- Email: [email protected]
High efficiency and high energy parametric wavelength conversion using a large aperture periodically poled MgO:LiNbO3
Available online at www.sciencedirect.com
Optics Communications 281 (2008) 3902–3905 www.elsevier.com/locate/optcom
High efficiency and high energy parametric wavelength conversion using a large aperture periodically poled MgO:LiNbO3 Ravi Bhushan a,*, Hidetsugu Yoshida a, Koji Tsubakimoto a, Hisanori Fujita a, Masahiro Nakatsuka a, Nobuaki Miyanaga a, Yasukazu Izawa a, Hideki Ishizuki b, Takunori Taira b b
a Institute of Laser Engineering, Osaka University, 2-6 Yamada-oka, Suita, Osaka 565-0871, Japan Laser Research Center for Molecular Science, Institute for Molecular Science, 38 Nishigonaka, Myodaiji, Okazaki 444-8585, Japan
Received 27 November 2007; received in revised form 15 February 2008; accepted 20 March 2008
Abstract We have demonstrated an efficient high energy 2 lm laser generation with a 36 mm long large aperture 5 mol% MgO-doped periodically poled LiNbO3 (PPMgLN) nonlinear optical crystal. A high power Q-switched Nd:YAG laser (1.064 lm) was used to pump the quasi-phase matched (QPM) optical parametric oscillator (OPO). A total output energy of 186 mJ with 58% slope efficiency was obtained in two separate beams at 2 lm. Ó 2008 Elsevier B.V. All rights reserved. PACS: 42.65.Yj; 42.65.Lm
1. Introduction Lasers emitting in the 2 lm spectral region have many important applications in the field of medicine, spectroscopy, remote sensing, nonlinear optics, and many scientific applications. For the realization of high power robust 2 lm laser source, there are two principal techniques, a Tm or Ho based laser [1] and a 1lm laser with an OPO for wavelength conversion [2]. Tm or Ho doped YAG or YLF solid state laser requires cryogenic cooling and complex pumping schemes, while Tm:Ho doped silica fiber laser is comparatively less complex and a high power output with good beam quality in cw operation is realized [3]. High power pulse operation is difficult because of its small core size. The later approach has advantages because of its simple technology but its effectiveness depends on the overall conversion efficiency of the OPO system.
*
Corresponding author. Tel.: +81 6 6879 8761; fax: +81 6 6877 0900. E-mail address: [email protected] (R. Bhushan).
0030-4018/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2008.03.035
Ferroelectric materials like KTiOPO4 (KTP) and LiNbO3 (LN) are attractive nonlinear optical materials for efficient wavelength conversion such as second harmonics generation (SHG), difference frequency generation (DFG), and OPO. Periodically-poled ferroelectric materials are advantageous for nonlinear optical wavelength conversion by quasi-phase matching (QPM) over birefringent phase matching technique. QPM utilizes a higher nonlinear optical coefficient in type–I configuration, circumvents the problem of walk-off and provides higher optical conversion efficiency. Periodically-poled LiNbO3 (PPLN) crystal has been commonly used for quasi-phase matching because of its large nonlinear optical coefficient (d33 = 26 pm/V at 1064 nm) high optical quality and a wide transparency from UV to mid-IR region (0.32–5 lm) [4]. However its high coercive force (21 KV/mm at room temperature) and sensitivity to photorefractive effect limit its use for high power applications. A 5 mol% MgO doped periodically-poled LN (PPMgLN) has high resistance to the photorefractive effect, low IR absorption and low coercive force
R. Bhushan et al. / Optics Communications 281 (2008) 3902–3905
