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Rough surface absorption of Nd:YAG laser rods
October 1995
ELSEVIER
Optical Materials 4 (1995) 811-814
Rough surface absorption of Nd:YAG laser rods Guido Mann a, Gerd Phillipps b a Optisches Institut, Technische Universitiit Berlin, Strasse des 17. Juni 135, D-10623 Berlin, Germany b Festkoerper-Laser-lnstitut Berlin GmbH, Strasse des 17. Juni 135, D-10623 Berlin, Germany
Received 1 January 1995; accepted 26 April 1995
Abstract The rough surface of Nd:YAG-laser rods exhibits a broad spectral absorption due to impurities. The optical density of the investigated surfaces was determined up to several percent in the near infrared and visible spectral range. The characteristics of the rough surface absorption was investigated.
Nd:YAG-rod
1. Introduction The rough surface of commercial N d : Y A G rods, manufactured by ultrasonic drilling of the rods from the boule or treatment with diamond cutting systems, avoid parasitic oscillations inside the rods at high optical pumping levels [ 1 ]. On the first view no absorption in the near infrared and visible spectral region is expected with exception of the Neodym absorption lines. Nevertheless, we found out that photoinduced reactions between impurities and pumping light of Xeflashlamps take place on the rough surface which lead to a remarkable amount of parasitic absorption.
2. Experimental Two kinds of measurements were necessary to separate bulk and surface absorption of the Nd:YAG-rods. In the first experiment, which is shown in Fig. 1, the sum of bulk absorption and the rough surface absorption was measured. A flashlamp pumped Cr,Nd:GGG laser was used to provide the probe beam with an average power of typically 40 W at a wavelength of 1062 nm. A system of cylinder lenses was used to generate 0925-3467/95/$09.50 © 1995 Elsevier ScienceB.V. All rights reserved SSD10925-3467 (95)00017-8
I Cr,Nd:GGG.Laser I
1062nm > 'lensCylindrical[i J \L/) /~ I ......
r°ugh/" I I surface [ ~ J
Pho~odlode '~633nm ; plate
He-Ne ~1 teser [~ Fig. 1. Experimental setup to measure intefferometrically the heat generation caused by a transverse passage of a Cr,Nd:GGG probe beam through a Nd:YAGrod. a line focus on the s i d e o f the Nd:YAG test rod. The passage of the laser light through the test rod caused a heat generation due to the parasitic surface absorption and color center absorption in the bulk. Heat generation due to Neodym absorption could be neglected, since the Neodym absorption lines in YAG don't match the wavelength of the Cr,Nd:GGG laser beam. By neglecting multiscattering processes and Fresnel losses at the rough surface, the optical absorption density a D for side transition can be estimated by a D = 2Ots6 + abD,
( 1)
812
G. Mann, G. Phillipps / Optical Materials 4 (1995) 811~14 Powerrneter
where D is the diameter of the Nd:YAG rod, as and are the absorption coefficient and the geometrical thickness of the rough surface, respectively, ab is the absorption coefficient of stable color centers at 1062 nm in the bulk [ 2,3 ]. The factor 2 describes the transmission through the both surfaces of the rod. In Eq. (1) it is supposed that the thickness 8 is much smaller than the diameter D of the test rod. The optical absorption density aD was determined by measuring the heat generation inside the Nd:YAGrod during the irradiation with the Cr,Nd:GGG laser beam. The heat which was generated at the rough surface of the rod was assumed to flow due to the heat gradient to the inside of the rod. The heat power was measured interferometrically with an HeNe-laser in the same kind as described in Ref. [3]. The plane surfaces of the Nd:YAG-rod reflected some part of the HeNe-laser beam and acted as a Fabry-Prrot interferometer. The temperature increase of the test rod during irradiation caused a change of the optical path length and modulated the interference fringes periodically, which was detected by a photodiode. Before the measurement was started it was proved that the Nd:YAG rod was in thermal equilibrium with the enviroment of the air. This was ensured by observing the interference signal. Then the Cr,Nd:GGG-laser was switched on. Due to the produced heat power in the Nd:YAG rod the interference fringes started moving. The frequency of the fringe motion kept constant within the first l0 s after switching on the laser. Therefore heat dissipation to the enviroment air and the rod holder, which consists of two thin wedge shaped glassplates, could be neglected. From the number of the interference maxima per time interval the heat power can be determined. The ratio of the heat power PH to the average output power P of the Cr,Nd:GGG-laser delivered the optical density ctD. All material parameters for Nd:YAG and formulas which are needed for the calculation of the heat power and absorption coefficient are summarized in Ref. [3]. The relative statistical uncertainty of the experimental results was + 3%. With a similar measurement, which is shown in Fig. 2, the bulk absorption coefficient ab was measured separately [ 3 ]. The Nd:YAG-rod was probed with the Cr,Nd:GGG-laser beam across the rod axis without grazing the rough surface. We assumed that the heat
1062 nm
0 ,-Ii
V/
°m
Diaphragm
Fig. 2. Experimental setup to measure the bulk absorption at 1062 nm in Nd:YAG. The interferometric method is the same as in Fig. 1. BS: beam splitter: HR 1062 nm at 45 °, HT 633 nm at 0 °.
