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This question is related to that one: Can laser diode be operated with lower than specified voltage?

I am driving a laser diode in a current controlled fashion. What actually happens below the lasing threshold?

In experiments, I can clearly observe some "fluorescence". The fluorescence intensity makes up for only ~0.2% of electrical power. Even more interestingly, the fluorescence intensity stagnates at a nearly constant level, independently of current. Also the voltage remains at a constant level.

Can one theoretically predict some sort of curve for the I / V - characteristics of a Laser Diode below threshold? Similar to the shockley equation for ideal diodes?

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  • \$\begingroup\$ Not sure fluorescence is the right term, but I've observed this phenomenon as well, and it's an interesting question. For my purposes I never had a need (or time) to explain or characterize it though. \$\endgroup\$ – John D May 11 '17 at 12:23
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It's normal for a laser diode to emit (very little) light below threshold current (cf. "Below the threshold current very little light is emitted by the laser structure."). I don't know, though, if there is a (simple) formula describing the P/I dependency in that operating region and also why one would care.

The fact that voltage stagnates is also no surprise: it's just the forward voltage of the laser diode which depends above some thresold only very little on current like in other diodes. So yes, the Shockley equation is a (more or less) suitable I/V equation. Such equations are always based on simplifications/presuppositions (simplified models of reality). Whether they are good enough for your purpose or not depends on your requirements.

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  • \$\begingroup\$ Thank you for your answer. I was just curious about what is going on here, since I observed this several times... \$\endgroup\$ – AnatraIlDuck May 13 '17 at 12:53

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