(4.5 KV/mm at room temperature) [5]. These characteristics collectively make PPMgLN a suitable material for the high power applications. In recent years, several efforts have been made for the fabrication of large aperture 5 mol% doped PPMgLN crystal. In previous papers, fabrication and poling characteristics of 3 mm and 5 mm thick 5 mol% doped PPMgLN crystals has been reported [5,6]. A total output energy of 22 mJ in 2 lm region was achieved by a quasi-phase matched OPO using PPMgLN crystal of 30 mm long and 3 3 mm2 poled volume and 32.1 lm grating period [7]. A high energy QPM optical parametric oscillation of 77 mJ near degeneracy and 52 mJ at degeneracy was demonstrated using a large aperture PPMgLN of 36 mm long and 5 5 mm2 QPM pattern with 32.1 and 32.3 lm grating period, respectively [8,9]. This is, to the best of our knowledge, the largest poled aperture available in a single PPMgLN crystal. A 5 mm wide and 36 mm long aluminum electrode was used for poling [8]. Large poled aperture in the single QPM crystal may be realized by increasing the width of the electrode. In this paper, we report the experimental results of an efficient high energy and compact quasi-phase matched OPO based on a large aperture 5 mol% MgO doped periodically poled LiNbO3, which has two 5 5 mm2 poled apertures with 32.1 lm grating period. PPMgLN crystal with two poled aperture was used to scale the OPO output power. The main aim of this work was to realize a high energy and high average power nanosecond laser source in 2 lm region. We obtained a total of 186 mJ of output pulse energy in two separate beams at 2 lm with 58% of slope efficiency. For efficient quasi-phase matched OPO operation, the output wavelengths were decided by the energy conservation and phase matching conditions [10], 1 1 1 ¼ þ kp ks ki np ðkp ; T Þ ns ðks ; T Þ ni ðki ; T Þ 1 Dk ¼ 2p kp ks ki KðT Þ
3903
Where Dk is the phase mismatching and must be zero for efficient OPO operation, kp, ks and ki are respectively pump, signal and idler wavelengths, nj(kj, T) (j = p, s, i) are the temperature dependent refractive index of the crystal for pump, signal and idler waves respectively, and K(T) is temperature dependent grating period of the crystal. The output wavelength of the OPO can be tuned by varying either the QPM crystal temperature or the QPM grating period. 2. Experiments and discussion We adopt a linear cavity configuration for high power QPM OPO, as shown in Fig. 1. A high power Q-switched Nd:YAG laser is used to pump the QPM OPO, which generates a single longitudinal and TEM00 mode. The pump source can produce a maximum output energy of 5 J at 1.064 lm with 10 ns pulses at a repetition rate of 10 Hz. To utilize the maximum advantage of the poled apertures and for uniform pumping, two 5 5 mm2 hard apertures separated by 2 mm (horizontally) was placed at the exit of the pump laser and the beam images at the aperture plane were projected inside the QPM crystal by an image relaying optics. The imaging optics constitutes of two identical imaging lenes, L1 and L2 (f = 1000 mm). The polarization of the pump beam was set parallel to the z-axis of the crystal for collinear interaction. A z-cut 36 mm long PPMgLN with two 5 5 mm2 QPM patterns was used for high energy OPO experiment to generate two beams at 2 lm. Both the input and output surfaces of the PPMgLN crystal were antireflection coated for the pump (1.064 lm) and oscillation wavelengths (1.9–2.2 lm). The PPMgLN crystal was housed in a temperature controlled oven with temperature stability of ±0.5 °C. The standard linear OPO cavity constitutes of two flat mirrors M1 and M2, were separated by 75 mm with a single pass pump– beam configuration. The QPM crystal was placed between the mirrors and kept at 80 °C during the experiment. The input mirror, M1 had more than 98% transmission at 1.064 lm and more than 99% reflectivity at 1.9–2.2 lm.
ð1Þ ð2Þ
OPO cavity L1
Oven
L2
Q-Switched Nd:YAG Laser 10 ns, 10 Hz
M3 1.064μm
Aperture
HWP
M1
M2 PPMgLN QPM crystal 2μm M4 Power meter
Fig. 1. Experimental set-up for a high energy wavelength converter based on a large aperture PPMgLN. L1, L2: imaging lenses, HWP: half wave plate, M1: input coupler, M2: output coupler, M3, M4: highly reflective mirrors to separate OPO output.
