Simultaneous measurement of absorption and scattering losses in bulk glass andoptical fibers by a microcalorimetric methodAlcibiade Zaganiaris Citation: Applied Physics Letters 25, 345 (1974); doi: 10.1063/1.1655502 View online: http://dx.doi.org/10.1063/1.1655502 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/25/6?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Novel method to measure bulk absorption in optically transparent materials Rev. Sci. Instrum. 71, 2279 (2000); 10.1063/1.1150441 A fiberopticsbased light scattering instrument for timeresolved simultaneous static and dynamicmeasurements Rev. Sci. Instrum. 67, 540 (1996); 10.1063/1.1146633 Scattering loss characteristics of selenidebased chalcogenide glass optical fibers J. Appl. Phys. 71, 4132 (1992); 10.1063/1.350843 Ultraviolet absorption measurements in singlemode optical glass fibers Appl. Phys. Lett. 60, 1791 (1992); 10.1063/1.107166 Method for the simultaneous measurement of surface and bulk conductance in semiconductors Appl. Phys. Lett. 44, 225 (1984); 10.1063/1.94679

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!lIb):::: : I -I I-

15.8nsee

TIME-

(a) Radiated Intensity ( b) Beam Current

FIG. 2. Upper trace: Time­resolved power measurement of the laser output at 10 Torr partial pressure of N2 from a 14-cm cavity having 8. 8% loss per round-trip transit. Peak power corresponds to 9.1 kW. The horizontal scale is 15.8 nsec/div. Lower trace: Time-resolved record of the total electron-beam current collected with a Faraday cup. Peak current density corresponds to 1.4 kA/cm2 at 1 MV. The first maximum is an L(di/dt) artifact.

shown is 9.1 kW and therefore corresponds to an emit­ted energy denSity of 0.27 J/liter. A summary of out­put energy as a function of nitrogen partial pressure and cavity loss per round -trip transit is shown in Table I.

In the case of the first entry in Table I, the larger mirror losses prevented lasing until later in the after­glow period. This was seen as a pronounced displace­ment of the output pulse to later time, This placed a lower limit on the small signal gain of 0,01 cm- 1 aver­aged over the 20 cm path length for a round -trip transit.

The more nearly optimal combination of pressure and cavity losses corresponds to the trace reproduced in Fig, 2, The decay of the pulse can be seen to corre­spond to the ringing down time of the cavity and indi­cates that the total energy of the output pulse is extract­ed prior to the pulse maximum at 7 nsec, The energy lost by the beam during the corresponding first 7 nsec

TABLE 1. Summary of nitrogen ion laser output at 427 nm for 7 atm of helium and various partial pressures of nitrogen.

N2 pressure Mirror losses Pulse width Energy denSity (Torr) flo rt-1) (nsec) (J /liter)

2.2 19 6 0.004 2.2 11.8 10 0.012 2.2 8.8 14.2 0.048 5.1 8.8 15.8 0.260

10 8.8 15 0.272 20 8.8 11 0.106

can be calculated from range- energy considerations to be 14,4 J/liter, This gives an output efficiency for 427-nm radiation of L ~ relative to beam energy lost in the radiating volume, This is to be compared with the theo­reticallimit of 6,5% for delta function, electron-beam excitation at a time early enough for reaction (1) to go to completion,

In conclusion, it appears the work reported here con­firms the importance of charge transfer reactions as laser pumping mechanisms. In particular, the high efficiency for the emission of 427 -nm laser radiation from the system discussed above evidently points to the importance of the nitrogen ion laser as a device of con­siderable significance,

*Research supported by the U. S. Advanced Research Projects Agency of the Department of Defense and monitored by ONR under contract No. N00014-67-A-0310-0007.

1C.B. Collins, A.J. Cunningham, S.M. Curry, B.W. Johnson, and M. Stockton, Appl. Phys. Lett. 24, 477 (1974).

Simultaneous measurement of absorption and scattering losses in bulk glass and optical fibers by a microcalorimetric method

Alcibiade Zaganiaris

Centre National d'Etudes des Telecommunications, Lannion-22301. France (Received 23 May 1974)

When light is launched into either an optical fiber or a bulk glass rod, discrimination of scattered and absorbed energies is possible through the different thermo kinetics of these phenomena. The design and performance of a differential calorimetric arrangement are presented and simultaneous measurements of the various loss coefficients are reported. At the present time, sensitivity is of the order of 1 dB/km and can be further improved.

The advent of low-loss glasses and fibers has spurred interest in the development of better techniques for measuring optical attenuation at very low levels,

Roughly speaking, total attenuation can be considered as conSisting of two parts with quite different mecha­nisms: absorption and scattering, so that in terms of linear loss coefficients1 the following equation holes:

(1)

Some representative methods for measuring the vari-

345 Applied Physics Letters, Vol. 25, No.6, 15 September 1974

ous loss coeffiCients in fiber and bulk glass samples are listed in Table I. Little is known about direct mea­surement of absorption loss in fibers. On the other hand, most existing techniques differ greatly in principle and sensitivity from one another; thus comparison of their results is often meaningless.

We report here a new method providing simultaneous and accurate measurements of absorption, scattering, and total losses in fibers and bulk glass. This method can also be applied to express bending loss in fibers

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~---------..-

CJ---I

CJ---I

.0

[h91ttt -----r110fl0f100nO:JoonOflOTO '\...------------ t, 0+ £

.0

-----r110flOnOOno~00nOflOTO'---------------t-T

.0 t /\ /"".. [hutJttt vv\... .

---'0000000000 t, T

FIG. 1. Illustration of the different kinetics between absorption and scattering. Scattered photons will reach point 0 immedi­ately after the beginning of the light pulse whereas absorbed energy will be detected later. The amplitude of the signal out­put is shown as a function of time.

in terms of radius of curvature and to determine the losses in fiber connections.

