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Highly efficient third-harmonic generation with electro-optically Q-switched diode-end-pumped Nd:YVO4 slab laser
Optics Communications 272 (2007) 192–196 www.elsevier.com/locate/optcom
Highly efficient third-harmonic generation with electro-optically Q-switched diode-end-pumped Nd:YVO4 slab laser Xiaomeng Liu
b
a,c,*,1 ,
Daijun Li b, Peng Shi a, Claus Ru¨diger Haas a, Alexander Schell a, Nianle Wu c, Keming Du a
a EdgeWave GmbH, Schumanstr. 18B, Wu¨rselen 52146, Germany Institute for Technology of Optical System RWTH Aachen, Steinbachstr. 15, 52074 Aachen, Germany c Department of Physics, Tsinghua University, Beijing 100084, China
Received 13 April 2006; received in revised form 17 June 2006; accepted 30 October 2006
Abstract We have achieved efficient third-harmonic generation (THG) with an electro-optically Q-switched diode-end-pumped slab laser by cascading second-harmonic and sum-frequency generation in a lithium triborate (LBO) crystal. The high conversion efficiency, short pulse length and high pulse energy is the characteristic of the output 355 nm light. An average power of 11.1 W at a repetition rate of 10 kHz was achieved. The pulse energy is 1.1 mJ and the pulse length is 5 ns. The peak power of pulse is 0.22 MW. The conversion efficiency from 1064 nm to 355 nm reached 44.4% which is to our knowledge the highest conversion efficiency. Furthermore, the 355 nm light is near TEM00 mode. The beam quality is M2 < 1.5. In this paper, the experimental setup, results and the factors which can affect the conversion efficiency are discussed. 2006 Elsevier B.V. All rights reserved.
1. Introduction In recent years, many people show interest in the thirdharmonic generation (THG). Third harmonic of Nd:YVO4 lasers with wavelength of 355 nm is interesting for micro precision processing in electronic industry (via drilling), photovoltaic (scribing) and display (soft marking) because of its high absorption by materials used. A lot of experiments have been carried on to produce THG, such as sum-frequency generation in a periodic structure [1], in compositionally graded films [2], in a two-dimensional photonic crystal [3], etc. Although these methods have their advantages, the traditional way to do THG in nonlinear crystals still has its incomparable superiority in this domain. In the year of 2003, 355 nm UV light was gener*
Corresponding author. Tel.: +86 13331090867; fax: +86 1062781604. E-mail address: [email protected] (X. Liu). 1 Xiaomeng Liu thanks the support of the Internationale Bu¨ro des Bundesministeriums fu¨r Bildung, Wissenschaft, Forschung und Technologie beim DLR through a research fellowship in EdgeWave GmbH. 0030-4018/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2006.10.080
ated by use of a type II CBO crystal as a sum frequency of the fundamental light and the second harmonic of a nanosecond Nd:YVO4 laser. The conversion efficiency from the fundamental light to the third harmonic reached 30% [4]. A study of second- and third-harmonic generation in BiB3O6 (BiBO) have been reported in 2005 [5]. They got a conversion efficiency as high as 34%. Nearly at the same time, a 17.7 W average power output at 355 nm by the THG of 1064 nm light have been obtained with the nonlinear optical crystal CsB3O5 (CBO) [6]. The energy conversion efficiency P355 nm/P1064 nm + P532 nm is 13.3%. This paper gives the experiment result of 44.4% conversion efficiency from 1064 nm to 355 nm which is to our knowledge the highest conversion efficiency. LiB3O5 (LBO) is a widely used nonlinear crystal in harmonic generations. It has high damage threshold, wide acceptance angle and small walk-off. It also has high conversion efficiency in second-harmonic generation (SHG) [7,8]. In this paper, we use it to get a high conversion efficiency not only in SHG, but also in THG.
