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New method for measuring extremely low optical absorptions
Journal of Non-Crystalline Solids 40 (1980) 605-610 © North-Holland Publishing Company
NEW METHOD FOR MEASURING EXTREMELY LOW OPTICAL ABSORPTIONS
A. BUBENZER, S. HUNKLINGER and K. DRANSFELD Max-Planck-lnstitut fftr Festk6rperforschung, Heisenbergstr. 1, D-7000 Stuttgart 80. FR G
A new highly sensitive method is described for the measurement of optical absorption and its wavelength dependence in extremely low loss materials (e.g. starting materials for optical fibres). The optical absorption - as distinct from scattering losses - is measured calorimetricaUy at low temperatures via the temperature rise due to the absorbed heat. Low temperature as compared with room temperature calorimetry is several orders of magnitude more sensitive due to the drastically decreased specific heat and the higher resolution of the temperature measurements. We have measured an optical absorption coefficient as low as 3 dB km -1 (7 × 10 -6 cm -1 ) in a Suprasil I glass rod of only 5 cm in length. In principle, by our mgthod absorption coefficients even smaller than 10 -6 cm -1 can be determined in a sample of the same length. Since the optical power necessary for this method amounts to only a few mW no single strong laser lines are required as in room temperature calorimetry and also low power light sources which are continuously tunable can be used. Our method is also sensitive enough to distinguish between absorption in the bulk and at the surfaces. Measurements on Suprasil I samples show an absorption peak around 600 nm probably due to OH-impurities in the bulk and a background absorption caused by dissipative processes at the two surfaces.
1. I n t r o d u c t i o n High p u r i t y glasses with extremely low optical absorption coefficient are currently available and are used in optical c o m m u n i c a t i o n fibres. It is o f considerable technical interest to k n o w this basic a b s o r p t i o n before i n t r o d u c i n g further losses b y the fibre-drawing process. Therefore, a m e t h o d is needed to determine extremely low optical absorption coefficients - typically o f the order o f 1 dB k m -1 (2.3 × 10 -6 cm -1) - even in relatively short b u l k samples. It is possible to measure optical a t t e n u a t i o n o f low loss b u l k glasses in fairly long rods (l -~ 30 cm) in a special s p e c t r o p h o t o m e t e r [1]. In general it is, however, simpler to apply calorimetric m e t h o d s . There the optical a b s o r p t i o n is measured via the t e m p e r a t u r e rise due to the absorbed energy. In the calorimetric m e t h o d s k n o w n so far, high i n t e n s i t y light sources are needed to o b t a i n measurable temperature variations. Therefore, o n l y strong laser lines can be used supplying a power of a b o u t 1 W [ 2 - 4 ] . C o n s e q u e n t l y , c o n t i n u o u s m e a s u r e m e n t s o f the a b s o r p t i o n spect r u m have n o t been possible so far. In addition, at such high laser powers the danger of optical bleaching c a n n o t be excluded. 605
606
A. Bubenzer et al. / Extremely low optical absorptions
The calorimetric method described here, however, avoids these problems. The sensitivity in our experiment is strongly enhanced by carrying out the measurements at low temperatures (T ~- 1.3 K): Firstly, the heat capacity of solids decreases drastically with temperature. In the case of fused quartz, for example, the specific heat is reduced by a factor of l0 s on cooling from room temperature to 1.3 K. Secondly, small temperature differences can be measured at least two orders of magnitude more sensitively at low temperatures. Consequently, the cryogenic method is, for the same heat input, about seven orders of magnitude more sensitive than room temperature calorimetry. The strongly increased sensitivity of our set-up allows us to use a lower input laser power, typically three orders of magnitude lower than is commonly used in room temperature calorimetric measurements, and still remain several orders of magnitude more sensitive. So it is possible to use a continuously tunable light source and to measure the wavelength dependence of the optical absorption.
