How Sensitive are Lasers to Back Reflections of their Beam?

I was recently using my 1.2W red laser when the beam (it was focused to infinity and super divergent by virtue of being from a red laser) became reflected by a shiny object in my house and came back into the laser. This interaction would have lasted for less than second and I'd say the spot was around 10 inches in width by the time it reached the laser

Don't worry I was wearing goggles and am fine!

Long story short, the laser is now damaged and only outputs 0.4W... This made me wonder, do all lasers become damaged when their own beam (even if it is quite dispersed) comes back to its point of origin? Or is this some kind of inherent sensitivity that only some of the more finicky diodes have?

It should be noted that I tested this (beam back reflection) with one of my weaker reds and saw no damage

On a side note, does anybody know of an affordable supplier of one-way beam optics to protect a laser from its own beam? I know they exist and have real-world applications (like fiber optic communication laser protection, powerful laser cutters, etc.) but have yet to see a consumer-friendly solution

Looking forward to hearing your thoughts LPF!

Experience tells me that not all lasers are damaged by their own reflection. Over the years, I have constructed many non-diode type lasers. Most of them use HR/OC resonators. The first step in resonator alignment is bore sighting the alignment laser. The second step is to adjust the HR mirror to reflect back into the source. I have used a variety of diode lasers and HeNes for this and have never had any problems.

I would not expect a beam that dispersed could do any damage to your laser.

Typically red diodes are at risk the most out of the standard diodes/lasers most of you will have (that said I almost only have experience with large, high power arrays of single mode reds in this manner). Blue diodes are pretty resilient to this as I have done some experiments to narrow linewidth and that requires reflecting quite a bit of laser.. you are almost making an external cavity.
That said some high power blues definitely don't have much left in the kitty for coating degradation/damage at the power levels many of you run, I have seen some very interesting data on this. So I would be careful also with higher power blue.

Beyond that when you are getting into external cavities and the rest at high power you should use an isolator. I have killed a PBS on one of them at ~130W xD.

Ah so I suppose my red diode is just super finicky... rip

gazer101 said:

Ah so I suppose my red diode is just super finicky... rip

Click to expand...

No, this is something I've seen reported on the forum many times over the years, including the very old tech 650-660 nm diodes from DVD-RW drives. I've killed at least one red diode with a full reflection from a mirror (just to try it). It appears to be common with red diodes.
I don't make a habit of reflecting beams toward myself (because it's dangerous) or back into the laser (because it's known to be bad for the laser) so I can't say from personal experience how many or which diode lasers can be killed this way.

Are there no 1-way coated optics on the market to prevent back reflections from re-entering the laser?
I wouldn't mind hooking up said optics to my driver if they need to act as faraday isolators, but they can't be these huge bulky industrial units

gazer101 said:

Are there no 1-way coated optics on the market to prevent back reflections from re-entering the laser?
I wouldn't mind hooking up said optics to my driver if they need to act as faraday isolators, but they can't be these huge bulky industrial units

Click to expand...

The least expensive and most compact solution is behavior modification.

Jokes aside - I can't imagine a demand for optical isolators outside of industry and research.

Hey it was an accident, but yeah I don't see how companies have much of an incentive to produce small-scale isolators. It'd be nice if they existed though

There are small isolators and fibre in-line ones but a decent isolator typically costs more than a high end laser pointer or most single diodes..

I’ve killed a BDR-209 before by accidentally reflecting light off the lens spring. Some are more sensitive than others and sometimes you just get lucky.

I could make my own DIY isolator, but it wouldn't be able to comfortably fit onto a handheld

Brewster window and a waveplate. Frankly just be more careful

Sounds like an interesting idea, would you be able to sketch out how such a thin isolator would operate?

It's the angle that counts. you can use a microscope slide or a coated window. The idea is at this angle the highest transmission occurs. A back reflection would likely be reflected off at an angle. Plus if you put a rotator in there it will not be in phase if it passes back through the rotator so you can use a polarity sensitive window. I really think this is over kill for a pointer.

window say P/rotator----> S Window /Rotator (reflected instead of transmitted) <--------S(likely random really)

Unless you are running power or Q peaks or ECLD or such I doubt you should have a problem worth this kind of effort.

Detail in this I am assuming the window is polarized.

Ah, so there would still need to be a rotator in there somewhere... Is there any way to make such a rotator like razor thin?
AFAIK it basically needs to be a strong permanent magnet around some sort of transparent ferromagnetic material in the center. But after using the strongest possible magnet (Neodymium) and center material the magnitude of the rotation seems to be directly influenced by the length of the tunnel

Would even a 1 degree polarization deflection be enough to build an isolator?