generation originated from the color center absorption in the bulk at 1062 nm, since no parasitic absorption from the polished end surfaces, which were antireflective coated for 1064 nm, was expected. Similar to the experiment described above, the ratio between heat power to average power P delivers the optical density for bulk absorption ab" L, where L is the geometrical length of the test rod. With the knowledge of a b the optical density of the rough surface 2as~ was calculated with Eq. ( 1 ).
3. Results and discussions The first measurements were done with three Nd:YAG rods with 0.25 inch in diameter and 4 inch in length. The doping level with Neodym ions was 1.1 at% specified by manufactures. In our laboratory these rods were initially heated in an oven at a temperature of 400°C for 5 h to remove possible stable color centers in the bulk. In the next step we measured the optical density of the bulk and the rough surface. The results of these measurements are shown in Table 1, column 1 and 2. The three rods exhibited an optical density of the rough surface of = 1.6 × 10 -2. As discussed later, the thickness ~ was assumed to be in the range of micrometer. Since the bulk absorption coefficient as of the three rods is only in the order of = 0 . 4 × 10 -3 cm -1, it is important to note that the absorption coefficient of the rough surface is several orders of magnitude higher than the absorption coefficient of the bulk. Then the Nd:YAG rods were put in a double elliptic cavity together with two Xe-fiashlamps. The length of the pumping pulses was 275 ItS (FWHM). Deionised water was used as cooling liquid. With this apparatus the test rods were pumped with an electrical pumping energy of 400 J in both lamps and 500 pulses, and the
G. Mann, G. Phillipps / Optical Materials 4 (1995) 811-814 Table 1 Optical density of the rough surface 2c~6 and bulk absorption coefficient for radiation with 1062 nm of Nd:YAG-rods after different treatments ( see text). The values of 2~$6 and c~bare given in dimensions of [ 10 2] and [ 10- • cm l ], respectively. 1
2
initial heated
rod#1 rod#2 rod#3
2c~6 1.64 1.51 1.52
ab 0.49 0.29 0.35
3
4
optical pumped
2cq6 3.4 3.1 3.8
ab 9.3 10.0 11.0
5
6
further heating
2a~6 3.8 4.5 3.5
ab 0.55 0.35 0.38
measurements of the bulk and rough surface absorption were repeated. The results are shown in the columns 3 and 4 of Table I. The ultraviolet part of the flashlamp light created stable color centers, which increase the bulk absorption by approximately one order of magnitude. The optical density for side pumping increased by a factor two. In the next step the crystals were tempered again up to a temperature of 400°C for 5 h to remove the stable color centers. The values of the final bulk and surface absorption are listed in the columns 5 and 6 of Table 1. The color center absorption in the bulk decreased to the intial value before pumping. In contrast to this the rough surface absorption was not influenced by the heating process in the case of the rods # 1 and #3, and increased in the case of rod #2. With these results we concluded, that the rough surface absorption is not caused by stable color centers. To reduce the rough surface absorption the Nd:YAG rods were cleaned in different ultrasonic baths in turn, one of them was filled with deionised water and the other was filled with a solution of 20% RBS and deionised water at a temperature of 70°C. This procedure reduced the surface absorption of the three rods from initial values of 2 c ~ 6 = 4 × 10 2 down to values between 0.7-1 × 10 -2. This demonstrates that the origin of the absorption are impurities onto the rough surface. The increase of the rough surface absorption by irradiation with Xe-flashlamp light can be explained by photoinduced reactions. Since C~bDwas much smaller than the rough surface absorption 2a~6 (see Eq. ( 1 ) ), we neglected the influence of the bulk absorption for the further investigations to simplify the experiments: ~D = 2c~s& The spectral behaviour of the absorption was investigated with an undoped YAG rod. The crystal was
813
