PHYS4121: Laser Physics Resources

This demo allows you to vary the parameters in a Fabry-Perot Etalon (Lecture 12), which in a simple form consists of two semi-reflecting mirrors of refectivity $R_1$ and $R_2$ separated by a distance $L$. Light is partially transmitted and reflected each time it strikes the mirror inside the etalon. The medium inside the cavity has refractive index $\eta$. An interference pattern is thus produced inside the cavity which depends on the frequency of the light, since this changes the phase difference $\delta$ between successive waves.

Lecture 12 shows that the transmission from such a device is proportional to the Airy function $A(\delta)$ which depends on the coefficient of finesse $F = \frac{\pi\sqrt{R}}{1-R}$
($R$ = $\sqrt{R_1R_2}$ when the mirrors are not identical): $$ I_T = I_0 \frac{1}{1+ \left(\frac{4F^2}{\pi^2}\right)\mathrm{sin}(\frac{\delta}{2})} $$ The phase difference $\delta$ is calculated from frequency $\omega$ using $\delta = 2\pi\omega / \omega_{FSR}$ where the free spectral range of the cavity is given by $\omega_{FSR} = 2\pi \frac{c}{2\eta L}$.
Changing $L$ and $\eta$ below also changes the spacing of the peaks of the plot of $I_T/I_0$ on the right due to their relationship with the free spectral range.

The plot on the left shows a plot of etalon reflectivity $R$ against finesse $F$ and highlights the current value of finesse determined by the selected values of $R_1$ and $R_2$. The finesse affects the FWHM of the plot of $I_T/I_0$ on the right (see Lecture 12). At low reflectivities, this phenomenon causes the the inhomogeneously broadened lineshapes associated with cavity modes to widen when the etalon FWHM begins to dominate over the natural linewidth. This would be visible in the the spectral hole burning demo if the reflectivity could be varied.