School project with laser diode

[bolle91]

Hi,

I'm studying engineering: applied physics..

In our project, we need to make a device that detects a concentration of black soot in the air..

To do that, we need to have a laser diode that shoots a particle, which will then send out sound waves if the laser has the right frequency.


Anyway, we need to start with the signal of a sound card (computer) and from there we need to make the electronics to make the laser diode work.

We know we need to add a current with a circuit with opamps and transistors and it should remain a sine-function.. Any help?? Thx.

I already found this:

http://www.bcae1.com/opamp.htm (opamp as a regulator) Could that be the right answer?

 

[chuckey]

" have a laser diode that shoots a particle", lasers fire out a beam of light, which is silent. Does your laser beam explode the soot particles?. Why do the particles send out sound in relationship to the laser frequency? Most lasers are fixed frequency.
If somehow this thing works, Why are you starting with an output from a PC? Most laser diodes are switched on or off so they need a pulse wave form. Its the best way to get a high peak light output with a low average power so the diode does not overheat.
Frank

 

[bolle91]

" have a laser diode that shoots a particle", lasers fire out a beam of light, which is silent. Does your laser beam explode the soot particles?. Why do the particles send out sound in relationship to the laser frequency? Most lasers are fixed frequency.
If somehow this thing works, Why are you starting with an output from a PC? Most laser diodes are switched on or off so they need a pulse wave form. Its the best way to get a high peak light output with a low average power so the diode does not overheat.
Frank

Well, it's called the photo-accoustic effect.. a soot particle absorbes light of a particulair wavelength.. The temperature rises and when the laser is turned off again, it cools down. Warming up means expanding and cooling down the opposite. That gives a pressure wave, thus a soudn wave..

But start form a sine-function (voltage) and need to have a sine-function as a current output. And it should be raised with a certain constant current..

a bit like this:



but when i put it in pspice, it gives a current of 0.. really strange.

 

[albbg]

The circuit shown in the article is a classic and works properly. Limitation are usually due to high frequency performances; in this case both op-amp and transistor characteristics must be choosen with care.
However I think in your case, since the current is always zero, there should be some major bug in the simulation circuits. Please check the power supply, the input voltage range (it has to be inside the voltage supply rails), the frequency of the sinusoid, that inverting and non-inverting inputs of the op-amp are not swapped.

 

[chuckey]

I looked at laser diodes, some - rated at 4A do not produce any light until their current is above .5 A, this will upset your sine function. In your simulator you have to have >.8V as your Vdc at the input.
I believe that you will have to go into pulsing mode, diode on for 5%, off for 95%, with a fairly low PRF - as high as is consistent with the soot particle cooling down. Couple this with a microphone channel that is tuned to this frequency, to filter out extraneous sound pickup. If your PRF is high enough you can mount your microphone in the end of an "organ pipe" to provide acoustic filtering and directivity.
Frank

 

[bolle91]

I looked at laser diodes, some - rated at 4A do not produce any light until their current is above .5 A, this will upset your sine function. In your simulator you have to have >.8V as your Vdc at the input.
I believe that you will have to go into pulsing mode, diode on for 5%, off for 95%, with a fairly low PRF - as high as is consistent with the soot particle cooling down. Couple this with a microphone channel that is tuned to this frequency, to filter out extraneous sound pickup. If your PRF is high enough you can mount your microphone in the end of an "organ pipe" to provide acoustic filtering and directivity.
Frank

The physics are no problem. We already got a nice set up with 3 caves in a cilinder 2 for destructive resonance and 1 for constructive. Like in a pax (photo-accoustic extentiometer).

But we know almost nothing of electronics yet. So what you are saying about the pulsing mode and PRF and such are not really clear to me. But wouldn't it work with the network in my link?? i don't think we need to have a perfect sine-function. But something with a on-off period will be enough.

And i already checked everything in Pspice, it seems to be right, but still gives a zero current..

 

[albbg]

I just did now a fast check with microcap (same engine of pspice) and it works without any problem.

I used: 100 ohm resistive load from 15V to the collector
op-amp LF147 supplied +15V GND
1k sensing resistor
transistor 2n2222
Offset 1V
sinusoid amplitude 0.1V
frequency 1k (but also 100k)

 

[FvM]

Well, it's called the photo-accoustic effect.. a soot particle absorbes light of a particulair wavelength

I was surprized about PAS (photo acoustic spectroscopy) methods used for soot measurement, because soot is perfectly black and doesn't expose spectroscopic properties. But I found several references in literature, using various laser wavelength from far IR (CO2 laser) up to green (532 nm). Apparently PAS is used as a sensitive absorption measurement method.

Because the laser is operated with low kHz modulation, drive methods are similar to variable power CW applications. I don't think that drive methods without monitoring diode feedback are reasonable. Controlled optical output power is a requirement for safe operation of most power laser diodes.

With a monitoring diode, a sine shaped modulation won't be a problem. I'm not sure, if has advantages against simple squarewave drive.

You should tell some details about the intended laser diode.

 

[bolle91]

I was surprized about PAS (photo acoustic spectroscopy) methods used for soot measurement, because soot is perfectly black and doesn't expose spectroscopic properties. But I found several references in literature, using various laser wavelength from far IR (CO2 laser) up to green (532 nm). Apparently PAS is used as a sensitive absorption measurement method.

Because the laser is operated with low kHz modulation, drive methods are similar to variable power CW applications. I don't think that drive methods without monitoring diode feedback are reasonable. Controlled optical output power is a requirement for safe operation of most power laser diodes.

With a monitoring diode, a sine shaped modulation won't be a problem. I'm not sure, if has advantages against simple squarewave drive.

You should tell some details about the intended laser diode.

First one: **broken link removed**

If not strong enough: **broken link removed**

Thx for the help guys, i'll make pspice work one way or another

 

[keith1200rs]

Why does the modulation need to be sinusoidal? The 500mW laser module you linked to has a TTL modulation input so if it can be used fast enough that should do.

If you do want a sinusoidal modulation you will probably have to design your own laser driver. The easiest way is probably to base it on a normal closed loop circuit with photodiode monitoring but ensure that the time constant of the feedback loop is slow enough that it cannot respond to the modulation. Then connect your sinusoidal source to the laser diode directly through a resistor. The resistor should be chosen so it adds and subtracts current from the laser drive. The intention is to drive the laser current up and down the steep part of the current/light curve and not go past the "knee". While it can take a few hundred mA to get the laser started, once you get past that point the slope efficiency is quite high - you only need a very small current change to give a large power output change.

Keith.

 

[bolle91]

Yes indeed, the steep part of the diode characteristic is the reason i asked how to add a current. And also why we would want a sine-function. So that we could get a power of 0 to a certain point and back. (so that's on off on off ...)
Our professor told us to use a circuit with opamp's and transistor's, so i think the laser driver will be to much work for nothing, and the network i found on that website will do if i can get it running on pspice.

Thx for the help. and sorry for the clumsy explanation, we're really new to electronics and lasers etc..

 

[keith1200rs]

That circuit is not much use to you. As I explained, you need a feedback loop using the monitor photodiode. The modulation can then be done with a simple resistor. First you need to choose a laser before you can start the design.

You haven't explained why you need a sine wave. On-off modulation would seem to be perfectly acceptable from what you have said.

Keith

 

[bolle91]

maybe that will be the next task in our process then..