X. Liu et al. / Optics Communications 272 (2007) 192–196
Partially end-pumped slab laser with hybrid resonators was proposed in 1998 [9]. It has been shown to be efficient, lightweight, compact and reliable sources. And it is favorable for power scaling at high beam quality and efficiency, as they have the properties of both the high overlapping efficiency of end-pumped rod lasers and the excellent cooling conductivity of slab lasers [10]. It can provide TEM00 mode fundamental light for the harmonic generation. 2. Properties of fundamental mode operation and design consideration Partially end-pumped slab laser with hybrid resonators has unique features of high beam quality, short pulse length and high peak power. This makes them most suitable for nonlinear frequency conversion, such as SHG and THG. The detail of Nd:YVO4 slab laser was shown in Fig. 1. The pump source was a 4-bar diode stack; the radiation emitted by each diode laser bar was individually collimated by micro-lens. After a set of lenses, the pump light was shaped to be an even thin line on the Nd:YVO4 crystal [11]. The slab Nd:YVO4 crystal was 0.4 at.% doped. It had dimension of 1 mm · 10 mm · 12 mm and was a-cut and its c axis was parallel to its 12 mm edge. The laser has a positive-branch, confocal unstable resonator in the plane of pumping line (X–Z plane in Fig. 1). The cavity length was 90 mm. The rear mirror M1 and the output mirror M2 are cylindrical with curvature in the X–Z plane. The radii of curvature of the totally reflecting rear mirror and the output mirror were 500 mm and 350 mm, respectively. The magnification is M = jR1/R2j = 1.4. The effective out coupling is 30%. The output beam size at the position of output mirror approximate 3.5 mm · 0.25 mm (X · Y). This laser provided the fundamental light source for THG. It had 30.4 W average power with the pulse length of 6.1 ns and a repetition rate of 10 kHz at the pump
a
Nd: : YVO 4
193
power of 127 W. And the M2 1.3. The average power and the pulse length of the fundamental light would be different when the repetition rate changed. This is shown in Fig. 2. The peak power is 0.5 MW. Such high peak power is of essential importance to get high conversion without tightly focussing of laser beam. In the X–Z plane, the beam was near-diffraction-limited. The light intensity distribution of the beam in far-field is near gaussian distribution. But there was some side lobes. In order to get beam for the following SHG and THG, we used a spatial filter block the side lobes. The intensity distribution in the far field of output beam after the spatial filter is shown in Fig. 3. This process lost 17.8% average power of the output beam at 10 kHz repetition rate. Only 25 W of average power at 1064 nm were incident on the SHG LBO crystal at 10 kHz repetition rate. The photon number match is important to the THG conversion efficiency. That is to say, we should make the photo number at second harmonic equal to the photo numbers at the fundamental wavelength. If the photon number of 1064 nm light and the 532 nm light is 1:1, the conversion efficiency will reach the highest [12,13]. That is to say, the SHG conversion efficiency should reach to 66.7%. In order to get this best conversion efficiency in SHG process. We should design our experiment parameters by the following calculation. The SHG conversion efficiency can be calculate by [14] " # 1=2 P 2x sin Dkl=2 2 1=2 P x g¼ ¼ tanh lK ; ð1Þ Dkl=2 Px A pffiffiffiffiffiffiffiffiffiffiffiffiffi : where K ¼ 2c3 x2 d 2eff , c ¼ l0 =e0 e ¼ 377=no . We approximately consider that the beam is parallel light in the SHG process and the phase matched. That is to say, Dk = 0. The equation can be simplified " 1=2 # P 2x 2 1=2 P x g¼ ¼ tanh lK : ð2Þ Px A
Pockel Cell
LD X
Polarizer
Z
b
M1
Y
M2
Z Fig. 1. Schematic of an electro-optically Q-switched Nd:YVO4 slab laser with a hybrid resonator, (a) is in X–Z plane, and (b) is in Y–Z plane, M1 and M2 are cavity mirrors.
Fig. 2. The average power and the pulse length of 1064 nm fundamental light.
194
X. Liu et al. / Optics Communications 272 (2007) 192–196
3. SHG and THG experiment As Fig. 4 shows, we used two LBO crystals in our THG scheme. One was type I for SHG and the other was type II for THG. The SHG LBO had dimension of 3 mm · 3 mm · 15 mm and was cut by h = 90 and / = 10.8. Its surfaces were both coated with AR1064 nm and AR532 nm. Since the conversion efficiency depends strongly on the quality and peak power of the fundamental light, we insert a beam shaping system before focus the light to the SHG LBO crystal. The beam shaping system was made up of three lenses. As Fig. 4 shows, the focus length of L1 and L2 are 100 mm and 20 mm in X–Z plane, respectively. L3
Fig. 3. The fundamental light intensity distribution of the beam in farfield on the SHG LBO.
