High power tunable femtosecond ultraviolet laser source based on an Yb-fiber-laser pumped optical parametric oscillator Chenglin Gu,1 Minglie Hu,*,1 Jintao Fan,1 Youjian Song,1 Bowen Liu,1 Lu Chai,1 Chingyue Wang,1 and Derryck T. Reid2 1
2
Ultrafast Laser Laboratory, Key Laboratory of Opto-electronics Information Technology (Ministry of Education), College of Precision Instruments and Opto-electronics Engineering, Tianjin University, 300072 Tianjin, China Scottish Universities Physics Alliance (SUPA), Department of Physics, School of Engineering and Physical Sciences, Heriot-Watt University, Riccarton, Edinburgh EH14 4AS, UK * [email protected]
Abstract: We report a high average power tunable 51 MHz femtosecond ultraviolet (UV) laser source based on an intra-cavity sum frequency mixing optical parametric oscillator (OPO) pumped by a fiber laser. The UV laser is generated by sum frequency generation (SFG) between the second harmonic of a mode-locked Yb-fiber laser and the signal of the OPO. A non-collinear configuration is used in the SFG to compensate the group velocity mismatch, and to increase the SFG conversion efficiency dramatically. Tunable ultraviolet pulses within the wavelength range from 385 to 400 nm have been produced with a maximum average power of 402 mW and a pulse width of 286 fs at 2 W Yb-fiber laser pump, corresponding to 20.1% near-infrared to UV conversion efficiency at 387 nm. To our knowledge, this is the first demonstration of tunable femtosecond UV pulse generation from a fiber laser pumped OPO, and is also the highest average power tunable UV femtosecond pulses from an OPO. ©2015 Optical Society of America OCIS codes: (190.4970) Parametric oscillators and amplifiers; (190.4360) Nonlinear optics, devices; (190.7110) Ultrafast nonlinear optics; (320.7110) Ultrafast nonlinear optics; (140.3610) Lasers, ultraviolet.
References and links 1.
J. Yin, J. G. Ren, H. Lu, Y. Cao, H. L. Yong, Y. P. Wu, C. Liu, S. K. Liao, F. Zhou, Y. Jiang, X. D. Cai, P. Xu, G. S. Pan, J. J. Jia, Y. M. Huang, H. Yin, J. Y. Wang, Y. A. Chen, C. Z. Peng, and J. W. Pan, “Quantum teleportation and entanglement distribution over 100-kilometre free-space channels,” Nature 488(7410), 185–188 (2012). 2. C. Consani, G. Auböck, F. van Mourik, and M. Chergui, “Ultrafast tryptophan-to-heme electron transfer in myoglobins revealed by UV 2D spectroscopy,” Science 339(6127), 1586–1589 (2013). 3. M. Ghotbi, A. Esteban-Martin, and M. Ebrahim-Zadeh, “Tunable, high-repetition-rate, femtosecond pulse generation in the ultraviolet,” Opt. Lett. 33(4), 345–347 (2008). 4. S. C. Kumar, G. K. Samanta, K. Devi, S. Sanguinetti, and M. Ebrahim-Zadeh, “Single-frequency, high-power, continuous-wave fiber-laser-pumped Ti:sapphire laser,” Appl. Opt. 51(1), 15–20 (2012). 5. J. Limpert, F. Roser, T. Schreiber, and A. Tunnermann, “High-Power Ultrafast Fiber Laser Systems,” IEEE J. Sel. Top. Quantum Electron. 12(2), 233–244 (2006). 6. T. P. Lamour, L. Kornaszewski, J. H. Sun, and D. T. Reid, “Yb:fiber-laser-pumped high-energy picosecond optical parametric oscillator,” Opt. Express 17(16), 14229–14234 (2009). 7. C. L. Gu, M. L. Hu, J. T. Fan, Y. J. Song, B. W. Liu, and C. Y. Wang, “High-power, dual-wavelength femtosecond LiB3O5 optical parametric oscillator pumped by fiber laser,” Opt. Lett. 39(13), 3896–3899 (2014). 8. H. G. Liu, M. L. Hu, B. W. Liu, Y. J. Song, L. Chai, A. M. Zheltikov, and C. Y. Wang, “Compact high-power multiwavelength photonic crystal-fiber-based laser source of femtosecond pulses in the infrared–visible– ultraviolet range,” J. Opt. Soc. Am. B 27(11), 2284–2289 (2010). 9. G. K. Samanta, S. C. Kumar, A. Aadhi, and M. Ebrahim-Zadeh, “Yb-fiber-laser-pumped, high-repetition-rate picosecond optical parametric oscillator tunable in the ultraviolet,” Opt. Express 22(10), 11476–11487 (2014). 10. C. L. Gu, M. L. Hu, L. M. Zhang, J. T. Fan, Y. J. Song, C. Y. Wang, and D. T. Reid, “High average power, widely tunable femtosecond laser source from red to mid-infrared based on an Yb-fiber-laser-pumped optical parametric oscillator,” Opt. Lett. 38(11), 1820–1822 (2013).
