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When I measured the optical pulse using photo-detector and oscilloscope, some negative voltage was measured after strong peak. What did I get negative voltage? How is it possible?

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    \$\begingroup\$ There is a generic answer to this but it would be easier to answer in more detail if you could show your measurement setup and tell us about your detector and oscilloscope, the type of probes used etc. \$\endgroup\$ – pipe Nov 21 '18 at 9:19