placed with the rough side in front of the entrance slit of a monochromator. A tungsten lamp was used as light source. The transmitted light through the rod was measured behind the monochromator with a spectral resolution of 5 nm. Since in this spectrum no line absorption could be seen, the rough surface absorption is a spectral broad band absorption. The optical density of the rough surface at 1062 nm was determined to be 2a~6= 4.2 × 10 - 2 . In a further experiment we determined the rough surface absorption at a wavelength of 596 nm by replacing the Cr,Nd:GGG-laser by a flashlamp pumped dye laser. The optical density at 596 nm of the rough surface absorption was determined to be 6.4 × 10 - 2, in agreement with the foregoing result, that the absorption is spectral broad. Additionally to the results described above we observed an unexpected behaviour of the rough surface absorption: When we pumped rod # 3 again, the optical density of the surface absorption increased from 0.92 × 10 -2 to 2.7 × 10 - 2 a s expected. But thereafter the cleaning procedure in the ultrasonic bath reduced the optical density only to an amount of 2.2× 10 -2 . Then we heated the crystal in the oven at 400°C for 5 h. The heating process reduced the surface absorption by a factor of 10 to an amount of only 0.23 × 10 2 in contrast to the behaviour described above. Touching the surface of the crystals with unprotected fingers changed the absorption dramatically: The optical density of the rough surface absorption increased by touching the undoped YAG-rod from 0.55 × 10 -2 to 2.6 x l 0 - 2 . Simple cleaning of the surface with aceton and deionised water reduced the optical density of the rough surface absorption only to an amount of 1.89× 10 -2. When we put the rod for 4 h into a tube with flowing destilled water at 50°C, the rough surface absorption was reduced only by an negligible amount down to a value of 1.74× 10 - 2 , tOO. The essential compounds of the perspiration onto the fingers consists of Na, Ca, K, Mg, Cl, several groups of amino acids, saturated and unsaturated fatty acids and other organic combinations [4]. In further experiments we contacted the rough surface of a test rod with different solutions of sodium chlorid, iron and oil, and measured the absorption coefficient of the surface before and after optical pumping. Only a mixture of iron oxyd in water and linseed oil changed the rough surface absorption: The optical density increased from
814
G. Mann, G. Phillipps / Optical Materials 4 (1995) 811~14
0.44X 10 -2 up to a value of 2.5 X 10 - 2 after optical pumping. A further attempt to identify the impurities which were responsible for the rough surface absorption was done by using a raster electron microscope (Hitachi $2700). This system is able to identify impurities within a layer of approximately 2 p,m and a doping level of approximately 1 weight percent with the EDSsystem (X-ray structure analytic with an energy dispersive spectrometer), operating with an electron acceleration voltage of 20-30 kV. We investigated two Nd:YAG rods, which exhibited a rough surface absorption of 0.94 X 10 - 2 and 5.8 X 10 -2. Besides the huge amount of yttrium, aluminium, oxygen and neodymium we only found two very small irregular peaks in the X-ray spectrum in the order of the detection limit. In the case these peaks represent real detection signals they indicate the presence of iron and sodium. Further research in finding impurities with the electron raster microscope was not successful. Therefore we conclude that either the density of the absorbing compounds was smaller than 1 weight percent or the thickness of the absorbing layer was much smaller than 2 I~m, which was the emission depth of the X-ray photons for the microscope. The last argument was supported by the following fact: The rough surface of the Nd:YAG rods was coated with a small layer of carbon with a thickness of 10-50 nm as conducting material for working with the electron raster microscope. The surface of the rods appeared dark in the visible region. The carbon layer increased the optical density of the rough surface at 1062 nm from 0.058 up to 0.08. Nevertheless we could not detect carbon atoms with the EDS system of the electron raster microscope, since the thickness of the carbon layer was too small. After the experiments with the electron microscope we removed the carbon layer from the surface by heating the crystal for 21 h in an oven at 400°C. After this procedure, the surface seemed clear, and the optical
density of the rough surface absorption amounted only 0.56 x 10 -2.