#231233 - $15.00 USD © 2015 OSA
Received 22 Dec 2014; revised 22 Feb 2015; accepted 23 Feb 2015; published 27 Feb 2015 9 Mar 2015 | Vol. 23, No. 5 | DOI:10.1364/OE.23.006181 | OPTICS EXPRESS 6181
11. Y. Nabekawa and K. Midorikawa, “Broadband sum frequency mixing using noncollinear angularly dispersed geometry for indirect phase control of sub-20-femtosecond UV pulses,” Opt. Express 11(4), 324–338 (2003). 12. Y. Nabekawa and K. Midorikawa, “Group-delay-dispersion-matched sum-frequency mixing for the indirect phase control of deep ultraviolet pulses in the sub-20-fs regime,” Appl. Phys. B 78(5), 569–581 (2004). 13. R. J. Ellingson and C. L. Tang, “High-power, high-repetition-rate femtosecond pulses tunable in the visible,” Opt. Lett. 18(6), 438–440 (1993). 14. A. Borsutzky, R. Brunger, C. Huang, and R. Wallenstein, “Harmonic and sum-frequency generation of pulsed laser radiation in BBO, LBO and KDP,” Appl. Phys. B 52(1), 55–62 (1991). 15. A. P. Baronavski, H. D. Ladouceur, and J. K. Shaw, “Analysis of cross correlation, phase velocity mismatch and group velocity mismatches in sum-frequency generation,” IEEE J. Quantum Electron. 29(2), 580–589 (1993). 16. X. Q. Zhang, H. He, J. T. Fan, C. L. Gu, X. Yan, M. L. Hu, X. Zhang, X. M. Ren, and C. Y. Wang, “Sum frequency generation in pure zinc-blende GaAs nanowires,” Opt. Express 21(23), 28432–28437 (2013).
1. Introduction Tunable, high-repetition-rate, femtosecond ultraviolet laser sources are of great importance in many applications ranging from quantum optics [1] to biology [2]. A previous example of such a source uses an optical parametric oscillator (OPO) with intra-cavity frequency doubling pumped by the second-harmonic light from a Ti:sapphire laser [3]. However, the relatively high complexity, large size and high cost, in part due to the need for water-cooling and bulk solid-state pump lasers, impose critical limitations [4]. Fortunately, recent developments in fiber lasers offer an alternative to Ti:sapphire lasers because they are compact, robust and can provide a high average power [5]. Fiber laser pumped OPOs with both high average power and high pulse energy have already been demonstrated [6, 7]. External frequency tripling or quadrupling of an Yb-fiber laser in a β-barium borate (BBO) crystal has been demonstrated as a source of femtosecond and picosecond pulses in the ultraviolet (UV) spectral region, but with a fixed wavelength [8]. Recently, a picosecond laser source tunable in the UV based on an Yb-fiber laser with an average power up to 30 mW is reported [9]. A natural progression is therefore to exploit the high power and efficiency provided by Yb-fiber-pumped OPOs to produce a tunable femtosecond ultraviolet source. Previously, we reported a high power femtosecond laser source, widely tunable from the red to the mid-infrared [10]. Now, in this Letter, we extend the wavelength range to the UV region. The technique was based on intra-cavity sum frequency generation (SFG) between the signal pulses in a femtosecond OPO and the second harmonics of its pump laser, providing femtosecond UV pulses tunable from 385 to 400 nm. A multi-grating quasi-phase-matching 5% MgO-doped periodically poled LiNbO3 (MgO:PPLN) crystal was used as the nonlinear crystal of the OPO. Wavelength tuning was achieved in the usual way by adjusting the OPO cavity length and changing the gratings of the MgO:PPLN. In order to obtain efficient UV pulse generation, non-collinear SFG was used to compensate the group-velocity-mismatch between the OPO signal and the second-harmonic pump pulses [11, 12]. We obtained 402 mW UV pulses with a pulse duration of 286 fs centered at 387.5 nm using a 2 W Yb-fiber laser pump, corresponding to an conversion efficiency of 20.1% from near-infrared (NIR) to UV. An alternative way to generate UV from a Yb-fiber laser is using an OPO with intracavity frequency doubling pumped by a second-harmonic green laser [9]. But the disadvantage is that no quasi-phase-matching crystal with a high nonlinear index is suitable for the OPO pumped by a green femtosecond laser due to photorefractive effects. Therefore, it leads to a relatively low NIR to UV conversion efficiency. 2. Experiment The configuration of the intra-cavity SFG femtosecond OPO is shown in Fig. 1. A homemade high power Yb-doped large-mode-area photonics crystal fiber laser operating at a repetition rate of 51 MHz with 2 W output power was used as the pump source for the OPO. Its central wavelength was 1040 nm with a full width at half maximum (FWHM) of 40 nm and duration of 70 fs. The beam diameter of the fiber laser is around 2 mm. L1 and L2 are anti-reflection coated convex lenses with a focal length of 100 mm for second harmonic generation (SHG). L3 and L4 form a telescope for expanding the SHG beam to achieve optimum mode matching with the intra-cavity mode. The OPO cavity is a bifocal ring; M1
#231233 - $15.00 USD © 2015 OSA
Received 22 Dec 2014; revised 22 Feb 2015; accepted 23 Feb 2015; published 27 Feb 2015 9 Mar 2015 | Vol. 23, No. 5 | DOI:10.1364/OE.23.006181 | OPTICS EXPRESS 6182
and M2 are gold-coated concave mirrors with a curvature of 150 mm and focus the pump and signal light into the OPO crystal. M6 is a silver coated concave mirror with the same curvature that focuses the second-harmonic green pump and signal light for SFG. M3, M4 and M5 are dielectric mirrors with 99% reflectivity over 1400-2100 nm. M3 (95% transmission at 520 nm) is used to couple in the second-harmonic pump pulses. M5 is the UV output coupling mirror with 95% transmission over 380-400 nm and 90% transmission at 520 nm for residual SHG green pulse output.
