Pressure Pulse Detection by Frustrated Internal ReflectionBoye Ahlborn and William W. Zuzak Citation: Review of Scientific Instruments 38, 194 (1967); doi: 10.1063/1.1771354 View online: http://dx.doi.org/10.1063/1.1771354 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/38/2?ver=pdfcov Published by the AIP Publishing Articles you may be interested in A simple demonstration of frustrated total internal reflection Am. J. Phys. 76, 746 (2008); 10.1119/1.2904473 Application of frustrated total internal reflection of millimeter waves for detection and evaluation of disbondsin dielectric joints Appl. Phys. Lett. 92, 094101 (2008); 10.1063/1.2890054 Frustrated total internal reflection: A simple application and demonstration Am. J. Phys. 71, 494 (2003); 10.1119/1.1523075 Frustrated total internal reflection: A demonstration and review Am. J. Phys. 54, 601 (1986); 10.1119/1.14514 Frustrated Total Internal Reflection Am. J. Phys. 40, 1038 (1972); 10.1119/1.1986740

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THE REVIEW OF SCIENTIFIC INSTRUMENTS VOLUME 38. NUMBER 2 FEBRUARY 1967

Pressure Pulse Detection by Frustrated Internal Reflection

BOYE AHLBORN AND WILLIAM W. ZUZAK

Department of Physics, University of British Columbia, Vancouver 8, Canada

(Received 22 July 1966; and in final form, 3 October 1966)

The development of a membrane type pressure probe is reported in which the position of the membrane is re­corded by frustrated internal reflection. The probe has a risetime of not more than 5 ,usee, and records pulses of any length, as well as static pressure. It can have a static sensitivity of several volts per atm, and is relatively unaffected by electrical pickup noise. Examples for application are given, and the probe is compared with a piezo probe.

INTRODUCTION

I T is well knownl that light penetrates into the medium of lower refractive index (n2) when internally reflected,

for angles of incidence, a, exceeding the critical value acrit

(see Fig. 1). If another transparent body of refractive index na with nS>n2 is brought close to the internally reflecting surface S2, a fraction It of the incident light 10 will be transmitted, and the rest is reflected. This phenomenon is sometimes called" frustrated internal reflection" (FIR). It has been extensively studied during the last century.2-6 According to Goos and Hanchen,4 the intensity of the transmitted light (It) depends on the separation d of the surfaces S2 and Sa, and on the incident angle a. Their ex­perimental data are plotted in Fig. 2. For example, with a=51.3°, a variation of d from 0 to 10-5 em reduces It by a factor 2. Thus FIR can be used to convert very small mechanical motions into large light signals. We applied

FIG. 1. Principle of frustrated internal reflection. Io-background light; I.-light transmitted by FIR for distance d and incident angle a.

·1 F. A. Jenkins and H. E. White, Fundamentals of Optics (McGraw-Hill Book Company, Inc., New York, 1957), p. 519.

2 G. Quincke, Pogg. Ann. 121, 1 (1866). a E. E. Hall, Phys Rev. 15, 73 (1902). 4 F. Goos and H. Hanchen, Ann. PhYSik 1, 33 (1947). 6 K. Artmann, Ann. Physik 2, 87 (1948). 6 Remi H. Renard, J. Opt. Soc. Am. 54, 1190 (1964).

194

this principle to determine the location of the membrane in a pressure gauge (hereafter called FIR probe)

DESIGN OF THE FIR PROBE

In our design, the unknown pressure is applied to a Mylar diaphragm (0.003 cm thick, mass per unit area 0.004 g/cm2), which is located opposite to the internally reflect­ing surface of a 90° glass prism. The Mylar serves two func­tions. It is displaced by the unknown pressure, and at the same time it acts as detector for frustrated internal reflec­tion. The back surface (equivalent to S4 in Fig. 1) of the Mylar is scratched with fine sand paper, so that the light is diffusely refracted, rather than internally reflected inside the Mylar. If the motion of the Mylar film is controlled by its inertia, a pressure step of 0.5 atm amplitude should move the film more than 10-0 em in 0.5 ~sec. Since the in­tensity of the transmitted light would then change by a factor 2 (see above), risetimes of about 1 JJ-sec appear feasible.

As seen in Fig. 3, the Mylar is sandwiched between the prism and a holding frame H. With no excess pressure p applied to the Mylar detector, the prism and Mylar touch at a few points, and the background light is transmitted at

0.8

0.6

5£ 0

r 0.2

0 0 , 6 8 10

--.... d FIG. 2. Fraction of transmitted light as function of d and a. Values

from Goos and Hanchen4 ; background light>. == 5893 A.

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PRESSURE PULSE DETECTION 195

mylar-,

P

[t:f! ~

FIG. 3. FIR 1. L-laser light source; S-spacer; H-holding frame; M-monochromator; It-transmitted light; p-pressure.

these points only. Upon application of a pressure, the surfaces are forced together, so that the net transmitted intensity is increased. Irregularities and corrugations of the Mylar film allow elastic forces to counterbalance the applied pressure within a limited region. If p is too large, the Mylar will be pressed completely against the prism, saturating the probe signal. The saturation can be eliminated by interposing a thin spacer S (e.g., 0.003 em thick) between the membrane and the prism. The pressure then will bow the membrane and activate stress forces inside the Mylar in addition to the shear forces. By this method, the saturation of the FIR probe can be shifted to higher pressures; however, the probe then will no longer respond in a lower pressure range.

p

FIG. 4. FIR II. Io-background light; Ir-light internally reflected; It-light transmitted into the Mylar and reflected at the aluminum coating.

t2r-------------------------------. volts

0.8

~'

/ 0~0----~a~2----~a~.'~--~a~6----~a~.8~---;t~o~at~m~~

---...... ~ p FIG. 5. Static sensitivity of a FIR probe.