3904
R. Bhushan et al. / Optics Communications 281 (2008) 3902–3905
We tested three types of uncoated substrates for suitability as an output coupler, namely BK-7, fused silica glass and Lithium Niobate (LN) wafer, and the total reflectivity of these uncoated substrates for oscillation wavelength were approximately 8, 6 and 14%, respectively. Fig. 2 shows the total output energy from the quasi-phase matched PPMgLN OPO as a function of the input energy at 80 °C crystal temperature. With BK-7 output coupler, a total output energy of 186 mJ (two beams) at 2 lm with slope efficiency of 58% was achieved for a Nd:YAG laser pump energy of 430 mJ (two beams) per pulse, which corresponds to an average pump irradiance of 86 MW/cm2, well below the surface damage threshold (about 150 MW/cm2) of the QPM crystal. A highly sensitive CCD camera (Electrophysics Micronviewer 7290) was used to record the OPO output intensity distributions, shown in Fig. 2. For reliable measurement, a set of edge filters (transmittance range from 1.3–2.5 lm) was used. Almost two square beam patterns (about 5 5 mm2) separated by 2 mm (approx.) were observed at the output plane. Under similar operating conditions, a total of 179 mJ of output with 53% of slope efficiency was observed with fused silica glass substrate. While with LN wafer, 159 mJ of output energy was measured with 60% of slope efficiency at 315 mJ of pump energy. Unfortunately, LN wafer was damaged at 63 MW/cm2, while no damage was observed on the uncoated BK-7 and fused silica glass substrates upto 90 MW/cm2 of pump irradiance. The optical conversion efficiency of the PPMgLN OPO system was measured as 43, 42 and 51% with an uncoated BK-7, fused silica glass, and LN sub-
strates, respectively. Higher conversion efficiency with uncoated BK-7 and fused silica glass substrates can be achieved at high pump irradiance. Considering low oscillation threshold, output stability and high power operation, the performance of an uncoated BK-7 and fused silca glass substrate as an output coupler was almost same. Under low pump irradiance LN wafer performs best as an output coupler. Though, the OPO was configured for infrared output ( 2 lm), visible (green and red) lights were also observed, as the SFG (sum frequency generation) between the pump and the OPO output wavelengths and DFG between the SHG of pump and the OPO output wavelengths. Generation of low SHG of the pump beam makes DFG weaker than SFG. Two highly reflective mirrors (R > 99% at 1.9– 2.2 lm), M3 and M4 were installed to separate the 2 lm OPO output. Fig. 3 shows, the OPO output spectrum as the SFG between the pump and the OPO output wavelengths at 80 °C crystal temperature. The spectrum was recorded from the direct output (before mirror, M3) with a grating monochromator (C5094, Hamamatsu Photonics, Japan). The resolution of the monochromator was 0.40 nm for a 20 lm slit-width setting. We observed 698 (signal) and 724 nm (idler) as the SFG between the pump and the OPO output wavelengths, respectively shown in Fig. 3. Consequently, the signal and idler waves had a center wavelength at 2.03 lm and 2.26 lm at 80 °C, respectively. The observed signal beam was more than 3 times intense than the corresponding idler. This was because of the absorption of the output coupler near 2.2 lm. The OPO output was
225 BK-7 (R~8%) Fused silica glass (R~6%)
200
LN wafer (R~14%)
175
Output energy mJ
150
Signal + Idler Slope : 53%
Signal + Idler Slope : 60%
125 100
Signal + Idler Slope : 58%
75
5 mm
50
5 mm
25
2 mm
0 0
100
200
300
400
500
Pump energy mJ Fig. 2. Performance of PPMgLN OPO pumped by Nd:YAG laser with uncoated BK-7, fused silica glass and LN wafer as an output coupler operated at 80 °C. Inset, intensity distribution of the quasi-phase matched PPMgLN OPO output.
