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High-power end-pumped CW Nd:GdVO4 laser formed with a flat–flat cavity
Optics & Laser Technology 31 (1999) 459±462
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High-power end-pumped CW Nd:GdVO4 laser formed with a ¯at±¯at cavity Junhai Liu*, Zongshu Shao, Huaijin Zhang, Xianlin Meng, Li Zhu, Minhua Jiang National Laboratory of Crystal Materials, Shandong University, Jinan 250100, China Received 9 September 1999; accepted 6 October 1999
Abstract Formed with a ¯at±¯at resonator, a diode-laser-array end-pumped CW Nd:GdVO4 laser at 1.06 mm, capable of generating 8.6 W of TEM00 output power with optical conversion eciency of 43% and slope eciency of 48%, has been developed. The laser beam was nearly diraction limited, with the beam quality factor measured to be M 2=1.22. Under the conditions of multimode operation, the laser was able to produce 11.2 W of low-order transverse mode radiation (M 2 < 2) at the incident pump power of 22 W, giving an optical conversion eciency of 51%, and a slope eciency of 55%. # 2000 Elsevier Science Ltd. All rights reserved. Keywords: Nd:GdVO4 laser; End-pumped; Flat±¯at cavity
1. Introduction Nd:YVO4 has been widely used in diode endpumped solid-state lasers because of its suitability for diode pumping. However, the relatively low thermal conductivity and thermal fracture strength greatly limit its applications in the regime of high-power operation. As a new laser medium, Nd:GdVO4, ®rst developed by Zagumennyi et al. in 1992 [1], has some advantages over Nd:YVO4 such as higher absorption coecient; larger emission cross section; and, most importantly, much higher thermal conductivity along h110i directions which is comparable to that of Nd:YAG [2±4]. As a result, it is reasonable to predict that Nd:GdVO4 will become a strong rival to Nd:YVO4, especially in the ®eld of high-power diode pumped lasers. Very recently, signi®cant progress has been made in the power scaling of end-pumped Nd:GdVO4 lasers [5±7]. By the use of lightly-doped Nd:GdVO4 crystal, we have demonstrated an end-
* Corresponding author. Tel.: +86-531-856-4848; fax: +86-531856-5403. E-mail address: [email protected] (J. Liu).
pumped laser formed with a concave-plano resonator, from which over 14 W of CW multi-mode radiation at 1.06 mm was achieved [7]. Flat±¯at resonators are usually thought of as being quite sensitive to misalignment, and consequently ®nd few applications in diode end-pumped lasers [8], except for the cases of microchip lasers. Under high pump power conditions, however, this characteristic will be greatly changed. In fact, the characteristics of ¯at±¯at resonators employed in end-pumped lasers, including the mode sizes and stability, will be dominated by the pump-induced thermal lensing in the laser medium. The stable state of ¯at±¯at resonators, nominally located at the stability boundary, will be driven deep into the stable region by the thermal lensing. Besides this, the ¯at±¯at cavity also provides larger average mode size in comparison with that of the commonly used concave±plano cavity having the same cavity length. Utilizing a ¯at±¯at cavity, Zhang et al. demonstrated an end-pumped Nd:YVO4 laser capable of producing 9 W of TEM00 output at 1.06 mm [9]. Feugnet et al. also demonstrated a longitudinally pumped Nd:YVO4 laser formed with a ¯at±¯at resonator, from which a nearly diraction limited output beam (M 2 < 1.1) was obtained [10].
