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26 Mar 2009
Scientists needing to make sensitive displacement measurements will benefit from a simple way to increase the resolution of an interferometer.
Researchers in France have discovered a clever way to increase the resolution of a standard two-beam interferometer. Their new approach could be used to make ultrasensitive displacement measurements as well as to perform ultraprecise atom localization (Optics Letters 34 755).
The resolution of an interferometer is directly linked to the fringe spacing of the recorded interferogram. Now, Hugues Guillet de Chatellus and colleagues have found a way to reduce the fringe spacing by a factor of 2, increasing the resolution of the interferometer.
"We take advantage of atomic physics to demonstrate that a fringe at the output of an interferometer can be far smaller than half a wavelength," Guillet de Chatellus told optics.org. "The usual detection of the interference signal when both beams are recombined is replaced with the detection of the fluorescence signal emitted by a specific atomic transition resonantly excited by the laser."
The challenge was to excite a suitable atomic transition with a suitable laser. "You need to excite a J=n/2 --> J=n/2 transition where n is an odd integer, such as the D1 line in alkali atoms," explained Guillet de Chatellus. "In general, the hyperfine structure of alkali metals is a problem, since the ground level is split. The solution is a broadband laser that excites the whole Doppler and hyperfine structure of the transition."
The group used a broadband modeless laser tuned to sodium's D1 line. "We use a commercial dye laser in which we insert an acousto-optic modulator and a Fabry-Perot etalon to narrow the spectral width," said Guillet de Chatellus. "In our case the laser is continuous wave and delivers up to 100 mW at 589.6 nm."
The interferometer set-up is then straightforward. A polarizing beam splitter divides the laser output into two orthogonally polarized modes that make up the reference and variable arms of a Michelson interferometer. In the variable arm, the team uses a translation plate to introduce a phase shift before recombining the beams and sending them through a sodium cell. Before entering the sodium cell, a small fraction of the beam is diverted to a photodiode.
In the final step, a multimode optical fibre collects the fluorescence signal from the sodium cell. The fluorescence signal can then be compared with the signal collected by the photodiode. It was this comparison that led Guillet de Chatellus to discover that the fluorescence signal shows a reduction in the fringe spacing by a factor of 2.
"It is interesting to note that the resolution of the device depends on the detection process: the usual beat signal versus the fluorescence of the atomic vapour," commented Guillet de Chatellus. "We are now working on a pulsed modeless laser to increase the saturation parameter and demonstrate ultranarrow patterns in the fluorescence. We also want to confine the atoms in the laser beam by adding a buffer gas."
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