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Development of a cw polycrystalline Cr2+ : ZnSe laser
Optics and Lasers in Engineering 39 (2003) 305–308
Development of a cw polycrystalline Cr2+ : ZnSe laser A. Di Lieto*, M. Tonelli " Istituto Nazionale di Fisica della Materia - Sezione di Pisa, Dipartimento di Fisica dell’Universita, Via Buonarroti 2, I-56127 Pisa, Italy Received 8 January 2001; received in revised form 23 May 2001; accepted 28 May 2001
Abstract We present the preliminary results regarding the operation of a Cr2+ : ZnSe laser, based on a polycrystalline sample that was prepared at the Dipartimento di Fisica of the Universit"a di Pisa. The performance of this active medium for laser action is compared with that of a single crystal, showing the actual possibility of developing good quality systems based on polycrystalline samples. r 2002 Elsevier Science Ltd. All rights reserved.
1. Introduction Cr2+-doped crystals are very attractive for the development of mid-infrared solidstate lasers. Among these, II–VI chalcogenide materials are receiving great interest, after the first proposals made in 1996 [1], for their room temperature operation [2,3]. Actually, these lasers are promising sources for many research fields, spanning from time-resolved molecular spectroscopy and gas analysis, to medical and biological applications, because of their large tunability, which includes the major absorption bands of molecules containing the OH group. A crucial point in the development of these lasers is the choice of the pump source, given that Cr2+ has an absorption band centered on 1.77 mm, and continuous and powerful sources in this spectral region are not common. Another interesting point is the possibility of developing a source by using not only a single crystal element, but also polycrystalline samples, because of the enormous advantages in producing them. *Corresponding author. E-mail address: [email protected] (A. Di Lieto). 0143-8166/03/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 1 4 3 - 8 1 6 6 ( 0 1 ) 0 0 1 2 6 - 9
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A. Di Lieto, M. Tonelli / Optics and Lasers in Engineering 39 (2003) 305–308
In this work we present the preliminary results obtained with a polycrystalline sample, compared with one of the best single crystal samples recently presented [3]. The comparison is meaningful only about the ‘‘overall’’ performance, because the two samples were prepared in different laboratories, but our interest is focused on demonstrating the feasibility of good laser systems based on polycrystalline samples.
2. Experimental apparatus The first sample was a single crystal ZnSe, diffusion doped with Cr2+ at Lawrence Livermore National Laboratory (USA). Its shape was a parallelogram, with a thickness of 2.7 mm, and its wider surfacesFabout 4 8 mm2Fwere polished to laser quality. The polycrystalline sample was prepared starting from a ZnSe sample, obtained by cutting a 2 mm thick optical window. The Cr2+ doping has been obtained by leaving it in an oven at a temperature of about 8001C for several days, in the atmosphere generated by the evaporation of a metallic high purity Cr powder. The Cr2+ concentration has been estimated to B2 1019 ions/cm3 by comparing the absorption coefficient of our sample to the absorption cross section reported in Ref. [4]. The final dimensions of the sample were 6 6 mm2. Both the samples were optically polished to laser quality by a standard technique. The experimental apparatus for studying the laser performances of our samples was composed of an astigmatically compensated X-folded four-mirror cavity, where the crystal was placed at Brewster angle on a copper block, without any active cooling. The cavity had two curved dichroic mirrors (radius of curvature, respectively, 75 and 100 mm), a final high reflectivity plane mirror, and an output coupler. The folding angles and the length of the two long arms were chosen to compensate for the astigmatism introduced by the crystal and to have a stable resonator. The pump radiation was provided by a cw cryogenic Co : MgF2 tunable laser, developed in our laboratory [5]. This laser can deliver up to 1.9 W at the Cr2+ pumping wavelength when pumped with a multimode Nd : YAG laser at 1.32 mm, and tuned with an intracavity birefringent plate. The effective power coupled into the Cr2+ : ZnSe cavity was adjustable by means of a variable attenuator, built with four Brewster plates. The pump beam was focused with an uncoated lens to produce a spot size within the crystal comparable with the spot size of the four-mirrors resonator: we estimated a pump spot size of B60 140 mm, and a cavity mode size of B70 170 mm. The focal length has been changed from 40 to70 mm, and the results shown here were obtained with the 60 mm one. The transmission of the input mirror has been measured for the pumping radiation tuned to the wavelength of the maximum absorption: we obtained the approximate value of 90%.