0030-3992/99/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 3 0 - 3 9 9 2 ( 9 9 ) 0 0 0 9 6 - 1
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J. Liu et al. / Optics & Laser Technology 31 (1999) 459±462
In this paper, we report on our results obtained with a diode-laser-array end-pumped CW Nd:GdVO4 laser formed with a ¯at±¯at resonator. The output characteristics of the laser, operated under dierent couplings, as well as for dierent cavity lengths, have been investigated in detail. An 8.6 W output of nearly diffraction limited TEM00 was achieved, which is, to our knowledge, the highest TEM00 output power ever reported for Nd:GdVO4 lasers. 2. Experimental laser setup Fig. 1 gives a schematic diagram of the experimental laser arrangement. M1 was a ¯at mirror, with one surface anti-re¯ection (AR) coated for 808 nm; and the other high re¯ectance (HR) coated at 1.06 mm and high transmittance (HT) coated at 808 nm. M2 was also a ¯at mirror with dierent transmissions at 1.06 mm of 5%, 15%, and 25%, and served as the output coupler. The 0.5 at. % Nd:GdVO4 crystal, which was grown by our group using the Czochralski technique, was cut with its crystallographic a-axis parallel to the optical axis of the laser system, and was of the dimensions 3 3 4 mm. To reduce the internal losses it added, the crystal was AR coated for both 1.06 mm and 808 nm on the pumped face, and AR coated for 1.06 mm on the opposite face. In order to extract from the crystal the heat generated under high pump power levels, we developed an ecient cooling device in which the crystal, wrapped with indium foil, was held in a copper block which was cooled by using thermoelectric coolers. It was therefore possible to keep the crystal surface temperature at some pretty low values even with 20 W of pump power incident upon the crystal, hence greatly reducing the risk of thermal fracture. The pump source employed in the laser was a ®ber-coupled diode-laser-array (OPC-D030-FCHS, Opto-Power Corp.) capable of providing a maximum output power of 30 W with the center wavelength around 807 nm. The output beam from the bundle end, which is 1.55 mm in diameter, was focused into the laser crystal with a spot radius of about 0.38 mm and numerical aperture of 0.22 by focusing optics with a coupling eciency of about 84%.
Fig. 1. Schematic diagram of the experimental laser setup.
In the resonator con®guration utilized in our experiment shown in Fig. 1, the laser crystal was placed very close to mirror M1, making M1 several millimeters apart from the pumped face of the crystal at the most. Under this condition, the TEM00 mode radius and the laser stability will be determined by the thermal lensing and the distance between the center of the crystal and the mirror M2 (termed L2). Fig. 2 gives the calculated TEM00 mode radius as a function of the focal length of the thermal lens (termed fT). It can be seen that the variation of the mode size with the thermal lensing is fairly smooth; and that the mode size goes to in®nity at the stability boundary. It can also be noticed that, with longer L2, larger mode size can be reached, but at the price of reducing the stability range. 3. Results and discussion To study the in¯uence of the magnitude of L2 on the performance of the laser, we tested with three dierent L2 during our experiment. At ®rst, we adopted a short cavity with L2 of 60 mm. As shown in Fig. 2, this cavity remains stable over a wide range extending from fT=1 to fT=60 mm, making it possible for the laser to operate under very high pump power conditions. The TEM00 mode radius, away from the stability boundary, was calculated to vary from 0.28 to 0.2 mm, depending on the magnitude of the thermal lensing. Fig. 3 shows the output characteristics of the laser. When the 5% transmission coupler was employed, the oscillation threshold was about 1 W. After the pump power was over 5 W, the 15% transmission coupler became the optimal. At the incident pump power of 22 W, an output power of 11.2 W was obtained, leading to an optical conversion eciency of 51%, while the slope eciency was determined to be 55%. The output beam quality factor
Fig. 2. Fundamental mode radius as a function of thermal focal length for three dierent magnitudes of L2.
J. Liu et al. / Optics & Laser Technology 31 (1999) 459±462
Fig. 3. Output power at 1.06 mm vs incident pump power for three dierent couplings and for L2=60 mm.
(M 2), measured at the output power of 10 W, was 1.98, showing the low-order transverse mode operation. The output power of the laser could be further scaled by increasing the incident pump power, but at the risk of thermal fracture of the crystal, since it was found that beyond the incident pump power of 22 W, the crystal surface temperature was rising continually, and was dicult to control at some low ®xed value. By using more powerful thermoelectric coolers to remove the heat from the crystal more eciently, this diculty should be avoided to some extent. When the planar cavity was made suciently long, the laser could be operated in the TEM00 mode. Fig. 4 shows the TEM00 output power as a function of the incident pump power, the corresponding L2 was 135 mm. In this case, the TEM00 mode radius in the laser crystal was computed to be ranging from 0.35 to 0.3 mm, varying with the magnitude of the thermal lensing. At the incident pump power of 20 W, 8.6 W of TEM00 output power was obtained, giving an optical conversion eciency of 43%, and a slope eciency of 48%. Since temperature controlling of the crystal became dicult if the incident pump power exceeded
Fig. 4. TEM00 output power at 1.06 mm vs incident pump power for three dierent couplings and for L2=135 mm.