At 10 kHz and the fundamental average power of 25 W, the cross-section of the focussed beam on SHG LBO is 4.4 · 103 cm2. For a circle focal spot, the radii is x0 = 375 lm. The fundamental average power density on LBO surface is about 100 MW/cm2. This is much less than the damage threshold of the surface film coating on LBO crystal of 500 MW/cm2. The Rayleigh length is also be estimated to px20 ¼ 415 mm: ð3Þ k To be noted that this estimation is based on the strict condition of M2 = 1. For the M2 1.3 laser, the actual Rayleigh length must about 319 mm. Within Rayleigh length, the beam radius keeps within root 2 of x0 and the power density is within factor of 2. If we put the SHG LBO on the waist of fundamental light, that means we have a range of about 319 mm to put the THG LBO behind the SHG LBO. If the length of SHG LBO and the THG LBO are both much shorter than the Rayleigh length, the fundamental light waist on the THG LBO would keep near x0. And the light power density can be as high as on the SHG LBO. So we do not need to refocus the light before the THG LBO. This makes the THG experiment construction (Fig. 4 shows) more simple and efficient.
Z0 ¼
1064 nm & 532 nm
Fig. 5. The focal spot on the SHG LBO crystal.
355 nm
Beam Shaping System
Nd:YVO4 Slab Laser
1064 nm
1064 nm L1
L2
L3
SHG
THG
LBO
LBO 1064 nm & 532 nm & 355 nm
Fig. 4. Schematic of experimental setup for THG.
1064 nm & 532 nm
Fig. 6. The average power and the pulse length of 532 nm light.
X. Liu et al. / Optics Communications 272 (2007) 192–196
is a sphere lens with the focus of 250 mm. They made the waists in the stable direction and in the unstable direction of the fundamental light at the same place. And focus the fundamental light on the SHG LBO crystal and the focal spot size is 4.4 · 103 cm2 as the theory calculation. The shape of the focal spot on the SHG LBO crystal is shown in Fig. 5. The average power and the pulse length of 532 nm light at different repetition rates are shown in Fig. 6. The conversion efficiency in the SHG process is about 59.2% at 10 kHz. This value is close to the theory calculation value. The THG LBO crystal had dimension of 3 mm · 3 mm · 14 mm and was cut by h = 43 and / = 90. These angles were fit for the type II phase matching (PM) of THG. Its incident surfaces was coated with AR1064 nm and 532 nm film. And its emission surface was coated with AR355 nm film. The output beam have the three harmonics. To get the pure 355 nm light, we used two mirrors with HR355 nm film to filter the lights and only keeps 355 nm THG light. Then we got 11.1 W average power 355 nm
195
ultraviolet (UV) light. At repetition rate of 10 kHz, the pulse length is 5 ns. The conversion efficiency from 532 nm to 355 nm was 75%. The average power, the pulse length and the intensity distribution in the far field of 355 nm light is shown in Figs. 7 and 8, M2 < 1.5. Fig. 9
Fig. 9. A typical pulse profile of 355 nm light.
Fig. 7. The average power and the pulse length of 355 nm light.
Fig. 8. The intensity distribution in the far field of 355 nm light.
Fig. 10. (a) is the power of 532 nm SHG light vs temperature of SHG crystal at 10 kHz repetition rate, (b) is the power of 355 nm THG light vs temperature of THG crystal at 10 kHz repetition rate.
