The apparatus you will use in this experiment is shown in Figure 5. The components will be moved to the side when you start your experiment.

4.1 Light Source

The light source is voltage-controlled 650-nm diode laser. The laser power is kept as low as possible so that the eye-injury hazard level is as low as possible. As long as you don’t stare into the laser, then the chance of eye injury is negligible. Keep the laser voltage below 2.5 V.

    Two mirrors are mounted downstream from the laser to provide complete control over the position and angle of the beam. The first mirror controls where the beam hits the second mirror, then the second mirror controls the angle of the output beam.

    The divergence of the laser is very small – on the order of milliradians.

4.2 Beam Expander

Two lenses, one mounted on a translational stage for better control of the output beam, make up the beam expander. The first lens is pretty small, so take extra care to make sure the laser is going through the center of it. The focal lengths of the lenses have been lost to the annals of time, but by looking at the beam width as a function of distance, we can place the components in Figure 4 at the correct spots.

    The second lens and pinhole are mounted on a translational stage to allow for near-perfect positioning in , , and .

Figure 5: Optics components used to make a beam expander with perfect Gaussian output beam. The razor edge and detector are used to measure the beam profile. Note that you may be using a different laser and have the attenuator in a different position.

4.3 Profile Measurement

The profile measurement is accomplished with a razor edge on a translational stage and a photodiode. The stage can be moved by fractions of a millimeter at a time, thereby blocking the beam and reducing the amount of light measured at the photodiode. The photodiode generates an amount of current that is proportional to the amount of light that hits it. That small current is passed through a large resistor to generate a measurable amount of voltage that is directly proportional to the amount of light hitting it. By slowly passing the razor through the beam profile, the intensity as a function of radius can be measured and the beam profile inferred.