How to Avoid Back Reflection and Laser Damage

Invisible laser beam is revealed by Rayleigh scattering, water vapour, and dust particles.
Working with lasers is dangerous.

Be careful when working with lasers, back-reflection can damage to your equipment and your eyes. All lasers are susceptible to back-reflections, which occurs when laser beams are reflected back into the laser cavity. This increases noise which, in turn, causes the laser beam to scatter onto highly reflective surfaces or the human eye. If your laser system uses amplifiers, these back-reflections can be very destructive, damaging things and harming people in its path.

Exposure to laser for as little as one microsecond (or 0.000001 sec) cause serious injury to your eye. The following precautions and methods can be used to avoid laser related accidents.

1. Wear your goggles! It’s common for companies to call for protective eyewear during alignment procedures, or when using high-power lasers. Nevertheless, it would be prudent to wear it at all times.

2. Use a Faraday Rotator. It allows light to pass in one direction but severely attenuates reflected light from the opposite direction. When the laser go through a polarizer, only oscillations with a 45° orientation are allowed to propagate. Any reflected beam is rotated by another 45° resulting in a total rotation of 90°. Any back-reflection is therefore blocked from reentering the polarizer.

3. Clean your optics regularly. When vaporized by a beam, dirt will easily scratch and pit sensitive optics. When dirt absorbs heat from the laser beam, it damages optical coatings, rendering the optics useless. Dirty optics cause severe back-reflection damage to the laser, as well as very bright scattering light can injure the eye.

4. Don’t send a full-power beam through your optical system until you have verified that the alignment is correct. If you (or another person) repositions an optic, this must be communicated to others before beginning any work.

5. Use a beam tube. The longer the wavelength, the more dangerous back-reflection becomes for you and your equipment. When working with a longwave lasers, use a beam tube or tip the optical part so that the reflection is directed at some other optical mount. Track back-reflections continually and make certain that the beams are reflected downward to prevent eye injuries. Should you drop anything on the floor, remember to turn your back to the optical desk when bending down to pick it up. If your back is not facing the optical desk, use your hand to block all possible lines of sight from the optical equipment to your eyes.

6. Be cautious with vertical beams. Check for back reflections. Put an optical beam dump below eye level to trap the beam. In addition, all vertical beams should have a barrier or a shroud covering the receiving optic.

7. Know the proper use and hazards for each optical device. Polarizers are extremely dangerous and have been involved in more laser eye injuries than any other type of optic. It is important to secure your optics to the table and to the optical mount. If you knock over an optic, don’t pick it up immediately. Turn the laser off first. Block the beam before reaching for or moving any optic.

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