4. Interpretation We suppose, that carbon atoms take part in the creation of the rough surface absorption due to several reasons: The contact of the carbon layer to the YAGsurface is very strong, and therefore removing is difficult, which would explain our observations that simple cleaning of the rough surface doesn't change the absorption. The detection of a small amount of carbon is not possible with the raster electron microscope. Touching of the rough surface of the rod with fingers increases the absorption, where carbon atoms are present in form of acids and fat. The photochemical reactions which increase the rough surface absorption can be explained by cracking chemical combinations, which absorb. This discussion shows, that further investigations are necessary to find out the origin of the rough surface absorption.
Acknowledgements We thank Haas Laser GmbH in Germany for providing us Nd:YAG-rods. Many thanks to Achim Vater for his help realising some experiments. This work was supported by the Bundesministerium fiir Forschung und Technologie, Germany.
References [ 1] W. Koechner, Solid-State Laser Engineering, 3rd Ed. (Springer, Berlin) p. 187. [2] M. Bass and A.E. Paladino, J. Appl. Phys. 38 (1967) 2706. [3] G. Phillipps and J. Vater, Appl. Optics 32 (1993) 3210. [4] O. Braun-Falco, G. Plewig and H.H. Wolff, Dermatologie und Venerologie, 3rd Ed. (Springer, Berlin, 1984) pp. 630, 650651.
ELSEVIER
Optical Materials 4 (1995) 811-814
Rough surface absorption of Nd:YAG laser rods Guido Mann a, Gerd Phillipps b a Optisches Institut, Technische Universitiit Berlin, Strasse des 17. Juni 135, D-10623 Berlin, Germany b Festkoerper-Laser-lnstitut Berlin GmbH, Strasse des 17. Juni 135, D-10623 Berlin, Germany
Received 1 January 1995; accepted 26 April 1995
Abstract The rough surface of Nd:YAG-laser rods exhibits a broad spectral absorption due to impurities. The optical density of the investigated surfaces was determined up to several percent in the near infrared and visible spectral range. The characteristics of the rough surface absorption was investigated.
Nd:YAG-rod
1. Introduction The rough surface of commercial N d : Y A G rods, manufactured by ultrasonic drilling of the rods from the boule or treatment with diamond cutting systems, avoid parasitic oscillations inside the rods at high optical pumping levels [ 1 ]. On the first view no absorption in the near infrared and visible spectral region is expected with exception of the Neodym absorption lines. Nevertheless, we found out that photoinduced reactions between impurities and pumping light of Xeflashlamps take place on the rough surface which lead to a remarkable amount of parasitic absorption.
2. Experimental Two kinds of measurements were necessary to separate bulk and surface absorption of the Nd:YAG-rods. In the first experiment, which is shown in Fig. 1, the sum of bulk absorption and the rough surface absorption was measured. A flashlamp pumped Cr,Nd:GGG laser was used to provide the probe beam with an average power of typically 40 W at a wavelength of 1062 nm. A system of cylinder lenses was used to generate 0925-3467/95/$09.50 © 1995 Elsevier ScienceB.V. All rights reserved SSD10925-3467 (95)00017-8
I Cr,Nd:GGG.Laser I
1062nm > 'lensCylindrical[i J \L/) /~ I ......
