Fabry-Pérot study : EXP200110
Référence : EXP 200 110
The Fabry-Perot interferometer consists of two parallel semi-reflecting plates (beamsplitters), separated by a distance L (The plate reflection coefficient is greater than 90%). A ray of light entering between the two plates is reflected very many times before it leaves. On each reflection, a portion R of the intensity is reflected and a portion (1 – R) is transmitted. The outgoing rays of light interfere at infinity or in the focal plane of a lens. Multiple interference can be constructive only if all rays are approximately in phase, particularly if there is a large number of rays and thus if R is large.
Rays are in phase only for some specific θ angles depending on λ and L.
SPECTROMETRY
The mercury vapour lamp (Hg) contains a spectral doublet made up of two wavelengths very close to one another. Use of a Fabry-Perot interferometer allows these two wavelengths to be clearly distinguished and their spectral gap to be
measured. For this purpose, we shall use an interference filter in order to select only this doublet amongst the spectral range of the mercury lamp.
FABRY PEROT STUDY - FINENESS
To be able to separate the different rings more efficiently, they need to be as fine as possible. This is equivalent to refining the peaks in the previous curve, i.e. reduce Δλ with respect to δλ.
Thus, a good quality interferometer will exhibit a far lower Δλ than δλ .The greater the fineness, the finer the rings. To increase fineness, the surfaces making up the cavity can be made highly reflective. Indeed, we can show that fineness increases at the same time as the surface reflection coefficient.
Consequently, commercially available Fabry-Pérot interferometers can have finenesses of a few dozens or a few
hundreds. In research, this can even attain a few hundred thousands. This high degree of fineness is a major asset of this type of interferometers with respect to Michelson’s interferometer, which has a fineness of 2. Thus, the number of oscillations N carried out by light in the cavity increases at the same time as fineness :
SUBJECTS APPROACHED
»»Interferences
»»Fabry Pérot study
»»Wavelength
»»Spectrometry
NECESSARY EQUIPMENT
| Reference | Description | quantity | | POF020200 | Fabry-Pérot Interferometer | 1 | | POD013210 | HeNe laser | 1 | | POD067040 | 10x lens | 1 | | POD002192 | Half-moon stand | 2 | | POD010002 | Millimetric metal screen | 1 | | POD010057 | Mercury spectral lamp, high pressure | 1 | | POD010572 | Filtre interférentiel vert - 546 nm | 1 | | POD010056 | Stand for spectral lamp | 1 |
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