Fig. 1. Experimental setup for tunable UV generation; DM: Dichroic mirror; M1-M6: mirrors; L1- L5: lens; HWP: half-wave plate.
The nonlinear crystal for OPO is a 1 mm long, 8.5 mm wide, 1 mm thick MgO:PPLN crystal (HC Photonics Corp. Taiwan). In our experiment, the crystal was housed in an oven and its temperature was adjustable from room temperature to 200 °C. It was maintained at 80 °C in order to avoid photorefractive effects. The crystal contained seven gratings with periods ranging from 28.5 to 31.5 μm, in steps of 0.5 μm. The crystal used for SHG of the Yb-fiber laser was a 2 mm thick LBO crystal cut at ϕ =90 , θ =12.9 for type-I interaction, and the crystal for UV SFG was a 1 mm thick BBO crystal, cut at θ =25 , also for type-I interaction. Within the OPO cavity, the pump, signal, idler and pump SHG pulses were all horizontally polarized while the UV SFG pulses were vertically polarized. 3. Results and discussion To configure the system we initially inserted a beam splitter (BS) into the OPO as a signal output coupler and optimized the cavity to make it operate with the maximum signal output power, which also corresponded to the maximum intra-cavity signal power. The output coupler, a K9 thin slice, was inserted under a small tilt angle inside the cavity with an output coupling ratio about 4%. Then we removed the BS and inserted the SFG BBO crystal and slightly shortened the cavity length by moving M6 to restart the oscillation. We measured the signal spectral coverage by tuning the cavity length for each of the seven MgO:PPLN gratings. The pump beam diameter inside the PPLN was calculated to be around 100 μm, and the signal beam diameter was 90 μm calculated from the transfer matrix of the cavity. Figure 2(a) shows the signal spectra obtained in this way, with the center wavelength continuously tunable from 1450 to 2050 nm. The filled black circles indicate the power at each wavelength at 1.45 W residual pump power after SHG, and the maximum output power is 270 mW at #231233 - $15.00 USD © 2015 OSA
Received 22 Dec 2014; revised 22 Feb 2015; accepted 23 Feb 2015; published 27 Feb 2015 9 Mar 2015 | Vol. 23, No. 5 | DOI:10.1364/OE.23.006181 | OPTICS EXPRESS 6183
1514 nm. Figure 2(b) shows a typical autocorrelation trace of the signal pulses at 1514 nm, with the pulse duration estimated to be 144 fs.
Fig. 2. (a) Wavelength tuning property and corresponding power of the signal; (b) Typical autocorrelation trace of the signal; (c) SH power as a function of the fiber-laser output power, and the inset shows the beam profile; (d) Spectrum (black curve) and depleted spectrum (red curve) of the green beam.
For the SHG of the Yb-fiber laser, we used a 2 mm thick LBO crystal phase-matched for type-I interaction, which had a relatively large acceptance angle [13]. This provided a relatively symmetric TEM00 spatial mode, enabling good spatial overlap between the SHG mode and the OPO intra-cavity mode. Figure 2(c) shows the second-harmonic power as a function of the fundamental fiber laser power. The inset shows the excellent beam profile of the SH mode. The SH power is not the maximum obtainable output because the residual fundamental power is needed to pump the OPO. We control the SHG efficiency by adjusting the focus distance between the lens L1 and the LBO crystal. The durations of the 520 nm green pulses are estimated to be 200 fs considering the group velocity mismatch (GVM) in the LBO. Figure 2(d) shows the SH spectrum (black curve). We introduced the SH beam into the OPO cavity through M3 to overlap with the intra-cavity signal mode. The delay of the second-harmonic pulses was then adjusted to temporally overlap with the signal pulses in the BBO crystal for optimum UV generation. The diameter of the signal beam inside the BBO was calculated to be around 100 μm. We then adjusted the diameter of the green beam inside the BBO by changing the collimator distance between the lenses L3 and L4, so as to match with the signal beam. BBO is a useful crystal for second order frequency conversion processes in the NIR because of the weak dependence of the phase-matching angle on the wavelength and its small GVM [14]. However, SFG between the green and NIR pulses suffers from strong GVM that makes these pulses walk off after only a small distance, leading to a poor conversion efficiency. In our experiment, we used a non-collinear configuration that effectively compensated the GVM between the green and NIR pulses. Firstly, we generated the UV pulses in a collinear interaction, obtaining a maximum UV output power by adjusting the delay of SH green pulses and optimizing the BBO phase-matching angle. In this situation, the UV power was relatively low due to the substantial temporal walk-off. Due to the large acceptance angle, we could adjust the green light in the horizontal plane lying perpendicular to the optical axis without strongly affecting the existing phase-matching condition. When the
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Received 22 Dec 2014; revised 22 Feb 2015; accepted 23 Feb 2015; published 27 Feb 2015 9 Mar 2015 | Vol. 23, No. 5 | DOI:10.1364/OE.23.006181 | OPTICS EXPRESS 6184
GVM is compensated perfectly, the non-collinear angle α between the SH and the signal is VS = VSH·cosα, where VS and VSH are the group velocities of the signal and the SH pulses in the BBO crystal, respectively.
