Electronic Design

Headed For Extinction Ratio

International standards, such as SONET and SDH, were established to specify performance criteria for high-speed transmission of telecommunications signals over fiber-optic cables. This standardization has resulted in a number of benefits, including reduced cost, improved reliability, and multivendor interoperability.

A key indicator of the performance of an optical-fiber-based communications system is extinction ratio. The extinction ratio is used to describe the efficiency with which the transmitted optical power is modulated over the fiber-optic transport. It's simply the relationship of the power used in transmitting a logic level "1" (P1) to the power used in transmitting a logic level "0" (P0). Extinction ratio can be defined as a linear ratio, P1/P0; as a power measurement, 10 × log(P1/P0); or as a percentage, (P0/P1) × 100. The convention adopted for logic levels is that transmission of light (laser on) represents a logic "1" and no light transmission (laser off) represents a logic "0."

Note that while in theory no light is transmitted for a logic level "0," in practice the laser is always "on" (although at a reduced intensity for the "0" state). This avoids the performance degradation (wavelength shift, waveform distortion) that would result from modulating the laser through the lasing threshold.

The measurement system typically consists of a digital storage oscilloscope or wideband digital-sampling oscilloscope and optical-to-electrical (O/E) converter. The standard requires this measurement system to be calibrated as an optical reference receiver. The transfer function is specified to approximate the performance of a receiver in a transmission system.

An accurate determination of extinction ratio seems to be a relatively straightforward task: Measure the two light levels and calculate the result, right? Wrong! In practice, an accurate determination of extinction ratio is notoriously difficult to make. Significant errors can be induced by minor measurement inaccuracies. There are two major challenges. The first is getting the modulated lightstream into the measurement system with minimum signal degradation (due to measurement system noise and temperature-related drift and offset). The other is extracting and processing the information with consideration to the potential sources of error. This includes the statistical processing of the acquired data and maximum scaling of the signal to the oscilloscope's ADC.

Consider the following example: The mean launched optical power into a transmission system is −14.54 dBm (35.2 µW). The mean P0 level is measured at −30.0 dBm (1 µW), and the mean P1 level is measured at −11.6 dBm (69.2 µW). What is the extinction ratio?

The extinction ratio is 10 × log(69.2/1) = 18.4 dB

A common source of measurement error is the offset due to the "dark current" of the O/E converter's photodiode. It's the result of an output voltage generated by the photodiode in the absence of any optical signal power at its input. Consider the effect on the previous example if a dark current offset of −24 dBm (4 µW) is present. The mean P1 level (including offset) measures −14.1dBm (39.2 µW), and the mean P0 level measures −23 dBm (5 µW). The resulting extinction ratio is 10 × log(39.2/5) = 8.9 dB. Failing to properly account for the dark current or any other minor measurement inaccuracies (particularly with respect to the P0 level) can result in significant error.

The benefit to proper characterization and optimization of extinction ratio is significant in terms of system cost and performance. Cost reduction is realized when a fiber-optic-based system can reliably transmit and receive a signal over a greater distance. This directly impacts the number of regenerators or optical amplifiers that are required in the system. This, in turn, can influence performance, because system-timing jitter will accumulate as a function of the number of active components in the transmission path. Furthermore, a correlation exists between improving the extinction ratio and a reduced bit error rate. Paul Fowler, LeCroy Corp.

See associated figure.

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