by Anette Zimmermann
While bandwidth rates rise, costs must continue to fallâ��including the cost to test. This is particularly true when testing such multiport components as multiplexers and splitters. New technology and processes address this requirement.
As consumers' interest in YouTube, online gaming, and video on demand continues to proliferate and bandwidth demands grow, companies are expanding their networks via long-distance metro connections into the access networks. Heavy Reading estimates that by the end of 2012, nearly 90 million households will have fiber to the home (“FTTH Worldwide Technology Update & Market Forecast,” Vol. 6, No. 1, February 2008), driving the industry to heighten its focus on optical connections and transmissions (Figure 1).
The cost of the build out, which lies in cable and fiber deployment, has led service providers to manage their networks by employing optical components that offer flexibility, lower costs, and higher speed. To reduce operating expenditures and deploy more bandwidth-hungry applications and services, a better management of the fiber links is required. This will be achieved through optical network elements such as the conventional multiplexers used in static networks and the reconfigurable optical add/drop multiplexers (ROADMs) used in reconfigurable networks. Other typical components are optical splitters for PONs used in FTTH installations. All of these components require extensive optical testing in production.
Manufacturers of these components are experiencing a dramatic need to reduce the cost per device, since the goal in this type of environment is to maximize throughput and achieve very high yield while minimizing production costs. The problem that arises is how to ensure yield but not sacrifice throughput. Quality management always relies on tools that test the output (such as the product itself) of each major manufacturing process, and verification or characterization tests are important and used as common tools. However, each test procedure reduces the throughput by introducing test time, increasing the cost per product.
It is obvious that optical component test platforms must address these manufacturing requirements. Flexibility in the application is as important as easy and fast operation of complex test procedures. Fast and accurate measurements can reduce the impact on throughput and, at the same time, enable quality control.
One of the challenges in testing multiport devices is the fiber handling and connection to the test instrumentation. For example, a 40-channel DWDM multiplexer or a 32-channel PON splitter requires substantial time to connect all of the output fibers to optical power meters using standard fiber-optic connectors.
Care is also required to correctly allocate the fibers to the power meters and make good connections. While these connections are made, the instrument is not available for measurements. If the operator must be able to reach in among the fibers and make connections, the space required for these ports is also an issue.
Essential test time can be saved, while the test instrument measures another device connected to a second adapter, via up-front connection of the individual fibers to a multiport adapter (see the photo). Repeatable high-precision connections result from a quick-locking mechanism to snap the multiport adapter onto the measurement instrument.
Use of a multiport adapter can also facilitate the alignment of the connector keys. This makes it easier to connect ports in the desired order, which helps avoid errors and connector damage. This approach even makes the use of bare fiber holders possible, which could remain on the fibers of the tested component through the complete process.
The spectral measurement of optical insertion loss (IL) and polarization-dependent loss (PDL) provides the primary performance data for verifying passive fiber-optic components. The spectral dependence of these parameters is especially important for testing the abovementioned multiport components. At the same time, faster characterization is required while maintaining low uncertainty of these measurement parameters.
In the past, if the PDL was required to be measured over a wide wavelength range with high resolution, the Mueller method was applied. The calculation of the PDL is based on the Mueller-Stokes calculus, and the method obtains the Mueller matrix through the measurement of the device under test (DUT) transmission at only four well-defined states of polarization. However, this measurement principle posts several stringent requirements on the measurement setup to reduce the uncertainty in the measurement.
To overcome these shortcomings a newly introduced single-scan Mueller method can be used. This method is based on a single-scan technique in which all measurement data is collected in a single wavelength sweep of the tunable laser. The measurement setup is shown in Figure 2.
Compared to the traditional Mueller method setup, the polarization controller has been replaced by a polarization synthesizer that includes a controller consisting of various waveplates. It also features an in-line polarimeter, which measures the actual state of polarization of the optical signal. This polarization synthesizer is capable of switching the setting of the waveplates within a few microseconds.
While the laser is sweeping, the polarization is switched with high speed to a set of four or six polarization states. High-speed sampling ensures the measurement data are collected accurately for every polarization state. Using six polarization states instead of four enables additional averaging and increases the accuracy of the measurement. The exact set of Stokes parameters for each polarization state is determined by the polarimeter that is integrated in the synthesizer.
The benefits of the single-scan Mueller method include a high measurement speed, as only one wavelength sweep is required, as well as increased loss and PDL measurement accuracy.
For the characterization of multiport components, a multiport power meter that measures several ports simultaneously can save significant time. However, once the multiport device is attached to the power meter, the measurements produce large amounts of data.
In the past, the bottleneck for test throughput was often the traditional GPIB connection for uploading high-resolution spectra from many channels. Modern power meters offer high-speed measurement data acquisition and transfer of up to 1 million samples per channel. Using a PC with USB or a LAN speeds up the data transfer and overcomes the restriction. To optimize the measurement speed, a power meter with a high dynamic range should be used for filter measurements, as this makes “stitching” of two power range traces unnecessary.
The use of high-speed measurement data acquisition, fast data transfer for post-processing, and a high dynamic range, combined with the novel method to measure PDL, results in higher test reliability and throughput for testing multiport fiber-optic components.
Such a test approach can typically characterize a 40-channel optical component for IL and PDL over a range of 50 nm in nearly 10 sec.
This performance, combined with a multiport adapter that allows connecting the individual fibers of a multichannel component up front, provides users with increased throughput and operational efficiency to meet today's challenges in manufacturing.
Anette Zimmermann is product manager, digital photonic test, Digital Test Division, at Agilent Technologies (www.agilent.com).
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