- Email: [email protected]
High-energy, sub-nanosecond linearly polarized passively Q-switched MOPA laser system
Optics and Laser Technology 95 (2017) 81–85
Contents lists available at ScienceDirect
Optics and Laser Technology journal homepage: www.elsevier.com/locate/jolt
Full length article
High-energy, sub-nanosecond linearly polarized passively Q-switched MOPA laser system Hee Chul Lee ⇑, Dong Wook Chang, Eun Jung Lee, Hyun Woong Yoon Lutronic Center, 219 Sowon-ro, Deogyang-gu, Goyang-si, Gyeonggi-do, South Korea
a r t i c l e
i n f o
Article history: Received 4 October 2016 Accepted 24 April 2017 Available online 5 May 2017 Keywords: Lasers Q-switched Laser amplifiers Diode-pumped
a b s t r a c t This study introduces a linearly polarized laser with a 0.6 J output energy and a 420 ps pulse width. Accordingly, [1 1 1]-cut Nd:YAG and [1 1 0]-cut Cr4+:YAG crystals are used to fabricate a linearly polarized seed laser. One side of both crystals is configured with a Brewster angle to enhance the extinction ratio of polarization. The output energy and the pulse duration of the seed laser are 2.51 mJ and 552 ps, respectively. The seed laser pulse is compressed to 420 ps using a fused-silica block, while its energy is amplified to 600 mJ. The output energy instability over a 90 min operation is ±4.7%. Ó 2017 Elsevier Ltd. All rights reserved.
1. Introduction Many researchers recently reported on passively Q-switched lasers with high peak powers at the megawatt level. High-peakpower, yet compact, laser sources are attractive for many applications, including microprocessing, remote sensing, and laser ignition. They can also be used for efficient wavelength conversion to green and UV wavelengths in various applications, such as photoionization and pulsed-laser deposition. Notably, a compact, passively Q-switched Nd:YAG laser was developed for the Mercury laser altimeter. This altimeter is an instrument to be placed on the surface of Mercury as part of the Space Environment, Geochemistry, and Ranging mission to the planet. A compact laserspark device based on a passively Q-switched Nd:YAG laser with Cr4+:YAG has recently been investigated by many research groups and appears to be close to realization. This spark device improves engine stability relative to conventional ignition systems that use spark plugs [1–8]. The [1 0 0]-cut Cr4+:YAG is generally used as a saturable absorber for a passively Q-switched laser. However, many groups reported excellent results by using [1 1 0]-cut Cr4+: YAG and matching the polarization direction to the crystallographic axis to increase the energy stability and attain a high extinction ratio of polarization. A [1 1 0]-cut Cr4+:YAG laser requires the use of a crystallographic axis on the plane perpendicular to the direction of the incident light. The output polarization mainly depends on the pump polarization.
⇑ Corresponding author. E-mail address: [email protected] (H.C. Lee). http://dx.doi.org/10.1016/j.optlastec.2017.04.024 0030-3992/Ó 2017 Elsevier Ltd. All rights reserved.
Therefore, the polarization state of a pump beam would still play a major role in attaining a highly stable polarized output beam even if a stabilized linear polarization could be achieved using [1 1 0]-cut Cr4+:YAG [3,9–12]. Achieving a linearly polarized pump beam for a multimode fiber-coupled laser diode [LD] is difficult and results in inefficiency. Hence, the insertion of a polarizer into the cavity is first considered. Unfortunately, increasing the cavity length causes the pulse width to increase. Thus, [1 1 1]-cut Nd: YAG and [1 1 0]-cut Cr4+:YAG crystals are prepared by cutting the Brewster’s angle on one side of each to achieve an improved extinction ratio of polarization. Compact, passively Q-switched microchip lasers are widely used as the master oscillator [MO] in high-average-power or high-energy systems [5,13–15]. This study presents a sub-nanosecond polarized Nd:YAG laser system in a master oscillator power amplifier [MOPA] configuration. The seed laser is compressed by a stimulated Brillouin scattering [SBS] process in a fused silica block. The laser-induced damage threshold of the antireflection coating film for the Nd:YAG used in this study is >3 GW/cm2. Therefore, the output energy is limited to 600 mJ to avoid the optical damage caused by the 2.86 GW/cm2 power density. The laser system exhibits an energy stability of less than ±5% over a 90 min operation. Sub-nanosecond laser systems with gigawatt peak powers are currently FDA-approved for the treatment of wrinkles, acne scarring, and removal of tattoos and pigmented lesions. Therefore, the laser system developed in this study could also be used in dermatology applications.
