Electronic – Laser related questions

laser

I have never designed a laser device, but I have tried to read extensively on them. I'm aware of the different classes of lasers, and the dangers associated with them. For instance, I know about the requirement for an interlock that shuts down the emitter whenever the case is opened. I know class 2 lasers must be visible so that they instigate the "blink reflex". I know the diodes need a lot of protection, from overvoltage, overcurrent, ESD, and so forth. They need regulated current and closed-loop feedback to maintain constant optical power.

My questions are:

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?

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?

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

For additional application information, know that I don't actually need coherent or collimated light (in terms of safety it's probably better if it wasn't coherent or collimated). Rather, what I need is a very powerful bulk light source (1 mW or better). The light source must be able to turn on and off, but it doesn't need to be modulated and the source itself will not encode any information. Thus, if there is perhaps some other device that can provide 1 mW into POF I will be very willing to investigate it, but at this point I am investigating the laser approach, because it seems like most LEDs aren't even capable of 500 uW.

Best Answer

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.

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