Fig. 3. (a) Wavelength tuning property of UV; (b) UV power dependence on wavelength, the cross-correlation trace between UV and fiber laser; (c) UV pulse durations and time-bandwidth product (TBP) as a function of wavelength; (d) UV and residual green beam profiles.
In the collinear condition, the two pulses overlap perfectly in the spatial domain but walkoff rapidly in the time domain due to the large GVM. Conversely, the GVM is compensated in the non-collinear condition, and the two pulses overlap perfectly in the time domain but walk-off in the spatial domain. Consequently, there is an optimum non-collinear angle between zero and α, which provides the best combination of temporal and spatial overlaps. In our experiment, the nonlinear angle between the SH green beam and the signal of OPO was around 3°. The spectrum and corresponding power of the UV SFG is shown in Fig. 3. The UV spectral tuning was recorded by signal spectral tuning in each of the seven gratings of MgO:PPLN. As shown in Fig. 3(a), the UV spectrum can be tuned over 385-400 nm with bandwidths varying from 3.0 to 4.3 nm, which is consistent with the behavior of the signal spectrum. The UV spectral tuning range can be expected from 383 nm to 415 nm corresponding to the tuning range of the signal, but the wavelengths longer than 400 nm are beyond the wavelength range of our UV output coupler M5. Figure 3(b) shows the UV output power dependence on the wavelength. The solid line and the dashed line show the noncollinear and collinear cases, respectively. Remarkable UV power increase is obtained using the non-collinear configuration. The total fiber laser input for the OPO and SHG was 2 W. At the maximum UV output power, the OPO pump power was 1.45 W while the SHG power was 550 mW. With the non-collinear setup, the maximum UV output power was 402 mW at 387.5 nm, corresponding to 20.1% NIR to UV conversion efficiency. The corresponding maximum green depletion was 58% as shown in Fig. 2(d). Across the whole tuning range, the UV power consistently exceeded 100 mW. The decrease of the generated UV power with increasing wavelength was mainly due to the decrease of the intra-cavity signal power as described earlier. The durations of the UV pulses were experimentally estimated by cross-correlation measurements through the SFG process in a 100 μm thick BBO crystal cut for type-I phase matching. The cross-correlation trace was recorded by measuring the intensity of the
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Received 22 Dec 2014; revised 22 Feb 2015; accepted 23 Feb 2015; published 27 Feb 2015 9 Mar 2015 | Vol. 23, No. 5 | DOI:10.1364/OE.23.006181 | OPTICS EXPRESS 6185
difference-frequency signal as a function of the delay time between the UV and the reference fiber laser pulse (inset of Fig. 3(b)). In the regime free of GVM, the FWHM τcc of the crosscorrelation trace is given by [15] τccq = τuvq + τrefq with q = 1.615 for hyperbolic secant pulses, where τuv and τref are the FWHM pulse widths of the UV and the reference. The crosscorrelation trace of the UV pulses at 387.5 nm is shown in the inset of Fig. 3(b). The pulse width of the reference pulse τref is estimated as 70 fs in an independent autocorrelation measurement. With τcc measured to be 304 fs, τuv is calculated to be 286 fs, corresponding to a time-bandwidth product of ΔυΔτ = 1.7. The solid line in Fig. 3(c) shows the durations of the UV pulses at different wavelengths; and the dashed line shows the corresponding timebandwidth products. The chirp of the UV pulses is mainly attributed to the GVM between the UV and green pulses. The UV beam profile is shown in Fig. 3(d), which indicates a good transverse mode. The M2 values of the UV beams are around M2x = 1.15 and M2y = 1.12 at 387.5 nm, and hardly vary with the wavelength. The residual green profile is also shown in Fig. 3(d), and the depletion feature of the green beam shows a clear path that the signal beam traveled through. This is a strong evidence of non-collinear SFG process in our experiment. The OPO system is put in a closed box to avoid the environmental disturbance, and stable for hours of operation with 0.5 nm wavelength drifting. A similar system is stable for application [16]. An active feedback stabilization system is going to be set up to further improve our OPO. 4. Conclusion We have demonstrated a new source of femtosecond UV pulses with practical output powers enabled by using a Yb-fiber laser as the OPO pump source. By using non-collinear intracavity SFM between the NIR OPO signal pulses and SH pulses from the Yb-fiber pump laser, we have generated femtosecond pulses tunable from 385 to 400 nm with a power up to 402 mW and a duration of 286 fs using 2W pump power, corresponding to 20.1% NIR to UV conversion efficiency. Such a high power UV femtosecond fiber-laser-based source is promising for a wide variety of applications, such as bio-imaging, micro-spectroscopy, and frequency-comb metrology. Acknowledgments This work was supported by National Basic Research Program of China (Grant Nos. 2011CB808101 and 2010CB327604), National Natural Science Foundation of China (Grant Nos. 61322502, 61227010, 61205131, and 11274239) and Tianjin City S&T Project (Grant No.13RCGFGX01122).