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2. Experimental setup and results 2.1. Sub-nanosecond MO A fiber-coupled LD with a 600 um core diameter, 120 W QCW, and 808 nm center wavelength was used herein as the end-pump source. The LD was operated at 10 Hz in all our experiments, while the temperature was maintained at 25 °C ± 0.5 °C. Two lens pairs were used for the collimation (not shown in Fig. 1) of the pump beam emitted from the optical fiber. The collimated pump beam was focused by an f = 50 mm convex lens producing a focused spot size of 900 um. A double-sided half-inch flat mirror was coated on one side with an 808 nm antireflection film. The other side exhibited 99.5% reflectivity at 1064 nm and 97% transmissivity at 808 nm. Many authors reported satellite pulse generation because it is related to high-pulse-energy or high-average-power passively Q-switched lasers [16–18]. The OC was set to have a free spectral range of 17.09 GHz (thickness = 6 mm; reflectivity at 1064.15 nm = 34.7% and at 1064.09 nm = 47.3%) to prevent the satellite pulse generation. The center thicknesses of the square [1 1 1]-cut Nd:YAG (1.1% doped) and [1 1 0]-cut Cr4+:YAG crystals were 5 mm and 4.7 mm, respectively. The [1 0 0] crystal axis of Cr4+:YAG was designed to be parallel to the Y-axis (Fig. 1) considering the anisotropic transmission direction of Cr4+:YAG with a high-intensity signal. The two crystals were mounted in a copper block. The flat side of the Nd:YAG was given 808 and 1064 nm antireflection coating. The Brewster angle surface of the Cr4+:YAG exhibited a 95% reflectivity at 808 nm. The flat side of the Cr4+: YAG was treated with a 1064 nm antireflection coating. The Nd: YAG and Cr4+:YAG crystals were mounted in a copper block, and their temperatures were not controlled. The initial transmission of T0 for the Cr4+:YAG changed with its thickness. The T0 value at the center of the Cr4+:YAG crystal was 25%, while the total cavity length was 12 mm. 2.2. Experimental results for MO Fig. 2 shows the measured pulse width and beam profile. The pulse width was measured using a 6 GHz oscilloscope and a single-mode, fiber-coupled 5-GHz InGaAs photodiode with rise times of 65 ps and 70 ps. The measured pulse width was 552 ps. We checked the beam quality defined by the M2 factor using CMOS beam profiler and convex lens (f = 500 mm) to record the diameter of the laser along the beam propagation direction. The results of which revealed an almost single transverse mode with a calculated M2 value of 1.17. The small circular diffraction pattern in the beam profile was caused by minor imperfections and dust on the absorption filter of the beam profiler. An output energy of 2.51 mJ was obtained with 19 mJ pumping energy and 200 ls pumping pulse width.
Fig. 2. Measured pulse width and beam profile [inset].
A cube-polarizing beam splitter (PBS, extinction ratio >1000:1) was installed in front of the OC to check the polarization state stability and the extinction ratio of the output beam polarization. An extinction ratio polarization greater than 1000:1 was obtained from the average transmission value of the P- and S-polarizations for 1 k shots of the output beam. Fig. 3 presents the measured energy over time (i.e., approximately 40 min). We obtained energy instability with a standard deviation of 1.95% at a room temperature of 25 °C without any temperature control.
2.3. Pulse compression results for MO The SBS liquid media required a fine filtration of any dissolved impurities, and must be carefully handled because of their toxicity. Meanwhile, given their high optical homogeneity, solid SBS materials (e.g., fused silica) only required polished incident surfaces. The compressed pulse width was shown simply as 2.3sB/IpgBL, where sB is the phonon lifetime of the fused silica; Ip is the input pulse intensity; gB is the SBS gain coefficient; and L is the interaction length [19]. A convex lens of f = 300 mm was used to control the beam divergence of the MO, thereby allowing us to form a 1 mm quasi-collimated beam. A 30 cm-long fused silica was employed to achieve pulse compression. The 30 cm length was not enough to achieve pulse compression because of the fused silica’s low SBS gain. Therefore, a right-angle prism was installed at the end of the fused silica block to double the propagation. The calculated value of the compressed pulse width was 421 ps. The fused silica’s phonon lifetime of 4 ns and an SBS gain of 0.91 cm/GW were used to estimate the compressed pulse width. Fig. 4 shows the measured compressed pulse width. The pulse width for 1 k shots varied from 410 ps to 470 ps with a 440 ps mean pulse width.