461
20 W, the laser was not operated beyond this level. The output beam was found to be nearly diraction limited, with a measured M 2=1.22 (at the output power of 8 W). It is noticed from Fig. 4 that unlike the short cavity situation (L2=60 mm), in the present case, the highest eciency was achieved by using the 25% transmission coupler in the high pump powers over 15 W. It is also found that the laser output tended to saturate at high pump powers when the 5% transmission coupler was employed. These results suggest that the internal losses were higher than those of the short cavity. Fig. 5 gives the output characteristics of the laser when the cavity length was selected as 240 mm. As shown in Fig. 2, over the whole stable range, the TEM00 mode radius is above 0.4 mm, greater than the spot radius of the pump beam (0.38 mm). In this case, the laser worked less eciently than in the former case, particularly when it was operated under high pump power conditions. A maximum output power of 6.1 W was obtained with 20 W of pump power incident upon the crystal, the corresponding conversion eciency was 30.5%. Despite the lower eciency, the output beam quality was improved, with a measured M2=1.15, due to the perfect mode matching between the TEM00 mode volume and the pump light. When the pump power was further increased, the laser output began to drop, as the increased thermal lensing had driven the resonator out of its stable region. It is easy to derive that in the above resonator con®guration, the situation where the resonator begins to get unstable occurs when fT=L2. This conclusion can be used to determine the thermal lens of end-pumped solid-state lasers experimentally [11]. It is also shown in Fig. 5 that above the incident pump power of 10 W, the slope eciency decreased remarkably, even with the high transmission coupling of 25%. The reason for this is attributed to the diraction losses caused by the spherical aberation accompanying the thermal lens. As can be seen from Fig. 2, in the range of thermal focal
Fig. 5. TEM00 output power at 1.06 mm vs incident pump power for three dierent couplings and for L2=240 mm.
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J. Liu et al. / Optics & Laser Technology 31 (1999) 459±462
length shorter than 400 mm, the fundamental mode radius increases rapidly with the strengthening of the thermal lensing. Therefore, the thermally induced diffraction losses became increasingly high [12], leading to the much lower slope eciency. 4. Conclusions In summary, we have demonstrated an end-pumped CW Nd:GdVO4 laser at 1.06 mm formed with a ¯at± ¯at resonator. Nearly diraction limited TEM00 output of 8.6 W was obtained with an optical conversion eciency of 43%, and a slope eciency of 48%. In multi-mode operation, the laser was capable of producing 11.2 W of low-order transverse mode radiation with an optical conversion eciency of 51%. Acknowledgements This work was supported by the High Technology Development Project of China.
References [1] Zagumennyi AI, Ostroumov VG, Shcherbakov IA, Jensen T, Meyn JP, Huber G. Sov J Quantum Electron 1992;22:1071±2. [2] Jensen T, Ostroumov VG, Meyn JP, Huber G, Zagumennyi AI, Shcherbakov IA. Appl Phys B 1994;58:373±9. [3] Shimamura K, Uda S, Kochurikhin VV, Taniuchi T, Fukuda T. Jpn J Appl Phys 1996;35:1832±5. [4] Studenikin PA, Zagumennyi AI, Zavartsev YD, Popov PA, Shcherbakov IA. Quantum Electron 1995;25:1162±5. [5] Wyss Chr P, Luthy W, Weber HP, Vlasov VI, Zavartsev YuD, Studenikin PA, Zagumennyi AI, Shcherbakov IA. Appl Phys B 1999;68:659±61. [6] Liu J, Shao Z, Meng X, Zhang H, Zhu L, Jiang M. Opt Commun 1999;164:199±202. [7] Liu J, Shao Z, Zhang H, Meng X, Zhu L, Jiang M. Appl Phys B 1999;69:241±3. [8] Agnesi A, Dell'Acqua S, Reali GC, Gobbi PG, Ragazzi D. Appl Opt 1997;36:597±601. [9] Zhang J, Quade M, Du KM, Liao Y, Falter S, Baumann M, Loosen P, Poprawe P. Electron Lett 1997;33:775±6. [10] Feugnet G, Bussac C, Larat C, Schwarz M, Pocholle JP. Opt Lett 1995;20:157±9. [11] Liu J, Lu JR, Lu JH, Shao Z, Jiang M. Chin Phys Lett 1999;16:181±3. [12] Chen YF, Huang TM, Kao CF, Wang CL, Wang SC. IEEE J Quantum Electron 1997;33:1424±9.