A. Di Lieto, M. Tonelli / Optics and Lasers in Engineering 39 (2003) 305–308
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3. Results The first test on the sample quality was made by measuring the absorption spectrum of both samples in a CARY 500 spectrophotometer, from 400 to 3000 nm. In particular, we checked the absorption coefficient of the band at 1.77 mm. For the monocrystalline sample, we noticed a strong dependence of the maximum value on the position of the exploring beam: the value reported here corresponds to the position actually used in the subsequent laser measurements. The reproducibility of the crystal positioning limits the accuracy of such a measurement, so we report only a maximum value of the absorption coefficient B9.770.3 cm 1. A completely different landscape has been observed for the polycrystalline sample. In this case, we did not find any dependence of the absorption coefficient on the position, but a much stronger value: the peak at 1772 nm was 16.170.1 cm 1. The second test was carried out by measuring the slope efficiency in a lasing experiment. The measurements were made first on the single crystal sample and thenF without any substantial adjustment of the cavityFwith the polycrystalline one. Fig. 1 shows the output power as a function of the input power, measured for both samples when the output coupler had a nominal transmission of 12%, and the pumping layout consisted of only a single pass beam. The two sets of data have practically the same slope, and the main difference is that the polycrystalline sample had a higher threshold value. The fitted slope efficiency are 30% for both samples, and the thresholds are 330 and 380 mW, respectively. All these values are referred to the input power, and they do not take into account the real absorbed power. Due to the different absorption, the slope efficiencies referred to the absorbed power are, respectively, 32% and 31% for the mono- and polycrystalline samples. The emitted wavelength was 2.47 mm, a value obtained without any wavelengthselective device inside the cavity. The measured threshold and slope efficiency for the monocrystalline sample should be compared with the best values we obtained in an optimized cavity: the present values are, respectively, 330 mW and 32%, while in Ref. [6] we measured
Fig. 1. Output power as a function of input power for both mono- and polycrystalline samples.
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A. Di Lieto, M. Tonelli / Optics and Lasers in Engineering 39 (2003) 305–308
180 mW for the threshold and 63% for the slope. Very recently, the highest slope efficiency reported in the literature has been further increased to a 73% value [7], but with a different cavity layout and pumping conditions. The results shown in the figure did not represent the best output power that can be obtained in a similar laser system, but we deliberately avoided any readjustment of the laser cavity to highlight the comparison between mono- and polycrystalline samples. The substantial similarity of the two samples performances is also confirmed from the measurement of the intrinsic losses made with the Caird method based on the analysis of the inverse slope efficiency [8]. For the monocrystalline sample, we obtained a 5% cm 1 for the single pass losses, while for the polycrystalline sample we measured an 8% cm 1. The 5% value itself is not a very exciting result: it should be compared with the best value of 3% we measured in our previous work [6] in an optimized layout, and the 2.5% reported in Ref. [7]. 4. Conclusions The preliminary results reported here on the cw laser action of a polycrystalline sample are the first proof of the feasibility of laser systems based on polycrystalline samples of ZnSe doped with Cr2+. In order to compare the best performances of mono- and polycrystalline samples, further investigations are needed, by studying samples doped under the same conditions. If the moderate reduction in the performance of a poly-with respect to the monocrystalline sample will be confirmed, the large impact factor of low cost in producing polycrystalline samples will largely compensate the downgrading. 5. Acknowledgements The authors wish to thank Ilaria Grassini and Fabio Torri for their invaluable support during the development of this work. References [1] De Loach LD, Page RH, Wilke GD, Payne SA, Krupke WF. IEEE J Quantum Electron 1996;32:885. [2] Wagner GJ, Carrig TJ, Page RH, Schaffers KI, Ndap J, Ma X, Burger A. Opt Lett 1999;24:19. [3] Sorokina IT, Sorokin E, Di Lieto A, Tonelli M, Page RH, Schaffers KI. in: Injeyan H, Keller U, Marshall C, editors. OSA Trends in optics and photonics. Vol. 34. advanced solid-state lasers, Washington, DC: Optical Society of America, 2000 p. 188. [4] Page RH, Schaffers KI, DeLoach LD, Wilke GW, Patel FD, Tassano JB, Payne SA, Krupke WF, Chen K, Burger A. IEEE J Quantum Electron 1997;332:609. [5] Di Lieto A, Development of a cw Co : MgF2 laser, Opt Laser Eng, (Optics in Italy Ms#28) 2001, in press. [6] Sorokina IT, Sorokin E, Di Lieto A, Tonelli M, Page RH, Schaffers KI, J Opt Soc Am 2001; B 18 926. [7] Podlipensky AV, Shcherbitsky VG, Kuleshov NV, Levchenko VI, Yakimovich VN, Mond M, Heumann E, Huber G, Kretschmann H, Kuck . S. Appl Phys 2001;B 72:253. [8] Caird JA, Payne SA, Staver PR, Ramponi AJ, Chase LL, Krupke WF. IEEE J Quantum Electron 1988;24:1077.