Fig. 1. Experimental layout of MO.
H.C. Lee et al. / Optics and Laser Technology 95 (2017) 81–85
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Fig. 5. Schematic of the MOPA system.
Fig. 3. Output energy stability performance of MO.
Fig. 6. Theoretical amplified energy from AMP2 as a function of the input energy. Fig. 4. Measured pulse width after pulse compression using the fused silica block.
2.4. Two-stage amplifier Fig. 5 illustrates the experimental setup of the two-stage amplifier. Each Nd:YAG crystal in the amplifier stages was pumped by two Xe 450 Torr flashlamps. 3X and 1.4X are means beam expansion ratio. Stage 1 of the amplifier [AMP1] consisted of a PBS, a Faraday rotator [FR], a Nd:YAG rod (Nd3+ ion dopant: 1.1%, 8 mm diameter, and 130 mm length), and an HR (High Reflector) to achieve a two-pass amplification. The energy input to AMP1 was 1.8 mJ. The main loss was caused by the Fresnel loss at the uncoated surface of the fused silica block and the iris used to cut the side beam. The Nd:YAG rod (1.1% dopant) for stage 2 of the amplifier [AMP2] had dimensions of 10 mm diameter and 130 mm length. The energy of an amplified pulse after a single pass can be calculated as follows using Eq. (1) [20]:
Eout ¼ Es ln f1 þ ½expðEin =Es Þ 1G0 g
ð1Þ
where G0 is the small signal single-pass gain, G0 = exp(g0l); l is the length of the active medium; g0 is the small signal gain coefficient; ES is the saturation fluence; and Ein is the input energy density. The input beam size for AMP2 was 8 mm, while the pumping region was 100 mm long. Our aim was to obtain a 600 mJ output energy with a sub-nanosecond pulse width. Fig. 6 shows the amplified energy as a function of the input energy to estimate the required energy. The calculated results indicated that AMP1 was required to provide 140 mJ of energy to obtain 600 mJ.
Fig. 7. Theoretical amplified energy for the single- and double-pass energy from AMP1 as a function of the input energy.
The output fluence E0 out from a two-pass amplifier can be calculated as follows using Eq. (2) [20]:
E0out ¼ Es ln 1 þ ½expðEout =Es Þ 1G00
G00 ¼ expðg00 lÞ;
g00 ¼ ð1 gE Þg0 :
ð2Þ
The extraction efficiency of gE = (Eout Ein)/g0lEs. Fig. 7 shows the amplified energy through the single- and double-pass energy from AMP1. The input beam size for AMP1
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Fig. 10. Stability performance of the amplified energy. Fig. 8. Generated 532 nm pulse energy versus 1064 nm pulse energy incident on the KTP crystal. The error bars represent the standard deviation of the mean value.
was 5.6 mm, while the pumping region was 100 mm long. Eq. (2) indicates that the single-pass amplified energy must be in excess of 23 mJ when the input energy is 1.8 mJ to obtain 140 mJ through the double-pass amplification. The output energy was measured at each AMP stage. We obtained 140 mJ from AMP1 with a 50 J electrical energy supplied to the flashlamp at a 10 Hz repetition rate. However, we could not obtain 600 mJ from AMP2 under the same operating conditions. We only obtained an output energy of 600 mJ from AMP2 as the electrical energy was increased to 55 J. After which, the output energy from AMP1 was 205 mJ. Comparing the theoretical data to the experimental data, the main reason for the low extracted energy efficiency from AMP2 was the low Nd ion-doping concentration of AMP2. Nd:YAG had an Nd ion concentration tolerance of ±10%. Another reason could be that the input beam did not have a uniform spatial beam distribution, or the gain distribution in the Nd:YAG rod was not uniform because of the non-uniform pumping. The measured pulse width was 420 ps. The energy fluence doubled at the output surface of the AMP2 rod because of the 8 mm spot size. Therefore, the peak power at the AMP2 output surface was 2.86 GW/cm2. We did not try to obtain an energy level of more than 600 mJ because the manufacturer cannot guarantee the absence of an optical damage at intensities of 3 GW/cm2 at 1064 nm. We obtained a 532 nm beam using the 15 mm 15 mm 4.5 mm KTP (h = 90°; U = 23.5°). The incident
