Optical Istruments

Tandem Interferometer TFP-1

Description

The TFP-1 spectrometer is based on a tandem 3+3 pass Fabry-Perot interferometer arrangement. The mirror spacing is selectable in the range 0.1 ÷ 30 mm. Mirrors are scanned by means of piezoelectric transducers up to about 2.5 µm around the rest position. 

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 500 cm-1) may be measured. The construction allows a change of mirror spacing with a minor loss of alignment, so that full alignment may be achieved again within seconds.

Isolation from environmental vibration is of great importance in high-resolution interferometry. 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. In the TFP-1 spectrometer, the interferometer stage is mounted over the active isolation system AVI-35 LPR, based on customised TableStable vibration isolation devices. The mounts are attached directly to the base plate, which itself should be attached as rigidly as possible to the floor. Adopting this solution, even pumps mounted directly on the optical table will not affect the operation of the interferometer.

The spectrometer includes an automatic stabilisation mechanism based on the double shutter system LM2, which allows the introduction of a fixed reference beam for stabilisation purposes. Using this method the alignment of mirrors can be maintained for an indefinite time.

The spectrometer is provided with all the optics necessary to operate, factory aligned and enclosed in a protective black enclosure, mounted on a sturdy aluminium base plate, suitable for installation on imperial and metric standard optical workbenches. A detector choice is available, as described below. A preliminary instruction course for use and basic check and alignment procedures of the spectrometer is included in the price of the spectrometer. Alternatively, the device can be installed on site by trained TableStable/JRS personnel.


Instrument Features :

  • - Scanning combination of two Fabry-Perot cavities in tandem configuration on a common translation stage; triple pass on each interferometer.
  • - Standard mirrors, flat to lambda/200 and coated with a highly resistant dielectric coating, are usable in the range 480 ÷ 540 nm. Custom coatings for different wavelengths are available on request.
  • - Easy change of mirror spacing, set by motor control of the stage within the range 30 µm to 30 mm with very little loss of alignment. Mirror spacing can be read directly by means of a dial gauge.
  • - Parallelogram translation stage prevents tilt of mirrors during scan.
  • - Feedback loop using a capacitive transducer gives highly linear scan.
  • - Stable against temperature changes - does not require construction from low expansion materials.
  • - Two embedded active vibration isolation units guarantee vibration suppression on the interferometer mirrors and improve system alignment stability.
  • - Scanning range 0 ÷ 2.5 µm using a deformable parallelogram scanning stage.
  • - Contrast of the instrumental response function higher than 1010.
  • - Maximum mirror tilt during scan: 10-8 rad.
  • - Maximum jitter during scan: 1 Å.
  • - Each interferometer is equipped with remote controlled (coarse) mechanical and (fine) piezoelectric alignment controls.
  • - Linear scan: departure from linearity max. 10-4 of scan amplitude.
  • - The optical plate for the instrument includes all the components required to analyse an incoming signal beam and to output the light to a light detector.
  • - Selection of input and output pinholes to match the range of mirror spacing. Input aperture of the instrument is f/18.
  • - Motorised switch of the instrument configuration from transmission to reflection, to ease alignment.
  • - Control unit of the instrument provides manual PZT alignment control of the mirrors, long time stabilisation using feedback loops on a reference laser beam, input shutter operation, selectable amplitude of scan, photon output counting through serial interface and electronic remote control of a subset of the functions. Available data acquisition software provides remote interaction capabilities.
     

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
 

Requirements for Brillouin scattering equipment

In order to obtain the best possible results from TableStable/JRS spectrometers, a series of general guidelines can be identified:

Location - a natural reduction of vibrations and thermal fluctuations is obtained when the spectrometer is placed at the lower floors of a building or in the basement, as far as possible from road and other sources of vibration. The temperature of the environment should be regulated to better than ± 2 °C over a 24 hour period. Larger variations may result in stabilisation problems and a slight loss of finesse.

Table top - a thick honeycomb table with a minimum dimension of about 100x200 cm is recommended. The use of pneumatic legs is not advisable, since these may degrade the performance of the active vibration isolation system.

Optics - optical equipment external to the spectrometer should be rigidly mounted on the table. Special damped components are not required. All lenses should be achromatic doublets 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.

Detectors - a detector designed for single photon counting with low dark count rate (DCR) is required. A high quantum efficiency (QE) means shorter measurement time. Aim for DCR/QE < 20 cps, if possible. Customers are free to adopt their own detector, yet TableStable/JRS provides a choice of detectors, picked from the best ones available on the market.

At the moment, the default detector is the Hamamatsu C11202-50 detector, having a peak efficiency of about 65% at 500 nm and a typical noise level of 7 cps. Tablestable offers this sensor together with a suitable mount and optics. 

The Lasercomponents COUNT®-10 blue series detector from LaserComponents, with a typical QE of 70% at 532 nm and <10 cps dark noise, is also supported. Tablestable/JRS can provide a mount and optics for this sensor, but not the sensor itsefl which should be purchased directly from the manufacturer. 

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. An Argon ion laser with internal etalon may also be used. Note: single frequency is essential – TEM00 is NOT single frequency!

We often receive request for suggestion on laser brands and models. The Excelsior by SpectraPhysics is a good choice, with very low noise and excellent stability. Other suitable laser include the Torus from Laser Quantum, the Slim by Oxxius. Please note that this list of laser sources is not meant to be complete. We are willing to test new sources in order to characterise their behaviour.

Documentation links:

Procedures and troubleshooting:

Available accessories (optional):

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