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Extinction Ratio Test of Optical Emitter

Extinction Ratio Test of Optical Emitter

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The extinction ratio (ER) of an optical emitter is measured as the ratio of the optical power for a logic '1' to that for a logic '0', and it is a key parameter affecting system performance and bit-error rate.

Definition and Importance

The extinction ratio (ER) quantifies how effectively an optical transmitter distinguishes between the high (logic '1') and low (logic '0') power levels. It is defined as:

ER=P1P0

where P1 is the average optical power for a logic '1' and P0 is the average power for a logic '0' . A higher ER indicates better modulation efficiency and lower bit-error rates (BER), while a low ER can degrade system performance and limit transmission distance .

Experimental Setup

  1. Optical Transmitter and Biasing: The laser or optical emitter should be biased near its threshold so that P0>0 , ensuring a finite low-level power output .
  2. Receiver and Oscilloscope: Use a high-speed photodetector connected to an oscilloscope capable of eye-diagram analysis. The oscilloscope should have a low-pass filter, typically a fourth-order Bessel-Thomson (BT-4) filter with a 3 dB cutoff at 0.75 times the bit rate, to minimize jitter and high-frequency noise .
  3. Eye-Diagram Analysis: Capture the NRZ (non-return-to-zero) waveform on the oscilloscope. The central 20% of the eye diagram is commonly used to compute the mean power levels for '1' and '0' .
  4. Calibration: Perform a dark level calibration to correct for offsets in the photodiode or oscilloscope electronics, which can affect the measured ER .

Measurement Procedure

  • Record the optical waveform and identify the average power levels P1 and P0 from the eye diagram.
  • Compute the ER using the ratio P1/P0 . It can also be expressed in decibels (dB) as ERdB=10log10(P1/P0) , .
  • Repeat measurements to ensure consistency and account for variability due to receiver characteristics or noise .

Considerations and Standards

  • ER affects bit-error ratio (BER) and system power penalty; lower ER requires higher average power to maintain the same BER .
  • Minimum ER values are standardized depending on the application, typically ranging from 8.2 to 10 dB for systems like Gigabit Ethernet, SONET, SDH, or Fibre Channel .
  • Measurement consistency can be improved by using traceable reference receivers and standardized oscilloscope settings .

Summary

Measuring the extinction ratio involves biasing the optical emitter, capturing the eye diagram, calibrating the system, and computing the ratio of logic '1' to logic '0' power levels. Accurate ER measurement is critical for evaluating transmitter performance, ensuring reliable digital optical communication, and minimizing BER power penalties .

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