Laserdiode

[kinglite]

Hi,

I developed an driver for my laserdiode an it works fine, but not fine enough.
I'm new with laserdiods and my collegue told me that the LD only lights up when the threshold is reached. My LD has a threshold at 80mA (max 200mA). I connected a multimeter to messure the current. I configurated everything to get close to these 80mA, but the LD lights up anyway. I have to get down to 3mA untill the LD has no visible light.
Now I want to know if there are other specifications besids the threshold which are important for the light of the LD like the fall of voltage at the LD.

Thanks for reading

(Sorry for the (probably) bad english)

[buffo]

Just because a laser diode is emitting light doesn't mean it's lasing. That is, it's probably not emitting monochromatic light. Many diodes will act similar to an LED below their threshold. (The output will be very dim, however.)

The voltage drop across the diode should be fairly constant. That is why current regulation is so important. Once the diode starts to conduct, it only takes a *tiny* voltage change to cause a large change in current flow.

By the way, welcome to PhotonLexicon. Your English is just fine!

Adam

[kinglite]

Ok, I just tested it again and meassured the voltage at the LD.
At around 5mA it's 1.8V. At 80mA it's 2.2V. And at 200mA it's 2.6V.
At 5mA there is an acceptable low light, but arround 70/80mA it's way to bright for the fact that it's at the threshold.

Suddenly my LD works fine. It lights up at ~80mA, but once the threshold was reached and I turn my modulation at 0V it falls back in the old conditions.
I'm kind of confused now. Is my LD broken? Or is the LD probably wrong packed and I've got an other type of LD than it's written? I have the feeling that the LD isn't at full power when I turn it up till 200mA, it's not that bright as I thought it would be. But I'm also afraid of damaging the LD (if I haven't already ) by going much higher than 200mA.

[buffo]

I'm not sure I understand... You said it is very dim until you reach about 80 ma of current, at which point it turns on and is very bright. Then it gets even brighter as you increase the current up to 200 ma. As you reduce the current, it gets dimmer again, right?

I would test it by running the diode at between 80 and 100 ma. It should be lasing. Now change the modulation input and see what happens. Does it dim down in a linear fashion? If so, that's what it's supposed to do. (Assuming you have an analog-modulating driver.)

Can you post a picture of the diode in operation and the driver?

Adam

[kinglite]

Sorry for this long break, but I had some PC issues.

I try to explain it again, I'm still confused.

I connect the LD and power on the source. The modulation ist at 0V, but my LD is gliming (7mA through LD). Not much but enough to see something. My collegue said that this isn't normal. There only has to be some light past the threshold, which is at 80mA.
Then I rise my modulation till 5V. The current rises linear until it reaches 210mA. The LD is brightening up too, but I can't say if it's linear to,but it looks like that (same backwards).

I measured the voltage at the LD at 0V modulation: 1,9V
The voltage at the LD at 5V modulation: 2,9V
The voltage at the LD with disconnected modulation: 1,9V

With disconnected modulaiton I reduced the operating voltage from 5V untill the light was gone. At 2V operating voltage it faded (1,5V at LD).

It seems that the LD lights up because of the voltage and not of the current.

I dont have a camera, so I can't make some pictures. But I can show you my driver shematic.

//Edit:A circuit before that makes out of the modulation voltage 0-5V a voltage from 0-1,25V. I have only a trial version of spice so I coudn't use that much parts. So I minimized it to that what you can see below.
How you can see I connected the amperemeter at the LD to see the current. Is this maybe the mistake I did?
The LD also has no monitoring diode if this is somehow important to this.
I think the LD is performing like a LED (like you mentioned before). How can this be interrupted/stopped?

Hope this is enough for the start. I now have less time (not my PC) so I might have forgot something.

Greets

[ElektroFreak]

You'll get a little light before the diode crosses it's threshold, but it will be dim and incoherent like a low-powered LED. Then when you cross threshold the light will suddenly become many times brighter.

You already know that when modulating a diode it is good idea to allow the current to fall just below threshold rather than no current at all. You will still have a tiny glow from the diode, though. All my analog reds still produce a faint light even at 0V on the modulation line.

[buffo]

It seems that the LD lights up because of the voltage and not of the current.

Don't forget that increasing the voltage increases the current. The difference is that the voltage drop across the laser diode doesn't change very much between threshold and max output. Thus it only takes a tiny voltage change to force a *large* current change.

Your test results look fine to me. The fact that you have some light coming from the diode even with a small amount of current flowing is not surprising. Most laser diodes do this.

Adam

[mixedgas]

Don't forget that increasing the voltage increases the current. The difference is that the voltage drop across the laser diode doesn't change very much between threshold and max output. Thus it only takes a tiny voltage change to force a *large* current change.

Your test results look fine to me. The fact that you have some light coming from the diode even with a small amount of current flowing is not surprising. Most laser diodes do this.

Adam

I'd beef up Q1, 2n2222 is good part, but not strong enough for this application
ie 200 or 450 mW max depending on the part. You'd like at least 3x that and darlington.

Steve

[kinglite]

Ok, thanks. But my collegue said to me that he has no light output below the threshold with this LD. I didn't see that by my self yet, but I'll ask him to show me if I meet him.
I borowed a camera to make some pictures.
Light at 0V:


Light at 5V:


There is not much difference so see like in reality (in real there is a little more to see, but not much). Maybe the angle is bad or I mixed up the pictures (forgot to set a mark or something in the pictures to distinguish).

You always say that some light is fine, but I think that this is much more than that. If only you where here to see it in person. I also don't want to make a direct picture of it (very expensive camera).

You'd like at least 3x that and darlington.

Maybe I insert a TIP101 instead

Greets

[ElektroFreak]

Based on your picture I'd say that's about right for below-threshold light. I do hope you're not running that diode up to full power just sitting there bare like that..