- Email: [email protected]
Broadly tunable KNbO3 OPOs pumped by Ti:sapphire lasers
15 October 1997
OPTICS COMMUNICATIONS ELSEVIER
Optics Communications 142 (1997) 262-264
Broadly tunable KNbO, OPOs pumped by Ti:sapphire lasers P. Rambaldi
a, M. Douard a, B. Vezin a, J.P. Wolf a,*, D. Rytz b
a LASH4 (lJMR5579), Uniuersite’ Claude Bernard L.von 1. 43. Bd da 11 Nouembre 1918, 69622 Villeurbanne Cedex, France b Forschungsinstitutfr
Edelsteine/ Edelmetalle (FEE), Struthstrasse 2. 55743 Idar-Oberstein. Germany
Received 27 November 1996; accepted 17 June 1997
Abstract We present the first broadly tunable KNbO, OPO in No-Tracking Configuration (NTC), pumped by a flashlamp-pumped Ti:sapphire laser. Tuning the pump laser from 733 to 841 nm yielded a tuning range from 908 nm to 1402 nm (for the signal), and 2103 to 3803 nm (for the idler). This range was limited by the mirror coatings, and continuous tuning should be achievable up to and beyond 4000 nm. Threshold was as low as 15 MW/cm2 and efficiencies up to 10% have been observed without AR-coatings on the crystal. 0 1997 Elsevier Science B.V.
Broadly tunable IR sources (l-5 pm) are needed for numerous spectroscopic applications, especially in the field of atmospheric sciences. The detection of the newly regulated Volatile Organic Compounds (VOCs) and air toxics, for example, is still limited to a large extent by the lack of convenient IR sources. Optical Parametric Oscillators (OPOs) certainly constitute the most promising laser candidates because of their broad tuning range and high efficiency. However, most of the OPOs are still limited to 3 pm, due to the transparency characteristics of the used non-linear crystals (KTP, BBO) [1.2]. Few results above 3 pm, especially in the C-H stretching band at 3.3 pm, have been reported so far using KTA 131, KNbO, [4] and LiNbO, [5]. All these systems were pumped by Nd:YAG lasers and tuned by angular tuning or temperature phase matching. An attractive alternative to these tuning techniques is the “No-Tracking Configuration” (NTC) which uses as pumping source a tunable laser such as Ti:sapphire [6]. Phase matching conditions, and thus tuning of the OPO, are in this case fulfilled just by changing the pump wavelength, without any action on the non-linear crystal. This leads to efficient, compact, rugged, and user-friendly de-
* Corresponding author. E-mail: [email protected]
.fr.
vices. Most of the NTC results have been reported using KTP, in OPOs [6-81 alone or OPO-OPA tandems [9]. However, until now, the tuning range was limited to 3 pm, preventing use for VOCs detection. Several attempts were dedicated to the TFC pumping of KNbO,, with limited success due to very high oscillation thresholds [8,9]. No runable output of KNbO, based TFC OPOs has been reported so far. We present here decisive improvements in the fabrication of KNbO, leading to high quality crystals, which allowed us to obtain the first broadly tunable KNbO, OPO pumped by a Ti:sapphire laser. The control of the phase transition during the cooling operation was considerably improved. So, a better homogeneity in the crystal is obtained. KNbO, crystals are grown by the top-seeded solution growth technique [ 10,l l] at FEE. Scaling the process from 150 g to 300 g of melt yields homogeneous crystals up to 50 g. The homogeneity of such crystals has been tested while scanning large single domain samples of a-cut KNbO, in a second harmonic generation geometry where the KNbO, sample doubles the 860 nm output of a cw Ti:sapphire laser. Samples for OPO applications were subsequently cut with the appropriate orientation. The KNbO, crystal was cut at 40.4” with respect to the b axis in the bc plane, in order to optimize the output tuning range while pumping between 720 and 900 nm.