size of the 1064 nm beam on the KTP was 11 mm. Figs. 8 and 9 show the generated 532 nm pulse energy versus the 1064 nm pulse energy and beam profiles, respectively. Considering the loss incurred by the 1064 nm cut-off filter in the green spectrum region, the conversion efficiencies for a green energy level of 235 mJ and 313 mJ were 62% and 55%, respectively. The main reason for the decreased conversion efficiency was the depolarization in the Nd:YAG crystal caused by the thermal loading. Fig. 10 presents the amplified energy stability over 90 min. The energy level gradually increased from 600 mJ to 625 mJ during the first 10 min. The exact reason for this energy increase was not determined, but appeared related to the temperature change inside the laser system. The initial cooling water temperature was 26 °C, which gradually increased to 35 °C over 10 min. After which, the temperature remained at 35 °C ± 0.8 °C. The energy stability for the overall measured time was ±4.7%. 3. Conclusion This study reported on a sub-nanosecond polarized Nd:YAG laser system in a MOPA configuration. The MO consisted of two crystals: [1 1 1]-cut Nd:YAG and [1 1 0]-cut Cr4+:YAG. Both had a Brewster angle cut surface to attain an enhanced extinction ratio of polarization. The seed-laser pulse from the MO was compressed by a 30 cm fused silica block, such that the 552 ps pulse width could be reduced to 421 ps by an SBS process. The 1.8 mJ seed energy from the MO was amplified to 600 mJ using a two-stage
Fig. 9. 532 nm beam profile of (a) 235 mJ and (b) 313 mJ.
H.C. Lee et al. / Optics and Laser Technology 95 (2017) 81–85
amplifier. The measured pulse widths of the maximum, minimum, and mean values were 530 ps, 360 ps, and 420 ps, respectively. Meanwhile, the energy instability at the maximum energy of 600 mJ was 9.32% for a 90 min operation time. Conventional megawatt nanosecond laser systems used in dermatology are gradually being replaced with gigawatt subnanosecond laser systems. Therefore, the laser system developed in this study could be applied to the field of dermatology.
References [1] H. Sakai, H. Kan, T. Taira, >1 MW peak power single-mode high-brightness passively Q-switched Nd3+:YAG microchip laser, Opt. Express 16 (2008) 19891–19899. [2] R. Bhandari, T. Taira, >6 MW peak power at 532 nm from passively Q-switched Nd:YAG/Cr4+:YAG microchip laser, Opt. Express 19 (2011) 19135–19141. [3] H. Sakai, A. Sone, H. Kan, T. Taira, Polarization stabilizing for diode-pumped passively Q-switched Nd:YAG microchip lasers, in: Advanced Solid-State Photonics, Technical Digest, Optical Society of America, 2006 (paper MD2). [4] C.Y. Cho, H.P. Cheng, Y.C. Chang, C.Y. Tang, Y.F. Chen, An energy adjustable linearly polarized passively Q-switched bulk laser with a wedged diffusion bonded Nd:YAG/Cr4+:YAG crystal, Opt. Express 23 (2015) 8162–8169. [5] D.J. Krebs, A.-M. Novo-Gradac, S.X. Li, S.J. Lindauer, R.S. Afzal, A.W. Yu, Compact, passively Q-switched Nd:YAG laser for the MESSENGER mission to Mercury, Appl. Opt. 44 (2005) 1715–1718. [6] M. Tsunekane, T. Inohara, A. Ando, N. Kido, K. Kanehara, T. Taira, High peak power, passively Q-switched microlaser for ignition of engines, IEEE J. Quantum Electron. 46 (2010) 277–284. [7] N. Pavel, M. Tsunekane, T. Taira, Composite, all-ceramics, high-peak power Nd: YAG/Cr4+:YAG monolithic micro-laser with multiple-beam output for engine ignition, Opt. Express 19 (2011) 9378–9384.
