# Laser related questions

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.

• 1. What laser drive chip do you have in mind (link to datasheet?)? Most of the ones I know are designed for high-speed modulation --- not relevant to your application. – The Photon May 15 '12 at 5:09
• Also, what is a "three-terminal diode"? – The Photon May 15 '12 at 5:41
• First rule of lasers: Don't look into the bright light with your remaining eye. – Olin Lathrop May 15 '12 at 11:48
• In the future, questions with multiple sub-questions should be split into individual posts (like "How can I drive a laser diode?", "What certification or regulations are necessary for a product containing a laser", and "Are there any known issues using lasers with 1mm core plastic optical fiber?"). However, ThePhoton has already provided a central answer to all three questions so we'll leave this for now if no one feels too strongly about it. Plus, it's featured in this week's newsletter. – Kevin Vermeer May 15 '12 at 14:01
• @ThePhoton, by "three terminal" I mean common, laser diode cathode, photo diode anode. The photo diode would is used to close the feedback loop. – ajs410 May 15 '12 at 16:05

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.

• Thanks for the excellent comment. Yes, POF attenuation is ~0.2 dB/m, but it is much easier to work with, and less expensive. We have spent much time looking for 1mW LEDs but they are very rare if not obsolete when we find one. I wish I could give you examples of laser diodes and driver chips, but that's just it; I don't know examples. You say "NPN BJT", I say "2n2222" and we haggle over manufacturer. I say "DC laser", you say...? I could perhaps go into digikey and type "laser driver", and select one ala "eenie meenie miney moe", but I'm not sure if that would be productive – ajs410 May 15 '12 at 16:44
• How to choose a laser would be a new question ...There's no "bog-standard" device that will work almost anywhere. The best pricing will be on parts designed for use in mass-market devices like CD players (red) or DVD-ROMs (blue?). The major manufactures are giant Japanese conglomerates like Mitsubishi. – The Photon May 15 '12 at 16:51
• I tried looking for "laser apc" because it seems like I want Automatic Power Control, but that doesn't turn up anything on digikey. A search of laser driver on digikey implies MAX3735 seems popular, but it does seem very much geared toward communication (differential inputs, modulation current specs, etc). Regarding safety, I am fairly paranoid about anyone (me, assembler, or user) getting blinded. But at least an LED isn't coherent, right? Regarding regulations, doesn't anyone enforce regulations (similar to FCC for RF)? And what about EU? – ajs410 May 15 '12 at 18:15
• Coherent or not has no impact on the eye hazard. – The Photon May 15 '12 at 18:19
• And, unfortunately, I don't know much about EU eye safety certifications. I believe that self-certification is the norm, but don't quote me. – The Photon May 15 '12 at 18:22
• You do not need a LASER - an LED will do what you need, there are many sutable LEDs available and they are easier to drive.

As you note that you do not need the attributes that a LASER specifically provides, use of an LED will make your task easier overall. There are a significant number of LEDs that will easily exceed your needs.

Assumption: efficiency = (Radiometric Power) / (DC input) is not of vast concern, bu higher is better.

You mention 650 nM (deep red) so I'll start there.

Most LEDs have a wide radiation half power total angle - 40 degrees to 160 degrees for most of the ones listed below and 6 degrees in one case. "Launching" his energy into plastic optical fibre will result in (probably) substantial energy loss but the levels available compared to your targt make such losses of no great consequence. Specialist hardware is available to convert wide angle radiation into a 1mm (POF) or 0.2mm (HCS) optical feed.

To start with vast overkill, a Luxeon Rebel deep red LED with ~= 1 Watt input will produce 250 mW to 350 mW of radiometric output at around 35% - 45% optical/DC efficiency depending on model chosen and bin. Coupling 1 mW of this into an optical fibre should 'not be too challenging'. In practice a far lower power level will be adequate.

Luxeon Rebel & Rebel ES colour portfolio - see Table 1 page 5.

At the other extreme here is an LED with integral 1mm POF "launcher" which produces approaching 1 mW max OUT of the far end of 0.5m of 1mm POF or 5m of 0.2mm HCS fibre with 60 mA LED current. See Table and note 3 on page 7 of data sheet below. As that is max rating (0.8 dBm max) it's somewhat below your spec but shows how achieveable the requirement is

Avago HFBR-1527ETZ LED transmitter datasheet

$US13.67/1 from Digikey In between: Advanced Photonics 950 nM emitter, 18 mW min 22 mW typical optical out at 50 mA LED current. SOT23 package. 140 degree emission angle. Osram 850 nM emitters 25 mW/steradian at 20 mA through 950 mW at 1A in (120 degrees, Platinum Dragon) Example - OSRAM chipled FH4056, 850 nM, 44 degrees, about 0.6 mW out per mA , <= 70 mA continuous at 25C derating to ~= 20 mA at 85C (still giving ~~= 10 mW out.) Roithner LASER diodes - IR & UV - some lovely stuff - but sit down before reading prices. LED suitability: It appears that a suitably powerful "deep red" LED will meet your need well. The Luxeon Rebel Deep Red example below will provide several Watts if required but cn be operated at much lower power levels. It will be representative of what can be achieved from LEDs from other top LED makers. (including Osram, Avago, Seoul Semi, Nichia, Cree, ...) At about 1 mW out for 10 mW in you'd expect ~- 5 mW from a 20 mA LED and going to much higher input powers costs only a few dollars. Example only - Luxeon Rebel - about 1 mW optical out per mA. This is at 350 mA, ~+ 1 Watt and will be slightly better as mA decrease And wavelength is as required • Note the Avago transmitter you linked provides only about 1/10 the power OP says he needs when used as recommended. Driven close to its abs max input current it could reach 1/2 mW. In general, parts designed for communications (aside from long-distance telecom parts) will be strictly limited to less than 1 mW to avoid the need for open fiber control. And at$13.67 I'd think he could get a CD laser for less (but of course need a more fussy circuit to avoid blowing it up). – The Photon May 15 '12 at 16:27
• @ThePhoton - Yes, what you say about the Avago part is entirely consistent with what I said, so we are entirelely agreed. As noted, the part makes 0.8 dBm MAX best case - which is below spec already, and you must design to min not max so it is below spec. He is probably not set on 1 mW out the far end of the fibre but still too low. The point was to show two extremes - one a 350 mW light out LED with no launcher and the other a launcher but below power. Then some stuff in between, Clearly he can achieve what he wants just by looking around and/or a little DIY. ... – Russell McMahon May 15 '12 at 17:17
• @ThePhoton - ... the SFH4056 is Prices: $US0.79/1 at Digikey and Datasheet . A look at the radiation pattern at the bottom of page 4 suggests it would not be too hard to couple substantial power into a 1mm fibre. – Russell McMahon May 15 '12 at 17:21 •$.79 is a good price, but standard POF has around 2 dB / m attenuation at 850 nm (according to a quick google search)...Without more info from OP about what he really wants we can't know if he'd be okay with that or not. – The Photon May 15 '12 at 18:08
• Thanks for the examples, they provide an excellent starting point for me to expand my own search. However, 850 nm is definitely out, you can't see the beam, and attenuation is way too high in POF (60 dB vs. 6 dB for our typical distance). 650nm is good, 500nm would be even better for POF except the receivers tend to be better around 650nm, which is why we chose that wavelength. – ajs410 May 15 '12 at 18:50