High sensitivity apparatus for low optical absorption measurementsL. C. Nistor, S. V. Nistor, and V. Teodorescu Citation: Journal of Applied Physics 56, 6 (1984); doi: 10.1063/1.333729 View online: http://dx.doi.org/10.1063/1.333729 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/56/1?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Highly sensitive frequency metrology for optical anisotropy measurements Rev. Sci. Instrum. 81, 033105 (2010); 10.1063/1.3356731 HIGH—SENSITIVITY THERMAL LENS OPTIMIZED TECHNIQUE TO MEASURE LOW LINEAR ABSORPTIONCOEFFICIENTS AIP Conf. Proc. 992, 1195 (2008); 10.1063/1.2926817 Highly sensitive plasma absorption probe for measuring low-density high-pressure plasmas J. Vac. Sci. Technol. A 21, 325 (2003); 10.1116/1.1532740 Susceptibility Measurements at High Frequency: A Versatile and Sensitive Apparatus Rev. Sci. Instrum. 44, 899 (1973); 10.1063/1.1686273 Apparatus for the Measurement of Optical Rotation of Solutions at High Pressure Rev. Sci. Instrum. 35, 1281 (1964); 10.1063/1.1718724

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High sensitivity apparatus for low optical absorption measurements L. C. Nistor, S. V. Nistor, and V. Teodorescu Central Institute of Physics, 1. F. T. M.. C P. MG-7 Miigurele. Bucuresti. Romania. 76900

(Received 29 June 1983; accepted for publication 24 October 1983)

The paper describes a laser calorimetric apparatus constructed and calibrated for measuring very low absorption coefficients of transparent materials. By using miniature thermistors as temperature sensors the sensitivity has been increased by at least two orders of magnitude compared with thermocouple calorimetry. Such a high sensitivity opens new experimental possibilities, like performing absorption measurements on highly transparent materials with low power cw lasers or with a single laser shot in the pulsed regime. .

I. INTRODUCTION

The progress in producing low-loss optical materials for optical fibers and windows for high power lasers has made necessary I the development of techniques for measuring bulk and surface absorption in the 10-4_10- 6

em-I range. Of various techniques developed for such purpose, ca­

lorimetry, where the heating due to the absorbed radiation from a laser is measured, has been the most widely em­ployed.2

•3 The most direct approach is to determine the tem­

perature change with the help of a thermal sensor. Several sensors have been used,4-13 of which thermocouples are pre­fered by most authors. The sample is placed inside a chamber, which is often evacuated, a laser beam is transmit­ted through the sample and the change in temperature is monitored by a thermocouple which is attached to the sam­ple. In the adiabatic technique the temperature increase dur­ing the irradiation is directly measured. It is assumed in this case that the temperature increase is uniform throughout the sampJ,e, which is correct for large thermal diffusivity and small heat losses, although it is possible to make adjustments for these factors. 14-18

The sensitivity of the method. can be imprOVed by in­creasing the laser power, controlling the ambient temperture during the experiment and increasing the sensor sensitivity. By using differential thermocouples and reference samples

to compensate for temperature variations, a maximum sensi­tivity of 10-6 cm- I using cw lasers with 1.0-100 W power output has been attained.4

•6

This paper describes a laser calorimetric apparatus con­structed and calibrated for measuring very low absorption coefficients in transparent materials. By using miniature thermistors as temperature sensors it has been possible to raise the sensitivity by at least two orders of magnitude. In this way it becomes possible to perform high sensitivity mea­surements using lasers with only a few watts output, to mea­sure small samples (like short optical fibers) or to perform absorption measurments in the pulsed regime with a single laser shot.

It should be pointed out that thermistors have been ear­lier used in laser calorimetric measurements of optical losses in mirrors. 19 .. 20 However, to our knowledge, such sensors have not yet been employed for measuring optical absorp­tion in transparent materials.