R. Bhushan et al. / Optics Communications 281 (2008) 3902–3905
3905
OPO Wavelength nm 1950
Intensity (a.u.)
120
2025
2100
2175
2250
2325
2400 12 0
105
10 5
90
90
75
75
Δλ ~ 4nm
60
60
45
45
30
30
Δλ ~ 5nm
15
15
0
0 690
700
710
720
730
SFG Wavelength nm Fig. 3. Output spectrum (signal at 2.03 lm, and idler at 2.27 lm) of 1.064 lm pumped PPMgLN OPO at 80 °C QPM crystal temperature.
considered as total output of signal and idler. OPO output wavelengths derived from the observed SFG wavelengths do not satisfy the energy conservation relation (1). When signal wavelength is 2.03 lm, the idler wavelength should be 2.24 lm, as given by Eq. (1). On the otherhand, idler wavelength of 2.26 lm is derived from the observed SFG wavelength of 724 nm. This discrepancy may be come from the absorption of the output coupler which decreases towards the shorter part of the idler spectrum. The OPO output wavelength could be tuned from 1.94 to 2.36 lm by varying the crystal temperature from 65 to 90 °C, and the degeneracy was observed at 2.13 lm at 90 °C which is in agreement with the theoretical predictions based on temperature-dependent Sellmeier equation [11]. 3. Conclusion We have demonstrated an efficient high energy parametric down conversion of a nanosecond pulse of Q-switched Nd:YAG laser based on a large aperture 36 mm long PPMgLN OPO. A total output energy of 186 mJ at 2 lm with a conversion efficiency of 43% was achieved with good beam quality. Experimental results show that an image relaying optics used for uniform pumping enhances the OPO system performance, and a high energy two beams 2 lm laser source was realized at 86 MW/cm2 of pump power irradiance, which is well below the damage
threshold of the PPMgLN crystal. Further, the number of output beams can be increased by increasing the QPM pattern in the crystal, consequently increases the stocking power of the system and these beams can be combined into a single beam to generate a high power 2 lm laser radiation. This system can also be used as a pump source for the development of mid-infrared laser source based on optical parametric oscillators and optical parametric amplifier. References [1] P.A. Budni, LA. Pomeranz, M.L. Lemons, C.A. Miller, J.R. Mosto, E.P. Chicklis, J. Opt. Soc. Am. B. 17 (2000) 723. [2] J.A. Giordmaine, R.C. Miller, Phys. Rev. Lett. 14 (1965) 973. [3] S.D. Jackson, A. Sabella, A. Hemming, S. Bennetts, D.G. Lancaster, Opt. Lett. 32 (2007) 241. [4] I. Shoji, T. Kondo, A. Kitamoto, M. Shirane, R. Ito, J. Opt. Soc. Am. B 14 (1997) 2268. [5] H. Ishizuki, I. Shoji, T. Taira, Appl. Phys. Lett. 82 (2003) 4062. [6] H. Ishizuki, T. Taira, Baltimore 2005 CFC7, Dig. Tech. Papers, CLEO 2005. [7] H. Ishizuki, I. Shoji, T. Taira, Opt. Lett. 29 (2004) 2527. [8] H. Ishizuki, T. Taira, Opt. Lett. 30 (2005) 2918. [9] J. Saikawa, M. Fujii, H. Ishizuki, T. Taira, Opt. Lett. 31 (2006) 3149. [10] L.E. Myers, R.C. Eckardt, M.M. Fejer, R.L. Byer, W.R. Bosenberg, J.W. Pierce, J. Opt. Soc. Am. B 12 (1995) 2102. [11] Y. Hirano, S. Yamamoto, H. Taniguchi, Baltimore 2001 CFH2, Dig. Tech. Papers, CLEO 2001.
