just to make sure....

[Vizier87]

Hi guys,
I'm building this regulator for my laser pump which needs to be maintained at 2V , 200mA as shown below:
View attachment 61564

It's an AP1117 regulator, which gives out 1A and can be adjusted like an LM317. So as the output current is 1A, I'm limiting it using the 2n3904 which has Ic rated at 200 max.

These are the components I already have, so I'd rather not use anything else.

The question is: Does it stay at the rated Ic if I saturate the BJT's base? Do I need to add any resistor in series for the laser?

Thanks for the input.

Vizier87

 

[Sceadwian]

Adjustable voltage regulators can be configured as constant current outputs by simply using a sense resistor of the proper value. The PDF for your regulator should have an example circuit, the 3904 is not needed.

 

[Vizier87]

err. Anyway, I need to have the capability to switch it. So will the 3904 limit it to a constant (or nearly constant) 200 mA?

Thanks Scead.

 

[kinarfi]

As I see it, what you have drawn is a 2.5 volt supply with no current regulation, unless it's internal to the ap1117, that set up is for voltage regulation. With the 2n3904 saturated, spec sheet says ~.25v, so your getting 2.25 volts across the laser.

 

[Vizier87]

Hi kinarfi, thanks for the reply.

Yes it is at 2.5V, it's from the datasheet. However I'll make the necessary adjustments when I place the parts. the voltage isn't really much of a problem. I can adjust it using a pot.

But right now I just want to know if the 3904 will act as a current regulator if it's saturated accordingly? Or do I need to have a more intelligent circuit which controls the parameters?

Thanks.
Vizier87.

 

[Sceadwian]

This is an LED diode module... you don't touch the voltage you simply limit the current, you can't do both at the same time. You need the linear regulator configured as a constant current source (read the datasheet for your regulator, there's a diagram in there that will show you how to do it) then use the 3904 to switch output on/off.

There is nothing at all in your circuit that will limit the current.

 

[Vizier87]

alright dude. Thanks for the input. The reason is the laser costs around 300 usd... and I'd like to go careful on this one.

Thanks Scead.

 

[Sceadwian]

I just revisited the PDF for your laser pump and noticed how simple the schematic you posted is (meaning missing hyper critical components), where is the feedback loop to your current controller, which by the way doesn't exist =)?

The pump you posted doesn't have any internal feedback, you have to use opamp circuitry to monitor the photodetector output so that the current can be controlled due to variation in both the diode with temperature and it's own lasing efficiency you need a tight control loop to keep the output stable.

What specifically is this for? The module you posted is a communications laser and it's only 70mw, if you just want a powerful laser you can purchase 100-300mw visible laser for under 100 dollars.

 

[Vizier87]

hmm... I just need to drive the laser to the fullest to produce a broadband source. I suppose that doesn't require any monitoring does it?

I'm gurgling for "voltage controlled current regulator". Is that the right place to look at?

Thanks Scead.

 

[Sceadwian]

No.. No... not at all.... You're misunderstanding something fundamental to what you're trying to do, and what you're doing it with.

It's a LED laser diode.... There will be nothing broadband about it at all! it will emit light at it's SPECIFIC wavelength and no other, any modulation will be amplitude, laser diodes are extremely frequency stable.

You need a constant current source, the photo diode output is to allow the constant current source to track the diode's actual output and adjust it for stability, these are meant for high end long range communication through fiber optics cable in impossible to reach conditions.

Unless you're trying to create an underwater fiber optics network I'm wonder how you came across such a module because it's not suited for much else, especially not for that price!

 

[Vizier87]

that's a whole different story here man. I'm using the laser to go through EDFA and a whole other optical circuits, and that will produce the BB source. It takes discipline in photonics to understand, but either way, I just have to turn it on constantly at the rated current.

Have a look at "ring laser EDFA" anyway if you need to look at it.

Thanks for the input anyway.

Vizier87.

 

[Sceadwian]

Is AM modulation tolerable in the output? Without using the feedback photo detectors you won't get a constant amplitude output, it will oscillate uncontrollably and unpredictably, possibly fatal to the laser module itself.

This does require discipline, it also requires highly specific explanations of the desired output. You posted a circuit that drives an LED laser module from a constant voltage source which incorporates no form of current control or feedback from the beam output which are critical to it's operation let alone longevity.

If you power the circuit up as described so far all it will take is a few degrees of thermal drift positive to fry the laser module, not to mention the AM properties of the output will be up to the parasitic components of construction and totally out of control, likely leading in the module to fail.

 

[Vizier87]

err that is the part which I'm still carrying out experiments with. The optical circuit is halfway done (or a quarter-way done ) I'm hoping to adjust the current using a pot and see the spectrum (I've got an spectral analyzer at my lab, FYI) and from there I should be able to determine the threshold of the laser.

Also, I tested the laser last time, and observed that at around 50 mA ( I think) the photodetector hardly changes it's output, so I suppose at low power the laser does not need a feedback control?

Anyway, can you suggest any references on how do I construct this control loop? Think I'll need it anyway.

Thanks Scead.

 

[Sceadwian]

50ma is 1/8th it's rated maximum output (tested short term), obviously you're not straining it! You can not derive full operational circuitry from something like that.

I'm not sure of the methods in the final output after your optics have been integrated. You don't seem to want to answer my questions of inquiry.

It is an absolute statement of fact that unless there is feedback on a laser diode that it will oscillate, or otherwise operate outside of it's SOA. Simple laser modules incorporate this on the die, your module is however a communication module, it needs an integrated control loop. This is again all beside the point if you don't describe what effect AM modulation of this pump module will have on your final optics, because you can get away with a constant current driver if AM modulation isn't a problem.

 

[Vizier87]

I'm not sure of the methods in the final output after your optics have been integrated. You don't seem to want to answer my questions of inquiry.

er which part of your question did you feel I dodn't answer? perhaps I was reading too fast. =)
Like I said, my setup is still halfway done, but the principals of the operation is explained by the "ring laser EDFA" theory, for instance this paper might tell you some stuff.

There are some parts I need it to be P&C but otherwise the electronics part is pretty straightforward, I suppose.

 

[ronv]

Laser Driver

If you hook up your regulator this way it will limit the current. The voltage will rise to the diode voltage. The transistor will "steal" the current to shut off the diode.

 

[Vizier87]

Simple laser modules incorporate this on the die, your module is however a communication module, it needs an integrated control loop. This is again all beside the point if you don't describe what effect AM modulation of this pump module will have on your final optics, because you can get away with a constant current driver if AM modulation isn't a problem.

I forgot to mention; it is NOT for communications, it's just for pumping the laser into an EDFA. I'm not modulating anything there.

Ronv: thanks for the input. I think it's pretty similar to my setup in my first post isn't it?

Guys, however, when I power it up, the dBm of the laser always fluctuates like hell. I wonder if there's a specific value I need to fix to it to get a stable output.

 

[Vizier87]

okay.. I think problem's solved. I plugged in 150mA at 2V... the detector stayed at a constant value. So hopefully no control loop is needed.

 

[Sceadwian]

the detector stayed at a constant value. So hopefully no control loop is needed

If you're happy with the results you've seen from the photo detector then the results are what count, just something to keep in mind later down the road if you start seeing amplitude drift on the final output.

What's the end goal of your project? My knowledge of lasers is pretty low but it's always nice to hear what a users end desire is.

 

[Vizier87]

well, I wanted a portable ASE source, and most of these things are made with minimal documentation. So the "ring laser" config will help realize that. I'm halfway done here. Will post pics if you like soon.