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A NEW PHOTOELECTRIC FABRY-PEROT
SPECTROMETER FOR LOW LIGHT INTENSITIES
A. Steudel, H. Walther
To cite this version:
JOURNAL DE PHYSIQUE Colloque C 2, suppliment au no 3-4, Tome 28, mars-avril1967, page C 2
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255A
NEW PHOTOELECTRIC FABRY-PEROT SPECTROMETER
FOR LOW LIGHT INTENSITIES
By A. STEUDEL and H. WALTHER
Institut fiir Experimentalphysik A, Technische Hochschule Hannover, Allemagne
Abstract. - In order to increase the signal to noise ratio of a Fabry-Perot spectrometer the follo- wing method was used to detect the photoelectric signals. The pressure in a Fabry-Perot interfero- meter was increased and lowered periodically with a period of about one minute thereby running through one order of the interferometer many times in opposite directions. For each period the signal is measured at 1 024 points and added to the respective values of the previous sweeps in a core memory. Thus a higher signal to noise ratio and a higher accuracy than with conventional methods is achieved in those cases where the noise of the photoelectric signal has a nonstatistical contribution such as an intensity drift or fluctuations of the light source. The averaged signal is given out by a recorder and punched into paper-tape so that the evaluation of the measurements can easily be done by a computer.
RCsumB. - Pour augmenter le rapport signal/bruit d'un spectromktre Fabry-Perot on a utilis6 la mkthode suivante pour detecter les signaux photoClectriques. On fait croitre et diminuer p6rio- diquement la pression dans un interferomktre Fabry-Pkrot avec une pkriode d'une minute environ, ce qui permet de balayer un ordre de l'interferomktre de nombreuses fois dans les deux sens. Pour chaque pbriode, le signal est mesurk en 1 024 points et ajoute a m valeurs respectives des balayages antkrieurs dans une memoire a ferrites. On peut ainsi obtenir un rapport signallbruit plus eleve et une precision meilleure qu'avec les methodes conventionnelles dans le cas oh le bruit du signal photoelectrique a une contribution non statistique telle que la derive d'intensite ou les fluctuations de la source de lumikre. Le signal moyen est fourni par un enregistreur bande perfo- rke et le resultat des mesures est obtenu facilement & l'aide d'une calculatrice electronique.
I. Introduction.
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In order to reduce the noise of the photoelectric signal of a Fabry-Perot spectrometer usually DC amplifiers with extremely small bandwidths are used. Further improvement of the signal to noise ratio by a factor of 2 n can be achieved by modulating the light intensity, amplifying the resulting photo- electric signal by an AC amplifier, and rectifying it phase sensitively. The small bandwidth of the detection system demands that the line structure is followed through very slowly in order to avoid deformations which reduce the accuracy. Therefore the time needed for the measurements becomes very large and a high stability of all experimental parameters especially of the light source is required. But this can be achieved only to a certain degree. With modern data averaging techniques developed in the last decade it is possible to remove most of these difficulties which is desirable for many spectroscopic investigations especially for those where only small amounts of substance are available as for instance for measurements with radioactive isotopes.The construction of the apparatus described below
is completed. The first measurements with it are in progress.
11. Principle of the Signal Enhancing System.
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I n this chapter a survey of the signal enhancing method will be given. The different parts of the apparatus will be described below in more detail.The air in a cylinder which is connected to the Fabry- Perot and to one chamber of a Jamin interferometer is compressed by a piston driven over a gear by a syn- chronous motor (Fig. 1). The fringes of the Jamin interferometer pass before a slit and are counted electronically. When a certain number has passed, the motor is reversed and the piston is moving in the opposite direction expanding the air in the cylinder. After the same number of fringes the whole cycle is repeated. In this way the optical path inside the Fabry- Perot is changed periodically between two preset values.
The photoelectric signal of the Fabry-Perot is amplified and transmitted to a Nuclear Data Enhan- cetron. In this instrument the signal is integrated over
A. STEUDEL AND H. WALTHER
Jamin interferometer
I I
Fobry Perot interferometermotor
FT
contra,FIG. 1. - Schematic diagram of the apparatus. The length of the Jamin interferometer is 40 cm. The motor control also has control over the pulse counter and the Nuclear Data Enhancetron. The various parts of the apparatus are shown in more detail in Fig. 2
to 4. The parts necessary for the digital output are omitted in this figure.
I I I
a certain constant time interval, digitized and stored consecutively in the core memory of the instrument. One part of the core memory (512 places) is used to store the signal during the pressure increase half period and the other part (also 512 places) during the pressure decrease.
The optical path in the Fabry-Perot interferometer is not changed linear in time by the method applied. Therefore the refractive index must be measured independently and assigned to the different storage points. For this purpose the number of data points per interference fringe of the Jamin interferometer is measured for each sweep. The assignment for the individual storage points is made by interpolating for these values.
sign01
amplif~er
111. The Pressure Variation.
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The cylinder with piston used for the pressure variation of the Fabry-Perot interferometer is shown in Fig. 2. The volume of the cylinder is about 2.5 liters. By the female thread in the second gear-wheel the piston is moved up and down. The piston needs about 30 seconds to move from the upper to the lower position or vice versa. The size of the pressure variation is changed by varying the absolute pressure in the whole system.