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[8] N. Pavel, T. Dascalu, G. Salamu, M. Dinca, N. Boicea, A. Birtas, Ignition of an automobile engine by high-peak power Nd:YAG/Cr4+:YAG laser-spark devices, Opt. Express 23 (2015) 33028–33037. [9] N.N. Il’ichev, A.V. Kir’yanov, P.P. Pashinin, S.M. Shpuga, Investigation of nonlinear-absorption anisotropy in YAG:Cr4+, JETP 78 (1994) 768–776. [10] A.V. Kir’yanov, V. Aboites, N.N. Il’ichev, A polarization-bistable neodymium laser with a Cr4+:YAG passive switch the weak resonant signal control, Opt. Commun. 169 (1999) 309–316. [11] Z. Sun, Q. Cheng, Y. Hui, M. Jiang, H. Lei, Q. Li, Enhancing extinction ratio of polarization and pulse stability simultaneously from passively Q-switched [100]-Nd:YAG/[1 1 0]-Cr4+:YAG laser, Opt. Commun. 335 (2015) 245–249. [12] Z. Sun, Q. Li, Y.L. Su, H. Lei, M.H. Jiang, Y.L. Hu, Controllable polarization for passively Q-switched Nd:YAG/Cr+4:YAG laser, Opt. Laser Technol. 56 (2014) 269–272. [13] H. Sakai, A. Sone, H. Kan, T. Taira, High brightness diode-pumped passively Qswitched Nd:YAG microchip laser with amplifier, in: Advanced Solid-State Photonics OSA Technical Digest Series (CD), Optical Society of America, 2008 (paper WE38). [14] P. Peuser, W. Platz, G. Holl, Miniaturized, high-power diode-pumped, Qswitched Nd:YAG laser oscillator–amplifier, Appl. Opt. 50 (2011) 399–404. [15] D. Chuchumishev, A. Gaydardzhiev, A. Trifonov, I. Buchvarov, Single-frequency MOPA system with near-diffraction-limited beam quality, Quantum Electron. 6 (2012) 528–530. [16] O.P. Kimmelma, I. Tittonen, S.C. Buchter, Short pulse, diode pumped, passively Q-switched Nd:YAG laser at 946 nm quadrupled for UV production, J. Eur. Opt. Soc. Rapid. Publ. 3 (2008) 1–5. [17] M. Wei, C. Cheng, S. Wu, Instability and satellite pulse of passively Q-switching Nd:LuVO4 laser with Cr4+:YAG saturable absorber, Opt. Commun. 281 (2008) 3527–3531. [18] J. Dong, K. Ueda, P. Yang, Multi-pulse oscillation and instabilities in microchip self-Q-switched transverse-mode laser, Opt. Express 17 (2009) 16986–16993. [19] H. Yoshida, H. Fujita, M. Nakatsuka, A. Fujinoki, Temporal compression by stimulated Brillouin scattering of Q-switched pulse with fused quartz glass, Jpn. J. Appl. Phys. 43 (2004) 1103–1105. [20] W. Koechner, Solid State Laser Engineering, 6th ed., Springer-Verlag, Berlin, 2006 (Chapter 4).
Contents lists available at ScienceDirect
Optics and Laser Technology journal homepage: www.elsevier.com/locate/jolt
Full length article
High-energy, sub-nanosecond linearly polarized passively Q-switched MOPA laser system Hee Chul Lee ⇑, Dong Wook Chang, Eun Jung Lee, Hyun Woong Yoon Lutronic Center, 219 Sowon-ro, Deogyang-gu, Goyang-si, Gyeonggi-do, South Korea
a r t i c l e
i n f o
Article history: Received 4 October 2016 Accepted 24 April 2017 Available online 5 May 2017 Keywords: Lasers Q-switched Laser amplifiers Diode-pumped
a b s t r a c t This study introduces a linearly polarized laser with a 0.6 J output energy and a 420 ps pulse width. Accordingly, [1 1 1]-cut Nd:YAG and [1 1 0]-cut Cr4+:YAG crystals are used to fabricate a linearly polarized seed laser. One side of both crystals is configured with a Brewster angle to enhance the extinction ratio of polarization. The output energy and the pulse duration of the seed laser are 2.51 mJ and 552 ps, respectively. The seed laser pulse is compressed to 420 ps using a fused-silica block, while its energy is amplified to 600 mJ. The output energy instability over a 90 min operation is ±4.7%. Ó 2017 Elsevier Ltd. All rights reserved.
