I like to build a laser pointer driver circuit that I can drive from an 8 bit uC. However, I like to get light output fairly constant. This requires for me to look at the photo detector (these lasers come with 3 pins, gnd, laser cathode, pd cathode) current and adjust laser input current to stabilize light output.

Ideally, I need to drive this from a GPIO of 8bit uC so that I can make it oscillate at different frequencies at different times. (100-300Khz range nothing very fast) I looked around and the analog circuits that are available are for DC, I couldn't come up with a way to drive them from GPIO.

I want to ask masters here, is there a way to do this with a few transistors etc. if not, what type of ICs I can use for this purpose. (A simple current driver with some feedback perhaps?)

  • \$\begingroup\$ What wavelength diode will you be using? Red,blue? The bias voltages on these are (significantly) different and most circuits designed for a red diode will not work with the shorter wavelength diodes. \$\endgroup\$ – gallamine Aug 5 '11 at 15:50
  • \$\begingroup\$ 850nm vcsel or edge emitter. \$\endgroup\$ – Frank Aug 6 '11 at 2:58
  1. What Olin says technically (as almost always :-)). Also, as he says - you need to be clearer in describing your need if you want a good answer.

  2. Lots of circuits here plus various pictures (this is just a Google image search - but useful). Be aware that the quality of the offerings will range from very good through rubbish. Caveat Emptor.

    Here are a few samples from above. No guarantees !!!

  3. This is a superb LASER resource - hav a good look at what it offers. Sam's LASER FAQ is a long time well known site with much good information. Here is his Diode LASER power supplies page.

    From the above, see at the end of this post "Care and feeding of LASER diodes".

  4. Commercial LASER Diode Driver PCBs. These LOOK like they could be modestly priced if they chose to make them so - but they are probably horriobly over priced.

  5. This is for information only - worth a look. It shows what goies into a commercial LASER diode driver block diagram - scary stuff.

From Sam's LASER Diode page - reformated:

Care and feeding of LASER diodes.

The following must be achieved to properly drive a laser diode and not ruin it in short order:

  • Absolute current limiting. This includes immunity to power line transients as well as those that may occur during power-on and power-off cycling. The parameters of many electronic components like ICs are rarely specified during periods of changing input power. Special laser diode drive chips are available which meet these requirements but a common op-amp may not be suitable without extreme care in circuit design - if at all.

  • Current regulation. Efficiency and optical power output of a laser diode goes up with decreasing temperature. This means that without optical feedback, a laser diode switched on and adjusted at room temperature will have reduced output once it warms up. Conversely, if the current is set up after the laser diode has warmed up, it will likely blow out the next time it is switched on at room temperature if there is no optical feedback based regulation.

  • Note that the damage from improper drive is not only due to thermal effects (though overheating is also possible) but due to exceeding the maximum optical power density (E/M field gradients?) at one of the end facets (mirrors) - and thus the nearly iSnstantaneous nature of the risk.

  • The optical output of a laser diode also declines as it heats up. This is reversible as long as no actual thermal damage has taken place. However, facet damage due to exceeding the optical output specifications is permanent. The result may be an expensive LED or (possibly greatly) reduced laser emission.

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    \$\begingroup\$ While I appreciate many of the (also many) links you often provide in your answers, I fail to see the relevance of the Google images link; it's just noise. \$\endgroup\$ – stevenvh Aug 4 '11 at 13:26

It sounds like you want two things: A circuit that modulates the laser diode current as necessary to achieve a specific light level, and a way of switching that on and off from a digital signal.

Make a basic current driver controlled by the output of a opamp, then have the measured light level provide negative feedback. The positive input to the circuit will be the digital output from the micro. Arrange the feedback so that you get the desired current at the digital high level from the micro. In other words, the circuit will consider the signal from the micro to be analog, but it will always be just one of two states. One is off, and the other results in your target current.

For more detail, you'll have to give us more relevant information.


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