A laser's optical cavity supports longitudinal modes spaced by Δν = c / 2L, where L is the mirror separation. A gain medium has a fixed gain bandwidth; your job is to set the cavity length L so that the mode spacing matches a target value — this is exactly the design trade-off behind single-mode vs multimode laser cavities. Adjust L, watch the mode comb move, and submit when you've matched the target spacing.
Your mode spacing: MHz
The gain-medium bandwidth requires a longitudinal mode spacing of:
Δν = c / 2L (c = 3×108 m/s).
Shorter cavities give wider-spaced modes (easier single-mode operation); longer cavities pack modes
closer together.