196
X. Liu et al. / Optics Communications 272 (2007) 192–196
shows a typical pulse profile measured by a fast photodiode and a digital oscilloscope. In the experiment, we can move the THG LBO crystal at a range of about 100 mm behind the SHG LBO. In this range, The THG conversion efficiency is constant. If the distance between the two LBO crystals is larger than 250 mm, the conversion efficiency will be a little lower. The PM in SHG and THG were both sensitive to the crystal temperature. The conversion efficiency would change with the crystal temperature. This can be seen in Fig. 10. In this figure, we can easily see that the THG process are more sensitively to the crystal temperature than the SHG process. The SHG and THG temperature tuning bandwidth are 4.6 and 2.6, respectively. In the page of 1992–1993 of the paper of J.A. Armstrong [15], we can see the detail method of calculating the conversion efficiency of three coupled waves in anisotropic crystals. By this method, we can get the curves of conversion efficiency vs temperature of SHG process and SFG (sum-frequency generation) process (Note: In this experiment, this process happened in THG LBO) in theory. 4. Conclusion In this paper, we introduce a THG experiment. The partially end-pumped slab laser with hybrid resonator, which provide the fundamental light to the THG system, is introduced briefly. We also analyze the factors that effect the harmonic generation. We got 11.1 W average power of TEM00 mode 355 nm light at 10 kHz repetition rate. The conversion efficiency
reached 44.4% from 1064 nm fundamental light to 355 nm THG light, which is to our knowledge the highest conversion efficiency. The energy per pulse and pulse length of the UV light were 1.11 mJ and 5 ns, respectively. The beam quality is M2 < 1.5.
References [1] C. Huang, Y. Zhu, Phys. Status Solidi B – Basic Solid State Phys. 242 (2005) 1694. [2] L. Gao, Phys. Rev. E 71 (2005) 067601. [3] D. Coquillat, G. Vecchi, C. Comaschi, A. Malvezzi, J. Torres, M.L. d’Yerville, Appl. Phys. Lett. 87 (2005) 101106. [4] H. Kitano, T. Matsui, K. Sato, N. Ushiyama, M. Yoshimura, Y. Mori, T. Sasaki, Opt. Lett. 28 (2003) 263. [5] M. Peltz, J. Bartschke, A. Borsutzky, R. Wallenstein, S. Vernay, T. Salva, D. Rytz, Appl. Phys B.81 (2005) 487. [6] Y. Wu, F. Chang, P. Fu, C. Chen, G. Wang, A. Geng, Y. Bo, D. Cui, Z. Xu, Chinese Phys. Lett. 22 (2005) 1426. [7] C. Chen, Y. Wu, A. Jiang, B. Wu, G. You, R. Li, S. Lin, J. Opt. Soc. Am. B 6 (1989) 616. [8] S. Lin, Z. sim, B. Wu, C. chen, J. Appl. Phys. 67 (1990) 634. [9] K.M. Du, N.L. Wu, J.D. Xu, J. Giesekus, P. Loosen, R. Poprawe, Opt. Lett. 23 (1998) 370. [10] K.M. Du, D.J. Li, H.L. Zhang, P. Shi, X.Y. Wei, R. Diart, Opt. Lett. 28 (2003) 87. [11] H. Zhang, P. Shi, D. Li, K. Du, Appl. Opt. 42 (2003) 1681. [12] R. Craxton, Opt. Commun. 34 (1980) 474. [13] W. Seka, S. Jacobs, J. Rizzo, R. Boni, R. Craxton, Opt. Commun. 34 (1980) 469. [14] Walter Koechner, Solid-State Laser Engineering, fifth revised and updated edition, Springer, 1999, (Chapter 10). [15] J.A. Armstrong, N. Bloembergen, J. Ducuing, P.S. Pershan, Phys. Rev. 127 (1962) 1918.
Highly efficient third-harmonic generation with electro-optically Q-switched diode-end-pumped Nd:YVO4 slab laser Xiaomeng Liu
b
a,c,*,1 ,
Daijun Li b, Peng Shi a, Claus Ru¨diger Haas a, Alexander Schell a, Nianle Wu c, Keming Du a
a EdgeWave GmbH, Schumanstr. 18B, Wu¨rselen 52146, Germany Institute for Technology of Optical System RWTH Aachen, Steinbachstr. 15, 52074 Aachen, Germany c Department of Physics, Tsinghua University, Beijing 100084, China
Received 13 April 2006; received in revised form 17 June 2006; accepted 30 October 2006
Abstract We have achieved efficient third-harmonic generation (THG) with an electro-optically Q-switched diode-end-pumped slab laser by cascading second-harmonic and sum-frequency generation in a lithium triborate (LBO) crystal. The high conversion efficiency, short pulse length and high pulse energy is the characteristic of the output 355 nm light. An average power of 11.1 W at a repetition rate of 10 kHz was achieved. The pulse energy is 1.1 mJ and the pulse length is 5 ns. The peak power of pulse is 0.22 MW. The conversion efficiency from 1064 nm to 355 nm reached 44.4% which is to our knowledge the highest conversion efficiency. Furthermore, the 355 nm light is near TEM00 mode. The beam quality is M2 < 1.5. In this paper, the experimental setup, results and the factors which can affect the conversion efficiency are discussed. 2006 Elsevier B.V. All rights reserved.