Optics Communications 281 (2008) 3902–3905 www.elsevier.com/locate/optcom
High efficiency and high energy parametric wavelength conversion using a large aperture periodically poled MgO:LiNbO3 Ravi Bhushan a,*, Hidetsugu Yoshida a, Koji Tsubakimoto a, Hisanori Fujita a, Masahiro Nakatsuka a, Nobuaki Miyanaga a, Yasukazu Izawa a, Hideki Ishizuki b, Takunori Taira b b
a Institute of Laser Engineering, Osaka University, 2-6 Yamada-oka, Suita, Osaka 565-0871, Japan Laser Research Center for Molecular Science, Institute for Molecular Science, 38 Nishigonaka, Myodaiji, Okazaki 444-8585, Japan
Received 27 November 2007; received in revised form 15 February 2008; accepted 20 March 2008
Abstract We have demonstrated an efficient high energy 2 lm laser generation with a 36 mm long large aperture 5 mol% MgO-doped periodically poled LiNbO3 (PPMgLN) nonlinear optical crystal. A high power Q-switched Nd:YAG laser (1.064 lm) was used to pump the quasi-phase matched (QPM) optical parametric oscillator (OPO). A total output energy of 186 mJ with 58% slope efficiency was obtained in two separate beams at 2 lm. Ó 2008 Elsevier B.V. All rights reserved. PACS: 42.65.Yj; 42.65.Lm
1. Introduction Lasers emitting in the 2 lm spectral region have many important applications in the field of medicine, spectroscopy, remote sensing, nonlinear optics, and many scientific applications. For the realization of high power robust 2 lm laser source, there are two principal techniques, a Tm or Ho based laser [1] and a 1lm laser with an OPO for wavelength conversion [2]. Tm or Ho doped YAG or YLF solid state laser requires cryogenic cooling and complex pumping schemes, while Tm:Ho doped silica fiber laser is comparatively less complex and a high power output with good beam quality in cw operation is realized [3]. High power pulse operation is difficult because of its small core size. The later approach has advantages because of its simple technology but its effectiveness depends on the overall conversion efficiency of the OPO system.
*
Corresponding author. Tel.: +81 6 6879 8761; fax: +81 6 6877 0900. E-mail address: [email protected] (R. Bhushan).
0030-4018/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2008.03.035
Ferroelectric materials like KTiOPO4 (KTP) and LiNbO3 (LN) are attractive nonlinear optical materials for efficient wavelength conversion such as second harmonics generation (SHG), difference frequency generation (DFG), and OPO. Periodically-poled ferroelectric materials are advantageous for nonlinear optical wavelength conversion by quasi-phase matching (QPM) over birefringent phase matching technique. QPM utilizes a higher nonlinear optical coefficient in type–I configuration, circumvents the problem of walk-off and provides higher optical conversion efficiency. Periodically-poled LiNbO3 (PPLN) crystal has been commonly used for quasi-phase matching because of its large nonlinear optical coefficient (d33 = 26 pm/V at 1064 nm) high optical quality and a wide transparency from UV to mid-IR region (0.32–5 lm) [4]. However its high coercive force (21 KV/mm at room temperature) and sensitivity to photorefractive effect limit its use for high power applications. A 5 mol% MgO doped periodically-poled LN (PPMgLN) has high resistance to the photorefractive effect, low IR absorption and low coercive force
R. Bhushan et al. / Optics Communications 281 (2008) 3902–3905