Development of a cw polycrystalline Cr2+ : ZnSe laser A. Di Lieto*, M. Tonelli " Istituto Nazionale di Fisica della Materia - Sezione di Pisa, Dipartimento di Fisica dell’Universita, Via Buonarroti 2, I-56127 Pisa, Italy Received 8 January 2001; received in revised form 23 May 2001; accepted 28 May 2001
Abstract We present the preliminary results regarding the operation of a Cr2+ : ZnSe laser, based on a polycrystalline sample that was prepared at the Dipartimento di Fisica of the Universit"a di Pisa. The performance of this active medium for laser action is compared with that of a single crystal, showing the actual possibility of developing good quality systems based on polycrystalline samples. r 2002 Elsevier Science Ltd. All rights reserved.
1. Introduction Cr2+-doped crystals are very attractive for the development of mid-infrared solidstate lasers. Among these, II–VI chalcogenide materials are receiving great interest, after the first proposals made in 1996 [1], for their room temperature operation [2,3]. Actually, these lasers are promising sources for many research fields, spanning from time-resolved molecular spectroscopy and gas analysis, to medical and biological applications, because of their large tunability, which includes the major absorption bands of molecules containing the OH group. A crucial point in the development of these lasers is the choice of the pump source, given that Cr2+ has an absorption band centered on 1.77 mm, and continuous and powerful sources in this spectral region are not common. Another interesting point is the possibility of developing a source by using not only a single crystal element, but also polycrystalline samples, because of the enormous advantages in producing them. *Corresponding author. E-mail address: [email protected] (A. Di Lieto). 0143-8166/03/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 1 4 3 - 8 1 6 6 ( 0 1 ) 0 0 1 2 6 - 9
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A. Di Lieto, M. Tonelli / Optics and Lasers in Engineering 39 (2003) 305–308
In this work we present the preliminary results obtained with a polycrystalline sample, compared with one of the best single crystal samples recently presented [3]. The comparison is meaningful only about the ‘‘overall’’ performance, because the two samples were prepared in different laboratories, but our interest is focused on demonstrating the feasibility of good laser systems based on polycrystalline samples.
2. Experimental apparatus The first sample was a single crystal ZnSe, diffusion doped with Cr2+ at Lawrence Livermore National Laboratory (USA). Its shape was a parallelogram, with a thickness of 2.7 mm, and its wider surfacesFabout 4 8 mm2Fwere polished to laser quality. The polycrystalline sample was prepared starting from a ZnSe sample, obtained by cutting a 2 mm thick optical window. The Cr2+ doping has been obtained by leaving it in an oven at a temperature of about 8001C for several days, in the atmosphere generated by the evaporation of a metallic high purity Cr powder. The Cr2+ concentration has been estimated to B2 1019 ions/cm3 by comparing the absorption coefficient of our sample to the absorption cross section reported in Ref. [4]. The final dimensions of the sample were 6 6 mm2. Both the samples were optically polished to laser quality by a standard technique. The experimental apparatus for studying the laser performances of our samples was composed of an astigmatically compensated X-folded four-mirror cavity, where the crystal was placed at Brewster angle on a copper block, without any active cooling. The cavity had two curved dichroic mirrors (radius of curvature, respectively, 75 and 100 mm), a final high reflectivity plane mirror, and an output coupler. The folding angles and the length of the two long arms were chosen to compensate for the astigmatism introduced by the crystal and to have a stable resonator. The pump radiation was provided by a cw cryogenic Co : MgF2 tunable laser, developed in our laboratory [5]. This laser can deliver up to 1.9 W at the Cr2+ pumping wavelength when pumped with a multimode Nd : YAG laser at 1.32 mm, and tuned with an intracavity birefringent plate. The effective power coupled into the Cr2+ : ZnSe cavity was adjustable by means of a variable attenuator, built with four Brewster plates. The pump beam was focused with an uncoated lens to produce a spot size within the crystal comparable with the spot size of the four-mirrors resonator: we estimated a pump spot size of B60 140 mm, and a cavity mode size of B70 170 mm. The focal length has been changed from 40 to70 mm, and the results shown here were obtained with the 60 mm one. The transmission of the input mirror has been measured for the pumping radiation tuned to the wavelength of the maximum absorption: we obtained the approximate value of 90%.
