# Driving of a Laser Diode

Discussion in 'LEDs and Optoelectronics' started by Thyrex, Jan 23, 2011.

1. ### Thyrex

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Dec 21, 2010
Hi,

I'm trying to understand how Laser Diodes (LD) work and their driving requirements, and wondered if someone could please give me a few pointers with the following.

I believe LDs are current driven, not voltage driven. I noted by experimentation that whilst the voltage across an LD was less than about 1.4V, I recorded no current through it. Is this correct? One could argue that a small drift current would be expected due to the applied voltage, or otherwise, that the junction's built in potential blocks all current.

Many Thanks.

2. ### Resqueline

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Jul 31, 2009
Yes, I also believe LD's are current driven like all other LED's are, and that they have a forward voltage knee point below which there flows "no" current. Define "no" current.

3. ### Thyrex

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Dec 21, 2010
Thanks for the reply. My problem is with the definition of "no" current, as I can think of a reason why there might be a very small (~uA) current and also why there might be none at all, as mentioned in my first post.

Do you know which one is correct please, or indeed if both are wrong?

Many Thanks

4. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,505
2,849
Jan 21, 2010
Ideally no current passes before the knee voltage, and then the diode has a voltage drop independent of voltage.

Practically, a very small current flows below the knee, one that increases with voltage.

Around the knee, it's a bit mushy with current increasing more and more rapidly with voltage.

Past the knee, the same thing happens, with current increasing extremely rapidly with voltage.

Incidentally, the same sort of thing happens in the reverse direction as well.

So there's really no obvious voltage to pick, and no real "zero" current either. Practically speaking, you might say zero is less than 1pA, or 1nA, or 1uA, or even 1mA. And that may be what Resqueline meant by 'define "no" current'.

5. ### Thyrex

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Dec 21, 2010
Ok, thanks Steve. That makes sense. It might be a bit complex, but on an atomic level what causes the very small current?

Also, sorry Resqueline if I misunderstoof you're response.

6. ### Resqueline

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Jul 31, 2009
No problem, I was maybe a bit short in my reply.
You said you measured no current; so my remark was aimed at getting out of you what kind of gear you measured it with and how small currents can it register?

I'm sorry but it's so long ago I had semiconductor theory at school that I can't really explain it without it requiring too much time & effort on my behalf to dredge it up again.