2. Principle of measurement As in most calorimetric measurements of the optical absorption the sample is thermally insulated and illuminated by a laser pulse having a duration of about a second. The absorbed energy Q causes the temperature to rise by AT: AT = a/mc
,
(1)
where m is the mass of the sample and c its specific heat. In order to measure the temperature variations we use carbon resistors which have a high temperature coefficient. First an optical pulse is sent through the sample whose temperature rise is detected by a carbon resistor R1. Then an electrical pulse of the same duration as the light pulse and of known electrical power is applied to a second carbon resistor R2 also attached to the sample (see fig. 1) and again the temperature rise of the sample is measured by R1. In this way the heating due to the absorbed light is simulated by an electrical heating. Via this calibration of the system the absolute magnitude of the heat input Q is now easily determined without knowing the values for AT, m and c. At low absorption levels ( a l ~ 1) the absorption coefficient a can be written as a = ( O / E i n ) 1-1 ,
(2)
where l is the length of the sample and E i n the energy input of the laser pulse. When using a sample of about 5 cm in length absorption coefficients as low as 10 -7 c m -1 or 0.04 dB cm -1 can be measured by our method. At these low absorption levels surface absorption may become comparable ,with bulk absorption. It is difficult to discriminate quantitatively between these two processes via the diffusion time of the heat from the surface to the middle of the sample since at low temperatures this
A. Bubenzer et al. /Extremely low optical absorptions
607
Tube for
Carbon resistor radiatic shields i
Powel r
Copper wires
Sample
Fig. 1. Schematic of the experimental arrangement. Nitrogen shields are omitted.
time is much shorter than at room temperature. The best way to separate both contributions is to measure samples of different lengths.
3. Technical As can be seen in fig. 1 the glass sample is suspended in the evacuated sample chamber of an optical 4He cryostat. Thermal contact of the sample to the environment is kept as low as possible. For suspension three nylon threads are used and thin constantan wires are used for electrical connections. Care has to be taken to prevent the heating of the sample due to radiation through the cryostat windows and due to mechanical vibrations, as for example caused by mechanical pumps. The cryostat windows, therefore, have a coating to block off IR radiation and the pumping tubes are mechanically decoupled by heavy metal blocks. The carbon resistors are thermally attached to the sample via copper wires soldered to the glass sample by an indium droplet. The resistors are located at a distance of 2 cm from the glass rod and surrounded by a radiation shield in order to protect them against scattered light. The light source is a dye laser pumped by an argon-ion laser. Compared with an ordinary tamp a laser has the advantage that optimum focusing conditions can be obtained and therefore, false signals due to scattered light can be avoided. A mechanical shutter forms a laser pulse of approximately 1 s duration and its power, of the order of a few mW, is registered by a power meter. A temperature rise leads to a decrease of the resistivity of the carbon resistors. Typically, the temperature rise is a few mK leading to resistivity variations in the order of K~2 which will finally give voltage variations in the range of 0.1 mV in our electrical circuit. These signals are stored in a transient recorder and displayed by a scope or plotter.
608
A. Bubenzer et al. / Extremely low optical absorptions
¢]o
.6 o.
~s ~C cJ o. 0
S upro.si[ 1 tength 50mm
soo Wavelength
(nrnJ
Fig. 2. Absorption spectrum of a Suprasil I sample (l = 50 mm). 4. E x p e r i m e n t a l results
Absorption spectra of different samples of vitreous silica were studied in the spectral range of our dye-laser (using as a dye Rhodamine 6G) which was continuously tunable between about 574 and 634 nm. In addition all the lines of the argon laser between 514.5 and 454.5 nm were used. In fig. 2 we show the absorption spectrum of a vitreous silica Suprasil I sample of 50 mm length at a temperature of 1.3 K. The dominant feature is the sharp peak around 595 nm. In order to make sure that this peak is due to bulk absorption and not caused by the surface, we have also measured a shorter and a longer rod of the same material having lengths of 25 mm and 100 mm, respectively. The results of these experiments are shown in fig. 3. An analysis of the data shows that we can
•
Y \
~-=15
Suprosit 1 • tength tOOrnm • length 2Smm
o
2a.c . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
s~o
s~o
~o
~
6~o
Wavetength
~3o
(nm)
Fig. 3. Ratio of the total absorbed to incident energy (Qtot/Ein) in Suprasil I samples of 100 mm and 25 mm lengths, respectively. The absorption caused by the two surfaces as calculated by extrapolation to zero length is indicated by a dashed line.
