Characteristics of a Laser Diode Below Threshold

Question

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?

Answer 1

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.