Tandem Fabry-Perot Interferometer TFP-1

The instrument is intended primarily for the study of Brillouin spectra, particularly of many, or broad, features over an extended spectral range. The flexibility is such that even Raman spectra (out to about 500cm^-1) may be measured. The novel construction allows a change of mirror spacing without more than minor loss of alignment, so that full alignment may be achieved again within seconds.
The important features of this type of interferometer are listed below.

·         Automatic synchronisation of tandem scan by use of a translation stage common to both interferometers

·         Easy change of mirror spacing.

·         Parallelogram translation stage prevents tilt of mirrors during scan.

·         Feedback loop using a capacitative transducer gives highly linear scan.

·         Stable against temperature changes - does not require construction from low expansion materials.

References

J.R.Sandercock, in Proc. 7th Int. Conf. on Raman Spectroscopy, Ottawa 1980

S.Lindsay et al., Rev. Sci. Instr. 52, 1478, 1981

J.R.Sandercock, Topics in Applied Physics (Springer Verlag 1982) Vol. 51 p.173

Notes on Brillouin Scattering Equipment

Table top - a thick honeycomb table with a minimum dimension of about 100x200 cm is recommended. See notes on vibration isolation on next page.

Optical equipment external to the spectrometer should be rigidly mounted on the table. Special damped components are not required. All lenses should be achromats or better (for reduced spherical aberration). While protected aluminium mirrors are adequate, it is preferable to use dielectric mirrors in the scattered light path to avoid loss of signal.

Photomultipliers - a tube designed for photon counting with low dark count rate (DCR) is required. A high quantum efficiency (QE) means shorter measurement time. Aim for DCR/QE < 20 cts/sec, if possible. Complete photon counters are available with built-in HV source, amplifier and discriminator. The MP983 manufactured by PerkinElmer, although based on a standard photomultiplier with QE about 10%, has an extremely low DCR and is currently about the best choice. An alternative is the Hamamatsu photon counter H6420. 

The most efficient device currently available, with a QE  as high as 70%, is the SPCM-AQR from PerkinElmer. Unfortunately the DCR is rather high and so this device is not suitable for measuring very weak signals. We can supply a housing for this detector with lens and shutter if required.

Multichannel Analyser - in conjunction with the University of Perugia JRS has produced GHOST, a versatile MCA with curve fitting and calibration features. This is built into the control unit as standard but can also be supplied as a separate unit for existing systems.

Laser – for Brillouin scattering measurements a single frequency laser is essential.. A power of 200 mW is normally adequate - more power heats the sample too much. The frequency doubled Nd-YAG laser is now a very good option. The Excelsior by SpectraPhysics is a good choice, with very low noise and excellent stability. An Argon ion laser with internal etalon may also be used. Note: single frequency is essential – TEM₀₀ is NOT single frequency!

Interferometer Stabilisation - the best solution is to use the double shutter system LM2, which allows the introduction of a fixed reference beam for stabilisation purposes.

The temperature of the environment should be regulated to better than +/- 2°C over a 24 hour period. Larger variations may result in some problems with the stabilisation and a slight loss of finesse.

Isolation of Interferometer from external vibrations

The interferometer will measure directly relative movements of the mirrors as small as 1Å. Movements as large as 10Å will degrade the performance of the instrument. Such movements can be caused by non-resonant distortions of the structure due to accelerations associated with building vibrations. An adequate solution to prevent such distortions is to mount the interferometer on a good optical table supported on pneumatic legs (resonant frequency around 1 Hz). In this case the instrument is however only isolated against building vibrations and not against vibrations introduced directly into the table by, for example, lasers, pumps etc.

A far more satisfactory solution is to support the interferometer on the active isolation system MOD-2. The mounts are attached directly to the optical table, which itself should be attached as rigidly as possible to the floor. With this solution even pumps mounted directly on the optical table will not affect the operation of the interferometer.

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