Tech Scan

December 5, 2002
High speed swept wavelength cut cost in Component Test

Mark Albert

Faster and more efficient testing has become a key enabling technology for manufacturers who design, develop and produce optical components. It has become significant for network operators too, who install and maintain them.

The rapidly expanding volume of data traffic is driving network operators to adopt new generations of optical systems, with more capacity and higher data rates. These new network systems rely heavily on futuristic optical components with more channels, higher operating rates, narrower bandwidths and tighter tolerances.

Designers and manufacturers of optical components need test methods that will help them optimise new designs and confirm specifications of manufactured parts. Testing must also be all- encompassing and accurate to ensure optimum operating performance.

Optical component developers are being challenged to respond with a wide variety of devices. Their advanced product designs require higher test accuracy and stability. While the shorter product development cycles and increased production volume call for faster throughput. Test time and accuracy have become major considerations.

Older traditional test methods are no longer acceptable. Traditional optical test methodologies can provide fast testing or accurate testing - but not both.

New Swept Wavelength Methodology Eliminates the Compromise
A unique methodology for swept wavelength measurements has been developed to eliminate the need to compromise test speed for accuracy. The basic approach uses a swept tunable laser source, a swept wavelength meter and multiple optical power meters.

In a single high-speed scan, the swept system analyses the insertion loss of every channel as well as the return loss of the device. It also corrects wavelength and drift errors at every data point, with traceable calibration. The key elements of this new methodology are real-time wavelength calibration and the synchronous acquisition of time-series data from all channels.

Swept Wavelength Meter (SWM) is a unique and new method for real-time wavelength calibration, that supports high test throughput without compromising accuracy. It has no moving parts. It is designed to perform exclusively in a swept system environment, in conjunction with a swept laser source and optical power meters.

SWM provides traceable wavelength calibration of a swept laser at every point in the scan, with picometer uncertainties and resolution in near real time. This solution yields the same wavelength accuracy obtainable when using a traditional wavelength meter for 100 percent of the data points. But it does not limit the sweep speed or make assumptions about the laser's accuracy or repeatability. In addition to all these, a typical test cycle from acquisition to analysis can be completed for multiple channels in a few seconds, with picometer wavelength uncertainty at every data point.

Like traditional wavelength meters, SWM uses an interferometer and a NIST-traceable reference for wavelength calibration. However, rather than using an emission source as the reference, the SWM uses a gas-absorption cell. And instead of a dynamic Michelson interferometer, the SWM uses a static Mach-Zehnder interferometer. This allows the changing wavelength of the swept laser to generate a sinusoidal interferometric signal.

The Mach-Zehnder interferometer in the SWM is built with one optical-fiber path longer than the other. Optical transmission through this unbalanced configuration is a sine-squared function of the wavenumber. Hence a simple calculation of the inverse sine-squared function results in a phase number, that is a function of the wave number at each sample point.

The absorption cell is used to calculate linear coefficients that equate the phase of interferometer with the wave number of laser. Since data is acquired synchronously, the sample points of the cell's absorption peaks correlate with the sine-squared function from the interferometer. By performing a least-squares fit to the wave number-phase data, the absolute wavenumber is calculated at every sample point.

The start of laser sweep triggers the measurements. As laser sweeps, all of the power meter detectors (including those on the SWM) are synchronised to acquire data in parallel. Each device making power measurements at the same discrete points in time and storing them in local memory. After the laser has finished sweeping and all data has been acquired, the power meters send their stored measurements to CPU over a high-speed PCI data bus.

The swept wavelength methodology is an inherently cost effective approach. It takes advantage of high density digital signal processing techniques to compute wave numbers rather than use electromechanical elements with tight tolerances. By completing all of the tests in a few seconds and correcting all of the data points in synchronism, lower cost tunable laser sources can be used. In fact their long term stability and drift are no longer critical to the accuracy of test results.

The rapid characterization made possible by the new methodology saves test time and money. Even comprehensive test suites can be executed in a fraction of the time previously spent stepping through a single scan. It introduces a whole new level of interactivity to design optimization in lab, bringing production testing up to full speed, and troubleshooting of problems on site.

(The author is the Product Marketing Manager Optical Test, Tektronix, Inc)