HP/Agilent 5517 Zeeman-Split HeNe Laser Mode Sweep Sam’s Laser FAQ ( Copyright © , Samuel M. Goldwasser, All Rights Reserved. The following animations illustrate how the longitudinal modes of an HP/Agilent 5517 Zeeman-split metrology laser behave as its cavity length changes compared with those of a normal HeNe laser and a 5517 tube with its magnet removed.

Longitudinal Modes Cavity modes of any laser that fall within the region of the gain curve which is greater than the lasing threshold (net gain > 1) will result in laser oscillation and a lasing mode with an output power approximately proportional to its amplitude compared to the lasing threshold. These are the longitudinal modes of the laser. The total output power of the laser is equal to the sum of all the longitudinal modes. Sam’s Laser FAQ ( Copyright © , Samuel M. Goldwasser, All Rights Reserved.

Sam’s Laser FAQ ( Copyright © , Samuel M. Goldwasser, All Rights Reserved. Mode Sweep As the cavity length increases due to thermal expansion, the wavelengths of the cavity modes (and longitudinal modes) increase (moving to the left). When a cavity mode passes the point on the left (longer wavelengths) where the gain becomes less than one, a new mode will eventually appear on the right to replace it. Should the cavity length decrease, the opposite will happen.

Zeeman-Split HeNe Laser The HP/Agilent 5517 is a Zeeman-split HeNe laser which uses an axial magnetic field from a powerful permanent magnet to split the neon gain curve into two components shifted up and down in optical frequency by several hundred MHz. The laser is stabilized using a feedback system that thermally controls cavity length. The output is a single beam containing a pair of optical frequencies differing by several MHz Sam’s Laser FAQ ( Copyright © , Samuel M. Goldwasser, All Rights Reserved.

Effects of Magnetic Field Sam’s Laser FAQ ( Copyright © , Samuel M. Goldwasser, All Rights Reserved. The normally symmetric single lasing mode power curve for the 5517 tube is split, shifted, skewed, and distorted due to the magnetic field and mode competition. Depending on the exact mode position on the gain curve, there may be normal modes only, Zeeman modes only, or both Zeeman modes and normal modes present.

Sam’s Laser FAQ ( Copyright © , Samuel M. Goldwasser, All Rights Reserved. Polarization Lasing modes which draw from the two split populations of excited neon atoms have opposite handed circular polarizations out of the laser tube. These are converted to (orthogonal) horizontal and vertical linear polarization by a ¼ waveplate. Thus, the longitudinal modes of a typical HP/Agilent 5517 metrology laser behave quite differently than those of a normal HeNe laser as the cavity length changes.

Split Frequency When a longitudinal mode is near the center of the Zeeman-split neon gain curves, it splits into left and right-circularly polarized components that differ in optical frequency by a few MHz. After being converted to linear polarized components by the ¼ waveplate, the difference or split frequency can be detected by a fast photodiode behind a polarizer oriented at 45 degrees to the polarization axes. The split frequency becomes REF for the laser and the interferometer. Sam’s Laser FAQ ( Copyright © , Samuel M. Goldwasser, All Rights Reserved.

HP/Agilent 5517 with No Magnet With the glass 5517 HeNe laser tube removed from the magnet, mode sweep differs dramatically compared to both the intact 5517 laser and the normal tube. The shape of the gain and lasing curves is more flat-top than Gaussian and the gain bandwidth is reduced even compared to that of the normal tube. The mode sweep is also very sensitive to stray magnetic fields and unstable tending to be partially polarized. The animation assumes stable random polarized laser behavior since otherwise it would be too screwy. Sam’s Laser FAQ ( Copyright © , Samuel M. Goldwasser, All Rights Reserved.

The End Sam’s Laser FAQ ( Copyright © , Samuel M. Goldwasser, All Rights Reserved.