#231233 - $15.00 USD © 2015 OSA
Received 22 Dec 2014; revised 22 Feb 2015; accepted 23 Feb 2015; published 27 Feb 2015 9 Mar 2015 | Vol. 23, No. 5 | DOI:10.1364/OE.23.006181 | OPTICS EXPRESS 6186
2
Ultrafast Laser Laboratory, Key Laboratory of Opto-electronics Information Technology (Ministry of Education), College of Precision Instruments and Opto-electronics Engineering, Tianjin University, 300072 Tianjin, China Scottish Universities Physics Alliance (SUPA), Department of Physics, School of Engineering and Physical Sciences, Heriot-Watt University, Riccarton, Edinburgh EH14 4AS, UK * [email protected]
Abstract: We report a high average power tunable 51 MHz femtosecond ultraviolet (UV) laser source based on an intra-cavity sum frequency mixing optical parametric oscillator (OPO) pumped by a fiber laser. The UV laser is generated by sum frequency generation (SFG) between the second harmonic of a mode-locked Yb-fiber laser and the signal of the OPO. A non-collinear configuration is used in the SFG to compensate the group velocity mismatch, and to increase the SFG conversion efficiency dramatically. Tunable ultraviolet pulses within the wavelength range from 385 to 400 nm have been produced with a maximum average power of 402 mW and a pulse width of 286 fs at 2 W Yb-fiber laser pump, corresponding to 20.1% near-infrared to UV conversion efficiency at 387 nm. To our knowledge, this is the first demonstration of tunable femtosecond UV pulse generation from a fiber laser pumped OPO, and is also the highest average power tunable UV femtosecond pulses from an OPO. ©2015 Optical Society of America OCIS codes: (190.4970) Parametric oscillators and amplifiers; (190.4360) Nonlinear optics, devices; (190.7110) Ultrafast nonlinear optics; (320.7110) Ultrafast nonlinear optics; (140.3610) Lasers, ultraviolet.
References and links 1.
J. Yin, J. G. Ren, H. Lu, Y. Cao, H. L. Yong, Y. P. Wu, C. Liu, S. K. Liao, F. Zhou, Y. Jiang, X. D. Cai, P. Xu, G. S. Pan, J. J. Jia, Y. M. Huang, H. Yin, J. Y. Wang, Y. A. Chen, C. Z. Peng, and J. W. Pan, “Quantum teleportation and entanglement distribution over 100-kilometre free-space channels,” Nature 488(7410), 185–188 (2012). 2. C. Consani, G. Auböck, F. van Mourik, and M. Chergui, “Ultrafast tryptophan-to-heme electron transfer in myoglobins revealed by UV 2D spectroscopy,” Science 339(6127), 1586–1589 (2013). 3. M. Ghotbi, A. Esteban-Martin, and M. Ebrahim-Zadeh, “Tunable, high-repetition-rate, femtosecond pulse generation in the ultraviolet,” Opt. Lett. 33(4), 345–347 (2008). 4. S. C. Kumar, G. K. Samanta, K. Devi, S. Sanguinetti, and M. Ebrahim-Zadeh, “Single-frequency, high-power, continuous-wave fiber-laser-pumped Ti:sapphire laser,” Appl. Opt. 51(1), 15–20 (2012). 5. J. Limpert, F. Roser, T. Schreiber, and A. Tunnermann, “High-Power Ultrafast Fiber Laser Systems,” IEEE J. Sel. Top. Quantum Electron. 12(2), 233–244 (2006). 6. T. P. Lamour, L. Kornaszewski, J. H. Sun, and D. T. Reid, “Yb:fiber-laser-pumped high-energy picosecond optical parametric oscillator,” Opt. Express 17(16), 14229–14234 (2009). 7. C. L. Gu, M. L. Hu, J. T. Fan, Y. J. Song, B. W. Liu, and C. Y. Wang, “High-power, dual-wavelength femtosecond LiB3O5 optical parametric oscillator pumped by fiber laser,” Opt. Lett. 39(13), 3896–3899 (2014). 8. H. G. Liu, M. L. Hu, B. W. Liu, Y. J. Song, L. Chai, A. M. Zheltikov, and C. Y. Wang, “Compact high-power multiwavelength photonic crystal-fiber-based laser source of femtosecond pulses in the infrared–visible– ultraviolet range,” J. Opt. Soc. Am. B 27(11), 2284–2289 (2010). 9. G. K. Samanta, S. C. Kumar, A. Aadhi, and M. Ebrahim-Zadeh, “Yb-fiber-laser-pumped, high-repetition-rate picosecond optical parametric oscillator tunable in the ultraviolet,” Opt. Express 22(10), 11476–11487 (2014). 10. C. L. Gu, M. L. Hu, L. M. Zhang, J. T. Fan, Y. J. Song, C. Y. Wang, and D. T. Reid, “High average power, widely tunable femtosecond laser source from red to mid-infrared based on an Yb-fiber-laser-pumped optical parametric oscillator,” Opt. Lett. 38(11), 1820–1822 (2013).