(4.5 KV/mm at room temperature) [5]. These characteristics collectively make PPMgLN a suitable material for the high power applications. In recent years, several efforts have been made for the fabrication of large aperture 5 mol% doped PPMgLN crystal. In previous papers, fabrication and poling characteristics of 3 mm and 5 mm thick 5 mol% doped PPMgLN crystals has been reported [5,6]. A total output energy of 22 mJ in 2 lm region was achieved by a quasi-phase matched OPO using PPMgLN crystal of 30 mm long and 3 3 mm2 poled volume and 32.1 lm grating period [7]. A high energy QPM optical parametric oscillation of 77 mJ near degeneracy and 52 mJ at degeneracy was demonstrated using a large aperture PPMgLN of 36 mm long and 5 5 mm2 QPM pattern with 32.1 and 32.3 lm grating period, respectively [8,9]. This is, to the best of our knowledge, the largest poled aperture available in a single PPMgLN crystal. A 5 mm wide and 36 mm long aluminum electrode was used for poling [8]. Large poled aperture in the single QPM crystal may be realized by increasing the width of the electrode. In this paper, we report the experimental results of an efficient high energy and compact quasi-phase matched OPO based on a large aperture 5 mol% MgO doped periodically poled LiNbO3, which has two 5 5 mm2 poled apertures with 32.1 lm grating period. PPMgLN crystal with two poled aperture was used to scale the OPO output power. The main aim of this work was to realize a high energy and high average power nanosecond laser source in 2 lm region. We obtained a total of 186 mJ of output pulse energy in two separate beams at 2 lm with 58% of slope efficiency. For efficient quasi-phase matched OPO operation, the output wavelengths were decided by the energy conservation and phase matching conditions [10], 1 1 1 ¼ þ kp ks ki np ðkp ; T Þ ns ðks ; T Þ ni ðki ; T Þ 1 Dk ¼ 2p kp ks ki KðT Þ
3903
Where Dk is the phase mismatching and must be zero for efficient OPO operation, kp, ks and ki are respectively pump, signal and idler wavelengths, nj(kj, T) (j = p, s, i) are the temperature dependent refractive index of the crystal for pump, signal and idler waves respectively, and K(T) is temperature dependent grating period of the crystal. The output wavelength of the OPO can be tuned by varying either the QPM crystal temperature or the QPM grating period. 2. Experiments and discussion We adopt a linear cavity configuration for high power QPM OPO, as shown in Fig. 1. A high power Q-switched Nd:YAG laser is used to pump the QPM OPO, which generates a single longitudinal and TEM00 mode. The pump source can produce a maximum output energy of 5 J at 1.064 lm with 10 ns pulses at a repetition rate of 10 Hz. To utilize the maximum advantage of the poled apertures and for uniform pumping, two 5 5 mm2 hard apertures separated by 2 mm (horizontally) was placed at the exit of the pump laser and the beam images at the aperture plane were projected inside the QPM crystal by an image relaying optics. The imaging optics constitutes of two identical imaging lenes, L1 and L2 (f = 1000 mm). The polarization of the pump beam was set parallel to the z-axis of the crystal for collinear interaction. A z-cut 36 mm long PPMgLN with two 5 5 mm2 QPM patterns was used for high energy OPO experiment to generate two beams at 2 lm. Both the input and output surfaces of the PPMgLN crystal were antireflection coated for the pump (1.064 lm) and oscillation wavelengths (1.9–2.2 lm). The PPMgLN crystal was housed in a temperature controlled oven with temperature stability of ±0.5 °C. The standard linear OPO cavity constitutes of two flat mirrors M1 and M2, were separated by 75 mm with a single pass pump– beam configuration. The QPM crystal was placed between the mirrors and kept at 80 °C during the experiment. The input mirror, M1 had more than 98% transmission at 1.064 lm and more than 99% reflectivity at 1.9–2.2 lm.
ð1Þ ð2Þ
OPO cavity L1
Oven
L2
Q-Switched Nd:YAG Laser 10 ns, 10 Hz
M3 1.064μm
Aperture
HWP
M1
M2 PPMgLN QPM crystal 2μm M4 Power meter
Fig. 1. Experimental set-up for a high energy wavelength converter based on a large aperture PPMgLN. L1, L2: imaging lenses, HWP: half wave plate, M1: input coupler, M2: output coupler, M3, M4: highly reflective mirrors to separate OPO output.