A. Bubenzer et al. / Extremely low optical absorptions g 10
i
i
/
F
o\..)..,p~o
609
t .-----r-~. -_
i
,,
_~?:"%. ~ ~s
,-'J
/
"~
__o----/
~°,,o,
°~°
8
o
i
~o
L
sgo
6~o
~
81o Wavele~jth
~3o (nm)
Fig. 4. Absorption spectrum of a Suprasil W (containing less than 5 ppm OH) and a Suprasil I sample (1200 ppm OH). Both samples have the same length (l = 50 mm). decompose the ratio of the total absorbed energy Qtot to the incident energy ~Tin into Otot/Ein = 2a s + OtBl '
(3)
where as is the surface absorption and aBl gives the dissipation in the bulk. The "background" absorption Qs/Ein = 2~s caused by the two surfaces has been calculated by extrapolation to zero length and is indicated in fig. 3 by a dashed line. From these data it is obvious that the absorption peak around 595 nm is proportional to the sample length and therefore arises entirely in the bulk. Since the absorption peak cannot be assigned to any transition element commonly present in Suprasil as an impurity [5] it is probably due to OH-impurities. The occurrence of such an absorption' line at 585 nm as the 4th harmonic of the fundamental OH vibrations at 2.73/~m has been predicted by Keck et al. [6]. This assessment is supported by the fact that the first three harmonics of the same OHvibrations have already been observed [6] and that this absorption line is absent in Suprasil W containing less than 5 ppm OH (see fig. 4). The overall absorption in Suprasil W is, however, higher than in Suprasil I probably due to Other impurities and defects not present in Suprasil I, in agreement with measurements by Heitmann [7]. Although all of our absorption coefficients were determined at low temperatures they agree at selected wavelengths fairly well with the room temperature data on similar material by Keck et al. [6] who reported 5 dB k i n - 1 at 633 nm and Heitmann [8] who gave an intrinsic value of 2.55 dB km -1 at 600 nm. 5. Conclusions We describe a low temperature calorimetric method which, for the first time, allows the measurement of the optical absorption spectrum of extremely low loss
610
A. Bubenzer et al. / Extremely low optical absorptions
materials. Our method is sensitive enough for measuring an absorption coefficient as low as 10 -7 cm -1 in a sample o f about 5 cm in length. Absorption spectra o f Suprasil I obtained b y this m e t h o d show an absorption peak at 595 nm probably due to OH-impurities in the bulk and a broad absorption occurring at the two surfaces.
Acknowledgements It is a pleasure to acknowledge clarifying discussions with D. Krause, particularly about the OH-vibrations in fused silica. We are grateful to Jenaer Glaswerke, Schott and Gen. for supplying us with the glass samples and for coating our cryostat windows. These experiments would not have been possible without the expert technical help o f P. Wimmer.
References [1] D. Krause, Optical Properties of Highly Transparent Solids, eds. S.S. Mitra and B. Bendow (Plenum, New York, 1975) p. 483. [2] M. Hass, J.M. Davisson, H.B. Rosenstock, J.A. Slinkman and J. Babiskin, Optical Properties of Highly Transparent Solids, eds. S.S. Mitra and B. Bendow (Plenum, New York, 1975) p. 435. [3] G. Gibbs and H.L. Lewis, J. Phys. E: Sci. Instrum. 11 (1978) 304. [4] K.J. White and H.J. Midwinter, Opt. Electron. 5 (1973) 323. [5] P.G. Schultz, J. Am. Ceram. Soc. 57 (1974) 309. [6] D.B. Keck, R.D. Maurer and P.G. Schultz, J. Am. Ceram. Soc. 57 (1974) 309. [7] W. Heitmann, Appl. Opt. 15 (1976) 256. [8] W. Heitmann, Nachrichtentechn. Z. 30 (1977) 503.
