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Fabry-Perot Cavity and Frequency Stabilization

Lasers are often billed as being monochromatic light sources, but that is not really the case for any real device. While it is true that most lasers are very precise, there is always some residual uncertainty in the wavelength of light they put out. This uncertainty, or linewidth, often sets the resolution limit of modern experiments in atomic physics, precision interferometry, and even some condensed-matter disciplines. The way to get around this limitation is to find some reference that the laser can be compared to that has a narrower linewidth than the laser itself. If you have such a standard, you can use feedback control to “lock” the laser to your standard and thus stabilize its wavelength. Two widely-used standards are Doppler-free atomic spectral lines and optical Fabry-Perot cavities. In this lab we will explore the physics of a Fabry-Perot cavity and practice locking a laser to it using two feedback-control techniques, side locking and Pound-Drever-Hall locking. Both techniques are common in modern research labs, and Pound-Drever-Hall locking has become especially widespread in the last few years.

Participants will use a modified Teachspin laser-diode apparatus to explore the physics of Fabry-Perot cavities, including stability criteria, standing-wave resonances, the concepts of linewidth and finesse, and Laguerre-Gaussian spatial modes. They will learn the basics of side and Pound-Drever-Hall locking, and we will cover enough feedback control theory to remove the mysterious “black-art” air the subject often has among physicists.