1. Introduction Many researchers recently reported on passively Q-switched lasers with high peak powers at the megawatt level. High-peakpower, yet compact, laser sources are attractive for many applications, including microprocessing, remote sensing, and laser ignition. They can also be used for efficient wavelength conversion to green and UV wavelengths in various applications, such as photoionization and pulsed-laser deposition. Notably, a compact, passively Q-switched Nd:YAG laser was developed for the Mercury laser altimeter. This altimeter is an instrument to be placed on the surface of Mercury as part of the Space Environment, Geochemistry, and Ranging mission to the planet. A compact laserspark device based on a passively Q-switched Nd:YAG laser with Cr4+:YAG has recently been investigated by many research groups and appears to be close to realization. This spark device improves engine stability relative to conventional ignition systems that use spark plugs [1–8]. The [1 0 0]-cut Cr4+:YAG is generally used as a saturable absorber for a passively Q-switched laser. However, many groups reported excellent results by using [1 1 0]-cut Cr4+: YAG and matching the polarization direction to the crystallographic axis to increase the energy stability and attain a high extinction ratio of polarization. A [1 1 0]-cut Cr4+:YAG laser requires the use of a crystallographic axis on the plane perpendicular to the direction of the incident light. The output polarization mainly depends on the pump polarization.
⇑ Corresponding author. E-mail address: [email protected] (H.C. Lee). http://dx.doi.org/10.1016/j.optlastec.2017.04.024 0030-3992/Ó 2017 Elsevier Ltd. All rights reserved.
Therefore, the polarization state of a pump beam would still play a major role in attaining a highly stable polarized output beam even if a stabilized linear polarization could be achieved using [1 1 0]-cut Cr4+:YAG [3,9–12]. Achieving a linearly polarized pump beam for a multimode fiber-coupled laser diode [LD] is difficult and results in inefficiency. Hence, the insertion of a polarizer into the cavity is first considered. Unfortunately, increasing the cavity length causes the pulse width to increase. Thus, [1 1 1]-cut Nd: YAG and [1 1 0]-cut Cr4+:YAG crystals are prepared by cutting the Brewster’s angle on one side of each to achieve an improved extinction ratio of polarization. Compact, passively Q-switched microchip lasers are widely used as the master oscillator [MO] in high-average-power or high-energy systems [5,13–15]. This study presents a sub-nanosecond polarized Nd:YAG laser system in a master oscillator power amplifier [MOPA] configuration. The seed laser is compressed by a stimulated Brillouin scattering [SBS] process in a fused silica block. The laser-induced damage threshold of the antireflection coating film for the Nd:YAG used in this study is >3 GW/cm2. Therefore, the output energy is limited to 600 mJ to avoid the optical damage caused by the 2.86 GW/cm2 power density. The laser system exhibits an energy stability of less than ±5% over a 90 min operation. Sub-nanosecond laser systems with gigawatt peak powers are currently FDA-approved for the treatment of wrinkles, acne scarring, and removal of tattoos and pigmented lesions. Therefore, the laser system developed in this study could also be used in dermatology applications.
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2. Experimental setup and results 2.1. Sub-nanosecond MO A fiber-coupled LD with a 600 um core diameter, 120 W QCW, and 808 nm center wavelength was used herein as the end-pump source. The LD was operated at 10 Hz in all our experiments, while the temperature was maintained at 25 °C ± 0.5 °C. Two lens pairs were used for the collimation (not shown in Fig. 1) of the pump beam emitted from the optical fiber. The collimated pump beam was focused by an f = 50 mm convex lens producing a focused spot size of 900 um. A double-sided half-inch flat mirror was coated on one side with an 808 nm antireflection film. The other side exhibited 99.5% reflectivity at 1064 nm and 97% transmissivity at 808 nm. Many authors reported satellite pulse generation because it is related to high-pulse-energy or high-average-power passively Q-switched lasers [16–18]. The OC was set to have a free spectral range of 17.09 GHz (thickness = 6 mm; reflectivity at 1064.15 nm = 34.7% and at 1064.09 nm = 47.3%) to prevent the satellite pulse generation. The center thicknesses of the square [1 1 1]-cut Nd:YAG (1.1% doped) and [1 1 0]-cut Cr4+:YAG crystals were 5 mm and 4.7 mm, respectively. The [1 0 0] crystal axis of Cr4+:YAG was designed to be parallel to the Y-axis (Fig. 1) considering the anisotropic transmission direction of Cr4+:YAG with a high-intensity signal. The two crystals were mounted in a copper block. The flat side of the Nd:YAG was given 808 and 1064 nm antireflection coating. The Brewster angle surface of the Cr4+:YAG exhibited a 95% reflectivity at 808 nm. The flat side of the Cr4+: YAG was treated with a 1064 nm antireflection coating. The Nd: YAG and Cr4+:YAG crystals were mounted in a copper block, and their temperatures were not controlled. The initial transmission of T0 for the Cr4+:YAG changed with its thickness. The T0 value at the center of the Cr4+:YAG crystal was 25%, while the total cavity length was 12 mm. 2.2. Experimental results for MO Fig. 2 shows the measured pulse width and beam profile. The pulse width was measured using a 6 GHz oscilloscope and a single-mode, fiber-coupled 5-GHz InGaAs photodiode with rise times of 65 ps and 70 ps. The measured pulse width was 552 ps. We checked the beam quality defined by the M2 factor using CMOS beam profiler and convex lens (f = 500 mm) to record the diameter of the laser along the beam propagation direction. The results of which revealed an almost single transverse mode with a calculated M2 value of 1.17. The small circular diffraction pattern in the beam profile was caused by minor imperfections and dust on the absorption filter of the beam profiler. An output energy of 2.51 mJ was obtained with 19 mJ pumping energy and 200 ls pumping pulse width.