#231233 - $15.00 USD © 2015 OSA
Received 22 Dec 2014; revised 22 Feb 2015; accepted 23 Feb 2015; published 27 Feb 2015 9 Mar 2015 | Vol. 23, No. 5 | DOI:10.1364/OE.23.006181 | OPTICS EXPRESS 6181
11. Y. Nabekawa and K. Midorikawa, “Broadband sum frequency mixing using noncollinear angularly dispersed geometry for indirect phase control of sub-20-femtosecond UV pulses,” Opt. Express 11(4), 324–338 (2003). 12. Y. Nabekawa and K. Midorikawa, “Group-delay-dispersion-matched sum-frequency mixing for the indirect phase control of deep ultraviolet pulses in the sub-20-fs regime,” Appl. Phys. B 78(5), 569–581 (2004). 13. R. J. Ellingson and C. L. Tang, “High-power, high-repetition-rate femtosecond pulses tunable in the visible,” Opt. Lett. 18(6), 438–440 (1993). 14. A. Borsutzky, R. Brunger, C. Huang, and R. Wallenstein, “Harmonic and sum-frequency generation of pulsed laser radiation in BBO, LBO and KDP,” Appl. Phys. B 52(1), 55–62 (1991). 15. A. P. Baronavski, H. D. Ladouceur, and J. K. Shaw, “Analysis of cross correlation, phase velocity mismatch and group velocity mismatches in sum-frequency generation,” IEEE J. Quantum Electron. 29(2), 580–589 (1993). 16. X. Q. Zhang, H. He, J. T. Fan, C. L. Gu, X. Yan, M. L. Hu, X. Zhang, X. M. Ren, and C. Y. Wang, “Sum frequency generation in pure zinc-blende GaAs nanowires,” Opt. Express 21(23), 28432–28437 (2013).
1. Introduction Tunable, high-repetition-rate, femtosecond ultraviolet laser sources are of great importance in many applications ranging from quantum optics [1] to biology [2]. A previous example of such a source uses an optical parametric oscillator (OPO) with intra-cavity frequency doubling pumped by the second-harmonic light from a Ti:sapphire laser [3]. However, the relatively high complexity, large size and high cost, in part due to the need for water-cooling and bulk solid-state pump lasers, impose critical limitations [4]. Fortunately, recent developments in fiber lasers offer an alternative to Ti:sapphire lasers because they are compact, robust and can provide a high average power [5]. Fiber laser pumped OPOs with both high average power and high pulse energy have already been demonstrated [6, 7]. External frequency tripling or quadrupling of an Yb-fiber laser in a β-barium borate (BBO) crystal has been demonstrated as a source of femtosecond and picosecond pulses in the ultraviolet (UV) spectral region, but with a fixed wavelength [8]. Recently, a picosecond laser source tunable in the UV based on an Yb-fiber laser with an average power up to 30 mW is reported [9]. A natural progression is therefore to exploit the high power and efficiency provided by Yb-fiber-pumped OPOs to produce a tunable femtosecond ultraviolet source. Previously, we reported a high power femtosecond laser source, widely tunable from the red to the mid-infrared [10]. Now, in this Letter, we extend the wavelength range to the UV region. The technique was based on intra-cavity sum frequency generation (SFG) between the signal pulses in a femtosecond OPO and the second harmonics of its pump laser, providing femtosecond UV pulses tunable from 385 to 400 nm. A multi-grating quasi-phase-matching 5% MgO-doped periodically poled LiNbO3 (MgO:PPLN) crystal was used as the nonlinear crystal of the OPO. Wavelength tuning was achieved in the usual way by adjusting the OPO cavity length and changing the gratings of the MgO:PPLN. In order to obtain efficient UV pulse generation, non-collinear SFG was used to compensate the group-velocity-mismatch between the OPO signal and the second-harmonic pump pulses [11, 12]. We obtained 402 mW UV pulses with a pulse duration of 286 fs centered at 387.5 nm using a 2 W Yb-fiber laser pump, corresponding to an conversion efficiency of 20.1% from near-infrared (NIR) to UV. An alternative way to generate UV from a Yb-fiber laser is using an OPO with intracavity frequency doubling pumped by a second-harmonic green laser [9]. But the disadvantage is that no quasi-phase-matching crystal with a high nonlinear index is suitable for the OPO pumped by a green femtosecond laser due to photorefractive effects. Therefore, it leads to a relatively low NIR to UV conversion efficiency. 2. Experiment The configuration of the intra-cavity SFG femtosecond OPO is shown in Fig. 1. A homemade high power Yb-doped large-mode-area photonics crystal fiber laser operating at a repetition rate of 51 MHz with 2 W output power was used as the pump source for the OPO. Its central wavelength was 1040 nm with a full width at half maximum (FWHM) of 40 nm and duration of 70 fs. The beam diameter of the fiber laser is around 2 mm. L1 and L2 are anti-reflection coated convex lenses with a focal length of 100 mm for second harmonic generation (SHG). L3 and L4 form a telescope for expanding the SHG beam to achieve optimum mode matching with the intra-cavity mode. The OPO cavity is a bifocal ring; M1
#231233 - $15.00 USD © 2015 OSA
Received 22 Dec 2014; revised 22 Feb 2015; accepted 23 Feb 2015; published 27 Feb 2015 9 Mar 2015 | Vol. 23, No. 5 | DOI:10.1364/OE.23.006181 | OPTICS EXPRESS 6182
and M2 are gold-coated concave mirrors with a curvature of 150 mm and focus the pump and signal light into the OPO crystal. M6 is a silver coated concave mirror with the same curvature that focuses the second-harmonic green pump and signal light for SFG. M3, M4 and M5 are dielectric mirrors with 99% reflectivity over 1400-2100 nm. M3 (95% transmission at 520 nm) is used to couple in the second-harmonic pump pulses. M5 is the UV output coupling mirror with 95% transmission over 380-400 nm and 90% transmission at 520 nm for residual SHG green pulse output.