Cathode resistance 10 kQ.

Two types of FIR probes were studied. FIR I is shown in Fig. 3. The light from a He-Ne CW laser impinges at an angle a slightly larger than acrit on the surface S2 (compare Fig. 1). The transmitted light is scattered by the rough­ened Mylar surface and is focused by a lens on the entrance slit of a monochromator M. The monochromator permits light at the laser wavelength to reach the detector (a IP21 photomultiplier), and discriminates against light which may be emitted by the pressure source. The sensitive spot of this probe has an area of about 0.1 cm2• Although FIR I worked satisfactorily, it is a bulky device, and is difficult to mount because the laser beam has to travel through the region of unknown pressure.

FIR II was therefore designed, and is shown schemati­cally in Fig. 4. The most significant difference from FIR I is a thin aluminum coating, which is evaporated onto the rough surface of the Mylar film using standard techniques. The coating reflects the transmitted light It back through the Mylar and prism, in random directions. Light reflected at small angles (3 (Fig. 4) is then focused with a lens onto the cathode of a photomultiplier. The internally reflected light I r is found only in the direction a' (a' about 10° larger than (3). Thus It and Ir can be easily separated. For the FIR II probe, the CW laser was replaced by a lens and a com­mercially available 2 Ware spot lamp (Cenco Cat No. 87324) with flat, ground and polished window. (The arc

FIG. 6. Slow pres­sure variations in a gas flow. Horizontal 0.2 sec/div. Upper trace FIR II probe, 0.5 V /div., cathode resistance 10 kQ. Lower trace piezo probe 0.001 V /div.

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196 BOYE AHLBORN AND WILLIAM W. ZUZAK

FIG. 7. Stagnation pressures modulated by chopping the air stream with a slotted wheel. FIR I, 0.5 V /div. Horizontal 50 msec/div.

lamp supply unfortunately has a considerable ripple, so that the light is modulated by line frequency.)

PERFORMANCE OF FIR I AND FIR II

The response of the FIR probes to static pressure was investigated by mounting the Mylar membrane as part of the wall of a dosed chamber in which the pressure could be set at given values. The sensitivity depends on the angles a, {J, the thickness of the spacer S, and, of course, on the photomultiplier and the cathode resistor Rc in the multiplier circuit. A typical curve for Rc= 10 kU is shown in Fig. 5. (By taking a larger R e, the sensitivity could easily be increased by a factor 10.) The response is non­linear, and in this particular case reached saturation at a gauge pressure of about 0.7 atm. The values were repro­ducible over several runs, and no hysteresis was observed.

Very slow pressure variations are recorded in Fig. 6. They were produced by chopping a slow gas flow with a slotted disk. FIR II (upper trace) and a piezo probe7

(lower trace) face the gas flow upstream of the chopper. The FIR probe signal is about 1000 times larger than the piezo signal, and is modulated by the ripple in the back­ground light mentioned above. The piezo probe with C = 256 pI" working across the 'scope resistance of 1 M~2 is obviously not capable of recording the true shape of these pulses.

FIG. 8. M = 1.2 shock wave recorded with FIR II probe (uPl?er trace, 0.5 V / div.); piezo probe (lower trace, 0.5 V / div.); 200 p.sec/div.

7 Pressure transducer LD-15/89 of the Atlantic Research Corp., Alexandria, Va.

FIG. 9. Front of M = 1.2 shock wave. Upper trace: FIR pro be, lower trace: piezo probe. 0.5 V / div.; 50/lsec/div.

A sequence of faster pulses is shown in Fig. 7. This oscil­logram records stagnation pressure variations produced with a chopped air stream which was blown against the membrane of FIR 1. The chopping wheel should geome­trically produce pulses of 0.22 T (T is the period of the chopping wheel), 0.04 T, and 0.002 T. The two longer pulses indicate identical stagnation pressures, whereas the airstream apparently is not fully developed through the smallest slit, so that the signal is reduced by a factor of five.

FIR II was finally tested with a step shock produced in an electromagnetically driven shock tube (operated in air at atmospheric pressure, M = 1.2, thus p~/ PI = 1.5). The probe and a piezo-crystal were placed diametrically oppo­site each other, and flush with the walls of the shock tube. Figure 8, upper trace, is the FIR probe signal; the lower trace is recorded by the piezo probe. Figure 9 shows the pres­sure rise on an expanded time scale, indicating a rise time of 5 J,lsec for the FIR probe. This corresponds roughly to the time required for the shock front to traverse the 0.2 em effective dimensions of the probe. The risetime of the piezo probe is about 20 J,lsec, since its effective diameter is about 0.8cm.

The experiments show that the FIR probe has a high sensitivity and a good response to pressure signals of any length. For short signals, the probe seems to be comparable with a piezo probe, while for long signals it is superior. We have, however, not yet attempted to obtain an absolute calibration for short pressure pulses.

ACKNOWLEDGMENT

We gratefully acknowledge the help of our colleagues in the plasma physics group, especially from Dr. F. L. Curzon, who made valuable comments on the manuscript, and from Dr. J. H. Williamson, who allowed us to use his shock tube equipIl).ent. The work was supported by a research grant from the Atomic Energy Control Board of Canada.

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