Tandem Interferometer TFP-2
Description
The recently developed TFP-2 HC is a high contrast evolution of the TFP-1 spectrometer. While maintaining the advanced and stable interferometer stage arrangement, a completely new optical plate has been developed to increase the contrast by at least 4 orders of magnitude with respect to the previous model.This brings the device to unprecedented level of selectivity, and is particularly advantageous for applications where a strong elastic light component is mixed with the Brillouin signal, or when it is necessary to perform measurements very close to the elastic line.
The standard triple pass tandem optical system used in the interferometer TFP-1 has its limitations. The contrast is limited to about 1011, which is way below the theoretically expected value, and some asymmetry is present in the transmitted peaks.
In a multipass interferometer, one would like the individual passes to be independent of each other. In other words, there should be no interferometric coupling between the passes. In the standard optics this is approximated by introducing some absorption and some misalignment between the passes. This results in the slight asymmetry and loss of contrast.
Furthermore the standard optical system is laid out as a 3-pass tandem arrangement. The first tandem pass inevitably then lies close to the third pass leading to some cross-talk which again reduces the contrast.
The new high contrast optics avoids these problems in two ways:
- - Firstly the optical arrangement is a tandem arrangement of two triple pass interferometers. A spatial filter separates the two interferometers and so eliminates any cross-talk between the first and last passes.
- - Secondly and more importantly, the coupling between passes is eliminated using quarter wave antireflection techniques. This requires no misalignment so that the transmission peaks remain completely symmetrical. Because the multiple reflections between passes are totally eliminated, the contrast is much higher. A contrast of 1015 is achieved.
Features
All the features of TFP-1 are maintained, with the following additions/changes:
- - Contrast of the instrumental response function has been estimated to be at least 1015.
- - The optical transmission of the device is slightly larger
- - Due to the polarising optics used inside the spectrometer, only vertically polarised light entering the pinhole is efficiently analysed
- - The response function of the interferometer is highly symmetric and closer to the ideal Airy function theoretically expected in a Fabry Perot device.
An upgrade of the optic plate of an old TFP-1 to TFP-2 HC is possible.
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 choice of detector, yet TableStable/JRS provides a choice of fevices picked from the best ones available on the market.
The current best suggestion for 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 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:
TFP-2 HC operator manual
JRS interferometry products brochure
A brief introduction to Brillouin Light Scattering using a Tablestable TFP spectrometer
Contrast assessment in the TFP-2 interferometer
Procedures and troubleshooting:
Troubleshooting: sinusoidal ripple in the spectrum
Procedure: alignment of the reference beam splitter
Procedure: mirror spacing calibration
Procedure: how to clean the interferometer mirrors
Suitable light source for alignment
Available accessories (optional):
Holder for LaserComponents Count(R) detector
Confocal microscope CM-1
Fiber optics input for reference beam (FC/PC connector)