Fig. 2. Measured pulse width and beam profile [inset].
A cube-polarizing beam splitter (PBS, extinction ratio >1000:1) was installed in front of the OC to check the polarization state stability and the extinction ratio of the output beam polarization. An extinction ratio polarization greater than 1000:1 was obtained from the average transmission value of the P- and S-polarizations for 1 k shots of the output beam. Fig. 3 presents the measured energy over time (i.e., approximately 40 min). We obtained energy instability with a standard deviation of 1.95% at a room temperature of 25 °C without any temperature control.
2.3. Pulse compression results for MO The SBS liquid media required a fine filtration of any dissolved impurities, and must be carefully handled because of their toxicity. Meanwhile, given their high optical homogeneity, solid SBS materials (e.g., fused silica) only required polished incident surfaces. The compressed pulse width was shown simply as 2.3sB/IpgBL, where sB is the phonon lifetime of the fused silica; Ip is the input pulse intensity; gB is the SBS gain coefficient; and L is the interaction length [19]. A convex lens of f = 300 mm was used to control the beam divergence of the MO, thereby allowing us to form a 1 mm quasi-collimated beam. A 30 cm-long fused silica was employed to achieve pulse compression. The 30 cm length was not enough to achieve pulse compression because of the fused silica’s low SBS gain. Therefore, a right-angle prism was installed at the end of the fused silica block to double the propagation. The calculated value of the compressed pulse width was 421 ps. The fused silica’s phonon lifetime of 4 ns and an SBS gain of 0.91 cm/GW were used to estimate the compressed pulse width. Fig. 4 shows the measured compressed pulse width. The pulse width for 1 k shots varied from 410 ps to 470 ps with a 440 ps mean pulse width.
Fig. 1. Experimental layout of MO.
H.C. Lee et al. / Optics and Laser Technology 95 (2017) 81–85
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Fig. 5. Schematic of the MOPA system.
Fig. 3. Output energy stability performance of MO.
Fig. 6. Theoretical amplified energy from AMP2 as a function of the input energy. Fig. 4. Measured pulse width after pulse compression using the fused silica block.
2.4. Two-stage amplifier Fig. 5 illustrates the experimental setup of the two-stage amplifier. Each Nd:YAG crystal in the amplifier stages was pumped by two Xe 450 Torr flashlamps. 3X and 1.4X are means beam expansion ratio. Stage 1 of the amplifier [AMP1] consisted of a PBS, a Faraday rotator [FR], a Nd:YAG rod (Nd3+ ion dopant: 1.1%, 8 mm diameter, and 130 mm length), and an HR (High Reflector) to achieve a two-pass amplification. The energy input to AMP1 was 1.8 mJ. The main loss was caused by the Fresnel loss at the uncoated surface of the fused silica block and the iris used to cut the side beam. The Nd:YAG rod (1.1% dopant) for stage 2 of the amplifier [AMP2] had dimensions of 10 mm diameter and 130 mm length. The energy of an amplified pulse after a single pass can be calculated as follows using Eq. (1) [20]:
Eout ¼ Es ln f1 þ ½expðEin =Es Þ 1G0 g
ð1Þ
where G0 is the small signal single-pass gain, G0 = exp(g0l); l is the length of the active medium; g0 is the small signal gain coefficient; ES is the saturation fluence; and Ein is the input energy density. The input beam size for AMP2 was 8 mm, while the pumping region was 100 mm long. Our aim was to obtain a 600 mJ output energy with a sub-nanosecond pulse width. Fig. 6 shows the amplified energy as a function of the input energy to estimate the required energy. The calculated results indicated that AMP1 was required to provide 140 mJ of energy to obtain 600 mJ.