1. Introduction In recent years, many people show interest in the thirdharmonic generation (THG). Third harmonic of Nd:YVO4 lasers with wavelength of 355 nm is interesting for micro precision processing in electronic industry (via drilling), photovoltaic (scribing) and display (soft marking) because of its high absorption by materials used. A lot of experiments have been carried on to produce THG, such as sum-frequency generation in a periodic structure [1], in compositionally graded films [2], in a two-dimensional photonic crystal [3], etc. Although these methods have their advantages, the traditional way to do THG in nonlinear crystals still has its incomparable superiority in this domain. In the year of 2003, 355 nm UV light was gener*
Corresponding author. Tel.: +86 13331090867; fax: +86 1062781604. E-mail address: [email protected] (X. Liu). 1 Xiaomeng Liu thanks the support of the Internationale Bu¨ro des Bundesministeriums fu¨r Bildung, Wissenschaft, Forschung und Technologie beim DLR through a research fellowship in EdgeWave GmbH. 0030-4018/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2006.10.080
ated by use of a type II CBO crystal as a sum frequency of the fundamental light and the second harmonic of a nanosecond Nd:YVO4 laser. The conversion efficiency from the fundamental light to the third harmonic reached 30% [4]. A study of second- and third-harmonic generation in BiB3O6 (BiBO) have been reported in 2005 [5]. They got a conversion efficiency as high as 34%. Nearly at the same time, a 17.7 W average power output at 355 nm by the THG of 1064 nm light have been obtained with the nonlinear optical crystal CsB3O5 (CBO) [6]. The energy conversion efficiency P355 nm/P1064 nm + P532 nm is 13.3%. This paper gives the experiment result of 44.4% conversion efficiency from 1064 nm to 355 nm which is to our knowledge the highest conversion efficiency. LiB3O5 (LBO) is a widely used nonlinear crystal in harmonic generations. It has high damage threshold, wide acceptance angle and small walk-off. It also has high conversion efficiency in second-harmonic generation (SHG) [7,8]. In this paper, we use it to get a high conversion efficiency not only in SHG, but also in THG.
X. Liu et al. / Optics Communications 272 (2007) 192–196
Partially end-pumped slab laser with hybrid resonators was proposed in 1998 [9]. It has been shown to be efficient, lightweight, compact and reliable sources. And it is favorable for power scaling at high beam quality and efficiency, as they have the properties of both the high overlapping efficiency of end-pumped rod lasers and the excellent cooling conductivity of slab lasers [10]. It can provide TEM00 mode fundamental light for the harmonic generation. 2. Properties of fundamental mode operation and design consideration Partially end-pumped slab laser with hybrid resonators has unique features of high beam quality, short pulse length and high peak power. This makes them most suitable for nonlinear frequency conversion, such as SHG and THG. The detail of Nd:YVO4 slab laser was shown in Fig. 1. The pump source was a 4-bar diode stack; the radiation emitted by each diode laser bar was individually collimated by micro-lens. After a set of lenses, the pump light was shaped to be an even thin line on the Nd:YVO4 crystal [11]. The slab Nd:YVO4 crystal was 0.4 at.% doped. It had dimension of 1 mm · 10 mm · 12 mm and was a-cut and its c axis was parallel to its 12 mm edge. The laser has a positive-branch, confocal unstable resonator in the plane of pumping line (X–Z plane in Fig. 1). The cavity length was 90 mm. The rear mirror M1 and the output mirror M2 are cylindrical with curvature in the X–Z plane. The radii of curvature of the totally reflecting rear mirror and the output mirror were 500 mm and 350 mm, respectively. The magnification is M = jR1/R2j = 1.4. The effective out coupling is 30%. The output beam size at the position of output mirror approximate 3.5 mm · 0.25 mm (X · Y). This laser provided the fundamental light source for THG. It had 30.4 W average power with the pulse length of 6.1 ns and a repetition rate of 10 kHz at the pump