r°ugh/" I I surface [ ~ J
Pho~odlode '~633nm ; plate
He-Ne ~1 teser [~ Fig. 1. Experimental setup to measure intefferometrically the heat generation caused by a transverse passage of a Cr,Nd:GGG probe beam through a Nd:YAGrod. a line focus on the s i d e o f the Nd:YAG test rod. The passage of the laser light through the test rod caused a heat generation due to the parasitic surface absorption and color center absorption in the bulk. Heat generation due to Neodym absorption could be neglected, since the Neodym absorption lines in YAG don't match the wavelength of the Cr,Nd:GGG laser beam. By neglecting multiscattering processes and Fresnel losses at the rough surface, the optical absorption density a D for side transition can be estimated by a D = 2Ots6 + abD,
( 1)
812
G. Mann, G. Phillipps / Optical Materials 4 (1995) 811~14 Powerrneter
where D is the diameter of the Nd:YAG rod, as and are the absorption coefficient and the geometrical thickness of the rough surface, respectively, ab is the absorption coefficient of stable color centers at 1062 nm in the bulk [ 2,3 ]. The factor 2 describes the transmission through the both surfaces of the rod. In Eq. (1) it is supposed that the thickness 8 is much smaller than the diameter D of the test rod. The optical absorption density aD was determined by measuring the heat generation inside the Nd:YAGrod during the irradiation with the Cr,Nd:GGG laser beam. The heat which was generated at the rough surface of the rod was assumed to flow due to the heat gradient to the inside of the rod. The heat power was measured interferometrically with an HeNe-laser in the same kind as described in Ref. [3]. The plane surfaces of the Nd:YAG-rod reflected some part of the HeNe-laser beam and acted as a Fabry-Prrot interferometer. The temperature increase of the test rod during irradiation caused a change of the optical path length and modulated the interference fringes periodically, which was detected by a photodiode. Before the measurement was started it was proved that the Nd:YAG rod was in thermal equilibrium with the enviroment of the air. This was ensured by observing the interference signal. Then the Cr,Nd:GGG-laser was switched on. Due to the produced heat power in the Nd:YAG rod the interference fringes started moving. The frequency of the fringe motion kept constant within the first l0 s after switching on the laser. Therefore heat dissipation to the enviroment air and the rod holder, which consists of two thin wedge shaped glassplates, could be neglected. From the number of the interference maxima per time interval the heat power can be determined. The ratio of the heat power PH to the average output power P of the Cr,Nd:GGG-laser delivered the optical density ctD. All material parameters for Nd:YAG and formulas which are needed for the calculation of the heat power and absorption coefficient are summarized in Ref. [3]. The relative statistical uncertainty of the experimental results was + 3%. With a similar measurement, which is shown in Fig. 2, the bulk absorption coefficient ab was measured separately [ 3 ]. The Nd:YAG-rod was probed with the Cr,Nd:GGG-laser beam across the rod axis without grazing the rough surface. We assumed that the heat
1062 nm
0 ,-Ii
V/
°m
Diaphragm
Fig. 2. Experimental setup to measure the bulk absorption at 1062 nm in Nd:YAG. The interferometric method is the same as in Fig. 1. BS: beam splitter: HR 1062 nm at 45 °, HT 633 nm at 0 °.
generation originated from the color center absorption in the bulk at 1062 nm, since no parasitic absorption from the polished end surfaces, which were antireflective coated for 1064 nm, was expected. Similar to the experiment described above, the ratio between heat power to average power P delivers the optical density for bulk absorption ab" L, where L is the geometrical length of the test rod. With the knowledge of a b the optical density of the rough surface 2as~ was calculated with Eq. ( 1 ).