II. EXPERIMENT

The laser calorimetric system for measuring very low absorption coefficients is shown schematically in Fig. 1. It consists of a metallic chamber which can be evacuated to better than 10-4 Torr in order to reduce heat losses from the sample and to protect the measuring system. The sample and

He-Ne LASER •• ~----L ,'''''' VACUUM CHAMBER CHART

RECORDER L. _____ ..I - - -..."

APERTURES

CO 2 LASER

I I-

I / \

_~_1+1_ M T LENS T I

WINDOW

RESISTANCE BRIDGE

PREAMPLIFIER

6 J. Appl. Phys. 56 (1). 1 July 1984

SAMPLE

REF, SAMPLE

RESISTANCE BRIDGE

SCHEMATIC

0021-8979/84/130006-04$02.40

FIG. I. Experimental apparatus for calorimetric measurements of extremely low absorption coeffi· cients.

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FIG. 2. Calibration curves for the thermistors (R 1 and R 2) used as tempera­ture sensors.

reference rods, both of the same material and similar sizes, are mounted on the sharp edges of a Plexiglas holder in order to reduce heat leakage through the specimen holder.

A pair of thermistors of similar characteristics made in our Institute have been used as temperature sensors. Their calibration curves, obtained by means of an ultrathermostat, exhibit (Fig. 2) a linear temperature versus resistance depen­dence in the 18-28 ·C temperature range. Their temperature coefficients are almost equal (1 KJ} 11 ·C ± 2 % and 0.95 Kfl 11 ·C ± 2%, respectively). The thermistors are contained in glass spheres of 0.3 mm diameter. During the measurements they are inserted, at the periphery of the specimen sample, respectively, reference sample, at mid­point, 1 mm deep, in holes drilled for this purpose. To ensure a better thermal contact the holes are filled with thermocon­ductive vaseline. The sample holder allows the alternative centering of each sample rod in the laser beam path by using a He-Ne laser rod and two mirrors. With this arrangement absorption measurements can be performed successively on both rods. Due to the small volume of the thermistors, the mass of the material removed by their insertion is so small (-0.8 mg) compared to the rods mass, that it can be neglect­ed in the heat flow analysis.

The thermistors are differentially connected in a dc re­sistance bridge, with a ceB as reference voltage source. The connection schematic is shown in the lower right corner of Fig. 1. The dc voltage resulting from the imbalance of the bridge is amplified by a PAR mode1113 preamplifier, mea­sured on a digital voltmeter and recorded on a strip-chart recorder. Due to the preamplifier high input resistance (Ra ~ 1 Gil), the voltage between points A and B of the bridge does not change by connecting the preamplifier.

A simple calulation of the imbalance signal caused by the thermal variation of the thermistor resistance yields the following formula:

7 J. Appl. Phys., Vol. 56, No.1, 1 July 1984

.au = E·.aR , (1)

2R(2+ R +S) Ra

where E = 1.58 Vis the cell voltage,.aR the resistance vari­ation of the themistor embedded in the sample rod, R = R 1 = R2 = 23.5 Kfl the thermistors resistances, and S = Sl = S2 = 20 Kfl the values of the other resistances of the bridge. As Ra > > R,s, Eq. (1) can be approximated by

(2)

By taking into consideration the temperature coeffi­cients of our themistors, as well as the other constant param­eters (E and R ), Eq. (2) can be expressed in the form

(3)

where.::1 Tis the temperature variation in the sample rod and .::1 U the corresponding differential voltage, determined from the experimental rise curve recorded on the strip chart re­corder.

According to Eq. (3) a variation of 1 ·C in the sample temperature will result in an imbalance signal .::1 Us = 16.8 m V. One of the most sensitive thermocouples (copper-constantan), used in previous calorimetric experi­ments, has a temperature coefficient of 0.06 m V iC21 which is 280 times less than the thermistor bridge temperature coefficent. Consequently, by simply using thermistors in­stead of thermocouples as temperature sensors, it results in an increase in the sensitivity of measuring small temperature variations of two orders of magnitude.

