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HeNe Laser Mode Sweep
The following set of 30 slides illustrate how the longitudinal modes of a typical 1-1.5 mW HeNe laser sweep or cycle through the neon gain curve as the laser warms up and the cavity length increases due to thermal expansion.
140 mm (~5.5 Inch) Cavity Length
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Cavity Modes and Lasing Modes
• Cavity modes that fall within the region of the neon gain curve which is greater than the lasing threshold (net gain > 1) will result 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.
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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 1, a new mode will eventually appear on the right to replace it.
• For this short HeNe laser, at most 2 modes are oscillating at any given time, and ofter there is only a single mode.
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Polarization
• Adjacent longitudinal modes in random polarized HeNe lasers are usually orthogonally polarized with respect to an orientation determined by the laser tube construction. These are denoted red and blue.
• Longitudinal modes in a linearly polarized HeNe laser will all have the same polarization orientation but similar amplitudes for the same size laser.
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Start slide show to view mode sweep.
(Left and right arrows may be used to change mode sweep speed or to simulate the feedback control of a stabilized HeNe laser by centering one mode on the neon gain curve or by balancing two modes on either side of the neon gain curve)
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