3904
R. Bhushan et al. / Optics Communications 281 (2008) 3902–3905
We tested three types of uncoated substrates for suitability as an output coupler, namely BK-7, fused silica glass and Lithium Niobate (LN) wafer, and the total reflectivity of these uncoated substrates for oscillation wavelength were approximately 8, 6 and 14%, respectively. Fig. 2 shows the total output energy from the quasi-phase matched PPMgLN OPO as a function of the input energy at 80 °C crystal temperature. With BK-7 output coupler, a total output energy of 186 mJ (two beams) at 2 lm with slope efficiency of 58% was achieved for a Nd:YAG laser pump energy of 430 mJ (two beams) per pulse, which corresponds to an average pump irradiance of 86 MW/cm2, well below the surface damage threshold (about 150 MW/cm2) of the QPM crystal. A highly sensitive CCD camera (Electrophysics Micronviewer 7290) was used to record the OPO output intensity distributions, shown in Fig. 2. For reliable measurement, a set of edge filters (transmittance range from 1.3–2.5 lm) was used. Almost two square beam patterns (about 5 5 mm2) separated by 2 mm (approx.) were observed at the output plane. Under similar operating conditions, a total of 179 mJ of output with 53% of slope efficiency was observed with fused silica glass substrate. While with LN wafer, 159 mJ of output energy was measured with 60% of slope efficiency at 315 mJ of pump energy. Unfortunately, LN wafer was damaged at 63 MW/cm2, while no damage was observed on the uncoated BK-7 and fused silica glass substrates upto 90 MW/cm2 of pump irradiance. The optical conversion efficiency of the PPMgLN OPO system was measured as 43, 42 and 51% with an uncoated BK-7, fused silica glass, and LN sub-
strates, respectively. Higher conversion efficiency with uncoated BK-7 and fused silica glass substrates can be achieved at high pump irradiance. Considering low oscillation threshold, output stability and high power operation, the performance of an uncoated BK-7 and fused silca glass substrate as an output coupler was almost same. Under low pump irradiance LN wafer performs best as an output coupler. Though, the OPO was configured for infrared output ( 2 lm), visible (green and red) lights were also observed, as the SFG (sum frequency generation) between the pump and the OPO output wavelengths and DFG between the SHG of pump and the OPO output wavelengths. Generation of low SHG of the pump beam makes DFG weaker than SFG. Two highly reflective mirrors (R > 99% at 1.9– 2.2 lm), M3 and M4 were installed to separate the 2 lm OPO output. Fig. 3 shows, the OPO output spectrum as the SFG between the pump and the OPO output wavelengths at 80 °C crystal temperature. The spectrum was recorded from the direct output (before mirror, M3) with a grating monochromator (C5094, Hamamatsu Photonics, Japan). The resolution of the monochromator was 0.40 nm for a 20 lm slit-width setting. We observed 698 (signal) and 724 nm (idler) as the SFG between the pump and the OPO output wavelengths, respectively shown in Fig. 3. Consequently, the signal and idler waves had a center wavelength at 2.03 lm and 2.26 lm at 80 °C, respectively. The observed signal beam was more than 3 times intense than the corresponding idler. This was because of the absorption of the output coupler near 2.2 lm. The OPO output was
225 BK-7 (R~8%) Fused silica glass (R~6%)
200
LN wafer (R~14%)
175
Output energy mJ
150
Signal + Idler Slope : 53%
Signal + Idler Slope : 60%
125 100
Signal + Idler Slope : 58%
75
5 mm
50
5 mm
25
2 mm
0 0
100
200
300
400
500
Pump energy mJ Fig. 2. Performance of PPMgLN OPO pumped by Nd:YAG laser with uncoated BK-7, fused silica glass and LN wafer as an output coupler operated at 80 °C. Inset, intensity distribution of the quasi-phase matched PPMgLN OPO output.