The differential mounting of the thermistors in the re­sistance bridge ensures a balance in their background noise versus the output signal to be amplified. In working with this measuring system, at high sensitivities, special precautions should be taken to minimize the pick-up noise, by carefully grounding the bridge and the connecting cables. In doing high sensitivity measurements the noise induced by thermal stray radiation from the surrounding becomes troublesome. However, the noise level can be reduced by covering with aluminium foil and cardboard the windows of the vacuum chamber (excepting the one through which the laser beam passes) during the measurements. Another source of noise is the laser radiation scattered on the sample faces. It can be substantially reduced by a careful alignment of the laser beam along the sample rod axis and by achieving a high parallelism of the end faces in the polishing process. With all these precautions taken, we have reduced the noise on the strip-chart recorder down to 0.1 p. V, allowing the observa­tion of signals with amplitude of 0.3 p. V, which corresponds to temperature changes in the sample as small as 2 X 10-s·C. Such a sensitivity makes possible absorption co­efficient measurements in the 10-6-10- 7 cm- 1 range using a cw laser with maximum output power of 5 W.

As shown in Fig. 1, in order to assure a narrow laser beam, a KO lens with I-m focal length and two apertures were placed between the laser and the vacuum chamber. To measure the energy lpower of the laser beam emerging from the sample rod the thermopile head of a Quantronix 504

Nistor, Nistor, and Teodorescu 7

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17 • air

16 lIE vacuum

15 n

14 UJ

13

12

11

10

8

7 0.2 0.4

FIG. 3. Calibration curves of the energy/power meter with the sensor placed in the vacuum chamber. (·~vacuated chamber; (e) chamber at atmosphere pressure.

energy/power meter was placed in the vacuum chamber, behind the sample rod. In this way any supplementary ab­sorption/or reflection in the output window of the vacuum chamber is avoided. However, this arrangement makes nec­essary a new calibration of the energy/power meter with the sensor in vacuum. The calibration was performed by using a second energy/power meter with identical performances outside the vacuum chamber. With this arrangement the en­ergy of the laser beam has been measured on both instru­ments at various energy values and in the same period of time. Figure 3 shows the calibration curves.

Curve A represents the energy (E)) of the sensor placed in the vacuum chamber, with the chamber at normal atmo­sphere, versus the energy (E2 ) measured by the sensor placed outside it. Curve B represents the result of similar measure­ments, with the chamber evacuated at 10-4 Torr. The differ­ence between the two curves at different laser energies gives the correction factor for the measured energy.

The signal from the energy/power meter was also mon­itored on a strip-chart recorder using the same time base like the one used to measure the temperature variation of the sample rod.

m. RESULTS AND OBSCUSSBON

The apparatus described in this paper has been already used for measuring optical absorption coefficients using ei­ther cw or single shot pulsed CO2 lasers.9,22 In all cases use w~ made of the long rod geometry23 which allows the separ­atIon of bulk and surface absorptions. The measurements were performed on KCl earlier grown in our laboratory9 of cylindrical shape, 2 em diameter and 6.2 cm long.

For the first measurements, made with the dual pur­pose of checking the operation of the above presented appa­ratus and to determine the absorption coefficient of KCl samples grown in our laboratory,9 use was made of a small

8 J. Appl. Phys., Vol. 56, No.1, 1 July 1984

> f")

I

~05 ::l <l

laser of f

laser on I 50

~

"" I o

2S :: I­<l

O~~~~~~ __ ~~~~-~-~-~-~~_~_~~_~_~~O o 50 100 150 200 250 300 350

t (sec)

FIG. 4. Typical thermal rise curve from measuring optical absorption in KCI rods using a cw CO2 laser with 4W output power.

cw CO2 laser with output power of tess than 5 W. The thermal rise curve exhibits (Fig. 4), like in the case

of thermocouple calorimetry, a two slope shape. The first slope is determined by the bulk absorption /3v and the sec­ond, which is steeper, by the combined bulk and surface ab­sorptions /3 = /3v + 2/3s· The curve presents an initial and final jump at the moment when the laser was switched on and off, respectively. With the sample available we have ob­tained the following values of the absorption coefficients: P=(5.3±0.3)X1O-3 cm-l and /3v = (3.0±0.3) X 10-3 cm- I

• For such relatively high absorption we did not have to use a high sensitivity, the measurements being performed at low amplification levels.