The method takes advantage of the different thermo­kinetics between absorbed and scattered energies: scattered photons propagate at light velocity whereas absorbed energy is converted into heat which moves much more slowly. So, when a light beam is launched into a fiber, an observer would detect the absorbed energy long after the scattered one (Fig. 1). The delay time T depends on the nature of the surroundings. It becomes infinite if the sample is in vacuum, in which case the observer would detect only scattered energy. In practice T is easily monitored by choOSing the appro­priate pressure in a hollow double-walled transparent cylinder, surrounding the sample.

Both scattered and absorbed energies are thermally detected using a modified microcalorimetric element of the Calvet type, shown in Fig. 2. This element con­sists of a hollow heat sink and a thin metal cylinder, both made of a good thermal conductor. The latter is

EVACUATED TRANSPARENT - CYLINDER ______

THERMAL INSULATOR

CAPILLARY

FIBER SAMPLE

'\Ifi;!i~"

MODE STRIPPER

~--1-1 LASER

TABLE 1. Several methods for measuring losses in bulk glass and fibers.

Loss measured Bulk glass Fiber

Total attenuation (at)

Single or double- Breaking method beam spectrophoto- (destructive) meter (Ref. 2,3)

Scattering (as)

Absorption (O'a)

Two-beam optical bridge (Ref. 4)

Scattering photo­meter (Ref. 5)

Calorimetric methods (Ref. 6,7)

Ac detection tech-nique (Ref. 8)

Integrating cube scattering detector (Ref. 9)

Ulbricht's sphere Ref. 10)

Integrating sphere (Ref. 11)

completely surrounded by a great number of identical thermoelectric junctions; its internal surface is optically blackened in order to convert the incident light into heat. The cold junctions are in good thermal contact with the large heat sink.

The thermocouples being connected in series, the total electromotive force E generated is the sum of the individual emf's. E is proved to be always proportional to the total heat flux cP crossing the internal wall and independent of the temperature distribution in the cavity12:

E= (£u/C)cp, (2) where £ is the thermoelectric power of the thermo­couple used, C is the thermal conductivity of the ther­mocouple wires, and u is the fraction of the total heat flux conducted away by the thermocouple wires.

Typical recorded responses to a 20-sec light pulse are reproduced in Fig. 3. The output is generally the sum of two different responses corresponding to absorp­tion and scattering13 [Fig. 3(a)]. The response due to ab­sorbed energy can be cancelled out by putting the sam-

OUTPUT

",.IIt:::... EXTERNAl BOUNDARY III -I COLD JUNCTIONS)

INTERNAL BOUNDARY 1 HOT JUNCTIONS)

INNER SURFACE OPTICALLY BLACKENED

INSULATORS

LABORATORY ELEMENT REFERENCE ELEMENT

FIG. 2. Schematic of the experimental apparatus, here shown with fiber samples. The two identical microcalorimetric elements are symmetrically imbedded in a large heat sink.

346 Appl. Phys. Lett., Vol. 25, No.6, 15 September 1974 Alcibiade Zaganiaris 346 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

128.123.44.23 On: Sat, 20 Dec 2014 00:53:12

100.-------------------------------,

100.--,----------------------------,

100.-----------------------------~

FIG. 3. Recorded output signals corresponding to a 20-sec light pulse, for several values of the pressure in the hollow cylinder. Ca) 760 Torr, superposition of absorption and scat­tering; (b) 10-6 Torr: Scattered energy is alone detected; (c) 100 Torr: the two peaks are far apart.

pIe in vacuuml4 [Fig. 3(a)]. The two different effects can be visualized separately by choosing the appropriate medium between the sample and the detector [Fig. 3(c)).

All these curves are generally sums of many exponen­tial terms.

The absorbed and scattered fractions of the energy and the total loss are proportional to the area under the corresponding curves, A., A., and A. + As, respectively, and they do not depend on the shape of the output:

O!. =KA.,

O!s=KAs,

O!t = K(A;As),

where K is a proportionality constant.

Calibration is made electrically by means of Joule heating so that absolute values of O!., o!s, and O!t can be achieved by integrating the corresponding curves.

347 Appl. Phys. Lett., Vol. 25, No.6, 15 September 1974

(3)

Thus, the various loss coefficients are achieved simultaneously through the same principle, using the same detector. So the numerical values of O!. and (Y.

can be compared in order to define the part of absorp­tion and scattering to total loss.

The experimental setup is shown in Fig. 2; it employs two twin microcalorimetric elements, imbedded in the same heat sink and connected in opposition. Such a differential arrangement provides an experimental zero which is quite stable over long time periods.

Measurements are carried out using monochromatic light pulses from an Ar-Kr ion laser (Coherent Radiation model No. 52 G-MG).

SenSitivity is of the order of dB/km with multi mode fibers and bulk glass rods, provided that samples at least 12 cm long are available.

Besides, the sensor is not in contact with the sample and therefore it provides no thermal disturbance to the region being monitored, contrary to most calorimetric devices.

Further study to design more sensitive microcalori­metric elements is presently in progress. Two main alternatives are conSidered: (i) utilization of semicon­ductor surface thermocouples, some 35 times more sensitive than chromel-constantan wire thermocouples.1s

(ii) Deposition of a great number of thin-film thermo­couples in series, 16 completely surrounding the internal boundary of the element.

Acknowledgment is made to Professor M. Laffitte for support and encouragement and to R. Chastel for helpful collaboration.

lKnowing C/ in cm-l , the conversion into the loss coefficient {3 in dB/km is made by the relation {3 = 4.35 x105 c/.

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