a
Nd: : YVO 4
193
power of 127 W. And the M2 1.3. The average power and the pulse length of the fundamental light would be different when the repetition rate changed. This is shown in Fig. 2. The peak power is 0.5 MW. Such high peak power is of essential importance to get high conversion without tightly focussing of laser beam. In the X–Z plane, the beam was near-diffraction-limited. The light intensity distribution of the beam in far-field is near gaussian distribution. But there was some side lobes. In order to get beam for the following SHG and THG, we used a spatial filter block the side lobes. The intensity distribution in the far field of output beam after the spatial filter is shown in Fig. 3. This process lost 17.8% average power of the output beam at 10 kHz repetition rate. Only 25 W of average power at 1064 nm were incident on the SHG LBO crystal at 10 kHz repetition rate. The photon number match is important to the THG conversion efficiency. That is to say, we should make the photo number at second harmonic equal to the photo numbers at the fundamental wavelength. If the photon number of 1064 nm light and the 532 nm light is 1:1, the conversion efficiency will reach the highest [12,13]. That is to say, the SHG conversion efficiency should reach to 66.7%. In order to get this best conversion efficiency in SHG process. We should design our experiment parameters by the following calculation. The SHG conversion efficiency can be calculate by [14] " # 1=2 P 2x sin Dkl=2 2 1=2 P x g¼ ¼ tanh lK ; ð1Þ Dkl=2 Px A pffiffiffiffiffiffiffiffiffiffiffiffiffi : where K ¼ 2c3 x2 d 2eff , c ¼ l0 =e0 e ¼ 377=no . We approximately consider that the beam is parallel light in the SHG process and the phase matched. That is to say, Dk = 0. The equation can be simplified " 1=2 # P 2x 2 1=2 P x g¼ ¼ tanh lK : ð2Þ Px A
Pockel Cell
LD X
Polarizer
Z
b
M1
Y
M2
Z Fig. 1. Schematic of an electro-optically Q-switched Nd:YVO4 slab laser with a hybrid resonator, (a) is in X–Z plane, and (b) is in Y–Z plane, M1 and M2 are cavity mirrors.
Fig. 2. The average power and the pulse length of 1064 nm fundamental light.
194
X. Liu et al. / Optics Communications 272 (2007) 192–196
3. SHG and THG experiment As Fig. 4 shows, we used two LBO crystals in our THG scheme. One was type I for SHG and the other was type II for THG. The SHG LBO had dimension of 3 mm · 3 mm · 15 mm and was cut by h = 90 and / = 10.8. Its surfaces were both coated with AR1064 nm and AR532 nm. Since the conversion efficiency depends strongly on the quality and peak power of the fundamental light, we insert a beam shaping system before focus the light to the SHG LBO crystal. The beam shaping system was made up of three lenses. As Fig. 4 shows, the focus length of L1 and L2 are 100 mm and 20 mm in X–Z plane, respectively. L3
Fig. 3. The fundamental light intensity distribution of the beam in farfield on the SHG LBO.
At 10 kHz and the fundamental average power of 25 W, the cross-section of the focussed beam on SHG LBO is 4.4 · 103 cm2. For a circle focal spot, the radii is x0 = 375 lm. The fundamental average power density on LBO surface is about 100 MW/cm2. This is much less than the damage threshold of the surface film coating on LBO crystal of 500 MW/cm2. The Rayleigh length is also be estimated to px20 ¼ 415 mm: ð3Þ k To be noted that this estimation is based on the strict condition of M2 = 1. For the M2 1.3 laser, the actual Rayleigh length must about 319 mm. Within Rayleigh length, the beam radius keeps within root 2 of x0 and the power density is within factor of 2. If we put the SHG LBO on the waist of fundamental light, that means we have a range of about 319 mm to put the THG LBO behind the SHG LBO. If the length of SHG LBO and the THG LBO are both much shorter than the Rayleigh length, the fundamental light waist on the THG LBO would keep near x0. And the light power density can be as high as on the SHG LBO. So we do not need to refocus the light before the THG LBO. This makes the THG experiment construction (Fig. 4 shows) more simple and efficient.
