Highly efficient tunable mid-infrared optical ... - OSA Publishing

Highly efficient tunable mid-infrared optical parametric oscillator pumped by a wavelength locked, Q-switched Er:YAG laser Jun Liu,1,4 Pinghua Tang,2,4 Yu Chen,1 Chujun Zhao,1,2,* Deyuan Shen,3 Shuangchun Wen,2 and Dianyuan Fan1 1 SZU-NUS Collaborative Innovation Center for Optoelectronic Science & Technology, Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education and Guangdong Province, College of Optoelectronic Engineering, Shenzhen University, Shenzhen 518060, China 2 Laboratory for Micro-/Nano- Optoelectronic Devices of Ministry of Education, IFSA Collaborative Innovation Center, School of Physics and Electronics, Hunan University, Changsha 410082, China 3 Key Laboratory of Micro and Nano Photonic Structures, Department of Optical Science and Engineering, Fudan University, Shanghai 200433, China 4 These authors contributed equally * [email protected]

Abstract: A highly efficient and stable mid-infrared optical parametric oscillator is demonstrated, pumped by an electro-optic Q-switched Er:YAG laser with operating wavelength locked at 1645 nm by a volume Bragg grating. The oscillator, based on MgO-doped periodically poled lithium niobate (MgO:PPLN) crystal, yields a maximum overall average output power in excess of 1 W, corresponding to a conversion efficiency of 35.5% and a slope efficiency of 43.6%. The signal and idler wavelengths of the OPO are around ~2.7 μm and ~4.3 μm, respectively, corresponding to the two peak absorption bands of CO2. Lasing characteristics of the oscillator, including the time evolution of the pump, signal and idler pulses at different pump power levels, are also investigated. Temperature tuning of the MgO:PPLN crystal gives signal and idler ranges of 2.67 to 2.72 μm and 4.17 to 4.31 μm, respectively. ©2015 Optical Society of America OCIS codes: (140.3070) Infrared and far-infrared lasers; (140.3580) Lasers, solid-state; (190.4970) Parametric oscillators and amplifiers.

References and links 1.

M. Tanaka and T. Yamanouchi, “Absorption properties of the near infrared CO2 bands,” J. Quant. Spectrosc. Radiat. Transfer 17(3), 421–432 (1977). 2. J. G. Kim, L. Shterengas, R. U. Martinelli, and G. L. Belenky, “High-power room-temperature continuous wave operation of 2.7 and 2.8 μm In(Al)GaAsSb/GaSb diode lasers,” Appl. Phys. Lett. 83(10), 1926–1928 (2003). 3. H. Jelínková, M. E. Doroshenko, M. Jelínek, J. Šulc, V. V. Osiko, V. V. Badikov, and D. V. Badikov, “Dysprosium-doped PbGa2S4 laser generating at 4.3 μm directly pumped by 1.7 μm laser diode,” Opt. Lett. 38(16), 3040–3043 (2013). 4. U. Siegenthaler, T. F. Stocker, E. Monnin, D. Lüthi, J. Schwander, B. Stauffer, D. Raynaud, J. M. Barnola, H. Fischer, V. Masson-Delmotte, and J. Jouzel, “Stable carbon cycle-climate relationship during the late Pleistocene,” Science 310(5752), 1313–1317 (2005). 5. C. D. Keeling, T. P. Whorf, M. Wahlen, and J. van der Plichtt, “Interannual extremes in the rate of rise of atmospheric carbon dioxide since 1980,” Nature 375(6533), 666–670 (1995). 6. M. Wang, L. Zhu, W. Chen, and D. Fan, “Efficient all-solid-state mid-infrared optical parametric oscillator based on resonantly pumped 1.645 μm Er:YAG laser,” Opt. Lett. 37(13), 2682–2684 (2012). 7. M. Vainio, J. Peltola, S. Persijn, F. J. M. Harren, and L. Halonen, “Singly resonant cw OPO with simple wavelength tuning,” Opt. Express 16(15), 11141–11146 (2008). 8. J. Liu, D. Shen, H. Huang, C. Zhao, X. Zhang, and D. Fan, “High-power and highly efficient operation of wavelength-tunable Raman fiber lasers based on volume Bragg gratings,” Opt. Express 22(6), 6605–6612 (2014). 9. J. Liu, W. Yao, C. Zhao, D. Shen, and D. Fan, “Volume Bragg grating narrowed high-power and highly efficient cladding-pumped Raman fiber laser,” Appl. Opt. 53(35), 8256–8260 (2014). 10. M. Eichhorn, S. T. Fredrich-Thornton, E. Heumann, and G. Huber, “Spectroscopic properties of Er3+:YAG at 300-550 K and their effects on the 1.6 μm laser transitions,” Appl. Phys. B 91(2), 249–256 (2008).