NEW METHOD FOR MEASURING EXTREMELY LOW OPTICAL ABSORPTIONS
A. BUBENZER, S. HUNKLINGER and K. DRANSFELD Max-Planck-lnstitut fftr Festk6rperforschung, Heisenbergstr. 1, D-7000 Stuttgart 80. FR G
A new highly sensitive method is described for the measurement of optical absorption and its wavelength dependence in extremely low loss materials (e.g. starting materials for optical fibres). The optical absorption - as distinct from scattering losses - is measured calorimetricaUy at low temperatures via the temperature rise due to the absorbed heat. Low temperature as compared with room temperature calorimetry is several orders of magnitude more sensitive due to the drastically decreased specific heat and the higher resolution of the temperature measurements. We have measured an optical absorption coefficient as low as 3 dB km -1 (7 × 10 -6 cm -1 ) in a Suprasil I glass rod of only 5 cm in length. In principle, by our mgthod absorption coefficients even smaller than 10 -6 cm -1 can be determined in a sample of the same length. Since the optical power necessary for this method amounts to only a few mW no single strong laser lines are required as in room temperature calorimetry and also low power light sources which are continuously tunable can be used. Our method is also sensitive enough to distinguish between absorption in the bulk and at the surfaces. Measurements on Suprasil I samples show an absorption peak around 600 nm probably due to OH-impurities in the bulk and a background absorption caused by dissipative processes at the two surfaces.
1. I n t r o d u c t i o n High p u r i t y glasses with extremely low optical absorption coefficient are currently available and are used in optical c o m m u n i c a t i o n fibres. It is o f considerable technical interest to k n o w this basic a b s o r p t i o n before i n t r o d u c i n g further losses b y the fibre-drawing process. Therefore, a m e t h o d is needed to determine extremely low optical absorption coefficients - typically o f the order o f 1 dB k m -1 (2.3 × 10 -6 cm -1) - even in relatively short b u l k samples. It is possible to measure optical a t t e n u a t i o n o f low loss b u l k glasses in fairly long rods (l -~ 30 cm) in a special s p e c t r o p h o t o m e t e r [1]. In general it is, however, simpler to apply calorimetric m e t h o d s . There the optical a b s o r p t i o n is measured via the t e m p e r a t u r e rise due to the absorbed energy. In the calorimetric m e t h o d s k n o w n so far, high i n t e n s i t y light sources are needed to o b t a i n measurable temperature variations. Therefore, o n l y strong laser lines can be used supplying a power of a b o u t 1 W [ 2 - 4 ] . C o n s e q u e n t l y , c o n t i n u o u s m e a s u r e m e n t s o f the a b s o r p t i o n spect r u m have n o t been possible so far. In addition, at such high laser powers the danger of optical bleaching c a n n o t be excluded. 605
606
A. Bubenzer et al. / Extremely low optical absorptions
The calorimetric method described here, however, avoids these problems. The sensitivity in our experiment is strongly enhanced by carrying out the measurements at low temperatures (T ~- 1.3 K): Firstly, the heat capacity of solids decreases drastically with temperature. In the case of fused quartz, for example, the specific heat is reduced by a factor of l0 s on cooling from room temperature to 1.3 K. Secondly, small temperature differences can be measured at least two orders of magnitude more sensitively at low temperatures. Consequently, the cryogenic method is, for the same heat input, about seven orders of magnitude more sensitive than room temperature calorimetry. The strongly increased sensitivity of our set-up allows us to use a lower input laser power, typically three orders of magnitude lower than is commonly used in room temperature calorimetric measurements, and still remain several orders of magnitude more sensitive. So it is possible to use a continuously tunable light source and to measure the wavelength dependence of the optical absorption.
