions. The final test of multiplexing/demultiplexing devices relies on high measurement accuracy because device specifications are based on the measurement results. Swept-wavelength and multichannel loss measurements reduce test cycle times, while providing a comprehensive set of measurement and analysis parameters, including insertion loss, PDL, and return loss. A photonics test library contains functions to perform swept-wavelength Mueller method PDL measurements and analyze spectral-loss measurements for specific channel characteristics such as center wavelength, n-dB bandwidth, and cross talk. The modular platform provides the flexibility and scalability needed to adapt the solution to the specific test needs (see Table on page this page).
As transmission bit rates increase, new test parameters must be determined to qualify a passive optical component. The all-parameter test solution for passive components is capable of measuring insertion, PDL and return loss, and phase parameters (CD, PMD) by connecting the component under test just once. As a consequence, the measurement uncertainty induced by multiple connection steps can be significantly reduced and the test throughput increased.
Receivers, transponders, 40-Gbit/sec forward error correction, and multiplexing functions are implemented in ICs. These devices are so complex that they push the boundaries of semiconductor technology.
Handling 40-Gbit/sec multiplexing represents a technological challenge, because traditional materials may not be able to propagate electrons at such high speeds. As designers move to 40 Gbits/sec, cost and reliability become major concerns.
Isolating functions into discrete components can significantly raise the cost of a line card and add complexity by requiring high-speed interconnection among various chipsets. That can affect the overall size of a 40-Gbit/sec design as well as its power dissipation and heat. Multiplexer implementations comprise 4:1 (10.8-Gbit/sec to 43.2-Gbit/sec) or 16:1 (2.7-Gbit/sec to 43.2-Gbit/sec) implementations.
The ideal way to thoroughly characterize multiplexers/demultiplexers while reducing test time is with a parallel bit-error-rate (BER) test. The benefit is that manual connects, disconnects, or programmed switching of serial equipment is no longer necessary. Deteriorating effects such as skew and jitter also can be identified quickly. An ideal ParBERT, which is an integrated BER measurement system, offers optical capabilities in C-band and/or L-band with wavelength agility of 1530-1565 nm or 1570-1600 nm to enable an integrated test for 40-Gbit/sec modules such as transponders and transceivers.
On the tributary 16x2.7-Gbit/sec side of the multiplexer/demultiplexer, the fast eye mask software accelerates manufacturing tests. With the possibility of measuring the BER of a predefined number of points-1 to 32-within the eye, the quick pass/fail result is ideal for manufacturing.
The photonic technologies associated with most fiber network solutions rely on optical amplifiers (OAs) to maintain signal strength throughout the network. Erbium-doped fiber amplifiers are currently the most mature OA technology, but research in Raman amplifiers is rapidly making its transition from the lab onto the production floor. Technologies in the early stages of development include semiconductor optical amplifiers and erbium-doped waveguide amplifiers. These emerging technologies promise to reduce the cost and size of the hardware, eventually bringing affordable high-speed network access to residential customers.
Forward-looking companies must be committed to providing research and manufacturing environments with an extensive set of standard and custom test solutions. The test methods include OA laser pump control, environmental chamber control, multiple OA parallel test schemes, precision WDM source input comb leveling, and high source powers. These options can be configured to meet customer-specific requirements. It is also important to implement customer-specific measurement algorithms, including noise gain profile, noise figure, and polarization dependent gain.
The emerging Raman and semiconductor OAs cannot rely on the modulated techniques used for characterizing erbium-doped amplifiers. A combination optical and electrical solution is needed.
The advances in fiber-optic technology naturally influence the development of optical test solutions. An increasing variety of components points to the need for more versatile test solutions.
At the same time, test solutions must support the cost-reduction goals in component manufacturing. By enabling automation in production environments and reducing cost of test, intelligent solutions will enhance the next-generation manufacturing floor.
Gunnar Stolze is application engineer at Agilent Technologies Deutschland GmbH (Böblingen, Germany).