# Protecting laser diode that is transmitting an audio signal

I would like to first state that I do not really know anything about circuit design- I am very much a beginner.

That being said, I am working on a project where I am using a laser to play music. I have the audio source (my iPod) going through an audio transformer (do not know the specifications, the circuit was given to me by my professor). I am powering the circuit with 4.5V and I have two LEDs that absorb excess voltage. The laser I am using is a 650nm 5mW laser. I measured the current through the laser diode and got 14mA. (I would like to add that I have tested this and at 4.5V, the laser is at its brightest and I have run it for over two hours, but I need a very robust circuit that can last forever essentially).

For some reason, the laser diodes keep burning out and I do not know why. I have done research and I found that a heat sink would help and so would a constant current source.

I want to operate the laser at the highest power without burning it out, so how do I do this? And when they say use a 'constant current source' what does that actually mean? When I look them up, I either find really large power sources where the lowest they go is 1A (I need 14mA). Or I find these small components that clearly need something else to them (like the LM317). What would you suggest for my problem?

Also, because the laser is transmitting the audio signal, the voltage fluctuates ever so slightly to create the signal- will this affect the solution?

• Frankly, I think you need to throw yourself on the mercy of the professor. You simply haven't the faintest idea what you are doing, to the point that you cannot learn from your mistakes - you simply haven't the intellectual framework to identify them. You should go to the professor and ask him for advice on how to acquire the basic knowledge you need. Alternatively, approach some of the other students in you class, and perhaps one of them will take pity on you. – WhatRoughBeast Nov 9 '17 at 21:29
• What kind of ESD precautions are you taking as you work with this laser? – The Photon Nov 9 '17 at 21:57
• @WhatRoughBeast I actually have not run into the problem of burning any laser diodes out- but my professor wants me to make this system last forever, and apparently whatever I have (which does not seem to have any issues) is not good enough. There are no other students working on anything like this and they know less than I do. – Lauren Wougk Nov 10 '17 at 4:12
• @ThePhoton I did research on ESD and I found the Lasorb component: lasorb.com/lasorb-overview – Lauren Wougk Nov 10 '17 at 4:13
• @LaurenWougk - From the OP, " the laser diodes keep burning out and I do not know why", and from your comment, "I actually have not run into the problem of burning any laser diodes out". Apparently your statements cannot be believed. And the professor apparently has issues with your circuit, but he won't tell you what they are? Like I say, you have no idea what you are doing. And no ability to communicate your problems. – WhatRoughBeast Nov 10 '17 at 20:37

## 2 Answers

I want to operate the laser at the highest power without burning it out, so how do I do this?

You are probably over driving the laser diode:

An often overlooked factor in handling diode lasers is the influence of temperature on the relationship between optical output power and operating current. While the threshold current raises with temperature the optical output power and differential efficiency decrease. The driver circuit thus should have a safety feature that ensures that a significant temperature increase will not destroy the laser.

Any driver circuit for diode lasers should include a well-filtered power supply that, as efficiently as possible, blocks inductive loads and other sources of interference. Battery operation circumvents the problem but is not an option in many industrial applications. Keeping the connections between the diode laser and the driver circuit short generally will help reduce interference.

Integrated driver circuits offer a variety of functions and safety measures, and they require few additional components. A simple resistor trimmer can usually set the operating point. It is important that diode lasers always have a regulated driver in either automatic current control or automatic power control operation. A standard laboratory power supply is not suitable for driving them directly.

Source: Driving Diode Lasers

So make sure you have clean power, get a good diode laser driver circuit and cool down your laser diode if necessary.

And when they say use a 'constant current source' what does that actually mean? When I look them up, I either find really large power sources where the lowest they go is 1A (I need 14mA). Or I find these small components that clearly need something else to them (like the LM317). What would you suggest for my problem?

See also this question: why-is-a-laser-supply-so-tricky

A constant current driver could be as something as simple as a transistor current mirror or with an opamp regulating it. In your case you would want to make sure that the CC circuit would not deliver too much current to the diode.

Here is more info on some good circuits for constant current circuits.

Also, because the laser is transmitting the audio signal, the voltage fluctuates ever so slightly to create the signal- will this affect the solution?

Yes, if your modulating the current it will change the brightness of the laser. Since a laser diode has a linear voltage-current curve you'll need to change the current up and down with a constant current circuit with an input that you can control. If you change the + terminal out for a voltage you can control in the circuit shown above with a voltage source then you can modulate the current. Again, with the circuit above you need to calculate the current output and make sure the voltage input stays below a value that will not exceed the rating for the current of your diode. The equations are listed on the circuit.

Use a diode with integrated driver and TTL interface.

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