Fig. 1. Experimental setup for tunable UV generation; DM: Dichroic mirror; M1-M6: mirrors; L1- L5: lens; HWP: half-wave plate.
The nonlinear crystal for OPO is a 1 mm long, 8.5 mm wide, 1 mm thick MgO:PPLN crystal (HC Photonics Corp. Taiwan). In our experiment, the crystal was housed in an oven and its temperature was adjustable from room temperature to 200 °C. It was maintained at 80 °C in order to avoid photorefractive effects. The crystal contained seven gratings with periods ranging from 28.5 to 31.5 μm, in steps of 0.5 μm. The crystal used for SHG of the Yb-fiber laser was a 2 mm thick LBO crystal cut at ϕ =90 , θ =12.9 for type-I interaction, and the crystal for UV SFG was a 1 mm thick BBO crystal, cut at θ =25 , also for type-I interaction. Within the OPO cavity, the pump, signal, idler and pump SHG pulses were all horizontally polarized while the UV SFG pulses were vertically polarized. 3. Results and discussion To configure the system we initially inserted a beam splitter (BS) into the OPO as a signal output coupler and optimized the cavity to make it operate with the maximum signal output power, which also corresponded to the maximum intra-cavity signal power. The output coupler, a K9 thin slice, was inserted under a small tilt angle inside the cavity with an output coupling ratio about 4%. Then we removed the BS and inserted the SFG BBO crystal and slightly shortened the cavity length by moving M6 to restart the oscillation. We measured the signal spectral coverage by tuning the cavity length for each of the seven MgO:PPLN gratings. The pump beam diameter inside the PPLN was calculated to be around 100 μm, and the signal beam diameter was 90 μm calculated from the transfer matrix of the cavity. Figure 2(a) shows the signal spectra obtained in this way, with the center wavelength continuously tunable from 1450 to 2050 nm. The filled black circles indicate the power at each wavelength at 1.45 W residual pump power after SHG, and the maximum output power is 270 mW at #231233 - $15.00 USD © 2015 OSA
Received 22 Dec 2014; revised 22 Feb 2015; accepted 23 Feb 2015; published 27 Feb 2015 9 Mar 2015 | Vol. 23, No. 5 | DOI:10.1364/OE.23.006181 | OPTICS EXPRESS 6183
1514 nm. Figure 2(b) shows a typical autocorrelation trace of the signal pulses at 1514 nm, with the pulse duration estimated to be 144 fs.
Fig. 2. (a) Wavelength tuning property and corresponding power of the signal; (b) Typical autocorrelation trace of the signal; (c) SH power as a function of the fiber-laser output power, and the inset shows the beam profile; (d) Spectrum (black curve) and depleted spectrum (red curve) of the green beam.
For the SHG of the Yb-fiber laser, we used a 2 mm thick LBO crystal phase-matched for type-I interaction, which had a relatively large acceptance angle [13]. This provided a relatively symmetric TEM00 spatial mode, enabling good spatial overlap between the SHG mode and the OPO intra-cavity mode. Figure 2(c) shows the second-harmonic power as a function of the fundamental fiber laser power. The inset shows the excellent beam profile of the SH mode. The SH power is not the maximum obtainable output because the residual fundamental power is needed to pump the OPO. We control the SHG efficiency by adjusting the focus distance between the lens L1 and the LBO crystal. The durations of the 520 nm green pulses are estimated to be 200 fs considering the group velocity mismatch (GVM) in the LBO. Figure 2(d) shows the SH spectrum (black curve). We introduced the SH beam into the OPO cavity through M3 to overlap with the intra-cavity signal mode. The delay of the second-harmonic pulses was then adjusted to temporally overlap with the signal pulses in the BBO crystal for optimum UV generation. The diameter of the signal beam inside the BBO was calculated to be around 100 μm. We then adjusted the diameter of the green beam inside the BBO by changing the collimator distance between the lenses L3 and L4, so as to match with the signal beam. BBO is a useful crystal for second order frequency conversion processes in the NIR because of the weak dependence of the phase-matching angle on the wavelength and its small GVM [14]. However, SFG between the green and NIR pulses suffers from strong GVM that makes these pulses walk off after only a small distance, leading to a poor conversion efficiency. In our experiment, we used a non-collinear configuration that effectively compensated the GVM between the green and NIR pulses. Firstly, we generated the UV pulses in a collinear interaction, obtaining a maximum UV output power by adjusting the delay of SH green pulses and optimizing the BBO phase-matching angle. In this situation, the UV power was relatively low due to the substantial temporal walk-off. Due to the large acceptance angle, we could adjust the green light in the horizontal plane lying perpendicular to the optical axis without strongly affecting the existing phase-matching condition. When the
#231233 - $15.00 USD © 2015 OSA
Received 22 Dec 2014; revised 22 Feb 2015; accepted 23 Feb 2015; published 27 Feb 2015 9 Mar 2015 | Vol. 23, No. 5 | DOI:10.1364/OE.23.006181 | OPTICS EXPRESS 6184
GVM is compensated perfectly, the non-collinear angle α between the SH and the signal is VS = VSH·cosα, where VS and VSH are the group velocities of the signal and the SH pulses in the BBO crystal, respectively.
