# Round trip time calculator

## About this calculator

This calculator computes the round trip time and repetition frequency of a pulsed laser cavity. The cavity can consist of up to four different materials with different group indexes and with either single or double pass through them. A ring cavity is an example of a single-pass cavity while a linear cavity is a double-pass cavity.

First, the physical lengths and group indexes of the materials comprising the cavity are used to calculate the optical path length of a single round trip as follows

$OPL = 2L_1n_{g1} + 2L_2n_{g2} + L_3n_{g3} + L_4n_{g4}$

The round trip time $\tau$ is then the optical path length divided by the speed of light in vacuum $c$

$\tau = \frac{OPL}{c}$

and the repetition frequency is the reciprocal of the round trip time

$f_{rep} = \frac{1}{\tau}$