IV. The Electronic Set-up.
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The signal of the photomultiplier I (Fig. 1 and Fig. 3) is amplified to about 80 V amplitude. The noise of the photomultiplier is reduced by a low pass filter. The low frequency cut- off of the amplifier is about 0.01 cps. In the next stage the amplitude of the signal is limited to 6 V. In this way an almost square wave voltage is obtained, which is fed into a Schmitt trigger producing sharp pulses at the zeros of the input voltage. The backlash of the Schmitt trigger is about 0.2 V. Nevertheless the errorA NEW PHOTOELECTRIC FABRY-PEROT SPECTROMETER
boll
motor drive
11 to the Fobry -Perot ond the Jomin interferometer
FIG. 2. - Cylinder for the pressure variation. To drive the
piston a synchronous motor is used. The copper tubing for the temperature control encircling the cylinder is omitted in this figure.
introduced by this is small because the input voltage of the Schmitt trigger is a square wave in good appro- ximation.
In this way each interference fringe produces two pulses, which are transmitted to a preset counter delivering a pulse when a preset number of pulses has been reached. This pulse together with a flip-flop, a power amplifier and relais I1 (Fig. 3) is used to reverse
the motor. Simultaneously the storing of the signals of photomultiplier I1 (Fig. 1 and Fig. 3) is switched from the first 512 channels to the second 512 channels of the Enhancetron or vice versa. So each half period of the measuring cycle is stored in its own part of the memory. Moreover the preset counter is reset to start anew the counting of the fringes produced while the
sipd d the photMult@erI
I
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AC o ~ iond t ~ nase Itlter t1
fn'wr pubes r I preset cwnterFIG. 3.
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Set-up for the generation of the control pulses forthe motor and the Enhancetron (see also Fig. 4).
motor is moving in the opposite direction. The pulses of the Schmitt trigger are used to synchronize the pressure variation and the sweep of the Enhancetron. The first pulse of every half period triggers the sweep (Fig. 4). The total sweep time is selected to be shorter than a half period of the measuring cycle ensuring that the storing of the signal is completed before the motor is reversed.
For the following reasons a correlation between the channel number of the Enhancetron and the change of the optical path in the Fabry-Perot is necessary :
1. nonlinearity of pressure variation with time, 2. unregularities of the motor speed.
Ideally the pulses of the Schmitt trigger should be used directly to control the change of the channels in the Enhancetron. But for this purpose over 200
C 2
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258 A. STEUDEL AND H. WALTHERFIG. 4.
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Block diagram of the Nuclear Data N D 800 Enhancetron in the function mode used for the purpose described in this paper. By the trigger input the pulser is started which controls the different parts of the instrument. The signal is integrated for a constant time interval digitized .and stored in the core memory. This cycle is repeated the values being stored consecutively. When the 512 (( channels 1) are followed through thesweep is stopped.
interference fringes would be necessary requiring a Jamin interferometer of a length of more than one meter assuming a spacer of more than 2.5 mm in the Fabry-Perot. Our method is a compromise but seems to be reasonably good. The pulses produced by the Enhancetron when changing from one channel to the next one (Fig. 4) are counted by a pulse counter (Fig. 5). The pukes of the Schmitt trigger are used to control the counter. The first pulse of a half period of a pres- sure variation starts the pulse counter, the next one stops it, causes the count rate to be punched into paper tape and restarts the counting. This is repeated for all following trigger pulses. Afterwards the values for the different sweeps but for corresponding interference fringes are averaged separately for the pressure increase and decrease half period. The averaged values are used for the assignment of the interference fringes to data points and for the subsequent interpolation of the points in between.
Just as the counting rates of the pulse counter the data points stored in the core memory of the Enhance- tron can be punched in paper tape permitting to do the evaluation on a computer. During the measure- ment the signal stored in the Enhancetron can be observed on the screen of an oscilloscope (Fig. 4 and Fig. 5).
V. Discussion of the Method. - Experiences with this method are not yet sufficient to tell to which extent the expected advantages are realized and to make a quantative comparison to conventional methods. Nevertheless the following points should be mentioned :
1. The information on a line structure usually
taken from many orders is obtained from the average of many measurements of one order of the Fabry- Perot (if the pressure variation of one half period is identical with the pressure variation necessary to
,
,Ier
,
FIG. 5.
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Schematic diagram of correlation between data points and pressure variation and of data output.change the optical path in the Fabry-Perot by half a wavelength). A high signal to noise ratio in this order can be achieved and the evaluation can be done with high accuracy.
A NEW PHOTOELECTRIC FABRY-PEROT SPECTROMETER C 2
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259large extent because the orders are followed through in opposite directions in each half period of the measuring cycle and the results of both measurements are available separately and can be compared to each other.
3. Because the change of the optical path is measu- red at the same time as the line structure it is not necessary to follow through a whole order of the Fabry-Perot because the wavenumber axis is known accurately enough. Therefore much time may be saved because very often only one part of an order includes the desired information but conventionally the whole has to be measured in order to get the wavenumber scale.
The support of the Deutsche Forschungsgemein- schaft is gratefully acknowledged.
We thank cand. phys. R. Sauer for the help with the electronic equipment.
INTERVENTIONS
G. HENDERSON.
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What was the relative inten- sity level of your fringes from the Jamin interfero-meter which triggered the square wave generator and did noise contribute any spurious triggering of your memory system ?
A. STEUDEL.
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The relative intensity level of the fringes is about 3 : 1. To reduce the noise of the pho- tomultiplier I (Fig. 3) a low pass filter in the ac amplifier is used to cut off frequencies higher than 30 cps. No further precautions are necessary to avoid a triggering by the noise.P. CONNES.