The high sensitivity of our apparatus has been fully em­ployed in measuring laser absorption in pulsed regime. It has been possible to perform for the first time measurements of optical absorption using single laser shots. 22 In the other published results of measurements with pulsed lasers, per­formed by thermocouple calorimetry, 12 trains of pulses were allowed to pass through the crystal in order to get a measura­ble increase in the temperature of the sample.

A typical experimental thermal rise curve obtained with a single laser shot of 0.264-J energy, delivered from a TEA pulsed CO2 laser in less than 2.5#s, is presented in Fig.5. The signal exhibits characteristics determined by the high sensitivity of the measuring system. At the beginning, after the laser pulse is fired, strong imbalances occur, which are associated with the strong transient electric fields pro­duced by the laser discharge. Moreover, all changes in the signal slope are imposed on a constant drift due to small but always present difference between the temperature of the

q--v-_~~---m-1::t t t tas~r on __ l.~ __ . .l. I· I j-02 t

o 50 100 150 200 250

- !ISE?c)

FIG. 5. Thermal rise curve obtained by irradiating a KCl rod with a single laser shot from a pulsed CO2 laser.

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sample and reference rod. We shall not discuss here the anal­ysis of the experimental results, which were presented else­where.22 We should like only to mention that in a typical experiment the temperature variation has been less than 2 X 10-4 ·C, corresponding to a signal amplitude of 3 J-l V, still one order of magnitude above the minimum observable signal.

Such temperature variations cannot be measured with a similar experimental arrangement using thermocouples in­stead of thermistors, as we have vainly attempted at the be­ginning of our studies.

Besides allowing to perform absorption measurements with single laser shots from a pulsed CO2 laser, the high sensitivity of the above presented apparatus opens new possi­bilities in measuring optical losses in transparent materials. With an experimental setup as simple as in the case of ther­mocouple calorimetry it is possible to measure extremely low optical absorptions. Such measurements would be of special interest at shorter wavelengths (below 5,um), where values of{3 less than lO-5cm -1 are expected. Moreover, it is possible to use shorter samples, which is of interest in mea­suring absorption in optical fibers, or to use smaller and less expensive lasers.

Concluding our results we should mention the possible sources of errors in the measurements at low absorption lev­els.

(1) Errors from thermistor calibration, about 2%. (2) Errors in reading A U on the strip chart recorder,

about 2%. (3) Errors in determining the transmitted energy

through the sample, ,,10%. (4) Errors due to radiation losses through the surfaces of

the sample rod by multiple reflexions inside the sample. Such losses occur from inherent small misalignments between the laser beam and the rod axis and from the un parallelism of the sample rod end faces. These errors are estimated to be about 5%

In conclusion the linear absorption coefficient can be determined with an approximate error of 15%-20% and a maximum sensitivity of 10- 7 cm -1. The accuracy could be even further improved by averaging the results of severa] measurements on the same sample.

9 J. Appl. Phys., Vol. 56, NO.1, 1 July 1984

ACKNOWLEDGMENTS

The authors would like to express their gratitude to Professor I. U rsu for continuous support and encourage­ment, to Dr. V. Ghiordanescu for helpful discussions in the early stages of this work and to Dr. I. Mihailescu for provid­ing the access at the pulsed CO2 laser.

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Nistor, Nistor, and Teodorescu 9

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