How to Collimate a Laser Beam

Divergent laser light can be collimated with a single lens.

A laser beam that has been directed through optics or a fiber can be approximately collimated with different lenses. A laser beam focused into a single-mode optical fiber can be coherently collimated. Mathematics can assist in collimating a laser based on beam diameter and focal length of the lens being used. You can also re-collimate a laser beam with a given lens by quick trial and error.

Things You'll Need

• Laser
• Plano-convex lens
• Lens holder
• Piece of white paper

Instructions

1. 

   Observe the beam diameter by placing a piece of paper in the light path.

   Identify the beam. Use the piece of white paper to see how quickly the laser light diverges with distance. Place the piece of paper perpendicular to the beam of light and move it slowly away from the source. The laser will appear as a dot on the paper and the diameter of the dot will expand as you move the paper away from the light source. Be careful not to look directly at the laser.

2. 

   Never exposure your eyes to direct laser radiation.

   Record the size of the beam as close to the source as possible. Place the piece of paper over the beam and trace the outline of the beam with a pencil. This is the smallest diameter of the collimated beam you can hope for. If the beam is too bright, use a neutral density filter or turn down the power to the laser.

3. Place the piece of paper as far away from the source as possible (the opposite wall of the lab if possible). Place your tracing of the beam size at the source in the center of the beam--that's your target size. For safety, you should be the only person in the room during this entire procedure.

4. Place the lens in the path of the laser approximately one focal length away from the source. Point the convex side of the lens toward the laser source. The spot size on the piece of paper should be smaller.

5. Move the lens slightly toward or away from the light source to make the spot size on the paper as small as possible. Make sure the lens is perfectly perpendicular to the beam at all times. Make sure that you do not accidentally reflect or refract laser light into your eyes.

6. Move the piece of paper toward the lens. The spot size should remain constant, indicating the laser beam is collimated. Adjust the distance of the lens from the source if it changes size until the beam diameter remains constant.

Tips & Warnings

• A laser that has gone through any optics except a single-mode fiber can be approximately re-collimated but will not be coherent (polarity and phase are changed). Use the mathematics and theory for a specific size collimated beam (see Resources).

• Follow all safety and hazard regulations regarding your particular laser. Safety goggles may be required. For UV lasers, a fluorescent index card might be necessary to observe the beam diameter.

References

• Hyperphysics: Quantum properties of light

Resources

• Newport Corporation: Focusing and Collimating