Z0 ¼
1064 nm & 532 nm
Fig. 5. The focal spot on the SHG LBO crystal.
355 nm
Beam Shaping System
Nd:YVO4 Slab Laser
1064 nm
1064 nm L1
L2
L3
SHG
THG
LBO
LBO 1064 nm & 532 nm & 355 nm
Fig. 4. Schematic of experimental setup for THG.
1064 nm & 532 nm
Fig. 6. The average power and the pulse length of 532 nm light.
X. Liu et al. / Optics Communications 272 (2007) 192–196
is a sphere lens with the focus of 250 mm. They made the waists in the stable direction and in the unstable direction of the fundamental light at the same place. And focus the fundamental light on the SHG LBO crystal and the focal spot size is 4.4 · 103 cm2 as the theory calculation. The shape of the focal spot on the SHG LBO crystal is shown in Fig. 5. The average power and the pulse length of 532 nm light at different repetition rates are shown in Fig. 6. The conversion efficiency in the SHG process is about 59.2% at 10 kHz. This value is close to the theory calculation value. The THG LBO crystal had dimension of 3 mm · 3 mm · 14 mm and was cut by h = 43 and / = 90. These angles were fit for the type II phase matching (PM) of THG. Its incident surfaces was coated with AR1064 nm and 532 nm film. And its emission surface was coated with AR355 nm film. The output beam have the three harmonics. To get the pure 355 nm light, we used two mirrors with HR355 nm film to filter the lights and only keeps 355 nm THG light. Then we got 11.1 W average power 355 nm
195
ultraviolet (UV) light. At repetition rate of 10 kHz, the pulse length is 5 ns. The conversion efficiency from 532 nm to 355 nm was 75%. The average power, the pulse length and the intensity distribution in the far field of 355 nm light is shown in Figs. 7 and 8, M2 < 1.5. Fig. 9
Fig. 9. A typical pulse profile of 355 nm light.
Fig. 7. The average power and the pulse length of 355 nm light.
Fig. 8. The intensity distribution in the far field of 355 nm light.
Fig. 10. (a) is the power of 532 nm SHG light vs temperature of SHG crystal at 10 kHz repetition rate, (b) is the power of 355 nm THG light vs temperature of THG crystal at 10 kHz repetition rate.
196
X. Liu et al. / Optics Communications 272 (2007) 192–196
shows a typical pulse profile measured by a fast photodiode and a digital oscilloscope. In the experiment, we can move the THG LBO crystal at a range of about 100 mm behind the SHG LBO. In this range, The THG conversion efficiency is constant. If the distance between the two LBO crystals is larger than 250 mm, the conversion efficiency will be a little lower. The PM in SHG and THG were both sensitive to the crystal temperature. The conversion efficiency would change with the crystal temperature. This can be seen in Fig. 10. In this figure, we can easily see that the THG process are more sensitively to the crystal temperature than the SHG process. The SHG and THG temperature tuning bandwidth are 4.6 and 2.6, respectively. In the page of 1992–1993 of the paper of J.A. Armstrong [15], we can see the detail method of calculating the conversion efficiency of three coupled waves in anisotropic crystals. By this method, we can get the curves of conversion efficiency vs temperature of SHG process and SFG (sum-frequency generation) process (Note: In this experiment, this process happened in THG LBO) in theory. 4. Conclusion In this paper, we introduce a THG experiment. The partially end-pumped slab laser with hybrid resonator, which provide the fundamental light to the THG system, is introduced briefly. We also analyze the factors that effect the harmonic generation. We got 11.1 W average power of TEM00 mode 355 nm light at 10 kHz repetition rate. The conversion efficiency
reached 44.4% from 1064 nm fundamental light to 355 nm THG light, which is to our knowledge the highest conversion efficiency. The energy per pulse and pulse length of the UV light were 1.11 mJ and 5 ns, respectively. The beam quality is M2 < 1.5.
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