3. Results and discussions The first measurements were done with three Nd:YAG rods with 0.25 inch in diameter and 4 inch in length. The doping level with Neodym ions was 1.1 at% specified by manufactures. In our laboratory these rods were initially heated in an oven at a temperature of 400°C for 5 h to remove possible stable color centers in the bulk. In the next step we measured the optical density of the bulk and the rough surface. The results of these measurements are shown in Table 1, column 1 and 2. The three rods exhibited an optical density of the rough surface of = 1.6 × 10 -2. As discussed later, the thickness ~ was assumed to be in the range of micrometer. Since the bulk absorption coefficient as of the three rods is only in the order of = 0 . 4 × 10 -3 cm -1, it is important to note that the absorption coefficient of the rough surface is several orders of magnitude higher than the absorption coefficient of the bulk. Then the Nd:YAG rods were put in a double elliptic cavity together with two Xe-fiashlamps. The length of the pumping pulses was 275 ItS (FWHM). Deionised water was used as cooling liquid. With this apparatus the test rods were pumped with an electrical pumping energy of 400 J in both lamps and 500 pulses, and the
G. Mann, G. Phillipps / Optical Materials 4 (1995) 811-814 Table 1 Optical density of the rough surface 2c~6 and bulk absorption coefficient for radiation with 1062 nm of Nd:YAG-rods after different treatments ( see text). The values of 2~$6 and c~bare given in dimensions of [ 10 2] and [ 10- • cm l ], respectively. 1
2
initial heated
rod#1 rod#2 rod#3
2c~6 1.64 1.51 1.52
ab 0.49 0.29 0.35
3
4
optical pumped
2cq6 3.4 3.1 3.8
ab 9.3 10.0 11.0
5
6
further heating
2a~6 3.8 4.5 3.5
ab 0.55 0.35 0.38
measurements of the bulk and rough surface absorption were repeated. The results are shown in the columns 3 and 4 of Table I. The ultraviolet part of the flashlamp light created stable color centers, which increase the bulk absorption by approximately one order of magnitude. The optical density for side pumping increased by a factor two. In the next step the crystals were tempered again up to a temperature of 400°C for 5 h to remove the stable color centers. The values of the final bulk and surface absorption are listed in the columns 5 and 6 of Table 1. The color center absorption in the bulk decreased to the intial value before pumping. In contrast to this the rough surface absorption was not influenced by the heating process in the case of the rods # 1 and #3, and increased in the case of rod #2. With these results we concluded, that the rough surface absorption is not caused by stable color centers. To reduce the rough surface absorption the Nd:YAG rods were cleaned in different ultrasonic baths in turn, one of them was filled with deionised water and the other was filled with a solution of 20% RBS and deionised water at a temperature of 70°C. This procedure reduced the surface absorption of the three rods from initial values of 2 c ~ 6 = 4 × 10 2 down to values between 0.7-1 × 10 -2. This demonstrates that the origin of the absorption are impurities onto the rough surface. The increase of the rough surface absorption by irradiation with Xe-flashlamp light can be explained by photoinduced reactions. Since C~bDwas much smaller than the rough surface absorption 2a~6 (see Eq. ( 1 ) ), we neglected the influence of the bulk absorption for the further investigations to simplify the experiments: ~D = 2c~s& The spectral behaviour of the absorption was investigated with an undoped YAG rod. The crystal was
813
placed with the rough side in front of the entrance slit of a monochromator. A tungsten lamp was used as light source. The transmitted light through the rod was measured behind the monochromator with a spectral resolution of 5 nm. Since in this spectrum no line absorption could be seen, the rough surface absorption is a spectral broad band absorption. The optical density of the rough surface at 1062 nm was determined to be 2a~6= 4.2 × 10 - 2 . In a further experiment we determined the rough surface absorption at a wavelength of 596 nm by replacing the Cr,Nd:GGG-laser by a flashlamp pumped dye laser. The optical density at 596 nm of the rough surface absorption was determined to be 6.4 × 10 - 2, in agreement with the foregoing result, that the absorption is spectral broad. Additionally to the results described above we observed an unexpected behaviour of the rough surface absorption: When we pumped rod # 3 again, the optical density of the surface absorption increased from 0.92 × 10 -2 to 2.7 × 10 - 2 a s expected. But thereafter the cleaning procedure in the ultrasonic bath reduced the optical density only to an amount of 2.2× 10 -2 . Then we heated the crystal in the oven at 400°C for 5 h. The