Aiming for accuracy

Optical transmitters used in high-speed digital communication systems are typically required to maintain a specific set of performance levels. One parameter, extinction ratio, describes optimal biasing conditions and how efficiently available laser power is converted to modulation power (see figure). Extinction ratio is a measure of the amplitude of the digital modulation on the optical carrier and affects the power penalty, or distance over which a fiber-optic system can reliably transmit and receive a signal while maintaining an acceptable bit error ratio or bit error rate (BER). Extinction ratio is defined as the average optical energy in a logic "one" bit divided by the average optical energy in a logic "zero" bit.

Although simple in concept, an accurate and repeatable extinction ratio measurement can be very difficult to achieve. Standards for communications systems such as synchronous optical networking (SONET), Gigabit Ethernet, and Fibre Channel specify minimum extinction ratio requirements for laser transmitters and the test system used for the measurement. It is one element in a set of complete physical layer tests that are intended to guarantee operability in an entire communications system. The level of extinction ratio varies according the data rate and test standard. Typical minimum values range from 7 to 10 (8.5 to 10 dB).

A high-speed laser waveform can be viewed in the form of an eye diagram. An eye diagram provides an easy analysis of the relative performance of signal levels by overlaying the various bit sequences on top of each other. Since the eye diagram shows many samples from several waveforms on a single display, it provides a view of the overall device or system performance for all data sequences and patterns. Once the eye diagram is generated, histograms are used to do a statistical analysis of the waveform. The histogram is analyzed to determine the mean '1' level and the mean '0' level, usually from the central region of the eye diagram. From these two values, the extinction ratio can be calculated.

The key elements of the test system for measuring eye-diagram extinction ratio measurements consist of an optical-to-electrical (O/E) converter, a low-pass electrical filter, and a digitizing sampling oscilloscope. The frequency response of the photodiode/filter combination is precisely specified to provide a consistent measurement method for characterizing optical transmitters. In practice, compliance measurements, including extinction ratio, are typically made in a filtered bandwidth as the filter performs an integrating effect to simulate the signal that will be seen by the decision circuit in a real system.

There are several factors that can potentially degrade an extinction ratio measurement. With no light signal applied to the input, photodiode receivers commonly generate a nonzero output signal (from photodiode dark currents or offset voltages from any electrical amplifiers in the O/E receiver chain) that results in an offset of the eye diagram. Digital communications analyzers, such as the Agilent 86100A Infiniium DCA, have a built-in calibration procedure to mathematically remove these dark levels from the extinction ratio measurement. After the user blocks any source of light from entering the optical receiver, the instrument measures the residual signals present with no input to the receiver, and then mathematically removes the offset from both the '1' and the '0' levels.

The frequency response of the measurement system, including the photodiode, any amplification, filtering, and the measuring oscilloscope can potentially lead to waveform distortion and degradation of the extinction ratio. In particular, any emphasis or de-emphasis of low frequencies will lead to an offset of the eye diagram. However, unlike dark levels, there is no simple technique to remove these offsets because they vary depending upon the frequency content of the signal being measured. The best technique for minimizing frequency response errors is to use a well-designed measurement system. Modern digital communication analyzers have integrated optical channels with built-in photodiodes and filters so that the entire measurement channel is calibrated to meet the strict frequency response requirements.

A systematic approach to reducing error in an extinction ratio measurement includes using the right algorithm, calibration, and measurement procedures, as well as test equipment that is specifically designed for the measurement.