Automated Testing for Optical Amplifiers

Author(s):
Scott Martinson - Anritsu Company

Industry:
Telecommunications

Products:
LabVIEW,

The Challenge:
Develop a system for testing Erbium Doped Fiber Amplifiers (EDFA). The system must have high accuracy, comprehensive data gathering, many user-defined measurements, quick measurement times, and simplicity for use in the manufacturing environment. Finally, the system must be able to accommodate user-written set-up, measurement, and report processing routines.

The Solution:
Create a National Instruments LabVIEW-based application for modularity and quick development speed, use the NI GPIB to control the various instruments, and develop it all on a PC for cost effectiveness.

Introduction
The Anritsu Company produces a broad range of optical products, including Optical Spectrum Analyzers (OSAs),laser light sources, and optical attenuators, but the unique testing requirements of an EDFA mandated the development of a special optical modulator box, some new measurement techniques, and a software application to control the numerous instruments. Using NI LabVIEW and a GPIB card on a personal computer, Anritsu leveraged their existing optical product line to bring a new system to market that is faster and less expensive than competing systems.

Requirements
While the system can be constructed entirely of Anritsu products, the software must also be flexible enough accommodate some non-Anritsu light sources and attenuators that the end-users may already own. The system must measure the gain and noise factor of each EDFA amplifier over a range of operating wavelengths, for a range of input signal powers, and for one or many laser signals on a single fiber. The major hardware requirement was an optical modulator capable of switching a laser signal off and on more than 100,000 times per second with an on/off extinction ratio greater than 70 dB. For maximum accuracy, the system may optionally include a 5x5 optical switch to simplify and speed up the calibration process.
The major components of the test system are an optical spectrum analyzer, tunable laser sources (TLS), broadband laser sources, optical attenuators and an optical modulator.

Measurement
The measurement technique chosen uses two stages of optical modulation. The first stage modulates the incoming signal 125,000 times per second to exploit the EDFA decay properties. The second modulation stage is synchronized with the first stage and allows the OSA to only look at the EDFA output when the incoming signal is switched off. The optical modulator box also contains optical switches and an optical coupler to allow measurement of the input signal, the output signal, the amplified spontaneous emission signal, and the optional probe signal. Calibration in this system entails using a fixed reference signal and measuring the optical loss in dB along every possible signal path.

To allow for different measurement environments, the software has three different measurement methods. The first method is used when the incoming fiber contains several different fixed wavelength signals and the user wishes to measure the noise figure and gain at each of the incoming wavelengths. Some users pack as many as 80 different laser signals on a single fiber. The system automatically locates the wavelength of each laser signal, measures the input light power, amplified light power, and amplified spontaneous emission (ASE) power. The noise figure and gain are calculated and displayed in graphical and tabular form. The second measurement method couples a low power laser probe with the incoming signal. The additional probe signal has a negligible effect on the EDFA characteristics, but allows the measurement of gain and noise figure at any wavelength, rather than only at the wavelengths of the incoming signal. The third measurement method allows the user to measure gain and noise factor across a wavelength range using a single tunable laser source and save the expense of several fixed light sources. The system software automatically sweeps the laser through the range at step intervals, collecting all the data.

All the measurement methods mentioned above also compute gain tilt, average gain, and gain flatness. The results can be displayed graphically in several different ways or in tabular form. The measurement results can be automatically saved as a disk file. The software performs all calculations while compensating for the calibrated path losses. The user may specify a range of input power levels and probe power levels that are automatically adjusted by the software using the laser sources or the optical attenuators under GPIB control.
At various points in each of the three measurement methods (e.g., during set-up, after measurement completion, and after report generation), the software utilizes LabVIEW’s ability to call external programs and calls predefined EXE file names. If the user wishes to customize the system, she can write her own programs using any language capable of creating an EXE, and insert their processing at these predefined places. This allows the user, for example, to control additional equipment not normally in the Anritsu system, or to process the report file and store the results in a proprietary data base system.

Software
Using LabVIEW and GPIB allowed us to concentrate on developing the actual measurement techniques instead of spending most of the time writing graphing and instrument communications routines. LabVIEW drivers were already available for all Anritsu equipment used, and we were able to support other manufacturers’ equipment with LabVIEW drivers that they had written. This allows the user to reuse hardware and save tens of thousands of dollars in capital equipment. The ability to use available drivers saved a great deal of development time and money, and all the graphing and file management features are built into LabVIEW. All of these features make the Anritsu EDFA test solution cost effective and easy to maintain.

Browse All Case Studies »