0030-4018/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved. PII SOO30-4018(97)00332-5
P. Rumbaldi et al./ Optics Communications Pock& Cell
LASER HEAD
363
142 (1997) 262-264
R 50% RCC -7 m
KNb03-OPO
Fig. I. The Tixapphire laser pumped KNbO, OPO in a No-Tracking Configuration (NT0
Aperture and length were respectively 4 X 4 mm and 10 mm. No AR coating was applied to the surfaces. The experimental OPO set-up is shown in Fig. 1. The pump laser is a modified flashlamp-pumped Ti:sapphire laser from Elight Laser Systems [ 121. It is based on a 6 mm diameter X 100 mm Ti:sapphire rod (doping 0.15% Ti by weight), pumped by four 6 mm diameter flashlamps in a diffuse, close-coupled, cavity. The resonator is constituted by a 4 Brewster prismatic tuning element, and a 50% output coupler with a - 7 m radius of curvature. A Pockels cell is used for Q-switching, leading to pulses of about 30 ns duration. Typical output energies are 100 mJ, at 10 Hz repetition rate (max. 20 Hz). Tuning range is 700-950 nm, with a bandwidth of 0.1 nm. The beam diameter of 6 mm was reduced for the present purpose to 3 mm by a refractive telescope. The OPO resonator is a singly resonant configuration, with two flat mirrors separated by 4 cm. The input mirror has a 99% reflectance from 1.1 to 1.35 pm (signal), and a 90% transmission from 700 to 900 nm (pump). The output coupler has a 90% reflectance from 1.1 to 1.35 pm, a 95% transmission from 700 to 900 nm, and a 75% transmission from 2 to 3.5 ym (idler). Note that broadband low damage threshold coatings have been chosen, in order to obtain the broadest tuning range with one set of mirrors. Maximum output power of our device was thus limited by the damage threshold of these soft coatings. At the OPO output, a dichroic mirror was used to reject the pump wavelength, and signal wavelengths were measured with a H20 Jobin Yvon spectrometer and an 88XL Photodyne Germanium detector. Fig. ‘2 shows the KNbO, OPO output energy at the signal wavelength, as a function of pump energy (@ 800 nm). The measured oscillation threshold is as low as 30 mJ, corresponding to 15 MW/cm’. This very good result has to be compared to the high thresholds ( 140 MW/cm’) reported so far by Vezin et al. [9], Bosenberg et al. [13] and the recent results of Zenzie et al. [8] (30 MW/cm’) for a Nd:YAG pumped OPO and an AR coated crystal. This dramatic improvement is definitely a consequence of the quality of the crystal, since the same arrangement was formerly used by Vezin et al. [9]. In this configuration, the threshold is comparable to KTP [9], although KNbO, was not in a non-critical phasematching configuration. Note furthermore that, as mentioned above, no AR-coatings were applied on the crystal,
and that Fresnel losses are about 10% at each face. Much better results may then be easily achieved by applying such coatings. The same remark can be mentioned for the slope efficiency, which is 7% for the signal alone, corresponding to approximately 10% for the overall conversion. The maximum signal output was 3 mJ. limited by the damage threshold of the mirror coatings. No saturation effects are observed, so that much higher energies should be obtained using a set of several narrowband high power mirrors. Furthermore, no thermal effects have been observed up to a repetition rate of IO Hz, the output energy remaining stable, although no active stabilization was used. The tuning range of our KNbO, OPO is presented in Fig. 3. With this set of mirrors, a tunable output from 908 nm to 1402 nm has been obtained for the signal, and 2103 to 3803 nm for the idler. The Ti:sapphire laser was scanned for this purpose from 733 nm to 841 nm. This result constitutes the first broadly tunable KNbO, OPO pumped by a Ti:sapphire laser. It is also the first TFC OPO operating above 3 km. The bandwidth of the signal was measured with an OMA (Chromex spectrometer of Princeton Instrument). A bandwidth of 1.2 nm with many Fabry-Perot interferences between both faces of the crystal, faces of the mirrors, etc. was found. This corresponds to an idler bandwidth of approximately 20 nm. By adjusting the different elements in the OPO cavity, the Fabry-Perot modes can mismatch each other and the signal bandwidth falls to 0.06 nm. and the idler bandwidth becomes 3.5 nm.
6
:
Fig. 2. Signal output energy of the NTC KNbO, OPO.
264
P. Rambaldi et al. / Optics Communications
I42 (1997) 262-264
Acknowledgements
The authors from LASIM wish to sincerely acknowledge the ECOTECH program, the ADEME and the CNRS for actively supporting this project. FEE acknowledges the support of the European Commission under a BRITEEURAM feasibility study.
a p
2000
P
References 700
750
800 pump
wavelength
850
900
in nm
Fig. 3. Tuning range: dots are experimental data, while solid curve is the result of a simulation using Sellmeier’s equations of Zysset et al. 114).