#239886 (C) 2015 OSA

Received 29 Apr 2015; revised 17 Jul 2015; accepted 26 Jul 2015; published 31 Jul 2015 10 Aug 2015 | Vol. 23, No. 16 | DOI:10.1364/OE.23.020812 | OPTICS EXPRESS 20812

11. D. Y. Shen, J. K. Sahu, and W. A. Clarkson, “Highly efficient in-band pumped Er:YAG laser with 60 W of output at 1645 nm,” Opt. Lett. 31(6), 754–756 (2006). 12. X. F. Yang, D. Y. Shen, T. Zhao, H. Chen, J. Zhou, J. Li, H. M. Kou, and Y. B. Pan, “In-band pumped Er:YAG ceramic laser with 11 W of output power at 1645 nm,” Laser Phys. 21(6), 1013–1016 (2011). 13. M. Eichhorn, “Transient wavelength performance of 1.53 μm InP laser diodes for pumping of Er3+-doped solidstate lasers,” Appl. Opt. 47(17), 3129–3133 (2008). 14. L. N. Zhu, C. Q. Gao, R. Wang, Y. Zheng, and M. W. Gao, “Fiber-bulk hybrid Er:YAG laser with 1617 nm single frequency laser output,” Laser Phys. Lett. 9(9), 674 (2012). 15. N. Vermeulen, P. Wasylczyk, S. Tonchev, P. Muys, H. Ottevaere, O. Parriaux, and H. Thienpont, “Highperformance wavelength tuning of a mid-infrared solid-state laser using a resonant diffraction grating,” Proc. SPIE 8433, 843307 (2012). 16. X. Zhang, D. Shen, H. Huang, J. Liu, X. Liu, J. Zhang, J. Zhang, D. Tang, and D. Fan, “Widely tunable, narrow bandwidth polycrystalline ceramic Er:YAG laser with a volume Bragg grating,” Opt. Express 22(6), 7154–7159 (2014). 17. L. E. Myers, G. D. Miller, R. C. Eckardt, M. M. Fejer, R. L. Byer, and W. R. Bosenberg, “Quasi-phase-matched 1.064-μm-pumped optical parametric oscillator in bulk periodically poled LiNbO3,” Opt. Lett. 20(1), 52–54 (1995). 18. A. Henderson and R. Stafford, “Low threshold, singly-resonant CW OPO pumped by an all-fiber pump source,” Opt. Express 14(2), 767–772 (2006). 19. B. Wu, J. Kong, and Y. Shen, “High-efficiency semi-external-cavity-structured periodically poled MgLN-based optical parametric oscillator with output power exceeding 9.2 W at 3.82 microm,” Opt. Lett. 35(8), 1118–1120 (2010). 20. M. V. O’Connor, M. A. Watson, D. P. Shepherd, D. C. Hanna, J. H. V. Price, A. Malinowski, J. Nilsson, N. G. R. Broderick, D. J. Richardson, and L. Lefort, “Synchronously pumped optical parametric oscillator driven by a femtosecond mode-locked fiber laser,” Opt. Lett. 27(12), 1052–1054 (2002). 21. O. Kokabee, A. Esteban-Martin, and M. Ebrahim-Zadeh, “Efficient, high-power, ytterbium-fiber-laser-pumped picosecond optical parametric oscillator,” Opt. Lett. 35(19), 3210–3212 (2010). 22. P. E. Britton, H. L. Offerhaus, D. J. Richardson, P. G. R. Smith, G. W. Ross, and D. C. Hanna, “Parametric oscillator directly pumped by a 1.55-mum erbium-fiber laser,” Opt. Lett. 24(14), 975–977 (1999). 23. Y. E. Young, S. D. Setzler, T. M. Pollak, and E. P. Chicklis, “Optical parametric oscillator pumped at 1645 nm by a 9 W, fiber-laser-pumped, Q-switched Er:YAG laser,” Advanced Solid-State Photonics (ASSP) TuC, TuC2 (2004). 24. M. W. Haakestad, H. Fonnum, G. Arisholm, E. Lippert, and K. Stenersen, “Mid-infrared optical parametric oscillator synchronously pumped by an erbium-doped fiber laser,” Opt. Express 18(24), 25379–25388 (2010). 25. P. Tang, J. Liu, B. Huang, C. Xu, C. Zhao, and S. Wen, “Stable and wavelength-locked Q-switched narrowlinewidth Er:YAG laser at 1645 nm,” Opt. Express 23(9), 11037–11042 (2015). 26. Y. Furukawa, K. Kitamura, S. Takekawa, A. Miyamoto, M. Terao, and N. Suda, “Photorefraction in LiNbO3 as a function of [Li]/[Nb] and MgO concentrations,” Appl. Phys. Lett. 77(16), 2494–2496 (2000).