2. Principle of measurement As in most calorimetric measurements of the optical absorption the sample is thermally insulated and illuminated by a laser pulse having a duration of about a second. The absorbed energy Q causes the temperature to rise by AT: AT = a/mc
,
(1)
where m is the mass of the sample and c its specific heat. In order to measure the temperature variations we use carbon resistors which have a high temperature coefficient. First an optical pulse is sent through the sample whose temperature rise is detected by a carbon resistor R1. Then an electrical pulse of the same duration as the light pulse and of known electrical power is applied to a second carbon resistor R2 also attached to the sample (see fig. 1) and again the temperature rise of the sample is measured by R1. In this way the heating due to the absorbed light is simulated by an electrical heating. Via this calibration of the system the absolute magnitude of the heat input Q is now easily determined without knowing the values for AT, m and c. At low absorption levels ( a l ~ 1) the absorption coefficient a can be written as a = ( O / E i n ) 1-1 ,
(2)
where l is the length of the sample and E i n the energy input of the laser pulse. When using a sample of about 5 cm in length absorption coefficients as low as 10 -7 c m -1 or 0.04 dB cm -1 can be measured by our method. At these low absorption levels surface absorption may become comparable ,with bulk absorption. It is difficult to discriminate quantitatively between these two processes via the diffusion time of the heat from the surface to the middle of the sample since at low temperatures this
A. Bubenzer et al. /Extremely low optical absorptions
607
Tube for
Carbon resistor radiatic shields i
Powel r
Copper wires
Sample
Fig. 1. Schematic of the experimental arrangement. Nitrogen shields are omitted.
time is much shorter than at room temperature. The best way to separate both contributions is to measure samples of different lengths.
3. Technical As can be seen in fig. 1 the glass sample is suspended in the evacuated sample chamber of an optical 4He cryostat. Thermal contact of the sample to the environment is kept as low as possible. For suspension three nylon threads are used and thin constantan wires are used for electrical connections. Care has to be taken to prevent the heating of the sample due to radiation through the cryostat windows and due to mechanical vibrations, as for example caused by mechanical pumps. The cryostat windows, therefore, have a coating to block off IR radiation and the pumping tubes are mechanically decoupled by heavy metal blocks. The carbon resistors are thermally attached to the sample via copper wires soldered to the glass sample by an indium droplet. The resistors are located at a distance of 2 cm from the glass rod and surrounded by a radiation shield in order to protect them against scattered light. The light source is a dye laser pumped by an argon-ion laser. Compared with an ordinary tamp a laser has the advantage that optimum focusing conditions can be obtained and therefore, false signals due to scattered light can be avoided. A mechanical shutter forms a laser pulse of approximately 1 s duration and its power, of the order of a few mW, is registered by a power meter. A temperature rise leads to a decrease of the resistivity of the carbon resistors. Typically, the temperature rise is a few mK leading to resistivity variations in the order of K~2 which will finally give voltage variations in the range of 0.1 mV in our electrical circuit. These signals are stored in a transient recorder and displayed by a scope or plotter.
608
A. Bubenzer et al. / Extremely low optical absorptions
¢]o
.6 o.
~s ~C cJ o. 0
S upro.si[ 1 tength 50mm
soo Wavelength
(nrnJ
Fig. 2. Absorption spectrum of a Suprasil I sample (l = 50 mm). 4. E x p e r i m e n t a l results
Absorption spectra of different samples of vitreous silica were studied in the spectral range of our dye-laser (using as a dye Rhodamine 6G) which was continuously tunable between about 574 and 634 nm. In addition all the lines of the argon laser between 514.5 and 454.5 nm were used. In fig. 2 we show the absorption spectrum of a vitreous silica Suprasil I sample of 50 mm length at a temperature of 1.3 K. The dominant feature is the sharp peak around 595 nm. In order to make sure that this peak is due to bulk absorption and not caused by the surface, we have also measured a shorter and a longer rod of the same material having lengths of 25 mm and 100 mm, respectively. The results of these experiments are shown in fig. 3. An analysis of the data shows that we can
•
Y \
~-=15
Suprosit 1 • tength tOOrnm • length 2Smm
o
2a.c . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
s~o
s~o
~o
~
6~o
Wavetength
~3o
(nm)
Fig. 3. Ratio of the total absorbed to incident energy (Qtot/Ein) in Suprasil I samples of 100 mm and 25 mm lengths, respectively. The absorption caused by the two surfaces as calculated by extrapolation to zero length is indicated by a dashed line.