Fig. 7. Theoretical amplified energy for the single- and double-pass energy from AMP1 as a function of the input energy.
The output fluence E0 out from a two-pass amplifier can be calculated as follows using Eq. (2) [20]:
E0out ¼ Es ln 1 þ ½expðEout =Es Þ 1G00
G00 ¼ expðg00 lÞ;
g00 ¼ ð1 gE Þg0 :
ð2Þ
The extraction efficiency of gE = (Eout Ein)/g0lEs. Fig. 7 shows the amplified energy through the single- and double-pass energy from AMP1. The input beam size for AMP1
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Fig. 10. Stability performance of the amplified energy. Fig. 8. Generated 532 nm pulse energy versus 1064 nm pulse energy incident on the KTP crystal. The error bars represent the standard deviation of the mean value.
was 5.6 mm, while the pumping region was 100 mm long. Eq. (2) indicates that the single-pass amplified energy must be in excess of 23 mJ when the input energy is 1.8 mJ to obtain 140 mJ through the double-pass amplification. The output energy was measured at each AMP stage. We obtained 140 mJ from AMP1 with a 50 J electrical energy supplied to the flashlamp at a 10 Hz repetition rate. However, we could not obtain 600 mJ from AMP2 under the same operating conditions. We only obtained an output energy of 600 mJ from AMP2 as the electrical energy was increased to 55 J. After which, the output energy from AMP1 was 205 mJ. Comparing the theoretical data to the experimental data, the main reason for the low extracted energy efficiency from AMP2 was the low Nd ion-doping concentration of AMP2. Nd:YAG had an Nd ion concentration tolerance of ±10%. Another reason could be that the input beam did not have a uniform spatial beam distribution, or the gain distribution in the Nd:YAG rod was not uniform because of the non-uniform pumping. The measured pulse width was 420 ps. The energy fluence doubled at the output surface of the AMP2 rod because of the 8 mm spot size. Therefore, the peak power at the AMP2 output surface was 2.86 GW/cm2. We did not try to obtain an energy level of more than 600 mJ because the manufacturer cannot guarantee the absence of an optical damage at intensities of 3 GW/cm2 at 1064 nm. We obtained a 532 nm beam using the 15 mm 15 mm 4.5 mm KTP (h = 90°; U = 23.5°). The incident
size of the 1064 nm beam on the KTP was 11 mm. Figs. 8 and 9 show the generated 532 nm pulse energy versus the 1064 nm pulse energy and beam profiles, respectively. Considering the loss incurred by the 1064 nm cut-off filter in the green spectrum region, the conversion efficiencies for a green energy level of 235 mJ and 313 mJ were 62% and 55%, respectively. The main reason for the decreased conversion efficiency was the depolarization in the Nd:YAG crystal caused by the thermal loading. Fig. 10 presents the amplified energy stability over 90 min. The energy level gradually increased from 600 mJ to 625 mJ during the first 10 min. The exact reason for this energy increase was not determined, but appeared related to the temperature change inside the laser system. The initial cooling water temperature was 26 °C, which gradually increased to 35 °C over 10 min. After which, the temperature remained at 35 °C ± 0.8 °C. The energy stability for the overall measured time was ±4.7%. 3. Conclusion This study reported on a sub-nanosecond polarized Nd:YAG laser system in a MOPA configuration. The MO consisted of two crystals: [1 1 1]-cut Nd:YAG and [1 1 0]-cut Cr4+:YAG. Both had a Brewster angle cut surface to attain an enhanced extinction ratio of polarization. The seed-laser pulse from the MO was compressed by a 30 cm fused silica block, such that the 552 ps pulse width could be reduced to 421 ps by an SBS process. The 1.8 mJ seed energy from the MO was amplified to 600 mJ using a two-stage
Fig. 9. 532 nm beam profile of (a) 235 mJ and (b) 313 mJ.
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amplifier. The measured pulse widths of the maximum, minimum, and mean values were 530 ps, 360 ps, and 420 ps, respectively. Meanwhile, the energy instability at the maximum energy of 600 mJ was 9.32% for a 90 min operation time. Conventional megawatt nanosecond laser systems used in dermatology are gradually being replaced with gigawatt subnanosecond laser systems. Therefore, the laser system developed in this study could be applied to the field of dermatology.
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