1. Introduction Tunable mid-infrared (mid-IR) laser sources operating in the 3~5 μm wavelength range have attracted considerable attention owing to their various applications, such as remote sensing, imaging, environmental monitoring, and so on. Particularly, laser emissions at around ~2.7 and ~4.3 μm are more desirable for atmospheric sensing application since both of these two mid-IR wavelengths correspond to the peak absorption of vibration-rotation bands of carbon dioxide (CO2), the primary greenhouse gas [1–3]. By using the differential absorption lidar (DIAL) technique, the CO2 can be monitored remotely and actively, which is critical for carbon cycle studies and climate predictions through environment models [4,5]. Two laser sources are usually required to monitor the gas at these two peak absorption bands. Recently, a novel and efficient lasing scheme has been proposed to yield these two mid-IR wavelengths simultaneously, by building an optical parametric oscillator (OPO) with appropriate crystal temperature and grating period selection pumped by a specific laser around 1.65 μm [6]. However, the output pulse shaping and evolution from the OPO system were not characterized in detail until now. In addition, the operation stability should be greatly improved and the actual output spectral linewidth needs to be confirmed. Therefore, the pump laser and the OPO resonator should be carefully designed and selected. Normally, the required pump sources around ~1.65 μm can be indirectly acquired through the nonlinear approaches such as OPO [7] or stimulated Raman scattering (SRS) effects [8,9]. Another attractive alternative method to directly generate this particular wavelength is based on the rare-earth ion Er-doped crystalline or ceramic solid-state lasers [10–12], where the direct resonant pumping regime is usually employed owing to the low quantum defect and

