Talk:Interferometry

I think Fabry-Perot interferometer which is used in laser, for instance, should be mentioned as a particularly accurate and precise instrument.

Contents

1 question 2 to do 3 Interferometry, point sources of light and coherency 4 Why monochromatic?

question

what properties of He-Ne lasers make them suitable for use in interferometry

simple, cheap and practically sufficient stabilization techniques (zeeman, two modes) because frequency of unstabilized lasers depends on resonator length and can change eg. with temperature

My understanding is that the advantage is narrow linewidth. It means long coherence length, meaning your interferometer can be a reasonable size while preserving fringe resolution. Laura Scudder 08:01, 7 Mar 2005 (UTC)

To be a little more precise, it is the difference in the pathlengths which can be greater if you use a narrow linewidth. If the difference in the pathlengths is identical you can still use a broad bandwidth, even if your interferometer is very large and the path lengths are long -- for example the MIDI interferometer could easily measure fringes using path lengths of 5 x 10²³m (i.e. 500 000 000 000 000 000 000 kilometers) using a very broad band light source because the two path lengths were equal (during observations of the astronomical source NGC 1068). The difference in the pathlengths (or the error in the pathlengths if you are trying to equalise them) which will still give you interference fringes is given by: (wavelength)/(fractional bandpass or linewidth). User:Rnt20 08:57, 8 Mar 2005 (UTC)

to do

add/check text for correlation interferometer to go with image:

Missing image
Correlation-interferometer.png
Image:Correlation-interferometer.png

-- 69.195.36.86 02:21, 4 Feb 2005 (UTC)

Interferometry, point sources of light and coherency

The article stated that for interference to occur the light that is used must be coherent light. The example of Newton's rings shows that coherency of the light is not a requirement for interference to occur. Newton's rings can be readily obtained with plain sunlight. Other examples of interference patterns from incoherent light are the colours of soap bubbles and of oil films on water; some of the frequencies of the daylight interfere constructively, others destructively. The destructive interference "suppresses" certain bands in the spectrum of the sunlight, so the eye sees colours.

The source of the light to be used for a Michelson-Morley type of interferometric experiment must be a point source or a single slit source if incoherent light is used, see Double-slit experiment, conditions for interference why it must be a point source. Coherency of the light does make interferometry easier, because is is not necessary to render it into a point source then. --Cleon Teunissen | Talk 16:03, 6 Apr 2005 (UTC)

Note that there is no difference between coherent and incoherent light, expect for the bandwidth (the range of frequencies in the light). Coherent and incoherent light sources can always be exchanged in any interferometric experiment, the only difference being that with an incoherent light source the interference fringes become less visible if there is a difference in the path lengths in the interferometer. With a coherent light source there can be a large difference in the path length, and fringes will still be seen. This means it is easier to set up experiments using coherent sources, as you do not need to worry about getting the path lengths exactly the same (anyone who has worked in an optical lab will know about this!). I fail to see how using coherent light makes any difference as to whether you can use point sources or not. Rnt20 17:09, 7 Apr 2005 (UTC)

In quantum electrodynamics Paul Dirac made the following observations about the quantum of action of the electromagnetic field, the photon "Each photon interferes only with itself. Interference between two different photons never occurs." (source: external link: Science week article on entanglement (http://scienceweek.com/2004/sa040827-5.htm) quoted from: Dirac, P. A. M. The Principles of Quantum Mechanics (Clarendon, Oxford, 1982))

To my knowledge: in laserlight there is a quantum entanglement of the photons. In this entangled state any photon can interfere with any other photon emitted by the same laser. So the quantum entanglement relaxes the constraints: if different paths are available, they do not have to be the same length. External link: Site with answers. See section: laser light is "in phase" light? (http://www.msu.edu/user/boswort9/attempt1/cep817web/amasci/scimis.htm)

It is unclear to me whether Newton's rings obtained with sunlight can be understood at all in terms of classical wave mechanics, as classical wave dynamics requires the waves to be coherent for interference to occur.

It seems to me that according to classical wave mechanics the waves involved are interacting with each other over the whole length of their "journey" as is illustrated by how water waves coming from two sources interact as they propagate parallel to the water surface. By contrast, photons do not interact during their "journey", the interference pattern is produced as the photon interacts with matter (the photon hits the screen.)

Interference patterns can be obtained from starlight. The individual photons of starlight were emitted by separate atoms. Photons do not interact during their "journey". When an interference patterns is obtained, each photon interacts with the interferometric setup individually. The photons entering the interferometric setup do not have the same phase on entering, but they don't have to, since each photon contributes to the interference patten individually. Quantum ElectroDynamics (QED) describes that even when a baseline of hundreds of meters is used, it is possible to obtain interference, and each quantum of action will have interfered with itself only.

It is unclear to me whether this can be interpreted in terms of classical wave dynamics. It seems to me that in classical wave dynamics it must be assumed that the wave phenomenon that light is assumed to be has self-interaction while on its journey, becoming coherent in the process. It cannot be assumed to have been coherent from the start, since it can come from anywhere on the surface of that sun.

I think that in the case of light classical wave mechanics is not up to the task of describing the physics that is going on. For example, in classical wave mechanics, a luminiferous ether is assumed. I think that coherency is a property of lightwaves propagating through the hypothesized luminiferous ether. --Cleon Teunissen | Talk 09:50, 8 Apr 2005 (UTC)

Why monochromatic?

The page makes a big deal of using monochromatic light sources, but many interferometers use white light (most early interferometers did). Coherent light sources (lasers) are very modern, whereas interferometers have been around since Young's slits! Rnt20 18:06, 3 May 2005 (UTC)

With near monochromatic light there are more discernable interference fringes. Each wavelength of light produces a different spacing of interference fringes. Indeed, in astronomical spectroscopy, the dispersion of the starlight is usually not obtained with the help of a prism, but with the help of a diffraction grating. So in an experiment in which the experimentors want to measure, say, a change in interference fringe spacing, then the more monochromatic the light, the more accurate the measurement reading. --Cleon Teunissen | Talk 20:02, 3 May 2005 (UTC)