How to Avoid Back Reflection and Laser Damage

Whenever you work with a laser, being alert to the presence of back-reflection, can prevent costly damage to your equipment and your eyes. Back-reflections occur when laser light is reflected backwards enters the laser cavity causing noise which, in turn, causes the laser beam to stray onto a highly reflective surface or the human eye. Moreover, if the laser system employs amplifiers, back-reflections can cause catastrophic and irreversible damage not covered under warranty, as well as partial or complete blindness.

According to Assessment of Alleged Retinal Laser Injuries, serious damage can be caused by a laser in as little as a microsecond (10-6 or.000001 sec). You may find the following list of potential causes of back-reflection can help you anticipate and avoid potential problems:

1. Wear your goggles! Although it’s typical for organizations to require protective eyewear during alignment procedures, or when high-power lasers, wearing the appropriate safety glasses at all times makes plain good sense.

2. Use a Faraday Rotator. The beam passes through a polarizer parallel to the incoming state of polarization. Only photons with a 45° orientation of the polarization are allowed to pass backwards. The polarization of the reflected beam is rotated by another 45° resulting in a total rotation of 90°, attenuating light and eliminating back-reflection.

3. Keep your optics clean. 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 which is harmful to the eyes.

4. Never send a full-power beam through your optical system unless you verify proper alignment. If you or someone else repositions an optic, this must be communicated to others prior to start of work.

5. Use a beam tube. The longer a laser beam, the more dangerous back-reflection becomes for you and your laser. When working with a long beam, use a beam tube or tilt the optical component so that the reflection goes into some other optical mount. Always track back-reflections and ensure the beams are reflected downward to avoid eye exposures. If you drop something on the floor, it’s always good practice is to turn your back to the optical table when bending down to pick up something from the floor. Alternatively, block your eye with your hand to avoid a direct line of sight with the optical.

6. Beware of vertical beams optics and check them for back reflections. Place a fixed beam block below eye height to trap the beam and to serve as a reminder In addition, all vertical beams should have a beam stop or a shroud covering the receiving optic.

7. Know the functionality and cautions associated with each optic. According to Laser Reference Guide, polarizers are extremely dangerous and have been involved in more laser eye injuries than any other type of optic. Secure your optics to the table and to the optical mount. If you knock over an optic, never pick it right back up. Turn the laser off first. Block the beam before reaching for or moving any optic.