R. Bhushan et al. / Optics Communications 281 (2008) 3902–3905
3905
OPO Wavelength nm 1950
Intensity (a.u.)
120
2025
2100
2175
2250
2325
2400 12 0
105
10 5
90
90
75
75
Δλ ~ 4nm
60
60
45
45
30
30
Δλ ~ 5nm
15
15
0
0 690
700
710
720
730
SFG Wavelength nm Fig. 3. Output spectrum (signal at 2.03 lm, and idler at 2.27 lm) of 1.064 lm pumped PPMgLN OPO at 80 °C QPM crystal temperature.
considered as total output of signal and idler. OPO output wavelengths derived from the observed SFG wavelengths do not satisfy the energy conservation relation (1). When signal wavelength is 2.03 lm, the idler wavelength should be 2.24 lm, as given by Eq. (1). On the otherhand, idler wavelength of 2.26 lm is derived from the observed SFG wavelength of 724 nm. This discrepancy may be come from the absorption of the output coupler which decreases towards the shorter part of the idler spectrum. The OPO output wavelength could be tuned from 1.94 to 2.36 lm by varying the crystal temperature from 65 to 90 °C, and the degeneracy was observed at 2.13 lm at 90 °C which is in agreement with the theoretical predictions based on temperature-dependent Sellmeier equation [11]. 3. Conclusion We have demonstrated an efficient high energy parametric down conversion of a nanosecond pulse of Q-switched Nd:YAG laser based on a large aperture 36 mm long PPMgLN OPO. A total output energy of 186 mJ at 2 lm with a conversion efficiency of 43% was achieved with good beam quality. Experimental results show that an image relaying optics used for uniform pumping enhances the OPO system performance, and a high energy two beams 2 lm laser source was realized at 86 MW/cm2 of pump power irradiance, which is well below the damage
threshold of the PPMgLN crystal. Further, the number of output beams can be increased by increasing the QPM pattern in the crystal, consequently increases the stocking power of the system and these beams can be combined into a single beam to generate a high power 2 lm laser radiation. This system can also be used as a pump source for the development of mid-infrared laser source based on optical parametric oscillators and optical parametric amplifier. References [1] P.A. Budni, LA. Pomeranz, M.L. Lemons, C.A. Miller, J.R. Mosto, E.P. Chicklis, J. Opt. Soc. Am. B. 17 (2000) 723. [2] J.A. Giordmaine, R.C. Miller, Phys. Rev. Lett. 14 (1965) 973. [3] S.D. Jackson, A. Sabella, A. Hemming, S. Bennetts, D.G. Lancaster, Opt. Lett. 32 (2007) 241. [4] I. Shoji, T. Kondo, A. Kitamoto, M. Shirane, R. Ito, J. Opt. Soc. Am. B 14 (1997) 2268. [5] H. Ishizuki, I. Shoji, T. Taira, Appl. Phys. Lett. 82 (2003) 4062. [6] H. Ishizuki, T. Taira, Baltimore 2005 CFC7, Dig. Tech. Papers, CLEO 2005. [7] H. Ishizuki, I. Shoji, T. Taira, Opt. Lett. 29 (2004) 2527. [8] H. Ishizuki, T. Taira, Opt. Lett. 30 (2005) 2918. [9] J. Saikawa, M. Fujii, H. Ishizuki, T. Taira, Opt. Lett. 31 (2006) 3149. [10] L.E. Myers, R.C. Eckardt, M.M. Fejer, R.L. Byer, W.R. Bosenberg, J.W. Pierce, J. Opt. Soc. Am. B 12 (1995) 2102. [11] Y. Hirano, S. Yamamoto, H. Taniguchi, Baltimore 2001 CFH2, Dig. Tech. Papers, CLEO 2001.