www.elsevier.com/locate/optlastec
High-power end-pumped CW Nd:GdVO4 laser formed with a ¯at±¯at cavity Junhai Liu*, Zongshu Shao, Huaijin Zhang, Xianlin Meng, Li Zhu, Minhua Jiang National Laboratory of Crystal Materials, Shandong University, Jinan 250100, China Received 9 September 1999; accepted 6 October 1999
Abstract Formed with a ¯at±¯at resonator, a diode-laser-array end-pumped CW Nd:GdVO4 laser at 1.06 mm, capable of generating 8.6 W of TEM00 output power with optical conversion eciency of 43% and slope eciency of 48%, has been developed. The laser beam was nearly diraction limited, with the beam quality factor measured to be M 2=1.22. Under the conditions of multimode operation, the laser was able to produce 11.2 W of low-order transverse mode radiation (M 2 < 2) at the incident pump power of 22 W, giving an optical conversion eciency of 51%, and a slope eciency of 55%. # 2000 Elsevier Science Ltd. All rights reserved. Keywords: Nd:GdVO4 laser; End-pumped; Flat±¯at cavity
1. Introduction Nd:YVO4 has been widely used in diode endpumped solid-state lasers because of its suitability for diode pumping. However, the relatively low thermal conductivity and thermal fracture strength greatly limit its applications in the regime of high-power operation. As a new laser medium, Nd:GdVO4, ®rst developed by Zagumennyi et al. in 1992 [1], has some advantages over Nd:YVO4 such as higher absorption coecient; larger emission cross section; and, most importantly, much higher thermal conductivity along h110i directions which is comparable to that of Nd:YAG [2±4]. As a result, it is reasonable to predict that Nd:GdVO4 will become a strong rival to Nd:YVO4, especially in the ®eld of high-power diode pumped lasers. Very recently, signi®cant progress has been made in the power scaling of end-pumped Nd:GdVO4 lasers [5±7]. By the use of lightly-doped Nd:GdVO4 crystal, we have demonstrated an end-
* Corresponding author. Tel.: +86-531-856-4848; fax: +86-531856-5403. E-mail address: [email protected] (J. Liu).
pumped laser formed with a concave-plano resonator, from which over 14 W of CW multi-mode radiation at 1.06 mm was achieved [7]. Flat±¯at resonators are usually thought of as being quite sensitive to misalignment, and consequently ®nd few applications in diode end-pumped lasers [8], except for the cases of microchip lasers. Under high pump power conditions, however, this characteristic will be greatly changed. In fact, the characteristics of ¯at±¯at resonators employed in end-pumped lasers, including the mode sizes and stability, will be dominated by the pump-induced thermal lensing in the laser medium. The stable state of ¯at±¯at resonators, nominally located at the stability boundary, will be driven deep into the stable region by the thermal lensing. Besides this, the ¯at±¯at cavity also provides larger average mode size in comparison with that of the commonly used concave±plano cavity having the same cavity length. Utilizing a ¯at±¯at cavity, Zhang et al. demonstrated an end-pumped Nd:YVO4 laser capable of producing 9 W of TEM00 output at 1.06 mm [9]. Feugnet et al. also demonstrated a longitudinally pumped Nd:YVO4 laser formed with a ¯at±¯at resonator, from which a nearly diraction limited output beam (M 2 < 1.1) was obtained [10].