Fig. 3. (a) Wavelength tuning property of UV; (b) UV power dependence on wavelength, the cross-correlation trace between UV and fiber laser; (c) UV pulse durations and time-bandwidth product (TBP) as a function of wavelength; (d) UV and residual green beam profiles.
In the collinear condition, the two pulses overlap perfectly in the spatial domain but walkoff rapidly in the time domain due to the large GVM. Conversely, the GVM is compensated in the non-collinear condition, and the two pulses overlap perfectly in the time domain but walk-off in the spatial domain. Consequently, there is an optimum non-collinear angle between zero and α, which provides the best combination of temporal and spatial overlaps. In our experiment, the nonlinear angle between the SH green beam and the signal of OPO was around 3°. The spectrum and corresponding power of the UV SFG is shown in Fig. 3. The UV spectral tuning was recorded by signal spectral tuning in each of the seven gratings of MgO:PPLN. As shown in Fig. 3(a), the UV spectrum can be tuned over 385-400 nm with bandwidths varying from 3.0 to 4.3 nm, which is consistent with the behavior of the signal spectrum. The UV spectral tuning range can be expected from 383 nm to 415 nm corresponding to the tuning range of the signal, but the wavelengths longer than 400 nm are beyond the wavelength range of our UV output coupler M5. Figure 3(b) shows the UV output power dependence on the wavelength. The solid line and the dashed line show the noncollinear and collinear cases, respectively. Remarkable UV power increase is obtained using the non-collinear configuration. The total fiber laser input for the OPO and SHG was 2 W. At the maximum UV output power, the OPO pump power was 1.45 W while the SHG power was 550 mW. With the non-collinear setup, the maximum UV output power was 402 mW at 387.5 nm, corresponding to 20.1% NIR to UV conversion efficiency. The corresponding maximum green depletion was 58% as shown in Fig. 2(d). Across the whole tuning range, the UV power consistently exceeded 100 mW. The decrease of the generated UV power with increasing wavelength was mainly due to the decrease of the intra-cavity signal power as described earlier. The durations of the UV pulses were experimentally estimated by cross-correlation measurements through the SFG process in a 100 μm thick BBO crystal cut for type-I phase matching. The cross-correlation trace was recorded by measuring the intensity of the
#231233 - $15.00 USD © 2015 OSA
Received 22 Dec 2014; revised 22 Feb 2015; accepted 23 Feb 2015; published 27 Feb 2015 9 Mar 2015 | Vol. 23, No. 5 | DOI:10.1364/OE.23.006181 | OPTICS EXPRESS 6185
difference-frequency signal as a function of the delay time between the UV and the reference fiber laser pulse (inset of Fig. 3(b)). In the regime free of GVM, the FWHM τcc of the crosscorrelation trace is given by [15] τccq = τuvq + τrefq with q = 1.615 for hyperbolic secant pulses, where τuv and τref are the FWHM pulse widths of the UV and the reference. The crosscorrelation trace of the UV pulses at 387.5 nm is shown in the inset of Fig. 3(b). The pulse width of the reference pulse τref is estimated as 70 fs in an independent autocorrelation measurement. With τcc measured to be 304 fs, τuv is calculated to be 286 fs, corresponding to a time-bandwidth product of ΔυΔτ = 1.7. The solid line in Fig. 3(c) shows the durations of the UV pulses at different wavelengths; and the dashed line shows the corresponding timebandwidth products. The chirp of the UV pulses is mainly attributed to the GVM between the UV and green pulses. The UV beam profile is shown in Fig. 3(d), which indicates a good transverse mode. The M2 values of the UV beams are around M2x = 1.15 and M2y = 1.12 at 387.5 nm, and hardly vary with the wavelength. The residual green profile is also shown in Fig. 3(d), and the depletion feature of the green beam shows a clear path that the signal beam traveled through. This is a strong evidence of non-collinear SFG process in our experiment. The OPO system is put in a closed box to avoid the environmental disturbance, and stable for hours of operation with 0.5 nm wavelength drifting. A similar system is stable for application [16]. An active feedback stabilization system is going to be set up to further improve our OPO. 4. Conclusion We have demonstrated a new source of femtosecond UV pulses with practical output powers enabled by using a Yb-fiber laser as the OPO pump source. By using non-collinear intracavity SFM between the NIR OPO signal pulses and SH pulses from the Yb-fiber pump laser, we have generated femtosecond pulses tunable from 385 to 400 nm with a power up to 402 mW and a duration of 286 fs using 2W pump power, corresponding to 20.1% NIR to UV conversion efficiency. Such a high power UV femtosecond fiber-laser-based source is promising for a wide variety of applications, such as bio-imaging, micro-spectroscopy, and frequency-comb metrology. Acknowledgments This work was supported by National Basic Research Program of China (Grant Nos. 2011CB808101 and 2010CB327604), National Natural Science Foundation of China (Grant Nos. 61322502, 61227010, 61205131, and 11274239) and Tianjin City S&T Project (Grant No.13RCGFGX01122).
#231233 - $15.00 USD © 2015 OSA
Received 22 Dec 2014; revised 22 Feb 2015; accepted 23 Feb 2015; published 27 Feb 2015 9 Mar 2015 | Vol. 23, No. 5 | DOI:10.1364/OE.23.006181 | OPTICS EXPRESS 6186
