Electronic – 3.1 volt 400 mA laser driver

1) Is it really as simple as grabbing a laser drive chip and a three-terminal diode from digikey and hooking them up according to the datasheet? Should the laser drive chip be able to handle all protection mechanisms necessary, or is there typically another device that's needed to handle some other form of protection?

The laser drive chips I'm familiar with are more about applying rapid modulation to the laser than providing DC power. Usually there's an additional power circuit required; and that power circuit is where the protection is normally implemented.

If you have a different type of drive chip in mind, please link the datasheet in your question.

2) Is there a central regulatory body that does any testing to determine what class of laser you have, and whether your product follows all the necessary regulations?

In the US, it's up to the laser manufacturer to self-certify their product. You may be able to find a consultant to assist you with that process if you don't have the expertise.

3) Are there any known issues using lasers with 1mm core plastic optical fiber? I know that POF has very different transmission windows vs. glass fiber, and I know that one of these optimal windows is 650nm.

Would the beam stay narrow inside the fiber, or would it begin to disperse?

The fiber is a waveguide, and the laser power will remain confined within the fiber core. It will attenuate (lose power over distance). There is also a process called dispersion which means different components of the laser power taking different amounts of time to traverse the fiber---but if you're not switching the signal quickly that's not likely to affect you.

Edit: A major difference between POF and glass fiber is that even in its transmission window, POF has much higher attenuation than glass. Attenuation in glass fiber is measured in tenths of dB per km. Attenuation in POF (last time I worked with it, several years ago) is measured in tenths or whole dB per meter.

Would it still be coherent and collimated after going through, say, 15 meters > of POF?

The signal will still be coherent, but the dispersion effect I mentioned above may reduce the coherence length if you've gone through a very long fiber.

The output beam will diverge at a substantial angle (not strictly collimated) when it exits the fiber. The divergence is a diffraction effect and the angle is inversely related to the fiber core diameter --- meaning POF will have a lower divergence angle than smaller-core fibers. In multi-mode fiber like POF the output divergence angle also depends on details of the fiber construction. In general the output divergence angle will be similar to the input acceptance angle.

I am investigating the laser approach, because it seems like most LEDs aren't even capable of 500 uW.

It doesn't matter much what most LEDs can do --- if you can find one LED that meets your needs, that is enough. And I think you should be able to find an LED to produce 1 mW and couple into POF, if you look long enough. But a laser should be able to do it more efficiently (but maybe more expensively).

Edit: Be aware that using an LED does not reduce your safety concerns. 1 mW is still 1 mW and can still be dangerous. You will want the same safety precautions (you mentioned open-fiber control) whether you use a laser or LED. Regulations have not all kept up with the improved capabilities of LEDs in recent years, but that doesn't mean you shouldn't protect yourself and your users.

Will the diode likely be destroyed with just DC current from two 1,5V batteries and no driver?

Bare laser diodes are fairly sensitive to over-voltage and over-current conditons. They can also be quite sensitive to ESD.

Two series AA cells (for example) might have enough internal resistance to keep from burning out a laser like yours (fairly high power). Two D cells might not. YMMV.

Is the point of the driver to only provide a steady current? Could not a suitable resistor do the same trick?

A good driver will also ensure there's no current or voltage spikes on power up and shut-down, and protect against ESD.

What is the point of the third pin on the laser diode?

Many laser diodes are packaged together with a photodiode to monitor the light output (usually from the "back facet" of the laser). This allows hooking up the laser driver in a control loop to achieve constant power output, even if the laser temperature changes, for example.

Typically the 3rd pin is a connection to the photodiode. Usually the laser and photodiode share a common pin (either "common anode" or "common cathode") and the laser datasheet will tell you which way your device is wired.

Is the light supposed to be spread out like that from a laser diode? I also bought a bag of cheapo 5mw laser diodes, that are mounted in a small brass housing and appear to have a simple driver attached. Would this housing be enough to focus the beam?

Bare laser diodes tend to emit light in an cone of 30 degrees or more half angle. This angle can be very different measured "in-plane" and "perpendicular" to the top surface of the laser chip. Some laser diodes are packaged with lenses to collimate the beam so that it has less spread. The datasheet for your device will tell you the expected emission angle.

Finally, in the case that I do need a driver to set this up correctly, would something like this do the trick?

The driver you linked looks well matched in terms of current and voltage capability.

Whether it provides adequate ESD protection, soft-start, etc., is not clear from the E-Bay listing.

Final Note: 380 mW is more than enough laser power to cause permanent eye damage in case of an accident. Be sure you understand the risks and choose appropriate safety precautions before working with this device and work safely.