#239886 (C) 2015 OSA


thus considerable thermal loading reduction. One typical resonant pumping approach is using high power Er or Er/Yb co-doped fiber lasers at ~1.53 µm with narrow linewidth (~1 nm) and high beam quality. This fiber-bulk hybrid laser scheme can offer efficient pump absorption and good mode-matching, but suffering from a cumbersome and complex configuration. In contrast, the known direct diode pumping approach contributes to a compact and robust Erdoped all-solid-state laser for the reduced number of optical components compared to the fiber laser pump regime. Particularly, the narrow linewidth, high-power GaAsP/InP-diodes in the ~1.53 µm range are available [13], which also allows much better pump absorption in the whole system, and therefore results in a higher overall lasing efficiency. On the other hand, the laser source required for pumping the OPO system should have a narrow spectral linewidth, offering the potential for efficient signal and idler laser generation together with a clean output spectrum. Output spectral narrowing in solid-state lasers is typically achieved by using etalons [14] or replica diffraction gratings [15] in the resonator. However, these spectral narrowing techniques suffer from either extra losses in the cavity or a cumbersome and complex laser configuration. Volume Bragg gratings (VBGs) recorded in photo-thermalrefractive glass combine the advantages of high diffraction efficiency, narrow spectral width, low insertion losses, a high damage threshold, and good thermal stability. These excellent properties make them very attractive as effective wavelength selection and spectral narrowing components in high power laser sources [16]. By employing the VBG as a wavelength selector and a pump input mirror simultaneously in the all-solid-state Er:YAG laser, the output spectral linewidth and laser stability can be further improved. So far, OPOs based on highly nonlinear and quasi-phase-matched materials such as periodically poled lithium niobate (PPLN), with appropriate choice of grating period and temperature, have been considered as an ideal approach to achieve laser with broad tunability in the 3~5 μm spectral region [17]. Most PPLN based OPO devices have been established by using ~1 μm lasers as the pump sources to achieve mid-IR generation in a variety of operation regimes, including single-frequency continuous wave (CW) [18], nanosecond pulses [19], and ultrashort pulses [20,21]. However, the conversion efficiency reported was greatly confined, much smaller than the quantum limit set by the Manley-Rowe relations. More recently, substantial studies have been implemented regarding the mid-IR OPOs pumped in the 1.5-1.6 µm eye-safe wavelength range, due to their potential high conversion efficiency and applications in the telecommunication industry [22–24]. Suffering from the large and cumbersome configuration based on fiber or fiber-bulk hybrid pump structure, their overall lasing efficiencies still have prospect to be further elevated. All-solid-state laser pumped OPO devices may be a possible solution to achieve high efficiency and reliability, resulting from their simplicity and compactness in size for mid-IR radiation. In this paper, a highly efficient tunable mid-infrared OPO has been demonstrated, the signal and idler wavelengths of which are in the ~2.7 and ~4.3 μm spectral region, respectively. The oscillator, based on MgO-doped PPLN (MgO:PPLN) crystal, was pumped by an all-solid-state electro-optic (EO) Q-switched Er:YAG laser with operating wavelength locked at 1645 nm. Lasing characteristics in terms of average output power and conversion efficiency were compared and investigated at different repetition rates. A maximum overall average output power in excess of 1 W was obtained at a repetition frequency of 2 kHz, corresponding to a conversion efficiency of 35.5% and a slope efficiency of 43.6%. We illustrated the temporal evolution of the pump, signal and idler pulses, regarding the pump depletion, lasing build-up and back-conversion process. Temperature tuning of the PPLN crystal is implemented, achieving a signal and idler wavelength tuning range of 2.67 to 2.72 μm and 4.17 to 4.31 μm, respectively. 2. Experimental setup 2.1 Pump laser The OPO was pumped by an in-house built all-solid-state EO Q-switched Er:YAG laser, the schematic diagram of which is shown in Fig. 1(a). The pump beam from a 35 W fiber-coupled

#239886 (C) 2015 OSA


laser diode at 1532 nm was focused onto a 40 mm-long 0.25 at.% doped Er:YAG crystal rod through a pair of lenses. A simple two-mirror cavity was employed for the Q-switched Er:YAG laser, which comprised a VBG (OptiGrate Corp.) with center wavelength of 1645 nm and spectral width (FWHM) of 0.37 nm as the pump input coupler (IC), and a 500 mm radiusof-curvature concave output coupler (OC) M1 with a transmittance of 13% at the lasing wavelength of 1645 nm and a high reflectivity (>99.8%) at the pump wavelength of 1532 nm. The operating wavelength was narrowed and locked at 1645 nm by the VBG. The EO Qswitcher in the cavity is the voltage-ON-type, consisting of a RTP Pockels cell together with a quarter-wave plate, and a Brewster-angled undoped YAG polarizer (high-reflection-coated for the s-polarized light). The overall cavity length of ~270 mm resulted in a calculated TEM00 mode radius of 360 μm in the middle of the laser medium, which is slightly larger than the focused pump laser spot radius of 320 μm for fundamental mode excitation with simultaneous excellent lasing efficiency. The intracavity YAG polarizer was used to ensure linearly ppolarized output. The proposed EO Q-switched Er:YAG laser can offer a simple way to obtain linearly polarized and relatively high-peak-power pulses for efficient pumping of a PPLN OPO. Maximum average output powers of 2.91 W and 2.96 W were obtained at the pulse repetition rates of 2 kHz and 3 kHz with the corresponding pulse duration of around 176 ns and 252 ns, respectively, for the same incident diode pump power of 23 W. However, at a repetition rate of 1 kHz, the diode laser power was restricted to 22 W in case of optical component damage, resulting in a maximum average output power of 2.54 W with a single pulse duration of 102 ns. The VBG-incorporated Er:YAG laser was less susceptible to the thermal effects. The measured output spectrum shows that the center wavelength was 1645.3 nm with a FWHM spectral width of 0.2 nm. The output laser beam was near-diffractionlimited with the measured M2 factor of 1.1 by the knife-edge scanning method. More details about the EO Q-switched Er:YAG pump source can be found in our previous publication [25]. 2.2 OPO resonator

Fig. 1. Experimental schematic of the OPO system: (a) the pump laser source; (b) the OPO cavity.

