Simultaneous measurement speeds the pace of production
Falling prices coupled with a market slowdown during the last 18 months have driven a major shift in component manufacturers' priorities. Reducing production costs has replaced speed to market as the chief concern. With production staff spending up to half their time on test-and-measurement tasks, the demand for more efficient and effective test solutions has become a key focus for reducing overall production costs.
Fortunately, innovative strategies and technologies enable many measurement parameters and many channels to be characterized simultaneously using techniques that also improve measurement speed and accuracy. In addition to direct improvements to the measurements themselves, greater levels of automation are being used to increase measurement speeds. Together these changes can reduce measurement times by more than 90%, and reduce the cost of advanced optical-component production by 20% to 40%.
This is particularly true in DWDM components such as multiplexers and interleavers, where several parameters have to be evaluated over many wavelength channels. Using novel techniques, the measurement speed, and thereby component throughput, is increased in three main ways-faster scanning through the wavelength range under test, testing of many measurement parameters with one wavelength scan, and evaluating the output of many channels simultaneously during the wavelength scan. The latest generation of instruments implement all these techniques to achieve lower-cost testing.
Fast scanning of the wavelength of tunable laser sources has seen major improvements in the last few years. In the past, a tunable laser source stepped through the wavelength range and required approximately one second to settle at each wavelength. This meant that a full scan through a wavelength band would take approximately 15 minutes-unacceptably long since it could then take hours to fully characterize a multiport component.
With the introduction of swept- wavelength lasers, the speed of scan for each wavelength band was reduced to tens of seconds. Test equipment manufacturers are now working closely with laser manufacturers to optimize the laser's scan speed, stability, and accuracy. The latest generation of tunable laser sources can sweep across both the C-and L-bands in 1 s and combine this speed with a wavelength accuracy of around 15 pm.
These laser sources also have features that allow improved measurement accuracy. Some can reach amplified spontaneous emission (ASE) levels of better than 70 dB, enabling deep features such as those seen in notch filters to be accurately evaluated. And a wavelength resolution of 0.1 pm allows many wavelength points to be measured in narrowband DWDM components so that fine structure, such as group delay ripple, can be determined.
Until recently, it was necessary for the component manufacturer to purchase multiple test systems to fully characterize the components being made. One test system would measure group delay, another would measure differential-group delay (DGD), and a third would perform loss measurements. These systems would have to be run independently with a test engineer connecting and removing the device under test (DUT) from each test system. However, connecting all of the ports of a typical 40-channel component can take 15 minutes, so it is essential to connect only once and use a switch network to route light to and from each instrument.
Although this is an effective approach, the optical switch itself then becomes part of the DUT. Polarization-dependent loss (PDL), insertion loss, and other characteristics of the switch are included in the measurement and must be calibrated out. In addition, the use of several different measurement instruments from different vendors means that each component manufacturer must perform the long task of mechanical and software integration to control these different systems.
Multiparameter instruments cover all the measurement parameters-group delay, chromatic dispersion, differential-group delay, polarization-mode dispersion, insertion loss, and in some cases, return loss-in a matter of seconds with a single wavelength scan. The measurement techniques fall into two main camps-modulation phase shift and swept-wavelength homodyne interferometry. Both techniques offer similar measurement speed and accuracy, but the modulation method also works with long, fiber-based components (see Fig. 1). An additional benefit of using a single measurement system is that the results for all the parameters can be formatted into one integrated data sheet that can either be stored as part of a quality control system or provided to the customer with the finished component.
A further significant change is the need to measure all the fiber channels in a DWDM multiplexer/demultiplexer component. Clearly, measuring all of the DUT channels at once realizes enormous savings in time and labor (see "AWG measurement benefits from advances in testing systems," p. 48). The latest instruments can, therefore, characterize every DUT channel simultaneously in a multichannel detection system.
The resulting ability to fully evaluate all channels during production (not previously possible without a considerable time penalty), together with faster R&D and type-approval qualification, are a clear competitive advantage for the user. These improvements in measurement technology have dramatically reduced the time required to test optical components (see Fig. 2).
Test equipment must also be flexible enough to adapt to the changing needs of the user. One of the best ways to achieve this is to make the system modular so that it can be adapted and upgraded as the user's product range broadens or the manufacturing volumes increase.
Growing numbers of test systems are being designed with modular architectures that enable either more measurement parameters or more measurement channels to be added as required. Users can purchase a basic system with few channels for their development process then expand it for production when high throughput is required. This represents a major benefit, as the ability to expand an existing system is cost-effective and eliminates new instrument purchases, reducing in turn the need for additional system integration and employee training.
Component manufacturers are evaluating the automation of fiber handling, pigtailing, and attachment as ways to reduce overall production process times. The latest test instruments are designed with consideration of this automation route and will easily allow an integrated manufacture/assembly/test process to be adopted when required.
The cost of testing optical components must be driven down to minimize the overall component test cost. Modern multiparameter, multichannel optical-component test solutions allow all the required tests to be performed within seconds. These systems lend themselves well to integration within automated component assembly stations, essential for when the market recovers and production volumes rise.
Dan Daly is business development manager at PerkinElmer Optoelectronics, Wokingham, RG41 2GY, England. He can be contacted at email@example.com.
AWG measurement benefits from advances in testing systems
The measurement of a 40-channel AWG demultiplexer demonstrates the dramatic improvements that have been achieved in component testing (see table). These planar lightwave circuits can have both high insertion loss and return loss and, in addition, can exhibit chromatic dispersion, DGD, and PDL. It is, therefore, important to measure all these parameters. But because of time constraints, dispersion measurements have historically been neglected in production and have only been undertaken during the development phase.
In calculating the table, we make the following assumptions: the cost of manufacture is $3000 and the component is tested after connectors have been added; and connecting the 40 channels to a piece of test equipment takes 15 min, including connector cleaning and mounting. We assume that 20 units are produced per 8-h work day, or 5000 units/yr, and that the labor rate (including overhead) for test engineers is $55/h. Test equipment is depreciated over three years. The component is tested at 10-pm wavelength steps over the C- and L-bands, measuring 10,000 wavelength points for each parameter. A stepped laser is estimated to take 1 s per wavelength point; a swept system scans in 120 s. We assume three single-parameter test systems are necessary to cover all parameters.
For the stepped-wavelength, single-parameter, single-channel measurement, the testing time is calculated to be approximately 14 days (40 channels x 3 test systems x 10,000 wavelength points x 1 s/wavelength point). In this case the cost of staff time alone is greater than the current market value of the component.
In this example, over 80% has been cut from the testing time and 13% from the production cost of the component with the latest test systems. The advantages of swept wavelength, multiparameter, multichannel test systems are clear.