0030-3992/99/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 3 0 - 3 9 9 2 ( 9 9 ) 0 0 0 9 6 - 1
460
J. Liu et al. / Optics & Laser Technology 31 (1999) 459±462
In this paper, we report on our results obtained with a diode-laser-array end-pumped CW Nd:GdVO4 laser formed with a ¯at±¯at resonator. The output characteristics of the laser, operated under dierent couplings, as well as for dierent cavity lengths, have been investigated in detail. An 8.6 W output of nearly diffraction limited TEM00 was achieved, which is, to our knowledge, the highest TEM00 output power ever reported for Nd:GdVO4 lasers. 2. Experimental laser setup Fig. 1 gives a schematic diagram of the experimental laser arrangement. M1 was a ¯at mirror, with one surface anti-re¯ection (AR) coated for 808 nm; and the other high re¯ectance (HR) coated at 1.06 mm and high transmittance (HT) coated at 808 nm. M2 was also a ¯at mirror with dierent transmissions at 1.06 mm of 5%, 15%, and 25%, and served as the output coupler. The 0.5 at. % Nd:GdVO4 crystal, which was grown by our group using the Czochralski technique, was cut with its crystallographic a-axis parallel to the optical axis of the laser system, and was of the dimensions 3 3 4 mm. To reduce the internal losses it added, the crystal was AR coated for both 1.06 mm and 808 nm on the pumped face, and AR coated for 1.06 mm on the opposite face. In order to extract from the crystal the heat generated under high pump power levels, we developed an ecient cooling device in which the crystal, wrapped with indium foil, was held in a copper block which was cooled by using thermoelectric coolers. It was therefore possible to keep the crystal surface temperature at some pretty low values even with 20 W of pump power incident upon the crystal, hence greatly reducing the risk of thermal fracture. The pump source employed in the laser was a ®ber-coupled diode-laser-array (OPC-D030-FCHS, Opto-Power Corp.) capable of providing a maximum output power of 30 W with the center wavelength around 807 nm. The output beam from the bundle end, which is 1.55 mm in diameter, was focused into the laser crystal with a spot radius of about 0.38 mm and numerical aperture of 0.22 by focusing optics with a coupling eciency of about 84%.
Fig. 1. Schematic diagram of the experimental laser setup.
In the resonator con®guration utilized in our experiment shown in Fig. 1, the laser crystal was placed very close to mirror M1, making M1 several millimeters apart from the pumped face of the crystal at the most. Under this condition, the TEM00 mode radius and the laser stability will be determined by the thermal lensing and the distance between the center of the crystal and the mirror M2 (termed L2). Fig. 2 gives the calculated TEM00 mode radius as a function of the focal length of the thermal lens (termed fT). It can be seen that the variation of the mode size with the thermal lensing is fairly smooth; and that the mode size goes to in®nity at the stability boundary. It can also be noticed that, with longer L2, larger mode size can be reached, but at the price of reducing the stability range. 3. Results and discussion To study the in¯uence of the magnitude of L2 on the performance of the laser, we tested with three dierent L2 during our experiment. At ®rst, we adopted a short cavity with L2 of 60 mm. As shown in Fig. 2, this cavity remains stable over a wide range extending from fT=1 to fT=60 mm, making it possible for the laser to operate under very high pump power conditions. The TEM00 mode radius, away from the stability boundary, was calculated to vary from 0.28 to 0.2 mm, depending on the magnitude of the thermal lensing. Fig. 3 shows the output characteristics of the laser. When the 5% transmission coupler was employed, the oscillation threshold was about 1 W. After the pump power was over 5 W, the 15% transmission coupler became the optimal. At the incident pump power of 22 W, an output power of 11.2 W was obtained, leading to an optical conversion eciency of 51%, while the slope eciency was determined to be 55%. The output beam quality factor
Fig. 2. Fundamental mode radius as a function of thermal focal length for three dierent magnitudes of L2.
J. Liu et al. / Optics & Laser Technology 31 (1999) 459±462
Fig. 3. Output power at 1.06 mm vs incident pump power for three dierent couplings and for L2=60 mm.
(M 2), measured at the output power of 10 W, was 1.98, showing the low-order transverse mode operation. The output power of the laser could be further scaled by increasing the incident pump power, but at the risk of thermal fracture of the crystal, since it was found that beyond the incident pump power of 22 W, the crystal surface temperature was rising continually, and was dicult to control at some low ®xed value. By using more powerful thermoelectric coolers to remove the heat from the crystal more eciently, this diculty should be avoided to some extent. When the planar cavity was made suciently long, the laser could be operated in the TEM00 mode. Fig. 4 shows the TEM00 output power as a function of the incident pump power, the corresponding L2 was 135 mm. In this case, the TEM00 mode radius in the laser crystal was computed to be ranging from 0.35 to 0.3 mm, varying with the magnitude of the thermal lensing. At the incident pump power of 20 W, 8.6 W of TEM00 output power was obtained, giving an optical conversion eciency of 43%, and a slope eciency of 48%. Since temperature controlling of the crystal became dicult if the incident pump power exceeded
Fig. 4. TEM00 output power at 1.06 mm vs incident pump power for three dierent couplings and for L2=135 mm.