The pump beam from the Q-switched Er:YAG laser passed through a half-wave plate and was finally directed toward the OPO system, as shown in Fig. 1(b). The OPO featured a 5 mol.% doped MgO:PPLN crystal (HC Photonics, Taiwan) with a single uniform grating period of 31.5 μm. The given grating period was selected to obtain the signal and idler wavelengths aiming at the two corresponding peak absorption bands of CO2 under nondegenerate operation over the convenient temperature range. Doping the PPLN crystal with MgO can effectively reduce the photorefraction damage that can further lead to output power degradation and instability [26]. The MgO:PPLN crystal had a dimension of 50 mm length, 5 mm width and 1 mm thickness. Both end facets of the PPLN crystal were polished and

#239886 (C) 2015 OSA


antireflection-coated at the pump wavelength of 1.645 μm, signal wavelength range of 2.5-3.0 μm, and idler wavelength range of 4.0-4.5 μm. The MgO:PPLN was carefully positioned in an oven to control its operating temperature that can be adjusted from room temperature to 200 °C with a precision of 0.1 °C. The OPO was designed to be singly resonant at the signal wavelength, formed by a simple plane-concave cavity comprising a plane pump IC (M2) with high reflectivity (>99%) in the signal and idler wavelength range and high transmission (>98%) at the pump wavelength (1645 nm), and a 120 mm radius-of-curvature concave OC (M3) with a transmittance of ~22% in the signal wavelength range and a high reflectivity (>99%) at the pump wavelength and high transmission (T>97%) in the idler wavelength range. The physical length of the OPO cavity was ~80 mm, resulting in a calculated TEM00 signal beam waist radius of ~250 μm at the center of_the crystal. The pump laser output was collimated and subsequently focused to a beam radius of ~300_μm for optimum modematching with the signal laser, corresponding to a confocal length of ~200 mm. A dichroic mirror with high transmission in the signal wavelength range and high reflectivity in the idler wavelength range was positioned at 45° outside the cavity to steer the signal from the idler output, allowing convenient power and temporal profile monitoring. The undepleted pump temporal profile can be detected and measured near the concave mirror M3 resulting from its reflection. 3. Results

Fig. 2. Output powers versus incident pump power for the OPO system at pulse repetition rates of 1, 2, and 3 kHz, respectively.

The power scaling capability of the OPO system at room temperature was first evaluated. The signal and idler average output powers as a function of the incident pump power at different pulse repetition rates are shown in Fig. 2. It can be seen that best lasing performance was obtained by employing the pump pulse repetition rate of 2 kHz in terms of output power and slope efficiency. A maximum overall output power of 1.03 W (0.7 W signal and 0.33 W idler) was generated at an incident pump power of 2.9 W, corresponding to a slope efficiency of 43.6% with respect to the incident pump power. Operating the OPO at higher repetition rates resulted in a significant drop in average output power and higher average pump power threshold, which can be attributed to the reduced conversion efficiency at higher repetition rates with decreasing pump pulse peak power. The maximum average output power fluctuations at different repetition rates remained below ~2% over a time scale of 5 minutes, which shows much better stability in contrast to Wang’s results in [6]. This is attributed to the less susceptibility of the VBG-incorporated Er:YAG pump source to the retro-reflections from the OPO system. It is worth noting that the overall average output power shows closely linear dependence on the incident pump power, suggesting that there is a considerable scope for further power scaling by simply increasing the incident pump power. However, the average #239886 (C) 2015 OSA


output power for the idler at the repetition rate of 1 kHz remained almost unchanged at higher pump power. This is attributed to the significant back-conversion from signal and idler to the pump in this situation, which will be discussed in detail in the following part. In addition, the output beam propagation factor M2 from the OPO system was measured to be ~1.59 with a moving knife-edge apparatus. There was no evidence of degraded performance resulting from photorefractive or thermal effects. Temporal evolution of the pump, signal and idler laser from the OPO system at room temperature was also investigated. A real time oscilloscope with a bandwidth of 4 GHz (Agilent Technol., DSO9404A) together with a InGaAs photoelectric detector (Thorlabs, DET10C/M) and a InAs detector (~3 ns rise-time at ~3 μm) were simultaneously used to monitor the residual pump pulse train, and the signal or idler pulse train, respectively.

Fig. 3. Pulse temporal profiles of the signal, idler and residual pump for different pump powers of (a) 0.2 W, (b) 1.5 W and (c) 2.5 W, respectively, at the repetition rate of 1 kHz.