A. Di Lieto, M. Tonelli / Optics and Lasers in Engineering 39 (2003) 305–308
307
3. Results The first test on the sample quality was made by measuring the absorption spectrum of both samples in a CARY 500 spectrophotometer, from 400 to 3000 nm. In particular, we checked the absorption coefficient of the band at 1.77 mm. For the monocrystalline sample, we noticed a strong dependence of the maximum value on the position of the exploring beam: the value reported here corresponds to the position actually used in the subsequent laser measurements. The reproducibility of the crystal positioning limits the accuracy of such a measurement, so we report only a maximum value of the absorption coefficient B9.770.3 cm 1. A completely different landscape has been observed for the polycrystalline sample. In this case, we did not find any dependence of the absorption coefficient on the position, but a much stronger value: the peak at 1772 nm was 16.170.1 cm 1. The second test was carried out by measuring the slope efficiency in a lasing experiment. The measurements were made first on the single crystal sample and thenF without any substantial adjustment of the cavityFwith the polycrystalline one. Fig. 1 shows the output power as a function of the input power, measured for both samples when the output coupler had a nominal transmission of 12%, and the pumping layout consisted of only a single pass beam. The two sets of data have practically the same slope, and the main difference is that the polycrystalline sample had a higher threshold value. The fitted slope efficiency are 30% for both samples, and the thresholds are 330 and 380 mW, respectively. All these values are referred to the input power, and they do not take into account the real absorbed power. Due to the different absorption, the slope efficiencies referred to the absorbed power are, respectively, 32% and 31% for the mono- and polycrystalline samples. The emitted wavelength was 2.47 mm, a value obtained without any wavelengthselective device inside the cavity. The measured threshold and slope efficiency for the monocrystalline sample should be compared with the best values we obtained in an optimized cavity: the present values are, respectively, 330 mW and 32%, while in Ref. [6] we measured
Fig. 1. Output power as a function of input power for both mono- and polycrystalline samples.
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A. Di Lieto, M. Tonelli / Optics and Lasers in Engineering 39 (2003) 305–308
180 mW for the threshold and 63% for the slope. Very recently, the highest slope efficiency reported in the literature has been further increased to a 73% value [7], but with a different cavity layout and pumping conditions. The results shown in the figure did not represent the best output power that can be obtained in a similar laser system, but we deliberately avoided any readjustment of the laser cavity to highlight the comparison between mono- and polycrystalline samples. The substantial similarity of the two samples performances is also confirmed from the measurement of the intrinsic losses made with the Caird method based on the analysis of the inverse slope efficiency [8]. For the monocrystalline sample, we obtained a 5% cm 1 for the single pass losses, while for the polycrystalline sample we measured an 8% cm 1. The 5% value itself is not a very exciting result: it should be compared with the best value of 3% we measured in our previous work [6] in an optimized layout, and the 2.5% reported in Ref. [7]. 4. Conclusions The preliminary results reported here on the cw laser action of a polycrystalline sample are the first proof of the feasibility of laser systems based on polycrystalline samples of ZnSe doped with Cr2+. In order to compare the best performances of mono- and polycrystalline samples, further investigations are needed, by studying samples doped under the same conditions. If the moderate reduction in the performance of a poly-with respect to the monocrystalline sample will be confirmed, the large impact factor of low cost in producing polycrystalline samples will largely compensate the downgrading. 5. Acknowledgements The authors wish to thank Ilaria Grassini and Fabio Torri for their invaluable support during the development of this work. References [1] De Loach LD, Page RH, Wilke GD, Payne SA, Krupke WF. IEEE J Quantum Electron 1996;32:885. [2] Wagner GJ, Carrig TJ, Page RH, Schaffers KI, Ndap J, Ma X, Burger A. Opt Lett 1999;24:19. [3] Sorokina IT, Sorokin E, Di Lieto A, Tonelli M, Page RH, Schaffers KI. in: Injeyan H, Keller U, Marshall C, editors. OSA Trends in optics and photonics. Vol. 34. advanced solid-state lasers, Washington, DC: Optical Society of America, 2000 p. 188. [4] Page RH, Schaffers KI, DeLoach LD, Wilke GW, Patel FD, Tassano JB, Payne SA, Krupke WF, Chen K, Burger A. IEEE J Quantum Electron 1997;332:609. [5] Di Lieto A, Development of a cw Co : MgF2 laser, Opt Laser Eng, (Optics in Italy Ms#28) 2001, in press. [6] Sorokina IT, Sorokin E, Di Lieto A, Tonelli M, Page RH, Schaffers KI, J Opt Soc Am 2001; B 18 926. [7] Podlipensky AV, Shcherbitsky VG, Kuleshov NV, Levchenko VI, Yakimovich VN, Mond M, Heumann E, Huber G, Kretschmann H, Kuck . S. Appl Phys 2001;B 72:253. [8] Caird JA, Payne SA, Staver PR, Ramponi AJ, Chase LL, Krupke WF. IEEE J Quantum Electron 1988;24:1077.