461
20 W, the laser was not operated beyond this level. The output beam was found to be nearly diraction limited, with a measured M 2=1.22 (at the output power of 8 W). It is noticed from Fig. 4 that unlike the short cavity situation (L2=60 mm), in the present case, the highest eciency was achieved by using the 25% transmission coupler in the high pump powers over 15 W. It is also found that the laser output tended to saturate at high pump powers when the 5% transmission coupler was employed. These results suggest that the internal losses were higher than those of the short cavity. Fig. 5 gives the output characteristics of the laser when the cavity length was selected as 240 mm. As shown in Fig. 2, over the whole stable range, the TEM00 mode radius is above 0.4 mm, greater than the spot radius of the pump beam (0.38 mm). In this case, the laser worked less eciently than in the former case, particularly when it was operated under high pump power conditions. A maximum output power of 6.1 W was obtained with 20 W of pump power incident upon the crystal, the corresponding conversion eciency was 30.5%. Despite the lower eciency, the output beam quality was improved, with a measured M2=1.15, due to the perfect mode matching between the TEM00 mode volume and the pump light. When the pump power was further increased, the laser output began to drop, as the increased thermal lensing had driven the resonator out of its stable region. It is easy to derive that in the above resonator con®guration, the situation where the resonator begins to get unstable occurs when fT=L2. This conclusion can be used to determine the thermal lens of end-pumped solid-state lasers experimentally [11]. It is also shown in Fig. 5 that above the incident pump power of 10 W, the slope eciency decreased remarkably, even with the high transmission coupling of 25%. The reason for this is attributed to the diraction losses caused by the spherical aberation accompanying the thermal lens. As can be seen from Fig. 2, in the range of thermal focal
Fig. 5. TEM00 output power at 1.06 mm vs incident pump power for three dierent couplings and for L2=240 mm.
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J. Liu et al. / Optics & Laser Technology 31 (1999) 459±462
length shorter than 400 mm, the fundamental mode radius increases rapidly with the strengthening of the thermal lensing. Therefore, the thermally induced diffraction losses became increasingly high [12], leading to the much lower slope eciency. 4. Conclusions In summary, we have demonstrated an end-pumped CW Nd:GdVO4 laser at 1.06 mm formed with a ¯at± ¯at resonator. Nearly diraction limited TEM00 output of 8.6 W was obtained with an optical conversion eciency of 43%, and a slope eciency of 48%. In multi-mode operation, the laser was capable of producing 11.2 W of low-order transverse mode radiation with an optical conversion eciency of 51%. Acknowledgements This work was supported by the High Technology Development Project of China.
References [1] Zagumennyi AI, Ostroumov VG, Shcherbakov IA, Jensen T, Meyn JP, Huber G. Sov J Quantum Electron 1992;22:1071±2. [2] Jensen T, Ostroumov VG, Meyn JP, Huber G, Zagumennyi AI, Shcherbakov IA. Appl Phys B 1994;58:373±9. [3] Shimamura K, Uda S, Kochurikhin VV, Taniuchi T, Fukuda T. Jpn J Appl Phys 1996;35:1832±5. [4] Studenikin PA, Zagumennyi AI, Zavartsev YD, Popov PA, Shcherbakov IA. Quantum Electron 1995;25:1162±5. [5] Wyss Chr P, Luthy W, Weber HP, Vlasov VI, Zavartsev YuD, Studenikin PA, Zagumennyi AI, Shcherbakov IA. Appl Phys B 1999;68:659±61. [6] Liu J, Shao Z, Meng X, Zhang H, Zhu L, Jiang M. Opt Commun 1999;164:199±202. [7] Liu J, Shao Z, Zhang H, Meng X, Zhu L, Jiang M. Appl Phys B 1999;69:241±3. [8] Agnesi A, Dell'Acqua S, Reali GC, Gobbi PG, Ragazzi D. Appl Opt 1997;36:597±601. [9] Zhang J, Quade M, Du KM, Liao Y, Falter S, Baumann M, Loosen P, Poprawe P. Electron Lett 1997;33:775±6. [10] Feugnet G, Bussac C, Larat C, Schwarz M, Pocholle JP. Opt Lett 1995;20:157±9. [11] Liu J, Lu JR, Lu JH, Shao Z, Jiang M. Chin Phys Lett 1999;16:181±3. [12] Chen YF, Huang TM, Kao CF, Wang CL, Wang SC. IEEE J Quantum Electron 1997;33:1424±9.