heating process reduced the surface absorption by a factor of 10 to an amount of only 0.23 × 10 2 in contrast to the behaviour described above. Touching the surface of the crystals with unprotected fingers changed the absorption dramatically: The optical density of the rough surface absorption increased by touching the undoped YAG-rod from 0.55 × 10 -2 to 2.6 x l 0 - 2 . Simple cleaning of the surface with aceton and deionised water reduced the optical density of the rough surface absorption only to an amount of 1.89× 10 -2. When we put the rod for 4 h into a tube with flowing destilled water at 50°C, the rough surface absorption was reduced only by an negligible amount down to a value of 1.74× 10 - 2 , tOO. The essential compounds of the perspiration onto the fingers consists of Na, Ca, K, Mg, Cl, several groups of amino acids, saturated and unsaturated fatty acids and other organic combinations [4]. In further experiments we contacted the rough surface of a test rod with different solutions of sodium chlorid, iron and oil, and measured the absorption coefficient of the surface before and after optical pumping. Only a mixture of iron oxyd in water and linseed oil changed the rough surface absorption: The optical density increased from
814
G. Mann, G. Phillipps / Optical Materials 4 (1995) 811~14
0.44X 10 -2 up to a value of 2.5 X 10 - 2 after optical pumping. A further attempt to identify the impurities which were responsible for the rough surface absorption was done by using a raster electron microscope (Hitachi $2700). This system is able to identify impurities within a layer of approximately 2 p,m and a doping level of approximately 1 weight percent with the EDSsystem (X-ray structure analytic with an energy dispersive spectrometer), operating with an electron acceleration voltage of 20-30 kV. We investigated two Nd:YAG rods, which exhibited a rough surface absorption of 0.94 X 10 - 2 and 5.8 X 10 -2. Besides the huge amount of yttrium, aluminium, oxygen and neodymium we only found two very small irregular peaks in the X-ray spectrum in the order of the detection limit. In the case these peaks represent real detection signals they indicate the presence of iron and sodium. Further research in finding impurities with the electron raster microscope was not successful. Therefore we conclude that either the density of the absorbing compounds was smaller than 1 weight percent or the thickness of the absorbing layer was much smaller than 2 I~m, which was the emission depth of the X-ray photons for the microscope. The last argument was supported by the following fact: The rough surface of the Nd:YAG rods was coated with a small layer of carbon with a thickness of 10-50 nm as conducting material for working with the electron raster microscope. The surface of the rods appeared dark in the visible region. The carbon layer increased the optical density of the rough surface at 1062 nm from 0.058 up to 0.08. Nevertheless we could not detect carbon atoms with the EDS system of the electron raster microscope, since the thickness of the carbon layer was too small. After the experiments with the electron microscope we removed the carbon layer from the surface by heating the crystal for 21 h in an oven at 400°C. After this procedure, the surface seemed clear, and the optical
density of the rough surface absorption amounted only 0.56 x 10 -2.
4. Interpretation We suppose, that carbon atoms take part in the creation of the rough surface absorption due to several reasons: The contact of the carbon layer to the YAGsurface is very strong, and therefore removing is difficult, which would explain our observations that simple cleaning of the rough surface doesn't change the absorption. The detection of a small amount of carbon is not possible with the raster electron microscope. Touching of the rough surface of the rod with fingers increases the absorption, where carbon atoms are present in form of acids and fat. The photochemical reactions which increase the rough surface absorption can be explained by cracking chemical combinations, which absorb. This discussion shows, that further investigations are necessary to find out the origin of the rough surface absorption.
Acknowledgements We thank Haas Laser GmbH in Germany for providing us Nd:YAG-rods. Many thanks to Achim Vater for his help realising some experiments. This work was supported by the Bundesministerium fiir Forschung und Technologie, Germany.
References [ 1] W. Koechner, Solid-State Laser Engineering, 3rd Ed. (Springer, Berlin) p. 187. [2] M. Bass and A.E. Paladino, J. Appl. Phys. 38 (1967) 2706. [3] G. Phillipps and J. Vater, Appl. Optics 32 (1993) 3210. [4] O. Braun-Falco, G. Plewig and H.H. Wolff, Dermatologie und Venerologie, 3rd Ed. (Springer, Berlin, 1984) pp. 630, 650651.