Very good agreement is obtained with simulations (full line), based on the Sellmeier equations of Zysset et al. [14]. This shows that with a proper choice of mirrors, continuous tunability should be obtained between 950 nm and 4 ym by tuning the Ti:sapphire laser from 740 nm to 860 nm only. From the KNbO, transparency characteristics, it may even be possible to extend the emission to and above 5 pm. The presented results open new perspectives for spectroscopic investigations in the infrared. This simple, compact, and very frequency-agile device appears to be a perfect source for atmospheric measurements, since numerous pollutants absorb in this spectral range, as for example, CH,, CO, CO,, HCl, HF, H,S, and VOCs. Further work will be dedicated to the injection seeding of this device to reduce its bandwidth, and its implementation in our already existing Differential Absorption Lidar (DIAL) system.
[I] K. Kate, IEEE .I. Quantum Electron. 27 (5) (1991) 1137. [2] Y.X. Fan, R.C. Eckardt, R.L. Byer, J. Nolting, R. Wallenstein, Appl. Phys. Lett. 53 (1988) 2014. [3] L.R. Marshall, J. Earl. A. Johnson, T. Pollak, P. Schunemann. OSA TOPS on Advanced Solid State Lasers, Eds. S.A. Payne, C. Pollock (1996). [4] R. Urschel, A. Fix, R. Wallenstein, D. Rytz. B. Zysset, J. Opt. Sot. Am. B 12 (4) (1995) 726. [5] M.J.T. Milton, T.D. Gardiner. G. Chourdakis, P.T. Woods, Optics Len. 19 (4) (1994) 281. [6] M. Douard, J.P. Wolf, P. Rairoux, J. Kolenda, L. WGste, M. Ulbricht, D. Weidauer, J. Phys. IV (1994) C4-683. [7] K. Kate, M. Matsutani, Optics Lett. 17 (1992) 178. [8] H.H. Zenzie, P.F. Moulton, Optics Lett. 19 (1994) 963. [9] B. Vezin, M. Douard, P. Rambaldi, J.P. Wolf. Appl. Phys. B 63 (1996) 199. [lo] Wu Xing, H. Looser, H. Wuest, H. Arend, J. Cryst. Growth 78 (1986) 4310. [ll] J. Hulliger, R. Gutmann, H. Wuest, J. Cry%. Growth 128 ( 1993) 8970. [12] Elight Laser Systems GmbH, Potsdamerstrasse 18A. 14513 Teltow/Berlin (Germany). [13] W.R. Bosenberg, R.H. Jarman, Optics Lett. 18 (1993) 1323. [14] B. Zysset, I. Biaggio. P. Giinter, J. Opt. Sot. Am. B 9 (3) (1992) 380.
OPTICS COMMUNICATIONS ELSEVIER
Optics Communications 142 (1997) 262-264
Broadly tunable KNbO, OPOs pumped by Ti:sapphire lasers P. Rambaldi
a, M. Douard a, B. Vezin a, J.P. Wolf a,*, D. Rytz b
a LASH4 (lJMR5579), Uniuersite’ Claude Bernard L.von 1. 43. Bd da 11 Nouembre 1918, 69622 Villeurbanne Cedex, France b Forschungsinstitutfr
Edelsteine/ Edelmetalle (FEE), Struthstrasse 2. 55743 Idar-Oberstein. Germany
Received 27 November 1996; accepted 17 June 1997
Abstract We present the first broadly tunable KNbO, OPO in No-Tracking Configuration (NTC), pumped by a flashlamp-pumped Ti:sapphire laser. Tuning the pump laser from 733 to 841 nm yielded a tuning range from 908 nm to 1402 nm (for the signal), and 2103 to 3803 nm (for the idler). This range was limited by the mirror coatings, and continuous tuning should be achievable up to and beyond 4000 nm. Threshold was as low as 15 MW/cm2 and efficiencies up to 10% have been observed without AR-coatings on the crystal. 0 1997 Elsevier Science B.V.
Broadly tunable IR sources (l-5 pm) are needed for numerous spectroscopic applications, especially in the field of atmospheric sciences. The detection of the newly regulated Volatile Organic Compounds (VOCs) and air toxics, for example, is still limited to a large extent by the lack of convenient IR sources. Optical Parametric Oscillators (OPOs) certainly constitute the most promising laser candidates because of their broad tuning range and high efficiency. However, most of the OPOs are still limited to 3 pm, due to the transparency characteristics of the used non-linear crystals (KTP, BBO) [1.2]. Few results above 3 pm, especially in the C-H stretching band at 3.3 pm, have been reported so far using KTA 131, KNbO, [4] and LiNbO, [5]. All these systems were pumped by Nd:YAG lasers and tuned by angular tuning or temperature phase matching. An attractive alternative to these tuning techniques is the “No-Tracking Configuration” (NTC) which uses as pumping source a tunable laser such as Ti:sapphire [6]. Phase matching conditions, and thus tuning of the OPO, are in this case fulfilled just by changing the pump wavelength, without any action on the non-linear crystal. This leads to efficient, compact, rugged, and user-friendly de-
* Corresponding author. E-mail: [email protected]
.fr.
