Extinction ratio test of high-speed optical communication signals

Next, let's look at the definition of the extinction ratio parameter and some factors that affect the results, and how to improve the test accuracy of the extinction ratio.
1: Definition of extinction ratio:
ER (Extinction Ratio) is the ratio of the average optical power when the optical module transmits a logic "1" to the average optical power when it transmits a logic "0". Generally expressed in logarithms, ER is a constant for a linear attenuation system. The higher the extinction ratio, the higher the sensitivity of the transmission system to external noise and the better the robustness.

– Measure the average value of the histogram at the center 20% of the optical eye diagram

Figure 1

Below is an example of a 9.953Gbps optical eye diagram and extinction ratio test:

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2. What is the reasonable range of extinction ratio?
Generally speaking, the higher the signal rate, the lower the extinction ratio of the signal will be, because when the signal rate is high, the signal-to-noise ratio is more difficult to control. At the same time, the difference in the attenuation of the optical signal and the loss in the transmission will also cause the extinction ratio to become lower. Therefore, a 2.5Gbps optical signal can reach 17-18dB, but a 10Gbps optical signal can reach 13-14dB, which is already very good. However, the extinction ratio does not require infinity, the higher the better. ER can indicate how much light is wasted (the biased optical power relative to the dark level does not carry any information). After ER is less than 5dB, the optical power loss increases sharply, which means that additional optical power is required. However, if ER is greater than 15, the extinction ratio has very little effect on performance. Therefore, if you define the extinction ratio of the product as greater than 20dB, it is actually meaningless. It will only increase the difficulty of debugging and product cost without improving the performance of the product. This can be seen from the following chart:

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3. What factors will affect the extinction ratio test?
1. The background noise of the instrument and the optical module. Because the background noise will affect the accurate calculation of P1/P0. Therefore, temperature compensation calibration of the host and the module is necessary. Sometimes after the optical module has been used for a long time, there will be some dust, so the input fiber and the tested fiber can be wiped with dust-free test paper to improve the test accuracy of the extinction ratio.

2. DC level calibration, or we call it dark level calibration, ER: Because
P0 is used as the denominator, any slight deviation of P0 will cause a large difference in the test results, so dark level calibration of the module is essential.

3. Frequency response curve of optical module. In optical signal testing, according to the specification, optical signals of different rates need to use 4th-order Bessel-Thompson filters of different bandwidths. We usually call it an optical reference receiver. Because the receiving end of the optical signal will use this optical reference receiver, the instrument must also use the same standard reference receiver to ensure that the signal "seen" by the instrument is consistent with the signal "seen" by the real optical receiving end. However, the frequency response curve of the filter has a certain tolerance in the specification. Even if the instrument manufacturer claims that its optical module is in compliance with the specification, there may be certain differences in the frequency response curves of different modules, so the consistency of the frequency response curve is difficult to guarantee.

Traditionally, to support different rates in a module, different filters must be cascaded, and different cascade modes must be switched to achieve different bandwidths. Due to the multi-pole cascade hardware, it is difficult to ensure the consistency of the response curves of different modules. The following figure is the frequency response curve of the 80C10B optical module of Tektronix for testing 40Gbps. It can achieve the following frequency response curve effect. It can be seen that the frequency response curves of 14 different optical modules are very consistent, and the entire curve varies within a very small range. This is because Tektronix uses a patented filter implementation technology. The bandwidth and frequency response curve are achieved by precisely adjusting the pulse width of the sampling head. Since the sampling pulse width is precisely adjustable, its frequency response curve can be strictly controlled.

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Traditional implementation method of optical test module filter

Figure 5

The implementation method of Tektronix's latest 80C10B optical module.

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4: The transient response characteristics of the optical module will also affect the test accuracy and consistency:
The following is the response characteristics of an older optical module from Tektronix. It can be seen that the differences in the response characteristics of different modules will affect the calculation results of the 1 level and the 0 level, thereby affecting the test differences.

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4. If I want to get a very accurate extinction ratio test result, how can I achieve it?
Tektronix can provide an ER calibration option. At this time, an additional option will be added to the test items of the oscilloscope, as shown in the figure below, Extinction Ratio (dB)-Calibrated. Provides the measurement results after ER calibration. So how is the result of this ER calibration obtained? At the Tektronix headquarters in the United States, there is a standard light source provided by the American National Standards Laboratory, which can output optical signals with different known extinction ratios. Then, the DSA8200+ corresponding optical module is used for testing. Then, the deviation measured under different extinction ratios is filled into the optical module for corresponding compensation, and the accurate extinction ratio measurement value corresponding to the standard light source can be obtained.

Figure 8

The calibrated extinction ratio test can provide very good repeatability and test accuracy:

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