= Scanning Fabry–Pérot measurement
{c}
An actual teaching experiment in LM-0300 Fabry Perot Spectrum Analyser by <Walter Luhs>, §§3.1.1–3.1.2, prepares a green <laser> beam by aligning it near and far along a rail, then centring back-reflected <light> from adjustable curved mirrors. An actuator using <piezoelectricity> moves one mirror. A screen first displays <interference fringes>; a focusing <optical lens> and <photodiode> then replace the viewing arrangement. The <oscilloscope> records the photodiode signal and the piezo drive reference on separate channels, using the latter as its trigger. Alignment and mirror separation are adjusted to suppress extra transverse <normal modes>; source <temperature> and drive <electric current> are controlled. Repeated peaks calibrate the scan using the appropriate <free spectral range>, and their spacing divided by their <full width at half maximum> estimates <finesse>.<LM-0300 Fabry Perot Spectrum Analyser by Walter Luhs>[LM-0300 Fabry Perot Spectrum Analyser, §1.0 pp. 3–4; §§3.1.1–3.1.2 pp. 20–21, figures 19–23]{ref}
This experiment uses a confocal curved-mirror cavity, whose repeated peak families can have spacing $c/(4d)$ in air; it must not be calibrated blindly with the parallel-mirror $c/(2d)$. Extra transverse <normal modes> can mimic source features. A finite measured peak width bounds source sharpness only after instrumental broadening is considered; it does not determine the source's intrinsic width by itself.<LM-0300 Fabry Perot Spectrum Analyser by Walter Luhs>[§3.1.2 p. 20: transverse modes, confocal calibration and source-linewidth inference]{ref}
For an independent ideal scan calculation, hold a narrow <laser> <wavelength> fixed and vary the parallel-mirror gap $d$ at <normal incidence>. Advancing one resonance order requires $\Delta d=\lambda/(2n)$. If $d$ changes uniformly over the measured interval, the <time> between repeated peaks divided by their <full width at half maximum> in <time> equals <finesse>. The cancellation of scan rate follows from $d(t)=d_0+vt$: both peak spacing and peak width are divided by the same $|v|$. It fails if the scan rate changes appreciably over that interval. A practical control in the University of Washington's motor-driven flat-mirror experiment is to check that successive peaks are equally spaced before using the time axis this way; its handout also warns that table strain and aggressive low-pass filtering alter the trace.<Fabry-Perot Interferometer, University of Washington Physics 331>[§2.2 pp. 9–11: central-spot scanning, nonuniform drive, table strain and filtering; §2.3 p. 11: widths and spacings in time]{ref}
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