LASER Voltage/Current Requirements for PWM

[rueffy]

Hi Guys,

My LASER diode dot module arrived from China yesterday, for those not familiar with my project I am modulating audio into PWM and then driving a LASER with the PWM signal, having a photodiode some distance away receive the light and demodulate it.

I don't know a great deal about my LASER module (no part number), other then its a 5mW device and its suitable for PWM as it has no supporting circuitry (e.g. capacitors to slow its response down). My question is, how do I determine the operating conditions for this laser without knowing them. I have a variable power supply, I'm guessing its a case of slowly increase voltage with a fixed resistance inline with the diode? Or can I calculate the requirements using the power spec provided?

Second question, the circuit I am going to use is simply a MOSFET switch connecting and disconnecting the power supply from the LASER as per the diagram below. Will this circuit work?

Thanks,
rueffy.

 

[mojo_1234]

Hi,

guess it doesn't work because the build-in driver of the LD module is too slow... if have some scope and light sensor (small solar panel etc.) you should be able to measure the slope time of the module.

my recommendation is to either build your own LD driver capable of analog/digital modulation or to use one of the already available LD drivers (e.g. available @ Roithner Lasers) for fiber communication etc.

some years back I've posted one of my first LD drivers based on a simple current source.

just search for "The real high power and full range DIY laser diode driver". :angel:

but there're also other designs available on this forum...

cheers,
-mo-

 

[Cyparagon]

The ideal way would be to use a driver with a modulation input. But since high regulation isn't necessary here, you can get away with omitting the driver and using a current-limiting resistor instead. This is low power - yes?

 

[rueffy]

Hey guys,

Haven't actually tested the circuit yet to say whether or not the LD module is having time response issues, my question was really regarding establishing the operating voltage/current for the laser.

I believe you cannot drive a laser with a changing voltage input (i.e. directly from PWM signal) the response time will then be an issue. Its more so a case of stopping and starting the current flow, hence my gate setup.

 

[rueffy]

The system is battery operated, the LASER is 5mW, there is a .25W audio amp in the circuit.. Yes I guessing its classed as low power.

 

[Cyparagon]

I believe you cannot drive a laser with a changing voltage input

Sure you can, as long as your op amp can source 20mA or so, and if you put a resistor in series with the driver to drop the extra voltage. If the driver is too slow (and I suspect it is) you can just get rid of it and drive the laser diode like you would an LED.

The low power simplifies things. It would be different if it were a high power diode.

 

[rueffy]

I have some fairly conflicting information on that, not that I'm disagreeing you but a lengthy tutorial on LASER PWM Comms specifically said not to vary the voltage, extract at the bottom. Am I interpreting this incorrectly?

Using laser pointers for voice communications

Do not attempt to modulate a laser diode by varying the voltage! Laser diodes, like plain, ordinary diodes, have voltage/current curves that can be extremely steep and vary with temperature: Even seemingly-identical devices from the same manufacturer can have significantly different operating characteristics. Like other semiconductor diodes, a laser diode will not seem to draw current at very low voltage until they start to conduct - at which point the amount of current that will flow will go up more-or-less exponentially with respect to voltage: The difference between a laser being "off" and being destroyed by too much current may be only a few 10's of millivolts and it is for this reason that all laser diodes have some sort of current regulation scheme incorporated within their operating circuitry.

 

[rueffy]

I've got the thing powered now, no luck with operation however, using the circuit in my first post, I have a nice 0/9V PWM signal on the MOSFET gate, the drain of the MOSFET is showing a constant 8.26V however, shouldn't the drain voltage / the voltage across the 100ohm resistor R11 be similar to the PWM waveform on the gate?

The ideal way would be to use a driver with a modulation input. But since high regulation isn't necessary here, you can get away with omitting the driver and using a current-limiting resistor instead. This is low power - yes?

Are you suggesting I omit the MOSFET and just have the LM311 output drive the LASER directly with a current limiting resistor in line?

 

[mojo_1234]

Don't recommend to just use a resistor in series with the LD (as with some LED). You'll need some quite high voltage drop to keep the current somehow stable.

If you're going to build a circuit like the one you've posted first why not building a PWM controlled current source? Main parts are an OPamp and a transistor (or MOSFET) acting as the current limiting element.

:beer:

-mo-

 

[123splat]

"The difference between a laser being "off" and being destroyed by too much current may be only a few 10's of millivolts and it is for this reason that all laser diodes have some sort of current regulation scheme incorporated within their operating circuitry. "

Hmmmm,,, OMG!!! Cyparagon, how much current do you think is in 'a few 10's of mV's' ??

Rueffy, why ask for advice and then refuse to believe it when you get it from a well respected member??
What do you think a PULSE is, if not 'varying voltage'??
0/9 volts on the gate and 8. something on the drain,,, Are you measuring with a scope or a meter (is the meter averaging, and if you are gating with 9 volts, what is the supply????) ?

Beam me up, Scotty.....

 

[Cyparagon]

No, I understand why there is confusion here. The article is saying you cannot drive any diode with just voltage. The same is true of LEDs. You need at least a resistor to limit the current. A series resistor increases the slope of the IV curve.

Are you suggesting I omit the MOSFET and just have the LM311 output drive the LASER directly with a current limiting resistor in line?

I looked at the datasheet to give you a more specific answer, and since this IC can only source ~130µA, you will need the fet.

the drain of the MOSFET is showing a constant 8.26V however, shouldn't the drain voltage / the voltage across the 100ohm resistor R11 be similar to the PWM waveform on the gate?

I think you mean the source. You have the drain connected to 9V. Are you measuring this with no load? Sounds like there is nothing to pull the voltage down on the source during the off-phase. For the sake of testing, change R11 to 1k and tie it to ground. Then check the source.

Don't recommend to just use a resistor in series with the LD (as with some LED). You'll need some quite high voltage drop to keep the current somehow stable.

And the voltage drop would be about 6V here. What's the problem?

 

[rueffy]

I think you mean the source. You have the drain connected to 9V. Are you measuring this with no load? Sounds like there is nothing to pull the voltage down on the source during the off-phase. For the sake of testing, change R11 to 1k and tie it to ground. Then check the source.

Bingo, you were spot on. I reversed my drain and source and threw in a 1k and grounded it, I've now got PWM on the LASER input, the LASER is a little bit dull, but it appears to have a sufficient response time and enough power to raise the receiver above the 'reception' threshold. The received audio is fairly distorted however, but its intermittent distortion...I'm not sure how to interpret this, it must be a bad solder connection or something. I've tested TX to RX directly without the LASER and the receiver circuit reproduces the audio perfectly. So something introduced during the incorporation of the LASER has introduced the distortion.

 

[Cyparagon]

Uh... where is the laser input now? Would you mind redrawing to show your current setup?

 

[rueffy]

This is the current configuration, I've checked all my connections, done a little re-soldering and now I just have a constant high pitch wail masking the majority of the audio (the audio is there, it can be heard). It's as if I'm picking up unfiltered PWM now.

 

[Cyparagon]

nono! You're putting too much voltage into the laser. The swing on the gate isn't correct either. It should go:

(9V)---(resistor)---(laser+)---(laser-)---(FET drain)---(FET source)---(ground)

Change resistor to something like 300-500. I only suggested 1k to ground for testing PWM output of the FET, not for driving the laser. Sorry I wasn't clear on that. Unfortunately, that configuration might have killed/degraded the laser.

 

[rueffy]

Oh I see, no you were clear, I just have a habit of getting excited and rushing things. So just to confirm here is the configuration you've suggested, Laser + and Laser - are next to each other, is that correct?