Fig. 4. Conversion efficiency versus incident pump power at the repetition rate of 1 kHz.

Figure 3 shows the pump, signal and idler pulse evolution for three different incident pump levels of 0.2 W, 1.5 W and 2.5 W at the repetition frequency of 1 kHz. One can see that for all situations the leading edge of the pump pulse had not been significantly depleted due to the low pump intensity where the signal pulse cannot be resonant without reaching the pump threshold value. In this case, the overall signal and idler intensity was not strong enough to deplete the leading edge of the pump pulse. The signal pulse is characteristic of a steep leading edge while the idler pulse exhibits a smooth rise and fall, which is attributed to the much lower responsivity at the idler wavelength of ~4.3 μm for the InAs photodetector operating at room temperature than at the signal wavelength of ~2.7 μm. #239886 (C) 2015 OSA


After the build-up is achieved, the signal and idler intensity increased significantly and the pump was depleted correspondingly. It can be seen in Fig. 3(a) that the trailing edge of the pump pulse was smoothly depleted at relatively low pump intensity and the residual pump pulse became asymmetric due to the depletion. The overall signal and idler output power increased further with the pump power. It can be seen in Fig. 4 that, at the pump power of around ~1.5 W, the conversion efficiency almost reached its maximum of 37.4%, while in the time domain an obvious small rise after an initial depletion began to appear in the trailing edge of the pump pulse, as shown in Fig. 3(b). This suggests that a portion of signal and idler photons was back-converted into pump photons through the sum frequency generation (SFG), called back-conversion process. With even higher pump power this back-conversion effect can be intensified, which resulted in a sharply increase in the pump temporal profile, as illustrated in Fig. 3(c). At the same time the conversion efficiency stopped growing and even decline slightly at the maximum pump level due to the serious back-conversion effect. It is also worth noting that when the OPO operated at a small incident pump power the signal pulse can be effectively compressed relative to the pump. Along with the pump power increase, the signal pulse broadened due to the back-conversion effect and then remained at a stable level regarding its pulse width. More obvious signal pulse shortening effect was observed at higher repetition rates of 2 kHz and 3 kHz owing to the weaker back-conversion effect.

Fig. 5. Output spectrum of the OPO laser system measured at room temperature.

The output spectrum of the OPO was measured using a calibrated monochromator at a resolution of 0.5 nm. The signal and idler’s spectra recorded at room temperature are shown in Fig. 5. The signal and idler center wavelengths were measured to be 2.67 and 4.31 μm at the maximum pump power with the corresponding FWHM spectral width of about 5 and 6 nm, respectively. Temperature tuning of the PPLN crystal was also implemented in our experiment to achieve a broad tuning range of the signal and idler beam. Figure 6 shows the wavelength-temperature tuning curve for the signal and idler, from which we can see that the signal and idler wavelengths can be continuously tuned from 2.67 to 2.72 μm and 4.17 to 4.31 μm by heating the crystal from 25 to 195 °C, respectively. The measured tuning curve agrees well with that calculated from the Sellmeier equations.

#239886 (C) 2015 OSA


Fig. 6. Temperature tuning curve (circles) of the signal and idler wavelength, compared with theoretical values (solid line) according to the Sellmeier equations.

4. Conclusion In conclusion, we report on a highly efficient and stable mid-infrared singly resonant OPO based on MgO:PPLN. The OPO was pumped by an all-solid-state EO Q-switched Er:YAG laser with wavelength locked and narrowed at 1645 nm by a VBG. The signal and idler wavelengths correspond to the two peak absorption bands of CO2. Lasing characteristics in terms of output power and slope efficiency for the OPO system were investigated at different repetition rates. A maximum overall average output power in excess of 1 W was yielded at a pulse repetition frequency of 2 kHz with a conversion efficiency of 35.5% and slope efficiency of 43.6%. The time evolution of the pump, signal and idler pulses for different pump power levels was also investigated and discussed, including the parametric build-up and back-conversion process. The signal and idler wavelengths were continuously tuned from 2.67 to 2.72 μm and 4.17 to 4.31 μm, respectively, through the temperature control of the MgO:PPLN crystal. Acknowledgments This work is partially supported by the National 973 Program of China (Grant No. 2012CB315701), the National Natural Science Fund Foundation of China (Grant Nos. 61205125 and 61475102).

#239886 (C) 2015 OSA