A. Bubenzer et al. / Extremely low optical absorptions g 10
i
i
/
F
o\..)..,p~o
609
t .-----r-~. -_
i
,,
_~?:"%. ~ ~s
,-'J
/
"~
__o----/
~°,,o,
°~°
8
o
i
~o
L
sgo
6~o
~
81o Wavele~jth
~3o (nm)
Fig. 4. Absorption spectrum of a Suprasil W (containing less than 5 ppm OH) and a Suprasil I sample (1200 ppm OH). Both samples have the same length (l = 50 mm). decompose the ratio of the total absorbed energy Qtot to the incident energy ~Tin into Otot/Ein = 2a s + OtBl '
(3)
where as is the surface absorption and aBl gives the dissipation in the bulk. The "background" absorption Qs/Ein = 2~s caused by the two surfaces has been calculated by extrapolation to zero length and is indicated in fig. 3 by a dashed line. From these data it is obvious that the absorption peak around 595 nm is proportional to the sample length and therefore arises entirely in the bulk. Since the absorption peak cannot be assigned to any transition element commonly present in Suprasil as an impurity [5] it is probably due to OH-impurities. The occurrence of such an absorption' line at 585 nm as the 4th harmonic of the fundamental OH vibrations at 2.73/~m has been predicted by Keck et al. [6]. This assessment is supported by the fact that the first three harmonics of the same OHvibrations have already been observed [6] and that this absorption line is absent in Suprasil W containing less than 5 ppm OH (see fig. 4). The overall absorption in Suprasil W is, however, higher than in Suprasil I probably due to Other impurities and defects not present in Suprasil I, in agreement with measurements by Heitmann [7]. Although all of our absorption coefficients were determined at low temperatures they agree at selected wavelengths fairly well with the room temperature data on similar material by Keck et al. [6] who reported 5 dB k i n - 1 at 633 nm and Heitmann [8] who gave an intrinsic value of 2.55 dB km -1 at 600 nm. 5. Conclusions We describe a low temperature calorimetric method which, for the first time, allows the measurement of the optical absorption spectrum of extremely low loss
610
A. Bubenzer et al. / Extremely low optical absorptions
materials. Our method is sensitive enough for measuring an absorption coefficient as low as 10 -7 cm -1 in a sample o f about 5 cm in length. Absorption spectra o f Suprasil I obtained b y this m e t h o d show an absorption peak at 595 nm probably due to OH-impurities in the bulk and a broad absorption occurring at the two surfaces.
Acknowledgements It is a pleasure to acknowledge clarifying discussions with D. Krause, particularly about the OH-vibrations in fused silica. We are grateful to Jenaer Glaswerke, Schott and Gen. for supplying us with the glass samples and for coating our cryostat windows. These experiments would not have been possible without the expert technical help o f P. Wimmer.
References [1] D. Krause, Optical Properties of Highly Transparent Solids, eds. S.S. Mitra and B. Bendow (Plenum, New York, 1975) p. 483. [2] M. Hass, J.M. Davisson, H.B. Rosenstock, J.A. Slinkman and J. Babiskin, Optical Properties of Highly Transparent Solids, eds. S.S. Mitra and B. Bendow (Plenum, New York, 1975) p. 435. [3] G. Gibbs and H.L. Lewis, J. Phys. E: Sci. Instrum. 11 (1978) 304. [4] K.J. White and H.J. Midwinter, Opt. Electron. 5 (1973) 323. [5] P.G. Schultz, J. Am. Ceram. Soc. 57 (1974) 309. [6] D.B. Keck, R.D. Maurer and P.G. Schultz, J. Am. Ceram. Soc. 57 (1974) 309. [7] W. Heitmann, Appl. Opt. 15 (1976) 256. [8] W. Heitmann, Nachrichtentechn. Z. 30 (1977) 503.