vices. Most of the NTC results have been reported using KTP, in OPOs [6-81 alone or OPO-OPA tandems [9]. However, until now, the tuning range was limited to 3 pm, preventing use for VOCs detection. Several attempts were dedicated to the TFC pumping of KNbO,, with limited success due to very high oscillation thresholds [8,9]. No runable output of KNbO, based TFC OPOs has been reported so far. We present here decisive improvements in the fabrication of KNbO, leading to high quality crystals, which allowed us to obtain the first broadly tunable KNbO, OPO pumped by a Ti:sapphire laser. The control of the phase transition during the cooling operation was considerably improved. So, a better homogeneity in the crystal is obtained. KNbO, crystals are grown by the top-seeded solution growth technique [ 10,l l] at FEE. Scaling the process from 150 g to 300 g of melt yields homogeneous crystals up to 50 g. The homogeneity of such crystals has been tested while scanning large single domain samples of a-cut KNbO, in a second harmonic generation geometry where the KNbO, sample doubles the 860 nm output of a cw Ti:sapphire laser. Samples for OPO applications were subsequently cut with the appropriate orientation. The KNbO, crystal was cut at 40.4” with respect to the b axis in the bc plane, in order to optimize the output tuning range while pumping between 720 and 900 nm.
0030-4018/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved. PII SOO30-4018(97)00332-5
P. Rumbaldi et al./ Optics Communications Pock& Cell
LASER HEAD
363
142 (1997) 262-264
R 50% RCC -7 m
KNb03-OPO
Fig. I. The Tixapphire laser pumped KNbO, OPO in a No-Tracking Configuration (NT0
Aperture and length were respectively 4 X 4 mm and 10 mm. No AR coating was applied to the surfaces. The experimental OPO set-up is shown in Fig. 1. The pump laser is a modified flashlamp-pumped Ti:sapphire laser from Elight Laser Systems [ 121. It is based on a 6 mm diameter X 100 mm Ti:sapphire rod (doping 0.15% Ti by weight), pumped by four 6 mm diameter flashlamps in a diffuse, close-coupled, cavity. The resonator is constituted by a 4 Brewster prismatic tuning element, and a 50% output coupler with a - 7 m radius of curvature. A Pockels cell is used for Q-switching, leading to pulses of about 30 ns duration. Typical output energies are 100 mJ, at 10 Hz repetition rate (max. 20 Hz). Tuning range is 700-950 nm, with a bandwidth of 0.1 nm. The beam diameter of 6 mm was reduced for the present purpose to 3 mm by a refractive telescope. The OPO resonator is a singly resonant configuration, with two flat mirrors separated by 4 cm. The input mirror has a 99% reflectance from 1.1 to 1.35 pm (signal), and a 90% transmission from 700 to 900 nm (pump). The output coupler has a 90% reflectance from 1.1 to 1.35 pm, a 95% transmission from 700 to 900 nm, and a 75% transmission from 2 to 3.5 ym (idler). Note that broadband low damage threshold coatings have been chosen, in order to obtain the broadest tuning range with one set of mirrors. Maximum output power of our device was thus limited by the damage threshold of these soft coatings. At the OPO output, a dichroic mirror was used to reject the pump wavelength, and signal wavelengths were measured with a H20 Jobin Yvon spectrometer and an 88XL Photodyne Germanium detector. Fig. ‘2 shows the KNbO, OPO output energy at the signal wavelength, as a function of pump energy (@ 800 nm). The measured oscillation threshold is as low as 30 mJ, corresponding to 15 MW/cm’. This very good result has to be compared to the high thresholds ( 140 MW/cm’) reported so far by Vezin et al. [9], Bosenberg et al. [13] and the recent results of Zenzie et al. [8] (30 MW/cm’) for a Nd:YAG pumped OPO and an AR coated crystal. This dramatic improvement is definitely a consequence of the quality of the crystal, since the same arrangement was formerly used by Vezin et al. [9]. In this configuration, the threshold is comparable to KTP [9], although KNbO, was not in a non-critical phasematching configuration. Note furthermore that, as mentioned above, no AR-coatings were applied on the crystal,
and that Fresnel losses are about 10% at each face. Much better results may then be easily achieved by applying such coatings. The same remark can be mentioned for the slope efficiency, which is 7% for the signal alone, corresponding to approximately 10% for the overall conversion. The maximum signal output was 3 mJ. limited by the damage threshold of the mirror coatings. No saturation effects are observed, so that much higher energies should be obtained using a set of several narrowband high power mirrors. Furthermore, no thermal effects have been observed up to a repetition rate of IO Hz, the output energy remaining stable, although no active stabilization was used. The tuning range of our KNbO, OPO is presented in Fig. 3. With this set of mirrors, a tunable output from 908 nm to 1402 nm has been obtained for the signal, and 2103 to 3803 nm for the idler. The Ti:sapphire laser was scanned for this purpose from 733 nm to 841 nm. This result constitutes the first broadly tunable KNbO, OPO pumped by a Ti:sapphire laser. It is also the first TFC OPO operating above 3 km. The bandwidth of the signal was measured with an OMA (Chromex spectrometer of Princeton Instrument). A bandwidth of 1.2 nm with many Fabry-Perot interferences between both faces of the crystal, faces of the mirrors, etc. was found. This corresponds to an idler bandwidth of approximately 20 nm. By adjusting the different elements in the OPO cavity, the Fabry-Perot modes can mismatch each other and the signal bandwidth falls to 0.06 nm. and the idler bandwidth becomes 3.5 nm.
