Forums

finding laser diode parameters

Started by Chris April 13, 2016
On Tuesday, April 19, 2016 at 6:15:55 PM UTC-4, Chris wrote:
> On Mon, 18 Apr 2016 06:22:31 -0700, George Herold wrote: > > > Those voltage numbers don't make much sense, unless there is some > > circuitry driving the diodes already. Are these bare diodes? > > Yes, I believe so. The only other component is what looks like an SMD > capacitor across the diode leads just before it goes into the brass > housing. I guess this is to iron out any voltage spikes?
Maybe a diode in reverse to prevent reverse voltage?
> > > The best analogy for a laser diode(LD) is an LED. > > The forward voltage is mostly set by the band gap.. color of light. > > There should still be an increase in the voltage as you increase the > > current (~60 mV for a decade increase in current... (I think)) > > That's what I'm seeing under test, actually. And despite the description, > all the diodes I@ve checked so far go into forward conduction at 3.3V.
I'm not sure what you mean. There's no current with 3.2 V? You can look up bare LD's on DK... check the spec sheet for typical forward voltages.
> > > I'm guess you've got diode with a built in controller (in the brass > > case.) > > I'm pretty sure these ones have no controller. Another batch from another > supplier just arrived. I'll check them tomorrow...
What's inside the brass case? (I'd bet a beer there an APC circuit in there.) George H.
On Wed, 20 Apr 2016 08:04:23 -0700, George Herold wrote:

> On Tuesday, April 19, 2016 at 6:15:55 PM UTC-4, Chris wrote: >> On Mon, 18 Apr 2016 06:22:31 -0700, George Herold wrote: >> >> > Those voltage numbers don't make much sense, unless there is some >> > circuitry driving the diodes already. Are these bare diodes? >> >> Yes, I believe so. The only other component is what looks like an SMD >> capacitor across the diode leads just before it goes into the brass >> housing. I guess this is to iron out any voltage spikes? > > Maybe a diode in reverse to prevent reverse voltage?
No, it's a cap (unless it's a diode with a junction capacitance of 10nF!)
>> >> > The best analogy for a laser diode(LD) is an LED. >> > The forward voltage is mostly set by the band gap.. color of light. >> > There should still be an increase in the voltage as you increase the >> > current (~60 mV for a decade increase in current... (I think)) >> >> That's what I'm seeing under test, actually. And despite the >> description, >> all the diodes I@ve checked so far go into forward conduction at 3.3V. > I'm not sure what you mean. There's no current with 3.2 V?
There is, but the current starts to increase hugely at 3.3V but the voltage then seems to stick around that level.
> What's inside the brass case? > (I'd bet a beer there an APC circuit in there.)
You owe me a beer. There's nothing but a lens in there. :) It's hardly surprising, though. These things are like DIRT cheap so I wouldn't expect any driver.
On Thursday, April 21, 2016 at 4:37:07 PM UTC-4, Chris wrote:
> On Wed, 20 Apr 2016 08:04:23 -0700, George Herold wrote: > > > On Tuesday, April 19, 2016 at 6:15:55 PM UTC-4, Chris wrote: > >> On Mon, 18 Apr 2016 06:22:31 -0700, George Herold wrote: > >> > >> > Those voltage numbers don't make much sense, unless there is some > >> > circuitry driving the diodes already. Are these bare diodes? > >> > >> Yes, I believe so. The only other component is what looks like an SMD > >> capacitor across the diode leads just before it goes into the brass > >> housing. I guess this is to iron out any voltage spikes? > > > > Maybe a diode in reverse to prevent reverse voltage? > > No, it's a cap (unless it's a diode with a junction capacitance of 10nF!) > > >> > >> > The best analogy for a laser diode(LD) is an LED. > >> > The forward voltage is mostly set by the band gap.. color of light. > >> > There should still be an increase in the voltage as you increase the > >> > current (~60 mV for a decade increase in current... (I think)) > >> > >> That's what I'm seeing under test, actually. And despite the > >> description, > >> all the diodes I@ve checked so far go into forward conduction at 3.3V. > > I'm not sure what you mean. There's no current with 3.2 V? > > There is, but the current starts to increase hugely at 3.3V but the > voltage then seems to stick around that level. > > > What's inside the brass case? > > (I'd bet a beer there an APC circuit in there.) > > You owe me a beer. There's nothing but a lens in there. :) It's hardly > surprising, though. These things are like DIRT cheap so I wouldn't expect > any driver.
OK, it is Friday. Can you stop by for Lunch and we'll grab a beer and a burger? The numbers don't make sense... but I don't know cheap diode lasers either. If it's a bare diode laser then you if you drive it with a current source. (a resistor and voltage source would work.) then you should see the laser output increase linearly with current.... I was just measuring the power out of my diode laser. Here's a crappy graph. https://www.dropbox.com/s/tcst4f09wf1arth/POWER.BMP?dl=0 The good news is I can get about 30 mW which is enough to make a MOT with. (MOT- Magneto-optical Trap.) George H.
On Fri, 22 Apr 2016 08:56:50 -0700, George Herold wrote:

> The numbers don't make sense... but I don't know cheap diode lasers > either. > If it's a bare diode laser then you if you drive it with a current > source. > (a resistor and voltage source would work.) then you should see the > laser output increase linearly with current.... I was just measuring the > power out of my diode laser. Here's a crappy graph. > https://www.dropbox.com/s/tcst4f09wf1arth/POWER.BMP?dl=0
Maybe we're at cross-purposes here. I was discussing voltage across the laser diode vs. the current through it. Not sure at all what you've graphed a photodiode's response into the equation here?
On Friday, April 22, 2016 at 2:57:02 PM UTC-4, Chris wrote:
> On Fri, 22 Apr 2016 08:56:50 -0700, George Herold wrote: > > > The numbers don't make sense... but I don't know cheap diode lasers > > either. > > If it's a bare diode laser then you if you drive it with a current > > source. > > (a resistor and voltage source would work.) then you should see the > > laser output increase linearly with current.... I was just measuring the > > power out of my diode laser. Here's a crappy graph. > > https://www.dropbox.com/s/tcst4f09wf1arth/POWER.BMP?dl=0 > > Maybe we're at cross-purposes here. I was discussing voltage across the > laser diode vs. the current through it. Not sure at all what you've > graphed a photodiode's response into the equation here?
Yeah I'm saying the voltage is not relevant to the light output. (if it's bare LD) (The bottom (X) axis is the LD current, the horizontal (Y) axis is the current from a photodiode looking at the laser. (about 0.6A/W so 20 mA is ~30 mW.) (It just happened to be something I was doing today.) George H.