How to Mix Laser Beams

Coaligning two or more lasers can add to the capabilities of a system.

Lasers create light that is monochromatic and coherent. These two characteristics mean that lasers can be tightly focussed, they can carry lots of information, and they can efficiently deliver a lot of optical power. Because lasers are monochromatic---that is, their light is all at a single wavelength---laser systems can be built with several lasers put together, aligned with each other to perform a task. For example, a low-energy red laser can be aligned with a high energy infrared welding laser. Then the pieces to be welded can be lined up safely using the red laser and only then the welding laser turned on. The steps to align laser beams are fairly straightforward.

Things You'll Need

• Two lasers
• Optical table
• Beam adjustment mechanism
• Dichroic beamsplitter
• Beamsplitter mount on translation stage
• Alignment target
• Laser goggles

Instructions

1. 

   The holes in an optical table provide a convenient alignment reference.

   Place the alignment target close to the output of the first laser centered on a line of reference holes on the optical table.

2. Turn on the first laser and adjust the laser beam position so it strikes the center of the alignment target.

3. 

   Translation stages, movable mirrors, and dichroic mirrors all make it easier to coalign two laser beams.

   Move the alignment target to the opposite end of the table along the same line of reference holes.

4. Adjust the laser beam angle so the beam strikes the center of the alignment target. Turn off the first laser.

5. Place the alignment target close to the output of the second laser centered on a line of reference holes on the table, a line perpendicular to the path of the first laser beam.

6. 

   Putting beams at right angles makes it easier to bring them together.

   Repeat Steps 2, 3, and 4 to align the second laser beam perpendicular to the first beam. Turn off the second laser.

7. Place the dichroic beamsplitter in its mount and attach the mount to the optical table so that the dichroic surface---the surface that reflects one laser beam and transmits the other---is right above the intersection of the two lines of reference holes where the lasers are aligned.

8. Turn on the two lasers. Move the dichroic beamsplitter until the two laser beams overlap on the dichroic surface.

Tips & Warnings

• It's possible to do the laser beam alignment just moving the lasers themselves, but it's usually easier to bounce the beam off a mirror or two to make the adjustments.

• To accurately align the beam to a line of reference holes, it's possible it will be necessary to repeat the alignment steps, refining each time. Making two alignment targets, with a hole in the middle of the near target, can make the refinement easier.

• Laser light can easily damage the human eye. Caution is necessary, especially when aligning laser beams, when stray reflections have not yet been identified and blocked. Use laser goggles and care when working with lasers.

References

• Institute for Solar Physics: Aligning Optical Tracks

Resources

• State University of New York: Understanding "Walking the Beam"
• Hamilton Thorne: About Laser Alignment and Beam Quality