6
:
Fig. 2. Signal output energy of the NTC KNbO, OPO.
264
P. Rambaldi et al. / Optics Communications
I42 (1997) 262-264
Acknowledgements
The authors from LASIM wish to sincerely acknowledge the ECOTECH program, the ADEME and the CNRS for actively supporting this project. FEE acknowledges the support of the European Commission under a BRITEEURAM feasibility study.
a p
2000
P
References 700
750
800 pump
wavelength
850
900
in nm
Fig. 3. Tuning range: dots are experimental data, while solid curve is the result of a simulation using Sellmeier’s equations of Zysset et al. 114).
Very good agreement is obtained with simulations (full line), based on the Sellmeier equations of Zysset et al. [14]. This shows that with a proper choice of mirrors, continuous tunability should be obtained between 950 nm and 4 ym by tuning the Ti:sapphire laser from 740 nm to 860 nm only. From the KNbO, transparency characteristics, it may even be possible to extend the emission to and above 5 pm. The presented results open new perspectives for spectroscopic investigations in the infrared. This simple, compact, and very frequency-agile device appears to be a perfect source for atmospheric measurements, since numerous pollutants absorb in this spectral range, as for example, CH,, CO, CO,, HCl, HF, H,S, and VOCs. Further work will be dedicated to the injection seeding of this device to reduce its bandwidth, and its implementation in our already existing Differential Absorption Lidar (DIAL) system.
[I] K. Kate, IEEE .I. Quantum Electron. 27 (5) (1991) 1137. [2] Y.X. Fan, R.C. Eckardt, R.L. Byer, J. Nolting, R. Wallenstein, Appl. Phys. Lett. 53 (1988) 2014. [3] L.R. Marshall, J. Earl. A. Johnson, T. Pollak, P. Schunemann. OSA TOPS on Advanced Solid State Lasers, Eds. S.A. Payne, C. Pollock (1996). [4] R. Urschel, A. Fix, R. Wallenstein, D. Rytz. B. Zysset, J. Opt. Sot. Am. B 12 (4) (1995) 726. [5] M.J.T. Milton, T.D. Gardiner. G. Chourdakis, P.T. Woods, Optics Len. 19 (4) (1994) 281. [6] M. Douard, J.P. Wolf, P. Rairoux, J. Kolenda, L. WGste, M. Ulbricht, D. Weidauer, J. Phys. IV (1994) C4-683. [7] K. Kate, M. Matsutani, Optics Lett. 17 (1992) 178. [8] H.H. Zenzie, P.F. Moulton, Optics Lett. 19 (1994) 963. [9] B. Vezin, M. Douard, P. Rambaldi, J.P. Wolf. Appl. Phys. B 63 (1996) 199. [lo] Wu Xing, H. Looser, H. Wuest, H. Arend, J. Cryst. Growth 78 (1986) 4310. [ll] J. Hulliger, R. Gutmann, H. Wuest, J. Cry%. Growth 128 ( 1993) 8970. [12] Elight Laser Systems GmbH, Potsdamerstrasse 18A. 14513 Teltow/Berlin (Germany). [13] W.R. Bosenberg, R.H. Jarman, Optics Lett. 18 (1993) 1323. [14] B. Zysset, I. Biaggio. P. Giinter, J. Opt. Sot. Am. B 9 (3) (1992) 380.